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diff --git a/.gitattributes b/.gitattributes new file mode 100644 index 0000000..6833f05 --- /dev/null +++ b/.gitattributes @@ -0,0 +1,3 @@ +* text=auto +*.txt text +*.md text diff --git a/3620-0.txt b/3620-0.txt new file mode 100644 index 0000000..9322559 --- /dev/null +++ b/3620-0.txt @@ -0,0 +1,14234 @@ +The Project Gutenberg eBook of Geological Observations on South America, by Charles Darwin + +This eBook is for the use of anyone anywhere in the United States and +most other parts of the world at no cost and with almost no restrictions +whatsoever. You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online at +www.gutenberg.org. If you are not located in the United States, you +will have to check the laws of the country where you are located before +using this eBook. + +Title: Geological Observations on South America + +Author: Charles Darwin + +Release Date: June 17, 2001 [eBook #3620] +[Most recently updated: December 12, 2021] + +Language: English + +Character set encoding: UTF-8 + + +*** START OF THE PROJECT GUTENBERG EBOOK GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA *** + + + + +Geological Observations on South America + +By Charles Darwin + + +CONTENTS + + EDITORIAL NOTE. + DETAILED TABLE OF CONTENTS. + GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA + CRITICAL INTRODUCTION. + CHAPTER I. ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA. + CHAPTER II. ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA. + CHAPTER III. ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS. + CHAPTER IV. ON THE FORMATIONS OF THE PAMPAS. + CHAPTER V. ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE. + CHAPTER VI. PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION. + CHAPTER VII. CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA. + CHAPTER VIII. NORTHERN CHILE.—CONCLUSION. + INDEX + + + + +EDITORIAL NOTE. + + +Although in some respects more technical in their subjects and style +than Darwin’s “Journal,” the books here reprinted will never lose their +value and interest for the originality of the observations they +contain. Many parts of them are admirably adapted for giving an insight +into problems regarding the structure and changes of the earth’s +surface, and in fact they form a charming introduction to physical +geology and physiography in their application to special domains. The +books themselves cannot be obtained for many times the price of the +present volume, and both the general reader, who desires to know more +of Darwin’s work, and the student of geology, who naturally wishes to +know how a master mind reasoned on most important geological subjects, +will be glad of the opportunity of possessing them in a convenient and +cheap form. + +The three introductions, which my friend Professor Judd has kindly +furnished, give critical and historical information which makes this +edition of special value. + +G.T.B. + + + + +DETAILED TABLE OF CONTENTS. + + +CRITICAL INTRODUCTION. + +CHAPTER I. +ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA. +Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and +Pumice-pebbles.—Step-formed plains of Patagonia, with upraised +shells.—Terrace-bounded valley of Santa Cruz, formerly a +sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth of +the elevated area.—Equability of the movements, as shown by the similar +heights of the plains.—Slowness of the elevatory process.—Mode of +formation of the step-formed plains.—Summary.- -Great shingle formation +of Patagonia; its extent, origin, and distribution.—Formation of +sea-cliffs. + +CHAPTER II. +ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA. +Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions +of the inhabitants on this subject.—Concepcion, earthquake and +elevation of.—VALPARAISO, great elevation of, upraised shells, earth or +marine origin, gradual rise of the land within the historical +period.—COQUIMBO, elevation of, in recent times; terraces of marine +origin, their inclination, their escarpments not horizontal.—Guasco, +gravel terraces of.—Copiapo.—PERU.— Upraised shells of Cobija, Iquique, +and Arica.—Lima, shell-beds and sea- beach on San Lorenzo.—Human +remains, fossil earthenware, earthquake debacle, recent subsidence.—On +the decay of upraised shells.—General summary. + +CHAPTER III. +ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS. +Basin-like plains of Chile; their drainage, their marine origin.—Marks +of sea-action on the eastern flanks of the Cordillera.—Sloping +terrace-like fringes of stratified shingle within the valleys of the +Cordillera; their marine origin.—Boulders in the valley of +Cachapual.—Horizontal elevation of the Cordillera.—Formation of +valleys.—Boulders moved by earthquake- waves.—Saline superficial +deposits.—Bed of nitrate of soda at Iquique.— Saline +incrustations.—Salt-lakes of La Plata and Patagonia; purity of the +salt; its origin. + +CHAPTER IV. +ON THE FORMATIONS OF THE PAMPAS. +Mineralogical constitution.—Microscopical structure.—Buenos Ayres, +shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—S. +Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta, +shells, bones, and infusoria of; co-existence of the recent shells and +extinct mammifers.— Buenos Ayres to St. Fe.—Skeletons of +Mastodon.—Infusoria.—Inferior marine tertiary strata, their +age.—Horse’s tooth. BANDA ORIENTAL.— Superficial Pampean +formation.—Inferior tertiary strata, variation of, connected with +volcanic action; Macrauchenia Patachonica at S. Julian in Patagonia, +age of, subsequent to living mollusca and to the erratic block period. +SUMMARY.—Area of Pampean formation.—Theories of origin.—Source of +sediment.—Estuary origin.—Contemporaneous with existing mollusca.— +Relations to underlying tertiary strata. Ancient deposit of estuary +origin.—Elevation and successive deposition of the Pampean formation.— +Number and state of the remains of mammifers; their habitation, food, +extinction, and range.—Conclusion.—Supplement on the thickness of the +Pampean formation.—Localities in Pampas at which mammiferous remains +have been found. + +CHAPTER V. +ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE. +Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with +infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P. Gallegos.— +Eastern Tierra del Fuego; leaves of extinct beech-trees.—Summary on the +Patagonian tertiary formations.—Tertiary formations of the Western +Coast.—Chonos and Chiloe groups, volcanic rocks +of.—Concepcion.—Navidad.— Coquimbo.—Summary.—Age of the tertiary +formations.—Lines of elevation.— Silicified wood.—Comparative ranges of +the extinct and living mollusca on the West Coast of S. +America.—Climate of the tertiary period.—On the causes of the absence +of recent conchiferous deposits on the coasts of South America.—On the +contemporaneous deposition and preservation of sedimentary formations. + +CHAPTER VI. +PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION. +Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of +foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in, +decomposition of.—La Plata, metamorphic and old volcanic rocks of.—S. +Ventana.—Claystone porphyry formation of Patagonia; singular +metamorphic rocks; pseudo-dikes.—Falkland Islands, palaeozoic fossils +of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of; +cleavage and foliation; form of land.—Chonos Archipelago, mica-schists, +foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes, +successive formation of.—Central and Northern Chile.—Concluding remarks +on cleavage and foliation.—Their close analogy and similar +origin.—Stratification of metamorphic schists.—Foliation of intrusive +rocks.—Relation of cleavage and foliation to the lines of tension +during metamorphosis. + +CHAPTER VII. +CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA. +Central Chile.—Basal formations of the Cordillera.—Origin of the +porphyritic clay-stone conglomerate.—Andesite.—Volcanic rocks.—Section +of the Cordillera by the Peuquenes or Portillo Pass.—Great gypseous +formation.—Peuquenes line; thickness of strata, fossils of.—Portillo +line.—Conglomerate, orthitic granite, mica-schist, volcanic rocks of.— +Concluding remarks on the denudation and elevation of the Portillo +line.— Section by the Cumbre, or Uspallata Pass.—Porphyries.—Gypseous +strata.— Section near the Puente del Inca; fossils of.—Great +subsidence.—Intrusive porphyries.—Plain of Uspallata.—Section of the +Uspallata chain.— Structure and nature of the strata.—Silicified +vertical trees.—Great subsidence.—Granitic rocks of axis.—Concluding +remarks on the Uspallata range; origin subsequent to that of the main +Cordillera; two periods of subsidence; comparison with the Portillo +chain. + +CHAPTER VIII. +NORTHERN CHILE.—CONCLUSION. +A Section from Illapel to Combarbala; gypseous formation with +silicified wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up +valley; fossils.—Guasco, fossils of.—Copiapo, section up valley; Las +Amolanas, silicified wood.—Conglomerates, nature of former land, +fossils, thickness of strata, great subsidence.—Valley of Despoblado, +fossils, tufaceous deposit, complicated dislocations of.—Relations +between ancient orifices of eruption and subsequent axes of +injection.—Iquique, Peru, fossils of, salt-deposits.—Metalliferous +veins.—Summary on the porphyritic conglomerate and gypseous +formations.—Great subsidence with partial elevations during the +cretaceo-oolitic period.—On the elevation and structure of the +Cordillera.—Recapitulation on the tertiary series.— Relation between +movements of subsidence and volcanic action.—Pampean formation.—Recent +elevatory movements.—Long-continued volcanic action in the +Cordillera.—Conclusion. + + + + +GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA + + + + +CRITICAL INTRODUCTION. + + +Of the remarkable “trilogy” constituted by Darwin’s writings which deal +with the geology of the “Beagle,” the member which has perhaps +attracted least attention, up to the present time is that which treats +of the geology of South America. The actual writing of this book +appears to have occupied Darwin a shorter period than either of the +other volumes of the series; his diary records that the work was +accomplished within ten months, namely, between July 1844 and April +1845; but the book was not actually issued till late in the year +following, the preface bearing the date “September 1846.” Altogether, +as Darwin informs us in his “Autobiography,” the geological books +“consumed four and a half years’ steady work,” most of the remainder of +the ten years that elapsed between the return of the “Beagle,” and the +completion of his geological books being, it is sad to relate, “lost +through illness!” + +Concerning the “Geological Observations on South America,” Darwin wrote +to his friend Lyell, as follows:—“My volume will be about 240 pages, +dreadfully dull, yet much condensed. I think whenever you have time to +look through it, you will think the collection of facts on the +elevation of the land and on the formation of terraces pretty good.” + +“Much condensed” is the verdict that everyone must endorse, on rising +from the perusal of this remarkable book; but by no means “dull.” The +three and a half years from April 1832 to September 1835, were spent by +Darwin in South America, and were devoted to continuous scientific +work; the problems he dealt with were either purely geological or those +which constitute the borderland between the geological and biological +sciences. It is impossible to read the journal which he kept during +this time without being impressed by the conviction that it contains +all the germs of thought which afterwards developed into the “Origin of +Species.” But it is equally evident that after his return to England, +biological speculations gradually began to exercise a more exclusive +sway over Darwin’s mind, and tended to dispossess geology, which during +the actual period of the voyage certainly engrossed most of his time +and attention. The wonderful series of observations made during those +three and a half years in South America could scarcely be done justice +to, in the 240 pages devoted to their exposition. That he executed the +work of preparing the book on South America in somewhat the manner of a +task, is shown by many references in his letters. Writing to Sir Joseph +Hooker in 1845, he says, “I hope this next summer to finish my South +American Geology, then to get out a little Zoology, and HURRAH FOR MY +SPECIES WORK!” + +It would seem that the feeling of disappointment, which Darwin so often +experienced in comparing a book when completed, with the observations +and speculations which had inspired it, was more keenly felt in the +case of his volume on South America than any other. To one friend he +writes, “I have of late been slaving extra hard, to the great +discomfiture of wretched digestive organs, at South America, and thank +all the fates, I have done three-fourths of it. Writing plain English +grows with me more and more difficult, and never attainable. As for +your pretending that you will read anything so dull as my pure +geological descriptions, lay not such a flattering unction on my soul, +for it is incredible.” To another friend he writes, “You do not know +what you threaten when you propose to read it—it is purely geological. +I said to my brother, ‘You will of course read it,’ and his answer was, +‘Upon my life, I would sooner even buy it.’” + +In spite of these disparaging remarks, however, we are strongly +inclined to believe that this book, despised by its author, and +neglected by his contemporaries, will in the end be admitted to be one +of Darwin’s chief titles to fame. It is, perhaps, an unfortunate +circumstance that the great success which he attained in biology by the +publication of the “Origin of Species” has, to some extent, +overshadowed the fact that Darwin’s claims as a geologist, are of the +very highest order. It is not too much to say that, had Darwin not been +a geologist, the “Origin of Species” could never have been written by +him. But apart from those geological questions, which have an important +bearing on biological thought and speculation, such as the proofs of +imperfection in the geological record, the relations of the later +tertiary faunas to the recent ones in the same areas, and the apparent +intermingling of types belonging to distant geological epochs, when we +study the palaeontology of remote districts,—there are other purely +geological problems, upon which the contributions made by Darwin are of +the very highest value. I believe that the verdict of the historians of +science will be that if Darwin had not taken a foremost place among the +biologists of this century, his position as a geologist would have been +an almost equally commanding one. + +But in the case of Darwin’s principal geological work—that relating to +the origin of the crystalline schists,—geologists were not at the time +prepared to receive his revolutionary teachings. The influence of +powerful authority was long exercised, indeed, to stifle his teaching, +and only now, when this unfortunate opposition has disappeared, is the +true nature and importance of Darwin’s purely geological work beginning +to be recognised. + +The two first chapters of the “Geological Observations on South +America,” deal with the proofs which exist of great, but frequently +interrupted, movements of elevation during very recent geological +times. In connection with this subject, Darwin’s particular attention +was directed to the relations between the great earthquakes of South +America—of some of which he had impressive experience—and the permanent +changes of elevation which were taking place. He was much struck by the +rapidity with which the evidence of such great earth movements is +frequently obliterated; and especially with the remarkable way in which +the action of rain-water, percolating through deposits on the earth’s +surface, removes all traces of shells and other calcareous organisms. +It was these considerations which were the parents of the +generalisation that a palaeontological record can only be preserved +during those periods in which long-continued slow subsidence is going +on. This in turn, led to the still wider and more suggestive conclusion +that the geological record as a whole is, and never can be more than, a +series of more or less isolated fragments. The recognition of this +important fact constitutes the keystone to any theory of evolution +which seeks to find a basis in the actual study of the types of life +that have formerly inhabited our globe. + +In his third chapter, Darwin gives a number of interesting facts, +collected during his visits to the plains and valleys of Chili, which +bear on the question of the origin of saliferous deposits—the +accumulation of salt, gypsum, and nitrate of soda. This is a problem +that has excited much discussion among geologists, and which, in spite +of many valuable observations, still remains to a great extent very +obscure. Among the important considerations insisted upon by Darwin is +that relating to the absence of marine shells in beds associated with +such deposits. He justly argues that if the strata were formed in +shallow waters, and then exposed by upheaval to subaerial action, all +shells and other calcareous organisms would be removed by solution. + +Following Lyell’s method, Darwin proceeds from the study of deposits +now being accumulated on the earth’s surface, to those which have been +formed during the more recent periods of the geological history. + +His account of the great Pampean formation, with its wonderful +mammalian remains—Mastodon, Toxodon, Scelidotherium, Macrauchenia, +Megatherium, Megalonyx, Mylodon, and Glyptodon—this full of interest. +His discovery of the remains of a true Equus afforded a remarkable +confirmation of the fact- -already made out in North America—that +species of horse had existed and become extinct in the New World, +before their introduction by the Spaniards in the sixteenth century. +Fully perceiving the importance of the microscope in studying the +nature and origin of such deposits as those of the Pampas, Darwin +submitted many of his specimens both to Dr. Carpenter in this country, +and to Professor Ehrenberg in Berlin. Many very important notes on the +microscopic organisms contained in the formation will be found +scattered through the chapter. + +Darwin’s study of the older tertiary formations, with their abundant +shells, and their relics of vegetable life buried under great sheets of +basalt, led him to consider carefully the question of climate during +these earlier periods. In opposition to prevalent views on this +subject, Darwin points out that his observations are opposed to the +conclusion that a higher temperature prevailed universally over the +globe during early geological periods. He argues that “the causes which +gave to the older tertiary productions of the quite temperate zones of +Europe a tropical character, WERE OF A LOCAL CHARACTER AND DID NOT +AFFECT THE WHOLE GLOBE.” In this, as in many similar instances, we see +the beneficial influence of extensive travel in freeing Darwin’s mind +from prevailing prejudices. It was this widening of experience which +rendered him so especially qualified to deal with the great problem of +the origin of species, and in doing so to emancipate himself from ideas +which were received with unquestioning faith by geologists whose +studies had been circumscribed within the limits of Western Europe. + +In the Cordilleras of Northern and Central Chili, Darwin, when studying +still older formations, clearly recognised that they contain an +admixture of the forms of life, which in Europe are distinctive of the +Cretaceous and Jurassic periods respectively. He was thus led to +conclude that the classification of geological periods, which fairly +well expresses the facts that had been discovered in the areas where +the science was first studied, is no longer capable of being applied +when we come to the study of widely distant regions. This important +conclusion led up to the further generalisation that each great +geological period has exhibited a geographical distribution of the +forms of animal and vegetable life, comparable to that which prevails +in the existing fauna and flora. To those who are familiar with the +extent to which the doctrine of universal formations has affected +geological thought and speculation, both long before and since the time +that Darwin wrote, the importance of this new standpoint to which he +was able to attain will be sufficiently apparent. Like the idea of the +extreme imperfection of the Geological Record, the doctrine of LOCAL +geological formations is found permeating and moulding all the +palaeontological reasonings of his great work. + +In one of Darwin’s letters, written while he was in South America, +there is a passage we have already quoted, in which he expresses his +inability to decide between the rival claims upon his attention of “the +old crystalline group of rocks,” and “the softer fossiliferous beds” +respectively. The sixth chapter of the work before us, entitled +“Plutonic and Metamorphic Rocks—Cleavage and Foliation,” contains a +brief summary of a series of observations and reasonings upon these +crystalline rocks, which are, we believe, calculated to effect a +revolution in geological science, and— though their value and +importance have long been overlooked—are likely to entitle Darwin in +the future to a position among geologists, scarcely, if at all, +inferior to that which he already occupies among biologists. + +Darwin’s studies of the great rock-masses of the Andes convinced him of +the close relations between the granitic or Plutonic rocks, and those +which were undoubtedly poured forth as lavas. Upon his return, he set +to work, with the aid of Professor Miller, to make a careful study of +the minerals composing the granites and those which occur in the lavas, +and he was able to show that in all essential respects they are +identical. He was further able to prove that there is a complete +gradation between the highly crystalline or granitic rock-masses, and +those containing more or less glassy matter between their crystals, +which constitute ordinary lavas. The importance of this conclusion will +be realised when we remember that it was then the common creed of +geologists—and still continues to be so on the Continent—that all +highly crystalline rocks are of great geological antiquity, and that +the igneous ejections which have taken place since the beginning of the +tertiary periods differ essentially, in their composition, their +structure, and their mode of occurrence, from those which have made +their appearance at earlier periods of the world’s history. + +Very completely have the conclusions of Darwin upon these subjects been +justified by recent researches. In England, the United States, and +Italy, examples of the gradual passage of rocks of truly granitic +structure into ordinary lavas have been described, and the reality of +the transition has been demonstrated by the most careful studies with +the microscope. Recent researches carried on in South America by +Professor Stelzner, have also shown the existence of a class of highly +crystalline rocks—the “Andengranites”—which combine in themselves many +of the characteristics which were once thought to be distinctive of the +so-called Plutonic and volcanic rocks. No one familiar with recent +geological literature—even in Germany and France, where the old views +concerning the distinction of igneous products of different ages have +been most stoutly maintained—can fail to recognise the fact that the +principles contended for by Darwin bid fair at no distant period to win +universal acceptance among geologists all over the globe. + +Still more important are the conclusions at which Darwin arrived with +respect to the origin of the schists and gneisses which cover so large +an area in South America. + +Carefully noting, by the aid of his compass and clinometer, at every +point which he visited, the direction and amount of inclination of the +parallel divisions in these rocks, he was led to a very important +generalisation— namely, that over very wide areas the direction +(strike) of the planes of cleavage in slates, and of foliation in +schists and gneisses, remained constant, though the amount of their +inclination (dip) often varied within wide limits. Further than this it +appeared that there was always a close correspondence between the +strike of the cleavage and foliation and the direction of the great +axes along which elevation had taken place in the district. + +In Tierra del Fuego, Darwin found striking evidence that the cleavage +intersecting great masses of slate-rocks was quite independent of their +original stratification, and could often, indeed, be seen cutting +across it at right angles. He was also able to verify Sedgwick’s +observation that, in some slates, glossy surfaces on the planes of +cleavage arise from the development of new minerals, chlorite, epidote +or mica, and that in this way a complete graduation from slates to true +schists may be traced. + +Darwin further showed that in highly schistose rocks, the folia bend +around and encircle any foreign bodies in the mass, and that in some +cases they exhibit the most tortuous forms and complicated puckerings. +He clearly saw that in all cases the forces by which these striking +phenomena must have been produced were persistent over wide areas, and +were connected with the great movements by which the rocks had been +upheaved and folded. + +That the distinct folia of quartz, feldspar, mica, and other minerals +composing the metamorphic schists could not have been separately +deposited as sediment was strongly insisted upon by Darwin; and in +doing so he opposed the view generally prevalent among geologists at +that time. He was thus driven to the conclusion that foliation, like +cleavage, is not an original, but a superinduced structure in +rock-masses, and that it is the result of re-crystallisation, under the +controlling influence of great pressure, of the materials of which the +rock was composed. + +In studying the lavas of Ascension, as we have already seen, Darwin was +led to recognise the circumstance that, when igneous rocks are +subjected to great differential movements during the period of their +consolidation, they acquire a foliated structure, closely analogous to +that of the crystalline schists. Like his predecessor in this field of +inquiry, Mr. Poulett Scrope, Charles Darwin seems to have been greatly +impressed by these facts, and he argued from them that the rocks +exhibiting the foliated structure must have been in a state of +plasticity, like that of a cooling mass of lava. At that time the +suggestive experiments of Tresca, Daubree, and others, showing that +solid masses under the influence of enormous pressure become actually +plastic, had not been published. Had Darwin been aware of these facts +he would have seen that it was not necessary to assume a state of +imperfect solidity in rock-masses in order to account for their having +yielded to pressure and tension, and, in doing so, acquiring the new +characters which distinguish the crystalline schists. + +The views put forward by Darwin on the origin of the crystalline +schists found an able advocate in Mr. Daniel Sharpe, who in 1852 and +1854 published two papers, dealing with the geology of the Scottish +Highlands and of the Alps respectively, in which he showed that the +principles arrived at by Darwin when studying the South American rocks +afford a complete explanation of the structure of the two districts in +question. + +But, on the other hand, the conclusions of Darwin and Sharpe were met +with the strongest opposition by Sir Roderick Murchison and Dr. A. +Geikie, who in 1861 read a paper before the Geological Society “On the +Coincidence between Stratification and Foliation in the Crystalline +Rocks of the Scottish Highlands,” in which they insisted that their +observations in Scotland tended to entirely disprove the conclusions of +Darwin that foliation in rocks is a secondary structure, and entirely +independent of the original stratification of the rock-masses. + +Now it is a most significant circumstance that, no sooner did the +officers of the Geological Survey commence the careful and detailed +study of the Scottish Highlands than they found themselves compelled to +make a formal retraction of the views which had been put forward by +Murchison and Geikie in opposition to the conclusions of Darwin. The +officers of the Geological Survey have completely abandoned the view +that the foliation of the Highland rocks has been determined by their +original stratification, and admit that the structure is the result of +the profound movements to which the rocks have been subjected. The same +conclusions have recently been supported by observations made in many +different districts—among which we may especially refer to those of Dr. +H. Reusch in Norway, and those of Dr. J. Lehmann in Saxony. At the +present time the arguments so clearly stated by Darwin in the work +before us, have, after enduring opposition or neglect for a whole +generation, begun to “triumph all along the line,” and we may look +forward confidently to the near future, when his claim to be regarded +as one of the greatest of geological discoverers shall be fully +vindicated. + +JOHN W. JUDD. + + + + +CHAPTER I. +ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA. + + +Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and +Pumice-pebbles.—Step-formed plains of Patagonia, with upraised +Shells.—Terrace-bounded Valley of Santa Cruz, formerly a +Sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth of +the elevated area.—Equability of the movements, as shown by the similar +heights of the plains.—Slowness of the elevatory process.—Mode of +formation of the step-formed plains.—Summary.—Great Shingle Formation +of Patagonia; its extent, origin, and distribution.—Formation of +sea-cliffs. + + +In the following Volume, which treats of the geology of South America, +and almost exclusively of the parts southward of the Tropic of +Capricorn, I have arranged the chapters according to the age of the +deposits, occasionally departing from this order, for the sake of +geographical simplicity. + +The elevation of the land within the recent period, and the +modifications of its surface through the action of the sea (to which +subjects I paid particular attention) will be first discussed; I will +then pass on to the tertiary deposits, and afterwards to the older +rocks. Only those districts and sections will be described in detail +which appear to me to deserve some particular attention; and I will, at +the end of each chapter, give a summary of the results. We will +commence with the proofs of the upheaval of the eastern coast of the +continent, from the Rio Plata southward; and, in the Second Chapter, +follow up the same subject along the shores of Chile and Peru. + +On the northern bank of the great estuary of the Rio Plata, near +Maldonado, I found at the head of a lake, sometimes brackish but +generally containing fresh water, a bed of muddy clay, six feet in +thickness, with numerous shells of species still existing in the Plata, +namely, the Azara labiata, d’Orbigny, fragments of Mytilus eduliformis, +d’Orbigny, Paludestrina Isabellei, d’Orbigny, and the Solen Caribaeus, +Lam., which last was embedded vertically in the position in which it +had lived. These shells lie at the height of only two feet above the +lake, nor would they have been worth mentioning, except in connection +with analogous facts. + +At Monte Video, I noticed near the town, and along the base of the +mount, beds of a living Mytilus, raised some feet above the surface of +the Plata: in a similar bed, at a height from thirteen to sixteen feet, +M. Isabelle collected eight species, which, according to M. d’Orbigny, +now live at the mouth of the estuary. (“Voyage dans l’Amerique Merid.: +Part. Geolog.” page 21.) At Colonia del Sacramiento, further westward, +I observed at the height of about fifteen feet above the river, there +of quite fresh water, a small bed of the same Mytilus, which lives in +brackish water at Monte Video. Near the mouth of Uruguay, and for at +least thirty-five miles northward, there are at intervals large sandy +tracts, extending several miles from the banks of the river, but not +raised much above its level, abounding with small bivalves, which occur +in such numbers that at the Agraciado they are sifted and burnt for +lime. Those which I examined near the A. S. Juan were much worn: they +consisted of Mactra Isabellei, d’Orbigny, mingled with few of Venus +sinuosa, Lam., both inhabiting, as I am informed by M. d’Orbigny, +brackish water at the mouth of the Plata, nearly or quite as salt as +the open sea. The loose sand, in which these shells are packed, is +heaped into low, straight, long lines of dunes, like those left by the +sea at the head of many bays. M. d’Orbigny has described an analogous +phenomenon on a greater scale, near San Pedro on the river Parana, +where he found widely extended beds and hillocks of sand, with vast +numbers of the Azara labiata, at the height of nearly 100 feet +(English) above the surface of that river. (Ibid page 43.) The Azara +inhabits brackish water, and is not known to be found nearer to San +Pedro than Buenos Ayres, distant above a hundred miles in a straight +line. Nearer Buenos Ayres, on the road from that place to San Isidro, +there are extensive beds, as I am informed by Sir Woodbine Parish, of +the Azara labiata, lying at about forty feet above the level of the +river, and distant between two and three miles from it. (“Buenos Ayres” +etc. by Sir Woodbine Parish page 168.) These shells are always found on +the highest banks in the district: they are embedded in a stratified +earthy mass, precisely like that of the great Pampean deposit hereafter +to be described. In one collection of these shells, there were some +valves of the Venus sinuosa, Lam., the same species found with the +Mactra on the banks of the Uruguay. South of Buenos Ayres, near +Ensenada, there are other beds of the Azara, some of which seem to have +been embedded in yellowish, calcareous, semi-crystalline matter; and +Sir W. Parish has given me from the banks of the Arroyo del Tristan, +situated in this same neighbourhood, at the distance of about a league +from the Plata, a specimen of a pale- reddish, calcereo-argillaceous +stone (precisely like parts of the Pampean deposit the importance of +which fact will be referred to in a succeeding chapter), abounding with +shells of an Azara, much worn, but which in general form and appearance +closely resemble, and are probably identical with, the A. labiata. +Besides these shells, cellular, highly crystalline rock, formed of the +casts of small bivalves, is found near Ensenada; and likewise beds of +sea-shells, which from their appearance appear to have lain on the +surface. Sir W. Parish has given me some of these shells, and M. +d’Orbigny pronounces them to be:— + +1. Buccinanops globulosum, d’Orbigny. + +2. Olivancillaria auricularia, d’Orbigny. + +3. Venus flexuosa, Lam. + +4. Cytheraea (imperfect). + +5. Mactra Isabellei, d’Orbigny. + +6. Ostrea pulchella, d’Orbigny. + +Besides these, Sir W. Parish procured (“Buenos Ayres” etc. by Sir W. +Parish page 168.) (as named by Mr. G.B. Sowerby) the following shells:— + +7. Voluta colocynthis. + +8. Voluta angulata. + +9. Buccinum (not spec.?). + +All these species (with, perhaps, the exception of the last) are +recent, and live on the South American coast. These shell-beds extend +from one league to six leagues from the Plata, and must lie many feet +above its level. I heard, also, of beds of shells on the Somborombon, +and on the Rio Salado, at which latter place, as M. d’Orbigny informs +me, the Mactra Isabellei and Venus sinuosa are found. + +During the elevation of the Provinces of La Plata, the waters of the +ancient estuary have but little affected (with the exception of the +sand- hills on the banks of the Parana and Uruguay) the outline of the +land. M. Parchappe, however, has described groups of sand dunes +scattered over the wide extent of the Pampas southward of Buenos Ayres +(D’Orbigny “Voyage Geolog.” page 44.), which M. d’Orbigny attributes +with much probability to the action of the sea, before the plains were +raised above its level. (Before proceeding to the districts southward +of La Plata, it may be worth while just to state, that there is some +evidence that the coast of Brazil has participated in a small amount of +elevation. Mr. Burchell informs me, that he collected at Santos +(latitude 24 degrees S.) oyster-shells, apparently recent, some miles +from the shore, and quite above the tidal action. Westward of Rio de +Janeiro, Captain Elliot is asserted (see Harlan “Med. and Phys. Res.” +page 35 and Dr. Meigs in “Transactions of the American Philosophical +Society”), to have found human bones, encrusted with sea-shells, +between fifteen and twenty feet above the level of the sea. Between Rio +de Janeiro and Cape Frio I crossed sandy tracts abounding with +sea-shells, at a distance of a league from the coast; but whether these +tracts have been formed by upheaval, or through the mere accumulation +of drift sand, I am not prepared to assert. At Bahia (latitude 13 +degrees S.), in some parts near the coast, there are traces of +sea-action at the height of about twenty feet above its present level; +there are also, in many parts, remnants of beds of sandstone and +conglomerate with numerous recent shells, raised a little above the +sea-level. I may add, that at the head of Bahia Bay there is a +formation, about forty feet in thickness, containing tertiary shells +apparently of fresh-water origin, now washed by the sea and encrusted +with Balini; this appears to indicate a small amount of subsidence +subsequent to its deposition. At Pernambuco (latitude 8 degrees S.), in +the alluvial or tertiary cliffs, surrounding the low land on which the +city stands, I looked in vain for organic remains, or other evidence of +changes in level.) + +SOUTHWARD OF THE PLATA. + +The coast as far as Bahia Blanca (in latitude 39 degrees S.) is formed +either of a horizontal range of cliffs, or of immense accumulations of +sand-dunes. Within Bahia Blanca, a small piece of tableland, about +twenty feet above high-water mark, called Punta Alta, is formed of +strata of cemented gravel and of red earthy mud, abounding with shells +(with others lying loose on the surface), and the bones of extinct +mammifers. These shells, twenty in number, together with a Balanus and +two corals, are all recent species, still inhabiting the neighbouring +seas. They will be enumerated in the Fourth Chapter, when describing +the Pampean formation; five of them are identical with the upraised +ones from near Buenos Ayres. The northern shore of Bahia Blanca is, in +main part, formed of immense sand-dunes, resting on gravel with recent +shells, and ranging in lines parallel to the shore. These ranges are +separated from each other by flat spaces, composed of stiff impure red +clay, in which, at the distance of about two miles from the coast, I +found by digging a few minute fragments of sea-shells. The sand-dunes +extend several miles inland, and stand on a plain, which slopes up to a +height of between one hundred and two hundred feet. Numerous, small, +well-rounded pebbles of pumice lie scattered both on the plain and +sand-hillocks: at Monte Hermoso, on the flat summit of a cliff, I found +many of them at a height of 120 feet (angular measurement) above the +level of the sea. These pumice pebbles, no doubt, were originally +brought down from the Cordillera by the rivers which cross the +continent, in the same way as the river Negro anciently brought down, +and still brings down, pumice, and as the river Chupat brings down +scoriae: when once delivered at the mouth of a river, they would +naturally have travelled along the coasts, and been cast up during the +elevation of the land, at different heights. The origin of the +argillaceous flats, which separate the parallel ranges of sand-dunes, +seems due to the tides here having a tendency (as I believe they have +on most shoal, protected coasts) to throw up a bar parallel to the +shore, and at some distance from it; this bar gradually becomes larger, +affording a base for the accumulation of sand- dunes, and the shallow +space within then becomes silted up with mud. The repetition of this +process, without any elevation of the land, would form a level plain +traversed by parallel lines of sand-hillocks; during a slow elevation +of the land, the hillocks would rest on a gently inclined surface, like +that on the northern shore of Bahia Blanca. I did not observe any +shells in this neighbourhood at a greater height than twenty feet; and +therefore the age of the sea-drifted pebbles of pumice, now standing at +the height of 120 feet, must remain uncertain. + +The main plain surrounding Bahia Blanca I estimated at from two hundred +to three hundred feet; it insensibly rises towards the distant Sierra +Ventana. There are in this neighbourhood some other and lower plains, +but they do not abut one at the foot of the other, in the manner +hereafter to be described, so characteristic of Patagonia. The plain on +which the settlement stands is crossed by many low sand-dunes, +abounding with the minute shells of the Paludestrina australis, +d’Orbigny, which now lives in the bay. This low plain is bounded to the +south, at the Cabeza del Buey, by the cliff-formed margin of a wide +plain of the Pampean formation, which I estimated at sixty feet in +height. On the summit of this cliff there is a range of high sand-dunes +extending several miles in an east and west line. + +Southward of Bahia Blanca, the river Colorado flows between two plains, +apparently from thirty to forty feet in height. Of these plains, the +southern one slopes up to the foot of the great sandstone plateau of +the Rio Negro; and the northern one against an escarpment of the +Pampean deposit; so that the Colorado flows in a valley fifty miles in +width, between the upper escarpments. I state this, because on the low +plain at the foot of the northern escarpment, I crossed an immense +accumulation of high sand-dunes, estimated by the Gauchos at no less +than eight miles in breadth. These dunes range westward from the coast, +which is twenty miles distant, to far inland, in lines parallel to the +valley; they are separated from each other by argillaceous flats, +precisely like those on the northern shore of Bahia Blanca. At present +there is no source whence this immense accumulation of sand could +proceed; but if, as I believe, the upper escarpments once formed the +shores of an estuary, in that case the sandstone formation of the river +Negro would have afforded an inexhaustible supply of sand, which would +naturally have accumulated on the northern shore, as on every part of +the coast open to the south winds between Bahia Blanca and Buenos +Ayres. + +At San Blas (40 degrees 40′ S.) a little south of the mouth of the +Colorado, M. d’Orbigny found fourteen species of existing shells (six +of them identical with those from Bahia Blanca), embedded in their +natural positions. (“Voyage” etc. page 54.) From the zone of depth +which these shells are known to inhabit, they must have been uplifted +thirty-two feet. He also found, at from fifteen to twenty feet above +this bed, the remains of an ancient beach. + +Ten miles southward, but 120 miles to the west, at Port S. Antonio, the +Officers employed on the Survey assured me that they saw many old sea- +shells strewed on the surface of the ground, similar to those found on +other parts of the coast of Patagonia. At San Josef, ninety miles south +in nearly the same longitude, I found, above the gravel, which caps an +old tertiary formation, an irregular bed and hillock of sand, several +feet in thickness, abounding with shells of Patella deaurita, Mytilus +Magellanicus, the latter retaining much of its colour; Fusus +Magellanicus (and a variety of the same), and a large Balanus (probably +B. Tulipa), all now found on this coast: I estimated this bed at from +eighty to one hundred feet above the level of the sea. To the westward +of this bay, there is a plain estimated at between two hundred and +three hundred feet in height: this plain seems, from many measurements, +to be a continuation of the sandstone platform of the river Negro. The +next place southward, where I landed, was at Port Desire, 340 miles +distant; but from the intermediate districts I received, through the +kindness of the Officers of the Survey, especially from Lieutenant +Stokes and Mr. King, many specimens and sketches, quite sufficient to +show the general uniformity of the whole line of coast. I may here +state, that the whole of Patagonia consists of a tertiary formation, +resting on and sometimes surrounding hills of porphyry and quartz: the +surface is worn into many wide valleys and into level step-formed +plains, rising one above another, all capped by irregular beds of +gravel, chiefly composed of porphyritic rocks. This gravel formation +will be separately described at the end of the chapter. + +My object in giving the following measurements of the plains, as taken +by the Officers of the Survey, is, as will hereafter be seen, to show +the remarkable equability of the recent elevatory movements. Round the +southern parts of Nuevo Gulf, as far as the River Chupat (seventy miles +southward of San Josef), there appear to be several plains, of which +the best defined are here represented. + +(In the following Diagrams: 1. Baseline is Level of sea. 2. Scale is +1/20 of inch to 100 feet vertical. 3. Height is shown in feet thus: An. +M. always stands for angular or trigonometrical measurement. Ba. M. +always stands for barometrical measurement. Est. always stands for +estimation by the Officers of the Survey. + +DIAGRAM 1. SECTION OF STEP-FORMED PLAINS SOUTH OF NUEVO GULF. + +From East (sea level) to West (high): Terrace 1. 80 Est. Terrace 2. +200-220 An. M. Terrace 3. 350 An. M.) + +The upper plain is here well defined (called Table Hills); its edge +forms a cliff or line of escarpment many miles in length, projecting +over a lower plain. The lowest plain corresponds with that at San Josef +with the recent shells on its surface. Between this lowest and the +uppermost plain, there is probably more than one step-formed terrace: +several measurements show the existence of the intermediate one of the +height given in Diagram 1. + +(DIAGRAM 2. SECTION OF PLAINS IN THE BAY OF ST. GEORGE. + +From East (sea level) to West (high): Terrace 1. 250 An. M. Terrace 2. +330 An. M. Terrace 3. 580 An. M. Terraces 4, 5 and 6 not measured. +Terrace 7. 1,200 Est.) + +Near the north headland of the great Bay of St. George (100 miles south +of the Chupat), two well-marked plains of 250 and 330 feet were +measured: these are said to sweep round a great part of the Bay. At its +south headland, 120 miles distant from the north headland, the 250 feet +plain was again measured. In the middle of the bay, a higher plain was +found at two neighbouring places (Tilli Roads and C. Marques) to be 580 +feet in height. Above this plain, towards the interior, Mr. Stokes +informs me that there were several other step-formed plains, the +highest of which was estimated at 1,200 feet, and was seen ranging at +apparently the same height for 150 miles northward. All these plains +have been worn into great valleys and much denuded. The section in +Diagram 3 is illustrative of the general structure of the great Bay of +St. George. At the south headland of the Bay of St. George (near C. +Three Points) the 250 plain is very extensive. + +(DIAGRAM 3. SECTION OF PLAINS AT PORT DESIRE. + +From East (sea level) to West (high): Terrace 1. 100 Est. Terrace 2. +245-255 Ba. M. Shells on surface. Terrace 3. 330 Ba. M. Shells on +surface. Terrace 4. Not measured.) + +At Port Desire (forty miles southward) I made several measurements with +the barometer of a plain, which extends along the north side of the +port and along the open coast, and which varies from 245 to 255 feet in +height: this plain abuts against the foot of a higher plain of 330 +feet, which extends also far northward along the coast, and likewise +into the interior. In the distance a higher inland platform was seen, +of which I do not know the height. In three separate places, I observed +the cliff of the 245-255 feet plain, fringed by a terrace or narrow +plain estimated at about one hundred feet in height. These plains are +represented in the section Diagram 3. + +In many places, even at the distance of three and four miles from the +coast, I found on the gravel-capped surface of the 245-255 feet, and of +the 330 feet plain, shells of Mytilus Magellanicus, M. edulis, Patella +deaurita, and another Patella, too much worn to be identified, but +apparently similar to one found abundantly adhering to the leaves of +the kelp. These species are the commonest now living on this coast. The +shells all appeared very old; the blue of the mussels was much faded; +and only traces of colour could be perceived in the Patellas, of which +the outer surfaces were scaling off. They lay scattered on the smooth +surface of the gravel, but abounded most in certain patches, especially +at the heads of the smaller valleys: they generally contained sand in +their insides; and I presume that they have been washed by alluvial +action out of thin sandy layers, traces of which may sometimes be seen +covering the gravel. The several plains have very level surfaces; but +all are scooped out by numerous broad, winding, flat-bottomed valleys, +in which, judging from the bushes, streams never flow. These remarks on +the state of the shells, and on the nature of the plains, apply to the +following cases, so need not be repeated. + +(DIAGRAM 4. SECTION OF PLAINS AT PORT S. JULIAN. + +From East (sea level) to West (high): Terrace 1. Shells on surface. 90 +Est. Terrace 2. 430 An. M. Terrace 3. 560 An. M. Terrace 4. 950 An. M.) + +Southward of Port Desire, the plains have been greatly denuded, with +only small pieces of tableland marking their former extension. But +opposite Bird Island, two considerable step-formed plains were +measured, and found respectively to be 350 and 590 feet in height. This +latter plain extends along the coast close to Port St. Julian (110 +miles south of Port Desire); see Diagram 4. + +The lowest plain was estimated at ninety feet: it is remarkable from +the usual gravel-bed being deeply worn into hollows, which are filled +up with, as well as the general surface covered by, sandy and reddish +earthy matter: in one of the hollows thus filled up, the skeleton of +the Macrauchenia Patachonica, as will hereafter be described, was +embedded. On the surface and in the upper parts of this earthy mass, +there were numerous shells of Mytilus Magellanicus and M. edulis, +Patella deaurita, and fragments of other species. This plain is +tolerably level, but not extensive; it forms a promontory seven or +eight miles long, and three or four wide. The upper plains in Diagram 4 +were measured by the Officers of the Survey; they were all capped by +thick beds of gravel, and were all more or less denuded; the 950 plain +consists merely of separate, truncated, gravel-capped hills, two of +which, by measurement, were found to differ only three feet. The 430 +feet plain extends, apparently with hardly a break, to near the +northern entrance of the Rio Santa Cruz (fifty miles to the south); but +it was there found to be only 330 feet in height. + +(DIAGRAM 5. SECTION OF PLAINS AT THE MOUTH OF THE RIO SANTA CRUZ. + +From East (sea level) to West (high): Terrace 1. (sloping) 355 Ba. M. +Shells on surface. 463 Ba. M. Terrace 2. 710 An. M. Terrace 3. 840 An. +M.) + +On the southern side of the mouth of the Santa Cruz we have Diagram 5, +which I am able to give with more detail than in the foregoing cases. + +The plain marked 355 feet (as ascertained by the barometer and by +angular measurement) is a continuation of the above-mentioned 330 feet +plain: it extends in a N.W. direction along the southern shores of the +estuary. It is capped by gravel, which in most parts is covered by a +thin bed of sandy earth, and is scooped out by many flat-bottomed +valleys. It appears to the eye quite level, but in proceeding in a +S.S.W. course, towards an escarpment distant about six miles, and +likewise ranging across the country in a N.W. line, it was found to +rise at first insensibly, and then for the last half-mile, sensibly, +close up to the base of the escarpment: at this point it was 463 feet +in height, showing a rise of 108 feet in the six miles. On this 355-463 +feet plain, I found several shells of Mytilus Magellanicus and of a +Mytilus, which Mr. Sowerby informs me is yet unnamed, though well-known +as recent on this coast; Patella deaurita; Fusus, I believe, +Magellanicus, but the specimen has been lost; and at the distance of +four miles from the coast, at the height of about four hundred feet, +there were fragments of the same Patella and of a Voluta (apparently V. +ancilla) partially embedded in the superficial sandy earth. All these +shells had the same ancient appearance with those from the foregoing +localities. As the tides along this part of the coast rise at the +Syzygal period forty feet, and therefore form a well-marked beach-line, +I particularly looked out for ridges in crossing this plain, which, as +we have seen, rises 108 feet in about six miles, but I could not see +any traces of such. The next highest plain is 710 feet above the sea; +it is very narrow, but level, and is capped with gravel; it abuts to +the foot of the 840 feet plain. This summit-plain extends as far as the +eye can range, both inland along the southern side of the valley of the +Santa Cruz, and southward along the Atlantic. + +THE VALLEY OF THE R. SANTA CRUZ. + +This valley runs in an east and west direction to the Cordillera, a +distance of about one hundred and sixty miles. It cuts through the +great Patagonian tertiary formation, including, in the upper half of +the valley, immense streams of basaltic lava, which as well as the +softer beds, are capped by gravel; and this gravel, high up the river, +is associated with a vast boulder formation. (I have described this +formation in a paper in the “Geological Transactions” volume 6 page +415.) In ascending the valley, the plain which at the mouth on the +southern side is 355 feet high, is seen to trend towards the +corresponding plain on the northern side, so that their escarpments +appear like the shores of a former estuary, larger than the existing +one: the escarpments, also, of the 840 feet summit-plain (with a +corresponding northern one, which is met with some way up the valley), +appear like the shores of a still larger estuary. Farther up the +valley, the sides are bounded throughout its entire length by level, +gravel-capped terraces, rising above each other in steps. The width +between the upper escarpments is on an average between seven and ten +miles; in one spot, however, where cutting through the basaltic lava, +it was only one mile and a half. Between the escarpments of the second +highest terrace the average width is about four or five miles. The +bottom of the valley, at the distance of 110 miles from its mouth, +begins sensibly to expand, and soon forms a considerable plain, 440 +feet above the level of the sea, through which the river flows in a gut +from twenty to forty feet in depth. I here found, at a point 140 miles +from the Atlantic, and seventy miles from the nearest creek of the +Pacific, at the height of 410 feet, a very old and worn shell of +Patella deaurita. Lower down the valley, 105 miles from the Atlantic +(longitude 71 degrees W.), and at an elevation of about 300 feet, I +also found, in the bed of the river, two much worn and broken shells of +the Voluta ancilla, still retaining traces of their colours; and one of +the Patella deaurita. It appeared that these shells had been washed +from the banks into the river; considering the distance from the sea, +the desert and absolutely unfrequented character of the country, and +the very ancient appearance of the shells (exactly like those found on +the plains nearer the coast), there is, I think, no cause to suspect +that they could have been brought here by Indians. + +The plain at the head of the valley is tolerably level, but water-worn, +and with many sand-dunes on it like those on a sea-coast. At the +highest point to which we ascended, it was sixteen miles wide in a +north and south line; and forty-five miles in length in an east and +west line. It is bordered by the escarpments, one above the other, of +two plains, which diverge as they approach the Cordillera, and +consequently resemble, at two levels, the shores of great bays facing +the mountains; and these mountains are breached in front of the lower +plain by a remarkable gap. The valley, therefore, of the Santa Cruz +consists of a straight broad cut, about ninety miles in length, +bordered by gravel-capped terraces and plains, the escarpments of which +at both ends diverge or expand, one over the other, after the manner of +the shores of great bays. Bearing in mind this peculiar form of the +land—the sand-dunes on the plain at the head of the valley—the gap in +the Cordillera, in front of it—the presence in two places of very +ancient shells of existing species—and lastly, the circumstance of the +355-453 feet plain, with the numerous marine remains on its surface, +sweeping from the Atlantic coast, far up the valley, I think we must +admit, that within the recent period, the course of the Santa Cruz +formed a sea-strait intersecting the continent. At this period, the +southern part of South America consisted of an archipelago of islands +360 miles in a north and south line. We shall presently see, that two +other straits also, since closed, then cut through Tierra del Fuego; I +may add, that one of them must at that time have expanded at the foot +of the Cordillera into a great bay (now Otway Water) like that which +formerly covered the 440 feet plain at the head of the Santa Cruz. + +(DIAGRAM 6. NORTH AND SOUTH SECTION ACROSS THE TERRACES BOUNDING THE +VALLEY OF THE RIVER SANTA CRUZ, HIGH UP ITS COURSE. + +The height of each terrace, above the level of the river (furthest to +nearest to the river) in feet: + +A, north and south: 1,122 B, north and south: 869 C, north and south: +639 D, north: not measured. D, north? (suggest south): 185 E: 20 Bed of +River. + +Vertical scale 1/20 of inch to 100 feet; but terrace E, being only +twenty feet above the river, has necessarily been raised. The +horizontal distances much contracted; the distance from the edge of A +North to A South being on an average from seven to ten miles.) I have +said that the valley in its whole course is bordered by gravel- capped +plains. The section (Diagram 6), supposed to be drawn in a north and +south line across the valley, can scarcely be considered as more than +illustrative; for during our hurried ascent it was impossible to +measure all the plains at any one place. At a point nearly midway +between the Cordillera and the Atlantic, I found the plain (A north) +1,122 feet above the river; all the lower plains on this side were here +united into one great broken cliff: at a point sixteen miles lower down +the stream, I found by measurement and estimation that B (north) was +869 above the river: very near to where A (north) was measured, C +(north) was 639 above the same level: the terrace D (north) was nowhere +measured: the lowest E (north) was in many places about twenty feet +above the river. These plains or terraces were best developed where the +valley was widest; the whole five, like gigantic steps, occurred +together only at a few points. The lower terraces are less continuous +than the higher ones, and appear to be entirely lost in the upper third +of the valley. Terrace C (south), however was traced continuously for a +great distance. The terrace B (north), at a point fifty- five miles +from the mouth of the river, was four miles in width; higher up the +valley this terrace (or at least the second highest one, for I could +not always trace it continuously) was about eight miles wide. This +second plain was generally wider than the lower ones—as indeed follows +from the valley from A (north) to A (south) being generally nearly +double the width of from B (north) to B (south). Low down the valley, +the summit-plain A (south) is continuous with the 840 feet plain on the +coast, but it is soon lost or unites with the escarpment of B (south). +The corresponding plain A (north), on the north side of the valley, +appears to range continuously from the Cordillera to the head of the +present estuary of the Santa Cruz, where it trends northward towards +Port St. Julian. Near the Cordillera the summit-plain on both sides of +the valley is between 3,200 and 3,300 feet in height; at 100 miles from +the Atlantic, it is 1,416 feet, and on the coast 840 feet, all above +the sea-beach; so that in a distance of 100 miles the plain rises 576 +feet, and much more rapidly near to the Cordillera. The lower terraces +B and C also appear to rise as they run up the valley; thus D (north), +measured at two points twenty-four miles apart, was found to have risen +185 feet. From several reasons I suspect, that this gradual inclination +of the plains up the valley, has been chiefly caused by the elevation +of the continent in mass, having been the greater the nearer to the +Cordillera. + +All the terraces are capped with well-rounded gravel, which rests +either on the denuded and sometimes furrowed surface of the soft +tertiary deposits, or on the basaltic lava. The difference in height +between some of the lower steps or terraces seems to be entirely owing +to a difference in the thickness of the capping gravel. Furrows and +inequalities in the gravel, where such occur, are filled up and +smoothed over with sandy earth. The pebbles, especially on the higher +plains, are often whitewashed, and even cemented together by a white +aluminous substance, and I occasionally found this to be the case with +the gravel on the terrace D. I could not perceive any trace of a +similar deposition on the pebbles now thrown up by the river, and +therefore I do not think that terrace D was river-formed. As the +terrace E generally stands about twenty feet above the bed of the +river, my first impression was to doubt whether even this lowest one +could have been so formed; but it should always be borne in mind, that +the horizontal upheaval of a district, by increasing the total descent +of the streams, will always tend to increase, first near the sea-coast +and then further and further up the valley, their corroding and +deepening powers: so that an alluvial plain, formed almost on a level +with a stream, will, after an elevation of this kind, in time be cut +through, and left standing at a height never again to be reached by the +water. With respect to the three upper terraces of the Santa Cruz, I +think there can be no doubt, that they were modelled by the sea, when +the valley was occupied by a strait, in the same manner (hereafter to +be discussed) as the greater step-formed, shell- strewed plains along +the coast of Patagonia. + +To return to the shores of the Atlantic: the 840 feet plain, at the +mouth of the Santa Cruz, is seen extending horizontally far to the +south; and I am informed by the Officers of the Survey, that bending +round the head of Coy Inlet (sixty-five miles southward), it trends +inland. Outliers of apparently the same height are seen forty miles +farther south, inland of the river Gallegos; and a plain comes down to +Cape Gregory (thirty-five miles southward), in the Strait of Magellan, +which was estimated at between eight hundred and one thousand feet in +height, and which, rising towards the interior, is capped by the +boulder formation. South of the Strait of Magellan, there are large +outlying masses of apparently the same great tableland, extending at +intervals along the eastern coast of Tierra del Fuego: at two places +here, 110 miles a part, this plain was found to be 950 and 970 feet in +height. + +From Coy Inlet, where the high summit-plain trends inland, a plain +estimated at 350 feet in height, extends for forty miles to the river +Gallegos. From this point to the Strait of Magellan, and on each side +of that Strait, the country has been much denuded and is less level. It +consists chiefly of the boulder formation, which rises to a height of +between one hundred and fifty and two hundred and fifty feet, and is +often capped by beds of gravel. At N.S. Gracia, on the north side of +the Inner Narrows of the Strait of Magellan, I found on the summit of a +cliff, 160 feet in height, shells of existing Patellae and Mytili, +scattered on the surface and partially embedded in earth. On the +eastern coast, also, of Tierra del Fuego, in latitude 53 degrees 20′ +south, I found many Mytili on some level land, estimated at 200 feet in +height. Anterior to the elevation attested by these shells, it is +evident by the present form of the land, and by the distribution of the +great erratic boulders on the surface, that two sea-channels connected +the Strait of Magellan both with Sebastian Bay and with Otway Water. +(“Geological Transactions” volume 6 page 419.) + +CONCLUDING REMARKS ON THE RECENT ELEVATION OF THE SOUTH-EASTERN COASTS +OF AMERICA, AND ON THE ACTION OF THE SEA ON THE LAND. + +Upraised shells of species, still existing as the commonest kinds in +the adjoining sea, occur, as we have seen, at heights of between a few +feet and 410 feet, at intervals from latitude 33 degrees 40′ to 53 +degrees 20′ south. This is a distance of 1,180 geographical miles—about +equal from London to the North Cape of Sweden. As the boulder formation +extends with nearly the same height 150 miles south of 53 degrees 20′, +the most southern point where I landed and found upraised shells; and +as the level Pampas ranges many hundred miles northward of the point, +where M. d’Orbigny found at the height of 100 feet beds of the Azara, +the space in a north and south line, which has been uplifted within the +recent period, must have been much above the 1,180 miles. By the term +“recent,” I refer only to that period within which the now living +mollusca were called into existence; for it will be seen in the Fourth +Chapter, that both at Bahia Blanca and P. S. Julian, the mammiferous +quadrupeds which co-existed with these shells belong to extinct +species. I have said that the upraised shells were found only at +intervals on this line of coast, but this in all probability may be +attributed to my not having landed at the intermediate points; for +wherever I did land, with the exception of the river Negro, shells were +found: moreover, the shells are strewed on plains or terraces, which, +as we shall immediately see, extend for great distances with a uniform +height. I ascended the higher plains only in a few places, owing to the +distance at which their escarpments generally range from the coast, so +that I am far from knowing that 410 feet is the maximum of elevation of +these upraised remains. The shells are those now most abundant in a +living state in the adjoining sea. (Captain King “Voyages of +‘Adventure’ and ‘Beagle’” volume 1 pages 6 and 133.) All of them have +an ancient appearance; but some, especially the mussels, although lying +fully exposed to the weather, retain to a considerable extent their +colours: this circumstance appears at first surprising, but it is now +known that the colouring principle of the Mytilus is so enduring, that +it is preserved when the shell itself is completely disintegrated. (See +Mr. Lyell “Proofs of a Gradual Rising in Sweden” in the “Philosophical +Transactions” 1835 page 1. See also Mr. Smith of Jordan Hill in the +“Edinburgh New Philosophical Journal” volume 25 page 393.) Most of the +shells are broken; I nowhere found two valves united; the fragments are +not rounded, at least in none of the specimens which I brought home. + +With respect to the breadth of the upraised area in an east and west +line, we know from the shells found at the Inner Narrows of the Strait +of Magellan, that the entire width of the plain, although there very +narrow, has been elevated. It is probable that in this southernmost +part of the continent, the movement has extended under the sea far +eastward; for at the Falkland Islands, though I could not find any +shells, the bones of whales have been noticed by several competent +observers, lying on the land at a considerable distance from the sea, +and at the height of some hundred feet above it. (“Voyages of the +‘Adventure’ and ‘Beagle’” volume 2 page 227. And Bougainville’s +“Voyage” tome 1 page 112.) Moreover, we know that in Tierra del Fuego +the boulder formation has been uplifted within the recent period, and a +similar formation occurs on the north-western shores (Byron Sound) of +these islands. (I owe this fact to the kindness of Captain Sulivan, +R.N., a highly competent observer. I mention it more especially, as in +my Paper (page 427) on the Boulder Formation, I have, after having +examined the northern and middle parts of the eastern island, said that +the formation was here wholly absent.) The distance from this point to +the Cordillera of Tierra del Fuego, is 360 miles, which we may take as +the probable width of the recently upraised area. In the latitude of +the R. Santa Cruz, we know from the shells found at the mouth and head, +and in the middle of the valley, that the entire width (about 160 +miles) of the surface eastward of the Cordillera has been upraised. +From the slope of the plains, as shown by the course of the rivers, for +several degrees northward of the Santa Cruz, it is probable that the +elevation attested by the shells on the coast has likewise extended to +the Cordillera. When, however, we look as far northward as the +provinces of La Plata, this conclusion would be very hazardous; not +only is the distance from Maldonado (where I found upraised shells) to +the Cordillera great, namely, 760 miles, but at the head of the estuary +of the Plata, a N.N.E. and S.S.W. range of tertiary volcanic rocks has +been observed (This volcanic formation will be described in Chapter IV. +It is not improbable that the height of the upraised shells at the head +of the estuary of the Plata, being greater than at Bahia Blanca or at +San Blas, may be owing to the upheaval of these latter places having +been connected with the distant line of the Cordillera, whilst that of +the provinces of La Plata was in connection with the adjoining tertiary +volcanic axis.), which may well indicate an axis of elevation quite +distinct from that of the Andes. Moreover, in the centre of the Pampas +in the chain of Cordova, severe earthquakes have been felt (See Sir W. +Parish’s work on “La Plata” page 242. For a notice of an earthquake +which drained a lake near Cordova, see also Temple’s “Travels in Peru.” +Sir W. Parish informs me, that a town between Salta and Tucuman (north +of Cordova) was formerly utterly overthrown by an earthquake.); whereas +at Mendoza, at the eastern foot of the Cordillera, only gentle +oscillations, transmitted from the shores of the Pacific, have ever +been experienced. Hence the elevation of the Pampas may be due to +several distinct axes of movement; and we cannot judge, from the +upraised shells round the estuary of the Plata, of the breadth of the +area uplifted within the recent period. + +Not only has the above specified long range of coast been elevated +within the recent period, but I think it may be safely inferred from +the similarity in height of the gravel-capped plains at distant points, +that there has been a remarkable degree of equability in the elevatory +process. I may premise, that when I measured the plains, it was simply +to ascertain the heights at which shells occurred; afterwards, +comparing these measurements with some of those made during the Survey, +I was struck with their uniformity, and accordingly tabulated all those +which represented the summit-edges of plains. The extension of the 330 +to 355 feet plain is very striking, being found over a space of 500 +geographical miles in a north and south line. A table (Table 1) of the +measurements is given below. The angular measurements and all the +estimations (in feet) are by the Officers of the Survey; the +barometrical ones by myself:— + +TABLE 1. + +Gallegos River to Coy Inlet (partly angular partly estimation) 350 +South Side of Santa Cruz (angular and barometric) 355 North Side of +Santa Cruz (angular and barometric) 330 Bird Island, plain opposite to +(angular) 350 Port Desire, plain extending far along coast (barometric) +330 St. George’s Bay, north promontory (angular) 330 Table Land, south +of New Bay (angular) 350 + +A plain, varying from 245 to 255 feet, seems to extend with much +uniformity from Port Desire to the north of St. George’s Bay, a +distance of 170 miles; and some approximate measurements (in feet), +also given in Table 2 below, indicate the much greater extension of 780 +miles:— + +TABLE 2. + +Coy Inlet, south of (partly angular and partly estimation) 200 to 300 +Port Desire (barometric) 245 to 255 C. Blanco (angular) 250 North +Promontory of St. George’s Bay (angular) 250 South of New Bay (angular) +200 to 220 North of S. Josef (estimation) 200 to 300 Plain of Rio Negro +(angular) 200 to 220 Bahia Blanca (estimation) 200 to 300 + +The extension, moreover, of the 560 to 580, and of the 80 to 100 feet, +plains is remarkable, though somewhat less obvious than in the former +cases. Bearing in mind that I have not picked these measurements out of +a series, but have used all those which represented the edges of +plains, I think it scarcely possible that these coincidences in height +should be accidental. We must therefore conclude that the action, +whatever it may have been, by which these plains have been modelled +into their present forms, has been singularly uniform. + +These plains or great terraces, of which three and four often rise like +steps one behind the other, are formed by the denudation of the old +Patagonian tertiary beds, and by the deposition on their surfaces of a +mass of well-rounded gravel, varying, near the coast, from ten to +thirty-five feet in thickness, but increasing in thickness towards the +interior. The gravel is often capped by a thin irregular bed of sandy +earth. The plains slope up, though seldom sensibly to the eye, from the +summit edge of one escarpment to the foot of the next highest one. +Within a distance of 150 miles, between Santa Cruz to Port Desire, +where the plains are particularly well developed, there are at least +seven stages or steps, one above the other. On the three lower ones, +namely, those of 100 feet, 250 feet, and 350 feet in height, existing +littoral shells are abundantly strewed, either on the surface, or +partially embedded in the superficial sandy earth. By whatever action +these three lower plains have been modelled, so undoubtedly have all +the higher ones, up to a height of 950 feet at S. Julian, and of 1,200 +feet (by estimation) along St. George’s Bay. I think it will not be +disputed, considering the presence of the upraised marine shells, that +the sea has been the active power during stages of some kind in the +elevatory process. + +We will now briefly consider this subject: if we look at the existing +coast-line, the evidence of the great denuding power of the sea is very +distinct; for, from Cape St. Diego, in latitude 54 degrees 30′ to the +mouth of the Rio Negro, in latitude 31 degrees (a length of more than +eight hundred miles), the shore is formed, with singularly few +exceptions, of bold and naked cliffs: in many places the cliffs are +high; thus, south of the Santa Cruz, they are between eight and nine +hundred feet in height, with their horizontal strata abruptly cut off, +showing the immense mass of matter which has been removed. Nearly this +whole line of coast consists of a series of greater or lesser curves, +the horns of which, and likewise certain straight projecting portions, +are formed of hard rocks; hence the concave parts are evidently the +effect and the measure of the denuding action on the softer strata. At +the foot of all the cliffs, the sea shoals very gradually far outwards; +and the bottom, for a space of some miles, everywhere consists of +gravel. I carefully examined the bed of the sea off the Santa Cruz, and +found that its inclination was exactly the same, both in amount and in +its peculiar curvature, with that of the 355 feet plain at this same +place. If, therefore, the coast, with the bed of the adjoining sea, +were now suddenly elevated one or two hundred feet, an inland line of +cliffs, that is an escarpment, would be formed, with a gravel-capped +plain at its foot gently sloping to the sea, and having an inclination +like that of the existing 355 feet plain. From the denuding tendency of +the sea, this newly formed plain would in time be eaten back into a +cliff: and repetitions of this elevatory and denuding process would +produce a series of gravel-capped sloping terraces, rising one above +another, like those fronting the shores of Patagonia. + +The chief difficulty (for there are other inconsiderable ones) on this +view, is the fact,—as far as I can trust two continuous lines of +soundings carefully taken between Santa Cruz and the Falkland Islands, +and several scattered observations on this and other coasts,—that the +pebbles at the bottom of the sea QUICKLY and REGULARLY decrease in size +with the increasing depth and distance from the shore, whereas in the +gravel on the sloping plains, no such decrease in size was perceptible. + +Table 3 below gives the average result of many soundings off the Santa +Cruz:— TABLE 3. + +Under two miles from the shore, many of the pebbles were of large size, +mingled with some small ones. + +Column 1. Distance in miles from the shore. + +Column 2. Depth in fathoms. + +Column 3. Size of Pebbles. + +I particularly attended to the size of the pebbles on the 355 feet +Santa Cruz plain, and I noticed that on the summit-edge of the present +sea cliffs many were as large as half a man’s head; and in crossing +from these cliffs to the foot of the next highest escarpment, a +distance of six miles, I could not observe any increase in their size. +We shall presently see that the theory of a slow and almost insensible +rise of the land, will explain all the facts connected with the +gravel-capped terraces, better than the theory of sudden elevations of +from one to two hundred feet. + +M. d’Orbigny has argued, from the upraised shells at San Blas being +embedded in the positions in which they lived, and from the valves of +the Azara labiata high on the banks of the Parana being united and +unrolled, that the elevation of Northern Patagonia and of La Plata must +have been sudden; for he thinks, if it had been gradual, these shells +would all have been rolled on successive beach-lines. But in PROTECTED +bays, such as in that of Bahia Blanca, wherever the sea is accumulating +extensive mud-banks, or where the winds quietly heap up sand-dunes, +beds of shells might assuredly be preserved buried in the positions in +which they had lived, even whilst the land retained the same level; +any, the smallest, amount of elevation would directly aid in their +preservation. I saw a multitude of spots in Bahia Blanca where this +might have been effected; and at Maldonado it almost certainly has been +effected. In speaking of the elevation of the land having been slow, I +do not wish to exclude the small starts which accompany earthquakes, as +on the coast of Chile; and by such movements beds of shells might +easily be uplifted, even in positions exposed to a heavy surf, without +undergoing any attrition: for instance, in 1835, a rocky flat off the +island of Santa Maria was at one blow upheaved above high-water mark, +and was left covered with gaping and putrefying mussel-shells, still +attached to the bed on which they had lived. If M. d’Orbigny had been +aware of the many long parallel lines of sand-hillocks, with infinitely +numerous shells of the Mactra and Venus, at a low level near the +Uruguay; if he had seen at Bahia Blanca the immense sand-dunes, with +water-worn pebbles of pumice, ranging in parallel lines, one behind the +other, up a height of at least 120 feet; if he had seen the sand-dunes, +with the countless Paludestrinas, on the low plain near the Fort at +this place, and that long line on the edge of the cliff, sixty feet +higher up; if he had crossed that long and great belt of parallel +sand-dunes, eight miles in width, standing at the height of from forty +to fifty feet above the Colorado, where sand could not now collect,—I +cannot believe he would have thought that the elevation of this great +district had been sudden. Certainly the sand-dunes (especially when +abounding with shells), which stand in ranges at so many different +levels, must all have required long time for their accumulation; and +hence I do not doubt that the last 100 feet of elevation of La Plata +and Northern Patagonia has been exceedingly slow. + +If we extend this conclusion to Central and Southern Patagonia, the +inclination of the successively rising gravel-capped plains can be +explained quite as well, as by the more obvious view already given of a +few comparatively great and sudden elevations; in either case we must +admit long periods of rest, during which the sea ate deeply into the +land. Let us suppose the present coast to rise at a nearly equable, +slow rate, yet sufficiently quick to prevent the waves quite removing +each part as soon as brought up; in this case every portion of the +present bed of the sea will successively form a beach-line, and from +being exposed to a like action will be similarly affected. It cannot +matter to what height the tides rise, even if to forty feet as at Santa +Cruz, for they will act with equal force and in like manner on each +successive line. Hence there is no difficulty in the fact of the 355 +feet plain at Santa Cruz sloping up 108 feet to the foot of the next +highest escarpment, and yet having no marks of any one particular +beach-line on it; for the whole surface on this view has been a beach. +I cannot pretend to follow out the precise action of the tidal-waves +during a rise of the land, slow, yet sufficiently quick to prevent or +check denudation: but if it be analogous to what takes place on +protected parts of the present coast, where gravel is now accumulating +in large quantities, an inclined surface, thickly capped by +well-rounded pebbles of about the same size, would be ultimately left. +(On the eastern side of Chiloe, which island we shall see in the next +chapter is now rising, I observed that all the beaches and extensive +tidal-flats were formed of shingle.) On the gravel now accumulating, +the waves, aided by the wind, sometimes throw up a thin covering of +sand, together with the common coast-shells. Shells thus cast up by +gales, would, during an elevatory period, never again be touched by the +sea. Hence, on this view of a slow and gradual rising of the land, +interrupted by periods of rest and denudation, we can understand the +pebbles being of about the same size over the entire width of the +step-like plains,—the occasional thin covering of sandy earth,—and the +presence of broken, unrolled fragments of those shells, which now live +exclusively near the coast. + +A SUMMARY OF RESULTS. + +It may be concluded that the coast on this side of the continent, for a +space of at least 1,180 miles, has been elevated to a height of 100 +feet in La Plata, and of 400 feet in Southern Patagonia, within the +period of existing shells, but not of existing mammifers. That in La +Plata the elevation has been very slowly effected: that in Patagonia +the movement may have been by considerable starts, but much more +probably slow and quiet. In either case, there have been long +intervening periods of comparative rest, during which the sea corroded +deeply, as it is still corroding, into the land. (I say COMPARATIVE and +not ABSOLUTE rest, because the sea acts, as we have seen, with great +denuding power on this whole line of coast; and therefore, during an +elevation of the land, if excessively slow (and of course during a +subsidence of the land), it is quite possible that lines of cliff might +be formed.) That the periods of denudation and elevation were +contemporaneous and equable over great spaces of coast, as shown by the +equable heights of the plains; that there have been at least eight +periods of denudation, and that the land, up to a height of from 950 to +1,200 feet, has been similarly modelled and affected: that the area +elevated, in the southernmost part of the continent, extended in +breadth to the Cordillera, and probably seaward to the Falkland +Islands; that northward, in La Plata, the breadth is unknown, there +having been probably more than one axis of elevation; and finally, +that, anterior to the elevation attested by these upraised shells, the +land was divided by a Strait where the River Santa Cruz now flows, and +that further southward there were other sea-straits, since closed. I +may add, that at Santa Cruz, in latitude 50 degrees S., the plains have +been uplifted at least 1,400 feet, since the period when gigantic +boulders were transported between sixty and seventy miles from their +parent rock, on floating icebergs. + +Lastly, considering the great upward movements which this long line of +coast has undergone, and the proximity of its southern half to the +volcanic axis of the Cordillera, it is highly remarkable that in the +many fine sections exposed in the Pampean, Patagonian tertiary, and +Boulder formations, I nowhere observed the smallest fault or abrupt +curvature in the strata. + +GRAVEL FORMATION OF PATAGONIA. + +I will here describe in more detail than has been as yet incidentally +done, the nature, origin, and extent of the great shingle covering of +Patagonia: but I do not mean to affirm that all of this shingle, +especially that on the higher plains, belongs to the recent period. A +thin bed of sandy earth, with small pebbles of various porphyries and +of quartz, covering a low plain on the north side of the Rio Colorado, +is the extreme northern limit of this formation. These little pebbles +have probably been derived from the denudation of a more regular bed of +gravel, capping the old tertiary sandstone plateau of the Rio Negro. +The gravel-bed near the Rio Negro is, on an average, about ten or +twelve feet in thickness; and the pebbles are larger than on the +northern side of the Colorado, being from one or two inches in +diameter, and composed chiefly of rather dark-tinted porphyries. +Amongst them I here first noticed a variety often to be referred to, +namely, a peculiar gallstone-yellow siliceous porphyry, frequently, but +not invariably, containing grains of quartz. The pebbles are embedded +in a white, gritty, calcareous matrix, very like mortar, sometimes +merely coating with a whitewash the separate stones, and sometimes +forming the greater part of the mass. In one place I saw in the gravel +concretionary nodules (not rounded) of crystallised gypsum, some as +large as a man’s head. I traced this bed for forty-five miles inland, +and was assured that it extended far into the interior. As the surface +of the calcareo- argillaceous plain of Pampean formation, on the +northern side of the wide valley of the Colorado, stands at about the +same height with the mortar- like cemented gravel capping the sandstone +on the southern side, it is probable, considering the apparent +equability of the subterranean movements along this side of America, +that this gravel of the Rio Negro and the upper beds of the Pampean +formation northward of the Colorado, are of nearly contemporaneous +origin, and that the calcareous matter has been derived from the same +source. + +Southward of the Rio Negro, the cliffs along the great bay of S. +Antonio are capped with gravel: at San Josef, I found that the pebbles +closely resembled those on the plain of the Rio Negro, but that they +were not cemented by calcareous matter. Between San Josef and Port +Desire, I was assured by the Officers of the Survey that the whole face +of the country is coated with gravel. At Port Desire and over a space +of twenty-five miles inland, on the three step-formed plains and in the +valleys, I everywhere passed over gravel which, where thickest, was +between thirty and forty feet. Here, as in other parts of Patagonia, +the gravel, or its sandy covering, was, as we have seen, often strewed +with recent marine shells. The sandy covering sometimes fills up +furrows in the gravel, as does the gravel in the underlying tertiary +formations. The pebbles are frequently whitewashed and even cemented +together by a peculiar, white, friable, aluminous, fusible substance, +which I believe is decomposed feldspar. At Port Desire, the gravel +rested sometimes on the basal formation of porphyry, and sometimes on +the upper or the lower denuded tertiary strata. It is remarkable that +most of the porphyritic pebbles differ from those varieties of porphyry +which occur here abundantly in situ. The peculiar gallstone-yellow +variety was common, but less numerous than at Port S. Julian, where it +formed nearly one-third of the mass of the gravel; the remaining part +there consisting of pale grey and greenish porphyries with many +crystals of feldspar. At Port S. Julian, I ascended one of the flat- +topped hills, the denuded remnant of the highest plain, and found it, +at the height of 950 feet, capped with the usual bed of gravel. + +Near the mouth of the Santa Cruz, the bed of gravel on the 355 feet +plain is from twenty to about thirty-five feet in thickness. The +pebbles vary from minute ones to the size of a hen’s egg, and even to +that of half a man’s head; they consist of paler varieties of porphyry +than those found further northward, and there are fewer of the +gallstone-yellow kind; pebbles of compact black clay-slate were here +first observed. The gravel, as we have seen, covers the step-formed +plains at the mouth, head, and on the sides of the great valley of the +Santa Cruz. At a distance of 110 miles from the coast, the plain has +risen to the height of 1,416 feet above the sea; and the gravel, with +the associated great boulder formation, has attained a thickness of 212 +feet. The plain, apparently with its usual gravel covering, slopes up +to the foot of the Cordillera to the height of between 3,200 and 3,300 +feet. In ascending the valley, the gravel gradually becomes entirely +altered in character: high up, we have pebbles of crystalline +feldspathic rocks, compact clay-slate, quartzose schists, and +pale-coloured porphyries; these rocks, judging both from the gigantic +boulders in the surface and from some small pebbles embedded beneath +700 feet in thickness of the old tertiary strata, are the prevailing +kinds in this part of the Cordillera; pebbles of basalt from the +neighbouring streams of basaltic lava are also numerous; there are few +or none of the reddish or of the gallstone-yellow porphyries so common +near the coast. Hence the pebbles on the 350 feet plain at the mouth of +the Santa Cruz cannot have been derived (with the exception of those of +compact clay- slate, which, however, may equally well have come from +the south) from the Cordillera in this latitude; but probably, in chief +part, from farther north. + +Southward of the Santa Cruz, the gravel may be seen continuously +capping the great 840 feet plain: at the Rio Gallegos, where this plain +is succeeded by a lower one, there is, as I am informed by Captain +Sulivan, an irregular covering of gravel from ten to twelve feet in +thickness over the whole country. The district on each side of the +Strait of Magellan is covered up either with gravel or the boulder +formation: it was interesting to observe the marked difference between +the perfectly rounded state of the pebbles in the great shingle +formation of Patagonia, and the more or less angular fragments in the +boulder formation. The pebbles and fragments near the Strait of +Magellan nearly all belong to rocks known to occur in Fuegia. I was +therefore much surprised in dredging south of the Strait to find, in +latitude 54 degrees 10′ south, many pebbles of the gallstone-yellow +siliceous porphyry; I procured others from a great depth off Staten +Island, and others were brought me from the western extremity of the +Falkland Islands. (At my request, Mr. Kent collected for me a bag of +pebbles from the beach of White Rock harbour, in the northern part of +the sound, between the two Falkland Islands. Out of these well-rounded +pebbles, varying in size from a walnut to a hen’s egg, with some +larger, thirty-eight evidently belonged to the rocks of these islands; +twenty-six were similar to the pebbles of porphyry found on the +Patagonian plains, which rocks do not exist in situ in the Falklands; +one pebble belonged to the peculiar yellow siliceous porphyry; thirty +were of doubtful origin.) The distribution of the pebbles of this +peculiar porphyry, which I venture to affirm is not found in situ +either in Fuegia, the Falkland Islands, or on the coast of Patagonia, +is very remarkable, for they are found over a space of 840 miles in a +north and south line, and at the Falklands, 300 miles eastward of the +coast of Patagonia. Their occurrence in Fuegia and the Falklands may, +however, perhaps be due to the same ice-agency by which the boulders +have been there transported. + +We have seen that porphyritic pebbles of a small size are first met +with on the northern side of the Rio Colorado, the bed becoming well +developed near the Rio Negro: from this latter point I have every +reason to believe that the gravel extends uninterruptedly over the +plains and valleys of Patagonia for at least 630 nautical miles +southward to the Rio Gallegos. From the slope of the plains, from the +nature of the pebbles, from their extension at the Rio Negro far into +the interior, and at the Santa Cruz close up to the Cordillera, I think +it highly probable that the whole breadth of Patagonia is thus covered. +If so, the average width of the bed must be about two hundred miles. +Near the coast the gravel is generally from ten to thirty feet in +thickness; and as in the valley of Santa Cruz it attains, at some +distance from the Cordillera, a thickness of 214 feet, we may, I think, +safely assume its average thickness over the whole area of 630 by 200 +miles, at fifty feet! + +The transportal and origin of this vast bed of pebbles is an +interesting problem. From the manner in which they cap the step-formed +plains, worn by the sea within the period of existing shells, their +deposition, at least on the plains up to a height of 400 feet, must +have been a recent geological event. From the form of the continent, we +may feel sure that they have come from the westward, probably, in chief +part from the Cordillera, but, perhaps, partly from unknown rocky +ridges in the central districts of Patagonia. That the pebbles have not +been transported by rivers, from the interior towards the coast, we may +conclude from the fewness and smallness of the streams of Patagonia: +moreover, in the case of the one great and rapid river of Santa Cruz, +we have good evidence that its transporting power is very trifling. +This river is from two to three hundred yards in width, about seventeen +feet deep in its middle, and runs with a singular degree of uniformity +five knots an hour, with no lakes and scarcely any still reaches: +nevertheless, to give one instance of its small transporting power, +upon careful examination, pebbles of compact basalt could not be found +in the bed of the river at a greater distance than ten miles below the +point where the stream rushes over the debris of the great basaltic +cliffs forming its shore: fragments of the CELLULAR varieties have been +washed down twice or thrice as far. That the pebbles in Central and +Northern Patagonia have not been transported by ice-agency, as seems to +have been the case to a considerable extent farther south, and likewise +in the northern hemisphere, we may conclude, from the absence of all +angular fragments in the gravel, and from the complete contrast in many +other respects between the shingle and neighbouring boulder formation. + +Looking to the gravel on any one of the step-formed plains, I cannot +doubt, from the several reasons assigned in this chapter, that it has +been spread out and leveled by the long-continued action of the sea, +probably during the slow rise of the land. The smooth and perfectly +rounded condition of the innumerable pebbles alone would prove +long-continued action. But how the whole mass of shingle on the +coast-plains has been transported from the mountains of the interior, +is another and more difficult question. The following considerations, +however, show that the sea by its ordinary action has considerable +power in distributing pebbles. Table 3 above shows how very uniformly +and gradually the pebbles decrease in size with the gradually seaward +increasing depth and distance. (I may mention, that at the distance of +150 miles from the Patagonian shore I carefully examined the minute +rounded particles in the sand, and found them to be fusible like the +porphyries of the great shingle bed. I could even distinguish particles +of the gallstone-yellow porphyry. It was interesting to notice how +gradually the particles of white quartz increased, as we approached the +Falkland Islands, which are thus constituted. In the whole line of +soundings between these islands and the coast of Patagonia dead or +living organic remains were most rare. On the relations between the +depth of water and the nature of the bottom, see Martin White on +“Soundings in the Channel” pages 4, 6, 175; also Captain Beechey’s +“Voyage to the Pacific” chapter 18.) A series of this kind irresistibly +leads to the conclusion, that the sea has the power of sifting and +distributing the loose matter on its bottom. According to Martin White, +the bed of the British Channel is disturbed during gales at depths of +sixty-three and sixty-seven fathoms, and at thirty fathoms, shingle and +fragments of shells are often deposited, afterwards to be carried away +again. (“Soundings in the Channel” pages 4, 166. M. Siau states +(“Edinburgh New Philosophical Journal” volume 31 page 246), that he +found the sediment, at a depth of 188 metres, arranged in ripples of +different degrees of fineness. There are some excellent discussions on +this and allied subjects in Sir H. De la Beche’s “Theoretical +Researches.”) Groundswells, which are believed to be caused by distant +gales, seem especially to affect the bottom: at such times, according +to Sir R. Schomburgk, the sea to a great distance round the West Indian +Islands, at depths from five to fifteen fathoms, becomes discoloured, +and even the anchors of vessels have been moved. (“Journal of Royal +Geographical Society” volume 5 page 25. It appears from Mr. Scott +Russell’s investigations (see Mr. Murchison’s “Anniversary Address +Geological Society” 1843 page 40), that in waves of translation the +motion of the particles of water is nearly as great at the bottom as at +the top.) There are, however, some difficulties in understanding how +the sea can transport pebbles lying at the bottom, for, from +experiments instituted on the power of running water, it would appear +that the currents of the sea have not sufficient velocity to move +stones of even moderate size: moreover, I have repeatedly found in the +most exposed situations that the pebbles which lie at the bottom are +encrusted with full-grown living corallines, furnished with the most +delicate, yet unbroken spines: for instance, in ten fathoms water off +the mouth of the Santa Cruz, many pebbles, under half an inch in +diameter, were thus coated with Flustracean zoophytes. (A pebble, one +and a half inch square and half an inch thick, was given me, dredged up +from twenty-seven fathoms depth off the western end of the Falkland +Islands, where the sea is remarkably stormy, and subject to violent +tides. This pebble was encrusted on all sides by a delicate living +coralline. I have seen many pebbles from depths between forty and +seventy fathoms thus encrusted; one from the latter depth off Cape +Horn.) Hence we must conclude that these pebbles are not often +violently disturbed: it should, however, be borne in mind that the +growth of corallines is rapid. The view, propounded by Professor +Playfair, will, I believe, explain this apparent difficulty,—namely, +that from the undulations of the sea TENDING to lift up and down +pebbles or other loose bodies at the bottom, such are liable, when thus +quite or partially raised, to be moved even by a very small force, a +little onwards. We can thus understand how oceanic or tidal currents of +no great strength, or that recoil movement of the bottom-water near the +land, called by sailors the “undertow” (which I presume must extend out +seaward as far as the BREAKING waves impel the surface-water towards +the beach), may gain the power during storms of sifting and +distributing pebbles even of considerable size, and yet without so +violently disturbing them as to injure the encrusting corallines. (I +may take this opportunity of remarking on a singular, but very common +character in the form of the bottom, in the creeks which deeply +penetrate the western shores of Tierra del Fuego; namely, that they are +almost invariably much shallower close to the open sea at their mouths +than inland. Thus, Cook, in entering Christmas Sound, first had +soundings in thirty-seven fathoms, then in fifty, then in sixty, and a +little farther in no bottom with 170 fathoms. The sealers are so +familiar with this fact, that they always look out for anchorage near +the entrances of the creeks. See, also, on this subject, the “Voyages +of the ‘Adventure’ and ‘Beagle’” volume 1 page 375 and “Appendix” page +313. This Shoalness of the sea- channels near their entrances probably +results from the quantity of sediment formed by the wear and tear of +the outer rocks exposed to the full force of the open sea. I have no +doubt that many lakes, for instance in Scotland, which are very deep +within, and are separated from the sea apparently only by a tract of +detritus, were originally sea-channels with banks of this nature near +their mouths, which have since been upheaved.) + +The sea acts in another and distinct manner in the distribution of +pebbles, namely by the waves on the beach. Mr. Palmer, in his excellent +memoir on this subject, has shown that vast masses of shingle travel +with surprising quickness along lines of coast, according to the +direction with which the waves break on the beach and that this is +determined by the prevailing direction of the winds. (“Philosophical +Transactions” 1834 page 576.) This agency must be powerful in mingling +together and disseminating pebbles derived from different sources: we +may, perhaps, thus understand the wide distribution of the +gallstone-yellow porphyry; and likewise, perhaps, the great difference +in the nature of the pebbles at the mouth of the Santa Cruz from those +in the same latitude at the head of the valley. + +I will not pretend to assign to these several and complicated agencies +their shares in the distribution of the Patagonian shingle: but from +the several considerations given in this chapter, and I may add, from +the frequency of a capping of gravel on tertiary deposits in all parts +of the world, as I have myself observed and seen stated in the works of +various authors, I cannot doubt that the power of widely dispersing +gravel is an ordinary contingent on the action of the sea; and that +even in the case of the great Patagonian shingle-bed we have no +occasion to call in the aid of debacles. I at one time imagined that +perhaps an immense accumulation of shingle had originally been +collected at the foot of the Cordillera; and that this accumulation, +when upraised above the level of the sea, had been eaten into and +partially spread out (as off the present line of coast); and that the +newly-spread out bed had in its turn been upraised, eaten into, and +re-spread out; and so onwards, until the shingle, which was first +accumulated in great thickness at the foot of the Cordillera, had +reached in thinner beds its present extension. By whatever means the +gravel formation of Patagonia may have been distributed, the vastness +of its area, its thickness, its superficial position, its recent +origin, and the great degree of similarity in the nature of its +pebbles, all appear to me well deserving the attention of geologists, +in relation to the origin of the widely-spread beds of conglomerate +belonging to past epochs. + +FORMATION OF CLIFFS. + +(DIAGRAM 7.—SECTION OF COAST-CLIFFS AND BOTTOM OF SEA, OFF THE ISLAND +OF ST. HELENA. + +Height in feet above sea level. + +Depths in fathoms. + +Vertical and horizontal scale, two inches to a nautical mile. The point +marked 1,600 feet is at the foot of High Knoll; point marked 510 feet +is on the edge of Ladder Hill. The strata consist of basaltic streams. + +A Section left to right: + +Height at the foot of High Knoll: 1,600 at top of strata. + +Height on the edge of Ladder Hill: 510 at top of strata. + +Bottom at coast rocky only to a depth of five or six fathoms. + +30 fathoms: bottom mud and sand. + +100 fathoms sloping more sharply to 250 fathoms.) + +When viewing the sea-worn cliffs of Patagonia, in some parts between +eight hundred and nine hundred feet in height, and formed of horizontal +tertiary strata, which must once have extended far seaward—or again, +when viewing the lofty cliffs round many volcanic islands, in which the +gentle inclination of the lava-streams indicates the former extension +of the land, a difficulty often occurred to me, namely, how the strata +could possibly have been removed by the action of the sea at a +considerable depth beneath its surface. The section in Diagram 7, which +represents the general form of the land on the northern and leeward +side of St. Helena (taken from Mr. Seale’s large model and various +measurements), and of the bottom of the adjoining sea (taken chiefly +from Captain Austin’s survey and some old charts), will show the nature +of this difficulty. + +If, as seems probable, the basaltic streams were originally prolonged +with nearly their present inclination, they must, as shown by the +dotted line in the section, once have extended at least to a point, now +covered by the sea to a depth of nearly thirty fathoms: but I have +every reason to believe they extended considerably further, for the +inclination of the streams is less near the coast than further inland. +It should also be observed, that other sections on the coast of this +island would have given far more striking results, but I had not the +exact measurements; thus, on the windward side, the cliffs are about +two thousand feet in height and the cut-off lava streams very gently +inclined, and the bottom of the sea has nearly a similar slope all +round the island. How, then, has all the hard basaltic rock, which once +extended beneath the surface of the sea, been worn away? According to +Captain Austin, the bottom is uneven and rocky only to that very small +distance from the beach within which the depth is from five to six +fathoms; outside this line, to a depth of about one hundred fathoms, +the bottom is smooth, gently inclined, and formed of mud and sand; +outside the one hundred fathoms, it plunges suddenly into unfathomable +depths, as is so very commonly the case on all coasts where sediment is +accumulating. At greater depths than the five or six fathoms, it seems +impossible, under existing circumstances, that the sea can both have +worn away hard rock, in parts to a thickness of at least 150 feet, and +have deposited a smooth bed of fine sediment. Now, if we had any reason +to suppose that St. Helena had, during a long period, gone on slowly +subsiding, every difficulty would be removed: for looking at the +diagram, and imagining a fresh amount of subsidence, we can see that +the waves would then act on the coast-cliffs with fresh and unimpaired +vigour, whilst the rocky ledge near the beach would be carried down to +that depth, at which sand and mud would be deposited on its bare and +uneven surface: after the formation near the shore of a new rocky +shoal, fresh subsidence would carry it down and allow it to be smoothly +covered up. But in the case of the many cliff-bounded islands, for +instance in some of the Canary Islands and of Madeira, round which the +inclination of the strata shows that the land once extended far into +the depths of the sea, where there is no apparent means of hard rock +being worn away—are we to suppose that all these islands have slowly +subsided? Madeira, I may remark, has, according to Mr. Smith of Jordan +Hill, subsided. Are we to extend this conclusion to the high, cliff- +bound, horizontally stratified shores of Patagonia, off which, though +the water is not deep even at the distance of several miles, yet the +smooth bottom of pebbles gradually decreasing in size with the +increasing depth, and derived from a foreign source, seem to declare +that the sea is now a depositing and not a corroding agent? I am much +inclined to suspect, that we shall hereafter find in all such cases, +that the land with the adjoining bed of the sea has in truth subsided: +the time will, I believe, come, when geologists will consider it as +improbable, that the land should have retained the same level during a +whole geological period, as that the atmosphere should have remained +absolutely calm during an entire season. + + + + +CHAPTER II. +ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA. + + +Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions +of the inhabitants on this subject.—Concepcion, earthquake and +elevation of.—VALPARAISO, great elevation of, upraised shells, earth of +marine origin, gradual rise of the land within the historical +period.—COQUIMBO, elevation of, in recent times; terraces of marine +origin, their inclination, their escarpments not horizontal.—Guasco, +gravel terraces of.—Copiapo.—PERU.—Upraised shells of Cobija, Iquique, +and Arica.—Lima, shell-beds and sea-beach on San Lorenzo, human +remains, fossil earthenware, earthquake debacle, recent subsidence.—On +the decay of upraised shells.—General summary. + + +Commencing at the south and proceeding northward, the first place at +which I landed, was at Cape Tres Montes, in latitude 46 degrees 35′. +Here, on the shores of Christmas Cove, I observed in several places a +beach of pebbles with recent shells, about twenty feet above high-water +mark. Southward of Tres Montes (between latitude 47 and 48 degrees), +Byron remarks, “We thought it very strange, that upon the summits of +the highest hills were found beds of shells, a foot or two thick.” +(“Narrative of the Loss of the ‘Wager’.”) In the Chonos Archipelago, +the island of Lemus (latitude 44 degrees 30′) was, according to M. +Coste, suddenly elevated eight feet, during the earthquake of 1829: he +adds, “Des roches jadis toujours couvertes par la mer, restant +aujourd’hui constamment decouvertes.” (“Comptes Rendus” October 1838 +page 706.) In other parts of this archipelago, I observed two terraces +of gravel, abutting to the foot of each other: at Lowe’s Harbour (43 +degrees 48′), under a great mass of the boulder formation, about three +hundred feet in thickness, I found a layer of sand, with numerous +comminuted fragments of sea-shells, having a fresh aspect, but too +small to be identified. + +THE ISLAND OF CHILOE. + +The evidence of recent elevation is here more satisfactory. The bay of +San Carlos is in most parts bounded by precipitous cliffs from about +ten to forty feet in height, their bases being separated from the +present line of tidal action by a talus, a few feet in height, covered +with vegetation. In one sheltered creek (west of P. Arena), instead of +a loose talus, there was a bare sloping bank of tertiary mudstone, +perforated, above the line of the highest tides, by numerous shells of +a Pholas now common in the harbour. The upper extremities of these +shells, standing upright in their holes with grass growing out of them, +were abraded about a quarter of an inch, to the same level with the +surrounding worn strata. In other parts, I observed (as at Pudeto) a +great beach, formed of comminuted shells, twenty feet above the present +shore. In other parts again, there were small caves worn into the foot +of the low cliffs, and protected from the waves by the talus with its +vegetation: one such cave, which I examined, had its mouth about twenty +feet, and its bottom, which was filled with sand containing fragments +of shells and legs of crabs, from eight to ten feet above high-water +mark. From these several facts, and from the appearance of the upraised +shells, I inferred that the elevation had been quite recent; and on +inquiring from Mr. Williams, the Portmaster, he told me he was +convinced that the land had risen, or the sea fallen, four feet within +the last four years. During this period, there had been one severe +earthquake, but no particular change of level was then observed; from +the habits of the people who all keep boats in the protected creeks, it +is absolutely impossible that a rise of four feet could have taken +place suddenly and been unperceived. Mr. Williams believes that the +change has been quite gradual. Without the elevatory movement continues +at a quick rate, there can be no doubt that the sea will soon destroy +the talus of earth at the foot of the cliffs round the bay, and will +then reach its former lateral extension, but not of course its former +level: some of the inhabitants assured me that one such talus, with a +footpath on it, was even already sensibly decreasing in width. + +I received several accounts of beds of shells, existing at considerable +heights in the inland parts of Chiloe; and to one of these, near +Catiman, I was guided by a countryman. Here, on the south side of the +peninsula of Lacuy, there was an immense bed of the Venus costellata +and of an oyster, lying on the summit-edge of a piece of tableland, 350 +feet (by the barometer) above the level of the sea. The shells were +closely packed together, embedded in and covered by a very black, damp, +peaty mould, two or three feet in thickness, out of which a forest of +great trees was growing. Considering the nature and dampness of this +peaty soil, it is surprising that the fine ridges on the outside of the +Venus are perfectly preserved, though all the shells have a blackened +appearance. I did not doubt that the black soil, which when dry, cakes +hard, was entirely of terrestrial origin, but on examining it under the +microscope, I found many very minute rounded fragments of shells, +amongst which I could distinguish bits of Serpulae and mussels. The +Venus costellata, and the Ostrea (O. edulis, according to Captain King) +are now the commonest shells in the adjoining bays. In a bed of shells, +a few feet below the 350 feet bed, I found a horn of the little Cervus +humilis, which now inhabits Chiloe. + +The eastern or inland side of Chiloe, with its many adjacent islets, +consists of tertiary and boulder deposits, worn into irregular plains +capped by gravel. Near Castro, and for ten miles southward, and on the +islet of Lemuy, I found the surface of the ground to a height of +between twenty and thirty feet above high-water mark, and in several +places apparently up to fifty feet, thickly coated by much comminuted +shells, chiefly of the Venus costellata and Mytilus Chiloensis; the +species now most abundant on this line of coast. As the inhabitants +carry immense numbers of these shells inland, the continuity of the bed +at the same height was often the only means of recognising its natural +origin. Near Castro, on each side of the creek and rivulet of the +Gamboa, three distinct terraces are seen: the lowest was estimated at +about one hundred and fifty feet in height, and the highest at about +five hundred feet, with the country irregularly rising behind it; +obscure traces, also, of these same terraces could be seen along other +parts of the coast. There can be no doubt that their three escarpments +record pauses in the elevation of the island. I may remark that several +promontories have the word Huapi, which signifies in the Indian tongue, +island, appended to them, such as Huapilinao, Huapilacuy, Caucahuapi, +etc.; and these, according to Indian traditions, once existed as +islands. In the same manner the term Pulo in Sumatra is appended to the +names of promontories, traditionally said to have been islands +(Marsden’s “Sumatra” page 31.); in Sumatra, as in Chiloe, there are +upraised recent shells. The Bay of Carelmapu, on the mainland north of +Chiloe, according to Aguerros, was in 1643 a good harbour (“Descripcion +Hist. de la Provincia de Chiloe” page 78. From the account given by the +old Spanish writers, it would appear that several other harbours, +between this point and Concepcion, were formerly much deeper than they +now are.); it is now quite useless, except for boats. + +VALDIVIA. + +I did not observe here any distinct proofs of recent elevation; but in +a bed of very soft sandstone, forming a fringe-like plain, about sixty +feet in height, round the hills of mica-slate, there are shells of +Mytilus, Crepidula, Solen, Novaculina, and Cytheraea, too imperfect to +be specifically recognised. At Imperial, seventy miles north of +Valdivia, Aguerros states that there are large beds of shells, at a +considerable distance from the coast, which are burnt for lime. (Ibid +page 25.) The island of Mocha, lying a little north of Imperial, was +uplifted two feet, during the earthquake of 1835. (“Voyages of +‘Adventure’ and ‘Beagle’” volume 2 page 415.) + +CONCEPCION. + +I cannot add anything to the excellent account by Captain Fitzroy of +the elevation of the land at this place, which accompanied the +earthquake of 1835. (Ibid volume 2 page 412 et seq. In volume 5 page +601 of the “Geological Transactions” I have given an account of the +remarkable volcanic phenomena, which accompanied this earthquake. These +phenomena appear to me to prove that the action, by which large tracts +of land are uplifted, and by which volcanic eruptions are produced, is +in every respect identical.) I will only recall to the recollection of +geologists, that the southern end of the island of St. Mary was +uplifted eight feet, the central part nine, and the northern end ten +feet; and the whole island more than the surrounding districts. Great +beds of mussels, patellae, and chitons still adhering to the rocks were +upraised above high-water mark; and some acres of a rocky flat, which +was formerly always covered by the sea, was left standing dry, and +exhaled an offensive smell, from the many attached and putrefying +shells. It appears from the researches of Captain Fitzroy that both the +island of St. Mary and Concepcion (which was uplifted only four or five +feet) in the course of some weeks subsided, and lost part of their +first elevation. I will only add as a lesson of caution, that round the +sandy shores of the great Bay of Concepcion, it was most difficult, +owing to the obliterating effects of the great accompanying wave, to +recognise any distinct evidence of this considerable upheaval; one spot +must be excepted, where there was a detached rock which before the +earthquake had always been covered by the sea, but afterwards was left +uncovered. + +On the island of Quiriquina (in the Bay of Concepcion), I found, at an +estimated height of four hundred feet, extensive layers of shells, +mostly comminuted, but some perfectly preserved and closely packed in +black vegetable mould; they consisted of Concholepas, Fissurella, +Mytilus, Trochus, and Balanus. Some of these layers of shells rested on +a thick bed of bright-red, dry, friable earth, capping the surface of +the tertiary sandstone, and extending, as I observed whilst sailing +along the coast, for 150 miles southward: at Valparaiso, we shall +presently see that a similar red earthy mass, though quite like +terrestrial mould, is really in chief part of recent marine origin. On +the flanks of this island of Quiriquina, at a less height than the 400 +feet, there were spaces several feet square, thickly strewed with +fragments of similar shells. During a subsequent visit of the “Beagle” +to Concepcion, Mr. Kent, the assistant-surgeon, was so kind as to make +for me some measurements with the barometer: he found many marine +remains along the shores of the whole bay, at a height of about twenty +feet; and from the hill of Sentinella behind Talcahuano, at the height +of 160 feet, he collected numerous shells, packed together close +beneath the surface in black earth, consisting of two species of +Mytilus, two of Crepidula, one of Concholepas, of Fissurella, Venus, +Mactra, Turbo, Monoceros, and the Balanus psittacus. These shells were +bleached, and within some of the Balani other Balani were growing, +showing that they must have long lain dead in the sea. The above +species I compared with living ones from the bay, and found them +identical; but having since lost the specimens, I cannot give their +names: this is of little importance, as Mr. Broderip has examined a +similar collection, made during Captain Beechey’s expedition, and +ascertained that they consisted of ten recent species, associated with +fragments of Echini, crabs, and Flustrae; some of these remains were +estimated by Lieutenant Belcher to lie at the height of nearly a +thousand feet above the level of the sea. (“Zoology of Captain +Beechey’s Voyage” page 162.) In some places round the bay, Mr. Kent +observed that there were beds formed exclusively of the Mytilus +Chiloensis: this species now lives in parts never uncovered by the +tides. At considerable heights, Mr. Kent found only a few shells; but +from the summit of one hill, 625 feet high, he brought me specimens of +the Concholepas, Mytilus Chiloensis, and a Turbo. These shells were +softer and more brittle than those from the height of 164 feet; and +these latter had obviously a much more ancient appearance than the same +species from the height of only twenty feet. + +COAST NORTH OF CONCEPCION. + +The first point examined was at the mouth of the Rapel (160 miles north +of Concepcion and sixty miles south of Valparaiso), where I observed a +few shells at the height of 100 feet, and some barnacles adhering to +the rocks three or four feet above the highest tides: M. Gay found here +recent shells at the distance of two leagues from the shore. (“Annales +des Scienc. Nat.” Avril 1833.) Inland there are some wide, +gravel-capped plains, intersected by many broad, flat-bottomed valleys +(now carrying insignificant streamlets), with their sides cut into +successive wall-like escarpments, rising one above another, and in many +places, according to M. Gay, worn into caves. The one cave (C. del +Obispo) which I examined, resembled those formed on many sea-coasts, +with its bottom filled with shingle. These inland plains, instead of +sloping towards the coast, are inclined in an opposite direction +towards the Cordillera, like the successively rising terraces on the +inland or eastern side of Chiloe: some points of granite, which project +through the plains near the coast, no doubt once formed a chain of +outlying islands, on the inland shores of which the plains were +accumulated. At Bucalemu, a few miles northward of the Rapel, I +observed at the foot, and on the summit-edge of a plain, ten miles from +the coast, many recent shells, mostly comminuted, but some perfect. +There were, also, many at the bottom of the great valley of the Maypu. +At San Antonio, shells are said to be collected and burnt for lime. At +the bottom of a great ravine (Quebrada Onda, on the road to Casa +Blanca), at the distance of several miles from the coast, I noticed a +considerable bed, composed exclusively of Mesodesma donaciforme, Desh., +lying on a bed of muddy sand: this shell now lives associated together +in great numbers, on tidal-flats on the coast of Chile. + +VALPARAISO. + +During two successive years I carefully examined, part of the time in +company with Mr. Alison, into all the facts connected with the recent +elevation of this neighbourhood. In very many parts a beach of broken +shells, about fourteen or fifteen feet above high-water mark, may be +observed; and at this level the coast-rocks, where precipitous, are +corroded in a band. At one spot, Mr. Alison, by removing some birds’ +dung, found at this same level barnacles adhering to the rocks. For +several miles southward of the bay, almost every flat little headland, +between the heights of 60 and 230 feet (measured by the barometer), is +smoothly coated by a thick mass of comminuted shells, of the same +species, and apparently in the same proportional numbers with those +existing in the adjoining sea. The Concholepas is much the most +abundant, and the best preserved shell; but I extracted perfectly +preserved specimens of the Fissurella biradiata, a Trochus and Balanus +(both well-known, but according to Mr. Sowerby yet unnamed) and parts +of the Mytilus Chiloensis. Most of these shells, as well as an +encrusting Nullipora, partially retain their colour; but they are +brittle, and often stained red from the underlying brecciated mass of +primary rocks; some are packed together, either in black or reddish +moulds; some lie loose on the bare rocky surfaces. The total number of +these shells is immense; they are less numerous, though still far from +rare, up a height of 1,000 feet above the sea. On the summit of a hill, +measured 557 feet, there was a small horizontal band of comminuted +shells, of which MANY consisted (and likewise from lesser heights) of +very young and small specimens of the still living Concholepas, +Trochus, Patellae, Crepidulae, and of Mytilus Magellanicus (?) (Mr. +Cuming informs me that he does not think this species identical with, +though closely resembling, the true M. Magellanicus of the southern and +eastern coast of South America; it lives abundantly on the coast of +Chile.): several of these shells were under a quarter of an inch in +their greatest diameter. My attention was called to this circumstance +by a native fisherman, whom I took to look at these shell-beds; and he +ridiculed the notion of such small shells having been brought up for +food; nor could some of the species have adhered when alive to other +larger shells. On another hill, some miles distant, and 648 feet high, +I found shells of the Concholepas and Trochus, perfect, though very +old, with fragments of Mytilus Chiloensis, all embedded in +reddish-brown mould: I also found these same species, with fragments of +an Echinus and of Balanus psittacus, on a hill 1,000 feet high. Above +this height, shells became very rare, though on a hill 1,300 feet high +(Measured by the barometer: the highest point in the range behind +Valparaiso I found to be 1,626 feet above the level of the sea.), I +collected the Concholepas, Trochus, Fissurella, and a Patella. At these +greater heights the shells are almost invariably embedded in mould, and +sometimes are exposed only by tearing up bushes. These shells obviously +had a very much more ancient appearance than those from the lesser +heights; the apices of the Trochi were often worn down; the little +holes made by burrowing animals were greatly enlarged; and the +Concholepas was often perforated quite through, owing to the inner +plates of shell having scaled off. + +Many of these shells, as I have said, were packed in, and were quite +filled with, blackish or reddish-brown earth, resting on the granitic +detritus. I did not doubt until lately that this mould was of purely +terrestrial origin, when with a microscope examining some of it from +the inside of a Concholepas from the height of about one hundred feet, +I found that it was in considerable part composed of minute fragments +of the spines, mouth- bones, and shells of Echini, and of minute +fragments, of chiefly very young Patellae, Mytili, and other species. I +found similar microscopical fragments in earth filling up the central +orifices of some large Fissurellae. This earth when crushed emits a +sickly smell, precisely like that from garden-mould mixed with guano. +The earth accidentally preserved within the shells, from the greater +heights, has the same general appearance, but it is a little redder; it +emits the same smell when rubbed, but I was unable to detect with +certainty any marine remains in it. This earth resembles in general +appearance, as before remarked, that capping the rocks of Quiriquina in +the Bay of Concepcion, on which beds of sea-shells lay. I have, also, +shown that the black, peaty soil, in which the shells at the height of +350 feet at Chiloe were packed, contained many minute fragments of +marine animals. These facts appear to me interesting, as they show that +soils, which would naturally be considered of purely terrestrial +nature, may owe their origin in chief part to the sea. + +Being well aware from what I have seen at Chiloe and in Tierra del +Fuego, that vast quantities of shells are carried, during successive +ages, far inland, where the inhabitants chiefly subsist on these +productions, I am bound to state that at greater heights than 557 feet, +where the number of very young and small shells proved that they had +not been carried up for food, the only evidence of the shells having +been naturally left by the sea, consists in their invariable and +uniform appearance of extreme antiquity—in the distance of some of the +places from the coast, in others being inaccessible from the nearest +part of the beach, and in the absence of fresh water for men to +drink—in the shells NOT LYING IN HEAPS,—and, lastly, in the close +similarity of the soil in which they are embedded, to that which lower +down can be unequivocally shown to be in great part formed from the +debris of the sea animals. (In the “Proceedings of the Geological +Society” volume 2 page 446, I have given a brief account of the +upraised shells on the coast of Chile, and have there stated that the +proofs of elevation are not satisfactory above the height of 230 feet. +I had at that time unfortunately overlooked a separate page written +during my second visit to Valparaiso, describing the shells now in my +possession from the 557 feet hill; I had not then unpacked my +collections, and had not reconsidered the obvious appearance of greater +antiquity of the shells from the greater heights, nor had I at that +time discovered the marine origin of the earth in which many of the +shells are packed. Considering these facts, I do not now feel a shadow +of doubt that the shells, at the height of 1,300 feet, have been +upraised by natural causes into their present position.) + +With respect to the position in which the shells lie, I was repeatedly +struck here, at Concepcion, and at other places, with the frequency of +their occurrence on the summits and edges either of separate hills, or +of little flat headlands often terminating precipitously over the sea. +The several above-enumerated species of mollusca, which are found +strewed on the surface of the land from a few feet above the level of +the sea up to the height of 1,300 feet, all now live either on the +beach, or at only a few fathoms’ depth: Mr. Edmondston, in a letter to +Professor E. Forbes, states that in dredging in the Bay of Valparaiso, +he found the common species of Concholepas, Fissurella, Trochus, +Monoceros, Chitons, etc., living in abundance from the beach to a depth +of seven fathoms; and dead shells occurred only a few fathoms deeper. +The common Turritella cingulata was dredged up living at even from ten +to fifteen fathoms; but this is a species which I did not find here +amongst the upraised shells. Considering this fact of the species being +all littoral or sub-littoral, considering their occurrence at various +heights, their vast numbers, and their generally comminuted state, +there can be little doubt that they were left on successive beach-lines +during a gradual elevation of the land. The presence, however, of so +many whole and perfectly preserved shells appears at first a difficulty +on this view, considering that the coast is exposed to the full force +of an open ocean: but we may suppose, either that these shells were +thrown during gales on flat ledges of rock just above the level of +high-water mark, and that during the elevation of the land they are +never again touched by the waves, or, that during earthquakes, such as +those of 1822, 1835, and 1837, rocky reefs covered with marine-animals +were it one blow uplifted above the future reach of the sea. This +latter explanation is, perhaps, the most probable one with respect to +the beds at Concepcion entirely composed of the Mytilus Chiloensis, a +species which lives below the lowest tides; and likewise with respect +to the great beds occurring both north and south of Valparaiso, of the +Mesodesma donaciforme,—a shell which, as I am informed by Mr. Cuming, +inhabits sandbanks at the level of the lowest tides. But even in the +case of shells having the habits of this Mytilus and Mesodesma, beds of +them, wherever the sea gently throws up sand or mud, and thus protects +its own accumulations, might be upraised by the slowest movement, and +yet remain undisturbed by the waves of each new beach-line. + +It is worthy of remark, that nowhere near Valparaiso above the height +of twenty feet, or rarely of fifty feet, I saw any lines of erosion on +the solid rocks, or any beds of pebbles; this, I believe, may be +accounted for by the disintegrating tendency of most of the rocks in +this neighbourhood. Nor is the land here modelled into terraces: Mr. +Alison, however, informs me, that on both sides of one narrow ravine, +at the height of 300 feet above the sea, he found a succession of +rather indistinct step-formed beaches, composed of broken shells, which +together covered a space of about eighty feet vertical. + +I can add nothing to the accounts already published of the elevation of +the land at Valparaiso, which accompanied the earthquake of 1822 (Dr. +Meyen “Reise um Erde” Th. 1 s. 221, found in 1831 seaweed and other +bodies still adhering to some rocks which during the shock of 1822 were +lifted above the sea.): but I heard it confidently asserted, that a +sentinel on duty, immediately after the shock, saw a part of a fort, +which previously was not within the line of his vision, and this would +indicate that the uplifting was not horizontal: it would even appear +from some facts collected by Mr. Alison, that only the eastern half of +the bay was then elevated. Through the kindness of this same gentleman, +I am able to give an interesting account of the changes of level, which +have supervened here within historical periods: about the year 1680 a +long sea-wall (or Prefil) was built, of which only a few fragments now +remain; up to the year 1817, the sea often broke over it, and washed +the houses on the opposite side of the road (where the prison now +stands); and even in 1819, Mr. J. Martin remembers walking at the foot +of this wall, and being often obliged to climb over it to escape the +waves. There now stands (1834) on the seaward side of this wall, and +between it and the beach, in one part a single row of houses, and in +another part two rows with a street between them. This great extension +of the beach in so short a time cannot be attributed simply to the +accumulation of detritus; for a resident engineer measured for me the +height between the lowest part of the wall visible, and the present +beach-line at spring-tides, and the difference was eleven feet six +inches. The church of S. Augustin is believed to have been built in +1614, and there is a tradition that the sea formerly flowed very near +it; by levelling, its foundations were found to stand nineteen feet six +inches above the highest beach-line; so that we see in a period of 220 +years, the elevation cannot have been as much as nineteen feet six +inches. From the facts given with respect to the sea-wall, and from the +testimony of the elder inhabitants, it appears certain that the change +in level began to be manifest about the year 1817. The only sudden +elevation of which there is any record occurred in 1822, and this seems +to have been less than three feet. Since that year, I was assured by +several competent observers, that part of an old wreck, which is firmly +embedded near the beach, has sensibly emerged; hence here, as at +Chiloe, a slow rise of the land appears to be now in progress. It seems +highly probable that the rocks which are corroded in a band at the +height of fourteen feet above the sea were acted on during the period, +when by tradition the base of S. Augustin church, now nineteen feet six +inches above the highest water-mark, was occasionally washed by the +waves. + +VALPARAISO TO COQUIMBO. + +For the first seventy-five miles north of Valparaiso I followed the +coast- road, and throughout this space I observed innumerable masses of +upraised shells. About Quintero there are immense accumulations (worked +for lime) of the Mesodesma donaciforme, packed in sandy earth; they +abound chiefly about fifteen feet above high-water, but shells are here +found, according to Mr. Miers, to a height of 500 feet, and at a +distance of three leagues from the coast (“Travels in Chile” volume 1 +pages 395, 458. I received several similar accounts from the +inhabitants, and was assured that there are many shells on the plain of +Casa Blanca, between Valparaiso and Santiago, at the height of 800 +feet.): I here noticed barnacles adhering to the rocks three or four +feet above the highest tides. In the neighbourhood of Plazilla and +Catapilco, at heights of between two hundred and three hundred feet, +the number of comminuted shells, with some perfect ones, especially of +the Mesodesma, packed in layers, was truly immense: the land at +Plazilla had evidently existed as a bay, with abrupt rocky masses +rising out of it, precisely like the islets in the broken bays now +indenting this coast. On both sides of the rivers Ligua, Longotomo, +Guachen, and Quilimari, there are plains of gravel about two hundred +feet in height, in many parts absolutely covered with shells. Close to +Conchalee, a gravel-plain is fronted by a lower and similar plain about +sixty feet in height, and this again is separated from the beach by a +wide tract of low land: the surfaces of all three plains or terraces +were strewed with vast numbers of the Concholepas, Mesodesma, an +existing Venus, and other still existing littoral shells. The two upper +terraces closely resemble in miniature the plains of Patagonia; and +like them are furrowed by dry, flat-bottomed, winding valleys. +Northward of this place I turned inward; and therefore found no more +shells: but the valleys of Chuapa, Illapel, and Limari, are bounded by +gravel-capped plains, often including a lower terrace within. These +plains send bay-like arms between and into the surrounding hills; and +they are continuously united with other extensive gravel-capped plains, +separating the coast mountain-ranges from the Cordillera. + +COQUIMBO. + +A narrow fringe-like plain, gently inclined towards the sea, here +extends for eleven miles along the coast, with arms stretching up +between the coast-mountains, and likewise up the valley of Coquimbo: at +its southern extremity it is directly connected with the plain of +Limari, out of which hills abruptly rise like islets, and other hills +project like headlands on a coast. The surface of the fringe-like plain +appears level, but differs insensibly in height, and greatly in +composition, in different parts. + +At the mouth of the valley of Coquimbo, the surface consists wholly of +gravel, and stands from 300 to 350 feet above the level of the sea, +being about one hundred feet higher than in other parts. In these other +and lower parts the superficial beds consist of calcareous matter, and +rest on ancient tertiary deposits hereafter to be described. The +uppermost calcareous layer is cream-coloured, compact, +smooth-fractured, sub- stalactiform, and contains some sand, earthy +matter, and recent shells. It lies on, and sends wedge-like veins into, +a much more friable, calcareous, tuff-like variety; and both rest on a +mass about twenty feet in thickness, formed of fragments of recent +shells, with a few whole ones, and with small pebbles firmly cemented +together. (In many respects this upper hard, and the underlying more +friable, varieties, resemble the great superficial beds at King +George’s Sound in Australia, which I have described in my “Geological +Observations on Volcanic Islands.” There could be little doubt that the +upper layers there have been hardened by the action of rain on the +friable, calcareous matter, and that the whole mass has originated in +the decay of minutely comminuted sea-shells and corals.) This latter +rock is called by the inhabitants losa, and is used for building: in +many parts it is divided into strata, which dip at an angle of ten +degrees seaward, and appear as if they had originally been heaped in +successive layers (as may be seen on coral-reefs) on a steep beach. +This stone is remarkable from being in parts entirely formed of empty, +pellucid capsules or cells of calcareous matter, of the size of small +seeds: a series of specimens unequivocally showed that all these +capsules once contained minute rounded fragments of shells which have +since been gradually dissolved by water percolating through the mass. +(I have incidentally described this rock in the above work on Volcanic +Islands.) + +The shells embedded in the calcareous beds forming the surface of this +fringe-like plain, at the height of from 200 to 250 feet above the sea, +consist of:— + +1. Venus opaca. 2. Mulinia Byronensis. 3. Pecten purpuratus. 4. +Mesodesma donaciforme. 5. Turritella cingulata. 6. Monoceros costatum. +7. Concholepas Peruviana. 8. Trochus (common Valparaiso species). 9. +Calyptraea Byronensis. + +Although these species are all recent, and are all found in the +neighbouring sea, yet I was particularly struck with the difference in +the proportional numbers of the several species, and of those now cast +up on the present beach. I found only one specimen of the Concholepas, +and the Pecten was very rare, though both these shells are now the +commonest kinds, with the exception, perhaps, of the Calyptraea +radians, of which I did not find one in the calcareous beds. I will not +pretend to determine how far this difference in the proportional +numbers depends on the age of the deposit, and how far on the +difference in nature between the present sandy beaches and the +calcareous bottom, on which the embedded shells must have lived. + +(DIAGRAM 8.—SECTION OF PLAIN OF COQUIMBO. + +A Section through Plain B-B and Ravine A. + +Surface of plain 252 feet above sea. + +A. Stratified sand, with recent shells in same proportions as on the +beach, half filling up a ravine. + +B. Surface of plain, with scattered shells in nearly same proportions +as on the beach. + +C. Upper calcareous bed, and D. Lower calcareous sandy bed (Losa), both +with recent shells, but not in same proportions as on the beach. + +E. Upper ferrugino-sandy old tertiary stratum, and F. Lower old +tertiary stratum, both with all, or nearly all, extinct shells.) + +On the bare surface of the calcareous plain, or in a thin covering of +sand, there were lying, at a height from 200 to 252 feet, many recent +shells, which had a much fresher appearance than the embedded ones: +fragments of the Concholepas, and of the common Mytilus, still +retaining a tinge of its colour, were numerous, and altogether there +was manifestly a closer approach in proportional numbers to those now +lying on the beach. In a mass of stratified, slightly agglutinated +sand, which in some places covers up the lower half of the seaward +escarpment of the plain, the included shells appeared to be in exactly +the same proportional numbers with those on the beach. On one side of a +steep-sided ravine, cutting through the plain behind Herradura Bay, I +observed a narrow strip of stratified sand, containing similar shells +in similar proportional numbers; a section of the ravine is represented +in Diagram 8, which serves also to show the general composition of the +plain. I mention this case of the ravine chiefly because without the +evidence of the marine shells in the sand, any one would have supposed +that it had been hollowed out by simple alluvial action. + +The escarpment of the fringe-like plain, which stretches for eleven +miles along the coast, is in some parts fronted by two or three narrow, +step- formed terraces, one of which at Herradura Bay expands into a +small plain. Its surface was there formed of gravel, cemented together +by calcareous matter; and out of it I extracted the following recent +shells, which are in a more perfect condition than those from the upper +plain:— + +1. Calyptraea radians. 2. Turritella cingulata. 3. Oliva Peruviana. 4. +Murex labiosus, var. 5. Nassa (identical with a living species). 6. +Solen Dombeiana. 7. Pecten purpuratus. 8. Venus Chilensis. 9. +Amphidesma rugulosum. The small irregular wrinkles of the posterior +part of this shell are rather stronger than in the recent specimens of +this species from Coquimbo. (G.B. Sowerby.) 10. Balanus (identical with +living species). + +On the syenitic ridge, which forms the southern boundary of Herradura +Bay and Plain, I found the Concholepas and Turritella cingulata (mostly +in fragments), at the height of 242 feet above the sea. I could not +have told that these shells had not formerly been brought up by man, if +I had not found one very small mass of them cemented together in a +friable calcareous tuff. I mention this fact more particularly, because +I carefully looked, in many apparently favourable spots, at lesser +heights on the side of this ridge, and could not find even the smallest +fragment of a shell. This is only one instance out of many, proving +that the absence of sea-shells on the surface, though in many respects +inexplicable, is an argument of very little weight in opposition to +other evidence on the recent elevation of the land. The highest point +in this neighbourhood at which I found upraised shells of existing +species was on an inland calcareous plain, at the height of 252 feet +above the sea. + +It would appear from Mr. Caldcleugh’s researches, that a rise has taken +place here within the last century and a half (“Proceedings of the +Geological Society” volume 2 page 446.); and as no sudden change of +level has been observed during the not very severe earthquakes, which +have occasionally occurred here, the rising has probably been slow, +like that now, or quite lately, in progress at Chiloe and at +Valparaiso: there are three well-known rocks, called the Pelicans, +which in 1710, according to Feuillee, were a fleur d’eau, but now are +said to stand twelve feet above low-water mark: the spring-tides rise +here only five feet. There is another rock, now nine feet above +high-water mark, which in the time of Frezier and Feuillee rose only +five or six feet out of water. Mr. Caldcleugh, I may add, also shows +(and I received similar accounts) that there has been a considerable +decrease in the soundings during the last twelve years in the Bays of +Coquimbo, Concepcion, Valparaiso, and Guasco; but as in these cases it +is nearly impossible to distinguish between the accumulation of +sediment and the upheavement of the bottom, I have not entered into any +details. + +VALLEY OF COQUIMBO. + +(FIGURE 9. EAST AND WEST SECTION THROUGH THE TERRACES AT COQUIMBO, +WHERE THEY DEBOUCH FROM THE VALLEY, AND FRONT THE SEA. + +Vertical scale 1/10 of inch to 100 feet: horizontal scale much +contracted. + +Height of terrace in feet from east (high) to west (low): Terrace F. +364 Terrace E. 302 Terrace D. shown dotted, height not given. Terrace +C. 120 Terrace B. 70 Terrace A. 25 sloping down to level of sea at Town +of Coquimbo.) + +The narrow coast-plain sends, as before stated, an arm, or more +correctly a fringe, on both sides, but chiefly on the southern side, +several miles up the valley. These fringes are worn into steps or +terraces, which present a most remarkable appearance, and have been +compared (though not very correctly) by Captain Basil Hall, to the +parallel roads of Glen Roy in Scotland: their origin has been ably +discussed by Mr. Lyell. (“Principles of Geology” 1st edition volume 3 +page 131.) The first section which I will give (Figure 9), is not drawn +across the valley, but in an east and west line at its mouth, where the +step-formed terraces debouch and present their very gently inclined +surfaces towards the Pacific. + +The bottom plain (A) is about a mile in width, and rises quite +insensibly from the beach to a height of twenty-five feet at the foot +of the next plain; it is sandy, and abundantly strewed with shells. + +Plain or terrace B is of small extent, and is almost concealed by the +houses of the town, as is likewise the escarpment of terrace C. On both +sides of a ravine, two miles south of the town, there are two little +terraces, one above the other, evidently corresponding with B and C; +and on them marine remains of the species already enumerated were +plentiful. Terrace E is very narrow, but quite distinct and level; a +little southward of the town there were traces of a terrace D +intermediate between E and C. Terrace F is part of the fringe-like +plain, which stretches for the eleven miles along the coast; it is here +composed of shingle, and is 100 feet higher than where composed of +calcareous matter. This greater height is obviously due to the quantity +of shingle, which at some former period has been brought down the great +valley of Coquimbo. + +Considering the many shells strewed over the terraces A, B, and C, and +a few miles southward on the calcareous plain, which is continuously +united with the upper step-like plain F, there cannot, I apprehend, be +any doubt, that these six terraces have been formed by the action of +the sea; and that their five escarpments mark so many periods of +comparative rest in the elevatory movement, during which the sea wore +into the land. The elevation between these periods may have been sudden +and on AN AVERAGE not more than seventy-two feet each time, or it may +have been gradual and insensibly slow. From the shells on the three +lower terraces, and on the upper one, and I may add on the three +gravel-capped terraces at Conchalee, being all littoral and +sub-littoral species, and from the analogical facts given at +Valparaiso, and lastly from the evidence of a slow rising lately or +still in progress here, it appears to me far more probable that the +movement has been slow. The existence of these successive escarpments, +or old cliff- lines, is in another respect highly instructive, for they +show periods of comparative rest in the elevatory movement, and of +denudation, which would never even have been suspected from a close +examination of many miles of coast southward of Coquimbo. + +(FIGURE 10. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF COQUIMBO. + +From north F (high) through E?, D, C, B, A (low), B?, C, D?, E, F +(high). + +Vertical scale 1/10 of inch to 100 feet: horizontal scale much +contracted. + +Terraces marked with ? do not occur on that side of the valley, and are +introduced only to make the diagram more intelligible. A river and +bottom- plain of valley C, E, and F, on the south side of valley, are +respectively, 197, 377, and 420 feet above the level of the sea. + +AA. The bottom of the valley, believed to be 100 feet above the sea: it +is continuously united with the lowest plain A of Figure 9. + +B. This terrace higher up the valley expands considerably; seaward it +is soon lost, its escarpment being united with that of C: it is not +developed at all on the south side of the valley. + +C. This terrace, like the last, is considerably expanded higher up the +valley. These two terraces apparently correspond with B and C of Figure +9. + +D is not well developed in the line of this section; but seaward it +expands into a plain: it is not present on the south side of the +valley; but it is met with, as stated under the former section, a +little south of the town. + +E is well developed on the south side, but absent on the north side of +the valley: though not continuously united with E of Figure 9, it +apparently corresponds with it. + +F. This is the surface-plain, and is continuously united with that +which stretches like a fringe along the coast. In ascending the valley +it gradually becomes narrower, and is at last, at the distance of about +ten miles from the sea, reduced to a row of flat-topped patches on the +sides of the mountains. None of the lower terraces extend so far up the +valley.) + +We come now to the terraces on the opposite sides of the east and west +valley of Coquimbo: the section in Figure 10 is taken in a north and +south line across the valley at a point about three miles from the sea. +The valley measured from the edges of the escarpments of the upper +plain FF is about a mile in width; but from the bases of the bounding +mountains it is from three to four miles wide. The terraces marked with +an interrogative do not exist on that side of the valley, but are +introduced merely to render the diagram more intelligible. + +These five terraces are formed of shingle and sand; three of them, as +marked by Captain B. Hall (namely, B, C, and F), are much more +conspicuous than the others. From the marine remains copiously strewed +at the mouth of the valley on the lower terraces, and southward of the +town on the upper one, they are, as before remarked, undoubtedly of +marine origin; but within the valley, and this fact well deserves +notice, at a distance of from only a mile and a half to three or four +miles from the sea, I could not find even a fragment of a shell. + +ON THE INCLINATION OF THE TERRACES OF COQUIMBO, AND ON THE UPPER AND +BASAL EDGES OF THEIR ESCARPMENTS NOT BEING HORIZONTAL. + +The surfaces of these terraces slope in a slight degree, as shown by +the sections in Figures 9 and 10 taken conjointly, both towards the +centre of the valley, and seawards towards its mouth. This double or +diagonal inclination, which is not the same in the several terraces, +is, as we shall immediately see, of simple explanation. There are, +however, some other points which at first appear by no means +obvious,—namely, first, that each terrace, taken in its whole breadth +from the summit-edge of one escarpment to the base of that above it, +and followed up the valley, is not horizontal; nor have the several +terraces, when followed up the valley, all the same inclination; thus I +found the terraces C, E, and F, measured at a point about two miles +from the mouth of the valley, stood severally between fifty-six to +seventy-seven feet higher than at the mouth. Again, if we look to any +one line of cliff or escarpment, neither its summit-edge nor its base +is horizontal. On the theory of the terraces having been formed during +a slow and equable rise of the land, with as many intervals of rest as +there are escarpments, it appears at first very surprising that +horizontal lines of some kind should not have been left on the land. + +The direction of the diagonal inclination in the different terraces +being different,—in some being directed more towards the middle of the +valley, in others more towards its mouth,—naturally follows on the view +of each terrace, being an accumulation of successive beach-lines round +bays, which must have been of different forms and sizes when the land +stood at different levels: for if we look to the actual beach of a +narrow creek, its slope is directed towards the middle; whereas, in an +open bay, or slight concavity on a coast, the slope is towards the +mouth, that is, almost directly seaward; hence as a bay alters in form +and size, so will the direction of the inclination of its successive +beaches become changed. + +(FIGURE 11. DIAGRAM OF A BAY IN A DISTRICT WHICH HAS BEGUN SLOWLY +RISING) + +If it were possible to trace any one of the many beach-lines, composing +each sloping terrace, it would of course be horizontal; but the only +lines of demarcation are the summit and basal edges of the escarpments. +Now the summit-edge of one of these escarpments marks the furthest line +or point to which the sea has cut into a mass of gravel sloping +seaward; and as the sea will generally have greater power at the mouth +than at the protected head of the bay, so will the escarpment at the +mouth be cut deeper into the land, and its summit-edge be higher; +consequently it will not be horizontal. With respect to the basal or +lower edges of the escarpments, from picturing in one’s mind ancient +bays ENTIRELY surrounded at successive periods by cliff-formed shores, +one’s first impression is that they at least necessarily must be +horizontal, if the elevation has been horizontal. But here is a +fallacy: for after the sea has, during a cessation of the elevation, +worn cliffs all round the shores of a bay, when the movement +recommences, and especially if it recommences slowly, it might well +happen that, at the exposed mouth of the bay, the waves might continue +for some time wearing into the land, whilst in the protected and upper +parts successive beach-lines might be accumulating in a sloping surface +or terrace at the foot of the cliffs which had been lately reached: +hence, supposing the whole line of escarpment to be finally uplifted +above the reach of the sea, its basal line or foot near the mouth will +run at a lower level than in the upper and protected parts of the bay; +consequently this basal line will not be horizontal. And it has already +been shown that the summit-edges of each escarpment will generally be +higher near the mouth (from the seaward sloping land being there most +exposed and cut into) than near the head of the bay; therefore the +total height of the escarpments will be greatest near the mouth; and +further up the old bay or valley they will on both sides generally thin +out and die away: I have observed this thinning out of the successive +escarpment at other places besides Coquimbo; and for a long time I was +quite unable to understand its meaning. The rude diagram in Figure 11 +will perhaps render what I mean more intelligible; it represents a bay +in a district which has begun slowly rising. Before the movement +commenced, it is supposed that the waves had been enabled to eat into +the land and form cliffs, as far up, but with gradually diminishing +power, as the points AA: after the movement had commenced and gone on +for a little time, the sea is supposed still to have retained the +power, at the exposed mouth of the bay, of cutting down and into the +land as it slowly emerged; but in the upper parts of the bay it is +supposed soon to have lost this power, owing to the more protected +situation and to the quantity of detritus brought down by the river; +consequently low land was there accumulated. As this low land was +formed during a slow elevatory movement, its surface will gently slope +upwards from the beach on all sides. Now, let us imagine the bay, not +to make the diagram more complicated, suddenly converted into a valley: +the basal line of the cliffs will of course be horizontal, as far as +the beach is now seen extending in the diagram; but in the upper part +of the valley, this line will be higher, the level of the district +having been raised whilst the low land was accumulating at the foot of +the inland cliffs. If, instead of the bay in the diagram being suddenly +converted into a valley, we suppose with much more probability it to be +upraised slowly, then the waves in the upper parts of the bay will +continue very gradually to fail to reach the cliffs, which are now in +the diagram represented as washed by the sea, and which, consequently, +will be left standing higher and higher above its level; whilst at the +still exposed mouth, it might well happen that the waves might be +enabled to cut deeper and deeper, both down and into the cliffs, as the +land slowly rose. + +The greater or lesser destroying power of the waves at the mouths of +successive bays, comparatively with this same power in their upper and +protected parts, will vary as the bays become changed in form and size, +and therefore at different levels, at their mouths and heads, more or +less of the surfaces between the escarpments (that is, the accumulated +beach-lines or terraces) will be left undestroyed: from what has gone +before we can see that, according as the elevatory movements after each +cessation recommence more or less slowly, according to the amount of +detritus delivered by the river at the heads of the successive bays, +and according to the degree of protection afforded by their altered +forms, so will a greater or less extent of terrace be accumulated in +the upper part, to which there will be no surface at a corresponding +level at the mouth: hence we can perceive why no one terrace, taken in +its whole breadth and followed up the valley, is horizontal, though +each separate beach-line must have been so; and why the inclination of +the several terraces, both transversely, and longitudinally up the +valley, is not alike. + +I have entered into this case in some detail, for I was long perplexed +(and others have felt the same difficulty) in understanding how, on the +idea of an equable elevation with the sea at intervals eating into the +land, it came that neither the terraces nor the upper nor lower edges +of the escarpments were horizontal. Along lines of coast, even of great +lengths, such as that of Patagonia, if they are nearly uniformly +exposed, the corroding power of the waves will be checked and conquered +by the elevatory movement, as often as it recommences, at about the +same period; and hence the terraces, or accumulated beach-lines, will +commence being formed at nearly the same levels: at each succeeding +period of rest, they will, also, be eaten into at nearly the same rate, +and consequently there will be a much closer coincidence in their +levels and inclinations, than in the terraces and escarpments formed +round bays with their different parts very differently exposed to the +action of the sea. It is only where the waves are enabled, after a long +lapse of time, slowly to corrode hard rocks, or to throw up, owing to +the supply of sediment being small and to the surface being steeply +inclined, a narrow beach or mound, that we can expect, as at Glen Roy +in Scotland (“Philosophical Transactions” 1839 page 39.), a distinct +line marking an old sea-level, and which will be strictly horizontal, +if the subsequent elevatory movements have been so: for in these cases +no discernible effects will be produced, except during the long +intervening periods of rest; whereas in the case of step-formed coasts, +such as those described in this and the preceding chapter, the terraces +themselves are accumulated during the slow elevatory process, the +accumulation commencing sooner in protected than in exposed situations, +and sooner where there is copious supply of detritus than where there +is little; on the other hand, the steps or escarpments are formed +during the stationary periods, and are more deeply cut down and into +the coast-land in exposed than in protected situations;—the cutting +action, moreover, being prolonged in the most exposed parts, both +during the beginning and ending, if slow, of the upward movement. + +Although in the foregoing discussion I have assumed the elevation to +have been horizontal, it may be suspected, from the considerable +seaward slope of the terraces, both up the valley of S. Cruz and up +that of Coquimbo, that the rising has been greater inland than nearer +the coast. There is reason to believe (Mr. Place in the “Quarterly +Journal of Science” 1824 volume 17 page 42.), from the effects produced +on the water-course of a mill during the earthquake of 1822 in Chile, +that the upheaval one mile inland was nearly double, namely, between +five and seven feet, to what it was on the Pacific. We know, also, from +the admirable researches of M. Bravais, that in Scandinavia the ancient +sea-beaches gently slope from the interior mountain-ranges towards the +coast, and that they are not parallel one to the other (“Voyages de la +Comm. du Nord” etc. also “Comptes Rendus” October 1842.), showing that +the proportional difference in the amount of elevation on the coast and +in the interior, varied at different periods. + +COQUIMBO TO GUASCO. + +In this distance of ninety miles, I found in almost every part marine +shells up to a height of apparently from two hundred to three hundred +feet. The desert plain near Choros is thus covered; it is bounded by +the escarpment of a higher plain, consisting of pale-coloured, earthy, +calcareous stone, like that of Coquimbo, with the same recent shells +embedded in it. In the valley of Chaneral, a similar bed occurs in +which, differently from that of Coquimbo, I observed many shells of the +Concholepas: near Guasco the same calcareous bed is likewise met with. + +In the valley of Guasco, the step-formed terraces of gravel are +displaced in a more striking manner than at any other point. I followed +the valley for thirty-seven miles (as reckoned by the inhabitants) from +the coast to Ballenar; in nearly the whole of this distance, five grand +terraces, running at corresponding heights on both sides of the broad +valley, are more conspicuous than the three best-developed ones at +Coquimbo. They give to the landscape the most singular and formal +aspect; and when the clouds hung low, hiding the neighbouring +mountains, the valley resembled in the most striking manner that of +Santa Cruz. The whole thickness of these terraces or plains seems +composed of gravel, rather firmly aggregated together, with occasional +parting seams of clay: the pebbles on the upper plain are often +whitewashed with an aluminous substance, as in Patagonia. Near the +coast I observed many sea-shells on the lower plains. At Freyrina +(twelve miles up the valley), there are six terraces beside the bottom- +surface of the valley: the two lower ones are here only from two +hundred to three hundred yards in width, but higher up the valley they +expand into plains; the third terrace is generally narrow; the fourth I +saw only in one place, but there it was distinct for the length of a +mile; the fifth is very broad; the sixth is the summit-plain, which +expands inland into a great basin. Not having a barometer with me, I +did not ascertain the height of these plains, but they appeared +considerably higher than those at Coquimbo. Their width varies much, +sometimes being very broad, and sometimes contracting into mere fringes +of separate flat-topped projections, and then quite disappearing: at +the one spot, where the fourth terrace was visible, the whole six +terraces were cut off for a short space by one single bold escarpment. +Near Ballenar (thirty-seven miles from the mouth of the river), the +valley between the summit-edges of the highest escarpments is several +miles in width, and the five terraces on both sides are broadly +developed: the highest cannot be less than six hundred feet above the +bed of the river, which itself must, I conceive, be some hundred feet +above the sea. + +A north and south section across the valley in this part is represented +in Figure 12. + +(FIGURE 12. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF GUASCO, AND OF +A PLAIN NORTH OF IT. + +From left (north, high) to right (south, high) through plains B and A +and the River of Guasco at the Town of Ballenar.) + +On the northern side of the valley the summit-plain of gravel, A, has +two escarpments, one facing the valley, and the other a great +basin-like plain, B, which stretches for several leagues northward. +This narrow plain, A, with the double escarpment, evidently once formed +a spit or promontory of gravel, projecting into and dividing two great +bays, and subsequently was worn on both sides into steep cliffs. +Whether the several escarpments in this valley were formed during the +same stationary periods with those of Coquimbo, I will not pretend to +conjecture; but if so the intervening and subsequent elevatory +movements must have been here much more energetic, for these plains +certainly stand at a much higher level than do those of Coquimbo. + +COPIAPO. + +From Guasco to Copiapo, I followed the road near the foot of the +Cordillera, and therefore saw no upraised remains. At the mouth, +however, of the valley of Copiapo there is a plain, estimated by Meyen +(“Reise um die Erde” th. 1 s. 372 et seq.) between fifty and seventy +feet in height, of which the upper part consists chiefly of gravel, +abounding with recent shells, chiefly of the Concholepas, Venus +Dombeyi, and Calyptraea trochiformis. A little inland, on a plain +estimated by myself at nearly three hundred feet, the upper stratum was +formed of broken shells and sand cemented by white calcareous matter, +and abounding with embedded recent shells, of which the Mulinia +Byronensis and Pecten purpuratus were the most numerous. The lower +plain stretches for some miles southward, and for an unknown distance +northward, but not far up the valley; its seaward face, according to +Meyen, is worn into caves above the level of the present beach. The +valley of Copiapo is much less steeply inclined and less direct in its +course than any other valley which I saw in Chile; and its bottom does +not generally consist of gravel: there are no step-formed terraces in +it, except at one spot near the mouth of the great lateral valley of +the Despoblado where there are only two, one above the other: lower +down the valley, in one place I observed that the solid rock had been +cut into the shape of a beach, and was smoothed over with shingle. + +Northward of Copiapo, in latitude 26 degrees S., the old voyager Wafer +found immense numbers of sea-shells some miles from the coast. +(Burnett’s “Collection of Voyages” volume 4 page 193.) At Cobija +(latitude 22 degrees 34′) M. d’Orbigny observed beds of gravel and +broken shells, containing ten species of recent shells; he also found, +on projecting points of porphyry, at a height of 300 feet, shells of +Concholepas, Chiton, Calyptraea, Fissurella, and Patella, still +attached to the spots on which they had lived. M. d’Orbigny argues from +this fact, that the elevation must have been great and sudden (“Voyage, +Part Geolog.” page 94. M. d’Orbigny (page 98), in summing up, says: +“S’il est certain (as he believes) que tous les terrains en pente, +compris entre la mer et les montagnes sont l’ancien rivage de la mer, +on doit supposer, pour l’ensemble, un exhaussement que ce ne serait pas +moindre de deux cent metres; il faudrait supposer encore que ce +soulevement n’a point ete graduel;...mais qu’il resulterait d’une seule +et meme cause fortuite,” etc. Now, on this view, when the sea was +forming the beach at the foot of the mountains, many shells of +Concholepas, Chiton, Calyptraea, Fissurella, and Patella (which are +known to live close to the beach), were attached to rocks at a depth of +300 feet, and at a depth of 600 feet several of these same shells were +accumulating in great numbers in horizontal beds. From what I have +myself seen in dredging, I believe this to be improbable in the highest +degree, if not impossible; and I think everyone who has read Professor +E. Forbes’s excellent researches on the subject, will without +hesitation agree in this conclusion.): to me it appears far more +probable that the movement was gradual, with small starts as during the +earthquakes of 1822 and 1835, by which whole beds of shells attached to +the rocks were lifted above the subsequent reach of the waves. M. +d’Orbigny also found rolled pebbles extending up the mountain to a +height of at least six hundred feet. At Iquique (latitude 20 degrees +12′ S.), in a great accumulation of sand, at a height estimated between +one hundred and fifty and two hundred feet, I observed many large +sea-shells which I thought could not have been blown up by the wind to +that height. Mr. J.H. Blake has lately described these shells: he +states that “inland toward the mountains they form a compact uniform +bed, scarcely a trace of the original shells being discernible; but as +we approach the shore, the forms become gradually more distinct till we +meet with the living shells on the coast.” (“Silliman’s American +Journal of Science” volume 44 page 2.) This interesting observation, +showing by the gradual decay of the shells how slowly and gradually the +coast must have been uplifted, we shall presently see fully confirmed +at Lima. At Arica (latitude 18 degrees 28′), M. d’Orbigny found a great +range of sand-dunes, fourteen leagues in length, stretching towards +Tacna, including recent shells and bones of Cetacea, and reaching up to +a height of 300 feet above the sea. (“Voyage” etc. page 101.) +Lieutenant Freyer has given some more precise facts: he states (In a +letter to Mr. Lyell “Geological Proceedings” volume 2 page 179.) that +the Morro of Arica is about four hundred feet high; it is worn into +obscure terraces, on the bare rock of which he found Balini and +Milleporae adhering. At the height of between twenty and thirty feet +the shells and corals were in a quite fresh state, but at fifty feet +they were much abraded; there were, however, traces of organic remains +at greater heights. On the road from Tacna to Arequipa, between +Loquimbo and Moquegua, Mr. M. Hamilton found numerous recent sea shells +in sand, at a considerable distance from the sea. (“Edinburgh New +Philosophical Journal” volume 30 page 155.) + +LIMA. + +Northward of Arica, I know nothing of the coast for about a space of +five degrees of latitude; but near Callao, the port of Lima, there is +abundant and very curious evidence of the elevation of the land. The +island of San Lorenzo is upwards of one thousand feet high; the basset +edges of the strata composing the lower part are worn into three +obscure, narrow, sloping steps or ledges, which can be seen only when +standing on them: they probably resemble those described by Lieutenant +Freyer at Arica. The surface of the lower ledge, which extends from a +low cliff overhanging the sea to the foot of the next upper escarpment, +is covered by an enormous accumulation of recent shells. (M. Chevalier, +in the “Voyage of the ‘Bonite’” observed these shells; but his +specimens were lost.—“L’Institut” 1838 page 151.) The bed is level, and +in some parts more than two feet in thickness; I traced it over a space +of one mile in length, and heard of it in other places: the uppermost +part is eighty-five feet by the barometer above high-water mark. The +shells are packed together, but not stratified: they are mingled with +earth and stones, and are generally covered by a few inches of +detritus; they rest on a mass of nearly angular fragments of the +underlying sandstone, sometimes cemented together by common salt. I +collected eighteen species of shells of all ages and sizes. Several of +the univalves had evidently long lain dead at the bottom of the sea, +for their INSIDES were incrusted with Balani and Serpulae. All, +according to Mr. G.B. Sowerby, are recent species: they consist of:— + +1. Mytilus Magellanicus: same as that found at Valparaiso, and there +stated to be probably distinct from the true M. Magellanicus of the +east coast. + +2. Venus costellata, Sowerby “Zoological Proceedings.” + +3. Pecten purpuratus, Lam. + +4. Chama, probably echinulata, Brod. + +5. Calyptraea Byronensis, Gray. + +6. Calyptraea radians (Trochus, Lam.) + +7. Fissurella affinis, Gray. + +8. Fissurella biradiata, Trembly. + +9. Purpura chocolatta, Duclos. + +10. Purpura Peruviana, Gray. + +11. Purpura labiata, Gray. + +12. Purpura buxea (Murex, Brod.). + +13. Concholepas Peruviana. + +14. Nassa, related to reticulata. + +15. Triton rudis, Brod. + +16. Trochus, not yet described, but well-known and very common. + +17 and 18. Balanus, two species, both common on the coast. + +These upraised shells appear to be nearly in the same proportional +numbers- -with the exception of the Crepidulae being more numerous—with +those on the existing beach. The state of preservation of the different +species differed much; but most of them were much corroded, brittle, +and bleached: the upper and lower surfaces of the Concholepas had +generally quite scaled off: some of the Trochi and Fissurellae still +partially retain their colours. It is remarkable that these shells, +taken all together, have fully as ancient an appearance, although the +extremely arid climate appears highly favourable for their +preservation, as those from 1,300 feet at Valparaiso, and certainly a +more ancient appearance than those from five to six hundred feet from +Valparaiso and Concepcion; at which places I have seen grass and other +vegetables actually growing out of the shells. Many of the univalves +here at San Lorenzo were filled with, and united together by, pure +salt, probably left by the evaporation of the sea-spray, as the land +slowly emerged. (The underlying sandstone contains true layers of salt; +so that the salt may possibly have come from the beds in the higher +parts of the island; but I think more probably from the sea-spray. It +is generally asserted that rain never falls on the coast of Peru; but +this is not quite accurate; for, on several days, during our visit, the +so-called Peruvian dew fell in sufficient quantity to make the streets +muddy, and it would certainly have washed so deliquescent a substance +as salt into the soil. I state this because M. d’Orbigny, in discussing +an analogous subject, supposes that I had forgotten that it never rains +on this whole line of coast. See Ulloa’s “Voyage” volume 2 English +Translation page 67 for an account of the muddy streets of Lima, and on +the continuance of the mists during the whole winter. Rain, also, falls +at rare intervals even in the driest districts, as, for instance, +during forty days, in 1726, at Chocope (7 degrees 46′); this rain +entirely ruined (“Ulloa” etc. page 18) the mud houses of the +inhabitants.) On the highest parts of the ledge, small fragments of the +shells were mingled with, and evidently in process of reduction into, a +yellowish-white, soft, calcareous powder, tasting strongly of salt, and +in some places as fine as prepared medicinal chalk. + +FOSSIL-REMAINS OF HUMAN ART. + +In the midst of these shells on San Lorenzo, I found light corallines, +the horny ovule-cases of Mollusca, roots of seaweed (Mr. Smith of +Jordan Hill found pieces of seaweed in an upraised pleistocene deposit +in Scotland. See his admirable Paper in the “Edinburgh New +Philosophical Journal” volume 25 page 384.), bones of birds, the heads +of Indian corn and other vegetable matter, a piece of woven rushes, and +another of nearly decayed COTTON string. I extracted these remains by +digging a hole, on a level spot; and they had all indisputably been +embedded with the shells. I compared the plaited rush, the COTTON +string, and Indian corn, at the house of an antiquary, with similar +objects, taken from the Huacas or burial-grounds of the ancient +Peruvians, and they were undistinguishable; it should be observed that +the Peruvians used string only of cotton. The small quantity of sand or +gravel with the shells, the absence of large stones, the width and +thickness of the bed, and the time requisite for a ledge to be cut into +the sandstone, all show that these remains were not thrown high up by +an earthquake-wave: on the other hand, these facts, together with the +number of dead shells, and of floating objects, both marine and +terrestrial, both natural and human, render it almost certain that they +were accumulated on a true beach, since upraised eighty-five feet, and +upraised this much since INDIAN MAN INHABITED PERU. The elevation may +have been, either by several small sudden starts, or quite gradual; in +this latter case the unrolled shells having been thrown up during gales +beyond the reach of the waves which afterwards broke on the slowly +emerging land. I have made these remarks, chiefly because I was at +first surprised at the complete difference in nature, between this +broad, smooth, upraised bed of shells, and the present shingle-beach at +the foot of the low sandstone-cliffs; but a beach formed, when the sea +is cutting into the land, as is shown now to be the case by the low +bare sandstone-cliffs, ought not to be compared with a beach +accumulated on a gently inclined rocky surface, at a period when the +sea (probably owing to the elevatory movement in process) was not able +to eat into the land. With respect to the mass of nearly angular, salt- +cemented fragments of sandstone, which lie under the shells, and which +are so unlike the materials of an ordinary sea-beach; I think it +probable after having seen the remarkable effects of the earthquake of +1835 (I have described this in my “Journal of Researches” page 303 2nd +edition.), in absolutely shattering as if by gunpowder the SURFACE of +the primary rocks near Concepcion, that a smooth bare surface of stone +was left by the sea covered by the shelly mass, and that afterwards +when upraised, it was superficially shattered by the severe shocks so +often experienced here. + +The very low land surrounding the town of Callao, is to the south +joined by an obscure escarpment to a higher plain (south of Bella +Vista), which stretches along the coast for a length of about eight +miles. This plain appears to the eye quite level; but the sea-cliffs +show that its height varies (as far as I could estimate) from seventy +to one hundred and twenty feet. It is composed of thin, sometimes +waving, beds of clay, often of bright red and yellow colours, of layers +of impure sand, and in one part with a great stratified mass of +granitic pebbles. These beds are capped by a remarkable mass, varying +from two to six feet in thickness, of reddish loam or mud, containing +many scattered and broken fragments of recent marine shells, sometimes +though rarely single large round pebble, more frequently short +irregular layers of fine gravel, and very many pieces of red coarse +earthenware, which from their curvatures must once have formed parts of +large vessels. The earthenware is of Indian manufacture; and I found +exactly similar pieces accidentally included within the bricks, of +which the neighbouring ancient Peruvian burial-mounds are built. These +fragments abounded in such numbers in certain spots, that it appeared +as if waggon-loads of earthenware had been smashed to pieces. The +broken sea- shells and pottery are strewed both on the surface, and +throughout the whole thickness of this upper loamy mass. I found them +wherever I examined the cliffs, for a space of between two and three +miles, and for half a mile inland; and there can be little doubt that +this same bed extends with a smooth surface several miles further over +the entire plain. Besides the little included irregular layers of small +pebbles, there are occasionally very obscure traces of stratification. + +At one of the highest parts of the cliff, estimated 120 feet above the +sea, where a little ravine came down, there were two sections, at right +angles to each other, of the floor of a shed or building. In both +sections or faces, two rows, one over the other, of large round stones +could be distinctly seen; they were packed close together on an +artificial layer of sand two inches thick, which had been placed on the +natural clay-beds; the round stones were covered by three feet in +thickness of the loam with broken sea-shells and pottery. Hence, before +this widely spread-out bed of loam was deposited, it is certain that +the plain was inhabited; and it is probable, from the broken vessels +being so much more abundant in certain spots than in others, and from +the underlying clay being fitted for their manufacture, that the kilns +stood here. + +The smoothness and wide extent of the plain, the bulk of matter +deposited, and the obscure traces of stratification seem to indicate +that the loam was deposited under water; on the other hand, the +presence of sea-shells, their broken state, the pebbles of various +sizes, and the artificial floor of round stones, almost prove that it +must have originated in a rush of water from the sea over the land. The +height of the plain, namely, 120 feet, renders it improbable that an +earthquake-wave, vast as some have here been, could have broken over +the surface at its present level; but when the land stood eighty-five +feet lower, at the period when the shells were thrown up on the ledge +at S. Lorenzo, and when as we know man inhabited this district, such an +event might well have occurred; and if we may further suppose, that the +plain was at that time converted into a temporary lake, as actually +occurred, during the earthquakes of 1713 and 1746, in the case of the +low land round Callao owing to its being encircled by a high +shingle-beach, all the appearances above described will be perfectly +explained. I must add, that at a lower level near the point where the +present low land round Callao joins the higher plain, there are +appearances of two distinct deposits both apparently formed by +debacles: in the upper one, a horse’s tooth and a dog’s jaw were +embedded; so that both must have been formed after the settlement of +the Spaniards: according to Acosta, the earthquake-wave of 1586 rose +eighty-four feet. + +The inhabitants of Callao do not believe, as far as I could ascertain, +that any change in level is now in progress. The great fragments of +brickwork, which it is asserted can be seen at the bottom of the sea, +and which have been adduced as a proof of a late subsidence, are, as I +am informed by Mr. Gill, a resident engineer, loose fragments; this is +probable, for I found on the beach, and not near the remains of any +building, masses of brickwork, three and four feet square, which had +been washed into their present places, and smoothed over with shingle +during the earthquake of 1746. The spit of land, on which the ruins of +OLD Callao stand, is so extremely low and narrow, that it is improbable +in the highest degree that a town should have been founded on it in its +present state; and I have lately heard that M. Tschudi has come to the +conclusion, from a comparison of old with modern charts, that the coast +both south and north of Callao has subsided. (I am indebted for this +fact to Dr. E. Dieffenbach. I may add that there is a tradition, that +the islands of San Lorenzo and Fronton were once joined, and that the +channel between San Lorenzo and the mainland, now above two miles in +width, was so narrow that cattle used to swim over.) I have shown that +the island of San Lorenzo has been upraised eighty-five feet since the +Peruvians inhabited this country; and whatever may have been the amount +of recent subsidence, by so much more must the elevation have exceeded +the eighty-five feet. In several places in this neighbourhood, marks of +sea-action have been observed: Ulloa gives a detailed account of such +appearances at a point five leagues northward of Callao: Mr. Cruikshank +found near Lima successive lines of sea-cliffs, with rounded blocks at +their bases, at a height of 700 feet above the present level of the +sea. (“Observaciones sobre el Clima del Lima” par Dr. H. Unanue page +4.—Ulloa’s “Voyage” volume 2 English Translation page 97.—For Mr. +Cruikshank’s observations, see Mr. Lyell’s “Principles of Geology” 1st +edition volume 3 page 130.) ON THE DECAY OF UPRAISED SEA-SHELLS. + +I have stated that many of the shells on the lower inclined ledge or +terrace of San Lorenzo are corroded in a peculiar manner, and that they +have a much more ancient appearance than the same species at +considerably greater heights on the coast of Chile. I have, also, +stated that these shells in the upper part of the ledge, at the height +of eighty-five feet above the sea, are falling, and in some parts are +quite changed into a fine, soft, saline, calcareous powder. The finest +part of this powder has been analysed for me, at the request of Sir H. +De la Beche, by the kindness of Mr. Trenham Reeks of the Museum of +Economic Geology; it consists of carbonate of lime in abundance, of +sulphate and muriate of lime, and of muriate and sulphate of soda. The +carbonate of lime is obviously derived from the shells; and common salt +is so abundant in parts of the bed, that, as before remarked, the +univalves are often filled with it. The sulphate of lime may have been +derived, as has probably the common salt, from the evaporation of the +sea-spray, during the emergence of the land; for sulphate of lime is +now copiously deposited from the spray on the shores of Ascension. (See +“Volcanic Islands” etc. by the Author.) The other saline bodies may +perhaps have been partially thus derived, but chiefly, as I conclude +from the following facts, through a different means. + +On most parts of the second ledge or old sea-beach, at a height of 170 +feet, there is a layer of white powder of variable thickness, as much +in some parts as two inches, lying on the angular, salt-cemented +fragments of sandstone and under about four inches of earth, which +powder, from its close resemblance in nature to the upper and most +decayed parts of the shelly mass, I can hardly doubt originally existed +as a bed of shells, now much collapsed and quite disintegrated. I could +not discover with the microscope a trace of organic structure in it; +but its chemical constituents, according to Mr. Reeks, are the same as +in the powder extracted from amongst the decaying shells on the lower +ledge, with the marked exception that the carbonate of lime is present +in only very small quantity. On the third and highest ledge, I observed +some of this powder in a similar position, and likewise occasionally in +small patches at considerably greater heights near the summit of the +island. At Iquique, where the whole face of the country is covered by a +highly saliferous alluvium, and where the climate is extremely dry, we +have seen that, according to Mr. Blake, the shells which are perfect +near the beach become, in ascending, gradually less and less perfect, +until scarcely a trace of their original structure can be discovered. +It is known that carbonate of lime and common salt left in a mass +together, and slightly moistened, partially decompose each other (I am +informed by Dr. Kane, through Mr. Reeks, that a manufactory was +established on this principle in France, but failed from the small +quantity of carbonate of soda produced. Sprengel “Gardeners’ Chronicle” +1845 page 157, states, that salt and carbonate of lime are liable to +mutual decomposition in the soil. Sir H. De la Beche informs me, that +calcareous rocks washed by the spray of the sea, are often corroded in +a peculiar manner; see also on this latter subject “Gardeners’ +Chronicle” page 675 1844.): now we have at San Lorenzo and at Iquique, +in the shells and salt packed together, and occasionally moistened by +the so- called Peruvian dew, the proper elements for this action. We +can thus understand the peculiar corroded appearance of the shells on +San Lorenzo, and the great decrease of quantity in the carbonate of +lime in the powder on the upper ledge. There is, however, a great +difficulty on this view, for the resultant salts should be carbonate of +soda and muriate of lime; the latter is present, but not the carbonate +of soda. Hence I am led to the perhaps unauthorised conjecture (which I +shall hereafter have to refer to) that the carbonate of soda, by some +unexplained means, becomes converted into a sulphate. + +If the above remarks be just, we are led to the very unexpected +conclusion, that a dry climate, by leaving the salt from the sea-spray +undissolved, is much less favourable to the preservation of upraised +shells than a humid climate. However this may be, it is interesting to +know the manner in which masses of shells, gradually upraised above the +sea-level, decay and finally disappear. + +A SUMMARY ON THE RECENT ELEVATION OF THE WEST COAST OF SOUTH AMERICA. + +We have seen that upraised marine remains occur at intervals, and in +some parts almost continuously, from latitude 45 degrees 35′ to 12 +degrees S., along the shores of the Pacific. This is a distance, in a +north and south line, of 2,075 geographical miles. From Byron’s +observations, the elevation has no doubt extended sixty miles further +south; and from the similarity in the form of the country near Lima, it +has probably extended many leagues further north. (I may take this +opportunity of stating that in a MS. in the Geological Society by Mr. +Weaver, it is stated that beds of oysters and other recent shells are +found thirty feet above the level of the sea, in many parts of Tampico, +in the Gulf of Mexico.) Along this great line of coast, besides the +organic remains, there are in very many parts, marks of erosion, caves, +ancient beaches, sand-dunes, and successive terraces of gravel, all +above the present level of the sea. From the steepness of the land on +this side of the continent, shells have rarely been found at greater +distances inland than from two to three leagues; but the marks of +sea-action are evident farther from the coast; for instance, in the +valley of Guasco, at a distance of between thirty and forty miles. +Judging from the upraised shells alone, the elevation in Chiloe has +been 350 feet, at Concepcion certainly 625 feet; and by estimation +1,000 feet; at Valparaiso 1,300 feet; at Coquimbo 252 feet; northward +of this place, sea-shells have not, I believe, been found above 300 +feet; and at Lima they were falling into decay (hastened probably by +the salt) at 85 feet. Not only has this amount of elevation taken place +within the period of existing Mollusca and Cirripedes; but their +proportional numbers in the neighbouring sea have in most cases +remained the same. Near Lima, however, a small change in this respect +between the living and the upraised was observed: at Coquimbo this was +more evident, all the shells being existing species, but with those +embedded in the uppermost calcareous plain not approximating so closely +in proportional numbers, as do those that lie loose on its surface at +the height of 252 feet, and still less closely than those which are +strewed on the lower plains, which latter are identical in proportional +numbers with those now cast up on the beach. From this circumstance, +and from not finding, upon careful examination, near Coquimbo any +shells at a greater height than 252 feet, I believe that the recent +elevation there has been much less than at Valparaiso, where it has +been 1,300 feet, and I may add, than at Concepcion. This considerable +inequality in the amount of elevation at Coquimbo and Valparaiso, +places only 200 miles apart, is not improbable, considering, first, the +difference in the force and number of the shocks now yearly affecting +different parts of this coast; and, secondly, the fact of single areas, +such as that of the province of Concepcion, having been uplifted very +unequally during the same earthquake. It would, in most cases, be very +hazardous to infer an inequality of elevation, from shells being found +on the surface or in superficial beds at different heights; for we do +not know on what their rate of decay depends; and at Coquimbo one +instance out of many has been given, of a promontory, which, from the +occurrence of one very small collection of lime-cemented shells, has +indisputably been elevated 242 feet, and yet on which, not even a +fragment of shell could be found on careful examination between this +height and the beach, although many sites appeared very favourable for +the preservation of organic remains: the absence, also, of shells on +the gravel-terraces a short distance up the valley of Coquimbo, though +abundant on the corresponding terraces at its mouth, should be borne in +mind. + +There are other epochs, besides that of the existence of recent +Mollusca, by which to judge of the changes of level on this coast. At +Lima, as we have just seen, the elevation has been at least eighty-five +feet, within the Indo-human period; and since the arrival of the +Spaniards in 1530, there has apparently been a sinking of the surface. +At Valparaiso, in the course of 220 years, the rise must have been less +than nineteen feet; but it has been as much as from ten to eleven feet +in the seventeen years subsequently to 1817, and of this rise only a +part can be attributed to the earthquake of 1822, the remainder having +been insensible and apparently still, in 1834, in progress. At Chiloe +the elevation has been gradual, and about four feet during four years. +At Coquimbo, also, it has been gradual, and in the course of 150 years +has amounted to several feet. The sudden small upheavals, accompanied +by earthquakes, as in 1822 at Valparaiso, in 1835 at Concepcion, and in +1837 in the Chonos Archipelago, are familiar to most geologists, but +the gradual rising of the coast of Chile has been hardly noticed; it +is, however, very important, as connecting together these two orders of +events. + +The rise of Lima, having been eighty-five feet within the period of +man, is the more surprising if we refer to the eastern coast of the +continent, for at Port S. Julian, in Patagonia, there is good evidence +(as we shall hereafter see) that when the land stood ninety feet lower, +the Macrauchenia, a mammiferous beast, was alive; and at Bahia Blanca, +when it stood only a few feet lower than it now does, many gigantic +quadrupeds ranged over the adjoining country. But the coast of +Patagonia is some way distant from the Cordillera, and the movement at +Bahia Blanca is perhaps noways connected with this great range, but +rather with the tertiary volcanic rocks of Banda Oriental, and +therefore the elevation at these places may have been infinitely slower +than on the coast of Peru. All such speculations, however, must be +vague, for as we know with certainty that the elevation of the whole +coast of Patagonia has been interrupted by many and long pauses, who +will pretend to say that, in such cases, many and long periods of +subsidence may not also have been intercalated? + +In many parts of the coast of Chile and Peru there are marks of the +action of the sea at successive heights on the land, showing that the +elevation has been interrupted by periods of comparative rest in the +upward movement, and of denudation in the action of the sea. These are +plainest at Chiloe, where, in a height of about five hundred feet, +there are three escarpments,—at Coquimbo, where in a height of 364 +feet, there are five,— at Guasco, where there are six, of which five +may perhaps correspond with those at Coquimbo, but if so, the +subsequent and intervening elevatory movements have been here much more +energetic,—at Lima, where, in a height of about 250 feet there are +three terraces, and others, as it is asserted, at considerably greater +heights. The almost entire absence of ancient marks of sea-action at +defined levels along considerable spaces of coast, as near Valparaiso +and Concepcion, is highly instructive, for as it is improbable that the +elevation at these places alone should have been continuous, we must +attribute the absence of such marks to the nature and form of the +coast-rocks. Seeing over how many hundred miles of the coast of +Patagonia, and on how many places on the shores of the Pacific, the +elevatory process has been interrupted by periods of comparative rest, +we may conclude, conjointly with the evidence drawn from other quarters +of the world, that the elevation of the land is generally an +intermittent action. From the quantity of matter removed in the +formation of the escarpments, especially of those of Patagonia, it +appears that the periods of rest in the movement, and of denudation of +the land, have generally been very long. In Patagonia, we have seen +that the elevation has been equable, and the periods of denudation +synchronous over very wide spaces of coast; on the shores of the +Pacific, owing to the terraces chiefly occurring in the valleys, we +have not equal means of judging on this point; and the very different +heights of the upraised shells at Coquimbo, Valparaiso, and Concepcion +seem directly opposed to such a conclusion. + +Whether on this side of the continent the elevation, between the +periods of comparative rest when the escarpments were formed, has been +by small sudden starts, such as those accompanying recent earthquakes, +or, as is most probable, by such starts conjointly with a gradual +upward movement, or by great and sudden upheavals, I have no direct +evidence. But as on the eastern coast, I was led to think, from the +analogy of the last hundred feet of elevation in La Plata, and from the +nearly equal size of the pebbles over the entire width of the terraces, +and from the upraised shells being all littoral species, that the +elevation had been gradual; so do I on this western coast, from the +analogy of the movements now in progress, and from the vast numbers of +shells now living exclusively on or close to the beach, which are +strewed over the whole surface of the land up to very considerable +heights, conclude, that the movement here also has been slow and +gradual, aided probably by small occasional starts. We know at least +that at Coquimbo, where five escarpments occur in a height of 364 feet, +the successive elevations, if they have been sudden, cannot have been +very great. It has, I think, been shown that the occasional +preservation of shells, unrolled and unbroken, is not improbable even +during a quite gradual rising of the land; and their preservation, if +the movement has been aided by small starts, is quite conformable with +what actually takes place during recent earthquakes. + +Judging from the present action of the sea, along the shores of the +Pacific, on the deposits of its own accumulation, the present time +seems in most places to be one of comparative rest in the elevatory +movement, and of denudation of the land. Undoubtedly this is the case +along the whole great length of Patagonia. At Chiloe, however, we have +seen that a narrow sloping fringe, covered with vegetation, separates +the present sea-beach from a line of low cliffs, which the waves lately +reached; here, then, the land is gaining in breadth and height, and the +present period is not one of rest in the elevation and of contingent +denudation; but if the rising be not prolonged at a quick rate, there +is every probability that the sea will soon regain its former +horizontal limits. I observed similar low sloping fringes on several +parts of the coast, both northward of Valparaiso and near Coquimbo; but +at this latter place, from the change in form which the coast has +undergone since the old escarpments were worn, it may be doubted +whether the sea, acting for any length of time at its present level, +would eat into the land; for it now rather tends to throw up great +masses of sand. It is from facts such as these that I have generally +used the term COMPARATIVE rest, as applied to the elevation of the +land; the rest or cessation in the movement being comparative both with +what has preceded it and followed it, and with the sea’s power of +corrosion at each spot and at each level. Near Lima, the cliff-formed +shores of San Lorenzo, and on the mainland south of Callao, show that +the sea is gaining on the land; and as we have here some evidence that +its surface has lately subsided or is still sinking, the periods of +comparative rest in the elevation and of contingent denudation, may +probably in many cases include periods of subsidence. It is only, as +was shown in detail when discussing the terraces of Coquimbo, when the +sea with difficulty and after a long lapse of time has either corroded +a narrow ledge into solid rock, or has heaped up on a steep surface a +NARROW mound of detritus, that we can confidently assert that the land +at that level and at that period long remained absolutely stationary. +In the case of terraces formed of gravel or sand, although the +elevation may have been strictly horizontal, it may well happen that no +one level beach-line may be traceable, and that neither the terraces +themselves nor the summit nor basal edges of their escarpments may be +horizontal. + +Finally, comparing the extent of the elevated area, as deduced from the +upraised recent organic remains, on the two sides of the continent, we +have seen that on the Atlantic, shells have been found at intervals +from Eastern Tierra del Fuego for 1,180 miles northward, and on the +Pacific for a space of 2,075 miles. For a length of 775 miles, they +occur in the same latitudes on both sides of the continent. Without +taking this circumstance into consideration, it is probable from the +reasons assigned in the last chapter, that the entire breadth of the +continent in Central Patagonia has been uplifted in mass; but from +other reasons there given, it would be hazardous to extend this +conclusion to La Plata. From the continent being narrow in the +southern-most parts of Patagonia, and from the shells found at the +Inner Narrows of the Strait of Magellan, and likewise far up the valley +of the Santa Cruz, it is probable that the southern part of the western +coast, which was not visited by me, has been elevated within the period +of recent Mollusca: if so, the shores of the Pacific have been +continuously, recently, and in a geological sense synchronously +upraised, from Lima for a length of 2,480 nautical miles southward,—a +distance equal to that from the Red Sea to the North Cape of +Scandinavia! + + + + +CHAPTER III. +ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS. + + +Basin-like plains of Chile; their drainage, their marine origin.—Marks +of sea-action on the eastern flanks of the Cordillera.—Sloping +terrace-like fringes of stratified shingle within the valleys of the +Cordillera; their marine origin.—Boulders in the valley of +Cachapual.—Horizontal elevation of the Cordillera.—Formation of +valleys.—Boulders moved by earthquake-waves.—Saline superficial +deposits.—Bed of nitrate of soda at Iquique.—Saline +incrustations.—Salt-lakes of La Plata and Patagonia; purity of the +salt; its origin. + + +The space between the Cordillera and the coast of Chile is on a rude +average from eighty to above one hundred miles in width; it is formed, +either of an almost continuous mass of mountains, or more commonly of +several nearly parallel ranges, separated by plains; in the more +southern parts of this province the mountains are quite subordinate to +the plains; in the northern part the mountains predominate. + +The basin-like plains at the foot of the Cordillera are in several +respects remarkable; that on which the capital of Chile stands is +fifteen miles in width, in an east and west line, and of much greater +length in a north and south line; it stands 1,750 feet above the sea; +its surface appears smooth, but really falls and rises in wide gentle +undulations, the hollows corresponding with the main valleys of the +Cordillera: the striking manner in which it abruptly comes up to the +foot of this great range has been remarked by every author since the +time of Molina. (This plain is partially separated into two basins by a +range of hills; the southern half, according to Meyen (“Reise um Erde” +Th. 1 s. 274), falls in height, by an abrupt step, of between fifteen +and twenty feet.) Near the Cordillera it is composed of a stratified +mass of pebbles of all sizes, occasionally including rounded boulders: +near its western boundary, it consists of reddish sandy clay, +containing some pebbles and numerous fragments of pumice, and sometimes +passes into pure sand or into volcanic ashes. At Podaguel, on this +western side of the plain, beds of sand are capped by a calcareous +tuff, the uppermost layers being generally hard and substalagmitic, and +the lower ones white and friable, both together precisely resembling +the beds at Coquimbo, which contain recent marine shells. Abrupt, but +rounded, hummocks of rock rise out of this plain: those of Sta. Lucia +and S. Cristoval are formed of greenstone-porphyry almost entirely +denuded of its original covering of porphyritic claystone breccia; on +their summits, many fragments of rock (some of them kinds not found in +situ) are coated and united together by a white, friable, calcareous +tuff, like that found at Podaguel. When this matter was deposited on +the summit of S. Cristoval, the water must have stood 946 feet above +the surface of the surrounding plain. (Or 2,690 feet above the sea, as +measured barometrically by Mr. Eck. This tuff appears to the eye nearly +pure; but when placed in acid it leaves a considerable residue of sand +and broken crystals, apparently of feldspar. Dr. Meyen (“Reise” Th. 1 +s. 269) says he found a similar substance on the neighbouring hill of +Dominico (and I found it also on the Cerro Blanco), and he attributes +it to the weathering of the stone. In some places which I examined, its +bulk put this view of its origin quite out of the question; and I +should much doubt whether the decomposition of a porphyry would, in any +case, leave a crust chiefly composed of carbonate of lime. The white +crust, which is commonly seen on weathered feldspathic rocks, does not +appear to contain any free carbonate of lime.) + +To the south this basin-like plain contracts, and rising scarcely +perceptibly with a smooth surface, passes through a remarkable level +gap in the mountains, forming a true land-strait, and called the +Angostura. It then immediately expands into a second basin-formed +plain: this again to the south contracts into another land-strait, and +expands into a third basin, which, however, falls suddenly in level +about forty feet. This third basin, to the south, likewise contracts +into a strait, and then again opens into the great plain of San +Fernando, stretching so far south that the snowy peaks of the distant +Cordillera are seen rising above its horizon as above the sea. These +plains, near the Cordillera, are generally formed of a thick stratified +mass of shingle (The plain of San Fernando has, according to MM. Meyen +and Gay “Reise” etc. Th. 1 ss. 295 and 298, near the Cordillera, an +upper step-formed plain of clay, on the surface of which they found +numerous blocks of rocks, from two to three feet long, either lying +single or piled in heaps, but all arranged in nearly straight lines.); +in other parts, of a red sandy clay, often with an admixture of +pumiceous matter. Although these basins are connected together like a +necklace, in a north and south line, by smooth land-straits, the +streams which drain them do not all flow north and south, but mostly +westward, through breaches worn in the bounding mountains; and in the +case of the second basin, or that of Rancagua, there are two distinct +breaches. Each basin, moreover, is not drained singly; thus, to give +the most striking instance, but not the only one, in proceeding +southward over the plain of Rancagua, we first find the water flowing +northward to and through the northern land-strait; then, without +crossing any marked ridge or watershed, we see it flowing +south-westward towards the northern one of the two breaches in the +western mountainous boundary; and lastly, again without any ridge, it +flows towards the southern breach in these same mountains. Hence the +surface of this one basin-like plain, appearing to the eye so level, +has been modelled with great nicety, so that the drainage, without any +conspicuous watersheds, is directed towards three openings in the +encircling mountains. ((It appears from Captain Herbert’s account of +the Diluvium of the Himalaya, “Gleanings of Science” Calcutta volume 2 +page 164, that precisely similar remarks apply to the drainage of the +plains or valleys between those great mountains.) The streams flowing +from the southern basin-like plains, after passing through the breaches +to the west, unite and form the river Rapel, which enters the Pacific +near Navidad. I followed the southernmost branch of this river, and +found that the basin or plain of San Fernando is continuously and +smoothly united with those plains, which were described in the Second +Chapter, as being worn near the coast into successive cave-eaten +escarpments, and still nearer to the coast, as being strewed with +upraised recent marine remains. + +I might have given descriptions of numerous other plains of the same +general form, some at the foot of the Cordillera, some near the coast, +and some halfway between these points. I will allude only to one other, +namely, the plain of Uspallata, lying on the eastern or opposite side +of the Cordillera, between that great range and the parallel lower +range of Uspallata. According to Miers, its surface is 6,000 feet above +the level of the sea: it is from ten to fifteen miles in width, and is +said to extend with an unbroken surface for 180 miles northwards: it is +drained by two rivers passing through breaches in the mountains to the +east. On the banks of the River Mendoza it is seen to be composed of a +great accumulation of stratified shingle, estimated at 400 feet in +thickness. In general appearance, and in numerous points of structure, +this plain closely resembles those of Chile. + +The origin and manner of formation of the thick beds of gravel, sandy +clay, volcanic detritus, and calcareous tuff, composing these +basin-like plains, is very important; because, as we shall presently +show, they send arms or fringes far up the main valleys of the +Cordillera. Many of the inhabitants believe that these plains were once +occupied by lakes, suddenly drained; but I conceive that the number of +the separate breaches at nearly the same level in the mountains +surrounding them quite precludes this idea. Had not such distinguished +naturalists as MM. Meyen and Gay stated their belief that these +deposits were left by great debacles rushing down from the Cordillera, +I should not have noticed a view, which appears to me from many reasons +improbable in the highest degree—namely, from the vast accumulation of +WELL-ROUNDED PEBBLES—their frequent stratification with layers of +sand—the overlying beds of calcareous tuff—this same substance coating +and uniting the fragments of rock on the hummocks in the plain of +Santiago—and lastly even from the worn, rounded, and much denuded state +of these hummocks, and of the headlands which project from the +surrounding mountains. On the other hand, these several circumstances, +as well as the continuous union of the basins at the foot of the +Cordillera, with the great plain of the Rio Rapel which still retains +the marks of sea-action at various levels, and their general similarity +in form and composition with the many plains near the coast, which are +either similarly marked or are strewed with upraised marine remains, +fully convince me that the mountains bounding these basin-plains were +breached, their islet-like projecting rocks worn, and the loose +stratified detritus forming their now level surfaces deposited, by the +sea, as the land slowly emerged. It is hardly possible to state too +strongly the perfect resemblance in outline between these basin-like, +long, and narrow plains of Chile (especially when in the early morning +the mists hanging low represented water), and the creeks and fiords now +intersecting the southern and western shores of the continent. We can +on this view of the sea, when the land stood lower, having long and +tranquilly occupied the spaces between the mountain-ranges, understand +how the boundaries of the separate basins were breached in more than +one place; for we see that this is the general character of the inland +bays and channels of Tierra del Fuego; we there, also, see in the +sawing action of the tides, which flow with great force in the cross +channels, a power sufficient to keep the breaches open as the land +emerged. We can further see that the waves would naturally leave the +smooth bottom of each great bay or channel, as it became slowly +converted into land, gently inclined to as many points as there were +mouths, through which the sea finally retreated, thus forming so many +watersheds, without any marked ridges, on a nearly level surface. The +absence of marine remains in these high inland plains cannot be +properly adduced as an objection to their marine origin: for we may +conclude, from shells not being found in the great shingle beds of +Patagonia, though copiously strewed on their surfaces, and from many +other analogous facts, that such deposits are eminently unfavourable +for the embedment of such remains; and with respect to shells not being +found strewed on the surface of these basin-like plains, it was shown +in the last chapter that remains thus exposed in time decay and +disappear. + +(FIGURE 13. SECTION OF THE PLAIN AT THE EASTERN FOOT OF THE CHILEAN +CORDILLERA. + +From Cordillera (left) through Talus-plain and Level surface, 2,700 +feet above sea, to Gravel terraces (right).) + +I observed some appearances on the plains at the eastern and opposite +foot of the Cordillera which are worth notice, as showing that the sea +there long acted at nearly the same level as on the basin-plains of +Chile. The mountains on this eastern side are exceedingly abrupt; they +rise out of a smooth, talus-like, very gentle, slope, from five to ten +miles in width (as represented in Figure 13), entirely composed of +perfectly rounded pebbles, often white-washed with an aluminous +substance like decomposed feldspar. This sloping plain or talus blends +into a perfectly flat space a few miles in width, composed of reddish +impure clay, with small calcareous concretions as in the Pampean +deposit,—of fine white sand with small pebbles in layers,—and of the +above-mentioned white aluminous earth, all interstratified together. +This flat space runs as far as Mendoza, thirty miles northward, and +stands probably at about the same height, namely, 2,700 feet (Pentland +and Miers) above the sea. To the east it is bounded by an escarpment, +eighty feet in height, running for many miles north and south, and +composed of perfectly round pebbles, and loose, white-washed, or +embedded in the aluminous earth: behind this escarpment there is a +second and similar one of gravel. Northward of Mendoza, these +escarpments become broken and quite obliterated; and it does not appear +that they ever enclosed a lake-like area: I conclude, therefore, that +they were formed by the sea, when it reached the foot of the +Cordillera, like the similar escarpments occurring at so many points on +the coasts of Chile and Patagonia. + +The talus-like plain slopes up with a smooth surface into the great dry +valleys of the Cordillera. On each hand of the Portillo valley, the +mountains are formed of red granite, mica-slate, and basalt, which all +have suffered a truly astonishing amount of denudation; the gravel in +the valley, as well as on the talus-like plain in front of it, is +composed of these rocks; but at the mouth of the valley, in the middle +(height probably about three thousand five hundred feet above the sea), +a few small isolated hillocks of several varieties of porphyry project, +round which, on all sides, smooth and often white-washed pebbles of +these same porphyries, to the exclusion of all others, extend to a +circumscribed distance. Now, it is difficult to conceive any other +agency, except the quiet and long-continued action of the sea on these +hillocks, which could have rounded and whitewashed the fragments of +porphyry, and caused them to radiate from such small and quite +insignificant centres, in the midst of that vast stream of stones which +has descended from the main Cordillera. + +SLOPING TERRACES OF GRAVEL IN THE VALLEYS OF THE CORDILLERA. + +(FIGURE 14. GROUND-PLAN OF A BIFURCATING VALLEY IN THE CORDILLERA, +bordered by smooth, sloping gravel-fringes (AA), worn along the course +of the river into cliffs.) + +All the main valleys on both flanks of the Chilean Cordillera have +formerly had, or still have, their bottoms filled up to a considerable +thickness by a mass of rudely stratified shingle. In Central Chile the +greater part of this mass has been removed by the torrents; +cliff-bounded fringes, more or less continuous, being left at +corresponding heights on both sides of the valleys. These fringes, or +as they may be called terraces, have a smooth surface, and as the +valleys rise, they gently rise with them: hence they are easily +irrigated, and afford great facilities for the construction of the +roads. From their uniformity, they give a remarkable character to the +scenery of these grand, wild, broken valleys. In width, the fringes +vary much, sometimes being only broad enough for the roads, and +sometimes expanding into narrow plains. Their surfaces, besides gently +rising up the valley, are slightly inclined towards its centre in such +a manner as to show that the whole bottom must once have been filled up +with a smooth and slightly concave mass, as still are the dry +unfurrowed valleys of Northern Chile. Where two valleys unite into one, +these terraces are particularly well exhibited, as is represented in +Figure 14. The thickness of the gravel forming these fringes, on a rude +average, may be said to vary from thirty to sixty or eighty feet; but +near the mouths of the valleys it was in several places from two to +three hundred feet. The amount of matter removed by the torrents has +been immense; yet in the lower parts of the valleys the terraces have +seldom been entirely worn away on either side, nor has the solid +underlying rock been reached: higher up the valleys, the terraces have +frequently been removed on one or the other side, and sometimes on both +sides; but in this latter case they reappear after a short interval on +the line, which they would have held had they been unbroken. Where the +solid rock has been reached, it has been cut into deep and narrow +gorges. Still higher up the valleys, the terraces gradually become more +and more broken, narrower, and less thick, until, at a height of from +seven to nine thousand feet, they become lost, and blended with the +piles of fallen detritus. + +I carefully examined in many places the state of the gravel, and almost +everywhere found the pebbles equally and perfectly rounded, +occasionally with great blocks of rock, and generally distinctly +stratified, often with parting seams of sand. The pebbles were +sometimes coated with a white aluminous, and less frequently with a +calcareous, crust. At great heights up the valleys the pebbles become +less rounded; and as the terraces become obliterated, the whole mass +passes into the nature of ordinary detritus. I was repeatedly struck +with the great difference between this detritus high up the valleys, +and the gravel of the terraces low down, namely, in the greater number +of the quite angular fragments in the detritus,—in the unequal degree +to which the other fragments have been rounded,—in the quantity of +associated earth,—in the absence of stratification,—and in the +irregularity of the upper surfaces. This difference was likewise well +shown at points low down the valleys, where precipitous ravines, +cutting through mountains of highly coloured rock, have thrown down +wide, fan- shaped accumulations of detritus on the terraces: in such +cases, the line of separation between the detritus and the terrace +could be pointed out to within an inch or two; the detritus consisting +entirely of angular and only partially rounded fragments of the +adjoining coloured rocks; the stratified shingle (as I ascertained by +close inspection, especially in one case, in the valley of the River +Mendoza) containing only a small proportion of these fragments, and +those few well rounded. + +I particularly attended to the appearance of the terraces where the +valleys made abrupt and considerable bends, but I could perceive no +difference in their structure: they followed the bends with their usual +nearly equable inclination. I observed, also, in several valleys, that +wherever large blocks of any rock became numerous, either on the +surface of the terrace or embedded in it, this rock soon appeared +higher up in situ: thus I have noticed blocks of porphyry, of andesitic +syenite, of porphyry and of syenite, alternately becoming numerous, and +in each case succeeded by mountains thus constituted. There is, +however, one remarkable exception to this rule; for along the valley of +the Cachapual, M. Gay found numerous large blocks of white granite, +which does not occur in the neighbourhood. I observed these blocks, as +well as others of andesitic syenite (not occurring here in situ), near +the baths of Cauquenes at a height of between two and three hundred +feet above the river, and therefore quite above the terrace or fringe +which borders that river; some miles up the valleys there were other +blocks at about the same height. I also noticed, at a less height, just +above the terrace, blocks of porphyries (apparently not found in the +immediately impending mountains), arranged in rude lines, as on a +sea-beach. All these blocks were rounded, and though large, not +gigantic, like the true erratic boulders of Patagonia and Fuegia. M. +Gay states that the granite does not occur in situ within a distance of +twenty leagues (“Annales des Science Nat. “ 1 series tome 28. M. Gay, +as I was informed, penetrated the Cordillera by the great oblique +valley of Los Cupressos, and not by the most direct line.); I suspect, +for several reasons, that it will ultimately be found at a much less +distance, though certainly not in the immediate neighbourhood. The +boulders found by MM. Meyen and Gay on the upper plain of San Fernando +(mentioned in a previous note) probably belong to this same class of +phenomena. + +These fringes of stratified gravel occur along all the great valleys of +the Cordillera, as well as along their main branches; they are +strikingly developed in the valleys of the Maypu, Mendoza, Aconcagua, +Cachapual, and according to Meyen, in the Tinguirica. (“Reise” etc. Th. +1 s. 302.) In the valleys, however, of Northern Chile, and in some on +the eastern flank of the Cordillera, as in the Portillo Valley, where +streams have never flowed, or are quite insignificant in volume, the +presence of a mass of stratified gravel can be inferred only from the +smooth slightly concave form of the bottom. One naturally seeks for +some explanation of so general and striking a phenomenon; that the +matter forming the fringes along the valleys, or still filling up their +entire beds, has not fallen from the adjoining mountains like common +detritus, is evident from the complete contrast in every respect +between the gravel and the piles of detritus, whether seen high up the +valleys on their sides, or low down in front of the more precipitous +ravines; that the matter has not been deposited by debacles, even if we +could believe in debacles having rushed down EVERY valley, and all +their branches, eastward and westward from the central pinnacles of the +Cordillera, we must admit from the following reasons,—from the distinct +stratification of the mass,—its smooth upper surface,—the well-rounded +and sometimes encrusted state of the pebbles, so different from the +loose debris on the mountains,—and especially from the terraces +preserving their uniform inclination round the most abrupt bends. To +suppose that as the land now stands, the rivers deposited the shingle +along the course of every valley, and all their main branches, appears +to me preposterous, seeing that these same rivers not only are now +removing and have removed much of this deposit, but are everywhere +tending to cut deep and narrow gorges in the hard underlying rocks. + +I have stated that these fringes of gravel, the origin of which are +inexplicable on the notion of debacles or of ordinary alluvial action, +are directly continuous with the similarly-composed basin-like plains +at the foot of the Cordillera, which, from the several reasons before +assigned, I cannot doubt were modelled by the agency of the sea. Now if +we suppose that the sea formerly occupied the valleys of the Chilean +Cordillera, in precisely the same manner as it now does in the more +southern parts of the continent, where deep winding creeks penetrate +into the very heart of, and in the case of Obstruction Sound quite +through, this great range; and if we suppose that the mountains were +upraised in the same slow manner as the eastern and western coasts have +been upraised within the recent period, then the origin and formation +of these sloping, terrace-like fringes of gravel can be simply +explained. For every part of the bottom of each valley will, on this +view, have long stood at the head of a sea creek, into which the then +existing torrents will have delivered fragments of rocks, where, by the +action of the tides, they will have been rolled, sometimes encrusted, +rudely stratified, and the whole surface levelled by the blending +together of the successive beach lines. (Sloping terraces of precisely +similar structure have been described by me “Philosophical +Transactions” 1839 page 58, in the valleys of Lochaber in Scotland, +where, at higher levels, the parallel roads of Glen Roy show the marks +of the long and quiet residence of the sea. I have no doubt that these +sloping terraces would have been present in the valleys of most of the +European ranges, had not every trace of them, and all wrecks of +sea-action, been swept away by the glaciers which have since occupied +them. I have shown that this is the case with the mountains (“London +and Edinburgh Philosophical Journal” volume 21 page 187) of North +Wales.) As the land rose, the torrents in every valley will have tended +to have removed the matter which just before had been arrested on, or +near, the beach-lines; the torrents, also, having continued to gain in +force by the continued elevation increasing their total descent from +their sources to the sea. This slow rising of the Cordillera, which +explains so well the otherwise inexplicable origin and structure of the +terraces, judging from all known analogies, will probably have been +interrupted by many periods of rest; but we ought not to expect to find +any evidence of these periods in the structure of the gravel- terraces: +for, as the waves at the heads of deep creeks have little erosive +power, so the only effect of the sea having long remained at the same +level will be that the upper parts of the creeks will have become +filled up at such periods to the level of the water with gravel and +sand; and that afterwards the rivers will have thrown down on the +filled-up parts a talus of similar matter, of which the inclination (as +at the head of a partially filled-up lake) will have been determined by +the supply of detritus, and the force of the stream. (I have attempted +to explain this process in a more detailed manner, in a letter to Mr. +Maclaren, published in the “Edinburgh New Philosophical Journal” volume +35 page 288.) Hence, after the final conversion of the creeks into +valleys, almost the only difference in the terraces at those points at +which the sea stood long, will be a somewhat more gentle inclination, +with river-worn instead of sea-worn detritus on the surface. + +I know of only one difficulty on the foregoing view, namely, the far- +transported blocks of rock high on the sides of the valley of the +Cachapual: I will not attempt any explanation of this phenomenon, but I +may state my belief that a mountain-ridge near the Baths of Cauquenes +has been upraised long subsequently to all the other ranges in the +neighbourhood, and that when this was effected the whole face of the +country must have been greatly altered. In the course of ages, +moreover, in this and other valleys, events may have occurred like, but +even on a grander scale than, that described by Molina, when a slip +during the earthquake of 1762 banked up for ten days the great River +Lontue, which then bursting its barrier “inundated the whole country,” +and doubtless transported many great fragments of rock. (“Compendio de +la Hist.” etc. etc. tome 1 page 30. M. Brongniart, in his report on M. +Gay’s labours “Annales des Sciences” 1833, considers that the boulders +in the Cachapual belong to the same class with the erratic boulders of +Europe. As the blocks which I saw are not gigantic, and especially as +they are not angular, and as they have not been transported fairly +across low spaces or wide valleys, I am unwilling to class them with +those which, both in the northern and southern hemisphere “Geological +Transactions” volume 6 page 415, have been transported by ice. It is to +be hoped that when M. Gay’s long-continued and admirable labours in +Chile are published, more light will be thrown on this subject. +However, the boulders may have been primarily transported; the final +position of those of porphyry, which have been described as arranged at +the foot of the mountain in rude lines, I cannot doubt, has been due to +the action of waves on a beach. The valley of the Cachapual, in the +part where the boulders occur, bursts through the high ridge of +Cauquenes, which runs parallel to, but at some distance from, the +Cordillera. This ridge has been subjected to excessive violence; +trachytic lava has burst from it, and hot springs yet flow at its base. +Seeing the enormous amount of denudation of solid rock in the upper and +much broader parts of this valley where it enters the Cordillera, and +seeing to what extent the ridge of Cauquenes now protects the great +range, I could not help believing (as alluded to in the text) that this +ridge with its trachytic eruptions had been thrown up at a much later +period than the Cordillera. If this has been the case, the boulders, +after having been transported to a low level by the torrents (which +exhibit in every valley proofs of their power of moving great +fragments), may have been raised up to their present height, with the +land on which they rested.) Finally, notwithstanding this one case of +difficulty, I cannot entertain any doubt, that these terrace-like +fringes, which are continuously united with the basin-shaped plains at +the foot of the Cordillera, have been formed by the arrestment of +river-borne detritus at successive levels, in the same manner as we see +now taking place at the heads of all those many, deep, winding fiords +intersecting the southern coasts. To my mind, this has been one of the +most important conclusions to which my observations on the geology of +South America have led me; for we thus learn that one of the grandest +and most symmetrical mountain-chains in the world, with its several +parallel lines, has been together uplifted in mass between seven and +nine thousand feet, in the same gradual manner as have the eastern and +western coasts within the recent period. (I do not wish to affirm that +all the lines have been uplifted quite equally; slight differences in +the elevation would leave no perceptible effect on the terraces. It +may, however, be inferred, perhaps with one exception, that since the +period when the sea occupied these valleys, the several ranges have not +been dislocated by GREAT and ABRUPT faults or upheavals; for if such +had occurred, the terraces of gravel at these points would not have +been continuous. The one exception is at the lower end of a plain in +the Valle del Yeso (a branch of the Maypu), where, at a great height, +the terraces and valley appear to have been broken through by a line of +upheaval, of which the evidence is plain in the adjoining mountains; +this dislocation, perhaps, occurred AFTER THE ELEVATION of this part of +the valley above the level of the sea. The valley here is almost +blocked up by a pile about one thousand feet in thickness, formed, as +far as I could judge, from three sides, entirely, or at least in chief +part, of gravel and detritus. On the south side, the river has cut +quite through this mass; on the northern side, and on the very summit, +deep ravines, parallel to the line of the valley, are worn, as if the +drainage from the valley above had passed by these two lines before +following its present course.) + +FORMATION OF VALLEYS. + +The bulk of solid rock which has been removed in the lower parts of the +valleys of the Cordillera has been enormous. It is only by reflecting +on such cases as that of the gravel beds of Patagonia, covering so many +thousand square leagues of surface, and which, if heaped into a ridge, +would form a mountain-range almost equal to the Cordillera, that the +amount of denudation becomes credible. The valleys within this range +often follow anticlinal but rarely synclinal lines; that is, the strata +on the two sides more often dip from the line of valley than towards +it. On the flanks of the range, the valleys most frequently run neither +along anticlinal nor synclinal axes, but along lines of flexure or +faults: that is, the strata on both sides dip in the same direction, +but with different, though often only slightly different, inclinations. +As most of the nearly parallel ridges which together form the +Cordillera run approximately north and south, the east and west valleys +cross them in zig-zag lines, bursting through the points where the +strata have been least inclined. No doubt the greater part of the +denudation was affected at the periods when tidal- creeks occupied the +valleys, and when the outer flanks of the mountains were exposed to the +full force of an open ocean. I have already alluded to the power of the +tidal action in the channels connecting great bays; and I may here +mention that one of the surveying vessels in a channel of this kind, +though under sail, was whirled round and round by the force of the +current. We shall hereafter see, that of the two main ridges forming +the Chilean Cordillera, the eastern and loftiest one owes the greater +part of its ANGULAR upheaval to a period subsequent to the elevation of +the western ridge; and it is likewise probable that many of the other +parallel ridges have been angularly upheaved at different periods; +consequently many parts of the surfaces of these mountains must +formerly have been exposed to the full force of the waves, which, if +the Cordillera were now sunk into the sea, would be protected by +parallel chains of islands. The torrents in the valleys certainly have +great power in wearing the rocks; as could be told by the dull rattling +sound of the many fragments night and day hurrying downwards; and as +was attested by the vast size of certain fragments, which I was assured +had been carried onwards during floods; yet we have seen in the lower +parts of the valleys, that the torrents have seldom removed all the +sea-checked shingle forming the terraces, and have had time since the +last elevation in mass only to cut in the underlying rocks, gorges, +deep and narrow, but quite insignificant in dimensions compared with +the entire width and depth of the valleys. + +Along the shores of the Pacific, I never ceased during my many and long +excursions to feel astonished at seeing every valley, ravine, and even +little inequality of surface, both in the hard granitic and soft +tertiary districts, retaining the exact outline, which they had when +the sea left their surfaces coated with organic remains. When these +remains shall have decayed, there will be scarcely any difference in +appearance between this line of coast-land and most other countries, +which we are accustomed to believe have assumed their present features +chiefly through the agency of the weather and fresh-water streams. In +the old granitic districts, no doubt it would be rash to attribute all +the modifications of outline exclusively to the sea-action; for who can +say how often this lately submerged coast may not previously have +existed as land, worn by running streams and washed by rain? This +source of doubt, however, does not apply to the districts superficially +formed of the modern tertiary deposits. The valleys worn by the sea, +through the softer formations, both on the Atlantic and Pacific sides +of the continent, are generally broad, winding, and flat-bottomed: the +only district of this nature now penetrated by arms of the sea, is the +island of Chiloe. + +Finally, the conclusion at which I have arrived, with respect to the +relative powers of rain and sea water on the land, is, that the latter +is far the most efficient agent, and that its chief tendency is to +widen the valleys; whilst torrents and rivers tend to deepen them, and +to remove the wreck of the sea’s destroying action. As the waves have +more power, the more open and exposed the space may be, so will they +always tend to widen more and more the mouths of valleys compared with +their upper parts: hence, doubtless, it is, that most valleys expand at +their mouths,—that part, at which the rivers flowing in them, generally +have the least wearing power. + +When reflecting on the action of the sea on the land at former levels, +the effect of the great waves, which generally accompany earthquakes, +must not be overlooked: few years pass without a severe earthquake +occurring on some part of the west coast of South America; and the +waves thus caused have great power. At Concepcion, after the shock of +1835, I saw large slabs of sandstone, one of which was six feet long, +three in breadth, and two in thickness, thrown high up on the beach; +and from the nature of the marine animals still adhering to it, it must +have been torn up from a considerable depth. On the other hand, at +Callao, the recoil-wave of the earthquake of 1746 carried great masses +of brickwork, between three and four feet square, some way out seaward. +During the course of ages, the effect thus produced at each successive +level, cannot have been small; and in some of the tertiary deposits on +this line of coast, I observed great boulders of granite and other +neighbouring rocks, embedded in fine sedimentary layers, the +transportal of which, except by the means of earthquake-waves, always +appeared to me inexplicable. + +SUPERFICIAL SALINE DEPOSITS. + +This subject may be here conveniently treated of: I will begin with the +most interesting case, namely, the superficial saline beds near Iquique +in Peru. The porphyritic mountains on the coast rise abruptly to a +height of between one thousand nine hundred and three thousand feet: +between their summits and an inland plain, on which the celebrated +deposit of nitrate of soda lies, there is a high undulatory district, +covered by a remarkable superficial saliferous crust, chiefly composed +of common salt, either in white, hard, opaque nodules, or mingled with +sand, in this latter case forming a compact sandstone. This saliferous +superficial crust extends from the edge of the coast-escarpment, over +the whole face of the country; but never attains, as I am assured by +Mr. Bollaert (long resident here) any great thickness. Although a very +slight shower falls only at intervals of many years, yet small +funnel-shaped cavities show that the salt has been in some parts +dissolved. (It is singular how slowly, according to the observations of +M. Cordier on the salt-mountain of Cardona in Spain “Ann. des Mines, +Translation of Geolog. Mem.” by De la Beche page 60, salt is dissolved, +where the amount of rain is supposed to be as much as 31.4 of an inch +in the year. It is calculated that only five feet in thickness is +dissolved in the course of a century.) In several places I saw large +patches of sand, quite moist, owing to the quantity of muriate of lime +(as ascertained by Mr. T. Reeks) contained in them. From the compact +salt- cemented sand being either red, purplish, or yellow, according to +the colour of the rocky strata on which it rested, I imagined that this +substance had probably been derived through common alluvial action from +the layers of salt which occur interstratified in the surrounding +mountains (“Journal of Researches” page 444 first edition.): but from +the interesting details given by M. d’Orbigny, and from finding on a +fresh examination of this agglomerated sand, that it is not irregularly +cemented, but consists of thin layers of sand of different tints of +colour, alternating with excessively fine parallel layers of salt, I +conclude that it is not of alluvial origin. M. d’Orbigny observed +analogous saline beds extending from Cobija for five degrees of +latitude northward, and at heights varying from six hundred to nine +hundred feet (“Voyage” etc. page 102. M. d’Orbigny found this deposit +intersected, in many places, by deep ravines, in which there was no +salt. Streams must once, though historically unknown, have flowed in +them; and M. d’Orbigny argues from the presence of undissolved salt +over the whole surrounding country, that the streams must have arisen +from rain or snow having fallen, not in the adjoining country, but on +the now arid Cordillera. I may remark, that from having observed ruins +of Indian buildings in absolutely sterile parts of the Chilian +Cordillera (“Journal” 2nd edition page 357), I am led to believe that +the climate, at a time when Indian man inhabited this part of the +continent, was in some slight degree more humid than it is at +present.): from finding recent sea- shells strewed on these saliferous +beds, and under them, great well-rounded blocks, exactly like those on +the existing beach, he believes that the salt, which is invariably +superficial, has been left by the evaporation of the sea-water. This +same conclusion must, I now believe, be extended to the superficial +saliferous beds of Iquique, though they stand about three thousand feet +above the level of the sea. + +Associated with the salt in the superficial beds, there are numerous, +thin, horizontal layers of impure, dirty-white, friable, gypseous and +calcareous tuffs. The gypseous beds are very remarkable, from abounding +with, so as sometimes to be almost composed of, irregular concretions, +from the size of an egg to that of a man’s head, of very hard, compact, +heavy gypsum, in the form of anhydrite. This gypsum contains some +foreign particles of stone; it is stained, judging from its action with +borax, with iron, and it exhales a strong aluminous odour. The surfaces +of the concretions are marked by sharp, radiating, or bifurcating +ridges, as if they had been (but not really) corroded: internally they +are penetrated by branching veins (like those of calcareous spar in the +septaria of the London clay) of pure white anhydrite. These veins might +naturally have been thought to have been formed by subsequent +infiltration, had not each little embedded fragment of rock been +likewise edged in a very remarkable manner by a narrow border of the +same white anhydrite: this shows that the veins must have been formed +by a process of segregation, and not of infiltration. Some of the +little included and CRACKED fragments of foreign rock are penetrated by +the anhydrite, and portions have evidently been thus mechanically +displaced: at St. Helena, I observed that calcareous matter, deposited +by rain water, also had the power to separate small fragments of rock +from the larger masses. (“Volcanic Islands” etc. page 87.) I believe +the superficial gypseous deposit is widely extended: I received +specimens of it from Pisagua, forty miles north of Iquique, and +likewise from Arica, where it coats a layer of pure salt. M. d’Orbigny +found at Cobija a bed of clay, lying above a mass of upraised recent +shells, which was saturated with sulphate of soda, and included thin +layers of fibrous gypsum. (“Voyage Geolog.” etc. page 95.) These widely +extended, superficial, beds of salt and gypsum, appear to me an +interesting geological phenomenon, which could be presented only under +a very dry climate. + +The plain or basin, on the borders of which the famous bed of nitrate +of soda lies, is situated at the distance of about thirty miles from +the sea, being separated from it by the saliferous district just +described. It stands at a height of 3,300 feet; its surface is level, +and some leagues in width; it extends forty miles northward, and has a +total length (as I was informed by Mr. Belford Wilson, the +Consul-General at Lima) of 420 miles. In a well near the works, +thirty-six yards in depth, sand, earth, and a little gravel were found: +in another well, near Almonte, fifty yards deep, the whole consisted, +according to Mr. Blake, of clay, including a layer of sand two feet +thick, which rested on fine gravel, and this on coarse gravel, with +large rounded fragments of rock. (See an admirable paper “Geological +and Miscellaneous Notices of Tarapaca” in “Silliman’s American Journal” +volume 44 page 1.) In many parts of this now utterly desert plain, +rushes and large prostrate trees in a hardened state, apparently +Mimosas, are found buried, at a depth from three to six feet; according +to Mr. Blake, they have all fallen to the south-west. The bed of +nitrate of soda is said to extend for forty to fifty leagues along the +western margin of the plain, but is not found in its central parts: it +is from two to three feet in thickness, and is so hard that it is +generally blasted with gunpowder; it slopes gently upwards from the +edge of the plain to between ten and thirty feet above its level. It +rests on sand in which, it is said, vegetable remains and broken shells +have been found; shells have also been found, according to Mr. Blake, +both on and in the nitrate of soda. It is covered by a superficial mass +of sand, containing nodules of common salt, and, as I was assured by a +miner, much soft gypseous matter, precisely like that in the +superficial crust already described: certainly this crust, with its +characteristic concretions of anhydrite, comes close down to the edge +of the plain. + +The nitrate of soda varies in purity in different parts, and often +contains nodules of common salt. According to Mr. Blake, the proportion +of nitrate of soda varies from 20 to 75 per cent. An analysis by Mr. A. +Hayes, of an average specimen, gave:— + + Nitrate of Soda.... 64.98 + Sulphate of Soda.... 3.00 + Chloride of Soda... 28.69 + Iodic Salts......... 0.63 + Shells and Marl..... 2.60 + 99.90 + +The “mother-water” at some of the refineries is very rich in iodic +salts, and is supposed to contain much muriate of lime. (“Literary +Gazette” 1841 page 475.) In an unrefined specimen brought home by +myself, Mr. T. Reeks has ascertained that the muriate of lime is very +abundant. With respect to the origin of this saline mass, from the +manner in which the gently inclined, compact bed follows for so many +miles the sinuous margin of the plain, there can be no doubt that it +was deposited from a sheet of water: from the fragments of embedded +shells, from the abundant iodic salts, from the superficial saliferous +crust occurring at a higher level and being probably of marine origin, +and from the plain resembling in form those of Chile and that of +Uspallata, there can be little doubt that this sheet of water was, at +least originally, connected with the sea. (From an official document, +shown me by Mr. Belford Wilson, it appears that the first export of +nitrate of soda to Europe was in July 1830, on French account, in a +British ship:— + + In year, the entire export was in Quintals. + 1830............................ 17,300 + 1831............................ 40,885 + 1832............................ 51,400 + 1833............................ 91,335 + 1834........................... 149,538 + The Spanish quintal nearly equals 100 English pounds.) + +THIN, SUPERFICIAL, SALINE INCRUSTATIONS. + +These saline incrustations are common in many parts of America: +Humboldt met with them on the tableland of Mexico, and the Jesuit +Falkner and other authors state that they occur at intervals over the +vast plains extending from the mouth of the Plata to Rioja and +Catamarca. (Azara “Travels” volume 1 page 55, considers that the Parana +is the eastern boundary of the saliferous region; but I heard of +“salitrales” in the Province of Entre Rios.) Hence it is that during +droughts, most of the streams in the Pampas are saline. I nowhere met +with these incrustations so abundantly as near Bahia Blanca: square +miles of the mud-flats, which near that place are raised only a few +feet above the sea, just enough to protect them from being overflowed, +appear, after dry weather, whiter than the ground after the thickest +hoar-frost. After rain the salts disappear, and every puddle of water +becomes highly saline; as the surface dries, the capillary action draws +the moisture up pieces of broken earth, dead sticks, and tufts of +grass, where the salt effloresces. The incrustation, where thickest, +does not exceed a quarter of an inch. M. Parchappe has analysed it (M. +d’Orbigny “Voyage” etc. Part. Hist. tome 1 page 664.); and finds that +the specimens collected at the extreme head of the low plain, near the +River Manuello, consist of 93 per cent of sulphate of soda, and 7 of +common salt; whilst the specimens taken close to the coast contain only +63 per cent of the sulphate, and 37 of the muriate of soda. This +remarkable fact, together with our knowledge that the whole of this low +muddy plain has been covered by the sea within the recent period, must +lead to the suspicion that the common salt, by some unknown process, +becomes in time changed into the sulphate. Friable, calcareous matter +is here abundant, and the case of the apparent double decomposition of +the shells and salt on San Lorenzo, should not be forgotten. + +The saline incrustations, near Bahia Blanca, are not confined to, +though most abundant on, the low muddy flats; for I noticed some on a +calcareous plain between thirty and forty feet above the sea, and even +a little occurs in still higher valleys. Low alluvial tracts in the +valleys of the Rivers Negro and Colorado are also encrusted, and in the +latter valley such spaces appeared to be occasionally overflowed by the +river. I observed saline incrustations in some of the valleys of +Southern Patagonia. At Port Desire a low, flat, muddy valley was +thickly incrusted by salts, which on analysis by Mr. T. Reeks, are +found to consist of a mixture of sulphate and muriate of soda, with +carbonate of lime and earthy matter. On the western side of the +continent, the southern coasts are much too humid for this phenomenon; +but in Northern Chile I again met with similar incrustations. On the +hardened mud, in parts of the broad, flat-bottomed valley of Copiapo, +the saline matter encrusts the ground to the thickness of some inches: +specimens, sent by Mr. Bingley to Apothecaries’ Hall for analysis, were +said to consist of carbonate and sulphate of soda. Much sulphate of +soda is found in the desert of Atacama. In all parts of South America, +the saline incrustations occur most frequently on low damp surfaces of +mud, where the climate is rather dry; and these low surfaces have, in +almost every case, been upraised above the level of the sea, within the +recent period. + +SALT-LAKES OF PATAGONIA AND LA PLATA. + +Salinas, or natural salt-lakes, occur in various formations on the +eastern side of the continent,—in the argillaceo-calcareous deposit of +the Pampas, in the sandstone of the Rio Negro, where they are very +numerous, in the pumiceous and other beds of the Patagonian tertiary +formation, and in small primary districts in the midst of this latter +formation. Port S. Julian is the most southerly point (latitude 49 +degrees to 50 degrees) at which salinas are known to occur. (According +to Azara “Travels” volume 1 page 56, there are salt-lakes as far north +as Chaco (latitude 25 degrees), on the banks of the Vermejo. The +salt-lakes of Siberia appear (Pallas “Travels” English Translation +volume 1 page 284) to occur in very similar depressions to those of +Patagonia.) The depressions, in which these salt-lakes lie, are from a +few feet to sixty metres, as asserted by M. d’Orbigny, below the +surface of the surrounding plains (“Voyage Geolog.” page 63.); and, +according to this same author, near the Rio Negro they all trend, +either in the N.E. and S.W. or in E. and W. lines, coincident with the +general slope of the plain. These depressions in the plain generally +have one side lower than the others, but there are no outlets for +drainage. Under a less dry climate, an outlet would soon have been +formed, and the salt washed away. The salinas occur at different +elevations above the sea; they are often several leagues in diameter; +they are generally very shallow, but there is a deep one in a +quartz-rock formation near C. Blanco. In the wet season, the whole, or +a part, of the salt is dissolved, being redeposited during the +succeeding dry season. At this period the appearance of the snow-white +expanse of salt crystallised in great cubes, is very striking. In a +large salina, northward of the Rio Negro, the salt at the bottom, +during the whole year, is between two and three feet in thickness. + +The salt rests almost always on a thick bed of black muddy sand, which +is fetid, probably from the decay of the burrowing worms inhabiting it. +(Professor Ehrenberg examined some of this muddy sand, but was unable +to find in it any infusoria.) In a salina, situated about fifteen miles +above the town of El Carmen on the Rio Negro, and three or four miles +from the banks of that river, I observed that this black mud rested on +gravel with a calcareous matrix, similar to that spread over the whole +surrounding plains: at Port S. Julian the mud, also, rested on the +gravel: hence the depressions must have been formed anteriorly to, or +contemporaneously with, the spreading out of the gravel. I was informed +that one small salina occurs in an alluvial plain within the valley of +the Rio Negro, and therefore its origin must be subsequent to the +excavation of that valley. When I visited the salina, fifteen miles +above the town, the salt was beginning to crystallise, and on the muddy +bottom there were lying many crystals, generally placed crossways of +sulphate of soda (as ascertained by Mr. Reeks), and embedded in the +mud, numerous crystals of sulphate of lime, from one to three inches in +length: M. d’Orbigny states that some of these crystals are acicular +and more than even nine inches in length (“Voyage Geolog.” page 64.); +others are macled and of great purity: those I found all contained some +sand in their centres. As the black and fetid sand overlies the gravel, +and that overlies the regular tertiary strata, I think there can be no +doubt that these remarkable crystals of sulphate of lime have been +deposited from the waters of the lake. The inhabitants call the +crystals of selenite, the padre del sal, and those of the sulphate of +soda, the madre del sal; they assured me that both are found under the +same circumstances in several of the neighbouring salinas; and that the +sulphate of soda is annually dissolved, and is always crystallised +before the common salt on the muddy bottom. (This is what might have +been expected; for M. Ballard asserts “Acad. des Sciences” October 7, +1844, that sulphate of soda is precipitated from solution more readily +from water containing muriate of soda in excess, than from pure water.) +The association of gypsum and salt in this case, as well as in the +superficial deposits of Iquique, appears to me interesting, considering +how generally these substances are associated in the older stratified +formations. + +Mr. Reeks has analysed for me some of the salt from the salina near the +Rio Negro; he finds it composed entirely of chloride of sodium, with +the exception of 0.26 of sulphate of lime and of 0.22 of earthy matter: +there are no traces of iodic salts. Some salt from the salina +Chiquitos, in the Pampean formation, is equally pure. It is a singular +fact, that the salt from these salinas does not serve so well for +preserving meat, as sea-salt from the Cape de Verde Islands; and a +merchant at Buenos Ayres told me that he considered it as 50 per cent +less valuable. The purity of the Patagonian salt, or absence from it of +those other saline bodies found in all sea- water, is the only +assignable cause for this inferiority; a conclusion which is supported +by the fact lately ascertained, that those salts answer best for +preserving cheese which contain most of the deliquescent chlorides. +(“Horticultural and Agricultural Gazette” 1845 page 93.) (It would +probably well answer for the merchants of Buenos Ayres (considering the +great consumption there of salt for preserving meat) to import the +deliquescent chlorides to mix with the salt from the salinas: I may +call attention to the fact, that at Iquique, a large quantity of +muriate of lime, left in the MOTHER-WATER during the refinement of the +nitrate of soda, is annually thrown away.) + +With respect to the origin of the salt in the salinas, the foregoing +analysis seems opposed to the view entertained by M. d’Orbigny and +others, and which seems so probable considering the recent elevation of +this line of coast, namely, that it is due to the evaporation of +sea-water and to the drainage from the surrounding strata impregnated +with sea-salt. I was informed (I know not whether accurately) that on +the northern side of the salina on the Rio Negro, there is a small +brine spring which flows at all times of the year: if this be so, the +salt in this case at least, probably is of subterranean origin. It at +first appears very singular that fresh water can often be procured in +wells, and is sometimes found in small lakes, quite close to these +salinas. (Sir W. Parish states “Buenos Ayres” etc. pages 122 and 170, +that this is the case near the great salinas westward of the S. +Ventana. I have seen similar statements in an ancient MS. Journal +lately published by S. Angelis. At Iquique, where the surface is so +thickly encrusted with saline matter, I tasted water only slightly +brackish, procured in a well thirty-six yards deep; but here one feels +less surprise at its presence, as pure water might percolate under +ground from the not very distant Cordillera.) I am not aware that this +fact bears particularly on the origin of the salt; but perhaps it is +rather opposed to the view of the salt having been washed out of the +surrounding superficial strata, but not to its having been the residue +of sea-water, left in depressions as the land was slowly elevated. + + + + +CHAPTER IV. +ON THE FORMATIONS OF THE PAMPAS. + + +Mineralogical constitution.—Microscopical structure.—Buenos Ayres, +shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—San +Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta, +shells, bones and infusoria of; co-existence of the recent shells and +extinct mammifers.—Buenos Ayres to Santa Fé.—Skeletons of +Mastodon.—Infusoria.—Inferior marine tertiary strata, their +age.—Horse’s tooth.—BANDA ORIENTAL.—Superficial Pampean +formation.—Inferior tertiary strata, variation of, connected with +volcanic action; Macrauchenia Patachonica at San Julian in Patagonia, +age of, subsequent to living mollusca and to the erratic block period. +SUMMARY.—Area of Pampean formation.—Theories of origin.—Source of +sediment.—Estuary origin.—Contemporaneous with existing +mollusca.—Relations to underlying tertiary strata.—Ancient deposit of +estuary origin.—Elevation and successive deposition of the Pampean +formation.—Number and state of the remains of mammifers; their +habitation, food, extinction, and range.—Conclusion.—Localities in +Pampas at which mammiferous remains have been found. + + +The Pampean formation is highly interesting from its vast extent, its +disputed origin, and from the number of extinct gigantic mammifers +embedded in it. It has upon the whole a very uniform character: +consisting of a more or less dull reddish, slightly indurated, +argillaceous earth or mud, often, but not always, including in +horizontal lines concretions of marl, and frequently passing into a +compact marly rock. The mud, wherever I examined it, even close to the +concretions, did not contain any carbonate of lime. The concretions are +generally nodular, sometimes rough externally, sometimes +stalactiformed; they are of a compact structure, but often penetrated +(as well as the mud) by hair-like serpentine cavities, and occasionally +with irregular fissures in their centres, lined with minute crystals of +carbonate of lime; they are of white, brown, or pale pinkish tints, +often marked by black dendritic manganese or iron; they are either +darker or lighter tinted than the surrounding mass; they contain much +carbonate of lime, but exhale a strong aluminous odour, and leave, when +dissolved in acids, a large but varying residue, of which the greater +part consists of sand. These concretions often unite into irregular +strata; and over very large tracts of country, the entire mass consists +of a hard, but generally cavernous marly rock: some of the varieties +might be called calcareous tuffs. + +Dr. Carpenter has kindly examined under the microscope, sliced and +polished specimens of these concretions, and of the solid marl-rock, +collected in various places between the Colorado and Santa Fe Bajada. +In the greater number, Dr. Carpenter finds that the whole substance +presents a tolerably uniform amorphous character, but with traces of +incipient crystalline metamorphosis; in other specimens he finds +microscopically minute rounded concretions of an amorphous substance +(resembling in size those in oolitic rocks, but not having a concentric +structure), united by a cement which is often crystalline. In some, Dr. +Carpenter can perceive distinct traces of shells, corals, Polythalamia, +and rarely of spongoid bodies. For the sake of comparison, I sent Dr. +Carpenter specimens of the calcareous rock, formed chiefly of fragments +of recent shells, from Coquimbo in Chile: in one of these specimens, +Dr. Carpenter finds, besides the larger fragments, microscopical +particles of shells, and a varying quantity of opaque amorphous matter; +in another specimen from the same bed, he finds the whole composed of +the amorphous matter, with layers showing indications of an incipient +crystalline metamorphosis: hence these latter specimens, both in +external appearance and in microscopical structure, closely resemble +those of the Pampas. Dr. Carpenter informs me that it is well known +that chemical precipitation throws down carbonate of lime in the opaque +amorphous state; and he is inclined to believe that the long-continued +attrition of a calcareous body in a state of crystalline or +semi-crystalline aggregation (as, for instance, in the ordinary shells +of Mollusca, which, when sliced, are transparent) may yield the same +result. From the intimate relations between all the Coquimbo specimens, +I can hardly doubt that the amorphous carbonate of lime in them has +resulted from the attrition and decay of the larger fragments of shell: +whether the amorphous matter in the marly rocks of the Pampas has +likewise thus originated, it would be hazardous to conjecture. + +For convenience’ sake, I will call the marly rock by the name given to +it by the inhabitants, namely, Tosca-rock; and the reddish argillaceous +earth, Pampean mud. This latter substance, I may mention, has been +examined for me by Professor Ehrenberg, and the result of his +examination will be given under the proper localities. + +I will commence my descriptions at a central spot, namely, at Buenos +Ayres, and thence proceed first southward to the extreme limit of the +deposit, and afterwards northward. The plain on which Buenos Ayres +stands is from thirty to forty feet in height. The Pampean mud is here +of a rather pale colour, and includes small nearly white nodules, and +other irregular strata of an unusually arenaceous variety of +tosca-rock. In a well at the depth of seventy feet, according to +Ignatio Nunez, much tosca-rock was met with, and at several points, at +one hundred feet deep, beds of sand have been found. I have already +given a list of the recent marine and estuary shells found in many +parts on the surface near Buenos Ayres, as far as three or four leagues +from the Plata. Specimens from near Ensenada, given me by Sir W. +Parish, where the rock is quarried just beneath the surface of the +plain, consist of broken bivalves, cemented by and converted into white +crystalline carbonate of lime. I have already alluded, in the first +chapter, to a specimen (also given me by Sir W. Parish) from the A. del +Tristan, in which shells, resembling in every respect the Azara +labiata, d’Orbigny, as far as their worn condition permits of +comparison, are embedded in a reddish, softish, somewhat arenaceous +marly rock: after careful comparison, with the aid of a microscope and +acids, I can perceive no difference between the basis of this rock and +the specimens collected by me in many parts of the Pampas. I have also +stated, on the authority of Sir W. Parish, that northward of Buenos +Ayres, on the highest parts of the plain, about forty feet above the +Plata, and two or three miles from it, numerous shells of the Azara +labiata (and I believe of Venus sinuosa) occur embedded in a stratified +earthy mass, including small marly concretions, and said to be +precisely like the great Pampean deposit. Hence we may conclude that +the mud of the Pampas continued to be deposited to within the period of +this existing estuary shell. Although this formation is of such immense +extent, I know of no other instance of the presence of shells in it. + +BUENOS AYRES TO THE RIO COLORADO. + +With the exception of a few metamorphic ridges, the country between +these two points, a distance of 400 geographical miles, belongs to the +Pampean formation, and in the southern part is generally formed of the +harder and more calcareous varieties. I will briefly describe my route: +about twenty- five miles S.S.W. of the capital, in a well forty yards +in depth, the upper part, and, as I was assured, the entire thickness, +was formed of dark red Pampean mud without concretions. North of the +River Salado, there are many lakes; and on the banks of one (near the +Guardia) there was a little cliff similarly composed, but including +many nodular and stalactiform concretions: I found here a large piece +of tessellated armour, like that of the Glyptodon, and many fragments +of bones. The cliffs on the Salado consist of pale-coloured Pampean +mud, including and passing into great masses of tosca-rock: here a +skeleton of the Megatherium and the bones of other extinct quadrupeds +(see the list at the end of this chapter) were found. Large quantities +of crystallised gypsum (of which specimens were given me) occur in the +cliffs of this river; and likewise (as I was assured by Mr. Lumb) in +the Pampean mud on the River Chuelo, seven leagues from Buenos Ayres: I +mention this because M. d’Orbigny lays some stress on the supposed +absence of this mineral in the Pampean formation. + +Southward of the Salado the country is low and swampy, with tosca-rock +appearing at long intervals at the surface. On the banks, however, of +the Tapalguen (sixty miles south of the Salado) there is a large extent +of tosca-rock, some highly compact and even semi-crystalline, overlying +pale Pampean mud with the usual concretions. Thirty miles further +south, the small quartz-ridge of Tapalguen is fringed on its northern +and southern flank, by little, narrow, flat-topped hills of tosca-rock, +which stand higher than the surrounding plain. Between this ridge and +the Sierra of Guitru-gueyu, a distance of sixty miles, the country is +swampy, with the tosca-rock appearing only in four or five spots: this +sierra, precisely like that of Tapalguen, is bordered by horizontal, +often cliff-bounded, little hills of tosca-rock, higher than the +surrounding plain. Here, also, a new appearance was presented in some +extensive and level banks of alluvium or detritus of the neighbouring +metamorphic rocks; but I neglected to observe whether it was stratified +or not. Between Guitru-gueyu and the Sierra Ventana, I crossed a dry +plain of tosca-rock higher than the country hitherto passed over, and +with small pieces of denuded tableland of the same formation, standing +still higher. + +The marly or calcareous beds not only come up nearly horizontally to +the northern and southern foot of the great quartzose mountains of the +Sierra Ventana, but interfold between the parallel ranges. The +superficial beds (for I nowhere obtained sections more than twenty feet +deep) retain, even close to the mountains, their usual character: the +uppermost layer, however, in one place included pebbles of quartz, and +rested on a mass of detritus of the same rock. At the very foot of the +mountains, there were some few piles of quartz and tosca-rock detritus, +including land-shells; but at the distance of only half a mile from +these lofty, jagged, and battered mountains, I could not, to my great +surprise, find on the boundless surface of the calcareous plain even a +single pebble. Quartz- pebbles, however, of considerable size have at +some period been transported to a distance of between forty and fifty +miles to the shores of Bahia Blanca. (Schmidtmeyer “Travels in Chile” +page 150, states that he first noticed on the Pampas, very small bits +of red granite, when fifty miles distant from the southern extremity of +the mountains of Cordova, which project on the plain, like a reef into +the sea.) + +The highest peak of the St. Ventana is, by Captain Fitzroy’s +measurement, 3,340 feet, and the calcareous plain at its foot (from +observations taken by some Spanish officers) 840 feet above the +sea-level. (“La Plata” etc. by Sir W. Parish page 146.) On the flanks +of the mountains, at a height of three hundred or four hundred feet +above the plain, there were a few small patches of conglomerate and +breccia, firmly cemented by ferruginous matter to the abrupt and +battered face of the quartz—traces being thus exhibited of ancient +sea-action. The high plain round this range sinks quite insensibly to +the eye on all sides, except to the north, where its surface is broken +into low cliffs. Round the Sierras Tapalguen, Guitru-gueyu, and between +the latter and the Ventana we have seen (and shall hereafter see round +some hills in Banda Oriental), that the tosca-rock forms low, flat- +topped, cliff-bounded hills, higher than the surrounding plains of +similar composition. From the horizontal stratification and from the +appearance of the broken cliffs, the greater height of the Pampean +formation round these primary hills ought not to be altogether or in +chief part attributed to these several points having been uplifted more +energetically than the surrounding country, but to the +argillaceo-calcareous mud having collected round them, when they +existed as islets or submarine rocks, at a greater height, than at the +bottom of the adjoining open sea;—the cliffs having been subsequently +worn during the elevation of the whole country in mass. + +Southward of the Ventana, the plain extends farther than the eye can +range; its surface is not very level, having slight depressions with no +drainage exits; it is generally covered by a few feet in thickness of +sandy earth; and in some places, according to M. Parchappe, by beds of +clay two yards thick. (M. d’Orbigny “Voyage” Part Geolog. pages 47, +48.) On the banks of the Sauce, four leagues S.E. of the Ventana, there +is an imperfect section about two hundred feet in height, displaying in +the upper part tosca-rock and in the lower part red Pampean mud. At the +settlement of Bahia Blanca, the uppermost plain is composed of very +compact, stratified tosca-rock, containing rounded grains of quartz +distinguishable by the naked eye: the lower plain, on which the +fortress stands, is described by M. Parchappe as composed of solid +tosca-rock (Ibid.); but the sections which I examined appeared more +like a redeposited mass of this rock, with small pebbles and fragments +of quartz. I shall immediately return to the important sections on the +shores of Bahia Blanca. Twenty miles southward of this place, there is +a remarkable ridge extending W. by N. and E. by S., formed of small, +separate, flat-topped, steep-sided hills, rising between one hundred +and two hundred feet above the Pampean plain at its southern base, +which plain is a little lower than that to the north. The uppermost +stratum in this ridge consists of pale, highly calcareous, compact +tosca-rock, resting (as seen in one place) on reddish Pampean mud, and +this again on a paler kind: at the foot of the ridge, there is a well +in reddish clay or mud. I have seen no other instance of a chain of +hills belonging to the Pampean formation; and as the strata show no +signs of disturbance, and as the direction of the ridge is the same +with that common to all the metamorphic lines in this whole area, I +suspect that the Pampean sediment has in this instance been accumulated +on and over a ridge of hard rocks, instead of, as in the case of the +above-mentioned Sierras, round their submarine flanks. South of this +little chain of tosca-rock, a plain of Pampean mud declines towards the +banks of the Colorado: in the middle a well has been dug in red Pampean +mud, covered by two feet of white, softish, highly calcareous +tosca-rock, over which lies sand with small pebbles three feet in +thickness—the first appearance of that vast shingle formation described +in the First Chapter. In the first section after crossing the Colorado, +an old tertiary formation, namely, the Rio Negro sandstone (to be +described in the next chapter), is met with: but from the accounts +given me by the Gauchos, I believe that at the mouth of the Colorado +the Pampean formation extends a little further southwards. + +BAHIA BLANCA. + +To return to the shores of this bay. At Monte Hermoso there is a good +section, about one hundred feet in height, of four distinct strata, +appearing to the eye horizontal, but thickening a little towards the +N.W. The uppermost bed, about twenty feet in thickness, consists of +obliquely laminated, soft sandstone, including many pebbles of quartz, +and falling at the surface into loose sand. The second bed, only six +inches thick, is a hard, dark-coloured sandstone. The third bed is +pale-coloured Pampean mud; and the fourth is of the same nature, but +darker coloured, including in its lower part horizontal layers and +lines of concretions of not very compact pinkish tosca-rock. The bottom +of the sea, I may remark, to a distance of several miles from the +shore, and to a depth of between sixty and one hundred feet, was found +by the anchors to be composed of tosca-rock and reddish Pampean mud. +Professor Ehrenberg has examined for me specimens of the two lower +beds, and finds in them three Polygastrica and six Phytolitharia. + +(The following list is given in the “Monatsberichten der konig. Akad. +zu Berlin” April 1845:— POLYGASTRICA. Fragilaria rhabdosoma. +Gallionella distans. Pinnularia? + +PHYTOLITHARIA. Lithodontium Bursa. Lithodontium furcatum. +Lithostylidium exesum. Lithostylidium rude. Lithostylidium Serra. +Spongolithis Fustis?) + +Of these, only one (Spongolithis Fustis?) is a marine form; five of +them are identical with microscopical structures of brackish-water +origin, hereafter to be mentioned, which form a central point in the +Pampean formation. In these two beds, especially in the lower one, +bones of extinct mammifers, some embedded in their proper relative +positions and others single, are very numerous in a small extent of the +cliffs. These remains consist of, first, the head of Ctenomys antiquus, +allied to the living Ctenomys Braziliensis; secondly, a fragment of the +remains of a rodent; thirdly, molar teeth and other bones of a large +rodent, closely allied to, but distinct from, the existing species of +Hydrochoerus, and therefore probably an inhabitant of fresh water; +fourth and fifthly, portions of vertebrae, limbs, ribs, and other bones +of two rodents; sixthly, bones of the extremities of some great +megatheroid quadruped. (See “Fossil Mammalia” page 109 by Professor +Owen, in the “Zoology of the Voyage of the ‘Beagle’;” and Catalogue +page 36 of Fossil Remains in Museum of Royal College of Surgeons.) The +number of the remains of rodents gives to this collection a peculiar +character, compared with those found in any other locality. All these +bones are compact and heavy; many of them are stained red, with their +surfaces polished; some of the smaller ones are as black as jet. + +Monte Hermoso is between fifty and sixty miles distant in a S.E. line +from the Ventana, with the intermediate country gently rising towards +it, and all consisting of the Pampean formation. What relation, then, +do these beds, at the level of the sea and under it, bear to those on +the flanks of the Ventana, at the height of 840 feet, and on the flanks +of the other neighbouring sierras, which, from the reasons already +assigned, do not appear to owe their greater height to unequal +elevation? When the tosca- rock was accumulating round the Ventana, and +when, with the exception of a few small rugged primary islands, the +whole wide surrounding plains must have been under water, were the +strata at Monte Hermoso depositing at the bottom of a great open sea, +between eight hundred and one thousand feet in depth? I much doubt +this; for if so, the almost perfect carcasses of the several small +rodents, the remains of which are so very numerous in so limited a +space, must have been drifted to this spot from the distance of many +hundred miles. It appears to me far more probable, that during the +Pampean period this whole area had commenced slowly rising (and in the +cliffs, at several different heights we have proofs of the land having +been exposed to sea-action at several levels), and that tracts of land +had thus been formed of Pampean sediment round the Ventana and the +other primary ranges, on which the several rodents and other quadrupeds +lived, and that a stream (in which perhaps the extinct aquatic +Hydrochoerus lived) drifted their bodies into the adjoining sea, into +which the Pampean mud continued to be poured from the north. As the +land continued to rise, it appears that this source of sediment was cut +off; and in its place sand and pebbles were borne down by stronger +currents, and conformably deposited over the Pampean strata. + +(FIGURE 15. SECTION OF BEDS WITH RECENT SHELLS AND EXTINCT MAMMIFERS, +AT PUNTA ALTA IN BAHIA BLANCA. (Showing beds from bottom to top: A, B, +C, D.)) + +Punta Alta is situated about thirty miles higher up on the northern +side of this same bay: it consists of a small plain, between twenty and +thirty feet in height, cut off on the shore by a line of low cliffs +about a mile in length, represented in Figure 15 with its vertical +scale necessarily exaggerated. The lower bed (A) is more extensive than +the upper ones; it consists of stratified gravel or conglomerate, +cemented by calcareo- arenaceous matter, and is divided by curvilinear +layers of pinkish marl, of which some are precisely like tosca-rock, +and some more sandy. The beds are curvilinear, owing to the action of +currents, and dip in different directions; they include an +extraordinary number of bones of gigantic mammifers and many shells. +The pebbles are of considerable size, and are of hard sandstone, and of +quartz, like that of the Ventana: there are also a few well-rounded +masses of tosca-rock. + +The second bed B is about fifteen feet in thickness, but towards both +extremities of the cliff (not included in the diagram) it either thins +out and dies away, or passes insensibly into an overlying bed of +gravel. It consists of red, tough clayey mud, with minute linear +cavities; it is marked with faint horizontal shades of colour; it +includes a few pebbles, and rarely a minute particle of shell: in one +spot, the dermal armour and a few bones of a Dasypoid quadruped were +embedded in it: it fills up furrows in the underlying gravel. With the +exception of the few pebbles and particles of shells, this bed +resembles the true Pampean mud; but it still more closely resembles the +clayey flats (mentioned in the First Chapter) separating the +successively rising parallel ranges of sand-dunes. + +The bed C is of stratified gravel, like the lowest one; it fills up +furrows in the underlying red mud, and is sometimes interstratified +with it, and sometimes insensibly passes into it; as the red mud thins +out, this upper gravel thickens. Shells are more numerous in it than in +the lower gravel; but the bones, though some are still present, are +less numerous. In one part, however, where this gravel and the red mud +passed into each other, I found several bones and a tolerably perfect +head of the Megatherium. Some of the large Volutas, though embedded in +the gravel-bed C, were filled with the red mud, including great numbers +of the little recent Paludestrina australis. These three lower beds are +covered by an unconformable mantle D of stratified sandy earth, +including many pebbles of quartz, pumice and phonolite, land and +sea-shells. + +M. d’Orbigny has been so obliging as to name for me the twenty species +of Mollusca embedded in the two gravel beds: they consist of:— + +1. Volutella angulata, d’Orbigny, “Voyage” Mollusq. and Pal. 2. Voluta +Braziliana, Sol 3. Olicancilleria Braziliensis d’Orbigny. 4. +Olicancilleria auricularia, d’Orbigny. 5. Olivina puelchana, d’Orbigny. +6. Buccinanops cochlidium, d’Orbigny. 7. Buccinanops globulosum, +d’Orbigny. 8. Colombella sertulariarum, d’Orbigny. 9. Trochus +Patagonicus, and var. of ditto, d’Orbigny. 10. Paludestrina Australis, +d’Orbigny. 11. Fissurella Patagonica, d’Orbigny. 12. Crepidula +muricata, Lam. 13. Venus purpurata, Lam. 14. Venus rostrata, Phillippi. +15. Mytilus Darwinianus, d’Orbigny. 16. Nucula semiornata, d’Orbigny. +17. Cardita Patagonica, d’Orbigny. 18. Corbula Patagonica (?), +d’Orbigny. 19. Pecten tethuelchus, d’Orbigny. 20. Ostrea puelchana, +d’Orbigny. 21. A living species of Balanus. 22 and 23. An Astrae and +encrusting Flustra, apparently identical with species now living in the +bay. + +All these shells now live on this coast, and most of them in this same +bay. I was also struck with the fact, that the proportional numbers of +the different kinds appeared to be the same with those now cast up on +the beach: in both cases specimens of Voluta, Crepidula, Venus, and +Trochus are the most abundant. Four or five of the species are the same +with the upraised shells on the Pampas near Buenos Ayres. All the +specimens have a very ancient and bleached appearance, and do not emit, +when heated, an animal odour: some of them are changed throughout into +a white, soft, fibrous substance; others have the space between the +external walls, either hollow, or filled up with crystalline carbonate +of lime. (A Bulinus, mentioned in the Introduction to the “Fossil +Mammalia” in the “Zoology of the Voyage of the ‘Beagle’” has so much +fresher an appearance, than the marine species, that I suspect it must +have fallen amongst the others, and been collected by mistake.) + +The remains of the extinct mammiferous animals, from the two gravel +beds have been described by Professor Owen in the “Zoology of the +Voyage of the ‘Beagle’:” they consist of, 1st, one nearly perfect head +and three fragments of heads of the Megatherium Cuvierii; 2nd, a lower +jaw of Megalonyx Jeffersonii; 3rd, lower jaw of Mylodon Darwinii; 4th, +fragments of a head of some gigantic Edental quadruped; 5th, an almost +entire skeleton of the great Scelidotherium leptocephalum, with most of +the bones, including the head, vertebrae, ribs, some of the extremities +to the claw- bone, and even, as remarked by Professor Owen, the +knee-cap, all nearly in their proper relative positions; 6th, fragments +of the jaw and a separate tooth of a Toxodon, belonging either to T. +Platensis, or to a second species lately discovered near Buenos Ayres; +7th, a tooth of Equus curvidens; 8th, tooth of a Pachyderm, closely +allied to Palaeotherium, of which parts of the head have been lately +sent from Buenos Ayres to the British Museum; in all probability this +pachyderm is identical with the Macrauchenia Patagonica from Port S. +Julian, hereafter to be referred to. Lastly, and 9thly, in a cliff of +the red clayey bed B, there was a double piece, about three feet long +and two wide, of the bony armour of a large Dasypoid quadruped, with +the two sides pressed nearly close together: as the cliff is now +rapidly washing away, this fossil probably was lately much more +perfect; from between its doubled-up sides, I extracted the middle and +ungual phalanges, united together, of one of the feet, and likewise a +separate phalanx: hence one or more of the limbs must have been +attached to the dermal case, when it was embedded. Besides these +several remains in a distinguishable condition, there were very many +single bones: the greater number were embedded in a space 200 yards +square. The preponderance of the Edental quadrupeds is remarkable; as +is, in contrast with the beds of Monte Hermoso, the absence of Rodents. +Most of the bones are now in a soft and friable condition, and, like +the shells, do not emit when burnt an animal odour. The decayed state +of the bones may be partly owing to their late exposure to the air and +tidal-waves. Barnacles, Serpulae, and corallines are attached to many +of the bones, but I neglected to observe whether these might not have +grown on them since being exposed to the present tidal action (After +having packed up my specimens at Bahia Blanca, this point occurred to +me, and I noted it; but forgot it on my return, until the remains had +been cleaned and oiled: my attention has been lately called to the +subject by some remarks by M. d’Orbigny.); but I believe that some of +the barnacles must have grown on the Scelidotherium, soon after being +deposited, and before being WHOLLY covered up by the gravel. Besides +the remains in the condition here described, I found one single +fragment of bone very much rolled, and as black as jet, so as perfectly +to resemble some of the remains from Monte Hermoso. + +Very many of the bones had been broken, abraded, and rolled, before +being embedded. Others, even some of those included in the coarsest +parts of the the now hard conglomerate, still retain all their minutest +prominences perfectly preserved; so that I conclude that they probably +were protected by skin, flesh, or ligaments, whilst being covered up. +In the case of the Scelidotherium, it is quite certain that the whole +skeleton was held together by its ligaments, when deposited in the +gravel in which I found it. Some cervical vertebrae and a humerus of +corresponding size lay so close together, as did some ribs and the +bones of a leg, that I thought that they must originally have belonged +to two skeletons, and not have been washed in single; but as remains +were here very numerous, I will not lay much stress on these two cases. +We have just seen that the armour of the Dasypoid quadruped was +certainly embedded together with some of the bones of the feet. + +Professor Ehrenberg has examined for me specimens of the finer matter +from in contact with these mammiferous remains: he finds in them two +Polygastrica, decidedly marine forms; and six Phytolitharia, of which +one is probably marine, and the others either of fresh-water or +terrestrial origin. (“Monatsberichten der Akad. zu Berlin” April 1845. +The list consists of:— + +POLYGASTRICA. Gallionella sulcata. Stauroptera aspera? fragm. + +PHYTOLITHARIA. Lithasteriscus tuberculatus. Lithostylidium +Clepsammidium. Lithostylidium quadratum. Lithostylidium rude. +Lithostylidium unidentatum. Spongolithis acicularis.) + +Only one of these eight microscopical bodies is common to the nine from +Monte Hermoso: but five of them are in common with those from the +Pampean mud on the banks of the Parana. The presence of any fresh-water +infusoria, considering the aridity of the surrounding country, is here +remarkable: the most probable explanation appears to be, that these +microscopical organisms were washed out of the adjoining great Pampean +formation during its denudation, and afterwards redeposited. + +We will now see what conclusions may be drawn from the facts above +detailed. It is certain that the gravel-beds and intermediate red mud +were deposited within the period, when existing species of Mollusca +held to each other nearly the same relative proportions as they do on +the present coast. These beds, from the number of littoral species, +must have been accumulated in shallow water; but not, judging from the +stratification of the gravel and the layers of marl, on a beach. From +the manner in which the red clay fills up furrows in the underlying +gravel, and is in some parts itself furrowed by the overlying gravel, +whilst in other parts it either insensibly passes into, or alternates +with, this upper gravel, we may infer several local changes in the +currents, perhaps caused by slight changes, up or down, in the level of +the land. By the elevation of these beds, to which period the alluvial +mantle with pumice-pebbles, land and sea-shells belongs, the plain of +Punta Alta, from twenty to thirty feet in height, was formed. In this +neighbourhood there are other and higher sea-formed plains and lines of +cliffs in the Pampean formation worn by the denuding action of the +waves at different levels. Hence we can easily understand the presence +of rounded masses of tosca-rock in this lowest plain; and likewise, as +the cliffs at Monte Hermoso with their mammiferous remains stand at a +higher level, the presence of the one much-rolled fragment of bone +which was as black as jet: possibly some few of the other much-rolled +bones may have been similarly derived, though I saw only the one +fragment, in the same condition with those from Monte Hermoso. M. +d’Orbigny has suggested that all these mammiferous remains may have +been washed out of the Pampean formation, and afterwards redeposited +together with the recent shells. (“Voyage” Part. Geolog. page 49.) +Undoubtedly it is a marvellous fact that these numerous gigantic +quadrupeds, belonging, with the exception of the Equus curvidens, to +seven extinct genera, and one, namely, the Toxodon, not falling into +any existing family, should have co-existed with Mollusca, all of which +are still living species; but analogous facts have been observed in +North America and in Europe. In the first place, it should not be +overlooked, that most of the co-embedded shells have a more ancient and +altered appearance than the bones. In the second place, is it probable +that numerous bones not hardened by silex or any other mineral, could +have retained their delicate prominences and surfaces perfect if they +had been washed out of one deposit, and re-embedded in another:—this +later deposit being formed of large, hard pebbles, arranged by the +action of currents or breakers in shallow water into variously curved +and inclined layers? The bones which are now in so perfect a state of +preservation, must, I conceive, have been fresh and sound when +embedded, and probably were protected by skin, flesh, or ligaments. The +skeleton of the Scelidotherium indisputably was deposited entire: shall +we say that when held together by its matrix it was washed out of an +old gravel-bed (totally unlike in character to the Pampean formation), +and re-embedded in another gravel-bed, composed (I speak after careful +comparison) of exactly the same kind of pebbles, in the same kind of +cement? I will lay no stress on the two cases of several ribs and bones +of the extremities having APPARENTLY been embedded in their proper +relative position: but will any one be so bold as to affirm that it is +possible, that a piece of the thin tessellated armour of a Dasypoid +quadruped, at least three feet long and two in width, and now so tender +that I was unable with the utmost care to extract a fragment more than +two or three inches square, could have been washed out of one bed, and +re-embedded in another, together with some of the small bones of the +feet, without having been dashed into atoms? We must then wholly reject +M. d’Orbigny’s supposition, and admit as certain, that the +Scelidotherium and the large Dasypoid quadruped, and as highly +probable, that the Toxodon, Megatherium, etc., some of the bones of +which are perfectly preserved, were embedded for the first time, and in +a fresh condition, in the strata in which they were found entombed. +These gigantic quadrupeds, therefore, though belonging to extinct +genera and families, coexisted with the twenty above-enumerated +Mollusca, the barnacle and two corals, still living on this coast. From +the rolled fragment of black bone, and from the plain of Punta Alta +being lower than that of Monte Hermoso, I conclude that the coarse +sub-littoral deposits of Punta Alta, are of subsequent origin to the +Pampean mud of Monte Hermoso; and the beds at this latter place, as we +have seen, are probably of subsequent origin to the high tosca-plain +round the Sierra Ventana: we shall, however, return, at the end of this +chapter, to the consideration of these several stages in the great +Pampean formation. + +BUENOS AYRES TO ST. FE BAJADA, IN ENTRE RIOS. + +For some distance northward of Buenos Ayres, the escarpment of the +Pampean formation does not approach very near to the Plata, and it is +concealed by vegetation: but in sections on the banks of the Rios +Luxan, Areco, and Arrecifes, I observed both pale and dark reddish +Pampean mud, with small, whitish concretions of tosca; at all these +places mammiferous remains have been found. In the cliffs on the +Parana, at San Nicolas, the Pampean mud contains but little tosca; here +M. d’Orbigny found the remains of two rodents (Ctenomys Bonariensis and +Kerodon antiquus) and the jaw of a Canis: when on the river I could +clearly distinguish in this fine line of cliffs, “horizontal lines of +variation both in tint and compactness.” (I quote these words from my +note-book, as written down on the spot, on account of the general +absence of stratification in the Pampean formation having been insisted +on by M. d’Orbigny as a proof of the diluvial origin of this great +deposit.) The plain northward of this point is very level, but with +some depressions and lakes; I estimated its height at from forty to +sixty feet above the Parana. At the A. Medio the bright red Pampean mud +contains scarcely any tosca-rock; whilst at a short distance the stream +of the Pabon, forms a cascade, about twenty feet in height, over a +cavernous mass of two varieties of tosca-rock; of which one is very +compact and semi- crystalline, with seams of crystallised carbonate of +lime: similar compact varieties are met with on the Salidillo and Seco. +The absolute identity (I speak after a comparison of my specimens) +between some of these varieties, and those from Tapalguen, and from the +ridge south of Bahia Blanca, a distance of 400 miles of latitude, is +very striking. + +At Rosario there is but little tosca-rock: near this place I first +noticed at the edge of the river traces of an underlying formation, +which, twenty- five miles higher up in the estancia of Gorodona, +consists of a pale yellowish clay, abounding with concretionary +cylinders of a ferruginous sandstone. This bed, which is probably the +equivalent of the older tertiary marine strata, immediately to be +described in Entre Rios, only just rises above the level of the Parana +when low. The rest of the cliff at Gorodona, is formed of red Pampean +mud, with, in the lower part, many concretions of tosca, some +stalacti-formed, and with only a few in the upper part: at the height +of six feet above the river, two gigantic skeletons of the Mastodon +Andium were here embedded; their bones were scattered a few feet apart, +but many of them still held their proper relative positions: they were +much decayed and as soft as cheese, so that even one of the great molar +teeth fell into pieces in my hand. We here see that the Pampean deposit +contains mammiferous remains close to its base. On the banks of the +Carcarana, a few miles distant, the lowest bed visible was pale Pampean +mud, with masses of tosca-rock, in one of which I found a much decayed +tooth of the Mastodon: above this bed, there was a thin layer almost +composed of small concretions of white tosca, out of which I extracted +a well preserved, but slightly broken tooth of Toxodon Platensis: above +this there was an unusual bed of very soft impure sandstone. In this +neighbourhood I noticed many single embedded bones, and I heard of +others having been found in so perfect a state that they were long used +as gate-posts: the Jesuit Falkner found here the dermal armour of some +gigantic Edental quadruped. + +In some of the red mud scraped from a tooth of one of the Mastodons at +Gorodona, Professor Ehrenberg finds seven Polygastrica and thirteen +Phytolitharia, all of them, I believe, with two exceptions, already +known species. (“Monatsberichten der konig. Akad. zu Berlin” April +1845. The list consists of:— + +POLYGASTRICA. Campylodiscus clypeus. Coscinodiscus subtilis. +Coscinodiscus al. sp. Eunotia. Gallionella granulata. Himantidium +gracile. Pinnularia borealis.) + +Of these twenty, the preponderating number are of fresh-water origin; +only two species of Coscinodiscus and a Spongolithis show the direct +influence of the sea; therefore Professor Ehrenberg arrives at the +important conclusion that the deposit must have been of brackish-water +origin. Of the thirteen Phytolitharia, nine are met with in the two +deposits in Bahia Blanca, where there is evidence from two other +species of Polygastrica that the beds were accumulated in brackish +water. The traces of coral, sponges, and Polythalamia, found by Dr. +Carpenter in the tosca-rock (of which I must observe the greater number +of specimens were from the upper beds in the southern parts of the +formation), apparently show a more purely marine origin. + +At ST. FE BAJADA, in Entre Rios, the cliffs, estimated at between sixty +and seventy feet in height, expose an interesting section: the lower +half consists of tertiary strata with marine shells, and the upper half +of the Pampean formation. The lowest bed is an obliquely laminated, +blackish, indurated mud, with distinct traces of vegetable remains. (M. +d’Orbigny “Voyage” Part. Geolog. page 37, has given a detailed +description of this section, but as he does not mention this lowest +bed, it may have been concealed when he was there by the river. There +is a considerable discrepancy between his description and mine, which I +can only account for by the beds themselves varying considerably in +short distances.) Above this there is a thick bed of yellowish sandy +clay, with much crystallised gypsum and many shells of Ostreae, +Pectens, and Arcae: above this there generally comes an arenaceous +crystalline limestone, but there is sometimes interposed a bed, about +twelve feet thick, of dark green, soapy clay, weathering into small +angular fragments. The limestone, where purest, is white, highly +crystalline, and full of cavities: it includes small pebbles of quartz, +broken shells, teeth of sharks, and sometimes, as I was informed, large +bones: it often contains so much sand as to pass into a calcareous +sandstone, and in such parts the great Ostrea Patagonica chiefly +abounds. (Captain Sulivan, R.N., has given me a specimen of this shell, +which he found in the cliffs at Point Cerrito, between twenty and +thirty miles above the Bajada.) In the upper part, the limestone +alternates with layers of fine white sand. The shells included in these +beds have been named for me by M. d’Orbigny: they consist of:— + +1. Ostrea Patagonica, d’Orbigny, “Voyage” Part. Pal. 2. Ostrea +Alvarezii, d’Orbigny, “Voyage” Part. Pal. 3. Pecten Paranensis, +d’Orbigny, “Voyage” Part. Pal. 4. Pecten Darwinianus, d’Orbigny, +“Voyage” Part. Pal. 5. Venus Munsterii, d’Orbigny, “Voyage” Pal. 6. +Arca Bonplandiana, d’Orbigny, “Voyage” Pal. 7. Cardium Platense, +d’Orbigny, “Voyage” Pal. 8. Tellina, probably nov. species, but too +imperfect for description. + +PHYTOLITHARIA. + +Lithasteriscus tuberculatus. Lithodontium bursa. Lithodontium furcatum. +Lithodontium rostratum. Lithostylidium Amphiodon. Lithostylidium +Clepsammidium. Lithostylidium Hamus. Lithostylidium polyedrum. +Lithostylidium quadratum. Lithostylidium rude. Lithostylidium Serra. +Lithostylidium unidentatum. Spongolithis Fustis. + +These species are all extinct: the six first were found by M. d’Orbigny +and myself in the formations of the Rio Negro, S. Josef, and other +parts of Patagonia; and therefore, as first observed by M. d’Orbigny, +these beds certainly belong to the great Patagonian formation, which +will be described in the ensuing chapter, and which we shall see must +be considered as a very ancient tertiary one. North of the Bajada, M. +d’Orbigny found, in beds which he considers as lying beneath the strata +here described, remains of a Toxodon, which he has named as a distinct +species from the T. Platensis of the Pampean formation. Much silicified +wood is found on the banks of the Parana (and likewise on the Uruguay), +and I was informed that they come out of these lower beds; four +specimens collected by myself are dicotyledonous. + +The upper half of the cliff, to a thickness of about thirty feet, +consists of Pampean mud, of which the lower part is pale-coloured, and +the upper part of a brighter red, with some irregular layers of an +arenaceous variety of tosca, and a few small concretions of the +ordinary kind. Close above the marine limestone, there is a thin +stratum with a concretionary outline of white hard tosca-rock or marl, +which may be considered either as the uppermost bed of the inferior +deposits, or the lowest of the Pampean formation; at one time I +considered this bed as marking a passage between the two formations: +but I have since become convinced that I was deceived on this point. In +the section on the Parana, I did not find any mammiferous remains; but +at two miles distance on the A. Tapas (a tributary of the Conchitas), +they were extremely numerous in a low cliff of red Pampean mud with +small concretions, precisely like the upper bed on the Parana. Most of +the bones were solitary and much decayed; but I saw the dermal armour +of a gigantic Edental quadruped, forming a caldron-like hollow, four or +five feet in diameter, out of which, as I was informed, the almost +entire skeleton had been lately removed. I found single teeth of the +Mastodon Andium, Toxodon Platensis, and Equus curvidens, near to each +other. As this latter tooth approaches closely to that of the common +horse, I paid particular attention to its true embedment, for I did not +at that time know that there was a similar tooth hidden in the matrix +with the other mammiferous remains from Punta Alta. It is an +interesting circumstance, that Professor Owen finds that the teeth of +this horse approach more closely in their peculiar curvature to a +fossil specimen brought by Mr. Lyell from North America, than to those +of any other species of Equus. (Lyell “Travels in North America” volume +1 page 164 and “Proceedings of Geological Society” volume 4 page 39.) + +The underlying marine tertiary strata extend over a wide area: I was +assured that they can be traced in ravines in an east and west line +across Entre Rios to the Uruguay, a distance of about 135 miles. In a +S.E. direction I heard of their existence at the head of the R. Nankay; +and at P. Gorda in Banda Oriental, a distance of 170 miles, I found the +same limestone, containing the same fossil shells, lying at about the +same level above the river as at St. Fe. In a southerly direction, +these beds sink in height, for at another P. Gorda in Entre Rios, the +limestone is seen at a much less height; and there can be little doubt +that the yellowish sandy clay, on a level with the river, between the +Carcarana and S. Nicholas, belongs to this same formation; as perhaps +do the beds of sand at Buenos Ayres, which lie at the bottom of the +Pampean formation, about sixty feet beneath the surface of the Plata. +The southerly declination of these beds may perhaps be due, not to +unequal elevation, but to the original form of the bottom of the sea, +sloping from land situated to the north; for that land existed at no +great distance, we have evidence in the vegetable remains in the lowest +bed at St. Fe; and in the silicified wood and in the bones of Toxodon +Paranensis, found (according to M. d’Orbigny) in still lower strata. + +BANDA ORIENTAL. + +This province lies on the northern side of the Plata, and eastward of +the Uruguay: it has a gentle undulatory surface, with a basis of +primary rocks; and is in most parts covered up with an unstratified +mass, of no great thickness, of reddish Pampean mud. In the eastern +half, near Maldonado, this deposit is more arenaceous than in the +Pampas, it contains many though small concretions of marl or +tosca-rock, and others of highly ferruginous sandstone; in one section, +only a few yards in depth, it rested on stratified sand. Near Monte +Video this deposit in some spots appears to be of greater thickness; +and the remains of the Glyptodon and other extinct mammifers have been +found in it. In the long line of cliffs, between fifty and sixty feet +in height, called the Barrancas de S. Gregorio, which extend westward +of the Rio S. Lucia, the lower half is formed of coarse sand of quartz +and feldspar without mica, like that now cast up on the beach near +Maldonado; and the upper half of Pampean mud, varying in colour and +containing honeycombed veins of soft calcareous matter and small +concretions of tosca-rock arranged in lines, and likewise a few pebbles +of quartz. This deposit fills up hollows and furrows in the underlying +sand; appearing as if water charged with mud had invaded a sandy beach. +These cliffs extend far westward, and at a distance of sixty miles, +near Colonia del Sacramiento, I found the Pampean deposit resting in +some places on this sand, and in others on the primary rocks: between +the sand and the reddish mud, there appeared to be interposed, but the +section was not a very good one, a thin bed of shells of an existing +Mytilus, still partially retaining their colour. The Pampean formation +in Banda Oriental might readily be mistaken for an alluvial deposit: +compared with that of the Pampas, it is often more sandy, and contains +small fragments of quartz; the concretions are much smaller, and there +are no extensive masses of tosca-rock. + +In the extreme western parts of this province, between the Uruguay and +a line drawn from Colonia to the R. Perdido (a tributary of the R. +Negro), the formations are far more complicated. Besides primary rocks, +we meet with extensive tracts and many flat-topped, horizontally +stratified, cliff- bounded, isolated hills of tertiary strata, varying +extraordinarily in mineralogical nature, some identical with the old +marine beds of St. Fe Bajada, and some with those of the much more +recent Pampean formation. There are, also, extensive LOW tracts of +country covered with a deposit containing mammiferous remains, +precisely like that just described in the more eastern parts of the +province. Although from the smooth and unbroken state of the country, I +never obtained a section of this latter deposit close to the foot of +the higher tertiary hills, yet I have not the least doubt that it is of +quite subsequent origin; having been deposited after the sea had worn +the tertiary strata into the cliff-bounded hills. This later formation, +which is certainly the equivalent of that of the Pampas, is well seen +in the valleys in the estancia of Berquelo, near Mercedes; it here +consists of reddish earth, full of rounded grains of quartz, and with +some small concretions of tosca-rock arranged in horizontal lines, so +as perfectly to resemble, except in containing a little calcareous +matter, the formation in the eastern parts of Banda Oriental, in Entre +Rios, and at other places: in this estancia the skeleton of a great +Edental quadruped was found. In the valley of the Sarandis, at the +distance of only a few miles, this deposit has a somewhat different +character, being whiter, softer, finer-grained, and full of little +cavities, and consequently of little specific gravity; nor does it +contain any concretions or calcareous matter: I here procured a head, +which when first discovered must have been quite perfect, of the +Toxodon Platensis, another of a Mylodon (This head was at first +considered by Professor Owen (in the “Zoology of the ‘Beagle’s’ +Voyage”) as belonging to a distinct genus, namely, Glossotherium.), +perhaps M. Darwinii, and a large piece of dermal armour, differing from +that of the Glyptodon clavipes. These bones are remarkable from their +extraordinarily fresh appearance; when held over a lamp of spirits of +wine, they give out a strong odour and burn with a small flame; Mr. T. +Reeks has been so kind as to analyse some of the fragments, and he +finds that they contain about 7 per cent of animal matter, and 8 per +cent of water. (Liebig “Chemistry of Agriculture” page 194 states that +fresh dry bones contain from 32 to 33 per cent of dry gelatine. See +also Dr. Daubeny, in “Edinburgh New Philosophical Journal” volume 37 +page 293.) + +The older tertiary strata, forming the higher isolated hills and +extensive tracts of country, vary, as I have said, extraordinarily in +composition: within the distance of a few miles, I sometimes passed +over crystalline limestone with agate, calcareous tuffs, and marly +rocks, all passing into each other,—red and pale mud with concretions +of tosca-rock, quite like the Pampean formation,—calcareous +conglomerates and sandstones,—bright red sandstones passing either into +red conglomerate, or into white sandstone,—hard siliceous sandstones, +jaspery and chalcedonic rocks, and numerous other subordinate +varieties. I was unable to mark out the relations of all these strata, +and will describe only a few distinct sections:—in the cliffs between +P. Gorda on the Uruguay and the A. de Vivoras, the upper bed is +crystalline cellular limestone often passing into calcareous sandstone, +with impressions of some of the same shells as at St. Fe Bajada; at P. +Gorda, this limestone is interstratified with and rests on, white sand, +which covers a bed about thirty feet thick of pale-coloured clay, with +many shells of the great Ostrea Patagonica (In my “Journal” page 171 +1st edition, I have hastily and inaccurately stated that the Pampean +mud, which is found over the eastern part of B. Oriental, lies OVER the +limestone at P. Gorda; I should have said that there was reason to +infer that it was a subsequent or superior deposit.): beneath this, in +the vertical cliff, nearly on a level with the river, there is a bed of +red mud absolutely like the Pampean deposit, with numerous often large +concretions of perfectly characterised white, compact tosca-rock. At +the mouth of the Vivoras, the river flows over a pale cavernous +tosca-rock, quite like that in the Pampas, and this APPEARED to +underlie the crystalline limestone; but the section was not unequivocal +like that at P. Gorda. These beds now form only a narrow and much +denuded strip of land; but they must once have extended much further; +for on the next stream, south of the S. Juan, Captain Sulivan, R.N., +found a little cliff, only just above the surface of the river, with +numerous shells of the Venus Munsterii, D’Orbigny,—one of the species +occurring at St. Fe, and of which there are casts at P. Gorda: the line +of cliffs of the subsequently deposited true Pampean mud, extend from +Colonia to within half a mile of this spot, and no doubt once covered +up this denuded marine stratum. Again at Colonia, a Frenchman found, in +digging the foundations of a house, a great mass of the Ostrea +Patagonica (of which I saw many fragments), packed together just +beneath the surface, and directly superimposed on the gneiss. These +sections are important: M. d’Orbigny is unwilling to believe that beds +of the same nature with the Pampean formation ever underlie the ancient +marine tertiary strata; and I was as much surprised at it as he could +have been; but the vertical cliff at P. Gorda allowed of no mistake, +and I must be permitted to affirm, that after having examined the +country from the Colorado to St. Fe Bajada, I could not be deceived in +the mineralogical character of the Pampean deposit. + +Moreover, in a precipitous part of the ravine of Las Bocas, a red +sandstone is distinctly seen to overlie a thick bed of pale mud, also +quite like the Pampean formation, abounding with concretions of true +tosca-rock. This sandstone extends over many miles of country: it is as +red as the brightest volcanic scoriae; it sometimes passes into a +coarse red conglomerate composed of the underlying primary rocks; and +often passes into a soft white sandstone with red streaks. At the +Calera de los Huerfanos, only a quarter of a mile south of where I +first met with the red sandstone, the crystalline white limestone is +quarried: as this bed is the uppermost, and as it often passes into +calcareous sandstone, interstratified with pure sand; and as the red +sandstone likewise passes into soft white sandstone, and is also the +uppermost bed, I believe that these two beds, though so different, are +equivalents. A few leagues southward of these two places, on each side +of the low primary range of S. Juan, there are some flat-topped, +cliff-bounded, separate little hills, very similar to those fringing +the primary ranges in the great plain south of Buenos Ayres: they are +composed—1st, of calcareous tuff with many particles of quartz, +sometimes passing into a coarse conglomerate; 2nd, of a stone +undistinguishable on the closest inspection from the compacter +varieties of tosca-rock; and 3rd, of semi-crystalline limestone, +including nodules of agate: these three varieties pass insensibly into +each other, and as they form the uppermost stratum in this district, I +believe that they, also, are the equivalents of the pure crystalline +limestone, and of the red and white sandstones and conglomerates. + +Between these points and Mercedes on the Rio Negro, there are scarcely +any good sections, the road passing over limestone, tosca-rock, +calcareous and bright red sandstones, and near the source of the San +Salvador over a wide extent of jaspery rocks, with much milky agate, +like that in the limestone near San Juan. In the estancia of Berquelo, +the separate, flat-topped, cliff-bounded hills are rather higher than +in the other parts of the country; they range in a N.E. and S.W. +direction; their uppermost beds consist of the same bright red +sandstone, passing sometimes into a conglomerate, and in the lower part +into soft white sandstone, and even into loose sand: beneath this +sandstone, I saw in two places layers of calcareous and marly rocks, +and in one place red Pampean-like earth; at the base of these sections, +there was a hard, stratified, white sandstone, with chalcedonic layers. +Near Mercedes, beds of the same nature and apparently of the same age, +are associated with compact, white, crystalline limestone, including +much botryoidal agate, and singular masses, like porcelain, but really +composed of a calcareo-siliceous paste. In sinking wells in this +district the chalcedonic strata seem to be the lowest. Beds, such as +there described, occur over the whole of this neighbourhood; but twenty +miles further up the R. Negro, in the cliffs of Perika, which are about +fifty feet in height, the upper bed is a prettily variegated +chalcedony, mingled with a pure white tallowy limestone; beneath this +there is a conglomerate of quartz and granite; beneath this many +sandstones, some highly calcareous; and the whole lower two-thirds of +the cliff consists of earthy calcareous beds of various degrees of +purity, with one layer of reddish Pampean-like mud. + +When examining the agates, the chalcedonic and jaspery rocks, some of +the limestones, and even the bright red sandstones, I was forcibly +struck with their resemblance to deposits formed in the neighbourhood +of volcanic action. I now find that M. Isabelle, in his “Voyage a +Buenos Ayres,” has described closely similar beds on Itaquy and Ibicuy +(which enter the Uruguay some way north of the R. Negro) and these beds +include fragments of red decomposed true scoriae hardened by zeolite, +and of black retinite: we have then here good evidence of volcanic +action during our tertiary period. Still further north, near S. Anna, +where the Parana makes a remarkable bend, M. Bonpland found some +singular amygdaloidal rocks, which perhaps may belong to this same +epoch. (M. d’Orbigny “Voyage” Part. Geolog. page 29) I may remark that, +judging from the size and well-rounded condition of the blocks of rock +in the above-described conglomerates, masses of primary formation +probably existed at this tertiary period above water: there is, also, +according to M. Isabelle, much conglomerate further north, at Salto. + +From whatever source and through whatever means the great Pampean +formation originated, we here have, I must repeat, unequivocal evidence +of a similar action at a period before that of the deposition of the +marine tertiary strata with extinct shells, at Santa Fe and P. Gorda. +During also the deposition of these strata, we have in the intercalated +layers of red Pampean-like mud and tosca-rock, and in the passage near +S. Juan of the semi-crystalline limestones with agate into tosca +undistinguishable from that of the Pampas, evidence of the same action, +though continued only at intervals and in a feeble manner. We have +further seen that in this district, at a period not only subsequent to +the deposition of the tertiary strata, but to their upheavement and +most extensive denudation, true Pampean mud with its usual characters +and including mammiferous remains, was deposited round and between the +hills or islets formed of these tertiary strata, and over the whole +eastern and low primary districts of Banda Oriental. + +EARTHY MASS, WITH EXTINCT MAMMIFEROUS REMAINS, OVER THE PORPHYRITIC +GRAVEL AT S. JULIAN, LATITUDE 49 DEGREES 14′ S., IN PATAGONIA. + +(FIGURE 16. SECTION OF THE LOWEST PLAIN AT PORT S. JULIAN. + +(Section through beds from top to bottom: A, B, C, D, E, F.) + +AA. Superficial bed of reddish earth, with the remains of the +Macrauchenia, and with recent sea-shells on the surface. + +B. Gravel of porphyritic rocks. + +C. and D. Pumiceous mudstone.—Ancient tertiary formation. + +E. and F. Sandstone and argillaceous beds.—Ancient tertiary formation.) + +This case, though not coming strictly under the Pampean formation, may +be conveniently given here. On the south side of the harbour, there is +a nearly level plain (mentioned in the First Chapter) about seven miles +long, and three or four miles wide, estimated at ninety feet in height, +and bordered by perpendicular cliffs, of which a section is represented +in Figure 16. + +The lower old tertiary strata (to be described in the next chapter) are +covered by the usual gravel bed; and this by an irregular earthy, +sometimes sandy mass, seldom more than two or three feet in thickness, +except where it fills up furrows or gullies worn not only through the +underlying gravel, but even through the upper tertiary beds. This +earthy mass is of a pale reddish colour, like the less pure varieties +of Pampean mud in Banda Oriental; it includes small calcareous +concretions, like those of tosca- rock but more arenaceous, and other +concretions of a greenish, indurated argillaceous substance: a few +pebbles, also, from the underlying gravel-bed are also included in it, +and these being occasionally arranged in horizontal lines, show that +the mass is of sub-aqueous origin. On the surface and embedded in the +superficial parts, there are numerous shells, partially retaining their +colours, of three or four of the now commonest littoral species. Near +the bottom of one deep furrow (represented in Figure 16), filled up +with this earthy deposit, I found a large part of the skeleton of the +Macrauchenia Patachonica—a gigantic and most extraordinary pachyderm, +allied, according to Professor Owen, to the Palaeotherium, but with +affinities to the Ruminants, especially to the American division of the +Camelidae. Several of the vertebrae in a chain, and nearly all the +bones of one of the limbs, even to the smallest bones of the foot, were +embedded in their proper relative positions: hence the skeleton was +certainly united by its flesh or ligaments, when enveloped in the mud. +This earthy mass, with its concretions and mammiferous remains, filling +up furrows in the underlying gravel, certainly presents a very striking +resemblance to some of the sections (for instance, at P. Alta in B. +Blanca, or at the Barrancas de S. Gregorio) in the Pampean formation; +but I must believe that this resemblance is only accidental. I suspect +that the mud which at the present day is accumulating in deep and +narrow gullies at the head of the harbour, would, after elevation, +present a very similar appearance. The southernmost part of the true +Pampean formation, namely, on the Colorado, lies 560 miles of latitude +north of this point. (In the succeeding chapter I shall have to refer +to a great deposit of extinct mammiferous remains, lately discovered by +Captain Sulivan, R.N., at a point still further south, namely, at the +R. Gallegos; their age must at present remain doubtful.) + +With respect to the age of the Macrauchenia, the shells on the surface +prove that the mass in which the skeleton was enveloped has been +elevated above the sea within the recent period: I did not see any of +the shells embedded at a sufficient depth to assure me (though it be +highly probable) that the whole thickness of the mass was +contemporaneous with these INDIVIDUAL SPECIMENS. That the Macrauchenia +lived subsequently to the spreading out of the gravel on this plain is +certain; and that this gravel, at the height of ninety feet, was spread +out long after the existence of recent shells, is scarcely less +certain. For, it was shown in the First Chapter, that this line of +coast has been upheaved with remarkable equability, and that over a +vast space both north and south of S. Julian, recent species of shells +are strewed on (or embedded in) the surface of the 250 feet plain, and +of the 350 feet plain up to a height of 400 feet. These wide +step-formed plains have been formed by the denuding action of the +coast-waves on the old tertiary strata; and therefore, when the surface +of the 350 feet plain, with the shells on it, first rose above the +level of the sea, the 250 feet plain did not exist, and its formation, +as well as the spreading out of the gravel on its summit, must have +taken place subsequently. So also the denudation and the +gravel-covering of the 90 feet plain must have taken place subsequently +to the elevation of the 250 feet plain, on which recent shells are also +strewed. Hence there cannot be any doubt that the Macrauchenia, which +certainly was entombed in a fresh state, and which must have been alive +after the spreading out of the gravel on the 90 feet plain, existed, +not only subsequently to the upraised shells on the surface of the 250 +feet plain, but also to those on the 350 to 400 feet plain: these +shells, eight in number (namely, three species of Mytilus, two of +Patella, one Fusus, Voluta, and Balanus), are undoubtedly recent +species, and are the commonest kinds now living on this coast. At Punta +Alta in B. Blanca, I remarked how marvellous it was, that the Toxodon, +a mammifer so unlike to all known genera, should have co-existed with +twenty- three still living marine animals; and now we find that the +Macrauchenia, a quadruped only a little less anomalous than the +Toxodon, also co-existed with eight other still existing Mollusca: it +should, moreover, be borne in mind, that a tooth of a pachydermatous +animal was found with the other remains at Punta Alta, which Professor +Owen thinks almost certainly belonged to the Macrauchenia. + +Mr. Lyell has arrived at a highly important conclusion with respect to +the age of the North American extinct mammifers (many of which are +closely allied to, and even identical with, those of the Pampean +formation), namely, that they lived subsequently to the period when +erratic boulders were transported by the agency of floating ice in +temperate latitudes. (“Geological Proceedings” volume 4 page 36.) Now +in the valley of the Santa Cruz, only fifty miles of latitude south of +the spot where the Macrauchenia was entombed, vast numbers of gigantic, +angular boulders, which must have been transported from the Cordillera +on icebergs, lie strewed on the plain, at the height of 1,400 feet +above the level of the sea. In ascending to this level, several +step-formed plains must be crossed, all of which have necessarily +required long time for their formation; hence the lowest or ninety feet +plain, with its superficial bed containing the remains of the +Macrauchenia, must have been formed very long subsequently to the +period when the 1,400 feet plain was beneath the sea, and boulders were +dropped on it from floating masses of ice. (It must not be inferred +from these remarks, that the ice-action ceased in South America at this +comparatively ancient period; for in Tierra del Fuego boulders were +probably transported contemporaneously with, if not subsequently to, +the formation of the ninety feet plain at S. Julian, and at other parts +of the coast of Patagonia.) Mr. Lyell’s conclusion, therefore, is thus +far confirmed in the southern hemisphere; and it is the more important, +as one is naturally tempted to admit so simple an explanation, that it +was the ice-period that caused the extinction of the numerous great +mammifers which so lately swarmed over the two Americas. + +A SUMMARY AND CONCLUDING REMARKS ON THE PAMPEAN FORMATION. + +One of its most striking features is its great extent; I passed +continuously over it from the Colorado to St. Fe Bajada, a distance of +500 geographical miles; and M. d’Orbigny traced it for 250 miles +further north. In the latitude of the Plata, I examined this formation +at intervals over an east and west line of 300 miles from Maldonado to +the R. Carcarana; and M. d’Orbigny believes it extends 100 miles +further inland: from Mr. Caldcleugh’s travels, however, I should have +thought that it had extended, south of the Cordovese range, to near +Mendoza, and I may add that I heard of great bones having been found +high up the R. Quinto. Hence the area of the Pampean formation, as +remarked by M. d’Orbigny, is probably at least equal to that of France, +and perhaps twice or thrice as great. In a basin, surrounded by +gravel-cliff (at a height of nearly three thousand feet), south of +Mendoza, there is, as described in the Third Chapter, a deposit very +like the Pampean, interstratified with other matter; and again at S. +Julian’s, in Patagonia, 560 miles south of the Colorado, a small +irregular bed of a nearly similar nature contains, as we have just +seen, mammiferous remains. In the provinces of Moxos and Chiquitos +(1,000 miles northward of the Pampas), and in Bolivia, at a height of +4,000 metres, M. d’Orbigny has described similar deposits, which he +believes to have been formed by the same agency contemporaneously with +the Pampean formation. Considering the immense distances between these +several points, and their different heights, it appears to me +infinitely more probable, that this similarity has resulted not from +contemporaneousness of origin, but from the similarity of the rocky +framework of the continent: it is known that in Brazil an immense area +consists of gneissic rocks, and we shall hereafter see, over how great +a length the plutonic rocks of the Cordillera, the overlying purple +porphyries, and the trachytic ejections, are almost identical in +nature. + +Three theories on the origin of the Pampean formation have been +propounded:—First, that of a great debacle by M. d’Orbigny; this seems +founded chiefly on the absence of stratification, and on the number of +embedded remains of terrestrial quadrupeds. Although the Pampean +formation (like so many argillaceous deposits) is not divided into +distinct and separate strata, yet we have seen that in one good section +it was striped with horizontal zones of colour, and that in several +specified places the upper and lower parts differed, not only +considerably in colour, but greatly in constitution. In the southern +part of the Pampas the upper mass (to a certain extent stratified) +generally consists of hard tosca-rock, and the lower part of red +Pampean mud, often itself divided into two or more masses, varying in +colour and in the quantity of included calcareous matter. In Western +Banda Oriental, beds of a similar nature, but of a greater age, +conformably underlie and are intercalated with the regularly stratified +tertiary formation. As a general rule, the marly concretions are +arranged in horizontal lines, sometimes united into irregular strata: +surely, if the mud had been tumultuously deposited in mass, the +included calcareous matter would have segregated itself irregularly, +and not into nodules arranged in horizontal lines, one above the other +and often far apart: this arrangement appears to me to prove that mud, +differing slightly in composition, was successively and quietly +deposited. On the theory of a debacle, a prodigious amount of mud, +without a single pebble, is supposed to have been borne over the wide +surface of the Pampas, when under water: on the other hand, over the +whole of Patagonia, the same or another debacle is supposed to have +borne nothing but gravel,—the gravel and the fine mud in the +neighbourhood of the Rios Negro and Colorado having been borne to an +equal distance from the Cordillera, or imagined line of disturbance: +assuredly directly opposite effects ought not to be attributed to the +same agency. Where, again, could a mass of fine sediment, charged with +calcareous matter in a fit state for chemical segregation, and in +quantity sufficient to cover an area at least 750 miles long, and 400 +miles broad, to a depth of from twenty to thirty feet to a hundred +feet, have been accumulated, ready to be transported by the supposed +debacle? To my mind it is little short of demonstration, that a great +lapse of time was necessary for the production and deposition of the +enormous amount of mudlike matter forming the Pampas; nor should I have +noticed the theory of a debacle, had it not been adduced by a +naturalist so eminent as M. d’Orbigny. + +A second theory, first suggested, I believe, by Sir W. Parish, is that +the Pampean formation was thrown down on low and marshy plains by the +rivers of this country before they assumed their present courses. The +appearance and composition of the deposit, the manner in which it +slopes up and round the primary ranges, the nature of the underlying +marine beds, the estuary and sea-shells on the surface, the overlying +sandstone beds at M. Hermoso, are all quite opposed to this view. Nor +do I believe that there is a single instance of a skeleton of one of +the extinct mammifers having been found in an upright position, as if +it had been mired. + +The third theory, of the truth of which I cannot entertain the smallest +doubt, is that the Pampean formation was slowly accumulated at the +mouth of the former estuary of the Plata and in the sea adjoining it. I +have come to this conclusion from the reasons assigned against the two +foregoing theories, and from simple geographical considerations. From +the numerous shells of the Azara labiata lying loose on the surface of +the plains, and near Buenos Ayres embedded in the tosca-rock, we know +that this formation not only was formerly covered by, but that the +uppermost parts were deposited in, the brackish water of the ancient La +Plata. Southward and seaward of Buenos Ayres, the plains were upheaved +from under water inhabited by true marine shells. We further know from +Professor Ehrenberg’s examination of the twenty microscopical organisms +in the mud round the tooth of the Mastodon high up the course of the +Parana, that the bottom- most part of this formation was of +brackish-water origin. A similar conclusion must be extended to the +beds of like composition, at the level of the sea and under it, at M. +Hermoso in Bahia Blanca. Dr. Carpenter finds that the harder varieties +of tosca-rock, collected chiefly to the south, contain marine spongoid +bodies, minute fragments of shells, corals, and Polythalamia; these +perhaps may have been drifted inwards by the tides, from the more open +parts of the sea. The absence of shells, throughout this deposit, with +the exception of the uppermost layers near Buenos Ayres, is a +remarkable fact: can it be explained by the brackish condition of the +water, or by the deep mud at the bottom? I have stated that both the +reddish mud and the concretions of tosca-rock are often penetrated by +minute, linear cavities, such as frequently may be observed in +fresh-water calcareous deposits:—were they produced by the burrowing of +small worms? Only on this view of the Pampean formation having been of +estuary origin, can the extraordinary numbers (presently to be alluded +to) of the embedded mammiferous remains be explained. (It is almost +superfluous to give the numerous cases (for instance, in Sumatra; Lyell +“Principles” volume 3 page 325 sixth edition, of the carcasses of +animals having been washed out to sea by swollen rivers; but I may +refer to a recent account by Mr. Bettington “Asiatic Society” 1845 June +21st, of oxen, deer, and bears being carried into the Gulf of Cambray; +see also the account in my “Journal” 2nd edition page 133, of the +numbers of animals drowned in the Plata during the great, often +recurrent, droughts.) + +With respect to the first origin of the reddish mud, I will only +remark, that the enormous area of Brazil consists in chief part of +gneissic and other granitic rocks, which have suffered decomposition, +and been converted into a red, gritty, argillaceous mass, to a greater +depth than in any other country which I have seen. The mixture of +rounded grains, and even of small fragments and pebbles of quartz, in +the Pampean mud of Banda Oriental, is evidently due to the neighbouring +and underlying primary rocks. The estuary mud was drifted during the +Pampean period in a much more southerly course, owing probably to the +east and west primary ridges south of the Plata not having been then +elevated, than the mud of the Plata at present is; for it was formerly +deposited as far south as the Colorado. The quantity of calcareous +matter in this formation, especially in those large districts where the +whole mass passes into tosca-rock, is very great: I have already +remarked on the close resemblance in external and microscopical +appearance, between this tosca-rock and the strata at Coquimbo, which +have certainly resulted from the decay and attrition of recent shells: +I dare not, however, extend this conclusion to the calcareous rocks of +the Pampas, more especially as the underlying tertiary strata in +western Banda Oriental show that at that period there was a copious +emission of carbonate of lime, in connection with volcanic action. (I +may add, that there are nearly similar superficial calcareous beds at +King George’s Sound in Australia; and these undoubtedly have been +formed by the disintegration of marine remains see “Volcanic Islands” +etc. page 144. There is, however, something very remarkable in the +frequency of superficial, thin beds of earthy calcareous matter, in +districts where the surrounding rocks are not calcareous. Major +Charters, in a Paper read before the Geographical Society April 13, +1840 and abstracted in the “Athenaeum” page 317, states that this is +the case in parts of Mexico, and that he has observed similar +appearances in many parts of South Africa. The circumstance of the +uppermost stratum round the ragged Sierra Ventana, consisting of +calcareous or marly matter, without any covering of alluvial matter, +strikes me as very singular, in whatever manner we view the deposition +and elevation of the Pampean formation.) + +The Pampean formation, judging from its similar composition, and from +the apparent absolute specific identity of some of its mammiferous +remains, and from the generic resemblance of others, belongs over its +vast area— throughout Banda Oriental, Entre Rios, and the wide extent +of the Pampas as far south as the Colorado,—to the same geological +epoch. The mammiferous remains occur at all depths from the top to the +bottom of the deposit; and I may add that nowhere in the Pampas is +there any appearance of much superficial denudation: some bones which I +found near the Guardia del Monte were embedded close to the surface; +and this appears to have been the case with many of those discovered in +Banda Oriental: on the Matanzas, twenty miles south of Buenos Ayres, a +Glyptodon was embedded five feet beneath the surface; numerous remains +were found by S. Muniz, near Luxan, at an average depth of eighteen +feet; in Buenos Ayres a skeleton was disinterred at sixty feet depth, +and on the Parana I have described two skeletons of the Mastodon only +five or six feet above the very base of the deposit. With respect to +the age of this formation, as judged of by the ordinary standard of the +existence of Mollusca, the only evidence within the limits of the true +Pampas which is at all trustworthy, is afforded by the still living +Azara labiata being embedded in tosca-rock near Buenos Ayres. At Punta +Alta, however, we have seen that several of the extinct mammifers, most +characteristic of the Pampean formation, co-existed with twenty species +of Mollusca, a barnacle and two corals, all still living on this same +coast;— for when we remember that the shells have a more ancient +appearance than the bones; that many of the bones, though embedded in a +coarse conglomerate, are perfectly preserved; that almost all the parts +of the skeleton of the Scelidotherium, even to the knee-cap, were lying +in their proper relative positions; and that a large piece of the +fragile dermal armour of a Dasypoid quadruped, connected with some of +the bones of the foot, had been entombed in a condition allowing the +two sides to be doubled together, it must assuredly be admitted that +these mammiferous remains were embedded in a fresh state, and therefore +that the living animals co-existed with the co-embedded shells. +Moreover, the Macrauchenia Patachonica (of which, according to +Professor Owen, remains also occur in the Pampas of Buenos Ayres, and +at Punta Alta) has been shown by satisfactory evidence of another kind, +to have lived on the plains of Patagonia long after the period when the +adjoining sea was first tenanted by its present commonest molluscous +animals. We must, therefore, conclude that the Pampean formation +belongs, in the ordinary geological sense of the word, to the Recent +Period. (M. d’Orbigny believes “Voyage” Part. Geolog. page 81, that +this formation, though “tres voisine de la notre, est neanmoins de +beaucoup anterieure a notre creation.”) + +At St. Fe Bajada, the Pampean estuary formation, with its mammiferous +remains, conformably overlies the marine tertiary strata, which (as +first shown by M. d’Orbigny) are contemporaneous with those of +Patagonia, and which, as we shall hereafter see, belong to a very +ancient tertiary stage. When examining the junction between these two +formations, I thought that the concretionary layer of marl marked a +passage between the marine and estuary stages. M. d’Orbigny disputes +this view (as given in my “Journal”), and I admit that it is erroneous, +though in some degree excusable, from their conformability and from +both abounding with calcareous matter. It would, indeed, have been a +great anomaly if there had been a true passage between a deposit +contemporaneous with existing species of mollusca, and one in which all +the mollusca appear to be extinct. Northward of Santa Fe, M. d’Orbigny +met with ferruginous sandstones, marly rocks, and other beds, which he +considers as a distinct and lower formation; but the evidence that they +are not parts of the same with an altered mineralogical character, does +not appear to me quite satisfactory. + +In Western Banda Oriental, while the marine tertiary strata were +accumulating, there were volcanic eruptions, much silex and lime were +precipitated from solution, coarse conglomerates were formed, being +derived probably from adjoining land, and layers of red mud and marly +rocks, like those of the Pampean formation, were occasionally +deposited. The true Pampean deposit, with mammiferous remains, instead +of as at Santa Fe overlying conformably the tertiary strata, is here +seen at a lower level folding round and between the flat-topped, +cliff-bounded hills, formed by a upheaval and denudation of these same +tertiary strata. The upheaval, having occurred here earlier than at +Santa Fe, may be naturally accounted for by the contemporaneous +volcanic action. At the Barrancas de S. Gregorio, the Pampean deposit, +as we have seen, overlies and fills up furrows in coarse sand, +precisely like that now accumulating on the shores near the mouth of +the Plata. I can hardly believe that this loose and coarse sand is +contemporaneous with the old tertiary and often crystalline strata of +the more western parts of the province; and am induced to suspect that +it is of subsequent origin. If that section near Colonia could be +implicitly trusted, in which, at a height of only fifteen feet above +the Plata, a bed of fresh-looking mussels, of an existing littoral +species, appeared to lie between the sand and the Pampean mud, I should +conclude that Banda Oriental must have stood, when the coarse sand was +accumulating, at only a little below its present level, and had then +subsided, allowing the estuary Pampean mud to cover far and wide its +surface up to a height of some hundred feet; and that after this +subsidence the province had been uplifted to its present level. + +In Western Banda Oriental, we know, from two unequivocal sections that +there is a mass, absolutely undistinguishable from the true Pampean +deposit, beneath the old tertiary strata. This inferior mass must be +very much more ancient than the upper deposit with its mammiferous +remains, for it lies beneath the tertiary strata in which all the +shells are extinct. Nevertheless, the lower and upper masses, as well +as some intermediate layers, are so similar in mineralogical character, +that I cannot doubt that they are all of estuary origin, and have been +derived from the same great source. At first it appeared to me +extremely improbable, that mud of the same nature should have been +deposited on nearly the same spot, during an immense lapse of time, +namely, from a period equivalent perhaps to the Eocene of Europe to +that of the Pampean formation. But as, at the very commencement of the +Pampean period, if not at a still earlier period, the Sierra Ventana +formed a boundary to the south,—the Cordillera or the plains in front +of them to the west,—the whole province of Corrientes probably to the +north, for, according to M. d’Orbigny, it is not covered by the Pampean +deposit,—and Brazil, as known by the remains in the caves, to the +north-east; and as again, during the older tertiary period, land +already existed in Western Banda Oriental and near St. Fe Bajada, as +may be inferred from the vegetable debris, from the quantities of +silicified wood, and from the remains of a Toxodon found, according to +M. d’Orbigny, in still lower strata, we may conclude, that at this +ancient period a great expanse of water was surrounded by the same +rocky framework which now bounds the plains of Pampean formation. This +having been the case, the circumstance of sediment of the same nature +having been deposited in the same area during an immense lapse of time, +though highly remarkable, does not appear incredible. + +The elevation of the Pampas, at least of the southern parts, has been +slow and interrupted by several periods of rest, as may be inferred +from the plains, cliffs, and lines of sand-dunes (with shells and +pumice-pebbles) standing at different heights. I believe, also, that +the Pampean mud continued to be deposited, after parts of this +formation had already been elevated, in the same manner as mud would +continue to be deposited in the estuary of the Plata, if the mud-banks +on its shores were now uplifted and changed into plains: I believe in +this from the improbability of so many skeletons and bones having been +accumulated at one spot, where M. Hermoso now stands, at a depth of +between eight hundred and one thousand feet, and at a vast distance +from any land except small rocky islets,—as must have been the case, if +the high tosca-plain round the Ventana and adjoining Sierras, had not +been already uplifted and converted into land, supporting mammiferous +animals. At Punta Alta we have good evidence that the gravel- strata, +which certainly belong to the true Pampean period, were accumulated +after the elevation in that neighbourhood of the main part of the +Pampean deposit, whence the rounded masses of tosca-rock were derived, +and that rolled fragment of black bone in the same peculiar condition +with the remains at Monte Hermoso. + +The number of the mammiferous remains embedded in the Pampas is, as I +have remarked, wonderful: it should be borne in mind that they have +almost exclusively been found in the cliffs and steep banks of rivers, +and that, until lately, they excited no attention amongst the +inhabitants: I am firmly convinced that a deep trench could not be cut +in any line across the Pampas, without intersecting the remains of some +quadruped. It is difficult to form an opinion in what part of the +Pampas they are most numerous; in a limited spot they could not well +have been more numerous than they were at P. Alta; the number, however, +lately found by Senor F. Muniz, near Luxan, in a central spot in the +Pampas, is extraordinarily great: at the end of this chapter I will +give a list of all the localities at which I have heard of remains +having been discovered. Very frequently the remains consist of almost +perfect skeletons; but there are, also, numerous single bones, as for +instance at St. Fe. Their state of preservation varies much, even when +embedded near each other: I saw none others so perfectly preserved as +the heads of the Toxodon and Mylodon from the white soft earthy bed on +the Sarandis in Banda Oriental. It is remarkable that in two limited +sections I found no less than five teeth separately embedded, and I +heard of teeth having been similarly found in other parts: may we +suppose that the skeletons or heads were for a long time gently drifted +by currents over the soft muddy bottom, and that the teeth +occasionally, here and there, dropped out? + +It may be naturally asked, where did these numerous animals live? From +the remarkable discoveries of MM. Lund and Clausen, it appears that +some of the species found in the Pampas inhabited the highlands of +Brazil: the Mastodon Andium is embedded at great heights in the +Cordillera from north of the equator to at least as far south as Tarija +(Humboldt states that the Mastodon has been discovered in New Granada: +it has been found in Quito. When at Lima, I saw a tooth of a Mastodon +in the possession of Don M. Rivero, found at Playa Chica on the +Maranon, near the Guallaga. Every one has heard of the numerous remains +of Mastodon in Bolivia.); and as there is no higher land, there can be +little doubt that this Mastodon must have lived on the plains and +valleys of that great range. These countries, however, appear too far +distant for the habitation of the individuals entombed in the Pampas: +we must probably look to nearer points, for instance to the province of +Corrientes, which, as already remarked, is said not to be covered by +the Pampean formation, and may therefore, at the period of its +deposition, have existed as dry land. I have already given my reasons +for believing that the animals embedded at M. Hermoso and at P. Alta in +Bahia Blanca, lived on adjoining land, formed of parts of the already +elevated Pampean deposit. With respect to the food of these many great +extinct quadrupeds, I will not repeat the facts given in my “Journal” +(second edition page 85), showing that there is no correlation between +the luxuriance of the vegetation of a country and the size of its +mammiferous inhabitants. I do not doubt that large animals could now +exist, as far as the amount, not kind, of vegetation is concerned, on +the sterile plains of Bahia Blanca and of the R. Negro, as well as on +the equally, if not more sterile plains of Southern Africa. The +climate, however, may perhaps have somewhat deteriorated since the +mammifers embedded at Bahia Blanca lived there; for we must not infer, +from the continued existence of the same shells on the present coasts, +that there has been no change in climate; for several of these shells +now range northward along the shores of Brazil, where the most +luxuriant vegetation flourishes under a tropical temperature. With +respect to the extinction, which at first fills the mind with +astonishment, of the many great and small mammifers of this period, I +may also refer to the work above cited (second edition page 173), in +which I have endeavoured to show, that however unable we may be to +explain the precise cause, we ought not properly to feel more surprised +at a species becoming extinct than at one being rare; and yet we are +accustomed to view the rarity of any particular species as an ordinary +event, not requiring any extraordinary agency. + +The several mammifers embedded in the Pampean formation, which mostly +belong to extinct genera, and some even to extinct families or orders, +and which differ nearly, if not quite, as much as do the Eocene +mammifers of Europe from living quadrupeds having existed +contemporaneously with mollusca, all still inhabiting the adjoining +sea, is certainly a most striking fact. It is, however, far from being +an isolated one; for, during the late tertiary deposits of Britain, an +elephant, rhinoceros, and hippopotamus co-existed with many recent land +and fresh-water shells; and in North America, we have the best evidence +that a mastodon, elephant, megatherium, megalonyx, mylodon, an extinct +horse and ox, likewise co- existed with numerous land, fresh-water, and +marine recent shells. (Many original observations, and a summary on +this subject, are given in Mr. Lyell’s paper in the “Geological +Proceedings” volume 4 page 3 and in his “Travels in North America” +volume 1 page 164 and volume 2 page 60. For the European analogous +cases see Mr. Lyell’s “Principles of Geology” 6th edition volume 1 page +37.) The enumeration of these extinct North American animals naturally +leads me to refer to the former closer relation of the mammiferous +inhabitants of the two Americas, which I have discussed in my +“Journal,” and likewise to the vast extent of country over which some +of them ranged: thus the same species of the Megatherium, Megalonyx, +Equus (as far as the state of their remains permits of identification), +extended from the Southern United States of North America to Bahia +Blanca, in latitude 39 degrees S., on the coast of Patagonia. The fact +of these animals having inhabited tropical and temperate regions, does +not appear to me any great difficulty, seeing that at the Cape of Good +Hope several quadrupeds, such as the elephant and hippopotamus, range +from the equator to latitude 35 degrees south. The case of the Mastodon +Andium is one of more difficulty, for it is found from latitude 36 +degrees S., over, as I have reason to believe, nearly the whole of +Brazil, and up the Cordillera to regions which, according to M. +d’Orbigny, border on perpetual snow, and which are almost destitute of +vegetation: undoubtedly the climate of the Cordillera must have been +different when the mastodon inhabited it; but we should not forget the +case of the Siberian mammoth and rhinoceros, as showing how severe a +climate the larger pachydermata can endure; nor overlook the fact of +the guanaco ranging at the present day over the hot low deserts of +Peru, the lofty pinnacles of the Cordillera, and the damp forest-clad +land of Southern Tierra del Fuego; the puma, also, is found from the +equator to the Strait of Magellan, and I have seen its footsteps only a +little below the limits of perpetual snow in the Cordillera of Chile. + +At the period, so recent in a geological sense, when these extinct +mammifers existed, the two Americas must have swarmed with quadrupeds, +many of them of gigantic size; for, besides those more particularly +referred to in this chapter, we must include in this same period those +wonderfully numerous remains, some few of them specifically, and others +generically related to those of the Pampas, discovered by MM. Lund and +Clausen in the caves of Brazil. Finally, the facts here given show how +cautious we ought to be in judging of the antiquity of a formation from +even a great amount of difference between the extinct and living +species in any one class of animals;—we ought even to be cautious in +accepting the general proposition, that change in organic forms and +lapse of time are at all, necessarily, correlatives. + +LOCALITIES WITHIN THE REGION OF THE PAMPAS WHERE GREAT BONES HAVE BEEN +FOUND. + +The following list, which includes every account which I have hitherto +met with of the discovery of fossil mammiferous remains in the Pampas, +may be hereafter useful to a geologist investigating this region, and +it tends to show their extraordinary abundance. I heard of and saw many +fossils, the original position of which I could not ascertain; and I +received many statements too vague to be here inserted. Beginning to +the south:—we have the two stations in Bahia Blanca, described in this +chapter, where at P. Alta, the Megatherium, Megalonyx, Scelidotherium, +Mylodon, Holophractus (or an allied genus), Toxodon, Macrauchenia, and +an Equus were collected; and at M. Hermoso a Ctenomys, Hydrochaerus, +some other rodents and the bones of a great megatheroid quadruped. +Close north-east of the S. Tapalguen, we have the Rios ‘Huesos’ (i.e. +“bones”), which probably takes its name from large fossil bones. Near +Villa Nuevo, and at Las Averias, not far from the Salado, three nearly +perfect skeletons, one of the Megatherium, one of the Glyptodon +clavipes, and one of some great Dasypoid quadruped, were found by the +agent of Sir W. Parish (see his work “Buenos Ayres” etc. page 171). I +have seen the tooth of a Mastodon from the Salado; a little northward +of this river, on the borders of a lake near the G. del Monte, I saw +many bones, and one large piece of dermal armour; higher up the Salado, +there is a place called Monte “Huesos.” On the Matanzas, about twenty +miles south of Buenos Ayres, the skeleton (vide page 178 of “Buenos +Ayres” etc. by Sir W. Parish) of a Glyptodon was found about five feet +beneath the surface; here also (see Catalogue of Royal College of +Surgeons) remains of Glyptodon clavipes, G. ornatus, and G. reticulatus +were found. Signor Angelis, in a letter which I have seen, refers to +some great remains found in Buenos Ayres, at a depth of twenty varas +from the surface. Seven leagues north of this city the same author +found the skeletons of Mylodon robustus and Glyptodon ornatus. From +this neighbourhood he has lately sent to the British Museum the +following fossils:—Remains of three or four individuals of Megatherium; +of three species of Glyptodon; of three individuals of the Mastodon +Andium; of Macrauchenia; of a second species of Toxodon, different from +T. Platensis; and lastly, of the Machairodus, a wonderful large +carnivorous animal. M. d’Orbigny has lately received from the Recolate +“Voyage” Pal. page 144), near Buenos Ayres, a tooth of Toxodon +Platensis. + +Proceeding northward, along the west bank of the Parana, we come to the +Rio Luxan, where two skeletons of the Megatherium have been found; and +lately, within eight leagues of the town of Luxan, Dr. F. X. Muniz has +collected (“British Packet” Buenos Ayres September 25, 1841), from an +average depth of eighteen feet, very numerous remains, of no less than, +as he believes, nine distinct species of mammifers. At Areco, large +bones have been found, which are believed, by the inhabitants, to have +been changed from small bones, by the water of the river! At Arrecifes, +the Glyptodon, sent to the College of Surgeons, was found; and I have +seen two teeth of a Mastodon from this quarter. At S. Nicolas, M. +d’Orbigny found remains of a Canis, Ctenomys, and Kerodon; and M. +Isabelle (“Voyage” page 332) refers to a gigantic Armadillo found +there. At S. Carlos, I heard of great bones. A little below the mouth +of the Carcarana, the two skeletons of Mastodon were found; on the +banks of this river, near S. Miguel, I found teeth of the Mastodon and +Toxodon; and “Falkner” (page 55) describes the osseous armour of some +great animal; I heard of many other bones in this neighbourhood. I have +seen, I may add, in the possession of Mr. Caldcleugh, the tooth of a +Mastodon Andium, said to have been found in Paraguay; I may here also +refer to a statement in this gentleman’s travels (volume 1 page 48), of +a great skeleton having been found in the province of Bolivia in +Brazil, on the R. de las Contas. The furthest point westward in the +Pampas, at which I have HEARD of fossil bones, was high up on the banks +of R. Quinto. + +In Entre Rios, besides the remains of the Mastodon, Toxodon, Equus, and +a great Dasypoid quadruped near St. Fe Bajada, I received an account of +bones having been found a little S.E. of P. Gorda (on the Parana), and +of an entire skeleton at Matanzas, on the Arroyo del Animal. + +In Banda Oriental, besides the remains of the Toxodon, Mylodon, and two +skeletons of great animals with osseous armour (distinct from that of +the Glyptodon), found on the Arroyos Sarandis and Berquelo, M. Isabelle +(“Voyage” page 322) says, many bones have been found near the R. Negro, +and on the R. Arapey, an affluent of the Paraguay, in latitude 30 +degrees 40 minutes south. I heard of bones near the source of the A. +Vivoras. I saw the remains of a Dasypoid quadruped from the Arroyo +Seco, close to M. Video; and M. d’Orbigny refers (“Voyage” Geolog. page +24), to another found on the Pedernal, an affluent of the St. Lucia; +and Signor Angelis, in a letter, states that a third skeleton of this +family has been found, near Canelones. I saw a tooth of the Mastodon +from Talas, another affluent of the St. Lucia. The most eastern point +at which I heard of great bones having been found, was at Solis Grande, +between M. Video and Maldonado. + + + + +CHAPTER V. +ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE. + + +Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with +Infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P. +Gallegos.—Eastern Tierra del Fuego; leaves of extinct +beech-trees.—Summary on the Patagonian tertiary formations.—Tertiary +formations of the Western Coast.—Chonos and Chiloe groups, volcanic +rocks of.—Concepcion.—Navidad.—Coquimbo.—Summary.—Age of the tertiary +formations.—Lines of elevation.—Silicified wood.—Comparative ranges of +the extinct and living mollusca on the West Coast of—S. +America.—Climate of the tertiary period.—On the causes of the absence +of recent conchiferous deposits on the coast of S. America.—On the +contemporaneous deposition and preservation of sedimentary formations. + +RIO NEGRO. + +I can add little to the details given by M. d’Orbigny on the sandstone +formation of this district. (“Voyage” Part Geolog. pages 57-65.) The +cliffs to the south of the river are about two hundred feet in height, +and are composed of sandstone of various tints and degrees of hardness. +One layer, which thinned out at both ends, consisted of earthy matter, +of a pale reddish colour, with some gypsum, and very like (I speak +after comparison of the specimens brought home) Pampean mud: above this +was a layer of compact marly rock with dendritic manganese. Many blocks +of a conglomerate of pumice-pebbles embedded in hard sandstone were +strewed at the foot of the cliff, and had evidently fallen from above. +A few miles N.E. of the town, I found, low down in the sandstone, a +bed, a few inches in thickness, of a white, friable, harsh-feeling +sediment, which adheres to the tongue, is of easy fusibility, and of +little specific gravity; examined under the microscope, it is seen to +be pumiceous tuff, formed of broken transparent crystals. In the cliffs +south of the river, there is, also, a thin layer of nearly similar +nature, but finer grained, and not so white; it might easily have been +mistaken for a calcareous tuff, but it contains no lime: this substance +precisely resembles a most widely extended and thick formation in +Southern Patagonia, hereafter to be described, and which is remarkable +for being partially formed of infusoria. These beds, conjointly with +the conglomerate of pumice, are interesting, as showing the nature of +the volcanic action in the Cordillera during this old tertiary period. + +In a bed at the base of the southern cliffs, M. d’Orbigny found two +extinct fresh-water shells, namely, a Unio and Chilina. This bed rested +on one with bones of an extinct rodent, namely, the Megamys +Patagoniensis; and this again on another with extinct marine shells. +The species found by M. d’Orbigny in different parts of this formation +consist of:— + +1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fe, and +whole coast of Patagonia). 2. Ostrea Ferrarisi, d’Orbigny, “Voyage, +Pal.” 3. Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. Fe, +and S. Josef). 4. Pecten Patagoniensis, d’Orbigny, “Voyage, Pal.” 5. +Venus Munsterii, d’Orbigny, “Voyage, Pal.” (also at St. Fe). 6. Arca +Bonplandiana, d’Orbigny, “Voyage, Pal.” (also at St. Fe). + +According to M. d’Orbigny, the sandstone extends westward along the +coast as far as Port S. Antonio, and up the R. Negro far into the +interior: northward I traced it to the southern side of the Rio +Colorado, where it forms a low denuded plain. This formation, though +contemporaneous with that of the rest of Patagonia, is quite different +in mineralogical composition, being connected with it only by the one +thin white layer: this difference may be reasonably attributed to the +sediment brought down in ancient times by the Rio Negro; by which +agency, also, we can understand the presence of the fresh-water shells, +and of the bones of land animals. Judging from the identity of four of +the above shells, this formation is contemporaneous (as remarked by M. +d’Orbigny) with that under the Pampean deposit in Entre Rios and in +Banda Oriental. The gravel capping the sandstone plain, with its +calcareous cement and nodules of gypsum, is probably, from the reasons +given in the First Chapter, contemporaneous with the uppermost beds of +the Pampean formation on the upper plain north of the Colorado. + +SAN JOSEF. + +My examination here was very short: the cliffs are about a hundred feet +high; the lower third consists of yellowish-brown, soft, slightly +calcareous, muddy sandstone, parts of which when struck emit a fetid +smell. In this bed the great Ostraea Patagonica, often marked with +dendritic manganese and small coral-lines, were extraordinarily +numerous. I found here the following shells:— + +1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fe and +whole coast of Patagonia). 2. Ostrea Alvarezii, d’Orbigny, “Voyage, +Pal.” (also at St. Fe and R. Negro). 3. Pecten Paranensis, d’Orbigny, +“Voyage, Pal.” (also at St. Fe, S. Julian, and Port Desire). 4. Pecten +Darwinianus, d’Orbigny, “Voyage, Pal.” (also at St. Fe). 5. Pecten +actinodes, G.B. Sowerby. 6. Terebratula Patagonica, G.B. Sowerby (also +S. Julian). 7. Casts of a Turritella. + +The four first of these species occur at St. Fe in Entre Rios, and the +two first in the sandstone of the Rio Negro. Above this fossiliferous +mass, there is a stratum of very fine-grained, pale brown mudstone, +including numerous laminae of selenite. All the strata appear +horizontal, but when followed by the eye for a long distance, they are +seen to have a small easterly dip. On the surface we have the +porphyritic gravel, and on it sand with recent shells. + +NUEVO GULF. + +From specimens and notes given me by Lieutenant Stokes, it appears that +the lower bed consists of soft muddy sandstone, like that of S. Josef, +with many imperfect shells, including the Pecten Paranensis, d’Orbigny, +casts of a Turritella and Scutella. On this there are two strata of the +pale brown mudstone, also like that of S. Josef, separated by a +darker-coloured, more argillaceous variety, including the Ostrea +Patagonica. Professor Ehrenberg has examined this mudstone for me: he +finds in it three already known microscopic organisms, enveloped in a +fine-grained pumiceous tuff, which I shall have immediately to describe +in detail. Specimens brought to me from the uppermost bed, north of the +Rio Chupat, consist of this same substance, but of a whiter colour. + +Tertiary strata, such as here described, appear to extend along the +whole coast between Rio Chupat and Port Desire, except where +interrupted by the underlying claystone porphyry, and by some +metamorphic rocks; these hard rocks, I may add, are found at intervals +over a space of about five degrees of latitude, from Point Union to a +point between Port S. Julian and S. Cruz, and will be described in the +ensuing chapter. Many gigantic specimens of the Ostraea Patagonica were +collected in the Gulf of St. George. + +PORT DESIRE. + +A good section of the lowest fossiliferous mass, about forty feet in +thickness, resting on claystone porphyry, is exhibited a few miles +south of the harbour. The shells sufficiently perfect to be recognised +consist of:— + +1. Ostrea Patagonica, d’Orbigny, (also at St. Fe, and whole coast of +Patagonia). 2. Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at +St. Fe, S. Josef, S. Julian). 3. Pecten centralis, G.B. Sowerby (also +at S. Julian and S. Cruz). 4. Cucullaea alta, G.B. Sowerby (also at S. +Cruz). 5. Nucula ornata, G.B. Sowerby. 6. Turritella Patagonica, G.B. +Sowerby. + +The fossiliferous strata, when not denuded, are conformably covered by +a considerable thickness of the fine-grained pumiceous mudstone, +divided into two masses: the lower half is very fine-grained, slightly +unctuous, and so compact as to break with a semi-conchoidal fracture, +though yielding to the nail; it includes laminae of selenite: the upper +half precisely resembles the one layer at the Rio Negro, and with the +exception of being whiter, the upper beds at San Josef and Nuevo Gulf. +In neither mass is there any trace to the naked eye of organic forms. +Taking the entire deposit, it is generally quite white, or yellowish, +or feebly tinted with green; it is either almost friable under the +finger, or as hard as chalk; it is of easy fusibility, of little +specific gravity, is not harsh to the touch, adheres to the tongue, and +when breathed on exhales a strong aluminous odour; it sometimes +contains a very little calcareous matter, and traces (besides the +included laminae) of gypsum. Under the microscope, according to +Professor Ehrenberg, it consists of minute, triturated, cellular, +glassy fragments of pumice, with some broken crystals. +(“Monatsberichten de konig. Akad. zu Berlin” vom April 1845.) In the +minute glassy fragments, Professor Ehrenberg recognises organic +structures, which have been affected by volcanic heat: in the specimens +from this place, and from Port S. Julian, he finds sixteen Polygastrica +and twelve Phytolitharia. Of these organisms, seven are new forms, the +others being previously known: all are of marine, and chiefly of +oceanic, origin. This deposit to the naked eye resembles the crust +which often appears on weathered surfaces of feldspathic rocks; it +likewise resembles those beds of earthy feldspathic matter, sometimes +interstratified with porphyritic rocks, as is the case in this very +district with the underlying purple claystone porphyry. From examining +specimens under a common microscope, and comparing them with other +specimens undoubtedly of volcanic origin, I had come to the same +conclusion with Professor Ehrenberg, namely, that this great deposit, +in its first origin, is of volcanic nature. + +PORT S. JULIAN. + +(FIGURE 17. SECTION OF THE STRATA EXHIBITED IN THE CLIFFS OF THE NINETY +FEET PLAIN AT PORT S. JULIAN. + +(Section through beds from top to bottom: A, B, C, D, E, F.)) + +On the south side of the harbour, Figure 17 gives the nature of the +beds seen in the cliffs of the ninety feet plain. Beginning at the +top:— + +1st, the earthy mass (AA), including the remains of the Macrauchenia, +with recent shells on the surface. + +Second, the porphyritic shingle (B), which in its lower part is +interstratified (owing, I believe, to redisposition during denudation) +with the white pumiceous mudstone. + +Third, this white mudstone, about twenty feet in thickness, and divided +into two varieties (C and D), both closely resembling the lower, fine- +grained, more unctuous and compact kind at Port Desire; and, as at that +place, including much selenite. + +Fourth, a fossiliferous mass, divided into three main beds, of which +the uppermost is thin, and consists of ferruginous sandstone, with many +shells of the great oyster and Pecten Paranensis; the middle bed (E) is +a yellowish earthy sandstone abounding with Scutellae; and the lowest +bed (F) is an indurated, greenish, sandy clay, including large +concretions of calcareous sandstone, many shells of the great oyster, +and in parts almost made up of fragments of Balanidae. Out of these +three beds, I procured the following twelve species, of which the two +first were exceedingly numerous in individuals, as were the +Terebratulae and Turritellae in certain layers:— + +1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fe, and +whole coast of Patagonia). 2. Pecten Paranensis, d’Orbigny, “Voyage, +Pal.” (St. Fe, S. Josef, Port Desire). 3. Pecten centralis, G.B. +Sowerby (also at Port Desire and S. Cruz). 4. Pecten geminatus, G.B. +Sowerby. 5. Terebratula Patagonica, G.B. Sowerby (also S. Josef). 6. +Struthiolaria ornata, G.B. Sowerby (also S. Cruz). 7. Fusus +Patagonicus, G.B. Sowerby. 8. Fusus Noachinus, G.B. Sowerby. 9. +Scalaria rugulosa, G.B. Sowerby. 10. Turritella ambulacrum, G.B. +Sowerby (also S. Cruz). 11. Pyrula, cast of, like P. ventricosa of +Sowerby, Tank Cat. 12. Balanus varians, G.B. Sowerby. 13. Scutella, +differing from the species from Nuevo Gulf. + +At the head of the inner harbour of Port S. Julian, the fossiliferous +mass is not displayed, and the sea-cliffs from the water’s edge to a +height of between one and two hundred feet are formed of the white +pumiceous mudstone, which here includes innumerable, far-extended, +sometimes horizontal, sometimes inclined or vertical laminae of +transparent gypsum, often about an inch in thickness. Further inland, +with the exception of the superficial gravel, the whole thickness of +the truncated hills, which represent a formerly continuous plain 950 +feet in height, appears to be formed of this white mudstone: here and +there, however, at various heights, thin earthy layers, containing the +great oyster, Pecten Paranensis and Turritella ambulacrum, are +interstratified; thus showing that the whole mass belongs to the same +epoch. I nowhere found even a fragment of a shell actually in the white +deposit, and only a single cast of a Turritella. Out of the eighteen +microscopic organisms discovered by Ehrenberg in the specimens from +this place, ten are common to the same deposit at Port Desire. I may +add that specimens of this white mudstone, with the same identical +characters were brought me from two points,—one twenty miles north of +S. Julian, where a wide gravel-capped plain, 350 feet in height, is +thus composed; and the other forty miles south of S. Julian, where, on +the old charts, the cliffs are marked as “Chalk Hills.” + +SANTA CRUZ. + +The gravel-capped cliffs at the mouth of the river are 355 feet in +height: the lower part, to a thickness of fifty or sixty feet, consists +of a more or less hardened, darkish, muddy, or argillaceous sandstone +(like the lowest bed of Port Desire), containing very many shells, some +silicified and some converted into yellow calcareous spar. The great +oyster is here numerous in layers; the Trigonocelia and Turritella are +also very numerous: it is remarkable that the Pecten Paranensis, so +common in all other parts of the coast, is here absent: the shells +consist of:— + +1. Ostrea Patagonica, d’Orbigny; “Voyage Pal.” (also at St. Fe and +whole coast of Patagonia). +2. Pecten centralis, G.B. Sowerby (also P. Desire and S. Julian). +3. Venus meridionalis of G.B. Sowerby. +4. Crassatella Lyellii, G.B. Sowerby. +5. Cardium puelchum, G.B. Sowerby. +6. Cardita Patagonica, G.B. Sowerby. +7. Mactra rugata, G.B. Sowerby. +8. Mactra Darwinii, G.B. Sowerby. +9. Cucullaea alta, G.B. Sowerby (also P. Desire). +10. Trigonocelia insolita, G.B. Sowerby. +11. Nucula (?) glabra, G.B. Sowerby. +12. Crepidula gregaria, G.B. Sowerby. +13. Voluta alta, G.B. Sowerby. +14. Trochus collaris, G.B. Sowerby. +15. Natica solida (?), G.B. Sowerby +16. Struthiolaria ornata, G.B. Sowerby (also P. Desire). +17. Turritella ambulacrum, G.B. Sowerby (also P. S. Julian). +Imperfect fragments of the genera Byssoarca, Artemis, and Fusus. + + +The upper part of the cliff is generally divided into three great +strata, differing slightly in composition, but essentially resembling +the pumiceous mudstone of the places farther north; the deposit, +however, here is more arenaceous, of greater specific gravity, and not +so white: it is interlaced with numerous thin veins, partially or quite +filled with transverse fibres of gypsum; these fibres were too short to +reach across the vein, have their extremities curved or bent: in the +same veins with the gypsum, and likewise in separate veins as well as +in little nests, there is much powdery sulphate of magnesia (as +ascertained by Mr. Reeks) in an uncompressed form: I believe that this +salt has not heretofore been found in veins. Of the three beds, the +central one is the most compact, and more like ordinary sandstone: it +includes numerous flattened spherical concretions, often united like a +necklace, composed of hard calcareous sandstone, containing a few +shells: some of these concretions were four feet in diameter, and in a +horizontal line nine feet apart, showing that the calcareous matter +must have been drawn to the centres of attraction, from a distance of +four feet and a half on both sides. In the upper and lower +finer-grained strata, there were other concretions of a grey colour, +containing calcareous matter, and so fine-grained and compact, as +almost to resemble porcelain- rock: I have seen exactly similar +concretions in a volcanic tufaceous bed in Chiloe. Although in this +upper fine-grained strata, organic remains were very rare, yet I +noticed a few of the great oyster; and in one included soft ferruginous +layer, there were some specimens of the Cucullaea alta (found at Port +Desire in the lower fossiliferous mass) and of the Mactra rugata, which +latter shell has been partially converted into gypsum. + +(FIGURE 18. SECTION OF THE PLAINS OF PATAGONIA, ON THE BANKS OF THE S. +CRUZ. + +(Section through strata (from top to bottom)): Surface of plain with +erratic boulders; 1,146 feet above the sea. a. Gravel and boulders, 212 +feet thick. b. Basaltic lava, 322 feet thick. c, d and e. Sedimentary +layers, bed of small pebbles and talus respectively, total 592 feet +thick. River of S. Cruz; here 280 feet above sea.) + +In ascending the valley of the S. Cruz, the upper strata of the coast- +cliffs are prolonged, with nearly the same characters, for fifty miles: +at about this point, they begin in the most gradual and scarcely +perceptible manner, to be banded with white lines; and after ascending +ten miles farther, we meet with distinct thin layers of whitish, +greenish, and yellowish fine-grained, fusible sediments. At eighty +miles from the coast, in a cliff thus composed, there were a few layers +of ferruginous sandstone, and of an argillaceous sandstone with +concretions of marl like those in the Pampas. (At this spot, for a +space of three-quarters of a mile along the north side of the river, +and for a width of half a mile, there has been a great slip, which has +formed hills between sixty and seventy feet in height, and has tilted +the strata into highly inclined and even vertical positions. The strata +generally dipped at an angle of 45 degrees towards the cliff from which +they had slided. I have observed in slips, both on a small and large +scale, that this inward dip is very general. Is it due to the +hydrostatic pressure of water percolating with difficulty through the +strata acting with greater force at the base of the mass than against +the upper part?) At one hundred miles from the coast, that is at a +central point between the Atlantic and the Cordillera, we have the +section in Figure 18. + +The upper half of the sedimentary mass, under the basaltic lava, +consists of innumerable zones of perfectly white bright green, +yellowish and brownish, fine-grained, sometimes incoherent, sedimentary +matter. The white, pumiceous, trachytic tuff-like varieties are of +rather greater specific gravity than the pumiceous mudstone on the +coast to the north; some of the layers, especially the browner ones, +are coarser, so that the broken crystals are distinguishable with a +weak lens. The layers vary in character in short distances. With the +exception of a few of the Ostrea Patagonica, which appeared to have +rolled down from the cliff above, no organic remains were found. The +chief difference between these layers taken as a whole, and the upper +beds both at the mouth of the river and on the coast northward, seems +to lie in the occasional presence of more colouring matter, and in the +supply having been intermittent; these characters, as we have seen, +very gradually disappear in descending the valley, and this fact may +perhaps be accounted for by the currents of a more open sea having +blended together the sediment from a distant and intermittent source. + +The coloured layers in the foregoing section rest on a mass, apparently +of great thickness (but much hidden by the talus), of soft sandstone, +almost composed of minute pebbles, from one-tenth to two-tenths of an +inch in diameter, of the rocks (with the entire exception of the +basaltic lava) composing the great boulders on the surface of the +plain, and probably composing the neighbouring Cordillera. Five miles +higher up the valley, and again thirty miles higher up (that is twenty +miles from the nearest range of the Cordillera), the lower plain +included within the upper escarpments, is formed, as seen on the banks +of the river, of a nearly similar but finer-grained, more earthy, +laminated sandstone, alternating with argillaceous beds, and containing +numerous moderately sized pebbles of the same rocks, and some shells of +the great Ostrea Patagonica. (I found at both places, but not in situ, +quantities of coniferous and ordinary dicotyledonous silicified wood, +which was examined for me by Mr. R. Brown.) As most of these shells had +been rolled before being here embedded, their presence does not prove +that the sandstone belongs to the great Patagonian tertiary formation, +for they might have been redeposited in it, when the valley existed as +a sea-strait; but as amongst the pebbles there were none of basalt, +although the cliffs on both sides of the valley are composed of this +rock, I believe that the sandstone does belong to this formation. At +the highest point to which we ascended, twenty miles distant from the +nearest slope of the Cordillera, I could see the horizontally zoned +white beds, stretching under the black basaltic lava, close up to the +mountains; so that the valley of the S. Cruz gives a fair idea of the +constitution of the whole width of Patagonia. + +BASALTIC LAVA OF THE S. CRUZ. + +This formation is first met with sixty-seven miles from the mouth of +the river; thence it extends uninterruptedly, generally but not +exclusively on the northern side of the valley, close up to the +Cordillera. The basalt is generally black and fine-grained, but +sometimes grey and laminated; it contains some olivine, and high up the +valley much glassy feldspar, where, also, it is often amygdaloidal; it +is never highly vesicular, except on the sides of rents and on the +upper and lower, spherically laminated surfaces. It is often columnar; +and in one place I saw magnificent columns, each face twelve feet in +width, with their interstices filled up with calcareous tuff. The +streams rest conformably on the white sedimentary beds, but I nowhere +saw the actual junction; nor did I anywhere see the white beds actually +superimposed on the lava; but some way up the valley at the foot of the +uppermost escarpments, they must be thus superimposed. Moreover, at the +lowest point down the valley, where the streams thin out and terminate +in irregular projections, the spaces or intervals between these +projections are filled up to the level of the now denuded and +gravel-capped surfaces of the plains, with the white-zoned sedimentary +beds; proving that this matter continued to be deposited after the +streams had flowed. Hence we may conclude that the basalt is +contemporaneous with the upper parts of the great tertiary formation. + +The lava where first met with is 130 feet in thickness: it there +consists of two, three, or perhaps more streams, divided from each +other by vesicular spheroids like those on the surface. From the +streams having, as it appears, extended to different distances, the +terminal points are of unequal heights. Generally the surface of the +basalt is smooth them in one part high up the valley, it was so uneven +and hummocky, that until I afterwards saw the streams extending +continuously on both sides of the valley up to a height of about three +thousand feet close to the Cordillera, I thought that the craters of +eruption were probably close at hand. This hummocky surface I believe +to have been caused by the crossing and heaping up of different +streams. In one place, there were several rounded ridges about twenty +feet in height, some of them as broad as high, and some broader, which +certainly had been formed whilst the lava was fluid, for in transverse +sections each ridge was seen to be concentrically laminated, and to be +composed of imperfect columns radiating from common centres, like the +spokes of wheels. + +The basaltic mass where first met with is, as I have said, 130 feet in +thickness, and, thirty-five miles higher up the valley, it increases to +322 feet. In the first fourteen and a half miles of this distance, the +upper surface of the lava, judging from three measurements taken above +the level of the river (of which the apparently very uniform +inclination has been calculated from its total height at a point 135 +miles from the mouth), slopes towards the Atlantic at an angle of only +0 degrees 7 minutes twenty seconds: this must be considered only as an +approximate measurement, but it cannot be far wrong. Taking the whole +thirty-five miles, the upper surface slopes at an angle of 0 degrees 10 +minutes 53 seconds; but this result is of no value in showing the +inclination of any one stream, for halfway between the two points of +measurement, the surface suddenly rises between one hundred and two +hundred feet, apparently caused by some of the uppermost streams having +extended thus far and no farther. From the measurement made at these +two points, thirty-five miles apart, the mean inclination of the +sedimentary beds, over which the lava has flowed, is NOW (after +elevation from under the sea) only 0 degrees 7 minutes 52 seconds: for +the sake of comparison, it may be mentioned that the bottom of the +present sea in a line from the mouth of the S. Cruz to the Falkland +Islands, from a depth of seventeen fathoms to a depth of eighty-five +fathoms, declines at an angle of 0 degrees 1 minute 22 seconds; between +the beach and the depth of seventeen fathoms, the slope is greater. +From a point about half-way up the valley, the basaltic mass rises more +abruptly towards the foot of the Cordillera, namely, from a height of +1,204 feet, to about 3,000 feet above the sea. + +This great deluge of lava is worthy, in its dimensions, of the great +continent to which it belongs. The aggregate streams have flowed from +the Cordillera to a distance (unparalleled, I believe, in any case yet +known) of about one hundred geographical miles. Near their furthest +extremity their total thickness is 130 feet, which increase thirty-five +miles farther inland, as we have just seen, to 322 feet. The least +inclination given by M. E. de Beaumont of the upper surface of a +lava-stream, namely 0 degrees 30 minutes, is that of the great +subaerial eruption in 1783 from Skaptar Jukul in Iceland; and M. E. de +Beaumont shows that it must have flowed down a mean inclination of less +than 0 degrees 20 minutes. (“Memoires pour servir” etc. pages 178 and +217.) But we now see that under the pressure of the sea, successive +streams have flowed over a smooth bottom with a mean inclination of not +more than 0 degrees 7 minutes 52 seconds; and that the upper surface of +the terminal portion (over a space of fourteen and a half miles) has an +inclination of not more than 0 degrees 7 minutes 20 seconds. If the +elevation of Patagonia has been greater nearer the Cordillera than near +the Atlantic (as is probable), then these angles are now all too large. +I must repeat, that although the foregoing measurements, which were all +carefully taken with the barometer, may not be absolutely correct, they +cannot be widely erroneous. + +Southward of the S. Cruz, the cliffs of the 840 feet plain extend to +Coy Inlet, and owing to the naked patches of the white sediment, they +are said on the charts to be “like the coast of Kent.” At Coy Inlet the +high plain trends inland, leaving flat-topped outliers. At Port +Gallegos (latitude 51 degrees 35 minutes, and ninety miles south of S. +Cruz), I am informed by Captain Sulivan, R.N., that there is a +gravel-capped plain from two to three hundred feet in height, formed of +numerous strata, some fine-grained and pale-coloured, like the upper +beds at the mouth of the S. Cruz, others rather dark and coarser, so as +to resemble gritstones or tuffs; these latter include rather large +fragments of apparently decomposed volcanic rocks; there are, also, +included layers of gravel. This formation is highly remarkable, from +abounding with mammiferous remains, which have not as yet been examined +by Professor Owen, but which include some large, but mostly small, +species of Pachydermata, Edentata, and Rodentia. From the appearance of +the pale-coloured, fine-grained beds, I was inclined to believe that +they corresponded with the upper beds of the S. Cruz; but Professor +Ehrenberg, who has examined some of the specimens, informs me that the +included microscopical organisms are wholly different, being fresh and +brackish-water forms. Hence the two to three hundred feet plain at Port +Gallegos is of unknown age, but probably of subsequent origin to the +great Patagonian tertiary formation. + +EASTERN TIERRA DEL FUEGO. + +Judging from the height, the general appearance, and the white colour +of the patches visible on the hill sides, the uppermost plain, both on +the north and western side of the Strait of Magellan, and along the +eastern coast of Tierra del Fuego as far south as near Port St. +Polycarp, probably belongs to the great Patagonian tertiary formation, +These higher table- ranges are fringed by low, irregular, extensive +plains, belonging to the boulder formation (Described in the +“Geological Transactions” volume 6 page 415.), and composed of coarse +unstratified masses, sometimes associated (as north of C. Virgin’s) +with fine, laminated, muddy sandstones. The cliffs in Sebastian Bay are +200 feet in height, and are composed of fine sandstones, often in +curvilinear layers, including hard concretions of calcareous sandstone, +and layers of gravel. In these beds there are fragments of wood, legs +of crabs, barnacles encrusted with corallines still partially retaining +their colour, imperfect fragments of a Pholas distinct from any known +species, and of a Venus, approaching very closely to, but slightly +different in form from, the V. lenticularis, a species living on the +coast of Chile. Leaves of trees are numerous between the laminae of the +muddy sandstone; they belong, as I am informed by Dr. J.D. Hooker, to +three species of deciduous beech, different from the two species which +compose the great proportion of trees in this forest-clad land. +(“Botany of the Antarctic Voyage” page 212.) From these facts it is +difficult to conjecture, whether we here see the basal part of the +great Patagonian formation, or some later deposit. + +A SUMMARY ON THE PATAGONIAN TERTIARY FORMATION. + +Four out of the seven fossil shells, from St. Fe in Entre Rios, were +found by M. d’Orbigny in the sandstone of the Rio Negro, and by me at +San Josef. Three out of the six from San Josef are identical with those +from Port Desire and S. Julian, which two places have together fifteen +species, out of which three are common to both. Santa Cruz has +seventeen species, out of which five are common to Port Desire and S. +Julian. Considering the difference in latitude between these several +places, and the small number of species altogether collected, namely +thirty-six, I conceive the above proportional number of species in +common, is sufficient to show that the lower fossiliferous mass belongs +nearly, I do not say absolutely, to the same epoch. What this epoch may +be, compared with the European tertiary stages, M. d’Orbigny will not +pretend to determine. The thirty-six species (including those collected +by myself and by M. d’Orbigny) are all extinct, or at least unknown; +but it should be borne in mind, that the present coast consists of +shingle, and that no one, I believe, has dredged here for shells; hence +it is not improbable that some of the species may hereafter be found +living. Some few of the species are closely related with existing ones; +this is especially the case, according to M. d’Orbigny and Mr. Sowerby, +with the Fusus Patagonicus; and, according to Mr. Sowerby, with the +Pyrula, the Venus meridionalis, the Crepidula gregaria, and the +Turritella ambulacrum, and T. Patagonica. At least three of the genera, +namely, Cucullaea, Crassatella, and (as determined by Mr. Sowerby) +Struthiolaria, are not found in this quarter of the world; and +Trigonocelia is extinct. The evidence taken altogether indicates that +this great tertiary formation is of considerable antiquity; but when +treating of the Chilean beds, I shall have to refer again to this +subject. + +The white pumiceous mudstone, with its abundant gypsum, belongs to the +same general epoch with the underlying fossiliferous mass, as may be +inferred from the shells included in the intercalated layers at Nuevo +Gulf, S. Julian, and S. Cruz. Out of the twenty-seven marine +microscopic structures found by Professor Ehrenberg in the specimens +from S. Julian and Port Desire, ten are common to these two places: the +three found at Nuevo Gulf are distinct. I have minutely described this +deposit, from its remarkable characters and its wide extension. From +Coy Inlet to Port Desire, a distance of 230 miles, it is certainly +continuous; and I have reason to believe that it likewise extends to +the Rio Chupat, Nuevo Gulf, and San Josef, a distance of 570 miles: we +have, also, seen that a single layer occurs at the Rio Negro. At Port +S. Julian it is from eight to nine hundred feet in thickness; and at S. +Cruz it extends, with a slightly altered character, up to the +Cordillera. From its microscopic structure, and from its analogy with +other formations in volcanic districts, it must be considered as +originally of volcanic origin: it may have been formed by the +long-continued attrition of vast quantities of pumice, or judging from +the manner in which the mass becomes, in ascending the valley of S. +Cruz, divided into variously coloured layers, from the long-continued +eruption of clouds of fine ashes. In either case, we must conclude, +that the southern volcanic orifices of the Cordillera, now in a dormant +state, were at about this period over a wide space, and for a great +length of time, in action. We have evidence of this fact, in the +latitude of the Rio Negro, in the sandstone-conglomerate with pumice, +and demonstrative proof of it, at S. Cruz, in the vast deluges of +basaltic lava: at this same tertiary period, also, there is distinct +evidence of volcanic action in Western Banda Oriental. + +The Patagonian tertiary formation extends continuously, judging from +fossils alone, from S. Cruz to near the Rio Colorado, a distance of +above six hundred miles, and reappears over a wide area in Entre Rios +and Banda Oriental, making a total distance of 1,100 miles; but this +formation undoubtedly extends (though no fossils were collected) far +south of the S. Cruz, and, according to M. d’Orbigny, 120 miles north +of St. Fe. At S. Cruz we have seen that it extends across the +continent; being on the coast about eight hundred feet in thickness +(and rather more at S. Julian), and rising with the contemporaneous +lava-streams to a height of about three thousand feet at the base of +the Cordillera. It rests, wherever any underlying formation can be +seen, on plutonic and metamorphic rocks. Including the newer Pampean +deposit, and those strata in Eastern Tierra del Fuego of doubtful age, +as well as the boulder formation, we have a line of more than +twenty-seven degrees of latitude, equal to that from the Straits of +Gibraltar to the south of Iceland, continuously composed of tertiary +formations. Throughout this great space the land has been upraised, +without the strata having been in a single instance, as far as my means +of observation went, unequally tilted or dislocated by a fault. + +TERTIARY FORMATIONS ON THE WEST COAST. +CHONOS ARCHIPELAGO. + +The numerous islands of this group, with the exception of Lemus, Ypun, +consist of metamorphic schists; these two islands are formed of softish +grey and brown, fusible, often laminated sandstones, containing a few +pebbles, fragments of black lignite, and numerous mammillated +concretions of hard calcareous sandstone. Out of these concretions at +Ypun (latitude 40 degrees 30 minutes S.), I extracted the four +following extinct species of shells:— + +1. Turritella suturalis, G.B. Sowerby (also Navidad). 2. Sigaretus +subglobosus, G.B. Sowerby (also Navidad). 3. Cytheraea (?) sulculosa +(?), G.B. Sowerby (also Chiloe and Huafo?). 4. Voluta, fragments of. + +In the northern parts of this group there are some cliffs of gravel and +of the boulder formation. In the southern part (at P. Andres in Tres +Montes), there is a volcanic formation, probably of tertiary origin. +The lavas attain a thickness of from two to three hundred feet; they +are extremely variable in colour and nature, being compact, or +brecciated, or cellular, or amygdaloidal with zeolite, agate and bole, +or porphyritic with glassy albitic feldspar. There is also much +imperfect rubbly pitchstone, with the interstices charged with powdery +carbonate of lime apparently of contemporaneous origin. These lavas are +conformably associated with strata of breccia and of brown tuff +containing lignite. The whole mass has been broken up and tilted at an +angle of 45 degrees, by a series of great volcanic dikes, one of which +was thirty yards in breadth. This volcanic formation resembles one, +presently to be described, in Chiloe. + +HUAFO. + +This island lies between the Chonos and Chiloe groups: it is about +eight hundred feet high, and perhaps has a nucleus of metamorphic +rocks. The strata which I examined consisted of fine-grained muddy +sandstones, with fragments of lignite and concretions of calcareous +sandstone. I collected the following extinct shells, of which the +Turritella was in great numbers:— + +1. Bulla cosmophila, G.B. Sowerby. 2. Pleurotoma subaequalis, G.B. +Sowerby. 3. Fusus cleryanus, d’Orbigny, “Voyage Pal.” (also at +Coquimbo). 4. Triton leucostomoides, G.B. Sowerby. 5. Turritella +Chilensis, G.B. Sowerby (also Mocha). 6. Venus, probably a distinct +species, but very imperfect. 7. Cytheraea (?) sulculosa (?), probably a +distinct species, but very imperfect. 8. Dentalium majus, G.B. Sowerby. + +CHILOE. + +This fine island is about one hundred miles in length. The entire +southern part, and the whole western coast, consists of mica-schist, +which likewise is seen in the ravines of the interior. The central +mountains rise to a height of 3,000 feet, and are said to be partly +formed of granite and greenstone: there are two small volcanic +districts. The eastern coast, and large parts of the northern extremity +of the island are composed of gravel, the boulder formation, and +underlying horizontal strata. The latter are well displayed for twenty +miles north and south of Castro; they vary in character from common +sandstone to fine-grained, laminated mudstones: all the specimens which +I examined are easily fusible, and some of the beds might be called +volcanic grit-stones. These latter strata are perhaps related to a mass +of columnar trachyte which occurs behind Castro. The sandstone +occasionally includes pebbles, and many fragments and layers of +lignite; of the latter, some are apparently formed of wood and others +of leaves: one layer on the N.W. side of Lemuy is nearly two feet in +thickness. There is also much silicified wood, both common +dicotyledonous and coniferous: a section of one specimen in the +direction of the medullary rays has, as I am informed by Mr. R. Brown, +the discs in a double row placed alternately, and not opposite as in +the true Araucaria. I found marine remains only in one spot, in some +concretions of hard calcareous sandstone: in several other districts I +have observed that organic remains were exclusively confined to such +concretions; are we to account for this fact, by the supposition that +the shells lived only at these points, or is it not more probable that +their remains were preserved only where concretions were formed? The +shells here are in a bad state, they consist of:— + +1. Tellinides (?) oblonga, G.B. Sowerby (a solenella in M. d’Orbigny’s +opinion). 2. Natica striolata, G.B. Sowerby. 3. Natica (?) pumila, G.B. +Sowerby. 4. Cytheraea (?) sulculosa, G.B. Sowerby (also Ypun and +Huafo?). + +At the northern extremity of the island, near S. Carlos, there is a +large volcanic formation, between five and seven hundred feet in +thickness. The commonest lava is blackish-grey or brown, either +vesicular, or amygdaloidal with calcareous spar and bole: most even of +the darkest varieties fuse into a pale-coloured glass. The next +commonest variety is a rubbly, rarely well characterised pitchstone +(fusing into a white glass) which passes in the most irregular manner +into stony grey lavas. This pitchstone, as well as some purple +claystone porphyry, certainly flowed in the form of streams. These +various lavas often pass, at a considerable depth from the surface, in +the most abrupt and singular manner into wacke. Great masses of the +solid rock are brecciated, and it was generally impossible to discover +whether the recementing process had been an igneous or aqueous action. +(In a cliff of the hardest fragmentary mass, I found several tortuous, +vertical veins, varying in thickness from a few tenths of an inch to +one inch and a half, of a substance which I have not seen described. It +is glossy, and of a brown colour; it is thinly laminated, with the +laminae transparent and elastic; it is a little harder than calcareous +spar; it is infusible under the blowpipe, sometimes decrepitates, gives +out water, curls up, blackens, and becomes magnetic. Borax easily +dissolves a considerable quantity of it, and gives a glass tinged with +green. I have no idea what its true nature is. On first seeing it, I +mistook it for lignite!) The beds are obscurely separated from each +other; they are sometimes parted by seams of tuff and layers of +pebbles. In one place they rested on, and in another place were capped +by, tuffs and girt-stones, apparently of submarine origin. + +The neighbouring peninsula of Lacuy is almost wholly formed of +tufaceous deposits, connected probably in their origin with the +volcanic hills just described. The tuffs are pale-coloured, alternating +with laminated mudstones and sandstones (all easily fusible), and +passing sometimes into fine-grained white beds strikingly resembling +the great upper infusorial deposit of Patagonia, and sometimes into +brecciolas with pieces of pumice in the last stage of decay; these +again pass into ordinary coarse breccias and conglomerates of hard +rocks. Within very short distances, some of the finer tuffs often +passed into each other in a peculiar manner, namely, by irregular +polygonal concretions of one variety increasing so much and so suddenly +in size, that the second variety, instead of any longer forming the +entire mass, was left merely in thin veins between the concretions. In +a straight line of cliffs, at Point Tenuy, I examined the following +remarkable section (Figure 19):— + +(FIGURE 19.) + +On the left hand, the lower part (AA) consists of regular, alternating +strata of brown tuffs and greenish laminated mudstone, gently inclined +to the right, and conformably covered by a mass (B left) of a white, +tufaceous and brecciolated deposit. On the right hand, the whole cliff +(BB right) consists of the same white tufaceous matter, which on this +side presents scarcely a trace of stratification, but to the left +becomes very gradually and rather indistinctly divided into strata +quite conformable with the underlying beds (AA): moreover, a few +hundred yards further to the left, where the surface has been less +denuded, the tufaceous strata (B left) are conformably covered by +another set of strata, like the underlying ones (AA) of this section. +In the middle of the diagram, the beds (AA) are seen to be abruptly cut +off, and to abut against the tufaceous non-stratified mass; but the +line of junction has been accidentally not represented steep enough, +for I particularly noticed that before the beds had been tilted to the +right, this line must have been nearly vertical. It appears that a +current of water cut for itself a deep and steep submarine channel, and +at the same time or afterwards filled it up with the tufaceous and +brecciolated matter, and spread the same over the surrounding submarine +beds; the matter becoming stratified in these more distant and less +troubled parts, and being moreover subsequently covered up by other +strata (like AA) not shown in the diagram. It is singular that three of +the beds (of AA) are prolonged in their proper direction, as +represented, beyond the line of junction into the white tufaceous +matter: the prolonged portions of two of the beds are rounded; in the +third, the terminal fragment has been pushed upwards: how these beds +could have been left thus prolonged, I will not pretend to explain. In +another section on the opposite side of a promontory, there was at the +foot of this same line of junction, that is at the bottom of the old +submarine channel, a pile of fragments of the strata (AA), with their +interstices filled up with white tufaceous matter: this is exactly what +might have been anticipated under such circumstances. + +(FIGURE 20. GROUND PLAN SHOWING THE RELATION BETWEEN VEINS AND +CONCRETIONARY ZONES IN A MASS OF TUFF.) + +The various tufaceous and other beds at this northern end of Chiloe +probably belong to about the same age with those near Castro, and they +contain, as there, many fragments of black lignite and of silicified +and pyritous wood, often embedded close together. They also contain +many and singular concretions: some are of hard calcareous sandstone, +in which it would appear that broken volcanic crystals and scales of +mica have been better preserved (as in the case of the organic remains +near Castro) than in the surrounding mass. Other concretions in the +white brecciola are of a hard, ferruginous, yet fusible, nature; they +are as round as cannon-balls, and vary from two or three inches to two +feet in diameter; their insides generally consist either of fine, +scarcely coherent volcanic sand (The frequent tendency in iron to form +hollow concretions or shell containing incoherent matter is singular; +D’Aubuisson (“Traite de Geogn.” tome 1 page 318) remarks on this +circumstance.), or of an argillaceous tuff; in this latter case, the +external crust was quite thin and hard. Some of these spherical balls +were encircled in the line of their equators, by a necklace-like row of +smaller concretions. Again there were other concretions, irregularly +formed, and composed of a hard, compact, ash- coloured stone, with an +almost porcelainous fracture, adhesive to the tongue, and without any +calcareous matter. These beds are, also, interlaced by many veins, +containing gypsum, ferruginous matter, calcareous spar, and agate. It +was here seen with remarkable distinctness, how intimately +concretionary action and the production of fissures and veins are +related together. Figure 20 is an accurate representation of a +horizontal space of tuff, about four feet long by two and a half in +width: the double lines represent the fissures partially filled with +oxide of iron and agate: the curvilinear lines show the course of the +innumerable, concentric, concretionary zones of different shades of +colour and of coarseness in the particles of tuff. The symmetry and +complexity of the arrangement gave the surface an elegant appearance. +It may be seen how obviously the fissures determine (or have been +determined by) the shape, sometimes of the whole concretion, and +sometimes only of its central parts. The fissures also determine the +curvatures of the long undulating zones of concretionary action. From +the varying composition of the veins and concretions, the amount of +chemical action which the mass has undergone is surprisingly great; and +it would likewise appear from the difference in size in the particles +of the concretionary zones, that the mass, also, has been subjected to +internal mechanical movements. + +In the peninsula of Lacuy, the strata over a width of four miles have +been upheaved by three distinct, and some other indistinct, lines of +elevation, ranging within a point of north and south. One line, about +two hundred feet in height, is regularly anticlinal, with the strata +dipping away on both sides, at an angle of 15 degrees, from a central +“valley of elevation,” about three hundred yards in width. A second +narrow steep ridge, only sixty feet high, is uniclinal, the strata +throughout dipping westward; those on both flanks being inclined at an +angle of from ten to fifteen degrees; whilst those on the ridge dip in +the same direction at an angle of between thirty and forty degrees. +This ridge, traced northwards, dies away; and the beds at its terminal +point, instead of dipping westward, are inclined 12 degrees to the +north. This case interested me, as being the first in which I found in +South America, formations perhaps of tertiary origin, broken by lines +of elevation. + +VALDIVIA: ISLAND OF MOCHA. + +The formations of Chiloe seem to extend with nearly the same character +to Valdivia, and for some leagues northward of it: the underlying rocks +are micaceous schists, and are covered up with sandstone and other +sedimentary beds, including, as I was assured, in many places layers of +lignite. I did not land on Mocha (latitude 38 degrees 20 minutes), but +Mr. Stokes brought me specimens of the grey, fine-grained, slightly +calcareous sandstone, precisely like that of Huafo, containing lignite +and numerous Turritellae. The island is flat topped, 1,240 feet in +height, and appears like an outlier of the sedimentary beds on the +mainland. The few shells collected consist of:— + +1. Turritella Chilensis, G.B. Sowerby (also at Huafo). 2. Fusus, very +imperfect, somewhat resembling F. subreflexus of Navidad, but probably +different. 3. Venus, fragments of. + +CONCEPCION. + +Sailing northward from Valdivia, the coast-cliffs are seen, first to +assume near the R. Tolten, and thence for 150 miles northward, to be +continued with the same mineralogical characters, immediately to be +described at Concepcion. I heard in many places of beds of lignite, +some of it fine and glossy, and likewise of silicified wood; near the +Tolten the cliffs are low, but they soon rise in height; and the +horizontal strata are prolonged, with a nearly level surface, until +coming to a more lofty tract between points Rumena and Lavapie. Here +the beds have been broken up by at least eight or nine parallel lines +of elevation, ranging E. or E.N.E. and W. or W.S.W. These lines can be +followed with the eye many miles into the interior; they are all +uniclinal, the strata in each dipping to a point between S. and S.S.E. +with an inclination in the central lines of about forty degrees, and in +the outer ones of under twenty degrees. This band of symmetrically +troubled country is about eight miles in width. + +The island of Quiriquina, in the Bay of Concepcion, is formed of +various soft and often ferruginous sandstones, with bands of pebbles, +and with the lower strata sometimes passing into a conglomerate resting +on the underlying metamorphic schists. These beds include subordinate +layers of greenish impure clay, soft micaceous and calcareous +sandstones, and reddish friable earthy matter with white specks like +decomposed crystals of feldspar; they include, also, hard concretions, +fragments of shells, lignite, and silicified wood. In the upper part +they pass into white, soft sediments and brecciolas, very like those +described at Chiloe; as indeed is the whole formation. At Lirguen and +other places on the eastern side of the bay, there are good sections of +the lower sandstones, which are generally ferruginous, but which vary +in character, and even pass into an argillaceous nature; they contain +hard concretions, fragments of lignite, silicified wood, and pebbles +(of the same rocks with the pebbles in the sandstones of Quiriquina), +and they alternate with numerous, often very thin layers of imperfect +coal, generally of little specific gravity. The main bed here is three +feet thick; and only the coal of this one bed has a glossy fracture. +Another irregular, curvilinear bed of brown, compact lignite, is +remarkable for being included in a mass of coarse gravel. These +imperfect coals, when placed in a heap, ignite spontaneously. The +cliffs on this side of the bay, as well as on the island of Quiriquina, +are capped with red friable earth, which, as stated in the Second +Chapter, is of recent formation. The stratification in this +neighbourhood is generally horizontal; but near Lirguen the beds dip +N.W. at an angle of 23 degrees; near Concepcion they are also inclined: +at the northern end of Quiriquina they have been tilted at an angle of +30 degrees, and at the southern end at angles varying from 15 degrees +to 40 degrees: these dislocations must have taken place under the sea. + +A collection of shells, from the island of Quiriquina, has been +described by M. d’Orbigny: they are all extinct, and from their generic +character, M. d’Orbigny inferred that they were of tertiary origin: +they consist of:— + +1. Scalaria Chilensis, d’Orbigny, “Voyage, Part Pal.” 2. Natica +Araucana, d’Orbigny, “Voyage, Part Pal.” 3. Natica australis, +d’Orbigny, “Voyage, Part Pal.” 4. Fusus difficilis, d’Orbigny, “Voyage, +Part Pal.” 5. Pyrula longirostra, d’Orbigny, “Voyage, Part Pal.” 6. +Pleurotoma Araucana, d’Orbigny, “Voyage, Part Pal.” 7. Cardium auca, +d’Orbigny, “Voyage, Part Pal.” 8. Cardium acuticostatum, d’Orbigny, +“Voyage, Part Pal.” 9. Venus auca, d’Orbigny, “Voyage, Part Pal.” 10. +Mactra cecileana, d’Orbigny, “Voyage, Part Pal.” 11. Mactra Araucana, +d’Orbigny, “Voyage, Part Pal.” 12. Arca Araucana, d’Orbigny, “Voyage, +Part Pal.” 13. Nucula Largillierti, d’Orbigny, “Voyage, Part Pal.” 14. +Trigonia Hanetiana, d’Orbigny, “Voyage, Part Pal.” + +During a second visit of the “Beagle” to Concepcion, Mr. Kent collected +for me some silicified wood and shells out of the concretions in the +sandstone from Tome, situated a short distance north of Lirguen. They +consist of:— + +1. Natica australis, d’Orbigny, “Voyage, Part Pal.” 2. Mactra Araucana, +d’Orbigny, “Voyage, Part Pal.” 3. Trigonia Hanetiana, d’Orbigny, +“Voyage, Part Pal.” 4. Pecten, fragments of, probably two species, but +too imperfect for description. 5. Baculites vagina, E. Forbes. 6. +Nautilus d’Orbignyanus, E. Forbes. + +Besides these shells, Captain Belcher found here an Ammonite, nearly +three feet in diameter, and so heavy that he could not bring it away; +fragments are deposited at Haslar Hospital: he also found the +silicified vertebrae of some very large animal. (“Zoology of Captain +Beechey’s Voyage” page 163.) From the identity in mineralogical nature +of the rocks, and from Captain Belcher’s minute description of the +coast between Lirguen and Tome, the fossiliferous concretions at this +latter place certainly belong to the same formation with the beds +examined by myself at Lirguen; and these again are undoubtedly the same +with the strata of Quiriquina; moreover; the three first of the shells +from Tome, though associated in the same concretions with the Baculite, +are identical with the species from Quiriquina. Hence all the sandstone +and lignitiferous beds in this neighbourhood certainly belong to the +same formation. Although the generic character of the Quiriquina +fossils naturally led M. d’Orbigny to conceive that they were of +tertiary origin, yet as we now find them associated with the Baculites +vagina and with an Ammonite, we must, in the opinion of M. d’Orbigny, +and if we are guided by the analogy of the northern hemisphere, rank +them in the Cretaceous system. Moreover, the Baculites vagina, which is +in a tolerable state of preservation, appears to Professor E. Forbes +certainly to be identical with a species, so named by him, from +Pondicherry in India; where it is associated with numerous decidedly +cretaceous species, which approach most nearly to Lower Greensand or +Neocomian forms: this fact, considering the vast distance between Chile +and India, is truly surprising. Again, the Nautilus d’Orbignyanus, as +far as its imperfect state allows of comparison, resembles, as I am +informed by Professor Forbes, both in its general form and in that of +its chambers, two species from the Upper Greensand. It may be added +that every one of the above-named genera from Quiriquina, which have an +apparently tertiary character, are found in the Pondicherry strata. +There are, however, some difficulties on this view of the formations at +Concepcion being cretaceous, which I shall afterwards allude to; and I +will here only state that the Cardium auca is found also at Coquimbo, +the beds at which place, there can be no doubt, are tertiary. + +NAVIDAD. (I was guided to this locality by the Report on M. Gay’s +“Geological Researches” in the “Annales des Scienc. Nat.” 1st series +tome 28.) + +The Concepcion formation extends some distance northward, but how far I +know not; for the next point at which I landed was at Navidad, 160 +miles north of Concepcion, and 60 miles south of Valparaiso. The cliffs +here are about eight hundred feet in height: they consist, wherever I +could examine them, of fine-grained, yellowish, earthy sandstones, with +ferruginous veins, and with concretions of hard calcareous sandstone. +In one part, there were many pebbles of the common metamorphic +porphyries of the Cordillera: and near the base of the cliff, I +observed a single rounded boulder of greenstone, nearly a yard in +diameter. I traced this sandstone formation beneath the superficial +covering of gravel, for some distance inland: the strata are slightly +inclined from the sea towards the Cordillera, which apparently has been +caused by their having been accumulated against or round outlying +masses of granite, of which some points project near the coast. The +sandstone contains fragments of wood, either in the state of lignite or +partially silicified, sharks’ teeth, and shells in great abundance, +both high up and low down the sea-cliffs. Pectunculus and Oliva were +most numerous in individuals, and next to them Turritella and Fusus. I +collected in a short time, though suffering from illness, the following +thirty-one species, all of which are extinct, and several of the genera +do not now range (as we shall hereafter show) nearly so far south:— + +1. Gastridium cepa, G.B. Sowerby. 2. Monoceros, fragments of, +considered by M. d’Orbigny as a new species. 3. Voluta alta, G.B. +Sowerby (considered by M. d’Orbigny as distinct from the V. alta of +Santa Cruz). 4. Voluta triplicata, G.B. Sowerby. 5. Oliva dimidiata, +G.B. Sowerby. 6. Pleurotoma discors, G.B. Sowerby. 7. Pleurotoma +turbinelloides, G.B. Sowerby. 8. Fusus subreflexus, G.B. Sowerby. 9. +Fusus pyruliformis, G.B. Sowerby. 10. Fusus, allied to F. regularis +(considered by M. d’Orbigny as a distinct species). 11. Turritella +suturalis, G.B. Sowerby. 12. Turritella Patagonica, G.B. Sowerby +(fragments of). 13. Trochus laevis, G.B. Sowerby. 14. Trochus collaris, +G.B. Sowerby (considered by M. d’Orbigny as the young of the T. +laevis). 15. Cassis monilifer, G.B. Sowerby. 16. Pyrula distans, G.B. +Sowerby. 17. Triton verruculosus, G.B. Sowerby. 18. Sigaretus +subglobosus, G.B. Sowerby. 19. Natica solida, G.B. Sowerby. (It is +doubtful whether the Natica solida of S. Cruz is the same species with +this.) 20. Terebra undulifera, G.B. Sowerby. 21. Terebra costellata, +G.B. Sowerby. 22. Bulla (fragments of). 23. Dentalium giganteum, do. +24. Dentalium sulcosum, do. 25. Corbis (?) laevigata, do. 26. Cardium +multiradiatum, do. 27. Venus meridionalis, do. 28. Pectunculus dispar, +(?) Desh. (considered by M. d’Orbigny as a distinct species). 29, 30. +Cytheraea and Mactra, fragments of (considered by M. d’Orbigny as new +species). 31. Pecten, fragments of. + +COQUIMBO. +(FIGURE 21. SECTION OF THE TERTIARY FORMATION AT COQUIMBO. + +From Level of Sea to Surface of plain, 252 feet above sea, through +levels F, E, D and C: + +F.—Lower sandstone, with concretions and silicified bones, with fossil +shells, all, or nearly all, extinct. + +E.—Upper ferruginous sandstone, with numerous Balani, with fossil +shells, all, or nearly all, extinct. + +C and D.—Calcareous beds with recent shells. + +A.—Stratified sand in a ravine, also with recent shells.) + +For more than two hundred miles northward of Navidad, the coast +consists of plutonic and metamorphic rocks, with the exception of some +quite insignificant superficial beds of recent origin. At Tonguay, +twenty-five miles south of Coquimbo, tertiary beds recommence. I have +already minutely described in the Second Chapter, the step-formed +plains of Coquimbo, and the upper calcareous beds (from twenty to +thirty feet in thickness) containing shells of recent species, but in +different proportions from those on the beach. There remains to be +described only the underlying ancient tertiary beds, represented in +Figure 21 by the letters F and E:— + +I obtained good sections of bed F only in Herradura Bay: it consists of +soft whitish sandstone, with ferruginous veins, some pebbles of +granite, and concretionary layers of hard calcareous sandstone. These +concretions are remarkable from the great number of large silicified +bones, apparently of cetaceous animals, which they contain; and +likewise of a shark’s teeth, closely resembling those of the Carcharias +megalodon. Shells of the following species, of which the gigantic +Oyster and Perna are the most conspicuous, are numerously embedded in +the concretions:— + +1. Bulla ambigua, d’Orbigny “Voyage” Pal. 2. Monoceros Blainvillii, +d’Orbigny “Voyage” Pal. 3. Cardium auca, d’Orbigny “Voyage” Pal. 4. +Panopaea Coquimbensis, d’Orbigny “Voyage” Pal. 5. Perna Gaudichaudi, +d’Orbigny “Voyage” Pal. 6. Artemis ponderosa; Mr. Sowerby can find no +distinguishing character between this fossil and the recent A. +ponderosa; it is certainly an Artemis, as shown by the pallial +impression. 7. Ostrea Patagonica (?); Mr. Sowerby can point out no +distinguishing character between this species and that so eminently +characteristic of the great Patagonian formation; but he will not +pretend to affirm that they are identical. 8. Fragments of a Venus and +Natica. + +The cliffs on one side of Herradura Bay are capped by a mass of +stratified shingle, containing a little calcareous matter, and I did +not doubt that it belonged to the same recent formation with the gravel +on the surrounding plains, also cemented by calcareous matter, until to +my surprise, I found in the midst of it, a single thin layer almost +entirely composed of the above gigantic oyster. + +At a little distance inland, I obtained several sections of the bed E, +which, though different in appearance from the lower bed F, belongs to +the same formation: it consists of a highly ferruginous sandy mass, +almost composed, like the lowest bed at Port S. Julian, of fragments of +Balanidae; it includes some pebbles, and layers of yellowish-brown +mudstone. The embedded shells consist of:— + +1. Monoceros Blainvillii, d’Orbigny “Voyage” Pal. 2. Monoceros +ambiguus, G.B. Sowerby. 3. Anomia alternans, G.B. Sowerby. 4. Pecten +rudis, G.B. Sowerby. 5. Perna Gaudichaudi, d’Orbigny “Voyage” Pal. 6. +Ostrea Patagonica (?), d’Orbigny “Voyage” Pal. 7. Ostrea, small +species, in imperfect state; it appeared to me like a small kind now +living in, but very rare in the bay. 8. Mytilus Chiloensis; Mr. Sowerby +can find no distinguishing character between this fossil, as far as its +not very perfect condition allows of comparison, and the recent +species. 9. Balanus Coquimbensis, G.B. Sowerby. 10. Balanus psittacus? +King. This appears to Mr. Sowerby and myself identical with a very +large and common species now living on the coast. + +The uppermost layers of this ferrugino-sandy mass are conformably +covered by, and impregnated to the depth of several inches with, the +calcareous matter of the bed D called losa: hence I at one time +imagined that there was a gradual passage between them; but as all the +species are recent in the bed D, whilst the most characteristic shells +of the uppermost layers of E are the extinct Perna, Pecten, and +Monoceros, I agree with M. d’Orbigny, that this view is erroneous, and +that there is only a mineralogical passage between them, and no gradual +transition in the nature of their organic remains. Besides the fourteen +species enumerated from these two lower beds, M. d’Orbigny has +described ten other species given to him from this locality; namely:— + +1. Fusus Cleryanus, d’Orbigny “Voyage” Pal. 2. Fusus petitianus, +d’Orbigny “Voyage” Pal. 3. Venus hanetiana, d’Orbigny “Voyage” Pal. 4. +Venus incerta (?) d’Orbigny “Voyage” Pal. 5. Venus Cleryana, d’Orbigny +“Voyage” Pal. 6. Venus petitiana, d’Orbigny “Voyage” Pal. 7. Venus +Chilensis, d’Orbigny “Voyage” Pal. 8. Solecurtus hanetianus, d’Orbigny +“Voyage” Pal. 9. Mactra auca, d’Orbigny “Voyage” Pal. 10. Oliva serena, +d’Orbigny “Voyage” Pal. + +Of these twenty-four shells, all are extinct, except, according to Mr. +Sowerby, the Artemis ponderosa, Mytilus Chiloensis, and probably the +great Balanus. + +COQUIMBO TO COPIAPO. + +A few miles north of Coquimbo, I met with the ferruginous, balaniferous +mass E with many silicified bones; I was informed that these silicified +bones occur also at Tonguay, south of Coquimbo: their number is +certainly remarkable, and they seem to take the place of the silicified +wood, so common on the coast-formations of Southern Chile. In the +valley of Chaneral, I again saw this same formation, capped with the +recent calcareous beds. I here left the coast, and did not see any more +of the tertiary formations, until descending to the sea at Copiapo: +here in one place I found variously coloured layers of sand and soft +sandstone, with seams of gypsum, and in another place, a comminuted +shelly mass, with layers of rotten-stone and seams of gypsum, including +many of the extinct gigantic oyster: beds with these oysters are said +to occur at English Harbour, a few miles north of Copiapo. + +COAST OF PERU. + +With the exception of deposits containing recent shells and of quite +insignificant dimensions, no tertiary formations have been observed on +this coast, for a space of twenty-two degrees of latitude north of +Copiapo, until coming to Payta, where there is said to be a +considerable calcareous deposit: a few fossils have been described by +M. d’Orbigny from this place, namely:— + +1. Rostellaria Gaudichaudi, d’Orbigny “Voyage” Pal. 2. Pectunculus +Paytensis, d’Orbigny “Voyage” Pal. 3. Venus petitiana, d’Orbigny +“Voyage” Pal. 4. Ostrea Patagonica? This great oyster (of which +specimens have been given me) cannot be distinguished by Mr. Sowerby +from some of the varieties from Patagonia; though it would be hazardous +to assert it is the same with that species, or with that from Coquimbo. + +CONCLUDING REMARKS. + +The formations described in this chapter, have, in the case of Chiloe +and probably in that of Concepcion and Navidad, apparently been +accumulated in troughs formed by submarine ridges extending parallel to +the ancient shores of the continent; in the case of the islands of +Mocha and Huafo it is highly probable, and in that of Ypun and Lemus +almost certain, that they were accumulated round isolated rocky centres +or nuclei, in the same manner as mud and sand are now collecting round +the outlying islets and reefs in the West Indian Archipelago. Hence, I +may remark, it does not follow that the outlying tertiary masses of +Mocha and Huafo were ever continuously united at the same level with +the formations on the mainland, though they may have been of +contemporaneous origin, and been subsequently upraised to the same +height. In the more northern parts of Chile, the tertiary strata seem +to have been separately accumulated in bays, now forming the mouths of +valleys. + +The relation between these several deposits on the shores of the +Pacific, is not nearly so clear as in the case of the tertiary +formations on the Atlantic. Judging from the form and height of the +land (evidence which I feel sure is here much more trustworthy than it +can ever be in such broken continents as that of Europe), from the +identity of mineralogical composition, from the presence of fragments +of lignite and of silicified wood, and from the intercalated layers of +imperfect coal, I must believe that the coast-formations from Central +Chiloe to Concepcion, a distance of 400 miles, are of the same age: +from nearly similar reasons, I suspect that the beds of Mocha, Huafo, +and Ypun, belong also to the same period. The commonest shell in Mocha +and Huafo is the same species of Turritella; and I believe the same +Cytheraea is found on the islands of Huafo, Chiloe, and Ypun; but with +these trifling exceptions, the few organic remains found at these +places are distinct. The numerous shells from Navidad, with the +exception of two, namely, the Sigaretus and Turritella found at Ypun, +are likewise distinct from those found in any other part of this coast. +Coquimbo has Cardium auca in common with Concepcion, and Fusus +Cleryanus with Huafo; I may add, that Coquimbo has Venus petitiana, and +a gigantic oyster (said by M. d’Orbigny also to be found a little south +of Concepcion) in common with Payta, though this latter place is +situated twenty-two degrees northward of latitude 27 degrees, to which +point the Coquimbo formation extends. + +From these facts, and from the generic resemblance of the fossils from +the different localities, I cannot avoid the suspicion that they all +belong to nearly the same epoch, which epoch, as we shall immediately +see, must be a very ancient tertiary one. But as the Baculite, +especially considering its apparent identity with the Cretaceous +Pondicherry species, and the presence of an Ammonite, and the +resemblance of the Nautilus to two upper greensand species, together +afford very strong evidence that the formation of Concepcion is a +Secondary one; I will, in my remarks on the fossils from the other +localities, put on one side those from Concepcion and from Eastern +Chiloe, which, whatever their age may be, appear to me to belong to one +group. I must, however, again call attention to the fact that the +Cardium auca is found both at Concepcion and in the undoubtedly +tertiary strata of Coquimbo: nor should the possibility be overlooked, +that as Trigonia, though known in the northern hemisphere only as a +Secondary genus, has living representatives in the Australian seas, so +a Baculite, Ammonite, and Trigonia may have survived in this remote +part of the southern ocean to a somewhat later period than to the north +of the equator. + +Before passing in review the fossils from the other localities, there +are two points, with respect to the formations between Concepcion and +Chiloe, which deserve some notice. First, that though the strata are +generally horizontal, they have been upheaved in Chiloe in a set of +parallel anticlinal and uniclinal lines ranging north and south,—in the +district near P. Rumena by eight or nine far-extended, most +symmetrical, uniclinal lines ranging nearly east and west,—and in the +neighbourhood of Concepcion by less regular single lines, directed both +N.E. and S.W., and N.W. and S.E. This fact is of some interest, as +showing that within a period which cannot be considered as very ancient +in relation to the history of the continent, the strata between the +Cordillera and the Pacific have been broken up in the same variously +directed manner as have the old plutonic and metamorphic rocks in this +same district. The second point is, that the sandstone between +Concepcion and Southern Chiloe is everywhere lignitiferous, and +includes much silicified wood; whereas the formations in Northern Chile +do not include beds of lignite or coal, and in place of the fragments +of silicified wood there are silicified bones. Now, at the present day, +from Cape Horn to near Concepcion, the land is entirely concealed by +forests, which thin out at Concepcion, and in Central and Northern +Chile entirely disappear. This coincidence in the distribution of the +fossil wood and the living forests may be quite accidental; but I +incline to take a different view of it; for, as the difference in +climate, on which the presence of forests depends, is here obviously in +chief part due to the form of the land, and as the Cordillera +undoubtedly existed when the lignitiferous beds were accumulating, I +conceive it is not improbable that the climate, during the +lignitiferous period, varied on different parts of the coast in a +somewhat similar manner as it now does. Looking to an earlier epoch, +when the strata of the Cordillera were depositing, there were islands +which even in the latitude of Northern Chile, where now all is +irreclaimably desert, supported large coniferous forests. + +TABLE 4. + +Column 1. Genera, with living and tertiary species on the west coast of +South America. (M. d’Orbigny states that the genus Natica is not found +on the coast of Chile; but Mr. Cuming found it at Valparaiso. Scalaria +was found at Valparaiso; Arca, at Iquique, in latitude 20, by Mr. +Cuming; Arca, also, was found by Captain King, at Juan Fernandez, in +latitude 33 degrees 30′S.) + +Column 2. Latitudes, in which found fossil on the coasts of Chile and +Peru. (In degrees and minutes.) + +Column 3. Southernmost latitude, in which found living on the west +coast of South America. (In degrees and minutes.) + +Bulla : 30 to 43 30 : 12 near Lima. + +Cassis : 34 : 1 37. + +Pyrula : 34 (and 36 30 at Concepcion) : 5 Payta. + +Fusus : 30 and 43 30 : 23 Mexillones; reappears at the St. of Magellan. + +Pleurotoma : 34 to 43 30 : 2 18 St. Elena. + +Terebra : 34 : 5 Payta. + +Sigaretus : 34 to 44 30 : 12 Lima. + +Anomia : 30 : 7 48. + +Perna : 30 : 1 23 Xixappa. + +Cardium : 30 to 34 (and 36 30 at Concepcion) : 5 Payta. + +Artemis : 30 : 5 Payta. + +Voluta : 34 to 44 30 : Mr. Cuming does not know of any species living +on the west coast, between the equator and latitude 43 south; from this +latitude a species is found as far south as Tierra del Fuego. + +Seventy-nine species of fossil shells, in a tolerably recognisable +condition, from the coast of Chile and Peru, are described in this +volume, and in the Palaeontological part of M. d’Orbigny’s “Voyage”: if +we put on one side the twenty species exclusively found at Concepcion +and Chiloe, fifty-nine species from Navidad and the other specified +localities remain. Of these fifty-nine species only an Artemis, a +Mytilus and Balanus, all from Coquimbo, are (in the opinion of Mr. +Sowerby, but not in that of M. d’Orbigny) identical with living shells; +and it would certainly require a better series of specimens to render +this conclusion certain. Only the Turritella Chilensis from Huafo and +Mocha, the T. Patagonica and Venus meridionalis from Navidad, come very +near to recent South American shells, namely, the two Turritellas to T. +cingulata, and the Venus to V. exalbida: some few other species come +rather less near; and some few resemble forms in the older European +tertiary deposits: none of the species resemble secondary forms. Hence +I conceive there can be no doubt that these formations are tertiary,—a +point necessary to consider, after the case of Concepcion. The +fifty-nine species belong to thirty-two genera; of these, Gastridium is +extinct, and three or four of the genera (viz. Panopaea, Rostellaria, +Corbis (?), and I believe Solecurtus) are not now found on the west +coast of South America. Fifteen of the genera have on this coast living +representatives in about the same latitudes with the fossil species; +but twelve genera now range very differently to what they formerly did. +The idea of Table 4, in which the difference between the extension in +latitude of the fossil and existing species is shown, is taken from M. +d’Orbigny’s work; but the range of the living shells is given on the +authority of Mr. Cuming, whose long-continued researches on the +conchology of South America are well-known. + +When we consider that very few, if any, of the fifty-nine fossil shells +are identical with, or make any close approach to, living species; when +we consider that some of the genera do not now exist on the west coast +of South America, and that no less than twelve genera out of the +thirty-two formerly ranged very differently from the existing species +of the same genera, we must admit that these deposits are of +considerable antiquity, and that they probably verge on the +commencement of the tertiary era. May we not venture to believe, that +they are of nearly contemporaneous origin with the Eocene formations of +the northern hemisphere? + +Comparing the fossil remains from the coast of Chile (leaving out, as +before, Concepcion and Chiloe) with those from Patagonia, we may +conclude, from their generic resemblance, and from the small number of +the species which from either coast approach closely to living forms, +that the formations of both belong to nearly the same epoch; and this +is the opinion of M. D’Orbigny. Had not a single fossil shell been +common to the two coasts, it could not have been argued that the +formations belonged to different ages; for Messrs. Cuming and Hinds +have found, on the comparison of nearly two thousand living species +from the opposite sides of South America, only one in common, namely, +the Purpura lapillus from both sides of the Isthmus of Panama: even the +shells collected by myself amongst the Chonos Islands and on the coast +of Patagonia, are dissimilar, and we must descend to the apex of the +continent, to Tierra del Fuego, to find these two great conchological +provinces united into one. Hence it is remarkable that four or five of +the fossil shells from Navidad, namely, Voluta alta, Turritella +Patagonica, Trochus collaris, Venus meridionalis, perhaps Natica +solida, and perhaps the large oyster from Coquimbo, are considered by +Mr. Sowerby as identical with species from Santa Cruz and P. Desire. M. +d’Orbigny, however, admits the perfect identity only of the Trochus. + +ON THE TEMPERATURE OF THE TERTIARY PERIOD. + +As the number of the fossil species and genera from the western and +eastern coasts is considerable, it will be interesting to consider the +probable nature of the climate under which they lived. We will first +take the case of Navidad, in latitude 34 degrees, where thirty-one +species were collected, and which, as we shall presently see, must have +inhabited shallow water, and therefore will necessarily well exhibit +the effects of temperature. Referring to Table 4 we find that the +existing species of the genera Cassis, Pyrula, Pleurotoma, Terebra, and +Sigaretus, which are generally (though by no means invariably) +characteristic of warmer latitudes, do not at the present day range +nearly so far south on this line of coast as the fossil species +formerly did. Including Coquimbo, we have Perna in the same +predicament. The first impression from this fact is, that the climate +must formerly have been warmer than it now is; but we must be very +cautious in admitting this, for Cardium, Bulla, and Fusus (and, if we +include Coquimbo, Anomia and Artemis) likewise formerly ranged farther +south than they now do; and as these genera are far from being +characteristic of hot climates, their former greater southern range may +well have been owing to causes quite distinct from climate: Voluta, +again, though generally so tropical a genus, is at present confined on +the west coast to colder or more southern latitudes than it was during +the tertiary period. The Trochus collaris, moreover, and, as we have +just seen according to Mr. Sowerby, two or three other species, +formerly ranged from Navidad as far south as Santa Cruz in latitude 50 +degrees. If, instead of comparing the fossils of Navidad, as we have +hitherto done, with the shells now living on the west coast of South +America, we compare them with those found in other parts of the world, +under nearly similar latitudes; for instance, in the southern parts of +the Mediterranean or of Australia, there is no evidence that the sea +off Navidad was formerly hotter than what might have been expected from +its latitude, even if it was somewhat warmer than it now is when cooled +by the great southern polar current. Several of the most tropical +genera have no representative fossils at Navidad; and there are only +single species of Cassis, Pyrula, and Sigaretus, two of Pleurotoma and +two of Terebra, but none of these species are of conspicuous size. In +Patagonia, there is even still less evidence in the character of the +fossils, of the climate having been formerly warmer. (It may be worth +while to mention that the shells living at the present day on this +eastern side of South America, in latitude 40 degrees, have perhaps a +more tropical character than those in corresponding latitudes on the +shores of Europe: for at Bahia Blanca and S. Blas, there are two fine +species of Voluta and four of Oliva.) As from the various reasons +already assigned, there can be little doubt that the formations of +Patagonia and at least of Navidad and Coquimbo in Chile, are the +equivalents of an ancient stage in the tertiary formations of the +northern hemisphere, the conclusion that the climate of the southern +seas at this period was not hotter than what might have been expected +from the latitude of each place, appears to me highly important; for we +must believe, in accordance with the views of Mr. Lyell, that the +causes which gave to the older tertiary productions of the quite +temperate zones of Europe a tropical character, WERE OF A LOCAL +CHARACTER AND DID NOT AFFECT THE ENTIRE GLOBE. On the other hand, I +have endeavoured to show, in the “Geological Transactions,” that, at a +much later period, Europe and North and South America were nearly +contemporaneously subjected to ice- action, and consequently to a +colder, or at least more equable, climate than that now characteristic +of the same latitudes. + +ON THE ABSENCE OF EXTENSIVE MODERN CONCHIFEROUS DEPOSITS IN SOUTH +AMERICA; AND ON THE CONTEMPORANEOUSNESS OF THE OLDER TERTIARY DEPOSITS +AT DISTANT POINTS BEING DUE TO CONTEMPORANEOUS MOVEMENTS OF SUBSIDENCE. + +Knowing from the researches of Professor E. Forbes, that molluscous +animals chiefly abound within a depth of 100 fathoms and under, and +bearing in mind how many thousand miles of both coasts of South America +have been upraised within the recent period by a slow, long-continued, +intermittent movement,—seeing the diversity in nature of the shores and +the number of shells now living on them,—seeing also that the sea off +Patagonia and off many parts of Chile, was during the tertiary period +highly favourable to the accumulation of sediment,—the absence of +extensive deposits including recent shells over these vast spaces of +coast is highly remarkable. The conchiferous calcareous beds at +Coquimbo, and at a few isolated points northward, offer the most marked +exception to this statement; for these beds are from twenty to thirty +feet in thickness, and they stretch for some miles along shore, +attaining, however, only a very trifling breadth. At Valdivia there is +some sandstone with imperfect casts of shells, which POSSIBLY may +belong to the recent period: parts of the boulder formation and the +shingle-beds on the lower plains of Patagonia probably belong to this +same period, but neither are fossiliferous: it also so happens that the +great Pampean formation does not include, with the exception of the +Azara, any mollusca. There cannot be the smallest doubt that the +upraised shells along the shores of the Atlantic and Pacific, whether +lying on the bare surface, or embedded in mould or in sand-hillocks, +will in the course of ages be destroyed by alluvial action: this +probably will be the case even with the calcareous beds of Coquimbo, so +liable to dissolution by rain-water. If we take into consideration the +probability of oscillations of level and the consequent action of the +tidal-waves at different heights, their destruction will appear almost +certain. Looking to an epoch as far distant in futurity as we now are +from the past Miocene period, there seems to me scarcely a chance, +under existing conditions, of the numerous shells now living in those +zones of depths most fertile in life, and found exclusively on the +western and south-eastern coasts of South America, being preserved to +this imaginary distant epoch. A whole conchological series will in time +be swept away, with no memorials of their existence preserved in the +earth’s crust. + +Can any light be thrown on this remarkable absence of recent +conchiferous deposits on these coasts, on which, at an ancient tertiary +epoch, strata abounding with organic remains were extensively +accumulated? I think there can, namely, by considering the conditions +necessary for the preservation of a formation to a distant age. Looking +to the enormous amount of denudation which on all sides of us has been +effected,—as evidenced by the lofty cliffs cutting off on so many +coasts horizontal and once far-extended strata of no great antiquity +(as in the case of Patagonia),—as evidenced by the level surface of the +ground on both sides of great faults and dislocations,—by inland lines +of escarpments, by outliers, and numberless other facts, and by that +argument of high generality advanced by Mr. Lyell, namely, that every +SEDIMENTARY formation, whatever its thickness may be, and over however +many hundred square miles it may extend, is the result and the measure +of an equal amount of wear and tear of pre-existing formations; +considering these facts, we must conclude that, as an ordinary rule, a +formation to resist such vast destroying powers, and to last to a +distant epoch, must be of wide extent, and either in itself, or +together with superincumbent strata, be of great thickness. In this +discussion, we are considering only formations containing the remains +of marine animals, which, as before mentioned, live, with some +exceptions within (most of them much within) depths of 100 fathoms. +How, then, can a thick and widely extended formation be accumulated, +which shall include such organic remains? First, let us take the case +of the bed of the sea long remaining at a stationary level: under these +circumstances it is evident that CONCHIFEROUS strata can accumulate +only to the same thickness with the depth at which the shells can live; +on gently inclined coasts alone can they accumulate to any considerable +width; and from the want of superincumbent pressure, it is probable +that the sedimentary matter will seldom be much consolidated: such +formations have no very good chance, when in the course of time they +are upraised, of long resisting the powers of denudation. The chance +will be less if the submarine surface, instead of having remained +stationary, shall have gone on slowly rising during the deposition of +the strata, for in this case their total thickness must be less, and +each part, before being consolidated or thickly covered up by +superincumbent matter, will have had successively to pass through the +ordeal of the beach; and on most coasts, the waves on the beach tend to +wear down and disperse every object exposed to their action. Now, both +on the south-eastern and western shores of South America, we have had +clear proofs that the land has been slowly rising, and in the long +lines of lofty cliffs, we have seen that the tendency of the sea is +almost everywhere to eat into the land. Considering these facts, it +ceases, I think, to be surprising, that extensive recent conchiferous +deposits are entirely absent on the southern and western shores of +America. + +Let us take the one remaining case, of the bed of the sea slowly +subsiding during a length of time, whilst sediment has gone on being +deposited. It is evident that strata might thus accumulate to any +thickness, each stratum being deposited in shallow water, and +consequently abounding with those shells which cannot live at great +depths: the pressure, also, I may observe, of each fresh bed would aid +in consolidating all the lower ones. Even on a rather steep coast, +though such must ever be unfavourable to widely extended deposits, the +formations would always tend to increase in breadth from the water +encroaching on the land. Hence we may admit that periods of slow +subsidence will commonly be most favourable to the accumulation of +CONCHIFEROUS deposits, of sufficient thickness, extension, and +hardness, to resist the average powers of denudation. + +We have seen that at an ancient tertiary epoch, fossiliferous deposits +were extensively deposited on the coasts of South America; and it is a +very interesting fact, that there is evidence that these ancient +tertiary beds were deposited during a period of subsidence. Thus, at +Navidad, the strata are about eight hundred feet in thickness, and the +fossil shells are abundant both at the level of the sea and some way up +the cliffs; having sent a list of these fossils to Professor E. Forbes, +he thinks they must have lived in water between one and ten fathoms in +depth: hence the bottom of the sea on which these shells once lived +must have subsided at least 700 feet to allow of the superincumbent +matter being deposited. I must here remark, that, as all these and the +following fossil shells are extinct species, Professor Forbes +necessarily judges of the depths at which they lived only from their +generic character, and from the analogical distribution of shells in +the northern hemisphere; but there is no just cause from this to doubt +the general results. At Huafo the strata are about the same thickness, +namely, 800 feet, and Professor Forbes thinks the fossils found there +cannot have lived at a greater depth than fifty fathoms, or 300 feet. +These two points, namely, Navidad and Huafo, are 570 miles apart, but +nearly halfway between them lies Mocha, an island 1,200 feet in height, +apparently formed of tertiary strata up to its level summit, and with +many shells, including the same Turritella with that found at Huafo, +embedded close to the level of the sea. In Patagonia, shells are +numerous at Santa Cruz, at the foot of the 350 feet plain, which has +certainly been formed by the denudation of the 840 feet plain, and +therefore was originally covered by strata that number of feet in +thickness, and these shells, according to Professor Forbes, probably +lived at a depth of between seven and fifteen fathoms: at Port S. +Julian, sixty miles to the north, shells are numerous at the foot of +the ninety feet plain (formed by the denudation of the 950 feet plain), +and likewise occasionally at the height of several hundred feet in the +upper strata; these shells must have lived in water somewhere between +five and fifty fathoms in depth. Although in other parts of Patagonia I +have no direct evidence of shoal-water shells having been buried under +a great thickness of superincumbent submarine strata, yet it should be +borne in mind that the lower fossiliferous strata with several of the +same species of Mollusca, the upper tufaceous beds, and the high +summit-plain, stretch for a considerable distance southward, and for +hundreds of miles northward; seeing this uniformity of structure, I +conceive it may be fairly concluded that the subsidence by which the +shells at Santa Cruz and S. Julian were carried down and covered up, +was not confined to these two points, but was co-extensive with a +considerable portion of the Patagonian tertiary formation. In a +succeeding chapter it will be seen, that we are led to a similar +conclusion with respect to the secondary fossiliferous strata of the +Cordillera, namely, that they also were deposited during a long- +continued and great period of subsidence. From the foregoing reasoning, +and from the facts just given, I think we must admit the probability of +the following proposition: namely, that when the bed of the sea is +either stationary or rising, circumstances are far less favourable, +than when the level is sinking, to the accumulation of CONCHIFEROUS +deposits of sufficient thickness and extension to resist, when +upheaved, the average vast amount of denudation. This result appears to +me, in several respects, very interesting: every one is at first +inclined to believe that at innumerable points, wherever there is a +supply of sediment, fossiliferous strata are now forming, which at some +future distant epoch will be upheaved and preserved; but on the views +above given, we must conclude that this is far from being the case; on +the contrary, we require (1st), a long-continued supply of sediment; +(2nd), an extensive shallow area; and (3rd), that this area shall +slowly subside to a great depth, so as to admit the accumulation of a +widely extended thick mass of superincumbent strata. In how few parts +of the world, probably, do these conditions at the present day concur! +We can thus, also, understand the general want of that close sequence +in fossiliferous formations which we might theoretically have +anticipated; for, without we suppose a subsiding movement to go on at +the same spot during an enormous period, from one geological era to +another, and during the whole of this period sediment to accumulate at +the proper rate, so that the depth should not become too great for the +continued existence of molluscous animals, it is scarcely possible that +there should be a perfect sequence at the same spot in the fossil +shells of the two geological formations. (Professor H.D. Rogers, in his +excellent address to the Association of American Geologists +(“Silliman’s Journal” volume 47 page 277) makes the following remark: +“I question if we are at all aware how COMPLETELY the whole history of +all departed time lies indelibly recorded with the amplest minuteness +of detail in the successive sediments of the globe, how effectually, in +other words, every period of time HAS WRITTEN ITS OWN HISTORY, +carefully preserving every created form and every trace of action.” I +think the correctness of such remarks is more than doubtful, even if we +except (as I suppose he would) all those numerous organic forms which +contain no hard parts.) So far from a very long-continued subsidence +being probable, many facts lead to the belief that the earth’s surface +oscillates up and down; and we have seen that during the elevatory +movements there is but a small chance of DURABLE fossiliferous deposits +accumulating. + +Lastly, these same considerations appear to throw some light on the +fact that certain periods appear to have been favourable to the +deposition, or at least to the preservation, of contemporaneous +formations at very distant points. We have seen that in South America +an enormous area has been rising within the recent period; and in other +quarters of the globe immense spaces appear to have risen +contemporaneously. From my examination of the coral- reefs of the great +oceans, I have been led to conclude that the bed of the sea has gone on +slowly sinking within the present era, over truly vast areas: this, +indeed, is in itself probable, from the simple fact of the rising areas +having been so large. In South America we have distinct evidence that +at nearly the same tertiary period, the bed of the sea off parts of the +coast of Chile and off Patagonia was sinking, though these regions are +very remote from each other. If, then, it holds good, as a general +rule, that in the same quarter of the globe the earth’s crust tends to +sink and rise contemporaneously over vast spaces, we can at once see, +that we have at distant points, at the same period, those very +conditions which appear to be requisite for the accumulation of +fossiliferous masses of sufficient extension, thickness, and hardness, +to resist denudation, and consequently to last unto an epoch distant in +futurity. (Professor Forbes has some admirable remarks on this subject, +in his “Report on the Shells of the Aegean Sea.” In a letter to Mr. +Maclaren (“Edinburgh New Philosophical Journal” January 1843), I +partially entered into this discussion, and endeavoured to show that it +was highly improbable, that upraised atolls or barrier-reefs, though of +great thickness, should, owing to their small extension or breadth, be +preserved to a distant future period.) + + + + +CHAPTER VI. +PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION. + + +Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of +foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in, +decomposition of.—La Plata, metamorphic and old volcanic rocks of.—S. +Ventana.—Claystone porphyry formation of Patagonia; singular +metamorphic rocks; pseudo-dikes.—Falkland Islands, Palaeozoic fossils +of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of; +cleavage and foliation; form of land.—Chonos Archipelago, mica-schists, +foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes, +successive formation of.—Central and Northern Chile.—Concluding remarks +on cleavage and foliation.—Their close analogy and similar +origin.—Stratification of metamorphic schists.—Foliation of intrusive +rocks.—Relation of cleavage and foliation to the lines of tension +during metamorphosis. + + +The metamorphic and plutonic formations of the several districts +visited by the “Beagle” will be here chiefly treated of, but only such +cases as appear to me new, or of some special interest, will be +described in detail; at the end of the chapter I will sum up all the +facts on cleavage and foliation,— to which I particularly attended. + +BAHIA, BRAZIL: latitude 13 degrees south. + +The prevailing rock is gneiss, often passing, by the disappearance of +the quartz and mica, and by the feldspar losing its red colour, into a +brilliantly grey primitive greenstone. Not unfrequently quartz and +hornblende are arranged in layers in almost amorphous feldspar. There +is some fine-grained syenitic granite, orbicularly marked by +ferruginous lines, and weathering into vertical, cylindrical holes, +almost touching each other. In the gneiss, concretions of granular +feldspar and others of garnets with mica occur. The gneiss is traversed +by numerous dikes composed of black, finely crystallised, hornblendic +rock, containing a little glassy feldspar and sometimes mica, and +varying in thickness from mere threads to ten feet: these threads, +which are often curvilinear, could sometimes be traced running into the +larger dikes. One of these dikes was remarkable from having been in two +or three places laterally disjointed, with unbroken gneiss interposed +between the broken ends, and in one part with a portion of the gneiss +driven, apparently whilst in a softened state, into its side or wall. +In several neighbouring places, the gneiss included angular, well- +defined, sometimes bent, masses of hornblende rock, quite like, except +in being more perfectly crystallised, that forming the dikes, and, at +least in one instance, containing (as determined by Professor Miller) +augite as well as hornblende. In one or two cases these angular masses, +though now quite separate from each other by the solid gneiss, had, +from their exact correspondence in size and shape, evidently once been +united; hence I cannot doubt that most or all of the fragments have +been derived from the breaking up of the dikes, of which we see the +first stage in the above- mentioned laterally disjointed one. The +gneiss close to the fragments generally contained many large crystals +of hornblende, which are entirely absent or rare in other parts: its +folia or laminae were gently bent round the fragments, in the same +manner as they sometimes are round concretions. Hence the gneiss has +certainly been softened, its composition modified, and its folia +arranged, subsequently to the breaking up of the dikes, these latter +also having been at the same time bent and softened. (Professor +Hitchcock “Geology of Massachusetts” volume 2 page 673, gives a closely +similar case of a greenstone dike in syenite.) + +I must here take the opportunity of premising, that by the term +CLEAVAGE I imply those planes of division which render a rock, +appearing to the eye quite or nearly homogeneous, fissile. By the term +FOLIATION, I refer to the layers or plates of different mineralogical +nature of which most metamorphic schists are composed; there are, also, +often included in such masses, alternating, homogeneous, fissile layers +or folia, and in this case the rock is both foliated and has a +cleavage. By STRATIFICATION, as applied to these formations, I mean +those alternate, parallel, large masses of different composition, which +are themselves frequently either foliated or fissile,—such as the +alternating so-called strata of mica-slate, gneiss, glossy clay-slate, +and marble. + +The folia of the gneiss within a few miles round Bahia generally strike +irregularly, and are often curvilinear, dipping in all directions at +various angles: but where best defined, they extended most frequently +in a N.E. by N. (or East 50 degrees N.) and S.W. by S. line, +corresponding nearly with the coast-line northwards of the bay. I may +add that Mr. Gardner found in several parts of the province of Ceara, +which lies between four and five hundred miles north of Bahia, gneiss +with the folia extending E. 45 degrees N.; and in Guyana according to +Sir R. Schomburgk, the same rock strikes E. 57 degrees N. Again, +Humboldt describes the gneiss-granite over an immense area in Venezuela +and even in Colombia, as striking E. 50 degrees N., and dipping to the +N.W. at an angle of fifty degrees. (Gardner “Geological Section of the +British Association” 1840. For Sir R. Schomburgk’s observations see +“Geographical Journal” 1842 page 190. See also Humboldt’s discussion on +Loxodrism in the “Personal Narrative.”) Hence all the observations +hitherto made tend to show that the gneissic rocks over the whole of +this part of the continent have their folia extending generally within +almost a point of the compass of the same direction. (I landed at only +one place north of Bahia, namely, at Pernambuco. I found there only +soft, horizontally stratified matter, formed from disintegrated +granitic rocks, and some yellowish impure limestone, probably of a +tertiary epoch. I have described a most singular natural bar of hard +sandstone, which protects the harbour, in the 19th volume 1841 page 258 +of the “London and Edinburgh Philosophical Magazine.” + +ABROLHOS ISLETS, Latitude 18 degrees S. off the coast of Brazil. + +Although not strictly in place, I do not know where I can more +conveniently describe this little group of small islands. The lowest +bed is a sandstone with ferruginous veins; it weathers into an +extraordinary honeycombed mass; above it there is a dark-coloured +argillaceous shale; above this a coarser sandstone—making a total +thickness of about sixty feet; and lastly, above these sedimentary +beds, there is a fine conformable mass of greenstone, in some parts +having a columnar structure. All the strata, as well as the surface of +the land, dip at an angle of about 12 degrees to N. by W. Some of the +islets are composed entirely of the sedimentary, others of the trappean +rocks, generally, however, with the sandstone, cropping out on the +southern shores.) + +RIO DE JANEIRO. + +This whole district is almost exclusively formed of gneiss, abounding +with garnets, and porphyritic with large crystals, even three and four +inches in length, of orthoclase feldspar: in these crystals mica and +garnets are often enclosed. At the western base of the Corcovado, there +is some ferruginous carious quartz-rock; and in the Tijeuka range, much +fine- grained granite. I observed boulders of greenstone in several +places; and on the islet of Villegagnon, and likewise on the coast some +miles northward, two large trappean dikes. The porphyritic gneiss, or +gneiss- granite as it has been called by Humboldt, is only so far +foliated that the constituent minerals are arranged with a certain +degree of regularity, and may be said to have a “GRAIN,” but they are +not separated into distinct folia or laminae. There are, however, +several other varieties of gneiss regularly foliated, and alternating +with each other in so-called strata. The stratification and foliation +of the ordinary gneisses, and the foliation or “grain” of the +gneiss-granite, are parallel to each other, and generally strike within +a point of N.E. and S.W. dipping at a high angle (between 50 and 60 +degrees) generally to S.E.: so that here again we meet with the strike +so prevalent over the more northern parts of this continent. The +mountains of gneiss-granite are to a remarkable degree abruptly +conical, which seems caused by the rock tending to exfoliate in thick, +conically concentric layers: these peaks resemble in shape those of +phonolite and other injected rocks on volcanic islands; nor is the +grain or foliation (as we shall afterwards see) any difficulty on the +idea of the gneiss-granite having been an intrusive rather than a +metamorphic formation. The lines of mountains, but not always each +separate hill, range nearly in the same direction with the foliation +and so-called stratification, but rather more easterly. + +(FIGURE 22. FRAGMENT OF GNEISS EMBEDDED IN ANOTHER VARIETY OF THE SAME +ROCK.) + +On a bare gently inclined surface of the porphyritic gneiss in Botofogo +Bay, I observed the appearance represented in Figure 22. A fragment +seven yards long and two in width, with angular and distinctly defined +edges, composed of a peculiar variety of gneiss with dark layers of +mica and garnets, is surrounded on all sides by the ordinary gneiss- +granite; both having been dislocated by a granitic vein. The folia in +the fragment and in the surrounding rock strike in the same N.N.E. and +S.S.W. line; but in the fragment they are vertical, whereas in the +gneiss-granite they dip at a small angle, as shown by the arrows, to +S.S.E. This fragment, considering its great size, its solitary +position, and its foliated structure parallel to that of the +surrounding rock, is, as far as I know, a unique case: and I will not +attempt any explanation of its origin. + +The numerous travellers in this country, have all been greatly +surprised at the depth to which the gneiss and other granitic rocks, as +well as the talcose slates of the interior, have been decomposed. (Spix +and Martius have collected in an Appendix to their “Travels,” the +largest body of facts on this subject. See also some remarks by M. Lund +in his communications to the Academy at Copenhagen; and others by M. +Gaudichaud in Freycinet “Voyage.”) Near Rio, every mineral except the +quartz has been completely softened, in some places to a depth little +less than one hundred feet. (Dr. Benza describes granitic rock, “Madras +Journal of Literature” etc. October 183? page 246), in the +Neelgherries, decomposed to a depth of forty feet.) The minerals retain +their positions in folia ranging in the usual direction; and fractured +quartz veins may be traced from the solid rock, running for some +distance into the softened, mottled, highly coloured, argillaceous +mass. It is said that these decomposed rocks abound with gems of +various kinds, often in a fractured state, owing, as some have +supposed, to the collapse of geodes, and that they contain gold and +diamonds. At Rio, it appeared to me that the gneiss had been softened +before the excavation (no doubt by the sea) of the existing, broad, +flat-bottomed valleys; for the depth of decomposition did not appear at +all conformable with the present undulations of the surface. The +porphyritic gneiss, where now exposed to the air, seems to withstand +decomposition remarkably well; and I could see no signs of any tendency +to the production of argillaceous masses like those here described. I +was also struck with the fact, that where a bare surface of this rock +sloped into one of the quiet bays, there were no marks of erosion at +the level of the water, and the parts both beneath and above it +preserved a uniform curve. At Bahia, the gneiss rocks are similarly +decomposed, with the upper parts insensibly losing their foliation, and +passing, without any distinct line of separation, into a bright red +argillaceous earth, including partially rounded fragments of quartz and +granite. From this circumstance, and from the rocks appearing to have +suffered decomposition before the excavation of the valleys, I suspect +that here, as at Rio, the decomposition took place under the sea. The +subject appeared to me a curious one, and would probably well repay +careful examination by an able mineralogist. + +THE NORTHERN PROVINCES OF LA PLATA. + +According to some observations communicated to me by Mr. Fox, the coast +from Rio de Janeiro to the mouth of the Plata seems everywhere to be +granitic, with a few trappean dikes. At Port Alegre, near the boundary +of Brazil, there are porphyries and diorites. (M. Isabelle “Voyage a +Buenos Ayres” page 479.) At the mouth of the Plata, I examined the +country for twenty-five miles west, and for about seventy miles north +of Maldonado: near this town, there is some common gneiss, and much, in +all parts of the country, of a coarse-grained mixture of quartz and +reddish feldspar, often, however, assuming a little dark-green +imperfect hornblende, and then immediately becoming foliated. The +abrupt hillocks thus composed, as well as the highly inclined folia of +the common varieties of gneiss, strike N.N.E. or a little more +easterly, and S.S.W. Clay-slate is occasionally met with, and near the +L. del Potrero, there is white marble, rendered fissile from the +presence of hornblende, mica, and asbestus; the cleavage of these rocks +and their stratification, that is the alternating masses thus composed, +strike N.N.E. and S.S.W. like the foliated gneisses, and have an almost +vertical dip. The Sierra Larga, a low range five miles west of +Maldonado, consists of quartzite, often ferruginous, having an +arenaceous feel, and divided into excessively thin, almost vertical +laminae or folia by microscopically minute scales, apparently of mica, +and striking in the usual N.N.E. and S.S.W. direction. The range itself +is formed of one principal line with some subordinate ones; and it +extends with remarkable uniformity far northward (it is said even to +the confines of Brazil), in the same line with the vertically ribboned +quartz rock of which it is composed. The S. de Las Animas is the +highest range in the country; I estimated it at 1,000 feet; it runs +north and south, and is formed of feldspathic porphyry; near its base +there is a N.N.W. and S.S.E. ridge of a conglomerate in a highly +porphyritic basis. + +Northward of Maldonado, and south of Las Minas, there is an E. and W. +hilly band of country, some miles in width, formed of siliceous +clay-slate, with some quartz, rock, and limestone, having a tortuous +irregular cleavage, generally ranging east and west. E. and S.E. of Las +Minas there is a confused district of imperfect gneiss and laminated +quartz, with the hills ranging in various directions, but with each +separate hill generally running in the same line with the folia of the +rocks of which it is composed: this confusion appears to have been +caused by the intersection of the [E. and W.] and [N.N.E. and S.S.W.] +strikes. Northward of Las Minas, the more regular northerly ranges +predominate: from this place to near Polanco, we meet with the +coarse-grained mixture of quartz and feldspar, often with the imperfect +hornblende, and then becoming foliated in a N. and S. line—with +imperfect clay-slate, including laminae of red crystallised +feldspar—with white or black marble, sometimes containing asbestus and +crystals of gypsum—with quartz-rock—with syenite—and lastly, with much +granite. The marble and granite alternate repeatedly in apparently +vertical masses: some miles northward of the Polanco, a wide district +is said to be entirely composed of marble. It is remarkable, how rare +mica is in the whole range of country north and westward of Maldonado. +Throughout this district, the cleavage of the clay-slate and marble—the +foliation of the gneiss and the quartz—the stratification or +alternating masses of these several rocks—and the range of the hills, +all coincide in direction; and although the country is only hilly, the +planes of division are almost everywhere very highly inclined or +vertical. + +Some ancient submarine volcanic rocks are worth mentioning, from their +rarity on this eastern side of the continent. In the valley of the +Tapas (fifty or sixty miles N. of Maldonado) there is a tract three or +four miles in length, composed of various trappean rocks with glassy +feldspar—of apparently metamorphosed grit-stones—of purplish +amygdaloids with large kernels of carbonate of lime (Near the Pan de +Azucar there is some greenish porphyry, in one place amygdaloidal with +agate.)—and much of a harshish rock with glassy feldspar intermediate +in character between claystone porphyry and trachyte. This latter rock +was in one spot remarkable from being full of drusy cavities, lined +with quartz crystals, and arranged in planes, dipping at an angle of 50 +degrees to the east, and striking parallel to the foliation of an +adjoining hill composed of the common mixture of quartz, feldspar, and +imperfect hornblende: this fact perhaps indicates that these volcanic +rocks have been metamorphosed, and their constituent parts rearranged, +at the same time and according to the same laws, with the granitic and +metamorphic formations of this whole region. In the valley of the +Marmaraya, a few miles south of the Tapas, a band of trappean and +amygdaloidal rock is interposed between a hill of granite and an +extensive surrounding formation of red conglomerate, which (like that +at the foot of the S. Animas) has its basis porphyritic with crystals +of feldspar, and which hence has certainly suffered metamorphosis. + +MONTE VIDEO. + +The rocks here consist of several varieties of gneiss, with the +feldspar often yellowish, granular and imperfectly crystallised, +alternating with, and passing insensibly into, beds, from a few yards +to nearly a mile in thickness, of fine or coarse grained, dark-green +hornblendic slate; this again often passing into chloritic schist. +These passages seem chiefly due to changes in the mica, and its +replacement by other minerals. At Rat Island I examined a mass of +chloritic schist, only a few yards square, irregularly surrounded on +all sides by the gneiss, and intricately penetrated by many curvilinear +veins of quartz, which gradually BLEND into the gneiss: the cleavage of +the chloritic schist and the foliation of the gneiss were exactly +parallel. Eastward of the city there is much fine- grained, +dark-coloured gneiss, almost assuming the character of hornblende- +slate, which alternates in thin laminae with laminae of quartz, the +whole mass being transversely intersected by numerous large veins of +quartz: I particularly observed that these veins were absolutely +continuous with the alternating laminae of quartz. In this case and at +Rat Island, the passage of the gneiss into imperfect hornblendic or +into chloritic slate, seemed to be connected with the segregation of +the veins of quartz. (Mr. Greenough page 78 “Critical Examination” +etc., observes that quartz in mica-slate sometimes appears in beds and +sometimes in veins. Von Buch also in his “Travels in Norway” page 236, +remarks on alternating laminae of quartz and hornblende-slate replacing +mica-schist.) + +The Mount, a hill believed to be 450 feet in height, from which the +place takes its name, is much the highest land in this neighbourhood: +it consists of hornblendic slate, which (except on the eastern and +disturbed base) has an east and west nearly vertical cleavage; the +longer axis of the hill also ranges in this same line. Near the summit +the hornblende-slate gradually becomes more and more coarsely +crystallised, and less plainly laminated, until it passes into a heavy, +sonorous greenstone, with a slaty conchoidal fracture; the laminae on +the north and south sides near the summit dip inwards, as if this upper +part had expanded or bulged outwards. This greenstone must, I conceive, +be considered as metamorphosed hornblende- slate. The Cerrito, the next +highest, but much less elevated point, is almost similarly composed. In +the more western parts of the province, besides gneiss, there is +quartz-rock, syenite, and granite; and at Colla, I heard of marble. + +Near M. Video, the space which I more accurately examined was about +fifteen miles in an east and west line, and here I found the foliation +of the gneiss and the cleavage of the slates generally well developed, +and extending parallel to the alternating strata composed of the +gneiss, hornblendic and chloritic schists. These planes of division all +range within one point of east and west, frequently east by south and +west by north; their dip is generally almost vertical, and scarcely +anywhere under 45 degrees: this fact, considering how slightly +undulatory the surface of the country is, deserves attention. Westward +of M. Video, towards the Uruguay, wherever the gneiss is exposed, the +highly inclined folia are seen striking in the same direction; I must +except one spot where the strike was N.W. by W. The little Sierra de S. +Juan, formed of gneiss and laminated quartz, must also be excepted, for +it ranges between [N. to N.E.] and [S. to S.W.] and seems to belong to +the same system with the hills in the Maldonado district. Finally, we +have seen that, for many miles northward of Maldonado and for +twenty-five miles westward of it, as far as the S. de las Animas, the +foliation, cleavage, so-called stratification and lines of hills, all +range N.N.E. and S.S.W., which is nearly coincident with the adjoining +coast of the Atlantic. Westward of the S. de las Animas, as far as even +the Uruguay, the foliation, cleavage, and stratification (but not lines +of hills, for there are no defined ones) all range about E. by S. and +W. by N., which is nearly coincident with the direction of the northern +shore of the Plata; in the confused country near Las Minas, where these +two great systems appear to intersect each other, the cleavage, +foliation, and stratification run in various directions, but generally +coincide with the line of each separate hill. + +SOUTHERN LA PLATA. + +The first ridge, south of the Plata, which projects through the Pampean +formation, is the Sierra Tapalguen and Vulcan, situated 200 miles +southward of the district just described. This ridge is only a few +hundred feet in height, and runs from C. Corrientes in a W.N.W. line +for at least 150 miles into the interior: at Tapalguen, it is composed +of unstratified granular quartz, remarkable from forming tabular masses +and small plains, surrounded by precipitous cliffs: other parts of the +range are said to consist of granite: and marble is found at the S. +Tinta. It appears from M. Parchappe’s observations, that at Tandil +there is a range of quartzose gneiss, very like the rocks of the S. +Larga near Maldonado, running in the same N.N.E. and S.S.W. direction; +so that the framework of the country here is very similar to that on +the northern shore of the Plata. (M. d’Orbigny’s “Voyage” Part. Geolog. +page 46. I have given a short account of the peculiar forms of the +quartz hills of Tapalguen, so unusual in a metamorphic formation, in my +“Journal of Researches” 2nd edition page 116.) + +The Sierra Guitru-gueyu is situated sixty miles south of the S. +Tapalguen: it consists of numerous parallel, sometimes blended together +ridges, about twenty-three miles in width, and five hundred feet in +height above the plain, and extending in a N.W. and S.E. direction. +Skirting round the extreme S.E. termination, I ascended only a few +points, which were composed of a fine-grained gneiss, almost composed +of feldspar with a little mica, and passing in the upper parts of the +hills into a rather compact purplish clay-slate. The cleavage was +nearly vertical, striking in a N.W. by W. and S.E. by E. line, nearly, +though not quite, coincident with the direction of the parallel ridges. + +The Sierra Ventana lies close south of that of Guitru-gueyu; it is +remarkable from attaining a height, very unusual on this side of the +continent, of 3,340 feet. It consists up to its summit, of quartz, +generally pure and white, but sometimes reddish, and divided into thick +laminae or strata: in one part there is a little glossy clay-slate with +a tortuous cleavage. The thick layers of quartz strike in a W. 30 +degrees N. line, dipping southerly at an angle of 45 degrees and +upwards. The principal line of mountains, with some quite subordinate +parallel ridges, range about W. 45 degrees N.: but at their S.E. +termination, only W. 25 degrees N. This Sierra is said to extend +between twenty and thirty leagues into the interior. + +PATAGONIA. + +With the exception perhaps of the hill of S. Antonio (600 feet high) in +the Gulf of S. Matias, which has never been visited by a geologist, +crystalline rocks are not met with on the coast of Patagonia for a +space of 380 miles south of the S. Ventana. At this point (latitude 43 +degrees 50 minutes), at Points Union and Tombo, plutonic rocks are said +to appear, and are found, at rather wide intervals, beneath the +Patagonian tertiary formation for a space of about three hundred miles +southward, to near Bird Island, in latitude 48 degrees 56 minutes. +Judging from specimens kindly collected for me by Mr. Stokes, the +prevailing rock at Ports St. Elena, Camerones, Malaspina, and as far +south as the Paps of Pineda, is a purplish-pink or brownish claystone +porphyry, sometimes laminated, sometimes slightly vesicular, with +crystals of opaque feldspar and with a few grains of quartz; hence +these porphyries resemble those immediately to be described at Port +Desire, and likewise a series which I have seen from P. Alegre on the +southern confines of Brazil. This porphyritic formation further +resembles in a singularly close manner the lowest stratified formation +of the Cordillera of Chile, which, as we shall hereafter see, has a +vast range, and attains a great thickness. At the bottom of the Gulf of +St. George, only tertiary deposits appear to be present. At Cape +Blanco, there is quartz rock, very like that of the Falkland Islands, +and some hard, blue siliceous clay-slate. + +At Port Desire there is an extensive formation of the claystone +porphyry, stretching at least twenty-five miles into the interior: it +has been denuded and deeply worn into gullies before being covered up +by the tertiary deposits, through which it here and there projects in +hills; those north of the bay being 440 feet in height. The strata have +in several places been tilted at small angles, generally either to +N.N.W. or S.S.E. By gradual passages and alternations, the porphyries +change incessantly in nature. I will describe only some of the +principal mineralogical changes, which are highly instructive, and +which I carefully examined. The prevailing rock has a compact purplish +base, with crystals of earthy or opaque feldspar, and often with grains +of quartz. There are other varieties, with an almost truly trachytic +base, full of little angular vesicles and crystals of glassy feldspar; +and there are beds of black perfect pitchstone, as well as of a +concretionary imperfect variety. On a casual inspection, the whole +series would be thought to be of the same plutonic or volcanic nature +with the trachytic varieties and pitchstone; but this is far from being +the case, as much of the porphyry is certainly of metamorphic origin. +Besides the true porphyries, there are many beds of earthy, quite white +or yellowish, friable, easily fusible matter, resembling chalk, which +under the microscope is seen to consist of minute broken crystals, and +which, as remarked in a former chapter, singularly resembles the upper +tufaceous beds of the Patagonian tertiary formation. This earthy +substance often becomes coarser, and contains minute rounded fragments +of porphyries and rounded grains of quartz, and in one case so many of +the latter as to resemble a common sandstone. These beds are sometimes +marked with true lines of aqueous deposition, separating particles of +different degrees of coarseness; in other cases there are parallel +ferruginous lines not of true deposition, as shown by the arrangement +of the particles, though singularly resembling them. The more indurated +varieties often include many small and some larger angular cavities, +which appear due to the removal of earthy matter: some varieties +contain mica. All these earthy and generally white stones insensibly +pass into more indurated sonorous varieties, breaking with a conchoidal +fracture, yet of small specific gravity; many of these latter varieties +assume a pale purple tint, being singularly banded and veined with +different shades, and often become plainly porphyritic with crystals of +feldspar. The formation of these crystals could be most clearly traced +by minute angular and often partially hollow patches of earthy matter, +first assuming a FIBROUS STRUCTURE, then passing into opaque +imperfectly shaped crystals, and lastly, into perfect glassy crystals. +When these crystals have appeared, and when the basis has become +compact, the rock in many places could not be distinguished from a true +claystone porphyry without a trace of mechanical structure. + +In some parts, these earthy or tufaceous beds pass into jaspery and +into beautifully mottled and banded porcelain rocks, which break into +splinters, translucent at their edges, hard enough to scratch glass, +and fusible into white transparent beads: grains of quartz included in +the porcelainous varieties can be seen melting into the surrounding +paste. In other parts, the earthy or tufaceous beds either insensibly +pass into, or alternate with, breccias composed of large and small +fragments of various purplish porphyries, with the matrix generally +porphyritic: these breccias, though their subaqueous origin is in many +places shown both by the arrangement of their smaller particles and by +an oblique or current lamination, also pass into porphyries, in which +every trace of mechanical origin and stratification has been +obliterated. + +Some highly porphyritic though coarse-grained masses, evidently of +sedimentary origin, and divided into thin layers, differing from each +other chiefly in the number of embedded grains of quartz, interested me +much from the peculiar manner in which here and there some of the +layers terminated in abrupt points, quite unlike those produced by a +layer of sediment naturally thinning out, and apparently the result of +a subsequent process of metamorphic aggregation. In another common +variety of a finer texture, the aggregating process had gone further, +for the whole mass consisted of quite short, parallel, often slightly +curved layers or patches, of whitish or reddish finely +granulo-crystalline feldspathic matter, generally terminating at both +ends in blunt points; these layers or patches further tended to pass +into wedge or almond-shaped little masses, and these finally into true +crystals of feldspar, with their centres often slightly drusy. The +series was so perfect that I could not doubt that these large crystals, +which had their longer axes placed parallel to each other, had +primarily originated in the metamorphosis and aggregation of +alternating layers of tuff; and hence their parallel position must be +attributed (unexpected though the conclusion may be), not to laws of +chemical action, but to the original planes of deposition. I am tempted +briefly to describe three other singular allied varieties of rock; the +first without examination would have passed for a stratified +porphyritic breccia, but all the included angular fragments consisted +of a border of pinkish crystalline feldspathic matter, surrounding a +dark translucent siliceous centre, in which grains of quartz not quite +blended into the paste could be distinguished: this uniformity in the +nature of the fragments shows that they are not of mechanical, but of +concretionary origin, having resulted perhaps from the self-breaking up +and aggregation of layers of indurated tuff containing numerous grains +of quartz,—into which, indeed, the whole mass in one part passed. The +second variety is a reddish non-porphyritic claystone, quite full of +spherical cavities, about half an inch in diameter, each lined with a +collapsed crust formed of crystals of quartz. The third variety also +consists of a pale purple non-porphyritic claystone, almost wholly +formed of concretionary balls, obscurely arranged in layers, of a less +compact and paler coloured claystone; each ball being on one side +partly hollow and lined with crystals of quartz. + +PSEUDO-DIKES. + +Some miles up the harbour, in a line of cliffs formed of slightly +metamorphosed tufaceous and porphyritic claystone beds, I observed +three vertical dikes, so closely resembling in general appearance +ordinary volcanic dikes, that I did not doubt, until closely examining +their composition, that they had been injected from below. The first is +straight, with parallel sides, and about four feet wide; it consists of +whitish, indurated tufaceous matter, precisely like some of the beds +intersected by it. The second dike is more remarkable; it is slightly +tortuous, about eighteen inches thick, and can be traced for a +considerable distance along the beach; it is of a purplish-red or brown +colour, and is formed chiefly of ROUNDED grains of quartz, with broken +crystals of earthy feldspar, scales of black mica, and minute fragments +of claystone porphyry, all firmly united together in a hard sparing +base. The structure of this dike shows obviously that it is of +mechanical and sedimentary origin; yet it thinned out upwards, and did +not cut through the uppermost strata in the cliffs. This fact at first +appears to indicate that the matter could not have been washed in from +above (Upfilled fissures are known to occur both in volcanic and in +ordinary sedimentary formations. At the Galapagos Archipelago “Volcanic +Islands” etc., there are some striking examples of pseudo-dikes +composed of hard tuff.); but if we reflect on the suction which would +result from a deep-seated fissure being formed, we may admit that if +the fissure were in any part open to the surface, mud and water might +well be drawn into it along its whole course. The third dike consisted +of a hard, rough, white rock, almost composed of broken crystals of +glassy feldspar, with numerous scales of black mica, cemented in a +scanty base; there was little in the appearance of this rock, to +preclude the idea of its having been a true injected feldspathic dike. +The matter composing these three pseudo-dikes, especially the second +one, appears to have suffered, like the surrounding strata, a certain +degree of metamorphic action; and this has much aided the deceptive +appearance. At Bahia, in Brazil, we have seen that a true injected +hornblendic dike, not only has suffered metamorphosis, but has been +dislocated and even diffused in the surrounding gneiss, under the form +of separate crystals and of fragments. + +FALKLAND ISLANDS. + +I have described these islands in a paper published in the third volume +of the “Geological Journal.” The mountain-ridges consist of quartz, and +the lower country of clay-slate and sandstone, the latter containing +Palaeozoic fossils. These fossils have been separately described by +Messrs. Morris and Sharpe: some of them resemble Silurian, and others +Devonian forms. In the eastern part of the group the several parallel +ridges of quartz extend in a west and east line; but further westward +the line becomes W.N.W. and E.S.E., and even still more northerly. The +cleavage-planes of the clay- slate are highly inclined, generally at an +angle of above 50 degrees, and often vertical; they strike almost +invariably in the same direction with the quartz ranges. The outline of +the indented shores of the two main islands, and the relative positions +of the smaller islets, accord with the strike both of the main axes of +elevation and of the cleavage of the clay- slate. + +TIERRA DEL FUEGO. + +My notes on the geology of this country are copious, but as they are +unimportant, and as fossils were found only in one district, a brief +sketch will be here sufficient. The east coast from the S. of Magellan +(where the boulder formation is largely developed) to St. Polycarp’s +Bay is formed of horizontal tertiary strata, bounded some way towards +the interior by a broad mountainous band of clay-slate. This great +clay-slate formation extends from St. Le Maire westward for 140 miles, +along both sides of the Beagle Channel to near its bifurcation. South +of this channel, it forms all Navarin Island, and the eastern half of +Hoste Island and of Hardy Peninsula; north of the Beagle Channel it +extends in a north-west line on both sides of Admiralty Sound to +Brunswick Peninsula in the St. of Magellan, and I have reason to +believe, stretches far up the eastern side of the Cordillera. The +western and broken side of Tierra del Fuego towards the Pacific is +formed of metamorphic schists, granite and various trappean rocks: the +line of separation between the crystalline and clay-slate formations +can generally be distinguished, as remarked by Captain King, by the +parallelism in the clay-slate districts of the shores and channels, +ranging in a line between [W. 20 degrees to 40 degrees N.] and [E. 20 +degrees to 40 degrees S.]. (“Geographical Journal” volume 1 page 155.) + +The clay-slate is generally fissile, sometimes siliceous or +ferruginous, with veins of quartz and calcareous spar; it often +assumes, especially on the loftier mountains, an altered feldspathic +character, passing into feldspathic porphyry: occasionally it is +associated with breccia and grauwacke. At Good Success Bay, there is a +little intercalated black crystalline limestone. At Port Famine much of +the clay-slate is calcareous, and passes either into a mudstone or into +grauwacke, including odd-shaped concretions of dark argillaceous +limestone. Here alone, on the shore a few miles north of Port Famine, +and on the summit of Mount Tarn (2,600 feet high), I found organic +remains; they consist of:— + +1. Ancyloceras simplex, d’Orbigny “Pal Franc” Mount Tarn. 2. Fusus (in +imperfect state), d’Orbigny “Pal Franc” Mount Tarn. 3. Natica, +d’Orbigny “Pal Franc” Mount Tarn. 4. Pentacrimus, d’Orbigny “Pal Franc” +Mount Tarn. 5. Lucina excentrica, G.B. Sowerby, Port Famine. 6. Venus +(in imperfect state), G.B. Sowerby, Port Famine. 7. Turbinolia (?), +G.B. Sowerby, Port Famine. 8. Hamites elatior, G.B. Sowerby, Port +Famine. + +M. d’Orbigny states that MM. Hombron and Grange found in this +neighbourhood an Ancyloceras, perhaps A. simplex, an Ammonite, a +Plicatula and Modiola. (“Voyage” Part Geolog. page 242.) M. d’Orbigny +believes from the general character of these fossils, and from the +Ancyloceras being identical (as far as its imperfect condition allows +of comparison) with the A. simplex of Europe, that the formation +belongs to an early stage of the Cretaceous system. Professor E. +Forbes, judging only from my specimens, concurs in the probability of +this conclusion. The Hamites elatior of the above list, of which a +description has been given by Mr. Sowerby, and which is remarkable from +its large size, has not been seen either by M. d’Orbigny or Professor +E. Forbes, as, since my return to England, the specimens have been +lost. The great clay-slate formation of Tierra del Fuego being +cretaceous, is certainly a very interesting fact,—whether we consider +the appearance of the country, which, without the evidence afforded by +the fossils, would form the analogy of most known districts, probably +have been considered as belonging to the Palaeozoic series,—or whether +we view it as showing that the age of this terminal portion of the +great axis of South America, is the same (as will hereafter be seen) +with the Cordillera of Chile and Peru. + +The clay-slate in many parts of Tierra del Fuego, is broken by dikes +and by great masses of greenstone, often highly hornblendic (In a +greenstone-dike in the Magdalen Channel, the feldspar cleaved with the +angle of albite. This dike was crossed, as well as the surrounding +slate, by a large vein of quartz, a circumstance of unusual +occurrence.): almost all the small islets within the clay-slate +districts are thus composed. The slate near the dikes generally becomes +paler-coloured, harder, less fissile, of a feldspathic nature, and +passes into a porphyry or greenstone: in one case, however, it became +more fissile, of a red colour, and contained minute scales of mica, +which were absent in the unaltered rock. On the east side of Ponsonby +Sound some dikes composed of a pale sonorous feldspathic rock, +porphyritic with a little feldspar, were remarkable from their +number,—there being within the space of a mile at least one +hundred,—from their nearly equalling in bulk the intermediate +slate,—and more especially from the excessive fineness (like the finest +inlaid carpentry) and perfect parallelism of their junctions with the +almost vertical laminae of clay-slate. I was unable to persuade myself +that these great parallel masses had been injected, until I found one +dike which abruptly thinned out to half its thickness, and had one of +its walls jagged, with fragments of the slate embedded in it. + +In Southern Tierra del Fuego, the clay-slate towards its S.W. boundary, +becomes much altered and feldspathic. Thus on Wollaston Island slate +and grauwacke can be distinctly traced passing into feldspathic rocks +and greenstones, including iron pyrites and epidote, but still +retaining traces of cleavage with the usual strike and dip. One such +metamorphosed mass was traversed by large vein-like masses of a +beautiful mixture (as ascertained by Professor Miller) of green +epidote, garnets, and white calcareous spar. On the northern point of +this same island, there were various ancient submarine volcanic rocks, +consisting of amygdaloids with dark bole and agate,—of basalt with +decomposed olivine—of compact lava with glassy feldspar,—and of a +coarse conglomerate of red scoriae, parts being amygdaloidal with +carbonate of lime. The southern part of Wollaston Island and the whole +of Hermite and Horn Islands, seem formed of cones of greenstone; the +outlying islets of Il Defenso and D. Raminez are said to consist of +porphyritic lava. (Determined by Professor Jameson. Weddell’s “Voyage” +page 169.) In crossing Hardy Peninsula, the slate still retaining +traces of its usual cleavage, passes into columnar feldspathic rocks, +which are succeeded by an irregular tract of trappean and basaltic +rocks, containing glassy feldspar and much iron pyrites: there is, +also, some harsh red claystone porphyry, and an almost true trachyte, +with needles of hornblende, and in one spot a curious slaty rock +divided into quadrangular columns, having a base almost like trachyte, +with drusy cavities lined by crystals, too imperfect, according to +Professor Miller, to be measured, but resembling Zeagonite. (See Mr. +Brooke’s Paper in the “London Philosophical Magazine” volume 10. This +mineral occurs in an ancient volcanic rock near Rome.) In the midst of +these singular rocks, no doubt of ancient submarine volcanic origin, a +high hill of feldspathic clay-slate projected, retaining its usual +cleavage. Near this point, there was a small hillock, having the aspect +of granite, but formed of white albite, brilliant crystals of +hornblende (both ascertained by the reflecting goniometer) and mica; +but with no quartz. No recent volcanic district has been observed in +any part of Tierra del Fuego. + +Five miles west of the bifurcation of the Beagle Channel, the slate- +formation, instead of becoming, as in the more southern parts of Tierra +del Fuego, feldspathic, and associated with trappean or old volcanic +rocks, passes by alternations into a great underlying mass of fine +gneiss and glossy clay-slate, which at no great distance is succeeded +by a grand formation of mica-slate containing garnets. The folia of +these metamorphic schists strike parallel to the cleavage-planes of the +clay-slate, which have a very uniform direction over the whole of this +part of the country: the folia, however, are undulatory and tortuous, +whilst the cleavage- laminae of the slate are straight. These schists +compose the chief mountain-chain of Southern Tierra del Fuego, ranging +along the north side of the northern arm of the Beagle Channel, in a +short W.N.W. and E.S.E. line, with two points (Mounts Sarmiento and +Darwin) rising to heights of 6,800 and 6,900 feet. On the south-western +side of this northern arm of the Beagle Channel, the clay-slate is seen +with its STRATA dipping from the great chain, so that the metamorphic +schists here form a ridge bordered on each side by clay-slate. Further +north, however, to the west of this great range, there is no +clay-slate, but only gneiss, mica, and hornblendic slates, resting on +great barren hills of true granite, and forming a tract about sixty +miles in width. Again, westward of these rocks, the outermost islands +are of trappean formation, which, from information obtained during the +voyages of the “Adventure” and “Beagle,” seem, together with granite, +chiefly to prevail along the western coast as far north as the entrance +of the St. of Magellan (See the Paper by Captain King in the +“Geographical Journal”; also a Letter to Dr. Fitton in “Geological +Proceedings” volume 1 page 29; also some observations by Captain +Fitzroy “Voyages” volume 1 page 375. I am indebted also to Mr. Lyell +for a series of specimens collected by Lieutenant Graves.): a little +more inland, on the eastern side of Clarence Island and S. Desolation, +granite, greenstone, mica-slate, and gneiss appear to predominate. I am +tempted to believe, that where the clay-slate has been metamorphosed at +great depths beneath the surface, gneiss, mica- slate, and other allied +rocks have been formed, but where the action has taken place nearer the +surface, feldspathic porphyries, greenstones, etc., have resulted, +often accompanied by submarine volcanic eruptions. + +Only one other rock, met with in both arms of the Beagle Channel, +deserves any notice, namely a granulo-crystalline mixture of white +albite, black hornblende (ascertained by measurement of the crystals, +and confirmed by Professor Miller), and more or less of brown mica, but +without any quartz. This rock occurs in large masses, closely +resembling in external form granite or syenite: in the southern arm of +the Channel, one such mass underlies the mica-slate, on which +clay-slate was superimposed: this peculiar plutonic rock which, as we +have seen, occurs also in Hardy Peninsula, is interesting, from its +perfect similarity with that (hereafter often to be referred to under +the name of andesite) forming the great injected axes of the Cordillera +of Chile. + +The stratification of the clay-slate is generally very obscure, whereas +the cleavage is remarkably well defined: to begin with the extreme +eastern parts of Tierra del Fuego; the cleavage-planes near the St. of +Le Maire strike either W. and E. or W.S.W. and E.N.E., and are highly +inclined; the form of the land, including Staten Island, indicates that +the axes of elevation have run in this same line, though I was unable +to distinguish the planes of stratification. Proceeding westward, I +accurately examined the cleavage of the clay-slate on the northern, +eastern, and western sides (thirty-five miles apart) of Navarin Island, +and everywhere found the laminae ranging with extreme regularity, +W.N.W. and E.S.E., seldom varying more than one point of the compass +from this direction. (The clay-slate in this island was in many places +crossed by parallel smooth joints. Out of five cases, the angle of +intersection between the strike of these joints and that of the +cleavage-laminae was in two cases 45 degrees and in two others 79 +degrees.) Both on the east and west coasts, I crossed at right angles +the cleavage-planes for a space of about eight miles, and found them +dipping at an angle of between 45 degrees and 90 degrees, generally to +S.S.W., sometimes to N.N.E., and often quite vertically. The S.S.W. dip +was occasionally succeeded abruptly by a N.N.E. dip, and this by a +vertical cleavage, or again by the S.S.W. dip; as in a lofty cliff on +the eastern end of the island the laminae of slate were seen to be +folded into very large steep curves, ranging in the usual W.N.W. line, +I suspect that the varying and opposite dips may possibly be accounted +for by the cleavage- laminae, though to the eye appearing straight, +being parts of large abrupt curves, with their summits cut off and worn +down. + +In several places I was particularly struck with the fact, that the +fine laminae of the clay-slate, where cutting straight through the +bands of stratification, and therefore indisputably true +cleavage-planes, differed slightly in their greyish and greenish tints +of colour, in compactness, and in some of the laminae having a rather +more jaspery appearance than others. I have not seen this fact +recorded, and it appears to me important, for it shows that the same +cause which has produced the highly fissile structure, has altered in a +slight degree the mineralogical character of the rock in the same +planes. The bands of stratification, just alluded to, can be +distinguished in many places, especially in Navarin Island, but only on +the weathered surfaces of the slate; they consist of slightly +undulatory zones of different shades of colour and of thicknesses, and +resemble the marks (more closely than anything else to which I can +compare them) left on the inside of a vessel by the draining away of +some dirty slightly agitated liquid: no difference in composition, +corresponding with these zones, could be seen in freshly fractured +surfaces. In the more level parts of Navarin Island, these bands of +stratification were nearly horizontal; but on the flanks of the +mountains they were inclined from them, but in no instance that I saw +at a very high angle. There can, I think, be no doubt that these zones, +which appear only on the weathered surfaces, are the last vestiges of +the original planes of stratification, now almost obliterated by the +highly fissile and altered structure which the mass has assumed. + +The clay-slate cleaves in the same W.N.W. and E.S.E. direction, as on +Navarin Island, on both sides of the Beagle Channel, on the eastern +side of Hoste Island, on the N.E. side of Hardy Peninsula, and on the +northern point of Wollaston Island; although in these two latter +localities the cleavage has been much obscured by the metamorphosed and +feldspathic condition of the slate. Within the area of these several +islands, including Navarin Island, the direction of the stratification +and of the mountain- chains is very obscure; though the mountains in +several places appeared to range in the same W.N.W. line with the +cleavage: the outline of the coast, however, does not correspond with +this line. Near the bifurcation of the Beagle Channel, where the +underlying metamorphic schists are first seen, they are foliated (with +some irregularities), in this same W.N.W. line, and parallel, as before +stated, to the main mountain-axis of this part of the country. Westward +of this main range, the metamorphic schists are foliated, though less +plainly, in the same direction, which is likewise common to the zone of +old erupted trappean rocks, forming the outermost islets. Hence the +area, over which the cleavage of the slate and the foliation of the +metamorphic schists extends with an average W.N.W. and E.S.E. strike, +is about forty miles in a north and south line, and ninety miles in an +east and west line. + +Further northward, near Port Famine, the stratification of the +clay-slate and of the associated rocks, is well defined, and there +alone the cleavage and strata-planes are parallel. A little north of +this port there is an anticlinal axis ranging N.W. (or a little more +westerly) and S.E.: south of the port, as far as Admiralty Sound and +Gabriel Channel, the outline of the land clearly indicates the +existence of several lines of elevation in this same N.W. direction, +which, I may add, is so uniform in the western half of the St. of +Magellan, that, as Captain King has remarked, “a parallel ruler placed +on the map upon the projecting points of the south shore, and extended +across the strait, will also touch the headlands on the opposite +coast.” (“Geographical Journal” volume 1 page 170.) It would appear, +from Captain King’s observations, that over all this area the cleavage +extends in the same line. Deep-water channels, however, in all parts of +Tierra del Fuego have burst through the trammels both of stratification +and cleavage; most of them may have been formed during the elevation of +the land by long- continued erosion, but others, for instance the +Beagle Channel, which stretches like a narrow canal for 120 miles +obliquely through the mountains, can hardly have thus originated. + +Finally, we have seen that in the extreme eastern point of Tierra del +Fuego, the cleavage and coast-lines extend W. and E. and even W.S.W. +and E.N.E.: over a large area westward, the cleavage, the main range of +mountains, and some subordinate ranges, but not the outlines of the +coast, strike W.N.W., and E.S.E.: in the central and western parts of +the St. of Magellan, the stratification, the mountain-ranges, the +outlines of the coast, and the cleavage all strike nearly N.W. and S.E. +North of the strait, the outline of the coast, and the mountains on the +mainland, run nearly north and south. Hence we see, at this southern +point of the continent, how gradually the Cordillera bend, from their +north and south course of so many thousand miles in length, into an E. +and even E.N.E. direction. + +WEST COAST, FROM THE SOUTHERN CHONOS ISLANDS TO NORTHERN CHILE. + +The first place at which we landed north of the St. of Magellan was +near Cape Tres Montes, in latitude 47 degrees S. Between this point and +the Northern Chonos Islands, a distance of 200 miles, the “Beagle” +visited several points, and specimens were collected for me from the +intermediate spaces by Lieutenant Stokes. The predominant rock is +mica-slate, with thick folia of quartz, very frequently alternating +with and passing into a chloritic, or into a black, glossy, often +striated, slightly anthracitic schist, which soils paper, and becomes +white under a great heat, and then fuses. Thin layers of feldspar, +swelling at intervals into well crystallised kernels, are sometimes +included in these black schists; and I observed one mass of the +ordinary black variety insensibly lose its fissile structure, and pass +into a singular mixture of chlorite, epidote, feldspar, and mica. Great +veins of quartz are numerous in the mica-schists; wherever these occur +the folia are much convoluted. In the southern part of the Peninsula of +Tres Montes, a compact altered feldspathic rock with crystals of +feldspar and grains of quartz is the commonest variety; this rock +exhibits occasionally traces of an original brecciated structure, and +often presents (like the altered state of Tierra del Fuego) traces of +cleavage- planes, which strike in the same direction with the folia of +mica-schist further northward. (The peculiar, abruptly conical form of +the hills in this neighbourhood, would have led any one at first to +have supposed that they had been formed of injected or intrusive rocks. +At Inchemo Island, a similar rock gradually becomes granulo-crystalline +and acquires scales of mica; and this variety at S. Estevan becomes +highly laminated, and though still exhibiting some rounded grains of +quartz, passes into the black, glossy, slightly anthracitic schist, +which, as we have seen, repeatedly alternates with and passes into the +micaceous and chloritic schists. Hence all the rocks on this line of +coast belong to one series, and insensibly vary from an altered +feldspathic clay-slate into largely foliated, true mica-schist. + +The cleavage of the homogeneous schists, the foliation of those +composed of more or less distinct minerals in layers, and the planes of +alternation of the different varieties or so-called stratification, are +all parallel, and preserve over this 200 miles of coast a remarkable +degree of uniformity in direction. At the northern end of the group, at +Low’s Harbour, the well- defined folia of mica-schist everywhere ranged +within eight degrees (or less than one point of the compass) of N. 19 +degrees W. and S. 19 degrees E.; and even the point of dip varied very +little, being always directed to the west and generally at an angle of +forty degrees; I should mention that I had here good opportunities of +observation, for I followed the naked rock on the beach, transversely +to the strike, for a distance of four miles and a half, and all the way +attended to the dip. Along the outer islands for 100 miles south of +Low’s Harbour, Lieutenant Stokes, during his boat- survey, kindly +observed for me the strike of the foliation, and he assures me that it +was invariably northerly, and the dip with one single exception to the +west. Further south at Vallenar Bay, the strike was almost universally +N. 25 degrees W. and the dip, generally at an angle of about 40 degrees +to W. 25 degrees S., but in some places almost vertical. Still farther +south, in the neighbourhood of the harbours of Anna Pink, S. Estevan +and S. Andres, and (judging from a distance) along the southern part of +Tres Montes, the foliation and cleavage extended in a line between [N. +11 degrees to 22 degrees W.] and [S. 11 degrees to 22 degrees E.]; and +the planes dipped generally westerly, but often easterly, at angles +varying from a gentle inclination to vertical. At A. Pink’s Harbour, +where the schists generally dipped easterly, wherever the angle became +very high, the strike changed from N. 11 degrees W. to even as much as +N. 45 degrees W.: in an analogous manner at Vallenar Bay, where the dip +was westerly (viz. on an average directed to W. 25 degrees S.), as soon +as the angle became very high, the planes struck in a line more than 25 +degrees west of north. The average result from all the observations on +this 200 miles of coast, is a strike of N. 19 degrees W. and S. 19 +degrees E.: considering that in each specified place my examination +extended over an area of several miles, and that Lieutenant Stokes’ +observations apply to a length of 100 miles, I think this remarkable +uniformity is pretty well established. The prevalence, throughout the +northern half of this line of coast, of a dip in one direction, that is +to the west, instead of being sometimes west and sometimes east, is, +judging from what I have elsewhere seen, an unusual circumstance. In +Brazil, La Plata, the Falkland Islands, and Tierra del Fuego, there is +generally an obvious relation between the axis of elevation, the +outline of the coast, and the strike of the cleavage or foliation: in +the Chonos Archipelago, however, neither the minor details of the +coast-line, nor the chain of the Cordillera, nor the subordinate +transverse mountain-axes, accord with the strike of the foliation and +cleavage: the seaward face of the numerous islands composing this +Archipelago, and apparently the line of the Cordillera, range N. 11 +degrees E., whereas, as we have just seen, the average strike of the +foliation is N. 19 degrees W. + +There is one interesting exception to the uniformity in the strike of +the foliation. At the northern point of Tres Montes (latitude 45 +degrees 52 minutes) a bold chain of granite, between two and three +thousand feet in height, runs from the coast far into the interior, in +an E.S.E. line, or more strictly E. 28 degrees S. and W. 28 degrees N. +(In the distance, other mountains could be seen apparently ranging +N.N.E. and S.S.W. at right angles to this one. I may add, that not far +from Vallenar Bay there is a fine range, apparently of granite, which +has burst through the mica-slate in a N.E. by E. and S.W. by S. line.) +In a bay, at the northern foot of this range, there are a few islets of +mica-slate, with the folia in some parts horizontal, but mostly +inclined at an average angle of 20 degrees to the north. On the +northern steep flank of the range, there are a few patches (some quite +isolated, and not larger than half a-crown!) of the mica-schist, +foliated with the same northerly dip. On the broad summit, as far as +the southern crest, there is much mica-slate, in some places even 400 +feet in thickness, with the folia all dipping north, at angles varying +from 5 degrees to 20 degrees, but sometimes mounting up to 30 degrees. +The southern flank consists of bare granite. The mica-slate is +penetrated by small veins of granite, branching from the main body. +(The granite within these veins, as well as generally at the junction +with the mica-slate, is more quartzose than elsewhere. The granite, I +may add, is traversed by dikes running for a very great length in the +line of the mountains; they are composed of a somewhat laminated +eurite, containing crystals of feldspar, hornblende, and octagons of +quartz.) Leaving out of view the prevalent strike of the folia in other +parts of this Archipelago, it might have been expected that they would +have dipped N. 28 degrees E., that is directly from the ridge, and, +considering its abruptness, at a high inclination; but the real dip, as +we have just seen, both at the foot and on the northern flank, and over +the entire summit, is at a small angle, and directed nearly due north. +From these considerations it occurred to me, that perhaps we here had +the novel and curious case of already inclined laminae obliquely tilted +at a subsequent period by the granitic axis. Mr. Hopkins, so well known +from his mathematical investigations, has most kindly calculated the +problem: the proposition sent was,—Take a district composed of laminae, +dipping at an angle of 40 degrees to W. 19 degrees S., and let an axis +of elevation traverse it in an E. 28 degrees S. line, what will the +position of the laminae be on the northern flank after a tilt, we will +first suppose, of 45 degrees? Mr. Hopkins informs me, that the angle of +the dip will be 28 degrees 31 minutes, and its direction to north 30 +degrees 33 minutes west. (On the south side of the axis (where, +however, I did not see any mica-slate) the dip of the folia would be at +an angle of 77 degrees 55 minutes, directed to west 35 degrees 33 +minutes south. Hence the two points of dip on the opposite sides of the +range, instead of being as in ordinary cases directly opposed to each +other at an angle of 180 degrees, would here be only 86 degrees 50 +minutes apart.) By varying the supposed angle of the tilt, our +previously inclined folia can be thrown into any angle between 26 +degrees, which is the least possible angle, and 90 degrees; but if a +small inclination be thus given to them, their point of dip will depart +far from the north, and therefore not accord with the actual position +of the folia of mica-schist on our granitic range. Hence it appears +very difficult, without varying considerably the elements of the +problem, thus to explain the anomalous strike and dip of the foliated +mica- schist, especially in those parts, namely, at the base of the +range, where the folia are almost horizontal. Mr. Hopkins, however, +adds, that great irregularities and lateral thrusts might be expected +in every great line of elevation, and that these would account for +considerable deviations from the calculated results: considering that +the granitic axis, as shown by the veins, has indisputably been +injected after the perfect formation of the mica-slate, and considering +the uniformity of the strike of the folia throughout the rest of the +Archipelago, I cannot but still think that their anomalous position at +this one point is someway directly and mechanically related to the +intrusion of this W.N.W. and E.S.E. mountain-chain of granite. + +Dikes are frequent in the metamorphic schists of the Chonos Islands, +and seem feebly to represent that great band of trappean and ancient +volcanic rocks on the south-western coast of Tierra del Fuego. At S. +Andres I observed in the space of half-a-mile, seven broad, parallel +dikes, composed of three varieties of trap, running in a N.W. and S.E. +line, parallel to the neighbouring mountain-ranges of altered +clay-slate; but they must be of long subsequent origin to these +mountains; for they intersected the volcanic formation described in the +last chapter. North of Tres Montes, I noticed three dikes differing +from each other in composition, one of them having a euritic base +including large octagons of quartz; these dikes, as well as several of +porphyritic greenstone at Vallenar Bay, extended N.E. and S.W., nearly +at right angles to the foliation of the schists, but in the line of +their joints. At Low’s Harbour, however, a set of great parallel dikes, +one ninety yards and another sixty yards in width, have been guided by +the foliation of the mica-schist, and hence are inclined westward at an +angle of 45 degrees: these dikes are formed of various porphyritic +traps, some of which are remarkable from containing numerous rounded +grains of quartz. A porphyritic trap of this latter kind, passed in one +of the dikes into a most curious hornstone, perfectly white, with a +waxy fracture and pellucid edges, fusible, and containing many grains +of quartz and specks of iron pyrites. In the ninety-yard dike several +large, apparently now quite isolated, fragments of mica-slate were +embedded: but as their foliation was exactly parallel to that of the +surrounding solid rock, no doubt these new separate fragments +originally formed wedge-shaped depending portions of a continuous vault +or crust, once extending over the dike, but since worn down and +denuded. + +CHILOE, VALDIVIA, CONCEPCION. + +In Chiloe, a great formation of mica-schist strikingly resembles that +of the Chonos Islands. For a space of eleven miles on the S.E. coast, +the folia were very distinct, though slightly convoluted, and ranged +within a point of N.N.W. and S.S.E., dipping either E.N.E. or more +commonly W.S.W., at an average angle of 22 degrees (in one spot, +however, at 60 degrees), and therefore decidedly at a lesser +inclination than amongst the Chonos Islands. On the west and +north-western shores, the foliation was often obscure, though, where +best defined, it ranged within a point of N. by W. and S. by E., +dipping either easterly or westerly, at varying and generally very +small angles. Hence, from the southern part of Tres Montes to the +northern end of Chiloe, a distance of 300 miles, we have closely allied +rocks with their folia striking on an average in the same direction, +namely between N. 11 degrees and 22 degrees W. Again, at Valdivia, we +meet with the same mica-schist, exhibiting nearly the same +mineralogical passages as in the Chonos Archipelago, often, however, +becoming more ferruginous, and containing so much feldspar as to pass +into gneiss. The folia were generally well defined; but nowhere else in +South America did I see them varying so much in direction: this seemed +chiefly caused by their forming parts, as I could sometimes distinctly +trace, of large flat curves: nevertheless, both near the settlement and +towards the interior, a N.W. and S.E. strike seemed more frequent than +any other direction; the angle of the dip was generally small. At +Concepcion, a highly glossy clay-slate had its cleavage often slightly +curvilinear, and inclined, seldom at a high angle, towards various +points of the compass: but here, as at Valdivia, a N.W. and S.E. strike +seemed to be the most frequent one. ((FIGURE 23.) I observed in some +parts that the tops of the laminae of the clay-slate (b in Figure 23) +under the superficial detritus and soil (a) were bent, sometimes +without being broken, as represented in Figure 23, which is copied from +one given by Sir H. De la Beche (page 42 “Geological Manual”) of an +exactly similar phenomenon in Devonshire. Mr. R.A.C. Austen, also, in +his excellent paper on S.E. Devon (“Geological Transactions” volume 6 +page 437), has described this phenomenon; he attributes it to the +action of frosts, but at the same time doubts whether the frosts of the +present day penetrate to a sufficient depth. As it is known that +earthquakes particularly affect the surface of the ground, it occurred +to me that this appearance might perhaps be due, at least at +Concepcion, to their frequent occurrence; the superficial layers of +detritus being either jerked in one direction, or, where the surface +was inclined, pushed a little downwards during each strong vibration. +In North Wales I have seen a somewhat analogous but less regular +appearance, though on a greater scale (“London Philosophical Magazine” +volume 21 page 184), and produced by a quite different cause, namely, +by the stranding of great icebergs; this latter appearance has also +been observed in N. America.) + +In certain spots large quartz veins were numerous, and near them, the +cleavage, as was the case with the foliation of the schists in the +Chonos Archipelago, became extremely tortuous. + +At the northern end of Quiriquina Island, in the Bay of Concepcion, at +least eight rudely parallel dikes, which have been guided to a certain +extent by the cleavage of the slate, occur within the space of a +quarter of a mile. They vary much in composition, resembling in many +respects the dikes at Low’s Harbour: the greater number consist of +feldspathic porphyries, sometimes containing grains of quartz: one, +however, was black and brilliant, like an augitic rock, but really +formed of feldspar; others of a feldspathic nature were perfectly +white, with either an earthy or crystalline fracture, and including +grains and regular octagons of quartz; these white varieties passed +into ordinary greenstones. Although, both here and at Low’s Harbour, +the nature of the rock varied considerably in the same dike, yet I +cannot but think that at these two places and in other parts of the +Chonos group, where the dikes, though close to each other and running +parallel, are of different composition, that they must have been formed +at different periods. In the case of Quiriquina this is a rather +interesting conclusion, for these eight parallel dikes cut through the +metamorphic schists in a N.W. and S.E. line, and since their injection +the overlying cretaceous or tertiary strata have been tilted (whilst +still under the sea) from a N.W. by N. and S.E. by S. line; and again, +during the great earthquake of February 1835, the ground in this +neighbourhood was fissured in N.W. and S.E. lines; and from the manner +in which buildings were thrown down, it was evident that the surface +undulated in this same direction. (“Geological Transactions” volume 6 +pages 602 and 617. “Journal of Researches” 2nd edition page 307.) + +CENTRAL AND NORTHERN CHILE. + +Northward of Concepcion, as far as Copiapo, the shores of the Pacific +consist, with the exception of some small tertiary basins, of gneiss, +mica- schist, altered clay-slate, granite, greenstone and syenite: +hence the coast from Tres Montes to Copiapo, a distance of 1,200 miles, +and I have reason to believe for a much greater space, is almost +similarly constituted. + +Near Valparaiso the prevailing rock is gneiss, generally including much +hornblende: concretionary balls formed of feldspar, hornblende and +mica, from two or three feet in diameter, are in very many places +conformably enfolded by the foliated gneiss: veins of quartz and +feldspar, including black schorl and well-crystallised epidote, are +numerous. Epidote likewise occurs in the gneiss in thin layers, +parallel to the foliation of the mass. One large vein of a coarse +granitic character was remarkable from in one part quite changing its +character, and insensibly passing into a blackish porphyry, including +acicular crystals of glassy feldspar and of hornblende: I have never +seen any other such case. (Humboldt “Personal Narrative” volume 4 page +60, has described with much surprise, concretionary balls, with +concentric divisions, composed of partially vitreous feldspar, +hornblende, and garnets, included within great veins of gneiss, which +cut across the mica-slate near Venezuela.) + +I shall in the few following remarks on the rocks of Chile allude +exclusively to their foliation and cleavage. In the gneiss round +Valparaiso the strike of the foliation is very variable, but I think +about N. by W. and S. by E. is the commonest direction; this likewise +holds good with the cleavage of the altered feldspathic clay-slates, +occasionally met with on the coast for ninety miles north of +Valparaiso. Some feldspathic slate, alternating with strata of +claystone porphyry in the Bell of Quillota and at Jajuel, and +therefore, perhaps, belonging to a later period than the metamorphic +schists on the coast, cleaved in this same direction. In the Eastern +Cordillera, in the Portillo Pass, there is a grand mass of mica- slate, +foliated in a north and south line, and with a high westerly dip: in +the Uspallata range, clay-slate and grauwacke have a highly inclined, +nearly north and south cleavage, though in some parts the strike is +irregular: in the main or Cumbre range, the direction of the cleavage +in the feldspathic clay-slate is N.W. and S.E. + +Between Coquimbo and Guasco there are two considerable formations of +mica- slate, in one of which the rock passed sometimes into common +clay-slate and sometimes into a glossy black variety, very like that in +the Chonos Archipelago. The folia and cleavage of these rocks ranged +between [N. and N.W. by N.] and [S. and S.W. by S.]. Near the Port of +Guasco several varieties of altered clay-slate have a quite irregular +cleavage. Between Guasco and Copiapo, there are some siliceous and +talcaceous slates cleaving in a north and south line, with an easterly +dip of between 60 and 70 degrees: high up, also, the main valley of +Copiapo, there is mica-slate with a high easterly dip. In the whole +space between Valparaiso and Copiapo an easterly dip is much more +common than an opposite or westerly one. + +CONCLUDING REMARKS ON CLEAVAGE AND FOLIATION. + +In this southern part of the southern hemisphere, we have seen that the +cleavage-laminae range over wide areas with remarkable uniformity, +cutting straight through the planes of stratification, but yet being +parallel in strike to the main axes of elevation, and generally to the +outlines of the coast. (In my paper on the Falkland Islands “Geological +Journal” volume 3 page 267, I have given a curious case on the +authority of Captain Sulivan, R.N., of much folded beds of clay-slate, +in some of which the cleavage is perpendicular to the horizon, and in +others it is perpendicular to each curvature or fold of the bed: this +appears a new case.) The dip, however, is as variable, both in angle +and in direction (that is, sometimes being inclined to the one side and +sometimes to the directly opposite side), as the strike is uniform. In +all these respects there is a close agreement with the facts given by +Professor Sedgwick in his celebrated memoir in the “Geological +Transactions,” and by Sir R.I. Murchison in his various excellent +discussions on this subject. The Falkland Islands, and more especially +Tierra del Fuego, offer striking instances of the lines of cleavage, +the principle axes of elevation, and the outlines of the coast, +gradually changing together their courses. The direction which prevails +throughout Tierra del Fuego and the Falkland Islands, namely, from west +with some northing to east with some southing, is also common to the +several ridges in Northern Patagonia and in the western parts of Banda +Oriental: in this latter province, in the Sierra Tapalguen, and in the +Western Falkland Island, the W. by N., or W.N.W. and E.S.E., ridges, +are crossed at right angles by others ranging N.N.E. and S.S.W. + +The fact of the cleavage-laminae in the clay-slate of Tierra del Fuego, +where seen cutting straight through the planes of stratification, and +where consequently there could be no doubt about their nature, +differing slightly in colour, texture, and hardness, appears to me very +interesting. In a thick mass of laminated, feldspathic and altered +clay-slate, interposed between two great strata of porphyritic +conglomerate in Central Chile, and where there could be but little +doubt about the bedding, I observed similar slight differences in +composition, and likewise some distinct thin layers of epidote, +parallel to the highly inclined cleavage of the mass. Again, I +incidentally noticed in North Wales, where glaciers had passed over the +truncated edges of the highly inclined laminae of clay-slate, that the +surface, though smooth, was worn into small parallel undulations, +caused by the competent laminae being of slightly different degrees of +hardness. (“London Philosophical Magazine” volume 21 page 182.) With +reference to the slates of North Wales, Professor Sedgwick describes +the planes of cleavage, as “coated over with chlorite and +semi-crystalline matter, which not only merely define the planes in +question, but strike in parallel flakes through the whole mass of the +rock.” (“Geological Transactions” volume 3 page 471.) In some of those +glossy and hard varieties of clay-slate, which may often be seen +passing into mica-schist, it has appeared to me that the cleavage- +planes were formed of excessively thin, generally slighted convoluted, +folia, composed of microscopically minute scales of mica. From these +several facts, and more especially from the case of the clay-slate in +Tierra del Fuego, it must, I think, be concluded, that the same power +which has impressed on the slate its fissile structure or cleavage has +tended to modify its mineralogical character in parallel planes. + +Let us now turn to the foliation of the metamorphic schists, a subject +which has been much less attended to. As in the case of +cleavage-laminae, the folia preserve over very large areas a uniform +strike: thus Humboldt found for a distance of 300 miles in Venezuela, +and indeed over a much larger space, gneiss, granite, mica, and +clay-slate, striking very uniformly N.E. and S.W., and dipping at an +angle of between 60 and 70 degrees to N.W. (“Personal Narrative” volume +6 page 59 et seq.); it would even appear from the facts given in this +chapter, that the metamorphic rocks throughout the north-eastern part +of South America are generally foliated within two points of N.E. and +S.W. Over the eastern parts of Banda Oriental, the foliation strikes +with a high inclination, very uniformly N.N.E. to S.S.W., and over the +western parts, in a W. by N. and E. by S. line. For a space of 300 +miles on the shores of the Chonos and Chiloe Islands, we have seen that +the foliation seldom deviates more than a point of the compass from a +N. 19 degrees W. and S. 19 degrees E. strike. As in the case of +cleavage, the angle of the dip in foliated rocks is generally high but +variable, and alternates from one side of the line of strike to the +other side, sometimes being vertical: in the Northern Chonos Islands, +however, the folia are inclined almost always to the west; in nearly +the same manner, the cleavage-laminae in Southern Tierra del Fuego +certainly dip much more frequently to S.S.W. than to the opposite +point. In Eastern Banda Oriental, in parts of Brazil, and in some other +districts, the foliation runs in the same direction with the +mountain-ranges and adjoining coast-lines: amongst the Chonos Islands, +however, this coincidence fails, and I have given my reasons for +suspecting that one granitic axis has burst through and tilted the +already inclined folia of mica-schist: in the case of cleavage, the +coincidence between its strike and that of the main stratification +seems sometimes to fail. (Cases are given by Mr. Jukes in his “Geology +of Newfoundland” page 130.) Foliation and cleavage resemble each other +in the planes winding round concretions, and in becoming tortuous where +veins of quartz abound. (I have seen in Brazil and Chile concretions +thus enfolded by foliated gneiss; and Macculloch “Highlands” volume 1 +page 64, has described a similar case. For analogous cases in +clay-slate, see Professor Henslow’s Memoir in “Cambridge Philosophical +Transactions” volume 1 page 379, and Macculloch’s “Classification of +Rocks” page 351. With respect to both foliation and cleavage becoming +tortuous where quartz-veins abound, I have seen instances near Monte +Video, at Concepcion, and in the Chonos Islands. See also Mr. +Greenough’s “Critical Examination” page 78.) On the flanks of the +mountains both in Tierra del Fuego and in other countries, I have +observed that the cleavage-planes frequently dip at a high angle +inwards; and this was long ago observed by Von Buch to be the case in +Norway: this fact is perhaps analogous to the folded, fan-like or +radiating structure in the metamorphic schists of the Alps, in which +the folia in the central crests are vertical and on the two flanks +inclined inwards. (Studer in “Edinburgh New Philosophical Journal” +volume 23 page 144.) Where masses of fissile and foliated rocks +alternate together, the cleavage and foliation, in all cases which I +have seen, are parallel. Where in one district the rocks are fissile, +and in another adjoining district they are foliated, the planes of +cleavage and foliation are likewise generally parallel: this is the +case with the feldspathic homogeneous slates in the southern part of +the Chonos group, compared with the fine foliated mica-schists of the +northern part; so again the clay- slate of the whole eastern side of +Tierra del Fuego cleaves in exactly the same line with the foliated +gneiss and mica-slate of the western coast; other analogous instances +might have been adduced. (I have given a case in Australia. See my +“Volcanic Islands.”) + +With respect to the origin of the folia of quartz, mica, feldspar, and +other minerals composing the metamorphic schists, Professor Sedgwick, +Mr. Lyell, and most authors believe, that the constituent parts of each +layer were separately deposited as sediment, and then metamorphosed. +This view, in the majority of cases, I believe to be quite untenable. +In those not uncommon instances, where a mass of clay-slate, in +approaching granite, gradually passes into gneiss, we clearly see that +folia of distinct minerals can originate through the metamorphosis of a +homogeneous fissile rock. (I have described in “Volcanic Islands” a +good instance of such a passage at the Cape of Good Hope.) The +deposition, it may be remarked, of numberless alternations of pure +quartz, and of the elements of mica or feldspar does not appear a +probable event. (See some excellent remarks on this subject, in +D’Aubuisson’s “Traite de Geog.” tome 1 page 297. Also some remarks by +Mr. Dana in “Silliman’s American Journal” volume 45 page 108.) In those +districts in which the metamorphic schists are foliated in planes +parallel to the cleavage of the rocks in an adjoining district, are we +to believe that the folia are due to sedimentary layers, whilst the +cleavage- laminae, though parallel, have no relation whatever to such +planes of deposition? On this view, how can we reconcile the vastness +of the areas over which the strike of the foliation is uniform, with +what we see in disturbed districts composed of true strata: and +especially, how can we understand the high and even vertical dip +throughout many wide districts, which are not mountainous, and +throughout some, as in Western Banda Oriental, which are not even +hilly? Are we to admit that in the northern part of the Chonos +Archipelago, mica-slate was first accumulated in parallel horizontal +folia to a thickness of about four geographical miles, and then +upturned at an angle of forty degrees; whilst, in the southern part of +this same Archipelago, the cleavage-laminae of closely allied rocks, +which none would imagine had ever been horizontal, dip at nearly the +same angle, to nearly the same point? + +Seeing, then, that foliated schists indisputably are sometimes produced +by the metamorphosis of homogeneous fissile rocks; seeing that +foliation and cleavage are so closely analogous in the several +above-enumerated respects; seeing that some fissile and almost +homogeneous rocks show incipient mineralogical changes along the planes +of their cleavage, and that other rocks with a fissile structure +alternate with, and pass into varieties with a foliated structure, I +cannot doubt that in most cases foliation and cleavage are parts of the +same process: in cleavage there being only an incipient separation of +the constituent minerals; in foliation a much more complete separation +and crystallisation. + +The fact often referred to in this chapter, of the foliation and the +so- called strata in the metamorphic series,—that is, the alternating +masses of different varieties of gneiss, mica-schist, and +hornblende-slate, etc.,—being parallel to each other, at first appears +quite opposed to the view, that the folia have no relation to the +planes of original deposition. Where the so-called beds are not very +thick and of widely different mineralogical composition from each +other, I do not think that there is any difficulty in supposing that +they have originated in an analogous manner with the separate folia. We +should bear in mind what thick strata, in ordinary sedimentary masses, +have obviously been formed by a concretionary process. In a pile of +volcanic rocks on the Island of Ascension, there are strata, differing +quite as much in appearance as the ordinary varieties of the +metamorphic schists, which undoubtedly have been produced, not by +successive flowings of lava, but by internal molecular changes. Near +Monte Video, where the stratification, as it would be called, of the +metamorphic series is, in most parts, particularly well developed, +being as usual, parallel to the foliation, we have seen that a mass of +chloritic schist, netted with quartz-veins, is entangled in gneiss, in +such a manner as to show that it had certainly originated in some +process of segregation: again, in another spot, the gneiss tended to +pass into hornblendic schist by alternating with layers of quartz; but +these layers of quartz almost certainly had never been separately +deposited, for they were absolutely continuous with the numerous +intersecting veins of quartz. I have never had an opportunity of +tracing for any distance, along the line both of strike and of dip, the +so-called beds in the metamorphic schists, but I strongly suspect that +they would not be found to extend with the same character, very far in +the line either of their dip or strike. Hence I am led to believe, that +most of the so-called beds are of the nature of complex folia, and have +not been separately deposited. Of course, this view cannot be extended +to THICK masses included in the metamorphic series, which are of +totally different composition from the adjoining schists, and which are +far extended, as is sometimes the case with quartz and marble; these +must generally be of the nature of true strata. (Macculloch +“Classification of Rocks” page 364, states that primary limestones are +often found in irregular masses or great nodules, “which can scarcely +be said to possess a stratified shape!”) Such strata, however, will +almost always strike in the same direction with the folia, owing to the +axes of elevation being in most countries parallel to the strike of the +foliation; but they will generally dip at a different angle from that +of the foliation; and the angle of the foliation in itself almost +always varies much: hence, in crossing a metamorphosed schistose +district, it would require especial attention to discriminate between +true strata of deposition and complex foliated masses. The mere +presence of true strata in the midst of a set of metamorphic schists, +is no argument that the foliation is of sedimentary origin, without it +be further shown in each case, that the folia not only strike, but dip +throughout in parallel planes with those of the true stratification. + +As in some cases it appears that where a fissile rock has been exposed +to partial metamorphic action, for instance from the irruption of +granite, the foliation has supervened on the already existing +cleavage-planes; so perhaps in some instances, the foliation of a rock +may have been determined by the original planes of deposition or of +oblique current-laminae: I have, however, myself, never seen such a +case, and I must maintain that in most extensive metamorphic areas, the +foliation is the extreme result of that process, of which cleavage is +the first effect. That foliation may arise without any previous +structural arrangement in the mass, we may infer from injected, and +therefore once liquified, rocks, both of volcanic and plutonic origin, +sometimes having a “grain” (as expressed by Professor Sedgwick), and +sometimes being composed of distinct folia or laminae of different +compositions. In my work on “Volcanic Islands,” I have given several +instances of this structure in volcanic rocks, and it is not uncommonly +seen in plutonic masses—thus, in the Cordillera of Chile, there are +gigantic mountain-like masses of red granite, which have been injected +whilst liquified, and which, nevertheless, display in parts a decidedly +laminar structure. (As remarked in a former part of this chapter, I +suspect that the boldly conical mountains of gneiss-granite, near Rio +de Janeiro, in which the constituent minerals are arranged in parallel +planes, are of intrusive origin. We must not, however, forget the +lesson of caution taught by the curious claystone porphyries of Port +Desire, in which we have seen that the breaking up and aggregation of a +thinly stratified tufaceous mass, has yielded a rock semi-porphyritic +with crystals of feldspar, arranged in the planes of original +deposition.) + +Finally, we have seen that the planes of cleavage and of foliation, +that is, of the incipient process and of the final result, generally +strike parallel to the principal axes of elevation, and to the outline +of the land: the strike of the axes of elevation (that is, of the lines +of fissures with the strata on their edges upturned), according to the +reasoning of Mr. Hopkins, is determined by the form of the area +undergoing changes of level, and the consequent direction of the lines +of tension and fissure. Now, in that remarkable pile of volcanic rocks +at Ascension, which has several times been alluded to (and in some +other cases), I have endeavoured to show, that the lamination of the +several varieties, and their alternations, have been caused by the +moving mass, just before its final consolidation, having been subjected +(as in a glacier) to planes of different tension; this difference in +the tension affecting the crystalline and concretionary processes. (In +“Volcanic Islands.”) One of the varieties of rock thus produced at +Ascension, at first sight, singularly resembles a fine-grained gneiss; +it consists of quite straight and parallel zones of excessive tenuity, +of more or less coloured crystallised feldspar, of distinct crystals of +quartz, diopside, and oxide of iron. These considerations, +notwithstanding the experiments made by Mr. Fox, showing the influence +of electrical currents in producing a structure like that of cleavage, +and notwithstanding the apparently inexplicable variation, both in the +inclination of the cleavage-laminae and in their dipping first to one +side and then to the other side of the line of strike, lead me to +suspect that the planes of cleavage and foliation are intimately +connected with the planes of different tension, to which the area was +long subjected, AFTER the main fissures or axes of upheavement had been +formed, but BEFORE the final consolidation of the mass and the total +cessation of all molecular movement. + + + + +CHAPTER VII. +CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA. + + +Central Chile.—Basal formations of the Cordillera.—Origin of the +porphyritic clay-stone conglomerate.—Andesite.—Volcanic rocks.—Section +of the Cordillera by the Peuquenes are Portillo Pass.—Great gypseous +formation.—Peuquenes line; thickness of strata, fossils of.—Portillo +line.—Conglomerate, orthitic granite, mica-schist, volcanic rocks +of.—Concluding remarks on the denudation and elevation of the Portillo +line.—Section by the Cumbre, or Uspallata Pass.—Porphyries.—Gypseous +strata.—Section near the Puente del Inca; fossils of.—Great +subsidence.—Intrusive porphyries.—Plain of Uspallata.—Section of the +Uspallata chain.—Structure and nature of the strata.—Silicified +vertical trees.—Great subsidence.—Granitic rocks of axis.—Concluding +remarks on the Uspallata range; origin subsequent to that of the main +Cordillera; two periods of subsidence; comparison with the Portillo +chain.— + + +The district between the Cordillera and the Pacific, on a rude average, +is from about eighty to one hundred miles in width. It is crossed by +many chains of mountains, of which the principal ones, in the latitude +of Valparaiso and southward of it, range nearly north and south; but in +the more northern parts of the province, they run in almost every +possible direction. Near the Pacific, the mountain-ranges are generally +formed of syenite or granite, and or of an allied euritic porphyry; in +the low country, besides these granitic rocks and greenstone, and much +gneiss, there are, especially northward of Valparaiso, some +considerable districts of true clay-slate with quartz veins, passing +into a feldspathic and porphyritic slate; there is also some grauwacke +and quartzose and jaspery rocks, the latter occasionally assuming the +character of the basis of claystone porphyry: trap-dikes are numerous. +Nearer the Cordillera the ranges (such as those of S. Fernando, the +Prado (Meyen “Reise um Erde” th. 1 s. 235.), and Aconcagua) are formed +partly of granitic rocks, and partly of purple porphyritic +conglomerates, claystone porphyry, greenstone porphyry, and other +rocks, such as we shall immediately see, form the basal strata of the +main Cordillera. In the more northern parts of Chile, this porphyritic +series extends over large tracts of country far from the Cordillera; +and even in Central Chile such occasionally occur in outlying +positions. + +I will describe the Campana of Quillota, which stands only fifteen +miles from the Pacific, as an instance of one of these outlying masses. +This hill is conspicuous from rising to the height of 6,400 feet: its +summit shows a nucleus, uncovered for a height of 800 feet, of fine +greenstone, including epidote and octahedral magnetic iron ore; its +flanks are formed of great strata of porphyritic claystone conglomerate +associated with various true porphyries and amygdaloids, alternating +with thick masses of a highly feldspathic, sometimes porphyritic, +pale-coloured slaty rock, with its cleavage-laminae dipping inwards at +a high angle. At the base of the hill there are syenites, a granular +mixture of quartz and feldspar, and harsh quartzose rocks, all +belonging to the basal metamorphic series. I may observe that at the +foot of several hills of this class, where the porphyries are first +seen (as near S. Fernando, the Prado, Las Vacas, etc.), similar harsh +quartzose rocks and granular mixtures of quartz and feldspar occur, as +if the more fusible constituent parts of the granitic series had been +drawn off to form the overlying porphyries. + +In Central Chile, the flanks of the main Cordillera, into which I +penetrated by four different valleys, generally consist of distinctly +stratified rocks. The strata are inclined at angles varying from +sometimes even under ten, to twenty degrees, very rarely exceeding +forty degrees: in some, however, of the quite small, exterior, +spur-like ridges, the inclination was not unfrequently greater. The dip +of the strata in the main outer lines was usually outwards or from the +Cordillera, but in Northern Chile frequently inwards,—that is, their +basset-edges fronted the Pacific. Dikes occur in extraordinary numbers. +In the great, central, loftiest ridges, the strata, as we shall +presently see, are almost always highly inclined and often vertical. +Before giving a detailed account of my two sections across the +Cordillera, it will, I think, be convenient to describe the basal +strata as seen, often to a thickness of four or five thousand feet, on +the flanks of the outer lines. + +BASAL STRATA OF THE CORDILLERA. + +The prevailing rock is a purplish or greenish, porphyritic claystone +conglomerate. The embedded fragments vary in size from mere particles +to blocks as much as six or eight inches (rarely more) in diameter; in +many places, where the fragments were minute, the signs of aqueous +deposition were unequivocally distinct; where they were large, such +evidence could rarely be detected. The basis is generally porphyritic +with perfect crystals of feldspar, and resembles that of a true +injected claystone porphyry: often, however, it has a mechanical or +sedimentary aspect, and sometimes (as at Jajuel) is jaspery. The +included fragments are either angular, or partially or quite rounded +(Some of the rounded fragments in the porphyritic conglomerate near the +Baths of Cauquenes, were marked with radii and concentric zones of +different shades of colour: any one who did not know that pebbles, for +instance flint pebbles from the chalk, are sometimes zoned +concentrically with their worn and rounded surfaces, might have been +led to infer, that these balls of porphyry were not true pebbles, but +had originated in concretionary action.); in some parts the rounded, in +others the angular fragments prevail, and usually both kinds are mixed +together: hence the word BRECCIA ought strictly to be appended to the +term PORPHYRITIC CONGLOMERATE. The fragments consist of many varieties +of claystone porphyry, usually of nearly the same colour with the +surrounding basis, namely, purplish-reddish, brownish, mottled or +bright green; occasionally fragments of a laminated, pale-coloured, +feldspathic rock, like altered clay-slate are included; as are +sometimes grains of quartz, but only in one instance in Central Chile +(namely, at the mines of Jajuel) a few pebbles of quartz. I nowhere +observed mica in this formation, and rarely hornblende; where the +latter mineral did occur, I was generally in doubt whether the mass +really belonged to this formation, or was of intrusive origin. +Calcareous spar occasionally occurs in small cavities; and nests and +layers of epidote are common. In some few places in the finer-grained +varieties (for instance, at Quillota), there were short, interrupted +layers of earthy feldspar, which could be traced, exactly as at Port +Desire, passing into large crystals of feldspar: I doubt, however, +whether in this instance the layers had ever been separately deposited +as tufaceous sediment. + +All the varieties of porphyritic conglomerates and breccias pass into +each other, and by innumerable gradations into porphyries no longer +retaining the least trace of mechanical origin: the transition appears +to have been effected much more easily in the finer-grained, than in +the coarser-grained varieties. In one instance, near Cauquenes, I +noticed that a porphyritic conglomerate assumed a spheroidal structure, +and tended to become columnar. Besides the porphyritic conglomerates +and the perfectly characterised porphyries, of metamorphic origin, +there are other porphyries, which, though differing not at all or only +slightly in composition, certainly have had a different origin: these +consist of pink or purple claystone porphyries, sometimes including +grains of quartz,—of greenstone porphyry, and of other dusky rocks, all +generally porphyritic with fine, large, tabular, opaque crystals, often +placed crosswise, of feldspar cleaving like albite (judging from +several measurements), and often amygdaloidal with silex, agate, +carbonate of lime, green and brown bole. (This bole is a very common +mineral in the amygdaloidal rocks; it is generally of a greenish- brown +colour, with a radiating structure; externally it is black with an +almost metallic lustre, but often coated by a bright green film. It is +soft and can be scratched by a quill; under the blowpipe swells greatly +and becomes scaly, then fuses easily into a black magnetic bead. This +substance is evidently similar to that which often occurs in submarine +volcanic rocks. An examination of some very curious specimens of a fine +porphyry (from Jajuel) leads me to suspect that some of these +amygdaloidal balls, instead of having been deposited in pre-existing +air-vesicles, are of concretionary origin; for in these specimens, some +of the pea-shaped little masses (often externally marked with minute +pits) are formed of a mixture of green earth with stony matter, like +the basis of the porphyry, including minute imperfect crystals of +feldspar; and these pea-shaped little masses are themselves +amygdaloidal with minute spheres of the green earth, each enveloped by +a film of white, apparently feldspathic, earthy matter: so that the +porphyry is doubly amygdaloidal. It should not, however, be overlooked, +that all the strata here have undergone metamorphic action, which may +have caused crystals of feldspar to appear, and other changes to be +effected, in the originally simple amygdaloidal balls. Mr. J.D. Dana, +in an excellent paper on Trap-rocks “Edinburgh New Philosophical +Journal” volume 41 page 198, has argued with great force, that all +amygdaloidal minerals have been deposited by aqueous infiltration. I +may take this opportunity of alluding to a curious case, described in +my work on “Volcanic Islands,” of an amygdaloid with many of its cells +only half filled up with a mesotypic mineral. M. Rose has described an +amygdaloid, brought by Dr. Meyen “Reise um Erde” Th. 1. s. 316, from +Chile, as consisting of crystallised quartz, with crystals of stilbite +within, and lined externally by green earth.) These several porphyritic +and amygdaloidal varieties never show any signs of passing into masses +of sedimentary origin: they occur both in great and small intrusive +masses, and likewise in strata alternating with those of the +porphyritic conglomerate, and with the planes of junction often quite +distinct, yet not seldom blended together. In some of these intrusive +masses, the porphyries exhibit, more or less plainly, a brecciated +structure, like that often seen in volcanic masses. These brecciated +porphyries could generally be distinguished at once from the +metamorphosed, porphyritic breccia- conglomerates, by all the fragments +being angular and being formed of the same variety, and by the absence +of every trace of aqueous deposition. One of the porphyries above +specified, namely, the greenstone porphyry with large tabular crystals +of albite, is particularly abundant, and in some parts of the +Cordillera (as near St. Jago) seemed more common even than the purplish +porphyritic conglomerate. Numerous dikes likewise consist of this +greenstone porphyry; others are formed of various fine-grained trappean +rocks; but very few of claystone porphyry: I saw no true basaltic +dikes. + +In several places in the lower part of the series, but not everywhere, +thick masses of a highly feldspathic, often porphyritic, slaty rock +occur interstratified with the porphyritic conglomerate; I believe in +one or two cases blackish limestone has been found in a similar +position. The feldspathic rock is of a pale grey or greenish colour; it +is easily fusible; where porphyritic, the crystals of feldspar are +generally small and vitreous: it is distinctly laminated, and sometimes +includes parallel layers of epidote (This mineral is extremely common +in all the formations of Chile; in the gneiss near Valparaiso and in +the granitic veins crossing it, in the injected greenstone crowning the +C. of Quillota, in some granitic porphyries, in the porphyritic +conglomerate, and in the feldspathic clay-slates.); the lamination +appears to be distinct from stratification. Occasionally this rock is +somewhat curious; and at one spot, namely, at the C. of Quillota, it +had a brecciated structure. Near the mines of Jajuel, in a thick +stratum of this feldspathic, porphyritic slate, there was a layer of +hard, blackish, siliceous, infusible, compact clay-slate, such as I saw +nowhere else; at the same place I was able to follow for a considerable +distance the junction between the slate and the conformably underlying +porphyritic conglomerate, and they certainly passed gradually into each +other. Wherever these slaty feldspathic rocks abound, greenstone seems +common; at the C. of Quillota a bed of well-crystallised greenstone lay +conformably in the midst of the feldspathic slate, with the upper and +lower junctions passing insensibly into it. From this point, and from +the frequently porphyritic condition of the slate, I should perhaps +have considered this rock as an erupted one (like certain laminated +feldspathic lavas in the trachytic series), had I not seen in Tierra +del Fuego how readily true clay-slate becomes feldspathic and +porphyritic, and had I not seen at Jajuel the included layer of black, +siliceous clay-slate, which no one could have thought of igneous +origin. The gentle passage of the feldspathic slate, at Jajuel, into +the porphyritic conglomerate, which is certainly of aqueous origin, +should also be taken in account. + +The alternating strata of porphyries and porphyritic conglomerate, and +with the occasionally included beds of feldspathic slate, together make +a grand formation; in several places within the Cordillera, I estimated +its thickness at from six to seven thousand feet. It extends for many +hundred miles, forming the western flank of the Chilean Cordillera; and +even at Iquique in Peru, 850 miles north of the southernmost point +examined by me in Chile, the coast-escarpment which rises to a height +of between two and three thousand feet is thus composed. In several +parts of Northern Chile this formation extends much further towards the +Pacific, over the granitic and metamorphic lower rocks, than it does in +Central Chile; but the main Cordillera may be considered as its central +line, and its breadth in an east and west direction is never great. At +first the origin of this thick, massive, long but narrow formation, +appeared to me very anomalous: whence were derived, and how were +dispersed the innumerable fragments, often of large size, sometimes +angular and sometimes rounded, and almost invariably composed of +porphyritic rocks? Seeing that the interstratified porphyries are never +vesicular and often not even amygdaloidal, we must conclude that the +pile was formed in deep water; how then came so many fragments to be +well rounded and so many to remain angular, sometimes the two kinds +being equally mingled, sometimes one and sometimes the other +preponderating? That the claystone, greenstone, and other porphyries +and amygdaloids, which lie CONFORMABLY between the beds of +conglomerate, are ancient submarine lavas, I think there can be no +doubt; and I believe we must look to the craters whence these streams +were erupted, as the source of the breccia- conglomerate; after the +great explosion, we may fairly imagine that the water in the heated and +scarcely quiescent crater would remain for a considerable time +sufficiently agitated to triturate and round the loose fragments, few +or many in number, would be shot forth at the next eruption, associated +with few or many angular fragments, according to the strength of the +explosion. (This certainly seems to have taken place in some recent +volcanic archipelagos, as at the Galapagos, where numerous craters are +exclusively formed of tuff and fragments of lava.) The porphyritic +conglomerate being purple or reddish, even when alternating with dusty- +coloured or bright green porphyries and amygdaloids, is probably an +analogous circumstance to the scoriae of the blackish basalts being +often bright red. The ancient submarine orifices whence the porphyries +and their fragments were ejected having been arranged in a band, like +most still active volcanoes, accounts for the thickness, the +narrowness, and linear extension of this formation. + +This whole great pile of rock has suffered much metamorphic action, as +is very obvious in the gradual formation and appearance of the crystals +of albitic feldspar and of epidote—in the bending together of the +fragments— in the appearance of a laminated structure in the +feldspathic slate—and, lastly, in the disappearance of the planes of +stratification, which could sometimes be seen on the same mountain +quite distinct in the upper part, less and less plain on the flanks, +and quite obliterated at the base. Partly owing to this metamorphic +action, and partly to the close relationship in origin, I have seen +fragments of porphyries—taken from a metamorphosed conglomerate—from a +neighbouring stream of lava—from the nucleus or centre (as it appeared +to me) of the whole submarine volcano— and lastly from an intrusive +mass of quite subsequent origin, all of which were absolutely +undistinguishable in external characters. + +One other rock, of plutonic origin, and highly important in the history +of the Cordillera, from having been injected in most of the great axes +of elevation, and from having apparently been instrumental in +metamorphosing the superincumbent strata, may be conveniently described +in this preliminary discussion. It has been called by some authors +ANDESITE: it mainly consists of well-crystallised white albite (as +determined with the goniometer in numerous specimens both by Professor +Miller and myself), of less perfectly crystallised green hornblende, +often associated with much mica, with chlorite and epidote, and +occasionally with a few grains of quartz: in one instance in Northern +Chile, I found crystals of orthitic or potash feldspar, mingled with +those of albite. (I here, and elsewhere, call by this name, those +feldspathic minerals which cleave like albite: but it now appears +(“Edinburgh New Philosophical Journal” volume 24 page 181) that Abich +has analysed a mineral from the Cordillera, associated with hornblende +and quartz (probably the same rock with that here under discussion), +which cleaves like albite, but which is a new and distinct kind, called +by him ANDESINE. It is allied to leucite, with the greater proportion +of its potash replaced by lime and soda. This mineral seems scarcely +distinguishable from albite, except by analysis.) Where the mica and +quartz are abundant, the rock cannot be distinguished from granite; and +it may be called andesitic granite. Where these two minerals are quite +absent, and when, as often then happens, the crystals of albite are +imperfect and blend together, the rock may be called andesitic +porphyry, which bears nearly the same relation to andesitic granite +that euritic porphyry does to common granite. These andesitic rocks +form mountain masses of a white colour, which, in their general outline +and appearance—in their joints—in their occasionally including +dark-coloured, angular fragments, apparently of some pre-existing +rock—and in the great dikes branching from them into the superincumbent +strata, manifest a close and striking resemblance to masses of common +granite and syenite: I never, however, saw in these andesitic rocks, +those granitic veins of segregation which are so common in true +granites. We have seen that andesite occurs in three places in Tierra +del Fuego; in Chile, from S. Fernando to Copiapo, a distance of 450 +miles, I found it under most of the axes of elevation; in a collection +of specimens from the Cordillera of Lima in Peru, I immediately +recognised it; and Erman states that it occurs in Eastern Kamtschatka. +(“Geographical Journal” volume 9 page 510.) From its wide range, and +from the important part it has played in the history of the Cordillera, +I think this rock has well deserved its distinct name of Andesite. + +The few still active volcanoes in Chile are confined to the central and +loftiest ranges of the Cordillera; and volcanic matter, such as appears +to have been of subaerial eruption, is everywhere rare. According to +Meyen, there is a hill of pumice high up the valley of the Maypu, and +likewise a trachytic formation at Colina, a village situated north of +St. Jago. (“Reise um Erde” Th. 1 ss. 338 and 362.) Close to this latter +city, there are two hills formed of a pale feldspathic porphyry, +remarkable from being doubly columnar, great cylindrical columns being +subdivided into smaller four- or five-sided ones; and a third hillock +(Cerro Blanco) is formed of a fragmentary mass of rock, which I +believed to be of volcanic origin, intermediate in character between +the above feldspathic porphyry and common trachyte, and containing +needles of hornblende and granular oxide of iron. Near the Baths of +Cauquenes, between two short parallel lines of elevation, where they +are intersected by the valley, there is a small, though distinct +volcanic district; the rock is a dark grey (andesitic) trachyte, which +fuses into a greenish-grey bead, and is formed of long crystals of +fractured glassy albite (judging from one measurement) mingled with +well- formed crystals, often twin, of augite. The whole mass is +vesicular, but the surface is darker coloured and much more vesicular +than any other part. This trachyte forms a cliff-bounded, horizontal, +narrow strip on the steep southern side of the valley, at the height of +four or five hundred feet above the river-bed; judging from an +apparently corresponding line of cliff on the northern side, the valley +must once have been filled up to this height by a field of lava. On the +summit of a lofty mountain some leagues higher up this same valley of +the Cachapual, I found columnar pitchstone porphyritic with feldspar; I +do not suppose this rock to be of volcanic origin, and only mention it +here, from its being intersected by masses and dikes of a VESICULAR +rock, approaching in character to trachyte; in no other part of Chile +did I observe vesicular or amygdaloidal dikes, though these are so +common in ordinary volcanic districts. + +PASSAGE OF THE ANDES BY THE PORTILLO OR PEQUENES PASS. + +Although I crossed the Cordillera only once by this pass, and only once +by that of the Cumbre or Uspallata (presently to be described), riding +slowly and halting occasionally to ascend the mountains, there are many +circumstances favourable to obtaining a more faithful sketch of their +structure than would at first be thought possible from so short an +examination. The mountains are steep and absolutely bare of vegetation; +the atmosphere is resplendently clear; the stratification distinct; and +the rocks brightly and variously coloured: some of the natural sections +might be truly compared for distinctness to those coloured ones in +geological works. Considering how little is known of the structure of +this gigantic range, to which I particularly attended, most travellers +having collected only specimens of the rocks, I think my +sketch-sections, though necessarily imperfect, possess some interest. +Section 1/1 in Plate 1 which I will now describe in detail, is on a +horizontal scale of a third of an inch to a nautical mile, and on a +vertical scale of one inch to a mile (or 6,000 feet). The width of the +range (excluding a few outlying hillocks), from the plain on which St. +Jago the capital of Chile stands, to the Pampas, is sixty miles, as far +as I can judge from the maps, which differ from each other and are all +EXCEEDINGLY imperfect. The St. Jago plain at the mouth of the Maypu, I +estimate from adjoining known points at 2,300 feet, and the Pampas at +3,500 feet, both above the level of the sea. The height of the Pequenes +line, according to Dr. Gillies, is 13,210 feet (“Journal of Natural and +Geographical Science” August 1830.); and that of the Portillo line +(both in the gaps where the road crosses them) is 14,345 feet; the +lowest part of the intermediate valley of Tenuyan is 7,530 feet—all +above the level of the sea. + +The Cordillera here, and indeed I believe throughout Chile, consist of +several parallel, anticlinal and uniclinal mountain-lines, ranging +north, or north with a little westing, and south. Some exterior and +much lower ridges often vary considerably from this course, projecting +like oblique spurs from the main ranges: in the district towards the +Pacific, the mountains, as before remarked, extend in various +directions, even east and west. In the main exterior lines, the strata, +as also before remarked, are seldom inclined at a high angle; but in +the central lofty ridges they are almost always highly inclined, broken +by many great faults, and often vertical. As far as I could judge, few +of the ranges are of great length: and in the central parts of the +Cordillera, I was frequently able to follow with my eye a ridge +gradually becoming higher and higher, as the stratification increased +in inclination, from one end where its height was trifling and its +strata gently inclined to the other end where vertical strata formed +snow-clad pinnacles. Even outside the main Cordillera, near the baths +of Cauquenes, I observed one such case, where a north and south ridge +had its strata in the valley inclined at 37 degrees, and less than a +mile south of it at 67 degrees: another parallel and similarly inclined +ridge rose at the distance of about five miles, into a lofty mountain +with absolutely vertical strata. Within the Cordillera, the height of +the ridges and the inclination of the strata often became doubled and +trebled in much shorter distances than five miles; this peculiar form +of upheaval probably indicates that the stratified crust was thin, and +hence yielded to the underlying intrusive masses unequally, at certain +points on the lines of fissure. + +The valleys, by which the Cordillera are drained, follow the anticlinal +or rarely synclinal troughs, which deviate most from the usual north +and south course; or still more commonly those lines of faults or of +unequal curvature (that is, lines with the strata on both hands dipping +in the same direction, but at a somewhat different angle) which deviate +most from a northerly course. Occasionally the torrents run for some +distance in the north and south valleys, and then recover their eastern +or western course by bursting through the ranges at those points where +the strata have been least inclined and the height consequently is +less. Hence the valleys, along which the roads run, are generally +zigzag; and, in drawing an east and west section, it is necessary to +contract greatly that which is actually seen on the road. + +Commencing at the western end of Section 1/1 where the R. Maypu +debouches on the plain of St. Jago, we immediately enter on the +porphyritic conglomerate formation, and in the midst of it find some +hummocks [A] of granite and syenite, which probably (for I neglected to +collect specimens) belong to the andesitic class. These are succeeded +by some rugged hills [B] of dark-green, crystalline, feldspathic and in +some parts slaty rocks, which I believe belong to the altered +clay-slate formation. From this point, great mountains of purplish and +greenish, generally thinly stratified, highly porphyritic +conglomerates, including many strata of amygdaloidal and greenstone +porphyries, extend up the valley to the junction of the rivers Yeso and +Volcan. As the valley here runs in a very southerly course, the width +of the porphyritic conglomerate formation is quite conjectural; and +from the same cause, I was unable to make out much about the +stratification. In most of the exterior mountains the dip was gentle +and directed inwards; and at only one spot I observed an inclination as +high as 50 degrees. Near the junction of the R. Colorado with the main +stream, there is a hill of whitish, brecciated, partially decomposed +feldspathic porphyry, having a volcanic aspect but not being really of +that nature: at Tolla, however, in this valley, Dr. Meyen met with a +hill of pumice containing mica. (“Reise um Erde” Th.1 ss. 338, 341.) At +the junction of the Yeso and Volcan [D] there is an extensive mass, in +white conical hillocks, of andesite, containing some mica, and passing +either into andesitic granite, or into a spotted, semi-granular mixture +of albitic (?) feldspar and hornblende: in the midst of this formation +Dr. Meyen found true trachyte. The andesite is covered by strata of +dark-coloured, crystalline, obscurely porphyritic rocks, and above them +by the ordinary porphyritic conglomerates,—the strata all dipping away +at a small angle from the underlying mass. The surrounding lofty +mountains appear to be entirely composed of the porphyritic +conglomerate, and I estimated its thickness here at between six and +seven thousand feet. Beyond the junction of the Yeso and Volcan, the +porphyritic strata appear to dip towards the hillocks of andesite at an +angle of 40 degrees; but at some distant points on the same ridge they +are bent up and vertical. Following the valley of the Yeso, trending +N.E. (and therefore still unfavourable for our transverse section), the +same porphyritic conglomerate formation is prolonged to near the +Cuestadel Indio, situated at the western end of the basin (like a +drained lake) of Yeso. Some way before arriving at this point, distant +lofty pinnacles capped by coloured strata belonging to the great +gypseous formation could first be seen. From the summit of the Cuesta, +looking southward, there is a magnificent sectional view of a +mountain-mass, at least 2,000 feet in thickness [E], of fine andesite +granite (containing much black mica, a little chlorite and quartz), +which sends great white dikes far into the superincumbent, +dark-coloured, porphyritic conglomerates. At the line of junction the +two formations are wonderfully interlaced together: in the lower part +of the porphyritic conglomerate, the stratification has been quite +obliterated, whilst in the upper part it is very distinct, the beds +composing the crests of the surrounding mountains being inclined at +angles of between 70 and 80 degrees, and some being even vertical. On +the northern side of the valley, there is a great corresponding mass of +andesitic granite, which is encased by porphyritic conglomerate, +dipping both on the western and eastern sides, at about 80 degrees to +west, but on the eastern side with the tips of the strata bent in such +a manner, as to render it probable that the whole mass has been on that +side thrown over and inverted. + +In the valley basin of the Yeso, which I estimated at 7,000 feet above +the level of the sea, we first reach at [F] the gypseous formation. Its +thickness is very great. It consists in most parts of snow-white, hard, +compact gypsum, which breaks with a saccharine fracture, having +translucent edges; under the blowpipe gives out much vapour; it +frequently includes nests and exceedingly thin layers of crystallised, +blackish carbonate of lime. Large, irregularly shaped concretions +(externally still exhibiting lines of aqueous deposition) of +blackish-grey, but sometimes white, coarsely and brilliantly +crystallised, hard anhydrite, abound within the common gypsum. +Hillocks, formed of the hardest and purest varieties of the white +gypsum, stand up above the surrounding parts, and have their surfaces +cracked and marked, just like newly baked bread. There is much pale +brown, soft argillaceous gypsum; and there were some intercalated green +beds which I had not time to reach. I saw only one fragment of selenite +or transparent gypsum, and that perhaps may have come from some +subsequently formed vein. From the mineralogical characters here given, +it is probable that these gypseous beds have undergone some metamorphic +action. The strata are much hidden by detritus, but they appeared in +most parts to be highly inclined; and in an adjoining lofty pinnacle +they could be distinctly seen bending up, and becoming vertical, +conformably with the underlying porphyritic conglomerate. In very many +parts of the great mountain-face [F], composed of thin gypseous beds, +there were innumerable masses, irregularly shaped and not like dikes, +yet with well-defined edges, of an imperfectly granular, pale greenish, +or yellowish-white rock, essentially composed of feldspar, with a +little chlorite or hornblende, epidote, iron-pyrites, and ferruginous +powder: I believe that these curious trappean masses have been injected +from the not far distant mountain-mass [E] of andesite whilst still +fluid, and that owing to the softness of the gypseous strata they have +not acquired the ordinary forms of dikes. Subsequently to the injection +of these feldspathic rocks, a great dislocation has taken place; and +the much shattered gypseous strata here overlie a hillock [G], composed +of vertical strata of impure limestone and of black highly calcareous +shale including threads of gypsum: these rocks, as we shall presently +see, belong to the upper parts of the gypseous series, and hence must +here have been thrown down by a vast fault. + +Proceeding up the valley-basin of the Yeso, and taking our section +sometimes on one hand and sometimes on the other, we come to a great +hill of stratified porphyritic conglomerate [H] dipping at 45 degrees +to the west; and a few hundred yards farther on, we have a bed between +three or four hundred feet thick of gypsum [I] dipping eastward at a +very high angle: here then we have a fault and anticlinal axis. On the +opposite side of the valley, a vertical mass of red conglomerate, +conformably underlying the gypsum, appears gradually to lose its +stratification and passes into a mountain of porphyry. The gypsum [I] +is covered by a bed [K], at least 1,000 feet in thickness, of a +purplish-red, compact, heavy, fine-grained sandstone or mudstone, which +fuses easily into a white enamel, and is seen under a lens to contain +triturated crystals. This is succeeded by a bed [L], 1,000 feet thick +(I believe I understate the thickness) of gypsum, exactly like the beds +before described; and this again is capped by another great bed [M] of +purplish-red sandstone. All these strata dip eastward; but the +inclination becomes less and less, as we leave the first and almost +vertical bed [I] of gypsum. + +Leaving the basin-plain of Yeso, the road rapidly ascends, passing by +mountains composed of the gypseous and associated beds, with their +stratification greatly disturbed and therefore not easily intelligible: +hence this part of the section has been left uncoloured. Shortly before +reaching the great Pequenes ridge, the lowest stratum visible [N] is a +red sandstone or mudstone, capped by a vast thickness of black, +compact, calcareous, shaly rock [O], which has been thrown into four +lofty, though small ridges: looking northward, the strata in these +ridges are seen gradually to rise in inclination, becoming in some +distant pinnacles absolutely vertical. + +The ridge of Pequenes, which divides the waters flowing into the +Pacific and Atlantic Oceans, extends in a nearly N.N.W. and S.S.E. +line; its strata dip eastward at an angle of between 30 and 45 degrees, +but in the higher peaks bending up and becoming almost vertical. Where +the road crosses this range, the height is 13,210 feet above the +sea-level, and I estimated the neighbouring pinnacles at from fourteen +to fifteen thousand feet. The lowest stratum visible in this ridge is a +red stratified sandstone [P]; on it are superimposed two great masses +[Q and S] of black, hard, compact, even having a conchoidal fracture, +calcareous, more or less laminated shale, passing into limestone: this +rock contains organic remains, presently to be enumerated. The +compacter varieties fuse easily in a white glass; and this I may add is +a very general character with all the sedimentary beds in the +Cordillera: although this rock when broken is generally quite black, it +everywhere weathers into an ash-grey tint. Between these two great +masses [Q and S], a bed [R] of gypsum is interposed, about three +hundred feet in thickness, and having the same characters as heretofore +described. I estimated the total thickness of these three beds [Q, R, +S] at nearly three thousand feet; and to this must be added, as will be +immediately seen, a great overlying mass of red sandstone. + +In descending the eastern slope of this great central range, the +strata, which in the upper part dip eastward at about an angle of 40 +degrees, become more and more curved, till they are nearly vertical; +and a little further onwards there is seen on the further side of a +ravine, a thick mass of strata of bright red sandstone [T], with their +upper extremities slightly curved, showing that they were once +conformably prolonged over the beds [S]: on the southern and opposite +side of the road, this red sandstone and the underlying black shaly +rocks stand vertical, and in actual juxtaposition. Continuing to +descend, we come to a synclinal valley filled with rubbish, beyond +which we have the red sandstone [T2] corresponding with [T], and now +dipping, as is seen both north and south of the road, at 45 degrees to +the west; and under it, the beds [S2, R2, Q2, and I believe P2] in +corresponding order and of similar composition, with those on the +western flank of the Pequenes range, but dipping westward. Close to the +synclinal valley the dip of these strata is 45 degrees, but at the +eastern or farther end of the series it increases to 60 degrees. Here +the great gypseous formation abruptly terminates, and is succeeded +eastward by a pile of more modern strata. Considering how violently +these central ranges have been dislocated, and how very numerous dikes +are in the exterior and lower parts of the Cordillera, it is remarkable +that I did not here notice a single dike. The prevailing rock in this +neighbourhood is the black, calcareous, compact shale, whilst in the +valley-basin of the Yeso the purplish red sandstone or mudstone +predominates,—both being associated with gypseous strata of exactly the +same nature. It would be very difficult to ascertain the relative +superposition of these several masses, for we shall afterwards see in +the Cumbre Pass that the gypseous and intercalated beds are +lens-shaped, and that they thin out, even where very thick, and +disappear in short horizontal distances: it is quite possible that the +black shales and red sandstones may be contemporaneous, but it is more +probable that the former compose the uppermost parts of the series. + +The fossils above alluded to in the black calcareous shales are few in +number, and are in an imperfect condition; they consist, as named for +me by M. d’Orbigny, of:— + +1. Ammonite, indeterminable, near to A. recticostatus, d’Orbigny, “Pal. +Franc.” (Neocomian formation). 2. Gryphaea, near to G. Couloni +(Neocomian formations of France and Neufchatel). 3. Natica, +indeterminable. 4. Cyprina rostrata, d’Orbigny, “Pal. Franc.” +(Neocomian formation). 5. Rostellaria angulosa (?), d’Orbigny, “Pal. de +l’Amer. Mer.” 6. Terebratula (?). + +Some of the fragments of Ammonites were as thick as a man’s arm: the +Gryphaea is much the most abundant shell. These fossils M. d’Orbigny +considers as belonging to the Neocomian stage of the Cretaceous system. +Dr. Meyen, who ascended the valley of the Rio Volcan, a branch of the +Yeso, found a nearly similar, but apparently more calcareous formation, +with much gypsum, and no doubt the equivalent of that here described +(“Reise um Erde” etc. Th. 1 s. 355.): the beds were vertical, and were +prolonged up to the limits of perpetual snow; at the height of 9,000 +feet above the sea, they abounded with fossils, consisting, according +to Von Buch (“Descript. Phys. des Iles Canaries” page 471.), of:— + +1. Exogyra (Gryphaea) Couloni, absolutely identical with specimens from +the Jura and South of France. 2. Trigonia costata, identical with those +found in the upper Jurassic beds at Hildesheim. 3. Pecten striatus, +identical with those found in the upper Jurassic beds at Hildesheim. 4. +Cucullaea, corresponding in form to C. longirostris, so frequent in the +upper Jurassic beds of Westphalia. 5. Ammonites resembling A. biplex. + +Von Buch concludes that this formation is intermediate between the +limestone of the Jura and the chalk, and that it is analogous with the +uppermost Jurassic beds forming the plains of Switzerland. Hence M. +D’Orbigny and Von Buch, under different terms, compare these fossils to +those from the same late stage in the secondary formations of Europe. + +Some of the fossils which I collected were found a good way down the +western slope of the main ridge, and hence must originally have been +covered up by a great thickness of the black shaly rock, independently +of the now denuded, thick, overlying masses of red sandstone. I +neglected at the time to estimate how many hundred or rather thousand +feet thick the superincumbent strata must have been: and I will not now +attempt to do so. This, however, would have been a highly interesting +point, as indicative of a great amount of subsidence, of which we shall +hereafter find in other parts of the Cordillera analogous evidence +during this same period. The altitude of the Peuquenes Range, +considering its not great antiquity, is very remarkable; many of the +fossils were embedded at the height of 13,210 feet, and the same beds +are prolonged up to at least from fourteen to fifteen thousand feet +above the level of the sea. + +THE PORTILLO OR EASTERN CHAIN. + +The valley of Tenuyan, separating the Peuquenes and Portillo lines, is, +as estimated by Dr. Gillies and myself, about twenty miles in width; +the lowest part, where the road crosses the river, being 7,500 feet +above the sea-level. The pass on the Portillo line is 14,365 feet high +(1,100 feet higher than that on the Peuquenes), and the neighbouring +pinnacles must, I conceive, rise to nearly 16,000 feet above the sea. +The river draining the intermediate valley of Tenuyan, passes through +the Portillo line. To return to our section:—shortly after leaving the +lower beds [P2] of the gypseous formation, we come to grand masses of a +coarse, red conglomerate [V], totally unlike any strata hitherto seen +in the Cordillera. This conglomerate is distinctly stratified, some of +the beds being well defined by the greater size of the pebbles: the +cement is calcareous and sometimes crystalline, though the mass shows +no signs of having been metamorphosed. The included pebbles are either +perfectly or only partially rounded: they consist of purplish +sandstones, of various porphyries, of brownish limestone, of black +calcareous, compact shale precisely like that in situ in the Peuquenes +range, and CONTAINING SOME OF THE SAME FOSSIL SHELLS; also very many +pebbles of quartz, some of micaceous schist, and numerous, broken, +rounded crystals of a reddish orthitic or potash feldspar (as +determined by Professor Miller), and these from their size must have +been derived from a coarse-grained rock, probably granite. From this +feldspar being orthitic, and even from its external appearance, I +venture positively to affirm that it has not been derived from the +rocks of the western ranges; but, on the other hand, it may well have +come, together with the quartz and metamorphic schists, from the +eastern or Portillo line, for this line mainly consists of coarse +orthitic granite. The pebbles of the fossiliferous slate and of the +purple sandstone, certainly have been derived from the Peuquenes or +western ranges. + +The road crosses the valley of Tenuyan in a nearly east and west line, +and for several miles we have on both hands the conglomerate, +everywhere dipping west and forming separate great mountains. The +strata, where first met with, after leaving the gypseous formation, are +inclined westward at an angle of only 20 degrees, which further on +increases to about 45 degrees. The gypseous strata, as we have seen, +are also inclined westward: hence, when looking from the eastern side +of the valley towards the Peuquenes range, a most deceptive appearance +is presented, as if the newer beds of conglomerate dipped directly +under the much older beds of the gypseous formation. In the middle of +the valley, a bold mountain of unstratified lilac-coloured porphyry +(with crystals of hornblende) projects; and further on, a little south +of the road, there is another mountain, with its strata inclined at a +small angle eastwards, which in its general aspect and colour, +resembles the porphyritic conglomerate formation, so rare on this side +of the Peuquenes line and so grandly developed throughout the western +ranges. + +The conglomerate is of great thickness: I do not suppose that the +strata forming the separate mountain-masses [V,V,V] have ever been +prolonged over each other, but that one mass has been broken up by +several, distinct, parallel, uniclinal lines of elevation. Judging +therefore of the thickness of the conglomerate, as seen in the separate +mountain-masses, I estimated it at least from one thousand five hundred +to two thousand feet. The lower beds rest conformably on some +singularly coloured, soft strata [W], which I could not reach to +examine; and these again rest conformably on a thick mass of micaceous, +thinly laminated, siliceous sandstone [X], associated with a little +black clay-slate. These lower beds are traversed by several dikes of +decomposing porphyry. The laminated sandstone is directly superimposed +on the vast masses of granite [Y,Y] which mainly compose the Portillo +range. The line of junction between this latter rock, which is of a +bright red colour, and the whitish sandstone was beautifully distinct; +the sandstone being penetrated by numerous, great, tortuous dikes +branching from the granite, and having been converted into a granular +quartz rock (singularly like that of the Falkland Islands), containing +specks of an ochrey powder, and black crystalline atoms, apparently of +imperfect mica. The quartzose strata in one spot were folded into a +regular dome. + +The granite which composes the magnificent bare pinnacles and the steep +western flank of the Portillo chain, is of a brick-red colour, coarsely +crystallised, and composed of orthitic or potash feldspar, quartz, and +imperfect mica in small quantity, sometimes passing into chlorite. +These minerals occasionally assume a laminar or foliated arrangement. +The fact of the feldspar being orthitic in this range, is very +remarkable, considering how rare, or rather, as I believe, entirely +absent, this mineral is throughout the western ranges, in which +soda-feldspar, or at least a variety cleaving like albite, is so +extremely abundant. In one spot on the western flank, and on the +eastern flank near Los Manantiales and near the crest, I noticed some +great masses of a whitish granite, parts of it fine- grained, and parts +containing large crystals of feldspar; I neglected to collect +specimens, so I do not know whether this feldspar is also orthitic, +though I am inclined to think so from its general appearance. I saw +also some syenite and one mass which resembled andesite, but of which I +likewise neglected to collect specimens. From the manner in which the +whitish granites formed separate mountain-masses in the midst of the +brick-red variety, and from one such mass near the crest being +traversed by numerous veins of flesh-coloured and greenish eurite (into +which I occasionally observed the brick-red granite insensibly +passing), I conclude that the white granites probably belong to an +older formation, almost overwhelmed and penetrated by the red granite. + +On the crest I saw also, at a short distance, some coloured stratified +beds, apparently like those [W] at the western base, but was prevented +examining them by a snowstorm: Mr. Caldcleugh, however, collected here +specimens of ribboned jasper, magnesian limestone, and other minerals. +(“Travels” etc. volume 1 page 308.) A little way down the eastern slope +a few fragments of quartz and mica-slate are met with; but the great +formation of this latter rock [Z], which covers up much of the eastern +flank and base of the Portillo range, cannot be conveniently examined +until much lower down at a place called Mal Paso. The mica-schist here +consists of thick layers of quartz, with intervening folia of +finely-scaly mica, often passing into a substance like black glossy +clay-slate: in one spot, the layers of the quartz having disappeared, +the whole mass became converted into glossy clay-slate. Where the folia +were best defined, they were inclined at a high angle westward, that +is, towards the range. The line of junction between the dark mica-slate +and the coarse red granite was most clearly distinguishable from a vast +distance: the granite sent many small veins into the mica-slate, and +included some angular fragments of it. As the sandstone on the western +base has been converted by the red granite into a granular quartz-rock, +so this great formation of mica-schist may possibly have been +metamorphosed at the same time and by the same means; but I think it +more probable, considering its more perfect metamorphic character and +its well-pronounced foliation, that it belongs to an anterior epoch, +connected with the white granites: I am the more inclined to this view, +from having found at the foot of the range the mica-schist surrounding +a hummock [Y2], exclusively composed of white granite. Near Los +Arenales, the mountains on all sides are composed of the mica-slate; +and looking backwards from this point up to the bare gigantic peaks +above, the view was eminently interesting. The colours of the red +granite and the black mica-slate are so distinct, that with a bright +light these rocks could be readily distinguished even from the Pampas, +at a level of at least 9,000 feet below. The red granite, from being +divided by parallel joints, has weathered into sharp pinnacles, on some +of which, even on some of the loftiest, little caps of mica-schist +could be clearly seen: here and there isolated patches of this rock +adhered to the mountain-flanks, and these often corresponded in height +and position on the opposite sides of the immense valleys. Lower down +the schist prevailed more and more, with only a few quite small points +of granite projecting through. Looking at the entire eastern face of +the Portillo range, the red colour far exceeds in area the black; yet +it was scarcely possible to doubt that the granite had once been almost +wholly encased by the mica-schist. + +At Los Arenales, low down on the eastern flank, the mica-slate is +traversed by several closely adjoining, broad dikes, parallel to each +other and to the foliation of the schist. The dikes are formed of three +different varieties of rock, of which a pale brown feldspathic porphyry +with grains of quartz was much the most abundant. These dikes with +their granules of quartz, as well as the mica-schist itself, strikingly +resemble the rocks of the Chonos Archipelago. At a height of about +twelve hundred feet above the dikes, and perhaps connected with them, +there is a range of cliffs formed of successive lava-streams [AA], +between three and four hundred feet in thickness, and in places finely +columnar. The lava consists of dark- greyish, harsh rocks, intermediate +in character between trachyte and basalt, containing glassy feldspar, +olivine, and a little mica, and sometimes amygdaloidal with zeolite: +the basis is either quite compact, or crenulated with air-vesicles +arranged in laminae. The streams are separated from each other by beds +of fragmentary brown scoriae, firmly cemented together, and including a +few well-rounded pebbles of lava. From their general appearance, I +suspect that these lava-streams flowed at an ancient period under the +pressure of the sea, when the Atlantic covered the Pampas and washed +the eastern foot of the Cordillera. (This conclusion might, perhaps, +even have been anticipated, from the general rarity of volcanic action, +except near the sea or large bodies of water. Conformably with this +rule, at the present day, there are no active volcanoes on this eastern +side of the Cordillera; nor are severe earthquakes experienced here.) +On the opposite and northern side of the valley there is another line +of lava- cliffs at a corresponding height; the valley between being of +considerable breadth, and as nearly as I could estimate 1,500 feet in +depth. This field of lava is confined on both sides by the mountains of +mica-schist, and slopes down rapidly but irregularly to the edge of the +Pampas, where, having a thickness of about two hundred feet, it +terminates against a little range of claystone porphyry. The valley in +this lower part expands into a bay-like, gentle slope, bordered by the +cliffs of lava, which must certainly once have extended across this +wide expanse. The inclination of the streams from Los Arenales to the +mouth of the valley is so great, that at the time (though ignorant of +M. Elie de Beaumont’s researches on the extremely small slope over +which lava can flow, and yet retain a compact structure and +considerable thickness) I concluded that they must subsequently to +their flowing have been upheaved and tilted from the mountains; of this +conclusion I can now entertain not the smallest doubt. + +At the mouth of the valley, within the cliffs of the above lava-field, +there are remnants, in the form of separate small hillocks and of lines +of low cliffs, of a considerable deposit of compact white tuff +(quarried for filtering-stones), composed of broken pumice, volcanic +crystals, scales of mica, and fragments of lava. This mass has suffered +much denudation; and the hard mica-schist has been deeply worn, since +the period of its deposition; and this period must have been subsequent +to the denudation of the basaltic lava-streams, as attested by their +encircling cliffs standing at a higher level. At the present day, under +the existing arid climate, ages might roll past without a square yard +of rock of any kind being denuded, except perhaps in the rarely +moistened drainage-channel of the valley. Must we then look back to +that ancient period, when the waves of the sea beat against the eastern +foot of the Cordillera, for a power sufficient to denude extensively, +though superficially, this tufaceous deposit, soft although it be? + +There remains only to mention some little water-worn hillocks [BB], a +few hundred feet in height, and mere mole-hills compared with the +gigantic mountains behind them, which rise out of the sloping, +shingle-covered margin of the Pampas. The first little range is +composed of a brecciated purple porphyritic claystone, with obscurely +marked strata dipping at 70 degrees to the S.W.; the other ranges +consist of—a pale-coloured feldspathic porphyry,—a purple claystone +porphyry with grains of quartz,— and a rock almost exclusively composed +of brick-red crystals of feldspar. These outermost small lines of +elevation extend in a N.W. by W. and S.E. by S. direction. + +CONCLUDING REMARKS ON THE PORTILLO RANGE. + +When on the Pampas and looking southward, and whilst travelling +northward, I could see for very many leagues the red granite and dark +mica-schist forming the crest and eastern flank of the Portillo line. +This great range, according to Dr. Gillies, can be traced with little +interruption for 140 miles southward to the R. Diamante, where it +unites with the western ranges: northward, according to this same +author, it terminates where the R. Mendoza debouches from the +mountains; but a little further north in the eastern part of the Cumbre +section, there are, as we shall hereafter see, some mountain-masses of +a brick-red porphyry, the last injected amidst many other porphyries, +and having so close an analogy with the coarse red granite of the +Portillo line, that I am tempted to believe that they belong to the +same axis of injection; if so, the Portillo line is at least 200 miles +in length. Its height, even in the lowest gap in the road, is 14,365 +feet, and some of the pinnacles apparently attain an elevation of about +16,000 feet above the sea. The geological history of this grand chain +appears to me eminently interesting. We may safely conclude, that at a +former period the valley of Tenuyan existed as an arm of the sea, about +twenty-miles in width, bordered on one hand by a ridge or chain of +islets of the black calcareous shales and purple sandstones of the +gypseous formation; and on the other hand, by a ridge or chain of +islets composed of mica-slate, white granite, and perhaps to a partial +extent of red granite. These two chains, whilst thus bordering the old +sea-channel, must have been exposed for a vast lapse of time to +alluvial and littoral action, during which the rocks were shattered, +the fragments rounded, and the strata of conglomerate accumulated to a +thickness of at least fifteen hundred or two thousand feet. The red +orthitic granite now forms, as we have seen, the main part of the +Portillo chain: it is injected in dikes not only into the mica-schist +and white granites, but into the laminated sandstone, which it has +metamorphosed, and which it has thrown off, together with the +conformably overlying coloured beds and stratified conglomerate, at an +angle of forty-five degrees. To have thrown off so vast a pile of +strata at this angle, is a proof that the main part of the red granite +(whether or not portions, as perhaps is probable, previously existed) +was injected in a liquified state after the accumulation both of the +laminated sandstone and of the conglomerate; this conglomerate, we +know, was accumulated, not only after the deposition of the +fossiliferous strata of the Peuquenes line, but after their elevation +and long-continued denudation: and these fossiliferous strata belong to +the early part of the Cretaceous system. Late, therefore, in a +geological sense, as must be the age of the main part of the red +granite, I can conceive nothing more impressive than the eastern view +of this great range, as forcing the mind to grapple with the idea of +the thousands of thousands of years requisite for the denudation of the +strata which originally encased it,—for that the fluidified granite was +once encased, its mineralogical composition and structure, and the bold +conical shape of the mountain-masses, yield sufficient evidence. Of the +encasing strata we see the last vestiges in the coloured beds on the +crest, in the little caps of mica-schist on some of the loftiest +pinnacles, and in the isolated patches of this same rock at +corresponding heights on the now bare and steep flanks. + +The lava-streams at the eastern foot of the Portillo are interesting, +not so much from the great denudation which they have suffered at a +comparatively late period as from the evidence they afford by their +inclination taken conjointly with their thickness and compactness, that +after the great range had assumed its present general outline, it +continued to rise as an axis of elevation. The plains extending from +the base of the Cordillera to the Atlantic show that the continent has +been upraised in mass to a height of 3,500 feet, and probably to a much +greater height, for the smooth shingle-covered margin of the Pampas is +prolonged in a gentle unbroken slope far up many of the great valleys. +Nor let it be assumed that the Peuquenes and Portillo ranges have +undergone only movements of elevation; for we shall hereafter see, that +the bottom of the sea subsided several thousand feet during the +deposition of strata, occupying the same relative place in the +Cordillera, with those of the Peuquenes ridge; moreover, we shall see +from the unequivocal evidence of buried upright trees, that at a +somewhat later period, during the formation of the Uspallata chain, +which corresponds geographically with that of the Portillo, there was +another subsidence of many thousand feet: here, indeed, in the valley +of Tenuyan, the accumulation of the coarse stratified conglomerate to a +thickness of fifteen hundred or two thousand feet, offers strong +presumptive evidence of subsidence; for all existing analogies lead to +the belief that large pebbles can be transported only in shallow water, +liable to be affected by currents and movements of undulation—and if +so, the shallow bed of the sea on which the pebbles were first +deposited must necessarily have sunk to allow of the accumulation of +the superincumbent strata. What a history of changes of level, and of +wear and tear, all since the age of the latter secondary formations of +Europe, does the structure of this one great mountain-chain reveal! + +PASSAGE OF THE ANDES BY THE CUMBRE OR USPALLATA PASS. + +This Pass crosses the Andes about sixty miles north of that just +described: the section given in Plate 1, Section 1/2, is on the same +scale as before, namely, at one-third of an inch to a mile in distance, +and one inch to a mile (or 6,000 feet) in height. Like the last +section, it is a mere sketch, and cannot pretend to accuracy, though +made under favourable circumstances. We will commence as before, with +the western half, of which the main range bears the name of the Cumbre +(that is the Ridge), and corresponds to the Peuquenes line in the +former section; as does the Uspallata range, though on a much smaller +scale, to that of the Portillo. Near the point where the river +Aconcagua debouches on the basin plain of the same name, at a height of +about two thousand three hundred feet above the sea, we meet with the +usual purple and greenish porphyritic claystone conglomerate. Beds of +this nature, alternating with numerous compact and amygdaloidal +porphyries, which have flowed as submarine lavas, and associated with +great mountain- masses of various, injected, non-stratified porphyries, +are prolonged the whole distance up to the Cumbre or central ridge. One +of the commonest stratified porphyries is of a green colour, highly +amygdaloidal with the various minerals described in the preliminary +discussion, and including fine tabular crystals of albite. The +mountain-range north (often with a little westing) and south. The +stratification, wherever I could clearly distinguish it, was inclined +westward or towards the Pacific, and, except near the Cumbre, seldom at +angles above 25 degrees. Only at one spot on this western side, on a +lofty pinnacle not far from the Cumbre, I saw strata apparently +belonging to the gypseous formation, and conformably capping a pile of +stratified porphyries. Hence, both in composition and in +stratification, the structure of the mountains on this western side of +the divortium aquarum, is far more simple than in the corresponding +part of the Peuquenes section. In the porphyritic claystone +conglomerate, the mechanical structure and the planes of stratification +have generally been much obscured and even quite obliterated towards +the base of the series, whilst in the upper parts, near the summits of +the mountains, both are distinctly displayed. In these upper portions +the porphyries are generally lighter coloured. In three places [X, Y, +Z] masses of andesite are exposed: at [Y], this rock contained some +quartz, but the greater part consisted of andesitic porphyry, with only +a few well-developed crystals of albite, and forming a great white +mass, having the external aspect of granite, capped by much dark +unstratified porphyry. In many parts of the mountains, there are dikes +of a green colour, and other white ones, which latter probably spring +from underlying masses of andesite. + +The Cumbre, where the road crosses it, is, according to Mr. Pentland, +12,454 feet above the sea; and the neighbouring peaks, composed of dark +purple and whitish porphyries, some obscurely stratified with a +westerly dip, and others without a trace of stratification, must exceed +13,000 feet in height. Descending the eastern slope of the Cumbre, the +structure becomes very complicated, and generally differs on the two +sides of the east and west line of road and section. First we come to a +great mass [A] of nearly vertical, singularly contorted strata, +composed of highly compact red sandstones, and of often calcareous +conglomerates, and penetrated by green, yellow, and reddish dikes; but +I shall presently have an opportunity of describing in some detail an +analogous pile of strata. These vertical beds are abruptly succeeded by +others [B], of apparently nearly the same nature but more +metamorphosed, alternating with porphyries and limestones; these dip +for a short space westward, but there has been here an extraordinary +dislocation, which, on the north side of the road, appears to have +determined the excavation of the north and south valley of the R. de +las Cuevas. On this northern side of the road, the strata [B] are +prolonged till they come in close contact with a jagged lofty mountain +[D] of dark- coloured, unstratified, intrusive porphyry, where the beds +have been more highly inclined and still more metamorphosed. This +mountain of porphyry seems to form a short axis of elevation, for south +of the road in its line there is a hill [C] of porphyritic conglomerate +with absolutely vertical strata. + +We now come to the gypseous formation: I will first describe the +structure of the several mountains, and then give in one section a +detailed account of the nature of the rocks. On the north side of the +road, which here runs in an east and west valley, the mountain of +porphyry [D] is succeeded by a hill [E] formed of the upper gypseous +strata tilted, at an angle of between 70 and 80 degrees to the west, by +a uniclinal axis of elevation which does not run parallel to the other +neighbouring ranges, and which is of short length; for on the south +side of the valley its prolongation is marked only by a small flexure +in a pile of strata inclined by a quite separate axis. A little further +on the north and south valley of Horcones enters at right angles our +line of section; its western side is bounded by a hill of gypseous +strata [F] dipping westward at about 45 degrees, and its eastern side +by a mountain of similar strata [G] inclined westward at 70 degrees, +and superimposed by an oblique fault on another mass of the same strata +[H], also inclined westward, but at an angle of about 30 degrees: the +complicated relation of these three masses [F, G, H] is explained by +the structure of a great mountain-range lying some way to the north, in +which a regular anticlinal axis (represented in the section by dotted +lines) is seen, with the strata on its eastern side again bending up +and forming a distinct uniclinal axis, of which the beds marked [H] +form the lower part. This great uniclinal line is intersected, near the +Puente del Inca, by the valley along which the road runs, and the +strata composing it will be immediately described. On the south side of +the road, in the space corresponding with the mountains [E, F, and G], +the strata everywhere dip westward generally at an angle of 30 degrees, +occasionally mounting up to 45 degrees, but not in an unbroken line, +for there are several vertical faults, forming separate uniclinal +masses, all dipping in the same direction,—a form of elevation common +in the Cordillera. We thus see that within a narrow space, the gypseous +strata have been upheaved and crushed together by a great uniclinal, +anticlinal, and one lesser uniclinal line [E] of elevation; and that +between these three lines and the Cumbre, in the sandstones, +conglomerates and porphyritic formation, there have been at least two +or three other great elevatory axes. + +The uniclinal axis [I] intersected near the Puente del Inca (of which +the strata at [H] form a part) ranges N. by W. and S. by E., forming a +chain of mountains, apparently little inferior in height to the Cumbre: +the strata, as we have seen, dip at an average angle of 30 degrees to +the west. (At this place, there are some hot and cold springs, the +warmest having a temperature, according to Lieutenant Brand “Travels,” +page 240, of 91 degrees; they emit much gas. According to Mr. Brande, +of the Royal Institution, ten cubical inches contain forty-five grains +of solid matter, consisting chiefly of salt, gypsum, carbonate of lime, +and oxide of iron. The water is charged with carbonic acid and +sulphuretted hydrogen. These springs deposit much tufa in the form of +spherical balls. They burst forth, as do those of Cauquenes, and +probably those of Villa Vicencio, on a line of elevation.) The flanks +of the mountains are here quite bare and steep, affording an excellent +section; so that I was able to inspect the strata to a thickness of +about 4,000 feet, and could clearly distinguish their general nature +for 1,000 feet higher, making a total thickness of 5,000 feet, to which +must be added about 1,000 feet of the inferior strata seen a little +lower down the valley, I will describe this one section in detail, +beginning at the bottom. + +1st. The lowest mass is the altered clay-slate described in the +preliminary discussion, and which in this line of section was here +first met with. Lower down the valley, at the R. de las Vacas, I had a +better opportunity of examining it; it is there in some parts well +characterised, having a distinct, nearly vertical, tortuous cleavage, +ranging N.W. and S.E., and intersected by quartz veins: in most parts, +however, it is crystalline and feldspathic, and passes into a true +greenstone often including grains of quartz. The clay-slate, in its +upper half, is frequently brecciated, the embedded angular fragments +being of nearly the same nature with the paste. + +2nd. Several strata of purplish porphyritic conglomerate, of no very +great thickness, rest conformably upon the feldspathic slate. A thick +bed of fine, purple, claystone porphyry, obscurely brecciated (but not +of metamorphosed sedimentary origin), and capped by porphyritic +conglomerate, was the lowest bed actually examined in this section at +the Puente del Inca. + +3rd. A stratum, eighty feet thick, of hard and very compact impure +whitish limestone, weathering bright red, with included layers +brecciated and re- cemented. Obscure marks of shell are distinguishable +in it. + +4th. A red, quartzose, fine-grained conglomerate, with grains of +quartz, and with patches of white earthy feldspar, apparently due to +some process of concretionary crystalline action; this bed is more +compact and metamorphosed than any of the overlying conglomerates. + +5th. A whitish cherty limestone, with nodules of bluish argillaceous +limestone. + +6th. A white conglomerate, with many particles of quartz, almost +blending into the paste. + +7th. Highly siliceous, fine-grained white sandstone. + +8th and 9th. Red and white beds not examined. + +10th. Yellow, fine-grained, thinly stratified, magnesian (judging from +its slow dissolution in acids) limestone: it includes some white quartz +pebbles, and little cavities, lined with calcareous spar, some +retaining the form of shells. + +11th. A bed between twenty and thirty feet thick, quite conformable +with the underlying ones, composed of a hard basis, tinged lilac-grey +porphyritic with NUMEROUS crystals of whitish feldspar, with black mica +and little spots of soft ferruginous matter: evidently a submarine +lava. + +12th. Yellow magnesian limestone, as before, part-stained purple. + +13th. A most singular rock; basis purplish grey, obscurely crystalline, +easily fusible into a dark green glass, not hard, thickly speckled with +crystals more or less perfect of white carbonate of lime, of red +hydrous oxide of iron, of a white and transparent mineral like +analcime, and of a green opaque mineral like soap-stone; the basis is +moreover amygdaloidal with many spherical balls of white crystallised +carbonate of lime, of which some are coated with the red oxide of iron. +I have no doubt, from the examination of a superincumbent stratum (19), +that this is a submarine lava; though in Northern Chile, some of the +metamorphosed sedimentary beds are almost as crystalline, and of as +varied composition. + +14th. Red sandstone, passing in the upper part into a coarse, hard, red +conglomerate, 300 feet thick, having a calcareous cement, and including +grains of quartz and broken crystals of feldspar; basis infusible; the +pebbles consist of dull purplish porphyries, with some of quartz, from +the size of a nut to a man’s head. This is the coarsest conglomerate in +this part of the Cordillera: in the middle there was a white layer not +examined. + +15th. Grand thick bed, of a very hard, yellowish-white rock, with a +crystalline feldspathic base, including large crystals of white +feldspar, many little cavities mostly full of soft ferruginous matter, +and numerous hexagonal plates of black mica. The upper part of this +great bed is slightly cellular; the lower part compact: the thickness +varied a little in different parts. Manifestly a submarine lava; and is +allied to bed 11. + +16th and 17th. Dull purplish, calcareous, fine-grained, compact +sandstones, which pass into coarse white conglomerates with numerous +particles of quartz. + +18th. Several alternations of red conglomerate, purplish sandstone, and +submarine lava, like that singular rock forming bed 13. + +19th. A very heavy, compact, greenish-black stone, with a fine-grained +obviously crystalline basis, containing a few specks of white +calcareous spar, many specks of the crystallised hydrous red oxide of +iron, and some specks of a green mineral; there are veins and nests +filled with epidote: certainly a submarine lava. + +20th. Many thin strata of compact, fine-grained, pale purple sandstone. + +21st. Gypsum in a nearly pure state, about three hundred feet in +thickness: this bed, in its concretions of anhydrite and layers of +small blackish crystals of carbonate of lime, exactly resembles the +great gypseous beds in the Peuquenes range. + +22nd. Pale purple and reddish sandstone, as in bed 20: about three +hundred feet in thickness. + +23rd. A thick mass composed of layers, often as thin as paper and +convoluted, of pure gypsum with others very impure, of a purplish +colour. + +24th. Pure gypsum, thick mass. + +25th. Red sandstones, of great thickness. + +26th. Pure gypsum, of great thickness. + +27th. Alternating layers of pure and impure gypsum, of great thickness. + +I was not able to ascend to these few last great strata, which compose +the neighbouring loftiest pinnacles. The thickness, from the lowest to +the uppermost bed of gypsum, cannot be less than 2,000 feet: the beds +beneath I estimated at 3,000 feet, and this does not include either the +lower parts of the porphyritic conglomerate, or the altered clay-slate; +I conceive the total thickness must be about six thousand feet. I +distinctly observed that not only the gypsum, but the alternating +sandstones and conglomerates were lens-shaped, and repeatedly thinned +out and replaced each other: thus in the distance of about a mile, a +bed 300 feet thick of sandstone between two beds of gypsum, thinned out +to nothing and disappeared. The lower part of this section differs +remarkably,—in the much greater diversity of its mineralogical +composition,—in the abundance of calcareous matter,—in the greater +coarseness of some of the conglomerates,—and in the numerous particles +and well-rounded pebbles, sometimes of large size, of quartz,— from any +other section hitherto described in Chile. From these peculiarities and +from the lens-form of the strata, it is probable that this great pile +of strata was accumulated on a shallow and very uneven bottom, near +some pre-existing land formed of various porphyries and quartz-rock. +The formation of porphyritic claystone conglomerate does not in this +section attain nearly its ordinary thickness; this may be PARTLY +attributed to the metamorphic action having been here much less +energetic than usual, though the lower beds have been affected to a +certain degree. If it had been as energetic as in most other parts of +Chile, many of the beds of sandstone and conglomerate, containing +rounded masses of porphyry, would doubtless have been converted into +porphyritic conglomerate; and these would have alternated with, and +even blended into, crystalline and porphyritic strata without a trace +of mechanical structure,—namely, into those which, in the present state +of the section, we see are unquestionably submarine lavas. + +The beds of gypsum, together with the red alternating sandstones and +conglomerates, present so perfect and curious a resemblance with those +seen in our former section in the basin-valley of Yeso, that I cannot +doubt the identity of the two formations: I may add, that a little +westward of the P. del Inca, a mass of gypsum passed into a +fine-grained, hard, brown sandstone, which contained some layers of +black, calcareous, compact, shaly rock, precisely like that seen in +such vast masses on the Peuquenes range. + +Near the Puente del Inca, numerous fragments of limestone, containing +some fossil remains, were scattered on the ground: these fragments so +perfectly resemble the limestone of bed No. 3, in which I saw +impressions of shells, that I have no doubt they have fallen from it. +The yellow magnesian limestone of bed No. 10, which also includes +traces of shells, has a different appearance. These fossils (as named +by M. d’Orbigny) consist of:— + +Gryphaea, near to G. Couloni (Neocomian formation). Arca, perhaps A. +Gabrielis, d’Orbigny, “Pal. Franc.” (Neocomian formation). + +Mr. Pentland made a collection of shells from this same spot, and Von +Buch considers them as consisting of:— + +Trigonia, resembling in form T. costata. Pholadomya, like one found by +M. Dufresnoy near Alencon. Isocardi excentrica, Voltz., identical with +that from the Jura. (“Description Phys. des Iles Can.” page 472.) + +Two of these shells, namely, the Gryphaea and Trigonia, appear to be +identical with species collected by Meyen and myself on the Peuquenes +range; and in the opinion of Von Buch and M. d’Orbigny, the two +formations belong to the same age. I must here add, that Professor E. +Forbes, who has examined my specimens from this place and from the +Peuquenes range, has likewise a strong impression that they indicate +the Cretaceous period, and probably an early epoch in it: so that all +the palaeontologists who have seen these fossils nearly coincide in +opinion regarding their age. The limestone, however, with these fossils +here lies at the very base of the formation, just above the porphyritic +conglomerate, and certainly several thousand feet lower in the series, +than the equivalent, fossiliferous, black, shaly rocks high up on the +Peuquenes range. + +It is well worthy of remark that these shells, or at least those of +which I saw impressions in the limestone (bed No. 3), must have been +covered up, on the LEAST computation, by 4,000 feet of strata: now we +know from Professor E. Forbes’s researches, that the sea at greater +depths than 600 feet becomes exceedingly barren of organic beings,—a +result quite in accordance with what little I have seen of deep-sea +soundings. Hence, after this limestone with its shells was deposited, +the bottom of the sea where the main line of the Cordillera now stands, +must have subsided some thousand feet to allow of the deposition of the +superincumbent submarine strata. Without supposing a movement of this +kind, it would, moreover, be impossible to understand the accumulation +of the several lower strata of COARSE, well-rounded conglomerates, +which it is scarcely possible to believe were spread out in profoundly +deep water, and which, especially those containing pebbles of quartz, +could hardly have been rounded in submarine craters and afterwards +ejected from them, as I believe to have been the case with much of the +porphyritic conglomerate formation. I may add that, in Professor +Forbes’s opinion, the above-enumerated species of mollusca probably did +not live at a much greater depth than twenty fathoms, that is only 120 +feet. + +To return to our section down the valley; standing on the great N. by +W. and S. by E. uniclinal axis of the Puente del Inca, of which a +section has just been given, and looking north-east, greater tabular +masses of gypseous formation (KK) could be seen in the distance, very +slightly inclined towards the east. Lower down the valley, the +mountains are almost exclusively composed of porphyries, many of them +of intrusive origin and non-stratified, others stratified, but with the +stratification seldom distinguishable except in the upper parts. +Disregarding local disturbances, the beds are either horizontal or +inclined at a small angle eastwards: hence, when standing on the plain +of Uspallata and looking to the west or backwards, the Cordillera +appear composed of huge, square, nearly horizontal, tabular masses: so +wide a space, with such lofty mountains so equably elevated, is rarely +met with within the Cordillera. In this line of section, the interval +between the Puente del Inca and the neighbourhood of the Cumbre, +includes all the chief axes of dislocation. + +The altered clay-slate formation, already described, is seen in several +parts of the valley as far down as Las Vacas, underlying the +porphyritic conglomerate. At the Casa de Pujios [L], there is a hummock +of (andesitic?) granite; and the stratification of the surrounding +mountains here changes from W. by S. to S.W. Again, near the R. Vacas +there is a larger formation of (andesitic?) granite [M], which sends a +meshwork of veins into the superincumbent clay-slate, and which locally +throws off the strata, on one side to N.W. and on the other to S.E. but +not at a high angle: at the junction, the clay-slate is altered into +fine-grained greenstone. This granitic axis is intersected by a green +dike, which I mention, because I do not remember having elsewhere seen +dikes in this lowest and latest intrusive rock. From the R. Vacas to +the plain of Uspallata, the valley runs N.E., so that I have had to +contract my section; it runs exclusively through porphyritic rocks. As +far as the Pass of Jaula, the claystone conglomerate formation, in most +parts highly porphyritic, and crossed by numerous dikes of greenstone +porphyry, attains a great thickness: there is also much intrusive +porphyry. From the Jaula to the plain, the stratification has been in +most places obliterated, except near the tops of some of the mountains; +and the metamorphic action has been extremely great. In this space, the +number and bulk of the intrusive masses of differently coloured +porphyries, injected one into another and intersected by dikes, is +truly extraordinary. I saw one mountain of whitish porphyry, from which +two huge dikes, thinning out, branched DOWNWARDS into an adjoining +blackish porphyry. Another hill of white porphyry, which had burst +through dark- coloured strata, was itself injected by a purple, +brecciated, and recemented porphyry, both being crossed by a green +dike, and both having been upheaved and injected by a granitic dome. +One brick-red porphyry, which above the Jaula forms an isolated mass in +the midst of the porphyritic conglomerate formation, and lower down the +valley a magnificent group of peaked mountains, differs remarkably from +all the other porphyries. It consists of a red feldspathic base, +including some rather large crystals of red feldspar, numerous large +angular grains of quartz, and little bits of a soft green mineral +answering in most of its characters to soapstone. The crystals of red +feldspar resemble in external appearance those of orthite, though, from +being partially decomposed, I was unable to measure them; and they +certainly are quite unlike the variety, so abundantly met with in +almost all the other rocks of this line of section, and which, wherever +I tried it, cleaved like albite. This brick-red porphyry appears to +have burst through all the other porphyries, and numerous red dikes +traversing the neighbouring mountains have proceeded from it: in some +few places, however, it was intersected by white dikes. From this +posteriority of intrusive origin,—from the close general resemblance +between this red porphyry and the red granite of the Portillo line, the +only difference being that the feldspar here is less perfectly +granular, and that soapstone replaces the mica, which is there +imperfect and passes into chlorite,—and from the Portillo line a little +southward of this point appearing to blend (according to Dr. Gillies) +into the western ranges,—I am strongly urged to believe (as formerly +remarked) that the grand mountain-masses composed of this brick-red +porphyry belong to the same axis of injection with the granite of the +Portillo line; if so, the injection of this porphyry probably took +place, as long subsequently to the several axes of elevation in the +gypseous formation near the Cumbre, as the injection of the Portillo +granite has been shown to have been subsequent to the elevation of the +gypseous strata composing the Peuquenes range; and this interval, we +have seen, must have been a very long one. + +The Plain of Uspallata has been briefly described in Chapter 3; it +resembles the basin-plains of Chile; it is ten or fifteen miles wide, +and is said to extend for 180 miles northward; its surface is nearly +six thousand feet above the sea; it is composed, to a thickness of some +hundred feet of loosely aggregated, stratified shingle, which is +prolonged with a gently sloping surface up the valleys in the mountains +on both sides. One section in this plain [Z] is interesting, from the +unusual circumstance of alternating layers of almost loose red and +white sand with lines of pebbles (from the size of a nut to that of an +apple), and beds of gravel, being inclined at an angle of 45 degrees, +and in some spots even at a higher angle. (I find that Mr. Smith of +Jordan Hill has described (“Edinburgh New Philosophical Journal” volume +25 page 392) beds of sand and gravel, near Edinburgh, tilted at an +angle of 60 degrees, and dislocated by miniature faults.) These beds +are dislocated by small faults: and are capped by a thick mass of +horizontally stratified gravel, evidently of subaqueous origin. Having +been accustomed to observe the irregularities of beds accumulated under +currents, I feel sure that the inclination here has not been thus +produced. The pebbles consist chiefly of the brick-red porphyry just +described and of white granite, both probably derived from the ranges +to the west, and of altered clay-slate and of certain porphyries, +apparently belonging to the rocks of the Uspallata chain. This plain +corresponds geographically with the valley of Tenuyan between the +Portillo and Peuquenes ranges; but in that valley the shingle, which +likewise has been derived both from the eastern and western ranges, has +been cemented into a hard conglomerate, and has been throughout tilted +at a considerable inclination; the gravel there apparently attains a +much greater thickness, and is probably of higher antiquity. + +THE USPALLATA RANGE. + +The road by the Villa Vicencio Pass does not strike directly across the +range, but runs for some leagues northward along its western base: and +I must briefly describe the rocks here seen, before continuing with the +coloured east and west section. At the mouth of the valley of Canota, +and at several points northwards, there is an extensive formation of a +glossy and harsh, and of a feldspathic clay-slate, including strata of +grauwacke, and having a tortuous, nearly vertical cleavage, traversed +by numerous metalliferous veins and others of quartz. The clay-slate is +in many parts capped by a thick mass of fragments of the same rock, +firmly recemented; and both together have been injected and broken up +by very numerous hillocks, ranging north and south, of lilac, white, +dark and salmon- coloured porphyries: one steep, now denuded, hillock +of porphyry had its face as distinctly impressed with the angles of a +fragmentary mass of the slate, with some of the points still remaining +embedded, as sealing-wax could be by a seal. At the mouth of this same +valley of Canota, in a fine escarpment having the strata dipping from +50 to 60 degrees to the N.E. (Nearly opposite to this escarpment, there +is another corresponding one, with the strata dipping not to the +exactly opposite point, or S.W., but to S.S.W.: consequently the two +escarpments trend towards each other, and some miles southward they +become actually united: this is a form of elevation which I have not +elsewhere seen.), the clay-slate formation is seen to be covered +by—(1st) a purple, claystone porphyry resting unconformably in some +parts on the solid slate, and in others on a thick fragmentary mass; +(2nd), a conformable stratum of compact blackish rock, having a +spheroidal structure, full of minute acicular crystals of glassy +feldspar, with red spots of oxide of iron; (3rd), a great stratum of +purplish-red claystone porphyry, abounding with crystals of opaque +feldspar, and laminated with thin, parallel, often short, layers, and +likewise with great irregular patches of white, earthy, +semi-crystalline feldspar; this rock (which I noticed in other +neighbouring places) perfectly resembles a curious variety described at +Port Desire, and occasionally occurs in the great porphyritic +conglomerate formation of Chile; (4th), a thin stratum of greenish +white, indurated tuff, fusible and containing broken crystals and +particles of porphyries; (5th), a grand mass, imperfectly columnar and +divided into three parallel and closely joined strata, of +cream-coloured claystone porphyry; (6th), a thick stratum of +lilac-coloured porphyry, which I could see was capped by another bed of +the cream-coloured variety; I was unable to examine the still higher +parts of the escarpment. These conformably stratified porphyries, +though none are either vesicular are amygdaloidal, have evidently +flowed as submarine lavas: some of them are separated from each other +by seams of indurated tuff, which, however, are quite insignificant in +thickness compared with the porphyries. This whole pile resembles, but +not very closely, some of the less brecciated parts of the great +porphyritic conglomerate formation of Chile; but it does not probably +belong to the same age, as the porphyries here rest unconformably on +the altered feldspathic clay-slate, whereas the porphyritic +conglomerate formation alternates with and rests conformably on it. +These porphyries, moreover, with the exception of the one blackish +stratum, and of the one indurated, white tufaceous bed, differ from the +beds composing the Uspallata range in the line of the Villa Vicencio +Pass. + +I will now give, first, a sketch of the structure of the range, as +represented in the section, and will then describe its composition and +interesting history. At its western foot, a hillock [N] is seen to rise +out of the plain, with its strata dipping at 70 degrees to the west, +fronted by strata [O] inclined at 45 degrees to the east, thus forming +a little north and south anticlinal axis. Some other little hillocks of +similar composition, with their strata highly inclined, range N.E. and +S.W., obliquely to the main Uspallata line. The cause of these +dislocations, which, though on a small scale, have been violent and +complicated, is seen to lie in hummocks of lilac, purple and red +porphyries, which have been injected in a liquified state through and +into the underlying clay-slate formation. Several dykes were exposed +here, but in no other part, that I saw of this range. As the strata +consist of black, white, greenish and brown-coloured rocks, and as the +intrusive porphyries are so brightly tinted, a most extraordinary view +was presented, like a coloured geological drawing. On the gently +inclined main western slope [PP], above the little anticlinal ridges +just mentioned, the strata dip at an average angle of 25 degrees to the +west; the inclination in some places being only 19 degrees, in some few +others as much as 45 degrees. The masses having these different +inclinations, are separated from each other by parallel vertical faults +[as represented at Pa], often giving rise to separate, parallel, +uniclinal ridges. The summit of the main range is broad and undulatory, +with the stratification undulatory and irregular: in a few places +granitic and porphyritic masses [Q] protrude, which, from the small +effect they have locally produced in deranging the strata, probably +form the upper points of a regular, great underlying dome. These +denuded granitic points, I estimated at about nine thousand feet in +height above the sea. On the eastern slope, the strata in the upper +part are regularly inclined at about 25 degrees to the east, so that +the summit of this chain, neglecting small irregularities, forms a +broad anticlinal axis. Lower down, however, near Los Hornillos [R], +there is a well-marked synclinal axis, beyond which the strata are +inclined at nearly the same angle, namely from 20 to 30 degrees, +inwards or westward. Owing to the amount of denudation which this chain +has suffered, the outline of the gently inclined eastern flank scarcely +offers the slightest indication of this synclinal axis. The stratified +beds, which we have hitherto followed across the range, a little +further down are seen to lie, I believe unconformably, on a broad +mountainous band of clay-slate and grauwacke. The strata and laminae of +this latter formation, on the extreme eastern flank, are generally +nearly vertical; further inwards they become inclined from 45 to 80 +degrees to the west: near Villa Vicencio [S] there is apparently an +anticlinal axis, but the structure of this outer part of the clay-slate +formation is so obscure, that I have not marked the planes of +stratification in the section. On the margin of the Pampas, some low, +much dislocated spurs of this same formation, project in a north- +easterly line, in the same oblique manner as do the ridges on the +western foot, and as is so frequently the case with those at the base +of the main Cordillera. + +I will now describe the nature of the beds, beginning at the base on +the eastern side. First, for the clay-slate formation: the slate is +generally hard and bluish, with the laminae coated by minute micaceous +scales; it alternates many times with a coarse-grained, greenish +grauwacke, containing rounded fragments of quartz and bits of slate in +a slightly calcareous basis. The slate in the upper part generally +becomes purplish, and the cleavage so irregular that the whole consists +of mere splinters. Transverse veins of quartz are numerous. At the +Calera, some leagues distant, there is a dark crystalline limestone, +apparently included in this formation. With the exception of the +grauwacke being here more abundant, and the clay-slate less altered, +this formation closely resembles that unconformably underlying the +porphyries at the western foot of this same range; and likewise that +alternating with the porphyritic conglomerate in the main Cordillera. +This formation is a considerable one, and extends several leagues +southward to near Mendoza: the mountains composed of it rise to a +height of about two thousand feet above the edge of the Pampas, or +about seven thousand feet above the sea. (I infer this from the height +of V. Vicencio, which was ascertained by Mr. Miers to be 5,328 feet +above the sea.) + +Secondly: the most usual bed on the clay-slate is a coarse, white, +slightly calcareous conglomerate, of no great thickness, including +broken crystals of feldspar, grains of quartz, and numerous pebbles of +brecciated claystone porphyry, but without any pebbles of the +underlying clay-slate. I nowhere saw the actual junction between this +bed and the clay-slate, though I spent a whole day in endeavouring to +discover their relations. In some places I distinctly saw the white +conglomerate and overlying beds inclined at from 25 to 30 degrees to +the west, and at the bottom of the same mountain, the clay-slate and +grauwacke inclined to the same point, but at an angle from 70 to 80 +degrees: in one instance, the clay-slate dipped not only at a different +angle, but to a different point from the overlying formation. In these +cases the two formations certainly appeared quite unconformable: +moreover, I found in the clay-slate one great, vertical, dike-like +fissure, filled up with an indurated whitish tuff, quite similar to +some of the upper beds presently to be described; and this shows that +the clay-slate must have been consolidated and dislocated before their +deposition. On the other hand, the stratification of the slate and +grauwacke, in some cases gradually and entirely disappeared in +approaching the overlying white conglomerate; in other cases the +stratification of the two formations became strictly conformable; and +again in other cases, there was some tolerably well characterised +clay-slate lying above the conglomerate. (The coarse, mechanical +structure of many grauwackes has always appeared to me a difficulty; +for the texture of the associated clay-slate and the nature of the +embedded organic remains where present, indicate that the whole has +been a deep-water deposit. Whence have the sometimes included angular +fragments of clay-slate, and the rounded masses of quartz and other +rocks, been derived? Many deep-water limestones, it is well known, have +been brecciated, and then firmly recemented.) The most probable +conclusion appears to be, that after the clay-slate formation had been +dislocated and tilted, but whilst under the sea, a fresh and more +recent deposition of clay-slate took place, on which the white +conglomerate was conformably deposited, with here and there a thin +intercalated bed of clay-slate. On this view the white conglomerates +and the presently to be described tuffs and lavas are really +unconformable to the main part of the clay-slate; and this, as we have +seen, certainly is the case with the clay-stone lavas in the valley of +Canota, at the western and opposite base of the range. + +Thirdly: on the white conglomerate, strata several hundred feet in +thickness are superimposed, varying much in nature in short distances: +the commonest variety is a white, much indurated tuff, sometimes +slightly calcareous, with ferruginous spots and water-lines, often +passing into whitish or purplish compact, fine-grained grit or +sandstones; other varieties become semi-porcellanic, and tinted faint +green or blue; others pass into an indurated shale: most of these +varieties are easily fusible. + +Fourthly: a bed, about one hundred feet thick of a compact, partially +columnar, pale-grey, feldspathic lava, stained with iron, including +very numerous crystals of opaque feldspar, and with some crystallised +and disseminated calcareous matter. The tufaceous stratum on which this +feldspathic lava rests is much hardened, stained purple, and has a +spherico-concretionary structure; it here contains a good many pebbles +of claystone porphyry. + +Fifthly: thin beds, 400 feet in thickness, varying much in nature, +consisting of white and ferruginous tuffs, in some parts having a +concretionary structure, in others containing rounded grains and a few +pebbles of quartz; also passing into hard gritstones and into greenish +mudstones: there is, also, much of a bluish-grey and green +semi-porcellanic stone. + +Sixthly: a volcanic stratum, 250 feet in thickness, of so varying a +nature that I do not believe a score of specimens would show all the +varieties; much is highly amygdaloidal, much compact; there are +greenish, blackish, purplish, and grey varieties, rarely including +crystals of green augite and minute acicular ones of feldspar, but +often crystals and amygdaloidal masses of white, red, and black +carbonate of lime. Some of the blackish varieties of this rock have a +conchoidal fracture and resemble basalt; others have an irregular +fracture. Some of the grey and purplish varieties are thickly speckled +with green earth and with white crystalline carbonate of lime; others +are largely amygdaloidal with green earth and calcareous spar. Again, +other earthy varieties, of greenish, purplish and grey tints, contain +much iron, and are almost half composed of amygdaloidal balls of dark +brown bole, of a whitish indurated feldspathic matter, of bright green +earth, of agate, and of black and white crystallised carbonate of lime. +All these varieties are easily fusible. Viewed from a distance, the +line of junction with the underlying semi-porcellanic strata was +distinct; but when examined closely, it was impossible to point out +within a foot where the lava ended and where the sedimentary mass +began: the rock at the time of junction was in most places hard, of a +bright green colour, and abounded with irregular amygdaloidal masses of +ferruginous and pure calcareous spar, and of agate. + +Seventhly: strata, eighty feet in thickness, of various indurated +tuffs, as before; many of the varieties have a fine basis including +rather coarse extraneous particles; some of them are compact and +semi-porcellanic, and include vegetable impressions. + +Eighthly: a bed, about fifty feet thick, of greenish-grey, compact, +feldspathic lava, with numerous small crystals of opaque feldspar, +black augite, and oxide of iron. The junction with the bed on which it +rested, was ill defined; balls and masses of the feldspathic rock being +enclosed in much altered tuff. + +Ninthly: indurated tuffs, as before. + +Tenthly: a conformable layer, less than two feet in thickness, of +pitchstone, generally brecciated, and traversed by veins of agate and +of carbonate of lime: parts are composed of apparently concretionary +fragments of a more perfect variety, arranged in horizontal lines in a +less perfectly characterised variety. I have much difficulty in +believing that this thin layer of pitchstone flowed as lava. + +Eleventhly: sedimentary and tufaceous beds as before, passing into +sandstone, including some conglomerate: the pebbles in the latter are +of claystone porphyry, well rounded, and some as large as +cricket-balls. + +Twelfthly: a bed of compact, sonorous, feldspathic lava, like that of +bed No. 8, divided by numerous joints into large angular blocks. + +Thirteenthly: sedimentary beds as before. + +Fourteenthly: a thick bed of greenish or greyish black, compact basalt +(fusing into a black enamel), with small crystals, occasionally +distinguishable, of feldspar and augite: the junction with the +underlying sedimentary bed, differently from that in most of the +foregoing streams, here was quite distinct:—the lava and tufaceous +matter preserving their perfect characters within two inches of each +other. This rock closely resembles certain parts of that varied and +singular lava-stream No. 6; it likewise resembles, as we shall +immediately see, many of the great upper beds on the western flank and +on the summit of this range. + +The pile of strata here described attains a great thickness; and above +the last-mentioned volcanic stratum, there were several other great +tufaceous beds alternating with submarine lavas, which I had not time +to examine; but a corresponding series, several thousand feet in +thickness, is well exhibited on the crest and western flank of the +range. Most of the lava- streams on the western side are of a jet-black +colour and basaltic nature; they are either compact and fine-grained, +including minute crystals of augite and feldspar, or they are +coarse-grained and abound with rather large coppery-brown crystals of +an augitic mineral. (Very easily fusible into a jet-black bead, +attracted by the magnet: the crystals are too much tarnished to be +measured by the goniometer.) Another variety was of a dull- red colour, +having a claystone brecciated basis, including specks of oxide of iron +and of calcareous spar, and amygdaloidal with green earth: there were +apparently several other varieties. These submarine lavas often exhibit +a spheroidal, and sometimes an imperfect columnar structure: their +upper junctions are much more clearly defined than their lower +junctions; but the latter are not so much blended into the underlying +sedimentary beds as is the case in the eastern flank. On the crest and +western flank of the range, the streams, viewed as a whole, are mostly +basaltic; whilst those on the eastern side, which stand lower in the +series, are, as we have seen, mostly feldspathic. + +The sedimentary strata alternating with the lavas on the crest and +western side, are of an almost infinitely varying nature; but a large +proportion of them closely resemble those already described on the +eastern flank: there are white and brown, indurated, easily fusible +tuffs,—some passing into pale blue and green semi-porcellanic +rocks,—others into brownish and purplish sandstones and gritstones, +often including grains of quartz,— others into mudstone containing +broken crystals and particles of rock, and occasionally single large +pebbles. There was one stratum of a bright red, coarse, volcanic +gritstone; another of conglomerate; another of a black, indurated, +carbonaceous shale marked with imperfect vegetable impressions; this +latter bed, which was thin, rested on a submarine lava, and followed +all the considerable inequalities of its upper surface. Mr. Miers +states that coal has been found in this range. Lastly, there was a bed +(like No. 10 on the eastern flank) evidently of sedimentary origin, and +remarkable from closely approaching in character to an imperfect +pitchstone, and from including extremely thin layers of perfect +pitchstone, as well as nodules and irregular fragments (but not +resembling extraneous fragments) of this same rock arranged in +horizontal lines: I conceive that this bed, which is only a few feet in +thickness, must have assumed its present state through metamorphic and +concretionary action. Most of these sedimentary strata are much +indurated, and no doubt have been partially metamorphosed: many of them +are extraordinarily heavy and compact; others have agate and +crystalline carbonate of lime disseminated throughout them. Some of the +beds exhibit a singular concretionary arrangement, with the curves +determined by the lines of fissure. There are many veins of agate and +calcareous spar, and innumerable ones of iron and other metals, which +have blackened and curiously affected the strata to considerable +distances on both sides. + +Many of these tufaceous beds resemble, with the exception of being more +indurated, the upper beds of the Great Patagonian tertiary formation, +especially those variously coloured layers high up the River Santa +Cruz, and in a remarkable degree the tufaceous formation at the +northern end of Chiloe. I was so much struck with this resemblance, +that I particularly looked out for silicified wood, and found it under +the following extraordinary circumstances. High up on this western +flank, at a height estimated at 7,000 feet above the sea, in a broken +escarpment of thin strata, composed of compact green gritstone passing +into a fine mudstone, and alternating with layers of coarser, brownish, +very heavy mudstone, including broken crystals and particles of rock +almost blended together, I counted the stumps of fifty-two trees. (For +the information of any future traveller, I will describe the spot in +detail. Proceeding eastward from the Agua del Zorro, and afterwards +leaving on the north side of the road a rancho attached to some old +goldmines, you pass through a gully with low but steep rocks on each +hand: the road then bends, and the ascent becomes steeper. A few +hundred yards farther on, a stone’s throw on the south side of the +road, the white calcareous stumps may be seen. The spot is about half a +mile east of the Agua del Zorro.) They projected between two and five +feet above the ground, and stood at exactly right angles to the strata, +which were here inclined at an angle of about 25 degrees to the west. +Eleven of these trees were silicified and well preserved; Mr. R. Brown +has been so kind as to examine the wood when sliced and polished; he +says it is coniferous, partaking of the characters of the Araucarian +tribe, with some curious points of affinity with the Yew. The bark +round the trunks must have been circularly furrowed with irregular +lines, for the mudstone round them is thus plainly marked. One cast +consisted of dark argillaceous limestone; and forty of them of coarsely +crystallised carbonate of lime, with cavities lined by quartz crystals: +these latter white calcareous columns do not retain any internal +structure, but their external form plainly shows their origin. All the +stumps have nearly the same diameter, varying from one foot to eighteen +inches; some of them stand within a yard of each other; they are +grouped in a clump within a space of about sixty yards across, with a +few scattered round at the distance of 150 yards. They all stand at +about the same level. The longest stump stood seven feet out of the +ground: the roots, if they are still preserved, are buried and +concealed. No one layer of the mudstone appeared much darker than the +others, as if it had formerly existed as soil, nor could this be +expected, for the same agents which replaced with silex and lime the +wood of the trees, would naturally have removed all vegetable matter +from the soil. Besides the fifty-two upright trees, there were a few +fragments, like broken branches, horizontally embedded. The surrounding +strata are crossed by veins of carbonate of lime, agate, and oxide of +iron; and a poor gold vein has been worked not far from the trees. + +The green and brown mudstone beds including the trees, are conformably +covered by much indurated, compact, white or ferruginous tuffs, which +pass upwards into a fine-grained, purplish sedimentary rock: these +strata, which, together, are from four to five hundred feet in +thickness, rest on a thick bed of submarine lava, and are conformably +covered by another great mass of fine-grained basalt, which I estimated +at 1,000 feet in thickness, and which probably has been formed by more +than one stream. (This rock is quite black, and fuses into a black +bead, attracted strongly by the magnet; it breaks with a conchoidal +fracture; the included crystals of augite are distinguishable by the +naked eye, but are not perfect enough to be measured: there are many +minute acicular crystals of glassy feldspar.) Above this mass I could +clearly distinguish five conformable alternations, each several hundred +feet in thickness, of stratified sedimentary rocks and lavas, such as +have been previously described. Certainly the upright trees have been +buried under several thousand feet in thickness of matter, accumulated +under the sea. As the trees obviously must once have grown on dry land, +what an enormous amount of subsidence is thus indicated! Nevertheless, +had it not been for the trees there was no appearance which would have +led any one even to have conjectured that these strata had subsided. As +the land, moreover, on which the trees grew, is formed of subaqueous +deposits, of nearly if not quite equal thickness with the +superincumbent strata, and as these deposits are regularly stratified +and fine-grained, not like the matter thrown up on a sea-beach, a +previous upward movement, aided no doubt by the great accumulation of +lavas and sediment, is also indicated. (At first I imagined, that the +strata with the trees might have been accumulated in a lake: but this +seems highly improbable; for, first, a very deep lake was necessary to +receive the matter below the trees, then it must have been drained for +their growth, and afterwards re-formed and made profoundly deep, so as +to receive a subsequent accumulation of matter SEVERAL THOUSAND feet in +thickness. And all this must have taken place necessarily before the +formation of the Uspallata range, and therefore on the margin of the +wide level expanse of the Pampas! Hence I conclude, that it is +infinitely more probable that the strata were accumulated under the +sea: the vast amount of denudation, moreover, which this range has +suffered, as shown by the wide valleys, by the exposure of the very +trees and by other appearances, could have been effected, I conceive, +only by the long-continued action of the sea; and this shows that the +range was either upheaved from under the sea, or subsequently let down +into it. From the natural manner in which the stumps (fifty-two in +number) are GROUPED IN A CLUMP, and from their all standing vertically +to the strata, it is superfluous to speculate on the chance of the +trees having been drifted from adjoining land, and deposited upright: I +may, however, mention that the late Dr. Malcolmson assured me, that he +once met in the Indian Ocean, fifty miles from land, several cocoa-nut +trees floating upright, owing to their roots being loaded with earth.) + +In nearly the middle of the range, there are some hills [Q], before +alluded to, formed of a kind of granite externally resembling andesite, +and consisting of a white, imperfectly granular, feldspathic basis, +including some perfect crystals apparently of albite (but I was unable +to measure them), much black mica, epidote in veins, and very little or +no quartz. Numerous small veins branch from this rock into the +surrounding strata; and it is a singular fact that these veins, though +composed of the same kind of feldspar and small scales of mica as in +the solid rock, abound with innumerable minute ROUNDED grains of +quartz: in the veins or dikes also, branching from the great granitic +axis in the peninsula of Tres Montes, I observed that quartz was more +abundant in them than in the main rock: I have heard of other analogous +cases: can we account for this fact, by the long-continued vicinity of +quartz when cooling, and by its having been thus more easily sucked +into fissures than the other constituent minerals of granite? (See a +paper by M. Elie de Beaumont, “Soc. Philomath.” May 1839 “L’Institut.” +1839 page 161.) The strata encasing the flanks of these granitic or +andesite masses, and forming a thick cap on one of their summits, +appear originally to have been of the same tufaceous nature with the +beds already described, but they are now changed into porcellanic, +jaspery, and crystalline rocks, and into others of a white colour with +a harsh texture, and having a siliceous aspect, though really of a +feldspathic nature and fusible. Both the granitic intrusive masses and +the encasing strata are penetrated by innumerable metallic veins, +mostly ferruginous and auriferous, but some containing copper-pyrites +and a few silver: near the veins, the rocks are blackened as if blasted +by gunpowder. The strata are only slightly dislocated close round these +hills, and hence, perhaps, it may be inferred that the granitic masses +form only the projecting points of a broad continuous axis-dome, which +has given to the upper parts of this range its anticlinal structure. + +CONCLUDING REMARKS ON THE USPALLATA RANGE. + +I will not attempt to estimate the total thickness of the pile of +strata forming this range, but it must amount to many thousand feet. +The sedimentary and tufaceous beds have throughout a general +similarity, though with infinite variations. The submarine lavas in the +lower part of the series are mostly feldspathic, whilst in the upper +part, on the summit and western flank, they are mostly basaltic. We are +thus reminded of the relative position in most recent volcanic +districts of the trachytic and basaltic lavas,—the latter from their +greater weight having sunk to a lower level in the earth’s crust, and +having consequently been erupted at a later period over the lighter and +upper lavas of the trachytic series. (See on this subject, “Volcanic +Islands” etc. by the Author.) Both the basaltic and feldspathic +submarine streams are very compact; none being vesicular, and only a +few amygdaloidal: the effects which some of them, especially those low +in the series, have produced on the tufaceous beds over which they have +flowed is highly curious. Independently of this local metamorphic +action, all the strata undoubtedly display an indurated and altered +character; and all the rocks of this range—the lavas, the alternating +sediments, the intrusive granite and porphyries, and the underlying +clay- slate—are intersected by metalliferous veins. The lava-strata can +often be seen extending for great distances, conformably with the under +and overlying beds; and it was obvious that they thickened towards the +west. Hence the points of eruption must have been situated westward of +the present range, in the direction of the main Cordillera: as, +however, the flanks of the Cordillera are entirely composed of various +porphyries, chiefly claystone and greenstone, some intrusive, and +others belonging to the porphyritic conglomerate formation, but all +quite unlike these submarine lava-streams, we must in all probability +look to the plain of Uspallata for the now deeply buried points of +eruption. + +Comparing our section of the Uspallata range with that of the Cumbre, +we see, with the exception of the underlying clay-slate, and perhaps of +the intrusive rocks of the axes, a striking dissimilarity in the strata +composing them. The great porphyritic conglomerate formation has not +extended as far as this range; nor have we here any of the gypseous +strata, the magnesian and other limestones, the red sandstones, the +siliceous beds with pebbles of quartz, and comparatively little of the +conglomerates, all of which form such vast masses over the basal series +in the main Cordillera. On the other hand, in the Cordillera, we do not +find those endless varieties of indurated tuffs, with their numerous +veins and concretionary arrangement, and those grit and mud stones, and +singular semi-porcellanic rocks, so abundant in the Uspallata range. +The submarine lavas, also, differ considerably; the feldspathic streams +of the Cordillera contain much mica, which is absent in those of the +Uspallata range: in this latter range we have seen on how grand a +scale, basaltic lava has been poured forth, of which there is not a +trace in the Cordillera. This dissimilarity is the more striking, +considering that these two parallel chains are separated by a plain +only between ten and fifteen miles in width; and that the Uspallata +lavas, as well as no doubt the alternating tufaceous beds, have +proceeded from the west, from points apparently between the two ranges. +To imagine that these two piles of strata were contemporaneously +deposited in two closely adjoining, very deep, submarine areas, +separated from each other by a lofty ridge, where a plain now extends, +would be a gratuitous hypothesis. And had they been contemporaneously +deposited, without any such dividing ridge, surely some of the gypseous +and other sedimentary matter forming such immensely thick masses in the +Cordillera, would have extended this short distance eastwards; and +surely some of the Uspallata tuffs and basalts also accumulated to so +great a thickness, would have extended a little westward. Hence I +conclude, that it is far from probable that these two series are not +contemporaneous; but that the strata of one of the chains were +deposited, and even the chain itself uplifted, before the formation of +the other:—which chain, then, is the oldest? Considering that in the +Uspallata range the lowest strata on the western flank lie +unconformably on the clay- slate, as probably is the case with those on +the eastern flank, whereas in the Cordillera all the overlying strata +lie conformably on this formation:—considering that in the Uspallata +range some of the beds, both low down and high up in the series, are +marked with vegetable impressions, showing the continued existence of +neighbouring land;—considering the close general resemblance between +the deposits of this range and those of tertiary origin in several +parts of the continent;—and lastly, even considering the lesser height +and outlying position of the Uspallata range,—I conclude that the +strata composing it are in all probability of subsequent origin, and +that they were accumulated at a period when a deep sea studded with +submarine volcanoes washed the eastern base of the already partially +elevated Cordillera. + +This conclusion is of much importance, for we have seen that in the +Cordillera, during the deposition of the Neocomian strata, the bed of +the sea must have subsided many thousand feet: we now learn that at a +later period an adjoining area first received a great accumulation of +strata, and was upheaved into land on which coniferous trees grew, and +that this area then subsided several thousand feet to receive the +superincumbent submarine strata, afterwards being broken up, denuded, +and elevated in mass to its present height. I am strengthened in this +conclusion of there having been two distinct, great periods of +subsidence, by reflecting on the thick mass of coarse stratified +conglomerate in the valley of Tenuyan, between the Peuquenes and +Portillo lines; for the accumulation of this mass seems to me, as +previously remarked, almost necessarily to have required a prolonged +subsidence; and this subsidence, from the pebbles in the conglomerate +having been to a great extent derived from the gypseous or Neocomian +strata of the Peuquenes line, we know must have been quite distinct +from, and subsequent to, that sinking movement which probably +accompanied the deposition of the Peuquenes strata, and which certainly +accompanied the deposition of the equivalent beds near the Puente del +Inca, in this line of section. + +The Uspallata chain corresponds in geographical position, though on a +small scale, with the Portillo line; and its clay-slate formation is +probably the equivalent of the mica-schist of the Portillo, there +metamorphosed by the old white granites and syenites. The coloured beds +under the conglomerate in the valley of Tenuyan, of which traces are +seen on the crest of the Portillo, and even the conglomerate itself, +may perhaps be synchronous with the tufaceous beds and submarine lavas +of the Uspallata range; an open sea and volcanic action in the latter +case, and a confined channel between two bordering chains of islets in +the former case, having been sufficient to account for the +mineralogical dissimilarity of the two series. From this correspondence +between the Uspallata and Portillo ranges, perhaps in age and certainly +in geographical position, one is tempted to consider the one range as +the prolongation of the other; but their axes are formed of totally +different intrusive rocks; and we have traced the apparent continuation +of the red granite of the Portillo in the red porphyries diverging into +the main Cordillera. Whether the axis of the Uspallata range was +injected before, or as perhaps is more probable, after that of the +Portillo line, I will not pretend to decide; but it is well to remember +that the highly inclined lava-streams on the eastern flank of the +Portillo line, prove that its angular upheavement was not a single and +sudden event; and therefore that the anticlinal elevation of the +Uspallata range may have been contemporaneous with some of the later +angular movements by which the gigantic Portillo range gained its +present height above the adjoining plain. + + + + +CHAPTER VIII. +NORTHERN CHILE. CONCLUSION. + + +Section from Illapel to Combarbala; gypseous formation with silicified +wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley; +fossils.—Guasco, fossils of.—Copiapo, section up valley; Las Amolanas, +silicified wood.—Conglomerates, nature of former land, fossils, +thickness of strata, great subsidence.—Valley of Despoblado, fossils, +tufaceous deposit, complicated dislocations of.—Relations between +ancient orifices of eruption and subsequent axes of injection.—Iquique, +Peru, fossils of, salt-deposits.—Metalliferous veins.—Summary on the +porphyritic conglomerate and gypseous formations.—Great subsidence with +partial elevations during the cretaceo-oolitic period.—On the elevation +and structure of the Cordillera.—Recapitulation on the tertiary +series.—Relation between movements of subsidence and volcanic +action.—Pampean formation.—Recent elevatory movements.—Long-continued +volcanic action in the Cordillera.—Conclusion. + +VALPARAISO TO COQUIMBO. + +I have already described the general nature of the rocks in the low +country north of Valparaiso, consisting of granites, syenites, +greenstones, and altered feldspathic clay-slate. Near Coquimbo there is +much hornblendic rock and various dusky-coloured porphyries. I will +describe only one section in this district, namely, from near Illapel +in a N.E. line to the mines of Los Hornos, and thence in a north by +east direction to Combarbala, at the foot of the main Cordillera. + +Near Illapel, after passing for some distance over granite, andesite, +and andesitic porphyry, we come to a greenish stratified feldspathic +rock, which I believe is altered clay-slate, conformably capped by +porphyries and porphyritic conglomerate of great thickness, dipping at +an average angle of 20 degrees to N.E. by N. The uppermost beds consist +of conglomerates and sandstone only a little metamorphosed, and +conformably covered by a gypseous formation of very great thickness, +but much denuded. This gypseous formation, where first met with, lies +in a broad valley or basin, a little southward of the mines of Los +Hornos: the lower half alone contains gypsum, not in great masses as in +the Cordillera, but in innumerable thin layers, seldom more than an +inch or two in thickness. The gypsum is either opaque or transparent, +and is associated with carbonate of lime. The layers alternate with +numerous varying ones of a calcareous clay-shale (with strong aluminous +odour, adhering to the tongue, easily fusible into a pale green glass), +more or less indurated, either earthy and cream-coloured, or greenish +and hard. The more indurated varieties have a compact, homogeneous, +almost crystalline fracture, and contain granules of crystallised oxide +of iron. Some of the varieties almost resemble honestones. There is +also a little black, hardly fusible, siliceo- calcareous clay-slate, +like some of the varieties alternating with gypsum on the Peuquenes +range. + +The upper half of this gypseous formation is mainly formed of the same +calcareous clay-shale rock, but without any gypsum, and varying +extremely in nature: it passes from a soft, coarse, earthy, ferruginous +state, including particles of quartz, into compact claystones with +crystallised oxide of iron,—into porcellanic layers, alternating with +seams of calcareous matter,—and into green porcelain-jasper, +excessively hard, but easily fusible. Strata of this nature alternate +with much black and brown siliceo-calcareous slate, remarkable from the +wonderful number of huge embedded logs of silicified wood. This wood, +according to Mr. R. Brown, is (judging from several specimens) all +coniferous. Some of the layers of the black siliceous slate contained +irregular angular fragments of imperfect pitchstone, which I believe, +as in the Uspallata range, has originated in a metamorphic process. +There was one bed of a marly tufaceous nature, and of little specific +gravity. Veins of agate and calcareous spar are numerous. The whole of +this gypseous formation, especially the upper half, has been injected, +metamorphosed, and locally contorted by numerous hillocks of intrusive +porphyries crowded together in an extraordinary manner. These hillocks +consist of purple claystone and of various other porphyries, and of +much white feldspathic greenstone passing into andesite; this latter +variety included in one case crystals of orthitic and albitic feldspar +touching each other, and others of hornblende, chlorite, and epidote. +The strata surrounding these intrusive hillocks at the mines of Los +Hornos, are intersected by many veins of copper-pyrites, associated +with much micaceous iron-ore, and by some of gold: in the neighbourhood +of these veins the rocks are blackened and much altered. The gypsum +near the intrusive masses is always opaque. One of these hillocks of +porphyry was capped by some stratified porphyritic conglomerate, which +must have been brought up from below, through the whole immense +thickness of the overlying gypseous formation. The lower beds of the +gypseous formation resemble the corresponding and probably +contemporaneous strata of the main Cordillera; whilst the upper beds in +several respects resemble those of the Uspallata chain, and possibly +may be contemporaneous with them; for I have endeavoured to show that +the Uspallata beds were accumulated subsequently to the gypseous or +Neocomian formations of the Cordillera. + +This pile of strata dips at an angle of about 20 degrees to N.E. by N., +close up to the foot of the Cuesta de Los Hornos, a crooked range of +mountains formed of intrusive rocks of the same nature with the above +described hillocks. Only in one or two places, on this south-eastern +side of the range, I noticed a narrow fringe of the upper gypseous +strata brushed up and inclined south-eastward from it. On its +north-eastern flank, and likewise on a few of the summits, the +stratified porphyritic conglomerate is inclined N.E.: so that, if we +disregard the very narrow anticlinal fringe of gypseous strata at its +S.E. foot, this range forms a second uniclinal axis of elevation. +Proceeding in a north-by-east direction to the village of Combarbala, +we come to a third escarpment of the porphyritic conglomerate, dipping +eastwards, and forming the outer range of the main Cordillera. The +lower beds were here more jaspery than usual, and they included some +white cherty strata and red sandstones, alternating with purple +claystone porphyry. Higher up in the Cordillera there appeared to be a +line of andesitic rocks; and beyond them, a fourth escarpment of the +porphyritic conglomerate, again dipping eastwards or inwards. The +overlying gypseous strata, if they ever existed here, have been +entirely removed. + +COPPER MINES OF PANUNCILLO. + +From Combarbala to Coquimbo, I traversed the country in a zigzag +direction, crossing and recrossing the porphyritic conglomerate and +finding in the granitic districts an unusual number of mountain-masses +composed of various intrusive, porphyritic rocks, many of them +andesitic. One common variety was greenish-black, with large crystals +of blackish albite. At Panuncillo a short N.N.W. and S.S.E. ridge, with +a nucleus formed of greenstone and of a slate-coloured porphyry +including crystals of glassy feldspar, deserves notice, from the very +singular nature of the almost vertical strata composing it. These +consist chiefly of a finer and coarser granular mixture, not very +compact, of white carbonate of lime, of protoxide of iron and of +yellowish garnets (ascertained by Professor Miller), each grain being +an almost perfect crystal. Some of the varieties consist exclusively of +granules of the calcareous spar; and some contain grains of copper ore, +and, I believe, of quartz. These strata alternate with a bluish, +compact, fusible, feldspathic rock. Much of the above granular mixture +has, also, a pseudo-brecciated structure, in which fragments are +obscurely arranged in planes parallel to those of the stratification, +and are conspicuous on the weathered surfaces. The fragments are +angular or rounded, small or large, and consist of bluish or reddish +compact feldspathic matter, in which a few acicular crystals of +feldspar can sometimes be seen. The fragments often blend at their +edges into the surrounding granular mass, and seem due to a kind of +concretionary action. + +These singular rocks are traversed by many copper veins, and appear to +rest conformably on the granular mixture (in parts as fine-grained as a +sandstone) of quartz, mica, hornblende, and feldspar; and this on fine- +grained, common gneiss; and this on a laminated mass, composed of +pinkish ORTHITIC feldspar, including a few specks of hornblende; and +lastly, this on granite, which together with andesitic rocks, form the +surrounding district. + +COQUIMBO: MINING DISTRICT OF ARQUEROS. + +At Coquimbo the porphyritic conglomerate formation approaches nearer to +the Pacific than in any other part of Chile visited by me, being +separated from the coast by a tract only a few miles broad of the usual +plutonic rocks, with the addition of a porphyry having a red euritic +base. In proceeding to the mines of Arqueros, the strata of porphyritic +conglomerate are at first nearly horizontal, an unusual circumstance, +and afterwards they dip gently to S.S.E. After having ascended to a +considerable height, we come to an undulatory district in which the +famous silver mines are situated; my examination was chiefly confined +to those of S. Rosa. Most of the rocks in this district are stratified, +dipping in various directions, and many of them are of so singular a +nature, that at the risk of being tedious I must briefly describe them. +The commonest variety is a dull-red, compact, finely brecciated stone, +containing much iron and innumerable white crystallised particles of +carbonate of lime, and minute extraneous fragments. Another variety is +almost equally common near S. Rosa; it has a bright green, scanty +basis, including distinct crystals and patches of white carbonate of +lime, and grains of red, semi-micaceous oxide of iron; in parts the +basis becomes dark green, and assumes an obscure crystalline +arrangement, and occasionally in parts it becomes soft and slightly +translucent like soapstone. These red and green rocks are often quite +distinct, and often pass into each other; the passage being sometimes +affected by a fine brecciated structure, particles of the red and green +matter being mingled together. Some of the varieties appear gradually +to become porphyritic with feldspar; and all of them are easily fusible +into pale or dark-coloured beads, strongly attracted by the magnet. I +should perhaps have mistaken several of these stratified rocks for +submarine lavas, like some of those described at the Puente del Inca, +had I not examined, a few leagues eastward of this point, a fine series +of analogous but less metamorphosed, sedimentary beds belonging to the +gypseous formation, and probably derived from a volcanic source. + +This formation is intersected by numerous metalliferous veins, running, +though irregularly, N.W. and S.E., and generally at right angles to the +many dikes. The veins consist of native silver, of muriate of silver, +an amalgam of silver, cobalt, antimony, and arsenic, generally embedded +in sulphate of barytes. (See the Report on M. Domeyko’s account of +those mines, in the “Comptes Rendus” tome 14 page 560.) I was assured +by Mr. Lambert, that native copper without a trace of silver has been +found in the same vein with native silver without a trace of copper. At +the mines of Aristeas, the silver veins are said to be unproductive as +soon as they pass into the green strata, whereas at S. Rosa, only two +or three miles distant, the reverse happens; and at the time of my +visit, the miners were working through a red stratum, in the hope of +the vein becoming productive in the underlying green sedimentary mass. +I have a specimen of one of these green rocks, with the usual granules +of white calcareous spar and red oxide of iron, abounding with +disseminated particles of glittering native and muriate of silver, yet +taken at the distance of one yard from any vein,—a circumstance, as I +was assured, of very rare occurrence. + +A SECTION EASTWARD, UP THE VALLEY OF COQUIMBO. + +After passing for a few miles over the coast granitic series, we come +to the porphyritic conglomerate, with its usual characters, and with +some of the beds distinctly displaying their mechanical origin. The +strata, where first met with, are, as before stated, only slightly +inclined; but near the Hacienda of Pluclaro, we come to an anticlinal +axis, with the beds much dislocated and shifted by a great fault, of +which not a trace is externally seen in the outline of the hill. I +believe that this anticlinal axis can be traced northwards, into the +district of Arqueros, where a conspicuous hill called Cerro Blanco, +formed of a harsh, cream-coloured euritic rock, including a few +crystals of reddish feldspar, and associated with some purplish +claystone porphyry, seems to fall on a line of elevation. In descending +from the Arqueros district, I crossed on the northern border of the +valley, strata inclined eastward from the Pluclaro axis: on the +porphyritic conglomerate there rested a mass, some hundred feet thick, +of brown argillaceous limestone, in parts crystalline, and in parts +almost composed of Hippurites Chilensis, d’Orbigny; above this came a +black calcareous shale, and on it a red conglomerate. In the brown +limestone, with the Hippurites, there was an impression of a Pecten and +a coral, and great numbers of a large Gryphaea, very like, and, +according to Professor E. Forbes, probably identical with G. +Orientalis, Forbes MS.,—a cretaceous species (probably upper greensand) +from Verdachellum, in Southern India. These fossils seem to occupy +nearly the same position with those at the Puente del Inca,—namely, at +the top of the porphyritic conglomerate, and at the base of the +gypseous formation. + +A little above the Hacienda of Pluclaro, I made a detour on the +northern side of the valley, to examine the superincumbent gypseous +strata, which I estimated at 6,000 feet in thickness. The uppermost +beds of the porphyritic conglomerate, on which the gypseous strata +conformably rest, are variously coloured, with one very singular and +beautiful stratum composed of purple pebbles of various kinds of +porphyry, embedded in white calcareous spar, including cavities lined +with bright-green crystallised epidote. The whole pile of strata +belonging to both formations is inclined, apparently from the +above-mentioned axis of Pluclaro, at an angle of between 20 and 30 +degrees to the east. I will here give a section of the principal beds +met with in crossing the entire thickness of the gypseous strata. + +Firstly: above the porphyritic conglomerate formation, there is a fine- +grained, red, crystalline sandstone. + +Secondly: a thick mass of smooth-grained, calcareo-aluminous, shaly +rock, often marked with dendritic manganese, and having, where most +compact, the external appearance of honestone. It is easily fusible. I +shall for the future, for convenience’ sake, call this variety +pseudo-honestone. Some of the varieties are quite black when freshly +broken, but all weather into a yellowish-ash coloured, soft, earthy +substance, precisely as is the case with the compact shaly rocks of the +Peuquenes range. This stratum is of the same general nature with many +of the beds near Los Hornos in the Illapel section. In this second bed, +or in the underlying red sandstone (for the surface was partially +concealed by detritus), there was a thick mass of gypsum, having the +same mineralogical characters with the great beds described in our +sections across the Cordillera. + +Thirdly: a thick stratum of fine-grained, red, sedimentary matter, +easily fusible into a white glass, like the basis of claystone +porphyry; but in parts jaspery, in parts brecciated, and including +crystalline specks of carbonate of lime. In some of the jaspery layers, +and in some of the black siliceous slaty bands, there were irregular +seams of imperfect pitchstone, undoubtedly of metamorphic origin, and +other seams of brown, crystalline limestone. Here, also, were masses, +externally resembling ill-preserved silicified wood. + +Fourthly and fifthly: calcareous pseudo-honestone; and a thick stratum +concealed by detritus. + +Sixthly: a thinly stratified mass of bright green, compact, +smooth-grained, calcareo-argillaceous stone, easily fusible, and +emitting a strong aluminous odour: the whole has a highly +angulo-concretionary structure; and it resembles, to a certain extent, +some of the upper tufaceo-infusorial deposits of the Patagonian +tertiary formation. It is in its nature allied to our pseudo-honestone, +and it includes well characterised layers of that variety; and other +layers of a pale green, harder, and brecciated variety; and others of +red sedimentary matter, like that of bed Three. Some pebbles of +porphyries are embedded in the upper part. + +Seventhly: red sedimentary matter or sandstone like that of bed One, +several hundred feet in thickness, and including jaspery layers, often +having a finely brecciated structure. + +Eighthly: white, much indurated, almost crystalline tuff, several +hundred feet in thickness, including rounded grains of quartz and +particles of green matter like that of bed Six. Parts pass into a very +pale green, semi- porcellanic stone. + +Ninthly: red or brown coarse conglomerate, three or four hundred feet +thick, formed chiefly of pebbles of porphyries, with volcanic +particles, in an arenaceous, non-calcareous, fusible basis: the upper +two feet are arenaceous without any pebbles. + +Tenthly: the last and uppermost stratum here exhibited, is a compact, +slate-coloured porphyry, with numerous elongated crystals of glassy +feldspar, from one hundred and fifty to two hundred feet in thickness; +it lies strictly conformably on the underlying conglomerate, and is +undoubtedly a submarine lava. + +This great pile of strata has been broken up in several places by +intrusive hillocks of purple claystone porphyry, and by dikes of +porphyritic greenstone: it is said that a few poor metalliferous veins +have been discovered here. From the fusible nature and general +appearance of the finer-grained strata, they probably owe their origin +(like the allied beds of the Uspallata range, and of the Upper +Patagonian tertiary formations), to gentle volcanic eruptions, and to +the abrasion of volcanic rocks. Comparing these beds with those in the +mining district of Arqueros, we see at both places rocks easily +fusible, of the same peculiar bright green and red colours, containing +calcareous matter, often having a finely brecciated structure, often +passing into each other, and often alternating together: hence I cannot +doubt that the only difference between them, lies in the Arqueros beds +having been more metamorphosed (in conformity with their more +dislocated and injected condition), and consequently in the calcareous +matter, oxide of iron and green colouring matter, having been +segregated under a more crystalline form. + +The strata are inclined, as before stated, from 20 to 30 degrees +eastward, towards an irregular north and south chain of andesitic +porphyry and of porphyritic greenstone, where they are abruptly cut +off. In the valley of Coquimbo, near to the H. of Gualliguaca, similar +plutonic rocks are met with, apparently a southern prolongation of the +above chain; and eastward of it we have an escarpment of the +porphyritic conglomerate, with the strata inclined at a small angle +eastward, which makes the third escarpment, including that nearest the +coast. Proceeding up the valley we come to another north and south line +of granite, andesite, and blackish porphyry, which seem to lie in an +irregular trough of the porphyritic conglomerate. Again, on the south +side of the R. Claro, there are some irregular granitic hills, which +have thrown off the strata of porphyritic conglomerate to the N.W. by +W.; but the stratification here has been much disturbed. I did not +proceed any farther up the valley, and this point is about two-thirds +of the distance between the Pacific and the main Cordillera. + +I will describe only one other section, namely, on the north side of +the R. Claro, which is interesting from containing fossils: the strata +are much dislocated by faults and dikes, and are inclined to the north, +towards a mountain of andesite and porphyry, into which they appear to +become almost blended. As the beds approach this mountain, their +inclination increases up to an angle of 70 degrees, and in the upper +part, the rocks become highly metamorphosed. The lowest bed visible in +this section, is a purplish hard sandstone. Secondly, a bed two or +three hundred feet thick, of a white siliceous sandstone, with a +calcareous cement, containing seams of slaty sandstone, and of hard +yellowish-brown (dolomitic?) limestone; numerous, well-rounded, little +pebbles of quartz are included in the sandstone. Thirdly, a dark +coloured limestone with some quartz pebbles, from fifty to sixty feet +in thickness, containing numerous silicified shells, presently to be +enumerated. Fourthly, very compact, calcareous, jaspery sandstone, +passing into (fifthly) a great bed, several hundred feet thick, of +conglomerate, composed of pebbles of white, red, and purple porphyries, +of sandstone and quartz, cemented by calcareous matter. I observed that +some of the finer parts of this conglomerate were much indurated within +a foot of a dike eight feet in width, and were rendered of a paler +colour with the calcareous matter segregated into white crystallised +particles; some parts were stained green from the colouring matter of +the dike. Sixthly, a thick mass, obscurely stratified, of a red +sedimentary stone or sandstone, full of crystalline calcareous matter, +imperfect crystals of oxide of iron, and I believe of feldspar, and +therefore closely resembling some of the highly metamorphosed beds at +Arqueros: this bed was capped by, and appeared to pass in its upper +part into, rocks similarly coloured, containing calcareous matter, and +abounding with minute crystals, mostly elongated and glassy, of reddish +albite. Seventhly, a conformable stratum of fine reddish porphyry with +large crystals of (albitic?) feldspar; probably a submarine lava. +Eighthly, another conformable bed of green porphyry, with specks of +green earth and cream-coloured crystals of feldspar. I believe that +there are other superincumbent crystalline strata and submarine lavas, +but I had not time to examine them. + +The upper beds in this section probably correspond with parts of the +great gypseous formation; and the lower beds of red sandstone +conglomerate and fossiliferous limestone no doubt are the equivalents +of the Hippurite stratum, seen in descending from Arqueros to Pluclaro, +which there lies conformably upon the porphyritic conglomerate +formation. The fossils found in the third bed, consist of:— + +Pecten Dufreynoyi, d’Orbigny, “Voyage, Part Pal.” This species, which +occurs here in vast numbers, according to M. D’Orbigny, resembles +certain cretaceous forms. + +Ostrea hemispherica, d’Orbigny, “Voyage” etc. + +Also resembles, according to the same author, cretaceous forms. + +Terebratula aenigma, d’Orbigny, “Voyage” etc. (Pl. 22 Figures 10-12.) + +Is allied, according to M. d’Orbigny, to T. concinna from the Forest +Marble. A series of this species, collected in several localities +hereafter to be referred to, has been laid before Professor Forbes; and +he informs me that many of the specimens are almost undistinguishable +from our oolitic T. tetraedra, and that the varieties amongst them are +such as are found in that variable species. Generally speaking, the +American specimens of T. aenigma may be distinguished from the British +T. tetraedra, by the surface having the ribs sharp and well-defined to +the beak, whilst in the British species they become obsolete and +smoothed down; but this difference is not constant. Professor Forbes +adds, that, possibly, internal characters may exist, which would +distinguish the American species from its European allies. + +Spirifer linguiferoides, E. Forbes. + +Professor Forbes states that this species is very near to S. linguifera +of Phillips (a carboniferous limestone fossil), but probably distinct. +M. d’Orbigny considers it as perhaps indicating the Jurassic period. + +Ammonites, imperfect impression of. + +M. Domeyko has sent to France a collection of fossils, which, I +presume, from the description given, must have come from the +neighbourhood of Arqueros; they consist of:— + +Pecten Dufreynoyi, d’Orbigny, “Voyage” Part Pal. Ostrea hemispherica, +d’Orbigny, “Voyage” Part Pal. Turritella Andii, d’Orbigny, “Voyage” +Part Pal. (Pleurotomaria Humboldtii of Von Buch). Hippurites Chilensis, +d’Orbigny, “Voyage” Part Pal. + +The specimens of this Hippurite, as well as those I collected in my +descent from Arqueros, are very imperfect; but in M. d’Orbigny’s +opinion they resemble, as does the Turritella Andii, cretaceous (upper +greensand) forms. + +Nautilus Domeykus, d’Orbigny, “Voyage” Part Pal. Terebratula aenigma, +d’Orbigny, “Voyage” Part Pal. Terebratula ignaciana, d’Orbigny, +“Voyage” Part Pal. + +This latter species was found by M. Domeyko in the same block of +limestone with the T. aenigma. According to M. d’Orbigny, it comes near +to T. ornithocephala from the Lias. A series of this species collected +at Guasco, has been examined by Professor E. Forbes, and he states that +it is difficult to distinguish between some of the specimens and the T. +hastata from the mountain limestone; and that it is equally difficult +to draw a line between them and some Marlstone Terebratulae. Without a +knowledge of the internal structure, it is impossible at present to +decide on their identity with analogous European forms. + +The remarks given on the several foregoing shells, show that, in M. +d’Orbigny’s opinion, the Pecten, Ostrea, Turritella, and Hippurite +indicate the cretaceous period; and the Gryphaea appears to Professor +Forbes to be identical with a species, associated in Southern India +with unquestionably cretaceous forms. On the other hand, the two +Terebratulae and the Spirifer point, in the opinion both of M. +d’Orbigny and Professor Forbes, to the oolitic series. Hence M. +d’Orbigny, not having himself examined this country, has concluded that +there are here two distinct formations; but the Spirifer and T. aenigma +were certainly included in the same bed with the Pecten and Ostrea, +whence I extracted them; and the geologist M. Domeyko sent home the two +Terebratulae with the other-named shells, from the same locality, +without specifying that they came from different beds. Again, as we +shall presently see, in a collection of shells given me from Guasco, +the same species, and others presenting analogous differences, are +mingled together, and are in the same condition; and lastly, in three +places in the valley of Copiapo, I found some of these same species +similarly grouped. Hence there cannot be any doubt, highly curious +though the fact be, that these several fossils, namely, the Hippurites, +Gryphaea, Ostrea, Pecten, Turritella, Nautilus, two Terebratulae, and +Spirifer all belong to the same formation, which would appear to form a +passage between the oolitic and cretaceous systems of Europe. Although +aware how unusual the term must sound, I shall, for convenience’ sake, +call this formation cretaceo- oolitic. Comparing the sections in this +valley of Coquimbo with those in the Cordillera described in the last +chapter, and bearing in mind the character of the beds in the +intermediate district of Los Hornos, there is certainly a close general +mineralogical resemblance between them, both in the underlying +porphyritic conglomerate, and in the overlying gypseous formation. +Considering this resemblance, and that the fossils from the Puente del +Inca at the base of the gypseous formation, and throughout the greater +part of its entire thickness on the Peuquenes range, indicate the +Neocomian period,—that is, the dawn of the cretaceous system, or, as +some have believed, a passage between this latter and the oolitic +series—I conclude that probably the gypseous and associated beds in all +the sections hitherto described, belong to the same great formation, +which I have denominated—cretaceo-oolitic. I may add, before leaving +Coquimbo, that M. Gay found in the neighbouring Cordillera, at the +height of 14,000 feet above the sea, a fossiliferous formation, +including a Trigonia and Pholadomya (D’Orbigny “Voyage” Part Geolog. +page 242.);—both of which genera occur at the Puente del Inca. + +COQUIMBO TO GUASCO. + +The rocks near the coast, and some way inland, do not differ from those +described northwards of Valparaiso: we have much greenstone, syenite, +feldspathic and jaspery slate, and grauwackes having a basis like that +of claystone; there are some large tracts of granite, in which the +constituent minerals are sometimes arranged in folia, thus composing an +imperfect gneiss. There are two large districts of mica-schists, +passing into glossy clay-slate, and resembling the great formation in +the Chonos Archipelago. In the valley of Guasco, an escarpment of +porphyritic conglomerate is first seen high up the valley, about two +leagues eastward of the town of Ballenar. I heard of a great gypseous +formation in the Cordillera; and a collection of shells made there was +given me. These shells are all in the same condition, and appear to +have come from the same bed: they consist of:— + +Turritella Andii, d’Orbigny, “Voyage” Part Pal. Pecten Dufreynoyi, +d’Orbigny, “Voyage” Part Pal. Terebatula ignaciana, d’Orbigny, “Voyage” +Part Pal. + +The relations of these species have been given under the head of +Coquimbo. + +Terebratula aenigma, d’Orbigny, “Voyage” Part Pal. + +This shell M. d’Orbigny does not consider identical with his T. +aenigma, but near to T. obsoleta. Professor Forbes thinks that it is +certainly a variety of T. aenigma: we shall meet with this variety +again at Copiapo. + +Spirifer Chilensis, E. Forbes. + +Professor Forbes remarks that this fossil resembles several +carboniferous limestone Spirifers; and that it is also related to some +liassic species, as S. Wolcotii. + +If these shells had been examined independently of the other +collections, they would probably have been considered, from the +characters of the two Terebratulae, and from the Spirifer, as oolitic; +but considering that the first species, and according to Professor +Forbes, the four first, are identical with those from Coquimbo, the two +formations no doubt are the same, and may, as I have said, be +provisionally called cretaceo-oolitic. + +VALLEY OF COPIAPO. + +The journey from Guasco to Copiapo, owing to the utterly desert nature +of the country, was necessarily so hurried, that I do not consider my +notes worth giving. In the valley of Copiapo some of the sections are +very interesting. From the sea to the town of Copiapo, a distance +estimated at thirty miles, the mountains are composed of greenstone, +granite, andesite, and blackish porphyry, together with some +dusky-green feldspathic rocks, which I believe to be altered +clay-slate: these mountains are crossed by many brown-coloured dikes, +running north and south. Above the town, the main valley runs in a +south-east and even more southerly course towards the Cordillera, where +it is divided into three great ravines, by the northern one of which, +called Jolquera, I penetrated for a short distance. The section, +Section 1/3 in Plate 1, gives an eye-sketch of the structure and +composition of the mountains on both sides of this valley: a straight +east and west line from the town to the Cordillera is perhaps not more +than thirty miles, but along the valley the distance is much greater. +Wherever the valley trended very southerly, I have endeavoured to +contract the section into its true proportion. This valley, I may add, +rises much more gently than any other valley which I saw in Chile. + +To commence with our section, for a short distance above the town we +have hills of the granitic series, together with some of that rock [A], +which I suspect to be altered clay-slate, but which Professor G. Rose, +judging from specimens collected by Meyen at P. Negro, states is +serpentine passing into greenstone. We then come suddenly to the great +gypseous formation [B], without having passed over, differently from, +in all the sections hitherto described, any of the porphyritic +conglomerate. The strata are at first either horizontal or gently +inclined westward; then highly inclined in various directions, and +contorted by underlying masses of intrusive rocks; and lastly, they +have a regular eastward dip, and form a tolerably well pronounced north +and south line of hills. This formation consists of thin strata, with +innumerable alternations, of black, calcareous slate-rock, of +calcareo-aluminous stones like those at Coquimbo, which I have called +pseudo-honestones of green jaspery layers, and of pale-purplish, +calcareous, soft rotten-stone, including seams and veins of gypsum. +These strata are conformably overlaid by a great thickness of thinly +stratified, compact limestone with included crystals of carbonate of +lime. At a place called Tierra Amarilla, at the foot of a mountain thus +composed there is a broad vein, or perhaps stratum, of a beautiful and +curious crystallised mixture, composed, according to Professor G. Rose, +of sulphate of iron under two forms, and of the sulphates of copper and +alumina (Meyen’s “Reise” etc. Th. 1, s. 394.): the section is so +obscure that I could not make out whether this vein or stratum occurred +in the gypseous formation, or more probably in some underlying masses +[A], which I believe are altered clay-slate. + +SECOND AXIS OF ELEVATION. + +After the gypseous masses [B], we come to a line of hills of +unstratified porphyry [C], which on their eastern side blend into +strata of great thickness of porphyritic conglomerate, dipping +eastward. This latter formation, however, here has not been nearly so +much metamorphosed as in most parts of Central Chile; it is composed of +beds of true purple claystone porphyry, repeatedly alternating with +thick beds of purplish-red conglomerate with the well-rounded, large +pebbles of various porphyries, not blended together. + +THIRD AXIS OF ELEVATION. + +Near the ravine of Los Hornitos, there is a well-marked line of +elevation, extending for many miles in a N.N.E. and S.S.W. direction, +with the strata dipping in most parts (as in the second axis) only in +one direction, namely, eastward at an average angle of between 30 and +40 degrees. Close to the mouth of the valley, however, there is, as +represented in the section, a steep and high mountain [D], composed of +various green and brown intrusive porphyries enveloped with strata, +apparently belonging to the upper parts of the porphyritic +conglomerate, and dipping both eastward and westward. I will describe +the section seen on the eastern side of this mountain [D], beginning at +the base with the lowest bed visible in the porphyritic conglomerate, +and proceeding upwards through the gypseous formation. Bed 1 consists +of reddish and brownish porphyry varying in character, and in many +parts highly amygdaloidal with carbonate of lime, and with bright green +and brown bole. Its upper surface is throughout clearly defined, but +the lower surface is in most parts indistinct, and towards the summit +of the mountain [D] quite blended into the intrusive porphyries. Bed 2, +a pale lilac, hard but not heavy stone, slightly laminated, including +small extraneous fragments, and imperfect as well as some perfect and +glassy crystals of feldspar; from one hundred and fifty to two hundred +feet in thickness. When examining it in situ, I thought it was +certainly a true porphyry, but my specimens now lead me to suspect that +it possibly may be a metamorphosed tuff. From its colour it could be +traced for a long distance, overlying in one part, quite conformably to +the porphyry of bed 1, and in another not distant part, a very thick +mass of conglomerate, composed of pebbles of a porphyry chiefly like +that of bed 1: this fact shows how the nature of the bottom formerly +varied in short horizontal distances. Bed 3, white, much indurated +tuff, containing minute pebbles, broken crystals, and scales of mica, +varies much in thickness. This bed is remarkable from containing many +globular and pear-shaped, externally rusty balls, from the size of an +apple to a man’s head, of very tough, slate-coloured porphyry, with +imperfect crystals of feldspar: in shape these balls do not resemble +pebbles, AND I BELIEVE THAT THEY ARE SUBAQUEOUS VOLCANIC BOMBS; they +differ from SUBAERIAL bombs only in not being vesicular. Bed 4; a dull +purplish-red, hard conglomerate, with crystallised particles and veins +of carbonate of lime, from three hundred to four hundred feet in +thickness. The pebbles are of claystone porphyries of many varieties; +they are tolerably well rounded, and vary in size from a large apple to +a man’s head. This bed includes three layers of coarse, black, +calcareous, somewhat slaty rock: the upper part passes into a compact +red sandstone. + +In a formation so highly variable in mineralogical nature, any division +not founded on fossil remains, must be extremely arbitrary: +nevertheless, the beds below the last conglomerate may, in accordance +with all the sections hitherto described, be considered as belonging to +the porphyritic conglomerate, and those above it to the gypseous +formation, marked [E] in the section. The part of the valley in which +the following beds are seen is near Potrero Seco. Bed 5, compact, +fine-grained, pale greenish-grey, non- calcareous, indurated mudstone, +easily fusible into a pale green and white glass. Bed 6, purplish, +coarse-grained, hard sandstone, with broken crystals of feldspar and +crystallised particles of carbonate of lime; it possesses a slightly +nodular structure. Bed 7, blackish-grey, much indurated, calcareous +mudstone, with extraneous particles of unequal size; the whole being in +parts finely brecciated. In this mass there is a stratum, twenty feet +in thickness, of impure gypsum. Bed 8, a greenish mudstone, with +several layers of gypsum. Bed 9, a highly indurated, easily fusible, +white tuff, thickly mottled with ferruginous matter, and including some +white semi-porcellanic layers, which are interlaced with ferruginous +veins. This stone closely resembles some of the commonest varieties in +the Uspallata chain. Bed 10, a thick bed of rather bright green, +indurated mudstone or tuff, with a concretionary nodular structure so +strongly developed that the whole mass consists of balls. I will not +attempt to estimate the thickness of the strata in the gypseous +formation hitherto described, but it must certainly be very many +hundred feet. Bed 11 is at least 800 feet in thickness: it consists of +thin layers of whitish, greenish, or more commonly brown, fine-grained, +indurated tuffs, which crumble into angular fragments: some of the +layers are semi-porcellanic, many of them highly ferruginous, and some +are almost composed of carbonate of lime and iron with drusy cavities +lined with quartzf-crystals. Bed 12, dull purplish or greenish or +dark-grey, very compact and much indurated mudstone: estimated at 1,500 +feet in thickness: in some parts this rock assumes the character of an +imperfect coarse clay-slate; but viewed under a lens, the basis always +has a mottled appearance, with the edges of the minute component +particles blending together. Parts are calcareous, and there are +numerous veins of highly crystalline carbonate of lime charged with +iron. The mass has a nodular structure, and is divided by only a few +planes of stratification: there are, however, two layers, each about +eighteen inches thick, of a dark brown, finer-grained stone, having a +conchoidal, semi-porcellanic fracture, which can be followed with the +eye for some miles across the country. + +I believe this last great bed is covered by other nearly similar +alternations; but the section is here obscured by a tilt from the next +porphyritic chain, presently to be described. I have given this section +in detail, as being illustrative of the general character of the +mountains in this neighbourhood; but it must not be supposed that any +one stratum long preserves the same character. At a distance of between +only two and three miles the green mudstones and white indurated tuffs +are to a great extent replaced by red sandstone and black calcareous +shaly rocks, alternating together. The white indurated tuff, bed 11, +here contains little or no gypsum, whereas on the northern and opposite +side of the valley, it is of much greater thickness and abounds with +layers of gypsum, some of them alternating with thin seams of +crystalline carbonate of lime. The uppermost, dark-coloured, hard +mudstone, bed 12, is in this neighbourhood the most constant stratum. +The whole series differs to a considerable extent, especially in its +upper part, from that met with at [BB], in the lower part of the +valley; nevertheless, I do not doubt that they are equivalents. + +FOURTH AXIS OF ELEVATION (VALLEY OF COPIAPO). + +This axis is formed of a chain of mountains [F], of which the central +masses (near La Punta) consist of andesite containing green hornblende +and coppery mica, and the outer masses of greenish and black +porphyries, together with some fine lilac-coloured claystone porphyry; +all these porphyries being injected and broken up by small hummocks of +andesite. The central great mass of this latter rock, is covered on the +eastern side by a black, fine-grained, highly micaceous slate, which, +together with the succeeding mountains of porphyry, are traversed by +numerous white dikes, branching from the andesite, and some of them +extending in straight lines, to a distance of at least two miles. The +mountains of porphyry eastward of the micaceous schist soon, but +gradually, assume (as observed in so many other cases) a stratified +structure, and can then be recognised as a part of the porphyritic +conglomerate formation. These strata [G] are inclined at a high angle +to the S.E., and form a mass from fifteen hundred to two thousand feet +in thickness. The gypseous masses to the west already described, dip +directly towards this axis, with the strata only in a few places (one +of which is represented in the section) thrown from it: hence this +fourth axis is mainly uniclinal towards the S.E., and just like our +third axis, only locally anticlinal. + +The above strata of porphyritic conglomerate [G] with their +south-eastward dip, come abruptly up against beds of the gypseous +formation [H], which are gently, but irregularly, inclined westward: so +that there is here a synclinal axis and great fault. Further up the +valley, here running nearly north and south, the gypseous formation is +prolonged for some distance; but the stratification is unintelligible, +the whole being broken up by faults, dikes, and metalliferous veins. +The strata consist chiefly of red calcareous sandstones, with numerous +veins in the place of layers, of gypsum; the sandstone is associated +with some black calcareous slate-rock, and with green +pseudo-honestones, passing into porcelain-jasper. Still further up the +valley, near Las Amolanas [I], the gypseous strata become more regular, +dipping at an angle of between 30 and 40 degrees to W.S.W., and +conformably overlying, near the mouth of the ravine of Jolquera, strata +[K] of porphyritic conglomerate. The whole series has been tilted by a +partially concealed axis [L], of granite, andesite, and a granitic +mixture of white feldspar, quartz, and oxide of iron. + +FIFTH AXIS OF ELEVATION (VALLEY OF COPIAPO, NEAR LOS AMOLANAS). + +I will describe in some detail the beds [I] seen here, which, as just +stated, dip to W.S.W., at an angle of from 30 to 40 degrees. I had not +time to examine the underlying porphyritic conglomerate, of which the +lowest beds, as seen at the mouth of the Jolquera, are highly compact, +with crystals of red oxide of iron; and I am not prepared to say +whether they are chiefly of volcanic or metamorphic origin. On these +beds there rests a coarse purplish conglomerate, very little +metamorphosed, composed of pebbles of porphyry, but remarkable from +containing one pebble of granite;—of which fact no instance has +occurred in the sections hitherto described. Above this conglomerate, +there is a black siliceous claystone, and above it numerous +alternations of dark-purplish and green porphyries, which may be +considered as the uppermost limit of the porphyritic conglomerate +formation. + +Above these porphyries comes a coarse, arenaceous conglomerate, the +lower half white and the upper half of a pink colour, composed chiefly +of pebbles of various porphyries, but with some of red sandstone and +jaspery rocks. In some of the more arenaceous parts of the +conglomerate, there was an oblique or current lamination; a +circumstance which I did not elsewhere observe. Above this +conglomerate, there is a vast thickness of thinly stratified, +pale-yellowish, siliceous sandstone, passing into a granular +quartz-rock, used for grindstones (hence the name of the place Las +Amolanas), and certainly belonging to the gypseous formation, as does +probably the immediately underlying conglomerate. In this yellowish +sandstone there are layers of white and pale-red siliceous +conglomerate; other layers with small, well-rounded pebbles of white +quartz, like the bed at the R. Claro at Coquimbo; others of a greenish, +fine-grained, less siliceous stone, somewhat resembling the +pseudo-honestones lower down the valley; and lastly, others of a black +calcareous shale-rock. In one of the layers of conglomerate, there was +embedded a fragment of mica-slate, of which this is the first instance; +hence perhaps, it is from a formation of mica-slate, that the numerous +small pebbles of quartz, both here and at Coquimbo, have been derived. +Not only does the siliceous sandstone include layers of the black, +thinly stratified, not fissile, calcareous shale-rock, but in one place +the whole mass, especially the upper part, was, in a marvellously short +horizontal distance, after frequent alternations, replaced by it. When +this occurred, a mountain-mass, several thousand feet in thickness was +thus composed; the black calcareous shale-rock, however, always +included some layers of the pale-yellowish siliceous sandstone, of the +red conglomerate, and of the greenish jaspery and pseudo-honestone +varieties. It likewise included three or four widely separated layers +of a brown limestone, abounding with shells immediately to be +described. This pile of strata was in parts traversed by many veins of +gypsum. The calcareous shale-rock, though when freshly broken quite +black, weathers into an ash- colour: in which respect and in general +appearance, it perfectly resembles those great fossiliferous beds of +the Peuquenes range, alternating with gypsum and red sandstone, +described in the last chapter. + +The shells out of the layers of brown limestone, included in the black +calcareous shale-rock, which latter, as just stated, replaces the white +siliceous sandstone, consist of:— + +Pecten Dufreynoyi, d’Orbigny, “Voyage” Part Pal. Turritella Andii, +d’Orbigny, “Voyage” Part Pal. + +Astarte Darwinii, E. Forbes. Gryphaea Darwinii, E. Forbes. + +An intermediate form between G. gigantea and G. incurva. + +Gryphaea nov. spec.?, E. Forbes. Perna Americana, E. Forbes. Avicula, +nov. spec. + +Considered by Mr. G.B. Sowerby as the A. echinata, by M. d’Orbigny as +certainly a new and distinct species, having a Jurassic aspect. The +specimen has been unfortunately lost. + +Terebratula aenigma, d’Orbigny, (var. of do. E. Forbes.) + +This is the same variety, with that from Guasco, considered by M. +D’Orbigny to be a distinct species from his T. aenigma, and related to +T. obsoleta. + +Plagiostoma and Ammonites, fragments of. + +The lower layers of the limestone contained thousands of the Gryphaea; +and the upper ones as many of the Turritella, with the Gryphaea (nov. +species) and Serpulae adhering to them; in all the layers, the +Terebratula and fragments of the Pecten were included. It was evident, +from the manner in which species were grouped together, that they had +lived where now embedded. Before making any further remarks, I may +state, that higher up this same valley we shall again meet with a +similar association of shells; and in the great Despoblado Valley, +which branches off near the town from that of Copiapo, the Pecten +Dufreynoyi, some Gryphites (I believe G. Darwinii), and the TRUE +Terebratula aenigma of d’Orbigny were found together in an equivalent +formation, as will be hereafter seen. A specimen also, I may add, of +the true T. aenigma, was given me from the neighbourhood of the famous +silver mines of Chanuncillo, a little south of the valley of the +Copiapo, and these mines, from their position, I have no doubt, lie +within the great gypseous formation: the rocks close to one of the +silver veins, judging from fragments shown me, resemble those singular +metamorphosed deposits from the mining district of Arqueros near +Coquimbo. + +I will reiterate the evidence on the association of these several +shells in the several localities. + +COQUIMBO. + +In the same bed, Rio Claro: Pecten Dufreynoyi. Ostrea hemispherica. +Terebratula aenigma. Spirifer linguiferoides. + +Same bed, near Arqueros: Hippurites Chilensis. Gryphaea orientalis. + +Collected by M. Domeyko from the same locality, apparently near +Arqueros: Terebratula aenigma and Terebratula ignaciana, in same block +of limestone: Pecten Dufreynoyi. Ostrea hemispherica. Hippurites +Chilensis. Turritella Andii. Nautilus Domeykus. + +GUASCO. + +In a collection from the Cordillera, given me: the specimens all in the +same condition: Pecten Dufreynoyi. Turritella Andii. Terebratula +ignaciana. Terebratula aenigma, var. Spirifer Chilensis. + +COPIAPO. + +Mingled together in alternating beds in the main valley of Copiapo near +Las Amolanas, and likewise higher up the valley: Pecten Dufreynoyi. +Turritella Andii. Terebratula aenigma, var. as at Guasco. Astarte +Darwinii. Gryphaea Darwinii. Gryphaea nov. species? Perna Americana. +Avicula, nov. species. + +Main valley of Copiapo, apparently same formation with that of +Amolanas: Terebratula aenigma (true). + +In the same bed, high up the great lateral valley of the Despoblado, in +the ravine of Maricongo: Terebratula aenigma (true). Pecten Dufreynoyi. +Gryphaea Darwinii? + +Considering this table, I think it is impossible to doubt that all +these fossils belong to the same formation. If, however, the species +from Las Amolanas, in the Valley of Copiapo, had, as in the case of +those from Guasco, been separately examined, they would probably have +been ranked as oolitic; for, although no Spirifers were found here, all +the other species, with the exception of the Pecten, Turritella, and +Astarte, have a more ancient aspect than cretaceous forms. On the other +hand, taking into account the evidence derived from the cretaceous +character of these three shells, and of the Hippurites, Gryphaea +orientalis, and Ostrea, from Coquimbo, we are driven back to the +provisional name already used of cretaceo-oolitic. From geological +evidence, I believe this formation to be the equivalent of the +Neocomian beds of the Cordillera of Central Chile. + +To return to our section near Las Amolanas:—Above the yellow siliceous +sandstone, or the equivalent calcareous slate-rock, with its bands of +fossil-shells, according as the one or other prevails, there is a pile +of strata, which cannot be less than from two to three thousand feet in +thickness, in main part composed of a coarse, bright red conglomerate, +with many intercalated beds of red sandstone, and some of green and +other coloured porcelain-jaspery layers. The included pebbles are +well-rounded, varying from the size of an egg to that of a +cricket-ball, with a few larger; and they consist chiefly of +porphyries. The basis of the conglomerate, as well as some of the +alternating thin beds, are formed of a red, rather harsh, easily +fusible sandstone, with crystalline calcareous particles. This whole +great pile is remarkable from the thousands of huge, embedded, +silicified trunks of trees, one of which was eight feet long, and +another eighteen feet in circumference: how marvellous it is, that +every vessel in so thick a mass of wood should have been converted into +silex! I brought home many specimens, and all of them, according to Mr. +R. Brown, present a coniferous structure. + +Above this great conglomerate, we have from two to three hundred feet +in thickness of red sandstone; and above this, a stratum of black +calcareous slate-rock, like that which alternates with and replaces the +underlying yellowish-white, siliceous sandstone. Close to the junction +between this upper black slate-rock and the upper red sandstone, I +found the Gryphaea Darwinii, the Turritella Andii, and vast numbers of +a bivalve, too imperfect to be recognised. Hence we see that, as far as +the evidence of these two shells serves—and the Turritella is an +eminently characteristic species—the whole thickness of this vast pile +of strata belongs to the same age. Again, above the last-mentioned +upper red sandstone, there were several alternations of the black, +calcareous slate-rock; but I was unable to ascend to them. All these +uppermost strata, like the lower ones, vary extremely in character in +short horizontal distances. The gypseous formation, as here seen, has a +coarser, more mechanical texture, and contains much more siliceous +matter than the corresponding beds lower down the valley. Its total +thickness, together with the upper beds of the porphyritic +conglomerate, I estimated at least at 8,000 feet; and only a small +portion of the porphyritic conglomerate, which on the eastern flank of +the fourth axis of elevation appeared to be from fifteen hundred to two +thousand feet thick, is here included. As corroborative of the great +thickness of the gypseous formation, I may mention that in the +Despoblado Valley (which branches from the main valley a little above +the town of Copiapo) I found a corresponding pile of red and white +sandstones, and of dark, calcareous, semi-jaspery mudstones, rising +from a nearly level surface and thrown into an absolutely vertical +position; so that, by pacing, I ascertained their thickness to be +nearly two thousand seven hundred feet; taking this as a standard of +comparison, I estimated the thickness of the strata ABOVE the +porphyritic conglomerate at 7,000 feet. + +The fossils before enumerated, from the limestone-layers in the whitish +siliceous sandstone, are now covered, on the least computation, by +strata from 5,000 to 6,000 feet in thickness. Professor E. Forbes +thinks that these shells probably lived at a depth of from about 30 to +40 fathoms, that is from 180 to 240 feet; anyhow, it is impossible that +they could have lived at the depth of from 5,000 to 6,000 feet. Hence +in this case, as in that of the Puente del Inca, we may safely conclude +that the bottom of the sea on which the shells lived, subsided, so as +to receive the superincumbent submarine strata: and this subsidence +must have taken place during the existence of these shells; for, as I +have shown, some of them occur high up as well as low down in the +series. That the bottom of the sea subsided, is in harmony with the +presence of the layers of coarse, well- rounded pebbles included +throughout this whole pile of strata, as well as of the great upper +mass of conglomerate from 2,000 to 3,000 feet thick; for coarse gravel +could hardly have been formed or spread out at the profound depths +indicated by the thickness of the strata. The subsidence, also, must +have been slow to have allowed of this often-recurrent spreading out of +the pebbles. Moreover, we shall presently see that the surfaces of some +of the streams of porphyritic lava beneath the gypseous formation, are +so highly amygdaloidal that it is scarcely possible to believe that +they flowed under the vast pressure of a deep ocean. The conclusion of +a great subsidence during the existence of these cretaceo-oolitic +fossils, may, I believe, be extended to the district of Coquimbo, +although owing to the fossiliferous beds there not being directly +covered by the upper gypseous strata, which in the section north of the +valley are about 6,000 feet in thickness, I did not there insist on +this conclusion. + +The pebbles in the above conglomerates, both in the upper and lower +beds, are all well rounded, and, though chiefly composed of various +porphyries, there are some of red sandstone and of a jaspery stone, +both like the rocks intercalated in layers in this same gypseous +formation; there was one pebble of mica-slate and some of quartz, +together with many particles of quartz. In these respects there is a +wide difference between the gypseous conglomerates and those of the +porphyritic-conglomerate formation, in which latter, angular and +rounded fragments, almost exclusively composed of porphyries, are +mingled together, and which, as already often remarked, probably were +ejected from craters deep under the sea. From these facts I conclude, +that during the formation of the conglomerates, land existed in the +neighbourhood, on the shores of which the innumerable pebbles were +rounded and thence dispersed, and on which the coniferous forests +flourished—for it is improbable that so many thousand logs of wood +should have drifted from any great distance. This land, probably +islands, must have been mainly formed of porphyries, with some +mica-slate, whence the quartz was derived, and with some red sandstone +and jaspery rocks. This latter fact is important, as it shows that in +this district, even previously to the deposition of the lower gypseous +or cretaceo-oolitic beds, strata of an analogous nature had elsewhere, +no doubt in the more central ranges of the Cordillera, been elevated; +thus recalling to our minds the relations of the Cumbre and Uspallata +chains. Having already referred to the great lateral valley of the +Despoblado, I may mention that above the 2,700 feet of red and white +sandstone and dark mudstone, there is a vast mass of coarse, hard, red +conglomerate, some thousand feet in thickness, which contains much +silicified wood, and evidently corresponds with the great upper +conglomerate at Las Amolanas: here, however, the conglomerate consists +almost exclusively of pebbles of granite, and of disintegrated crystals +of reddish feldspar and quartz firmly recemented together. In this +case, we may conclude that the land whence the pebbles were derived, +and on which the now silicified trees once flourished, was formed of +granite. + +The mountains near Las Amolanas, composed of the cretaceo-oolitic +strata, are interlaced with dikes like a spider’s web, to an extent +which I have never seen equalled, except in the denuded interior of a +volcanic crater: north and south lines, however, predominate. These +dikes are composed of green, white, and blackish rocks, all porphyritic +with feldspar, and often with large crystals of hornblende. The white +varieties approach closely in character to andesite, which composes as +we have seen, the injected axes of so many of the lines of elevation. +Some of the green varieties are finely laminated, parallel to the walls +of the dikes. + +SIXTH AXIS OF ELEVATION (VALLEY OF COPIAPO). + +This axis consists of a broad mountainous mass [O] of andesite, +composed of albite, brown mica, and chlorite, passing into andesitic +granite, with quartz: on its western side it has thrown off, at a +considerable angle, a thick mass of stratified porphyries, including +much epidote [NN], and remarkable only from being divided into very +thin beds, as highly amygdaloidal on their surfaces as subaerial +lava-streams are often vesicular. This porphyritic formation is +conformably covered, as seen some way up the ravine of Jolquera, by a +mere remnant of the lower part of the cretaceo-oolitic formation [MM], +which in one part encases, as represented in the coloured section, the +foot of the andesitic axis [L], of the already described fifth line, +and in another part entirely conceals it: in this latter case, the +gypseous or cretaceo-oolitic strata falsely appeared to dip under the +porphyritic conglomerate of the fifth axis. The lowest bed of the +gypseous formation, as seen here [M], is of yellowish siliceous +sandstone, precisely like that of Amolanas, interlaced in parts with +veins of gypsum, and including layers of the black, calcareous, +non-fissile slate-rock: the Turritella Andii, Pecten Dufreynoyi, +Terebratula aenigma, var., and some Gryphites were embedded in these +layers. The sandstone varies in thickness from only twenty to eighty +feet; and this variation is caused by the inequalities in the upper +surface of an underlying stream of purple claystone porphyry. Hence the +above fossils here lie at the very base of the gypseous or +cretaceo-oolitic formation, and hence they were probably once covered +up by strata about seven thousand feet in thickness: it is, however, +possible, though from the nature of all the other sections in this +district not probable, that the porphyritic claystone lava may in this +case have invaded a higher level in the series. Above the sandstone +there is a considerable mass of much indurated, purplish-black, +calcareous claystone, allied in nature to the often-mentioned black +calcareous slate- rock. Eastward of the broad andesitic axis of this +sixth line, and penetrated by many dikes from it, there is a great +formation [P] of mica-schist, with its usual variations, and passing in +one part into a ferruginous quartz-rock. The folia are curved and +highly inclined, generally dipping eastward. It is probable that this +mica-schist is an old formation, connected with the granitic rocks and +metamorphic schists near the coast; and that the one fragment of +mica-slate, and the pebbles of quartz low down in the gypseous +formation at Las Amolanas, have been derived from it. The mica-schist +is succeeded by stratified porphyritic conglomerate [Q] of great +thickness, dipping eastward with a high inclination: I have included +this latter mountain-mass in the same anticlinal axis with the +porphyritic streams [NN]; but I am far from sure that the two masses +may not have been independently upheaved. + +SEVENTH AXIS OF ELEVATION. + +Proceeding up the ravine, we come to another mass [R] of andesite; and +beyond this, we again have a very thick, stratified porphyritic +formation [S], dipping at a small angle eastward, and forming the basal +part of the main Cordillera. I did not ascend the ravine any higher; +but here, near Castano, I examined several sections, of which I will +not give the details, only observing, that the porphyritic beds, or +submarine lavas, preponderate greatly in bulk over the alternating +sedimentary layers, which have been but little metamorphosed: these +latter consist of fine-grained red tuffs and of whitish volcanic +grit-stones, together with much of a singular, compact rock, having an +almost crystalline basis, finely brecciated with red and green +fragments, and occasionally including a few large pebbles. The +porphyritic lavas are highly amygdaloidal, both on their upper and +lower surfaces; they consist chiefly of claystone porphyry, but with +one common variety, like some of the streams at the Puente del Inca, +having a grey mottled basis, abounding with crystals of red hydrous +oxide of iron, green ones apparently of epidote, and a few glassy ones +of feldspar. This pile of strata differs considerably from the basal +strata of the Cordillera in Central Chile, and may possibly belong to +the upper and gypseous series: I saw, however, in the bed of the +valley, one fragment of porphyritic breccia-conglomerate, exactly like +those great masses met with in the more southern parts of Chile. + +Finally, I must observe, that though I have described between the town +of Copiapo and the western flank of the main Cordillera seven or eight +axes of elevation, extending nearly north and south, it must not be +supposed that they all run continuously for great distances. As was +stated to be the case in our sections across the Cordillera of Central +Chile, so here most of the lines of elevation, with the exception of +the first, third, and fifth, are very short. The stratification is +everywhere disturbed and intricate; nowhere have I seen more numerous +faults and dikes. The whole district, from the sea to the Cordillera, +is more or less metalliferous; and I heard of gold, silver, copper, +lead, mercury, and iron veins. The metamorphic action, even in the +lower strata, has certainly been far less here than in Central Chile. + +VALLEY OF THE DESPOBLADO. + +This great barren valley, which has already been alluded to, enters the +main valley of Copiapo a little above the town: it runs at first +northerly, then N.E., and more easterly into the Cordillera; I followed +its dreary course to the foot of the first main ridge. I will not give +a detailed section, because it would be essentially similar to that +already given, and because the stratification is exceedingly +complicated. After leaving the plutonic hills near the town, I met +first, as in the main valley, with the gypseous formation, having the +same diversified character as before, and soon afterwards with masses +of porphyritic conglomerate, about one thousand feet in thickness. In +the lower part of this formation there were very thick beds composed of +fragments of claystone porphyries, both angular and rounded, with the +smaller ones partially blended together and the basis rendered +porphyritic; these beds separated distinct streams, from sixty to +eighty feet in thickness, of claystone lavas. Near Paipote, also, there +was much true porphyritic breccia-conglomerate: nevertheless, few of +these masses were metamorphosed to the same degree with the +corresponding formation in Central Chile. I did not meet in this valley +with any true andesite, but only with imperfect andesitic porphyry, +including large crystals of hornblende: numerous as have been the +varieties of intrusive porphyries already mentioned, there were here +mountains composed of a new kind, having a compact, smooth, +cream-coloured basis, including only a few crystals of feldspar, and +mottled with dendritic spots of oxide of iron. There were also some +mountains of a porphyry with a brick-red basis, containing irregular, +often lens-shaped, patches of compact feldspar, and crystals of +feldspar, which latter to my surprise I find to be orthite. + +At the foot of the first ridge of the main Cordillera, in the ravine of +Maricongo, and at an elevation which, from the extreme coldness and +appearance of the vegetation, I estimated at about ten thousand feet, I +found beds of white sandstone and of limestone including the Pecten +Dufreynoyi, Terebratula aenigma, and some Gryphites. This ridge throws +the water on the one hand into the Pacific, and on the other, as I was +informed, into a great gravel-covered, basin-like plain, including a +salt- lake, and without any drainage-exit. In crossing the Cordillera +by this Pass, it is said that three principal ridges must be traversed, +instead of two, or only one as in Central Chile. + +The crest of this first main ridge and the surrounding mountains, with +the exception of a few lofty pinnacles, are capped by a great thickness +of a horizontally stratified, tufaceous deposit. The lowest bed is of a +pale purple colour, hard, fine-grained, and full of broken crystals of +feldspar and scales of mica. The middle bed is coarser, and less hard, +and hence weathers into very sharp pinnacles; it includes very small +fragments of granite, and innumerable ones of all sizes of grey +vesicular trachyte, some of which were distinctly rounded. The +uppermost bed is about two hundred feet in thickness, of a darker +colour and apparently hard: but I had not time to ascend to it. These +three horizontal beds may be seen for the distance of many leagues, +especially westward or in the direction of the Pacific, capping the +summits of the mountains, and standing on the opposite sides of the +immense valleys at exactly corresponding heights. If united they would +form a plain, inclined very slightly towards the Pacific; the beds +become thinner in this direction, and the tuff (judging from one point +to which I ascended, some way down the valley) finer-grained and of +less specific gravity, though still compact and sonorous under the +hammer. The gently inclined, almost horizontal stratification, the +presence of some rounded pebbles, and the compactness of the lowest +bed, though rendering it probable, would not have convinced me that +this mass had been of subaqueous origin, for it is known that volcanic +ashes falling on land and moistened by rain often become hard and +stratified; but beds thus originating, and owing their consolidation to +atmospheric moisture, would have covered almost equally every +neighbouring summit, high and low, and would not have left those above +a certain exact level absolutely bare; this circumstance seems to me to +prove that the volcanic ejections were arrested at their present, +widely extended, equable level, and there consolidated by some other +means than simple atmospheric moisture; and this no doubt must have +been a sheet of water. A lake at this great height, and without a +barrier on any one side, is out of the question; consequently we must +conclude that the tufaceous matter was anciently deposited beneath the +sea. It was certainly deposited before the excavation of the valleys, +or at least before their final enlargement (I have endeavoured to show +in my “Journal” etc. (2nd edition) page 355, that this arid valley was +left by the retreating sea, as the land slowly rose, in the state in +which we now see it.); and I may add, that Mr. Lambert, a gentleman +well acquainted with this country, informs me, that in ascending the +ravine of Santandres (which branches off from the Despoblado) he met +with streams of lava and much erupted matter capping all the hills of +granite and porphyry, with the exception of some projecting points; he +also remarked that the valleys had been excavated subsequently to these +eruptions. + +This volcanic formation, which I am informed by Mr. Lambert extends far +northward, is of interest, as typifying what has taken place on a +grander scale on the corresponding western side of the Cordillera of +Peru. Under another point of view, however, it possesses a far higher +interest, as confirming that conclusion drawn from the structure of the +fringes of stratified shingle which are prolonged from the plains at +the foot of the Cordillera far up the valleys,—namely, that this great +range has been elevated in mass to a height of between eight and nine +thousand feet (I may here mention that on the south side of the main +valley of Copiapo, near Potrero Seco, the mountains are capped by a +thick mass of horizontally stratified shingle, at a height which I +estimated at between fifteen hundred and two thousand feet above the +bed of the valley. This shingle, I believe, forms the edge of a wide +plain, which stretches southwards between two mountain ranges.); and +now, judging from this tufaceous deposit, we may conclude that the +horizontal elevation has been in the district of Copiapo about ten +thousand feet. + +(FIGURE 24.) + +In the valley of the Despoblado, the stratification, as before remarked +has been much disturbed, and in some points to a greater degree than I +have anywhere else seen. I will give two cases: a very thick mass of +thinly stratified red sandstone, including beds of conglomerate, has +been crushed together (as represented in Figure 24) into a yoke or +urn-formed trough, so that the strata on both sides have been folded +inwards: on the right hand the properly underlying porphyritic +claystone conglomerate is seen overlying the sandstone, but it soon +becomes vertical, and then is inclined towards the trough, so that the +beds radiate like the spokes of a wheel: on the left hand, the inverted +porphyritic conglomerate also assumes a dip towards the trough, not +gradually, as on the right hand, but by means of a vertical fault and +synclinal break; and a little still further on towards the left, there +is a second great oblique fault (both shown by the arrow- lines), with +the strata dipping to a directly opposite point; these mountains are +intersected by infinitely numerous dikes, some of which can be seen to +rise from hummocks of greenstone, and can be traced for thousands of +feet. In the second case, two low ridges trend together and unite at +the head of a little wedge-shaped valley: throughout the right- hand +ridge, the strata dip at 45 degrees to the east; in the left-hand +ridge, we have the very same strata and at first with exactly the same +dip; but in following this ridge up the valley, the strata are seen +very regularly to become more and more inclined until they stand +vertical, they then gradually fall over (the basset edges forming +symmetrical serpentine lines along the crest), till at the very head of +the valley they are reversed at an angle of 45 degrees: so that at this +point the beds have been turned through an angle of 135 degrees; and +here there is a kind of anticlinal axis, with the strata on both sides +dipping to opposite points at an angle of 45 degrees, but those on the +left hand upside down. + +ON THE ERUPTIVE SOURCES OF THE PORPHYRITIC CLAYSTONE AND GREENSTONE +LAVAS. + +In Central Chile, from the extreme metamorphic action, it is in most +parts difficult to distinguish between the streams of porphyritic lava +and the porphyritic breccia-conglomerate, but here, at Copiapo, they +are generally perfectly distinct, and in the Despoblado, I saw for the +first time, two great strata of purple claystone porphyry, after having +been for a considerable space closely united together, one above the +other, become separated by a mass of fragmentary matter, and then both +thin out;—the lower one more rapidly than the upper and greater stream. +Considering the number and thickness of the streams of porphyritic +lava, and the great thickness of the beds of breccia-conglomerate, +there can be little doubt that the sources of eruption must originally +have been numerous: nevertheless, it is now most difficult even to +conjecture the precise point of any one of the ancient submarine +craters. I have repeatedly observed mountains of porphyries, more or +less distinctly stratified towards their summits or on their flanks, +without a trace of stratification in their central and basal parts: in +most cases, I believe this is simply due either to the obliterating +effects of metamorphic action, or to such parts having been mainly +formed of intrusive porphyries, or to both causes conjoined; in some +instances, however, it appeared to me very probable that the great +central unstratified masses of porphyry were the now partially denuded +nuclei of the old submarine volcanoes, and that the stratified parts +marked the points whence the streams flowed. In one case alone, and it +was in this Valley of the Despoblado, I was able actually to trace a +thick stratum of purplish porphyry, which for a space of some miles +conformably overlay the usual alternating beds of breccia-conglomerates +and claystone lavas, until it became united with, and blended into, a +mountainous mass of various unstratified porphyries. + +The difficulty of tracing the streams of porphyries to their ancient +and doubtless numerous eruptive sources, may be partly explained by the +very general disturbance which the Cordillera in most parts has +suffered; but I strongly suspect that there is a more specific cause, +namely, THAT THE ORIGINAL POINTS OF ERUPTION TEND TO BECOME THE POINTS +OF INJECTION. This in itself does not seem improbable; for where the +earth’s crust has once yielded, it would be liable to yield again, +though the liquified intrusive matter might not be any longer enabled +to reach the submarine surface and flow as lava. I have been led to +this conclusion, from having so frequently observed that, where part of +an unstratified mountain-mass resembled in mineralogical character the +adjoining streams or strata, there were several other kinds of +intrusive porphyries and andesitic rocks injected into the same point. +As these intrusive mountain-masses form most of the axes-lines in the +Cordillera, whether anticlinal, uniclinal, or synclinal, and as the +main valleys have generally been hollowed out along these lines, the +intrusive masses have generally suffered much denudation. Hence they +are apt to stand in some degree isolated, and to be situated at the +points where the valleys abruptly bend, or where the main tributaries +enter. On this view of there being a tendency in the old points of +eruption to become the points of subsequent injection and disturbance, +and consequently of denudation, it ceases to be surprising that the +streams of lava in the porphyritic claystone conglomerate formation, +and in other analogous cases, should most rarely be traceable to their +actual sources. + +IQUIQUE, SOUTHERN PERU. + +Differently from what we have seen throughout Chile, the coast here is +formed not by the granitic series, but by an escarpment of the +porphyritic conglomerate formation, between two and three thousand feet +in height. (The lowest point, where the road crosses the +coast-escarpment, is 1,900 feet by the barometer above the level of the +sea.) I had time only for a very short examination; the chief part of +the escarpment appears to be composed of various reddish and purple, +sometimes laminated, porphyries, resembling those of Chile; and I saw +some of the porphyritic breccia-conglomerate; the stratification +appeared but little inclined. The uppermost part, judging from the +rocks near the famous silver mine of Huantajaya, consists of laminated, +impure, argillaceous, purplish-grey limestone, associated, I believe, +with some purple sandstone. (Mr. Bollaert has described “Geological +Proceedings” volume 2 page 598, a singular mass of stratified detritus, +gravel, and sand, eighty-one yards in thickness, overlying the +limestone, and abounding with loose masses of silver ore. The miners +believe that they can attribute these masses to their proper veins.) In +the limestone shells are found: the three following species were given +me:— + +Lucina Americana, E. Forbes. Terebratula inca, E. Forbes. Terebratula +aenigma, D’Orbigny. + +This latter species we have seen associated with the fossils of which +lists have been given in this chapter, in two places in the valley of +Coquimbo, and in the ravine of Maricongo at Copiapo. Considering this +fact, and the superposition of these beds on the porphyritic +conglomerate formation; and, as we shall immediately see, from their +containing much gypsum, and from their otherwise close general +resemblance in mineralogical nature with the strata described in the +valley of Copiapo, I have little doubt that these fossiliferous beds of +Iquique belong to the great cretaceo-oolitic formation of Northern +Chile. Iquique is situated seven degrees latitude north of Copiapo; and +I may here mention, that an Ammonites, nov. species, and an Astarte, +nov. species, were given me from the Cerro Pasco, about ten degrees of +latitude north of Iquique, and M. D’Orbigny thinks that they probably +indicate a Neocomian formation. Again, fifteen degrees of latitude +northward, in Colombia, there is a grand fossiliferous deposit, now +well known from the labours of Von Buch, Lea, d’Orbigny, and Forbes, +which belongs to the earlier stages of the cretaceous system. Hence, +bearing in mind the character of the few fossils from Tierra del Fuego, +there is some evidence that a great portion of the stratified deposits +of the whole vast range of the South American Cordillera belongs to +about the same geological epoch. + +Proceeding from the coast escarpment inwards, I crossed, in a space of +about thirty miles, an elevated undulatory district, with the beds +dipping in various directions. The rocks are of many kinds,—white +laminated, sometimes siliceous sandstone,—purple and red sandstone, +sometimes so highly calcareous as to have a crystalline +fracture,—argillaceous limestone,—black calcareous slate-rock, like +that so often described at Copiapo and other places,—thinly laminated, +fine-grained, greenish, indurated, sedimentary, fusible rocks, +approaching in character to the so- called pseudo-honestone of Chile, +including thin contemporaneous veins of gypsum,—and lastly, much +calcareous, laminated porcelain jasper, of a green colour, with red +spots, and of extremely easy fusibility: I noticed one conformable +stratum of a freckled-brown, feldspathic lava. I may here mention that +I heard of great beds of gypsum in the Cordillera. The only novel point +in this formation, is the presence of innumerable thin layers of +rock-salt, alternating with the laminated and hard, but sometimes +earthy, yellowish, or bright red and ferruginous sandstones. The +thickest layer of salt was only two inches, and it thinned out at both +ends. On one of these saliferous masses I noticed a stratum about +twelve feet thick, of dark-brown, hard brecciated, easily fusible rock, +containing grains of quartz and of black oxide of iron, together with +numerous imperfect fragments of shells. The problem of the origin of +salt is so obscure, that every fact, even geographical position, is +worth recording. (It is well known that stratified salt is found in +several places on the shores of Peru. The island of San Lorenzo, off +Lima, is composed of a pile of thin strata, about eight hundred feet in +thickness, composed of yellowish and purplish, hard siliceous, or +earthy sandstones, alternating with thin layers of shale, which in +places passes into a greenish, semi-porcellanic, fusible rock. There +are some thin beds of reddish mudstone, and soft ferruginous +rotten-stones, with layers of gypsum. In nearly all these varieties, +especially in the softer sandstones, there are numerous thin seams of +rock-salt: I was informed that one layer has been found two inches in +thickness. The manner in which the minutest fissures of the dislocated +beds have been penetrated by the salt, apparently by subsequent +infiltration, is very curious. On the south side of the island, layers +of coal and of impure limestone have been discovered. Hence we here +have salt, gypsum, and coal associated together. The strata include +veins of quartz, carbonate of lime, and iron pyrites; they have been +dislocated by an injected mass of greenish-brown feldspathic trap. Not +only is salt abundant on the extreme western limits of the district +between the Cordillera and the Pacific, but, according to Helms, it is +found in the outlying low hills on the eastern flank of the Cordillera. +These facts appear to me opposed to the theory, that rock-salt is due +to the sinking of water, charged with salt, in mediterranean spaces of +the ocean. The general character of the geology of these countries +would rather lead to the opinion, that its origin is in some way +connected with volcanic heat at the bottom of the sea: see on this +subject Sir R. Murchison “Anniversary Address to the Geological +Society” 1843 page 65.) With the exception of these saliferous beds, +most of the rocks as already remarked, present a striking general +resemblance with the upper parts of the gypseous or cretaceo-oolitic +formation of Chile. + +METALLIFEROUS VEINS. + +I have only a few remarks to make on this subject: in nine mining +districts, some of them of considerable extent, which I visited in +CENTRAL Chile, I found the PRINCIPAL veins running from between [N. and +N.W.] to [S. and S.E.] (These mining districts are Yaquil near +Nancagua, where the direction of the chief veins, to which only in all +cases I refer, is north and south; in the Uspallata range, the +prevailing line is N.N.W. and S.S.E.; in the C. de Prado, it is N.N.W. +and S.S.E.; near Illapel, it is N. by W. and S. by E.; at Los Hornos +the direction varies from between [N. and N.W.] to [S. and S.E.]; at +the C. de los Hornos (further northward), it is N.N.W. and S.S.E.; at +Panuncillo, it is N.N.W. and S.S.E.; and, lastly, at Arqueros, the +direction is N.W. and S.E.): in some other places, however, their +courses appeared quite irregular, as is said to be generally the case +in the whole valley of Copiapo: at Tambillos, south of Coquimbo, I saw +one large copper vein extending east and west. It is worthy of notice, +that the foliation of the gneiss and mica-slate, where such rocks +occur, certainly tend to run like the metalliferous veins, though often +irregularly, in a direction a little westward of north. At Yaquil, I +observed that the principal auriferous veins ran nearly parallel to the +grain or imperfect cleavage of the surrounding GRANITIC rocks. With +respect to the distribution of the different metals, copper, gold, and +iron are generally associated together, and are most frequently found +(but with many exceptions, as we shall presently see) in the rocks of +the lower series, between the Cordillera and the Pacific, namely, in +granite, syenite, altered feldspathic clay-slate, gneiss, and as near +Guasco mica-schist. The copper-ores consist of sulphurets, oxides, and +carbonates, sometimes with laminae of native metal: I was assured that +in some cases (as at Panuncillo S.E. of Coquimbo), the upper part of +the same vein contains oxides, and the lower part sulphurets of copper. +(The same fact has been observed by Mr. Taylor in Cuba: “London +Philosophical Journal” volume 11 page 21.) Gold occurs in its native +form; it is believed that, in many cases, the upper part of the vein is +the most productive part: this fact probably is connected with the +abundance of this metal in the stratified detritus of Chile, which must +have been chiefly derived from the degradation of the upper portions of +the rocks. These superficial beds of well-rounded gravel and sand, +containing gold, appeared to me to have been formed under the sea close +to the beach, during the slow elevation of the land: Schmidtmeyer +remarks that in Chile gold is sought for in shelving banks at the +height of some feet on the sides of the streams, and not in their beds, +as would have been the case had this metal been deposited by common +alluvial action. (“Travels in Chile” page 29.) Very frequently the +copper-ores, including some gold, are associated with abundant +micaceous specular iron. Gold is often found in iron-pyrites: at two +gold mines at Yaquil (near Nancagua), I was informed by the proprietor +that in one the gold was always associated with copper-pyrites, and in +the other with iron-pyrites: in this latter case, it is said that if +the vein ceases to contain iron-pyrites, it is yet worth while to +continue the search, but if the iron-pyrites, when it reappears, is not +auriferous, it is better at once to give up working the vein. Although +I believe copper and gold are most frequently found in the lower +granitic and metamorphic schistose series, yet these metals occur both +in the porphyritic conglomerate formation (as on the flanks of the Bell +of Quillota and at Jajuel), and in the superincumbent strata. At Jajuel +I was informed that the copper-ore, with some gold, is found only in +the greenstones and altered feldspathic clay-slate, which alternate +with the purple porphyritic conglomerate. Several gold veins and some +of copper- ore are worked in several parts of the Uspallata range, both +in the metamorphosed strata, which have been shown to have been of +probably subsequent origin to the Neocomian or gypseous formation of +the main Cordillera, and in the intrusive andesitic rocks of that +range. At Los Hornos (N.E. of Illapel), likewise, there are numerous +veins of copper- pyrites and of gold, both in the strata of the +gypseous formation and in the injected hills of andesite and various +porphyries. + +Silver, in the form of a chloride, sulphuret, or an amalgam, or in its +native state, and associated with lead and other metals, and at +Arqueros with pure native copper, occurs chiefly in the upper great +gypseous or cretaceo-oolitic formation which forms probably the richest +mass in Chile. We may instance the mining districts of Arqueros near +Coquimbo, and of nearly the whole valley of Copiapo, and of Iquique +(where the principal veins run N.E. by E. and S.W. by W.), in Peru. +Hence comes Molina’s remark, that silver is born in the cold and +solitary deserts of the Upper Cordillera. There are, however, +exceptions to this rule: at Paral (S.E. of Coquimbo) silver is found in +the porphyritic conglomerate formation; as I suspect is likewise the +case at S. Pedro de Nolasko in the Peuquenes Pass. Rich argentiferous +lead is found in the clay-slate of the Uspallata range; and I saw an +old silver-mine in a hill of syenite at the foot of the Bell of +Quillota: I was also assured that silver has been found in the +andesitic and porphyritic region between the town of Copiapo and the +Pacific. I have stated in a previous part of this chapter, that in two +neighbouring mines at Arqueros the veins in one were productive when +they traversed the singular green sedimentary beds, and unproductive +when crossing the reddish beds; whereas at the other mine exactly the +reverse takes place; I have also described the singular and rare case +of numerous particles of native silver and of the chloride being +disseminated in the green rock at the distance of a yard from the vein. +Mercury occurs with silver both at Arqueros and at Copiapo: at the base +of C. de los Hornos (S.E. of Coquimbo, a different place from Los +Hornos, before mentioned) I saw in a syenitic rock numerous quartzose +veins, containing a little cinnabar in nests: there were here other +parallel veins of copper and of a ferrugino-auriferous ore. I believe +tin has never been found in Chile. + +From information given me by Mr. Nixon of Yaquil (At the Durazno mine, +the gold is associated with copper-pyrites, and the veins contain large +prisms of plumbago. Crystallised carbonate of lime is one of the +commonest minerals in the matrix of the Chilean veins.), and by others, +it appears that in Chile those veins are generally most permanently +productive, which, consisting of various minerals (sometimes differing +but slightly from the surrounding rocks), include parallel strings RICH +in metals; such a vein is called a veta real. More commonly the mines +are worked only where one, two, or more thin veins or strings running +in a different direction, intersect a POOR “veta real:” it is +unanimously believed that at such points of intersection (cruceros), +the quantity of metal is much greater than that contained in other +parts of the intersecting veins. In some cruceros or points of +intersection, the metals extend even beyond the walls of the main, +broad, stony vein. It is said that the greater the angle of +intersection, the greater the produce; and that nearly parallel strings +attract each other; in the Uspallata range, I observed that numerous +thin auri-ferruginous veins repeatedly ran into knots, and then +branched out again. I have already described the remarkable manner in +which rocks of the Uspallata range are indurated and blackened (as if +by a blast of gunpowder) to a considerable distance from the metallic +veins. + +Finally, I may observe, that the presence of metallic veins seems +obviously connected with the presence of intrusive rocks, and with the +degree of metamorphic action which the different districts of Chile +have undergone. (Sir R. Murchison and his fellow travellers have given +some striking facts on this subject in their account of the Ural +Mountains (“Geological Proceedings” volume 3 page 748.) Such +metamorphosed areas are generally accompanied by numerous dikes and +injected masses of andesite and various porphyries: I have in several +places traced the metalliferous veins from the intrusive masses into +the encasing strata. Knowing that the porphyritic conglomerate +formation consists of alternate streams of submarine lavas and of the +debris of anciently erupted rocks, and that the strata of the upper +gypseous formation sometimes include submarine lavas, and are composed +of tuffs, mudstones, and mineral substances, probably due to volcanic +exhalations,—the richness of these strata is highly remarkable when +compared with the erupted beds, often of submarine origin, but NOT +METAMORPHOSED, which compose the numerous islands in the Pacific, +Indian, and Atlantic Oceans; for in these islands metals are entirely +absent, and their nature even unknown to the aborigines. + +A SUMMARY OF THE GEOLOGICAL HISTORY OF THE CHILEAN CORDILLERA, AND OF +THE SOUTHERN PARTS OF SOUTH AMERICA. + +We have seen that the shores of the Pacific, for a space of 1,200 miles +from Tres Montes to Copiapo, and I believe for a very much greater +distance, are composed, with the exception of the tertiary basins, of +metamorphic schists, plutonic rocks, and more or less altered +clay-slate. On the floor of the ocean thus constituted, vast streams of +various purplish claystone and greenstone porphyries were poured forth, +together with great alternating piles of angular and rounded fragments +of similar rocks ejected from the submarine craters. From the +compactness of the streams and fragments, it is probable that, with the +exception of some districts in Northern Chile, the eruptions took place +in profoundly deep water. The orifices of eruption appear to have been +studded over a breadth, with some outliers, of from fifty to one +hundred miles: and closely enough together, both north and south, and +east and west, for the ejected matter to form a continuous mass, which +in Central Chile is more than a mile in thickness. I traced this +mould-like mass, for only 450 miles; but judging from what I saw at +Iquique, from specimens, and from published accounts, it appears to +have a manifold greater length. In the basal parts of the series, and +especially towards the flanks of the range, mud, since converted into a +feldspathic slaty rock, and sometimes into greenstone, was occasionally +deposited between the beds of erupted matter: with this exception the +uniformity of the porphyritic rocks is very remarkable. + +At the period when the claystone and greenstone porphyries nearly or +quite ceased being erupted, that great pile of strata which, from often +abounding with gypsum, I have generally called the gypseous formation +was deposited, and feldspathic lavas, together with other singular +volcanic rocks, were occasionally poured forth: I am far from +pretending that any distinct line of demarcation can be drawn between +this formation and the underlying porphyries and porphyritic +conglomerate, but in a mass of such great thickness, and between beds +of such widely different mineralogical nature, some division was +necessary. At about the commencement of the gypseous period, the bottom +of the sea here seems first to have been peopled by shells, not many in +kind, but abounding in individuals. At the P. del Inca the fossils are +embedded near the base of the formation; in the Peuquenes range, at +different levels, halfway up, and even higher in the series; hence, in +these sections, the whole pile of strata belongs to the same period: +the same remark is applicable to the beds at Copiapo, which attain a +thickness of between seven and eight thousand feet. The fossil shells +in the Cordillera of Central Chile, in the opinion of all the +palaeontologists who have examined them, belong to the earlier stages +of the cretaceous system; whilst in Northern Chile there is a most +singular mixture of cretaceous and oolitic forms: from the geological +relations, however, of these two districts, I cannot but think that +they all belong to nearly the same epoch, which I have provisionally +called cretaceo-oolitic. + +The strata in this formation, composed of black calcareous shaly-rocks +of red and white, and sometimes siliceous sandstone, of coarse +conglomerates, limestones, tuffs, dark mudstones, and those singular +fine-grained rocks which I have called pseudo-honestones, vast beds of +gypsum, and many other jaspery and scarcely describable varieties, vary +and replace each other in short horizontal distances, to an extent, I +believe, unequalled even in any tertiary basin. Most of these +substances are easily fusible, and have apparently been derived either +from volcanoes still in quiet action, or from the attrition of volcanic +products. If we picture to ourselves the bottom of the sea, rendered +uneven in an extreme degree, with numerous craters, some few +occasionally in eruption, but the greater number in the state of +solfataras, discharging calcareous, siliceous, ferruginous matters, and +gypsum or sulphuric acid to an amount surpassing, perhaps, even the +existing sulphureous volcanoes of Java (Von Buch’s “Description +Physique des Iles Canaries” page 428.), we shall probably understand +the circumstances under which this singular pile of varying strata was +accumulated. The shells appear to have lived at the quiescent periods +when only limestone or calcareo-argillaceous matter was depositing. +From Dr. Gillies’ account, this gypseous or cretaceo-oolitic formation +extends as far south as the Pass of Planchon, and I followed it +northward at intervals for 500 miles: judging from the character of the +beds with the Terebratula aenigma, at Iquique, it extends from four to +five hundred miles further: and perhaps even for ten degrees of +latitude north of Iquique to the Cerro Pasco, not far from Lima: again, +we know that a cretaceous formation, abounding with fossils, is largely +developed north of the equator, in Colombia: in Tierra del Fuego, at +about this same period, a wide district of clay-slate was deposited, +which in its mineralogical characters and external features, might be +compared to the Silurian regions of North Wales. The gypseous +formation, like that of the porphyritic breccia- conglomerate on which +it rests, is of inconsiderable breadth; though of greater breadth in +Northern than in Central Chile. + +As the fossil shells in this formation are covered, in the Peuquenes +ridge, by a great thickness of strata; at the Puente del Inca, by at +least five thousand feet; at Coquimbo, though the superposition there +is less plainly seen, by about six thousand feet; and at Copiapo, +certainly by five or six thousand, and probably by seven thousand feet +(the same species there recurring in the upper and lower parts of the +series), we may feel confident that the bottom of the sea subsided +during this cretaceo-oolitic period, so as to allow of the accumulation +of the superincumbent submarine strata. This conclusion is confirmed +by, or perhaps rather explains, the presence of the many beds at many +levels of coarse conglomerate, the well- rounded pebbles in which we +cannot believe were transported in very deep water. Even the underlying +porphyries at Copiapo. with their highly amygdaloidal surfaces, do not +appear to have flowed under great pressure. The great sinking movement +thus plainly indicated, must have extended in a north and south line +for at least four hundred miles, and probably was co- extensive with +the gypseous formation. + +The beds of conglomerate just referred to, and the extraordinarily +numerous silicified trunks of fir-trees at Los Hornos, perhaps at +Coquimbo and at two distant points in the valley of Copiapo, indicate +that land existed at this period in the neighbourhood. This land, or +islands, in the northern part of the district of Copiapo, must have +been almost exclusively composed, judging from the nature of the +pebbles of granite: in the southern parts of Copiapo, it must have been +mainly formed of claystone porphyries, with some mica-schist, and with +much sandstone and jaspery rocks exactly like the rocks in the gypseous +formation, and no doubt belonging to its basal series. In several other +places also, during the accumulation of the gypseous formation, its +basal parts and the underlying porphyritic conglomerate must likewise +have been already partially upheaved and exposed to wear and tear; near +the Puente del Inca and at Coquimbo, there must have existed masses of +mica-schist or some such rock, whence were derived the many small +pebbles of opaque quartz. It follows from these facts, that in some +parts of the Cordillera the upper beds of the gypseous formation must +lie unconformably on the lower beds; and the whole gypseous formation, +in parts, unconformably on the porphyritic conglomerate; although I saw +no such cases, yet in many places the gypseous formation is entirely +absent; and this, although no doubt generally caused by quite +subsequent denudation, may in others be due to the underlying +porphyritic conglomerate having been locally upheaved before the +deposition of the gypseous strata, and thus having become the source of +the pebbles of porphyry embedded in them. In the porphyritic +conglomerate formation, in its lower and middle parts, there is very +rarely any evidence, with the exception of the small quartz pebbles at +Jajuel near Aconcagua, and of the single pebble of granite at Copiapo, +of the existence of neighbouring land: in the upper parts, however, and +especially in the district of Copiapo, the number of thoroughly +well-rounded pebbles of compact porphyries make me believe, that, as +during the prolonged accumulation of the gypseous formation the lower +beds had already been locally upheaved and exposed to wear and tear, so +it was with the porphyritic conglomerate. Hence in following thus far +the geological history of the Cordillera, it may be inferred that the +bed of a deep and open, or nearly open, ocean was filled up by +porphyritic eruptions, aided probably by some general and some local +elevations, to that comparatively shallow level at which the cretaceo- +oolitic shells first lived. At this period, the submarine craters +yielded at intervals a prodigious supply of gypsum and other mineral +exhalations, and occasionally, in certain places poured forth lavas, +chiefly of a feldspathic nature: at this period, islands clothed with +fir-trees and composed of porphyries, primary rocks, and the lower +gypseous strata had already been locally upheaved, and exposed to the +action of the waves;—the general movement, however, at this time having +been over a very wide area, one of slow subsidence, prolonged till the +bed of the sea sank several thousand feet. + +In Central Chile, after the deposition of a great thickness of the +gypseous strata, and after their upheaval, by which the Cumbre and +adjoining ranges were formed, a vast pile of tufaceous matter and +submarine lava was accumulated, where the Uspallata chain now stands; +also after the deposition and upheaval of the equivalent gypseous +strata of the Peuquenes range, the great thick mass of conglomerate in +the valley of Tenuyan was accumulated: during the deposition of the +Uspallata strata, we know absolutely, from the buried vertical trees, +that there was a subsidence of some thousand feet; and we may infer +from the nature of the conglomerate in the valley of Tenuyan, that a +similar and perhaps contemporaneous movement there took place. We have, +then, evidence of a second great period of subsidence; and, as in the +case of the subsidence which accompanied the accumulation of the +cretaceo-oolitic strata, so this latter subsidence appears to have been +complicated by alternate or local elevatory movement— for the vertical +trees, buried in the midst of the Uspallata strata, must have grown on +dry land, formed by the upheaval of the lower submarine beds. Presently +I shall have to recapitulate the facts, showing that at a still later +period, namely, at nearly the commencement of the old tertiary deposits +of Patagonia and of Chile, the continent stood at nearly its present +level, and then, for the third time, slowly subsided to the amount of +several hundred feet, and was afterwards slowly re-uplifted to its +present level. + +The highest peaks of the Cordillera appear to consist of active or more +commonly dormant volcanoes,—such as Tupungato, Maypu, and Aconcagua, +which latter stands 23,000 feet above the level of the sea, and many +others. The next highest peaks are formed of the gypseous and +porphyritic strata, thrown into vertical or highly inclined positions. +Besides the elevation thus gained by angular displacements, I infer, +without any hesitation—from the stratified gravel-fringes which gently +slope up the valleys of the Cordillera from the gravel-capped plains at +their base, which latter are connected with the plains, still covered +with recent shells on the coast— that this great range has been +upheaved in mass by a slow movement, to an amount of at least 8,000 +feet. In the Despoblado Valley, north of Copiapo, the horizontal +elevation, judging from the compact, stratified tufaceous deposit, +capping the distant mountains at corresponding heights, was about ten +thousand feet. It is very possible, or rather probable, that this +elevation in mass may not have been strictly horizontal, but more +energetic under the Cordillera, than towards the coast on either side; +nevertheless, movements of this kind may be conveniently distinguished +from those by which strata have been abruptly broken and upturned. When +viewing the Cordillera, before having read Mr. Hopkins’s profound +“Researches on Physical Geology,” the conviction was impressed on me, +that the angular dislocations, however violent, were quite subordinate +in importance to the great upward movement in mass, and that they had +been caused by the edges of the wide fissures, which necessarily +resulted from the tension of the elevated area, having yielded to the +inward rush of fluidified rock, and having thus been upturned. + +The ridges formed by the angularly upheaved strata are seldom of great +length: in the central parts of the Cordillera they are generally +parallel to each other, and run in north and south lines; but towards +the flanks they often extend more or less obliquely. The angular +displacement has been much more violent in the central than in the +exterior MAIN lines; but it has likewise been violent in some of the +MINOR lines on the extreme flanks. The violence has been very unequal +on the same short lines; the crust having apparently tended to yield on +certain points along the lines of fissures. These points, I have +endeavoured to show, were probably first foci of eruption, and +afterwards of injected masses of porphyry and andesite. (Sir R. +Murchison and his companions state “Geological Proceedings” volume 3 +page 747, that no true granite appears in the higher Ural Mountains; +but that syenitic greenstone—a rock closely analogous to our +andesite—is far the most abundant of the intrusive masses.) The close +similarity of the andesitic granites and porphyries, throughout Chile, +Tierra del Fuego, and even in Peru, is very remarkable. The prevalence +of feldspar cleaving like albite, is common not only to the andesites, +but (as I infer from the high authority of Professor G. Rose, as well +as from my own measurements) to the various claystone and greenstone +porphyries, and to the trachytic lavas of the Cordillera. The andesitic +rocks have in most cases been the last injected ones, and they probably +form a continuous dome under this great range: they stand in intimate +relationship with the modern lavas; and they seem to have been the +immediate agent in metamorphosing the porphyritic conglomerate +formation, and often likewise the gypseous strata, to the extraordinary +extent to which they have suffered. + +With respect to the age at which the several parallel ridges composing +the Cordillera were upthrown, I have little evidence. Many of them may +have been contemporaneously elevated and injected in the same manner as +in volcanic archipelagoes lavas are contemporaneously ejected on the +parallel lines of fissure. (“Volcanic Islands” etc.) But the pebbles +apparently derived from the wear and tear of the porphyritic +conglomerate formation, which are occasionally present in the upper +parts of this same formation, and are often present in the gypseous +formation, together with the pebbles from the basal parts of the latter +formation in its upper strata, render it almost certain that portions, +we may infer ridges, of these two formations were successively +upheaved. In the case of the gigantic Portillo range, we may feel +almost certain that a preexisting granitic line was upraised (not by a +single blow, as shown by the highly inclined basaltic streams in the +valley on its eastern flank) at a period long subsequent to the +upheavement of the parallel Peuquenes range. (I have endeavoured to +show in my “Journal” 2nd edition page 321, that the singular fact of +the river, which drains the valley between these two ranges, passing +through the Portillo and higher line, is explained by its slow and +subsequent elevation. There are many analogous cases in the drainage of +rivers: see “Edinburgh New Philosophical Journal” volume 28 pages 33 +and 44.) Again, subsequently to the upheavement of the Cumbre chain, +that of Uspallata was formed and elevated; and afterwards, I may add, +in the plain of Uspallata, beds of sand and gravel were violently +upthrown. The manner in which the various kinds of porphyries and +andesites have been injected one into the other, and in which the +infinitely numerous dikes of various composition intersect each other, +plainly show that the stratified crust has been stretched and yielded +many times over the same points. With respect to the age of the axes of +elevation between the Pacific and the Cordillera, I know little: but +there are some lines which must—namely, those running north and south +in Chiloe, those eight or nine east and west, parallel, far-extended, +most symmetrical uniclinal lines at P. Rumena, and the short N.W.-S.E. +and N.E.- S.W. lines at Concepcion—have been upheaved long after the +formation of the Cordillera. Even during the earthquake of 1835, when +the linear north and south islet of St. Mary was uplifted several feet +above the surrounding area, we perhaps see one feeble step in the +formation of a subordinate mountain-axis. In some cases, moreover, for +instance, near the baths of Cauquenes, I was forcibly struck with the +small size of the breaches cut through the exterior mountain-ranges, +compared with the size of the same valleys higher up where entering the +Cordillera; and this circumstance appeared to me scarcely explicable, +except on the idea of the exterior lines having been subsequently +upthrown, and therefore having been exposed to a less amount of +denudation. From the manner in which the fringes of gravel are +prolonged in unbroken slopes up the valleys of the Cordillera, I infer +that most of the greater dislocations took place during the earlier +parts of the great elevation in mass: I have, however, elsewhere given +a case, and M. de Tschudi has given another, of a ridge thrown up in +Peru across the bed of a river, and consequently after the final +elevation of the country above the level of the sea. (“Reise in Peru” +Band 2 s.8: Author’s “Journal” 2nd edition page 359.) + +Ascending to the older tertiary formations, I will not again +recapitulate the remarks already given at the end of the Fifth +Chapter,—on their great extent, especially along the shores of the +Atlantic—on their antiquity, perhaps corresponding with that of the +eocene deposits of Europe,—on the almost entire dissimilarity, though +the formations are apparently contemporaneous, of the fossils from the +eastern and western coasts, as is likewise the case, even in a still +more marked degree, with the shells now living in these opposite though +approximate seas,—on the climate of this period not having been more +tropical than what might have been expected from the latitudes of the +places under which the deposits occur; a circumstance rendered well +worthy of notice, from the contrast with what is known to have been the +case during the older tertiary periods of Europe, and likewise from the +fact of the southern hemisphere having suffered at a much later period, +apparently at the same time with the northern hemisphere, a colder or +more equable temperature, as shown by the zones formerly affected by +ice-action. Nor will I recapitulate the proofs of the bottom of the +sea, both on the eastern and western coast, having subsided seven or +eight hundred feet during this tertiary period; the movement having +apparently been co-extensive, or nearly co-extensive, with the deposits +of this age. Nor will I again give the facts and reasoning on which the +proposition was founded, that when the bed of the sea is either +stationary or rising, circumstances are far less favourable than when +its level is sinking, to the accumulation of conchiferous deposits of +sufficient thickness, extension, and hardness to resist, when upheaved, +the ordinary vast amount of denudation. We have seen that the highly +remarkable fact of the absence of any EXTENSIVE formations containing +recent shells, either on the eastern or western coasts of the +continent,—though these coasts now abound with living mollusca,—though +they are, and apparently have always been, as favourable for the +deposition of sediment as they were when the tertiary formations were +copiously deposited,—and though they have been upheaved to an amount +quite sufficient to bring up strata from the depths the most fertile +for animal life—can be explained in accordance with the above +proposition. As a deduction, it was also attempted to be shown, first, +that the want of close sequence in the fossils of successive +formations, and of successive stages in the same formation, would +follow from the improbability of the same area continuing slowly to +subside from one whole period to another, or even during a single +entire period; and secondly, that certain epochs having been favourable +at distant points, in the same quarter of the world for the synchronous +accumulation of fossiliferous strata, would follow from movements of +subsidence having apparently, like those of elevation, +contemporaneously affected very large areas. + +There is another point which deserves some notice, namely, the analogy +between the upper parts of the Patagonian tertiary formation, as well +as of the upper possibly contemporaneous beds at Chiloe and Concepcion, +with the great gypseous formation of Cordillera; for in both +formations, the rocks, in their fusible nature, in their containing +gypsum, and in many other characters, show a connection, either +intimate or remote, with volcanic action; and as the strata in both +were accumulated during subsidence, it appears at first natural to +connect this sinking movement with a state of high activity in the +neighbouring volcanoes. During the cretaceo-oolitic period this +certainly appears to have been the case at the Puente del Inca, judging +from the number of intercalated lava-streams in the lower 3,000 feet of +strata; but generally, the volcanic orifices seem at this time to have +existed as submarine solfataras, and were certainly quiescent compared +with their state during the accumulation of the porphyritic +conglomerate formation. During the deposition of the tertiary strata we +know that at S. Cruz, deluges of basaltic lava were poured forth; but +as these lie in the upper part of the series, it is possible that the +subsidence may at that time have ceased: at Chiloe, I was unable to +ascertain to what part of the series the pile of lavas belonged. The +Uspallata tuffs and great streams of submarine lavas, were probably +intermediate in age between the cretaceo- oolitic and older tertiary +formations, and we know from the buried trees that there was a great +subsidence during their accumulation; but even in this case, the +subsidence may not have been strictly contemporaneous with the great +volcanic eruptions, for we must believe in at least one intercalated +period of elevation, during which the ground was upraised on which the +now buried trees grew. I have been led to make these remarks, and to +throw some doubt on the strict contemporaneousness of high volcanic +activity and movements of subsidence, from the conviction impressed on +my mind by the study of coral formations, that these two actions do not +generally go on synchronously;—on the contrary, that in volcanic +districts, subsidence ceases as soon as the orifices burst forth into +renewed action, and only recommences when they again have become +dormant. (“The Structure and Distribution of Coral Reefs.”) + +At a later period, the Pampean mud, of estuary origin, was deposited +over a wide area,—in one district conformably on the underlying old +tertiary strata, and in another district unconformably on them, after +their upheaval and denudation. During and before the accumulation, +however, of these old tertiary strata, and, therefore, at a very remote +period, sediment, strikingly resembling that of the Pampas, was +deposited; showing during how long a time in this case the same +agencies were at work in the same area. The deposition of the Pampean +estuary mud was accompanied, at least in the southern parts of the +Pampas, by an elevatory movement, so that the M. Hermoso beds probably +were accumulated after the upheaval of those round the S. Ventana; and +those at P. Alta after the upheaval of the M. Hermoso strata; but there +is some reason to suspect that one period of subsidence intervened, +during which mud was deposited over the coarse sand of the Barrancas de +S. Gregorio, and on the higher parts of Banda Oriental. The mammiferous +animals characteristic of this formation, many of which differ as much +from the present inhabitants of South America, as do the eocene mammals +of Europe from the present ones of that quarter of the globe, certainly +co-existed at B. Blanca with twenty species of mollusca, one balanus, +and two corals, all now living in the adjoining sea: this is likewise +the case in Patagonia with the Macrauchenia, which co-existed with +eight shells, still the commonest kinds on that coast. I will not +repeat what I have elsewhere said, on the place of habitation, food, +wide range, and extinction of the numerous gigantic mammifers, which at +this late period inhabited the two Americas. + +The nature and grouping of the shells embedded in the old tertiary +formations of Patagonia and Chile show us, that the continent at that +period must have stood only a few fathoms below its present level, and +that afterwards it subsided over a wide area, seven or eight hundred +feet. The manner in which it has since been rebrought up to its actual +level, was described in detail in the First and Second Chapters. It was +there shown that recent shells are found on the shores of the Atlantic, +from Tierra del Fuego northward for a space of at least 1,180 nautical +miles, and at the height of about 100 feet in La Plata, and of 400 feet +in Patagonia. The elevatory movements on this side of the continent +have been slow; and the coast of Patagonia, up to the height in one +part of 950 feet and in another of 1,200 feet, is modelled into eight +great, step-like, gravel-capped plains, extending for hundreds of miles +with the same heights; this fact shows that the periods of denudation +(which, judging from the amount of matter removed, must have been long +continued) and of elevation were synchronous over surprisingly great +lengths of coasts. On the shores of the Pacific, upraised shells of +recent species, generally, though not always, in the same proportional +numbers as in the adjoining sea, have actually been found over a north +and south space of 2,075 miles, and there is reason to believe that +they occur over a space of 2,480 miles. The elevation on this western +side of the continent has not been equable; at Valparaiso, within the +period during which upraised shells have remained undecayed on the +surface, it has been 1,300 feet, whilst at Coquimbo, 200 miles +northward, it has been within this same period only 252 feet. At Lima, +the land has been uplifted at least 80 feet since Indian man inhabited +that district; but the level within historical times apparently has +subsided. At Coquimbo, in a height of 364 feet, the elevation has been +interrupted by five periods of comparative rest. At several places the +land has been lately, or still is, rising both insensibly and by sudden +starts of a few feet during earthquake-shocks; this shows that these +two kinds of upward movement are intimately connected together. For a +space of 775 miles, upraised recent shells are found on the two +opposite sides of the continent; and in the southern half of this +space, it may be safely inferred from the slope of the land up to the +Cordillera, and from the shells found in the central part of Tierra del +Fuego, and high up the River Santa Cruz, that the entire breadth of the +continent has been uplifted. From the general occurrence on both coasts +of successive lines of escarpments, of sand-dunes and marks of erosion, +we must conclude that the elevatory movement has been normally +interrupted by periods, when the land either was stationary, or when it +rose at so slow a rate as not to resist the average denuding power of +the waves, or when it subsided. In the case of the present high +sea-cliffs of Patagonia and in other analogous instances, we have seen +that the difficulty in understanding how strata can be removed at those +depths under the sea, at which the currents and oscillations of the +water are depositing a smooth surface of mud, sand, and sifted pebbles, +leads to the suspicion that the formation or denudation of such cliffs +has been accompanied by a sinking movement. + +In South America, everything has taken place on a grand scale, and all +geological phenomena are still in active operation. We know how violent +at the present day the earthquakes are, we have seen how great an area +is now rising, and the plains of tertiary origin are of vast +dimensions; an almost straight line can be drawn from Tierra del Fuego +for 1,600 miles northward, and probably for a much greater distance, +which shall intersect no formation older than the Patagonian deposits; +so equable has been the upheaval of the beds, that throughout this long +line, not a fault in the stratification or abrupt dislocation was +anywhere observable. Looking to the basal, metamorphic, and plutonic +rocks of the continent, the areas formed of them are likewise vast; and +their planes of cleavage and foliation strike over surprisingly great +spaces in uniform directions. The Cordillera, with its pinnacles here +and there rising upwards of twenty thousand feet above the level of the +sea, ranges in an unbroken line from Tierra del Fuego, apparently to +the Arctic circle. This grand range has suffered both the most violent +dislocations, and slow, though grand, upward and downward movements in +mass; I know not whether the spectacle of its immense valleys, with +mountain-masses of once liquified and intrusive rocks now bared and +intersected, or whether the view of those plains, composed of shingle +and sediment hence derived, which stretch to the borders of the +Atlantic Ocean, is best adapted to excite our astonishment at the +amount of wear and tear which these mountains have undergone. + +The Cordillera from Tierra del Fuego to Mexico, is penetrated by +volcanic orifices, and those now in action are connected in great +trains. The intimate relation between their recent eruptions and the +slow elevation of the continent in mass, appears to me highly +important, for no explanation of the one phenomenon can be considered +as satisfactory which is not applicable to the other. (On the +Connection of certain Volcanic Phenomena in South America: “Geological +Transactions” volume 5 page 609.) The permanence of the volcanic action +on this chain of mountains is, also, a striking fact; first, we have +the deluges of submarine lavas alternating with the porphyritic +conglomerate strata, then occasionally feldspathic streams and abundant +mineral exhalations during the gypseous or cretaceo- oolitic period: +then the eruptions of the Uspallata range, and at an ancient but +unknown period, when the sea came up to the eastern foot of the +Cordillera, streams of basaltic lava at the foot of the Portillo range; +then the old tertiary eruptions; and lastly, there are here and there +amongst the mountains, much worn and apparently very ancient volcanic +formations without any craters; there are, also, craters quite extinct, +and others in the condition of solfataras, and others occasionally or +habitually in fierce action. Hence it would appear that the Cordillera +has been, probably with some quiescent periods, a source of volcanic +matter from an epoch anterior to our cretaceo-oolitic formation to the +present day; and now the earthquakes, daily recurrent on some part of +the western coast, give little hope that the subterranean energy is +expended. + +Recurring to the evidence by which it was shown that some at least of +the parallel ridges, which together compose the Cordillera, were +successively and slowly upthrown at widely different periods; and that +the whole range certainly once, and almost certainly twice, subsided +some thousand feet, and being then brought up by a slow movement in +mass, again, during the old tertiary formations, subsided several +hundred feet, and again was brought up to its present level by a slow +and often interrupted movement; we see how opposed is this complicated +history of changes slowly effected, to the views of those geologists +who believe that this great mountain-chain was formed in late times by +a single blow. I have endeavoured elsewhere to show, that the +excessively disturbed condition of the strata in the Cordillera, so far +from indicating single periods of extreme violence, presents +insuperable difficulties, except on the admission that the masses of +once liquified rocks of the axes were repeatedly injected with +intervals sufficiently long for their successive cooling and +consolidation. (“Geological Transactions” volume 5 page 626.) Finally, +if we look to the analogies drawn from the changes now in progress in +the earth’s crust, whether to the manner in which volcanic matter is +erupted, or to the manner in which the land is historically known to +have risen and sunk: or again, if we look to the vast amount of +denudation which every part of the Cordillera has obviously suffered, +the changes through which it has been brought into its present +condition, will appear neither to have been too slowly effected, nor to +have been too complicated. + +NOTE. + +As, both in France and England, translations of a passage in Professor +Ehrenberg’s Memoir, often referred to in the Fourth Chapter of this +volume, have appeared, implying that Professor Ehrenberg believes, from +the character of the infusoria, that the Pampean formation was +deposited by a sea-debacle rushing over the land, I may state, on the +authority of a letter to me, that these translations are incorrect. The +following is the passage in question:— + +“Durch Beachtung der mikroscopischen Formen hat sich nun feststellen +lassen, das die Mastodonten-Lager am La Plata und die Knochen-Lager am +Monte Hermoso, who wie die der Riesen-Gurtelthiere in den Dunenhugeln +bei Bahia Blanca, beides in Patagonien, unveranderte brakische +Susswasserbildungen sind, die einst wohl sammtlich zum obersten +Fluthgebiethe des Meeres im tieferen Festlande +gehorten.”—“Monatsberichten der konigl. Akad. etc.” zu Berlin vom April +1845. + + + + +INDEX. + + +Abich, on a new variety of feldspar. + +Abrolhos islands. + +Absence of recent formations on the S. American coasts. + +Aguerros on elevation of Imperial. + +Albite, constituent mineral in andesite. —in rocks of Tierra del Fuego. +—in porphyries. —crystals of, with orthite. + +Alison, Mr., on elevation of Valparaiso. + +Alumina, sulphate of. + +Ammonites from Concepcion. + +Amolanas, Las. + +Amygdaloid, curious varieties of. + +Amygdaloids of the Uspallata range. —of Copiapo. + +Andesite of Chile. —in the valley of Maypu. —of the Cumbre pass. —of +the Uspallata range. —of Los Hornos. —of Copiapo. + +Anhydrite, concretions of. + +Araucaria, silicified wood of. Arica, elevation of. + +Arqueros, mines of. + +Ascension, gypsum deposited on. —laminated volcanic rocks of. + +Augite in fragments, in gneiss. —with albite, in lava. + +Austin, Mr. R.A.C., on bent cleavage lamina. + +Austin, Captain, on sea-bottom. + +Australia, foliated rocks of. + +Azara labiata, beds of, at San Pedro. + +Baculites vagina. + +Bahia Blanca, elevation of. —formations near. —character of living +shells of. + +Bahia (Brazil), elevation near. —crystalline rocks of. + +Ballard, M., on the precipitation of sulphate of soda. + +Banda Oriental, tertiary formations of. —crystalline rocks of. + +Barnacles above sea-level. —adhering to upraised shells. + +Basalt of S. Cruz. —streams of, in the Portillo range. —in the +Uspallata range. + +Basin chains of Chile. + +Beagle Channel. + +Beaumont, Elie de, on inclination of lava-streams. —on viscid +quartz-rocks. + +Beech-tree, leaves of fossil. + +Beechey, Captain, on sea-bottom. + +Belcher, Lieutenant, on elevated shells from Concepcion. + +Bella Vista, plain of. + +Benza, Dr., on decomposed granite. + +Bettington, Mr., on quadrupeds transported by rivers. + +Blake, Mr., on the decay of elevated shells near Iquique. —on nitrate +of soda. + +Bole. + +Bollaert, Mr., on mines of Iquique. + +Bones, silicified. —fossil, fresh condition of. + +Bottom of sea off Patagonia. + +Bougainville, on elevation of the Falkland islands. + +Boulder formation of S. Cruz. —of Falkland islands. —anterior to +certain extinct quadrupeds. —of Tierra del Fuego. + +Boulders in the Cordillera. —transported by earthquake-waves. —in +fine-grained tertiary deposits. + +Brande, Mr., on a mineral spring. + +Bravais, M., on elevation of Scandinavia. + +Brazil, elevation of. —crystalline rocks of. + +Broderip, Mr., on elevated shells from Concepcion. + +Brown, Mr. R., on silicified wood of Uspallata range. + +Brown, on silicified wood. + +Bucalema, elevated shells near. + +Buch, Von, on cleavage. —on cretaceous fossils of the Cordillera. —on +the sulphureous volcanoes of Java. + +Buenos Ayres. + +Burchell, Mr., on elevated shells of Brazil. + +Byron, on elevated shells. + +Cachapual, boulders in valley of. + +Caldcleugh, Mr., on elevation of Coquimbo. —on rocks of the Portillo +range. + +Callao, elevation near. —old town of. + +Cape of Good Hope, metamorphic rocks of. + +Carcharias megalodon. + +Carpenter, Dr., on microscopic organisms. + +Castro (Chiloe), beds near. + +Cauquenes Baths, boulders near. —pebbles in porphyry near. —volcanic +formation near. —stratification near. + +Caves above sea-level. + +Cervus pumilus, fossil-horns of. + +Chevalier, M., on elevation near Lima. + +Chile, structure of country between the Cordillera and the Pacific. +—tertiary formations of. —crystalline rocks in. —central, geology of. +—northern, geology of. + +Chiloe, gravel on coast. —elevation of. —tertiary formation of. +—crystalline rocks of. + +Chlorite-schist, near M. Video. + +Chonos archipelago, tertiary formations of. —crystalline rocks of. + +Chupat, Rio, scoriae transported by. + +Claro, Rio, fossiliferous beds of. + +Clay-shale of Los Hornos. + +Clay-slate, formation of, Tierra del Fuego. —of Concepcion. +—feldspathic, of Chile. — —of the Uspallata range. —black siliceous, +band of, in porphyritic formations of Chile. + +Claystone porphyry, formation of, in Chile. —origin of. —eruptive +sources of. + +Cleavage, definition of. —at Bahia. —Rio de Janeiro. —Maldonado. —Monte +Video. —S. Guitru-gueyu. —Falkland I. —Tierra del Fuego. —Chonos I. +—Chiloe. —Concepcion. —Chile. —discussion on. + +Cleavage-laminae superficially bent. + +Cliffs, formation of. + +Climate, late changes in. —of Chile during tertiary period. + +Coal of Concepcion. —S. Lorenzo. + +Coast-denudation of St. Helena. + +Cobija, elevation of. + +Colombia, cretaceous formation of. + +Colonia del Sacramiento, elevation of. —Pampean formation near +Colorado, Rio, gravel of. —sand-dunes of. —Pampean formation near. + +Combarbala. + +Concepcion, elevation of. —deposits of. —crystalline rocks of. + +Conchalee, gravel-terraces of. + +Concretions of gypsum, at Iquique. —in sandstone at S. Cruz. —in +tufaceous tuff of Chiloe. —in gneiss. —in claystone-porphyry at Port +Desire. —in gneiss at Valparaiso. —in metamorphic rocks. —of anhydrite. +—relations of, to veins. + +Conglomerate claystone of Chile. —of Tenuyan. —of the Cumbre Pass. —of +Rio Claro. —of Copiapo. + +Cook, Captain, on form of sea-bottom. + +Copiapo, elevation of. —tertiary formations of. —secondary formations +of. + +Copper, sulphate of. —native, at Arqueros. —mines of, at Panuncillo. +—veins, distribution of. + +Coquimbo, elevation and terraces of. —tertiary formations of. +—secondary formations of. + +Corallines living on pebbles. + +Cordillera, valleys bordered by gravel fringes. —basal strata of. +—fossils of. —elevation of. —gypseous formations of. +—claystone-porphyries of. —andesitic rocks of. —volcanoes of. + +Coste, M., on elevation of Lemus. + +Coy inlet, tertiary formation of. + +Crassatella Lyellii. + +Cruickshanks, Mr., on elevation near Lima. + +Crystals of feldspar, gradual formation of, at Port Desire. + +Cumbre, Pass of, in Cordillera. + +Cuming, Mr., on habits of the Mesodesma. —on range of living shells on +west coast. + +Dana, Mr., on foliated rocks. —on amygdaloids. + +Darwin, Mount. + +D’Aubuisson, on concretions. —on foliated rocks. Decay, gradual, of +upraised shells. + +Decomposition of granite rocks. + +De la Beche, Sir H., his theoretical researches in geology. —on the +action of salt on calcareous rocks. —on bent cleavage-laminae. + +Denudation on coast of Patagonia. —great powers of. —of the Portillo +range. + +Deposits, saline. + +Despoblado, valley of. + +Detritus, nature of, in Cordillera. + +Devonshire, bent cleavage in. + +Dikes, in gneiss of Brazil. —near Rio de Janeiro. —pseudo, at Port +Desire. —in Tierra del Fuego. —in Chonos archipelago, containing +quartz. —near Concepcion, with quartz. —granitic-porphyritic, at +Valparaiso. —rarely vesicular in Cordillera. —absent in the central +ridges of the Portillo pass. —of the Portillo range, with grains of +quartz. —intersecting each other often. —numerous at Copiapo. + +Domeyko, M., on the silver mines of Coquimbo. on the fossils of +Coquimbo. + +D’Orbigny, M. A., on upraised shells of Monte Video. —on elevated +shells at St. Pedro. —on elevated shells near B. Ayres. —on elevation +of S. Blas. —on the sudden elevation of La Plata. —on elevated shells +near Cobija. —on elevated shells near Arica. —on the climate of Peru. +—on salt deposits of Cobija. —on crystals of gypsum in salt-lakes. —on +absence of gypsum in the Pampean formation. —on fossil remains from +Bahia Blanca. —on fossil remains from the banks of the Parana. —on the +geology of St. Fe. —on the age of Pampean formation. —on the Mastodon +Andium. —on the geology of the Rio Negro. —on the character of the +Patagonian fossils. —on fossils from Concepcion. — —from Coquimbo. — +—from Payta. —on fossil tertiary shells of Chile. —on cretaceous +fossils of Tierra del Fuego. — —from the Cordillera of Chile. + +Earth, marine origin of. + +Earthenware, fossil. + +Earthquake, effect of, at S. Maria. —elevation during, at Lemus. —of +1822, at Valparaiso. —effects of, in shattering surface. —fissures made +by. —probable effects on cleavage. + +Earthquakes in Pampas. + +Earthquake-waves, power of, in throwing up shells. —effects of, near +Lima. —power of, in transporting boulders. + +Edmonston, Mr., on depths at which shells live at Valparaiso. + +Ehrenberg, Professor, on infusoria in the Pampean formation. —on +infusoria in the Patagonian formation. + +Elevation of La Plata. —Brazil. —Bahia Blanca. —San Blas. —Patagonia. +—Tierra del Fuego. —Falkland islands. —Pampas. —Chonos archipelago. +—Chiloe. —Chile. —Valparaiso. —Coquimbo. —Guasco. —Iquique. —Cobija. +—Lima. —sudden, at S. Maria. — —at Lemus. —insensible, at Chiloe. — —at +Valparaiso. — —at Coquimbo. —axes of, at Chiloe. — —at P. Rumena. —at +Concepcion. —unfavourable for the accumulation of permanent deposits. +—lines of, parallel to cleavage and foliation. —lines of, oblique to +foliation. —areas of, causing lines of elevation and cleavage. —lines +of, in the Cordillera. —slow, in the Portillo range. —two periods of, +in Cordillera of Central Chile. —of the Uspallata range. —two periods +of, in Cumbre Pass. —horizontal, in the Cordillera of Copiapo. —axes +of, coincident with volcanic orifices. —of the Cordillera, summary on. + +Elliott, Captain, on human remains. + +Ensenada, elevated shells of. + +Entre Rios, geology of. + +Equus curvidens. + +Epidote in Tierra del Fuego. —in gneiss. —frequent in Chile. —in the +Uspallata range. —in porphyry of Coquimbo. + +Erman, M., on andesite. Escarpments, recent, of Patagonia. + +Extinction of fossil mammifers. + +Falkland islands, elevation of. —pebbles on coast. —geology of. + +Falkner, on saline incrustations. + +Faults, great, in Cordillera. + +Feldspar, earthy, metamorphosis of, at Port Desire. —albitic. —crystals +of, with albite. —orthitic, in conglomerate of Tenuyan. —in granite of +Portillo range. —in porphyries in the Cumbre Pass. + +Feuillee on sea-level at Coquimbo. + +Fissures, relations of, to concretions. —upfilled, at Port Desire. —in +clay-slate. + +Fitton, Dr., on the geology of Tierra del Fuego. + +Fitzroy, Captain, on the elevation of the Falkland islands. —on the +elevation of Concepcion. + +Foliation, definition of. —of rocks at Bahia. —Rio de Janeiro. +—Maldonado. —Monte Video. —S. Guitru-gueyu. —Falkland I. —Tierra del +Fuego. —Chonos archipelago. —Chiloe. —Concepcion. —Chile. —discussion +on. + +Forbes, Professor E., on cretaceous fossils of Concepcion. —on +cretaceous fossils and subsidence in Cumbre Pass. —on fossils from +Guasco. — —from Coquimbo. — —from Copiapo. —on depths at which shells +live. + +Formation, Pampean. — —area of. — —estuary origin. —tertiary of Entre +Rios. —of Banda Oriental. —volcanic, in Banda Oriental. —of Patagonia. +—summary on. —tertiary of Tierra del Fuego. — —of the Chonos +archipelago. — —of Chiloe. — —of Chile. — —of Concepcion. — —of +Navidad. — —of Coquimbo. — —of Peru. — —subsidence during. —volcanic, +of Tres Montes. — —of Chiloe. — —old, near Maldonado. — —with laminar +structure. — —ancient, in Tierra del Fuego. —recent, absent on S. +American coast. —metamorphic, of claystone-porphyry of Patagonia. +—foliation of. —plutonic, with laminar structure. —palaeozoic, of the +Falkland I. —claystone, at Concepcion. —Jurassic, of Cordillera. +—Neocomian, of the Portillo Pass. —volcanic, of Cumbre Pass. —gypseous, +of Los Hornos. — —of Coquimbo. — —of Guasco. — —of Copiapo. — —of +Iquique. —cretaceo-oolitic, of Coquimbo. — —of Guasco. — —of Copiapo. — +—of Iquique. + +Fossils, Neocomian, of Portillo Pass. — —of Cumbre Pass. —secondary, of +Coquimbo. — —of Guasco. — —of Copiapo. — —of Iquique. —palaeozoic, from +the Falklands. + +Fragments of hornblende-rock in gneiss. —of gneiss in gneiss. + +Freyer, Lieutenant, on elevated shells of Arica. + +Frezier on sea-level at Coquimbo. + +Galapagos archipelago, pseudo-dikes of. + +Gallegos, Port, tertiary formation of. + +Garnets in gneiss. —in mica-slate. —at Panuncillo. + +Gardichaud, M., on granites of Brazil. + +Gay, M., on elevated shells. —on boulders in the Cordillera. —on +fossils from Cordillera of Coquimbo. + +Gill, Mr., on brickwork transported by an earthquake-wave. + +Gillies, Dr., on heights in the Cordillera. —on extension of the +Portillo range. + +Glen Roy, parallel roads of. —sloping terraces of. + +Gneiss, near Bahia. —of Rio de Janeiro. —decomposition of. + +Gold, distribution of. + +Gorodona, formations near. Granite, axis of oblique, to foliation. +—andesitic. —of Portillo range. —veins of, quartzose. —pebble of, in +porphyritic conglomerate. —conglomerate. + +Grauwacke of Uspallata range. + +Gravel at bottom of sea. —formation of, in Patagonia. —means of +transportation of. —strata of, inclined. + +Gravel-terraces in Cordillera. + +Greenough, Mr., on quartz veins. + +Greenstone, resulting from metamorphose hornblende-rock. —of Tierra del +Fuego. —on the summit of the Campana of Quillota. —porphyry. —relation +of, to clay-slate. + +Gryphaea orientalis. + +Guasco, elevation of. —secondary formation of. + +Guitru-gueyu, Sierra. + +Guyana, gneissic rocks of. + +Gypsum, nodules of, in gravel at Rio Negro. —deposited from sea-water. +—deposits of, at Iquique. —crystals of, in salt lakes. —in Pampean +formation. —in tertiary formation of Patagonia. —great formation of, in +the Portillo Pass. — —in the Cumbre Pass. — —near Los Hornos. — —at +Coquimbo. — —at Copiapo. — —near Iquique. —of San Lorenzo. + +Hall, Captain, on terraces at Coquimbo. + +Hamilton, Mr., on elevation near Tacna. + +Harlan, Dr., on human remains. + +Hayes, Mr. A., on nitrate of soda. + +Henslow, Professor, on concretions. + +Herbert, Captain, on valleys in the Himalaya. + +Herradura Bay, elevated shells of. —tertiary formations of. + +Himalaya, valleys in. + +Hippurites Chilensis. + +Hitchcock, Professor, on dikes. + +Honestones, pseudo, of Coquimbo. —of Copiapo. + +Hooker, Dr. J.D., on fossil beech-leaves. + +Hopkins, Mr., on axes of elevation oblique to foliation. —on origin of +lines of elevation. + +Hornblende-rock, fragments of, in gneiss. + +Hornblende-schist, near M. Video. + +Hornos, Los, section near. + +Hornstone, dike of. + +Horse, fossil tooth of. + +Huafo island. —subsidence at. + +Huantajaya, mines of. + +Humboldt, on saline incrustations. —on foliations of gneiss. —on +concretions in gneiss. + +Icebergs, action on cleavage. + +Illapel, section near. + +Imperial, beds of shells near. + +Incrustations, saline. + +Infusoria in Pampean formation. —in Patagonian formation. + +Iodine, salts of. + +Iquique, elevation of. —saliferous deposits of. —cretaceo-oolitic +formation of. + +Iron, oxide of, in lavas. —in sedimentary beds. —tendency in, to +produce hollow concretions. —sulphate of. + +Isabelle, M., on volcanic rocks of Banda Oriental. + +Joints in clay-slate. + +Jukes, Mr., on cleavage in Newfoundland. + +Kamtschatka, andesite of. + +Kane, Dr., on the production of carbonate of soda. + +King George’s sound, calcareous beds of. + +Lakes, origin of. —fresh-water, near salt lakes. + +Lava, basaltic, of S. Cruz. —claystone-porphyry, at Chiloe. — —ancient +submarine. —basaltic, of the Portillo range. —feldspathic, of the +Cumbre Pass. —submarine, of the Uspallata range. —basaltic, of the +Uspallata range. —submarine, of Coquimbo. —of Copiapo. + +Lemus island. + +Lemuy islet. + +Lignite of Chiloe. —of Concepcion. + +Lima, elevation of. + +Lime, muriate of. + +Limestone of Cumbre Pass. —of Coquimbo. —of Copiapo. + +Lund and Clausen on remains of caves in Brazil. + +Lund, M., on granites of Brazil. + +Lyell, M., on upraised shells retaining their colours. —on terraces at +Coquimbo. —on elevation near Lima. —on fossil horse’s tooth. —on the +boulder-formation being anterior to the extinction of North American +mammifers. —on quadrupeds washed down by floods. —on age of American +fossil mammifers. —on changes of climate. —on denudation. —on +foliation. + +MacCulloch, Dr., on concretions. —on beds of marble. + +Maclaren, Mr., letter to, on coral-formations. + +Macrauchenia Patachonica. + +Madeira, subsidence of. + +Magellan, Strait, elevation near, of. + +Magnesia, sulphate of, in veins. + +Malcolmson, Dr., on trees carried out to sea. + +Maldonado, elevation of. —Pampean formation of. —crystalline rocks of. + +Mammalia, fossil, of Bahia Blanca. — —near St. Fe. — —of Banda +Oriental. — —of St. Julian. — —at Port Gallegos. —washed down by +floods. —number of remains of, and range of, in Pampas. + +Man, skeletons of (Brazil). —remains of, near Lima. —Indian, antiquity +of. + +Marble, beds of. + +Maricongo, ravine of. + +Marsden, on elevation of Sumatra. + +Mastodon Andium, remains of. —range of. + +Maypu, Rio, mouth of, with upraised shells. —gravel fringes of. +—debouchement from the Cordillera. + +Megalonyx, range of. + +Megatherium, range of. + +Miers, Mr., on elevated shells. —on the height of the Uspallata plain. + +Minas, Las. + +Mocha Island, elevation of. —tertiary form of. —subsidence at. + +Molina, on a great flood. + +Monte Hermoso, elevation of. —fossils of. + +Monte Video, elevation of. —Pampean formation of. —crystalline rocks +of. + +Morris and Sharpe, Messrs., on the palaeozoic fossils of the Falklands. + +Mud, Pampean. —long deposited on the same area. + +Murchison, Sir R., on cleavage. —on waves transporting gravel. —on +origin of salt formations. —on the relations of metalliferous veins and +intrusive rocks. —on the absence of granite in the Ural. + +Nautilus d’Orbignyanus. + +Navidad, tertiary formations of, subsidence of. + +Negro, Rio, pumice of pebbles of. —gravel of. —salt lakes of. —tertiary +strata of. + +North America, fossil remains of. + +North Wales, sloping terraces absent in. —bent cleavage of. + +Neuvo Gulf, plains of. —tertiary formation of. + +Owen, Professor, on fossil mammiferous remains. + +Palmer, Mr., on transportation of gravel. + +Pampas, elevation of. —earthquakes of. —formation of. —localities in +which fossil mammifers have been found. + +Panuncillo, mines of. + +Parana, Rio, on saline incrustations. —Pampean formations near. —on the +S. Tandil. + +Parish, Sir W., on elevated shells near Buenos Ayres. —on earthquakes +in the Pampas. —on fresh-water near salt lakes. —on origin of Pampean +formation. + +Patagonia, elevation and plains of. —denudation of. —gravel-formation +of. —sea-cliffs of. —subsidence during tertiary period. —crystalline +rocks of. + +Payta, tertiary formations of. + +Pebbles of pumice. —decrease in size on the coast of Patagonia. —means +of transportation. —encrusted with living corallines. —distribution of, +at the eastern foot of Cordillera. —dispersal of, in the Pampas. —zoned +with colour. + +Pentland, Mr., on heights in the Cordillera. —on fossils of the +Cordillera. + +Pernambuco. + +Peru, tertiary formations of. + +Peuquenes, Pass of, in the Cordillera. —ridge of. + +Pholas, elevated shells of. + +Pitchstone of Chiloe. —of Port Desire. —near Cauquenes. —layers of, in +the Uspallata range. —of Los Hornos. —of Coquimbo. + +Plains of Patagonia. —of Chiloe. —of Chile. —of Uspallata. —on eastern +foot of Cordillera. —of Iquique. + +Plata, La, elevation of. —tertiary formation of. —crystalline rocks of. + +Playfair, Professor, on the transportation of gravel. + +Pluclaro, axis of. + +Pondicherry, fossils of. + +Porcelain rocks of Port Desire. —of the Uspallata range. + +Porphyry, pebbles of, strewed over Patagonia. + +Porphyry, claystone, of Chiloe, — —of Patagonia. — —of Chile. +—greenstone, of Chile. —doubly columnar. —claystone, rare, on the +eastern side of the Portillo Pass. —brick-red and orthitic, of Cumbre +Pass. —intrusive, repeatedly injected. —claystone of the Uspallata +range. — —of Copiapo. — —eruptive sources of. + +Port Desire, elevation and plains of. —tertiary formation of. +—porphyries of. + +Portillo Pass in the Cordillera. + +Portillo chain. —compared with that of the Uspallata. + +Prefil or sea-wall of Valparaiso. + +Puente del Inca, section of. + +Pumice, pebbles of. —conglomerate of R. Negro. —hills of, in the +Cordillera. + +Punta Alta, elevation of. —beds of. + +Quartz-rock of the S. Ventana. —C. Blanco. —Falkland islands. —Portillo +range. —viscidity of. —veins of, near Monte Video. — —in dike of +greenstone. —grains of, in mica slate. — —in dikes. —veins of, +relations to cleavage. + +Quillota, Campana of. + +Quintero, elevation of. + +Quiriquina, elevation of. —deposits of. + +Rancagua, plain of. + +Rapel, R. elevation near. + +Reeks, Mr. T., his analysis of decomposed shells. —his analysis of +salts. + +Remains, human. + +Rio de Janeiro, elevation near. —crystalline rocks of. + +Rivers, small power of transporting pebbles. —small power of, in +forming valleys. —drainage of, in the Cordillera. + +Roads, parallel, of Glen Roy. + +Rocks, volcanic, of Banda Oriental. —Tres Montes. —Chiloe. —Tierra del +Fuego. —with laminar structure. + +Rodents, fossil, remains of. + +Rogers, Professor, address to Association of American Geologists. + +Rose, Professor G., on sulphate of iron at Copiapo. + +S. Blas, elevation of. + +S. Cruz, elevation and plains of. —valley of. —nature of gravel in +valley of. —boulder formation of. —tertiary formation of. —subsidence +at. + +S. Fe Bajada, formations of. + +S. George’s bay, plains of. + +S. Helena island, sea-cliffs, and subsidence of. + +S. Josef, elevation of. —tertiary formation of. + +S. Juan, elevation near. + +S. Julian, elevation and plains of. —salt lake of. —earthy deposit with +mammiferous remains. —tertiary formations of. —subsidence at. + +S. Lorenzo, elevation of. —old salt formation of. + +S. Mary, island of, elevation of. + +S. Pedro, elevation of. + +Salado, R., elevated shells of. —Pampean formation of. + +Salines. + +Salt, with upraised shell. —lakes of. —purity of, in salt lakes. +—deliquescent, necessary for the preservation of meat. —ancient +formation of, at Iquique. — —at S. Lorenzo. —strata of, origin of. + +Salts, superficial deposits of. + +Sand-dunes of the Uruguay. —of the Pampas. —near Bahia Blanca. —of the +Colorado. —of S. Cruz. —of Arica. + +Sarmiento, Mount. + +Schmidtmeyer on auriferous detritus. + +Schomburghk, Sir R., on sea-bottom. —on the rocks of Guyana. + +Scotland, sloping terraces of. + +Sea, nature of bottom of, off Patagonia. —power of, in forming valleys. + +Sea cliffs, formation of. + +Seale, Mr., model of St. Helena. + +Sebastian Bay, tertiary formation of. + +Sedgwick, Professor, on cleavage. + +Serpentine of Copiapo. + +Serpulae, on upraised rocks. + +Shale-rock, of the Portillo Pass. —of Copiapo. + +Shells, upraised state of, in Patagonia. —elevated, too small for human +food. —transported far inland, for food. —upraised, proportional +numbers varying. — —gradual decay of. — —absent on high plains of +Chile. — —near Bahia Blanca. —preserved in concretions. —living and +fossil range of, on west coast. —living, different on the east and west +coast. + +Shingle of Patagonia. + +Siau, M., on sea-bottom. + +Silver mines of Arqueros. —of Chanuncillo. —of Iquique. —distribution +of. + +Slip, great, at S. Cruz. + +Smith, Mr., of Jordan Hill, on upraised shells retaining their colours. +—on Madeira. —on elevated seaweed. —on inclined gravel beds. + +Soda, nitrate of. —sulphate of, near Bahia Blanca. —carbonate of. + +Soundings off Patagonia. —in Tierra del Fuego. + +Spirifers. + +Spix and Martius on Brazil. Sprengel on the production of carbonate of +soda. + +Springs, mineral, in the Cumbre Pass. + +Stratification of sandstone in metamorphic rocks. —of clay-slate in +Tierra del Fuego. —of the Cordillera of Central Chile. —little +disturbed in Cumbre Pass. —disturbance of, near Copiapo. + +Streams of lava at S. Cruz, inclination of. —in the Portillo range. + +String of cotton with fossil-shells. + +Struthiolaria ornata. + +Studer, M., on metamorphic rocks. + +Subsidence during formation of sea-cliffs. —near Lima. —probable, +during Pampean formation. —necessary for the accumulation of permanent +deposits. —during the tertiary formations of Chile and Patagonia. +—probable during the Neocomian formation of the Portillo Pass. +—probable during the formation of conglomerate of Tenuyan. —during the +Neocomian formation of the Cumbre Pass. —of the Uspallata range. +—great, at Copiapo. — —during the formation of the Cordillera. + +Sulphur, volcanic exhalations of. + +Sumatra, promontories of. + +A Summary on the recent elevatory movements. —on the Pampean formation. +—on the tertiary formations of Patagonia and Chile. —on the Chilean +Cordillera. —on the cretaceo-oolitic formation. —on the subsidences of +the Cordillera. —on the elevation of the Cordillera. + +Tacna, elevation of. + +Tampico, elevated shells near. + +Tandil, crystalline rocks of. + +Tapalguen, Pampean formation of. —crystalline rocks of. + +Taylor, Mr., on copper veins of Cuba. + +Temperature of Chile during the tertiary period. + +Tension, lines of, origin of, axes of elevation and of cleavage. + +Tenuy Point, singular section of. + +Tenuyan, valley of. + +Terraces of the valley of S. Cruz. —of equable heights throughout +Patagonia. —of Patagonia, formation of. —of Chiloe. —at Conchalee. —of +Coquimbo. —not horizontal at Coquimbo. —of Guasco. —of S. Lorenzo. —of +gravel within the Cordillera. + +Theories on the origin of the Pampean formation. + +Tierra Amarilla. + +Tierra del Fuego, form of sea-bottom. —tertiary formations of. +—clay-slate formation of. —cretaceous formation of. —crystalline rocks +of. —cleavage of clay-slate. + +Tosca rock. + +Trachyte of Chiloe. —of Port Desire. —in the Cordillera. + +Traditions of promontories having been islands. —on changes of level +near Lima. + +Trees buried in plain of Iquique. —silicified, vertical, of the +Uspallata range. + +Tres Montes, elevation of. —volcanic rocks of. + +Trigonocelia insolita. + +Tristan Arroyo, elevated shells of. + +Tschudi, Mr., on subsidence near Lima. + +Tuff, calcareous, at Coquimbo. —on basin-plain near St. Jago. +—structure of, in Pampas. —origin of, in Pampas. —pumiceous, of R. +Negro. —Nuevo Gulf. —Port Desire. —S. Cruz. —Patagonia, summary on +Chiloe. —formation of, in Portillo chain. —great deposit of, at +Copiapo. + +Tuffs, volcanic, metamorphic, of Uspallata. —of Coquimbo. + +Ulloa, on rain in Peru. —on elevation near Lima. + +Uruguay, Rio, elevation of country near. + +Uspallata, plain of. —pass of. —range of. —concluding remarks on. + +Valdivia, tertiary beds of. —mica-slate of. + +Valley of S. Cruz, structure of. —Coquimbo. —Guasco, structure of. +—Copiapo, structure of. —S. Cruz, tertiary formations of. —Coquimbo, +geology of. —Guasco, secondary formations of. —Copiapo, secondary +formations of. —Despoblado. + +Valleys in the Cordillera bordered by gravel fringes. —formation of. +—in the Cordillera. + +Valparaiso, elevation of. —gneiss of. + +Vein of quartz near Monte Video. —in mica-slate. —relations of, to +cleavage. —in a trap dike. —of granite, quartzose. —remarkable, in +gneiss, near Valparaiso. + +Veins, relations of, to concretions. —metalliferous, of the Uspallata +range. —metalliferous, discussion on. + +Venezuela, gneissic rocks of. + +Ventana, Sierra, Pampean formation near. —quartz-rock of. + +Villa Vincencio Pass. + +Volcan, Rio, mouth of. —fossils of. + +Volcanoes of the Cordillera. —absent, except near bodies of water. +—ancient submarine, in Cordillera. —action of, in relation to changes +of level. —long action of, in the Cordillera. + +Wafer on elevated shells. + +Waves caused by earthquakes, power of, in transporting boulders. —power +of, in throwing up shells. + +Weaver, Mr., on elevated shells. + +White, Martin., on sea-bottom. + +Wood, silicified, of Entre Rios. —S. Cruz. —Chiloe. —Uspallata range. +—Los Hornos. —Copiapo. + +Yeso, Rio, and plain of. + +Ypun Island, tertiary formation of. + +Zeagonite. + + + + +*** END OF THE PROJECT GUTENBERG EBOOK GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA *** + +Updated editions will replace the previous one--the old editions will +be renamed. + +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the +United States without permission and without paying copyright +royalties. Special rules, set forth in the General Terms of Use part +of this license, apply to copying and distributing Project +Gutenberg-tm electronic works to protect the PROJECT GUTENBERG-tm +concept and trademark. 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You may copy it, give it away or re-use it under the terms +of the Project Gutenberg License included with this eBook or online +at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you +are not located in the United States, you will have to check the laws of the +country where you are located before using this eBook. +</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: Geological Observations on South America</div> +<div style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Author: Charles Darwin</div> +<div style='display:block; margin:1em 0'>Release Date: June 17, 2001 [eBook #3620]<br /> +[Most recently updated: December 12, 2021]</div> +<div style='display:block; margin:1em 0'>Language: English</div> +<div style='display:block; margin:1em 0'>Character set encoding: UTF-8</div> +<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA ***</div> + +<h1>Geological Observations on South America</h1> + +<h2 class="no-break">By Charles Darwin</h2> + +<hr /> + +<h2>CONTENTS</h2> + +<table summary="" style=""> + +<tr> +<td> <a href="#link2H_4_0001">EDITORIAL NOTE.</a></td> +</tr> + +<tr> +<td> <a href="#link2H_4_0002">DETAILED TABLE OF CONTENTS.</a></td> +</tr> + +<tr> +<td> <a href="#link2H_4_0003">GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA</a></td> +</tr> + +<tr> +<td> <a href="#link2H_4_0004">CRITICAL INTRODUCTION.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0001">CHAPTER I. ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0002">CHAPTER II. ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0003">CHAPTER III. ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0004">CHAPTER IV. ON THE FORMATIONS OF THE PAMPAS.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0005">CHAPTER V. ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0006">CHAPTER VI. PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0007">CHAPTER VII. CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0008">CHAPTER VIII. NORTHERN CHILE.—CONCLUSION.</a></td> +</tr> + +<tr> +<td> <a href="#link2HCH0009">INDEX</a></td> +</tr> + +</table> + +<div class="chapter"> + +<h2><a name="link2H_4_0001"></a> +EDITORIAL NOTE.</h2> + +<p> +Although in some respects more technical in their subjects and style than +Darwin’s “Journal,” the books here reprinted will never lose +their value and interest for the originality of the observations they contain. +Many parts of them are admirably adapted for giving an insight into problems +regarding the structure and changes of the earth’s surface, and in fact +they form a charming introduction to physical geology and physiography in their +application to special domains. The books themselves cannot be obtained for +many times the price of the present volume, and both the general reader, who +desires to know more of Darwin’s work, and the student of geology, who +naturally wishes to know how a master mind reasoned on most important +geological subjects, will be glad of the opportunity of possessing them in a +convenient and cheap form. +</p> + +<p> +The three introductions, which my friend Professor Judd has kindly furnished, +give critical and historical information which makes this edition of special +value. +</p> + +<p class="right"> +G.T.B. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2H_4_0002"></a> +DETAILED TABLE OF CONTENTS.</h2> + +<p class="noindent"> +<a href="#link2H_4_0004">CRITICAL INTRODUCTION.</a><br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0001">CHAPTER I.</a><br/> +ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.<br/> +Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and +Pumice-pebbles.—Step-formed plains of Patagonia, with upraised +shells.—Terrace-bounded valley of Santa Cruz, formerly a +sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth +of the elevated area.—Equability of the movements, as shown by the +similar heights of the plains.—Slowness of the elevatory +process.—Mode of formation of the step-formed plains.—Summary.- +-Great shingle formation of Patagonia; its extent, origin, and +distribution.—Formation of sea-cliffs.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0002">CHAPTER II.</a><br/> +ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.<br/> +Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions +of the inhabitants on this subject.—Concepcion, earthquake and elevation +of.—VALPARAISO, great elevation of, upraised shells, earth or marine origin, +gradual rise of the land within the historical period.—COQUIMBO, +elevation of, in recent times; terraces of marine origin, their inclination, +their escarpments not horizontal.—Guasco, gravel terraces +of.—Copiapo.—PERU.— Upraised shells of Cobija, Iquique, and +Arica.—Lima, shell-beds and sea- beach on San Lorenzo.—Human +remains, fossil earthenware, earthquake debacle, recent subsidence.—On +the decay of upraised shells.—General summary.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0003">CHAPTER III.</a><br/> +ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS.<br/> +Basin-like plains of Chile; their drainage, their marine origin.—Marks of +sea-action on the eastern flanks of the Cordillera.—Sloping terrace-like +fringes of stratified shingle within the valleys of the Cordillera; their +marine origin.—Boulders in the valley of Cachapual.—Horizontal +elevation of the Cordillera.—Formation of valleys.—Boulders moved +by earthquake- waves.—Saline superficial deposits.—Bed of nitrate +of soda at Iquique.— Saline incrustations.—Salt-lakes of La Plata +and Patagonia; purity of the salt; its origin.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0004">CHAPTER IV.</a><br/> +ON THE FORMATIONS OF THE PAMPAS.<br/> +Mineralogical constitution.—Microscopical structure.—Buenos Ayres, +shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—S. +Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta, +shells, bones, and infusoria of; co-existence of the recent shells and extinct +mammifers.— Buenos Ayres to St. Fe.—Skeletons of +Mastodon.—Infusoria.—Inferior marine tertiary strata, their +age.—Horse’s tooth. BANDA ORIENTAL.— Superficial Pampean +formation.—Inferior tertiary strata, variation of, connected with +volcanic action; Macrauchenia Patachonica at S. Julian in Patagonia, age of, +subsequent to living mollusca and to the erratic block period. +SUMMARY.—Area of Pampean formation.—Theories of +origin.—Source of sediment.—Estuary origin.—Contemporaneous +with existing mollusca.— Relations to underlying tertiary strata. Ancient +deposit of estuary origin.—Elevation and successive deposition of the +Pampean formation.— Number and state of the remains of mammifers; their +habitation, food, extinction, and range.—Conclusion.—Supplement on +the thickness of the Pampean formation.—Localities in Pampas at which +mammiferous remains have been found.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0005">CHAPTER V.</a><br/> +ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.<br/> +Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with +infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P. +Gallegos.— Eastern Tierra del Fuego; leaves of extinct +beech-trees.—Summary on the Patagonian tertiary +formations.—Tertiary formations of the Western Coast.—Chonos and Chiloe +groups, volcanic rocks of.—Concepcion.—Navidad.— +Coquimbo.—Summary.—Age of the tertiary formations.—Lines of +elevation.— Silicified wood.—Comparative ranges of the extinct and +living mollusca on the West Coast of S. America.—Climate of the tertiary +period.—On the causes of the absence of recent conchiferous deposits on +the coasts of South America.—On the contemporaneous deposition and +preservation of sedimentary formations.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0006">CHAPTER VI.</a><br/> +PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.<br/> +Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of +foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in, +decomposition of.—La Plata, metamorphic and old volcanic rocks +of.—S. Ventana.—Claystone porphyry formation of Patagonia; singular +metamorphic rocks; pseudo-dikes.—Falkland Islands, palaeozoic fossils +of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of; +cleavage and foliation; form of land.—Chonos Archipelago, mica-schists, +foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, +dikes, successive formation of.—Central and Northern +Chile.—Concluding remarks on cleavage and foliation.—Their close +analogy and similar origin.—Stratification of metamorphic +schists.—Foliation of intrusive rocks.—Relation of cleavage and +foliation to the lines of tension during metamorphosis.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0007">CHAPTER VII.</a><br/> +CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.<br/> +Central Chile.—Basal formations of the Cordillera.—Origin of the +porphyritic clay-stone conglomerate.—Andesite.—Volcanic +rocks.—Section of the Cordillera by the Peuquenes or Portillo +Pass.—Great gypseous formation.—Peuquenes line; thickness of +strata, fossils of.—Portillo line.—Conglomerate, orthitic granite, +mica-schist, volcanic rocks of.— Concluding remarks on the denudation and +elevation of the Portillo line.— Section by the Cumbre, or Uspallata +Pass.—Porphyries.—Gypseous strata.— Section near the Puente +del Inca; fossils of.—Great subsidence.—Intrusive +porphyries.—Plain of Uspallata.—Section of the Uspallata +chain.— Structure and nature of the strata.—Silicified vertical +trees.—Great subsidence.—Granitic rocks of axis.—Concluding +remarks on the Uspallata range; origin subsequent to that of the main +Cordillera; two periods of subsidence; comparison with the Portillo chain.<br/><br/> +</p> + +<p class="noindent"> +<a href="#link2HCH0008">CHAPTER VIII.</a><br/> +NORTHERN CHILE.—CONCLUSION.<br/> +A Section from Illapel to Combarbala; gypseous formation with silicified +wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley; +fossils.—Guasco, fossils of.—Copiapo, section up valley; Las +Amolanas, silicified wood.—Conglomerates, nature of former land, fossils, +thickness of strata, great subsidence.—Valley of Despoblado, fossils, +tufaceous deposit, complicated dislocations of.—Relations between ancient +orifices of eruption and subsequent axes of injection.—Iquique, Peru, +fossils of, salt-deposits.—Metalliferous veins.—Summary on the +porphyritic conglomerate and gypseous formations.—Great subsidence with +partial elevations during the cretaceo-oolitic period.—On the elevation +and structure of the Cordillera.—Recapitulation on the tertiary +series.— Relation between movements of subsidence and volcanic +action.—Pampean formation.—Recent elevatory +movements.—Long-continued volcanic action in the +Cordillera.—Conclusion. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2H_4_0003"></a> +GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2H_4_0004"></a> +CRITICAL INTRODUCTION.</h2> + +<p> +Of the remarkable “trilogy” constituted by Darwin’s writings +which deal with the geology of the “Beagle,” the member which has +perhaps attracted least attention, up to the present time is that which treats +of the geology of South America. The actual writing of this book appears to +have occupied Darwin a shorter period than either of the other volumes of the +series; his diary records that the work was accomplished within ten months, +namely, between July 1844 and April 1845; but the book was not actually issued +till late in the year following, the preface bearing the date “September +1846.” Altogether, as Darwin informs us in his +“Autobiography,” the geological books “consumed four and a +half years’ steady work,” most of the remainder of the ten years +that elapsed between the return of the “Beagle,” and the completion +of his geological books being, it is sad to relate, “lost through +illness!” +</p> + +<p> +Concerning the “Geological Observations on South America,” Darwin +wrote to his friend Lyell, as follows:—“My volume will be about 240 +pages, dreadfully dull, yet much condensed. I think whenever you have time to +look through it, you will think the collection of facts on the elevation of the +land and on the formation of terraces pretty good.” +</p> + +<p> +“Much condensed” is the verdict that everyone must endorse, on +rising from the perusal of this remarkable book; but by no means +“dull.” The three and a half years from April 1832 to September +1835, were spent by Darwin in South America, and were devoted to continuous +scientific work; the problems he dealt with were either purely geological or +those which constitute the borderland between the geological and biological +sciences. It is impossible to read the journal which he kept during this time +without being impressed by the conviction that it contains all the germs of +thought which afterwards developed into the “Origin of Species.” +But it is equally evident that after his return to England, biological +speculations gradually began to exercise a more exclusive sway over +Darwin’s mind, and tended to dispossess geology, which during the actual +period of the voyage certainly engrossed most of his time and attention. The +wonderful series of observations made during those three and a half years in +South America could scarcely be done justice to, in the 240 pages devoted to +their exposition. That he executed the work of preparing the book on South +America in somewhat the manner of a task, is shown by many references in his +letters. Writing to Sir Joseph Hooker in 1845, he says, “I hope this next +summer to finish my South American Geology, then to get out a little Zoology, +and HURRAH FOR MY SPECIES WORK!” +</p> + +<p> +It would seem that the feeling of disappointment, which Darwin so often +experienced in comparing a book when completed, with the observations and +speculations which had inspired it, was more keenly felt in the case of his +volume on South America than any other. To one friend he writes, “I have +of late been slaving extra hard, to the great discomfiture of wretched +digestive organs, at South America, and thank all the fates, I have done +three-fourths of it. Writing plain English grows with me more and more +difficult, and never attainable. As for your pretending that you will read +anything so dull as my pure geological descriptions, lay not such a flattering +unction on my soul, for it is incredible.” To another friend he writes, +“You do not know what you threaten when you propose to read it—it +is purely geological. I said to my brother, ‘You will of course read +it,’ and his answer was, ‘Upon my life, I would sooner even buy +it.’” +</p> + +<p> +In spite of these disparaging remarks, however, we are strongly inclined to +believe that this book, despised by its author, and neglected by his +contemporaries, will in the end be admitted to be one of Darwin’s chief +titles to fame. It is, perhaps, an unfortunate circumstance that the great +success which he attained in biology by the publication of the “Origin of +Species” has, to some extent, overshadowed the fact that Darwin’s +claims as a geologist, are of the very highest order. It is not too much to say +that, had Darwin not been a geologist, the “Origin of Species” +could never have been written by him. But apart from those geological +questions, which have an important bearing on biological thought and +speculation, such as the proofs of imperfection in the geological record, the +relations of the later tertiary faunas to the recent ones in the same areas, +and the apparent intermingling of types belonging to distant geological epochs, +when we study the palaeontology of remote districts,—there are other +purely geological problems, upon which the contributions made by Darwin are of +the very highest value. I believe that the verdict of the historians of science +will be that if Darwin had not taken a foremost place among the biologists of +this century, his position as a geologist would have been an almost equally +commanding one. +</p> + +<p> +But in the case of Darwin’s principal geological work—that relating +to the origin of the crystalline schists,—geologists were not at the time +prepared to receive his revolutionary teachings. The influence of powerful +authority was long exercised, indeed, to stifle his teaching, and only now, +when this unfortunate opposition has disappeared, is the true nature and +importance of Darwin’s purely geological work beginning to be recognised. +</p> + +<p> +The two first chapters of the “Geological Observations on South +America,” deal with the proofs which exist of great, but frequently +interrupted, movements of elevation during very recent geological times. In +connection with this subject, Darwin’s particular attention was directed +to the relations between the great earthquakes of South America—of some +of which he had impressive experience—and the permanent changes of +elevation which were taking place. He was much struck by the rapidity with +which the evidence of such great earth movements is frequently obliterated; and +especially with the remarkable way in which the action of rain-water, +percolating through deposits on the earth’s surface, removes all traces +of shells and other calcareous organisms. It was these considerations which +were the parents of the generalisation that a palaeontological record can only +be preserved during those periods in which long-continued slow subsidence is +going on. This in turn, led to the still wider and more suggestive conclusion +that the geological record as a whole is, and never can be more than, a series +of more or less isolated fragments. The recognition of this important fact +constitutes the keystone to any theory of evolution which seeks to find a basis +in the actual study of the types of life that have formerly inhabited our +globe. +</p> + +<p> +In his third chapter, Darwin gives a number of interesting facts, collected +during his visits to the plains and valleys of Chili, which bear on the +question of the origin of saliferous deposits—the accumulation of salt, +gypsum, and nitrate of soda. This is a problem that has excited much discussion +among geologists, and which, in spite of many valuable observations, still +remains to a great extent very obscure. Among the important considerations +insisted upon by Darwin is that relating to the absence of marine shells in +beds associated with such deposits. He justly argues that if the strata were +formed in shallow waters, and then exposed by upheaval to subaerial action, all +shells and other calcareous organisms would be removed by solution. +</p> + +<p> +Following Lyell’s method, Darwin proceeds from the study of deposits now +being accumulated on the earth’s surface, to those which have been formed +during the more recent periods of the geological history. +</p> + +<p> +His account of the great Pampean formation, with its wonderful mammalian +remains—Mastodon, Toxodon, Scelidotherium, Macrauchenia, Megatherium, +Megalonyx, Mylodon, and Glyptodon—this full of interest. His discovery of +the remains of a true Equus afforded a remarkable confirmation of the fact- +-already made out in North America—that species of horse had existed and +become extinct in the New World, before their introduction by the Spaniards in +the sixteenth century. Fully perceiving the importance of the microscope in +studying the nature and origin of such deposits as those of the Pampas, Darwin +submitted many of his specimens both to Dr. Carpenter in this country, and to +Professor Ehrenberg in Berlin. Many very important notes on the microscopic +organisms contained in the formation will be found scattered through the +chapter. +</p> + +<p> +Darwin’s study of the older tertiary formations, with their abundant +shells, and their relics of vegetable life buried under great sheets of basalt, +led him to consider carefully the question of climate during these earlier +periods. In opposition to prevalent views on this subject, Darwin points out +that his observations are opposed to the conclusion that a higher temperature +prevailed universally over the globe during early geological periods. He argues +that “the causes which gave to the older tertiary productions of the +quite temperate zones of Europe a tropical character, WERE OF A LOCAL CHARACTER +AND DID NOT AFFECT THE WHOLE GLOBE.” In this, as in many similar +instances, we see the beneficial influence of extensive travel in freeing +Darwin’s mind from prevailing prejudices. It was this widening of +experience which rendered him so especially qualified to deal with the great +problem of the origin of species, and in doing so to emancipate himself from +ideas which were received with unquestioning faith by geologists whose studies +had been circumscribed within the limits of Western Europe. +</p> + +<p> +In the Cordilleras of Northern and Central Chili, Darwin, when studying still +older formations, clearly recognised that they contain an admixture of the +forms of life, which in Europe are distinctive of the Cretaceous and Jurassic +periods respectively. He was thus led to conclude that the classification of +geological periods, which fairly well expresses the facts that had been +discovered in the areas where the science was first studied, is no longer +capable of being applied when we come to the study of widely distant regions. +This important conclusion led up to the further generalisation that each great +geological period has exhibited a geographical distribution of the forms of +animal and vegetable life, comparable to that which prevails in the existing +fauna and flora. To those who are familiar with the extent to which the +doctrine of universal formations has affected geological thought and +speculation, both long before and since the time that Darwin wrote, the +importance of this new standpoint to which he was able to attain will be +sufficiently apparent. Like the idea of the extreme imperfection of the +Geological Record, the doctrine of LOCAL geological formations is found +permeating and moulding all the palaeontological reasonings of his great work. +</p> + +<p> +In one of Darwin’s letters, written while he was in South America, there +is a passage we have already quoted, in which he expresses his inability to +decide between the rival claims upon his attention of “the old +crystalline group of rocks,” and “the softer fossiliferous +beds” respectively. The sixth chapter of the work before us, entitled +“Plutonic and Metamorphic Rocks—Cleavage and Foliation,” +contains a brief summary of a series of observations and reasonings upon these +crystalline rocks, which are, we believe, calculated to effect a revolution in +geological science, and— though their value and importance have long been +overlooked—are likely to entitle Darwin in the future to a position among +geologists, scarcely, if at all, inferior to that which he already occupies +among biologists. +</p> + +<p> +Darwin’s studies of the great rock-masses of the Andes convinced him of +the close relations between the granitic or Plutonic rocks, and those which +were undoubtedly poured forth as lavas. Upon his return, he set to work, with +the aid of Professor Miller, to make a careful study of the minerals composing +the granites and those which occur in the lavas, and he was able to show that +in all essential respects they are identical. He was further able to prove that +there is a complete gradation between the highly crystalline or granitic +rock-masses, and those containing more or less glassy matter between their +crystals, which constitute ordinary lavas. The importance of this conclusion +will be realised when we remember that it was then the common creed of +geologists—and still continues to be so on the Continent—that all +highly crystalline rocks are of great geological antiquity, and that the +igneous ejections which have taken place since the beginning of the tertiary +periods differ essentially, in their composition, their structure, and their +mode of occurrence, from those which have made their appearance at earlier +periods of the world’s history. +</p> + +<p> +Very completely have the conclusions of Darwin upon these subjects been +justified by recent researches. In England, the United States, and Italy, +examples of the gradual passage of rocks of truly granitic structure into +ordinary lavas have been described, and the reality of the transition has been +demonstrated by the most careful studies with the microscope. Recent researches +carried on in South America by Professor Stelzner, have also shown the +existence of a class of highly crystalline rocks—the +“Andengranites”—which combine in themselves many of the +characteristics which were once thought to be distinctive of the so-called +Plutonic and volcanic rocks. No one familiar with recent geological +literature—even in Germany and France, where the old views concerning the +distinction of igneous products of different ages have been most stoutly +maintained—can fail to recognise the fact that the principles contended +for by Darwin bid fair at no distant period to win universal acceptance among +geologists all over the globe. +</p> + +<p> +Still more important are the conclusions at which Darwin arrived with respect +to the origin of the schists and gneisses which cover so large an area in South +America. +</p> + +<p> +Carefully noting, by the aid of his compass and clinometer, at every point +which he visited, the direction and amount of inclination of the parallel +divisions in these rocks, he was led to a very important generalisation— +namely, that over very wide areas the direction (strike) of the planes of +cleavage in slates, and of foliation in schists and gneisses, remained +constant, though the amount of their inclination (dip) often varied within wide +limits. Further than this it appeared that there was always a close +correspondence between the strike of the cleavage and foliation and the +direction of the great axes along which elevation had taken place in the +district. +</p> + +<p> +In Tierra del Fuego, Darwin found striking evidence that the cleavage +intersecting great masses of slate-rocks was quite independent of their +original stratification, and could often, indeed, be seen cutting across it at +right angles. He was also able to verify Sedgwick’s observation that, in +some slates, glossy surfaces on the planes of cleavage arise from the +development of new minerals, chlorite, epidote or mica, and that in this way a +complete graduation from slates to true schists may be traced. +</p> + +<p> +Darwin further showed that in highly schistose rocks, the folia bend around and +encircle any foreign bodies in the mass, and that in some cases they exhibit +the most tortuous forms and complicated puckerings. He clearly saw that in all +cases the forces by which these striking phenomena must have been produced were +persistent over wide areas, and were connected with the great movements by +which the rocks had been upheaved and folded. +</p> + +<p> +That the distinct folia of quartz, feldspar, mica, and other minerals composing +the metamorphic schists could not have been separately deposited as sediment +was strongly insisted upon by Darwin; and in doing so he opposed the view +generally prevalent among geologists at that time. He was thus driven to the +conclusion that foliation, like cleavage, is not an original, but a +superinduced structure in rock-masses, and that it is the result of +re-crystallisation, under the controlling influence of great pressure, of the +materials of which the rock was composed. +</p> + +<p> +In studying the lavas of Ascension, as we have already seen, Darwin was led to +recognise the circumstance that, when igneous rocks are subjected to great +differential movements during the period of their consolidation, they acquire a +foliated structure, closely analogous to that of the crystalline schists. Like +his predecessor in this field of inquiry, Mr. Poulett Scrope, Charles Darwin +seems to have been greatly impressed by these facts, and he argued from them +that the rocks exhibiting the foliated structure must have been in a state of +plasticity, like that of a cooling mass of lava. At that time the suggestive +experiments of Tresca, Daubree, and others, showing that solid masses under the +influence of enormous pressure become actually plastic, had not been published. +Had Darwin been aware of these facts he would have seen that it was not +necessary to assume a state of imperfect solidity in rock-masses in order to +account for their having yielded to pressure and tension, and, in doing so, +acquiring the new characters which distinguish the crystalline schists. +</p> + +<p> +The views put forward by Darwin on the origin of the crystalline schists found +an able advocate in Mr. Daniel Sharpe, who in 1852 and 1854 published two +papers, dealing with the geology of the Scottish Highlands and of the Alps +respectively, in which he showed that the principles arrived at by Darwin when +studying the South American rocks afford a complete explanation of the +structure of the two districts in question. +</p> + +<p> +But, on the other hand, the conclusions of Darwin and Sharpe were met with the +strongest opposition by Sir Roderick Murchison and Dr. A. Geikie, who in 1861 +read a paper before the Geological Society “On the Coincidence between +Stratification and Foliation in the Crystalline Rocks of the Scottish +Highlands,” in which they insisted that their observations in Scotland +tended to entirely disprove the conclusions of Darwin that foliation in rocks +is a secondary structure, and entirely independent of the original +stratification of the rock-masses. +</p> + +<p> +Now it is a most significant circumstance that, no sooner did the officers of +the Geological Survey commence the careful and detailed study of the Scottish +Highlands than they found themselves compelled to make a formal retraction of +the views which had been put forward by Murchison and Geikie in opposition to +the conclusions of Darwin. The officers of the Geological Survey have +completely abandoned the view that the foliation of the Highland rocks has been +determined by their original stratification, and admit that the structure is +the result of the profound movements to which the rocks have been subjected. +The same conclusions have recently been supported by observations made in many +different districts—among which we may especially refer to those of Dr. +H. Reusch in Norway, and those of Dr. J. Lehmann in Saxony. At the present time +the arguments so clearly stated by Darwin in the work before us, have, after +enduring opposition or neglect for a whole generation, begun to “triumph +all along the line,” and we may look forward confidently to the near +future, when his claim to be regarded as one of the greatest of geological +discoverers shall be fully vindicated. +</p> + +<p class="right"> +J<small>OHN</small> W. J<small>UDD</small>. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0001"></a> +CHAPTER I.<br /> +ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.</h2> + +<p class="letter"> +Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and +Pumice-pebbles.—Step-formed plains of Patagonia, with upraised +Shells.—Terrace-bounded Valley of Santa Cruz, formerly a +Sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth +of the elevated area.—Equability of the movements, as shown by the +similar heights of the plains.—Slowness of the elevatory +process.—Mode of formation of the step-formed +plains.—Summary.—Great Shingle Formation of Patagonia; its extent, +origin, and distribution.—Formation of sea-cliffs. +</p> + +<p> +In the following Volume, which treats of the geology of South America, and +almost exclusively of the parts southward of the Tropic of Capricorn, I have +arranged the chapters according to the age of the deposits, occasionally +departing from this order, for the sake of geographical simplicity. +</p> + +<p> +The elevation of the land within the recent period, and the modifications of +its surface through the action of the sea (to which subjects I paid particular +attention) will be first discussed; I will then pass on to the tertiary +deposits, and afterwards to the older rocks. Only those districts and sections +will be described in detail which appear to me to deserve some particular +attention; and I will, at the end of each chapter, give a summary of the +results. We will commence with the proofs of the upheaval of the eastern coast +of the continent, from the Rio Plata southward; and, in the Second Chapter, +follow up the same subject along the shores of Chile and Peru. +</p> + +<p> +On the northern bank of the great estuary of the Rio Plata, near Maldonado, I +found at the head of a lake, sometimes brackish but generally containing fresh +water, a bed of muddy clay, six feet in thickness, with numerous shells of +species still existing in the Plata, namely, the Azara labiata, +d’Orbigny, fragments of Mytilus eduliformis, d’Orbigny, +Paludestrina Isabellei, d’Orbigny, and the Solen Caribaeus, Lam., which +last was embedded vertically in the position in which it had lived. These +shells lie at the height of only two feet above the lake, nor would they have +been worth mentioning, except in connection with analogous facts. +</p> + +<p> +At Monte Video, I noticed near the town, and along the base of the mount, beds +of a living Mytilus, raised some feet above the surface of the Plata: in a +similar bed, at a height from thirteen to sixteen feet, M. Isabelle collected +eight species, which, according to M. d’Orbigny, now live at the mouth of +the estuary. (“Voyage dans l’Amerique Merid.: Part. Geolog.” +page 21.) At Colonia del Sacramiento, further westward, I observed at the +height of about fifteen feet above the river, there of quite fresh water, a +small bed of the same Mytilus, which lives in brackish water at Monte Video. +Near the mouth of Uruguay, and for at least thirty-five miles northward, there +are at intervals large sandy tracts, extending several miles from the banks of +the river, but not raised much above its level, abounding with small bivalves, +which occur in such numbers that at the Agraciado they are sifted and burnt for +lime. Those which I examined near the A. S. Juan were much worn: they consisted +of Mactra Isabellei, d’Orbigny, mingled with few of Venus sinuosa, Lam., +both inhabiting, as I am informed by M. d’Orbigny, brackish water at the +mouth of the Plata, nearly or quite as salt as the open sea. The loose sand, in +which these shells are packed, is heaped into low, straight, long lines of +dunes, like those left by the sea at the head of many bays. M. d’Orbigny +has described an analogous phenomenon on a greater scale, near San Pedro on the +river Parana, where he found widely extended beds and hillocks of sand, with +vast numbers of the Azara labiata, at the height of nearly 100 feet (English) +above the surface of that river. (Ibid page 43.) The Azara inhabits brackish +water, and is not known to be found nearer to San Pedro than Buenos Ayres, +distant above a hundred miles in a straight line. Nearer Buenos Ayres, on the +road from that place to San Isidro, there are extensive beds, as I am informed +by Sir Woodbine Parish, of the Azara labiata, lying at about forty feet above +the level of the river, and distant between two and three miles from it. +(“Buenos Ayres” etc. by Sir Woodbine Parish page 168.) These shells +are always found on the highest banks in the district: they are embedded in a +stratified earthy mass, precisely like that of the great Pampean deposit +hereafter to be described. In one collection of these shells, there were some +valves of the Venus sinuosa, Lam., the same species found with the Mactra on +the banks of the Uruguay. South of Buenos Ayres, near Ensenada, there are other +beds of the Azara, some of which seem to have been embedded in yellowish, +calcareous, semi-crystalline matter; and Sir W. Parish has given me from the +banks of the Arroyo del Tristan, situated in this same neighbourhood, at the +distance of about a league from the Plata, a specimen of a pale- reddish, +calcereo-argillaceous stone (precisely like parts of the Pampean deposit the +importance of which fact will be referred to in a succeeding chapter), +abounding with shells of an Azara, much worn, but which in general form and +appearance closely resemble, and are probably identical with, the A. labiata. +Besides these shells, cellular, highly crystalline rock, formed of the casts of +small bivalves, is found near Ensenada; and likewise beds of sea-shells, which +from their appearance appear to have lain on the surface. Sir W. Parish has +given me some of these shells, and M. d’Orbigny pronounces them to +be:— +</p> + +<p> +1. Buccinanops globulosum, d’Orbigny. +</p> + +<p> +2. Olivancillaria auricularia, d’Orbigny. +</p> + +<p> +3. Venus flexuosa, Lam. +</p> + +<p> +4. Cytheraea (imperfect). +</p> + +<p> +5. Mactra Isabellei, d’Orbigny. +</p> + +<p> +6. Ostrea pulchella, d’Orbigny. +</p> + +<p> +Besides these, Sir W. Parish procured (“Buenos Ayres” etc. by Sir +W. Parish page 168.) (as named by Mr. G.B. Sowerby) the following +shells:— +</p> + +<p> +7. Voluta colocynthis. +</p> + +<p> +8. Voluta angulata. +</p> + +<p> +9. Buccinum (not spec.?). +</p> + +<p> +All these species (with, perhaps, the exception of the last) are recent, and +live on the South American coast. These shell-beds extend from one league to +six leagues from the Plata, and must lie many feet above its level. I heard, +also, of beds of shells on the Somborombon, and on the Rio Salado, at which +latter place, as M. d’Orbigny informs me, the Mactra Isabellei and Venus +sinuosa are found. +</p> + +<p> +During the elevation of the Provinces of La Plata, the waters of the ancient +estuary have but little affected (with the exception of the sand- hills on the +banks of the Parana and Uruguay) the outline of the land. M. Parchappe, +however, has described groups of sand dunes scattered over the wide extent of +the Pampas southward of Buenos Ayres (D’Orbigny “Voyage +Geolog.” page 44.), which M. d’Orbigny attributes with much +probability to the action of the sea, before the plains were raised above its +level. (Before proceeding to the districts southward of La Plata, it may be +worth while just to state, that there is some evidence that the coast of Brazil +has participated in a small amount of elevation. Mr. Burchell informs me, that +he collected at Santos (latitude 24 degrees S.) oyster-shells, apparently +recent, some miles from the shore, and quite above the tidal action. Westward +of Rio de Janeiro, Captain Elliot is asserted (see Harlan “Med. and Phys. +Res.” page 35 and Dr. Meigs in “Transactions of the American +Philosophical Society”), to have found human bones, encrusted with +sea-shells, between fifteen and twenty feet above the level of the sea. Between +Rio de Janeiro and Cape Frio I crossed sandy tracts abounding with sea-shells, +at a distance of a league from the coast; but whether these tracts have been +formed by upheaval, or through the mere accumulation of drift sand, I am not +prepared to assert. At Bahia (latitude 13 degrees S.), in some parts near the +coast, there are traces of sea-action at the height of about twenty feet above +its present level; there are also, in many parts, remnants of beds of sandstone +and conglomerate with numerous recent shells, raised a little above the +sea-level. I may add, that at the head of Bahia Bay there is a formation, about +forty feet in thickness, containing tertiary shells apparently of fresh-water +origin, now washed by the sea and encrusted with Balini; this appears to +indicate a small amount of subsidence subsequent to its deposition. At +Pernambuco (latitude 8 degrees S.), in the alluvial or tertiary cliffs, +surrounding the low land on which the city stands, I looked in vain for organic +remains, or other evidence of changes in level.) +</p> + +<h3>SOUTHWARD OF THE PLATA.</h3> + +<p> +The coast as far as Bahia Blanca (in latitude 39 degrees S.) is formed either +of a horizontal range of cliffs, or of immense accumulations of sand-dunes. +Within Bahia Blanca, a small piece of tableland, about twenty feet above +high-water mark, called Punta Alta, is formed of strata of cemented gravel and +of red earthy mud, abounding with shells (with others lying loose on the +surface), and the bones of extinct mammifers. These shells, twenty in number, +together with a Balanus and two corals, are all recent species, still +inhabiting the neighbouring seas. They will be enumerated in the Fourth +Chapter, when describing the Pampean formation; five of them are identical with +the upraised ones from near Buenos Ayres. The northern shore of Bahia Blanca +is, in main part, formed of immense sand-dunes, resting on gravel with recent +shells, and ranging in lines parallel to the shore. These ranges are separated +from each other by flat spaces, composed of stiff impure red clay, in which, at +the distance of about two miles from the coast, I found by digging a few minute +fragments of sea-shells. The sand-dunes extend several miles inland, and stand +on a plain, which slopes up to a height of between one hundred and two hundred +feet. Numerous, small, well-rounded pebbles of pumice lie scattered both on the +plain and sand-hillocks: at Monte Hermoso, on the flat summit of a cliff, I +found many of them at a height of 120 feet (angular measurement) above the +level of the sea. These pumice pebbles, no doubt, were originally brought down +from the Cordillera by the rivers which cross the continent, in the same way as +the river Negro anciently brought down, and still brings down, pumice, and as +the river Chupat brings down scoriae: when once delivered at the mouth of a +river, they would naturally have travelled along the coasts, and been cast up +during the elevation of the land, at different heights. The origin of the +argillaceous flats, which separate the parallel ranges of sand-dunes, seems due +to the tides here having a tendency (as I believe they have on most shoal, +protected coasts) to throw up a bar parallel to the shore, and at some distance +from it; this bar gradually becomes larger, affording a base for the +accumulation of sand- dunes, and the shallow space within then becomes silted +up with mud. The repetition of this process, without any elevation of the land, +would form a level plain traversed by parallel lines of sand-hillocks; during a +slow elevation of the land, the hillocks would rest on a gently inclined +surface, like that on the northern shore of Bahia Blanca. I did not observe any +shells in this neighbourhood at a greater height than twenty feet; and +therefore the age of the sea-drifted pebbles of pumice, now standing at the +height of 120 feet, must remain uncertain. +</p> + +<p> +The main plain surrounding Bahia Blanca I estimated at from two hundred to +three hundred feet; it insensibly rises towards the distant Sierra Ventana. +There are in this neighbourhood some other and lower plains, but they do not +abut one at the foot of the other, in the manner hereafter to be described, so +characteristic of Patagonia. The plain on which the settlement stands is +crossed by many low sand-dunes, abounding with the minute shells of the +Paludestrina australis, d’Orbigny, which now lives in the bay. This low +plain is bounded to the south, at the Cabeza del Buey, by the cliff-formed +margin of a wide plain of the Pampean formation, which I estimated at sixty +feet in height. On the summit of this cliff there is a range of high sand-dunes +extending several miles in an east and west line. +</p> + +<p> +Southward of Bahia Blanca, the river Colorado flows between two plains, +apparently from thirty to forty feet in height. Of these plains, the southern +one slopes up to the foot of the great sandstone plateau of the Rio Negro; and +the northern one against an escarpment of the Pampean deposit; so that the +Colorado flows in a valley fifty miles in width, between the upper escarpments. +I state this, because on the low plain at the foot of the northern escarpment, +I crossed an immense accumulation of high sand-dunes, estimated by the Gauchos +at no less than eight miles in breadth. These dunes range westward from the +coast, which is twenty miles distant, to far inland, in lines parallel to the +valley; they are separated from each other by argillaceous flats, precisely +like those on the northern shore of Bahia Blanca. At present there is no source +whence this immense accumulation of sand could proceed; but if, as I believe, +the upper escarpments once formed the shores of an estuary, in that case the +sandstone formation of the river Negro would have afforded an inexhaustible +supply of sand, which would naturally have accumulated on the northern shore, +as on every part of the coast open to the south winds between Bahia Blanca and +Buenos Ayres. +</p> + +<p> +At San Blas (40 degrees 40′ S.) a little south of the mouth of the +Colorado, M. d’Orbigny found fourteen species of existing shells (six of +them identical with those from Bahia Blanca), embedded in their natural +positions. (“Voyage” etc. page 54.) From the zone of depth which +these shells are known to inhabit, they must have been uplifted thirty-two +feet. He also found, at from fifteen to twenty feet above this bed, the remains +of an ancient beach. +</p> + +<p> +Ten miles southward, but 120 miles to the west, at Port S. Antonio, the +Officers employed on the Survey assured me that they saw many old sea- shells +strewed on the surface of the ground, similar to those found on other parts of +the coast of Patagonia. At San Josef, ninety miles south in nearly the same +longitude, I found, above the gravel, which caps an old tertiary formation, an +irregular bed and hillock of sand, several feet in thickness, abounding with +shells of Patella deaurita, Mytilus Magellanicus, the latter retaining much of +its colour; Fusus Magellanicus (and a variety of the same), and a large Balanus +(probably B. Tulipa), all now found on this coast: I estimated this bed at from +eighty to one hundred feet above the level of the sea. To the westward of this +bay, there is a plain estimated at between two hundred and three hundred feet +in height: this plain seems, from many measurements, to be a continuation of +the sandstone platform of the river Negro. The next place southward, where I +landed, was at Port Desire, 340 miles distant; but from the intermediate +districts I received, through the kindness of the Officers of the Survey, +especially from Lieutenant Stokes and Mr. King, many specimens and sketches, +quite sufficient to show the general uniformity of the whole line of coast. I +may here state, that the whole of Patagonia consists of a tertiary formation, +resting on and sometimes surrounding hills of porphyry and quartz: the surface +is worn into many wide valleys and into level step-formed plains, rising one +above another, all capped by irregular beds of gravel, chiefly composed of +porphyritic rocks. This gravel formation will be separately described at the +end of the chapter. +</p> + +<p> +My object in giving the following measurements of the plains, as taken by the +Officers of the Survey, is, as will hereafter be seen, to show the remarkable +equability of the recent elevatory movements. Round the southern parts of Nuevo +Gulf, as far as the River Chupat (seventy miles southward of San Josef), there +appear to be several plains, of which the best defined are here represented. +</p> + +<p> +(In the following Diagrams: 1. Baseline is Level of sea. 2. Scale is 1/20 of +inch to 100 feet vertical. 3. Height is shown in feet thus: An. M. always +stands for angular or trigonometrical measurement. Ba. M. always stands for +barometrical measurement. Est. always stands for estimation by the Officers of +the Survey. +</p> + +<h3>DIAGRAM 1. SECTION OF STEP-FORMED PLAINS SOUTH OF NUEVO GULF.</h3> + +<p> +From East (sea level) to West (high): Terrace 1. 80 Est. Terrace 2. 200-220 An. +M. Terrace 3. 350 An. M.) +</p> + +<p> +The upper plain is here well defined (called Table Hills); its edge forms a +cliff or line of escarpment many miles in length, projecting over a lower +plain. The lowest plain corresponds with that at San Josef with the recent +shells on its surface. Between this lowest and the uppermost plain, there is +probably more than one step-formed terrace: several measurements show the +existence of the intermediate one of the height given in Diagram 1. +</p> + +<h3>(DIAGRAM 2. SECTION OF PLAINS IN THE BAY OF ST. GEORGE.</h3> + +<p> +From East (sea level) to West (high): Terrace 1. 250 An. M. Terrace 2. 330 An. +M. Terrace 3. 580 An. M. Terraces 4, 5 and 6 not measured. Terrace 7. 1,200 +Est.) +</p> + +<p> +Near the north headland of the great Bay of St. George (100 miles south of the +Chupat), two well-marked plains of 250 and 330 feet were measured: these are +said to sweep round a great part of the Bay. At its south headland, 120 miles +distant from the north headland, the 250 feet plain was again measured. In the +middle of the bay, a higher plain was found at two neighbouring places (Tilli +Roads and C. Marques) to be 580 feet in height. Above this plain, towards the +interior, Mr. Stokes informs me that there were several other step-formed +plains, the highest of which was estimated at 1,200 feet, and was seen ranging +at apparently the same height for 150 miles northward. All these plains have +been worn into great valleys and much denuded. The section in Diagram 3 is +illustrative of the general structure of the great Bay of St. George. At the +south headland of the Bay of St. George (near C. Three Points) the 250 plain is +very extensive. +</p> + +<h3>(DIAGRAM 3. SECTION OF PLAINS AT PORT DESIRE.</h3> + +<p> +From East (sea level) to West (high): Terrace 1. 100 Est. Terrace 2. 245-255 +Ba. M. Shells on surface. Terrace 3. 330 Ba. M. Shells on surface. Terrace 4. +Not measured.) +</p> + +<p> +At Port Desire (forty miles southward) I made several measurements with the +barometer of a plain, which extends along the north side of the port and along +the open coast, and which varies from 245 to 255 feet in height: this plain +abuts against the foot of a higher plain of 330 feet, which extends also far +northward along the coast, and likewise into the interior. In the distance a +higher inland platform was seen, of which I do not know the height. In three +separate places, I observed the cliff of the 245-255 feet plain, fringed by a +terrace or narrow plain estimated at about one hundred feet in height. These +plains are represented in the section Diagram 3. +</p> + +<p> +In many places, even at the distance of three and four miles from the coast, I +found on the gravel-capped surface of the 245-255 feet, and of the 330 feet +plain, shells of Mytilus Magellanicus, M. edulis, Patella deaurita, and another +Patella, too much worn to be identified, but apparently similar to one found +abundantly adhering to the leaves of the kelp. These species are the commonest +now living on this coast. The shells all appeared very old; the blue of the +mussels was much faded; and only traces of colour could be perceived in the +Patellas, of which the outer surfaces were scaling off. They lay scattered on +the smooth surface of the gravel, but abounded most in certain patches, +especially at the heads of the smaller valleys: they generally contained sand +in their insides; and I presume that they have been washed by alluvial action +out of thin sandy layers, traces of which may sometimes be seen covering the +gravel. The several plains have very level surfaces; but all are scooped out by +numerous broad, winding, flat-bottomed valleys, in which, judging from the +bushes, streams never flow. These remarks on the state of the shells, and on +the nature of the plains, apply to the following cases, so need not be +repeated. +</p> + +<h3>(DIAGRAM 4. SECTION OF PLAINS AT PORT S. JULIAN.</h3> + +<p> +From East (sea level) to West (high): Terrace 1. Shells on surface. 90 Est. +Terrace 2. 430 An. M. Terrace 3. 560 An. M. Terrace 4. 950 An. M.) +</p> + +<p> +Southward of Port Desire, the plains have been greatly denuded, with only small +pieces of tableland marking their former extension. But opposite Bird Island, +two considerable step-formed plains were measured, and found respectively to be +350 and 590 feet in height. This latter plain extends along the coast close to +Port St. Julian (110 miles south of Port Desire); see Diagram 4. +</p> + +<p> +The lowest plain was estimated at ninety feet: it is remarkable from the usual +gravel-bed being deeply worn into hollows, which are filled up with, as well as +the general surface covered by, sandy and reddish earthy matter: in one of the +hollows thus filled up, the skeleton of the Macrauchenia Patachonica, as will +hereafter be described, was embedded. On the surface and in the upper parts of +this earthy mass, there were numerous shells of Mytilus Magellanicus and M. +edulis, Patella deaurita, and fragments of other species. This plain is +tolerably level, but not extensive; it forms a promontory seven or eight miles +long, and three or four wide. The upper plains in Diagram 4 were measured by +the Officers of the Survey; they were all capped by thick beds of gravel, and +were all more or less denuded; the 950 plain consists merely of separate, +truncated, gravel-capped hills, two of which, by measurement, were found to +differ only three feet. The 430 feet plain extends, apparently with hardly a +break, to near the northern entrance of the Rio Santa Cruz (fifty miles to the +south); but it was there found to be only 330 feet in height. +</p> + +<h3>(DIAGRAM 5. SECTION OF PLAINS AT THE MOUTH OF THE RIO SANTA CRUZ.</h3> + +<p> +From East (sea level) to West (high): Terrace 1. (sloping) 355 Ba. M. Shells on +surface. 463 Ba. M. Terrace 2. 710 An. M. Terrace 3. 840 An. M.) +</p> + +<p> +On the southern side of the mouth of the Santa Cruz we have Diagram 5, which I +am able to give with more detail than in the foregoing cases. +</p> + +<p> +The plain marked 355 feet (as ascertained by the barometer and by angular +measurement) is a continuation of the above-mentioned 330 feet plain: it +extends in a N.W. direction along the southern shores of the estuary. It is +capped by gravel, which in most parts is covered by a thin bed of sandy earth, +and is scooped out by many flat-bottomed valleys. It appears to the eye quite +level, but in proceeding in a S.S.W. course, towards an escarpment distant +about six miles, and likewise ranging across the country in a N.W. line, it was +found to rise at first insensibly, and then for the last half-mile, sensibly, +close up to the base of the escarpment: at this point it was 463 feet in +height, showing a rise of 108 feet in the six miles. On this 355-463 feet +plain, I found several shells of Mytilus Magellanicus and of a Mytilus, which +Mr. Sowerby informs me is yet unnamed, though well-known as recent on this +coast; Patella deaurita; Fusus, I believe, Magellanicus, but the specimen has +been lost; and at the distance of four miles from the coast, at the height of +about four hundred feet, there were fragments of the same Patella and of a +Voluta (apparently V. ancilla) partially embedded in the superficial sandy +earth. All these shells had the same ancient appearance with those from the +foregoing localities. As the tides along this part of the coast rise at the +Syzygal period forty feet, and therefore form a well-marked beach-line, I +particularly looked out for ridges in crossing this plain, which, as we have +seen, rises 108 feet in about six miles, but I could not see any traces of +such. The next highest plain is 710 feet above the sea; it is very narrow, but +level, and is capped with gravel; it abuts to the foot of the 840 feet plain. +This summit-plain extends as far as the eye can range, both inland along the +southern side of the valley of the Santa Cruz, and southward along the +Atlantic. +</p> + +<h3>THE VALLEY OF THE R. SANTA CRUZ.</h3> + +<p> +This valley runs in an east and west direction to the Cordillera, a distance of +about one hundred and sixty miles. It cuts through the great Patagonian +tertiary formation, including, in the upper half of the valley, immense streams +of basaltic lava, which as well as the softer beds, are capped by gravel; and +this gravel, high up the river, is associated with a vast boulder formation. (I +have described this formation in a paper in the “Geological +Transactions” volume 6 page 415.) In ascending the valley, the plain +which at the mouth on the southern side is 355 feet high, is seen to trend +towards the corresponding plain on the northern side, so that their escarpments +appear like the shores of a former estuary, larger than the existing one: the +escarpments, also, of the 840 feet summit-plain (with a corresponding northern +one, which is met with some way up the valley), appear like the shores of a +still larger estuary. Farther up the valley, the sides are bounded throughout +its entire length by level, gravel-capped terraces, rising above each other in +steps. The width between the upper escarpments is on an average between seven +and ten miles; in one spot, however, where cutting through the basaltic lava, +it was only one mile and a half. Between the escarpments of the second highest +terrace the average width is about four or five miles. The bottom of the +valley, at the distance of 110 miles from its mouth, begins sensibly to expand, +and soon forms a considerable plain, 440 feet above the level of the sea, +through which the river flows in a gut from twenty to forty feet in depth. I +here found, at a point 140 miles from the Atlantic, and seventy miles from the +nearest creek of the Pacific, at the height of 410 feet, a very old and worn +shell of Patella deaurita. Lower down the valley, 105 miles from the Atlantic +(longitude 71 degrees W.), and at an elevation of about 300 feet, I also found, +in the bed of the river, two much worn and broken shells of the Voluta ancilla, +still retaining traces of their colours; and one of the Patella deaurita. It +appeared that these shells had been washed from the banks into the river; +considering the distance from the sea, the desert and absolutely unfrequented +character of the country, and the very ancient appearance of the shells +(exactly like those found on the plains nearer the coast), there is, I think, +no cause to suspect that they could have been brought here by Indians. +</p> + +<p> +The plain at the head of the valley is tolerably level, but water-worn, and +with many sand-dunes on it like those on a sea-coast. At the highest point to +which we ascended, it was sixteen miles wide in a north and south line; and +forty-five miles in length in an east and west line. It is bordered by the +escarpments, one above the other, of two plains, which diverge as they approach +the Cordillera, and consequently resemble, at two levels, the shores of great +bays facing the mountains; and these mountains are breached in front of the +lower plain by a remarkable gap. The valley, therefore, of the Santa Cruz +consists of a straight broad cut, about ninety miles in length, bordered by +gravel-capped terraces and plains, the escarpments of which at both ends +diverge or expand, one over the other, after the manner of the shores of great +bays. Bearing in mind this peculiar form of the land—the sand-dunes on +the plain at the head of the valley—the gap in the Cordillera, in front +of it—the presence in two places of very ancient shells of existing +species—and lastly, the circumstance of the 355-453 feet plain, with the +numerous marine remains on its surface, sweeping from the Atlantic coast, far +up the valley, I think we must admit, that within the recent period, the course +of the Santa Cruz formed a sea-strait intersecting the continent. At this +period, the southern part of South America consisted of an archipelago of +islands 360 miles in a north and south line. We shall presently see, that two +other straits also, since closed, then cut through Tierra del Fuego; I may add, +that one of them must at that time have expanded at the foot of the Cordillera +into a great bay (now Otway Water) like that which formerly covered the 440 +feet plain at the head of the Santa Cruz. +</p> + +<p> +(DIAGRAM 6. NORTH AND SOUTH SECTION ACROSS THE TERRACES BOUNDING THE VALLEY OF +THE RIVER SANTA CRUZ, HIGH UP ITS COURSE. +</p> + +<p> +The height of each terrace, above the level of the river (furthest to nearest +to the river) in feet: +</p> + +<p> +A, north and south: 1,122 B, north and south: 869 C, north and south: 639 D, +north: not measured. D, north? (suggest south): 185 E: 20 Bed of River. +</p> + +<p> +Vertical scale 1/20 of inch to 100 feet; but terrace E, being only twenty feet +above the river, has necessarily been raised. The horizontal distances much +contracted; the distance from the edge of A North to A South being on an +average from seven to ten miles.) I have said that the valley in its whole +course is bordered by gravel- capped plains. The section (Diagram 6), supposed +to be drawn in a north and south line across the valley, can scarcely be +considered as more than illustrative; for during our hurried ascent it was +impossible to measure all the plains at any one place. At a point nearly midway +between the Cordillera and the Atlantic, I found the plain (A north) 1,122 feet +above the river; all the lower plains on this side were here united into one +great broken cliff: at a point sixteen miles lower down the stream, I found by +measurement and estimation that B (north) was 869 above the river: very near to +where A (north) was measured, C (north) was 639 above the same level: the +terrace D (north) was nowhere measured: the lowest E (north) was in many places +about twenty feet above the river. These plains or terraces were best developed +where the valley was widest; the whole five, like gigantic steps, occurred +together only at a few points. The lower terraces are less continuous than the +higher ones, and appear to be entirely lost in the upper third of the valley. +Terrace C (south), however was traced continuously for a great distance. The +terrace B (north), at a point fifty- five miles from the mouth of the river, +was four miles in width; higher up the valley this terrace (or at least the +second highest one, for I could not always trace it continuously) was about +eight miles wide. This second plain was generally wider than the lower +ones—as indeed follows from the valley from A (north) to A (south) being +generally nearly double the width of from B (north) to B (south). Low down the +valley, the summit-plain A (south) is continuous with the 840 feet plain on the +coast, but it is soon lost or unites with the escarpment of B (south). The +corresponding plain A (north), on the north side of the valley, appears to +range continuously from the Cordillera to the head of the present estuary of +the Santa Cruz, where it trends northward towards Port St. Julian. Near the +Cordillera the summit-plain on both sides of the valley is between 3,200 and +3,300 feet in height; at 100 miles from the Atlantic, it is 1,416 feet, and on +the coast 840 feet, all above the sea-beach; so that in a distance of 100 miles +the plain rises 576 feet, and much more rapidly near to the Cordillera. The +lower terraces B and C also appear to rise as they run up the valley; thus D +(north), measured at two points twenty-four miles apart, was found to have +risen 185 feet. From several reasons I suspect, that this gradual inclination +of the plains up the valley, has been chiefly caused by the elevation of the +continent in mass, having been the greater the nearer to the Cordillera. +</p> + +<p> +All the terraces are capped with well-rounded gravel, which rests either on the +denuded and sometimes furrowed surface of the soft tertiary deposits, or on the +basaltic lava. The difference in height between some of the lower steps or +terraces seems to be entirely owing to a difference in the thickness of the +capping gravel. Furrows and inequalities in the gravel, where such occur, are +filled up and smoothed over with sandy earth. The pebbles, especially on the +higher plains, are often whitewashed, and even cemented together by a white +aluminous substance, and I occasionally found this to be the case with the +gravel on the terrace D. I could not perceive any trace of a similar deposition +on the pebbles now thrown up by the river, and therefore I do not think that +terrace D was river-formed. As the terrace E generally stands about twenty feet +above the bed of the river, my first impression was to doubt whether even this +lowest one could have been so formed; but it should always be borne in mind, +that the horizontal upheaval of a district, by increasing the total descent of +the streams, will always tend to increase, first near the sea-coast and then +further and further up the valley, their corroding and deepening powers: so +that an alluvial plain, formed almost on a level with a stream, will, after an +elevation of this kind, in time be cut through, and left standing at a height +never again to be reached by the water. With respect to the three upper +terraces of the Santa Cruz, I think there can be no doubt, that they were +modelled by the sea, when the valley was occupied by a strait, in the same +manner (hereafter to be discussed) as the greater step-formed, shell- strewed +plains along the coast of Patagonia. +</p> + +<p> +To return to the shores of the Atlantic: the 840 feet plain, at the mouth of +the Santa Cruz, is seen extending horizontally far to the south; and I am +informed by the Officers of the Survey, that bending round the head of Coy +Inlet (sixty-five miles southward), it trends inland. Outliers of apparently +the same height are seen forty miles farther south, inland of the river +Gallegos; and a plain comes down to Cape Gregory (thirty-five miles southward), +in the Strait of Magellan, which was estimated at between eight hundred and one +thousand feet in height, and which, rising towards the interior, is capped by +the boulder formation. South of the Strait of Magellan, there are large +outlying masses of apparently the same great tableland, extending at intervals +along the eastern coast of Tierra del Fuego: at two places here, 110 miles a +part, this plain was found to be 950 and 970 feet in height. +</p> + +<p> +From Coy Inlet, where the high summit-plain trends inland, a plain estimated at +350 feet in height, extends for forty miles to the river Gallegos. From this +point to the Strait of Magellan, and on each side of that Strait, the country +has been much denuded and is less level. It consists chiefly of the boulder +formation, which rises to a height of between one hundred and fifty and two +hundred and fifty feet, and is often capped by beds of gravel. At N.S. Gracia, +on the north side of the Inner Narrows of the Strait of Magellan, I found on +the summit of a cliff, 160 feet in height, shells of existing Patellae and +Mytili, scattered on the surface and partially embedded in earth. On the +eastern coast, also, of Tierra del Fuego, in latitude 53 degrees 20′ +south, I found many Mytili on some level land, estimated at 200 feet in height. +Anterior to the elevation attested by these shells, it is evident by the +present form of the land, and by the distribution of the great erratic boulders +on the surface, that two sea-channels connected the Strait of Magellan both +with Sebastian Bay and with Otway Water. (“Geological Transactions” +volume 6 page 419.) +</p> + +<p> +CONCLUDING REMARKS ON THE RECENT ELEVATION OF THE SOUTH-EASTERN COASTS OF +AMERICA, AND ON THE ACTION OF THE SEA ON THE LAND. +</p> + +<p> +Upraised shells of species, still existing as the commonest kinds in the +adjoining sea, occur, as we have seen, at heights of between a few feet and 410 +feet, at intervals from latitude 33 degrees 40′ to 53 degrees 20′ +south. This is a distance of 1,180 geographical miles—about equal from +London to the North Cape of Sweden. As the boulder formation extends with +nearly the same height 150 miles south of 53 degrees 20′, the most +southern point where I landed and found upraised shells; and as the level +Pampas ranges many hundred miles northward of the point, where M. +d’Orbigny found at the height of 100 feet beds of the Azara, the space in +a north and south line, which has been uplifted within the recent period, must +have been much above the 1,180 miles. By the term “recent,” I refer +only to that period within which the now living mollusca were called into +existence; for it will be seen in the Fourth Chapter, that both at Bahia Blanca +and P. S. Julian, the mammiferous quadrupeds which co-existed with these shells +belong to extinct species. I have said that the upraised shells were found only +at intervals on this line of coast, but this in all probability may be +attributed to my not having landed at the intermediate points; for wherever I +did land, with the exception of the river Negro, shells were found: moreover, +the shells are strewed on plains or terraces, which, as we shall immediately +see, extend for great distances with a uniform height. I ascended the higher +plains only in a few places, owing to the distance at which their escarpments +generally range from the coast, so that I am far from knowing that 410 feet is +the maximum of elevation of these upraised remains. The shells are those now +most abundant in a living state in the adjoining sea. (Captain King +“Voyages of ‘Adventure’ and ‘Beagle’” +volume 1 pages 6 and 133.) All of them have an ancient appearance; but some, +especially the mussels, although lying fully exposed to the weather, retain to +a considerable extent their colours: this circumstance appears at first +surprising, but it is now known that the colouring principle of the Mytilus is +so enduring, that it is preserved when the shell itself is completely +disintegrated. (See Mr. Lyell “Proofs of a Gradual Rising in +Sweden” in the “Philosophical Transactions” 1835 page 1. See +also Mr. Smith of Jordan Hill in the “Edinburgh New Philosophical +Journal” volume 25 page 393.) Most of the shells are broken; I nowhere +found two valves united; the fragments are not rounded, at least in none of the +specimens which I brought home. +</p> + +<p> +With respect to the breadth of the upraised area in an east and west line, we +know from the shells found at the Inner Narrows of the Strait of Magellan, that +the entire width of the plain, although there very narrow, has been elevated. +It is probable that in this southernmost part of the continent, the movement +has extended under the sea far eastward; for at the Falkland Islands, though I +could not find any shells, the bones of whales have been noticed by several +competent observers, lying on the land at a considerable distance from the sea, +and at the height of some hundred feet above it. (“Voyages of the +‘Adventure’ and ‘Beagle’” volume 2 page 227. And +Bougainville’s “Voyage” tome 1 page 112.) Moreover, we know +that in Tierra del Fuego the boulder formation has been uplifted within the +recent period, and a similar formation occurs on the north-western shores +(Byron Sound) of these islands. (I owe this fact to the kindness of Captain +Sulivan, R.N., a highly competent observer. I mention it more especially, as in +my Paper (page 427) on the Boulder Formation, I have, after having examined the +northern and middle parts of the eastern island, said that the formation was +here wholly absent.) The distance from this point to the Cordillera of Tierra +del Fuego, is 360 miles, which we may take as the probable width of the +recently upraised area. In the latitude of the R. Santa Cruz, we know from the +shells found at the mouth and head, and in the middle of the valley, that the +entire width (about 160 miles) of the surface eastward of the Cordillera has +been upraised. From the slope of the plains, as shown by the course of the +rivers, for several degrees northward of the Santa Cruz, it is probable that +the elevation attested by the shells on the coast has likewise extended to the +Cordillera. When, however, we look as far northward as the provinces of La +Plata, this conclusion would be very hazardous; not only is the distance from +Maldonado (where I found upraised shells) to the Cordillera great, namely, 760 +miles, but at the head of the estuary of the Plata, a N.N.E. and S.S.W. range +of tertiary volcanic rocks has been observed (This volcanic formation will be +described in Chapter IV. It is not improbable that the height of the upraised +shells at the head of the estuary of the Plata, being greater than at Bahia +Blanca or at San Blas, may be owing to the upheaval of these latter places +having been connected with the distant line of the Cordillera, whilst that of +the provinces of La Plata was in connection with the adjoining tertiary +volcanic axis.), which may well indicate an axis of elevation quite distinct +from that of the Andes. Moreover, in the centre of the Pampas in the chain of +Cordova, severe earthquakes have been felt (See Sir W. Parish’s work on +“La Plata” page 242. For a notice of an earthquake which drained a +lake near Cordova, see also Temple’s “Travels in Peru.” Sir +W. Parish informs me, that a town between Salta and Tucuman (north of Cordova) +was formerly utterly overthrown by an earthquake.); whereas at Mendoza, at the +eastern foot of the Cordillera, only gentle oscillations, transmitted from the +shores of the Pacific, have ever been experienced. Hence the elevation of the +Pampas may be due to several distinct axes of movement; and we cannot judge, +from the upraised shells round the estuary of the Plata, of the breadth of the +area uplifted within the recent period. +</p> + +<p> +Not only has the above specified long range of coast been elevated within the +recent period, but I think it may be safely inferred from the similarity in +height of the gravel-capped plains at distant points, that there has been a +remarkable degree of equability in the elevatory process. I may premise, that +when I measured the plains, it was simply to ascertain the heights at which +shells occurred; afterwards, comparing these measurements with some of those +made during the Survey, I was struck with their uniformity, and accordingly +tabulated all those which represented the summit-edges of plains. The extension +of the 330 to 355 feet plain is very striking, being found over a space of 500 +geographical miles in a north and south line. A table (Table 1) of the +measurements is given below. The angular measurements and all the estimations +(in feet) are by the Officers of the Survey; the barometrical ones by +myself:— +</p> + +<h3>TABLE 1.</h3> + +<p> +Gallegos River to Coy Inlet (partly angular partly estimation) 350 South Side +of Santa Cruz (angular and barometric) 355 North Side of Santa Cruz (angular +and barometric) 330 Bird Island, plain opposite to (angular) 350 Port Desire, +plain extending far along coast (barometric) 330 St. George’s Bay, north +promontory (angular) 330 Table Land, south of New Bay (angular) 350 +</p> + +<p> +A plain, varying from 245 to 255 feet, seems to extend with much uniformity +from Port Desire to the north of St. George’s Bay, a distance of 170 +miles; and some approximate measurements (in feet), also given in Table 2 +below, indicate the much greater extension of 780 miles:— +</p> + +<h3>TABLE 2.</h3> + +<p> +Coy Inlet, south of (partly angular and partly estimation) 200 to 300 Port +Desire (barometric) 245 to 255 C. Blanco (angular) 250 North Promontory of St. +George’s Bay (angular) 250 South of New Bay (angular) 200 to 220 North of +S. Josef (estimation) 200 to 300 Plain of Rio Negro (angular) 200 to 220 Bahia +Blanca (estimation) 200 to 300 +</p> + +<p> +The extension, moreover, of the 560 to 580, and of the 80 to 100 feet, plains +is remarkable, though somewhat less obvious than in the former cases. Bearing +in mind that I have not picked these measurements out of a series, but have +used all those which represented the edges of plains, I think it scarcely +possible that these coincidences in height should be accidental. We must +therefore conclude that the action, whatever it may have been, by which these +plains have been modelled into their present forms, has been singularly +uniform. +</p> + +<p> +These plains or great terraces, of which three and four often rise like steps +one behind the other, are formed by the denudation of the old Patagonian +tertiary beds, and by the deposition on their surfaces of a mass of +well-rounded gravel, varying, near the coast, from ten to thirty-five feet in +thickness, but increasing in thickness towards the interior. The gravel is +often capped by a thin irregular bed of sandy earth. The plains slope up, +though seldom sensibly to the eye, from the summit edge of one escarpment to +the foot of the next highest one. Within a distance of 150 miles, between Santa +Cruz to Port Desire, where the plains are particularly well developed, there +are at least seven stages or steps, one above the other. On the three lower +ones, namely, those of 100 feet, 250 feet, and 350 feet in height, existing +littoral shells are abundantly strewed, either on the surface, or partially +embedded in the superficial sandy earth. By whatever action these three lower +plains have been modelled, so undoubtedly have all the higher ones, up to a +height of 950 feet at S. Julian, and of 1,200 feet (by estimation) along St. +George’s Bay. I think it will not be disputed, considering the presence +of the upraised marine shells, that the sea has been the active power during +stages of some kind in the elevatory process. +</p> + +<p> +We will now briefly consider this subject: if we look at the existing +coast-line, the evidence of the great denuding power of the sea is very +distinct; for, from Cape St. Diego, in latitude 54 degrees 30′ to the +mouth of the Rio Negro, in latitude 31 degrees (a length of more than eight +hundred miles), the shore is formed, with singularly few exceptions, of bold +and naked cliffs: in many places the cliffs are high; thus, south of the Santa +Cruz, they are between eight and nine hundred feet in height, with their +horizontal strata abruptly cut off, showing the immense mass of matter which +has been removed. Nearly this whole line of coast consists of a series of +greater or lesser curves, the horns of which, and likewise certain straight +projecting portions, are formed of hard rocks; hence the concave parts are +evidently the effect and the measure of the denuding action on the softer +strata. At the foot of all the cliffs, the sea shoals very gradually far +outwards; and the bottom, for a space of some miles, everywhere consists of +gravel. I carefully examined the bed of the sea off the Santa Cruz, and found +that its inclination was exactly the same, both in amount and in its peculiar +curvature, with that of the 355 feet plain at this same place. If, therefore, +the coast, with the bed of the adjoining sea, were now suddenly elevated one or +two hundred feet, an inland line of cliffs, that is an escarpment, would be +formed, with a gravel-capped plain at its foot gently sloping to the sea, and +having an inclination like that of the existing 355 feet plain. From the +denuding tendency of the sea, this newly formed plain would in time be eaten +back into a cliff: and repetitions of this elevatory and denuding process would +produce a series of gravel-capped sloping terraces, rising one above another, +like those fronting the shores of Patagonia. +</p> + +<p> +The chief difficulty (for there are other inconsiderable ones) on this view, is +the fact,—as far as I can trust two continuous lines of soundings +carefully taken between Santa Cruz and the Falkland Islands, and several +scattered observations on this and other coasts,—that the pebbles at the +bottom of the sea QUICKLY and REGULARLY decrease in size with the increasing +depth and distance from the shore, whereas in the gravel on the sloping plains, +no such decrease in size was perceptible. +</p> + +<p> +Table 3 below gives the average result of many soundings off the Santa +Cruz:— TABLE 3. +</p> + +<p> +Under two miles from the shore, many of the pebbles were of large size, mingled +with some small ones. +</p> + +<p> +Column 1. Distance in miles from the shore. +</p> + +<p> +Column 2. Depth in fathoms. +</p> + +<p> +Column 3. Size of Pebbles. +</p> + +<p> +I particularly attended to the size of the pebbles on the 355 feet Santa Cruz +plain, and I noticed that on the summit-edge of the present sea cliffs many +were as large as half a man’s head; and in crossing from these cliffs to +the foot of the next highest escarpment, a distance of six miles, I could not +observe any increase in their size. We shall presently see that the theory of a +slow and almost insensible rise of the land, will explain all the facts +connected with the gravel-capped terraces, better than the theory of sudden +elevations of from one to two hundred feet. +</p> + +<p> +M. d’Orbigny has argued, from the upraised shells at San Blas being +embedded in the positions in which they lived, and from the valves of the Azara +labiata high on the banks of the Parana being united and unrolled, that the +elevation of Northern Patagonia and of La Plata must have been sudden; for he +thinks, if it had been gradual, these shells would all have been rolled on +successive beach-lines. But in PROTECTED bays, such as in that of Bahia Blanca, +wherever the sea is accumulating extensive mud-banks, or where the winds +quietly heap up sand-dunes, beds of shells might assuredly be preserved buried +in the positions in which they had lived, even whilst the land retained the +same level; any, the smallest, amount of elevation would directly aid in their +preservation. I saw a multitude of spots in Bahia Blanca where this might have +been effected; and at Maldonado it almost certainly has been effected. In +speaking of the elevation of the land having been slow, I do not wish to +exclude the small starts which accompany earthquakes, as on the coast of Chile; +and by such movements beds of shells might easily be uplifted, even in +positions exposed to a heavy surf, without undergoing any attrition: for +instance, in 1835, a rocky flat off the island of Santa Maria was at one blow +upheaved above high-water mark, and was left covered with gaping and putrefying +mussel-shells, still attached to the bed on which they had lived. If M. +d’Orbigny had been aware of the many long parallel lines of +sand-hillocks, with infinitely numerous shells of the Mactra and Venus, at a +low level near the Uruguay; if he had seen at Bahia Blanca the immense +sand-dunes, with water-worn pebbles of pumice, ranging in parallel lines, one +behind the other, up a height of at least 120 feet; if he had seen the +sand-dunes, with the countless Paludestrinas, on the low plain near the Fort at +this place, and that long line on the edge of the cliff, sixty feet higher up; +if he had crossed that long and great belt of parallel sand-dunes, eight miles +in width, standing at the height of from forty to fifty feet above the +Colorado, where sand could not now collect,—I cannot believe he would +have thought that the elevation of this great district had been sudden. +Certainly the sand-dunes (especially when abounding with shells), which stand +in ranges at so many different levels, must all have required long time for +their accumulation; and hence I do not doubt that the last 100 feet of +elevation of La Plata and Northern Patagonia has been exceedingly slow. +</p> + +<p> +If we extend this conclusion to Central and Southern Patagonia, the inclination +of the successively rising gravel-capped plains can be explained quite as well, +as by the more obvious view already given of a few comparatively great and +sudden elevations; in either case we must admit long periods of rest, during +which the sea ate deeply into the land. Let us suppose the present coast to +rise at a nearly equable, slow rate, yet sufficiently quick to prevent the +waves quite removing each part as soon as brought up; in this case every +portion of the present bed of the sea will successively form a beach-line, and +from being exposed to a like action will be similarly affected. It cannot +matter to what height the tides rise, even if to forty feet as at Santa Cruz, +for they will act with equal force and in like manner on each successive line. +Hence there is no difficulty in the fact of the 355 feet plain at Santa Cruz +sloping up 108 feet to the foot of the next highest escarpment, and yet having +no marks of any one particular beach-line on it; for the whole surface on this +view has been a beach. I cannot pretend to follow out the precise action of the +tidal-waves during a rise of the land, slow, yet sufficiently quick to prevent +or check denudation: but if it be analogous to what takes place on protected +parts of the present coast, where gravel is now accumulating in large +quantities, an inclined surface, thickly capped by well-rounded pebbles of +about the same size, would be ultimately left. (On the eastern side of Chiloe, +which island we shall see in the next chapter is now rising, I observed that +all the beaches and extensive tidal-flats were formed of shingle.) On the +gravel now accumulating, the waves, aided by the wind, sometimes throw up a +thin covering of sand, together with the common coast-shells. Shells thus cast +up by gales, would, during an elevatory period, never again be touched by the +sea. Hence, on this view of a slow and gradual rising of the land, interrupted +by periods of rest and denudation, we can understand the pebbles being of about +the same size over the entire width of the step-like plains,—the +occasional thin covering of sandy earth,—and the presence of broken, +unrolled fragments of those shells, which now live exclusively near the coast. +</p> + +<h3>A SUMMARY OF RESULTS.</h3> + +<p> +It may be concluded that the coast on this side of the continent, for a space +of at least 1,180 miles, has been elevated to a height of 100 feet in La Plata, +and of 400 feet in Southern Patagonia, within the period of existing shells, +but not of existing mammifers. That in La Plata the elevation has been very +slowly effected: that in Patagonia the movement may have been by considerable +starts, but much more probably slow and quiet. In either case, there have been +long intervening periods of comparative rest, during which the sea corroded +deeply, as it is still corroding, into the land. (I say COMPARATIVE and not +ABSOLUTE rest, because the sea acts, as we have seen, with great denuding power +on this whole line of coast; and therefore, during an elevation of the land, if +excessively slow (and of course during a subsidence of the land), it is quite +possible that lines of cliff might be formed.) That the periods of denudation +and elevation were contemporaneous and equable over great spaces of coast, as +shown by the equable heights of the plains; that there have been at least eight +periods of denudation, and that the land, up to a height of from 950 to 1,200 +feet, has been similarly modelled and affected: that the area elevated, in the +southernmost part of the continent, extended in breadth to the Cordillera, and +probably seaward to the Falkland Islands; that northward, in La Plata, the +breadth is unknown, there having been probably more than one axis of elevation; +and finally, that, anterior to the elevation attested by these upraised shells, +the land was divided by a Strait where the River Santa Cruz now flows, and that +further southward there were other sea-straits, since closed. I may add, that +at Santa Cruz, in latitude 50 degrees S., the plains have been uplifted at +least 1,400 feet, since the period when gigantic boulders were transported +between sixty and seventy miles from their parent rock, on floating icebergs. +</p> + +<p> +Lastly, considering the great upward movements which this long line of coast +has undergone, and the proximity of its southern half to the volcanic axis of +the Cordillera, it is highly remarkable that in the many fine sections exposed +in the Pampean, Patagonian tertiary, and Boulder formations, I nowhere observed +the smallest fault or abrupt curvature in the strata. +</p> + +<h3>GRAVEL FORMATION OF PATAGONIA.</h3> + +<p> +I will here describe in more detail than has been as yet incidentally done, the +nature, origin, and extent of the great shingle covering of Patagonia: but I do +not mean to affirm that all of this shingle, especially that on the higher +plains, belongs to the recent period. A thin bed of sandy earth, with small +pebbles of various porphyries and of quartz, covering a low plain on the north +side of the Rio Colorado, is the extreme northern limit of this formation. +These little pebbles have probably been derived from the denudation of a more +regular bed of gravel, capping the old tertiary sandstone plateau of the Rio +Negro. The gravel-bed near the Rio Negro is, on an average, about ten or twelve +feet in thickness; and the pebbles are larger than on the northern side of the +Colorado, being from one or two inches in diameter, and composed chiefly of +rather dark-tinted porphyries. Amongst them I here first noticed a variety +often to be referred to, namely, a peculiar gallstone-yellow siliceous +porphyry, frequently, but not invariably, containing grains of quartz. The +pebbles are embedded in a white, gritty, calcareous matrix, very like mortar, +sometimes merely coating with a whitewash the separate stones, and sometimes +forming the greater part of the mass. In one place I saw in the gravel +concretionary nodules (not rounded) of crystallised gypsum, some as large as a +man’s head. I traced this bed for forty-five miles inland, and was +assured that it extended far into the interior. As the surface of the calcareo- +argillaceous plain of Pampean formation, on the northern side of the wide +valley of the Colorado, stands at about the same height with the mortar- like +cemented gravel capping the sandstone on the southern side, it is probable, +considering the apparent equability of the subterranean movements along this +side of America, that this gravel of the Rio Negro and the upper beds of the +Pampean formation northward of the Colorado, are of nearly contemporaneous +origin, and that the calcareous matter has been derived from the same source. +</p> + +<p> +Southward of the Rio Negro, the cliffs along the great bay of S. Antonio are +capped with gravel: at San Josef, I found that the pebbles closely resembled +those on the plain of the Rio Negro, but that they were not cemented by +calcareous matter. Between San Josef and Port Desire, I was assured by the +Officers of the Survey that the whole face of the country is coated with +gravel. At Port Desire and over a space of twenty-five miles inland, on the +three step-formed plains and in the valleys, I everywhere passed over gravel +which, where thickest, was between thirty and forty feet. Here, as in other +parts of Patagonia, the gravel, or its sandy covering, was, as we have seen, +often strewed with recent marine shells. The sandy covering sometimes fills up +furrows in the gravel, as does the gravel in the underlying tertiary +formations. The pebbles are frequently whitewashed and even cemented together +by a peculiar, white, friable, aluminous, fusible substance, which I believe is +decomposed feldspar. At Port Desire, the gravel rested sometimes on the basal +formation of porphyry, and sometimes on the upper or the lower denuded tertiary +strata. It is remarkable that most of the porphyritic pebbles differ from those +varieties of porphyry which occur here abundantly in situ. The peculiar +gallstone-yellow variety was common, but less numerous than at Port S. Julian, +where it formed nearly one-third of the mass of the gravel; the remaining part +there consisting of pale grey and greenish porphyries with many crystals of +feldspar. At Port S. Julian, I ascended one of the flat- topped hills, the +denuded remnant of the highest plain, and found it, at the height of 950 feet, +capped with the usual bed of gravel. +</p> + +<p> +Near the mouth of the Santa Cruz, the bed of gravel on the 355 feet plain is +from twenty to about thirty-five feet in thickness. The pebbles vary from +minute ones to the size of a hen’s egg, and even to that of half a +man’s head; they consist of paler varieties of porphyry than those found +further northward, and there are fewer of the gallstone-yellow kind; pebbles of +compact black clay-slate were here first observed. The gravel, as we have seen, +covers the step-formed plains at the mouth, head, and on the sides of the great +valley of the Santa Cruz. At a distance of 110 miles from the coast, the plain +has risen to the height of 1,416 feet above the sea; and the gravel, with the +associated great boulder formation, has attained a thickness of 212 feet. The +plain, apparently with its usual gravel covering, slopes up to the foot of the +Cordillera to the height of between 3,200 and 3,300 feet. In ascending the +valley, the gravel gradually becomes entirely altered in character: high up, we +have pebbles of crystalline feldspathic rocks, compact clay-slate, quartzose +schists, and pale-coloured porphyries; these rocks, judging both from the +gigantic boulders in the surface and from some small pebbles embedded beneath +700 feet in thickness of the old tertiary strata, are the prevailing kinds in +this part of the Cordillera; pebbles of basalt from the neighbouring streams of +basaltic lava are also numerous; there are few or none of the reddish or of the +gallstone-yellow porphyries so common near the coast. Hence the pebbles on the +350 feet plain at the mouth of the Santa Cruz cannot have been derived (with +the exception of those of compact clay- slate, which, however, may equally well +have come from the south) from the Cordillera in this latitude; but probably, +in chief part, from farther north. +</p> + +<p> +Southward of the Santa Cruz, the gravel may be seen continuously capping the +great 840 feet plain: at the Rio Gallegos, where this plain is succeeded by a +lower one, there is, as I am informed by Captain Sulivan, an irregular covering +of gravel from ten to twelve feet in thickness over the whole country. The +district on each side of the Strait of Magellan is covered up either with +gravel or the boulder formation: it was interesting to observe the marked +difference between the perfectly rounded state of the pebbles in the great +shingle formation of Patagonia, and the more or less angular fragments in the +boulder formation. The pebbles and fragments near the Strait of Magellan nearly +all belong to rocks known to occur in Fuegia. I was therefore much surprised in +dredging south of the Strait to find, in latitude 54 degrees 10′ south, +many pebbles of the gallstone-yellow siliceous porphyry; I procured others from +a great depth off Staten Island, and others were brought me from the western +extremity of the Falkland Islands. (At my request, Mr. Kent collected for me a +bag of pebbles from the beach of White Rock harbour, in the northern part of +the sound, between the two Falkland Islands. Out of these well-rounded pebbles, +varying in size from a walnut to a hen’s egg, with some larger, +thirty-eight evidently belonged to the rocks of these islands; twenty-six were +similar to the pebbles of porphyry found on the Patagonian plains, which rocks +do not exist in situ in the Falklands; one pebble belonged to the peculiar +yellow siliceous porphyry; thirty were of doubtful origin.) The distribution of +the pebbles of this peculiar porphyry, which I venture to affirm is not found +in situ either in Fuegia, the Falkland Islands, or on the coast of Patagonia, +is very remarkable, for they are found over a space of 840 miles in a north and +south line, and at the Falklands, 300 miles eastward of the coast of Patagonia. +Their occurrence in Fuegia and the Falklands may, however, perhaps be due to +the same ice-agency by which the boulders have been there transported. +</p> + +<p> +We have seen that porphyritic pebbles of a small size are first met with on the +northern side of the Rio Colorado, the bed becoming well developed near the Rio +Negro: from this latter point I have every reason to believe that the gravel +extends uninterruptedly over the plains and valleys of Patagonia for at least +630 nautical miles southward to the Rio Gallegos. From the slope of the plains, +from the nature of the pebbles, from their extension at the Rio Negro far into +the interior, and at the Santa Cruz close up to the Cordillera, I think it +highly probable that the whole breadth of Patagonia is thus covered. If so, the +average width of the bed must be about two hundred miles. Near the coast the +gravel is generally from ten to thirty feet in thickness; and as in the valley +of Santa Cruz it attains, at some distance from the Cordillera, a thickness of +214 feet, we may, I think, safely assume its average thickness over the whole +area of 630 by 200 miles, at fifty feet! +</p> + +<p> +The transportal and origin of this vast bed of pebbles is an interesting +problem. From the manner in which they cap the step-formed plains, worn by the +sea within the period of existing shells, their deposition, at least on the +plains up to a height of 400 feet, must have been a recent geological event. +From the form of the continent, we may feel sure that they have come from the +westward, probably, in chief part from the Cordillera, but, perhaps, partly +from unknown rocky ridges in the central districts of Patagonia. That the +pebbles have not been transported by rivers, from the interior towards the +coast, we may conclude from the fewness and smallness of the streams of +Patagonia: moreover, in the case of the one great and rapid river of Santa +Cruz, we have good evidence that its transporting power is very trifling. This +river is from two to three hundred yards in width, about seventeen feet deep in +its middle, and runs with a singular degree of uniformity five knots an hour, +with no lakes and scarcely any still reaches: nevertheless, to give one +instance of its small transporting power, upon careful examination, pebbles of +compact basalt could not be found in the bed of the river at a greater distance +than ten miles below the point where the stream rushes over the debris of the +great basaltic cliffs forming its shore: fragments of the CELLULAR varieties +have been washed down twice or thrice as far. That the pebbles in Central and +Northern Patagonia have not been transported by ice-agency, as seems to have +been the case to a considerable extent farther south, and likewise in the +northern hemisphere, we may conclude, from the absence of all angular fragments +in the gravel, and from the complete contrast in many other respects between +the shingle and neighbouring boulder formation. +</p> + +<p> +Looking to the gravel on any one of the step-formed plains, I cannot doubt, +from the several reasons assigned in this chapter, that it has been spread out +and leveled by the long-continued action of the sea, probably during the slow +rise of the land. The smooth and perfectly rounded condition of the innumerable +pebbles alone would prove long-continued action. But how the whole mass of +shingle on the coast-plains has been transported from the mountains of the +interior, is another and more difficult question. The following considerations, +however, show that the sea by its ordinary action has considerable power in +distributing pebbles. Table 3 above shows how very uniformly and gradually the +pebbles decrease in size with the gradually seaward increasing depth and +distance. (I may mention, that at the distance of 150 miles from the Patagonian +shore I carefully examined the minute rounded particles in the sand, and found +them to be fusible like the porphyries of the great shingle bed. I could even +distinguish particles of the gallstone-yellow porphyry. It was interesting to +notice how gradually the particles of white quartz increased, as we approached +the Falkland Islands, which are thus constituted. In the whole line of +soundings between these islands and the coast of Patagonia dead or living +organic remains were most rare. On the relations between the depth of water and +the nature of the bottom, see Martin White on “Soundings in the +Channel” pages 4, 6, 175; also Captain Beechey’s “Voyage to +the Pacific” chapter 18.) A series of this kind irresistibly leads to the +conclusion, that the sea has the power of sifting and distributing the loose +matter on its bottom. According to Martin White, the bed of the British Channel +is disturbed during gales at depths of sixty-three and sixty-seven fathoms, and +at thirty fathoms, shingle and fragments of shells are often deposited, +afterwards to be carried away again. (“Soundings in the Channel” +pages 4, 166. M. Siau states (“Edinburgh New Philosophical Journal” +volume 31 page 246), that he found the sediment, at a depth of 188 metres, +arranged in ripples of different degrees of fineness. There are some excellent +discussions on this and allied subjects in Sir H. De la Beche’s +“Theoretical Researches.”) Groundswells, which are believed to be +caused by distant gales, seem especially to affect the bottom: at such times, +according to Sir R. Schomburgk, the sea to a great distance round the West +Indian Islands, at depths from five to fifteen fathoms, becomes discoloured, +and even the anchors of vessels have been moved. (“Journal of Royal +Geographical Society” volume 5 page 25. It appears from Mr. Scott +Russell’s investigations (see Mr. Murchison’s “Anniversary +Address Geological Society” 1843 page 40), that in waves of translation +the motion of the particles of water is nearly as great at the bottom as at the +top.) There are, however, some difficulties in understanding how the sea can +transport pebbles lying at the bottom, for, from experiments instituted on the +power of running water, it would appear that the currents of the sea have not +sufficient velocity to move stones of even moderate size: moreover, I have +repeatedly found in the most exposed situations that the pebbles which lie at +the bottom are encrusted with full-grown living corallines, furnished with the +most delicate, yet unbroken spines: for instance, in ten fathoms water off the +mouth of the Santa Cruz, many pebbles, under half an inch in diameter, were +thus coated with Flustracean zoophytes. (A pebble, one and a half inch square +and half an inch thick, was given me, dredged up from twenty-seven fathoms +depth off the western end of the Falkland Islands, where the sea is remarkably +stormy, and subject to violent tides. This pebble was encrusted on all sides by +a delicate living coralline. I have seen many pebbles from depths between forty +and seventy fathoms thus encrusted; one from the latter depth off Cape Horn.) +Hence we must conclude that these pebbles are not often violently disturbed: it +should, however, be borne in mind that the growth of corallines is rapid. The +view, propounded by Professor Playfair, will, I believe, explain this apparent +difficulty,—namely, that from the undulations of the sea TENDING to lift +up and down pebbles or other loose bodies at the bottom, such are liable, when +thus quite or partially raised, to be moved even by a very small force, a +little onwards. We can thus understand how oceanic or tidal currents of no +great strength, or that recoil movement of the bottom-water near the land, +called by sailors the “undertow” (which I presume must extend out +seaward as far as the BREAKING waves impel the surface-water towards the +beach), may gain the power during storms of sifting and distributing pebbles +even of considerable size, and yet without so violently disturbing them as to +injure the encrusting corallines. (I may take this opportunity of remarking on +a singular, but very common character in the form of the bottom, in the creeks +which deeply penetrate the western shores of Tierra del Fuego; namely, that +they are almost invariably much shallower close to the open sea at their mouths +than inland. Thus, Cook, in entering Christmas Sound, first had soundings in +thirty-seven fathoms, then in fifty, then in sixty, and a little farther in no +bottom with 170 fathoms. The sealers are so familiar with this fact, that they +always look out for anchorage near the entrances of the creeks. See, also, on +this subject, the “Voyages of the ‘Adventure’ and +‘Beagle’” volume 1 page 375 and “Appendix” page +313. This Shoalness of the sea- channels near their entrances probably results +from the quantity of sediment formed by the wear and tear of the outer rocks +exposed to the full force of the open sea. I have no doubt that many lakes, for +instance in Scotland, which are very deep within, and are separated from the +sea apparently only by a tract of detritus, were originally sea-channels with +banks of this nature near their mouths, which have since been upheaved.) +</p> + +<p> +The sea acts in another and distinct manner in the distribution of pebbles, +namely by the waves on the beach. Mr. Palmer, in his excellent memoir on this +subject, has shown that vast masses of shingle travel with surprising quickness +along lines of coast, according to the direction with which the waves break on +the beach and that this is determined by the prevailing direction of the winds. +(“Philosophical Transactions” 1834 page 576.) This agency must be +powerful in mingling together and disseminating pebbles derived from different +sources: we may, perhaps, thus understand the wide distribution of the +gallstone-yellow porphyry; and likewise, perhaps, the great difference in the +nature of the pebbles at the mouth of the Santa Cruz from those in the same +latitude at the head of the valley. +</p> + +<p> +I will not pretend to assign to these several and complicated agencies their +shares in the distribution of the Patagonian shingle: but from the several +considerations given in this chapter, and I may add, from the frequency of a +capping of gravel on tertiary deposits in all parts of the world, as I have +myself observed and seen stated in the works of various authors, I cannot doubt +that the power of widely dispersing gravel is an ordinary contingent on the +action of the sea; and that even in the case of the great Patagonian +shingle-bed we have no occasion to call in the aid of debacles. I at one time +imagined that perhaps an immense accumulation of shingle had originally been +collected at the foot of the Cordillera; and that this accumulation, when +upraised above the level of the sea, had been eaten into and partially spread +out (as off the present line of coast); and that the newly-spread out bed had +in its turn been upraised, eaten into, and re-spread out; and so onwards, until +the shingle, which was first accumulated in great thickness at the foot of the +Cordillera, had reached in thinner beds its present extension. By whatever +means the gravel formation of Patagonia may have been distributed, the vastness +of its area, its thickness, its superficial position, its recent origin, and +the great degree of similarity in the nature of its pebbles, all appear to me +well deserving the attention of geologists, in relation to the origin of the +widely-spread beds of conglomerate belonging to past epochs. +</p> + +<h3>FORMATION OF CLIFFS.</h3> + +<p> +(DIAGRAM 7.—SECTION OF COAST-CLIFFS AND BOTTOM OF SEA, OFF THE ISLAND OF +ST. HELENA. +</p> + +<p> +Height in feet above sea level. +</p> + +<p> +Depths in fathoms. +</p> + +<p> +Vertical and horizontal scale, two inches to a nautical mile. The point marked +1,600 feet is at the foot of High Knoll; point marked 510 feet is on the edge +of Ladder Hill. The strata consist of basaltic streams. +</p> + +<p> +A Section left to right: +</p> + +<p> +Height at the foot of High Knoll: 1,600 at top of strata. +</p> + +<p> +Height on the edge of Ladder Hill: 510 at top of strata. +</p> + +<p> +Bottom at coast rocky only to a depth of five or six fathoms. +</p> + +<p> +30 fathoms: bottom mud and sand. +</p> + +<p> +100 fathoms sloping more sharply to 250 fathoms.) +</p> + +<p> +When viewing the sea-worn cliffs of Patagonia, in some parts between eight +hundred and nine hundred feet in height, and formed of horizontal tertiary +strata, which must once have extended far seaward—or again, when viewing +the lofty cliffs round many volcanic islands, in which the gentle inclination +of the lava-streams indicates the former extension of the land, a difficulty +often occurred to me, namely, how the strata could possibly have been removed +by the action of the sea at a considerable depth beneath its surface. The +section in Diagram 7, which represents the general form of the land on the +northern and leeward side of St. Helena (taken from Mr. Seale’s large +model and various measurements), and of the bottom of the adjoining sea (taken +chiefly from Captain Austin’s survey and some old charts), will show the +nature of this difficulty. +</p> + +<p> +If, as seems probable, the basaltic streams were originally prolonged with +nearly their present inclination, they must, as shown by the dotted line in the +section, once have extended at least to a point, now covered by the sea to a +depth of nearly thirty fathoms: but I have every reason to believe they +extended considerably further, for the inclination of the streams is less near +the coast than further inland. It should also be observed, that other sections +on the coast of this island would have given far more striking results, but I +had not the exact measurements; thus, on the windward side, the cliffs are +about two thousand feet in height and the cut-off lava streams very gently +inclined, and the bottom of the sea has nearly a similar slope all round the +island. How, then, has all the hard basaltic rock, which once extended beneath +the surface of the sea, been worn away? According to Captain Austin, the bottom +is uneven and rocky only to that very small distance from the beach within +which the depth is from five to six fathoms; outside this line, to a depth of +about one hundred fathoms, the bottom is smooth, gently inclined, and formed of +mud and sand; outside the one hundred fathoms, it plunges suddenly into +unfathomable depths, as is so very commonly the case on all coasts where +sediment is accumulating. At greater depths than the five or six fathoms, it +seems impossible, under existing circumstances, that the sea can both have worn +away hard rock, in parts to a thickness of at least 150 feet, and have +deposited a smooth bed of fine sediment. Now, if we had any reason to suppose +that St. Helena had, during a long period, gone on slowly subsiding, every +difficulty would be removed: for looking at the diagram, and imagining a fresh +amount of subsidence, we can see that the waves would then act on the +coast-cliffs with fresh and unimpaired vigour, whilst the rocky ledge near the +beach would be carried down to that depth, at which sand and mud would be +deposited on its bare and uneven surface: after the formation near the shore of +a new rocky shoal, fresh subsidence would carry it down and allow it to be +smoothly covered up. But in the case of the many cliff-bounded islands, for +instance in some of the Canary Islands and of Madeira, round which the +inclination of the strata shows that the land once extended far into the depths +of the sea, where there is no apparent means of hard rock being worn +away—are we to suppose that all these islands have slowly subsided? +Madeira, I may remark, has, according to Mr. Smith of Jordan Hill, subsided. +Are we to extend this conclusion to the high, cliff- bound, horizontally +stratified shores of Patagonia, off which, though the water is not deep even at +the distance of several miles, yet the smooth bottom of pebbles gradually +decreasing in size with the increasing depth, and derived from a foreign +source, seem to declare that the sea is now a depositing and not a corroding +agent? I am much inclined to suspect, that we shall hereafter find in all such +cases, that the land with the adjoining bed of the sea has in truth subsided: +the time will, I believe, come, when geologists will consider it as improbable, +that the land should have retained the same level during a whole geological +period, as that the atmosphere should have remained absolutely calm during an +entire season. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0002"></a> +CHAPTER II.<br /> +ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.</h2> + +<p class="letter"> +Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions +of the inhabitants on this subject.—Concepcion, earthquake and elevation +of.—VALPARAISO, great elevation of, upraised shells, earth of marine +origin, gradual rise of the land within the historical period.—COQUIMBO, +elevation of, in recent times; terraces of marine origin, their inclination, +their escarpments not horizontal.—Guasco, gravel terraces +of.—Copiapo.—PERU.—Upraised shells of Cobija, Iquique, and +Arica.—Lima, shell-beds and sea-beach on San Lorenzo, human remains, +fossil earthenware, earthquake debacle, recent subsidence.—On the decay +of upraised shells.—General summary. +</p> + +<p> +Commencing at the south and proceeding northward, the first place at which I +landed, was at Cape Tres Montes, in latitude 46 degrees 35′. Here, on +the shores of Christmas Cove, I observed in several places a beach of pebbles +with recent shells, about twenty feet above high-water mark. Southward of Tres +Montes (between latitude 47 and 48 degrees), Byron remarks, “We thought +it very strange, that upon the summits of the highest hills were found beds of +shells, a foot or two thick.” (“Narrative of the Loss of the +‘Wager’.”) In the Chonos Archipelago, the island of Lemus +(latitude 44 degrees 30′) was, according to M. Coste, suddenly elevated +eight feet, during the earthquake of 1829: he adds, “Des roches jadis +toujours couvertes par la mer, restant aujourd’hui constamment +decouvertes.” (“Comptes Rendus” October 1838 page 706.) In +other parts of this archipelago, I observed two terraces of gravel, abutting to +the foot of each other: at Lowe’s Harbour (43 degrees 48′), under +a great mass of the boulder formation, about three hundred feet in thickness, I +found a layer of sand, with numerous comminuted fragments of sea-shells, having +a fresh aspect, but too small to be identified. +</p> + +<h3>THE ISLAND OF CHILOE.</h3> + +<p> +The evidence of recent elevation is here more satisfactory. The bay of San +Carlos is in most parts bounded by precipitous cliffs from about ten to forty +feet in height, their bases being separated from the present line of tidal +action by a talus, a few feet in height, covered with vegetation. In one +sheltered creek (west of P. Arena), instead of a loose talus, there was a bare +sloping bank of tertiary mudstone, perforated, above the line of the highest +tides, by numerous shells of a Pholas now common in the harbour. The upper +extremities of these shells, standing upright in their holes with grass growing +out of them, were abraded about a quarter of an inch, to the same level with +the surrounding worn strata. In other parts, I observed (as at Pudeto) a great +beach, formed of comminuted shells, twenty feet above the present shore. In +other parts again, there were small caves worn into the foot of the low cliffs, +and protected from the waves by the talus with its vegetation: one such cave, +which I examined, had its mouth about twenty feet, and its bottom, which was +filled with sand containing fragments of shells and legs of crabs, from eight +to ten feet above high-water mark. From these several facts, and from the +appearance of the upraised shells, I inferred that the elevation had been quite +recent; and on inquiring from Mr. Williams, the Portmaster, he told me he was +convinced that the land had risen, or the sea fallen, four feet within the last +four years. During this period, there had been one severe earthquake, but no +particular change of level was then observed; from the habits of the people who +all keep boats in the protected creeks, it is absolutely impossible that a rise +of four feet could have taken place suddenly and been unperceived. Mr. Williams +believes that the change has been quite gradual. Without the elevatory movement +continues at a quick rate, there can be no doubt that the sea will soon destroy +the talus of earth at the foot of the cliffs round the bay, and will then reach +its former lateral extension, but not of course its former level: some of the +inhabitants assured me that one such talus, with a footpath on it, was even +already sensibly decreasing in width. +</p> + +<p> +I received several accounts of beds of shells, existing at considerable heights +in the inland parts of Chiloe; and to one of these, near Catiman, I was guided +by a countryman. Here, on the south side of the peninsula of Lacuy, there was +an immense bed of the Venus costellata and of an oyster, lying on the +summit-edge of a piece of tableland, 350 feet (by the barometer) above the +level of the sea. The shells were closely packed together, embedded in and +covered by a very black, damp, peaty mould, two or three feet in thickness, out +of which a forest of great trees was growing. Considering the nature and +dampness of this peaty soil, it is surprising that the fine ridges on the +outside of the Venus are perfectly preserved, though all the shells have a +blackened appearance. I did not doubt that the black soil, which when dry, +cakes hard, was entirely of terrestrial origin, but on examining it under the +microscope, I found many very minute rounded fragments of shells, amongst which +I could distinguish bits of Serpulae and mussels. The Venus costellata, and the +Ostrea (O. edulis, according to Captain King) are now the commonest shells in +the adjoining bays. In a bed of shells, a few feet below the 350 feet bed, I +found a horn of the little Cervus humilis, which now inhabits Chiloe. +</p> + +<p> +The eastern or inland side of Chiloe, with its many adjacent islets, consists +of tertiary and boulder deposits, worn into irregular plains capped by gravel. +Near Castro, and for ten miles southward, and on the islet of Lemuy, I found +the surface of the ground to a height of between twenty and thirty feet above +high-water mark, and in several places apparently up to fifty feet, thickly +coated by much comminuted shells, chiefly of the Venus costellata and Mytilus +Chiloensis; the species now most abundant on this line of coast. As the +inhabitants carry immense numbers of these shells inland, the continuity of the +bed at the same height was often the only means of recognising its natural +origin. Near Castro, on each side of the creek and rivulet of the Gamboa, three +distinct terraces are seen: the lowest was estimated at about one hundred and +fifty feet in height, and the highest at about five hundred feet, with the +country irregularly rising behind it; obscure traces, also, of these same +terraces could be seen along other parts of the coast. There can be no doubt +that their three escarpments record pauses in the elevation of the island. I +may remark that several promontories have the word Huapi, which signifies in +the Indian tongue, island, appended to them, such as Huapilinao, Huapilacuy, +Caucahuapi, etc.; and these, according to Indian traditions, once existed as +islands. In the same manner the term Pulo in Sumatra is appended to the names +of promontories, traditionally said to have been islands (Marsden’s +“Sumatra” page 31.); in Sumatra, as in Chiloe, there are upraised +recent shells. The Bay of Carelmapu, on the mainland north of Chiloe, according +to Aguerros, was in 1643 a good harbour (“Descripcion Hist. de la +Provincia de Chiloe” page 78. From the account given by the old Spanish +writers, it would appear that several other harbours, between this point and +Concepcion, were formerly much deeper than they now are.); it is now quite +useless, except for boats. +</p> + +<h3>VALDIVIA.</h3> + +<p> +I did not observe here any distinct proofs of recent elevation; but in a bed of +very soft sandstone, forming a fringe-like plain, about sixty feet in height, +round the hills of mica-slate, there are shells of Mytilus, Crepidula, Solen, +Novaculina, and Cytheraea, too imperfect to be specifically recognised. At +Imperial, seventy miles north of Valdivia, Aguerros states that there are large +beds of shells, at a considerable distance from the coast, which are burnt for +lime. (Ibid page 25.) The island of Mocha, lying a little north of Imperial, +was uplifted two feet, during the earthquake of 1835. (“Voyages of +‘Adventure’ and ‘Beagle’” volume 2 page 415.) +</p> + +<h3>CONCEPCION.</h3> + +<p> +I cannot add anything to the excellent account by Captain Fitzroy of the +elevation of the land at this place, which accompanied the earthquake of 1835. +(Ibid volume 2 page 412 et seq. In volume 5 page 601 of the “Geological +Transactions” I have given an account of the remarkable volcanic +phenomena, which accompanied this earthquake. These phenomena appear to me to +prove that the action, by which large tracts of land are uplifted, and by which +volcanic eruptions are produced, is in every respect identical.) I will only +recall to the recollection of geologists, that the southern end of the island +of St. Mary was uplifted eight feet, the central part nine, and the northern +end ten feet; and the whole island more than the surrounding districts. Great +beds of mussels, patellae, and chitons still adhering to the rocks were +upraised above high-water mark; and some acres of a rocky flat, which was +formerly always covered by the sea, was left standing dry, and exhaled an +offensive smell, from the many attached and putrefying shells. It appears from +the researches of Captain Fitzroy that both the island of St. Mary and +Concepcion (which was uplifted only four or five feet) in the course of some +weeks subsided, and lost part of their first elevation. I will only add as a +lesson of caution, that round the sandy shores of the great Bay of Concepcion, +it was most difficult, owing to the obliterating effects of the great +accompanying wave, to recognise any distinct evidence of this considerable +upheaval; one spot must be excepted, where there was a detached rock which +before the earthquake had always been covered by the sea, but afterwards was +left uncovered. +</p> + +<p> +On the island of Quiriquina (in the Bay of Concepcion), I found, at an +estimated height of four hundred feet, extensive layers of shells, mostly +comminuted, but some perfectly preserved and closely packed in black vegetable +mould; they consisted of Concholepas, Fissurella, Mytilus, Trochus, and +Balanus. Some of these layers of shells rested on a thick bed of bright-red, +dry, friable earth, capping the surface of the tertiary sandstone, and +extending, as I observed whilst sailing along the coast, for 150 miles +southward: at Valparaiso, we shall presently see that a similar red earthy +mass, though quite like terrestrial mould, is really in chief part of recent +marine origin. On the flanks of this island of Quiriquina, at a less height +than the 400 feet, there were spaces several feet square, thickly strewed with +fragments of similar shells. During a subsequent visit of the +“Beagle” to Concepcion, Mr. Kent, the assistant-surgeon, was so +kind as to make for me some measurements with the barometer: he found many +marine remains along the shores of the whole bay, at a height of about twenty +feet; and from the hill of Sentinella behind Talcahuano, at the height of 160 +feet, he collected numerous shells, packed together close beneath the surface +in black earth, consisting of two species of Mytilus, two of Crepidula, one of +Concholepas, of Fissurella, Venus, Mactra, Turbo, Monoceros, and the Balanus +psittacus. These shells were bleached, and within some of the Balani other +Balani were growing, showing that they must have long lain dead in the sea. The +above species I compared with living ones from the bay, and found them +identical; but having since lost the specimens, I cannot give their names: this +is of little importance, as Mr. Broderip has examined a similar collection, +made during Captain Beechey’s expedition, and ascertained that they +consisted of ten recent species, associated with fragments of Echini, crabs, +and Flustrae; some of these remains were estimated by Lieutenant Belcher to lie +at the height of nearly a thousand feet above the level of the sea. +(“Zoology of Captain Beechey’s Voyage” page 162.) In some +places round the bay, Mr. Kent observed that there were beds formed exclusively +of the Mytilus Chiloensis: this species now lives in parts never uncovered by +the tides. At considerable heights, Mr. Kent found only a few shells; but from +the summit of one hill, 625 feet high, he brought me specimens of the +Concholepas, Mytilus Chiloensis, and a Turbo. These shells were softer and more +brittle than those from the height of 164 feet; and these latter had obviously +a much more ancient appearance than the same species from the height of only +twenty feet. +</p> + +<h3>COAST NORTH OF CONCEPCION.</h3> + +<p> +The first point examined was at the mouth of the Rapel (160 miles north of +Concepcion and sixty miles south of Valparaiso), where I observed a few shells +at the height of 100 feet, and some barnacles adhering to the rocks three or +four feet above the highest tides: M. Gay found here recent shells at the +distance of two leagues from the shore. (“Annales des Scienc. Nat.” +Avril 1833.) Inland there are some wide, gravel-capped plains, intersected by +many broad, flat-bottomed valleys (now carrying insignificant streamlets), with +their sides cut into successive wall-like escarpments, rising one above +another, and in many places, according to M. Gay, worn into caves. The one cave +(C. del Obispo) which I examined, resembled those formed on many sea-coasts, +with its bottom filled with shingle. These inland plains, instead of sloping +towards the coast, are inclined in an opposite direction towards the +Cordillera, like the successively rising terraces on the inland or eastern side +of Chiloe: some points of granite, which project through the plains near the +coast, no doubt once formed a chain of outlying islands, on the inland shores +of which the plains were accumulated. At Bucalemu, a few miles northward of the +Rapel, I observed at the foot, and on the summit-edge of a plain, ten miles +from the coast, many recent shells, mostly comminuted, but some perfect. There +were, also, many at the bottom of the great valley of the Maypu. At San +Antonio, shells are said to be collected and burnt for lime. At the bottom of a +great ravine (Quebrada Onda, on the road to Casa Blanca), at the distance of +several miles from the coast, I noticed a considerable bed, composed +exclusively of Mesodesma donaciforme, Desh., lying on a bed of muddy sand: this +shell now lives associated together in great numbers, on tidal-flats on the +coast of Chile. +</p> + +<h3>VALPARAISO.</h3> + +<p> +During two successive years I carefully examined, part of the time in company +with Mr. Alison, into all the facts connected with the recent elevation of this +neighbourhood. In very many parts a beach of broken shells, about fourteen or +fifteen feet above high-water mark, may be observed; and at this level the +coast-rocks, where precipitous, are corroded in a band. At one spot, Mr. +Alison, by removing some birds’ dung, found at this same level barnacles +adhering to the rocks. For several miles southward of the bay, almost every +flat little headland, between the heights of 60 and 230 feet (measured by the +barometer), is smoothly coated by a thick mass of comminuted shells, of the +same species, and apparently in the same proportional numbers with those +existing in the adjoining sea. The Concholepas is much the most abundant, and +the best preserved shell; but I extracted perfectly preserved specimens of the +Fissurella biradiata, a Trochus and Balanus (both well-known, but according to +Mr. Sowerby yet unnamed) and parts of the Mytilus Chiloensis. Most of these +shells, as well as an encrusting Nullipora, partially retain their colour; but +they are brittle, and often stained red from the underlying brecciated mass of +primary rocks; some are packed together, either in black or reddish moulds; +some lie loose on the bare rocky surfaces. The total number of these shells is +immense; they are less numerous, though still far from rare, up a height of +1,000 feet above the sea. On the summit of a hill, measured 557 feet, there was +a small horizontal band of comminuted shells, of which MANY consisted (and +likewise from lesser heights) of very young and small specimens of the still +living Concholepas, Trochus, Patellae, Crepidulae, and of Mytilus Magellanicus +(?) (Mr. Cuming informs me that he does not think this species identical with, +though closely resembling, the true M. Magellanicus of the southern and eastern +coast of South America; it lives abundantly on the coast of Chile.): several of +these shells were under a quarter of an inch in their greatest diameter. My +attention was called to this circumstance by a native fisherman, whom I took to +look at these shell-beds; and he ridiculed the notion of such small shells +having been brought up for food; nor could some of the species have adhered +when alive to other larger shells. On another hill, some miles distant, and 648 +feet high, I found shells of the Concholepas and Trochus, perfect, though very +old, with fragments of Mytilus Chiloensis, all embedded in reddish-brown mould: +I also found these same species, with fragments of an Echinus and of Balanus +psittacus, on a hill 1,000 feet high. Above this height, shells became very +rare, though on a hill 1,300 feet high (Measured by the barometer: the highest +point in the range behind Valparaiso I found to be 1,626 feet above the level +of the sea.), I collected the Concholepas, Trochus, Fissurella, and a Patella. +At these greater heights the shells are almost invariably embedded in mould, +and sometimes are exposed only by tearing up bushes. These shells obviously had +a very much more ancient appearance than those from the lesser heights; the +apices of the Trochi were often worn down; the little holes made by burrowing +animals were greatly enlarged; and the Concholepas was often perforated quite +through, owing to the inner plates of shell having scaled off. +</p> + +<p> +Many of these shells, as I have said, were packed in, and were quite filled +with, blackish or reddish-brown earth, resting on the granitic detritus. I did +not doubt until lately that this mould was of purely terrestrial origin, when +with a microscope examining some of it from the inside of a Concholepas from +the height of about one hundred feet, I found that it was in considerable part +composed of minute fragments of the spines, mouth- bones, and shells of Echini, +and of minute fragments, of chiefly very young Patellae, Mytili, and other +species. I found similar microscopical fragments in earth filling up the +central orifices of some large Fissurellae. This earth when crushed emits a +sickly smell, precisely like that from garden-mould mixed with guano. The earth +accidentally preserved within the shells, from the greater heights, has the +same general appearance, but it is a little redder; it emits the same smell +when rubbed, but I was unable to detect with certainty any marine remains in +it. This earth resembles in general appearance, as before remarked, that +capping the rocks of Quiriquina in the Bay of Concepcion, on which beds of +sea-shells lay. I have, also, shown that the black, peaty soil, in which the +shells at the height of 350 feet at Chiloe were packed, contained many minute +fragments of marine animals. These facts appear to me interesting, as they show +that soils, which would naturally be considered of purely terrestrial nature, +may owe their origin in chief part to the sea. +</p> + +<p> +Being well aware from what I have seen at Chiloe and in Tierra del Fuego, that +vast quantities of shells are carried, during successive ages, far inland, +where the inhabitants chiefly subsist on these productions, I am bound to state +that at greater heights than 557 feet, where the number of very young and small +shells proved that they had not been carried up for food, the only evidence of +the shells having been naturally left by the sea, consists in their invariable +and uniform appearance of extreme antiquity—in the distance of some of +the places from the coast, in others being inaccessible from the nearest part +of the beach, and in the absence of fresh water for men to drink—in the +shells NOT LYING IN HEAPS,—and, lastly, in the close similarity of the +soil in which they are embedded, to that which lower down can be unequivocally +shown to be in great part formed from the debris of the sea animals. (In the +“Proceedings of the Geological Society” volume 2 page 446, I have +given a brief account of the upraised shells on the coast of Chile, and have +there stated that the proofs of elevation are not satisfactory above the height +of 230 feet. I had at that time unfortunately overlooked a separate page +written during my second visit to Valparaiso, describing the shells now in my +possession from the 557 feet hill; I had not then unpacked my collections, and +had not reconsidered the obvious appearance of greater antiquity of the shells +from the greater heights, nor had I at that time discovered the marine origin +of the earth in which many of the shells are packed. Considering these facts, I +do not now feel a shadow of doubt that the shells, at the height of 1,300 feet, +have been upraised by natural causes into their present position.) +</p> + +<p> +With respect to the position in which the shells lie, I was repeatedly struck +here, at Concepcion, and at other places, with the frequency of their +occurrence on the summits and edges either of separate hills, or of little flat +headlands often terminating precipitously over the sea. The several +above-enumerated species of mollusca, which are found strewed on the surface of +the land from a few feet above the level of the sea up to the height of 1,300 +feet, all now live either on the beach, or at only a few fathoms’ depth: +Mr. Edmondston, in a letter to Professor E. Forbes, states that in dredging in +the Bay of Valparaiso, he found the common species of Concholepas, Fissurella, +Trochus, Monoceros, Chitons, etc., living in abundance from the beach to a +depth of seven fathoms; and dead shells occurred only a few fathoms deeper. The +common Turritella cingulata was dredged up living at even from ten to fifteen +fathoms; but this is a species which I did not find here amongst the upraised +shells. Considering this fact of the species being all littoral or +sub-littoral, considering their occurrence at various heights, their vast +numbers, and their generally comminuted state, there can be little doubt that +they were left on successive beach-lines during a gradual elevation of the +land. The presence, however, of so many whole and perfectly preserved shells +appears at first a difficulty on this view, considering that the coast is +exposed to the full force of an open ocean: but we may suppose, either that +these shells were thrown during gales on flat ledges of rock just above the +level of high-water mark, and that during the elevation of the land they are +never again touched by the waves, or, that during earthquakes, such as those of +1822, 1835, and 1837, rocky reefs covered with marine-animals were it one blow +uplifted above the future reach of the sea. This latter explanation is, +perhaps, the most probable one with respect to the beds at Concepcion entirely +composed of the Mytilus Chiloensis, a species which lives below the lowest +tides; and likewise with respect to the great beds occurring both north and +south of Valparaiso, of the Mesodesma donaciforme,—a shell which, as I am +informed by Mr. Cuming, inhabits sandbanks at the level of the lowest tides. +But even in the case of shells having the habits of this Mytilus and Mesodesma, +beds of them, wherever the sea gently throws up sand or mud, and thus protects +its own accumulations, might be upraised by the slowest movement, and yet +remain undisturbed by the waves of each new beach-line. +</p> + +<p> +It is worthy of remark, that nowhere near Valparaiso above the height of twenty +feet, or rarely of fifty feet, I saw any lines of erosion on the solid rocks, +or any beds of pebbles; this, I believe, may be accounted for by the +disintegrating tendency of most of the rocks in this neighbourhood. Nor is the +land here modelled into terraces: Mr. Alison, however, informs me, that on both +sides of one narrow ravine, at the height of 300 feet above the sea, he found a +succession of rather indistinct step-formed beaches, composed of broken shells, +which together covered a space of about eighty feet vertical. +</p> + +<p> +I can add nothing to the accounts already published of the elevation of the +land at Valparaiso, which accompanied the earthquake of 1822 (Dr. Meyen +“Reise um Erde” Th. 1 s. 221, found in 1831 seaweed and other +bodies still adhering to some rocks which during the shock of 1822 were lifted +above the sea.): but I heard it confidently asserted, that a sentinel on duty, +immediately after the shock, saw a part of a fort, which previously was not +within the line of his vision, and this would indicate that the uplifting was +not horizontal: it would even appear from some facts collected by Mr. Alison, +that only the eastern half of the bay was then elevated. Through the kindness +of this same gentleman, I am able to give an interesting account of the changes +of level, which have supervened here within historical periods: about the year +1680 a long sea-wall (or Prefil) was built, of which only a few fragments now +remain; up to the year 1817, the sea often broke over it, and washed the houses +on the opposite side of the road (where the prison now stands); and even in +1819, Mr. J. Martin remembers walking at the foot of this wall, and being often +obliged to climb over it to escape the waves. There now stands (1834) on the +seaward side of this wall, and between it and the beach, in one part a single +row of houses, and in another part two rows with a street between them. This +great extension of the beach in so short a time cannot be attributed simply to +the accumulation of detritus; for a resident engineer measured for me the +height between the lowest part of the wall visible, and the present beach-line +at spring-tides, and the difference was eleven feet six inches. The church of +S. Augustin is believed to have been built in 1614, and there is a tradition +that the sea formerly flowed very near it; by levelling, its foundations were +found to stand nineteen feet six inches above the highest beach-line; so that +we see in a period of 220 years, the elevation cannot have been as much as +nineteen feet six inches. From the facts given with respect to the sea-wall, +and from the testimony of the elder inhabitants, it appears certain that the +change in level began to be manifest about the year 1817. The only sudden +elevation of which there is any record occurred in 1822, and this seems to have +been less than three feet. Since that year, I was assured by several competent +observers, that part of an old wreck, which is firmly embedded near the beach, +has sensibly emerged; hence here, as at Chiloe, a slow rise of the land appears +to be now in progress. It seems highly probable that the rocks which are +corroded in a band at the height of fourteen feet above the sea were acted on +during the period, when by tradition the base of S. Augustin church, now +nineteen feet six inches above the highest water-mark, was occasionally washed +by the waves. +</p> + +<h3>VALPARAISO TO COQUIMBO.</h3> + +<p> +For the first seventy-five miles north of Valparaiso I followed the coast- +road, and throughout this space I observed innumerable masses of upraised +shells. About Quintero there are immense accumulations (worked for lime) of the +Mesodesma donaciforme, packed in sandy earth; they abound chiefly about fifteen +feet above high-water, but shells are here found, according to Mr. Miers, to a +height of 500 feet, and at a distance of three leagues from the coast +(“Travels in Chile” volume 1 pages 395, 458. I received several +similar accounts from the inhabitants, and was assured that there are many +shells on the plain of Casa Blanca, between Valparaiso and Santiago, at the +height of 800 feet.): I here noticed barnacles adhering to the rocks three or +four feet above the highest tides. In the neighbourhood of Plazilla and +Catapilco, at heights of between two hundred and three hundred feet, the number +of comminuted shells, with some perfect ones, especially of the Mesodesma, +packed in layers, was truly immense: the land at Plazilla had evidently existed +as a bay, with abrupt rocky masses rising out of it, precisely like the islets +in the broken bays now indenting this coast. On both sides of the rivers Ligua, +Longotomo, Guachen, and Quilimari, there are plains of gravel about two hundred +feet in height, in many parts absolutely covered with shells. Close to +Conchalee, a gravel-plain is fronted by a lower and similar plain about sixty +feet in height, and this again is separated from the beach by a wide tract of +low land: the surfaces of all three plains or terraces were strewed with vast +numbers of the Concholepas, Mesodesma, an existing Venus, and other still +existing littoral shells. The two upper terraces closely resemble in miniature +the plains of Patagonia; and like them are furrowed by dry, flat-bottomed, +winding valleys. Northward of this place I turned inward; and therefore found +no more shells: but the valleys of Chuapa, Illapel, and Limari, are bounded by +gravel-capped plains, often including a lower terrace within. These plains send +bay-like arms between and into the surrounding hills; and they are continuously +united with other extensive gravel-capped plains, separating the coast +mountain-ranges from the Cordillera. +</p> + +<h3>COQUIMBO.</h3> + +<p> +A narrow fringe-like plain, gently inclined towards the sea, here extends for +eleven miles along the coast, with arms stretching up between the +coast-mountains, and likewise up the valley of Coquimbo: at its southern +extremity it is directly connected with the plain of Limari, out of which hills +abruptly rise like islets, and other hills project like headlands on a coast. +The surface of the fringe-like plain appears level, but differs insensibly in +height, and greatly in composition, in different parts. +</p> + +<p> +At the mouth of the valley of Coquimbo, the surface consists wholly of gravel, +and stands from 300 to 350 feet above the level of the sea, being about one +hundred feet higher than in other parts. In these other and lower parts the +superficial beds consist of calcareous matter, and rest on ancient tertiary +deposits hereafter to be described. The uppermost calcareous layer is +cream-coloured, compact, smooth-fractured, sub- stalactiform, and contains some +sand, earthy matter, and recent shells. It lies on, and sends wedge-like veins +into, a much more friable, calcareous, tuff-like variety; and both rest on a +mass about twenty feet in thickness, formed of fragments of recent shells, with +a few whole ones, and with small pebbles firmly cemented together. (In many +respects this upper hard, and the underlying more friable, varieties, resemble +the great superficial beds at King George’s Sound in Australia, which I +have described in my “Geological Observations on Volcanic Islands.” +There could be little doubt that the upper layers there have been hardened by +the action of rain on the friable, calcareous matter, and that the whole mass +has originated in the decay of minutely comminuted sea-shells and corals.) This +latter rock is called by the inhabitants losa, and is used for building: in +many parts it is divided into strata, which dip at an angle of ten degrees +seaward, and appear as if they had originally been heaped in successive layers +(as may be seen on coral-reefs) on a steep beach. This stone is remarkable from +being in parts entirely formed of empty, pellucid capsules or cells of +calcareous matter, of the size of small seeds: a series of specimens +unequivocally showed that all these capsules once contained minute rounded +fragments of shells which have since been gradually dissolved by water +percolating through the mass. (I have incidentally described this rock in the +above work on Volcanic Islands.) +</p> + +<p> +The shells embedded in the calcareous beds forming the surface of this +fringe-like plain, at the height of from 200 to 250 feet above the sea, consist +of:— +</p> + +<p> +1. Venus opaca. 2. Mulinia Byronensis. 3. Pecten purpuratus. 4. Mesodesma +donaciforme. 5. Turritella cingulata. 6. Monoceros costatum. 7. Concholepas +Peruviana. 8. Trochus (common Valparaiso species). 9. Calyptraea Byronensis. +</p> + +<p> +Although these species are all recent, and are all found in the neighbouring +sea, yet I was particularly struck with the difference in the proportional +numbers of the several species, and of those now cast up on the present beach. +I found only one specimen of the Concholepas, and the Pecten was very rare, +though both these shells are now the commonest kinds, with the exception, +perhaps, of the Calyptraea radians, of which I did not find one in the +calcareous beds. I will not pretend to determine how far this difference in the +proportional numbers depends on the age of the deposit, and how far on the +difference in nature between the present sandy beaches and the calcareous +bottom, on which the embedded shells must have lived. +</p> + +<h3>(DIAGRAM 8.—SECTION OF PLAIN OF COQUIMBO.</h3> + +<p> +A Section through Plain B-B and Ravine A. +</p> + +<p> +Surface of plain 252 feet above sea. +</p> + +<p> +A. Stratified sand, with recent shells in same proportions as on the beach, +half filling up a ravine. +</p> + +<p> +B. Surface of plain, with scattered shells in nearly same proportions as on the +beach. +</p> + +<p> +C. Upper calcareous bed, and D. Lower calcareous sandy bed (Losa), both with +recent shells, but not in same proportions as on the beach. +</p> + +<p> +E. Upper ferrugino-sandy old tertiary stratum, and F. Lower old tertiary +stratum, both with all, or nearly all, extinct shells.) +</p> + +<p> +On the bare surface of the calcareous plain, or in a thin covering of sand, +there were lying, at a height from 200 to 252 feet, many recent shells, which +had a much fresher appearance than the embedded ones: fragments of the +Concholepas, and of the common Mytilus, still retaining a tinge of its colour, +were numerous, and altogether there was manifestly a closer approach in +proportional numbers to those now lying on the beach. In a mass of stratified, +slightly agglutinated sand, which in some places covers up the lower half of +the seaward escarpment of the plain, the included shells appeared to be in +exactly the same proportional numbers with those on the beach. On one side of a +steep-sided ravine, cutting through the plain behind Herradura Bay, I observed +a narrow strip of stratified sand, containing similar shells in similar +proportional numbers; a section of the ravine is represented in Diagram 8, +which serves also to show the general composition of the plain. I mention this +case of the ravine chiefly because without the evidence of the marine shells in +the sand, any one would have supposed that it had been hollowed out by simple +alluvial action. +</p> + +<p> +The escarpment of the fringe-like plain, which stretches for eleven miles along +the coast, is in some parts fronted by two or three narrow, step- formed +terraces, one of which at Herradura Bay expands into a small plain. Its surface +was there formed of gravel, cemented together by calcareous matter; and out of +it I extracted the following recent shells, which are in a more perfect +condition than those from the upper plain:— +</p> + +<p> +1. Calyptraea radians. 2. Turritella cingulata. 3. Oliva Peruviana. 4. Murex +labiosus, var. 5. Nassa (identical with a living species). 6. Solen Dombeiana. +7. Pecten purpuratus. 8. Venus Chilensis. 9. Amphidesma rugulosum. The small +irregular wrinkles of the posterior part of this shell are rather stronger than +in the recent specimens of this species from Coquimbo. (G.B. Sowerby.) 10. +Balanus (identical with living species). +</p> + +<p> +On the syenitic ridge, which forms the southern boundary of Herradura Bay and +Plain, I found the Concholepas and Turritella cingulata (mostly in fragments), +at the height of 242 feet above the sea. I could not have told that these +shells had not formerly been brought up by man, if I had not found one very +small mass of them cemented together in a friable calcareous tuff. I mention +this fact more particularly, because I carefully looked, in many apparently +favourable spots, at lesser heights on the side of this ridge, and could not +find even the smallest fragment of a shell. This is only one instance out of +many, proving that the absence of sea-shells on the surface, though in many +respects inexplicable, is an argument of very little weight in opposition to +other evidence on the recent elevation of the land. The highest point in this +neighbourhood at which I found upraised shells of existing species was on an +inland calcareous plain, at the height of 252 feet above the sea. +</p> + +<p> +It would appear from Mr. Caldcleugh’s researches, that a rise has taken +place here within the last century and a half (“Proceedings of the +Geological Society” volume 2 page 446.); and as no sudden change of level +has been observed during the not very severe earthquakes, which have +occasionally occurred here, the rising has probably been slow, like that now, +or quite lately, in progress at Chiloe and at Valparaiso: there are three +well-known rocks, called the Pelicans, which in 1710, according to Feuillee, +were a fleur d’eau, but now are said to stand twelve feet above low-water +mark: the spring-tides rise here only five feet. There is another rock, now +nine feet above high-water mark, which in the time of Frezier and Feuillee rose +only five or six feet out of water. Mr. Caldcleugh, I may add, also shows (and +I received similar accounts) that there has been a considerable decrease in the +soundings during the last twelve years in the Bays of Coquimbo, Concepcion, +Valparaiso, and Guasco; but as in these cases it is nearly impossible to +distinguish between the accumulation of sediment and the upheavement of the +bottom, I have not entered into any details. +</p> + +<h3>VALLEY OF COQUIMBO.</h3> + +<p> +(FIGURE 9. EAST AND WEST SECTION THROUGH THE TERRACES AT COQUIMBO, WHERE THEY +DEBOUCH FROM THE VALLEY, AND FRONT THE SEA. +</p> + +<p> +Vertical scale 1/10 of inch to 100 feet: horizontal scale much contracted. +</p> + +<p> +Height of terrace in feet from east (high) to west (low): Terrace F. 364 +Terrace E. 302 Terrace D. shown dotted, height not given. Terrace C. 120 +Terrace B. 70 Terrace A. 25 sloping down to level of sea at Town of Coquimbo.) +</p> + +<p> +The narrow coast-plain sends, as before stated, an arm, or more correctly a +fringe, on both sides, but chiefly on the southern side, several miles up the +valley. These fringes are worn into steps or terraces, which present a most +remarkable appearance, and have been compared (though not very correctly) by +Captain Basil Hall, to the parallel roads of Glen Roy in Scotland: their origin +has been ably discussed by Mr. Lyell. (“Principles of Geology” 1st +edition volume 3 page 131.) The first section which I will give (Figure 9), is +not drawn across the valley, but in an east and west line at its mouth, where +the step-formed terraces debouch and present their very gently inclined +surfaces towards the Pacific. +</p> + +<p> +The bottom plain (A) is about a mile in width, and rises quite insensibly from +the beach to a height of twenty-five feet at the foot of the next plain; it is +sandy, and abundantly strewed with shells. +</p> + +<p> +Plain or terrace B is of small extent, and is almost concealed by the houses of +the town, as is likewise the escarpment of terrace C. On both sides of a +ravine, two miles south of the town, there are two little terraces, one above +the other, evidently corresponding with B and C; and on them marine remains of +the species already enumerated were plentiful. Terrace E is very narrow, but +quite distinct and level; a little southward of the town there were traces of a +terrace D intermediate between E and C. Terrace F is part of the fringe-like +plain, which stretches for the eleven miles along the coast; it is here +composed of shingle, and is 100 feet higher than where composed of calcareous +matter. This greater height is obviously due to the quantity of shingle, which +at some former period has been brought down the great valley of Coquimbo. +</p> + +<p> +Considering the many shells strewed over the terraces A, B, and C, and a few +miles southward on the calcareous plain, which is continuously united with the +upper step-like plain F, there cannot, I apprehend, be any doubt, that these +six terraces have been formed by the action of the sea; and that their five +escarpments mark so many periods of comparative rest in the elevatory movement, +during which the sea wore into the land. The elevation between these periods +may have been sudden and on AN AVERAGE not more than seventy-two feet each +time, or it may have been gradual and insensibly slow. From the shells on the +three lower terraces, and on the upper one, and I may add on the three +gravel-capped terraces at Conchalee, being all littoral and sub-littoral +species, and from the analogical facts given at Valparaiso, and lastly from the +evidence of a slow rising lately or still in progress here, it appears to me +far more probable that the movement has been slow. The existence of these +successive escarpments, or old cliff- lines, is in another respect highly +instructive, for they show periods of comparative rest in the elevatory +movement, and of denudation, which would never even have been suspected from a +close examination of many miles of coast southward of Coquimbo. +</p> + +<h3>(FIGURE 10. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF COQUIMBO.</h3> + +<p> +From north F (high) through E?, D, C, B, A (low), B?, C, D?, E, F (high). +</p> + +<p> +Vertical scale 1/10 of inch to 100 feet: horizontal scale much contracted. +</p> + +<p> +Terraces marked with ? do not occur on that side of the valley, and are +introduced only to make the diagram more intelligible. A river and bottom- +plain of valley C, E, and F, on the south side of valley, are respectively, +197, 377, and 420 feet above the level of the sea. +</p> + +<p> +AA. The bottom of the valley, believed to be 100 feet above the sea: it is +continuously united with the lowest plain A of Figure 9. +</p> + +<p> +B. This terrace higher up the valley expands considerably; seaward it is soon +lost, its escarpment being united with that of C: it is not developed at all on +the south side of the valley. +</p> + +<p> +C. This terrace, like the last, is considerably expanded higher up the valley. +These two terraces apparently correspond with B and C of Figure 9. +</p> + +<p> +D is not well developed in the line of this section; but seaward it expands +into a plain: it is not present on the south side of the valley; but it is met +with, as stated under the former section, a little south of the town. +</p> + +<p> +E is well developed on the south side, but absent on the north side of the +valley: though not continuously united with E of Figure 9, it apparently +corresponds with it. +</p> + +<p> +F. This is the surface-plain, and is continuously united with that which +stretches like a fringe along the coast. In ascending the valley it gradually +becomes narrower, and is at last, at the distance of about ten miles from the +sea, reduced to a row of flat-topped patches on the sides of the mountains. +None of the lower terraces extend so far up the valley.) +</p> + +<p> +We come now to the terraces on the opposite sides of the east and west valley +of Coquimbo: the section in Figure 10 is taken in a north and south line across +the valley at a point about three miles from the sea. The valley measured from +the edges of the escarpments of the upper plain FF is about a mile in width; +but from the bases of the bounding mountains it is from three to four miles +wide. The terraces marked with an interrogative do not exist on that side of +the valley, but are introduced merely to render the diagram more intelligible. +</p> + +<p> +These five terraces are formed of shingle and sand; three of them, as marked by +Captain B. Hall (namely, B, C, and F), are much more conspicuous than the +others. From the marine remains copiously strewed at the mouth of the valley on +the lower terraces, and southward of the town on the upper one, they are, as +before remarked, undoubtedly of marine origin; but within the valley, and this +fact well deserves notice, at a distance of from only a mile and a half to +three or four miles from the sea, I could not find even a fragment of a shell. +</p> + +<p> +ON THE INCLINATION OF THE TERRACES OF COQUIMBO, AND ON THE UPPER AND BASAL +EDGES OF THEIR ESCARPMENTS NOT BEING HORIZONTAL. +</p> + +<p> +The surfaces of these terraces slope in a slight degree, as shown by the +sections in Figures 9 and 10 taken conjointly, both towards the centre of the +valley, and seawards towards its mouth. This double or diagonal inclination, +which is not the same in the several terraces, is, as we shall immediately see, +of simple explanation. There are, however, some other points which at first +appear by no means obvious,—namely, first, that each terrace, taken in +its whole breadth from the summit-edge of one escarpment to the base of that +above it, and followed up the valley, is not horizontal; nor have the several +terraces, when followed up the valley, all the same inclination; thus I found +the terraces C, E, and F, measured at a point about two miles from the mouth of +the valley, stood severally between fifty-six to seventy-seven feet higher than +at the mouth. Again, if we look to any one line of cliff or escarpment, neither +its summit-edge nor its base is horizontal. On the theory of the terraces +having been formed during a slow and equable rise of the land, with as many +intervals of rest as there are escarpments, it appears at first very surprising +that horizontal lines of some kind should not have been left on the land. +</p> + +<p> +The direction of the diagonal inclination in the different terraces being +different,—in some being directed more towards the middle of the valley, +in others more towards its mouth,—naturally follows on the view of each +terrace, being an accumulation of successive beach-lines round bays, which must +have been of different forms and sizes when the land stood at different levels: +for if we look to the actual beach of a narrow creek, its slope is directed +towards the middle; whereas, in an open bay, or slight concavity on a coast, +the slope is towards the mouth, that is, almost directly seaward; hence as a +bay alters in form and size, so will the direction of the inclination of its +successive beaches become changed. +</p> + +<h3>(FIGURE 11. DIAGRAM OF A BAY IN A DISTRICT WHICH HAS BEGUN SLOWLY RISING) +</h3> + +<p> +If it were possible to trace any one of the many beach-lines, composing each +sloping terrace, it would of course be horizontal; but the only lines of +demarcation are the summit and basal edges of the escarpments. Now the +summit-edge of one of these escarpments marks the furthest line or point to +which the sea has cut into a mass of gravel sloping seaward; and as the sea +will generally have greater power at the mouth than at the protected head of +the bay, so will the escarpment at the mouth be cut deeper into the land, and +its summit-edge be higher; consequently it will not be horizontal. With respect +to the basal or lower edges of the escarpments, from picturing in one’s +mind ancient bays ENTIRELY surrounded at successive periods by cliff-formed +shores, one’s first impression is that they at least necessarily must be +horizontal, if the elevation has been horizontal. But here is a fallacy: for +after the sea has, during a cessation of the elevation, worn cliffs all round +the shores of a bay, when the movement recommences, and especially if it +recommences slowly, it might well happen that, at the exposed mouth of the bay, +the waves might continue for some time wearing into the land, whilst in the +protected and upper parts successive beach-lines might be accumulating in a +sloping surface or terrace at the foot of the cliffs which had been lately +reached: hence, supposing the whole line of escarpment to be finally uplifted +above the reach of the sea, its basal line or foot near the mouth will run at a +lower level than in the upper and protected parts of the bay; consequently this +basal line will not be horizontal. And it has already been shown that the +summit-edges of each escarpment will generally be higher near the mouth (from +the seaward sloping land being there most exposed and cut into) than near the +head of the bay; therefore the total height of the escarpments will be greatest +near the mouth; and further up the old bay or valley they will on both sides +generally thin out and die away: I have observed this thinning out of the +successive escarpment at other places besides Coquimbo; and for a long time I +was quite unable to understand its meaning. The rude diagram in Figure 11 will +perhaps render what I mean more intelligible; it represents a bay in a district +which has begun slowly rising. Before the movement commenced, it is supposed +that the waves had been enabled to eat into the land and form cliffs, as far +up, but with gradually diminishing power, as the points AA: after the movement +had commenced and gone on for a little time, the sea is supposed still to have +retained the power, at the exposed mouth of the bay, of cutting down and into +the land as it slowly emerged; but in the upper parts of the bay it is supposed +soon to have lost this power, owing to the more protected situation and to the +quantity of detritus brought down by the river; consequently low land was there +accumulated. As this low land was formed during a slow elevatory movement, its +surface will gently slope upwards from the beach on all sides. Now, let us +imagine the bay, not to make the diagram more complicated, suddenly converted +into a valley: the basal line of the cliffs will of course be horizontal, as +far as the beach is now seen extending in the diagram; but in the upper part of +the valley, this line will be higher, the level of the district having been +raised whilst the low land was accumulating at the foot of the inland cliffs. +If, instead of the bay in the diagram being suddenly converted into a valley, +we suppose with much more probability it to be upraised slowly, then the waves +in the upper parts of the bay will continue very gradually to fail to reach the +cliffs, which are now in the diagram represented as washed by the sea, and +which, consequently, will be left standing higher and higher above its level; +whilst at the still exposed mouth, it might well happen that the waves might be +enabled to cut deeper and deeper, both down and into the cliffs, as the land +slowly rose. +</p> + +<p> +The greater or lesser destroying power of the waves at the mouths of successive +bays, comparatively with this same power in their upper and protected parts, +will vary as the bays become changed in form and size, and therefore at +different levels, at their mouths and heads, more or less of the surfaces +between the escarpments (that is, the accumulated beach-lines or terraces) will +be left undestroyed: from what has gone before we can see that, according as +the elevatory movements after each cessation recommence more or less slowly, +according to the amount of detritus delivered by the river at the heads of the +successive bays, and according to the degree of protection afforded by their +altered forms, so will a greater or less extent of terrace be accumulated in +the upper part, to which there will be no surface at a corresponding level at +the mouth: hence we can perceive why no one terrace, taken in its whole breadth +and followed up the valley, is horizontal, though each separate beach-line must +have been so; and why the inclination of the several terraces, both +transversely, and longitudinally up the valley, is not alike. +</p> + +<p> +I have entered into this case in some detail, for I was long perplexed (and +others have felt the same difficulty) in understanding how, on the idea of an +equable elevation with the sea at intervals eating into the land, it came that +neither the terraces nor the upper nor lower edges of the escarpments were +horizontal. Along lines of coast, even of great lengths, such as that of +Patagonia, if they are nearly uniformly exposed, the corroding power of the +waves will be checked and conquered by the elevatory movement, as often as it +recommences, at about the same period; and hence the terraces, or accumulated +beach-lines, will commence being formed at nearly the same levels: at each +succeeding period of rest, they will, also, be eaten into at nearly the same +rate, and consequently there will be a much closer coincidence in their levels +and inclinations, than in the terraces and escarpments formed round bays with +their different parts very differently exposed to the action of the sea. It is +only where the waves are enabled, after a long lapse of time, slowly to corrode +hard rocks, or to throw up, owing to the supply of sediment being small and to +the surface being steeply inclined, a narrow beach or mound, that we can +expect, as at Glen Roy in Scotland (“Philosophical Transactions” +1839 page 39.), a distinct line marking an old sea-level, and which will be +strictly horizontal, if the subsequent elevatory movements have been so: for in +these cases no discernible effects will be produced, except during the long +intervening periods of rest; whereas in the case of step-formed coasts, such as +those described in this and the preceding chapter, the terraces themselves are +accumulated during the slow elevatory process, the accumulation commencing +sooner in protected than in exposed situations, and sooner where there is +copious supply of detritus than where there is little; on the other hand, the +steps or escarpments are formed during the stationary periods, and are more +deeply cut down and into the coast-land in exposed than in protected +situations;—the cutting action, moreover, being prolonged in the most +exposed parts, both during the beginning and ending, if slow, of the upward +movement. +</p> + +<p> +Although in the foregoing discussion I have assumed the elevation to have been +horizontal, it may be suspected, from the considerable seaward slope of the +terraces, both up the valley of S. Cruz and up that of Coquimbo, that the +rising has been greater inland than nearer the coast. There is reason to +believe (Mr. Place in the “Quarterly Journal of Science” 1824 +volume 17 page 42.), from the effects produced on the water-course of a mill +during the earthquake of 1822 in Chile, that the upheaval one mile inland was +nearly double, namely, between five and seven feet, to what it was on the +Pacific. We know, also, from the admirable researches of M. Bravais, that in +Scandinavia the ancient sea-beaches gently slope from the interior +mountain-ranges towards the coast, and that they are not parallel one to the +other (“Voyages de la Comm. du Nord” etc. also “Comptes +Rendus” October 1842.), showing that the proportional difference in the +amount of elevation on the coast and in the interior, varied at different +periods. +</p> + +<h3>COQUIMBO TO GUASCO.</h3> + +<p> +In this distance of ninety miles, I found in almost every part marine shells up +to a height of apparently from two hundred to three hundred feet. The desert +plain near Choros is thus covered; it is bounded by the escarpment of a higher +plain, consisting of pale-coloured, earthy, calcareous stone, like that of +Coquimbo, with the same recent shells embedded in it. In the valley of +Chaneral, a similar bed occurs in which, differently from that of Coquimbo, I +observed many shells of the Concholepas: near Guasco the same calcareous bed is +likewise met with. +</p> + +<p> +In the valley of Guasco, the step-formed terraces of gravel are displaced in a +more striking manner than at any other point. I followed the valley for +thirty-seven miles (as reckoned by the inhabitants) from the coast to Ballenar; +in nearly the whole of this distance, five grand terraces, running at +corresponding heights on both sides of the broad valley, are more conspicuous +than the three best-developed ones at Coquimbo. They give to the landscape the +most singular and formal aspect; and when the clouds hung low, hiding the +neighbouring mountains, the valley resembled in the most striking manner that +of Santa Cruz. The whole thickness of these terraces or plains seems composed +of gravel, rather firmly aggregated together, with occasional parting seams of +clay: the pebbles on the upper plain are often whitewashed with an aluminous +substance, as in Patagonia. Near the coast I observed many sea-shells on the +lower plains. At Freyrina (twelve miles up the valley), there are six terraces +beside the bottom- surface of the valley: the two lower ones are here only from +two hundred to three hundred yards in width, but higher up the valley they +expand into plains; the third terrace is generally narrow; the fourth I saw +only in one place, but there it was distinct for the length of a mile; the +fifth is very broad; the sixth is the summit-plain, which expands inland into a +great basin. Not having a barometer with me, I did not ascertain the height of +these plains, but they appeared considerably higher than those at Coquimbo. +Their width varies much, sometimes being very broad, and sometimes contracting +into mere fringes of separate flat-topped projections, and then quite +disappearing: at the one spot, where the fourth terrace was visible, the whole +six terraces were cut off for a short space by one single bold escarpment. Near +Ballenar (thirty-seven miles from the mouth of the river), the valley between +the summit-edges of the highest escarpments is several miles in width, and the +five terraces on both sides are broadly developed: the highest cannot be less +than six hundred feet above the bed of the river, which itself must, I +conceive, be some hundred feet above the sea. +</p> + +<p> +A north and south section across the valley in this part is represented in +Figure 12. +</p> + +<p> +(FIGURE 12. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF GUASCO, AND OF A PLAIN +NORTH OF IT. +</p> + +<p> +From left (north, high) to right (south, high) through plains B and A and the +River of Guasco at the Town of Ballenar.) +</p> + +<p> +On the northern side of the valley the summit-plain of gravel, A, has two +escarpments, one facing the valley, and the other a great basin-like plain, B, +which stretches for several leagues northward. This narrow plain, A, with the +double escarpment, evidently once formed a spit or promontory of gravel, +projecting into and dividing two great bays, and subsequently was worn on both +sides into steep cliffs. Whether the several escarpments in this valley were +formed during the same stationary periods with those of Coquimbo, I will not +pretend to conjecture; but if so the intervening and subsequent elevatory +movements must have been here much more energetic, for these plains certainly +stand at a much higher level than do those of Coquimbo. +</p> + +<h3>COPIAPO.</h3> + +<p> +From Guasco to Copiapo, I followed the road near the foot of the Cordillera, +and therefore saw no upraised remains. At the mouth, however, of the valley of +Copiapo there is a plain, estimated by Meyen (“Reise um die Erde” +th. 1 s. 372 et seq.) between fifty and seventy feet in height, of which the +upper part consists chiefly of gravel, abounding with recent shells, chiefly of +the Concholepas, Venus Dombeyi, and Calyptraea trochiformis. A little inland, +on a plain estimated by myself at nearly three hundred feet, the upper stratum +was formed of broken shells and sand cemented by white calcareous matter, and +abounding with embedded recent shells, of which the Mulinia Byronensis and +Pecten purpuratus were the most numerous. The lower plain stretches for some +miles southward, and for an unknown distance northward, but not far up the +valley; its seaward face, according to Meyen, is worn into caves above the +level of the present beach. The valley of Copiapo is much less steeply inclined +and less direct in its course than any other valley which I saw in Chile; and +its bottom does not generally consist of gravel: there are no step-formed +terraces in it, except at one spot near the mouth of the great lateral valley +of the Despoblado where there are only two, one above the other: lower down the +valley, in one place I observed that the solid rock had been cut into the shape +of a beach, and was smoothed over with shingle. +</p> + +<p> +Northward of Copiapo, in latitude 26 degrees S., the old voyager Wafer found +immense numbers of sea-shells some miles from the coast. (Burnett’s +“Collection of Voyages” volume 4 page 193.) At Cobija (latitude 22 +degrees 34′) M. d’Orbigny observed beds of gravel and broken +shells, containing ten species of recent shells; he also found, on projecting +points of porphyry, at a height of 300 feet, shells of Concholepas, Chiton, +Calyptraea, Fissurella, and Patella, still attached to the spots on which they +had lived. M. d’Orbigny argues from this fact, that the elevation must +have been great and sudden (“Voyage, Part Geolog.” page 94. M. +d’Orbigny (page 98), in summing up, says: “S’il est certain +(as he believes) que tous les terrains en pente, compris entre la mer et les +montagnes sont l’ancien rivage de la mer, on doit supposer, pour +l’ensemble, un exhaussement que ce ne serait pas moindre de deux cent +metres; il faudrait supposer encore que ce soulevement n’a point ete +graduel;...mais qu’il resulterait d’une seule et meme cause +fortuite,” etc. Now, on this view, when the sea was forming the beach at +the foot of the mountains, many shells of Concholepas, Chiton, Calyptraea, +Fissurella, and Patella (which are known to live close to the beach), were +attached to rocks at a depth of 300 feet, and at a depth of 600 feet several of +these same shells were accumulating in great numbers in horizontal beds. From +what I have myself seen in dredging, I believe this to be improbable in the +highest degree, if not impossible; and I think everyone who has read Professor +E. Forbes’s excellent researches on the subject, will without hesitation +agree in this conclusion.): to me it appears far more probable that the +movement was gradual, with small starts as during the earthquakes of 1822 and +1835, by which whole beds of shells attached to the rocks were lifted above the +subsequent reach of the waves. M. d’Orbigny also found rolled pebbles +extending up the mountain to a height of at least six hundred feet. At Iquique +(latitude 20 degrees 12′ S.), in a great accumulation of sand, at a +height estimated between one hundred and fifty and two hundred feet, I observed +many large sea-shells which I thought could not have been blown up by the wind +to that height. Mr. J.H. Blake has lately described these shells: he states +that “inland toward the mountains they form a compact uniform bed, +scarcely a trace of the original shells being discernible; but as we approach +the shore, the forms become gradually more distinct till we meet with the +living shells on the coast.” (“Silliman’s American Journal of +Science” volume 44 page 2.) This interesting observation, showing by the +gradual decay of the shells how slowly and gradually the coast must have been +uplifted, we shall presently see fully confirmed at Lima. At Arica (latitude 18 +degrees 28′), M. d’Orbigny found a great range of sand-dunes, +fourteen leagues in length, stretching towards Tacna, including recent shells +and bones of Cetacea, and reaching up to a height of 300 feet above the sea. +(“Voyage” etc. page 101.) Lieutenant Freyer has given some more +precise facts: he states (In a letter to Mr. Lyell “Geological +Proceedings” volume 2 page 179.) that the Morro of Arica is about four +hundred feet high; it is worn into obscure terraces, on the bare rock of which +he found Balini and Milleporae adhering. At the height of between twenty and +thirty feet the shells and corals were in a quite fresh state, but at fifty +feet they were much abraded; there were, however, traces of organic remains at +greater heights. On the road from Tacna to Arequipa, between Loquimbo and +Moquegua, Mr. M. Hamilton found numerous recent sea shells in sand, at a +considerable distance from the sea. (“Edinburgh New Philosophical +Journal” volume 30 page 155.) +</p> + +<h3>LIMA.</h3> + +<p> +Northward of Arica, I know nothing of the coast for about a space of five +degrees of latitude; but near Callao, the port of Lima, there is abundant and +very curious evidence of the elevation of the land. The island of San Lorenzo +is upwards of one thousand feet high; the basset edges of the strata composing +the lower part are worn into three obscure, narrow, sloping steps or ledges, +which can be seen only when standing on them: they probably resemble those +described by Lieutenant Freyer at Arica. The surface of the lower ledge, which +extends from a low cliff overhanging the sea to the foot of the next upper +escarpment, is covered by an enormous accumulation of recent shells. (M. +Chevalier, in the “Voyage of the ‘Bonite’” observed +these shells; but his specimens were lost.—“L’Institut” +1838 page 151.) The bed is level, and in some parts more than two feet in +thickness; I traced it over a space of one mile in length, and heard of it in +other places: the uppermost part is eighty-five feet by the barometer above +high-water mark. The shells are packed together, but not stratified: they are +mingled with earth and stones, and are generally covered by a few inches of +detritus; they rest on a mass of nearly angular fragments of the underlying +sandstone, sometimes cemented together by common salt. I collected eighteen +species of shells of all ages and sizes. Several of the univalves had evidently +long lain dead at the bottom of the sea, for their INSIDES were incrusted with +Balani and Serpulae. All, according to Mr. G.B. Sowerby, are recent species: +they consist of:— +</p> + +<p> +1. Mytilus Magellanicus: same as that found at Valparaiso, and there stated to +be probably distinct from the true M. Magellanicus of the east coast. +</p> + +<p> +2. Venus costellata, Sowerby “Zoological Proceedings.” +</p> + +<p> +3. Pecten purpuratus, Lam. +</p> + +<p> +4. Chama, probably echinulata, Brod. +</p> + +<p> +5. Calyptraea Byronensis, Gray. +</p> + +<p> +6. Calyptraea radians (Trochus, Lam.) +</p> + +<p> +7. Fissurella affinis, Gray. +</p> + +<p> +8. Fissurella biradiata, Trembly. +</p> + +<p> +9. Purpura chocolatta, Duclos. +</p> + +<p> +10. Purpura Peruviana, Gray. +</p> + +<p> +11. Purpura labiata, Gray. +</p> + +<p> +12. Purpura buxea (Murex, Brod.). +</p> + +<p> +13. Concholepas Peruviana. +</p> + +<p> +14. Nassa, related to reticulata. +</p> + +<p> +15. Triton rudis, Brod. +</p> + +<p> +16. Trochus, not yet described, but well-known and very common. +</p> + +<p> +17 and 18. Balanus, two species, both common on the coast. +</p> + +<p> +These upraised shells appear to be nearly in the same proportional numbers- +-with the exception of the Crepidulae being more numerous—with those on +the existing beach. The state of preservation of the different species differed +much; but most of them were much corroded, brittle, and bleached: the upper and +lower surfaces of the Concholepas had generally quite scaled off: some of the +Trochi and Fissurellae still partially retain their colours. It is remarkable +that these shells, taken all together, have fully as ancient an appearance, +although the extremely arid climate appears highly favourable for their +preservation, as those from 1,300 feet at Valparaiso, and certainly a more +ancient appearance than those from five to six hundred feet from Valparaiso and +Concepcion; at which places I have seen grass and other vegetables actually +growing out of the shells. Many of the univalves here at San Lorenzo were +filled with, and united together by, pure salt, probably left by the +evaporation of the sea-spray, as the land slowly emerged. (The underlying +sandstone contains true layers of salt; so that the salt may possibly have come +from the beds in the higher parts of the island; but I think more probably from +the sea-spray. It is generally asserted that rain never falls on the coast of +Peru; but this is not quite accurate; for, on several days, during our visit, +the so-called Peruvian dew fell in sufficient quantity to make the streets +muddy, and it would certainly have washed so deliquescent a substance as salt +into the soil. I state this because M. d’Orbigny, in discussing an +analogous subject, supposes that I had forgotten that it never rains on this +whole line of coast. See Ulloa’s “Voyage” volume 2 English +Translation page 67 for an account of the muddy streets of Lima, and on the +continuance of the mists during the whole winter. Rain, also, falls at rare +intervals even in the driest districts, as, for instance, during forty days, in +1726, at Chocope (7 degrees 46′); this rain entirely ruined +(“Ulloa” etc. page 18) the mud houses of the inhabitants.) On the +highest parts of the ledge, small fragments of the shells were mingled with, +and evidently in process of reduction into, a yellowish-white, soft, calcareous +powder, tasting strongly of salt, and in some places as fine as prepared +medicinal chalk. +</p> + +<h3>FOSSIL-REMAINS OF HUMAN ART.</h3> + +<p> +In the midst of these shells on San Lorenzo, I found light corallines, the +horny ovule-cases of Mollusca, roots of seaweed (Mr. Smith of Jordan Hill found +pieces of seaweed in an upraised pleistocene deposit in Scotland. See his +admirable Paper in the “Edinburgh New Philosophical Journal” volume +25 page 384.), bones of birds, the heads of Indian corn and other vegetable +matter, a piece of woven rushes, and another of nearly decayed COTTON string. I +extracted these remains by digging a hole, on a level spot; and they had all +indisputably been embedded with the shells. I compared the plaited rush, the +COTTON string, and Indian corn, at the house of an antiquary, with similar +objects, taken from the Huacas or burial-grounds of the ancient Peruvians, and +they were undistinguishable; it should be observed that the Peruvians used +string only of cotton. The small quantity of sand or gravel with the shells, +the absence of large stones, the width and thickness of the bed, and the time +requisite for a ledge to be cut into the sandstone, all show that these remains +were not thrown high up by an earthquake-wave: on the other hand, these facts, +together with the number of dead shells, and of floating objects, both marine +and terrestrial, both natural and human, render it almost certain that they +were accumulated on a true beach, since upraised eighty-five feet, and upraised +this much since INDIAN MAN INHABITED PERU. The elevation may have been, either +by several small sudden starts, or quite gradual; in this latter case the +unrolled shells having been thrown up during gales beyond the reach of the +waves which afterwards broke on the slowly emerging land. I have made these +remarks, chiefly because I was at first surprised at the complete difference in +nature, between this broad, smooth, upraised bed of shells, and the present +shingle-beach at the foot of the low sandstone-cliffs; but a beach formed, when +the sea is cutting into the land, as is shown now to be the case by the low +bare sandstone-cliffs, ought not to be compared with a beach accumulated on a +gently inclined rocky surface, at a period when the sea (probably owing to the +elevatory movement in process) was not able to eat into the land. With respect +to the mass of nearly angular, salt- cemented fragments of sandstone, which lie +under the shells, and which are so unlike the materials of an ordinary +sea-beach; I think it probable after having seen the remarkable effects of the +earthquake of 1835 (I have described this in my “Journal of +Researches” page 303 2nd edition.), in absolutely shattering as if by +gunpowder the SURFACE of the primary rocks near Concepcion, that a smooth bare +surface of stone was left by the sea covered by the shelly mass, and that +afterwards when upraised, it was superficially shattered by the severe shocks +so often experienced here. +</p> + +<p> +The very low land surrounding the town of Callao, is to the south joined by an +obscure escarpment to a higher plain (south of Bella Vista), which stretches +along the coast for a length of about eight miles. This plain appears to the +eye quite level; but the sea-cliffs show that its height varies (as far as I +could estimate) from seventy to one hundred and twenty feet. It is composed of +thin, sometimes waving, beds of clay, often of bright red and yellow colours, +of layers of impure sand, and in one part with a great stratified mass of +granitic pebbles. These beds are capped by a remarkable mass, varying from two +to six feet in thickness, of reddish loam or mud, containing many scattered and +broken fragments of recent marine shells, sometimes though rarely single large +round pebble, more frequently short irregular layers of fine gravel, and very +many pieces of red coarse earthenware, which from their curvatures must once +have formed parts of large vessels. The earthenware is of Indian manufacture; +and I found exactly similar pieces accidentally included within the bricks, of +which the neighbouring ancient Peruvian burial-mounds are built. These +fragments abounded in such numbers in certain spots, that it appeared as if +waggon-loads of earthenware had been smashed to pieces. The broken sea- shells +and pottery are strewed both on the surface, and throughout the whole thickness +of this upper loamy mass. I found them wherever I examined the cliffs, for a +space of between two and three miles, and for half a mile inland; and there can +be little doubt that this same bed extends with a smooth surface several miles +further over the entire plain. Besides the little included irregular layers of +small pebbles, there are occasionally very obscure traces of stratification. +</p> + +<p> +At one of the highest parts of the cliff, estimated 120 feet above the sea, +where a little ravine came down, there were two sections, at right angles to +each other, of the floor of a shed or building. In both sections or faces, two +rows, one over the other, of large round stones could be distinctly seen; they +were packed close together on an artificial layer of sand two inches thick, +which had been placed on the natural clay-beds; the round stones were covered +by three feet in thickness of the loam with broken sea-shells and pottery. +Hence, before this widely spread-out bed of loam was deposited, it is certain +that the plain was inhabited; and it is probable, from the broken vessels being +so much more abundant in certain spots than in others, and from the underlying +clay being fitted for their manufacture, that the kilns stood here. +</p> + +<p> +The smoothness and wide extent of the plain, the bulk of matter deposited, and +the obscure traces of stratification seem to indicate that the loam was +deposited under water; on the other hand, the presence of sea-shells, their +broken state, the pebbles of various sizes, and the artificial floor of round +stones, almost prove that it must have originated in a rush of water from the +sea over the land. The height of the plain, namely, 120 feet, renders it +improbable that an earthquake-wave, vast as some have here been, could have +broken over the surface at its present level; but when the land stood +eighty-five feet lower, at the period when the shells were thrown up on the +ledge at S. Lorenzo, and when as we know man inhabited this district, such an +event might well have occurred; and if we may further suppose, that the plain +was at that time converted into a temporary lake, as actually occurred, during +the earthquakes of 1713 and 1746, in the case of the low land round Callao +owing to its being encircled by a high shingle-beach, all the appearances above +described will be perfectly explained. I must add, that at a lower level near +the point where the present low land round Callao joins the higher plain, there +are appearances of two distinct deposits both apparently formed by debacles: in +the upper one, a horse’s tooth and a dog’s jaw were embedded; so +that both must have been formed after the settlement of the Spaniards: +according to Acosta, the earthquake-wave of 1586 rose eighty-four feet. +</p> + +<p> +The inhabitants of Callao do not believe, as far as I could ascertain, that any +change in level is now in progress. The great fragments of brickwork, which it +is asserted can be seen at the bottom of the sea, and which have been adduced +as a proof of a late subsidence, are, as I am informed by Mr. Gill, a resident +engineer, loose fragments; this is probable, for I found on the beach, and not +near the remains of any building, masses of brickwork, three and four feet +square, which had been washed into their present places, and smoothed over with +shingle during the earthquake of 1746. The spit of land, on which the ruins of +OLD Callao stand, is so extremely low and narrow, that it is improbable in the +highest degree that a town should have been founded on it in its present state; +and I have lately heard that M. Tschudi has come to the conclusion, from a +comparison of old with modern charts, that the coast both south and north of +Callao has subsided. (I am indebted for this fact to Dr. E. Dieffenbach. I may +add that there is a tradition, that the islands of San Lorenzo and Fronton were +once joined, and that the channel between San Lorenzo and the mainland, now +above two miles in width, was so narrow that cattle used to swim over.) I have +shown that the island of San Lorenzo has been upraised eighty-five feet since +the Peruvians inhabited this country; and whatever may have been the amount of +recent subsidence, by so much more must the elevation have exceeded the +eighty-five feet. In several places in this neighbourhood, marks of sea-action +have been observed: Ulloa gives a detailed account of such appearances at a +point five leagues northward of Callao: Mr. Cruikshank found near Lima +successive lines of sea-cliffs, with rounded blocks at their bases, at a height +of 700 feet above the present level of the sea. (“Observaciones sobre el +Clima del Lima” par Dr. H. Unanue page 4.—Ulloa’s +“Voyage” volume 2 English Translation page 97.—For Mr. +Cruikshank’s observations, see Mr. Lyell’s “Principles of +Geology” 1st edition volume 3 page 130.) ON THE DECAY OF UPRAISED +SEA-SHELLS. +</p> + +<p> +I have stated that many of the shells on the lower inclined ledge or terrace of +San Lorenzo are corroded in a peculiar manner, and that they have a much more +ancient appearance than the same species at considerably greater heights on the +coast of Chile. I have, also, stated that these shells in the upper part of the +ledge, at the height of eighty-five feet above the sea, are falling, and in +some parts are quite changed into a fine, soft, saline, calcareous powder. The +finest part of this powder has been analysed for me, at the request of Sir H. +De la Beche, by the kindness of Mr. Trenham Reeks of the Museum of Economic +Geology; it consists of carbonate of lime in abundance, of sulphate and muriate +of lime, and of muriate and sulphate of soda. The carbonate of lime is +obviously derived from the shells; and common salt is so abundant in parts of +the bed, that, as before remarked, the univalves are often filled with it. The +sulphate of lime may have been derived, as has probably the common salt, from +the evaporation of the sea-spray, during the emergence of the land; for +sulphate of lime is now copiously deposited from the spray on the shores of +Ascension. (See “Volcanic Islands” etc. by the Author.) The other +saline bodies may perhaps have been partially thus derived, but chiefly, as I +conclude from the following facts, through a different means. +</p> + +<p> +On most parts of the second ledge or old sea-beach, at a height of 170 feet, +there is a layer of white powder of variable thickness, as much in some parts +as two inches, lying on the angular, salt-cemented fragments of sandstone and +under about four inches of earth, which powder, from its close resemblance in +nature to the upper and most decayed parts of the shelly mass, I can hardly +doubt originally existed as a bed of shells, now much collapsed and quite +disintegrated. I could not discover with the microscope a trace of organic +structure in it; but its chemical constituents, according to Mr. Reeks, are the +same as in the powder extracted from amongst the decaying shells on the lower +ledge, with the marked exception that the carbonate of lime is present in only +very small quantity. On the third and highest ledge, I observed some of this +powder in a similar position, and likewise occasionally in small patches at +considerably greater heights near the summit of the island. At Iquique, where +the whole face of the country is covered by a highly saliferous alluvium, and +where the climate is extremely dry, we have seen that, according to Mr. Blake, +the shells which are perfect near the beach become, in ascending, gradually +less and less perfect, until scarcely a trace of their original structure can +be discovered. It is known that carbonate of lime and common salt left in a +mass together, and slightly moistened, partially decompose each other (I am +informed by Dr. Kane, through Mr. Reeks, that a manufactory was established on +this principle in France, but failed from the small quantity of carbonate of +soda produced. Sprengel “Gardeners’ Chronicle” 1845 page 157, +states, that salt and carbonate of lime are liable to mutual decomposition in +the soil. Sir H. De la Beche informs me, that calcareous rocks washed by the +spray of the sea, are often corroded in a peculiar manner; see also on this +latter subject “Gardeners’ Chronicle” page 675 1844.): now we +have at San Lorenzo and at Iquique, in the shells and salt packed together, and +occasionally moistened by the so- called Peruvian dew, the proper elements for +this action. We can thus understand the peculiar corroded appearance of the +shells on San Lorenzo, and the great decrease of quantity in the carbonate of +lime in the powder on the upper ledge. There is, however, a great difficulty on +this view, for the resultant salts should be carbonate of soda and muriate of +lime; the latter is present, but not the carbonate of soda. Hence I am led to +the perhaps unauthorised conjecture (which I shall hereafter have to refer to) +that the carbonate of soda, by some unexplained means, becomes converted into a +sulphate. +</p> + +<p> +If the above remarks be just, we are led to the very unexpected conclusion, +that a dry climate, by leaving the salt from the sea-spray undissolved, is much +less favourable to the preservation of upraised shells than a humid climate. +However this may be, it is interesting to know the manner in which masses of +shells, gradually upraised above the sea-level, decay and finally disappear. +</p> + +<h3>A SUMMARY ON THE RECENT ELEVATION OF THE WEST COAST OF SOUTH AMERICA. </h3> + +<p> +We have seen that upraised marine remains occur at intervals, and in some parts +almost continuously, from latitude 45 degrees 35′ to 12 degrees S., +along the shores of the Pacific. This is a distance, in a north and south line, +of 2,075 geographical miles. From Byron’s observations, the elevation has +no doubt extended sixty miles further south; and from the similarity in the +form of the country near Lima, it has probably extended many leagues further +north. (I may take this opportunity of stating that in a MS. in the Geological +Society by Mr. Weaver, it is stated that beds of oysters and other recent +shells are found thirty feet above the level of the sea, in many parts of +Tampico, in the Gulf of Mexico.) Along this great line of coast, besides the +organic remains, there are in very many parts, marks of erosion, caves, ancient +beaches, sand-dunes, and successive terraces of gravel, all above the present +level of the sea. From the steepness of the land on this side of the continent, +shells have rarely been found at greater distances inland than from two to +three leagues; but the marks of sea-action are evident farther from the coast; +for instance, in the valley of Guasco, at a distance of between thirty and +forty miles. Judging from the upraised shells alone, the elevation in Chiloe +has been 350 feet, at Concepcion certainly 625 feet; and by estimation 1,000 +feet; at Valparaiso 1,300 feet; at Coquimbo 252 feet; northward of this place, +sea-shells have not, I believe, been found above 300 feet; and at Lima they +were falling into decay (hastened probably by the salt) at 85 feet. Not only +has this amount of elevation taken place within the period of existing Mollusca +and Cirripedes; but their proportional numbers in the neighbouring sea have in +most cases remained the same. Near Lima, however, a small change in this +respect between the living and the upraised was observed: at Coquimbo this was +more evident, all the shells being existing species, but with those embedded in +the uppermost calcareous plain not approximating so closely in proportional +numbers, as do those that lie loose on its surface at the height of 252 feet, +and still less closely than those which are strewed on the lower plains, which +latter are identical in proportional numbers with those now cast up on the +beach. From this circumstance, and from not finding, upon careful examination, +near Coquimbo any shells at a greater height than 252 feet, I believe that the +recent elevation there has been much less than at Valparaiso, where it has been +1,300 feet, and I may add, than at Concepcion. This considerable inequality in +the amount of elevation at Coquimbo and Valparaiso, places only 200 miles +apart, is not improbable, considering, first, the difference in the force and +number of the shocks now yearly affecting different parts of this coast; and, +secondly, the fact of single areas, such as that of the province of Concepcion, +having been uplifted very unequally during the same earthquake. It would, in +most cases, be very hazardous to infer an inequality of elevation, from shells +being found on the surface or in superficial beds at different heights; for we +do not know on what their rate of decay depends; and at Coquimbo one instance +out of many has been given, of a promontory, which, from the occurrence of one +very small collection of lime-cemented shells, has indisputably been elevated +242 feet, and yet on which, not even a fragment of shell could be found on +careful examination between this height and the beach, although many sites +appeared very favourable for the preservation of organic remains: the absence, +also, of shells on the gravel-terraces a short distance up the valley of +Coquimbo, though abundant on the corresponding terraces at its mouth, should be +borne in mind. +</p> + +<p> +There are other epochs, besides that of the existence of recent Mollusca, by +which to judge of the changes of level on this coast. At Lima, as we have just +seen, the elevation has been at least eighty-five feet, within the Indo-human +period; and since the arrival of the Spaniards in 1530, there has apparently +been a sinking of the surface. At Valparaiso, in the course of 220 years, the +rise must have been less than nineteen feet; but it has been as much as from +ten to eleven feet in the seventeen years subsequently to 1817, and of this +rise only a part can be attributed to the earthquake of 1822, the remainder +having been insensible and apparently still, in 1834, in progress. At Chiloe +the elevation has been gradual, and about four feet during four years. At +Coquimbo, also, it has been gradual, and in the course of 150 years has +amounted to several feet. The sudden small upheavals, accompanied by +earthquakes, as in 1822 at Valparaiso, in 1835 at Concepcion, and in 1837 in +the Chonos Archipelago, are familiar to most geologists, but the gradual rising +of the coast of Chile has been hardly noticed; it is, however, very important, +as connecting together these two orders of events. +</p> + +<p> +The rise of Lima, having been eighty-five feet within the period of man, is the +more surprising if we refer to the eastern coast of the continent, for at Port +S. Julian, in Patagonia, there is good evidence (as we shall hereafter see) +that when the land stood ninety feet lower, the Macrauchenia, a mammiferous +beast, was alive; and at Bahia Blanca, when it stood only a few feet lower than +it now does, many gigantic quadrupeds ranged over the adjoining country. But +the coast of Patagonia is some way distant from the Cordillera, and the +movement at Bahia Blanca is perhaps noways connected with this great range, but +rather with the tertiary volcanic rocks of Banda Oriental, and therefore the +elevation at these places may have been infinitely slower than on the coast of +Peru. All such speculations, however, must be vague, for as we know with +certainty that the elevation of the whole coast of Patagonia has been +interrupted by many and long pauses, who will pretend to say that, in such +cases, many and long periods of subsidence may not also have been intercalated? +</p> + +<p> +In many parts of the coast of Chile and Peru there are marks of the action of +the sea at successive heights on the land, showing that the elevation has been +interrupted by periods of comparative rest in the upward movement, and of +denudation in the action of the sea. These are plainest at Chiloe, where, in a +height of about five hundred feet, there are three escarpments,—at +Coquimbo, where in a height of 364 feet, there are five,— at Guasco, +where there are six, of which five may perhaps correspond with those at +Coquimbo, but if so, the subsequent and intervening elevatory movements have +been here much more energetic,—at Lima, where, in a height of about 250 +feet there are three terraces, and others, as it is asserted, at considerably +greater heights. The almost entire absence of ancient marks of sea-action at +defined levels along considerable spaces of coast, as near Valparaiso and +Concepcion, is highly instructive, for as it is improbable that the elevation +at these places alone should have been continuous, we must attribute the +absence of such marks to the nature and form of the coast-rocks. Seeing over +how many hundred miles of the coast of Patagonia, and on how many places on the +shores of the Pacific, the elevatory process has been interrupted by periods of +comparative rest, we may conclude, conjointly with the evidence drawn from +other quarters of the world, that the elevation of the land is generally an +intermittent action. From the quantity of matter removed in the formation of +the escarpments, especially of those of Patagonia, it appears that the periods +of rest in the movement, and of denudation of the land, have generally been +very long. In Patagonia, we have seen that the elevation has been equable, and +the periods of denudation synchronous over very wide spaces of coast; on the +shores of the Pacific, owing to the terraces chiefly occurring in the valleys, +we have not equal means of judging on this point; and the very different +heights of the upraised shells at Coquimbo, Valparaiso, and Concepcion seem +directly opposed to such a conclusion. +</p> + +<p> +Whether on this side of the continent the elevation, between the periods of +comparative rest when the escarpments were formed, has been by small sudden +starts, such as those accompanying recent earthquakes, or, as is most probable, +by such starts conjointly with a gradual upward movement, or by great and +sudden upheavals, I have no direct evidence. But as on the eastern coast, I was +led to think, from the analogy of the last hundred feet of elevation in La +Plata, and from the nearly equal size of the pebbles over the entire width of +the terraces, and from the upraised shells being all littoral species, that the +elevation had been gradual; so do I on this western coast, from the analogy of +the movements now in progress, and from the vast numbers of shells now living +exclusively on or close to the beach, which are strewed over the whole surface +of the land up to very considerable heights, conclude, that the movement here +also has been slow and gradual, aided probably by small occasional starts. We +know at least that at Coquimbo, where five escarpments occur in a height of 364 +feet, the successive elevations, if they have been sudden, cannot have been +very great. It has, I think, been shown that the occasional preservation of +shells, unrolled and unbroken, is not improbable even during a quite gradual +rising of the land; and their preservation, if the movement has been aided by +small starts, is quite conformable with what actually takes place during recent +earthquakes. +</p> + +<p> +Judging from the present action of the sea, along the shores of the Pacific, on +the deposits of its own accumulation, the present time seems in most places to +be one of comparative rest in the elevatory movement, and of denudation of the +land. Undoubtedly this is the case along the whole great length of Patagonia. +At Chiloe, however, we have seen that a narrow sloping fringe, covered with +vegetation, separates the present sea-beach from a line of low cliffs, which +the waves lately reached; here, then, the land is gaining in breadth and +height, and the present period is not one of rest in the elevation and of +contingent denudation; but if the rising be not prolonged at a quick rate, +there is every probability that the sea will soon regain its former horizontal +limits. I observed similar low sloping fringes on several parts of the coast, +both northward of Valparaiso and near Coquimbo; but at this latter place, from +the change in form which the coast has undergone since the old escarpments were +worn, it may be doubted whether the sea, acting for any length of time at its +present level, would eat into the land; for it now rather tends to throw up +great masses of sand. It is from facts such as these that I have generally used +the term COMPARATIVE rest, as applied to the elevation of the land; the rest or +cessation in the movement being comparative both with what has preceded it and +followed it, and with the sea’s power of corrosion at each spot and at +each level. Near Lima, the cliff-formed shores of San Lorenzo, and on the +mainland south of Callao, show that the sea is gaining on the land; and as we +have here some evidence that its surface has lately subsided or is still +sinking, the periods of comparative rest in the elevation and of contingent +denudation, may probably in many cases include periods of subsidence. It is +only, as was shown in detail when discussing the terraces of Coquimbo, when the +sea with difficulty and after a long lapse of time has either corroded a narrow +ledge into solid rock, or has heaped up on a steep surface a NARROW mound of +detritus, that we can confidently assert that the land at that level and at +that period long remained absolutely stationary. In the case of terraces formed +of gravel or sand, although the elevation may have been strictly horizontal, it +may well happen that no one level beach-line may be traceable, and that neither +the terraces themselves nor the summit nor basal edges of their escarpments may +be horizontal. +</p> + +<p> +Finally, comparing the extent of the elevated area, as deduced from the +upraised recent organic remains, on the two sides of the continent, we have +seen that on the Atlantic, shells have been found at intervals from Eastern +Tierra del Fuego for 1,180 miles northward, and on the Pacific for a space of +2,075 miles. For a length of 775 miles, they occur in the same latitudes on +both sides of the continent. Without taking this circumstance into +consideration, it is probable from the reasons assigned in the last chapter, +that the entire breadth of the continent in Central Patagonia has been uplifted +in mass; but from other reasons there given, it would be hazardous to extend +this conclusion to La Plata. From the continent being narrow in the +southern-most parts of Patagonia, and from the shells found at the Inner +Narrows of the Strait of Magellan, and likewise far up the valley of the Santa +Cruz, it is probable that the southern part of the western coast, which was not +visited by me, has been elevated within the period of recent Mollusca: if so, +the shores of the Pacific have been continuously, recently, and in a geological +sense synchronously upraised, from Lima for a length of 2,480 nautical miles +southward,—a distance equal to that from the Red Sea to the North Cape of +Scandinavia! +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0003"></a> +CHAPTER III.<br /> +ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS.</h2> + +<p class="letter"> +Basin-like plains of Chile; their drainage, their marine origin.—Marks of +sea-action on the eastern flanks of the Cordillera.—Sloping terrace-like +fringes of stratified shingle within the valleys of the Cordillera; their +marine origin.—Boulders in the valley of Cachapual.—Horizontal +elevation of the Cordillera.—Formation of valleys.—Boulders moved +by earthquake-waves.—Saline superficial deposits.—Bed of nitrate of +soda at Iquique.—Saline incrustations.—Salt-lakes of La Plata and +Patagonia; purity of the salt; its origin. +</p> + +<p> +The space between the Cordillera and the coast of Chile is on a rude average +from eighty to above one hundred miles in width; it is formed, either of an +almost continuous mass of mountains, or more commonly of several nearly +parallel ranges, separated by plains; in the more southern parts of this +province the mountains are quite subordinate to the plains; in the northern +part the mountains predominate. +</p> + +<p> +The basin-like plains at the foot of the Cordillera are in several respects +remarkable; that on which the capital of Chile stands is fifteen miles in +width, in an east and west line, and of much greater length in a north and +south line; it stands 1,750 feet above the sea; its surface appears smooth, but +really falls and rises in wide gentle undulations, the hollows corresponding +with the main valleys of the Cordillera: the striking manner in which it +abruptly comes up to the foot of this great range has been remarked by every +author since the time of Molina. (This plain is partially separated into two +basins by a range of hills; the southern half, according to Meyen (“Reise +um Erde” Th. 1 s. 274), falls in height, by an abrupt step, of between +fifteen and twenty feet.) Near the Cordillera it is composed of a stratified +mass of pebbles of all sizes, occasionally including rounded boulders: near its +western boundary, it consists of reddish sandy clay, containing some pebbles +and numerous fragments of pumice, and sometimes passes into pure sand or into +volcanic ashes. At Podaguel, on this western side of the plain, beds of sand +are capped by a calcareous tuff, the uppermost layers being generally hard and +substalagmitic, and the lower ones white and friable, both together precisely +resembling the beds at Coquimbo, which contain recent marine shells. Abrupt, +but rounded, hummocks of rock rise out of this plain: those of Sta. Lucia and +S. Cristoval are formed of greenstone-porphyry almost entirely denuded of its +original covering of porphyritic claystone breccia; on their summits, many +fragments of rock (some of them kinds not found in situ) are coated and united +together by a white, friable, calcareous tuff, like that found at Podaguel. +When this matter was deposited on the summit of S. Cristoval, the water must +have stood 946 feet above the surface of the surrounding plain. (Or 2,690 feet +above the sea, as measured barometrically by Mr. Eck. This tuff appears to the +eye nearly pure; but when placed in acid it leaves a considerable residue of +sand and broken crystals, apparently of feldspar. Dr. Meyen +(“Reise” Th. 1 s. 269) says he found a similar substance on the +neighbouring hill of Dominico (and I found it also on the Cerro Blanco), and he +attributes it to the weathering of the stone. In some places which I examined, +its bulk put this view of its origin quite out of the question; and I should +much doubt whether the decomposition of a porphyry would, in any case, leave a +crust chiefly composed of carbonate of lime. The white crust, which is commonly +seen on weathered feldspathic rocks, does not appear to contain any free +carbonate of lime.) +</p> + +<p> +To the south this basin-like plain contracts, and rising scarcely perceptibly +with a smooth surface, passes through a remarkable level gap in the mountains, +forming a true land-strait, and called the Angostura. It then immediately +expands into a second basin-formed plain: this again to the south contracts +into another land-strait, and expands into a third basin, which, however, falls +suddenly in level about forty feet. This third basin, to the south, likewise +contracts into a strait, and then again opens into the great plain of San +Fernando, stretching so far south that the snowy peaks of the distant +Cordillera are seen rising above its horizon as above the sea. These plains, +near the Cordillera, are generally formed of a thick stratified mass of shingle +(The plain of San Fernando has, according to MM. Meyen and Gay +“Reise” etc. Th. 1 ss. 295 and 298, near the Cordillera, an upper +step-formed plain of clay, on the surface of which they found numerous blocks +of rocks, from two to three feet long, either lying single or piled in heaps, +but all arranged in nearly straight lines.); in other parts, of a red sandy +clay, often with an admixture of pumiceous matter. Although these basins are +connected together like a necklace, in a north and south line, by smooth +land-straits, the streams which drain them do not all flow north and south, but +mostly westward, through breaches worn in the bounding mountains; and in the +case of the second basin, or that of Rancagua, there are two distinct breaches. +Each basin, moreover, is not drained singly; thus, to give the most striking +instance, but not the only one, in proceeding southward over the plain of +Rancagua, we first find the water flowing northward to and through the northern +land-strait; then, without crossing any marked ridge or watershed, we see it +flowing south-westward towards the northern one of the two breaches in the +western mountainous boundary; and lastly, again without any ridge, it flows +towards the southern breach in these same mountains. Hence the surface of this +one basin-like plain, appearing to the eye so level, has been modelled with +great nicety, so that the drainage, without any conspicuous watersheds, is +directed towards three openings in the encircling mountains. ((It appears from +Captain Herbert’s account of the Diluvium of the Himalaya, +“Gleanings of Science” Calcutta volume 2 page 164, that precisely +similar remarks apply to the drainage of the plains or valleys between those +great mountains.) The streams flowing from the southern basin-like plains, +after passing through the breaches to the west, unite and form the river Rapel, +which enters the Pacific near Navidad. I followed the southernmost branch of +this river, and found that the basin or plain of San Fernando is continuously +and smoothly united with those plains, which were described in the Second +Chapter, as being worn near the coast into successive cave-eaten escarpments, +and still nearer to the coast, as being strewed with upraised recent marine +remains. +</p> + +<p> +I might have given descriptions of numerous other plains of the same general +form, some at the foot of the Cordillera, some near the coast, and some halfway +between these points. I will allude only to one other, namely, the plain of +Uspallata, lying on the eastern or opposite side of the Cordillera, between +that great range and the parallel lower range of Uspallata. According to Miers, +its surface is 6,000 feet above the level of the sea: it is from ten to fifteen +miles in width, and is said to extend with an unbroken surface for 180 miles +northwards: it is drained by two rivers passing through breaches in the +mountains to the east. On the banks of the River Mendoza it is seen to be +composed of a great accumulation of stratified shingle, estimated at 400 feet +in thickness. In general appearance, and in numerous points of structure, this +plain closely resembles those of Chile. +</p> + +<p> +The origin and manner of formation of the thick beds of gravel, sandy clay, +volcanic detritus, and calcareous tuff, composing these basin-like plains, is +very important; because, as we shall presently show, they send arms or fringes +far up the main valleys of the Cordillera. Many of the inhabitants believe that +these plains were once occupied by lakes, suddenly drained; but I conceive that +the number of the separate breaches at nearly the same level in the mountains +surrounding them quite precludes this idea. Had not such distinguished +naturalists as MM. Meyen and Gay stated their belief that these deposits were +left by great debacles rushing down from the Cordillera, I should not have +noticed a view, which appears to me from many reasons improbable in the highest +degree—namely, from the vast accumulation of WELL-ROUNDED +PEBBLES—their frequent stratification with layers of sand—the +overlying beds of calcareous tuff—this same substance coating and uniting +the fragments of rock on the hummocks in the plain of Santiago—and lastly +even from the worn, rounded, and much denuded state of these hummocks, and of +the headlands which project from the surrounding mountains. On the other hand, +these several circumstances, as well as the continuous union of the basins at +the foot of the Cordillera, with the great plain of the Rio Rapel which still +retains the marks of sea-action at various levels, and their general similarity +in form and composition with the many plains near the coast, which are either +similarly marked or are strewed with upraised marine remains, fully convince me +that the mountains bounding these basin-plains were breached, their islet-like +projecting rocks worn, and the loose stratified detritus forming their now +level surfaces deposited, by the sea, as the land slowly emerged. It is hardly +possible to state too strongly the perfect resemblance in outline between these +basin-like, long, and narrow plains of Chile (especially when in the early +morning the mists hanging low represented water), and the creeks and fiords now +intersecting the southern and western shores of the continent. We can on this +view of the sea, when the land stood lower, having long and tranquilly occupied +the spaces between the mountain-ranges, understand how the boundaries of the +separate basins were breached in more than one place; for we see that this is +the general character of the inland bays and channels of Tierra del Fuego; we +there, also, see in the sawing action of the tides, which flow with great force +in the cross channels, a power sufficient to keep the breaches open as the land +emerged. We can further see that the waves would naturally leave the smooth +bottom of each great bay or channel, as it became slowly converted into land, +gently inclined to as many points as there were mouths, through which the sea +finally retreated, thus forming so many watersheds, without any marked ridges, +on a nearly level surface. The absence of marine remains in these high inland +plains cannot be properly adduced as an objection to their marine origin: for +we may conclude, from shells not being found in the great shingle beds of +Patagonia, though copiously strewed on their surfaces, and from many other +analogous facts, that such deposits are eminently unfavourable for the +embedment of such remains; and with respect to shells not being found strewed +on the surface of these basin-like plains, it was shown in the last chapter +that remains thus exposed in time decay and disappear. +</p> + +<p> +(FIGURE 13. SECTION OF THE PLAIN AT THE EASTERN FOOT OF THE CHILEAN CORDILLERA. +</p> + +<p> +From Cordillera (left) through Talus-plain and Level surface, 2,700 feet above +sea, to Gravel terraces (right).) +</p> + +<p> +I observed some appearances on the plains at the eastern and opposite foot of +the Cordillera which are worth notice, as showing that the sea there long acted +at nearly the same level as on the basin-plains of Chile. The mountains on this +eastern side are exceedingly abrupt; they rise out of a smooth, talus-like, +very gentle, slope, from five to ten miles in width (as represented in Figure +13), entirely composed of perfectly rounded pebbles, often white-washed with an +aluminous substance like decomposed feldspar. This sloping plain or talus +blends into a perfectly flat space a few miles in width, composed of reddish +impure clay, with small calcareous concretions as in the Pampean +deposit,—of fine white sand with small pebbles in layers,—and of +the above-mentioned white aluminous earth, all interstratified together. This +flat space runs as far as Mendoza, thirty miles northward, and stands probably +at about the same height, namely, 2,700 feet (Pentland and Miers) above the +sea. To the east it is bounded by an escarpment, eighty feet in height, running +for many miles north and south, and composed of perfectly round pebbles, and +loose, white-washed, or embedded in the aluminous earth: behind this escarpment +there is a second and similar one of gravel. Northward of Mendoza, these +escarpments become broken and quite obliterated; and it does not appear that +they ever enclosed a lake-like area: I conclude, therefore, that they were +formed by the sea, when it reached the foot of the Cordillera, like the similar +escarpments occurring at so many points on the coasts of Chile and Patagonia. +</p> + +<p> +The talus-like plain slopes up with a smooth surface into the great dry valleys +of the Cordillera. On each hand of the Portillo valley, the mountains are +formed of red granite, mica-slate, and basalt, which all have suffered a truly +astonishing amount of denudation; the gravel in the valley, as well as on the +talus-like plain in front of it, is composed of these rocks; but at the mouth +of the valley, in the middle (height probably about three thousand five hundred +feet above the sea), a few small isolated hillocks of several varieties of +porphyry project, round which, on all sides, smooth and often white-washed +pebbles of these same porphyries, to the exclusion of all others, extend to a +circumscribed distance. Now, it is difficult to conceive any other agency, +except the quiet and long-continued action of the sea on these hillocks, which +could have rounded and whitewashed the fragments of porphyry, and caused them +to radiate from such small and quite insignificant centres, in the midst of +that vast stream of stones which has descended from the main Cordillera. +</p> + +<h3>SLOPING TERRACES OF GRAVEL IN THE VALLEYS OF THE CORDILLERA.</h3> + +<p> +(FIGURE 14. GROUND-PLAN OF A BIFURCATING VALLEY IN THE CORDILLERA, bordered by +smooth, sloping gravel-fringes (AA), worn along the course of the river into +cliffs.) +</p> + +<p> +All the main valleys on both flanks of the Chilean Cordillera have formerly +had, or still have, their bottoms filled up to a considerable thickness by a +mass of rudely stratified shingle. In Central Chile the greater part of this +mass has been removed by the torrents; cliff-bounded fringes, more or less +continuous, being left at corresponding heights on both sides of the valleys. +These fringes, or as they may be called terraces, have a smooth surface, and as +the valleys rise, they gently rise with them: hence they are easily irrigated, +and afford great facilities for the construction of the roads. From their +uniformity, they give a remarkable character to the scenery of these grand, +wild, broken valleys. In width, the fringes vary much, sometimes being only +broad enough for the roads, and sometimes expanding into narrow plains. Their +surfaces, besides gently rising up the valley, are slightly inclined towards +its centre in such a manner as to show that the whole bottom must once have +been filled up with a smooth and slightly concave mass, as still are the dry +unfurrowed valleys of Northern Chile. Where two valleys unite into one, these +terraces are particularly well exhibited, as is represented in Figure 14. The +thickness of the gravel forming these fringes, on a rude average, may be said +to vary from thirty to sixty or eighty feet; but near the mouths of the valleys +it was in several places from two to three hundred feet. The amount of matter +removed by the torrents has been immense; yet in the lower parts of the valleys +the terraces have seldom been entirely worn away on either side, nor has the +solid underlying rock been reached: higher up the valleys, the terraces have +frequently been removed on one or the other side, and sometimes on both sides; +but in this latter case they reappear after a short interval on the line, which +they would have held had they been unbroken. Where the solid rock has been +reached, it has been cut into deep and narrow gorges. Still higher up the +valleys, the terraces gradually become more and more broken, narrower, and less +thick, until, at a height of from seven to nine thousand feet, they become +lost, and blended with the piles of fallen detritus. +</p> + +<p> +I carefully examined in many places the state of the gravel, and almost +everywhere found the pebbles equally and perfectly rounded, occasionally with +great blocks of rock, and generally distinctly stratified, often with parting +seams of sand. The pebbles were sometimes coated with a white aluminous, and +less frequently with a calcareous, crust. At great heights up the valleys the +pebbles become less rounded; and as the terraces become obliterated, the whole +mass passes into the nature of ordinary detritus. I was repeatedly struck with +the great difference between this detritus high up the valleys, and the gravel +of the terraces low down, namely, in the greater number of the quite angular +fragments in the detritus,—in the unequal degree to which the other +fragments have been rounded,—in the quantity of associated +earth,—in the absence of stratification,—and in the irregularity of +the upper surfaces. This difference was likewise well shown at points low down +the valleys, where precipitous ravines, cutting through mountains of highly +coloured rock, have thrown down wide, fan- shaped accumulations of detritus on +the terraces: in such cases, the line of separation between the detritus and +the terrace could be pointed out to within an inch or two; the detritus +consisting entirely of angular and only partially rounded fragments of the +adjoining coloured rocks; the stratified shingle (as I ascertained by close +inspection, especially in one case, in the valley of the River Mendoza) +containing only a small proportion of these fragments, and those few well +rounded. +</p> + +<p> +I particularly attended to the appearance of the terraces where the valleys +made abrupt and considerable bends, but I could perceive no difference in their +structure: they followed the bends with their usual nearly equable inclination. +I observed, also, in several valleys, that wherever large blocks of any rock +became numerous, either on the surface of the terrace or embedded in it, this +rock soon appeared higher up in situ: thus I have noticed blocks of porphyry, +of andesitic syenite, of porphyry and of syenite, alternately becoming +numerous, and in each case succeeded by mountains thus constituted. There is, +however, one remarkable exception to this rule; for along the valley of the +Cachapual, M. Gay found numerous large blocks of white granite, which does not +occur in the neighbourhood. I observed these blocks, as well as others of +andesitic syenite (not occurring here in situ), near the baths of Cauquenes at +a height of between two and three hundred feet above the river, and therefore +quite above the terrace or fringe which borders that river; some miles up the +valleys there were other blocks at about the same height. I also noticed, at a +less height, just above the terrace, blocks of porphyries (apparently not found +in the immediately impending mountains), arranged in rude lines, as on a +sea-beach. All these blocks were rounded, and though large, not gigantic, like +the true erratic boulders of Patagonia and Fuegia. M. Gay states that the +granite does not occur in situ within a distance of twenty leagues +(“Annales des Science Nat. “ 1 series tome 28. M. Gay, as I was +informed, penetrated the Cordillera by the great oblique valley of Los +Cupressos, and not by the most direct line.); I suspect, for several reasons, +that it will ultimately be found at a much less distance, though certainly not +in the immediate neighbourhood. The boulders found by MM. Meyen and Gay on the +upper plain of San Fernando (mentioned in a previous note) probably belong to +this same class of phenomena. +</p> + +<p> +These fringes of stratified gravel occur along all the great valleys of the +Cordillera, as well as along their main branches; they are strikingly developed +in the valleys of the Maypu, Mendoza, Aconcagua, Cachapual, and according to +Meyen, in the Tinguirica. (“Reise” etc. Th. 1 s. 302.) In the +valleys, however, of Northern Chile, and in some on the eastern flank of the +Cordillera, as in the Portillo Valley, where streams have never flowed, or are +quite insignificant in volume, the presence of a mass of stratified gravel can +be inferred only from the smooth slightly concave form of the bottom. One +naturally seeks for some explanation of so general and striking a phenomenon; +that the matter forming the fringes along the valleys, or still filling up +their entire beds, has not fallen from the adjoining mountains like common +detritus, is evident from the complete contrast in every respect between the +gravel and the piles of detritus, whether seen high up the valleys on their +sides, or low down in front of the more precipitous ravines; that the matter +has not been deposited by debacles, even if we could believe in debacles having +rushed down EVERY valley, and all their branches, eastward and westward from +the central pinnacles of the Cordillera, we must admit from the following +reasons,—from the distinct stratification of the mass,—its smooth +upper surface,—the well-rounded and sometimes encrusted state of the +pebbles, so different from the loose debris on the mountains,—and +especially from the terraces preserving their uniform inclination round the +most abrupt bends. To suppose that as the land now stands, the rivers deposited +the shingle along the course of every valley, and all their main branches, +appears to me preposterous, seeing that these same rivers not only are now +removing and have removed much of this deposit, but are everywhere tending to +cut deep and narrow gorges in the hard underlying rocks. +</p> + +<p> +I have stated that these fringes of gravel, the origin of which are +inexplicable on the notion of debacles or of ordinary alluvial action, are +directly continuous with the similarly-composed basin-like plains at the foot +of the Cordillera, which, from the several reasons before assigned, I cannot +doubt were modelled by the agency of the sea. Now if we suppose that the sea +formerly occupied the valleys of the Chilean Cordillera, in precisely the same +manner as it now does in the more southern parts of the continent, where deep +winding creeks penetrate into the very heart of, and in the case of Obstruction +Sound quite through, this great range; and if we suppose that the mountains +were upraised in the same slow manner as the eastern and western coasts have +been upraised within the recent period, then the origin and formation of these +sloping, terrace-like fringes of gravel can be simply explained. For every part +of the bottom of each valley will, on this view, have long stood at the head of +a sea creek, into which the then existing torrents will have delivered +fragments of rocks, where, by the action of the tides, they will have been +rolled, sometimes encrusted, rudely stratified, and the whole surface levelled +by the blending together of the successive beach lines. (Sloping terraces of +precisely similar structure have been described by me “Philosophical +Transactions” 1839 page 58, in the valleys of Lochaber in Scotland, +where, at higher levels, the parallel roads of Glen Roy show the marks of the +long and quiet residence of the sea. I have no doubt that these sloping +terraces would have been present in the valleys of most of the European ranges, +had not every trace of them, and all wrecks of sea-action, been swept away by +the glaciers which have since occupied them. I have shown that this is the case +with the mountains (“London and Edinburgh Philosophical Journal” +volume 21 page 187) of North Wales.) As the land rose, the torrents in every +valley will have tended to have removed the matter which just before had been +arrested on, or near, the beach-lines; the torrents, also, having continued to +gain in force by the continued elevation increasing their total descent from +their sources to the sea. This slow rising of the Cordillera, which explains so +well the otherwise inexplicable origin and structure of the terraces, judging +from all known analogies, will probably have been interrupted by many periods +of rest; but we ought not to expect to find any evidence of these periods in +the structure of the gravel- terraces: for, as the waves at the heads of deep +creeks have little erosive power, so the only effect of the sea having long +remained at the same level will be that the upper parts of the creeks will have +become filled up at such periods to the level of the water with gravel and +sand; and that afterwards the rivers will have thrown down on the filled-up +parts a talus of similar matter, of which the inclination (as at the head of a +partially filled-up lake) will have been determined by the supply of detritus, +and the force of the stream. (I have attempted to explain this process in a +more detailed manner, in a letter to Mr. Maclaren, published in the +“Edinburgh New Philosophical Journal” volume 35 page 288.) Hence, +after the final conversion of the creeks into valleys, almost the only +difference in the terraces at those points at which the sea stood long, will be +a somewhat more gentle inclination, with river-worn instead of sea-worn +detritus on the surface. +</p> + +<p> +I know of only one difficulty on the foregoing view, namely, the far- +transported blocks of rock high on the sides of the valley of the Cachapual: I +will not attempt any explanation of this phenomenon, but I may state my belief +that a mountain-ridge near the Baths of Cauquenes has been upraised long +subsequently to all the other ranges in the neighbourhood, and that when this +was effected the whole face of the country must have been greatly altered. In +the course of ages, moreover, in this and other valleys, events may have +occurred like, but even on a grander scale than, that described by Molina, when +a slip during the earthquake of 1762 banked up for ten days the great River +Lontue, which then bursting its barrier “inundated the whole +country,” and doubtless transported many great fragments of rock. +(“Compendio de la Hist.” etc. etc. tome 1 page 30. M. Brongniart, +in his report on M. Gay’s labours “Annales des Sciences” +1833, considers that the boulders in the Cachapual belong to the same class +with the erratic boulders of Europe. As the blocks which I saw are not +gigantic, and especially as they are not angular, and as they have not been +transported fairly across low spaces or wide valleys, I am unwilling to class +them with those which, both in the northern and southern hemisphere +“Geological Transactions” volume 6 page 415, have been transported +by ice. It is to be hoped that when M. Gay’s long-continued and admirable +labours in Chile are published, more light will be thrown on this subject. +However, the boulders may have been primarily transported; the final position +of those of porphyry, which have been described as arranged at the foot of the +mountain in rude lines, I cannot doubt, has been due to the action of waves on +a beach. The valley of the Cachapual, in the part where the boulders occur, +bursts through the high ridge of Cauquenes, which runs parallel to, but at some +distance from, the Cordillera. This ridge has been subjected to excessive +violence; trachytic lava has burst from it, and hot springs yet flow at its +base. Seeing the enormous amount of denudation of solid rock in the upper and +much broader parts of this valley where it enters the Cordillera, and seeing to +what extent the ridge of Cauquenes now protects the great range, I could not +help believing (as alluded to in the text) that this ridge with its trachytic +eruptions had been thrown up at a much later period than the Cordillera. If +this has been the case, the boulders, after having been transported to a low +level by the torrents (which exhibit in every valley proofs of their power of +moving great fragments), may have been raised up to their present height, with +the land on which they rested.) Finally, notwithstanding this one case of +difficulty, I cannot entertain any doubt, that these terrace-like fringes, +which are continuously united with the basin-shaped plains at the foot of the +Cordillera, have been formed by the arrestment of river-borne detritus at +successive levels, in the same manner as we see now taking place at the heads +of all those many, deep, winding fiords intersecting the southern coasts. To my +mind, this has been one of the most important conclusions to which my +observations on the geology of South America have led me; for we thus learn +that one of the grandest and most symmetrical mountain-chains in the world, +with its several parallel lines, has been together uplifted in mass between +seven and nine thousand feet, in the same gradual manner as have the eastern +and western coasts within the recent period. (I do not wish to affirm that all +the lines have been uplifted quite equally; slight differences in the elevation +would leave no perceptible effect on the terraces. It may, however, be +inferred, perhaps with one exception, that since the period when the sea +occupied these valleys, the several ranges have not been dislocated by GREAT +and ABRUPT faults or upheavals; for if such had occurred, the terraces of +gravel at these points would not have been continuous. The one exception is at +the lower end of a plain in the Valle del Yeso (a branch of the Maypu), where, +at a great height, the terraces and valley appear to have been broken through +by a line of upheaval, of which the evidence is plain in the adjoining +mountains; this dislocation, perhaps, occurred AFTER THE ELEVATION of this part +of the valley above the level of the sea. The valley here is almost blocked up +by a pile about one thousand feet in thickness, formed, as far as I could +judge, from three sides, entirely, or at least in chief part, of gravel and +detritus. On the south side, the river has cut quite through this mass; on the +northern side, and on the very summit, deep ravines, parallel to the line of +the valley, are worn, as if the drainage from the valley above had passed by +these two lines before following its present course.) +</p> + +<h3>FORMATION OF VALLEYS.</h3> + +<p> +The bulk of solid rock which has been removed in the lower parts of the valleys +of the Cordillera has been enormous. It is only by reflecting on such cases as +that of the gravel beds of Patagonia, covering so many thousand square leagues +of surface, and which, if heaped into a ridge, would form a mountain-range +almost equal to the Cordillera, that the amount of denudation becomes credible. +The valleys within this range often follow anticlinal but rarely synclinal +lines; that is, the strata on the two sides more often dip from the line of +valley than towards it. On the flanks of the range, the valleys most frequently +run neither along anticlinal nor synclinal axes, but along lines of flexure or +faults: that is, the strata on both sides dip in the same direction, but with +different, though often only slightly different, inclinations. As most of the +nearly parallel ridges which together form the Cordillera run approximately +north and south, the east and west valleys cross them in zig-zag lines, +bursting through the points where the strata have been least inclined. No doubt +the greater part of the denudation was affected at the periods when tidal- +creeks occupied the valleys, and when the outer flanks of the mountains were +exposed to the full force of an open ocean. I have already alluded to the power +of the tidal action in the channels connecting great bays; and I may here +mention that one of the surveying vessels in a channel of this kind, though +under sail, was whirled round and round by the force of the current. We shall +hereafter see, that of the two main ridges forming the Chilean Cordillera, the +eastern and loftiest one owes the greater part of its ANGULAR upheaval to a +period subsequent to the elevation of the western ridge; and it is likewise +probable that many of the other parallel ridges have been angularly upheaved at +different periods; consequently many parts of the surfaces of these mountains +must formerly have been exposed to the full force of the waves, which, if the +Cordillera were now sunk into the sea, would be protected by parallel chains of +islands. The torrents in the valleys certainly have great power in wearing the +rocks; as could be told by the dull rattling sound of the many fragments night +and day hurrying downwards; and as was attested by the vast size of certain +fragments, which I was assured had been carried onwards during floods; yet we +have seen in the lower parts of the valleys, that the torrents have seldom +removed all the sea-checked shingle forming the terraces, and have had time +since the last elevation in mass only to cut in the underlying rocks, gorges, +deep and narrow, but quite insignificant in dimensions compared with the entire +width and depth of the valleys. +</p> + +<p> +Along the shores of the Pacific, I never ceased during my many and long +excursions to feel astonished at seeing every valley, ravine, and even little +inequality of surface, both in the hard granitic and soft tertiary districts, +retaining the exact outline, which they had when the sea left their surfaces +coated with organic remains. When these remains shall have decayed, there will +be scarcely any difference in appearance between this line of coast-land and +most other countries, which we are accustomed to believe have assumed their +present features chiefly through the agency of the weather and fresh-water +streams. In the old granitic districts, no doubt it would be rash to attribute +all the modifications of outline exclusively to the sea-action; for who can say +how often this lately submerged coast may not previously have existed as land, +worn by running streams and washed by rain? This source of doubt, however, does +not apply to the districts superficially formed of the modern tertiary +deposits. The valleys worn by the sea, through the softer formations, both on +the Atlantic and Pacific sides of the continent, are generally broad, winding, +and flat-bottomed: the only district of this nature now penetrated by arms of +the sea, is the island of Chiloe. +</p> + +<p> +Finally, the conclusion at which I have arrived, with respect to the relative +powers of rain and sea water on the land, is, that the latter is far the most +efficient agent, and that its chief tendency is to widen the valleys; whilst +torrents and rivers tend to deepen them, and to remove the wreck of the +sea’s destroying action. As the waves have more power, the more open and +exposed the space may be, so will they always tend to widen more and more the +mouths of valleys compared with their upper parts: hence, doubtless, it is, +that most valleys expand at their mouths,—that part, at which the rivers +flowing in them, generally have the least wearing power. +</p> + +<p> +When reflecting on the action of the sea on the land at former levels, the +effect of the great waves, which generally accompany earthquakes, must not be +overlooked: few years pass without a severe earthquake occurring on some part +of the west coast of South America; and the waves thus caused have great power. +At Concepcion, after the shock of 1835, I saw large slabs of sandstone, one of +which was six feet long, three in breadth, and two in thickness, thrown high up +on the beach; and from the nature of the marine animals still adhering to it, +it must have been torn up from a considerable depth. On the other hand, at +Callao, the recoil-wave of the earthquake of 1746 carried great masses of +brickwork, between three and four feet square, some way out seaward. During the +course of ages, the effect thus produced at each successive level, cannot have +been small; and in some of the tertiary deposits on this line of coast, I +observed great boulders of granite and other neighbouring rocks, embedded in +fine sedimentary layers, the transportal of which, except by the means of +earthquake-waves, always appeared to me inexplicable. +</p> + +<h3>SUPERFICIAL SALINE DEPOSITS.</h3> + +<p> +This subject may be here conveniently treated of: I will begin with the most +interesting case, namely, the superficial saline beds near Iquique in Peru. The +porphyritic mountains on the coast rise abruptly to a height of between one +thousand nine hundred and three thousand feet: between their summits and an +inland plain, on which the celebrated deposit of nitrate of soda lies, there is +a high undulatory district, covered by a remarkable superficial saliferous +crust, chiefly composed of common salt, either in white, hard, opaque nodules, +or mingled with sand, in this latter case forming a compact sandstone. This +saliferous superficial crust extends from the edge of the coast-escarpment, +over the whole face of the country; but never attains, as I am assured by Mr. +Bollaert (long resident here) any great thickness. Although a very slight +shower falls only at intervals of many years, yet small funnel-shaped cavities +show that the salt has been in some parts dissolved. (It is singular how +slowly, according to the observations of M. Cordier on the salt-mountain of +Cardona in Spain “Ann. des Mines, Translation of Geolog. Mem.” by +De la Beche page 60, salt is dissolved, where the amount of rain is supposed to +be as much as 31.4 of an inch in the year. It is calculated that only five feet +in thickness is dissolved in the course of a century.) In several places I saw +large patches of sand, quite moist, owing to the quantity of muriate of lime +(as ascertained by Mr. T. Reeks) contained in them. From the compact salt- +cemented sand being either red, purplish, or yellow, according to the colour of +the rocky strata on which it rested, I imagined that this substance had +probably been derived through common alluvial action from the layers of salt +which occur interstratified in the surrounding mountains (“Journal of +Researches” page 444 first edition.): but from the interesting details +given by M. d’Orbigny, and from finding on a fresh examination of this +agglomerated sand, that it is not irregularly cemented, but consists of thin +layers of sand of different tints of colour, alternating with excessively fine +parallel layers of salt, I conclude that it is not of alluvial origin. M. +d’Orbigny observed analogous saline beds extending from Cobija for five +degrees of latitude northward, and at heights varying from six hundred to nine +hundred feet (“Voyage” etc. page 102. M. d’Orbigny found this +deposit intersected, in many places, by deep ravines, in which there was no +salt. Streams must once, though historically unknown, have flowed in them; and +M. d’Orbigny argues from the presence of undissolved salt over the whole +surrounding country, that the streams must have arisen from rain or snow having +fallen, not in the adjoining country, but on the now arid Cordillera. I may +remark, that from having observed ruins of Indian buildings in absolutely +sterile parts of the Chilian Cordillera (“Journal” 2nd edition page +357), I am led to believe that the climate, at a time when Indian man inhabited +this part of the continent, was in some slight degree more humid than it is at +present.): from finding recent sea- shells strewed on these saliferous beds, +and under them, great well-rounded blocks, exactly like those on the existing +beach, he believes that the salt, which is invariably superficial, has been +left by the evaporation of the sea-water. This same conclusion must, I now +believe, be extended to the superficial saliferous beds of Iquique, though they +stand about three thousand feet above the level of the sea. +</p> + +<p> +Associated with the salt in the superficial beds, there are numerous, thin, +horizontal layers of impure, dirty-white, friable, gypseous and calcareous +tuffs. The gypseous beds are very remarkable, from abounding with, so as +sometimes to be almost composed of, irregular concretions, from the size of an +egg to that of a man’s head, of very hard, compact, heavy gypsum, in the +form of anhydrite. This gypsum contains some foreign particles of stone; it is +stained, judging from its action with borax, with iron, and it exhales a strong +aluminous odour. The surfaces of the concretions are marked by sharp, +radiating, or bifurcating ridges, as if they had been (but not really) +corroded: internally they are penetrated by branching veins (like those of +calcareous spar in the septaria of the London clay) of pure white anhydrite. +These veins might naturally have been thought to have been formed by subsequent +infiltration, had not each little embedded fragment of rock been likewise edged +in a very remarkable manner by a narrow border of the same white anhydrite: +this shows that the veins must have been formed by a process of segregation, +and not of infiltration. Some of the little included and CRACKED fragments of +foreign rock are penetrated by the anhydrite, and portions have evidently been +thus mechanically displaced: at St. Helena, I observed that calcareous matter, +deposited by rain water, also had the power to separate small fragments of rock +from the larger masses. (“Volcanic Islands” etc. page 87.) I +believe the superficial gypseous deposit is widely extended: I received +specimens of it from Pisagua, forty miles north of Iquique, and likewise from +Arica, where it coats a layer of pure salt. M. d’Orbigny found at Cobija +a bed of clay, lying above a mass of upraised recent shells, which was +saturated with sulphate of soda, and included thin layers of fibrous gypsum. +(“Voyage Geolog.” etc. page 95.) These widely extended, +superficial, beds of salt and gypsum, appear to me an interesting geological +phenomenon, which could be presented only under a very dry climate. +</p> + +<p> +The plain or basin, on the borders of which the famous bed of nitrate of soda +lies, is situated at the distance of about thirty miles from the sea, being +separated from it by the saliferous district just described. It stands at a +height of 3,300 feet; its surface is level, and some leagues in width; it +extends forty miles northward, and has a total length (as I was informed by Mr. +Belford Wilson, the Consul-General at Lima) of 420 miles. In a well near the +works, thirty-six yards in depth, sand, earth, and a little gravel were found: +in another well, near Almonte, fifty yards deep, the whole consisted, according +to Mr. Blake, of clay, including a layer of sand two feet thick, which rested +on fine gravel, and this on coarse gravel, with large rounded fragments of +rock. (See an admirable paper “Geological and Miscellaneous Notices of +Tarapaca” in “Silliman’s American Journal” volume 44 +page 1.) In many parts of this now utterly desert plain, rushes and large +prostrate trees in a hardened state, apparently Mimosas, are found buried, at a +depth from three to six feet; according to Mr. Blake, they have all fallen to +the south-west. The bed of nitrate of soda is said to extend for forty to fifty +leagues along the western margin of the plain, but is not found in its central +parts: it is from two to three feet in thickness, and is so hard that it is +generally blasted with gunpowder; it slopes gently upwards from the edge of the +plain to between ten and thirty feet above its level. It rests on sand in +which, it is said, vegetable remains and broken shells have been found; shells +have also been found, according to Mr. Blake, both on and in the nitrate of +soda. It is covered by a superficial mass of sand, containing nodules of common +salt, and, as I was assured by a miner, much soft gypseous matter, precisely +like that in the superficial crust already described: certainly this crust, +with its characteristic concretions of anhydrite, comes close down to the edge +of the plain. +</p> + +<p> +The nitrate of soda varies in purity in different parts, and often contains +nodules of common salt. According to Mr. Blake, the proportion of nitrate of +soda varies from 20 to 75 per cent. An analysis by Mr. A. Hayes, of an average +specimen, gave:— +</p> + +<pre xml:space="preserve"> +Nitrate of Soda.... 64.98 +Sulphate of Soda.... 3.00 +Chloride of Soda... 28.69 +Iodic Salts......... 0.63 +Shells and Marl..... 2.60 + 99.90 +</pre> + +<p> +The “mother-water” at some of the refineries is very rich in iodic +salts, and is supposed to contain much muriate of lime. (“Literary +Gazette” 1841 page 475.) In an unrefined specimen brought home by myself, +Mr. T. Reeks has ascertained that the muriate of lime is very abundant. With +respect to the origin of this saline mass, from the manner in which the gently +inclined, compact bed follows for so many miles the sinuous margin of the +plain, there can be no doubt that it was deposited from a sheet of water: from +the fragments of embedded shells, from the abundant iodic salts, from the +superficial saliferous crust occurring at a higher level and being probably of +marine origin, and from the plain resembling in form those of Chile and that of +Uspallata, there can be little doubt that this sheet of water was, at least +originally, connected with the sea. (From an official document, shown me by Mr. +Belford Wilson, it appears that the first export of nitrate of soda to Europe +was in July 1830, on French account, in a British ship:— +</p> + +<pre xml:space="preserve"> +In year, the entire export was in Quintals. +1830............................ 17,300 +1831............................ 40,885 +1832............................ 51,400 +1833............................ 91,335 +1834........................... 149,538 +The Spanish quintal nearly equals 100 English pounds.) +</pre> + +<h3>THIN, SUPERFICIAL, SALINE INCRUSTATIONS.</h3> + +<p> +These saline incrustations are common in many parts of America: Humboldt met +with them on the tableland of Mexico, and the Jesuit Falkner and other authors +state that they occur at intervals over the vast plains extending from the +mouth of the Plata to Rioja and Catamarca. (Azara “Travels” volume +1 page 55, considers that the Parana is the eastern boundary of the saliferous +region; but I heard of “salitrales” in the Province of Entre Rios.) +Hence it is that during droughts, most of the streams in the Pampas are saline. +I nowhere met with these incrustations so abundantly as near Bahia Blanca: +square miles of the mud-flats, which near that place are raised only a few feet +above the sea, just enough to protect them from being overflowed, appear, after +dry weather, whiter than the ground after the thickest hoar-frost. After rain +the salts disappear, and every puddle of water becomes highly saline; as the +surface dries, the capillary action draws the moisture up pieces of broken +earth, dead sticks, and tufts of grass, where the salt effloresces. The +incrustation, where thickest, does not exceed a quarter of an inch. M. +Parchappe has analysed it (M. d’Orbigny “Voyage” etc. Part. +Hist. tome 1 page 664.); and finds that the specimens collected at the extreme +head of the low plain, near the River Manuello, consist of 93 per cent of +sulphate of soda, and 7 of common salt; whilst the specimens taken close to the +coast contain only 63 per cent of the sulphate, and 37 of the muriate of soda. +This remarkable fact, together with our knowledge that the whole of this low +muddy plain has been covered by the sea within the recent period, must lead to +the suspicion that the common salt, by some unknown process, becomes in time +changed into the sulphate. Friable, calcareous matter is here abundant, and the +case of the apparent double decomposition of the shells and salt on San +Lorenzo, should not be forgotten. +</p> + +<p> +The saline incrustations, near Bahia Blanca, are not confined to, though most +abundant on, the low muddy flats; for I noticed some on a calcareous plain +between thirty and forty feet above the sea, and even a little occurs in still +higher valleys. Low alluvial tracts in the valleys of the Rivers Negro and +Colorado are also encrusted, and in the latter valley such spaces appeared to +be occasionally overflowed by the river. I observed saline incrustations in +some of the valleys of Southern Patagonia. At Port Desire a low, flat, muddy +valley was thickly incrusted by salts, which on analysis by Mr. T. Reeks, are +found to consist of a mixture of sulphate and muriate of soda, with carbonate +of lime and earthy matter. On the western side of the continent, the southern +coasts are much too humid for this phenomenon; but in Northern Chile I again +met with similar incrustations. On the hardened mud, in parts of the broad, +flat-bottomed valley of Copiapo, the saline matter encrusts the ground to the +thickness of some inches: specimens, sent by Mr. Bingley to Apothecaries’ +Hall for analysis, were said to consist of carbonate and sulphate of soda. Much +sulphate of soda is found in the desert of Atacama. In all parts of South +America, the saline incrustations occur most frequently on low damp surfaces of +mud, where the climate is rather dry; and these low surfaces have, in almost +every case, been upraised above the level of the sea, within the recent period. +</p> + +<h3>SALT-LAKES OF PATAGONIA AND LA PLATA.</h3> + +<p> +Salinas, or natural salt-lakes, occur in various formations on the eastern side +of the continent,—in the argillaceo-calcareous deposit of the Pampas, in +the sandstone of the Rio Negro, where they are very numerous, in the pumiceous +and other beds of the Patagonian tertiary formation, and in small primary +districts in the midst of this latter formation. Port S. Julian is the most +southerly point (latitude 49 degrees to 50 degrees) at which salinas are known +to occur. (According to Azara “Travels” volume 1 page 56, there are +salt-lakes as far north as Chaco (latitude 25 degrees), on the banks of the +Vermejo. The salt-lakes of Siberia appear (Pallas “Travels” English +Translation volume 1 page 284) to occur in very similar depressions to those of +Patagonia.) The depressions, in which these salt-lakes lie, are from a few feet +to sixty metres, as asserted by M. d’Orbigny, below the surface of the +surrounding plains (“Voyage Geolog.” page 63.); and, according to +this same author, near the Rio Negro they all trend, either in the N.E. and +S.W. or in E. and W. lines, coincident with the general slope of the plain. +These depressions in the plain generally have one side lower than the others, +but there are no outlets for drainage. Under a less dry climate, an outlet +would soon have been formed, and the salt washed away. The salinas occur at +different elevations above the sea; they are often several leagues in diameter; +they are generally very shallow, but there is a deep one in a quartz-rock +formation near C. Blanco. In the wet season, the whole, or a part, of the salt +is dissolved, being redeposited during the succeeding dry season. At this +period the appearance of the snow-white expanse of salt crystallised in great +cubes, is very striking. In a large salina, northward of the Rio Negro, the +salt at the bottom, during the whole year, is between two and three feet in +thickness. +</p> + +<p> +The salt rests almost always on a thick bed of black muddy sand, which is +fetid, probably from the decay of the burrowing worms inhabiting it. (Professor +Ehrenberg examined some of this muddy sand, but was unable to find in it any +infusoria.) In a salina, situated about fifteen miles above the town of El +Carmen on the Rio Negro, and three or four miles from the banks of that river, +I observed that this black mud rested on gravel with a calcareous matrix, +similar to that spread over the whole surrounding plains: at Port S. Julian the +mud, also, rested on the gravel: hence the depressions must have been formed +anteriorly to, or contemporaneously with, the spreading out of the gravel. I +was informed that one small salina occurs in an alluvial plain within the +valley of the Rio Negro, and therefore its origin must be subsequent to the +excavation of that valley. When I visited the salina, fifteen miles above the +town, the salt was beginning to crystallise, and on the muddy bottom there were +lying many crystals, generally placed crossways of sulphate of soda (as +ascertained by Mr. Reeks), and embedded in the mud, numerous crystals of +sulphate of lime, from one to three inches in length: M. d’Orbigny states +that some of these crystals are acicular and more than even nine inches in +length (“Voyage Geolog.” page 64.); others are macled and of great +purity: those I found all contained some sand in their centres. As the black +and fetid sand overlies the gravel, and that overlies the regular tertiary +strata, I think there can be no doubt that these remarkable crystals of +sulphate of lime have been deposited from the waters of the lake. The +inhabitants call the crystals of selenite, the padre del sal, and those of the +sulphate of soda, the madre del sal; they assured me that both are found under +the same circumstances in several of the neighbouring salinas; and that the +sulphate of soda is annually dissolved, and is always crystallised before the +common salt on the muddy bottom. (This is what might have been expected; for M. +Ballard asserts “Acad. des Sciences” October 7, 1844, that sulphate +of soda is precipitated from solution more readily from water containing +muriate of soda in excess, than from pure water.) The association of gypsum and +salt in this case, as well as in the superficial deposits of Iquique, appears +to me interesting, considering how generally these substances are associated in +the older stratified formations. +</p> + +<p> +Mr. Reeks has analysed for me some of the salt from the salina near the Rio +Negro; he finds it composed entirely of chloride of sodium, with the exception +of 0.26 of sulphate of lime and of 0.22 of earthy matter: there are no traces +of iodic salts. Some salt from the salina Chiquitos, in the Pampean formation, +is equally pure. It is a singular fact, that the salt from these salinas does +not serve so well for preserving meat, as sea-salt from the Cape de Verde +Islands; and a merchant at Buenos Ayres told me that he considered it as 50 per +cent less valuable. The purity of the Patagonian salt, or absence from it of +those other saline bodies found in all sea- water, is the only assignable cause +for this inferiority; a conclusion which is supported by the fact lately +ascertained, that those salts answer best for preserving cheese which contain +most of the deliquescent chlorides. (“Horticultural and Agricultural +Gazette” 1845 page 93.) (It would probably well answer for the merchants +of Buenos Ayres (considering the great consumption there of salt for preserving +meat) to import the deliquescent chlorides to mix with the salt from the +salinas: I may call attention to the fact, that at Iquique, a large quantity of +muriate of lime, left in the MOTHER-WATER during the refinement of the nitrate +of soda, is annually thrown away.) +</p> + +<p> +With respect to the origin of the salt in the salinas, the foregoing analysis +seems opposed to the view entertained by M. d’Orbigny and others, and +which seems so probable considering the recent elevation of this line of coast, +namely, that it is due to the evaporation of sea-water and to the drainage from +the surrounding strata impregnated with sea-salt. I was informed (I know not +whether accurately) that on the northern side of the salina on the Rio Negro, +there is a small brine spring which flows at all times of the year: if this be +so, the salt in this case at least, probably is of subterranean origin. It at +first appears very singular that fresh water can often be procured in wells, +and is sometimes found in small lakes, quite close to these salinas. (Sir W. +Parish states “Buenos Ayres” etc. pages 122 and 170, that this is +the case near the great salinas westward of the S. Ventana. I have seen similar +statements in an ancient MS. Journal lately published by S. Angelis. At +Iquique, where the surface is so thickly encrusted with saline matter, I tasted +water only slightly brackish, procured in a well thirty-six yards deep; but +here one feels less surprise at its presence, as pure water might percolate +under ground from the not very distant Cordillera.) I am not aware that this +fact bears particularly on the origin of the salt; but perhaps it is rather +opposed to the view of the salt having been washed out of the surrounding +superficial strata, but not to its having been the residue of sea-water, left +in depressions as the land was slowly elevated. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0004"></a> +CHAPTER IV.<br /> +ON THE FORMATIONS OF THE PAMPAS.</h2> + +<p class="letter"> +Mineralogical constitution.—Microscopical structure.—Buenos Ayres, +shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—San +Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta, +shells, bones and infusoria of; co-existence of the recent shells and extinct +mammifers.—Buenos Ayres to Santa Fé.—Skeletons of +Mastodon.—Infusoria.—Inferior marine tertiary strata, their +age.—Horse’s tooth.—BANDA ORIENTAL.—Superficial Pampean +formation.—Inferior tertiary strata, variation of, connected with +volcanic action; Macrauchenia Patachonica at San Julian in Patagonia, age of, +subsequent to living mollusca and to the erratic block period. +SUMMARY.—Area of Pampean formation.—Theories of +origin.—Source of sediment.—Estuary origin.—Contemporaneous +with existing mollusca.—Relations to underlying tertiary +strata.—Ancient deposit of estuary origin.—Elevation and successive +deposition of the Pampean formation.—Number and state of the remains of +mammifers; their habitation, food, extinction, and +range.—Conclusion.—Localities in Pampas at which mammiferous +remains have been found. +</p> + +<p> +The Pampean formation is highly interesting from its vast extent, its disputed +origin, and from the number of extinct gigantic mammifers embedded in it. It +has upon the whole a very uniform character: consisting of a more or less dull +reddish, slightly indurated, argillaceous earth or mud, often, but not always, +including in horizontal lines concretions of marl, and frequently passing into +a compact marly rock. The mud, wherever I examined it, even close to the +concretions, did not contain any carbonate of lime. The concretions are +generally nodular, sometimes rough externally, sometimes stalactiformed; they +are of a compact structure, but often penetrated (as well as the mud) by +hair-like serpentine cavities, and occasionally with irregular fissures in +their centres, lined with minute crystals of carbonate of lime; they are of +white, brown, or pale pinkish tints, often marked by black dendritic manganese +or iron; they are either darker or lighter tinted than the surrounding mass; +they contain much carbonate of lime, but exhale a strong aluminous odour, and +leave, when dissolved in acids, a large but varying residue, of which the +greater part consists of sand. These concretions often unite into irregular +strata; and over very large tracts of country, the entire mass consists of a +hard, but generally cavernous marly rock: some of the varieties might be called +calcareous tuffs. +</p> + +<p> +Dr. Carpenter has kindly examined under the microscope, sliced and polished +specimens of these concretions, and of the solid marl-rock, collected in +various places between the Colorado and Santa Fe Bajada. In the greater number, +Dr. Carpenter finds that the whole substance presents a tolerably uniform +amorphous character, but with traces of incipient crystalline metamorphosis; in +other specimens he finds microscopically minute rounded concretions of an +amorphous substance (resembling in size those in oolitic rocks, but not having +a concentric structure), united by a cement which is often crystalline. In +some, Dr. Carpenter can perceive distinct traces of shells, corals, +Polythalamia, and rarely of spongoid bodies. For the sake of comparison, I sent +Dr. Carpenter specimens of the calcareous rock, formed chiefly of fragments of +recent shells, from Coquimbo in Chile: in one of these specimens, Dr. Carpenter +finds, besides the larger fragments, microscopical particles of shells, and a +varying quantity of opaque amorphous matter; in another specimen from the same +bed, he finds the whole composed of the amorphous matter, with layers showing +indications of an incipient crystalline metamorphosis: hence these latter +specimens, both in external appearance and in microscopical structure, closely +resemble those of the Pampas. Dr. Carpenter informs me that it is well known +that chemical precipitation throws down carbonate of lime in the opaque +amorphous state; and he is inclined to believe that the long-continued +attrition of a calcareous body in a state of crystalline or semi-crystalline +aggregation (as, for instance, in the ordinary shells of Mollusca, which, when +sliced, are transparent) may yield the same result. From the intimate relations +between all the Coquimbo specimens, I can hardly doubt that the amorphous +carbonate of lime in them has resulted from the attrition and decay of the +larger fragments of shell: whether the amorphous matter in the marly rocks of +the Pampas has likewise thus originated, it would be hazardous to conjecture. +</p> + +<p> +For convenience’ sake, I will call the marly rock by the name given to it +by the inhabitants, namely, Tosca-rock; and the reddish argillaceous earth, +Pampean mud. This latter substance, I may mention, has been examined for me by +Professor Ehrenberg, and the result of his examination will be given under the +proper localities. +</p> + +<p> +I will commence my descriptions at a central spot, namely, at Buenos Ayres, and +thence proceed first southward to the extreme limit of the deposit, and +afterwards northward. The plain on which Buenos Ayres stands is from thirty to +forty feet in height. The Pampean mud is here of a rather pale colour, and +includes small nearly white nodules, and other irregular strata of an unusually +arenaceous variety of tosca-rock. In a well at the depth of seventy feet, +according to Ignatio Nunez, much tosca-rock was met with, and at several +points, at one hundred feet deep, beds of sand have been found. I have already +given a list of the recent marine and estuary shells found in many parts on the +surface near Buenos Ayres, as far as three or four leagues from the Plata. +Specimens from near Ensenada, given me by Sir W. Parish, where the rock is +quarried just beneath the surface of the plain, consist of broken bivalves, +cemented by and converted into white crystalline carbonate of lime. I have +already alluded, in the first chapter, to a specimen (also given me by Sir W. +Parish) from the A. del Tristan, in which shells, resembling in every respect +the Azara labiata, d’Orbigny, as far as their worn condition permits of +comparison, are embedded in a reddish, softish, somewhat arenaceous marly rock: +after careful comparison, with the aid of a microscope and acids, I can +perceive no difference between the basis of this rock and the specimens +collected by me in many parts of the Pampas. I have also stated, on the +authority of Sir W. Parish, that northward of Buenos Ayres, on the highest +parts of the plain, about forty feet above the Plata, and two or three miles +from it, numerous shells of the Azara labiata (and I believe of Venus sinuosa) +occur embedded in a stratified earthy mass, including small marly concretions, +and said to be precisely like the great Pampean deposit. Hence we may conclude +that the mud of the Pampas continued to be deposited to within the period of +this existing estuary shell. Although this formation is of such immense extent, +I know of no other instance of the presence of shells in it. +</p> + +<h3>BUENOS AYRES TO THE RIO COLORADO.</h3> + +<p> +With the exception of a few metamorphic ridges, the country between these two +points, a distance of 400 geographical miles, belongs to the Pampean formation, +and in the southern part is generally formed of the harder and more calcareous +varieties. I will briefly describe my route: about twenty- five miles S.S.W. of +the capital, in a well forty yards in depth, the upper part, and, as I was +assured, the entire thickness, was formed of dark red Pampean mud without +concretions. North of the River Salado, there are many lakes; and on the banks +of one (near the Guardia) there was a little cliff similarly composed, but +including many nodular and stalactiform concretions: I found here a large piece +of tessellated armour, like that of the Glyptodon, and many fragments of bones. +The cliffs on the Salado consist of pale-coloured Pampean mud, including and +passing into great masses of tosca-rock: here a skeleton of the Megatherium and +the bones of other extinct quadrupeds (see the list at the end of this chapter) +were found. Large quantities of crystallised gypsum (of which specimens were +given me) occur in the cliffs of this river; and likewise (as I was assured by +Mr. Lumb) in the Pampean mud on the River Chuelo, seven leagues from Buenos +Ayres: I mention this because M. d’Orbigny lays some stress on the +supposed absence of this mineral in the Pampean formation. +</p> + +<p> +Southward of the Salado the country is low and swampy, with tosca-rock +appearing at long intervals at the surface. On the banks, however, of the +Tapalguen (sixty miles south of the Salado) there is a large extent of +tosca-rock, some highly compact and even semi-crystalline, overlying pale +Pampean mud with the usual concretions. Thirty miles further south, the small +quartz-ridge of Tapalguen is fringed on its northern and southern flank, by +little, narrow, flat-topped hills of tosca-rock, which stand higher than the +surrounding plain. Between this ridge and the Sierra of Guitru-gueyu, a +distance of sixty miles, the country is swampy, with the tosca-rock appearing +only in four or five spots: this sierra, precisely like that of Tapalguen, is +bordered by horizontal, often cliff-bounded, little hills of tosca-rock, higher +than the surrounding plain. Here, also, a new appearance was presented in some +extensive and level banks of alluvium or detritus of the neighbouring +metamorphic rocks; but I neglected to observe whether it was stratified or not. +Between Guitru-gueyu and the Sierra Ventana, I crossed a dry plain of +tosca-rock higher than the country hitherto passed over, and with small pieces +of denuded tableland of the same formation, standing still higher. +</p> + +<p> +The marly or calcareous beds not only come up nearly horizontally to the +northern and southern foot of the great quartzose mountains of the Sierra +Ventana, but interfold between the parallel ranges. The superficial beds (for I +nowhere obtained sections more than twenty feet deep) retain, even close to the +mountains, their usual character: the uppermost layer, however, in one place +included pebbles of quartz, and rested on a mass of detritus of the same rock. +At the very foot of the mountains, there were some few piles of quartz and +tosca-rock detritus, including land-shells; but at the distance of only half a +mile from these lofty, jagged, and battered mountains, I could not, to my great +surprise, find on the boundless surface of the calcareous plain even a single +pebble. Quartz- pebbles, however, of considerable size have at some period been +transported to a distance of between forty and fifty miles to the shores of +Bahia Blanca. (Schmidtmeyer “Travels in Chile” page 150, states +that he first noticed on the Pampas, very small bits of red granite, when fifty +miles distant from the southern extremity of the mountains of Cordova, which +project on the plain, like a reef into the sea.) +</p> + +<p> +The highest peak of the St. Ventana is, by Captain Fitzroy’s measurement, +3,340 feet, and the calcareous plain at its foot (from observations taken by +some Spanish officers) 840 feet above the sea-level. (“La Plata” +etc. by Sir W. Parish page 146.) On the flanks of the mountains, at a height of +three hundred or four hundred feet above the plain, there were a few small +patches of conglomerate and breccia, firmly cemented by ferruginous matter to +the abrupt and battered face of the quartz—traces being thus exhibited of +ancient sea-action. The high plain round this range sinks quite insensibly to +the eye on all sides, except to the north, where its surface is broken into low +cliffs. Round the Sierras Tapalguen, Guitru-gueyu, and between the latter and +the Ventana we have seen (and shall hereafter see round some hills in Banda +Oriental), that the tosca-rock forms low, flat- topped, cliff-bounded hills, +higher than the surrounding plains of similar composition. From the horizontal +stratification and from the appearance of the broken cliffs, the greater height +of the Pampean formation round these primary hills ought not to be altogether +or in chief part attributed to these several points having been uplifted more +energetically than the surrounding country, but to the argillaceo-calcareous +mud having collected round them, when they existed as islets or submarine +rocks, at a greater height, than at the bottom of the adjoining open +sea;—the cliffs having been subsequently worn during the elevation of the +whole country in mass. +</p> + +<p> +Southward of the Ventana, the plain extends farther than the eye can range; its +surface is not very level, having slight depressions with no drainage exits; it +is generally covered by a few feet in thickness of sandy earth; and in some +places, according to M. Parchappe, by beds of clay two yards thick. (M. +d’Orbigny “Voyage” Part Geolog. pages 47, 48.) On the banks +of the Sauce, four leagues S.E. of the Ventana, there is an imperfect section +about two hundred feet in height, displaying in the upper part tosca-rock and +in the lower part red Pampean mud. At the settlement of Bahia Blanca, the +uppermost plain is composed of very compact, stratified tosca-rock, containing +rounded grains of quartz distinguishable by the naked eye: the lower plain, on +which the fortress stands, is described by M. Parchappe as composed of solid +tosca-rock (Ibid.); but the sections which I examined appeared more like a +redeposited mass of this rock, with small pebbles and fragments of quartz. I +shall immediately return to the important sections on the shores of Bahia +Blanca. Twenty miles southward of this place, there is a remarkable ridge +extending W. by N. and E. by S., formed of small, separate, flat-topped, +steep-sided hills, rising between one hundred and two hundred feet above the +Pampean plain at its southern base, which plain is a little lower than that to +the north. The uppermost stratum in this ridge consists of pale, highly +calcareous, compact tosca-rock, resting (as seen in one place) on reddish +Pampean mud, and this again on a paler kind: at the foot of the ridge, there is +a well in reddish clay or mud. I have seen no other instance of a chain of +hills belonging to the Pampean formation; and as the strata show no signs of +disturbance, and as the direction of the ridge is the same with that common to +all the metamorphic lines in this whole area, I suspect that the Pampean +sediment has in this instance been accumulated on and over a ridge of hard +rocks, instead of, as in the case of the above-mentioned Sierras, round their +submarine flanks. South of this little chain of tosca-rock, a plain of Pampean +mud declines towards the banks of the Colorado: in the middle a well has been +dug in red Pampean mud, covered by two feet of white, softish, highly +calcareous tosca-rock, over which lies sand with small pebbles three feet in +thickness—the first appearance of that vast shingle formation described +in the First Chapter. In the first section after crossing the Colorado, an old +tertiary formation, namely, the Rio Negro sandstone (to be described in the +next chapter), is met with: but from the accounts given me by the Gauchos, I +believe that at the mouth of the Colorado the Pampean formation extends a +little further southwards. +</p> + +<h3>BAHIA BLANCA.</h3> + +<p> +To return to the shores of this bay. At Monte Hermoso there is a good section, +about one hundred feet in height, of four distinct strata, appearing to the eye +horizontal, but thickening a little towards the N.W. The uppermost bed, about +twenty feet in thickness, consists of obliquely laminated, soft sandstone, +including many pebbles of quartz, and falling at the surface into loose sand. +The second bed, only six inches thick, is a hard, dark-coloured sandstone. The +third bed is pale-coloured Pampean mud; and the fourth is of the same nature, +but darker coloured, including in its lower part horizontal layers and lines of +concretions of not very compact pinkish tosca-rock. The bottom of the sea, I +may remark, to a distance of several miles from the shore, and to a depth of +between sixty and one hundred feet, was found by the anchors to be composed of +tosca-rock and reddish Pampean mud. Professor Ehrenberg has examined for me +specimens of the two lower beds, and finds in them three Polygastrica and six +Phytolitharia. +</p> + +<p> +(The following list is given in the “Monatsberichten der konig. Akad. zu +Berlin” April 1845:— POLYGASTRICA. Fragilaria rhabdosoma. +Gallionella distans. Pinnularia? +</p> + +<p> +PHYTOLITHARIA. Lithodontium Bursa. Lithodontium furcatum. Lithostylidium +exesum. Lithostylidium rude. Lithostylidium Serra. Spongolithis Fustis?) +</p> + +<p> +Of these, only one (Spongolithis Fustis?) is a marine form; five of them are +identical with microscopical structures of brackish-water origin, hereafter to +be mentioned, which form a central point in the Pampean formation. In these two +beds, especially in the lower one, bones of extinct mammifers, some embedded in +their proper relative positions and others single, are very numerous in a small +extent of the cliffs. These remains consist of, first, the head of Ctenomys +antiquus, allied to the living Ctenomys Braziliensis; secondly, a fragment of +the remains of a rodent; thirdly, molar teeth and other bones of a large +rodent, closely allied to, but distinct from, the existing species of +Hydrochoerus, and therefore probably an inhabitant of fresh water; fourth and +fifthly, portions of vertebrae, limbs, ribs, and other bones of two rodents; +sixthly, bones of the extremities of some great megatheroid quadruped. (See +“Fossil Mammalia” page 109 by Professor Owen, in the “Zoology +of the Voyage of the ‘Beagle’;” and Catalogue page 36 of +Fossil Remains in Museum of Royal College of Surgeons.) The number of the +remains of rodents gives to this collection a peculiar character, compared with +those found in any other locality. All these bones are compact and heavy; many +of them are stained red, with their surfaces polished; some of the smaller ones +are as black as jet. +</p> + +<p> +Monte Hermoso is between fifty and sixty miles distant in a S.E. line from the +Ventana, with the intermediate country gently rising towards it, and all +consisting of the Pampean formation. What relation, then, do these beds, at the +level of the sea and under it, bear to those on the flanks of the Ventana, at +the height of 840 feet, and on the flanks of the other neighbouring sierras, +which, from the reasons already assigned, do not appear to owe their greater +height to unequal elevation? When the tosca- rock was accumulating round the +Ventana, and when, with the exception of a few small rugged primary islands, +the whole wide surrounding plains must have been under water, were the strata +at Monte Hermoso depositing at the bottom of a great open sea, between eight +hundred and one thousand feet in depth? I much doubt this; for if so, the +almost perfect carcasses of the several small rodents, the remains of which are +so very numerous in so limited a space, must have been drifted to this spot +from the distance of many hundred miles. It appears to me far more probable, +that during the Pampean period this whole area had commenced slowly rising (and +in the cliffs, at several different heights we have proofs of the land having +been exposed to sea-action at several levels), and that tracts of land had thus +been formed of Pampean sediment round the Ventana and the other primary ranges, +on which the several rodents and other quadrupeds lived, and that a stream (in +which perhaps the extinct aquatic Hydrochoerus lived) drifted their bodies into +the adjoining sea, into which the Pampean mud continued to be poured from the +north. As the land continued to rise, it appears that this source of sediment +was cut off; and in its place sand and pebbles were borne down by stronger +currents, and conformably deposited over the Pampean strata. +</p> + +<p> +(FIGURE 15. SECTION OF BEDS WITH RECENT SHELLS AND EXTINCT MAMMIFERS, AT PUNTA +ALTA IN BAHIA BLANCA. (Showing beds from bottom to top: A, B, C, D.)) +</p> + +<p> +Punta Alta is situated about thirty miles higher up on the northern side of +this same bay: it consists of a small plain, between twenty and thirty feet in +height, cut off on the shore by a line of low cliffs about a mile in length, +represented in Figure 15 with its vertical scale necessarily exaggerated. The +lower bed (A) is more extensive than the upper ones; it consists of stratified +gravel or conglomerate, cemented by calcareo- arenaceous matter, and is divided +by curvilinear layers of pinkish marl, of which some are precisely like +tosca-rock, and some more sandy. The beds are curvilinear, owing to the action +of currents, and dip in different directions; they include an extraordinary +number of bones of gigantic mammifers and many shells. The pebbles are of +considerable size, and are of hard sandstone, and of quartz, like that of the +Ventana: there are also a few well-rounded masses of tosca-rock. +</p> + +<p> +The second bed B is about fifteen feet in thickness, but towards both +extremities of the cliff (not included in the diagram) it either thins out and +dies away, or passes insensibly into an overlying bed of gravel. It consists of +red, tough clayey mud, with minute linear cavities; it is marked with faint +horizontal shades of colour; it includes a few pebbles, and rarely a minute +particle of shell: in one spot, the dermal armour and a few bones of a Dasypoid +quadruped were embedded in it: it fills up furrows in the underlying gravel. +With the exception of the few pebbles and particles of shells, this bed +resembles the true Pampean mud; but it still more closely resembles the clayey +flats (mentioned in the First Chapter) separating the successively rising +parallel ranges of sand-dunes. +</p> + +<p> +The bed C is of stratified gravel, like the lowest one; it fills up furrows in +the underlying red mud, and is sometimes interstratified with it, and sometimes +insensibly passes into it; as the red mud thins out, this upper gravel +thickens. Shells are more numerous in it than in the lower gravel; but the +bones, though some are still present, are less numerous. In one part, however, +where this gravel and the red mud passed into each other, I found several bones +and a tolerably perfect head of the Megatherium. Some of the large Volutas, +though embedded in the gravel-bed C, were filled with the red mud, including +great numbers of the little recent Paludestrina australis. These three lower +beds are covered by an unconformable mantle D of stratified sandy earth, +including many pebbles of quartz, pumice and phonolite, land and sea-shells. +</p> + +<p> +M. d’Orbigny has been so obliging as to name for me the twenty species of +Mollusca embedded in the two gravel beds: they consist of:— +</p> + +<p> +1. Volutella angulata, d’Orbigny, “Voyage” Mollusq. and Pal. +2. Voluta Braziliana, Sol 3. Olicancilleria Braziliensis d’Orbigny. 4. +Olicancilleria auricularia, d’Orbigny. 5. Olivina puelchana, +d’Orbigny. 6. Buccinanops cochlidium, d’Orbigny. 7. Buccinanops +globulosum, d’Orbigny. 8. Colombella sertulariarum, d’Orbigny. 9. +Trochus Patagonicus, and var. of ditto, d’Orbigny. 10. Paludestrina +Australis, d’Orbigny. 11. Fissurella Patagonica, d’Orbigny. 12. +Crepidula muricata, Lam. 13. Venus purpurata, Lam. 14. Venus rostrata, +Phillippi. 15. Mytilus Darwinianus, d’Orbigny. 16. Nucula semiornata, +d’Orbigny. 17. Cardita Patagonica, d’Orbigny. 18. Corbula +Patagonica (?), d’Orbigny. 19. Pecten tethuelchus, d’Orbigny. 20. +Ostrea puelchana, d’Orbigny. 21. A living species of Balanus. 22 and 23. +An Astrae and encrusting Flustra, apparently identical with species now living +in the bay. +</p> + +<p> +All these shells now live on this coast, and most of them in this same bay. I +was also struck with the fact, that the proportional numbers of the different +kinds appeared to be the same with those now cast up on the beach: in both +cases specimens of Voluta, Crepidula, Venus, and Trochus are the most abundant. +Four or five of the species are the same with the upraised shells on the Pampas +near Buenos Ayres. All the specimens have a very ancient and bleached +appearance, and do not emit, when heated, an animal odour: some of them are +changed throughout into a white, soft, fibrous substance; others have the space +between the external walls, either hollow, or filled up with crystalline +carbonate of lime. (A Bulinus, mentioned in the Introduction to the +“Fossil Mammalia” in the “Zoology of the Voyage of the +‘Beagle’” has so much fresher an appearance, than the marine +species, that I suspect it must have fallen amongst the others, and been +collected by mistake.) +</p> + +<p> +The remains of the extinct mammiferous animals, from the two gravel beds have +been described by Professor Owen in the “Zoology of the Voyage of the +‘Beagle’:” they consist of, 1st, one nearly perfect head and +three fragments of heads of the Megatherium Cuvierii; 2nd, a lower jaw of +Megalonyx Jeffersonii; 3rd, lower jaw of Mylodon Darwinii; 4th, fragments of a +head of some gigantic Edental quadruped; 5th, an almost entire skeleton of the +great Scelidotherium leptocephalum, with most of the bones, including the head, +vertebrae, ribs, some of the extremities to the claw- bone, and even, as +remarked by Professor Owen, the knee-cap, all nearly in their proper relative +positions; 6th, fragments of the jaw and a separate tooth of a Toxodon, +belonging either to T. Platensis, or to a second species lately discovered near +Buenos Ayres; 7th, a tooth of Equus curvidens; 8th, tooth of a Pachyderm, +closely allied to Palaeotherium, of which parts of the head have been lately +sent from Buenos Ayres to the British Museum; in all probability this pachyderm +is identical with the Macrauchenia Patagonica from Port S. Julian, hereafter to +be referred to. Lastly, and 9thly, in a cliff of the red clayey bed B, there +was a double piece, about three feet long and two wide, of the bony armour of a +large Dasypoid quadruped, with the two sides pressed nearly close together: as +the cliff is now rapidly washing away, this fossil probably was lately much +more perfect; from between its doubled-up sides, I extracted the middle and +ungual phalanges, united together, of one of the feet, and likewise a separate +phalanx: hence one or more of the limbs must have been attached to the dermal +case, when it was embedded. Besides these several remains in a distinguishable +condition, there were very many single bones: the greater number were embedded +in a space 200 yards square. The preponderance of the Edental quadrupeds is +remarkable; as is, in contrast with the beds of Monte Hermoso, the absence of +Rodents. Most of the bones are now in a soft and friable condition, and, like +the shells, do not emit when burnt an animal odour. The decayed state of the +bones may be partly owing to their late exposure to the air and tidal-waves. +Barnacles, Serpulae, and corallines are attached to many of the bones, but I +neglected to observe whether these might not have grown on them since being +exposed to the present tidal action (After having packed up my specimens at +Bahia Blanca, this point occurred to me, and I noted it; but forgot it on my +return, until the remains had been cleaned and oiled: my attention has been +lately called to the subject by some remarks by M. d’Orbigny.); but I +believe that some of the barnacles must have grown on the Scelidotherium, soon +after being deposited, and before being WHOLLY covered up by the gravel. +Besides the remains in the condition here described, I found one single +fragment of bone very much rolled, and as black as jet, so as perfectly to +resemble some of the remains from Monte Hermoso. +</p> + +<p> +Very many of the bones had been broken, abraded, and rolled, before being +embedded. Others, even some of those included in the coarsest parts of the the +now hard conglomerate, still retain all their minutest prominences perfectly +preserved; so that I conclude that they probably were protected by skin, flesh, +or ligaments, whilst being covered up. In the case of the Scelidotherium, it is +quite certain that the whole skeleton was held together by its ligaments, when +deposited in the gravel in which I found it. Some cervical vertebrae and a +humerus of corresponding size lay so close together, as did some ribs and the +bones of a leg, that I thought that they must originally have belonged to two +skeletons, and not have been washed in single; but as remains were here very +numerous, I will not lay much stress on these two cases. We have just seen that +the armour of the Dasypoid quadruped was certainly embedded together with some +of the bones of the feet. +</p> + +<p> +Professor Ehrenberg has examined for me specimens of the finer matter from in +contact with these mammiferous remains: he finds in them two Polygastrica, +decidedly marine forms; and six Phytolitharia, of which one is probably marine, +and the others either of fresh-water or terrestrial origin. +(“Monatsberichten der Akad. zu Berlin” April 1845. The list +consists of:— +</p> + +<p> +POLYGASTRICA. Gallionella sulcata. Stauroptera aspera? fragm. +</p> + +<p> +PHYTOLITHARIA. Lithasteriscus tuberculatus. Lithostylidium Clepsammidium. +Lithostylidium quadratum. Lithostylidium rude. Lithostylidium unidentatum. +Spongolithis acicularis.) +</p> + +<p> +Only one of these eight microscopical bodies is common to the nine from Monte +Hermoso: but five of them are in common with those from the Pampean mud on the +banks of the Parana. The presence of any fresh-water infusoria, considering the +aridity of the surrounding country, is here remarkable: the most probable +explanation appears to be, that these microscopical organisms were washed out +of the adjoining great Pampean formation during its denudation, and afterwards +redeposited. +</p> + +<p> +We will now see what conclusions may be drawn from the facts above detailed. It +is certain that the gravel-beds and intermediate red mud were deposited within +the period, when existing species of Mollusca held to each other nearly the +same relative proportions as they do on the present coast. These beds, from the +number of littoral species, must have been accumulated in shallow water; but +not, judging from the stratification of the gravel and the layers of marl, on a +beach. From the manner in which the red clay fills up furrows in the underlying +gravel, and is in some parts itself furrowed by the overlying gravel, whilst in +other parts it either insensibly passes into, or alternates with, this upper +gravel, we may infer several local changes in the currents, perhaps caused by +slight changes, up or down, in the level of the land. By the elevation of these +beds, to which period the alluvial mantle with pumice-pebbles, land and +sea-shells belongs, the plain of Punta Alta, from twenty to thirty feet in +height, was formed. In this neighbourhood there are other and higher sea-formed +plains and lines of cliffs in the Pampean formation worn by the denuding action +of the waves at different levels. Hence we can easily understand the presence +of rounded masses of tosca-rock in this lowest plain; and likewise, as the +cliffs at Monte Hermoso with their mammiferous remains stand at a higher level, +the presence of the one much-rolled fragment of bone which was as black as jet: +possibly some few of the other much-rolled bones may have been similarly +derived, though I saw only the one fragment, in the same condition with those +from Monte Hermoso. M. d’Orbigny has suggested that all these mammiferous +remains may have been washed out of the Pampean formation, and afterwards +redeposited together with the recent shells. (“Voyage” Part. +Geolog. page 49.) Undoubtedly it is a marvellous fact that these numerous +gigantic quadrupeds, belonging, with the exception of the Equus curvidens, to +seven extinct genera, and one, namely, the Toxodon, not falling into any +existing family, should have co-existed with Mollusca, all of which are still +living species; but analogous facts have been observed in North America and in +Europe. In the first place, it should not be overlooked, that most of the +co-embedded shells have a more ancient and altered appearance than the bones. +In the second place, is it probable that numerous bones not hardened by silex +or any other mineral, could have retained their delicate prominences and +surfaces perfect if they had been washed out of one deposit, and re-embedded in +another:—this later deposit being formed of large, hard pebbles, arranged +by the action of currents or breakers in shallow water into variously curved +and inclined layers? The bones which are now in so perfect a state of +preservation, must, I conceive, have been fresh and sound when embedded, and +probably were protected by skin, flesh, or ligaments. The skeleton of the +Scelidotherium indisputably was deposited entire: shall we say that when held +together by its matrix it was washed out of an old gravel-bed (totally unlike +in character to the Pampean formation), and re-embedded in another gravel-bed, +composed (I speak after careful comparison) of exactly the same kind of +pebbles, in the same kind of cement? I will lay no stress on the two cases of +several ribs and bones of the extremities having APPARENTLY been embedded in +their proper relative position: but will any one be so bold as to affirm that +it is possible, that a piece of the thin tessellated armour of a Dasypoid +quadruped, at least three feet long and two in width, and now so tender that I +was unable with the utmost care to extract a fragment more than two or three +inches square, could have been washed out of one bed, and re-embedded in +another, together with some of the small bones of the feet, without having been +dashed into atoms? We must then wholly reject M. d’Orbigny’s +supposition, and admit as certain, that the Scelidotherium and the large +Dasypoid quadruped, and as highly probable, that the Toxodon, Megatherium, +etc., some of the bones of which are perfectly preserved, were embedded for the +first time, and in a fresh condition, in the strata in which they were found +entombed. These gigantic quadrupeds, therefore, though belonging to extinct +genera and families, coexisted with the twenty above-enumerated Mollusca, the +barnacle and two corals, still living on this coast. From the rolled fragment +of black bone, and from the plain of Punta Alta being lower than that of Monte +Hermoso, I conclude that the coarse sub-littoral deposits of Punta Alta, are of +subsequent origin to the Pampean mud of Monte Hermoso; and the beds at this +latter place, as we have seen, are probably of subsequent origin to the high +tosca-plain round the Sierra Ventana: we shall, however, return, at the end of +this chapter, to the consideration of these several stages in the great Pampean +formation. +</p> + +<h3>BUENOS AYRES TO ST. FE BAJADA, IN ENTRE RIOS.</h3> + +<p> +For some distance northward of Buenos Ayres, the escarpment of the Pampean +formation does not approach very near to the Plata, and it is concealed by +vegetation: but in sections on the banks of the Rios Luxan, Areco, and +Arrecifes, I observed both pale and dark reddish Pampean mud, with small, +whitish concretions of tosca; at all these places mammiferous remains have been +found. In the cliffs on the Parana, at San Nicolas, the Pampean mud contains +but little tosca; here M. d’Orbigny found the remains of two rodents +(Ctenomys Bonariensis and Kerodon antiquus) and the jaw of a Canis: when on the +river I could clearly distinguish in this fine line of cliffs, +“horizontal lines of variation both in tint and compactness.” (I +quote these words from my note-book, as written down on the spot, on account of +the general absence of stratification in the Pampean formation having been +insisted on by M. d’Orbigny as a proof of the diluvial origin of this +great deposit.) The plain northward of this point is very level, but with some +depressions and lakes; I estimated its height at from forty to sixty feet above +the Parana. At the A. Medio the bright red Pampean mud contains scarcely any +tosca-rock; whilst at a short distance the stream of the Pabon, forms a +cascade, about twenty feet in height, over a cavernous mass of two varieties of +tosca-rock; of which one is very compact and semi- crystalline, with seams of +crystallised carbonate of lime: similar compact varieties are met with on the +Salidillo and Seco. The absolute identity (I speak after a comparison of my +specimens) between some of these varieties, and those from Tapalguen, and from +the ridge south of Bahia Blanca, a distance of 400 miles of latitude, is very +striking. +</p> + +<p> +At Rosario there is but little tosca-rock: near this place I first noticed at +the edge of the river traces of an underlying formation, which, twenty- five +miles higher up in the estancia of Gorodona, consists of a pale yellowish clay, +abounding with concretionary cylinders of a ferruginous sandstone. This bed, +which is probably the equivalent of the older tertiary marine strata, +immediately to be described in Entre Rios, only just rises above the level of +the Parana when low. The rest of the cliff at Gorodona, is formed of red +Pampean mud, with, in the lower part, many concretions of tosca, some +stalacti-formed, and with only a few in the upper part: at the height of six +feet above the river, two gigantic skeletons of the Mastodon Andium were here +embedded; their bones were scattered a few feet apart, but many of them still +held their proper relative positions: they were much decayed and as soft as +cheese, so that even one of the great molar teeth fell into pieces in my hand. +We here see that the Pampean deposit contains mammiferous remains close to its +base. On the banks of the Carcarana, a few miles distant, the lowest bed +visible was pale Pampean mud, with masses of tosca-rock, in one of which I +found a much decayed tooth of the Mastodon: above this bed, there was a thin +layer almost composed of small concretions of white tosca, out of which I +extracted a well preserved, but slightly broken tooth of Toxodon Platensis: +above this there was an unusual bed of very soft impure sandstone. In this +neighbourhood I noticed many single embedded bones, and I heard of others +having been found in so perfect a state that they were long used as gate-posts: +the Jesuit Falkner found here the dermal armour of some gigantic Edental +quadruped. +</p> + +<p> +In some of the red mud scraped from a tooth of one of the Mastodons at +Gorodona, Professor Ehrenberg finds seven Polygastrica and thirteen +Phytolitharia, all of them, I believe, with two exceptions, already known +species. (“Monatsberichten der konig. Akad. zu Berlin” April 1845. +The list consists of:— +</p> + +<p> +POLYGASTRICA. Campylodiscus clypeus. Coscinodiscus subtilis. Coscinodiscus al. +sp. Eunotia. Gallionella granulata. Himantidium gracile. Pinnularia borealis.) +</p> + +<p> +Of these twenty, the preponderating number are of fresh-water origin; only two +species of Coscinodiscus and a Spongolithis show the direct influence of the +sea; therefore Professor Ehrenberg arrives at the important conclusion that the +deposit must have been of brackish-water origin. Of the thirteen Phytolitharia, +nine are met with in the two deposits in Bahia Blanca, where there is evidence +from two other species of Polygastrica that the beds were accumulated in +brackish water. The traces of coral, sponges, and Polythalamia, found by Dr. +Carpenter in the tosca-rock (of which I must observe the greater number of +specimens were from the upper beds in the southern parts of the formation), +apparently show a more purely marine origin. +</p> + +<p> +At ST. FE BAJADA, in Entre Rios, the cliffs, estimated at between sixty and +seventy feet in height, expose an interesting section: the lower half consists +of tertiary strata with marine shells, and the upper half of the Pampean +formation. The lowest bed is an obliquely laminated, blackish, indurated mud, +with distinct traces of vegetable remains. (M. d’Orbigny +“Voyage” Part. Geolog. page 37, has given a detailed description of +this section, but as he does not mention this lowest bed, it may have been +concealed when he was there by the river. There is a considerable discrepancy +between his description and mine, which I can only account for by the beds +themselves varying considerably in short distances.) Above this there is a +thick bed of yellowish sandy clay, with much crystallised gypsum and many +shells of Ostreae, Pectens, and Arcae: above this there generally comes an +arenaceous crystalline limestone, but there is sometimes interposed a bed, +about twelve feet thick, of dark green, soapy clay, weathering into small +angular fragments. The limestone, where purest, is white, highly crystalline, +and full of cavities: it includes small pebbles of quartz, broken shells, teeth +of sharks, and sometimes, as I was informed, large bones: it often contains so +much sand as to pass into a calcareous sandstone, and in such parts the great +Ostrea Patagonica chiefly abounds. (Captain Sulivan, R.N., has given me a +specimen of this shell, which he found in the cliffs at Point Cerrito, between +twenty and thirty miles above the Bajada.) In the upper part, the limestone +alternates with layers of fine white sand. The shells included in these beds +have been named for me by M. d’Orbigny: they consist of:— +</p> + +<p> +1. Ostrea Patagonica, d’Orbigny, “Voyage” Part. Pal. 2. +Ostrea Alvarezii, d’Orbigny, “Voyage” Part. Pal. 3. Pecten +Paranensis, d’Orbigny, “Voyage” Part. Pal. 4. Pecten +Darwinianus, d’Orbigny, “Voyage” Part. Pal. 5. Venus +Munsterii, d’Orbigny, “Voyage” Pal. 6. Arca Bonplandiana, +d’Orbigny, “Voyage” Pal. 7. Cardium Platense, +d’Orbigny, “Voyage” Pal. 8. Tellina, probably nov. species, +but too imperfect for description. +</p> + +<h3>PHYTOLITHARIA.</h3> + +<p> +Lithasteriscus tuberculatus. Lithodontium bursa. Lithodontium furcatum. +Lithodontium rostratum. Lithostylidium Amphiodon. Lithostylidium Clepsammidium. +Lithostylidium Hamus. Lithostylidium polyedrum. Lithostylidium quadratum. +Lithostylidium rude. Lithostylidium Serra. Lithostylidium unidentatum. +Spongolithis Fustis. +</p> + +<p> +These species are all extinct: the six first were found by M. d’Orbigny +and myself in the formations of the Rio Negro, S. Josef, and other parts of +Patagonia; and therefore, as first observed by M. d’Orbigny, these beds +certainly belong to the great Patagonian formation, which will be described in +the ensuing chapter, and which we shall see must be considered as a very +ancient tertiary one. North of the Bajada, M. d’Orbigny found, in beds +which he considers as lying beneath the strata here described, remains of a +Toxodon, which he has named as a distinct species from the T. Platensis of the +Pampean formation. Much silicified wood is found on the banks of the Parana +(and likewise on the Uruguay), and I was informed that they come out of these +lower beds; four specimens collected by myself are dicotyledonous. +</p> + +<p> +The upper half of the cliff, to a thickness of about thirty feet, consists of +Pampean mud, of which the lower part is pale-coloured, and the upper part of a +brighter red, with some irregular layers of an arenaceous variety of tosca, and +a few small concretions of the ordinary kind. Close above the marine limestone, +there is a thin stratum with a concretionary outline of white hard tosca-rock +or marl, which may be considered either as the uppermost bed of the inferior +deposits, or the lowest of the Pampean formation; at one time I considered this +bed as marking a passage between the two formations: but I have since become +convinced that I was deceived on this point. In the section on the Parana, I +did not find any mammiferous remains; but at two miles distance on the A. Tapas +(a tributary of the Conchitas), they were extremely numerous in a low cliff of +red Pampean mud with small concretions, precisely like the upper bed on the +Parana. Most of the bones were solitary and much decayed; but I saw the dermal +armour of a gigantic Edental quadruped, forming a caldron-like hollow, four or +five feet in diameter, out of which, as I was informed, the almost entire +skeleton had been lately removed. I found single teeth of the Mastodon Andium, +Toxodon Platensis, and Equus curvidens, near to each other. As this latter +tooth approaches closely to that of the common horse, I paid particular +attention to its true embedment, for I did not at that time know that there was +a similar tooth hidden in the matrix with the other mammiferous remains from +Punta Alta. It is an interesting circumstance, that Professor Owen finds that +the teeth of this horse approach more closely in their peculiar curvature to a +fossil specimen brought by Mr. Lyell from North America, than to those of any +other species of Equus. (Lyell “Travels in North America” volume 1 +page 164 and “Proceedings of Geological Society” volume 4 page 39.) +</p> + +<p> +The underlying marine tertiary strata extend over a wide area: I was assured +that they can be traced in ravines in an east and west line across Entre Rios +to the Uruguay, a distance of about 135 miles. In a S.E. direction I heard of +their existence at the head of the R. Nankay; and at P. Gorda in Banda +Oriental, a distance of 170 miles, I found the same limestone, containing the +same fossil shells, lying at about the same level above the river as at St. Fe. +In a southerly direction, these beds sink in height, for at another P. Gorda in +Entre Rios, the limestone is seen at a much less height; and there can be +little doubt that the yellowish sandy clay, on a level with the river, between +the Carcarana and S. Nicholas, belongs to this same formation; as perhaps do +the beds of sand at Buenos Ayres, which lie at the bottom of the Pampean +formation, about sixty feet beneath the surface of the Plata. The southerly +declination of these beds may perhaps be due, not to unequal elevation, but to +the original form of the bottom of the sea, sloping from land situated to the +north; for that land existed at no great distance, we have evidence in the +vegetable remains in the lowest bed at St. Fe; and in the silicified wood and +in the bones of Toxodon Paranensis, found (according to M. d’Orbigny) in +still lower strata. +</p> + +<h3>BANDA ORIENTAL.</h3> + +<p> +This province lies on the northern side of the Plata, and eastward of the +Uruguay: it has a gentle undulatory surface, with a basis of primary rocks; and +is in most parts covered up with an unstratified mass, of no great thickness, +of reddish Pampean mud. In the eastern half, near Maldonado, this deposit is +more arenaceous than in the Pampas, it contains many though small concretions +of marl or tosca-rock, and others of highly ferruginous sandstone; in one +section, only a few yards in depth, it rested on stratified sand. Near Monte +Video this deposit in some spots appears to be of greater thickness; and the +remains of the Glyptodon and other extinct mammifers have been found in it. In +the long line of cliffs, between fifty and sixty feet in height, called the +Barrancas de S. Gregorio, which extend westward of the Rio S. Lucia, the lower +half is formed of coarse sand of quartz and feldspar without mica, like that +now cast up on the beach near Maldonado; and the upper half of Pampean mud, +varying in colour and containing honeycombed veins of soft calcareous matter +and small concretions of tosca-rock arranged in lines, and likewise a few +pebbles of quartz. This deposit fills up hollows and furrows in the underlying +sand; appearing as if water charged with mud had invaded a sandy beach. These +cliffs extend far westward, and at a distance of sixty miles, near Colonia del +Sacramiento, I found the Pampean deposit resting in some places on this sand, +and in others on the primary rocks: between the sand and the reddish mud, there +appeared to be interposed, but the section was not a very good one, a thin bed +of shells of an existing Mytilus, still partially retaining their colour. The +Pampean formation in Banda Oriental might readily be mistaken for an alluvial +deposit: compared with that of the Pampas, it is often more sandy, and contains +small fragments of quartz; the concretions are much smaller, and there are no +extensive masses of tosca-rock. +</p> + +<p> +In the extreme western parts of this province, between the Uruguay and a line +drawn from Colonia to the R. Perdido (a tributary of the R. Negro), the +formations are far more complicated. Besides primary rocks, we meet with +extensive tracts and many flat-topped, horizontally stratified, cliff- bounded, +isolated hills of tertiary strata, varying extraordinarily in mineralogical +nature, some identical with the old marine beds of St. Fe Bajada, and some with +those of the much more recent Pampean formation. There are, also, extensive LOW +tracts of country covered with a deposit containing mammiferous remains, +precisely like that just described in the more eastern parts of the province. +Although from the smooth and unbroken state of the country, I never obtained a +section of this latter deposit close to the foot of the higher tertiary hills, +yet I have not the least doubt that it is of quite subsequent origin; having +been deposited after the sea had worn the tertiary strata into the +cliff-bounded hills. This later formation, which is certainly the equivalent of +that of the Pampas, is well seen in the valleys in the estancia of Berquelo, +near Mercedes; it here consists of reddish earth, full of rounded grains of +quartz, and with some small concretions of tosca-rock arranged in horizontal +lines, so as perfectly to resemble, except in containing a little calcareous +matter, the formation in the eastern parts of Banda Oriental, in Entre Rios, +and at other places: in this estancia the skeleton of a great Edental quadruped +was found. In the valley of the Sarandis, at the distance of only a few miles, +this deposit has a somewhat different character, being whiter, softer, +finer-grained, and full of little cavities, and consequently of little specific +gravity; nor does it contain any concretions or calcareous matter: I here +procured a head, which when first discovered must have been quite perfect, of +the Toxodon Platensis, another of a Mylodon (This head was at first considered +by Professor Owen (in the “Zoology of the ‘Beagle’s’ +Voyage”) as belonging to a distinct genus, namely, Glossotherium.), +perhaps M. Darwinii, and a large piece of dermal armour, differing from that of +the Glyptodon clavipes. These bones are remarkable from their extraordinarily +fresh appearance; when held over a lamp of spirits of wine, they give out a +strong odour and burn with a small flame; Mr. T. Reeks has been so kind as to +analyse some of the fragments, and he finds that they contain about 7 per cent +of animal matter, and 8 per cent of water. (Liebig “Chemistry of +Agriculture” page 194 states that fresh dry bones contain from 32 to 33 +per cent of dry gelatine. See also Dr. Daubeny, in “Edinburgh New +Philosophical Journal” volume 37 page 293.) +</p> + +<p> +The older tertiary strata, forming the higher isolated hills and extensive +tracts of country, vary, as I have said, extraordinarily in composition: within +the distance of a few miles, I sometimes passed over crystalline limestone with +agate, calcareous tuffs, and marly rocks, all passing into each +other,—red and pale mud with concretions of tosca-rock, quite like the +Pampean formation,—calcareous conglomerates and sandstones,—bright +red sandstones passing either into red conglomerate, or into white +sandstone,—hard siliceous sandstones, jaspery and chalcedonic rocks, and +numerous other subordinate varieties. I was unable to mark out the relations of +all these strata, and will describe only a few distinct sections:—in the +cliffs between P. Gorda on the Uruguay and the A. de Vivoras, the upper bed is +crystalline cellular limestone often passing into calcareous sandstone, with +impressions of some of the same shells as at St. Fe Bajada; at P. Gorda, this +limestone is interstratified with and rests on, white sand, which covers a bed +about thirty feet thick of pale-coloured clay, with many shells of the great +Ostrea Patagonica (In my “Journal” page 171 1st edition, I have +hastily and inaccurately stated that the Pampean mud, which is found over the +eastern part of B. Oriental, lies OVER the limestone at P. Gorda; I should have +said that there was reason to infer that it was a subsequent or superior +deposit.): beneath this, in the vertical cliff, nearly on a level with the +river, there is a bed of red mud absolutely like the Pampean deposit, with +numerous often large concretions of perfectly characterised white, compact +tosca-rock. At the mouth of the Vivoras, the river flows over a pale cavernous +tosca-rock, quite like that in the Pampas, and this APPEARED to underlie the +crystalline limestone; but the section was not unequivocal like that at P. +Gorda. These beds now form only a narrow and much denuded strip of land; but +they must once have extended much further; for on the next stream, south of the +S. Juan, Captain Sulivan, R.N., found a little cliff, only just above the +surface of the river, with numerous shells of the Venus Munsterii, +D’Orbigny,—one of the species occurring at St. Fe, and of which +there are casts at P. Gorda: the line of cliffs of the subsequently deposited +true Pampean mud, extend from Colonia to within half a mile of this spot, and +no doubt once covered up this denuded marine stratum. Again at Colonia, a +Frenchman found, in digging the foundations of a house, a great mass of the +Ostrea Patagonica (of which I saw many fragments), packed together just beneath +the surface, and directly superimposed on the gneiss. These sections are +important: M. d’Orbigny is unwilling to believe that beds of the same +nature with the Pampean formation ever underlie the ancient marine tertiary +strata; and I was as much surprised at it as he could have been; but the +vertical cliff at P. Gorda allowed of no mistake, and I must be permitted to +affirm, that after having examined the country from the Colorado to St. Fe +Bajada, I could not be deceived in the mineralogical character of the Pampean +deposit. +</p> + +<p> +Moreover, in a precipitous part of the ravine of Las Bocas, a red sandstone is +distinctly seen to overlie a thick bed of pale mud, also quite like the Pampean +formation, abounding with concretions of true tosca-rock. This sandstone +extends over many miles of country: it is as red as the brightest volcanic +scoriae; it sometimes passes into a coarse red conglomerate composed of the +underlying primary rocks; and often passes into a soft white sandstone with red +streaks. At the Calera de los Huerfanos, only a quarter of a mile south of +where I first met with the red sandstone, the crystalline white limestone is +quarried: as this bed is the uppermost, and as it often passes into calcareous +sandstone, interstratified with pure sand; and as the red sandstone likewise +passes into soft white sandstone, and is also the uppermost bed, I believe that +these two beds, though so different, are equivalents. A few leagues southward +of these two places, on each side of the low primary range of S. Juan, there +are some flat-topped, cliff-bounded, separate little hills, very similar to +those fringing the primary ranges in the great plain south of Buenos Ayres: +they are composed—1st, of calcareous tuff with many particles of quartz, +sometimes passing into a coarse conglomerate; 2nd, of a stone undistinguishable +on the closest inspection from the compacter varieties of tosca-rock; and 3rd, +of semi-crystalline limestone, including nodules of agate: these three +varieties pass insensibly into each other, and as they form the uppermost +stratum in this district, I believe that they, also, are the equivalents of the +pure crystalline limestone, and of the red and white sandstones and +conglomerates. +</p> + +<p> +Between these points and Mercedes on the Rio Negro, there are scarcely any good +sections, the road passing over limestone, tosca-rock, calcareous and bright +red sandstones, and near the source of the San Salvador over a wide extent of +jaspery rocks, with much milky agate, like that in the limestone near San Juan. +In the estancia of Berquelo, the separate, flat-topped, cliff-bounded hills are +rather higher than in the other parts of the country; they range in a N.E. and +S.W. direction; their uppermost beds consist of the same bright red sandstone, +passing sometimes into a conglomerate, and in the lower part into soft white +sandstone, and even into loose sand: beneath this sandstone, I saw in two +places layers of calcareous and marly rocks, and in one place red Pampean-like +earth; at the base of these sections, there was a hard, stratified, white +sandstone, with chalcedonic layers. Near Mercedes, beds of the same nature and +apparently of the same age, are associated with compact, white, crystalline +limestone, including much botryoidal agate, and singular masses, like +porcelain, but really composed of a calcareo-siliceous paste. In sinking wells +in this district the chalcedonic strata seem to be the lowest. Beds, such as +there described, occur over the whole of this neighbourhood; but twenty miles +further up the R. Negro, in the cliffs of Perika, which are about fifty feet in +height, the upper bed is a prettily variegated chalcedony, mingled with a pure +white tallowy limestone; beneath this there is a conglomerate of quartz and +granite; beneath this many sandstones, some highly calcareous; and the whole +lower two-thirds of the cliff consists of earthy calcareous beds of various +degrees of purity, with one layer of reddish Pampean-like mud. +</p> + +<p> +When examining the agates, the chalcedonic and jaspery rocks, some of the +limestones, and even the bright red sandstones, I was forcibly struck with +their resemblance to deposits formed in the neighbourhood of volcanic action. I +now find that M. Isabelle, in his “Voyage a Buenos Ayres,” has +described closely similar beds on Itaquy and Ibicuy (which enter the Uruguay +some way north of the R. Negro) and these beds include fragments of red +decomposed true scoriae hardened by zeolite, and of black retinite: we have +then here good evidence of volcanic action during our tertiary period. Still +further north, near S. Anna, where the Parana makes a remarkable bend, M. +Bonpland found some singular amygdaloidal rocks, which perhaps may belong to +this same epoch. (M. d’Orbigny “Voyage” Part. Geolog. page +29) I may remark that, judging from the size and well-rounded condition of the +blocks of rock in the above-described conglomerates, masses of primary +formation probably existed at this tertiary period above water: there is, also, +according to M. Isabelle, much conglomerate further north, at Salto. +</p> + +<p> +From whatever source and through whatever means the great Pampean formation +originated, we here have, I must repeat, unequivocal evidence of a similar +action at a period before that of the deposition of the marine tertiary strata +with extinct shells, at Santa Fe and P. Gorda. During also the deposition of +these strata, we have in the intercalated layers of red Pampean-like mud and +tosca-rock, and in the passage near S. Juan of the semi-crystalline limestones +with agate into tosca undistinguishable from that of the Pampas, evidence of +the same action, though continued only at intervals and in a feeble manner. We +have further seen that in this district, at a period not only subsequent to the +deposition of the tertiary strata, but to their upheavement and most extensive +denudation, true Pampean mud with its usual characters and including +mammiferous remains, was deposited round and between the hills or islets formed +of these tertiary strata, and over the whole eastern and low primary districts +of Banda Oriental. +</p> + +<p> +EARTHY MASS, WITH EXTINCT MAMMIFEROUS REMAINS, OVER THE PORPHYRITIC GRAVEL AT +S. JULIAN, LATITUDE 49 DEGREES 14′ S., IN PATAGONIA. +</p> + +<h3>(FIGURE 16. SECTION OF THE LOWEST PLAIN AT PORT S. JULIAN.</h3> + +<p> +(Section through beds from top to bottom: A, B, C, D, E, F.) +</p> + +<p> +AA. Superficial bed of reddish earth, with the remains of the Macrauchenia, and +with recent sea-shells on the surface. +</p> + +<p> +B. Gravel of porphyritic rocks. +</p> + +<p> +C. and D. Pumiceous mudstone.—Ancient tertiary formation. +</p> + +<p> +E. and F. Sandstone and argillaceous beds.—Ancient tertiary formation.) +</p> + +<p> +This case, though not coming strictly under the Pampean formation, may be +conveniently given here. On the south side of the harbour, there is a nearly +level plain (mentioned in the First Chapter) about seven miles long, and three +or four miles wide, estimated at ninety feet in height, and bordered by +perpendicular cliffs, of which a section is represented in Figure 16. +</p> + +<p> +The lower old tertiary strata (to be described in the next chapter) are covered +by the usual gravel bed; and this by an irregular earthy, sometimes sandy mass, +seldom more than two or three feet in thickness, except where it fills up +furrows or gullies worn not only through the underlying gravel, but even +through the upper tertiary beds. This earthy mass is of a pale reddish colour, +like the less pure varieties of Pampean mud in Banda Oriental; it includes +small calcareous concretions, like those of tosca- rock but more arenaceous, +and other concretions of a greenish, indurated argillaceous substance: a few +pebbles, also, from the underlying gravel-bed are also included in it, and +these being occasionally arranged in horizontal lines, show that the mass is of +sub-aqueous origin. On the surface and embedded in the superficial parts, there +are numerous shells, partially retaining their colours, of three or four of the +now commonest littoral species. Near the bottom of one deep furrow (represented +in Figure 16), filled up with this earthy deposit, I found a large part of the +skeleton of the Macrauchenia Patachonica—a gigantic and most +extraordinary pachyderm, allied, according to Professor Owen, to the +Palaeotherium, but with affinities to the Ruminants, especially to the American +division of the Camelidae. Several of the vertebrae in a chain, and nearly all +the bones of one of the limbs, even to the smallest bones of the foot, were +embedded in their proper relative positions: hence the skeleton was certainly +united by its flesh or ligaments, when enveloped in the mud. This earthy mass, +with its concretions and mammiferous remains, filling up furrows in the +underlying gravel, certainly presents a very striking resemblance to some of +the sections (for instance, at P. Alta in B. Blanca, or at the Barrancas de S. +Gregorio) in the Pampean formation; but I must believe that this resemblance is +only accidental. I suspect that the mud which at the present day is +accumulating in deep and narrow gullies at the head of the harbour, would, +after elevation, present a very similar appearance. The southernmost part of +the true Pampean formation, namely, on the Colorado, lies 560 miles of latitude +north of this point. (In the succeeding chapter I shall have to refer to a +great deposit of extinct mammiferous remains, lately discovered by Captain +Sulivan, R.N., at a point still further south, namely, at the R. Gallegos; +their age must at present remain doubtful.) +</p> + +<p> +With respect to the age of the Macrauchenia, the shells on the surface prove +that the mass in which the skeleton was enveloped has been elevated above the +sea within the recent period: I did not see any of the shells embedded at a +sufficient depth to assure me (though it be highly probable) that the whole +thickness of the mass was contemporaneous with these INDIVIDUAL SPECIMENS. That +the Macrauchenia lived subsequently to the spreading out of the gravel on this +plain is certain; and that this gravel, at the height of ninety feet, was +spread out long after the existence of recent shells, is scarcely less certain. +For, it was shown in the First Chapter, that this line of coast has been +upheaved with remarkable equability, and that over a vast space both north and +south of S. Julian, recent species of shells are strewed on (or embedded in) +the surface of the 250 feet plain, and of the 350 feet plain up to a height of +400 feet. These wide step-formed plains have been formed by the denuding action +of the coast-waves on the old tertiary strata; and therefore, when the surface +of the 350 feet plain, with the shells on it, first rose above the level of the +sea, the 250 feet plain did not exist, and its formation, as well as the +spreading out of the gravel on its summit, must have taken place subsequently. +So also the denudation and the gravel-covering of the 90 feet plain must have +taken place subsequently to the elevation of the 250 feet plain, on which +recent shells are also strewed. Hence there cannot be any doubt that the +Macrauchenia, which certainly was entombed in a fresh state, and which must +have been alive after the spreading out of the gravel on the 90 feet plain, +existed, not only subsequently to the upraised shells on the surface of the 250 +feet plain, but also to those on the 350 to 400 feet plain: these shells, eight +in number (namely, three species of Mytilus, two of Patella, one Fusus, Voluta, +and Balanus), are undoubtedly recent species, and are the commonest kinds now +living on this coast. At Punta Alta in B. Blanca, I remarked how marvellous it +was, that the Toxodon, a mammifer so unlike to all known genera, should have +co-existed with twenty- three still living marine animals; and now we find that +the Macrauchenia, a quadruped only a little less anomalous than the Toxodon, +also co-existed with eight other still existing Mollusca: it should, moreover, +be borne in mind, that a tooth of a pachydermatous animal was found with the +other remains at Punta Alta, which Professor Owen thinks almost certainly +belonged to the Macrauchenia. +</p> + +<p> +Mr. Lyell has arrived at a highly important conclusion with respect to the age +of the North American extinct mammifers (many of which are closely allied to, +and even identical with, those of the Pampean formation), namely, that they +lived subsequently to the period when erratic boulders were transported by the +agency of floating ice in temperate latitudes. (“Geological +Proceedings” volume 4 page 36.) Now in the valley of the Santa Cruz, only +fifty miles of latitude south of the spot where the Macrauchenia was entombed, +vast numbers of gigantic, angular boulders, which must have been transported +from the Cordillera on icebergs, lie strewed on the plain, at the height of +1,400 feet above the level of the sea. In ascending to this level, several +step-formed plains must be crossed, all of which have necessarily required long +time for their formation; hence the lowest or ninety feet plain, with its +superficial bed containing the remains of the Macrauchenia, must have been +formed very long subsequently to the period when the 1,400 feet plain was +beneath the sea, and boulders were dropped on it from floating masses of ice. +(It must not be inferred from these remarks, that the ice-action ceased in +South America at this comparatively ancient period; for in Tierra del Fuego +boulders were probably transported contemporaneously with, if not subsequently +to, the formation of the ninety feet plain at S. Julian, and at other parts of +the coast of Patagonia.) Mr. Lyell’s conclusion, therefore, is thus far +confirmed in the southern hemisphere; and it is the more important, as one is +naturally tempted to admit so simple an explanation, that it was the ice-period +that caused the extinction of the numerous great mammifers which so lately +swarmed over the two Americas. +</p> + +<h3>A SUMMARY AND CONCLUDING REMARKS ON THE PAMPEAN FORMATION.</h3> + +<p> +One of its most striking features is its great extent; I passed continuously +over it from the Colorado to St. Fe Bajada, a distance of 500 geographical +miles; and M. d’Orbigny traced it for 250 miles further north. In the +latitude of the Plata, I examined this formation at intervals over an east and +west line of 300 miles from Maldonado to the R. Carcarana; and M. +d’Orbigny believes it extends 100 miles further inland: from Mr. +Caldcleugh’s travels, however, I should have thought that it had +extended, south of the Cordovese range, to near Mendoza, and I may add that I +heard of great bones having been found high up the R. Quinto. Hence the area of +the Pampean formation, as remarked by M. d’Orbigny, is probably at least +equal to that of France, and perhaps twice or thrice as great. In a basin, +surrounded by gravel-cliff (at a height of nearly three thousand feet), south +of Mendoza, there is, as described in the Third Chapter, a deposit very like +the Pampean, interstratified with other matter; and again at S. Julian’s, +in Patagonia, 560 miles south of the Colorado, a small irregular bed of a +nearly similar nature contains, as we have just seen, mammiferous remains. In +the provinces of Moxos and Chiquitos (1,000 miles northward of the Pampas), and +in Bolivia, at a height of 4,000 metres, M. d’Orbigny has described +similar deposits, which he believes to have been formed by the same agency +contemporaneously with the Pampean formation. Considering the immense distances +between these several points, and their different heights, it appears to me +infinitely more probable, that this similarity has resulted not from +contemporaneousness of origin, but from the similarity of the rocky framework +of the continent: it is known that in Brazil an immense area consists of +gneissic rocks, and we shall hereafter see, over how great a length the +plutonic rocks of the Cordillera, the overlying purple porphyries, and the +trachytic ejections, are almost identical in nature. +</p> + +<p> +Three theories on the origin of the Pampean formation have been +propounded:—First, that of a great debacle by M. d’Orbigny; this +seems founded chiefly on the absence of stratification, and on the number of +embedded remains of terrestrial quadrupeds. Although the Pampean formation +(like so many argillaceous deposits) is not divided into distinct and separate +strata, yet we have seen that in one good section it was striped with +horizontal zones of colour, and that in several specified places the upper and +lower parts differed, not only considerably in colour, but greatly in +constitution. In the southern part of the Pampas the upper mass (to a certain +extent stratified) generally consists of hard tosca-rock, and the lower part of +red Pampean mud, often itself divided into two or more masses, varying in +colour and in the quantity of included calcareous matter. In Western Banda +Oriental, beds of a similar nature, but of a greater age, conformably underlie +and are intercalated with the regularly stratified tertiary formation. As a +general rule, the marly concretions are arranged in horizontal lines, sometimes +united into irregular strata: surely, if the mud had been tumultuously +deposited in mass, the included calcareous matter would have segregated itself +irregularly, and not into nodules arranged in horizontal lines, one above the +other and often far apart: this arrangement appears to me to prove that mud, +differing slightly in composition, was successively and quietly deposited. On +the theory of a debacle, a prodigious amount of mud, without a single pebble, +is supposed to have been borne over the wide surface of the Pampas, when under +water: on the other hand, over the whole of Patagonia, the same or another +debacle is supposed to have borne nothing but gravel,—the gravel and the +fine mud in the neighbourhood of the Rios Negro and Colorado having been borne +to an equal distance from the Cordillera, or imagined line of disturbance: +assuredly directly opposite effects ought not to be attributed to the same +agency. Where, again, could a mass of fine sediment, charged with calcareous +matter in a fit state for chemical segregation, and in quantity sufficient to +cover an area at least 750 miles long, and 400 miles broad, to a depth of from +twenty to thirty feet to a hundred feet, have been accumulated, ready to be +transported by the supposed debacle? To my mind it is little short of +demonstration, that a great lapse of time was necessary for the production and +deposition of the enormous amount of mudlike matter forming the Pampas; nor +should I have noticed the theory of a debacle, had it not been adduced by a +naturalist so eminent as M. d’Orbigny. +</p> + +<p> +A second theory, first suggested, I believe, by Sir W. Parish, is that the +Pampean formation was thrown down on low and marshy plains by the rivers of +this country before they assumed their present courses. The appearance and +composition of the deposit, the manner in which it slopes up and round the +primary ranges, the nature of the underlying marine beds, the estuary and +sea-shells on the surface, the overlying sandstone beds at M. Hermoso, are all +quite opposed to this view. Nor do I believe that there is a single instance of +a skeleton of one of the extinct mammifers having been found in an upright +position, as if it had been mired. +</p> + +<p> +The third theory, of the truth of which I cannot entertain the smallest doubt, +is that the Pampean formation was slowly accumulated at the mouth of the former +estuary of the Plata and in the sea adjoining it. I have come to this +conclusion from the reasons assigned against the two foregoing theories, and +from simple geographical considerations. From the numerous shells of the Azara +labiata lying loose on the surface of the plains, and near Buenos Ayres +embedded in the tosca-rock, we know that this formation not only was formerly +covered by, but that the uppermost parts were deposited in, the brackish water +of the ancient La Plata. Southward and seaward of Buenos Ayres, the plains were +upheaved from under water inhabited by true marine shells. We further know from +Professor Ehrenberg’s examination of the twenty microscopical organisms +in the mud round the tooth of the Mastodon high up the course of the Parana, +that the bottom- most part of this formation was of brackish-water origin. A +similar conclusion must be extended to the beds of like composition, at the +level of the sea and under it, at M. Hermoso in Bahia Blanca. Dr. Carpenter +finds that the harder varieties of tosca-rock, collected chiefly to the south, +contain marine spongoid bodies, minute fragments of shells, corals, and +Polythalamia; these perhaps may have been drifted inwards by the tides, from +the more open parts of the sea. The absence of shells, throughout this deposit, +with the exception of the uppermost layers near Buenos Ayres, is a remarkable +fact: can it be explained by the brackish condition of the water, or by the +deep mud at the bottom? I have stated that both the reddish mud and the +concretions of tosca-rock are often penetrated by minute, linear cavities, such +as frequently may be observed in fresh-water calcareous deposits:—were +they produced by the burrowing of small worms? Only on this view of the Pampean +formation having been of estuary origin, can the extraordinary numbers +(presently to be alluded to) of the embedded mammiferous remains be explained. +(It is almost superfluous to give the numerous cases (for instance, in Sumatra; +Lyell “Principles” volume 3 page 325 sixth edition, of the +carcasses of animals having been washed out to sea by swollen rivers; but I may +refer to a recent account by Mr. Bettington “Asiatic Society” 1845 +June 21st, of oxen, deer, and bears being carried into the Gulf of Cambray; see +also the account in my “Journal” 2nd edition page 133, of the +numbers of animals drowned in the Plata during the great, often recurrent, +droughts.) +</p> + +<p> +With respect to the first origin of the reddish mud, I will only remark, that +the enormous area of Brazil consists in chief part of gneissic and other +granitic rocks, which have suffered decomposition, and been converted into a +red, gritty, argillaceous mass, to a greater depth than in any other country +which I have seen. The mixture of rounded grains, and even of small fragments +and pebbles of quartz, in the Pampean mud of Banda Oriental, is evidently due +to the neighbouring and underlying primary rocks. The estuary mud was drifted +during the Pampean period in a much more southerly course, owing probably to +the east and west primary ridges south of the Plata not having been then +elevated, than the mud of the Plata at present is; for it was formerly +deposited as far south as the Colorado. The quantity of calcareous matter in +this formation, especially in those large districts where the whole mass passes +into tosca-rock, is very great: I have already remarked on the close +resemblance in external and microscopical appearance, between this tosca-rock +and the strata at Coquimbo, which have certainly resulted from the decay and +attrition of recent shells: I dare not, however, extend this conclusion to the +calcareous rocks of the Pampas, more especially as the underlying tertiary +strata in western Banda Oriental show that at that period there was a copious +emission of carbonate of lime, in connection with volcanic action. (I may add, +that there are nearly similar superficial calcareous beds at King +George’s Sound in Australia; and these undoubtedly have been formed by +the disintegration of marine remains see “Volcanic Islands” etc. +page 144. There is, however, something very remarkable in the frequency of +superficial, thin beds of earthy calcareous matter, in districts where the +surrounding rocks are not calcareous. Major Charters, in a Paper read before +the Geographical Society April 13, 1840 and abstracted in the +“Athenaeum” page 317, states that this is the case in parts of +Mexico, and that he has observed similar appearances in many parts of South +Africa. The circumstance of the uppermost stratum round the ragged Sierra +Ventana, consisting of calcareous or marly matter, without any covering of +alluvial matter, strikes me as very singular, in whatever manner we view the +deposition and elevation of the Pampean formation.) +</p> + +<p> +The Pampean formation, judging from its similar composition, and from the +apparent absolute specific identity of some of its mammiferous remains, and +from the generic resemblance of others, belongs over its vast area— +throughout Banda Oriental, Entre Rios, and the wide extent of the Pampas as far +south as the Colorado,—to the same geological epoch. The mammiferous +remains occur at all depths from the top to the bottom of the deposit; and I +may add that nowhere in the Pampas is there any appearance of much superficial +denudation: some bones which I found near the Guardia del Monte were embedded +close to the surface; and this appears to have been the case with many of those +discovered in Banda Oriental: on the Matanzas, twenty miles south of Buenos +Ayres, a Glyptodon was embedded five feet beneath the surface; numerous remains +were found by S. Muniz, near Luxan, at an average depth of eighteen feet; in +Buenos Ayres a skeleton was disinterred at sixty feet depth, and on the Parana +I have described two skeletons of the Mastodon only five or six feet above the +very base of the deposit. With respect to the age of this formation, as judged +of by the ordinary standard of the existence of Mollusca, the only evidence +within the limits of the true Pampas which is at all trustworthy, is afforded +by the still living Azara labiata being embedded in tosca-rock near Buenos +Ayres. At Punta Alta, however, we have seen that several of the extinct +mammifers, most characteristic of the Pampean formation, co-existed with twenty +species of Mollusca, a barnacle and two corals, all still living on this same +coast;— for when we remember that the shells have a more ancient +appearance than the bones; that many of the bones, though embedded in a coarse +conglomerate, are perfectly preserved; that almost all the parts of the +skeleton of the Scelidotherium, even to the knee-cap, were lying in their +proper relative positions; and that a large piece of the fragile dermal armour +of a Dasypoid quadruped, connected with some of the bones of the foot, had been +entombed in a condition allowing the two sides to be doubled together, it must +assuredly be admitted that these mammiferous remains were embedded in a fresh +state, and therefore that the living animals co-existed with the co-embedded +shells. Moreover, the Macrauchenia Patachonica (of which, according to +Professor Owen, remains also occur in the Pampas of Buenos Ayres, and at Punta +Alta) has been shown by satisfactory evidence of another kind, to have lived on +the plains of Patagonia long after the period when the adjoining sea was first +tenanted by its present commonest molluscous animals. We must, therefore, +conclude that the Pampean formation belongs, in the ordinary geological sense +of the word, to the Recent Period. (M. d’Orbigny believes +“Voyage” Part. Geolog. page 81, that this formation, though +“tres voisine de la notre, est neanmoins de beaucoup anterieure a notre +creation.”) +</p> + +<p> +At St. Fe Bajada, the Pampean estuary formation, with its mammiferous remains, +conformably overlies the marine tertiary strata, which (as first shown by M. +d’Orbigny) are contemporaneous with those of Patagonia, and which, as we +shall hereafter see, belong to a very ancient tertiary stage. When examining +the junction between these two formations, I thought that the concretionary +layer of marl marked a passage between the marine and estuary stages. M. +d’Orbigny disputes this view (as given in my “Journal”), and +I admit that it is erroneous, though in some degree excusable, from their +conformability and from both abounding with calcareous matter. It would, +indeed, have been a great anomaly if there had been a true passage between a +deposit contemporaneous with existing species of mollusca, and one in which all +the mollusca appear to be extinct. Northward of Santa Fe, M. d’Orbigny +met with ferruginous sandstones, marly rocks, and other beds, which he +considers as a distinct and lower formation; but the evidence that they are not +parts of the same with an altered mineralogical character, does not appear to +me quite satisfactory. +</p> + +<p> +In Western Banda Oriental, while the marine tertiary strata were accumulating, +there were volcanic eruptions, much silex and lime were precipitated from +solution, coarse conglomerates were formed, being derived probably from +adjoining land, and layers of red mud and marly rocks, like those of the +Pampean formation, were occasionally deposited. The true Pampean deposit, with +mammiferous remains, instead of as at Santa Fe overlying conformably the +tertiary strata, is here seen at a lower level folding round and between the +flat-topped, cliff-bounded hills, formed by a upheaval and denudation of these +same tertiary strata. The upheaval, having occurred here earlier than at Santa +Fe, may be naturally accounted for by the contemporaneous volcanic action. At +the Barrancas de S. Gregorio, the Pampean deposit, as we have seen, overlies +and fills up furrows in coarse sand, precisely like that now accumulating on +the shores near the mouth of the Plata. I can hardly believe that this loose +and coarse sand is contemporaneous with the old tertiary and often crystalline +strata of the more western parts of the province; and am induced to suspect +that it is of subsequent origin. If that section near Colonia could be +implicitly trusted, in which, at a height of only fifteen feet above the Plata, +a bed of fresh-looking mussels, of an existing littoral species, appeared to +lie between the sand and the Pampean mud, I should conclude that Banda Oriental +must have stood, when the coarse sand was accumulating, at only a little below +its present level, and had then subsided, allowing the estuary Pampean mud to +cover far and wide its surface up to a height of some hundred feet; and that +after this subsidence the province had been uplifted to its present level. +</p> + +<p> +In Western Banda Oriental, we know, from two unequivocal sections that there is +a mass, absolutely undistinguishable from the true Pampean deposit, beneath the +old tertiary strata. This inferior mass must be very much more ancient than the +upper deposit with its mammiferous remains, for it lies beneath the tertiary +strata in which all the shells are extinct. Nevertheless, the lower and upper +masses, as well as some intermediate layers, are so similar in mineralogical +character, that I cannot doubt that they are all of estuary origin, and have +been derived from the same great source. At first it appeared to me extremely +improbable, that mud of the same nature should have been deposited on nearly +the same spot, during an immense lapse of time, namely, from a period +equivalent perhaps to the Eocene of Europe to that of the Pampean formation. +But as, at the very commencement of the Pampean period, if not at a still +earlier period, the Sierra Ventana formed a boundary to the south,—the +Cordillera or the plains in front of them to the west,—the whole province +of Corrientes probably to the north, for, according to M. d’Orbigny, it +is not covered by the Pampean deposit,—and Brazil, as known by the +remains in the caves, to the north-east; and as again, during the older +tertiary period, land already existed in Western Banda Oriental and near St. Fe +Bajada, as may be inferred from the vegetable debris, from the quantities of +silicified wood, and from the remains of a Toxodon found, according to M. +d’Orbigny, in still lower strata, we may conclude, that at this ancient +period a great expanse of water was surrounded by the same rocky framework +which now bounds the plains of Pampean formation. This having been the case, +the circumstance of sediment of the same nature having been deposited in the +same area during an immense lapse of time, though highly remarkable, does not +appear incredible. +</p> + +<p> +The elevation of the Pampas, at least of the southern parts, has been slow and +interrupted by several periods of rest, as may be inferred from the plains, +cliffs, and lines of sand-dunes (with shells and pumice-pebbles) standing at +different heights. I believe, also, that the Pampean mud continued to be +deposited, after parts of this formation had already been elevated, in the same +manner as mud would continue to be deposited in the estuary of the Plata, if +the mud-banks on its shores were now uplifted and changed into plains: I +believe in this from the improbability of so many skeletons and bones having +been accumulated at one spot, where M. Hermoso now stands, at a depth of +between eight hundred and one thousand feet, and at a vast distance from any +land except small rocky islets,—as must have been the case, if the high +tosca-plain round the Ventana and adjoining Sierras, had not been already +uplifted and converted into land, supporting mammiferous animals. At Punta Alta +we have good evidence that the gravel- strata, which certainly belong to the +true Pampean period, were accumulated after the elevation in that neighbourhood +of the main part of the Pampean deposit, whence the rounded masses of +tosca-rock were derived, and that rolled fragment of black bone in the same +peculiar condition with the remains at Monte Hermoso. +</p> + +<p> +The number of the mammiferous remains embedded in the Pampas is, as I have +remarked, wonderful: it should be borne in mind that they have almost +exclusively been found in the cliffs and steep banks of rivers, and that, until +lately, they excited no attention amongst the inhabitants: I am firmly +convinced that a deep trench could not be cut in any line across the Pampas, +without intersecting the remains of some quadruped. It is difficult to form an +opinion in what part of the Pampas they are most numerous; in a limited spot +they could not well have been more numerous than they were at P. Alta; the +number, however, lately found by Senor F. Muniz, near Luxan, in a central spot +in the Pampas, is extraordinarily great: at the end of this chapter I will give +a list of all the localities at which I have heard of remains having been +discovered. Very frequently the remains consist of almost perfect skeletons; +but there are, also, numerous single bones, as for instance at St. Fe. Their +state of preservation varies much, even when embedded near each other: I saw +none others so perfectly preserved as the heads of the Toxodon and Mylodon from +the white soft earthy bed on the Sarandis in Banda Oriental. It is remarkable +that in two limited sections I found no less than five teeth separately +embedded, and I heard of teeth having been similarly found in other parts: may +we suppose that the skeletons or heads were for a long time gently drifted by +currents over the soft muddy bottom, and that the teeth occasionally, here and +there, dropped out? +</p> + +<p> +It may be naturally asked, where did these numerous animals live? From the +remarkable discoveries of MM. Lund and Clausen, it appears that some of the +species found in the Pampas inhabited the highlands of Brazil: the Mastodon +Andium is embedded at great heights in the Cordillera from north of the equator +to at least as far south as Tarija (Humboldt states that the Mastodon has been +discovered in New Granada: it has been found in Quito. When at Lima, I saw a +tooth of a Mastodon in the possession of Don M. Rivero, found at Playa Chica on +the Maranon, near the Guallaga. Every one has heard of the numerous remains of +Mastodon in Bolivia.); and as there is no higher land, there can be little +doubt that this Mastodon must have lived on the plains and valleys of that +great range. These countries, however, appear too far distant for the +habitation of the individuals entombed in the Pampas: we must probably look to +nearer points, for instance to the province of Corrientes, which, as already +remarked, is said not to be covered by the Pampean formation, and may +therefore, at the period of its deposition, have existed as dry land. I have +already given my reasons for believing that the animals embedded at M. Hermoso +and at P. Alta in Bahia Blanca, lived on adjoining land, formed of parts of the +already elevated Pampean deposit. With respect to the food of these many great +extinct quadrupeds, I will not repeat the facts given in my +“Journal” (second edition page 85), showing that there is no +correlation between the luxuriance of the vegetation of a country and the size +of its mammiferous inhabitants. I do not doubt that large animals could now +exist, as far as the amount, not kind, of vegetation is concerned, on the +sterile plains of Bahia Blanca and of the R. Negro, as well as on the equally, +if not more sterile plains of Southern Africa. The climate, however, may +perhaps have somewhat deteriorated since the mammifers embedded at Bahia Blanca +lived there; for we must not infer, from the continued existence of the same +shells on the present coasts, that there has been no change in climate; for +several of these shells now range northward along the shores of Brazil, where +the most luxuriant vegetation flourishes under a tropical temperature. With +respect to the extinction, which at first fills the mind with astonishment, of +the many great and small mammifers of this period, I may also refer to the work +above cited (second edition page 173), in which I have endeavoured to show, +that however unable we may be to explain the precise cause, we ought not +properly to feel more surprised at a species becoming extinct than at one being +rare; and yet we are accustomed to view the rarity of any particular species as +an ordinary event, not requiring any extraordinary agency. +</p> + +<p> +The several mammifers embedded in the Pampean formation, which mostly belong to +extinct genera, and some even to extinct families or orders, and which differ +nearly, if not quite, as much as do the Eocene mammifers of Europe from living +quadrupeds having existed contemporaneously with mollusca, all still inhabiting +the adjoining sea, is certainly a most striking fact. It is, however, far from +being an isolated one; for, during the late tertiary deposits of Britain, an +elephant, rhinoceros, and hippopotamus co-existed with many recent land and +fresh-water shells; and in North America, we have the best evidence that a +mastodon, elephant, megatherium, megalonyx, mylodon, an extinct horse and ox, +likewise co- existed with numerous land, fresh-water, and marine recent shells. +(Many original observations, and a summary on this subject, are given in Mr. +Lyell’s paper in the “Geological Proceedings” volume 4 page 3 +and in his “Travels in North America” volume 1 page 164 and volume +2 page 60. For the European analogous cases see Mr. Lyell’s +“Principles of Geology” 6th edition volume 1 page 37.) The +enumeration of these extinct North American animals naturally leads me to refer +to the former closer relation of the mammiferous inhabitants of the two +Americas, which I have discussed in my “Journal,” and likewise to +the vast extent of country over which some of them ranged: thus the same +species of the Megatherium, Megalonyx, Equus (as far as the state of their +remains permits of identification), extended from the Southern United States of +North America to Bahia Blanca, in latitude 39 degrees S., on the coast of +Patagonia. The fact of these animals having inhabited tropical and temperate +regions, does not appear to me any great difficulty, seeing that at the Cape of +Good Hope several quadrupeds, such as the elephant and hippopotamus, range from +the equator to latitude 35 degrees south. The case of the Mastodon Andium is +one of more difficulty, for it is found from latitude 36 degrees S., over, as I +have reason to believe, nearly the whole of Brazil, and up the Cordillera to +regions which, according to M. d’Orbigny, border on perpetual snow, and +which are almost destitute of vegetation: undoubtedly the climate of the +Cordillera must have been different when the mastodon inhabited it; but we +should not forget the case of the Siberian mammoth and rhinoceros, as showing +how severe a climate the larger pachydermata can endure; nor overlook the fact +of the guanaco ranging at the present day over the hot low deserts of Peru, the +lofty pinnacles of the Cordillera, and the damp forest-clad land of Southern +Tierra del Fuego; the puma, also, is found from the equator to the Strait of +Magellan, and I have seen its footsteps only a little below the limits of +perpetual snow in the Cordillera of Chile. +</p> + +<p> +At the period, so recent in a geological sense, when these extinct mammifers +existed, the two Americas must have swarmed with quadrupeds, many of them of +gigantic size; for, besides those more particularly referred to in this +chapter, we must include in this same period those wonderfully numerous +remains, some few of them specifically, and others generically related to those +of the Pampas, discovered by MM. Lund and Clausen in the caves of Brazil. +Finally, the facts here given show how cautious we ought to be in judging of +the antiquity of a formation from even a great amount of difference between the +extinct and living species in any one class of animals;—we ought even to +be cautious in accepting the general proposition, that change in organic forms +and lapse of time are at all, necessarily, correlatives. +</p> + +<p> +LOCALITIES WITHIN THE REGION OF THE PAMPAS WHERE GREAT BONES HAVE BEEN FOUND. +</p> + +<p> +The following list, which includes every account which I have hitherto met with +of the discovery of fossil mammiferous remains in the Pampas, may be hereafter +useful to a geologist investigating this region, and it tends to show their +extraordinary abundance. I heard of and saw many fossils, the original position +of which I could not ascertain; and I received many statements too vague to be +here inserted. Beginning to the south:—we have the two stations in Bahia +Blanca, described in this chapter, where at P. Alta, the Megatherium, +Megalonyx, Scelidotherium, Mylodon, Holophractus (or an allied genus), Toxodon, +Macrauchenia, and an Equus were collected; and at M. Hermoso a Ctenomys, +Hydrochaerus, some other rodents and the bones of a great megatheroid +quadruped. Close north-east of the S. Tapalguen, we have the Rios +‘Huesos’ (i.e. “bones”), which probably takes its name +from large fossil bones. Near Villa Nuevo, and at Las Averias, not far from the +Salado, three nearly perfect skeletons, one of the Megatherium, one of the +Glyptodon clavipes, and one of some great Dasypoid quadruped, were found by the +agent of Sir W. Parish (see his work “Buenos Ayres” etc. page 171). +I have seen the tooth of a Mastodon from the Salado; a little northward of this +river, on the borders of a lake near the G. del Monte, I saw many bones, and +one large piece of dermal armour; higher up the Salado, there is a place called +Monte “Huesos.” On the Matanzas, about twenty miles south of Buenos +Ayres, the skeleton (vide page 178 of “Buenos Ayres” etc. by Sir W. +Parish) of a Glyptodon was found about five feet beneath the surface; here also +(see Catalogue of Royal College of Surgeons) remains of Glyptodon clavipes, G. +ornatus, and G. reticulatus were found. Signor Angelis, in a letter which I +have seen, refers to some great remains found in Buenos Ayres, at a depth of +twenty varas from the surface. Seven leagues north of this city the same author +found the skeletons of Mylodon robustus and Glyptodon ornatus. From this +neighbourhood he has lately sent to the British Museum the following +fossils:—Remains of three or four individuals of Megatherium; of three +species of Glyptodon; of three individuals of the Mastodon Andium; of +Macrauchenia; of a second species of Toxodon, different from T. Platensis; and +lastly, of the Machairodus, a wonderful large carnivorous animal. M. +d’Orbigny has lately received from the Recolate “Voyage” Pal. +page 144), near Buenos Ayres, a tooth of Toxodon Platensis. +</p> + +<p> +Proceeding northward, along the west bank of the Parana, we come to the Rio +Luxan, where two skeletons of the Megatherium have been found; and lately, +within eight leagues of the town of Luxan, Dr. F. X. Muniz has collected +(“British Packet” Buenos Ayres September 25, 1841), from an average +depth of eighteen feet, very numerous remains, of no less than, as he believes, +nine distinct species of mammifers. At Areco, large bones have been found, +which are believed, by the inhabitants, to have been changed from small bones, +by the water of the river! At Arrecifes, the Glyptodon, sent to the College of +Surgeons, was found; and I have seen two teeth of a Mastodon from this quarter. +At S. Nicolas, M. d’Orbigny found remains of a Canis, Ctenomys, and +Kerodon; and M. Isabelle (“Voyage” page 332) refers to a gigantic +Armadillo found there. At S. Carlos, I heard of great bones. A little below the +mouth of the Carcarana, the two skeletons of Mastodon were found; on the banks +of this river, near S. Miguel, I found teeth of the Mastodon and Toxodon; and +“Falkner” (page 55) describes the osseous armour of some great +animal; I heard of many other bones in this neighbourhood. I have seen, I may +add, in the possession of Mr. Caldcleugh, the tooth of a Mastodon Andium, said +to have been found in Paraguay; I may here also refer to a statement in this +gentleman’s travels (volume 1 page 48), of a great skeleton having been +found in the province of Bolivia in Brazil, on the R. de las Contas. The +furthest point westward in the Pampas, at which I have HEARD of fossil bones, +was high up on the banks of R. Quinto. +</p> + +<p> +In Entre Rios, besides the remains of the Mastodon, Toxodon, Equus, and a great +Dasypoid quadruped near St. Fe Bajada, I received an account of bones having +been found a little S.E. of P. Gorda (on the Parana), and of an entire skeleton +at Matanzas, on the Arroyo del Animal. +</p> + +<p> +In Banda Oriental, besides the remains of the Toxodon, Mylodon, and two +skeletons of great animals with osseous armour (distinct from that of the +Glyptodon), found on the Arroyos Sarandis and Berquelo, M. Isabelle +(“Voyage” page 322) says, many bones have been found near the R. +Negro, and on the R. Arapey, an affluent of the Paraguay, in latitude 30 +degrees 40 minutes south. I heard of bones near the source of the A. Vivoras. I +saw the remains of a Dasypoid quadruped from the Arroyo Seco, close to M. +Video; and M. d’Orbigny refers (“Voyage” Geolog. page 24), to +another found on the Pedernal, an affluent of the St. Lucia; and Signor +Angelis, in a letter, states that a third skeleton of this family has been +found, near Canelones. I saw a tooth of the Mastodon from Talas, another +affluent of the St. Lucia. The most eastern point at which I heard of great +bones having been found, was at Solis Grande, between M. Video and Maldonado. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0005"></a> +CHAPTER V.<br /> +ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.</h2> + +<p class="letter"> +Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with +Infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P. +Gallegos.—Eastern Tierra del Fuego; leaves of extinct +beech-trees.—Summary on the Patagonian tertiary +formations.—Tertiary formations of the Western Coast.—Chonos and +Chiloe groups, volcanic rocks +of.—Concepcion.—Navidad.—Coquimbo.—Summary.—Age +of the tertiary formations.—Lines of elevation.—Silicified +wood.—Comparative ranges of the extinct and living mollusca on the West +Coast of—S. America.—Climate of the tertiary period.—On the +causes of the absence of recent conchiferous deposits on the coast of S. +America.—On the contemporaneous deposition and preservation of +sedimentary formations. +</p> + +<h3>RIO NEGRO.</h3> + +<p> +I can add little to the details given by M. d’Orbigny on the sandstone +formation of this district. (“Voyage” Part Geolog. pages 57-65.) +The cliffs to the south of the river are about two hundred feet in height, and +are composed of sandstone of various tints and degrees of hardness. One layer, +which thinned out at both ends, consisted of earthy matter, of a pale reddish +colour, with some gypsum, and very like (I speak after comparison of the +specimens brought home) Pampean mud: above this was a layer of compact marly +rock with dendritic manganese. Many blocks of a conglomerate of pumice-pebbles +embedded in hard sandstone were strewed at the foot of the cliff, and had +evidently fallen from above. A few miles N.E. of the town, I found, low down in +the sandstone, a bed, a few inches in thickness, of a white, friable, +harsh-feeling sediment, which adheres to the tongue, is of easy fusibility, and +of little specific gravity; examined under the microscope, it is seen to be +pumiceous tuff, formed of broken transparent crystals. In the cliffs south of +the river, there is, also, a thin layer of nearly similar nature, but finer +grained, and not so white; it might easily have been mistaken for a calcareous +tuff, but it contains no lime: this substance precisely resembles a most widely +extended and thick formation in Southern Patagonia, hereafter to be described, +and which is remarkable for being partially formed of infusoria. These beds, +conjointly with the conglomerate of pumice, are interesting, as showing the +nature of the volcanic action in the Cordillera during this old tertiary +period. +</p> + +<p> +In a bed at the base of the southern cliffs, M. d’Orbigny found two +extinct fresh-water shells, namely, a Unio and Chilina. This bed rested on one +with bones of an extinct rodent, namely, the Megamys Patagoniensis; and this +again on another with extinct marine shells. The species found by M. +d’Orbigny in different parts of this formation consist of:— +</p> + +<p> +1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. +Fe, and whole coast of Patagonia). 2. Ostrea Ferrarisi, d’Orbigny, +“Voyage, Pal.” 3. Ostrea Alvarezii, d’Orbigny, “Voyage, +Pal.” (also at St. Fe, and S. Josef). 4. Pecten Patagoniensis, +d’Orbigny, “Voyage, Pal.” 5. Venus Munsterii, +d’Orbigny, “Voyage, Pal.” (also at St. Fe). 6. Arca +Bonplandiana, d’Orbigny, “Voyage, Pal.” (also at St. Fe). +</p> + +<p> +According to M. d’Orbigny, the sandstone extends westward along the coast +as far as Port S. Antonio, and up the R. Negro far into the interior: northward +I traced it to the southern side of the Rio Colorado, where it forms a low +denuded plain. This formation, though contemporaneous with that of the rest of +Patagonia, is quite different in mineralogical composition, being connected +with it only by the one thin white layer: this difference may be reasonably +attributed to the sediment brought down in ancient times by the Rio Negro; by +which agency, also, we can understand the presence of the fresh-water shells, +and of the bones of land animals. Judging from the identity of four of the +above shells, this formation is contemporaneous (as remarked by M. +d’Orbigny) with that under the Pampean deposit in Entre Rios and in Banda +Oriental. The gravel capping the sandstone plain, with its calcareous cement +and nodules of gypsum, is probably, from the reasons given in the First +Chapter, contemporaneous with the uppermost beds of the Pampean formation on +the upper plain north of the Colorado. +</p> + +<h3>SAN JOSEF.</h3> + +<p> +My examination here was very short: the cliffs are about a hundred feet high; +the lower third consists of yellowish-brown, soft, slightly calcareous, muddy +sandstone, parts of which when struck emit a fetid smell. In this bed the great +Ostraea Patagonica, often marked with dendritic manganese and small +coral-lines, were extraordinarily numerous. I found here the following +shells:— +</p> + +<p> +1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. +Fe and whole coast of Patagonia). 2. Ostrea Alvarezii, d’Orbigny, +“Voyage, Pal.” (also at St. Fe and R. Negro). 3. Pecten Paranensis, +d’Orbigny, “Voyage, Pal.” (also at St. Fe, S. Julian, and +Port Desire). 4. Pecten Darwinianus, d’Orbigny, “Voyage, +Pal.” (also at St. Fe). 5. Pecten actinodes, G.B. Sowerby. 6. Terebratula +Patagonica, G.B. Sowerby (also S. Julian). 7. Casts of a Turritella. +</p> + +<p> +The four first of these species occur at St. Fe in Entre Rios, and the two +first in the sandstone of the Rio Negro. Above this fossiliferous mass, there +is a stratum of very fine-grained, pale brown mudstone, including numerous +laminae of selenite. All the strata appear horizontal, but when followed by the +eye for a long distance, they are seen to have a small easterly dip. On the +surface we have the porphyritic gravel, and on it sand with recent shells. +</p> + +<h3>NUEVO GULF.</h3> + +<p> +From specimens and notes given me by Lieutenant Stokes, it appears that the +lower bed consists of soft muddy sandstone, like that of S. Josef, with many +imperfect shells, including the Pecten Paranensis, d’Orbigny, casts of a +Turritella and Scutella. On this there are two strata of the pale brown +mudstone, also like that of S. Josef, separated by a darker-coloured, more +argillaceous variety, including the Ostrea Patagonica. Professor Ehrenberg has +examined this mudstone for me: he finds in it three already known microscopic +organisms, enveloped in a fine-grained pumiceous tuff, which I shall have +immediately to describe in detail. Specimens brought to me from the uppermost +bed, north of the Rio Chupat, consist of this same substance, but of a whiter +colour. +</p> + +<p> +Tertiary strata, such as here described, appear to extend along the whole coast +between Rio Chupat and Port Desire, except where interrupted by the underlying +claystone porphyry, and by some metamorphic rocks; these hard rocks, I may add, +are found at intervals over a space of about five degrees of latitude, from +Point Union to a point between Port S. Julian and S. Cruz, and will be +described in the ensuing chapter. Many gigantic specimens of the Ostraea +Patagonica were collected in the Gulf of St. George. +</p> + +<h3>PORT DESIRE.</h3> + +<p> +A good section of the lowest fossiliferous mass, about forty feet in thickness, +resting on claystone porphyry, is exhibited a few miles south of the harbour. +The shells sufficiently perfect to be recognised consist of:— +</p> + +<p> +1. Ostrea Patagonica, d’Orbigny, (also at St. Fe, and whole coast of +Patagonia). 2. Pecten Paranensis, d’Orbigny, “Voyage, Pal.” +(also at St. Fe, S. Josef, S. Julian). 3. Pecten centralis, G.B. Sowerby (also +at S. Julian and S. Cruz). 4. Cucullaea alta, G.B. Sowerby (also at S. Cruz). +5. Nucula ornata, G.B. Sowerby. 6. Turritella Patagonica, G.B. Sowerby. +</p> + +<p> +The fossiliferous strata, when not denuded, are conformably covered by a +considerable thickness of the fine-grained pumiceous mudstone, divided into two +masses: the lower half is very fine-grained, slightly unctuous, and so compact +as to break with a semi-conchoidal fracture, though yielding to the nail; it +includes laminae of selenite: the upper half precisely resembles the one layer +at the Rio Negro, and with the exception of being whiter, the upper beds at San +Josef and Nuevo Gulf. In neither mass is there any trace to the naked eye of +organic forms. Taking the entire deposit, it is generally quite white, or +yellowish, or feebly tinted with green; it is either almost friable under the +finger, or as hard as chalk; it is of easy fusibility, of little specific +gravity, is not harsh to the touch, adheres to the tongue, and when breathed on +exhales a strong aluminous odour; it sometimes contains a very little +calcareous matter, and traces (besides the included laminae) of gypsum. Under +the microscope, according to Professor Ehrenberg, it consists of minute, +triturated, cellular, glassy fragments of pumice, with some broken crystals. +(“Monatsberichten de konig. Akad. zu Berlin” vom April 1845.) In +the minute glassy fragments, Professor Ehrenberg recognises organic structures, +which have been affected by volcanic heat: in the specimens from this place, +and from Port S. Julian, he finds sixteen Polygastrica and twelve +Phytolitharia. Of these organisms, seven are new forms, the others being +previously known: all are of marine, and chiefly of oceanic, origin. This +deposit to the naked eye resembles the crust which often appears on weathered +surfaces of feldspathic rocks; it likewise resembles those beds of earthy +feldspathic matter, sometimes interstratified with porphyritic rocks, as is the +case in this very district with the underlying purple claystone porphyry. From +examining specimens under a common microscope, and comparing them with other +specimens undoubtedly of volcanic origin, I had come to the same conclusion +with Professor Ehrenberg, namely, that this great deposit, in its first origin, +is of volcanic nature. +</p> + +<h3>PORT S. JULIAN.</h3> + +<p> +(FIGURE 17. SECTION OF THE STRATA EXHIBITED IN THE CLIFFS OF THE NINETY FEET +PLAIN AT PORT S. JULIAN. +</p> + +<p> +(Section through beds from top to bottom: A, B, C, D, E, F.)) +</p> + +<p> +On the south side of the harbour, Figure 17 gives the nature of the beds seen +in the cliffs of the ninety feet plain. Beginning at the top:— +</p> + +<p> +1st, the earthy mass (AA), including the remains of the Macrauchenia, with +recent shells on the surface. +</p> + +<p> +Second, the porphyritic shingle (B), which in its lower part is interstratified +(owing, I believe, to redisposition during denudation) with the white pumiceous +mudstone. +</p> + +<p> +Third, this white mudstone, about twenty feet in thickness, and divided into +two varieties (C and D), both closely resembling the lower, fine- grained, more +unctuous and compact kind at Port Desire; and, as at that place, including much +selenite. +</p> + +<p> +Fourth, a fossiliferous mass, divided into three main beds, of which the +uppermost is thin, and consists of ferruginous sandstone, with many shells of +the great oyster and Pecten Paranensis; the middle bed (E) is a yellowish +earthy sandstone abounding with Scutellae; and the lowest bed (F) is an +indurated, greenish, sandy clay, including large concretions of calcareous +sandstone, many shells of the great oyster, and in parts almost made up of +fragments of Balanidae. Out of these three beds, I procured the following +twelve species, of which the two first were exceedingly numerous in +individuals, as were the Terebratulae and Turritellae in certain layers:— +</p> + +<p> +1. Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. +Fe, and whole coast of Patagonia). 2. Pecten Paranensis, d’Orbigny, +“Voyage, Pal.” (St. Fe, S. Josef, Port Desire). 3. Pecten +centralis, G.B. Sowerby (also at Port Desire and S. Cruz). 4. Pecten geminatus, +G.B. Sowerby. 5. Terebratula Patagonica, G.B. Sowerby (also S. Josef). 6. +Struthiolaria ornata, G.B. Sowerby (also S. Cruz). 7. Fusus Patagonicus, G.B. +Sowerby. 8. Fusus Noachinus, G.B. Sowerby. 9. Scalaria rugulosa, G.B. Sowerby. +10. Turritella ambulacrum, G.B. Sowerby (also S. Cruz). 11. Pyrula, cast of, +like P. ventricosa of Sowerby, Tank Cat. 12. Balanus varians, G.B. Sowerby. 13. +Scutella, differing from the species from Nuevo Gulf. +</p> + +<p> +At the head of the inner harbour of Port S. Julian, the fossiliferous mass is +not displayed, and the sea-cliffs from the water’s edge to a height of +between one and two hundred feet are formed of the white pumiceous mudstone, +which here includes innumerable, far-extended, sometimes horizontal, sometimes +inclined or vertical laminae of transparent gypsum, often about an inch in +thickness. Further inland, with the exception of the superficial gravel, the +whole thickness of the truncated hills, which represent a formerly continuous +plain 950 feet in height, appears to be formed of this white mudstone: here and +there, however, at various heights, thin earthy layers, containing the great +oyster, Pecten Paranensis and Turritella ambulacrum, are interstratified; thus +showing that the whole mass belongs to the same epoch. I nowhere found even a +fragment of a shell actually in the white deposit, and only a single cast of a +Turritella. Out of the eighteen microscopic organisms discovered by Ehrenberg +in the specimens from this place, ten are common to the same deposit at Port +Desire. I may add that specimens of this white mudstone, with the same +identical characters were brought me from two points,—one twenty miles +north of S. Julian, where a wide gravel-capped plain, 350 feet in height, is +thus composed; and the other forty miles south of S. Julian, where, on the old +charts, the cliffs are marked as “Chalk Hills.” +</p> + +<h3>SANTA CRUZ.</h3> + +<p> +The gravel-capped cliffs at the mouth of the river are 355 feet in height: the +lower part, to a thickness of fifty or sixty feet, consists of a more or less +hardened, darkish, muddy, or argillaceous sandstone (like the lowest bed of +Port Desire), containing very many shells, some silicified and some converted +into yellow calcareous spar. The great oyster is here numerous in layers; the +Trigonocelia and Turritella are also very numerous: it is remarkable that the +Pecten Paranensis, so common in all other parts of the coast, is here absent: +the shells consist of:— +</p> + +<p class="letter"> +1. Ostrea Patagonica, d’Orbigny; “Voyage Pal.” (also at St. +Fe and whole coast of Patagonia).<br/> +2. Pecten centralis, G.B. Sowerby (also P. Desire and S. Julian).<br/> +3. Venus meridionalis of G.B. Sowerby.<br/> +4. Crassatella Lyellii, G.B. Sowerby.<br/> +5. Cardium puelchum, G.B. Sowerby.<br/> +6. Cardita Patagonica, G.B. Sowerby.<br/> +7. Mactra rugata, G.B. Sowerby.<br/> +8. Mactra Darwinii, G.B. Sowerby.<br/> +9. Cucullaea alta, G.B. Sowerby (also P. Desire).<br/> +10. Trigonocelia insolita, G.B. Sowerby.<br/> +11. Nucula (?) glabra, G.B. Sowerby.<br/> +12. Crepidula gregaria, G.B. Sowerby.<br/> +13. Voluta alta, G.B. Sowerby.<br/> +14. Trochus collaris, G.B. Sowerby.<br/> +15. Natica solida (?), G.B. Sowerby<br/> +16. Struthiolaria ornata, G.B. Sowerby (also P. Desire).<br/> +17. Turritella ambulacrum, G.B. Sowerby (also P. S. Julian).<br/> +Imperfect fragments of the genera Byssoarca, Artemis, and Fusus. +</p> + +<p> +The upper part of the cliff is generally divided into three great strata, +differing slightly in composition, but essentially resembling the pumiceous +mudstone of the places farther north; the deposit, however, here is more +arenaceous, of greater specific gravity, and not so white: it is interlaced +with numerous thin veins, partially or quite filled with transverse fibres of +gypsum; these fibres were too short to reach across the vein, have their +extremities curved or bent: in the same veins with the gypsum, and likewise in +separate veins as well as in little nests, there is much powdery sulphate of +magnesia (as ascertained by Mr. Reeks) in an uncompressed form: I believe that +this salt has not heretofore been found in veins. Of the three beds, the +central one is the most compact, and more like ordinary sandstone: it includes +numerous flattened spherical concretions, often united like a necklace, +composed of hard calcareous sandstone, containing a few shells: some of these +concretions were four feet in diameter, and in a horizontal line nine feet +apart, showing that the calcareous matter must have been drawn to the centres +of attraction, from a distance of four feet and a half on both sides. In the +upper and lower finer-grained strata, there were other concretions of a grey +colour, containing calcareous matter, and so fine-grained and compact, as +almost to resemble porcelain- rock: I have seen exactly similar concretions in +a volcanic tufaceous bed in Chiloe. Although in this upper fine-grained strata, +organic remains were very rare, yet I noticed a few of the great oyster; and in +one included soft ferruginous layer, there were some specimens of the Cucullaea +alta (found at Port Desire in the lower fossiliferous mass) and of the Mactra +rugata, which latter shell has been partially converted into gypsum. +</p> + +<p> +(FIGURE 18. SECTION OF THE PLAINS OF PATAGONIA, ON THE BANKS OF THE S. CRUZ. +</p> + +<p> +(Section through strata (from top to bottom)): Surface of plain with erratic +boulders; 1,146 feet above the sea. a. Gravel and boulders, 212 feet thick. b. +Basaltic lava, 322 feet thick. c, d and e. Sedimentary layers, bed of small +pebbles and talus respectively, total 592 feet thick. River of S. Cruz; here +280 feet above sea.) +</p> + +<p> +In ascending the valley of the S. Cruz, the upper strata of the coast- cliffs +are prolonged, with nearly the same characters, for fifty miles: at about this +point, they begin in the most gradual and scarcely perceptible manner, to be +banded with white lines; and after ascending ten miles farther, we meet with +distinct thin layers of whitish, greenish, and yellowish fine-grained, fusible +sediments. At eighty miles from the coast, in a cliff thus composed, there were +a few layers of ferruginous sandstone, and of an argillaceous sandstone with +concretions of marl like those in the Pampas. (At this spot, for a space of +three-quarters of a mile along the north side of the river, and for a width of +half a mile, there has been a great slip, which has formed hills between sixty +and seventy feet in height, and has tilted the strata into highly inclined and +even vertical positions. The strata generally dipped at an angle of 45 degrees +towards the cliff from which they had slided. I have observed in slips, both on +a small and large scale, that this inward dip is very general. Is it due to the +hydrostatic pressure of water percolating with difficulty through the strata +acting with greater force at the base of the mass than against the upper part?) +At one hundred miles from the coast, that is at a central point between the +Atlantic and the Cordillera, we have the section in Figure 18. +</p> + +<p> +The upper half of the sedimentary mass, under the basaltic lava, consists of +innumerable zones of perfectly white bright green, yellowish and brownish, +fine-grained, sometimes incoherent, sedimentary matter. The white, pumiceous, +trachytic tuff-like varieties are of rather greater specific gravity than the +pumiceous mudstone on the coast to the north; some of the layers, especially +the browner ones, are coarser, so that the broken crystals are distinguishable +with a weak lens. The layers vary in character in short distances. With the +exception of a few of the Ostrea Patagonica, which appeared to have rolled down +from the cliff above, no organic remains were found. The chief difference +between these layers taken as a whole, and the upper beds both at the mouth of +the river and on the coast northward, seems to lie in the occasional presence +of more colouring matter, and in the supply having been intermittent; these +characters, as we have seen, very gradually disappear in descending the valley, +and this fact may perhaps be accounted for by the currents of a more open sea +having blended together the sediment from a distant and intermittent source. +</p> + +<p> +The coloured layers in the foregoing section rest on a mass, apparently of +great thickness (but much hidden by the talus), of soft sandstone, almost +composed of minute pebbles, from one-tenth to two-tenths of an inch in +diameter, of the rocks (with the entire exception of the basaltic lava) +composing the great boulders on the surface of the plain, and probably +composing the neighbouring Cordillera. Five miles higher up the valley, and +again thirty miles higher up (that is twenty miles from the nearest range of +the Cordillera), the lower plain included within the upper escarpments, is +formed, as seen on the banks of the river, of a nearly similar but +finer-grained, more earthy, laminated sandstone, alternating with argillaceous +beds, and containing numerous moderately sized pebbles of the same rocks, and +some shells of the great Ostrea Patagonica. (I found at both places, but not in +situ, quantities of coniferous and ordinary dicotyledonous silicified wood, +which was examined for me by Mr. R. Brown.) As most of these shells had been +rolled before being here embedded, their presence does not prove that the +sandstone belongs to the great Patagonian tertiary formation, for they might +have been redeposited in it, when the valley existed as a sea-strait; but as +amongst the pebbles there were none of basalt, although the cliffs on both +sides of the valley are composed of this rock, I believe that the sandstone +does belong to this formation. At the highest point to which we ascended, +twenty miles distant from the nearest slope of the Cordillera, I could see the +horizontally zoned white beds, stretching under the black basaltic lava, close +up to the mountains; so that the valley of the S. Cruz gives a fair idea of the +constitution of the whole width of Patagonia. +</p> + +<h3>BASALTIC LAVA OF THE S. CRUZ.</h3> + +<p> +This formation is first met with sixty-seven miles from the mouth of the river; +thence it extends uninterruptedly, generally but not exclusively on the +northern side of the valley, close up to the Cordillera. The basalt is +generally black and fine-grained, but sometimes grey and laminated; it contains +some olivine, and high up the valley much glassy feldspar, where, also, it is +often amygdaloidal; it is never highly vesicular, except on the sides of rents +and on the upper and lower, spherically laminated surfaces. It is often +columnar; and in one place I saw magnificent columns, each face twelve feet in +width, with their interstices filled up with calcareous tuff. The streams rest +conformably on the white sedimentary beds, but I nowhere saw the actual +junction; nor did I anywhere see the white beds actually superimposed on the +lava; but some way up the valley at the foot of the uppermost escarpments, they +must be thus superimposed. Moreover, at the lowest point down the valley, where +the streams thin out and terminate in irregular projections, the spaces or +intervals between these projections are filled up to the level of the now +denuded and gravel-capped surfaces of the plains, with the white-zoned +sedimentary beds; proving that this matter continued to be deposited after the +streams had flowed. Hence we may conclude that the basalt is contemporaneous +with the upper parts of the great tertiary formation. +</p> + +<p> +The lava where first met with is 130 feet in thickness: it there consists of +two, three, or perhaps more streams, divided from each other by vesicular +spheroids like those on the surface. From the streams having, as it appears, +extended to different distances, the terminal points are of unequal heights. +Generally the surface of the basalt is smooth them in one part high up the +valley, it was so uneven and hummocky, that until I afterwards saw the streams +extending continuously on both sides of the valley up to a height of about +three thousand feet close to the Cordillera, I thought that the craters of +eruption were probably close at hand. This hummocky surface I believe to have +been caused by the crossing and heaping up of different streams. In one place, +there were several rounded ridges about twenty feet in height, some of them as +broad as high, and some broader, which certainly had been formed whilst the +lava was fluid, for in transverse sections each ridge was seen to be +concentrically laminated, and to be composed of imperfect columns radiating +from common centres, like the spokes of wheels. +</p> + +<p> +The basaltic mass where first met with is, as I have said, 130 feet in +thickness, and, thirty-five miles higher up the valley, it increases to 322 +feet. In the first fourteen and a half miles of this distance, the upper +surface of the lava, judging from three measurements taken above the level of +the river (of which the apparently very uniform inclination has been calculated +from its total height at a point 135 miles from the mouth), slopes towards the +Atlantic at an angle of only 0 degrees 7 minutes twenty seconds: this must be +considered only as an approximate measurement, but it cannot be far wrong. +Taking the whole thirty-five miles, the upper surface slopes at an angle of 0 +degrees 10 minutes 53 seconds; but this result is of no value in showing the +inclination of any one stream, for halfway between the two points of +measurement, the surface suddenly rises between one hundred and two hundred +feet, apparently caused by some of the uppermost streams having extended thus +far and no farther. From the measurement made at these two points, thirty-five +miles apart, the mean inclination of the sedimentary beds, over which the lava +has flowed, is NOW (after elevation from under the sea) only 0 degrees 7 +minutes 52 seconds: for the sake of comparison, it may be mentioned that the +bottom of the present sea in a line from the mouth of the S. Cruz to the +Falkland Islands, from a depth of seventeen fathoms to a depth of eighty-five +fathoms, declines at an angle of 0 degrees 1 minute 22 seconds; between the +beach and the depth of seventeen fathoms, the slope is greater. From a point +about half-way up the valley, the basaltic mass rises more abruptly towards the +foot of the Cordillera, namely, from a height of 1,204 feet, to about 3,000 +feet above the sea. +</p> + +<p> +This great deluge of lava is worthy, in its dimensions, of the great continent +to which it belongs. The aggregate streams have flowed from the Cordillera to a +distance (unparalleled, I believe, in any case yet known) of about one hundred +geographical miles. Near their furthest extremity their total thickness is 130 +feet, which increase thirty-five miles farther inland, as we have just seen, to +322 feet. The least inclination given by M. E. de Beaumont of the upper surface +of a lava-stream, namely 0 degrees 30 minutes, is that of the great subaerial +eruption in 1783 from Skaptar Jukul in Iceland; and M. E. de Beaumont shows +that it must have flowed down a mean inclination of less than 0 degrees 20 +minutes. (“Memoires pour servir” etc. pages 178 and 217.) But we +now see that under the pressure of the sea, successive streams have flowed over +a smooth bottom with a mean inclination of not more than 0 degrees 7 minutes 52 +seconds; and that the upper surface of the terminal portion (over a space of +fourteen and a half miles) has an inclination of not more than 0 degrees 7 +minutes 20 seconds. If the elevation of Patagonia has been greater nearer the +Cordillera than near the Atlantic (as is probable), then these angles are now +all too large. I must repeat, that although the foregoing measurements, which +were all carefully taken with the barometer, may not be absolutely correct, +they cannot be widely erroneous. +</p> + +<p> +Southward of the S. Cruz, the cliffs of the 840 feet plain extend to Coy Inlet, +and owing to the naked patches of the white sediment, they are said on the +charts to be “like the coast of Kent.” At Coy Inlet the high plain +trends inland, leaving flat-topped outliers. At Port Gallegos (latitude 51 +degrees 35 minutes, and ninety miles south of S. Cruz), I am informed by +Captain Sulivan, R.N., that there is a gravel-capped plain from two to three +hundred feet in height, formed of numerous strata, some fine-grained and +pale-coloured, like the upper beds at the mouth of the S. Cruz, others rather +dark and coarser, so as to resemble gritstones or tuffs; these latter include +rather large fragments of apparently decomposed volcanic rocks; there are, +also, included layers of gravel. This formation is highly remarkable, from +abounding with mammiferous remains, which have not as yet been examined by +Professor Owen, but which include some large, but mostly small, species of +Pachydermata, Edentata, and Rodentia. From the appearance of the pale-coloured, +fine-grained beds, I was inclined to believe that they corresponded with the +upper beds of the S. Cruz; but Professor Ehrenberg, who has examined some of +the specimens, informs me that the included microscopical organisms are wholly +different, being fresh and brackish-water forms. Hence the two to three hundred +feet plain at Port Gallegos is of unknown age, but probably of subsequent +origin to the great Patagonian tertiary formation. +</p> + +<h3>EASTERN TIERRA DEL FUEGO.</h3> + +<p> +Judging from the height, the general appearance, and the white colour of the +patches visible on the hill sides, the uppermost plain, both on the north and +western side of the Strait of Magellan, and along the eastern coast of Tierra +del Fuego as far south as near Port St. Polycarp, probably belongs to the great +Patagonian tertiary formation, These higher table- ranges are fringed by low, +irregular, extensive plains, belonging to the boulder formation (Described in +the “Geological Transactions” volume 6 page 415.), and composed of +coarse unstratified masses, sometimes associated (as north of C. +Virgin’s) with fine, laminated, muddy sandstones. The cliffs in Sebastian +Bay are 200 feet in height, and are composed of fine sandstones, often in +curvilinear layers, including hard concretions of calcareous sandstone, and +layers of gravel. In these beds there are fragments of wood, legs of crabs, +barnacles encrusted with corallines still partially retaining their colour, +imperfect fragments of a Pholas distinct from any known species, and of a +Venus, approaching very closely to, but slightly different in form from, the V. +lenticularis, a species living on the coast of Chile. Leaves of trees are +numerous between the laminae of the muddy sandstone; they belong, as I am +informed by Dr. J.D. Hooker, to three species of deciduous beech, different +from the two species which compose the great proportion of trees in this +forest-clad land. (“Botany of the Antarctic Voyage” page 212.) From +these facts it is difficult to conjecture, whether we here see the basal part +of the great Patagonian formation, or some later deposit. +</p> + +<h3>A SUMMARY ON THE PATAGONIAN TERTIARY FORMATION.</h3> + +<p> +Four out of the seven fossil shells, from St. Fe in Entre Rios, were found by +M. d’Orbigny in the sandstone of the Rio Negro, and by me at San Josef. +Three out of the six from San Josef are identical with those from Port Desire +and S. Julian, which two places have together fifteen species, out of which +three are common to both. Santa Cruz has seventeen species, out of which five +are common to Port Desire and S. Julian. Considering the difference in latitude +between these several places, and the small number of species altogether +collected, namely thirty-six, I conceive the above proportional number of +species in common, is sufficient to show that the lower fossiliferous mass +belongs nearly, I do not say absolutely, to the same epoch. What this epoch may +be, compared with the European tertiary stages, M. d’Orbigny will not +pretend to determine. The thirty-six species (including those collected by +myself and by M. d’Orbigny) are all extinct, or at least unknown; but it +should be borne in mind, that the present coast consists of shingle, and that +no one, I believe, has dredged here for shells; hence it is not improbable that +some of the species may hereafter be found living. Some few of the species are +closely related with existing ones; this is especially the case, according to +M. d’Orbigny and Mr. Sowerby, with the Fusus Patagonicus; and, according +to Mr. Sowerby, with the Pyrula, the Venus meridionalis, the Crepidula +gregaria, and the Turritella ambulacrum, and T. Patagonica. At least three of +the genera, namely, Cucullaea, Crassatella, and (as determined by Mr. Sowerby) +Struthiolaria, are not found in this quarter of the world; and Trigonocelia is +extinct. The evidence taken altogether indicates that this great tertiary +formation is of considerable antiquity; but when treating of the Chilean beds, +I shall have to refer again to this subject. +</p> + +<p> +The white pumiceous mudstone, with its abundant gypsum, belongs to the same +general epoch with the underlying fossiliferous mass, as may be inferred from +the shells included in the intercalated layers at Nuevo Gulf, S. Julian, and S. +Cruz. Out of the twenty-seven marine microscopic structures found by Professor +Ehrenberg in the specimens from S. Julian and Port Desire, ten are common to +these two places: the three found at Nuevo Gulf are distinct. I have minutely +described this deposit, from its remarkable characters and its wide extension. +From Coy Inlet to Port Desire, a distance of 230 miles, it is certainly +continuous; and I have reason to believe that it likewise extends to the Rio +Chupat, Nuevo Gulf, and San Josef, a distance of 570 miles: we have, also, seen +that a single layer occurs at the Rio Negro. At Port S. Julian it is from eight +to nine hundred feet in thickness; and at S. Cruz it extends, with a slightly +altered character, up to the Cordillera. From its microscopic structure, and +from its analogy with other formations in volcanic districts, it must be +considered as originally of volcanic origin: it may have been formed by the +long-continued attrition of vast quantities of pumice, or judging from the +manner in which the mass becomes, in ascending the valley of S. Cruz, divided +into variously coloured layers, from the long-continued eruption of clouds of +fine ashes. In either case, we must conclude, that the southern volcanic +orifices of the Cordillera, now in a dormant state, were at about this period +over a wide space, and for a great length of time, in action. We have evidence +of this fact, in the latitude of the Rio Negro, in the sandstone-conglomerate +with pumice, and demonstrative proof of it, at S. Cruz, in the vast deluges of +basaltic lava: at this same tertiary period, also, there is distinct evidence +of volcanic action in Western Banda Oriental. +</p> + +<p> +The Patagonian tertiary formation extends continuously, judging from fossils +alone, from S. Cruz to near the Rio Colorado, a distance of above six hundred +miles, and reappears over a wide area in Entre Rios and Banda Oriental, making +a total distance of 1,100 miles; but this formation undoubtedly extends (though +no fossils were collected) far south of the S. Cruz, and, according to M. +d’Orbigny, 120 miles north of St. Fe. At S. Cruz we have seen that it +extends across the continent; being on the coast about eight hundred feet in +thickness (and rather more at S. Julian), and rising with the contemporaneous +lava-streams to a height of about three thousand feet at the base of the +Cordillera. It rests, wherever any underlying formation can be seen, on +plutonic and metamorphic rocks. Including the newer Pampean deposit, and those +strata in Eastern Tierra del Fuego of doubtful age, as well as the boulder +formation, we have a line of more than twenty-seven degrees of latitude, equal +to that from the Straits of Gibraltar to the south of Iceland, continuously +composed of tertiary formations. Throughout this great space the land has been +upraised, without the strata having been in a single instance, as far as my +means of observation went, unequally tilted or dislocated by a fault. +</p> + +<h3>TERTIARY FORMATIONS ON THE WEST COAST.</h3> <h3>CHONOS ARCHIPELAGO. </h3> + +<p> +The numerous islands of this group, with the exception of Lemus, Ypun, consist +of metamorphic schists; these two islands are formed of softish grey and brown, +fusible, often laminated sandstones, containing a few pebbles, fragments of +black lignite, and numerous mammillated concretions of hard calcareous +sandstone. Out of these concretions at Ypun (latitude 40 degrees 30 minutes +S.), I extracted the four following extinct species of shells:— +</p> + +<p> +1. Turritella suturalis, G.B. Sowerby (also Navidad). 2. Sigaretus subglobosus, +G.B. Sowerby (also Navidad). 3. Cytheraea (?) sulculosa (?), G.B. Sowerby (also +Chiloe and Huafo?). 4. Voluta, fragments of. +</p> + +<p> +In the northern parts of this group there are some cliffs of gravel and of the +boulder formation. In the southern part (at P. Andres in Tres Montes), there is +a volcanic formation, probably of tertiary origin. The lavas attain a thickness +of from two to three hundred feet; they are extremely variable in colour and +nature, being compact, or brecciated, or cellular, or amygdaloidal with +zeolite, agate and bole, or porphyritic with glassy albitic feldspar. There is +also much imperfect rubbly pitchstone, with the interstices charged with +powdery carbonate of lime apparently of contemporaneous origin. These lavas are +conformably associated with strata of breccia and of brown tuff containing +lignite. The whole mass has been broken up and tilted at an angle of 45 +degrees, by a series of great volcanic dikes, one of which was thirty yards in +breadth. This volcanic formation resembles one, presently to be described, in +Chiloe. +</p> + +<h3>HUAFO.</h3> + +<p> +This island lies between the Chonos and Chiloe groups: it is about eight +hundred feet high, and perhaps has a nucleus of metamorphic rocks. The strata +which I examined consisted of fine-grained muddy sandstones, with fragments of +lignite and concretions of calcareous sandstone. I collected the following +extinct shells, of which the Turritella was in great numbers:— +</p> + +<p> +1. Bulla cosmophila, G.B. Sowerby. 2. Pleurotoma subaequalis, G.B. Sowerby. 3. +Fusus cleryanus, d’Orbigny, “Voyage Pal.” (also at Coquimbo). +4. Triton leucostomoides, G.B. Sowerby. 5. Turritella Chilensis, G.B. Sowerby +(also Mocha). 6. Venus, probably a distinct species, but very imperfect. 7. +Cytheraea (?) sulculosa (?), probably a distinct species, but very imperfect. +8. Dentalium majus, G.B. Sowerby. +</p> + +<h3>CHILOE.</h3> + +<p> +This fine island is about one hundred miles in length. The entire southern +part, and the whole western coast, consists of mica-schist, which likewise is +seen in the ravines of the interior. The central mountains rise to a height of +3,000 feet, and are said to be partly formed of granite and greenstone: there +are two small volcanic districts. The eastern coast, and large parts of the +northern extremity of the island are composed of gravel, the boulder formation, +and underlying horizontal strata. The latter are well displayed for twenty +miles north and south of Castro; they vary in character from common sandstone +to fine-grained, laminated mudstones: all the specimens which I examined are +easily fusible, and some of the beds might be called volcanic grit-stones. +These latter strata are perhaps related to a mass of columnar trachyte which +occurs behind Castro. The sandstone occasionally includes pebbles, and many +fragments and layers of lignite; of the latter, some are apparently formed of +wood and others of leaves: one layer on the N.W. side of Lemuy is nearly two +feet in thickness. There is also much silicified wood, both common +dicotyledonous and coniferous: a section of one specimen in the direction of +the medullary rays has, as I am informed by Mr. R. Brown, the discs in a double +row placed alternately, and not opposite as in the true Araucaria. I found +marine remains only in one spot, in some concretions of hard calcareous +sandstone: in several other districts I have observed that organic remains were +exclusively confined to such concretions; are we to account for this fact, by +the supposition that the shells lived only at these points, or is it not more +probable that their remains were preserved only where concretions were formed? +The shells here are in a bad state, they consist of:— +</p> + +<p> +1. Tellinides (?) oblonga, G.B. Sowerby (a solenella in M. +d’Orbigny’s opinion). 2. Natica striolata, G.B. Sowerby. 3. Natica +(?) pumila, G.B. Sowerby. 4. Cytheraea (?) sulculosa, G.B. Sowerby (also Ypun +and Huafo?). +</p> + +<p> +At the northern extremity of the island, near S. Carlos, there is a large +volcanic formation, between five and seven hundred feet in thickness. The +commonest lava is blackish-grey or brown, either vesicular, or amygdaloidal +with calcareous spar and bole: most even of the darkest varieties fuse into a +pale-coloured glass. The next commonest variety is a rubbly, rarely well +characterised pitchstone (fusing into a white glass) which passes in the most +irregular manner into stony grey lavas. This pitchstone, as well as some purple +claystone porphyry, certainly flowed in the form of streams. These various +lavas often pass, at a considerable depth from the surface, in the most abrupt +and singular manner into wacke. Great masses of the solid rock are brecciated, +and it was generally impossible to discover whether the recementing process had +been an igneous or aqueous action. (In a cliff of the hardest fragmentary mass, +I found several tortuous, vertical veins, varying in thickness from a few +tenths of an inch to one inch and a half, of a substance which I have not seen +described. It is glossy, and of a brown colour; it is thinly laminated, with +the laminae transparent and elastic; it is a little harder than calcareous +spar; it is infusible under the blowpipe, sometimes decrepitates, gives out +water, curls up, blackens, and becomes magnetic. Borax easily dissolves a +considerable quantity of it, and gives a glass tinged with green. I have no +idea what its true nature is. On first seeing it, I mistook it for lignite!) +The beds are obscurely separated from each other; they are sometimes parted by +seams of tuff and layers of pebbles. In one place they rested on, and in +another place were capped by, tuffs and girt-stones, apparently of submarine +origin. +</p> + +<p> +The neighbouring peninsula of Lacuy is almost wholly formed of tufaceous +deposits, connected probably in their origin with the volcanic hills just +described. The tuffs are pale-coloured, alternating with laminated mudstones +and sandstones (all easily fusible), and passing sometimes into fine-grained +white beds strikingly resembling the great upper infusorial deposit of +Patagonia, and sometimes into brecciolas with pieces of pumice in the last +stage of decay; these again pass into ordinary coarse breccias and +conglomerates of hard rocks. Within very short distances, some of the finer +tuffs often passed into each other in a peculiar manner, namely, by irregular +polygonal concretions of one variety increasing so much and so suddenly in +size, that the second variety, instead of any longer forming the entire mass, +was left merely in thin veins between the concretions. In a straight line of +cliffs, at Point Tenuy, I examined the following remarkable section (Figure +19):— +</p> + +<h3>(FIGURE 19.)</h3> + +<p> +On the left hand, the lower part (AA) consists of regular, alternating strata +of brown tuffs and greenish laminated mudstone, gently inclined to the right, +and conformably covered by a mass (B left) of a white, tufaceous and +brecciolated deposit. On the right hand, the whole cliff (BB right) consists of +the same white tufaceous matter, which on this side presents scarcely a trace +of stratification, but to the left becomes very gradually and rather +indistinctly divided into strata quite conformable with the underlying beds +(AA): moreover, a few hundred yards further to the left, where the surface has +been less denuded, the tufaceous strata (B left) are conformably covered by +another set of strata, like the underlying ones (AA) of this section. In the +middle of the diagram, the beds (AA) are seen to be abruptly cut off, and to +abut against the tufaceous non-stratified mass; but the line of junction has +been accidentally not represented steep enough, for I particularly noticed that +before the beds had been tilted to the right, this line must have been nearly +vertical. It appears that a current of water cut for itself a deep and steep +submarine channel, and at the same time or afterwards filled it up with the +tufaceous and brecciolated matter, and spread the same over the surrounding +submarine beds; the matter becoming stratified in these more distant and less +troubled parts, and being moreover subsequently covered up by other strata +(like AA) not shown in the diagram. It is singular that three of the beds (of +AA) are prolonged in their proper direction, as represented, beyond the line of +junction into the white tufaceous matter: the prolonged portions of two of the +beds are rounded; in the third, the terminal fragment has been pushed upwards: +how these beds could have been left thus prolonged, I will not pretend to +explain. In another section on the opposite side of a promontory, there was at +the foot of this same line of junction, that is at the bottom of the old +submarine channel, a pile of fragments of the strata (AA), with their +interstices filled up with white tufaceous matter: this is exactly what might +have been anticipated under such circumstances. +</p> + +<p> +(FIGURE 20. GROUND PLAN SHOWING THE RELATION BETWEEN VEINS AND CONCRETIONARY +ZONES IN A MASS OF TUFF.) +</p> + +<p> +The various tufaceous and other beds at this northern end of Chiloe probably +belong to about the same age with those near Castro, and they contain, as +there, many fragments of black lignite and of silicified and pyritous wood, +often embedded close together. They also contain many and singular concretions: +some are of hard calcareous sandstone, in which it would appear that broken +volcanic crystals and scales of mica have been better preserved (as in the case +of the organic remains near Castro) than in the surrounding mass. Other +concretions in the white brecciola are of a hard, ferruginous, yet fusible, +nature; they are as round as cannon-balls, and vary from two or three inches to +two feet in diameter; their insides generally consist either of fine, scarcely +coherent volcanic sand (The frequent tendency in iron to form hollow +concretions or shell containing incoherent matter is singular; +D’Aubuisson (“Traite de Geogn.” tome 1 page 318) remarks on +this circumstance.), or of an argillaceous tuff; in this latter case, the +external crust was quite thin and hard. Some of these spherical balls were +encircled in the line of their equators, by a necklace-like row of smaller +concretions. Again there were other concretions, irregularly formed, and +composed of a hard, compact, ash- coloured stone, with an almost porcelainous +fracture, adhesive to the tongue, and without any calcareous matter. These beds +are, also, interlaced by many veins, containing gypsum, ferruginous matter, +calcareous spar, and agate. It was here seen with remarkable distinctness, how +intimately concretionary action and the production of fissures and veins are +related together. Figure 20 is an accurate representation of a horizontal space +of tuff, about four feet long by two and a half in width: the double lines +represent the fissures partially filled with oxide of iron and agate: the +curvilinear lines show the course of the innumerable, concentric, concretionary +zones of different shades of colour and of coarseness in the particles of tuff. +The symmetry and complexity of the arrangement gave the surface an elegant +appearance. It may be seen how obviously the fissures determine (or have been +determined by) the shape, sometimes of the whole concretion, and sometimes only +of its central parts. The fissures also determine the curvatures of the long +undulating zones of concretionary action. From the varying composition of the +veins and concretions, the amount of chemical action which the mass has +undergone is surprisingly great; and it would likewise appear from the +difference in size in the particles of the concretionary zones, that the mass, +also, has been subjected to internal mechanical movements. +</p> + +<p> +In the peninsula of Lacuy, the strata over a width of four miles have been +upheaved by three distinct, and some other indistinct, lines of elevation, +ranging within a point of north and south. One line, about two hundred feet in +height, is regularly anticlinal, with the strata dipping away on both sides, at +an angle of 15 degrees, from a central “valley of elevation,” about +three hundred yards in width. A second narrow steep ridge, only sixty feet +high, is uniclinal, the strata throughout dipping westward; those on both +flanks being inclined at an angle of from ten to fifteen degrees; whilst those +on the ridge dip in the same direction at an angle of between thirty and forty +degrees. This ridge, traced northwards, dies away; and the beds at its terminal +point, instead of dipping westward, are inclined 12 degrees to the north. This +case interested me, as being the first in which I found in South America, +formations perhaps of tertiary origin, broken by lines of elevation. +</p> + +<h3>VALDIVIA: ISLAND OF MOCHA.</h3> + +<p> +The formations of Chiloe seem to extend with nearly the same character to +Valdivia, and for some leagues northward of it: the underlying rocks are +micaceous schists, and are covered up with sandstone and other sedimentary +beds, including, as I was assured, in many places layers of lignite. I did not +land on Mocha (latitude 38 degrees 20 minutes), but Mr. Stokes brought me +specimens of the grey, fine-grained, slightly calcareous sandstone, precisely +like that of Huafo, containing lignite and numerous Turritellae. The island is +flat topped, 1,240 feet in height, and appears like an outlier of the +sedimentary beds on the mainland. The few shells collected consist of:— +</p> + +<p> +1. Turritella Chilensis, G.B. Sowerby (also at Huafo). 2. Fusus, very +imperfect, somewhat resembling F. subreflexus of Navidad, but probably +different. 3. Venus, fragments of. +</p> + +<h3>CONCEPCION.</h3> + +<p> +Sailing northward from Valdivia, the coast-cliffs are seen, first to assume +near the R. Tolten, and thence for 150 miles northward, to be continued with +the same mineralogical characters, immediately to be described at Concepcion. I +heard in many places of beds of lignite, some of it fine and glossy, and +likewise of silicified wood; near the Tolten the cliffs are low, but they soon +rise in height; and the horizontal strata are prolonged, with a nearly level +surface, until coming to a more lofty tract between points Rumena and Lavapie. +Here the beds have been broken up by at least eight or nine parallel lines of +elevation, ranging E. or E.N.E. and W. or W.S.W. These lines can be followed +with the eye many miles into the interior; they are all uniclinal, the strata +in each dipping to a point between S. and S.S.E. with an inclination in the +central lines of about forty degrees, and in the outer ones of under twenty +degrees. This band of symmetrically troubled country is about eight miles in +width. +</p> + +<p> +The island of Quiriquina, in the Bay of Concepcion, is formed of various soft +and often ferruginous sandstones, with bands of pebbles, and with the lower +strata sometimes passing into a conglomerate resting on the underlying +metamorphic schists. These beds include subordinate layers of greenish impure +clay, soft micaceous and calcareous sandstones, and reddish friable earthy +matter with white specks like decomposed crystals of feldspar; they include, +also, hard concretions, fragments of shells, lignite, and silicified wood. In +the upper part they pass into white, soft sediments and brecciolas, very like +those described at Chiloe; as indeed is the whole formation. At Lirguen and +other places on the eastern side of the bay, there are good sections of the +lower sandstones, which are generally ferruginous, but which vary in character, +and even pass into an argillaceous nature; they contain hard concretions, +fragments of lignite, silicified wood, and pebbles (of the same rocks with the +pebbles in the sandstones of Quiriquina), and they alternate with numerous, +often very thin layers of imperfect coal, generally of little specific gravity. +The main bed here is three feet thick; and only the coal of this one bed has a +glossy fracture. Another irregular, curvilinear bed of brown, compact lignite, +is remarkable for being included in a mass of coarse gravel. These imperfect +coals, when placed in a heap, ignite spontaneously. The cliffs on this side of +the bay, as well as on the island of Quiriquina, are capped with red friable +earth, which, as stated in the Second Chapter, is of recent formation. The +stratification in this neighbourhood is generally horizontal; but near Lirguen +the beds dip N.W. at an angle of 23 degrees; near Concepcion they are also +inclined: at the northern end of Quiriquina they have been tilted at an angle +of 30 degrees, and at the southern end at angles varying from 15 degrees to 40 +degrees: these dislocations must have taken place under the sea. +</p> + +<p> +A collection of shells, from the island of Quiriquina, has been described by M. +d’Orbigny: they are all extinct, and from their generic character, M. +d’Orbigny inferred that they were of tertiary origin: they consist +of:— +</p> + +<p> +1. Scalaria Chilensis, d’Orbigny, “Voyage, Part Pal.” 2. +Natica Araucana, d’Orbigny, “Voyage, Part Pal.” 3. Natica +australis, d’Orbigny, “Voyage, Part Pal.” 4. Fusus +difficilis, d’Orbigny, “Voyage, Part Pal.” 5. Pyrula +longirostra, d’Orbigny, “Voyage, Part Pal.” 6. Pleurotoma +Araucana, d’Orbigny, “Voyage, Part Pal.” 7. Cardium auca, +d’Orbigny, “Voyage, Part Pal.” 8. Cardium acuticostatum, +d’Orbigny, “Voyage, Part Pal.” 9. Venus auca, +d’Orbigny, “Voyage, Part Pal.” 10. Mactra cecileana, +d’Orbigny, “Voyage, Part Pal.” 11. Mactra Araucana, +d’Orbigny, “Voyage, Part Pal.” 12. Arca Araucana, +d’Orbigny, “Voyage, Part Pal.” 13. Nucula Largillierti, +d’Orbigny, “Voyage, Part Pal.” 14. Trigonia Hanetiana, +d’Orbigny, “Voyage, Part Pal.” +</p> + +<p> +During a second visit of the “Beagle” to Concepcion, Mr. Kent +collected for me some silicified wood and shells out of the concretions in the +sandstone from Tome, situated a short distance north of Lirguen. They consist +of:— +</p> + +<p> +1. Natica australis, d’Orbigny, “Voyage, Part Pal.” 2. Mactra +Araucana, d’Orbigny, “Voyage, Part Pal.” 3. Trigonia +Hanetiana, d’Orbigny, “Voyage, Part Pal.” 4. Pecten, +fragments of, probably two species, but too imperfect for description. 5. +Baculites vagina, E. Forbes. 6. Nautilus d’Orbignyanus, E. Forbes. +</p> + +<p> +Besides these shells, Captain Belcher found here an Ammonite, nearly three feet +in diameter, and so heavy that he could not bring it away; fragments are +deposited at Haslar Hospital: he also found the silicified vertebrae of some +very large animal. (“Zoology of Captain Beechey’s Voyage” +page 163.) From the identity in mineralogical nature of the rocks, and from +Captain Belcher’s minute description of the coast between Lirguen and +Tome, the fossiliferous concretions at this latter place certainly belong to +the same formation with the beds examined by myself at Lirguen; and these again +are undoubtedly the same with the strata of Quiriquina; moreover; the three +first of the shells from Tome, though associated in the same concretions with +the Baculite, are identical with the species from Quiriquina. Hence all the +sandstone and lignitiferous beds in this neighbourhood certainly belong to the +same formation. Although the generic character of the Quiriquina fossils +naturally led M. d’Orbigny to conceive that they were of tertiary origin, +yet as we now find them associated with the Baculites vagina and with an +Ammonite, we must, in the opinion of M. d’Orbigny, and if we are guided +by the analogy of the northern hemisphere, rank them in the Cretaceous system. +Moreover, the Baculites vagina, which is in a tolerable state of preservation, +appears to Professor E. Forbes certainly to be identical with a species, so +named by him, from Pondicherry in India; where it is associated with numerous +decidedly cretaceous species, which approach most nearly to Lower Greensand or +Neocomian forms: this fact, considering the vast distance between Chile and +India, is truly surprising. Again, the Nautilus d’Orbignyanus, as far as +its imperfect state allows of comparison, resembles, as I am informed by +Professor Forbes, both in its general form and in that of its chambers, two +species from the Upper Greensand. It may be added that every one of the +above-named genera from Quiriquina, which have an apparently tertiary +character, are found in the Pondicherry strata. There are, however, some +difficulties on this view of the formations at Concepcion being cretaceous, +which I shall afterwards allude to; and I will here only state that the Cardium +auca is found also at Coquimbo, the beds at which place, there can be no doubt, +are tertiary. +</p> + +<p> +NAVIDAD. (I was guided to this locality by the Report on M. Gay’s +“Geological Researches” in the “Annales des Scienc. +Nat.” 1st series tome 28.) +</p> + +<p> +The Concepcion formation extends some distance northward, but how far I know +not; for the next point at which I landed was at Navidad, 160 miles north of +Concepcion, and 60 miles south of Valparaiso. The cliffs here are about eight +hundred feet in height: they consist, wherever I could examine them, of +fine-grained, yellowish, earthy sandstones, with ferruginous veins, and with +concretions of hard calcareous sandstone. In one part, there were many pebbles +of the common metamorphic porphyries of the Cordillera: and near the base of +the cliff, I observed a single rounded boulder of greenstone, nearly a yard in +diameter. I traced this sandstone formation beneath the superficial covering of +gravel, for some distance inland: the strata are slightly inclined from the sea +towards the Cordillera, which apparently has been caused by their having been +accumulated against or round outlying masses of granite, of which some points +project near the coast. The sandstone contains fragments of wood, either in the +state of lignite or partially silicified, sharks’ teeth, and shells in +great abundance, both high up and low down the sea-cliffs. Pectunculus and +Oliva were most numerous in individuals, and next to them Turritella and Fusus. +I collected in a short time, though suffering from illness, the following +thirty-one species, all of which are extinct, and several of the genera do not +now range (as we shall hereafter show) nearly so far south:— +</p> + +<p> +1. Gastridium cepa, G.B. Sowerby. 2. Monoceros, fragments of, considered by M. +d’Orbigny as a new species. 3. Voluta alta, G.B. Sowerby (considered by +M. d’Orbigny as distinct from the V. alta of Santa Cruz). 4. Voluta +triplicata, G.B. Sowerby. 5. Oliva dimidiata, G.B. Sowerby. 6. Pleurotoma +discors, G.B. Sowerby. 7. Pleurotoma turbinelloides, G.B. Sowerby. 8. Fusus +subreflexus, G.B. Sowerby. 9. Fusus pyruliformis, G.B. Sowerby. 10. Fusus, +allied to F. regularis (considered by M. d’Orbigny as a distinct +species). 11. Turritella suturalis, G.B. Sowerby. 12. Turritella Patagonica, +G.B. Sowerby (fragments of). 13. Trochus laevis, G.B. Sowerby. 14. Trochus +collaris, G.B. Sowerby (considered by M. d’Orbigny as the young of the T. +laevis). 15. Cassis monilifer, G.B. Sowerby. 16. Pyrula distans, G.B. Sowerby. +17. Triton verruculosus, G.B. Sowerby. 18. Sigaretus subglobosus, G.B. Sowerby. +19. Natica solida, G.B. Sowerby. (It is doubtful whether the Natica solida of +S. Cruz is the same species with this.) 20. Terebra undulifera, G.B. Sowerby. +21. Terebra costellata, G.B. Sowerby. 22. Bulla (fragments of). 23. Dentalium +giganteum, do. 24. Dentalium sulcosum, do. 25. Corbis (?) laevigata, do. 26. +Cardium multiradiatum, do. 27. Venus meridionalis, do. 28. Pectunculus dispar, +(?) Desh. (considered by M. d’Orbigny as a distinct species). 29, 30. +Cytheraea and Mactra, fragments of (considered by M. d’Orbigny as new +species). 31. Pecten, fragments of. +</p> + +<h3>COQUIMBO.</h3> <h3>(FIGURE 21. SECTION OF THE TERTIARY FORMATION AT +COQUIMBO.</h3> + +<p> +From Level of Sea to Surface of plain, 252 feet above sea, through levels F, E, +D and C: +</p> + +<p> +F.—Lower sandstone, with concretions and silicified bones, with fossil +shells, all, or nearly all, extinct. +</p> + +<p> +E.—Upper ferruginous sandstone, with numerous Balani, with fossil shells, +all, or nearly all, extinct. +</p> + +<p> +C and D.—Calcareous beds with recent shells. +</p> + +<p> +A.—Stratified sand in a ravine, also with recent shells.) +</p> + +<p> +For more than two hundred miles northward of Navidad, the coast consists of +plutonic and metamorphic rocks, with the exception of some quite insignificant +superficial beds of recent origin. At Tonguay, twenty-five miles south of +Coquimbo, tertiary beds recommence. I have already minutely described in the +Second Chapter, the step-formed plains of Coquimbo, and the upper calcareous +beds (from twenty to thirty feet in thickness) containing shells of recent +species, but in different proportions from those on the beach. There remains to +be described only the underlying ancient tertiary beds, represented in Figure +21 by the letters F and E:— +</p> + +<p> +I obtained good sections of bed F only in Herradura Bay: it consists of soft +whitish sandstone, with ferruginous veins, some pebbles of granite, and +concretionary layers of hard calcareous sandstone. These concretions are +remarkable from the great number of large silicified bones, apparently of +cetaceous animals, which they contain; and likewise of a shark’s teeth, +closely resembling those of the Carcharias megalodon. Shells of the following +species, of which the gigantic Oyster and Perna are the most conspicuous, are +numerously embedded in the concretions:— +</p> + +<p> +1. Bulla ambigua, d’Orbigny “Voyage” Pal. 2. Monoceros +Blainvillii, d’Orbigny “Voyage” Pal. 3. Cardium auca, +d’Orbigny “Voyage” Pal. 4. Panopaea Coquimbensis, +d’Orbigny “Voyage” Pal. 5. Perna Gaudichaudi, d’Orbigny +“Voyage” Pal. 6. Artemis ponderosa; Mr. Sowerby can find no +distinguishing character between this fossil and the recent A. ponderosa; it is +certainly an Artemis, as shown by the pallial impression. 7. Ostrea Patagonica +(?); Mr. Sowerby can point out no distinguishing character between this species +and that so eminently characteristic of the great Patagonian formation; but he +will not pretend to affirm that they are identical. 8. Fragments of a Venus and +Natica. +</p> + +<p> +The cliffs on one side of Herradura Bay are capped by a mass of stratified +shingle, containing a little calcareous matter, and I did not doubt that it +belonged to the same recent formation with the gravel on the surrounding +plains, also cemented by calcareous matter, until to my surprise, I found in +the midst of it, a single thin layer almost entirely composed of the above +gigantic oyster. +</p> + +<p> +At a little distance inland, I obtained several sections of the bed E, which, +though different in appearance from the lower bed F, belongs to the same +formation: it consists of a highly ferruginous sandy mass, almost composed, +like the lowest bed at Port S. Julian, of fragments of Balanidae; it includes +some pebbles, and layers of yellowish-brown mudstone. The embedded shells +consist of:— +</p> + +<p> +1. Monoceros Blainvillii, d’Orbigny “Voyage” Pal. 2. +Monoceros ambiguus, G.B. Sowerby. 3. Anomia alternans, G.B. Sowerby. 4. Pecten +rudis, G.B. Sowerby. 5. Perna Gaudichaudi, d’Orbigny “Voyage” +Pal. 6. Ostrea Patagonica (?), d’Orbigny “Voyage” Pal. 7. +Ostrea, small species, in imperfect state; it appeared to me like a small kind +now living in, but very rare in the bay. 8. Mytilus Chiloensis; Mr. Sowerby can +find no distinguishing character between this fossil, as far as its not very +perfect condition allows of comparison, and the recent species. 9. Balanus +Coquimbensis, G.B. Sowerby. 10. Balanus psittacus? King. This appears to Mr. +Sowerby and myself identical with a very large and common species now living on +the coast. +</p> + +<p> +The uppermost layers of this ferrugino-sandy mass are conformably covered by, +and impregnated to the depth of several inches with, the calcareous matter of +the bed D called losa: hence I at one time imagined that there was a gradual +passage between them; but as all the species are recent in the bed D, whilst +the most characteristic shells of the uppermost layers of E are the extinct +Perna, Pecten, and Monoceros, I agree with M. d’Orbigny, that this view +is erroneous, and that there is only a mineralogical passage between them, and +no gradual transition in the nature of their organic remains. Besides the +fourteen species enumerated from these two lower beds, M. d’Orbigny has +described ten other species given to him from this locality; namely:— +</p> + +<p> +1. Fusus Cleryanus, d’Orbigny “Voyage” Pal. 2. Fusus +petitianus, d’Orbigny “Voyage” Pal. 3. Venus hanetiana, +d’Orbigny “Voyage” Pal. 4. Venus incerta (?) d’Orbigny +“Voyage” Pal. 5. Venus Cleryana, d’Orbigny +“Voyage” Pal. 6. Venus petitiana, d’Orbigny +“Voyage” Pal. 7. Venus Chilensis, d’Orbigny +“Voyage” Pal. 8. Solecurtus hanetianus, d’Orbigny +“Voyage” Pal. 9. Mactra auca, d’Orbigny “Voyage” +Pal. 10. Oliva serena, d’Orbigny “Voyage” Pal. +</p> + +<p> +Of these twenty-four shells, all are extinct, except, according to Mr. Sowerby, +the Artemis ponderosa, Mytilus Chiloensis, and probably the great Balanus. +</p> + +<h3>COQUIMBO TO COPIAPO.</h3> + +<p> +A few miles north of Coquimbo, I met with the ferruginous, balaniferous mass E +with many silicified bones; I was informed that these silicified bones occur +also at Tonguay, south of Coquimbo: their number is certainly remarkable, and +they seem to take the place of the silicified wood, so common on the +coast-formations of Southern Chile. In the valley of Chaneral, I again saw this +same formation, capped with the recent calcareous beds. I here left the coast, +and did not see any more of the tertiary formations, until descending to the +sea at Copiapo: here in one place I found variously coloured layers of sand and +soft sandstone, with seams of gypsum, and in another place, a comminuted shelly +mass, with layers of rotten-stone and seams of gypsum, including many of the +extinct gigantic oyster: beds with these oysters are said to occur at English +Harbour, a few miles north of Copiapo. +</p> + +<h3>COAST OF PERU.</h3> + +<p> +With the exception of deposits containing recent shells and of quite +insignificant dimensions, no tertiary formations have been observed on this +coast, for a space of twenty-two degrees of latitude north of Copiapo, until +coming to Payta, where there is said to be a considerable calcareous deposit: a +few fossils have been described by M. d’Orbigny from this place, +namely:— +</p> + +<p> +1. Rostellaria Gaudichaudi, d’Orbigny “Voyage” Pal. 2. +Pectunculus Paytensis, d’Orbigny “Voyage” Pal. 3. Venus +petitiana, d’Orbigny “Voyage” Pal. 4. Ostrea Patagonica? This +great oyster (of which specimens have been given me) cannot be distinguished by +Mr. Sowerby from some of the varieties from Patagonia; though it would be +hazardous to assert it is the same with that species, or with that from +Coquimbo. +</p> + +<h3>CONCLUDING REMARKS.</h3> + +<p> +The formations described in this chapter, have, in the case of Chiloe and +probably in that of Concepcion and Navidad, apparently been accumulated in +troughs formed by submarine ridges extending parallel to the ancient shores of +the continent; in the case of the islands of Mocha and Huafo it is highly +probable, and in that of Ypun and Lemus almost certain, that they were +accumulated round isolated rocky centres or nuclei, in the same manner as mud +and sand are now collecting round the outlying islets and reefs in the West +Indian Archipelago. Hence, I may remark, it does not follow that the outlying +tertiary masses of Mocha and Huafo were ever continuously united at the same +level with the formations on the mainland, though they may have been of +contemporaneous origin, and been subsequently upraised to the same height. In +the more northern parts of Chile, the tertiary strata seem to have been +separately accumulated in bays, now forming the mouths of valleys. +</p> + +<p> +The relation between these several deposits on the shores of the Pacific, is +not nearly so clear as in the case of the tertiary formations on the Atlantic. +Judging from the form and height of the land (evidence which I feel sure is +here much more trustworthy than it can ever be in such broken continents as +that of Europe), from the identity of mineralogical composition, from the +presence of fragments of lignite and of silicified wood, and from the +intercalated layers of imperfect coal, I must believe that the coast-formations +from Central Chiloe to Concepcion, a distance of 400 miles, are of the same +age: from nearly similar reasons, I suspect that the beds of Mocha, Huafo, and +Ypun, belong also to the same period. The commonest shell in Mocha and Huafo is +the same species of Turritella; and I believe the same Cytheraea is found on +the islands of Huafo, Chiloe, and Ypun; but with these trifling exceptions, the +few organic remains found at these places are distinct. The numerous shells +from Navidad, with the exception of two, namely, the Sigaretus and Turritella +found at Ypun, are likewise distinct from those found in any other part of this +coast. Coquimbo has Cardium auca in common with Concepcion, and Fusus Cleryanus +with Huafo; I may add, that Coquimbo has Venus petitiana, and a gigantic oyster +(said by M. d’Orbigny also to be found a little south of Concepcion) in +common with Payta, though this latter place is situated twenty-two degrees +northward of latitude 27 degrees, to which point the Coquimbo formation +extends. +</p> + +<p> +From these facts, and from the generic resemblance of the fossils from the +different localities, I cannot avoid the suspicion that they all belong to +nearly the same epoch, which epoch, as we shall immediately see, must be a very +ancient tertiary one. But as the Baculite, especially considering its apparent +identity with the Cretaceous Pondicherry species, and the presence of an +Ammonite, and the resemblance of the Nautilus to two upper greensand species, +together afford very strong evidence that the formation of Concepcion is a +Secondary one; I will, in my remarks on the fossils from the other localities, +put on one side those from Concepcion and from Eastern Chiloe, which, whatever +their age may be, appear to me to belong to one group. I must, however, again +call attention to the fact that the Cardium auca is found both at Concepcion +and in the undoubtedly tertiary strata of Coquimbo: nor should the possibility +be overlooked, that as Trigonia, though known in the northern hemisphere only +as a Secondary genus, has living representatives in the Australian seas, so a +Baculite, Ammonite, and Trigonia may have survived in this remote part of the +southern ocean to a somewhat later period than to the north of the equator. +</p> + +<p> +Before passing in review the fossils from the other localities, there are two +points, with respect to the formations between Concepcion and Chiloe, which +deserve some notice. First, that though the strata are generally horizontal, +they have been upheaved in Chiloe in a set of parallel anticlinal and uniclinal +lines ranging north and south,—in the district near P. Rumena by eight or +nine far-extended, most symmetrical, uniclinal lines ranging nearly east and +west,—and in the neighbourhood of Concepcion by less regular single +lines, directed both N.E. and S.W., and N.W. and S.E. This fact is of some +interest, as showing that within a period which cannot be considered as very +ancient in relation to the history of the continent, the strata between the +Cordillera and the Pacific have been broken up in the same variously directed +manner as have the old plutonic and metamorphic rocks in this same district. +The second point is, that the sandstone between Concepcion and Southern Chiloe +is everywhere lignitiferous, and includes much silicified wood; whereas the +formations in Northern Chile do not include beds of lignite or coal, and in +place of the fragments of silicified wood there are silicified bones. Now, at +the present day, from Cape Horn to near Concepcion, the land is entirely +concealed by forests, which thin out at Concepcion, and in Central and Northern +Chile entirely disappear. This coincidence in the distribution of the fossil +wood and the living forests may be quite accidental; but I incline to take a +different view of it; for, as the difference in climate, on which the presence +of forests depends, is here obviously in chief part due to the form of the +land, and as the Cordillera undoubtedly existed when the lignitiferous beds +were accumulating, I conceive it is not improbable that the climate, during the +lignitiferous period, varied on different parts of the coast in a somewhat +similar manner as it now does. Looking to an earlier epoch, when the strata of +the Cordillera were depositing, there were islands which even in the latitude +of Northern Chile, where now all is irreclaimably desert, supported large +coniferous forests. +</p> + +<h3>TABLE 4.</h3> + +<p> +Column 1. Genera, with living and tertiary species on the west coast of South +America. (M. d’Orbigny states that the genus Natica is not found on the +coast of Chile; but Mr. Cuming found it at Valparaiso. Scalaria was found at +Valparaiso; Arca, at Iquique, in latitude 20, by Mr. Cuming; Arca, also, was +found by Captain King, at Juan Fernandez, in latitude 33 degrees 30′S.) +</p> + +<p> +Column 2. Latitudes, in which found fossil on the coasts of Chile and Peru. (In +degrees and minutes.) +</p> + +<p> +Column 3. Southernmost latitude, in which found living on the west coast of +South America. (In degrees and minutes.) +</p> + +<p> +Bulla : 30 to 43 30 : 12 near Lima. +</p> + +<p> +Cassis : 34 : 1 37. +</p> + +<p> +Pyrula : 34 (and 36 30 at Concepcion) : 5 Payta. +</p> + +<p> +Fusus : 30 and 43 30 : 23 Mexillones; reappears at the St. of Magellan. +</p> + +<p> +Pleurotoma : 34 to 43 30 : 2 18 St. Elena. +</p> + +<p> +Terebra : 34 : 5 Payta. +</p> + +<p> +Sigaretus : 34 to 44 30 : 12 Lima. +</p> + +<p> +Anomia : 30 : 7 48. +</p> + +<p> +Perna : 30 : 1 23 Xixappa. +</p> + +<p> +Cardium : 30 to 34 (and 36 30 at Concepcion) : 5 Payta. +</p> + +<p> +Artemis : 30 : 5 Payta. +</p> + +<p> +Voluta : 34 to 44 30 : Mr. Cuming does not know of any species living on the +west coast, between the equator and latitude 43 south; from this latitude a +species is found as far south as Tierra del Fuego. +</p> + +<p> +Seventy-nine species of fossil shells, in a tolerably recognisable condition, +from the coast of Chile and Peru, are described in this volume, and in the +Palaeontological part of M. d’Orbigny’s “Voyage”: if we +put on one side the twenty species exclusively found at Concepcion and Chiloe, +fifty-nine species from Navidad and the other specified localities remain. Of +these fifty-nine species only an Artemis, a Mytilus and Balanus, all from +Coquimbo, are (in the opinion of Mr. Sowerby, but not in that of M. +d’Orbigny) identical with living shells; and it would certainly require a +better series of specimens to render this conclusion certain. Only the +Turritella Chilensis from Huafo and Mocha, the T. Patagonica and Venus +meridionalis from Navidad, come very near to recent South American shells, +namely, the two Turritellas to T. cingulata, and the Venus to V. exalbida: some +few other species come rather less near; and some few resemble forms in the +older European tertiary deposits: none of the species resemble secondary forms. +Hence I conceive there can be no doubt that these formations are +tertiary,—a point necessary to consider, after the case of Concepcion. +The fifty-nine species belong to thirty-two genera; of these, Gastridium is +extinct, and three or four of the genera (viz. Panopaea, Rostellaria, Corbis +(?), and I believe Solecurtus) are not now found on the west coast of South +America. Fifteen of the genera have on this coast living representatives in +about the same latitudes with the fossil species; but twelve genera now range +very differently to what they formerly did. The idea of Table 4, in which the +difference between the extension in latitude of the fossil and existing species +is shown, is taken from M. d’Orbigny’s work; but the range of the +living shells is given on the authority of Mr. Cuming, whose long-continued +researches on the conchology of South America are well-known. +</p> + +<p> +When we consider that very few, if any, of the fifty-nine fossil shells are +identical with, or make any close approach to, living species; when we consider +that some of the genera do not now exist on the west coast of South America, +and that no less than twelve genera out of the thirty-two formerly ranged very +differently from the existing species of the same genera, we must admit that +these deposits are of considerable antiquity, and that they probably verge on +the commencement of the tertiary era. May we not venture to believe, that they +are of nearly contemporaneous origin with the Eocene formations of the northern +hemisphere? +</p> + +<p> +Comparing the fossil remains from the coast of Chile (leaving out, as before, +Concepcion and Chiloe) with those from Patagonia, we may conclude, from their +generic resemblance, and from the small number of the species which from either +coast approach closely to living forms, that the formations of both belong to +nearly the same epoch; and this is the opinion of M. D’Orbigny. Had not a +single fossil shell been common to the two coasts, it could not have been +argued that the formations belonged to different ages; for Messrs. Cuming and +Hinds have found, on the comparison of nearly two thousand living species from +the opposite sides of South America, only one in common, namely, the Purpura +lapillus from both sides of the Isthmus of Panama: even the shells collected by +myself amongst the Chonos Islands and on the coast of Patagonia, are +dissimilar, and we must descend to the apex of the continent, to Tierra del +Fuego, to find these two great conchological provinces united into one. Hence +it is remarkable that four or five of the fossil shells from Navidad, namely, +Voluta alta, Turritella Patagonica, Trochus collaris, Venus meridionalis, +perhaps Natica solida, and perhaps the large oyster from Coquimbo, are +considered by Mr. Sowerby as identical with species from Santa Cruz and P. +Desire. M. d’Orbigny, however, admits the perfect identity only of the +Trochus. +</p> + +<h3>ON THE TEMPERATURE OF THE TERTIARY PERIOD.</h3> + +<p> +As the number of the fossil species and genera from the western and eastern +coasts is considerable, it will be interesting to consider the probable nature +of the climate under which they lived. We will first take the case of Navidad, +in latitude 34 degrees, where thirty-one species were collected, and which, as +we shall presently see, must have inhabited shallow water, and therefore will +necessarily well exhibit the effects of temperature. Referring to Table 4 we +find that the existing species of the genera Cassis, Pyrula, Pleurotoma, +Terebra, and Sigaretus, which are generally (though by no means invariably) +characteristic of warmer latitudes, do not at the present day range nearly so +far south on this line of coast as the fossil species formerly did. Including +Coquimbo, we have Perna in the same predicament. The first impression from this +fact is, that the climate must formerly have been warmer than it now is; but we +must be very cautious in admitting this, for Cardium, Bulla, and Fusus (and, if +we include Coquimbo, Anomia and Artemis) likewise formerly ranged farther south +than they now do; and as these genera are far from being characteristic of hot +climates, their former greater southern range may well have been owing to +causes quite distinct from climate: Voluta, again, though generally so tropical +a genus, is at present confined on the west coast to colder or more southern +latitudes than it was during the tertiary period. The Trochus collaris, +moreover, and, as we have just seen according to Mr. Sowerby, two or three +other species, formerly ranged from Navidad as far south as Santa Cruz in +latitude 50 degrees. If, instead of comparing the fossils of Navidad, as we +have hitherto done, with the shells now living on the west coast of South +America, we compare them with those found in other parts of the world, under +nearly similar latitudes; for instance, in the southern parts of the +Mediterranean or of Australia, there is no evidence that the sea off Navidad +was formerly hotter than what might have been expected from its latitude, even +if it was somewhat warmer than it now is when cooled by the great southern +polar current. Several of the most tropical genera have no representative +fossils at Navidad; and there are only single species of Cassis, Pyrula, and +Sigaretus, two of Pleurotoma and two of Terebra, but none of these species are +of conspicuous size. In Patagonia, there is even still less evidence in the +character of the fossils, of the climate having been formerly warmer. (It may +be worth while to mention that the shells living at the present day on this +eastern side of South America, in latitude 40 degrees, have perhaps a more +tropical character than those in corresponding latitudes on the shores of +Europe: for at Bahia Blanca and S. Blas, there are two fine species of Voluta +and four of Oliva.) As from the various reasons already assigned, there can be +little doubt that the formations of Patagonia and at least of Navidad and +Coquimbo in Chile, are the equivalents of an ancient stage in the tertiary +formations of the northern hemisphere, the conclusion that the climate of the +southern seas at this period was not hotter than what might have been expected +from the latitude of each place, appears to me highly important; for we must +believe, in accordance with the views of Mr. Lyell, that the causes which gave +to the older tertiary productions of the quite temperate zones of Europe a +tropical character, WERE OF A LOCAL CHARACTER AND DID NOT AFFECT THE ENTIRE +GLOBE. On the other hand, I have endeavoured to show, in the “Geological +Transactions,” that, at a much later period, Europe and North and South +America were nearly contemporaneously subjected to ice- action, and +consequently to a colder, or at least more equable, climate than that now +characteristic of the same latitudes. +</p> + +<p> +ON THE ABSENCE OF EXTENSIVE MODERN CONCHIFEROUS DEPOSITS IN SOUTH AMERICA; AND +ON THE CONTEMPORANEOUSNESS OF THE OLDER TERTIARY DEPOSITS AT DISTANT POINTS +BEING DUE TO CONTEMPORANEOUS MOVEMENTS OF SUBSIDENCE. +</p> + +<p> +Knowing from the researches of Professor E. Forbes, that molluscous animals +chiefly abound within a depth of 100 fathoms and under, and bearing in mind how +many thousand miles of both coasts of South America have been upraised within +the recent period by a slow, long-continued, intermittent movement,—seeing +the diversity in nature of the shores and the number of shells now living on +them,—seeing also that the sea off Patagonia and off many parts of Chile, +was during the tertiary period highly favourable to the accumulation of +sediment,—the absence of extensive deposits including recent shells over +these vast spaces of coast is highly remarkable. The conchiferous calcareous +beds at Coquimbo, and at a few isolated points northward, offer the most marked +exception to this statement; for these beds are from twenty to thirty feet in +thickness, and they stretch for some miles along shore, attaining, however, +only a very trifling breadth. At Valdivia there is some sandstone with +imperfect casts of shells, which POSSIBLY may belong to the recent period: +parts of the boulder formation and the shingle-beds on the lower plains of +Patagonia probably belong to this same period, but neither are fossiliferous: +it also so happens that the great Pampean formation does not include, with the +exception of the Azara, any mollusca. There cannot be the smallest doubt that +the upraised shells along the shores of the Atlantic and Pacific, whether lying +on the bare surface, or embedded in mould or in sand-hillocks, will in the +course of ages be destroyed by alluvial action: this probably will be the case +even with the calcareous beds of Coquimbo, so liable to dissolution by +rain-water. If we take into consideration the probability of oscillations of +level and the consequent action of the tidal-waves at different heights, their +destruction will appear almost certain. Looking to an epoch as far distant in +futurity as we now are from the past Miocene period, there seems to me scarcely +a chance, under existing conditions, of the numerous shells now living in those +zones of depths most fertile in life, and found exclusively on the western and +south-eastern coasts of South America, being preserved to this imaginary +distant epoch. A whole conchological series will in time be swept away, with no +memorials of their existence preserved in the earth’s crust. +</p> + +<p> +Can any light be thrown on this remarkable absence of recent conchiferous +deposits on these coasts, on which, at an ancient tertiary epoch, strata +abounding with organic remains were extensively accumulated? I think there can, +namely, by considering the conditions necessary for the preservation of a +formation to a distant age. Looking to the enormous amount of denudation which +on all sides of us has been effected,—as evidenced by the lofty cliffs +cutting off on so many coasts horizontal and once far-extended strata of no +great antiquity (as in the case of Patagonia),—as evidenced by the level +surface of the ground on both sides of great faults and dislocations,—by +inland lines of escarpments, by outliers, and numberless other facts, and by +that argument of high generality advanced by Mr. Lyell, namely, that every +SEDIMENTARY formation, whatever its thickness may be, and over however many +hundred square miles it may extend, is the result and the measure of an equal +amount of wear and tear of pre-existing formations; considering these facts, we +must conclude that, as an ordinary rule, a formation to resist such vast +destroying powers, and to last to a distant epoch, must be of wide extent, and +either in itself, or together with superincumbent strata, be of great +thickness. In this discussion, we are considering only formations containing +the remains of marine animals, which, as before mentioned, live, with some +exceptions within (most of them much within) depths of 100 fathoms. How, then, +can a thick and widely extended formation be accumulated, which shall include +such organic remains? First, let us take the case of the bed of the sea long +remaining at a stationary level: under these circumstances it is evident that +CONCHIFEROUS strata can accumulate only to the same thickness with the depth at +which the shells can live; on gently inclined coasts alone can they accumulate +to any considerable width; and from the want of superincumbent pressure, it is +probable that the sedimentary matter will seldom be much consolidated: such +formations have no very good chance, when in the course of time they are +upraised, of long resisting the powers of denudation. The chance will be less +if the submarine surface, instead of having remained stationary, shall have +gone on slowly rising during the deposition of the strata, for in this case +their total thickness must be less, and each part, before being consolidated or +thickly covered up by superincumbent matter, will have had successively to pass +through the ordeal of the beach; and on most coasts, the waves on the beach +tend to wear down and disperse every object exposed to their action. Now, both +on the south-eastern and western shores of South America, we have had clear +proofs that the land has been slowly rising, and in the long lines of lofty +cliffs, we have seen that the tendency of the sea is almost everywhere to eat +into the land. Considering these facts, it ceases, I think, to be surprising, +that extensive recent conchiferous deposits are entirely absent on the southern +and western shores of America. +</p> + +<p> +Let us take the one remaining case, of the bed of the sea slowly subsiding +during a length of time, whilst sediment has gone on being deposited. It is +evident that strata might thus accumulate to any thickness, each stratum being +deposited in shallow water, and consequently abounding with those shells which +cannot live at great depths: the pressure, also, I may observe, of each fresh +bed would aid in consolidating all the lower ones. Even on a rather steep +coast, though such must ever be unfavourable to widely extended deposits, the +formations would always tend to increase in breadth from the water encroaching +on the land. Hence we may admit that periods of slow subsidence will commonly +be most favourable to the accumulation of CONCHIFEROUS deposits, of sufficient +thickness, extension, and hardness, to resist the average powers of denudation. +</p> + +<p> +We have seen that at an ancient tertiary epoch, fossiliferous deposits were +extensively deposited on the coasts of South America; and it is a very +interesting fact, that there is evidence that these ancient tertiary beds were +deposited during a period of subsidence. Thus, at Navidad, the strata are about +eight hundred feet in thickness, and the fossil shells are abundant both at the +level of the sea and some way up the cliffs; having sent a list of these +fossils to Professor E. Forbes, he thinks they must have lived in water between +one and ten fathoms in depth: hence the bottom of the sea on which these shells +once lived must have subsided at least 700 feet to allow of the superincumbent +matter being deposited. I must here remark, that, as all these and the +following fossil shells are extinct species, Professor Forbes necessarily +judges of the depths at which they lived only from their generic character, and +from the analogical distribution of shells in the northern hemisphere; but +there is no just cause from this to doubt the general results. At Huafo the +strata are about the same thickness, namely, 800 feet, and Professor Forbes +thinks the fossils found there cannot have lived at a greater depth than fifty +fathoms, or 300 feet. These two points, namely, Navidad and Huafo, are 570 +miles apart, but nearly halfway between them lies Mocha, an island 1,200 feet +in height, apparently formed of tertiary strata up to its level summit, and +with many shells, including the same Turritella with that found at Huafo, +embedded close to the level of the sea. In Patagonia, shells are numerous at +Santa Cruz, at the foot of the 350 feet plain, which has certainly been formed +by the denudation of the 840 feet plain, and therefore was originally covered +by strata that number of feet in thickness, and these shells, according to +Professor Forbes, probably lived at a depth of between seven and fifteen +fathoms: at Port S. Julian, sixty miles to the north, shells are numerous at +the foot of the ninety feet plain (formed by the denudation of the 950 feet +plain), and likewise occasionally at the height of several hundred feet in the +upper strata; these shells must have lived in water somewhere between five and +fifty fathoms in depth. Although in other parts of Patagonia I have no direct +evidence of shoal-water shells having been buried under a great thickness of +superincumbent submarine strata, yet it should be borne in mind that the lower +fossiliferous strata with several of the same species of Mollusca, the upper +tufaceous beds, and the high summit-plain, stretch for a considerable distance +southward, and for hundreds of miles northward; seeing this uniformity of +structure, I conceive it may be fairly concluded that the subsidence by which +the shells at Santa Cruz and S. Julian were carried down and covered up, was +not confined to these two points, but was co-extensive with a considerable +portion of the Patagonian tertiary formation. In a succeeding chapter it will +be seen, that we are led to a similar conclusion with respect to the secondary +fossiliferous strata of the Cordillera, namely, that they also were deposited +during a long- continued and great period of subsidence. From the foregoing +reasoning, and from the facts just given, I think we must admit the probability +of the following proposition: namely, that when the bed of the sea is either +stationary or rising, circumstances are far less favourable, than when the +level is sinking, to the accumulation of CONCHIFEROUS deposits of sufficient +thickness and extension to resist, when upheaved, the average vast amount of +denudation. This result appears to me, in several respects, very interesting: +every one is at first inclined to believe that at innumerable points, wherever +there is a supply of sediment, fossiliferous strata are now forming, which at +some future distant epoch will be upheaved and preserved; but on the views +above given, we must conclude that this is far from being the case; on the +contrary, we require (1st), a long-continued supply of sediment; (2nd), an +extensive shallow area; and (3rd), that this area shall slowly subside to a +great depth, so as to admit the accumulation of a widely extended thick mass of +superincumbent strata. In how few parts of the world, probably, do these +conditions at the present day concur! We can thus, also, understand the general +want of that close sequence in fossiliferous formations which we might +theoretically have anticipated; for, without we suppose a subsiding movement to +go on at the same spot during an enormous period, from one geological era to +another, and during the whole of this period sediment to accumulate at the +proper rate, so that the depth should not become too great for the continued +existence of molluscous animals, it is scarcely possible that there should be a +perfect sequence at the same spot in the fossil shells of the two geological +formations. (Professor H.D. Rogers, in his excellent address to the Association +of American Geologists (“Silliman’s Journal” volume 47 page +277) makes the following remark: “I question if we are at all aware how +COMPLETELY the whole history of all departed time lies indelibly recorded with +the amplest minuteness of detail in the successive sediments of the globe, how +effectually, in other words, every period of time HAS WRITTEN ITS OWN HISTORY, +carefully preserving every created form and every trace of action.” I +think the correctness of such remarks is more than doubtful, even if we except +(as I suppose he would) all those numerous organic forms which contain no hard +parts.) So far from a very long-continued subsidence being probable, many facts +lead to the belief that the earth’s surface oscillates up and down; and +we have seen that during the elevatory movements there is but a small chance of +DURABLE fossiliferous deposits accumulating. +</p> + +<p> +Lastly, these same considerations appear to throw some light on the fact that +certain periods appear to have been favourable to the deposition, or at least +to the preservation, of contemporaneous formations at very distant points. We +have seen that in South America an enormous area has been rising within the +recent period; and in other quarters of the globe immense spaces appear to have +risen contemporaneously. From my examination of the coral- reefs of the great +oceans, I have been led to conclude that the bed of the sea has gone on slowly +sinking within the present era, over truly vast areas: this, indeed, is in +itself probable, from the simple fact of the rising areas having been so large. +In South America we have distinct evidence that at nearly the same tertiary +period, the bed of the sea off parts of the coast of Chile and off Patagonia +was sinking, though these regions are very remote from each other. If, then, it +holds good, as a general rule, that in the same quarter of the globe the +earth’s crust tends to sink and rise contemporaneously over vast spaces, +we can at once see, that we have at distant points, at the same period, those +very conditions which appear to be requisite for the accumulation of +fossiliferous masses of sufficient extension, thickness, and hardness, to +resist denudation, and consequently to last unto an epoch distant in futurity. +(Professor Forbes has some admirable remarks on this subject, in his +“Report on the Shells of the Aegean Sea.” In a letter to Mr. +Maclaren (“Edinburgh New Philosophical Journal” January 1843), I +partially entered into this discussion, and endeavoured to show that it was +highly improbable, that upraised atolls or barrier-reefs, though of great +thickness, should, owing to their small extension or breadth, be preserved to a +distant future period.) +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0006"></a> +CHAPTER VI.<br /> +PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.</h2> + +<p class="letter"> +Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of +foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in, +decomposition of.—La Plata, metamorphic and old volcanic rocks +of.—S. Ventana.—Claystone porphyry formation of Patagonia; singular +metamorphic rocks; pseudo-dikes.—Falkland Islands, Palaeozoic fossils +of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of; +cleavage and foliation; form of land.—Chonos Archipelago, mica-schists, +foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, +dikes, successive formation of.—Central and Northern +Chile.—Concluding remarks on cleavage and foliation.—Their close +analogy and similar origin.—Stratification of metamorphic +schists.—Foliation of intrusive rocks.—Relation of cleavage and +foliation to the lines of tension during metamorphosis. +</p> + +<p> +The metamorphic and plutonic formations of the several districts visited by the +“Beagle” will be here chiefly treated of, but only such cases as +appear to me new, or of some special interest, will be described in detail; at +the end of the chapter I will sum up all the facts on cleavage and +foliation,— to which I particularly attended. +</p> + +<p> +BAHIA, BRAZIL: latitude 13 degrees south. +</p> + +<p> +The prevailing rock is gneiss, often passing, by the disappearance of the +quartz and mica, and by the feldspar losing its red colour, into a brilliantly +grey primitive greenstone. Not unfrequently quartz and hornblende are arranged +in layers in almost amorphous feldspar. There is some fine-grained syenitic +granite, orbicularly marked by ferruginous lines, and weathering into vertical, +cylindrical holes, almost touching each other. In the gneiss, concretions of +granular feldspar and others of garnets with mica occur. The gneiss is +traversed by numerous dikes composed of black, finely crystallised, hornblendic +rock, containing a little glassy feldspar and sometimes mica, and varying in +thickness from mere threads to ten feet: these threads, which are often +curvilinear, could sometimes be traced running into the larger dikes. One of +these dikes was remarkable from having been in two or three places laterally +disjointed, with unbroken gneiss interposed between the broken ends, and in one +part with a portion of the gneiss driven, apparently whilst in a softened +state, into its side or wall. In several neighbouring places, the gneiss +included angular, well- defined, sometimes bent, masses of hornblende rock, +quite like, except in being more perfectly crystallised, that forming the +dikes, and, at least in one instance, containing (as determined by Professor +Miller) augite as well as hornblende. In one or two cases these angular masses, +though now quite separate from each other by the solid gneiss, had, from their +exact correspondence in size and shape, evidently once been united; hence I +cannot doubt that most or all of the fragments have been derived from the +breaking up of the dikes, of which we see the first stage in the above- +mentioned laterally disjointed one. The gneiss close to the fragments generally +contained many large crystals of hornblende, which are entirely absent or rare +in other parts: its folia or laminae were gently bent round the fragments, in +the same manner as they sometimes are round concretions. Hence the gneiss has +certainly been softened, its composition modified, and its folia arranged, +subsequently to the breaking up of the dikes, these latter also having been at +the same time bent and softened. (Professor Hitchcock “Geology of +Massachusetts” volume 2 page 673, gives a closely similar case of a +greenstone dike in syenite.) +</p> + +<p> +I must here take the opportunity of premising, that by the term CLEAVAGE I +imply those planes of division which render a rock, appearing to the eye quite +or nearly homogeneous, fissile. By the term FOLIATION, I refer to the layers or +plates of different mineralogical nature of which most metamorphic schists are +composed; there are, also, often included in such masses, alternating, +homogeneous, fissile layers or folia, and in this case the rock is both +foliated and has a cleavage. By STRATIFICATION, as applied to these formations, +I mean those alternate, parallel, large masses of different composition, which +are themselves frequently either foliated or fissile,—such as the +alternating so-called strata of mica-slate, gneiss, glossy clay-slate, and +marble. +</p> + +<p> +The folia of the gneiss within a few miles round Bahia generally strike +irregularly, and are often curvilinear, dipping in all directions at various +angles: but where best defined, they extended most frequently in a N.E. by N. +(or East 50 degrees N.) and S.W. by S. line, corresponding nearly with the +coast-line northwards of the bay. I may add that Mr. Gardner found in several +parts of the province of Ceara, which lies between four and five hundred miles +north of Bahia, gneiss with the folia extending E. 45 degrees N.; and in Guyana +according to Sir R. Schomburgk, the same rock strikes E. 57 degrees N. Again, +Humboldt describes the gneiss-granite over an immense area in Venezuela and +even in Colombia, as striking E. 50 degrees N., and dipping to the N.W. at an +angle of fifty degrees. (Gardner “Geological Section of the British +Association” 1840. For Sir R. Schomburgk’s observations see +“Geographical Journal” 1842 page 190. See also Humboldt’s +discussion on Loxodrism in the “Personal Narrative.”) Hence all the +observations hitherto made tend to show that the gneissic rocks over the whole +of this part of the continent have their folia extending generally within +almost a point of the compass of the same direction. (I landed at only one +place north of Bahia, namely, at Pernambuco. I found there only soft, +horizontally stratified matter, formed from disintegrated granitic rocks, and +some yellowish impure limestone, probably of a tertiary epoch. I have described +a most singular natural bar of hard sandstone, which protects the harbour, in +the 19th volume 1841 page 258 of the “London and Edinburgh Philosophical +Magazine.” +</p> + +<p> +ABROLHOS ISLETS, Latitude 18 degrees S. off the coast of Brazil. +</p> + +<p> +Although not strictly in place, I do not know where I can more conveniently +describe this little group of small islands. The lowest bed is a sandstone with +ferruginous veins; it weathers into an extraordinary honeycombed mass; above it +there is a dark-coloured argillaceous shale; above this a coarser +sandstone—making a total thickness of about sixty feet; and lastly, above +these sedimentary beds, there is a fine conformable mass of greenstone, in some +parts having a columnar structure. All the strata, as well as the surface of +the land, dip at an angle of about 12 degrees to N. by W. Some of the islets +are composed entirely of the sedimentary, others of the trappean rocks, +generally, however, with the sandstone, cropping out on the southern shores.) +</p> + +<h3>RIO DE JANEIRO.</h3> + +<p> +This whole district is almost exclusively formed of gneiss, abounding with +garnets, and porphyritic with large crystals, even three and four inches in +length, of orthoclase feldspar: in these crystals mica and garnets are often +enclosed. At the western base of the Corcovado, there is some ferruginous +carious quartz-rock; and in the Tijeuka range, much fine- grained granite. I +observed boulders of greenstone in several places; and on the islet of +Villegagnon, and likewise on the coast some miles northward, two large trappean +dikes. The porphyritic gneiss, or gneiss- granite as it has been called by +Humboldt, is only so far foliated that the constituent minerals are arranged +with a certain degree of regularity, and may be said to have a +“GRAIN,” but they are not separated into distinct folia or laminae. +There are, however, several other varieties of gneiss regularly foliated, and +alternating with each other in so-called strata. The stratification and +foliation of the ordinary gneisses, and the foliation or “grain” of +the gneiss-granite, are parallel to each other, and generally strike within a +point of N.E. and S.W. dipping at a high angle (between 50 and 60 degrees) +generally to S.E.: so that here again we meet with the strike so prevalent over +the more northern parts of this continent. The mountains of gneiss-granite are +to a remarkable degree abruptly conical, which seems caused by the rock tending +to exfoliate in thick, conically concentric layers: these peaks resemble in +shape those of phonolite and other injected rocks on volcanic islands; nor is +the grain or foliation (as we shall afterwards see) any difficulty on the idea +of the gneiss-granite having been an intrusive rather than a metamorphic +formation. The lines of mountains, but not always each separate hill, range +nearly in the same direction with the foliation and so-called stratification, +but rather more easterly. +</p> + +<p> +(FIGURE 22. FRAGMENT OF GNEISS EMBEDDED IN ANOTHER VARIETY OF THE SAME ROCK.) +</p> + +<p> +On a bare gently inclined surface of the porphyritic gneiss in Botofogo Bay, I +observed the appearance represented in Figure 22. A fragment seven yards long +and two in width, with angular and distinctly defined edges, composed of a +peculiar variety of gneiss with dark layers of mica and garnets, is surrounded +on all sides by the ordinary gneiss- granite; both having been dislocated by a +granitic vein. The folia in the fragment and in the surrounding rock strike in +the same N.N.E. and S.S.W. line; but in the fragment they are vertical, whereas +in the gneiss-granite they dip at a small angle, as shown by the arrows, to +S.S.E. This fragment, considering its great size, its solitary position, and +its foliated structure parallel to that of the surrounding rock, is, as far as +I know, a unique case: and I will not attempt any explanation of its origin. +</p> + +<p> +The numerous travellers in this country, have all been greatly surprised at the +depth to which the gneiss and other granitic rocks, as well as the talcose +slates of the interior, have been decomposed. (Spix and Martius have collected +in an Appendix to their “Travels,” the largest body of facts on +this subject. See also some remarks by M. Lund in his communications to the +Academy at Copenhagen; and others by M. Gaudichaud in Freycinet +“Voyage.”) Near Rio, every mineral except the quartz has been +completely softened, in some places to a depth little less than one hundred +feet. (Dr. Benza describes granitic rock, “Madras Journal of +Literature” etc. October 183? page 246), in the Neelgherries, decomposed +to a depth of forty feet.) The minerals retain their positions in folia ranging +in the usual direction; and fractured quartz veins may be traced from the solid +rock, running for some distance into the softened, mottled, highly coloured, +argillaceous mass. It is said that these decomposed rocks abound with gems of +various kinds, often in a fractured state, owing, as some have supposed, to the +collapse of geodes, and that they contain gold and diamonds. At Rio, it +appeared to me that the gneiss had been softened before the excavation (no +doubt by the sea) of the existing, broad, flat-bottomed valleys; for the depth +of decomposition did not appear at all conformable with the present undulations +of the surface. The porphyritic gneiss, where now exposed to the air, seems to +withstand decomposition remarkably well; and I could see no signs of any +tendency to the production of argillaceous masses like those here described. I +was also struck with the fact, that where a bare surface of this rock sloped +into one of the quiet bays, there were no marks of erosion at the level of the +water, and the parts both beneath and above it preserved a uniform curve. At +Bahia, the gneiss rocks are similarly decomposed, with the upper parts +insensibly losing their foliation, and passing, without any distinct line of +separation, into a bright red argillaceous earth, including partially rounded +fragments of quartz and granite. From this circumstance, and from the rocks +appearing to have suffered decomposition before the excavation of the valleys, +I suspect that here, as at Rio, the decomposition took place under the sea. The +subject appeared to me a curious one, and would probably well repay careful +examination by an able mineralogist. +</p> + +<h3>THE NORTHERN PROVINCES OF LA PLATA.</h3> + +<p> +According to some observations communicated to me by Mr. Fox, the coast from +Rio de Janeiro to the mouth of the Plata seems everywhere to be granitic, with +a few trappean dikes. At Port Alegre, near the boundary of Brazil, there are +porphyries and diorites. (M. Isabelle “Voyage a Buenos Ayres” page +479.) At the mouth of the Plata, I examined the country for twenty-five miles +west, and for about seventy miles north of Maldonado: near this town, there is +some common gneiss, and much, in all parts of the country, of a coarse-grained +mixture of quartz and reddish feldspar, often, however, assuming a little +dark-green imperfect hornblende, and then immediately becoming foliated. The +abrupt hillocks thus composed, as well as the highly inclined folia of the +common varieties of gneiss, strike N.N.E. or a little more easterly, and S.S.W. +Clay-slate is occasionally met with, and near the L. del Potrero, there is +white marble, rendered fissile from the presence of hornblende, mica, and +asbestus; the cleavage of these rocks and their stratification, that is the +alternating masses thus composed, strike N.N.E. and S.S.W. like the foliated +gneisses, and have an almost vertical dip. The Sierra Larga, a low range five +miles west of Maldonado, consists of quartzite, often ferruginous, having an +arenaceous feel, and divided into excessively thin, almost vertical laminae or +folia by microscopically minute scales, apparently of mica, and striking in the +usual N.N.E. and S.S.W. direction. The range itself is formed of one principal +line with some subordinate ones; and it extends with remarkable uniformity far +northward (it is said even to the confines of Brazil), in the same line with +the vertically ribboned quartz rock of which it is composed. The S. de Las +Animas is the highest range in the country; I estimated it at 1,000 feet; it +runs north and south, and is formed of feldspathic porphyry; near its base +there is a N.N.W. and S.S.E. ridge of a conglomerate in a highly porphyritic +basis. +</p> + +<p> +Northward of Maldonado, and south of Las Minas, there is an E. and W. hilly +band of country, some miles in width, formed of siliceous clay-slate, with some +quartz, rock, and limestone, having a tortuous irregular cleavage, generally +ranging east and west. E. and S.E. of Las Minas there is a confused district of +imperfect gneiss and laminated quartz, with the hills ranging in various +directions, but with each separate hill generally running in the same line with +the folia of the rocks of which it is composed: this confusion appears to have +been caused by the intersection of the [E. and W.] and [N.N.E. and S.S.W.] +strikes. Northward of Las Minas, the more regular northerly ranges predominate: +from this place to near Polanco, we meet with the coarse-grained mixture of +quartz and feldspar, often with the imperfect hornblende, and then becoming +foliated in a N. and S. line—with imperfect clay-slate, including laminae +of red crystallised feldspar—with white or black marble, sometimes +containing asbestus and crystals of gypsum—with quartz-rock—with +syenite—and lastly, with much granite. The marble and granite alternate +repeatedly in apparently vertical masses: some miles northward of the Polanco, +a wide district is said to be entirely composed of marble. It is remarkable, +how rare mica is in the whole range of country north and westward of Maldonado. +Throughout this district, the cleavage of the clay-slate and marble—the +foliation of the gneiss and the quartz—the stratification or alternating +masses of these several rocks—and the range of the hills, all coincide in +direction; and although the country is only hilly, the planes of division are +almost everywhere very highly inclined or vertical. +</p> + +<p> +Some ancient submarine volcanic rocks are worth mentioning, from their rarity +on this eastern side of the continent. In the valley of the Tapas (fifty or +sixty miles N. of Maldonado) there is a tract three or four miles in length, +composed of various trappean rocks with glassy feldspar—of apparently +metamorphosed grit-stones—of purplish amygdaloids with large kernels of +carbonate of lime (Near the Pan de Azucar there is some greenish porphyry, in +one place amygdaloidal with agate.)—and much of a harshish rock with +glassy feldspar intermediate in character between claystone porphyry and +trachyte. This latter rock was in one spot remarkable from being full of drusy +cavities, lined with quartz crystals, and arranged in planes, dipping at an +angle of 50 degrees to the east, and striking parallel to the foliation of an +adjoining hill composed of the common mixture of quartz, feldspar, and +imperfect hornblende: this fact perhaps indicates that these volcanic rocks +have been metamorphosed, and their constituent parts rearranged, at the same +time and according to the same laws, with the granitic and metamorphic +formations of this whole region. In the valley of the Marmaraya, a few miles +south of the Tapas, a band of trappean and amygdaloidal rock is interposed +between a hill of granite and an extensive surrounding formation of red +conglomerate, which (like that at the foot of the S. Animas) has its basis +porphyritic with crystals of feldspar, and which hence has certainly suffered +metamorphosis. +</p> + +<h3>MONTE VIDEO.</h3> + +<p> +The rocks here consist of several varieties of gneiss, with the feldspar often +yellowish, granular and imperfectly crystallised, alternating with, and passing +insensibly into, beds, from a few yards to nearly a mile in thickness, of fine +or coarse grained, dark-green hornblendic slate; this again often passing into +chloritic schist. These passages seem chiefly due to changes in the mica, and +its replacement by other minerals. At Rat Island I examined a mass of chloritic +schist, only a few yards square, irregularly surrounded on all sides by the +gneiss, and intricately penetrated by many curvilinear veins of quartz, which +gradually BLEND into the gneiss: the cleavage of the chloritic schist and the +foliation of the gneiss were exactly parallel. Eastward of the city there is +much fine- grained, dark-coloured gneiss, almost assuming the character of +hornblende- slate, which alternates in thin laminae with laminae of quartz, the +whole mass being transversely intersected by numerous large veins of quartz: I +particularly observed that these veins were absolutely continuous with the +alternating laminae of quartz. In this case and at Rat Island, the passage of +the gneiss into imperfect hornblendic or into chloritic slate, seemed to be +connected with the segregation of the veins of quartz. (Mr. Greenough page 78 +“Critical Examination” etc., observes that quartz in mica-slate +sometimes appears in beds and sometimes in veins. Von Buch also in his +“Travels in Norway” page 236, remarks on alternating laminae of +quartz and hornblende-slate replacing mica-schist.) +</p> + +<p> +The Mount, a hill believed to be 450 feet in height, from which the place takes +its name, is much the highest land in this neighbourhood: it consists of +hornblendic slate, which (except on the eastern and disturbed base) has an east +and west nearly vertical cleavage; the longer axis of the hill also ranges in +this same line. Near the summit the hornblende-slate gradually becomes more and +more coarsely crystallised, and less plainly laminated, until it passes into a +heavy, sonorous greenstone, with a slaty conchoidal fracture; the laminae on +the north and south sides near the summit dip inwards, as if this upper part +had expanded or bulged outwards. This greenstone must, I conceive, be +considered as metamorphosed hornblende- slate. The Cerrito, the next highest, +but much less elevated point, is almost similarly composed. In the more western +parts of the province, besides gneiss, there is quartz-rock, syenite, and +granite; and at Colla, I heard of marble. +</p> + +<p> +Near M. Video, the space which I more accurately examined was about fifteen +miles in an east and west line, and here I found the foliation of the gneiss +and the cleavage of the slates generally well developed, and extending parallel +to the alternating strata composed of the gneiss, hornblendic and chloritic +schists. These planes of division all range within one point of east and west, +frequently east by south and west by north; their dip is generally almost +vertical, and scarcely anywhere under 45 degrees: this fact, considering how +slightly undulatory the surface of the country is, deserves attention. Westward +of M. Video, towards the Uruguay, wherever the gneiss is exposed, the highly +inclined folia are seen striking in the same direction; I must except one spot +where the strike was N.W. by W. The little Sierra de S. Juan, formed of gneiss +and laminated quartz, must also be excepted, for it ranges between [N. to N.E.] +and [S. to S.W.] and seems to belong to the same system with the hills in the +Maldonado district. Finally, we have seen that, for many miles northward of +Maldonado and for twenty-five miles westward of it, as far as the S. de las +Animas, the foliation, cleavage, so-called stratification and lines of hills, +all range N.N.E. and S.S.W., which is nearly coincident with the adjoining +coast of the Atlantic. Westward of the S. de las Animas, as far as even the +Uruguay, the foliation, cleavage, and stratification (but not lines of hills, +for there are no defined ones) all range about E. by S. and W. by N., which is +nearly coincident with the direction of the northern shore of the Plata; in the +confused country near Las Minas, where these two great systems appear to +intersect each other, the cleavage, foliation, and stratification run in +various directions, but generally coincide with the line of each separate hill. +</p> + +<h3>SOUTHERN LA PLATA.</h3> + +<p> +The first ridge, south of the Plata, which projects through the Pampean +formation, is the Sierra Tapalguen and Vulcan, situated 200 miles southward of +the district just described. This ridge is only a few hundred feet in height, +and runs from C. Corrientes in a W.N.W. line for at least 150 miles into the +interior: at Tapalguen, it is composed of unstratified granular quartz, +remarkable from forming tabular masses and small plains, surrounded by +precipitous cliffs: other parts of the range are said to consist of granite: +and marble is found at the S. Tinta. It appears from M. Parchappe’s +observations, that at Tandil there is a range of quartzose gneiss, very like +the rocks of the S. Larga near Maldonado, running in the same N.N.E. and S.S.W. +direction; so that the framework of the country here is very similar to that on +the northern shore of the Plata. (M. d’Orbigny’s +“Voyage” Part. Geolog. page 46. I have given a short account of the +peculiar forms of the quartz hills of Tapalguen, so unusual in a metamorphic +formation, in my “Journal of Researches” 2nd edition page 116.) +</p> + +<p> +The Sierra Guitru-gueyu is situated sixty miles south of the S. Tapalguen: it +consists of numerous parallel, sometimes blended together ridges, about +twenty-three miles in width, and five hundred feet in height above the plain, +and extending in a N.W. and S.E. direction. Skirting round the extreme S.E. +termination, I ascended only a few points, which were composed of a +fine-grained gneiss, almost composed of feldspar with a little mica, and +passing in the upper parts of the hills into a rather compact purplish +clay-slate. The cleavage was nearly vertical, striking in a N.W. by W. and S.E. +by E. line, nearly, though not quite, coincident with the direction of the +parallel ridges. +</p> + +<p> +The Sierra Ventana lies close south of that of Guitru-gueyu; it is remarkable +from attaining a height, very unusual on this side of the continent, of 3,340 +feet. It consists up to its summit, of quartz, generally pure and white, but +sometimes reddish, and divided into thick laminae or strata: in one part there +is a little glossy clay-slate with a tortuous cleavage. The thick layers of +quartz strike in a W. 30 degrees N. line, dipping southerly at an angle of 45 +degrees and upwards. The principal line of mountains, with some quite +subordinate parallel ridges, range about W. 45 degrees N.: but at their S.E. +termination, only W. 25 degrees N. This Sierra is said to extend between twenty +and thirty leagues into the interior. +</p> + +<h3>PATAGONIA.</h3> + +<p> +With the exception perhaps of the hill of S. Antonio (600 feet high) in the +Gulf of S. Matias, which has never been visited by a geologist, crystalline +rocks are not met with on the coast of Patagonia for a space of 380 miles south +of the S. Ventana. At this point (latitude 43 degrees 50 minutes), at Points +Union and Tombo, plutonic rocks are said to appear, and are found, at rather +wide intervals, beneath the Patagonian tertiary formation for a space of about +three hundred miles southward, to near Bird Island, in latitude 48 degrees 56 +minutes. Judging from specimens kindly collected for me by Mr. Stokes, the +prevailing rock at Ports St. Elena, Camerones, Malaspina, and as far south as +the Paps of Pineda, is a purplish-pink or brownish claystone porphyry, +sometimes laminated, sometimes slightly vesicular, with crystals of opaque +feldspar and with a few grains of quartz; hence these porphyries resemble those +immediately to be described at Port Desire, and likewise a series which I have +seen from P. Alegre on the southern confines of Brazil. This porphyritic +formation further resembles in a singularly close manner the lowest stratified +formation of the Cordillera of Chile, which, as we shall hereafter see, has a +vast range, and attains a great thickness. At the bottom of the Gulf of St. +George, only tertiary deposits appear to be present. At Cape Blanco, there is +quartz rock, very like that of the Falkland Islands, and some hard, blue +siliceous clay-slate. +</p> + +<p> +At Port Desire there is an extensive formation of the claystone porphyry, +stretching at least twenty-five miles into the interior: it has been denuded +and deeply worn into gullies before being covered up by the tertiary deposits, +through which it here and there projects in hills; those north of the bay being +440 feet in height. The strata have in several places been tilted at small +angles, generally either to N.N.W. or S.S.E. By gradual passages and +alternations, the porphyries change incessantly in nature. I will describe only +some of the principal mineralogical changes, which are highly instructive, and +which I carefully examined. The prevailing rock has a compact purplish base, +with crystals of earthy or opaque feldspar, and often with grains of quartz. +There are other varieties, with an almost truly trachytic base, full of little +angular vesicles and crystals of glassy feldspar; and there are beds of black +perfect pitchstone, as well as of a concretionary imperfect variety. On a +casual inspection, the whole series would be thought to be of the same plutonic +or volcanic nature with the trachytic varieties and pitchstone; but this is far +from being the case, as much of the porphyry is certainly of metamorphic +origin. Besides the true porphyries, there are many beds of earthy, quite white +or yellowish, friable, easily fusible matter, resembling chalk, which under the +microscope is seen to consist of minute broken crystals, and which, as remarked +in a former chapter, singularly resembles the upper tufaceous beds of the +Patagonian tertiary formation. This earthy substance often becomes coarser, and +contains minute rounded fragments of porphyries and rounded grains of quartz, +and in one case so many of the latter as to resemble a common sandstone. These +beds are sometimes marked with true lines of aqueous deposition, separating +particles of different degrees of coarseness; in other cases there are parallel +ferruginous lines not of true deposition, as shown by the arrangement of the +particles, though singularly resembling them. The more indurated varieties +often include many small and some larger angular cavities, which appear due to +the removal of earthy matter: some varieties contain mica. All these earthy and +generally white stones insensibly pass into more indurated sonorous varieties, +breaking with a conchoidal fracture, yet of small specific gravity; many of +these latter varieties assume a pale purple tint, being singularly banded and +veined with different shades, and often become plainly porphyritic with +crystals of feldspar. The formation of these crystals could be most clearly +traced by minute angular and often partially hollow patches of earthy matter, +first assuming a FIBROUS STRUCTURE, then passing into opaque imperfectly shaped +crystals, and lastly, into perfect glassy crystals. When these crystals have +appeared, and when the basis has become compact, the rock in many places could +not be distinguished from a true claystone porphyry without a trace of +mechanical structure. +</p> + +<p> +In some parts, these earthy or tufaceous beds pass into jaspery and into +beautifully mottled and banded porcelain rocks, which break into splinters, +translucent at their edges, hard enough to scratch glass, and fusible into +white transparent beads: grains of quartz included in the porcelainous +varieties can be seen melting into the surrounding paste. In other parts, the +earthy or tufaceous beds either insensibly pass into, or alternate with, +breccias composed of large and small fragments of various purplish porphyries, +with the matrix generally porphyritic: these breccias, though their subaqueous +origin is in many places shown both by the arrangement of their smaller +particles and by an oblique or current lamination, also pass into porphyries, +in which every trace of mechanical origin and stratification has been +obliterated. +</p> + +<p> +Some highly porphyritic though coarse-grained masses, evidently of sedimentary +origin, and divided into thin layers, differing from each other chiefly in the +number of embedded grains of quartz, interested me much from the peculiar +manner in which here and there some of the layers terminated in abrupt points, +quite unlike those produced by a layer of sediment naturally thinning out, and +apparently the result of a subsequent process of metamorphic aggregation. In +another common variety of a finer texture, the aggregating process had gone +further, for the whole mass consisted of quite short, parallel, often slightly +curved layers or patches, of whitish or reddish finely granulo-crystalline +feldspathic matter, generally terminating at both ends in blunt points; these +layers or patches further tended to pass into wedge or almond-shaped little +masses, and these finally into true crystals of feldspar, with their centres +often slightly drusy. The series was so perfect that I could not doubt that +these large crystals, which had their longer axes placed parallel to each +other, had primarily originated in the metamorphosis and aggregation of +alternating layers of tuff; and hence their parallel position must be +attributed (unexpected though the conclusion may be), not to laws of chemical +action, but to the original planes of deposition. I am tempted briefly to +describe three other singular allied varieties of rock; the first without +examination would have passed for a stratified porphyritic breccia, but all the +included angular fragments consisted of a border of pinkish crystalline +feldspathic matter, surrounding a dark translucent siliceous centre, in which +grains of quartz not quite blended into the paste could be distinguished: this +uniformity in the nature of the fragments shows that they are not of +mechanical, but of concretionary origin, having resulted perhaps from the +self-breaking up and aggregation of layers of indurated tuff containing +numerous grains of quartz,—into which, indeed, the whole mass in one part +passed. The second variety is a reddish non-porphyritic claystone, quite full +of spherical cavities, about half an inch in diameter, each lined with a +collapsed crust formed of crystals of quartz. The third variety also consists +of a pale purple non-porphyritic claystone, almost wholly formed of +concretionary balls, obscurely arranged in layers, of a less compact and paler +coloured claystone; each ball being on one side partly hollow and lined with +crystals of quartz. +</p> + +<h3>PSEUDO-DIKES.</h3> + +<p> +Some miles up the harbour, in a line of cliffs formed of slightly metamorphosed +tufaceous and porphyritic claystone beds, I observed three vertical dikes, so +closely resembling in general appearance ordinary volcanic dikes, that I did +not doubt, until closely examining their composition, that they had been +injected from below. The first is straight, with parallel sides, and about four +feet wide; it consists of whitish, indurated tufaceous matter, precisely like +some of the beds intersected by it. The second dike is more remarkable; it is +slightly tortuous, about eighteen inches thick, and can be traced for a +considerable distance along the beach; it is of a purplish-red or brown colour, +and is formed chiefly of ROUNDED grains of quartz, with broken crystals of +earthy feldspar, scales of black mica, and minute fragments of claystone +porphyry, all firmly united together in a hard sparing base. The structure of +this dike shows obviously that it is of mechanical and sedimentary origin; yet +it thinned out upwards, and did not cut through the uppermost strata in the +cliffs. This fact at first appears to indicate that the matter could not have +been washed in from above (Upfilled fissures are known to occur both in +volcanic and in ordinary sedimentary formations. At the Galapagos Archipelago +“Volcanic Islands” etc., there are some striking examples of +pseudo-dikes composed of hard tuff.); but if we reflect on the suction which +would result from a deep-seated fissure being formed, we may admit that if the +fissure were in any part open to the surface, mud and water might well be drawn +into it along its whole course. The third dike consisted of a hard, rough, +white rock, almost composed of broken crystals of glassy feldspar, with +numerous scales of black mica, cemented in a scanty base; there was little in +the appearance of this rock, to preclude the idea of its having been a true +injected feldspathic dike. The matter composing these three pseudo-dikes, +especially the second one, appears to have suffered, like the surrounding +strata, a certain degree of metamorphic action; and this has much aided the +deceptive appearance. At Bahia, in Brazil, we have seen that a true injected +hornblendic dike, not only has suffered metamorphosis, but has been dislocated +and even diffused in the surrounding gneiss, under the form of separate +crystals and of fragments. +</p> + +<h3>FALKLAND ISLANDS.</h3> + +<p> +I have described these islands in a paper published in the third volume of the +“Geological Journal.” The mountain-ridges consist of quartz, and +the lower country of clay-slate and sandstone, the latter containing Palaeozoic +fossils. These fossils have been separately described by Messrs. Morris and +Sharpe: some of them resemble Silurian, and others Devonian forms. In the +eastern part of the group the several parallel ridges of quartz extend in a +west and east line; but further westward the line becomes W.N.W. and E.S.E., +and even still more northerly. The cleavage-planes of the clay- slate are +highly inclined, generally at an angle of above 50 degrees, and often vertical; +they strike almost invariably in the same direction with the quartz ranges. The +outline of the indented shores of the two main islands, and the relative +positions of the smaller islets, accord with the strike both of the main axes +of elevation and of the cleavage of the clay- slate. +</p> + +<h3>TIERRA DEL FUEGO.</h3> + +<p> +My notes on the geology of this country are copious, but as they are +unimportant, and as fossils were found only in one district, a brief sketch +will be here sufficient. The east coast from the S. of Magellan (where the +boulder formation is largely developed) to St. Polycarp’s Bay is formed +of horizontal tertiary strata, bounded some way towards the interior by a broad +mountainous band of clay-slate. This great clay-slate formation extends from +St. Le Maire westward for 140 miles, along both sides of the Beagle Channel to +near its bifurcation. South of this channel, it forms all Navarin Island, and +the eastern half of Hoste Island and of Hardy Peninsula; north of the Beagle +Channel it extends in a north-west line on both sides of Admiralty Sound to +Brunswick Peninsula in the St. of Magellan, and I have reason to believe, +stretches far up the eastern side of the Cordillera. The western and broken +side of Tierra del Fuego towards the Pacific is formed of metamorphic schists, +granite and various trappean rocks: the line of separation between the +crystalline and clay-slate formations can generally be distinguished, as +remarked by Captain King, by the parallelism in the clay-slate districts of the +shores and channels, ranging in a line between [W. 20 degrees to 40 degrees N.] +and [E. 20 degrees to 40 degrees S.]. (“Geographical Journal” +volume 1 page 155.) +</p> + +<p> +The clay-slate is generally fissile, sometimes siliceous or ferruginous, with +veins of quartz and calcareous spar; it often assumes, especially on the +loftier mountains, an altered feldspathic character, passing into feldspathic +porphyry: occasionally it is associated with breccia and grauwacke. At Good +Success Bay, there is a little intercalated black crystalline limestone. At +Port Famine much of the clay-slate is calcareous, and passes either into a +mudstone or into grauwacke, including odd-shaped concretions of dark +argillaceous limestone. Here alone, on the shore a few miles north of Port +Famine, and on the summit of Mount Tarn (2,600 feet high), I found organic +remains; they consist of:— +</p> + +<p> +1. Ancyloceras simplex, d’Orbigny “Pal Franc” Mount Tarn. 2. +Fusus (in imperfect state), d’Orbigny “Pal Franc” Mount Tarn. +3. Natica, d’Orbigny “Pal Franc” Mount Tarn. 4. Pentacrimus, +d’Orbigny “Pal Franc” Mount Tarn. 5. Lucina excentrica, G.B. +Sowerby, Port Famine. 6. Venus (in imperfect state), G.B. Sowerby, Port Famine. +7. Turbinolia (?), G.B. Sowerby, Port Famine. 8. Hamites elatior, G.B. Sowerby, +Port Famine. +</p> + +<p> +M. d’Orbigny states that MM. Hombron and Grange found in this +neighbourhood an Ancyloceras, perhaps A. simplex, an Ammonite, a Plicatula and +Modiola. (“Voyage” Part Geolog. page 242.) M. d’Orbigny +believes from the general character of these fossils, and from the Ancyloceras +being identical (as far as its imperfect condition allows of comparison) with +the A. simplex of Europe, that the formation belongs to an early stage of the +Cretaceous system. Professor E. Forbes, judging only from my specimens, concurs +in the probability of this conclusion. The Hamites elatior of the above list, +of which a description has been given by Mr. Sowerby, and which is remarkable +from its large size, has not been seen either by M. d’Orbigny or +Professor E. Forbes, as, since my return to England, the specimens have been +lost. The great clay-slate formation of Tierra del Fuego being cretaceous, is +certainly a very interesting fact,—whether we consider the appearance of +the country, which, without the evidence afforded by the fossils, would form +the analogy of most known districts, probably have been considered as belonging +to the Palaeozoic series,—or whether we view it as showing that the age +of this terminal portion of the great axis of South America, is the same (as +will hereafter be seen) with the Cordillera of Chile and Peru. +</p> + +<p> +The clay-slate in many parts of Tierra del Fuego, is broken by dikes and by +great masses of greenstone, often highly hornblendic (In a greenstone-dike in +the Magdalen Channel, the feldspar cleaved with the angle of albite. This dike +was crossed, as well as the surrounding slate, by a large vein of quartz, a +circumstance of unusual occurrence.): almost all the small islets within the +clay-slate districts are thus composed. The slate near the dikes generally +becomes paler-coloured, harder, less fissile, of a feldspathic nature, and +passes into a porphyry or greenstone: in one case, however, it became more +fissile, of a red colour, and contained minute scales of mica, which were +absent in the unaltered rock. On the east side of Ponsonby Sound some dikes +composed of a pale sonorous feldspathic rock, porphyritic with a little +feldspar, were remarkable from their number,—there being within the space +of a mile at least one hundred,—from their nearly equalling in bulk the +intermediate slate,—and more especially from the excessive fineness (like +the finest inlaid carpentry) and perfect parallelism of their junctions with +the almost vertical laminae of clay-slate. I was unable to persuade myself that +these great parallel masses had been injected, until I found one dike which +abruptly thinned out to half its thickness, and had one of its walls jagged, +with fragments of the slate embedded in it. +</p> + +<p> +In Southern Tierra del Fuego, the clay-slate towards its S.W. boundary, becomes +much altered and feldspathic. Thus on Wollaston Island slate and grauwacke can +be distinctly traced passing into feldspathic rocks and greenstones, including +iron pyrites and epidote, but still retaining traces of cleavage with the usual +strike and dip. One such metamorphosed mass was traversed by large vein-like +masses of a beautiful mixture (as ascertained by Professor Miller) of green +epidote, garnets, and white calcareous spar. On the northern point of this same +island, there were various ancient submarine volcanic rocks, consisting of +amygdaloids with dark bole and agate,—of basalt with decomposed +olivine—of compact lava with glassy feldspar,—and of a coarse +conglomerate of red scoriae, parts being amygdaloidal with carbonate of lime. +The southern part of Wollaston Island and the whole of Hermite and Horn +Islands, seem formed of cones of greenstone; the outlying islets of Il Defenso +and D. Raminez are said to consist of porphyritic lava. (Determined by +Professor Jameson. Weddell’s “Voyage” page 169.) In crossing +Hardy Peninsula, the slate still retaining traces of its usual cleavage, passes +into columnar feldspathic rocks, which are succeeded by an irregular tract of +trappean and basaltic rocks, containing glassy feldspar and much iron pyrites: +there is, also, some harsh red claystone porphyry, and an almost true trachyte, +with needles of hornblende, and in one spot a curious slaty rock divided into +quadrangular columns, having a base almost like trachyte, with drusy cavities +lined by crystals, too imperfect, according to Professor Miller, to be +measured, but resembling Zeagonite. (See Mr. Brooke’s Paper in the +“London Philosophical Magazine” volume 10. This mineral occurs in +an ancient volcanic rock near Rome.) In the midst of these singular rocks, no +doubt of ancient submarine volcanic origin, a high hill of feldspathic +clay-slate projected, retaining its usual cleavage. Near this point, there was +a small hillock, having the aspect of granite, but formed of white albite, +brilliant crystals of hornblende (both ascertained by the reflecting +goniometer) and mica; but with no quartz. No recent volcanic district has been +observed in any part of Tierra del Fuego. +</p> + +<p> +Five miles west of the bifurcation of the Beagle Channel, the slate- formation, +instead of becoming, as in the more southern parts of Tierra del Fuego, +feldspathic, and associated with trappean or old volcanic rocks, passes by +alternations into a great underlying mass of fine gneiss and glossy clay-slate, +which at no great distance is succeeded by a grand formation of mica-slate +containing garnets. The folia of these metamorphic schists strike parallel to +the cleavage-planes of the clay-slate, which have a very uniform direction over +the whole of this part of the country: the folia, however, are undulatory and +tortuous, whilst the cleavage- laminae of the slate are straight. These schists +compose the chief mountain-chain of Southern Tierra del Fuego, ranging along +the north side of the northern arm of the Beagle Channel, in a short W.N.W. and +E.S.E. line, with two points (Mounts Sarmiento and Darwin) rising to heights of +6,800 and 6,900 feet. On the south-western side of this northern arm of the +Beagle Channel, the clay-slate is seen with its STRATA dipping from the great +chain, so that the metamorphic schists here form a ridge bordered on each side +by clay-slate. Further north, however, to the west of this great range, there +is no clay-slate, but only gneiss, mica, and hornblendic slates, resting on +great barren hills of true granite, and forming a tract about sixty miles in +width. Again, westward of these rocks, the outermost islands are of trappean +formation, which, from information obtained during the voyages of the +“Adventure” and “Beagle,” seem, together with granite, +chiefly to prevail along the western coast as far north as the entrance of the +St. of Magellan (See the Paper by Captain King in the “Geographical +Journal”; also a Letter to Dr. Fitton in “Geological +Proceedings” volume 1 page 29; also some observations by Captain Fitzroy +“Voyages” volume 1 page 375. I am indebted also to Mr. Lyell for a +series of specimens collected by Lieutenant Graves.): a little more inland, on +the eastern side of Clarence Island and S. Desolation, granite, greenstone, +mica-slate, and gneiss appear to predominate. I am tempted to believe, that +where the clay-slate has been metamorphosed at great depths beneath the +surface, gneiss, mica- slate, and other allied rocks have been formed, but +where the action has taken place nearer the surface, feldspathic porphyries, +greenstones, etc., have resulted, often accompanied by submarine volcanic +eruptions. +</p> + +<p> +Only one other rock, met with in both arms of the Beagle Channel, deserves any +notice, namely a granulo-crystalline mixture of white albite, black hornblende +(ascertained by measurement of the crystals, and confirmed by Professor +Miller), and more or less of brown mica, but without any quartz. This rock +occurs in large masses, closely resembling in external form granite or syenite: +in the southern arm of the Channel, one such mass underlies the mica-slate, on +which clay-slate was superimposed: this peculiar plutonic rock which, as we +have seen, occurs also in Hardy Peninsula, is interesting, from its perfect +similarity with that (hereafter often to be referred to under the name of +andesite) forming the great injected axes of the Cordillera of Chile. +</p> + +<p> +The stratification of the clay-slate is generally very obscure, whereas the +cleavage is remarkably well defined: to begin with the extreme eastern parts of +Tierra del Fuego; the cleavage-planes near the St. of Le Maire strike either W. +and E. or W.S.W. and E.N.E., and are highly inclined; the form of the land, +including Staten Island, indicates that the axes of elevation have run in this +same line, though I was unable to distinguish the planes of stratification. +Proceeding westward, I accurately examined the cleavage of the clay-slate on +the northern, eastern, and western sides (thirty-five miles apart) of Navarin +Island, and everywhere found the laminae ranging with extreme regularity, +W.N.W. and E.S.E., seldom varying more than one point of the compass from this +direction. (The clay-slate in this island was in many places crossed by +parallel smooth joints. Out of five cases, the angle of intersection between +the strike of these joints and that of the cleavage-laminae was in two cases 45 +degrees and in two others 79 degrees.) Both on the east and west coasts, I +crossed at right angles the cleavage-planes for a space of about eight miles, +and found them dipping at an angle of between 45 degrees and 90 degrees, +generally to S.S.W., sometimes to N.N.E., and often quite vertically. The +S.S.W. dip was occasionally succeeded abruptly by a N.N.E. dip, and this by a +vertical cleavage, or again by the S.S.W. dip; as in a lofty cliff on the +eastern end of the island the laminae of slate were seen to be folded into very +large steep curves, ranging in the usual W.N.W. line, I suspect that the +varying and opposite dips may possibly be accounted for by the cleavage- +laminae, though to the eye appearing straight, being parts of large abrupt +curves, with their summits cut off and worn down. +</p> + +<p> +In several places I was particularly struck with the fact, that the fine +laminae of the clay-slate, where cutting straight through the bands of +stratification, and therefore indisputably true cleavage-planes, differed +slightly in their greyish and greenish tints of colour, in compactness, and in +some of the laminae having a rather more jaspery appearance than others. I have +not seen this fact recorded, and it appears to me important, for it shows that +the same cause which has produced the highly fissile structure, has altered in +a slight degree the mineralogical character of the rock in the same planes. The +bands of stratification, just alluded to, can be distinguished in many places, +especially in Navarin Island, but only on the weathered surfaces of the slate; +they consist of slightly undulatory zones of different shades of colour and of +thicknesses, and resemble the marks (more closely than anything else to which I +can compare them) left on the inside of a vessel by the draining away of some +dirty slightly agitated liquid: no difference in composition, corresponding +with these zones, could be seen in freshly fractured surfaces. In the more +level parts of Navarin Island, these bands of stratification were nearly +horizontal; but on the flanks of the mountains they were inclined from them, +but in no instance that I saw at a very high angle. There can, I think, be no +doubt that these zones, which appear only on the weathered surfaces, are the +last vestiges of the original planes of stratification, now almost obliterated +by the highly fissile and altered structure which the mass has assumed. +</p> + +<p> +The clay-slate cleaves in the same W.N.W. and E.S.E. direction, as on Navarin +Island, on both sides of the Beagle Channel, on the eastern side of Hoste +Island, on the N.E. side of Hardy Peninsula, and on the northern point of +Wollaston Island; although in these two latter localities the cleavage has been +much obscured by the metamorphosed and feldspathic condition of the slate. +Within the area of these several islands, including Navarin Island, the +direction of the stratification and of the mountain- chains is very obscure; +though the mountains in several places appeared to range in the same W.N.W. +line with the cleavage: the outline of the coast, however, does not correspond +with this line. Near the bifurcation of the Beagle Channel, where the +underlying metamorphic schists are first seen, they are foliated (with some +irregularities), in this same W.N.W. line, and parallel, as before stated, to +the main mountain-axis of this part of the country. Westward of this main +range, the metamorphic schists are foliated, though less plainly, in the same +direction, which is likewise common to the zone of old erupted trappean rocks, +forming the outermost islets. Hence the area, over which the cleavage of the +slate and the foliation of the metamorphic schists extends with an average +W.N.W. and E.S.E. strike, is about forty miles in a north and south line, and +ninety miles in an east and west line. +</p> + +<p> +Further northward, near Port Famine, the stratification of the clay-slate and +of the associated rocks, is well defined, and there alone the cleavage and +strata-planes are parallel. A little north of this port there is an anticlinal +axis ranging N.W. (or a little more westerly) and S.E.: south of the port, as +far as Admiralty Sound and Gabriel Channel, the outline of the land clearly +indicates the existence of several lines of elevation in this same N.W. +direction, which, I may add, is so uniform in the western half of the St. of +Magellan, that, as Captain King has remarked, “a parallel ruler placed on +the map upon the projecting points of the south shore, and extended across the +strait, will also touch the headlands on the opposite coast.” +(“Geographical Journal” volume 1 page 170.) It would appear, from +Captain King’s observations, that over all this area the cleavage extends +in the same line. Deep-water channels, however, in all parts of Tierra del +Fuego have burst through the trammels both of stratification and cleavage; most +of them may have been formed during the elevation of the land by long- +continued erosion, but others, for instance the Beagle Channel, which stretches +like a narrow canal for 120 miles obliquely through the mountains, can hardly +have thus originated. +</p> + +<p> +Finally, we have seen that in the extreme eastern point of Tierra del Fuego, +the cleavage and coast-lines extend W. and E. and even W.S.W. and E.N.E.: over +a large area westward, the cleavage, the main range of mountains, and some +subordinate ranges, but not the outlines of the coast, strike W.N.W., and +E.S.E.: in the central and western parts of the St. of Magellan, the +stratification, the mountain-ranges, the outlines of the coast, and the +cleavage all strike nearly N.W. and S.E. North of the strait, the outline of +the coast, and the mountains on the mainland, run nearly north and south. Hence +we see, at this southern point of the continent, how gradually the Cordillera +bend, from their north and south course of so many thousand miles in length, +into an E. and even E.N.E. direction. +</p> + +<h3>WEST COAST, FROM THE SOUTHERN CHONOS ISLANDS TO NORTHERN CHILE.</h3> + +<p> +The first place at which we landed north of the St. of Magellan was near Cape +Tres Montes, in latitude 47 degrees S. Between this point and the Northern +Chonos Islands, a distance of 200 miles, the “Beagle” visited +several points, and specimens were collected for me from the intermediate +spaces by Lieutenant Stokes. The predominant rock is mica-slate, with thick +folia of quartz, very frequently alternating with and passing into a chloritic, +or into a black, glossy, often striated, slightly anthracitic schist, which +soils paper, and becomes white under a great heat, and then fuses. Thin layers +of feldspar, swelling at intervals into well crystallised kernels, are +sometimes included in these black schists; and I observed one mass of the +ordinary black variety insensibly lose its fissile structure, and pass into a +singular mixture of chlorite, epidote, feldspar, and mica. Great veins of +quartz are numerous in the mica-schists; wherever these occur the folia are +much convoluted. In the southern part of the Peninsula of Tres Montes, a +compact altered feldspathic rock with crystals of feldspar and grains of quartz +is the commonest variety; this rock exhibits occasionally traces of an original +brecciated structure, and often presents (like the altered state of Tierra del +Fuego) traces of cleavage- planes, which strike in the same direction with the +folia of mica-schist further northward. (The peculiar, abruptly conical form of +the hills in this neighbourhood, would have led any one at first to have +supposed that they had been formed of injected or intrusive rocks. At Inchemo +Island, a similar rock gradually becomes granulo-crystalline and acquires +scales of mica; and this variety at S. Estevan becomes highly laminated, and +though still exhibiting some rounded grains of quartz, passes into the black, +glossy, slightly anthracitic schist, which, as we have seen, repeatedly +alternates with and passes into the micaceous and chloritic schists. Hence all +the rocks on this line of coast belong to one series, and insensibly vary from +an altered feldspathic clay-slate into largely foliated, true mica-schist. +</p> + +<p> +The cleavage of the homogeneous schists, the foliation of those composed of +more or less distinct minerals in layers, and the planes of alternation of the +different varieties or so-called stratification, are all parallel, and preserve +over this 200 miles of coast a remarkable degree of uniformity in direction. At +the northern end of the group, at Low’s Harbour, the well- defined folia +of mica-schist everywhere ranged within eight degrees (or less than one point +of the compass) of N. 19 degrees W. and S. 19 degrees E.; and even the point of +dip varied very little, being always directed to the west and generally at an +angle of forty degrees; I should mention that I had here good opportunities of +observation, for I followed the naked rock on the beach, transversely to the +strike, for a distance of four miles and a half, and all the way attended to +the dip. Along the outer islands for 100 miles south of Low’s Harbour, +Lieutenant Stokes, during his boat- survey, kindly observed for me the strike +of the foliation, and he assures me that it was invariably northerly, and the +dip with one single exception to the west. Further south at Vallenar Bay, the +strike was almost universally N. 25 degrees W. and the dip, generally at an +angle of about 40 degrees to W. 25 degrees S., but in some places almost +vertical. Still farther south, in the neighbourhood of the harbours of Anna +Pink, S. Estevan and S. Andres, and (judging from a distance) along the +southern part of Tres Montes, the foliation and cleavage extended in a line +between [N. 11 degrees to 22 degrees W.] and [S. 11 degrees to 22 degrees E.]; +and the planes dipped generally westerly, but often easterly, at angles varying +from a gentle inclination to vertical. At A. Pink’s Harbour, where the +schists generally dipped easterly, wherever the angle became very high, the +strike changed from N. 11 degrees W. to even as much as N. 45 degrees W.: in an +analogous manner at Vallenar Bay, where the dip was westerly (viz. on an +average directed to W. 25 degrees S.), as soon as the angle became very high, +the planes struck in a line more than 25 degrees west of north. The average +result from all the observations on this 200 miles of coast, is a strike of N. +19 degrees W. and S. 19 degrees E.: considering that in each specified place my +examination extended over an area of several miles, and that Lieutenant +Stokes’ observations apply to a length of 100 miles, I think this +remarkable uniformity is pretty well established. The prevalence, throughout +the northern half of this line of coast, of a dip in one direction, that is to +the west, instead of being sometimes west and sometimes east, is, judging from +what I have elsewhere seen, an unusual circumstance. In Brazil, La Plata, the +Falkland Islands, and Tierra del Fuego, there is generally an obvious relation +between the axis of elevation, the outline of the coast, and the strike of the +cleavage or foliation: in the Chonos Archipelago, however, neither the minor +details of the coast-line, nor the chain of the Cordillera, nor the subordinate +transverse mountain-axes, accord with the strike of the foliation and cleavage: +the seaward face of the numerous islands composing this Archipelago, and +apparently the line of the Cordillera, range N. 11 degrees E., whereas, as we +have just seen, the average strike of the foliation is N. 19 degrees W. +</p> + +<p> +There is one interesting exception to the uniformity in the strike of the +foliation. At the northern point of Tres Montes (latitude 45 degrees 52 +minutes) a bold chain of granite, between two and three thousand feet in +height, runs from the coast far into the interior, in an E.S.E. line, or more +strictly E. 28 degrees S. and W. 28 degrees N. (In the distance, other +mountains could be seen apparently ranging N.N.E. and S.S.W. at right angles to +this one. I may add, that not far from Vallenar Bay there is a fine range, +apparently of granite, which has burst through the mica-slate in a N.E. by E. +and S.W. by S. line.) In a bay, at the northern foot of this range, there are a +few islets of mica-slate, with the folia in some parts horizontal, but mostly +inclined at an average angle of 20 degrees to the north. On the northern steep +flank of the range, there are a few patches (some quite isolated, and not +larger than half a-crown!) of the mica-schist, foliated with the same northerly +dip. On the broad summit, as far as the southern crest, there is much +mica-slate, in some places even 400 feet in thickness, with the folia all +dipping north, at angles varying from 5 degrees to 20 degrees, but sometimes +mounting up to 30 degrees. The southern flank consists of bare granite. The +mica-slate is penetrated by small veins of granite, branching from the main +body. (The granite within these veins, as well as generally at the junction +with the mica-slate, is more quartzose than elsewhere. The granite, I may add, +is traversed by dikes running for a very great length in the line of the +mountains; they are composed of a somewhat laminated eurite, containing +crystals of feldspar, hornblende, and octagons of quartz.) Leaving out of view +the prevalent strike of the folia in other parts of this Archipelago, it might +have been expected that they would have dipped N. 28 degrees E., that is +directly from the ridge, and, considering its abruptness, at a high +inclination; but the real dip, as we have just seen, both at the foot and on +the northern flank, and over the entire summit, is at a small angle, and +directed nearly due north. From these considerations it occurred to me, that +perhaps we here had the novel and curious case of already inclined laminae +obliquely tilted at a subsequent period by the granitic axis. Mr. Hopkins, so +well known from his mathematical investigations, has most kindly calculated the +problem: the proposition sent was,—Take a district composed of laminae, +dipping at an angle of 40 degrees to W. 19 degrees S., and let an axis of +elevation traverse it in an E. 28 degrees S. line, what will the position of +the laminae be on the northern flank after a tilt, we will first suppose, of 45 +degrees? Mr. Hopkins informs me, that the angle of the dip will be 28 degrees +31 minutes, and its direction to north 30 degrees 33 minutes west. (On the +south side of the axis (where, however, I did not see any mica-slate) the dip +of the folia would be at an angle of 77 degrees 55 minutes, directed to west 35 +degrees 33 minutes south. Hence the two points of dip on the opposite sides of +the range, instead of being as in ordinary cases directly opposed to each other +at an angle of 180 degrees, would here be only 86 degrees 50 minutes apart.) By +varying the supposed angle of the tilt, our previously inclined folia can be +thrown into any angle between 26 degrees, which is the least possible angle, +and 90 degrees; but if a small inclination be thus given to them, their point +of dip will depart far from the north, and therefore not accord with the actual +position of the folia of mica-schist on our granitic range. Hence it appears +very difficult, without varying considerably the elements of the problem, thus +to explain the anomalous strike and dip of the foliated mica- schist, +especially in those parts, namely, at the base of the range, where the folia +are almost horizontal. Mr. Hopkins, however, adds, that great irregularities +and lateral thrusts might be expected in every great line of elevation, and +that these would account for considerable deviations from the calculated +results: considering that the granitic axis, as shown by the veins, has +indisputably been injected after the perfect formation of the mica-slate, and +considering the uniformity of the strike of the folia throughout the rest of +the Archipelago, I cannot but still think that their anomalous position at this +one point is someway directly and mechanically related to the intrusion of this +W.N.W. and E.S.E. mountain-chain of granite. +</p> + +<p> +Dikes are frequent in the metamorphic schists of the Chonos Islands, and seem +feebly to represent that great band of trappean and ancient volcanic rocks on +the south-western coast of Tierra del Fuego. At S. Andres I observed in the +space of half-a-mile, seven broad, parallel dikes, composed of three varieties +of trap, running in a N.W. and S.E. line, parallel to the neighbouring +mountain-ranges of altered clay-slate; but they must be of long subsequent +origin to these mountains; for they intersected the volcanic formation +described in the last chapter. North of Tres Montes, I noticed three dikes +differing from each other in composition, one of them having a euritic base +including large octagons of quartz; these dikes, as well as several of +porphyritic greenstone at Vallenar Bay, extended N.E. and S.W., nearly at right +angles to the foliation of the schists, but in the line of their joints. At +Low’s Harbour, however, a set of great parallel dikes, one ninety yards +and another sixty yards in width, have been guided by the foliation of the +mica-schist, and hence are inclined westward at an angle of 45 degrees: these +dikes are formed of various porphyritic traps, some of which are remarkable +from containing numerous rounded grains of quartz. A porphyritic trap of this +latter kind, passed in one of the dikes into a most curious hornstone, +perfectly white, with a waxy fracture and pellucid edges, fusible, and +containing many grains of quartz and specks of iron pyrites. In the ninety-yard +dike several large, apparently now quite isolated, fragments of mica-slate were +embedded: but as their foliation was exactly parallel to that of the +surrounding solid rock, no doubt these new separate fragments originally formed +wedge-shaped depending portions of a continuous vault or crust, once extending +over the dike, but since worn down and denuded. +</p> + +<h3>CHILOE, VALDIVIA, CONCEPCION.</h3> + +<p> +In Chiloe, a great formation of mica-schist strikingly resembles that of the +Chonos Islands. For a space of eleven miles on the S.E. coast, the folia were +very distinct, though slightly convoluted, and ranged within a point of N.N.W. +and S.S.E., dipping either E.N.E. or more commonly W.S.W., at an average angle +of 22 degrees (in one spot, however, at 60 degrees), and therefore decidedly at +a lesser inclination than amongst the Chonos Islands. On the west and +north-western shores, the foliation was often obscure, though, where best +defined, it ranged within a point of N. by W. and S. by E., dipping either +easterly or westerly, at varying and generally very small angles. Hence, from +the southern part of Tres Montes to the northern end of Chiloe, a distance of +300 miles, we have closely allied rocks with their folia striking on an average +in the same direction, namely between N. 11 degrees and 22 degrees W. Again, at +Valdivia, we meet with the same mica-schist, exhibiting nearly the same +mineralogical passages as in the Chonos Archipelago, often, however, becoming +more ferruginous, and containing so much feldspar as to pass into gneiss. The +folia were generally well defined; but nowhere else in South America did I see +them varying so much in direction: this seemed chiefly caused by their forming +parts, as I could sometimes distinctly trace, of large flat curves: +nevertheless, both near the settlement and towards the interior, a N.W. and +S.E. strike seemed more frequent than any other direction; the angle of the dip +was generally small. At Concepcion, a highly glossy clay-slate had its cleavage +often slightly curvilinear, and inclined, seldom at a high angle, towards +various points of the compass: but here, as at Valdivia, a N.W. and S.E. strike +seemed to be the most frequent one. ((FIGURE 23.) I observed in some parts that +the tops of the laminae of the clay-slate (b in Figure 23) under the +superficial detritus and soil (a) were bent, sometimes without being broken, as +represented in Figure 23, which is copied from one given by Sir H. De la Beche +(page 42 “Geological Manual”) of an exactly similar phenomenon in +Devonshire. Mr. R.A.C. Austen, also, in his excellent paper on S.E. Devon +(“Geological Transactions” volume 6 page 437), has described this +phenomenon; he attributes it to the action of frosts, but at the same time +doubts whether the frosts of the present day penetrate to a sufficient depth. +As it is known that earthquakes particularly affect the surface of the ground, +it occurred to me that this appearance might perhaps be due, at least at +Concepcion, to their frequent occurrence; the superficial layers of detritus +being either jerked in one direction, or, where the surface was inclined, +pushed a little downwards during each strong vibration. In North Wales I have +seen a somewhat analogous but less regular appearance, though on a greater +scale (“London Philosophical Magazine” volume 21 page 184), and +produced by a quite different cause, namely, by the stranding of great +icebergs; this latter appearance has also been observed in N. America.) +</p> + +<p> +In certain spots large quartz veins were numerous, and near them, the cleavage, +as was the case with the foliation of the schists in the Chonos Archipelago, +became extremely tortuous. +</p> + +<p> +At the northern end of Quiriquina Island, in the Bay of Concepcion, at least +eight rudely parallel dikes, which have been guided to a certain extent by the +cleavage of the slate, occur within the space of a quarter of a mile. They vary +much in composition, resembling in many respects the dikes at Low’s +Harbour: the greater number consist of feldspathic porphyries, sometimes +containing grains of quartz: one, however, was black and brilliant, like an +augitic rock, but really formed of feldspar; others of a feldspathic nature +were perfectly white, with either an earthy or crystalline fracture, and +including grains and regular octagons of quartz; these white varieties passed +into ordinary greenstones. Although, both here and at Low’s Harbour, the +nature of the rock varied considerably in the same dike, yet I cannot but think +that at these two places and in other parts of the Chonos group, where the +dikes, though close to each other and running parallel, are of different +composition, that they must have been formed at different periods. In the case +of Quiriquina this is a rather interesting conclusion, for these eight parallel +dikes cut through the metamorphic schists in a N.W. and S.E. line, and since +their injection the overlying cretaceous or tertiary strata have been tilted +(whilst still under the sea) from a N.W. by N. and S.E. by S. line; and again, +during the great earthquake of February 1835, the ground in this neighbourhood +was fissured in N.W. and S.E. lines; and from the manner in which buildings +were thrown down, it was evident that the surface undulated in this same +direction. (“Geological Transactions” volume 6 pages 602 and 617. +“Journal of Researches” 2nd edition page 307.) +</p> + +<h3>CENTRAL AND NORTHERN CHILE.</h3> + +<p> +Northward of Concepcion, as far as Copiapo, the shores of the Pacific consist, +with the exception of some small tertiary basins, of gneiss, mica- schist, +altered clay-slate, granite, greenstone and syenite: hence the coast from Tres +Montes to Copiapo, a distance of 1,200 miles, and I have reason to believe for +a much greater space, is almost similarly constituted. +</p> + +<p> +Near Valparaiso the prevailing rock is gneiss, generally including much +hornblende: concretionary balls formed of feldspar, hornblende and mica, from +two or three feet in diameter, are in very many places conformably enfolded by +the foliated gneiss: veins of quartz and feldspar, including black schorl and +well-crystallised epidote, are numerous. Epidote likewise occurs in the gneiss +in thin layers, parallel to the foliation of the mass. One large vein of a +coarse granitic character was remarkable from in one part quite changing its +character, and insensibly passing into a blackish porphyry, including acicular +crystals of glassy feldspar and of hornblende: I have never seen any other such +case. (Humboldt “Personal Narrative” volume 4 page 60, has +described with much surprise, concretionary balls, with concentric divisions, +composed of partially vitreous feldspar, hornblende, and garnets, included +within great veins of gneiss, which cut across the mica-slate near Venezuela.) +</p> + +<p> +I shall in the few following remarks on the rocks of Chile allude exclusively +to their foliation and cleavage. In the gneiss round Valparaiso the strike of +the foliation is very variable, but I think about N. by W. and S. by E. is the +commonest direction; this likewise holds good with the cleavage of the altered +feldspathic clay-slates, occasionally met with on the coast for ninety miles +north of Valparaiso. Some feldspathic slate, alternating with strata of +claystone porphyry in the Bell of Quillota and at Jajuel, and therefore, +perhaps, belonging to a later period than the metamorphic schists on the coast, +cleaved in this same direction. In the Eastern Cordillera, in the Portillo +Pass, there is a grand mass of mica- slate, foliated in a north and south line, +and with a high westerly dip: in the Uspallata range, clay-slate and grauwacke +have a highly inclined, nearly north and south cleavage, though in some parts +the strike is irregular: in the main or Cumbre range, the direction of the +cleavage in the feldspathic clay-slate is N.W. and S.E. +</p> + +<p> +Between Coquimbo and Guasco there are two considerable formations of mica- +slate, in one of which the rock passed sometimes into common clay-slate and +sometimes into a glossy black variety, very like that in the Chonos +Archipelago. The folia and cleavage of these rocks ranged between [N. and N.W. +by N.] and [S. and S.W. by S.]. Near the Port of Guasco several varieties of +altered clay-slate have a quite irregular cleavage. Between Guasco and Copiapo, +there are some siliceous and talcaceous slates cleaving in a north and south +line, with an easterly dip of between 60 and 70 degrees: high up, also, the +main valley of Copiapo, there is mica-slate with a high easterly dip. In the +whole space between Valparaiso and Copiapo an easterly dip is much more common +than an opposite or westerly one. +</p> + +<h3>CONCLUDING REMARKS ON CLEAVAGE AND FOLIATION.</h3> + +<p> +In this southern part of the southern hemisphere, we have seen that the +cleavage-laminae range over wide areas with remarkable uniformity, cutting +straight through the planes of stratification, but yet being parallel in strike +to the main axes of elevation, and generally to the outlines of the coast. (In +my paper on the Falkland Islands “Geological Journal” volume 3 page +267, I have given a curious case on the authority of Captain Sulivan, R.N., of +much folded beds of clay-slate, in some of which the cleavage is perpendicular +to the horizon, and in others it is perpendicular to each curvature or fold of +the bed: this appears a new case.) The dip, however, is as variable, both in +angle and in direction (that is, sometimes being inclined to the one side and +sometimes to the directly opposite side), as the strike is uniform. In all +these respects there is a close agreement with the facts given by Professor +Sedgwick in his celebrated memoir in the “Geological Transactions,” +and by Sir R.I. Murchison in his various excellent discussions on this subject. +The Falkland Islands, and more especially Tierra del Fuego, offer striking +instances of the lines of cleavage, the principle axes of elevation, and the +outlines of the coast, gradually changing together their courses. The direction +which prevails throughout Tierra del Fuego and the Falkland Islands, namely, +from west with some northing to east with some southing, is also common to the +several ridges in Northern Patagonia and in the western parts of Banda +Oriental: in this latter province, in the Sierra Tapalguen, and in the Western +Falkland Island, the W. by N., or W.N.W. and E.S.E., ridges, are crossed at +right angles by others ranging N.N.E. and S.S.W. +</p> + +<p> +The fact of the cleavage-laminae in the clay-slate of Tierra del Fuego, where +seen cutting straight through the planes of stratification, and where +consequently there could be no doubt about their nature, differing slightly in +colour, texture, and hardness, appears to me very interesting. In a thick mass +of laminated, feldspathic and altered clay-slate, interposed between two great +strata of porphyritic conglomerate in Central Chile, and where there could be +but little doubt about the bedding, I observed similar slight differences in +composition, and likewise some distinct thin layers of epidote, parallel to the +highly inclined cleavage of the mass. Again, I incidentally noticed in North +Wales, where glaciers had passed over the truncated edges of the highly +inclined laminae of clay-slate, that the surface, though smooth, was worn into +small parallel undulations, caused by the competent laminae being of slightly +different degrees of hardness. (“London Philosophical Magazine” +volume 21 page 182.) With reference to the slates of North Wales, Professor +Sedgwick describes the planes of cleavage, as “coated over with chlorite +and semi-crystalline matter, which not only merely define the planes in +question, but strike in parallel flakes through the whole mass of the +rock.” (“Geological Transactions” volume 3 page 471.) In some +of those glossy and hard varieties of clay-slate, which may often be seen +passing into mica-schist, it has appeared to me that the cleavage- planes were +formed of excessively thin, generally slighted convoluted, folia, composed of +microscopically minute scales of mica. From these several facts, and more +especially from the case of the clay-slate in Tierra del Fuego, it must, I +think, be concluded, that the same power which has impressed on the slate its +fissile structure or cleavage has tended to modify its mineralogical character +in parallel planes. +</p> + +<p> +Let us now turn to the foliation of the metamorphic schists, a subject which +has been much less attended to. As in the case of cleavage-laminae, the folia +preserve over very large areas a uniform strike: thus Humboldt found for a +distance of 300 miles in Venezuela, and indeed over a much larger space, +gneiss, granite, mica, and clay-slate, striking very uniformly N.E. and S.W., +and dipping at an angle of between 60 and 70 degrees to N.W. (“Personal +Narrative” volume 6 page 59 et seq.); it would even appear from the facts +given in this chapter, that the metamorphic rocks throughout the north-eastern +part of South America are generally foliated within two points of N.E. and S.W. +Over the eastern parts of Banda Oriental, the foliation strikes with a high +inclination, very uniformly N.N.E. to S.S.W., and over the western parts, in a +W. by N. and E. by S. line. For a space of 300 miles on the shores of the +Chonos and Chiloe Islands, we have seen that the foliation seldom deviates more +than a point of the compass from a N. 19 degrees W. and S. 19 degrees E. +strike. As in the case of cleavage, the angle of the dip in foliated rocks is +generally high but variable, and alternates from one side of the line of strike +to the other side, sometimes being vertical: in the Northern Chonos Islands, +however, the folia are inclined almost always to the west; in nearly the same +manner, the cleavage-laminae in Southern Tierra del Fuego certainly dip much +more frequently to S.S.W. than to the opposite point. In Eastern Banda +Oriental, in parts of Brazil, and in some other districts, the foliation runs +in the same direction with the mountain-ranges and adjoining coast-lines: +amongst the Chonos Islands, however, this coincidence fails, and I have given +my reasons for suspecting that one granitic axis has burst through and tilted +the already inclined folia of mica-schist: in the case of cleavage, the +coincidence between its strike and that of the main stratification seems +sometimes to fail. (Cases are given by Mr. Jukes in his “Geology of +Newfoundland” page 130.) Foliation and cleavage resemble each other in +the planes winding round concretions, and in becoming tortuous where veins of +quartz abound. (I have seen in Brazil and Chile concretions thus enfolded by +foliated gneiss; and Macculloch “Highlands” volume 1 page 64, has +described a similar case. For analogous cases in clay-slate, see Professor +Henslow’s Memoir in “Cambridge Philosophical Transactions” +volume 1 page 379, and Macculloch’s “Classification of Rocks” +page 351. With respect to both foliation and cleavage becoming tortuous where +quartz-veins abound, I have seen instances near Monte Video, at Concepcion, and +in the Chonos Islands. See also Mr. Greenough’s “Critical +Examination” page 78.) On the flanks of the mountains both in Tierra del +Fuego and in other countries, I have observed that the cleavage-planes +frequently dip at a high angle inwards; and this was long ago observed by Von +Buch to be the case in Norway: this fact is perhaps analogous to the folded, +fan-like or radiating structure in the metamorphic schists of the Alps, in +which the folia in the central crests are vertical and on the two flanks +inclined inwards. (Studer in “Edinburgh New Philosophical Journal” +volume 23 page 144.) Where masses of fissile and foliated rocks alternate +together, the cleavage and foliation, in all cases which I have seen, are +parallel. Where in one district the rocks are fissile, and in another adjoining +district they are foliated, the planes of cleavage and foliation are likewise +generally parallel: this is the case with the feldspathic homogeneous slates in +the southern part of the Chonos group, compared with the fine foliated +mica-schists of the northern part; so again the clay- slate of the whole +eastern side of Tierra del Fuego cleaves in exactly the same line with the +foliated gneiss and mica-slate of the western coast; other analogous instances +might have been adduced. (I have given a case in Australia. See my +“Volcanic Islands.”) +</p> + +<p> +With respect to the origin of the folia of quartz, mica, feldspar, and other +minerals composing the metamorphic schists, Professor Sedgwick, Mr. Lyell, and +most authors believe, that the constituent parts of each layer were separately +deposited as sediment, and then metamorphosed. This view, in the majority of +cases, I believe to be quite untenable. In those not uncommon instances, where +a mass of clay-slate, in approaching granite, gradually passes into gneiss, we +clearly see that folia of distinct minerals can originate through the +metamorphosis of a homogeneous fissile rock. (I have described in +“Volcanic Islands” a good instance of such a passage at the Cape of +Good Hope.) The deposition, it may be remarked, of numberless alternations of +pure quartz, and of the elements of mica or feldspar does not appear a probable +event. (See some excellent remarks on this subject, in +D’Aubuisson’s “Traite de Geog.” tome 1 page 297. Also +some remarks by Mr. Dana in “Silliman’s American Journal” +volume 45 page 108.) In those districts in which the metamorphic schists are +foliated in planes parallel to the cleavage of the rocks in an adjoining +district, are we to believe that the folia are due to sedimentary layers, +whilst the cleavage- laminae, though parallel, have no relation whatever to +such planes of deposition? On this view, how can we reconcile the vastness of +the areas over which the strike of the foliation is uniform, with what we see +in disturbed districts composed of true strata: and especially, how can we +understand the high and even vertical dip throughout many wide districts, which +are not mountainous, and throughout some, as in Western Banda Oriental, which +are not even hilly? Are we to admit that in the northern part of the Chonos +Archipelago, mica-slate was first accumulated in parallel horizontal folia to a +thickness of about four geographical miles, and then upturned at an angle of +forty degrees; whilst, in the southern part of this same Archipelago, the +cleavage-laminae of closely allied rocks, which none would imagine had ever +been horizontal, dip at nearly the same angle, to nearly the same point? +</p> + +<p> +Seeing, then, that foliated schists indisputably are sometimes produced by the +metamorphosis of homogeneous fissile rocks; seeing that foliation and cleavage +are so closely analogous in the several above-enumerated respects; seeing that +some fissile and almost homogeneous rocks show incipient mineralogical changes +along the planes of their cleavage, and that other rocks with a fissile +structure alternate with, and pass into varieties with a foliated structure, I +cannot doubt that in most cases foliation and cleavage are parts of the same +process: in cleavage there being only an incipient separation of the +constituent minerals; in foliation a much more complete separation and +crystallisation. +</p> + +<p> +The fact often referred to in this chapter, of the foliation and the so- called +strata in the metamorphic series,—that is, the alternating masses of +different varieties of gneiss, mica-schist, and hornblende-slate, etc.,—being +parallel to each other, at first appears quite opposed to the view, that the +folia have no relation to the planes of original deposition. Where the +so-called beds are not very thick and of widely different mineralogical +composition from each other, I do not think that there is any difficulty in +supposing that they have originated in an analogous manner with the separate +folia. We should bear in mind what thick strata, in ordinary sedimentary +masses, have obviously been formed by a concretionary process. In a pile of +volcanic rocks on the Island of Ascension, there are strata, differing quite as +much in appearance as the ordinary varieties of the metamorphic schists, which +undoubtedly have been produced, not by successive flowings of lava, but by +internal molecular changes. Near Monte Video, where the stratification, as it +would be called, of the metamorphic series is, in most parts, particularly well +developed, being as usual, parallel to the foliation, we have seen that a mass +of chloritic schist, netted with quartz-veins, is entangled in gneiss, in such +a manner as to show that it had certainly originated in some process of +segregation: again, in another spot, the gneiss tended to pass into hornblendic +schist by alternating with layers of quartz; but these layers of quartz almost +certainly had never been separately deposited, for they were absolutely +continuous with the numerous intersecting veins of quartz. I have never had an +opportunity of tracing for any distance, along the line both of strike and of +dip, the so-called beds in the metamorphic schists, but I strongly suspect that +they would not be found to extend with the same character, very far in the line +either of their dip or strike. Hence I am led to believe, that most of the +so-called beds are of the nature of complex folia, and have not been separately +deposited. Of course, this view cannot be extended to THICK masses included in +the metamorphic series, which are of totally different composition from the +adjoining schists, and which are far extended, as is sometimes the case with +quartz and marble; these must generally be of the nature of true strata. +(Macculloch “Classification of Rocks” page 364, states that primary +limestones are often found in irregular masses or great nodules, “which +can scarcely be said to possess a stratified shape!”) Such strata, +however, will almost always strike in the same direction with the folia, owing +to the axes of elevation being in most countries parallel to the strike of the +foliation; but they will generally dip at a different angle from that of the +foliation; and the angle of the foliation in itself almost always varies much: +hence, in crossing a metamorphosed schistose district, it would require +especial attention to discriminate between true strata of deposition and +complex foliated masses. The mere presence of true strata in the midst of a set +of metamorphic schists, is no argument that the foliation is of sedimentary +origin, without it be further shown in each case, that the folia not only +strike, but dip throughout in parallel planes with those of the true +stratification. +</p> + +<p> +As in some cases it appears that where a fissile rock has been exposed to +partial metamorphic action, for instance from the irruption of granite, the +foliation has supervened on the already existing cleavage-planes; so perhaps in +some instances, the foliation of a rock may have been determined by the +original planes of deposition or of oblique current-laminae: I have, however, +myself, never seen such a case, and I must maintain that in most extensive +metamorphic areas, the foliation is the extreme result of that process, of +which cleavage is the first effect. That foliation may arise without any +previous structural arrangement in the mass, we may infer from injected, and +therefore once liquified, rocks, both of volcanic and plutonic origin, +sometimes having a “grain” (as expressed by Professor Sedgwick), +and sometimes being composed of distinct folia or laminae of different +compositions. In my work on “Volcanic Islands,” I have given +several instances of this structure in volcanic rocks, and it is not uncommonly +seen in plutonic masses—thus, in the Cordillera of Chile, there are +gigantic mountain-like masses of red granite, which have been injected whilst +liquified, and which, nevertheless, display in parts a decidedly laminar +structure. (As remarked in a former part of this chapter, I suspect that the +boldly conical mountains of gneiss-granite, near Rio de Janeiro, in which the +constituent minerals are arranged in parallel planes, are of intrusive origin. +We must not, however, forget the lesson of caution taught by the curious +claystone porphyries of Port Desire, in which we have seen that the breaking up +and aggregation of a thinly stratified tufaceous mass, has yielded a rock +semi-porphyritic with crystals of feldspar, arranged in the planes of original +deposition.) +</p> + +<p> +Finally, we have seen that the planes of cleavage and of foliation, that is, of +the incipient process and of the final result, generally strike parallel to the +principal axes of elevation, and to the outline of the land: the strike of the +axes of elevation (that is, of the lines of fissures with the strata on their +edges upturned), according to the reasoning of Mr. Hopkins, is determined by +the form of the area undergoing changes of level, and the consequent direction +of the lines of tension and fissure. Now, in that remarkable pile of volcanic +rocks at Ascension, which has several times been alluded to (and in some other +cases), I have endeavoured to show, that the lamination of the several +varieties, and their alternations, have been caused by the moving mass, just +before its final consolidation, having been subjected (as in a glacier) to +planes of different tension; this difference in the tension affecting the +crystalline and concretionary processes. (In “Volcanic Islands.”) +One of the varieties of rock thus produced at Ascension, at first sight, +singularly resembles a fine-grained gneiss; it consists of quite straight and +parallel zones of excessive tenuity, of more or less coloured crystallised +feldspar, of distinct crystals of quartz, diopside, and oxide of iron. These +considerations, notwithstanding the experiments made by Mr. Fox, showing the +influence of electrical currents in producing a structure like that of +cleavage, and notwithstanding the apparently inexplicable variation, both in +the inclination of the cleavage-laminae and in their dipping first to one side +and then to the other side of the line of strike, lead me to suspect that the +planes of cleavage and foliation are intimately connected with the planes of +different tension, to which the area was long subjected, AFTER the main +fissures or axes of upheavement had been formed, but BEFORE the final +consolidation of the mass and the total cessation of all molecular movement. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0007"></a> +CHAPTER VII.<br /> +CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.</h2> + +<p class="letter"> +Central Chile.—Basal formations of the Cordillera.—Origin of the +porphyritic clay-stone conglomerate.—Andesite.—Volcanic +rocks.—Section of the Cordillera by the Peuquenes are Portillo +Pass.—Great gypseous formation.—Peuquenes line; thickness of +strata, fossils of.—Portillo line.—Conglomerate, orthitic granite, +mica-schist, volcanic rocks of.—Concluding remarks on the denudation and +elevation of the Portillo line.—Section by the Cumbre, or Uspallata +Pass.—Porphyries.—Gypseous strata.—Section near the Puente +del Inca; fossils of.—Great subsidence.—Intrusive +porphyries.—Plain of Uspallata.—Section of the Uspallata +chain.—Structure and nature of the strata.—Silicified vertical +trees.—Great subsidence.—Granitic rocks of axis.—Concluding +remarks on the Uspallata range; origin subsequent to that of the main +Cordillera; two periods of subsidence; comparison with the Portillo +chain.— +</p> + +<p> +The district between the Cordillera and the Pacific, on a rude average, is from +about eighty to one hundred miles in width. It is crossed by many chains of +mountains, of which the principal ones, in the latitude of Valparaiso and +southward of it, range nearly north and south; but in the more northern parts +of the province, they run in almost every possible direction. Near the Pacific, +the mountain-ranges are generally formed of syenite or granite, and or of an +allied euritic porphyry; in the low country, besides these granitic rocks and +greenstone, and much gneiss, there are, especially northward of Valparaiso, +some considerable districts of true clay-slate with quartz veins, passing into +a feldspathic and porphyritic slate; there is also some grauwacke and quartzose +and jaspery rocks, the latter occasionally assuming the character of the basis +of claystone porphyry: trap-dikes are numerous. Nearer the Cordillera the +ranges (such as those of S. Fernando, the Prado (Meyen “Reise um +Erde” th. 1 s. 235.), and Aconcagua) are formed partly of granitic rocks, +and partly of purple porphyritic conglomerates, claystone porphyry, greenstone +porphyry, and other rocks, such as we shall immediately see, form the basal +strata of the main Cordillera. In the more northern parts of Chile, this +porphyritic series extends over large tracts of country far from the +Cordillera; and even in Central Chile such occasionally occur in outlying +positions. +</p> + +<p> +I will describe the Campana of Quillota, which stands only fifteen miles from +the Pacific, as an instance of one of these outlying masses. This hill is +conspicuous from rising to the height of 6,400 feet: its summit shows a +nucleus, uncovered for a height of 800 feet, of fine greenstone, including +epidote and octahedral magnetic iron ore; its flanks are formed of great strata +of porphyritic claystone conglomerate associated with various true porphyries +and amygdaloids, alternating with thick masses of a highly feldspathic, +sometimes porphyritic, pale-coloured slaty rock, with its cleavage-laminae +dipping inwards at a high angle. At the base of the hill there are syenites, a +granular mixture of quartz and feldspar, and harsh quartzose rocks, all +belonging to the basal metamorphic series. I may observe that at the foot of +several hills of this class, where the porphyries are first seen (as near S. +Fernando, the Prado, Las Vacas, etc.), similar harsh quartzose rocks and +granular mixtures of quartz and feldspar occur, as if the more fusible +constituent parts of the granitic series had been drawn off to form the +overlying porphyries. +</p> + +<p> +In Central Chile, the flanks of the main Cordillera, into which I penetrated by +four different valleys, generally consist of distinctly stratified rocks. The +strata are inclined at angles varying from sometimes even under ten, to twenty +degrees, very rarely exceeding forty degrees: in some, however, of the quite +small, exterior, spur-like ridges, the inclination was not unfrequently +greater. The dip of the strata in the main outer lines was usually outwards or +from the Cordillera, but in Northern Chile frequently inwards,—that is, +their basset-edges fronted the Pacific. Dikes occur in extraordinary numbers. +In the great, central, loftiest ridges, the strata, as we shall presently see, +are almost always highly inclined and often vertical. Before giving a detailed +account of my two sections across the Cordillera, it will, I think, be +convenient to describe the basal strata as seen, often to a thickness of four +or five thousand feet, on the flanks of the outer lines. +</p> + +<h3>BASAL STRATA OF THE CORDILLERA.</h3> + +<p> +The prevailing rock is a purplish or greenish, porphyritic claystone +conglomerate. The embedded fragments vary in size from mere particles to blocks +as much as six or eight inches (rarely more) in diameter; in many places, where +the fragments were minute, the signs of aqueous deposition were unequivocally +distinct; where they were large, such evidence could rarely be detected. The +basis is generally porphyritic with perfect crystals of feldspar, and resembles +that of a true injected claystone porphyry: often, however, it has a mechanical +or sedimentary aspect, and sometimes (as at Jajuel) is jaspery. The included +fragments are either angular, or partially or quite rounded (Some of the +rounded fragments in the porphyritic conglomerate near the Baths of Cauquenes, +were marked with radii and concentric zones of different shades of colour: any +one who did not know that pebbles, for instance flint pebbles from the chalk, +are sometimes zoned concentrically with their worn and rounded surfaces, might +have been led to infer, that these balls of porphyry were not true pebbles, but +had originated in concretionary action.); in some parts the rounded, in others +the angular fragments prevail, and usually both kinds are mixed together: hence +the word BRECCIA ought strictly to be appended to the term PORPHYRITIC +CONGLOMERATE. The fragments consist of many varieties of claystone porphyry, +usually of nearly the same colour with the surrounding basis, namely, +purplish-reddish, brownish, mottled or bright green; occasionally fragments of +a laminated, pale-coloured, feldspathic rock, like altered clay-slate are +included; as are sometimes grains of quartz, but only in one instance in +Central Chile (namely, at the mines of Jajuel) a few pebbles of quartz. I +nowhere observed mica in this formation, and rarely hornblende; where the +latter mineral did occur, I was generally in doubt whether the mass really +belonged to this formation, or was of intrusive origin. Calcareous spar +occasionally occurs in small cavities; and nests and layers of epidote are +common. In some few places in the finer-grained varieties (for instance, at +Quillota), there were short, interrupted layers of earthy feldspar, which could +be traced, exactly as at Port Desire, passing into large crystals of feldspar: +I doubt, however, whether in this instance the layers had ever been separately +deposited as tufaceous sediment. +</p> + +<p> +All the varieties of porphyritic conglomerates and breccias pass into each +other, and by innumerable gradations into porphyries no longer retaining the +least trace of mechanical origin: the transition appears to have been effected +much more easily in the finer-grained, than in the coarser-grained varieties. +In one instance, near Cauquenes, I noticed that a porphyritic conglomerate +assumed a spheroidal structure, and tended to become columnar. Besides the +porphyritic conglomerates and the perfectly characterised porphyries, of +metamorphic origin, there are other porphyries, which, though differing not at +all or only slightly in composition, certainly have had a different origin: +these consist of pink or purple claystone porphyries, sometimes including +grains of quartz,—of greenstone porphyry, and of other dusky rocks, all +generally porphyritic with fine, large, tabular, opaque crystals, often placed +crosswise, of feldspar cleaving like albite (judging from several +measurements), and often amygdaloidal with silex, agate, carbonate of lime, +green and brown bole. (This bole is a very common mineral in the amygdaloidal +rocks; it is generally of a greenish- brown colour, with a radiating structure; +externally it is black with an almost metallic lustre, but often coated by a +bright green film. It is soft and can be scratched by a quill; under the +blowpipe swells greatly and becomes scaly, then fuses easily into a black +magnetic bead. This substance is evidently similar to that which often occurs +in submarine volcanic rocks. An examination of some very curious specimens of a +fine porphyry (from Jajuel) leads me to suspect that some of these amygdaloidal +balls, instead of having been deposited in pre-existing air-vesicles, are of +concretionary origin; for in these specimens, some of the pea-shaped little +masses (often externally marked with minute pits) are formed of a mixture of +green earth with stony matter, like the basis of the porphyry, including minute +imperfect crystals of feldspar; and these pea-shaped little masses are +themselves amygdaloidal with minute spheres of the green earth, each enveloped +by a film of white, apparently feldspathic, earthy matter: so that the porphyry +is doubly amygdaloidal. It should not, however, be overlooked, that all the +strata here have undergone metamorphic action, which may have caused crystals +of feldspar to appear, and other changes to be effected, in the originally +simple amygdaloidal balls. Mr. J.D. Dana, in an excellent paper on Trap-rocks +“Edinburgh New Philosophical Journal” volume 41 page 198, has +argued with great force, that all amygdaloidal minerals have been deposited by +aqueous infiltration. I may take this opportunity of alluding to a curious +case, described in my work on “Volcanic Islands,” of an amygdaloid +with many of its cells only half filled up with a mesotypic mineral. M. Rose +has described an amygdaloid, brought by Dr. Meyen “Reise um Erde” +Th. 1. s. 316, from Chile, as consisting of crystallised quartz, with crystals +of stilbite within, and lined externally by green earth.) These several +porphyritic and amygdaloidal varieties never show any signs of passing into +masses of sedimentary origin: they occur both in great and small intrusive +masses, and likewise in strata alternating with those of the porphyritic +conglomerate, and with the planes of junction often quite distinct, yet not +seldom blended together. In some of these intrusive masses, the porphyries +exhibit, more or less plainly, a brecciated structure, like that often seen in +volcanic masses. These brecciated porphyries could generally be distinguished +at once from the metamorphosed, porphyritic breccia- conglomerates, by all the +fragments being angular and being formed of the same variety, and by the +absence of every trace of aqueous deposition. One of the porphyries above +specified, namely, the greenstone porphyry with large tabular crystals of +albite, is particularly abundant, and in some parts of the Cordillera (as near +St. Jago) seemed more common even than the purplish porphyritic conglomerate. +Numerous dikes likewise consist of this greenstone porphyry; others are formed +of various fine-grained trappean rocks; but very few of claystone porphyry: I +saw no true basaltic dikes. +</p> + +<p> +In several places in the lower part of the series, but not everywhere, thick +masses of a highly feldspathic, often porphyritic, slaty rock occur +interstratified with the porphyritic conglomerate; I believe in one or two +cases blackish limestone has been found in a similar position. The feldspathic +rock is of a pale grey or greenish colour; it is easily fusible; where +porphyritic, the crystals of feldspar are generally small and vitreous: it is +distinctly laminated, and sometimes includes parallel layers of epidote (This +mineral is extremely common in all the formations of Chile; in the gneiss near +Valparaiso and in the granitic veins crossing it, in the injected greenstone +crowning the C. of Quillota, in some granitic porphyries, in the porphyritic +conglomerate, and in the feldspathic clay-slates.); the lamination appears to +be distinct from stratification. Occasionally this rock is somewhat curious; +and at one spot, namely, at the C. of Quillota, it had a brecciated structure. +Near the mines of Jajuel, in a thick stratum of this feldspathic, porphyritic +slate, there was a layer of hard, blackish, siliceous, infusible, compact +clay-slate, such as I saw nowhere else; at the same place I was able to follow +for a considerable distance the junction between the slate and the conformably +underlying porphyritic conglomerate, and they certainly passed gradually into +each other. Wherever these slaty feldspathic rocks abound, greenstone seems +common; at the C. of Quillota a bed of well-crystallised greenstone lay +conformably in the midst of the feldspathic slate, with the upper and lower +junctions passing insensibly into it. From this point, and from the frequently +porphyritic condition of the slate, I should perhaps have considered this rock +as an erupted one (like certain laminated feldspathic lavas in the trachytic +series), had I not seen in Tierra del Fuego how readily true clay-slate becomes +feldspathic and porphyritic, and had I not seen at Jajuel the included layer of +black, siliceous clay-slate, which no one could have thought of igneous origin. +The gentle passage of the feldspathic slate, at Jajuel, into the porphyritic +conglomerate, which is certainly of aqueous origin, should also be taken in +account. +</p> + +<p> +The alternating strata of porphyries and porphyritic conglomerate, and with the +occasionally included beds of feldspathic slate, together make a grand +formation; in several places within the Cordillera, I estimated its thickness +at from six to seven thousand feet. It extends for many hundred miles, forming +the western flank of the Chilean Cordillera; and even at Iquique in Peru, 850 +miles north of the southernmost point examined by me in Chile, the +coast-escarpment which rises to a height of between two and three thousand feet +is thus composed. In several parts of Northern Chile this formation extends +much further towards the Pacific, over the granitic and metamorphic lower +rocks, than it does in Central Chile; but the main Cordillera may be considered +as its central line, and its breadth in an east and west direction is never +great. At first the origin of this thick, massive, long but narrow formation, +appeared to me very anomalous: whence were derived, and how were dispersed the +innumerable fragments, often of large size, sometimes angular and sometimes +rounded, and almost invariably composed of porphyritic rocks? Seeing that the +interstratified porphyries are never vesicular and often not even amygdaloidal, +we must conclude that the pile was formed in deep water; how then came so many +fragments to be well rounded and so many to remain angular, sometimes the two +kinds being equally mingled, sometimes one and sometimes the other +preponderating? That the claystone, greenstone, and other porphyries and +amygdaloids, which lie CONFORMABLY between the beds of conglomerate, are +ancient submarine lavas, I think there can be no doubt; and I believe we must +look to the craters whence these streams were erupted, as the source of the +breccia- conglomerate; after the great explosion, we may fairly imagine that +the water in the heated and scarcely quiescent crater would remain for a +considerable time sufficiently agitated to triturate and round the loose +fragments, few or many in number, would be shot forth at the next eruption, +associated with few or many angular fragments, according to the strength of the +explosion. (This certainly seems to have taken place in some recent volcanic +archipelagos, as at the Galapagos, where numerous craters are exclusively +formed of tuff and fragments of lava.) The porphyritic conglomerate being +purple or reddish, even when alternating with dusty- coloured or bright green +porphyries and amygdaloids, is probably an analogous circumstance to the +scoriae of the blackish basalts being often bright red. The ancient submarine +orifices whence the porphyries and their fragments were ejected having been +arranged in a band, like most still active volcanoes, accounts for the +thickness, the narrowness, and linear extension of this formation. +</p> + +<p> +This whole great pile of rock has suffered much metamorphic action, as is very +obvious in the gradual formation and appearance of the crystals of albitic +feldspar and of epidote—in the bending together of the fragments— +in the appearance of a laminated structure in the feldspathic slate—and, +lastly, in the disappearance of the planes of stratification, which could +sometimes be seen on the same mountain quite distinct in the upper part, less +and less plain on the flanks, and quite obliterated at the base. Partly owing +to this metamorphic action, and partly to the close relationship in origin, I +have seen fragments of porphyries—taken from a metamorphosed +conglomerate—from a neighbouring stream of lava—from the nucleus or +centre (as it appeared to me) of the whole submarine volcano— and lastly +from an intrusive mass of quite subsequent origin, all of which were absolutely +undistinguishable in external characters. +</p> + +<p> +One other rock, of plutonic origin, and highly important in the history of the +Cordillera, from having been injected in most of the great axes of elevation, +and from having apparently been instrumental in metamorphosing the +superincumbent strata, may be conveniently described in this preliminary +discussion. It has been called by some authors ANDESITE: it mainly consists of +well-crystallised white albite (as determined with the goniometer in numerous +specimens both by Professor Miller and myself), of less perfectly crystallised +green hornblende, often associated with much mica, with chlorite and epidote, +and occasionally with a few grains of quartz: in one instance in Northern +Chile, I found crystals of orthitic or potash feldspar, mingled with those of +albite. (I here, and elsewhere, call by this name, those feldspathic minerals +which cleave like albite: but it now appears (“Edinburgh New +Philosophical Journal” volume 24 page 181) that Abich has analysed a +mineral from the Cordillera, associated with hornblende and quartz (probably +the same rock with that here under discussion), which cleaves like albite, but +which is a new and distinct kind, called by him ANDESINE. It is allied to +leucite, with the greater proportion of its potash replaced by lime and soda. +This mineral seems scarcely distinguishable from albite, except by analysis.) +Where the mica and quartz are abundant, the rock cannot be distinguished from +granite; and it may be called andesitic granite. Where these two minerals are +quite absent, and when, as often then happens, the crystals of albite are +imperfect and blend together, the rock may be called andesitic porphyry, which +bears nearly the same relation to andesitic granite that euritic porphyry does +to common granite. These andesitic rocks form mountain masses of a white +colour, which, in their general outline and appearance—in their +joints—in their occasionally including dark-coloured, angular fragments, +apparently of some pre-existing rock—and in the great dikes branching +from them into the superincumbent strata, manifest a close and striking +resemblance to masses of common granite and syenite: I never, however, saw in +these andesitic rocks, those granitic veins of segregation which are so common +in true granites. We have seen that andesite occurs in three places in Tierra +del Fuego; in Chile, from S. Fernando to Copiapo, a distance of 450 miles, I +found it under most of the axes of elevation; in a collection of specimens from +the Cordillera of Lima in Peru, I immediately recognised it; and Erman states +that it occurs in Eastern Kamtschatka. (“Geographical Journal” +volume 9 page 510.) From its wide range, and from the important part it has +played in the history of the Cordillera, I think this rock has well deserved +its distinct name of Andesite. +</p> + +<p> +The few still active volcanoes in Chile are confined to the central and +loftiest ranges of the Cordillera; and volcanic matter, such as appears to have +been of subaerial eruption, is everywhere rare. According to Meyen, there is a +hill of pumice high up the valley of the Maypu, and likewise a trachytic +formation at Colina, a village situated north of St. Jago. (“Reise um +Erde” Th. 1 ss. 338 and 362.) Close to this latter city, there are two +hills formed of a pale feldspathic porphyry, remarkable from being doubly +columnar, great cylindrical columns being subdivided into smaller four- or +five-sided ones; and a third hillock (Cerro Blanco) is formed of a fragmentary +mass of rock, which I believed to be of volcanic origin, intermediate in +character between the above feldspathic porphyry and common trachyte, and +containing needles of hornblende and granular oxide of iron. Near the Baths of +Cauquenes, between two short parallel lines of elevation, where they are +intersected by the valley, there is a small, though distinct volcanic district; +the rock is a dark grey (andesitic) trachyte, which fuses into a greenish-grey +bead, and is formed of long crystals of fractured glassy albite (judging from +one measurement) mingled with well- formed crystals, often twin, of augite. The +whole mass is vesicular, but the surface is darker coloured and much more +vesicular than any other part. This trachyte forms a cliff-bounded, horizontal, +narrow strip on the steep southern side of the valley, at the height of four or +five hundred feet above the river-bed; judging from an apparently corresponding +line of cliff on the northern side, the valley must once have been filled up to +this height by a field of lava. On the summit of a lofty mountain some leagues +higher up this same valley of the Cachapual, I found columnar pitchstone +porphyritic with feldspar; I do not suppose this rock to be of volcanic origin, +and only mention it here, from its being intersected by masses and dikes of a +VESICULAR rock, approaching in character to trachyte; in no other part of Chile +did I observe vesicular or amygdaloidal dikes, though these are so common in +ordinary volcanic districts. +</p> + +<h3>PASSAGE OF THE ANDES BY THE PORTILLO OR PEQUENES PASS.</h3> + +<p> +Although I crossed the Cordillera only once by this pass, and only once by that +of the Cumbre or Uspallata (presently to be described), riding slowly and +halting occasionally to ascend the mountains, there are many circumstances +favourable to obtaining a more faithful sketch of their structure than would at +first be thought possible from so short an examination. The mountains are steep +and absolutely bare of vegetation; the atmosphere is resplendently clear; the +stratification distinct; and the rocks brightly and variously coloured: some of +the natural sections might be truly compared for distinctness to those coloured +ones in geological works. Considering how little is known of the structure of +this gigantic range, to which I particularly attended, most travellers having +collected only specimens of the rocks, I think my sketch-sections, though +necessarily imperfect, possess some interest. Section 1/1 in Plate 1 which I +will now describe in detail, is on a horizontal scale of a third of an inch to +a nautical mile, and on a vertical scale of one inch to a mile (or 6,000 feet). +The width of the range (excluding a few outlying hillocks), from the plain on +which St. Jago the capital of Chile stands, to the Pampas, is sixty miles, as +far as I can judge from the maps, which differ from each other and are all +EXCEEDINGLY imperfect. The St. Jago plain at the mouth of the Maypu, I estimate +from adjoining known points at 2,300 feet, and the Pampas at 3,500 feet, both +above the level of the sea. The height of the Pequenes line, according to Dr. +Gillies, is 13,210 feet (“Journal of Natural and Geographical +Science” August 1830.); and that of the Portillo line (both in the gaps +where the road crosses them) is 14,345 feet; the lowest part of the +intermediate valley of Tenuyan is 7,530 feet—all above the level of the +sea. +</p> + +<p> +The Cordillera here, and indeed I believe throughout Chile, consist of several +parallel, anticlinal and uniclinal mountain-lines, ranging north, or north with +a little westing, and south. Some exterior and much lower ridges often vary +considerably from this course, projecting like oblique spurs from the main +ranges: in the district towards the Pacific, the mountains, as before remarked, +extend in various directions, even east and west. In the main exterior lines, +the strata, as also before remarked, are seldom inclined at a high angle; but +in the central lofty ridges they are almost always highly inclined, broken by +many great faults, and often vertical. As far as I could judge, few of the +ranges are of great length: and in the central parts of the Cordillera, I was +frequently able to follow with my eye a ridge gradually becoming higher and +higher, as the stratification increased in inclination, from one end where its +height was trifling and its strata gently inclined to the other end where +vertical strata formed snow-clad pinnacles. Even outside the main Cordillera, +near the baths of Cauquenes, I observed one such case, where a north and south +ridge had its strata in the valley inclined at 37 degrees, and less than a mile +south of it at 67 degrees: another parallel and similarly inclined ridge rose +at the distance of about five miles, into a lofty mountain with absolutely +vertical strata. Within the Cordillera, the height of the ridges and the +inclination of the strata often became doubled and trebled in much shorter +distances than five miles; this peculiar form of upheaval probably indicates +that the stratified crust was thin, and hence yielded to the underlying +intrusive masses unequally, at certain points on the lines of fissure. +</p> + +<p> +The valleys, by which the Cordillera are drained, follow the anticlinal or +rarely synclinal troughs, which deviate most from the usual north and south +course; or still more commonly those lines of faults or of unequal curvature +(that is, lines with the strata on both hands dipping in the same direction, +but at a somewhat different angle) which deviate most from a northerly course. +Occasionally the torrents run for some distance in the north and south valleys, +and then recover their eastern or western course by bursting through the ranges +at those points where the strata have been least inclined and the height +consequently is less. Hence the valleys, along which the roads run, are +generally zigzag; and, in drawing an east and west section, it is necessary to +contract greatly that which is actually seen on the road. +</p> + +<p> +Commencing at the western end of Section 1/1 where the R. Maypu debouches on +the plain of St. Jago, we immediately enter on the porphyritic conglomerate +formation, and in the midst of it find some hummocks [A] of granite and +syenite, which probably (for I neglected to collect specimens) belong to the +andesitic class. These are succeeded by some rugged hills [B] of dark-green, +crystalline, feldspathic and in some parts slaty rocks, which I believe belong +to the altered clay-slate formation. From this point, great mountains of +purplish and greenish, generally thinly stratified, highly porphyritic +conglomerates, including many strata of amygdaloidal and greenstone porphyries, +extend up the valley to the junction of the rivers Yeso and Volcan. As the +valley here runs in a very southerly course, the width of the porphyritic +conglomerate formation is quite conjectural; and from the same cause, I was +unable to make out much about the stratification. In most of the exterior +mountains the dip was gentle and directed inwards; and at only one spot I +observed an inclination as high as 50 degrees. Near the junction of the R. +Colorado with the main stream, there is a hill of whitish, brecciated, +partially decomposed feldspathic porphyry, having a volcanic aspect but not +being really of that nature: at Tolla, however, in this valley, Dr. Meyen met +with a hill of pumice containing mica. (“Reise um Erde” Th.1 ss. +338, 341.) At the junction of the Yeso and Volcan [D] there is an extensive +mass, in white conical hillocks, of andesite, containing some mica, and passing +either into andesitic granite, or into a spotted, semi-granular mixture of +albitic (?) feldspar and hornblende: in the midst of this formation Dr. Meyen +found true trachyte. The andesite is covered by strata of dark-coloured, +crystalline, obscurely porphyritic rocks, and above them by the ordinary +porphyritic conglomerates,—the strata all dipping away at a small angle +from the underlying mass. The surrounding lofty mountains appear to be entirely +composed of the porphyritic conglomerate, and I estimated its thickness here at +between six and seven thousand feet. Beyond the junction of the Yeso and +Volcan, the porphyritic strata appear to dip towards the hillocks of andesite +at an angle of 40 degrees; but at some distant points on the same ridge they +are bent up and vertical. Following the valley of the Yeso, trending N.E. (and +therefore still unfavourable for our transverse section), the same porphyritic +conglomerate formation is prolonged to near the Cuestadel Indio, situated at +the western end of the basin (like a drained lake) of Yeso. Some way before +arriving at this point, distant lofty pinnacles capped by coloured strata +belonging to the great gypseous formation could first be seen. From the summit +of the Cuesta, looking southward, there is a magnificent sectional view of a +mountain-mass, at least 2,000 feet in thickness [E], of fine andesite granite +(containing much black mica, a little chlorite and quartz), which sends great +white dikes far into the superincumbent, dark-coloured, porphyritic +conglomerates. At the line of junction the two formations are wonderfully +interlaced together: in the lower part of the porphyritic conglomerate, the +stratification has been quite obliterated, whilst in the upper part it is very +distinct, the beds composing the crests of the surrounding mountains being +inclined at angles of between 70 and 80 degrees, and some being even vertical. +On the northern side of the valley, there is a great corresponding mass of +andesitic granite, which is encased by porphyritic conglomerate, dipping both +on the western and eastern sides, at about 80 degrees to west, but on the +eastern side with the tips of the strata bent in such a manner, as to render it +probable that the whole mass has been on that side thrown over and inverted. +</p> + +<p> +In the valley basin of the Yeso, which I estimated at 7,000 feet above the +level of the sea, we first reach at [F] the gypseous formation. Its thickness +is very great. It consists in most parts of snow-white, hard, compact gypsum, +which breaks with a saccharine fracture, having translucent edges; under the +blowpipe gives out much vapour; it frequently includes nests and exceedingly +thin layers of crystallised, blackish carbonate of lime. Large, irregularly +shaped concretions (externally still exhibiting lines of aqueous deposition) of +blackish-grey, but sometimes white, coarsely and brilliantly crystallised, hard +anhydrite, abound within the common gypsum. Hillocks, formed of the hardest and +purest varieties of the white gypsum, stand up above the surrounding parts, and +have their surfaces cracked and marked, just like newly baked bread. There is +much pale brown, soft argillaceous gypsum; and there were some intercalated +green beds which I had not time to reach. I saw only one fragment of selenite +or transparent gypsum, and that perhaps may have come from some subsequently +formed vein. From the mineralogical characters here given, it is probable that +these gypseous beds have undergone some metamorphic action. The strata are much +hidden by detritus, but they appeared in most parts to be highly inclined; and +in an adjoining lofty pinnacle they could be distinctly seen bending up, and +becoming vertical, conformably with the underlying porphyritic conglomerate. In +very many parts of the great mountain-face [F], composed of thin gypseous beds, +there were innumerable masses, irregularly shaped and not like dikes, yet with +well-defined edges, of an imperfectly granular, pale greenish, or +yellowish-white rock, essentially composed of feldspar, with a little chlorite +or hornblende, epidote, iron-pyrites, and ferruginous powder: I believe that +these curious trappean masses have been injected from the not far distant +mountain-mass [E] of andesite whilst still fluid, and that owing to the +softness of the gypseous strata they have not acquired the ordinary forms of +dikes. Subsequently to the injection of these feldspathic rocks, a great +dislocation has taken place; and the much shattered gypseous strata here +overlie a hillock [G], composed of vertical strata of impure limestone and of +black highly calcareous shale including threads of gypsum: these rocks, as we +shall presently see, belong to the upper parts of the gypseous series, and +hence must here have been thrown down by a vast fault. +</p> + +<p> +Proceeding up the valley-basin of the Yeso, and taking our section sometimes on +one hand and sometimes on the other, we come to a great hill of stratified +porphyritic conglomerate [H] dipping at 45 degrees to the west; and a few +hundred yards farther on, we have a bed between three or four hundred feet +thick of gypsum [I] dipping eastward at a very high angle: here then we have a +fault and anticlinal axis. On the opposite side of the valley, a vertical mass +of red conglomerate, conformably underlying the gypsum, appears gradually to +lose its stratification and passes into a mountain of porphyry. The gypsum [I] +is covered by a bed [K], at least 1,000 feet in thickness, of a purplish-red, +compact, heavy, fine-grained sandstone or mudstone, which fuses easily into a +white enamel, and is seen under a lens to contain triturated crystals. This is +succeeded by a bed [L], 1,000 feet thick (I believe I understate the thickness) +of gypsum, exactly like the beds before described; and this again is capped by +another great bed [M] of purplish-red sandstone. All these strata dip eastward; +but the inclination becomes less and less, as we leave the first and almost +vertical bed [I] of gypsum. +</p> + +<p> +Leaving the basin-plain of Yeso, the road rapidly ascends, passing by mountains +composed of the gypseous and associated beds, with their stratification greatly +disturbed and therefore not easily intelligible: hence this part of the section +has been left uncoloured. Shortly before reaching the great Pequenes ridge, the +lowest stratum visible [N] is a red sandstone or mudstone, capped by a vast +thickness of black, compact, calcareous, shaly rock [O], which has been thrown +into four lofty, though small ridges: looking northward, the strata in these +ridges are seen gradually to rise in inclination, becoming in some distant +pinnacles absolutely vertical. +</p> + +<p> +The ridge of Pequenes, which divides the waters flowing into the Pacific and +Atlantic Oceans, extends in a nearly N.N.W. and S.S.E. line; its strata dip +eastward at an angle of between 30 and 45 degrees, but in the higher peaks +bending up and becoming almost vertical. Where the road crosses this range, the +height is 13,210 feet above the sea-level, and I estimated the neighbouring +pinnacles at from fourteen to fifteen thousand feet. The lowest stratum visible +in this ridge is a red stratified sandstone [P]; on it are superimposed two +great masses [Q and S] of black, hard, compact, even having a conchoidal +fracture, calcareous, more or less laminated shale, passing into limestone: +this rock contains organic remains, presently to be enumerated. The compacter +varieties fuse easily in a white glass; and this I may add is a very general +character with all the sedimentary beds in the Cordillera: although this rock +when broken is generally quite black, it everywhere weathers into an ash-grey +tint. Between these two great masses [Q and S], a bed [R] of gypsum is +interposed, about three hundred feet in thickness, and having the same +characters as heretofore described. I estimated the total thickness of these +three beds [Q, R, S] at nearly three thousand feet; and to this must be added, +as will be immediately seen, a great overlying mass of red sandstone. +</p> + +<p> +In descending the eastern slope of this great central range, the strata, which +in the upper part dip eastward at about an angle of 40 degrees, become more and +more curved, till they are nearly vertical; and a little further onwards there +is seen on the further side of a ravine, a thick mass of strata of bright red +sandstone [T], with their upper extremities slightly curved, showing that they +were once conformably prolonged over the beds [S]: on the southern and opposite +side of the road, this red sandstone and the underlying black shaly rocks stand +vertical, and in actual juxtaposition. Continuing to descend, we come to a +synclinal valley filled with rubbish, beyond which we have the red sandstone +[T2] corresponding with [T], and now dipping, as is seen both north and south +of the road, at 45 degrees to the west; and under it, the beds [S2, R2, Q2, and +I believe P2] in corresponding order and of similar composition, with those on +the western flank of the Pequenes range, but dipping westward. Close to the +synclinal valley the dip of these strata is 45 degrees, but at the eastern or +farther end of the series it increases to 60 degrees. Here the great gypseous +formation abruptly terminates, and is succeeded eastward by a pile of more +modern strata. Considering how violently these central ranges have been +dislocated, and how very numerous dikes are in the exterior and lower parts of +the Cordillera, it is remarkable that I did not here notice a single dike. The +prevailing rock in this neighbourhood is the black, calcareous, compact shale, +whilst in the valley-basin of the Yeso the purplish red sandstone or mudstone +predominates,—both being associated with gypseous strata of exactly the +same nature. It would be very difficult to ascertain the relative superposition +of these several masses, for we shall afterwards see in the Cumbre Pass that +the gypseous and intercalated beds are lens-shaped, and that they thin out, +even where very thick, and disappear in short horizontal distances: it is quite +possible that the black shales and red sandstones may be contemporaneous, but +it is more probable that the former compose the uppermost parts of the series. +</p> + +<p> +The fossils above alluded to in the black calcareous shales are few in number, +and are in an imperfect condition; they consist, as named for me by M. +d’Orbigny, of:— +</p> + +<p> +1. Ammonite, indeterminable, near to A. recticostatus, d’Orbigny, +“Pal. Franc.” (Neocomian formation). 2. Gryphaea, near to G. +Couloni (Neocomian formations of France and Neufchatel). 3. Natica, +indeterminable. 4. Cyprina rostrata, d’Orbigny, “Pal. Franc.” +(Neocomian formation). 5. Rostellaria angulosa (?), d’Orbigny, +“Pal. de l’Amer. Mer.” 6. Terebratula (?). +</p> + +<p> +Some of the fragments of Ammonites were as thick as a man’s arm: the +Gryphaea is much the most abundant shell. These fossils M. d’Orbigny +considers as belonging to the Neocomian stage of the Cretaceous system. Dr. +Meyen, who ascended the valley of the Rio Volcan, a branch of the Yeso, found a +nearly similar, but apparently more calcareous formation, with much gypsum, and +no doubt the equivalent of that here described (“Reise um Erde” +etc. Th. 1 s. 355.): the beds were vertical, and were prolonged up to the +limits of perpetual snow; at the height of 9,000 feet above the sea, they +abounded with fossils, consisting, according to Von Buch (“Descript. +Phys. des Iles Canaries” page 471.), of:— +</p> + +<p> +1. Exogyra (Gryphaea) Couloni, absolutely identical with specimens from the +Jura and South of France. 2. Trigonia costata, identical with those found in +the upper Jurassic beds at Hildesheim. 3. Pecten striatus, identical with those +found in the upper Jurassic beds at Hildesheim. 4. Cucullaea, corresponding in +form to C. longirostris, so frequent in the upper Jurassic beds of Westphalia. +5. Ammonites resembling A. biplex. +</p> + +<p> +Von Buch concludes that this formation is intermediate between the limestone of +the Jura and the chalk, and that it is analogous with the uppermost Jurassic +beds forming the plains of Switzerland. Hence M. D’Orbigny and Von Buch, +under different terms, compare these fossils to those from the same late stage +in the secondary formations of Europe. +</p> + +<p> +Some of the fossils which I collected were found a good way down the western +slope of the main ridge, and hence must originally have been covered up by a +great thickness of the black shaly rock, independently of the now denuded, +thick, overlying masses of red sandstone. I neglected at the time to estimate +how many hundred or rather thousand feet thick the superincumbent strata must +have been: and I will not now attempt to do so. This, however, would have been +a highly interesting point, as indicative of a great amount of subsidence, of +which we shall hereafter find in other parts of the Cordillera analogous +evidence during this same period. The altitude of the Peuquenes Range, +considering its not great antiquity, is very remarkable; many of the fossils +were embedded at the height of 13,210 feet, and the same beds are prolonged up +to at least from fourteen to fifteen thousand feet above the level of the sea. +</p> + +<h3>THE PORTILLO OR EASTERN CHAIN.</h3> + +<p> +The valley of Tenuyan, separating the Peuquenes and Portillo lines, is, as +estimated by Dr. Gillies and myself, about twenty miles in width; the lowest +part, where the road crosses the river, being 7,500 feet above the sea-level. +The pass on the Portillo line is 14,365 feet high (1,100 feet higher than that +on the Peuquenes), and the neighbouring pinnacles must, I conceive, rise to +nearly 16,000 feet above the sea. The river draining the intermediate valley of +Tenuyan, passes through the Portillo line. To return to our +section:—shortly after leaving the lower beds [P2] of the gypseous +formation, we come to grand masses of a coarse, red conglomerate [V], totally +unlike any strata hitherto seen in the Cordillera. This conglomerate is +distinctly stratified, some of the beds being well defined by the greater size +of the pebbles: the cement is calcareous and sometimes crystalline, though the +mass shows no signs of having been metamorphosed. The included pebbles are +either perfectly or only partially rounded: they consist of purplish +sandstones, of various porphyries, of brownish limestone, of black calcareous, +compact shale precisely like that in situ in the Peuquenes range, and +CONTAINING SOME OF THE SAME FOSSIL SHELLS; also very many pebbles of quartz, +some of micaceous schist, and numerous, broken, rounded crystals of a reddish +orthitic or potash feldspar (as determined by Professor Miller), and these from +their size must have been derived from a coarse-grained rock, probably granite. +From this feldspar being orthitic, and even from its external appearance, I +venture positively to affirm that it has not been derived from the rocks of the +western ranges; but, on the other hand, it may well have come, together with +the quartz and metamorphic schists, from the eastern or Portillo line, for this +line mainly consists of coarse orthitic granite. The pebbles of the +fossiliferous slate and of the purple sandstone, certainly have been derived +from the Peuquenes or western ranges. +</p> + +<p> +The road crosses the valley of Tenuyan in a nearly east and west line, and for +several miles we have on both hands the conglomerate, everywhere dipping west +and forming separate great mountains. The strata, where first met with, after +leaving the gypseous formation, are inclined westward at an angle of only 20 +degrees, which further on increases to about 45 degrees. The gypseous strata, +as we have seen, are also inclined westward: hence, when looking from the +eastern side of the valley towards the Peuquenes range, a most deceptive +appearance is presented, as if the newer beds of conglomerate dipped directly +under the much older beds of the gypseous formation. In the middle of the +valley, a bold mountain of unstratified lilac-coloured porphyry (with crystals +of hornblende) projects; and further on, a little south of the road, there is +another mountain, with its strata inclined at a small angle eastwards, which in +its general aspect and colour, resembles the porphyritic conglomerate +formation, so rare on this side of the Peuquenes line and so grandly developed +throughout the western ranges. +</p> + +<p> +The conglomerate is of great thickness: I do not suppose that the strata +forming the separate mountain-masses [V,V,V] have ever been prolonged over each +other, but that one mass has been broken up by several, distinct, parallel, +uniclinal lines of elevation. Judging therefore of the thickness of the +conglomerate, as seen in the separate mountain-masses, I estimated it at least +from one thousand five hundred to two thousand feet. The lower beds rest +conformably on some singularly coloured, soft strata [W], which I could not +reach to examine; and these again rest conformably on a thick mass of +micaceous, thinly laminated, siliceous sandstone [X], associated with a little +black clay-slate. These lower beds are traversed by several dikes of +decomposing porphyry. The laminated sandstone is directly superimposed on the +vast masses of granite [Y,Y] which mainly compose the Portillo range. The line +of junction between this latter rock, which is of a bright red colour, and the +whitish sandstone was beautifully distinct; the sandstone being penetrated by +numerous, great, tortuous dikes branching from the granite, and having been +converted into a granular quartz rock (singularly like that of the Falkland +Islands), containing specks of an ochrey powder, and black crystalline atoms, +apparently of imperfect mica. The quartzose strata in one spot were folded into +a regular dome. +</p> + +<p> +The granite which composes the magnificent bare pinnacles and the steep western +flank of the Portillo chain, is of a brick-red colour, coarsely crystallised, +and composed of orthitic or potash feldspar, quartz, and imperfect mica in +small quantity, sometimes passing into chlorite. These minerals occasionally +assume a laminar or foliated arrangement. The fact of the feldspar being +orthitic in this range, is very remarkable, considering how rare, or rather, as +I believe, entirely absent, this mineral is throughout the western ranges, in +which soda-feldspar, or at least a variety cleaving like albite, is so +extremely abundant. In one spot on the western flank, and on the eastern flank +near Los Manantiales and near the crest, I noticed some great masses of a +whitish granite, parts of it fine- grained, and parts containing large crystals +of feldspar; I neglected to collect specimens, so I do not know whether this +feldspar is also orthitic, though I am inclined to think so from its general +appearance. I saw also some syenite and one mass which resembled andesite, but +of which I likewise neglected to collect specimens. From the manner in which +the whitish granites formed separate mountain-masses in the midst of the +brick-red variety, and from one such mass near the crest being traversed by +numerous veins of flesh-coloured and greenish eurite (into which I occasionally +observed the brick-red granite insensibly passing), I conclude that the white +granites probably belong to an older formation, almost overwhelmed and +penetrated by the red granite. +</p> + +<p> +On the crest I saw also, at a short distance, some coloured stratified beds, +apparently like those [W] at the western base, but was prevented examining them +by a snowstorm: Mr. Caldcleugh, however, collected here specimens of ribboned +jasper, magnesian limestone, and other minerals. (“Travels” etc. +volume 1 page 308.) A little way down the eastern slope a few fragments of +quartz and mica-slate are met with; but the great formation of this latter rock +[Z], which covers up much of the eastern flank and base of the Portillo range, +cannot be conveniently examined until much lower down at a place called Mal +Paso. The mica-schist here consists of thick layers of quartz, with intervening +folia of finely-scaly mica, often passing into a substance like black glossy +clay-slate: in one spot, the layers of the quartz having disappeared, the whole +mass became converted into glossy clay-slate. Where the folia were best +defined, they were inclined at a high angle westward, that is, towards the +range. The line of junction between the dark mica-slate and the coarse red +granite was most clearly distinguishable from a vast distance: the granite sent +many small veins into the mica-slate, and included some angular fragments of +it. As the sandstone on the western base has been converted by the red granite +into a granular quartz-rock, so this great formation of mica-schist may +possibly have been metamorphosed at the same time and by the same means; but I +think it more probable, considering its more perfect metamorphic character and +its well-pronounced foliation, that it belongs to an anterior epoch, connected +with the white granites: I am the more inclined to this view, from having found +at the foot of the range the mica-schist surrounding a hummock [Y2], +exclusively composed of white granite. Near Los Arenales, the mountains on all +sides are composed of the mica-slate; and looking backwards from this point up +to the bare gigantic peaks above, the view was eminently interesting. The +colours of the red granite and the black mica-slate are so distinct, that with +a bright light these rocks could be readily distinguished even from the Pampas, +at a level of at least 9,000 feet below. The red granite, from being divided by +parallel joints, has weathered into sharp pinnacles, on some of which, even on +some of the loftiest, little caps of mica-schist could be clearly seen: here +and there isolated patches of this rock adhered to the mountain-flanks, and +these often corresponded in height and position on the opposite sides of the +immense valleys. Lower down the schist prevailed more and more, with only a few +quite small points of granite projecting through. Looking at the entire eastern +face of the Portillo range, the red colour far exceeds in area the black; yet +it was scarcely possible to doubt that the granite had once been almost wholly +encased by the mica-schist. +</p> + +<p> +At Los Arenales, low down on the eastern flank, the mica-slate is traversed by +several closely adjoining, broad dikes, parallel to each other and to the +foliation of the schist. The dikes are formed of three different varieties of +rock, of which a pale brown feldspathic porphyry with grains of quartz was much +the most abundant. These dikes with their granules of quartz, as well as the +mica-schist itself, strikingly resemble the rocks of the Chonos Archipelago. At +a height of about twelve hundred feet above the dikes, and perhaps connected +with them, there is a range of cliffs formed of successive lava-streams [AA], +between three and four hundred feet in thickness, and in places finely +columnar. The lava consists of dark- greyish, harsh rocks, intermediate in +character between trachyte and basalt, containing glassy feldspar, olivine, and +a little mica, and sometimes amygdaloidal with zeolite: the basis is either +quite compact, or crenulated with air-vesicles arranged in laminae. The streams +are separated from each other by beds of fragmentary brown scoriae, firmly +cemented together, and including a few well-rounded pebbles of lava. From their +general appearance, I suspect that these lava-streams flowed at an ancient +period under the pressure of the sea, when the Atlantic covered the Pampas and +washed the eastern foot of the Cordillera. (This conclusion might, perhaps, +even have been anticipated, from the general rarity of volcanic action, except +near the sea or large bodies of water. Conformably with this rule, at the +present day, there are no active volcanoes on this eastern side of the +Cordillera; nor are severe earthquakes experienced here.) On the opposite and +northern side of the valley there is another line of lava- cliffs at a +corresponding height; the valley between being of considerable breadth, and as +nearly as I could estimate 1,500 feet in depth. This field of lava is confined +on both sides by the mountains of mica-schist, and slopes down rapidly but +irregularly to the edge of the Pampas, where, having a thickness of about two +hundred feet, it terminates against a little range of claystone porphyry. The +valley in this lower part expands into a bay-like, gentle slope, bordered by +the cliffs of lava, which must certainly once have extended across this wide +expanse. The inclination of the streams from Los Arenales to the mouth of the +valley is so great, that at the time (though ignorant of M. Elie de +Beaumont’s researches on the extremely small slope over which lava can +flow, and yet retain a compact structure and considerable thickness) I +concluded that they must subsequently to their flowing have been upheaved and +tilted from the mountains; of this conclusion I can now entertain not the +smallest doubt. +</p> + +<p> +At the mouth of the valley, within the cliffs of the above lava-field, there +are remnants, in the form of separate small hillocks and of lines of low +cliffs, of a considerable deposit of compact white tuff (quarried for +filtering-stones), composed of broken pumice, volcanic crystals, scales of +mica, and fragments of lava. This mass has suffered much denudation; and the +hard mica-schist has been deeply worn, since the period of its deposition; and +this period must have been subsequent to the denudation of the basaltic +lava-streams, as attested by their encircling cliffs standing at a higher +level. At the present day, under the existing arid climate, ages might roll +past without a square yard of rock of any kind being denuded, except perhaps in +the rarely moistened drainage-channel of the valley. Must we then look back to +that ancient period, when the waves of the sea beat against the eastern foot of +the Cordillera, for a power sufficient to denude extensively, though +superficially, this tufaceous deposit, soft although it be? +</p> + +<p> +There remains only to mention some little water-worn hillocks [BB], a few +hundred feet in height, and mere mole-hills compared with the gigantic +mountains behind them, which rise out of the sloping, shingle-covered margin of +the Pampas. The first little range is composed of a brecciated purple +porphyritic claystone, with obscurely marked strata dipping at 70 degrees to +the S.W.; the other ranges consist of—a pale-coloured feldspathic +porphyry,—a purple claystone porphyry with grains of quartz,— and a +rock almost exclusively composed of brick-red crystals of feldspar. These +outermost small lines of elevation extend in a N.W. by W. and S.E. by S. +direction. +</p> + +<h3>CONCLUDING REMARKS ON THE PORTILLO RANGE.</h3> + +<p> +When on the Pampas and looking southward, and whilst travelling northward, I +could see for very many leagues the red granite and dark mica-schist forming +the crest and eastern flank of the Portillo line. This great range, according +to Dr. Gillies, can be traced with little interruption for 140 miles southward +to the R. Diamante, where it unites with the western ranges: northward, +according to this same author, it terminates where the R. Mendoza debouches +from the mountains; but a little further north in the eastern part of the +Cumbre section, there are, as we shall hereafter see, some mountain-masses of a +brick-red porphyry, the last injected amidst many other porphyries, and having +so close an analogy with the coarse red granite of the Portillo line, that I am +tempted to believe that they belong to the same axis of injection; if so, the +Portillo line is at least 200 miles in length. Its height, even in the lowest +gap in the road, is 14,365 feet, and some of the pinnacles apparently attain an +elevation of about 16,000 feet above the sea. The geological history of this +grand chain appears to me eminently interesting. We may safely conclude, that +at a former period the valley of Tenuyan existed as an arm of the sea, about +twenty-miles in width, bordered on one hand by a ridge or chain of islets of +the black calcareous shales and purple sandstones of the gypseous formation; +and on the other hand, by a ridge or chain of islets composed of mica-slate, +white granite, and perhaps to a partial extent of red granite. These two +chains, whilst thus bordering the old sea-channel, must have been exposed for a +vast lapse of time to alluvial and littoral action, during which the rocks were +shattered, the fragments rounded, and the strata of conglomerate accumulated to +a thickness of at least fifteen hundred or two thousand feet. The red orthitic +granite now forms, as we have seen, the main part of the Portillo chain: it is +injected in dikes not only into the mica-schist and white granites, but into +the laminated sandstone, which it has metamorphosed, and which it has thrown +off, together with the conformably overlying coloured beds and stratified +conglomerate, at an angle of forty-five degrees. To have thrown off so vast a +pile of strata at this angle, is a proof that the main part of the red granite +(whether or not portions, as perhaps is probable, previously existed) was +injected in a liquified state after the accumulation both of the laminated +sandstone and of the conglomerate; this conglomerate, we know, was accumulated, +not only after the deposition of the fossiliferous strata of the Peuquenes +line, but after their elevation and long-continued denudation: and these +fossiliferous strata belong to the early part of the Cretaceous system. Late, +therefore, in a geological sense, as must be the age of the main part of the +red granite, I can conceive nothing more impressive than the eastern view of +this great range, as forcing the mind to grapple with the idea of the thousands +of thousands of years requisite for the denudation of the strata which +originally encased it,—for that the fluidified granite was once encased, +its mineralogical composition and structure, and the bold conical shape of the +mountain-masses, yield sufficient evidence. Of the encasing strata we see the +last vestiges in the coloured beds on the crest, in the little caps of +mica-schist on some of the loftiest pinnacles, and in the isolated patches of +this same rock at corresponding heights on the now bare and steep flanks. +</p> + +<p> +The lava-streams at the eastern foot of the Portillo are interesting, not so +much from the great denudation which they have suffered at a comparatively late +period as from the evidence they afford by their inclination taken conjointly +with their thickness and compactness, that after the great range had assumed +its present general outline, it continued to rise as an axis of elevation. The +plains extending from the base of the Cordillera to the Atlantic show that the +continent has been upraised in mass to a height of 3,500 feet, and probably to +a much greater height, for the smooth shingle-covered margin of the Pampas is +prolonged in a gentle unbroken slope far up many of the great valleys. Nor let +it be assumed that the Peuquenes and Portillo ranges have undergone only +movements of elevation; for we shall hereafter see, that the bottom of the sea +subsided several thousand feet during the deposition of strata, occupying the +same relative place in the Cordillera, with those of the Peuquenes ridge; +moreover, we shall see from the unequivocal evidence of buried upright trees, +that at a somewhat later period, during the formation of the Uspallata chain, +which corresponds geographically with that of the Portillo, there was another +subsidence of many thousand feet: here, indeed, in the valley of Tenuyan, the +accumulation of the coarse stratified conglomerate to a thickness of fifteen +hundred or two thousand feet, offers strong presumptive evidence of subsidence; +for all existing analogies lead to the belief that large pebbles can be +transported only in shallow water, liable to be affected by currents and +movements of undulation—and if so, the shallow bed of the sea on which +the pebbles were first deposited must necessarily have sunk to allow of the +accumulation of the superincumbent strata. What a history of changes of level, +and of wear and tear, all since the age of the latter secondary formations of +Europe, does the structure of this one great mountain-chain reveal! +</p> + +<h3>PASSAGE OF THE ANDES BY THE CUMBRE OR USPALLATA PASS.</h3> + +<p> +This Pass crosses the Andes about sixty miles north of that just described: the +section given in Plate 1, Section 1/2, is on the same scale as before, namely, +at one-third of an inch to a mile in distance, and one inch to a mile (or 6,000 +feet) in height. Like the last section, it is a mere sketch, and cannot pretend +to accuracy, though made under favourable circumstances. We will commence as +before, with the western half, of which the main range bears the name of the +Cumbre (that is the Ridge), and corresponds to the Peuquenes line in the former +section; as does the Uspallata range, though on a much smaller scale, to that +of the Portillo. Near the point where the river Aconcagua debouches on the +basin plain of the same name, at a height of about two thousand three hundred +feet above the sea, we meet with the usual purple and greenish porphyritic +claystone conglomerate. Beds of this nature, alternating with numerous compact +and amygdaloidal porphyries, which have flowed as submarine lavas, and +associated with great mountain- masses of various, injected, non-stratified +porphyries, are prolonged the whole distance up to the Cumbre or central ridge. +One of the commonest stratified porphyries is of a green colour, highly +amygdaloidal with the various minerals described in the preliminary discussion, +and including fine tabular crystals of albite. The mountain-range north (often +with a little westing) and south. The stratification, wherever I could clearly +distinguish it, was inclined westward or towards the Pacific, and, except near +the Cumbre, seldom at angles above 25 degrees. Only at one spot on this western +side, on a lofty pinnacle not far from the Cumbre, I saw strata apparently +belonging to the gypseous formation, and conformably capping a pile of +stratified porphyries. Hence, both in composition and in stratification, the +structure of the mountains on this western side of the divortium aquarum, is +far more simple than in the corresponding part of the Peuquenes section. In the +porphyritic claystone conglomerate, the mechanical structure and the planes of +stratification have generally been much obscured and even quite obliterated +towards the base of the series, whilst in the upper parts, near the summits of +the mountains, both are distinctly displayed. In these upper portions the +porphyries are generally lighter coloured. In three places [X, Y, Z] masses of +andesite are exposed: at [Y], this rock contained some quartz, but the greater +part consisted of andesitic porphyry, with only a few well-developed crystals +of albite, and forming a great white mass, having the external aspect of +granite, capped by much dark unstratified porphyry. In many parts of the +mountains, there are dikes of a green colour, and other white ones, which +latter probably spring from underlying masses of andesite. +</p> + +<p> +The Cumbre, where the road crosses it, is, according to Mr. Pentland, 12,454 +feet above the sea; and the neighbouring peaks, composed of dark purple and +whitish porphyries, some obscurely stratified with a westerly dip, and others +without a trace of stratification, must exceed 13,000 feet in height. +Descending the eastern slope of the Cumbre, the structure becomes very +complicated, and generally differs on the two sides of the east and west line +of road and section. First we come to a great mass [A] of nearly vertical, +singularly contorted strata, composed of highly compact red sandstones, and of +often calcareous conglomerates, and penetrated by green, yellow, and reddish +dikes; but I shall presently have an opportunity of describing in some detail +an analogous pile of strata. These vertical beds are abruptly succeeded by +others [B], of apparently nearly the same nature but more metamorphosed, +alternating with porphyries and limestones; these dip for a short space +westward, but there has been here an extraordinary dislocation, which, on the +north side of the road, appears to have determined the excavation of the north +and south valley of the R. de las Cuevas. On this northern side of the road, +the strata [B] are prolonged till they come in close contact with a jagged +lofty mountain [D] of dark- coloured, unstratified, intrusive porphyry, where +the beds have been more highly inclined and still more metamorphosed. This +mountain of porphyry seems to form a short axis of elevation, for south of the +road in its line there is a hill [C] of porphyritic conglomerate with +absolutely vertical strata. +</p> + +<p> +We now come to the gypseous formation: I will first describe the structure of +the several mountains, and then give in one section a detailed account of the +nature of the rocks. On the north side of the road, which here runs in an east +and west valley, the mountain of porphyry [D] is succeeded by a hill [E] formed +of the upper gypseous strata tilted, at an angle of between 70 and 80 degrees +to the west, by a uniclinal axis of elevation which does not run parallel to +the other neighbouring ranges, and which is of short length; for on the south +side of the valley its prolongation is marked only by a small flexure in a pile +of strata inclined by a quite separate axis. A little further on the north and +south valley of Horcones enters at right angles our line of section; its +western side is bounded by a hill of gypseous strata [F] dipping westward at +about 45 degrees, and its eastern side by a mountain of similar strata [G] +inclined westward at 70 degrees, and superimposed by an oblique fault on +another mass of the same strata [H], also inclined westward, but at an angle of +about 30 degrees: the complicated relation of these three masses [F, G, H] is +explained by the structure of a great mountain-range lying some way to the +north, in which a regular anticlinal axis (represented in the section by dotted +lines) is seen, with the strata on its eastern side again bending up and +forming a distinct uniclinal axis, of which the beds marked [H] form the lower +part. This great uniclinal line is intersected, near the Puente del Inca, by +the valley along which the road runs, and the strata composing it will be +immediately described. On the south side of the road, in the space +corresponding with the mountains [E, F, and G], the strata everywhere dip +westward generally at an angle of 30 degrees, occasionally mounting up to 45 +degrees, but not in an unbroken line, for there are several vertical faults, +forming separate uniclinal masses, all dipping in the same direction,—a +form of elevation common in the Cordillera. We thus see that within a narrow +space, the gypseous strata have been upheaved and crushed together by a great +uniclinal, anticlinal, and one lesser uniclinal line [E] of elevation; and that +between these three lines and the Cumbre, in the sandstones, conglomerates and +porphyritic formation, there have been at least two or three other great +elevatory axes. +</p> + +<p> +The uniclinal axis [I] intersected near the Puente del Inca (of which the +strata at [H] form a part) ranges N. by W. and S. by E., forming a chain of +mountains, apparently little inferior in height to the Cumbre: the strata, as +we have seen, dip at an average angle of 30 degrees to the west. (At this +place, there are some hot and cold springs, the warmest having a temperature, +according to Lieutenant Brand “Travels,” page 240, of 91 degrees; +they emit much gas. According to Mr. Brande, of the Royal Institution, ten +cubical inches contain forty-five grains of solid matter, consisting chiefly of +salt, gypsum, carbonate of lime, and oxide of iron. The water is charged with +carbonic acid and sulphuretted hydrogen. These springs deposit much tufa in the +form of spherical balls. They burst forth, as do those of Cauquenes, and +probably those of Villa Vicencio, on a line of elevation.) The flanks of the +mountains are here quite bare and steep, affording an excellent section; so +that I was able to inspect the strata to a thickness of about 4,000 feet, and +could clearly distinguish their general nature for 1,000 feet higher, making a +total thickness of 5,000 feet, to which must be added about 1,000 feet of the +inferior strata seen a little lower down the valley, I will describe this one +section in detail, beginning at the bottom. +</p> + +<p> +1st. The lowest mass is the altered clay-slate described in the preliminary +discussion, and which in this line of section was here first met with. Lower +down the valley, at the R. de las Vacas, I had a better opportunity of +examining it; it is there in some parts well characterised, having a distinct, +nearly vertical, tortuous cleavage, ranging N.W. and S.E., and intersected by +quartz veins: in most parts, however, it is crystalline and feldspathic, and +passes into a true greenstone often including grains of quartz. The clay-slate, +in its upper half, is frequently brecciated, the embedded angular fragments +being of nearly the same nature with the paste. +</p> + +<p> +2nd. Several strata of purplish porphyritic conglomerate, of no very great +thickness, rest conformably upon the feldspathic slate. A thick bed of fine, +purple, claystone porphyry, obscurely brecciated (but not of metamorphosed +sedimentary origin), and capped by porphyritic conglomerate, was the lowest bed +actually examined in this section at the Puente del Inca. +</p> + +<p> +3rd. A stratum, eighty feet thick, of hard and very compact impure whitish +limestone, weathering bright red, with included layers brecciated and re- +cemented. Obscure marks of shell are distinguishable in it. +</p> + +<p> +4th. A red, quartzose, fine-grained conglomerate, with grains of quartz, and +with patches of white earthy feldspar, apparently due to some process of +concretionary crystalline action; this bed is more compact and metamorphosed +than any of the overlying conglomerates. +</p> + +<p> +5th. A whitish cherty limestone, with nodules of bluish argillaceous limestone. +</p> + +<p> +6th. A white conglomerate, with many particles of quartz, almost blending into +the paste. +</p> + +<p> +7th. Highly siliceous, fine-grained white sandstone. +</p> + +<p> +8th and 9th. Red and white beds not examined. +</p> + +<p> +10th. Yellow, fine-grained, thinly stratified, magnesian (judging from its slow +dissolution in acids) limestone: it includes some white quartz pebbles, and +little cavities, lined with calcareous spar, some retaining the form of shells. +</p> + +<p> +11th. A bed between twenty and thirty feet thick, quite conformable with the +underlying ones, composed of a hard basis, tinged lilac-grey porphyritic with +NUMEROUS crystals of whitish feldspar, with black mica and little spots of soft +ferruginous matter: evidently a submarine lava. +</p> + +<p> +12th. Yellow magnesian limestone, as before, part-stained purple. +</p> + +<p> +13th. A most singular rock; basis purplish grey, obscurely crystalline, easily +fusible into a dark green glass, not hard, thickly speckled with crystals more +or less perfect of white carbonate of lime, of red hydrous oxide of iron, of a +white and transparent mineral like analcime, and of a green opaque mineral like +soap-stone; the basis is moreover amygdaloidal with many spherical balls of +white crystallised carbonate of lime, of which some are coated with the red +oxide of iron. I have no doubt, from the examination of a superincumbent +stratum (19), that this is a submarine lava; though in Northern Chile, some of +the metamorphosed sedimentary beds are almost as crystalline, and of as varied +composition. +</p> + +<p> +14th. Red sandstone, passing in the upper part into a coarse, hard, red +conglomerate, 300 feet thick, having a calcareous cement, and including grains +of quartz and broken crystals of feldspar; basis infusible; the pebbles consist +of dull purplish porphyries, with some of quartz, from the size of a nut to a +man’s head. This is the coarsest conglomerate in this part of the +Cordillera: in the middle there was a white layer not examined. +</p> + +<p> +15th. Grand thick bed, of a very hard, yellowish-white rock, with a crystalline +feldspathic base, including large crystals of white feldspar, many little +cavities mostly full of soft ferruginous matter, and numerous hexagonal plates +of black mica. The upper part of this great bed is slightly cellular; the lower +part compact: the thickness varied a little in different parts. Manifestly a +submarine lava; and is allied to bed 11. +</p> + +<p> +16th and 17th. Dull purplish, calcareous, fine-grained, compact sandstones, +which pass into coarse white conglomerates with numerous particles of quartz. +</p> + +<p> +18th. Several alternations of red conglomerate, purplish sandstone, and +submarine lava, like that singular rock forming bed 13. +</p> + +<p> +19th. A very heavy, compact, greenish-black stone, with a fine-grained +obviously crystalline basis, containing a few specks of white calcareous spar, +many specks of the crystallised hydrous red oxide of iron, and some specks of a +green mineral; there are veins and nests filled with epidote: certainly a +submarine lava. +</p> + +<p> +20th. Many thin strata of compact, fine-grained, pale purple sandstone. +</p> + +<p> +21st. Gypsum in a nearly pure state, about three hundred feet in thickness: +this bed, in its concretions of anhydrite and layers of small blackish crystals +of carbonate of lime, exactly resembles the great gypseous beds in the +Peuquenes range. +</p> + +<p> +22nd. Pale purple and reddish sandstone, as in bed 20: about three hundred feet +in thickness. +</p> + +<p> +23rd. A thick mass composed of layers, often as thin as paper and convoluted, +of pure gypsum with others very impure, of a purplish colour. +</p> + +<p> +24th. Pure gypsum, thick mass. +</p> + +<p> +25th. Red sandstones, of great thickness. +</p> + +<p> +26th. Pure gypsum, of great thickness. +</p> + +<p> +27th. Alternating layers of pure and impure gypsum, of great thickness. +</p> + +<p> +I was not able to ascend to these few last great strata, which compose the +neighbouring loftiest pinnacles. The thickness, from the lowest to the +uppermost bed of gypsum, cannot be less than 2,000 feet: the beds beneath I +estimated at 3,000 feet, and this does not include either the lower parts of +the porphyritic conglomerate, or the altered clay-slate; I conceive the total +thickness must be about six thousand feet. I distinctly observed that not only +the gypsum, but the alternating sandstones and conglomerates were lens-shaped, +and repeatedly thinned out and replaced each other: thus in the distance of +about a mile, a bed 300 feet thick of sandstone between two beds of gypsum, +thinned out to nothing and disappeared. The lower part of this section differs +remarkably,—in the much greater diversity of its mineralogical +composition,—in the abundance of calcareous matter,—in the greater +coarseness of some of the conglomerates,—and in the numerous particles +and well-rounded pebbles, sometimes of large size, of quartz,— from any +other section hitherto described in Chile. From these peculiarities and from +the lens-form of the strata, it is probable that this great pile of strata was +accumulated on a shallow and very uneven bottom, near some pre-existing land +formed of various porphyries and quartz-rock. The formation of porphyritic +claystone conglomerate does not in this section attain nearly its ordinary +thickness; this may be PARTLY attributed to the metamorphic action having been +here much less energetic than usual, though the lower beds have been affected +to a certain degree. If it had been as energetic as in most other parts of +Chile, many of the beds of sandstone and conglomerate, containing rounded +masses of porphyry, would doubtless have been converted into porphyritic +conglomerate; and these would have alternated with, and even blended into, +crystalline and porphyritic strata without a trace of mechanical +structure,—namely, into those which, in the present state of the section, +we see are unquestionably submarine lavas. +</p> + +<p> +The beds of gypsum, together with the red alternating sandstones and +conglomerates, present so perfect and curious a resemblance with those seen in +our former section in the basin-valley of Yeso, that I cannot doubt the +identity of the two formations: I may add, that a little westward of the P. del +Inca, a mass of gypsum passed into a fine-grained, hard, brown sandstone, which +contained some layers of black, calcareous, compact, shaly rock, precisely like +that seen in such vast masses on the Peuquenes range. +</p> + +<p> +Near the Puente del Inca, numerous fragments of limestone, containing some +fossil remains, were scattered on the ground: these fragments so perfectly +resemble the limestone of bed No. 3, in which I saw impressions of shells, that +I have no doubt they have fallen from it. The yellow magnesian limestone of bed +No. 10, which also includes traces of shells, has a different appearance. These +fossils (as named by M. d’Orbigny) consist of:— +</p> + +<p> +Gryphaea, near to G. Couloni (Neocomian formation). Arca, perhaps A. Gabrielis, +d’Orbigny, “Pal. Franc.” (Neocomian formation). +</p> + +<p> +Mr. Pentland made a collection of shells from this same spot, and Von Buch +considers them as consisting of:— +</p> + +<p> +Trigonia, resembling in form T. costata. Pholadomya, like one found by M. +Dufresnoy near Alencon. Isocardi excentrica, Voltz., identical with that from +the Jura. (“Description Phys. des Iles Can.” page 472.) +</p> + +<p> +Two of these shells, namely, the Gryphaea and Trigonia, appear to be identical +with species collected by Meyen and myself on the Peuquenes range; and in the +opinion of Von Buch and M. d’Orbigny, the two formations belong to the +same age. I must here add, that Professor E. Forbes, who has examined my +specimens from this place and from the Peuquenes range, has likewise a strong +impression that they indicate the Cretaceous period, and probably an early +epoch in it: so that all the palaeontologists who have seen these fossils +nearly coincide in opinion regarding their age. The limestone, however, with +these fossils here lies at the very base of the formation, just above the +porphyritic conglomerate, and certainly several thousand feet lower in the +series, than the equivalent, fossiliferous, black, shaly rocks high up on the +Peuquenes range. +</p> + +<p> +It is well worthy of remark that these shells, or at least those of which I saw +impressions in the limestone (bed No. 3), must have been covered up, on the +LEAST computation, by 4,000 feet of strata: now we know from Professor E. +Forbes’s researches, that the sea at greater depths than 600 feet becomes +exceedingly barren of organic beings,—a result quite in accordance with +what little I have seen of deep-sea soundings. Hence, after this limestone with +its shells was deposited, the bottom of the sea where the main line of the +Cordillera now stands, must have subsided some thousand feet to allow of the +deposition of the superincumbent submarine strata. Without supposing a movement +of this kind, it would, moreover, be impossible to understand the accumulation +of the several lower strata of COARSE, well-rounded conglomerates, which it is +scarcely possible to believe were spread out in profoundly deep water, and +which, especially those containing pebbles of quartz, could hardly have been +rounded in submarine craters and afterwards ejected from them, as I believe to +have been the case with much of the porphyritic conglomerate formation. I may +add that, in Professor Forbes’s opinion, the above-enumerated species of +mollusca probably did not live at a much greater depth than twenty fathoms, +that is only 120 feet. +</p> + +<p> +To return to our section down the valley; standing on the great N. by W. and S. +by E. uniclinal axis of the Puente del Inca, of which a section has just been +given, and looking north-east, greater tabular masses of gypseous formation +(KK) could be seen in the distance, very slightly inclined towards the east. +Lower down the valley, the mountains are almost exclusively composed of +porphyries, many of them of intrusive origin and non-stratified, others +stratified, but with the stratification seldom distinguishable except in the +upper parts. Disregarding local disturbances, the beds are either horizontal or +inclined at a small angle eastwards: hence, when standing on the plain of +Uspallata and looking to the west or backwards, the Cordillera appear composed +of huge, square, nearly horizontal, tabular masses: so wide a space, with such +lofty mountains so equably elevated, is rarely met with within the Cordillera. +In this line of section, the interval between the Puente del Inca and the +neighbourhood of the Cumbre, includes all the chief axes of dislocation. +</p> + +<p> +The altered clay-slate formation, already described, is seen in several parts +of the valley as far down as Las Vacas, underlying the porphyritic +conglomerate. At the Casa de Pujios [L], there is a hummock of (andesitic?) +granite; and the stratification of the surrounding mountains here changes from +W. by S. to S.W. Again, near the R. Vacas there is a larger formation of +(andesitic?) granite [M], which sends a meshwork of veins into the +superincumbent clay-slate, and which locally throws off the strata, on one side +to N.W. and on the other to S.E. but not at a high angle: at the junction, the +clay-slate is altered into fine-grained greenstone. This granitic axis is +intersected by a green dike, which I mention, because I do not remember having +elsewhere seen dikes in this lowest and latest intrusive rock. From the R. +Vacas to the plain of Uspallata, the valley runs N.E., so that I have had to +contract my section; it runs exclusively through porphyritic rocks. As far as +the Pass of Jaula, the claystone conglomerate formation, in most parts highly +porphyritic, and crossed by numerous dikes of greenstone porphyry, attains a +great thickness: there is also much intrusive porphyry. From the Jaula to the +plain, the stratification has been in most places obliterated, except near the +tops of some of the mountains; and the metamorphic action has been extremely +great. In this space, the number and bulk of the intrusive masses of +differently coloured porphyries, injected one into another and intersected by +dikes, is truly extraordinary. I saw one mountain of whitish porphyry, from +which two huge dikes, thinning out, branched DOWNWARDS into an adjoining +blackish porphyry. Another hill of white porphyry, which had burst through +dark- coloured strata, was itself injected by a purple, brecciated, and +recemented porphyry, both being crossed by a green dike, and both having been +upheaved and injected by a granitic dome. One brick-red porphyry, which above +the Jaula forms an isolated mass in the midst of the porphyritic conglomerate +formation, and lower down the valley a magnificent group of peaked mountains, +differs remarkably from all the other porphyries. It consists of a red +feldspathic base, including some rather large crystals of red feldspar, +numerous large angular grains of quartz, and little bits of a soft green +mineral answering in most of its characters to soapstone. The crystals of red +feldspar resemble in external appearance those of orthite, though, from being +partially decomposed, I was unable to measure them; and they certainly are +quite unlike the variety, so abundantly met with in almost all the other rocks +of this line of section, and which, wherever I tried it, cleaved like albite. +This brick-red porphyry appears to have burst through all the other porphyries, +and numerous red dikes traversing the neighbouring mountains have proceeded +from it: in some few places, however, it was intersected by white dikes. From +this posteriority of intrusive origin,—from the close general resemblance +between this red porphyry and the red granite of the Portillo line, the only +difference being that the feldspar here is less perfectly granular, and that +soapstone replaces the mica, which is there imperfect and passes into +chlorite,—and from the Portillo line a little southward of this point +appearing to blend (according to Dr. Gillies) into the western ranges,—I +am strongly urged to believe (as formerly remarked) that the grand +mountain-masses composed of this brick-red porphyry belong to the same axis of +injection with the granite of the Portillo line; if so, the injection of this +porphyry probably took place, as long subsequently to the several axes of +elevation in the gypseous formation near the Cumbre, as the injection of the +Portillo granite has been shown to have been subsequent to the elevation of the +gypseous strata composing the Peuquenes range; and this interval, we have seen, +must have been a very long one. +</p> + +<p> +The Plain of Uspallata has been briefly described in Chapter 3; it resembles +the basin-plains of Chile; it is ten or fifteen miles wide, and is said to +extend for 180 miles northward; its surface is nearly six thousand feet above +the sea; it is composed, to a thickness of some hundred feet of loosely +aggregated, stratified shingle, which is prolonged with a gently sloping +surface up the valleys in the mountains on both sides. One section in this +plain [Z] is interesting, from the unusual circumstance of alternating layers +of almost loose red and white sand with lines of pebbles (from the size of a +nut to that of an apple), and beds of gravel, being inclined at an angle of 45 +degrees, and in some spots even at a higher angle. (I find that Mr. Smith of +Jordan Hill has described (“Edinburgh New Philosophical Journal” +volume 25 page 392) beds of sand and gravel, near Edinburgh, tilted at an angle +of 60 degrees, and dislocated by miniature faults.) These beds are dislocated +by small faults: and are capped by a thick mass of horizontally stratified +gravel, evidently of subaqueous origin. Having been accustomed to observe the +irregularities of beds accumulated under currents, I feel sure that the +inclination here has not been thus produced. The pebbles consist chiefly of the +brick-red porphyry just described and of white granite, both probably derived +from the ranges to the west, and of altered clay-slate and of certain +porphyries, apparently belonging to the rocks of the Uspallata chain. This +plain corresponds geographically with the valley of Tenuyan between the +Portillo and Peuquenes ranges; but in that valley the shingle, which likewise +has been derived both from the eastern and western ranges, has been cemented +into a hard conglomerate, and has been throughout tilted at a considerable +inclination; the gravel there apparently attains a much greater thickness, and +is probably of higher antiquity. +</p> + +<h3>THE USPALLATA RANGE.</h3> + +<p> +The road by the Villa Vicencio Pass does not strike directly across the range, +but runs for some leagues northward along its western base: and I must briefly +describe the rocks here seen, before continuing with the coloured east and west +section. At the mouth of the valley of Canota, and at several points +northwards, there is an extensive formation of a glossy and harsh, and of a +feldspathic clay-slate, including strata of grauwacke, and having a tortuous, +nearly vertical cleavage, traversed by numerous metalliferous veins and others +of quartz. The clay-slate is in many parts capped by a thick mass of fragments +of the same rock, firmly recemented; and both together have been injected and +broken up by very numerous hillocks, ranging north and south, of lilac, white, +dark and salmon- coloured porphyries: one steep, now denuded, hillock of +porphyry had its face as distinctly impressed with the angles of a fragmentary +mass of the slate, with some of the points still remaining embedded, as +sealing-wax could be by a seal. At the mouth of this same valley of Canota, in +a fine escarpment having the strata dipping from 50 to 60 degrees to the N.E. +(Nearly opposite to this escarpment, there is another corresponding one, with +the strata dipping not to the exactly opposite point, or S.W., but to S.S.W.: +consequently the two escarpments trend towards each other, and some miles +southward they become actually united: this is a form of elevation which I have +not elsewhere seen.), the clay-slate formation is seen to be covered +by—(1st) a purple, claystone porphyry resting unconformably in some parts +on the solid slate, and in others on a thick fragmentary mass; (2nd), a +conformable stratum of compact blackish rock, having a spheroidal structure, +full of minute acicular crystals of glassy feldspar, with red spots of oxide of +iron; (3rd), a great stratum of purplish-red claystone porphyry, abounding with +crystals of opaque feldspar, and laminated with thin, parallel, often short, +layers, and likewise with great irregular patches of white, earthy, +semi-crystalline feldspar; this rock (which I noticed in other neighbouring +places) perfectly resembles a curious variety described at Port Desire, and +occasionally occurs in the great porphyritic conglomerate formation of Chile; +(4th), a thin stratum of greenish white, indurated tuff, fusible and containing +broken crystals and particles of porphyries; (5th), a grand mass, imperfectly +columnar and divided into three parallel and closely joined strata, of +cream-coloured claystone porphyry; (6th), a thick stratum of lilac-coloured +porphyry, which I could see was capped by another bed of the cream-coloured +variety; I was unable to examine the still higher parts of the escarpment. +These conformably stratified porphyries, though none are either vesicular are +amygdaloidal, have evidently flowed as submarine lavas: some of them are +separated from each other by seams of indurated tuff, which, however, are quite +insignificant in thickness compared with the porphyries. This whole pile +resembles, but not very closely, some of the less brecciated parts of the great +porphyritic conglomerate formation of Chile; but it does not probably belong to +the same age, as the porphyries here rest unconformably on the altered +feldspathic clay-slate, whereas the porphyritic conglomerate formation +alternates with and rests conformably on it. These porphyries, moreover, with +the exception of the one blackish stratum, and of the one indurated, white +tufaceous bed, differ from the beds composing the Uspallata range in the line +of the Villa Vicencio Pass. +</p> + +<p> +I will now give, first, a sketch of the structure of the range, as represented +in the section, and will then describe its composition and interesting history. +At its western foot, a hillock [N] is seen to rise out of the plain, with its +strata dipping at 70 degrees to the west, fronted by strata [O] inclined at 45 +degrees to the east, thus forming a little north and south anticlinal axis. +Some other little hillocks of similar composition, with their strata highly +inclined, range N.E. and S.W., obliquely to the main Uspallata line. The cause +of these dislocations, which, though on a small scale, have been violent and +complicated, is seen to lie in hummocks of lilac, purple and red porphyries, +which have been injected in a liquified state through and into the underlying +clay-slate formation. Several dykes were exposed here, but in no other part, +that I saw of this range. As the strata consist of black, white, greenish and +brown-coloured rocks, and as the intrusive porphyries are so brightly tinted, a +most extraordinary view was presented, like a coloured geological drawing. On +the gently inclined main western slope [PP], above the little anticlinal ridges +just mentioned, the strata dip at an average angle of 25 degrees to the west; +the inclination in some places being only 19 degrees, in some few others as +much as 45 degrees. The masses having these different inclinations, are +separated from each other by parallel vertical faults [as represented at Pa], +often giving rise to separate, parallel, uniclinal ridges. The summit of the +main range is broad and undulatory, with the stratification undulatory and +irregular: in a few places granitic and porphyritic masses [Q] protrude, which, +from the small effect they have locally produced in deranging the strata, +probably form the upper points of a regular, great underlying dome. These +denuded granitic points, I estimated at about nine thousand feet in height +above the sea. On the eastern slope, the strata in the upper part are regularly +inclined at about 25 degrees to the east, so that the summit of this chain, +neglecting small irregularities, forms a broad anticlinal axis. Lower down, +however, near Los Hornillos [R], there is a well-marked synclinal axis, beyond +which the strata are inclined at nearly the same angle, namely from 20 to 30 +degrees, inwards or westward. Owing to the amount of denudation which this +chain has suffered, the outline of the gently inclined eastern flank scarcely +offers the slightest indication of this synclinal axis. The stratified beds, +which we have hitherto followed across the range, a little further down are +seen to lie, I believe unconformably, on a broad mountainous band of clay-slate +and grauwacke. The strata and laminae of this latter formation, on the extreme +eastern flank, are generally nearly vertical; further inwards they become +inclined from 45 to 80 degrees to the west: near Villa Vicencio [S] there is +apparently an anticlinal axis, but the structure of this outer part of the +clay-slate formation is so obscure, that I have not marked the planes of +stratification in the section. On the margin of the Pampas, some low, much +dislocated spurs of this same formation, project in a north- easterly line, in +the same oblique manner as do the ridges on the western foot, and as is so +frequently the case with those at the base of the main Cordillera. +</p> + +<p> +I will now describe the nature of the beds, beginning at the base on the +eastern side. First, for the clay-slate formation: the slate is generally hard +and bluish, with the laminae coated by minute micaceous scales; it alternates +many times with a coarse-grained, greenish grauwacke, containing rounded +fragments of quartz and bits of slate in a slightly calcareous basis. The slate +in the upper part generally becomes purplish, and the cleavage so irregular +that the whole consists of mere splinters. Transverse veins of quartz are +numerous. At the Calera, some leagues distant, there is a dark crystalline +limestone, apparently included in this formation. With the exception of the +grauwacke being here more abundant, and the clay-slate less altered, this +formation closely resembles that unconformably underlying the porphyries at the +western foot of this same range; and likewise that alternating with the +porphyritic conglomerate in the main Cordillera. This formation is a +considerable one, and extends several leagues southward to near Mendoza: the +mountains composed of it rise to a height of about two thousand feet above the +edge of the Pampas, or about seven thousand feet above the sea. (I infer this +from the height of V. Vicencio, which was ascertained by Mr. Miers to be 5,328 +feet above the sea.) +</p> + +<p> +Secondly: the most usual bed on the clay-slate is a coarse, white, slightly +calcareous conglomerate, of no great thickness, including broken crystals of +feldspar, grains of quartz, and numerous pebbles of brecciated claystone +porphyry, but without any pebbles of the underlying clay-slate. I nowhere saw +the actual junction between this bed and the clay-slate, though I spent a whole +day in endeavouring to discover their relations. In some places I distinctly +saw the white conglomerate and overlying beds inclined at from 25 to 30 degrees +to the west, and at the bottom of the same mountain, the clay-slate and +grauwacke inclined to the same point, but at an angle from 70 to 80 degrees: in +one instance, the clay-slate dipped not only at a different angle, but to a +different point from the overlying formation. In these cases the two formations +certainly appeared quite unconformable: moreover, I found in the clay-slate one +great, vertical, dike-like fissure, filled up with an indurated whitish tuff, +quite similar to some of the upper beds presently to be described; and this +shows that the clay-slate must have been consolidated and dislocated before +their deposition. On the other hand, the stratification of the slate and +grauwacke, in some cases gradually and entirely disappeared in approaching the +overlying white conglomerate; in other cases the stratification of the two +formations became strictly conformable; and again in other cases, there was +some tolerably well characterised clay-slate lying above the conglomerate. (The +coarse, mechanical structure of many grauwackes has always appeared to me a +difficulty; for the texture of the associated clay-slate and the nature of the +embedded organic remains where present, indicate that the whole has been a +deep-water deposit. Whence have the sometimes included angular fragments of +clay-slate, and the rounded masses of quartz and other rocks, been derived? +Many deep-water limestones, it is well known, have been brecciated, and then +firmly recemented.) The most probable conclusion appears to be, that after the +clay-slate formation had been dislocated and tilted, but whilst under the sea, +a fresh and more recent deposition of clay-slate took place, on which the white +conglomerate was conformably deposited, with here and there a thin intercalated +bed of clay-slate. On this view the white conglomerates and the presently to be +described tuffs and lavas are really unconformable to the main part of the +clay-slate; and this, as we have seen, certainly is the case with the +clay-stone lavas in the valley of Canota, at the western and opposite base of +the range. +</p> + +<p> +Thirdly: on the white conglomerate, strata several hundred feet in thickness +are superimposed, varying much in nature in short distances: the commonest +variety is a white, much indurated tuff, sometimes slightly calcareous, with +ferruginous spots and water-lines, often passing into whitish or purplish +compact, fine-grained grit or sandstones; other varieties become +semi-porcellanic, and tinted faint green or blue; others pass into an indurated +shale: most of these varieties are easily fusible. +</p> + +<p> +Fourthly: a bed, about one hundred feet thick of a compact, partially columnar, +pale-grey, feldspathic lava, stained with iron, including very numerous +crystals of opaque feldspar, and with some crystallised and disseminated +calcareous matter. The tufaceous stratum on which this feldspathic lava rests +is much hardened, stained purple, and has a spherico-concretionary structure; +it here contains a good many pebbles of claystone porphyry. +</p> + +<p> +Fifthly: thin beds, 400 feet in thickness, varying much in nature, consisting +of white and ferruginous tuffs, in some parts having a concretionary structure, +in others containing rounded grains and a few pebbles of quartz; also passing +into hard gritstones and into greenish mudstones: there is, also, much of a +bluish-grey and green semi-porcellanic stone. +</p> + +<p> +Sixthly: a volcanic stratum, 250 feet in thickness, of so varying a nature that +I do not believe a score of specimens would show all the varieties; much is +highly amygdaloidal, much compact; there are greenish, blackish, purplish, and +grey varieties, rarely including crystals of green augite and minute acicular +ones of feldspar, but often crystals and amygdaloidal masses of white, red, and +black carbonate of lime. Some of the blackish varieties of this rock have a +conchoidal fracture and resemble basalt; others have an irregular fracture. +Some of the grey and purplish varieties are thickly speckled with green earth +and with white crystalline carbonate of lime; others are largely amygdaloidal +with green earth and calcareous spar. Again, other earthy varieties, of +greenish, purplish and grey tints, contain much iron, and are almost half +composed of amygdaloidal balls of dark brown bole, of a whitish indurated +feldspathic matter, of bright green earth, of agate, and of black and white +crystallised carbonate of lime. All these varieties are easily fusible. Viewed +from a distance, the line of junction with the underlying semi-porcellanic +strata was distinct; but when examined closely, it was impossible to point out +within a foot where the lava ended and where the sedimentary mass began: the +rock at the time of junction was in most places hard, of a bright green colour, +and abounded with irregular amygdaloidal masses of ferruginous and pure +calcareous spar, and of agate. +</p> + +<p> +Seventhly: strata, eighty feet in thickness, of various indurated tuffs, as +before; many of the varieties have a fine basis including rather coarse +extraneous particles; some of them are compact and semi-porcellanic, and +include vegetable impressions. +</p> + +<p> +Eighthly: a bed, about fifty feet thick, of greenish-grey, compact, feldspathic +lava, with numerous small crystals of opaque feldspar, black augite, and oxide +of iron. The junction with the bed on which it rested, was ill defined; balls +and masses of the feldspathic rock being enclosed in much altered tuff. +</p> + +<p> +Ninthly: indurated tuffs, as before. +</p> + +<p> +Tenthly: a conformable layer, less than two feet in thickness, of pitchstone, +generally brecciated, and traversed by veins of agate and of carbonate of lime: +parts are composed of apparently concretionary fragments of a more perfect +variety, arranged in horizontal lines in a less perfectly characterised +variety. I have much difficulty in believing that this thin layer of pitchstone +flowed as lava. +</p> + +<p> +Eleventhly: sedimentary and tufaceous beds as before, passing into sandstone, +including some conglomerate: the pebbles in the latter are of claystone +porphyry, well rounded, and some as large as cricket-balls. +</p> + +<p> +Twelfthly: a bed of compact, sonorous, feldspathic lava, like that of bed No. +8, divided by numerous joints into large angular blocks. +</p> + +<p> +Thirteenthly: sedimentary beds as before. +</p> + +<p> +Fourteenthly: a thick bed of greenish or greyish black, compact basalt (fusing +into a black enamel), with small crystals, occasionally distinguishable, of +feldspar and augite: the junction with the underlying sedimentary bed, +differently from that in most of the foregoing streams, here was quite +distinct:—the lava and tufaceous matter preserving their perfect +characters within two inches of each other. This rock closely resembles certain +parts of that varied and singular lava-stream No. 6; it likewise resembles, as +we shall immediately see, many of the great upper beds on the western flank and +on the summit of this range. +</p> + +<p> +The pile of strata here described attains a great thickness; and above the +last-mentioned volcanic stratum, there were several other great tufaceous beds +alternating with submarine lavas, which I had not time to examine; but a +corresponding series, several thousand feet in thickness, is well exhibited on +the crest and western flank of the range. Most of the lava- streams on the +western side are of a jet-black colour and basaltic nature; they are either +compact and fine-grained, including minute crystals of augite and feldspar, or +they are coarse-grained and abound with rather large coppery-brown crystals of +an augitic mineral. (Very easily fusible into a jet-black bead, attracted by +the magnet: the crystals are too much tarnished to be measured by the +goniometer.) Another variety was of a dull- red colour, having a claystone +brecciated basis, including specks of oxide of iron and of calcareous spar, and +amygdaloidal with green earth: there were apparently several other varieties. +These submarine lavas often exhibit a spheroidal, and sometimes an imperfect +columnar structure: their upper junctions are much more clearly defined than +their lower junctions; but the latter are not so much blended into the +underlying sedimentary beds as is the case in the eastern flank. On the crest +and western flank of the range, the streams, viewed as a whole, are mostly +basaltic; whilst those on the eastern side, which stand lower in the series, +are, as we have seen, mostly feldspathic. +</p> + +<p> +The sedimentary strata alternating with the lavas on the crest and western +side, are of an almost infinitely varying nature; but a large proportion of +them closely resemble those already described on the eastern flank: there are +white and brown, indurated, easily fusible tuffs,—some passing into pale +blue and green semi-porcellanic rocks,—others into brownish and purplish +sandstones and gritstones, often including grains of quartz,— others into +mudstone containing broken crystals and particles of rock, and occasionally +single large pebbles. There was one stratum of a bright red, coarse, volcanic +gritstone; another of conglomerate; another of a black, indurated, carbonaceous +shale marked with imperfect vegetable impressions; this latter bed, which was +thin, rested on a submarine lava, and followed all the considerable +inequalities of its upper surface. Mr. Miers states that coal has been found in +this range. Lastly, there was a bed (like No. 10 on the eastern flank) +evidently of sedimentary origin, and remarkable from closely approaching in +character to an imperfect pitchstone, and from including extremely thin layers +of perfect pitchstone, as well as nodules and irregular fragments (but not +resembling extraneous fragments) of this same rock arranged in horizontal +lines: I conceive that this bed, which is only a few feet in thickness, must +have assumed its present state through metamorphic and concretionary action. +Most of these sedimentary strata are much indurated, and no doubt have been +partially metamorphosed: many of them are extraordinarily heavy and compact; +others have agate and crystalline carbonate of lime disseminated throughout +them. Some of the beds exhibit a singular concretionary arrangement, with the +curves determined by the lines of fissure. There are many veins of agate and +calcareous spar, and innumerable ones of iron and other metals, which have +blackened and curiously affected the strata to considerable distances on both +sides. +</p> + +<p> +Many of these tufaceous beds resemble, with the exception of being more +indurated, the upper beds of the Great Patagonian tertiary formation, +especially those variously coloured layers high up the River Santa Cruz, and in +a remarkable degree the tufaceous formation at the northern end of Chiloe. I +was so much struck with this resemblance, that I particularly looked out for +silicified wood, and found it under the following extraordinary circumstances. +High up on this western flank, at a height estimated at 7,000 feet above the +sea, in a broken escarpment of thin strata, composed of compact green gritstone +passing into a fine mudstone, and alternating with layers of coarser, brownish, +very heavy mudstone, including broken crystals and particles of rock almost +blended together, I counted the stumps of fifty-two trees. (For the information +of any future traveller, I will describe the spot in detail. Proceeding +eastward from the Agua del Zorro, and afterwards leaving on the north side of +the road a rancho attached to some old goldmines, you pass through a gully with +low but steep rocks on each hand: the road then bends, and the ascent becomes +steeper. A few hundred yards farther on, a stone’s throw on the south +side of the road, the white calcareous stumps may be seen. The spot is about +half a mile east of the Agua del Zorro.) They projected between two and five +feet above the ground, and stood at exactly right angles to the strata, which +were here inclined at an angle of about 25 degrees to the west. Eleven of these +trees were silicified and well preserved; Mr. R. Brown has been so kind as to +examine the wood when sliced and polished; he says it is coniferous, partaking +of the characters of the Araucarian tribe, with some curious points of affinity +with the Yew. The bark round the trunks must have been circularly furrowed with +irregular lines, for the mudstone round them is thus plainly marked. One cast +consisted of dark argillaceous limestone; and forty of them of coarsely +crystallised carbonate of lime, with cavities lined by quartz crystals: these +latter white calcareous columns do not retain any internal structure, but their +external form plainly shows their origin. All the stumps have nearly the same +diameter, varying from one foot to eighteen inches; some of them stand within a +yard of each other; they are grouped in a clump within a space of about sixty +yards across, with a few scattered round at the distance of 150 yards. They all +stand at about the same level. The longest stump stood seven feet out of the +ground: the roots, if they are still preserved, are buried and concealed. No +one layer of the mudstone appeared much darker than the others, as if it had +formerly existed as soil, nor could this be expected, for the same agents which +replaced with silex and lime the wood of the trees, would naturally have +removed all vegetable matter from the soil. Besides the fifty-two upright +trees, there were a few fragments, like broken branches, horizontally embedded. +The surrounding strata are crossed by veins of carbonate of lime, agate, and +oxide of iron; and a poor gold vein has been worked not far from the trees. +</p> + +<p> +The green and brown mudstone beds including the trees, are conformably covered +by much indurated, compact, white or ferruginous tuffs, which pass upwards into +a fine-grained, purplish sedimentary rock: these strata, which, together, are +from four to five hundred feet in thickness, rest on a thick bed of submarine +lava, and are conformably covered by another great mass of fine-grained basalt, +which I estimated at 1,000 feet in thickness, and which probably has been +formed by more than one stream. (This rock is quite black, and fuses into a +black bead, attracted strongly by the magnet; it breaks with a conchoidal +fracture; the included crystals of augite are distinguishable by the naked eye, +but are not perfect enough to be measured: there are many minute acicular +crystals of glassy feldspar.) Above this mass I could clearly distinguish five +conformable alternations, each several hundred feet in thickness, of stratified +sedimentary rocks and lavas, such as have been previously described. Certainly +the upright trees have been buried under several thousand feet in thickness of +matter, accumulated under the sea. As the trees obviously must once have grown +on dry land, what an enormous amount of subsidence is thus indicated! +Nevertheless, had it not been for the trees there was no appearance which would +have led any one even to have conjectured that these strata had subsided. As +the land, moreover, on which the trees grew, is formed of subaqueous deposits, +of nearly if not quite equal thickness with the superincumbent strata, and as +these deposits are regularly stratified and fine-grained, not like the matter +thrown up on a sea-beach, a previous upward movement, aided no doubt by the +great accumulation of lavas and sediment, is also indicated. (At first I +imagined, that the strata with the trees might have been accumulated in a lake: +but this seems highly improbable; for, first, a very deep lake was necessary to +receive the matter below the trees, then it must have been drained for their +growth, and afterwards re-formed and made profoundly deep, so as to receive a +subsequent accumulation of matter SEVERAL THOUSAND feet in thickness. And all +this must have taken place necessarily before the formation of the Uspallata +range, and therefore on the margin of the wide level expanse of the Pampas! +Hence I conclude, that it is infinitely more probable that the strata were +accumulated under the sea: the vast amount of denudation, moreover, which this +range has suffered, as shown by the wide valleys, by the exposure of the very +trees and by other appearances, could have been effected, I conceive, only by +the long-continued action of the sea; and this shows that the range was either +upheaved from under the sea, or subsequently let down into it. From the natural +manner in which the stumps (fifty-two in number) are GROUPED IN A CLUMP, and +from their all standing vertically to the strata, it is superfluous to +speculate on the chance of the trees having been drifted from adjoining land, +and deposited upright: I may, however, mention that the late Dr. Malcolmson +assured me, that he once met in the Indian Ocean, fifty miles from land, +several cocoa-nut trees floating upright, owing to their roots being loaded +with earth.) +</p> + +<p> +In nearly the middle of the range, there are some hills [Q], before alluded to, +formed of a kind of granite externally resembling andesite, and consisting of a +white, imperfectly granular, feldspathic basis, including some perfect crystals +apparently of albite (but I was unable to measure them), much black mica, +epidote in veins, and very little or no quartz. Numerous small veins branch +from this rock into the surrounding strata; and it is a singular fact that +these veins, though composed of the same kind of feldspar and small scales of +mica as in the solid rock, abound with innumerable minute ROUNDED grains of +quartz: in the veins or dikes also, branching from the great granitic axis in +the peninsula of Tres Montes, I observed that quartz was more abundant in them +than in the main rock: I have heard of other analogous cases: can we account +for this fact, by the long-continued vicinity of quartz when cooling, and by +its having been thus more easily sucked into fissures than the other +constituent minerals of granite? (See a paper by M. Elie de Beaumont, +“Soc. Philomath.” May 1839 “L’Institut.” 1839 +page 161.) The strata encasing the flanks of these granitic or andesite masses, +and forming a thick cap on one of their summits, appear originally to have been +of the same tufaceous nature with the beds already described, but they are now +changed into porcellanic, jaspery, and crystalline rocks, and into others of a +white colour with a harsh texture, and having a siliceous aspect, though really +of a feldspathic nature and fusible. Both the granitic intrusive masses and the +encasing strata are penetrated by innumerable metallic veins, mostly +ferruginous and auriferous, but some containing copper-pyrites and a few +silver: near the veins, the rocks are blackened as if blasted by gunpowder. The +strata are only slightly dislocated close round these hills, and hence, +perhaps, it may be inferred that the granitic masses form only the projecting +points of a broad continuous axis-dome, which has given to the upper parts of +this range its anticlinal structure. +</p> + +<h3>CONCLUDING REMARKS ON THE USPALLATA RANGE.</h3> + +<p> +I will not attempt to estimate the total thickness of the pile of strata +forming this range, but it must amount to many thousand feet. The sedimentary +and tufaceous beds have throughout a general similarity, though with infinite +variations. The submarine lavas in the lower part of the series are mostly +feldspathic, whilst in the upper part, on the summit and western flank, they +are mostly basaltic. We are thus reminded of the relative position in most +recent volcanic districts of the trachytic and basaltic lavas,—the latter +from their greater weight having sunk to a lower level in the earth’s +crust, and having consequently been erupted at a later period over the lighter +and upper lavas of the trachytic series. (See on this subject, “Volcanic +Islands” etc. by the Author.) Both the basaltic and feldspathic submarine +streams are very compact; none being vesicular, and only a few amygdaloidal: +the effects which some of them, especially those low in the series, have +produced on the tufaceous beds over which they have flowed is highly curious. +Independently of this local metamorphic action, all the strata undoubtedly +display an indurated and altered character; and all the rocks of this +range—the lavas, the alternating sediments, the intrusive granite and +porphyries, and the underlying clay- slate—are intersected by +metalliferous veins. The lava-strata can often be seen extending for great +distances, conformably with the under and overlying beds; and it was obvious +that they thickened towards the west. Hence the points of eruption must have +been situated westward of the present range, in the direction of the main +Cordillera: as, however, the flanks of the Cordillera are entirely composed of +various porphyries, chiefly claystone and greenstone, some intrusive, and +others belonging to the porphyritic conglomerate formation, but all quite +unlike these submarine lava-streams, we must in all probability look to the +plain of Uspallata for the now deeply buried points of eruption. +</p> + +<p> +Comparing our section of the Uspallata range with that of the Cumbre, we see, +with the exception of the underlying clay-slate, and perhaps of the intrusive +rocks of the axes, a striking dissimilarity in the strata composing them. The +great porphyritic conglomerate formation has not extended as far as this range; +nor have we here any of the gypseous strata, the magnesian and other +limestones, the red sandstones, the siliceous beds with pebbles of quartz, and +comparatively little of the conglomerates, all of which form such vast masses +over the basal series in the main Cordillera. On the other hand, in the +Cordillera, we do not find those endless varieties of indurated tuffs, with +their numerous veins and concretionary arrangement, and those grit and mud +stones, and singular semi-porcellanic rocks, so abundant in the Uspallata +range. The submarine lavas, also, differ considerably; the feldspathic streams +of the Cordillera contain much mica, which is absent in those of the Uspallata +range: in this latter range we have seen on how grand a scale, basaltic lava +has been poured forth, of which there is not a trace in the Cordillera. This +dissimilarity is the more striking, considering that these two parallel chains +are separated by a plain only between ten and fifteen miles in width; and that +the Uspallata lavas, as well as no doubt the alternating tufaceous beds, have +proceeded from the west, from points apparently between the two ranges. To +imagine that these two piles of strata were contemporaneously deposited in two +closely adjoining, very deep, submarine areas, separated from each other by a +lofty ridge, where a plain now extends, would be a gratuitous hypothesis. And +had they been contemporaneously deposited, without any such dividing ridge, +surely some of the gypseous and other sedimentary matter forming such immensely +thick masses in the Cordillera, would have extended this short distance +eastwards; and surely some of the Uspallata tuffs and basalts also accumulated +to so great a thickness, would have extended a little westward. Hence I +conclude, that it is far from probable that these two series are not +contemporaneous; but that the strata of one of the chains were deposited, and +even the chain itself uplifted, before the formation of the other:—which +chain, then, is the oldest? Considering that in the Uspallata range the lowest +strata on the western flank lie unconformably on the clay- slate, as probably +is the case with those on the eastern flank, whereas in the Cordillera all the +overlying strata lie conformably on this formation:—considering that in the +Uspallata range some of the beds, both low down and high up in the series, are +marked with vegetable impressions, showing the continued existence of +neighbouring land;—considering the close general resemblance between the +deposits of this range and those of tertiary origin in several parts of the +continent;—and lastly, even considering the lesser height and outlying +position of the Uspallata range,—I conclude that the strata composing it +are in all probability of subsequent origin, and that they were accumulated at +a period when a deep sea studded with submarine volcanoes washed the eastern +base of the already partially elevated Cordillera. +</p> + +<p> +This conclusion is of much importance, for we have seen that in the Cordillera, +during the deposition of the Neocomian strata, the bed of the sea must have +subsided many thousand feet: we now learn that at a later period an adjoining +area first received a great accumulation of strata, and was upheaved into land +on which coniferous trees grew, and that this area then subsided several +thousand feet to receive the superincumbent submarine strata, afterwards being +broken up, denuded, and elevated in mass to its present height. I am +strengthened in this conclusion of there having been two distinct, great +periods of subsidence, by reflecting on the thick mass of coarse stratified +conglomerate in the valley of Tenuyan, between the Peuquenes and Portillo +lines; for the accumulation of this mass seems to me, as previously remarked, +almost necessarily to have required a prolonged subsidence; and this +subsidence, from the pebbles in the conglomerate having been to a great extent +derived from the gypseous or Neocomian strata of the Peuquenes line, we know +must have been quite distinct from, and subsequent to, that sinking movement +which probably accompanied the deposition of the Peuquenes strata, and which +certainly accompanied the deposition of the equivalent beds near the Puente del +Inca, in this line of section. +</p> + +<p> +The Uspallata chain corresponds in geographical position, though on a small +scale, with the Portillo line; and its clay-slate formation is probably the +equivalent of the mica-schist of the Portillo, there metamorphosed by the old +white granites and syenites. The coloured beds under the conglomerate in the +valley of Tenuyan, of which traces are seen on the crest of the Portillo, and +even the conglomerate itself, may perhaps be synchronous with the tufaceous +beds and submarine lavas of the Uspallata range; an open sea and volcanic +action in the latter case, and a confined channel between two bordering chains +of islets in the former case, having been sufficient to account for the +mineralogical dissimilarity of the two series. From this correspondence between +the Uspallata and Portillo ranges, perhaps in age and certainly in geographical +position, one is tempted to consider the one range as the prolongation of the +other; but their axes are formed of totally different intrusive rocks; and we +have traced the apparent continuation of the red granite of the Portillo in the +red porphyries diverging into the main Cordillera. Whether the axis of the +Uspallata range was injected before, or as perhaps is more probable, after that +of the Portillo line, I will not pretend to decide; but it is well to remember +that the highly inclined lava-streams on the eastern flank of the Portillo +line, prove that its angular upheavement was not a single and sudden event; and +therefore that the anticlinal elevation of the Uspallata range may have been +contemporaneous with some of the later angular movements by which the gigantic +Portillo range gained its present height above the adjoining plain. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0008"></a> +CHAPTER VIII.<br /> +NORTHERN CHILE. CONCLUSION.</h2> + +<p class="letter"> +Section from Illapel to Combarbala; gypseous formation with silicified +wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley; +fossils.—Guasco, fossils of.—Copiapo, section up valley; Las +Amolanas, silicified wood.—Conglomerates, nature of former land, fossils, +thickness of strata, great subsidence.—Valley of Despoblado, fossils, +tufaceous deposit, complicated dislocations of.—Relations between ancient +orifices of eruption and subsequent axes of injection.—Iquique, Peru, +fossils of, salt-deposits.—Metalliferous veins.—Summary on the +porphyritic conglomerate and gypseous formations.—Great subsidence with +partial elevations during the cretaceo-oolitic period.—On the elevation +and structure of the Cordillera.—Recapitulation on the tertiary +series.—Relation between movements of subsidence and volcanic +action.—Pampean formation.—Recent elevatory +movements.—Long-continued volcanic action in the +Cordillera.—Conclusion. +</p> + +<h3>VALPARAISO TO COQUIMBO.</h3> + +<p> +I have already described the general nature of the rocks in the low country +north of Valparaiso, consisting of granites, syenites, greenstones, and altered +feldspathic clay-slate. Near Coquimbo there is much hornblendic rock and +various dusky-coloured porphyries. I will describe only one section in this +district, namely, from near Illapel in a N.E. line to the mines of Los Hornos, +and thence in a north by east direction to Combarbala, at the foot of the main +Cordillera. +</p> + +<p> +Near Illapel, after passing for some distance over granite, andesite, and +andesitic porphyry, we come to a greenish stratified feldspathic rock, which I +believe is altered clay-slate, conformably capped by porphyries and porphyritic +conglomerate of great thickness, dipping at an average angle of 20 degrees to +N.E. by N. The uppermost beds consist of conglomerates and sandstone only a +little metamorphosed, and conformably covered by a gypseous formation of very +great thickness, but much denuded. This gypseous formation, where first met +with, lies in a broad valley or basin, a little southward of the mines of Los +Hornos: the lower half alone contains gypsum, not in great masses as in the +Cordillera, but in innumerable thin layers, seldom more than an inch or two in +thickness. The gypsum is either opaque or transparent, and is associated with +carbonate of lime. The layers alternate with numerous varying ones of a +calcareous clay-shale (with strong aluminous odour, adhering to the tongue, +easily fusible into a pale green glass), more or less indurated, either earthy +and cream-coloured, or greenish and hard. The more indurated varieties have a +compact, homogeneous, almost crystalline fracture, and contain granules of +crystallised oxide of iron. Some of the varieties almost resemble honestones. +There is also a little black, hardly fusible, siliceo- calcareous clay-slate, +like some of the varieties alternating with gypsum on the Peuquenes range. +</p> + +<p> +The upper half of this gypseous formation is mainly formed of the same +calcareous clay-shale rock, but without any gypsum, and varying extremely in +nature: it passes from a soft, coarse, earthy, ferruginous state, including +particles of quartz, into compact claystones with crystallised oxide of +iron,—into porcellanic layers, alternating with seams of calcareous +matter,—and into green porcelain-jasper, excessively hard, but easily +fusible. Strata of this nature alternate with much black and brown +siliceo-calcareous slate, remarkable from the wonderful number of huge embedded +logs of silicified wood. This wood, according to Mr. R. Brown, is (judging from +several specimens) all coniferous. Some of the layers of the black siliceous +slate contained irregular angular fragments of imperfect pitchstone, which I +believe, as in the Uspallata range, has originated in a metamorphic process. +There was one bed of a marly tufaceous nature, and of little specific gravity. +Veins of agate and calcareous spar are numerous. The whole of this gypseous +formation, especially the upper half, has been injected, metamorphosed, and +locally contorted by numerous hillocks of intrusive porphyries crowded together +in an extraordinary manner. These hillocks consist of purple claystone and of +various other porphyries, and of much white feldspathic greenstone passing into +andesite; this latter variety included in one case crystals of orthitic and +albitic feldspar touching each other, and others of hornblende, chlorite, and +epidote. The strata surrounding these intrusive hillocks at the mines of Los +Hornos, are intersected by many veins of copper-pyrites, associated with much +micaceous iron-ore, and by some of gold: in the neighbourhood of these veins +the rocks are blackened and much altered. The gypsum near the intrusive masses +is always opaque. One of these hillocks of porphyry was capped by some +stratified porphyritic conglomerate, which must have been brought up from +below, through the whole immense thickness of the overlying gypseous formation. +The lower beds of the gypseous formation resemble the corresponding and +probably contemporaneous strata of the main Cordillera; whilst the upper beds +in several respects resemble those of the Uspallata chain, and possibly may be +contemporaneous with them; for I have endeavoured to show that the Uspallata +beds were accumulated subsequently to the gypseous or Neocomian formations of +the Cordillera. +</p> + +<p> +This pile of strata dips at an angle of about 20 degrees to N.E. by N., close +up to the foot of the Cuesta de Los Hornos, a crooked range of mountains formed +of intrusive rocks of the same nature with the above described hillocks. Only +in one or two places, on this south-eastern side of the range, I noticed a +narrow fringe of the upper gypseous strata brushed up and inclined +south-eastward from it. On its north-eastern flank, and likewise on a few of +the summits, the stratified porphyritic conglomerate is inclined N.E.: so that, +if we disregard the very narrow anticlinal fringe of gypseous strata at its +S.E. foot, this range forms a second uniclinal axis of elevation. Proceeding in +a north-by-east direction to the village of Combarbala, we come to a third +escarpment of the porphyritic conglomerate, dipping eastwards, and forming the +outer range of the main Cordillera. The lower beds were here more jaspery than +usual, and they included some white cherty strata and red sandstones, +alternating with purple claystone porphyry. Higher up in the Cordillera there +appeared to be a line of andesitic rocks; and beyond them, a fourth escarpment +of the porphyritic conglomerate, again dipping eastwards or inwards. The +overlying gypseous strata, if they ever existed here, have been entirely +removed. +</p> + +<h3>COPPER MINES OF PANUNCILLO.</h3> + +<p> +From Combarbala to Coquimbo, I traversed the country in a zigzag direction, +crossing and recrossing the porphyritic conglomerate and finding in the +granitic districts an unusual number of mountain-masses composed of various +intrusive, porphyritic rocks, many of them andesitic. One common variety was +greenish-black, with large crystals of blackish albite. At Panuncillo a short +N.N.W. and S.S.E. ridge, with a nucleus formed of greenstone and of a +slate-coloured porphyry including crystals of glassy feldspar, deserves notice, +from the very singular nature of the almost vertical strata composing it. These +consist chiefly of a finer and coarser granular mixture, not very compact, of +white carbonate of lime, of protoxide of iron and of yellowish garnets +(ascertained by Professor Miller), each grain being an almost perfect crystal. +Some of the varieties consist exclusively of granules of the calcareous spar; +and some contain grains of copper ore, and, I believe, of quartz. These strata +alternate with a bluish, compact, fusible, feldspathic rock. Much of the above +granular mixture has, also, a pseudo-brecciated structure, in which fragments +are obscurely arranged in planes parallel to those of the stratification, and +are conspicuous on the weathered surfaces. The fragments are angular or +rounded, small or large, and consist of bluish or reddish compact feldspathic +matter, in which a few acicular crystals of feldspar can sometimes be seen. The +fragments often blend at their edges into the surrounding granular mass, and +seem due to a kind of concretionary action. +</p> + +<p> +These singular rocks are traversed by many copper veins, and appear to rest +conformably on the granular mixture (in parts as fine-grained as a sandstone) +of quartz, mica, hornblende, and feldspar; and this on fine- grained, common +gneiss; and this on a laminated mass, composed of pinkish ORTHITIC feldspar, +including a few specks of hornblende; and lastly, this on granite, which +together with andesitic rocks, form the surrounding district. +</p> + +<h3>COQUIMBO: MINING DISTRICT OF ARQUEROS.</h3> + +<p> +At Coquimbo the porphyritic conglomerate formation approaches nearer to the +Pacific than in any other part of Chile visited by me, being separated from the +coast by a tract only a few miles broad of the usual plutonic rocks, with the +addition of a porphyry having a red euritic base. In proceeding to the mines of +Arqueros, the strata of porphyritic conglomerate are at first nearly +horizontal, an unusual circumstance, and afterwards they dip gently to S.S.E. +After having ascended to a considerable height, we come to an undulatory +district in which the famous silver mines are situated; my examination was +chiefly confined to those of S. Rosa. Most of the rocks in this district are +stratified, dipping in various directions, and many of them are of so singular +a nature, that at the risk of being tedious I must briefly describe them. The +commonest variety is a dull-red, compact, finely brecciated stone, containing +much iron and innumerable white crystallised particles of carbonate of lime, +and minute extraneous fragments. Another variety is almost equally common near +S. Rosa; it has a bright green, scanty basis, including distinct crystals and +patches of white carbonate of lime, and grains of red, semi-micaceous oxide of +iron; in parts the basis becomes dark green, and assumes an obscure crystalline +arrangement, and occasionally in parts it becomes soft and slightly translucent +like soapstone. These red and green rocks are often quite distinct, and often +pass into each other; the passage being sometimes affected by a fine brecciated +structure, particles of the red and green matter being mingled together. Some +of the varieties appear gradually to become porphyritic with feldspar; and all +of them are easily fusible into pale or dark-coloured beads, strongly attracted +by the magnet. I should perhaps have mistaken several of these stratified rocks +for submarine lavas, like some of those described at the Puente del Inca, had I +not examined, a few leagues eastward of this point, a fine series of analogous +but less metamorphosed, sedimentary beds belonging to the gypseous formation, +and probably derived from a volcanic source. +</p> + +<p> +This formation is intersected by numerous metalliferous veins, running, though +irregularly, N.W. and S.E., and generally at right angles to the many dikes. +The veins consist of native silver, of muriate of silver, an amalgam of silver, +cobalt, antimony, and arsenic, generally embedded in sulphate of barytes. (See +the Report on M. Domeyko’s account of those mines, in the “Comptes +Rendus” tome 14 page 560.) I was assured by Mr. Lambert, that native +copper without a trace of silver has been found in the same vein with native +silver without a trace of copper. At the mines of Aristeas, the silver veins +are said to be unproductive as soon as they pass into the green strata, whereas +at S. Rosa, only two or three miles distant, the reverse happens; and at the +time of my visit, the miners were working through a red stratum, in the hope of +the vein becoming productive in the underlying green sedimentary mass. I have a +specimen of one of these green rocks, with the usual granules of white +calcareous spar and red oxide of iron, abounding with disseminated particles of +glittering native and muriate of silver, yet taken at the distance of one yard +from any vein,—a circumstance, as I was assured, of very rare occurrence. +</p> + +<h3>A SECTION EASTWARD, UP THE VALLEY OF COQUIMBO.</h3> + +<p> +After passing for a few miles over the coast granitic series, we come to the +porphyritic conglomerate, with its usual characters, and with some of the beds +distinctly displaying their mechanical origin. The strata, where first met +with, are, as before stated, only slightly inclined; but near the Hacienda of +Pluclaro, we come to an anticlinal axis, with the beds much dislocated and +shifted by a great fault, of which not a trace is externally seen in the +outline of the hill. I believe that this anticlinal axis can be traced +northwards, into the district of Arqueros, where a conspicuous hill called +Cerro Blanco, formed of a harsh, cream-coloured euritic rock, including a few +crystals of reddish feldspar, and associated with some purplish claystone +porphyry, seems to fall on a line of elevation. In descending from the Arqueros +district, I crossed on the northern border of the valley, strata inclined +eastward from the Pluclaro axis: on the porphyritic conglomerate there rested a +mass, some hundred feet thick, of brown argillaceous limestone, in parts +crystalline, and in parts almost composed of Hippurites Chilensis, +d’Orbigny; above this came a black calcareous shale, and on it a red +conglomerate. In the brown limestone, with the Hippurites, there was an +impression of a Pecten and a coral, and great numbers of a large Gryphaea, very +like, and, according to Professor E. Forbes, probably identical with G. +Orientalis, Forbes MS.,—a cretaceous species (probably upper greensand) +from Verdachellum, in Southern India. These fossils seem to occupy nearly the +same position with those at the Puente del Inca,—namely, at the top of +the porphyritic conglomerate, and at the base of the gypseous formation. +</p> + +<p> +A little above the Hacienda of Pluclaro, I made a detour on the northern side +of the valley, to examine the superincumbent gypseous strata, which I estimated +at 6,000 feet in thickness. The uppermost beds of the porphyritic conglomerate, +on which the gypseous strata conformably rest, are variously coloured, with one +very singular and beautiful stratum composed of purple pebbles of various kinds +of porphyry, embedded in white calcareous spar, including cavities lined with +bright-green crystallised epidote. The whole pile of strata belonging to both +formations is inclined, apparently from the above-mentioned axis of Pluclaro, +at an angle of between 20 and 30 degrees to the east. I will here give a +section of the principal beds met with in crossing the entire thickness of the +gypseous strata. +</p> + +<p> +Firstly: above the porphyritic conglomerate formation, there is a fine- +grained, red, crystalline sandstone. +</p> + +<p> +Secondly: a thick mass of smooth-grained, calcareo-aluminous, shaly rock, often +marked with dendritic manganese, and having, where most compact, the external +appearance of honestone. It is easily fusible. I shall for the future, for +convenience’ sake, call this variety pseudo-honestone. Some of the +varieties are quite black when freshly broken, but all weather into a +yellowish-ash coloured, soft, earthy substance, precisely as is the case with +the compact shaly rocks of the Peuquenes range. This stratum is of the same +general nature with many of the beds near Los Hornos in the Illapel section. In +this second bed, or in the underlying red sandstone (for the surface was +partially concealed by detritus), there was a thick mass of gypsum, having the +same mineralogical characters with the great beds described in our sections +across the Cordillera. +</p> + +<p> +Thirdly: a thick stratum of fine-grained, red, sedimentary matter, easily +fusible into a white glass, like the basis of claystone porphyry; but in parts +jaspery, in parts brecciated, and including crystalline specks of carbonate of +lime. In some of the jaspery layers, and in some of the black siliceous slaty +bands, there were irregular seams of imperfect pitchstone, undoubtedly of +metamorphic origin, and other seams of brown, crystalline limestone. Here, +also, were masses, externally resembling ill-preserved silicified wood. +</p> + +<p> +Fourthly and fifthly: calcareous pseudo-honestone; and a thick stratum +concealed by detritus. +</p> + +<p> +Sixthly: a thinly stratified mass of bright green, compact, smooth-grained, +calcareo-argillaceous stone, easily fusible, and emitting a strong aluminous +odour: the whole has a highly angulo-concretionary structure; and it resembles, +to a certain extent, some of the upper tufaceo-infusorial deposits of the +Patagonian tertiary formation. It is in its nature allied to our +pseudo-honestone, and it includes well characterised layers of that variety; +and other layers of a pale green, harder, and brecciated variety; and others of +red sedimentary matter, like that of bed Three. Some pebbles of porphyries are +embedded in the upper part. +</p> + +<p> +Seventhly: red sedimentary matter or sandstone like that of bed One, several +hundred feet in thickness, and including jaspery layers, often having a finely +brecciated structure. +</p> + +<p> +Eighthly: white, much indurated, almost crystalline tuff, several hundred feet +in thickness, including rounded grains of quartz and particles of green matter +like that of bed Six. Parts pass into a very pale green, semi- porcellanic +stone. +</p> + +<p> +Ninthly: red or brown coarse conglomerate, three or four hundred feet thick, +formed chiefly of pebbles of porphyries, with volcanic particles, in an +arenaceous, non-calcareous, fusible basis: the upper two feet are arenaceous +without any pebbles. +</p> + +<p> +Tenthly: the last and uppermost stratum here exhibited, is a compact, +slate-coloured porphyry, with numerous elongated crystals of glassy feldspar, +from one hundred and fifty to two hundred feet in thickness; it lies strictly +conformably on the underlying conglomerate, and is undoubtedly a submarine +lava. +</p> + +<p> +This great pile of strata has been broken up in several places by intrusive +hillocks of purple claystone porphyry, and by dikes of porphyritic greenstone: +it is said that a few poor metalliferous veins have been discovered here. From +the fusible nature and general appearance of the finer-grained strata, they +probably owe their origin (like the allied beds of the Uspallata range, and of +the Upper Patagonian tertiary formations), to gentle volcanic eruptions, and to +the abrasion of volcanic rocks. Comparing these beds with those in the mining +district of Arqueros, we see at both places rocks easily fusible, of the same +peculiar bright green and red colours, containing calcareous matter, often +having a finely brecciated structure, often passing into each other, and often +alternating together: hence I cannot doubt that the only difference between +them, lies in the Arqueros beds having been more metamorphosed (in conformity +with their more dislocated and injected condition), and consequently in the +calcareous matter, oxide of iron and green colouring matter, having been +segregated under a more crystalline form. +</p> + +<p> +The strata are inclined, as before stated, from 20 to 30 degrees eastward, +towards an irregular north and south chain of andesitic porphyry and of +porphyritic greenstone, where they are abruptly cut off. In the valley of +Coquimbo, near to the H. of Gualliguaca, similar plutonic rocks are met with, +apparently a southern prolongation of the above chain; and eastward of it we +have an escarpment of the porphyritic conglomerate, with the strata inclined at +a small angle eastward, which makes the third escarpment, including that +nearest the coast. Proceeding up the valley we come to another north and south +line of granite, andesite, and blackish porphyry, which seem to lie in an +irregular trough of the porphyritic conglomerate. Again, on the south side of +the R. Claro, there are some irregular granitic hills, which have thrown off +the strata of porphyritic conglomerate to the N.W. by W.; but the +stratification here has been much disturbed. I did not proceed any farther up +the valley, and this point is about two-thirds of the distance between the +Pacific and the main Cordillera. +</p> + +<p> +I will describe only one other section, namely, on the north side of the R. +Claro, which is interesting from containing fossils: the strata are much +dislocated by faults and dikes, and are inclined to the north, towards a +mountain of andesite and porphyry, into which they appear to become almost +blended. As the beds approach this mountain, their inclination increases up to +an angle of 70 degrees, and in the upper part, the rocks become highly +metamorphosed. The lowest bed visible in this section, is a purplish hard +sandstone. Secondly, a bed two or three hundred feet thick, of a white +siliceous sandstone, with a calcareous cement, containing seams of slaty +sandstone, and of hard yellowish-brown (dolomitic?) limestone; numerous, +well-rounded, little pebbles of quartz are included in the sandstone. Thirdly, +a dark coloured limestone with some quartz pebbles, from fifty to sixty feet in +thickness, containing numerous silicified shells, presently to be enumerated. +Fourthly, very compact, calcareous, jaspery sandstone, passing into (fifthly) a +great bed, several hundred feet thick, of conglomerate, composed of pebbles of +white, red, and purple porphyries, of sandstone and quartz, cemented by +calcareous matter. I observed that some of the finer parts of this conglomerate +were much indurated within a foot of a dike eight feet in width, and were +rendered of a paler colour with the calcareous matter segregated into white +crystallised particles; some parts were stained green from the colouring matter +of the dike. Sixthly, a thick mass, obscurely stratified, of a red sedimentary +stone or sandstone, full of crystalline calcareous matter, imperfect crystals +of oxide of iron, and I believe of feldspar, and therefore closely resembling +some of the highly metamorphosed beds at Arqueros: this bed was capped by, and +appeared to pass in its upper part into, rocks similarly coloured, containing +calcareous matter, and abounding with minute crystals, mostly elongated and +glassy, of reddish albite. Seventhly, a conformable stratum of fine reddish +porphyry with large crystals of (albitic?) feldspar; probably a submarine lava. +Eighthly, another conformable bed of green porphyry, with specks of green earth +and cream-coloured crystals of feldspar. I believe that there are other +superincumbent crystalline strata and submarine lavas, but I had not time to +examine them. +</p> + +<p> +The upper beds in this section probably correspond with parts of the great +gypseous formation; and the lower beds of red sandstone conglomerate and +fossiliferous limestone no doubt are the equivalents of the Hippurite stratum, +seen in descending from Arqueros to Pluclaro, which there lies conformably upon +the porphyritic conglomerate formation. The fossils found in the third bed, +consist of:— +</p> + +<p> +Pecten Dufreynoyi, d’Orbigny, “Voyage, Part Pal.” This +species, which occurs here in vast numbers, according to M. D’Orbigny, +resembles certain cretaceous forms. +</p> + +<p> +Ostrea hemispherica, d’Orbigny, “Voyage” etc. +</p> + +<p> +Also resembles, according to the same author, cretaceous forms. +</p> + +<p> +Terebratula aenigma, d’Orbigny, “Voyage” etc. (Pl. 22 Figures +10-12.) +</p> + +<p> +Is allied, according to M. d’Orbigny, to T. concinna from the Forest +Marble. A series of this species, collected in several localities hereafter to +be referred to, has been laid before Professor Forbes; and he informs me that +many of the specimens are almost undistinguishable from our oolitic T. +tetraedra, and that the varieties amongst them are such as are found in that +variable species. Generally speaking, the American specimens of T. aenigma may +be distinguished from the British T. tetraedra, by the surface having the ribs +sharp and well-defined to the beak, whilst in the British species they become +obsolete and smoothed down; but this difference is not constant. Professor +Forbes adds, that, possibly, internal characters may exist, which would +distinguish the American species from its European allies. +</p> + +<p> +Spirifer linguiferoides, E. Forbes. +</p> + +<p> +Professor Forbes states that this species is very near to S. linguifera of +Phillips (a carboniferous limestone fossil), but probably distinct. M. +d’Orbigny considers it as perhaps indicating the Jurassic period. +</p> + +<p> +Ammonites, imperfect impression of. +</p> + +<p> +M. Domeyko has sent to France a collection of fossils, which, I presume, from +the description given, must have come from the neighbourhood of Arqueros; they +consist of:— +</p> + +<p> +Pecten Dufreynoyi, d’Orbigny, “Voyage” Part Pal. Ostrea +hemispherica, d’Orbigny, “Voyage” Part Pal. Turritella Andii, +d’Orbigny, “Voyage” Part Pal. (Pleurotomaria Humboldtii of +Von Buch). Hippurites Chilensis, d’Orbigny, “Voyage” Part +Pal. +</p> + +<p> +The specimens of this Hippurite, as well as those I collected in my descent +from Arqueros, are very imperfect; but in M. d’Orbigny’s opinion +they resemble, as does the Turritella Andii, cretaceous (upper greensand) +forms. +</p> + +<p> +Nautilus Domeykus, d’Orbigny, “Voyage” Part Pal. Terebratula +aenigma, d’Orbigny, “Voyage” Part Pal. Terebratula ignaciana, +d’Orbigny, “Voyage” Part Pal. +</p> + +<p> +This latter species was found by M. Domeyko in the same block of limestone with +the T. aenigma. According to M. d’Orbigny, it comes near to T. +ornithocephala from the Lias. A series of this species collected at Guasco, has +been examined by Professor E. Forbes, and he states that it is difficult to +distinguish between some of the specimens and the T. hastata from the mountain +limestone; and that it is equally difficult to draw a line between them and +some Marlstone Terebratulae. Without a knowledge of the internal structure, it +is impossible at present to decide on their identity with analogous European +forms. +</p> + +<p> +The remarks given on the several foregoing shells, show that, in M. +d’Orbigny’s opinion, the Pecten, Ostrea, Turritella, and Hippurite +indicate the cretaceous period; and the Gryphaea appears to Professor Forbes to +be identical with a species, associated in Southern India with unquestionably +cretaceous forms. On the other hand, the two Terebratulae and the Spirifer +point, in the opinion both of M. d’Orbigny and Professor Forbes, to the +oolitic series. Hence M. d’Orbigny, not having himself examined this +country, has concluded that there are here two distinct formations; but the +Spirifer and T. aenigma were certainly included in the same bed with the Pecten +and Ostrea, whence I extracted them; and the geologist M. Domeyko sent home the +two Terebratulae with the other-named shells, from the same locality, without +specifying that they came from different beds. Again, as we shall presently +see, in a collection of shells given me from Guasco, the same species, and +others presenting analogous differences, are mingled together, and are in the +same condition; and lastly, in three places in the valley of Copiapo, I found +some of these same species similarly grouped. Hence there cannot be any doubt, +highly curious though the fact be, that these several fossils, namely, the +Hippurites, Gryphaea, Ostrea, Pecten, Turritella, Nautilus, two Terebratulae, +and Spirifer all belong to the same formation, which would appear to form a +passage between the oolitic and cretaceous systems of Europe. Although aware +how unusual the term must sound, I shall, for convenience’ sake, call +this formation cretaceo- oolitic. Comparing the sections in this valley of +Coquimbo with those in the Cordillera described in the last chapter, and +bearing in mind the character of the beds in the intermediate district of Los +Hornos, there is certainly a close general mineralogical resemblance between +them, both in the underlying porphyritic conglomerate, and in the overlying +gypseous formation. Considering this resemblance, and that the fossils from the +Puente del Inca at the base of the gypseous formation, and throughout the +greater part of its entire thickness on the Peuquenes range, indicate the +Neocomian period,—that is, the dawn of the cretaceous system, or, as some +have believed, a passage between this latter and the oolitic series—I +conclude that probably the gypseous and associated beds in all the sections +hitherto described, belong to the same great formation, which I have +denominated—cretaceo-oolitic. I may add, before leaving Coquimbo, that M. +Gay found in the neighbouring Cordillera, at the height of 14,000 feet above +the sea, a fossiliferous formation, including a Trigonia and Pholadomya +(D’Orbigny “Voyage” Part Geolog. page 242.);—both of +which genera occur at the Puente del Inca. +</p> + +<h3>COQUIMBO TO GUASCO.</h3> + +<p> +The rocks near the coast, and some way inland, do not differ from those +described northwards of Valparaiso: we have much greenstone, syenite, +feldspathic and jaspery slate, and grauwackes having a basis like that of +claystone; there are some large tracts of granite, in which the constituent +minerals are sometimes arranged in folia, thus composing an imperfect gneiss. +There are two large districts of mica-schists, passing into glossy clay-slate, +and resembling the great formation in the Chonos Archipelago. In the valley of +Guasco, an escarpment of porphyritic conglomerate is first seen high up the +valley, about two leagues eastward of the town of Ballenar. I heard of a great +gypseous formation in the Cordillera; and a collection of shells made there was +given me. These shells are all in the same condition, and appear to have come +from the same bed: they consist of:— +</p> + +<p> +Turritella Andii, d’Orbigny, “Voyage” Part Pal. Pecten +Dufreynoyi, d’Orbigny, “Voyage” Part Pal. Terebatula +ignaciana, d’Orbigny, “Voyage” Part Pal. +</p> + +<p> +The relations of these species have been given under the head of Coquimbo. +</p> + +<p> +Terebratula aenigma, d’Orbigny, “Voyage” Part Pal. +</p> + +<p> +This shell M. d’Orbigny does not consider identical with his T. aenigma, +but near to T. obsoleta. Professor Forbes thinks that it is certainly a variety +of T. aenigma: we shall meet with this variety again at Copiapo. +</p> + +<p> +Spirifer Chilensis, E. Forbes. +</p> + +<p> +Professor Forbes remarks that this fossil resembles several carboniferous +limestone Spirifers; and that it is also related to some liassic species, as S. +Wolcotii. +</p> + +<p> +If these shells had been examined independently of the other collections, they +would probably have been considered, from the characters of the two +Terebratulae, and from the Spirifer, as oolitic; but considering that the first +species, and according to Professor Forbes, the four first, are identical with +those from Coquimbo, the two formations no doubt are the same, and may, as I +have said, be provisionally called cretaceo-oolitic. +</p> + +<h3>VALLEY OF COPIAPO.</h3> + +<p> +The journey from Guasco to Copiapo, owing to the utterly desert nature of the +country, was necessarily so hurried, that I do not consider my notes worth +giving. In the valley of Copiapo some of the sections are very interesting. +From the sea to the town of Copiapo, a distance estimated at thirty miles, the +mountains are composed of greenstone, granite, andesite, and blackish porphyry, +together with some dusky-green feldspathic rocks, which I believe to be altered +clay-slate: these mountains are crossed by many brown-coloured dikes, running +north and south. Above the town, the main valley runs in a south-east and even +more southerly course towards the Cordillera, where it is divided into three +great ravines, by the northern one of which, called Jolquera, I penetrated for +a short distance. The section, Section 1/3 in Plate 1, gives an eye-sketch of +the structure and composition of the mountains on both sides of this valley: a +straight east and west line from the town to the Cordillera is perhaps not more +than thirty miles, but along the valley the distance is much greater. Wherever +the valley trended very southerly, I have endeavoured to contract the section +into its true proportion. This valley, I may add, rises much more gently than +any other valley which I saw in Chile. +</p> + +<p> +To commence with our section, for a short distance above the town we have hills +of the granitic series, together with some of that rock [A], which I suspect to +be altered clay-slate, but which Professor G. Rose, judging from specimens +collected by Meyen at P. Negro, states is serpentine passing into greenstone. +We then come suddenly to the great gypseous formation [B], without having +passed over, differently from, in all the sections hitherto described, any of +the porphyritic conglomerate. The strata are at first either horizontal or +gently inclined westward; then highly inclined in various directions, and +contorted by underlying masses of intrusive rocks; and lastly, they have a +regular eastward dip, and form a tolerably well pronounced north and south line +of hills. This formation consists of thin strata, with innumerable +alternations, of black, calcareous slate-rock, of calcareo-aluminous stones +like those at Coquimbo, which I have called pseudo-honestones of green jaspery +layers, and of pale-purplish, calcareous, soft rotten-stone, including seams +and veins of gypsum. These strata are conformably overlaid by a great thickness +of thinly stratified, compact limestone with included crystals of carbonate of +lime. At a place called Tierra Amarilla, at the foot of a mountain thus +composed there is a broad vein, or perhaps stratum, of a beautiful and curious +crystallised mixture, composed, according to Professor G. Rose, of sulphate of +iron under two forms, and of the sulphates of copper and alumina (Meyen’s +“Reise” etc. Th. 1, s. 394.): the section is so obscure that I +could not make out whether this vein or stratum occurred in the gypseous +formation, or more probably in some underlying masses [A], which I believe are +altered clay-slate. +</p> + +<h3>SECOND AXIS OF ELEVATION.</h3> + +<p> +After the gypseous masses [B], we come to a line of hills of unstratified +porphyry [C], which on their eastern side blend into strata of great thickness +of porphyritic conglomerate, dipping eastward. This latter formation, however, +here has not been nearly so much metamorphosed as in most parts of Central +Chile; it is composed of beds of true purple claystone porphyry, repeatedly +alternating with thick beds of purplish-red conglomerate with the well-rounded, +large pebbles of various porphyries, not blended together. +</p> + +<h3>THIRD AXIS OF ELEVATION.</h3> + +<p> +Near the ravine of Los Hornitos, there is a well-marked line of elevation, +extending for many miles in a N.N.E. and S.S.W. direction, with the strata +dipping in most parts (as in the second axis) only in one direction, namely, +eastward at an average angle of between 30 and 40 degrees. Close to the mouth +of the valley, however, there is, as represented in the section, a steep and +high mountain [D], composed of various green and brown intrusive porphyries +enveloped with strata, apparently belonging to the upper parts of the +porphyritic conglomerate, and dipping both eastward and westward. I will +describe the section seen on the eastern side of this mountain [D], beginning +at the base with the lowest bed visible in the porphyritic conglomerate, and +proceeding upwards through the gypseous formation. Bed 1 consists of reddish +and brownish porphyry varying in character, and in many parts highly +amygdaloidal with carbonate of lime, and with bright green and brown bole. Its +upper surface is throughout clearly defined, but the lower surface is in most +parts indistinct, and towards the summit of the mountain [D] quite blended into +the intrusive porphyries. Bed 2, a pale lilac, hard but not heavy stone, +slightly laminated, including small extraneous fragments, and imperfect as well +as some perfect and glassy crystals of feldspar; from one hundred and fifty to +two hundred feet in thickness. When examining it in situ, I thought it was +certainly a true porphyry, but my specimens now lead me to suspect that it +possibly may be a metamorphosed tuff. From its colour it could be traced for a +long distance, overlying in one part, quite conformably to the porphyry of bed +1, and in another not distant part, a very thick mass of conglomerate, composed +of pebbles of a porphyry chiefly like that of bed 1: this fact shows how the +nature of the bottom formerly varied in short horizontal distances. Bed 3, +white, much indurated tuff, containing minute pebbles, broken crystals, and +scales of mica, varies much in thickness. This bed is remarkable from +containing many globular and pear-shaped, externally rusty balls, from the size +of an apple to a man’s head, of very tough, slate-coloured porphyry, with +imperfect crystals of feldspar: in shape these balls do not resemble pebbles, +AND I BELIEVE THAT THEY ARE SUBAQUEOUS VOLCANIC BOMBS; they differ from +SUBAERIAL bombs only in not being vesicular. Bed 4; a dull purplish-red, hard +conglomerate, with crystallised particles and veins of carbonate of lime, from +three hundred to four hundred feet in thickness. The pebbles are of claystone +porphyries of many varieties; they are tolerably well rounded, and vary in size +from a large apple to a man’s head. This bed includes three layers of +coarse, black, calcareous, somewhat slaty rock: the upper part passes into a +compact red sandstone. +</p> + +<p> +In a formation so highly variable in mineralogical nature, any division not +founded on fossil remains, must be extremely arbitrary: nevertheless, the beds +below the last conglomerate may, in accordance with all the sections hitherto +described, be considered as belonging to the porphyritic conglomerate, and +those above it to the gypseous formation, marked [E] in the section. The part +of the valley in which the following beds are seen is near Potrero Seco. Bed 5, +compact, fine-grained, pale greenish-grey, non- calcareous, indurated mudstone, +easily fusible into a pale green and white glass. Bed 6, purplish, +coarse-grained, hard sandstone, with broken crystals of feldspar and +crystallised particles of carbonate of lime; it possesses a slightly nodular +structure. Bed 7, blackish-grey, much indurated, calcareous mudstone, with +extraneous particles of unequal size; the whole being in parts finely +brecciated. In this mass there is a stratum, twenty feet in thickness, of +impure gypsum. Bed 8, a greenish mudstone, with several layers of gypsum. Bed +9, a highly indurated, easily fusible, white tuff, thickly mottled with +ferruginous matter, and including some white semi-porcellanic layers, which are +interlaced with ferruginous veins. This stone closely resembles some of the +commonest varieties in the Uspallata chain. Bed 10, a thick bed of rather +bright green, indurated mudstone or tuff, with a concretionary nodular +structure so strongly developed that the whole mass consists of balls. I will +not attempt to estimate the thickness of the strata in the gypseous formation +hitherto described, but it must certainly be very many hundred feet. Bed 11 is +at least 800 feet in thickness: it consists of thin layers of whitish, +greenish, or more commonly brown, fine-grained, indurated tuffs, which crumble +into angular fragments: some of the layers are semi-porcellanic, many of them +highly ferruginous, and some are almost composed of carbonate of lime and iron +with drusy cavities lined with quartzf-crystals. Bed 12, dull purplish or +greenish or dark-grey, very compact and much indurated mudstone: estimated at +1,500 feet in thickness: in some parts this rock assumes the character of an +imperfect coarse clay-slate; but viewed under a lens, the basis always has a +mottled appearance, with the edges of the minute component particles blending +together. Parts are calcareous, and there are numerous veins of highly +crystalline carbonate of lime charged with iron. The mass has a nodular +structure, and is divided by only a few planes of stratification: there are, +however, two layers, each about eighteen inches thick, of a dark brown, +finer-grained stone, having a conchoidal, semi-porcellanic fracture, which can +be followed with the eye for some miles across the country. +</p> + +<p> +I believe this last great bed is covered by other nearly similar alternations; +but the section is here obscured by a tilt from the next porphyritic chain, +presently to be described. I have given this section in detail, as being +illustrative of the general character of the mountains in this neighbourhood; +but it must not be supposed that any one stratum long preserves the same +character. At a distance of between only two and three miles the green +mudstones and white indurated tuffs are to a great extent replaced by red +sandstone and black calcareous shaly rocks, alternating together. The white +indurated tuff, bed 11, here contains little or no gypsum, whereas on the +northern and opposite side of the valley, it is of much greater thickness and +abounds with layers of gypsum, some of them alternating with thin seams of +crystalline carbonate of lime. The uppermost, dark-coloured, hard mudstone, bed +12, is in this neighbourhood the most constant stratum. The whole series +differs to a considerable extent, especially in its upper part, from that met +with at [BB], in the lower part of the valley; nevertheless, I do not doubt +that they are equivalents. +</p> + +<h3>FOURTH AXIS OF ELEVATION (VALLEY OF COPIAPO).</h3> + +<p> +This axis is formed of a chain of mountains [F], of which the central masses +(near La Punta) consist of andesite containing green hornblende and coppery +mica, and the outer masses of greenish and black porphyries, together with some +fine lilac-coloured claystone porphyry; all these porphyries being injected and +broken up by small hummocks of andesite. The central great mass of this latter +rock, is covered on the eastern side by a black, fine-grained, highly micaceous +slate, which, together with the succeeding mountains of porphyry, are traversed +by numerous white dikes, branching from the andesite, and some of them +extending in straight lines, to a distance of at least two miles. The mountains +of porphyry eastward of the micaceous schist soon, but gradually, assume (as +observed in so many other cases) a stratified structure, and can then be +recognised as a part of the porphyritic conglomerate formation. These strata +[G] are inclined at a high angle to the S.E., and form a mass from fifteen +hundred to two thousand feet in thickness. The gypseous masses to the west +already described, dip directly towards this axis, with the strata only in a +few places (one of which is represented in the section) thrown from it: hence +this fourth axis is mainly uniclinal towards the S.E., and just like our third +axis, only locally anticlinal. +</p> + +<p> +The above strata of porphyritic conglomerate [G] with their south-eastward dip, +come abruptly up against beds of the gypseous formation [H], which are gently, +but irregularly, inclined westward: so that there is here a synclinal axis and +great fault. Further up the valley, here running nearly north and south, the +gypseous formation is prolonged for some distance; but the stratification is +unintelligible, the whole being broken up by faults, dikes, and metalliferous +veins. The strata consist chiefly of red calcareous sandstones, with numerous +veins in the place of layers, of gypsum; the sandstone is associated with some +black calcareous slate-rock, and with green pseudo-honestones, passing into +porcelain-jasper. Still further up the valley, near Las Amolanas [I], the +gypseous strata become more regular, dipping at an angle of between 30 and 40 +degrees to W.S.W., and conformably overlying, near the mouth of the ravine of +Jolquera, strata [K] of porphyritic conglomerate. The whole series has been +tilted by a partially concealed axis [L], of granite, andesite, and a granitic +mixture of white feldspar, quartz, and oxide of iron. +</p> + +<h3>FIFTH AXIS OF ELEVATION (VALLEY OF COPIAPO, NEAR LOS AMOLANAS).</h3> + +<p> +I will describe in some detail the beds [I] seen here, which, as just stated, +dip to W.S.W., at an angle of from 30 to 40 degrees. I had not time to examine +the underlying porphyritic conglomerate, of which the lowest beds, as seen at +the mouth of the Jolquera, are highly compact, with crystals of red oxide of +iron; and I am not prepared to say whether they are chiefly of volcanic or +metamorphic origin. On these beds there rests a coarse purplish conglomerate, +very little metamorphosed, composed of pebbles of porphyry, but remarkable from +containing one pebble of granite;—of which fact no instance has occurred in +the sections hitherto described. Above this conglomerate, there is a black +siliceous claystone, and above it numerous alternations of dark-purplish and +green porphyries, which may be considered as the uppermost limit of the +porphyritic conglomerate formation. +</p> + +<p> +Above these porphyries comes a coarse, arenaceous conglomerate, the lower half +white and the upper half of a pink colour, composed chiefly of pebbles of +various porphyries, but with some of red sandstone and jaspery rocks. In some +of the more arenaceous parts of the conglomerate, there was an oblique or +current lamination; a circumstance which I did not elsewhere observe. Above +this conglomerate, there is a vast thickness of thinly stratified, +pale-yellowish, siliceous sandstone, passing into a granular quartz-rock, used +for grindstones (hence the name of the place Las Amolanas), and certainly +belonging to the gypseous formation, as does probably the immediately +underlying conglomerate. In this yellowish sandstone there are layers of white +and pale-red siliceous conglomerate; other layers with small, well-rounded +pebbles of white quartz, like the bed at the R. Claro at Coquimbo; others of a +greenish, fine-grained, less siliceous stone, somewhat resembling the +pseudo-honestones lower down the valley; and lastly, others of a black +calcareous shale-rock. In one of the layers of conglomerate, there was embedded +a fragment of mica-slate, of which this is the first instance; hence perhaps, +it is from a formation of mica-slate, that the numerous small pebbles of +quartz, both here and at Coquimbo, have been derived. Not only does the +siliceous sandstone include layers of the black, thinly stratified, not +fissile, calcareous shale-rock, but in one place the whole mass, especially the +upper part, was, in a marvellously short horizontal distance, after frequent +alternations, replaced by it. When this occurred, a mountain-mass, several +thousand feet in thickness was thus composed; the black calcareous shale-rock, +however, always included some layers of the pale-yellowish siliceous sandstone, +of the red conglomerate, and of the greenish jaspery and pseudo-honestone +varieties. It likewise included three or four widely separated layers of a +brown limestone, abounding with shells immediately to be described. This pile +of strata was in parts traversed by many veins of gypsum. The calcareous +shale-rock, though when freshly broken quite black, weathers into an ash- +colour: in which respect and in general appearance, it perfectly resembles +those great fossiliferous beds of the Peuquenes range, alternating with gypsum +and red sandstone, described in the last chapter. +</p> + +<p> +The shells out of the layers of brown limestone, included in the black +calcareous shale-rock, which latter, as just stated, replaces the white +siliceous sandstone, consist of:— +</p> + +<p> +Pecten Dufreynoyi, d’Orbigny, “Voyage” Part Pal. Turritella +Andii, d’Orbigny, “Voyage” Part Pal. +</p> + +<p> +Astarte Darwinii, E. Forbes. Gryphaea Darwinii, E. Forbes. +</p> + +<p> +An intermediate form between G. gigantea and G. incurva. +</p> + +<p> +Gryphaea nov. spec.?, E. Forbes. Perna Americana, E. Forbes. Avicula, nov. +spec. +</p> + +<p> +Considered by Mr. G.B. Sowerby as the A. echinata, by M. d’Orbigny as +certainly a new and distinct species, having a Jurassic aspect. The specimen +has been unfortunately lost. +</p> + +<p> +Terebratula aenigma, d’Orbigny, (var. of do. E. Forbes.) +</p> + +<p> +This is the same variety, with that from Guasco, considered by M. +D’Orbigny to be a distinct species from his T. aenigma, and related to T. +obsoleta. +</p> + +<p> +Plagiostoma and Ammonites, fragments of. +</p> + +<p> +The lower layers of the limestone contained thousands of the Gryphaea; and the +upper ones as many of the Turritella, with the Gryphaea (nov. species) and +Serpulae adhering to them; in all the layers, the Terebratula and fragments of +the Pecten were included. It was evident, from the manner in which species were +grouped together, that they had lived where now embedded. Before making any +further remarks, I may state, that higher up this same valley we shall again +meet with a similar association of shells; and in the great Despoblado Valley, +which branches off near the town from that of Copiapo, the Pecten Dufreynoyi, +some Gryphites (I believe G. Darwinii), and the TRUE Terebratula aenigma of +d’Orbigny were found together in an equivalent formation, as will be +hereafter seen. A specimen also, I may add, of the true T. aenigma, was given +me from the neighbourhood of the famous silver mines of Chanuncillo, a little +south of the valley of the Copiapo, and these mines, from their position, I +have no doubt, lie within the great gypseous formation: the rocks close to one +of the silver veins, judging from fragments shown me, resemble those singular +metamorphosed deposits from the mining district of Arqueros near Coquimbo. +</p> + +<p> +I will reiterate the evidence on the association of these several shells in the +several localities. +</p> + +<h3>COQUIMBO.</h3> + +<p> +In the same bed, Rio Claro: Pecten Dufreynoyi. Ostrea hemispherica. Terebratula +aenigma. Spirifer linguiferoides. +</p> + +<p> +Same bed, near Arqueros: Hippurites Chilensis. Gryphaea orientalis. +</p> + +<p> +Collected by M. Domeyko from the same locality, apparently near Arqueros: +Terebratula aenigma and Terebratula ignaciana, in same block of limestone: +Pecten Dufreynoyi. Ostrea hemispherica. Hippurites Chilensis. Turritella Andii. +Nautilus Domeykus. +</p> + +<h3>GUASCO.</h3> + +<p> +In a collection from the Cordillera, given me: the specimens all in the same +condition: Pecten Dufreynoyi. Turritella Andii. Terebratula ignaciana. +Terebratula aenigma, var. Spirifer Chilensis. +</p> + +<h3>COPIAPO.</h3> + +<p> +Mingled together in alternating beds in the main valley of Copiapo near Las +Amolanas, and likewise higher up the valley: Pecten Dufreynoyi. Turritella +Andii. Terebratula aenigma, var. as at Guasco. Astarte Darwinii. Gryphaea +Darwinii. Gryphaea nov. species? Perna Americana. Avicula, nov. species. +</p> + +<p> +Main valley of Copiapo, apparently same formation with that of Amolanas: +Terebratula aenigma (true). +</p> + +<p> +In the same bed, high up the great lateral valley of the Despoblado, in the +ravine of Maricongo: Terebratula aenigma (true). Pecten Dufreynoyi. Gryphaea +Darwinii? +</p> + +<p> +Considering this table, I think it is impossible to doubt that all these +fossils belong to the same formation. If, however, the species from Las +Amolanas, in the Valley of Copiapo, had, as in the case of those from Guasco, +been separately examined, they would probably have been ranked as oolitic; for, +although no Spirifers were found here, all the other species, with the +exception of the Pecten, Turritella, and Astarte, have a more ancient aspect +than cretaceous forms. On the other hand, taking into account the evidence +derived from the cretaceous character of these three shells, and of the +Hippurites, Gryphaea orientalis, and Ostrea, from Coquimbo, we are driven back +to the provisional name already used of cretaceo-oolitic. From geological +evidence, I believe this formation to be the equivalent of the Neocomian beds +of the Cordillera of Central Chile. +</p> + +<p> +To return to our section near Las Amolanas:—Above the yellow siliceous +sandstone, or the equivalent calcareous slate-rock, with its bands of +fossil-shells, according as the one or other prevails, there is a pile of +strata, which cannot be less than from two to three thousand feet in thickness, +in main part composed of a coarse, bright red conglomerate, with many +intercalated beds of red sandstone, and some of green and other coloured +porcelain-jaspery layers. The included pebbles are well-rounded, varying from +the size of an egg to that of a cricket-ball, with a few larger; and they +consist chiefly of porphyries. The basis of the conglomerate, as well as some +of the alternating thin beds, are formed of a red, rather harsh, easily fusible +sandstone, with crystalline calcareous particles. This whole great pile is +remarkable from the thousands of huge, embedded, silicified trunks of trees, +one of which was eight feet long, and another eighteen feet in circumference: +how marvellous it is, that every vessel in so thick a mass of wood should have +been converted into silex! I brought home many specimens, and all of them, +according to Mr. R. Brown, present a coniferous structure. +</p> + +<p> +Above this great conglomerate, we have from two to three hundred feet in +thickness of red sandstone; and above this, a stratum of black calcareous +slate-rock, like that which alternates with and replaces the underlying +yellowish-white, siliceous sandstone. Close to the junction between this upper +black slate-rock and the upper red sandstone, I found the Gryphaea Darwinii, +the Turritella Andii, and vast numbers of a bivalve, too imperfect to be +recognised. Hence we see that, as far as the evidence of these two shells +serves—and the Turritella is an eminently characteristic +species—the whole thickness of this vast pile of strata belongs to the +same age. Again, above the last-mentioned upper red sandstone, there were +several alternations of the black, calcareous slate-rock; but I was unable to +ascend to them. All these uppermost strata, like the lower ones, vary extremely +in character in short horizontal distances. The gypseous formation, as here +seen, has a coarser, more mechanical texture, and contains much more siliceous +matter than the corresponding beds lower down the valley. Its total thickness, +together with the upper beds of the porphyritic conglomerate, I estimated at +least at 8,000 feet; and only a small portion of the porphyritic conglomerate, +which on the eastern flank of the fourth axis of elevation appeared to be from +fifteen hundred to two thousand feet thick, is here included. As corroborative +of the great thickness of the gypseous formation, I may mention that in the +Despoblado Valley (which branches from the main valley a little above the town +of Copiapo) I found a corresponding pile of red and white sandstones, and of +dark, calcareous, semi-jaspery mudstones, rising from a nearly level surface +and thrown into an absolutely vertical position; so that, by pacing, I +ascertained their thickness to be nearly two thousand seven hundred feet; +taking this as a standard of comparison, I estimated the thickness of the +strata ABOVE the porphyritic conglomerate at 7,000 feet. +</p> + +<p> +The fossils before enumerated, from the limestone-layers in the whitish +siliceous sandstone, are now covered, on the least computation, by strata from +5,000 to 6,000 feet in thickness. Professor E. Forbes thinks that these shells +probably lived at a depth of from about 30 to 40 fathoms, that is from 180 to +240 feet; anyhow, it is impossible that they could have lived at the depth of +from 5,000 to 6,000 feet. Hence in this case, as in that of the Puente del +Inca, we may safely conclude that the bottom of the sea on which the shells +lived, subsided, so as to receive the superincumbent submarine strata: and this +subsidence must have taken place during the existence of these shells; for, as +I have shown, some of them occur high up as well as low down in the series. +That the bottom of the sea subsided, is in harmony with the presence of the +layers of coarse, well- rounded pebbles included throughout this whole pile of +strata, as well as of the great upper mass of conglomerate from 2,000 to 3,000 +feet thick; for coarse gravel could hardly have been formed or spread out at +the profound depths indicated by the thickness of the strata. The subsidence, +also, must have been slow to have allowed of this often-recurrent spreading out +of the pebbles. Moreover, we shall presently see that the surfaces of some of +the streams of porphyritic lava beneath the gypseous formation, are so highly +amygdaloidal that it is scarcely possible to believe that they flowed under the +vast pressure of a deep ocean. The conclusion of a great subsidence during the +existence of these cretaceo-oolitic fossils, may, I believe, be extended to the +district of Coquimbo, although owing to the fossiliferous beds there not being +directly covered by the upper gypseous strata, which in the section north of +the valley are about 6,000 feet in thickness, I did not there insist on this +conclusion. +</p> + +<p> +The pebbles in the above conglomerates, both in the upper and lower beds, are +all well rounded, and, though chiefly composed of various porphyries, there are +some of red sandstone and of a jaspery stone, both like the rocks intercalated +in layers in this same gypseous formation; there was one pebble of mica-slate +and some of quartz, together with many particles of quartz. In these respects +there is a wide difference between the gypseous conglomerates and those of the +porphyritic-conglomerate formation, in which latter, angular and rounded +fragments, almost exclusively composed of porphyries, are mingled together, and +which, as already often remarked, probably were ejected from craters deep under +the sea. From these facts I conclude, that during the formation of the +conglomerates, land existed in the neighbourhood, on the shores of which the +innumerable pebbles were rounded and thence dispersed, and on which the +coniferous forests flourished—for it is improbable that so many thousand +logs of wood should have drifted from any great distance. This land, probably +islands, must have been mainly formed of porphyries, with some mica-slate, +whence the quartz was derived, and with some red sandstone and jaspery rocks. +This latter fact is important, as it shows that in this district, even +previously to the deposition of the lower gypseous or cretaceo-oolitic beds, +strata of an analogous nature had elsewhere, no doubt in the more central +ranges of the Cordillera, been elevated; thus recalling to our minds the +relations of the Cumbre and Uspallata chains. Having already referred to the +great lateral valley of the Despoblado, I may mention that above the 2,700 feet +of red and white sandstone and dark mudstone, there is a vast mass of coarse, +hard, red conglomerate, some thousand feet in thickness, which contains much +silicified wood, and evidently corresponds with the great upper conglomerate at +Las Amolanas: here, however, the conglomerate consists almost exclusively of +pebbles of granite, and of disintegrated crystals of reddish feldspar and +quartz firmly recemented together. In this case, we may conclude that the land +whence the pebbles were derived, and on which the now silicified trees once +flourished, was formed of granite. +</p> + +<p> +The mountains near Las Amolanas, composed of the cretaceo-oolitic strata, are +interlaced with dikes like a spider’s web, to an extent which I have +never seen equalled, except in the denuded interior of a volcanic crater: north +and south lines, however, predominate. These dikes are composed of green, +white, and blackish rocks, all porphyritic with feldspar, and often with large +crystals of hornblende. The white varieties approach closely in character to +andesite, which composes as we have seen, the injected axes of so many of the +lines of elevation. Some of the green varieties are finely laminated, parallel +to the walls of the dikes. +</p> + +<h3>SIXTH AXIS OF ELEVATION (VALLEY OF COPIAPO).</h3> + +<p> +This axis consists of a broad mountainous mass [O] of andesite, composed of +albite, brown mica, and chlorite, passing into andesitic granite, with quartz: +on its western side it has thrown off, at a considerable angle, a thick mass of +stratified porphyries, including much epidote [NN], and remarkable only from +being divided into very thin beds, as highly amygdaloidal on their surfaces as +subaerial lava-streams are often vesicular. This porphyritic formation is +conformably covered, as seen some way up the ravine of Jolquera, by a mere +remnant of the lower part of the cretaceo-oolitic formation [MM], which in one +part encases, as represented in the coloured section, the foot of the andesitic +axis [L], of the already described fifth line, and in another part entirely +conceals it: in this latter case, the gypseous or cretaceo-oolitic strata +falsely appeared to dip under the porphyritic conglomerate of the fifth axis. +The lowest bed of the gypseous formation, as seen here [M], is of yellowish +siliceous sandstone, precisely like that of Amolanas, interlaced in parts with +veins of gypsum, and including layers of the black, calcareous, non-fissile +slate-rock: the Turritella Andii, Pecten Dufreynoyi, Terebratula aenigma, var., +and some Gryphites were embedded in these layers. The sandstone varies in +thickness from only twenty to eighty feet; and this variation is caused by the +inequalities in the upper surface of an underlying stream of purple claystone +porphyry. Hence the above fossils here lie at the very base of the gypseous or +cretaceo-oolitic formation, and hence they were probably once covered up by +strata about seven thousand feet in thickness: it is, however, possible, though +from the nature of all the other sections in this district not probable, that +the porphyritic claystone lava may in this case have invaded a higher level in +the series. Above the sandstone there is a considerable mass of much indurated, +purplish-black, calcareous claystone, allied in nature to the often-mentioned +black calcareous slate- rock. Eastward of the broad andesitic axis of this +sixth line, and penetrated by many dikes from it, there is a great formation +[P] of mica-schist, with its usual variations, and passing in one part into a +ferruginous quartz-rock. The folia are curved and highly inclined, generally +dipping eastward. It is probable that this mica-schist is an old formation, +connected with the granitic rocks and metamorphic schists near the coast; and +that the one fragment of mica-slate, and the pebbles of quartz low down in the +gypseous formation at Las Amolanas, have been derived from it. The mica-schist +is succeeded by stratified porphyritic conglomerate [Q] of great thickness, +dipping eastward with a high inclination: I have included this latter +mountain-mass in the same anticlinal axis with the porphyritic streams [NN]; +but I am far from sure that the two masses may not have been independently +upheaved. +</p> + +<h3>SEVENTH AXIS OF ELEVATION.</h3> + +<p> +Proceeding up the ravine, we come to another mass [R] of andesite; and beyond +this, we again have a very thick, stratified porphyritic formation [S], dipping +at a small angle eastward, and forming the basal part of the main Cordillera. I +did not ascend the ravine any higher; but here, near Castano, I examined +several sections, of which I will not give the details, only observing, that +the porphyritic beds, or submarine lavas, preponderate greatly in bulk over the +alternating sedimentary layers, which have been but little metamorphosed: these +latter consist of fine-grained red tuffs and of whitish volcanic grit-stones, +together with much of a singular, compact rock, having an almost crystalline +basis, finely brecciated with red and green fragments, and occasionally +including a few large pebbles. The porphyritic lavas are highly amygdaloidal, +both on their upper and lower surfaces; they consist chiefly of claystone +porphyry, but with one common variety, like some of the streams at the Puente +del Inca, having a grey mottled basis, abounding with crystals of red hydrous +oxide of iron, green ones apparently of epidote, and a few glassy ones of +feldspar. This pile of strata differs considerably from the basal strata of the +Cordillera in Central Chile, and may possibly belong to the upper and gypseous +series: I saw, however, in the bed of the valley, one fragment of porphyritic +breccia-conglomerate, exactly like those great masses met with in the more +southern parts of Chile. +</p> + +<p> +Finally, I must observe, that though I have described between the town of +Copiapo and the western flank of the main Cordillera seven or eight axes of +elevation, extending nearly north and south, it must not be supposed that they +all run continuously for great distances. As was stated to be the case in our +sections across the Cordillera of Central Chile, so here most of the lines of +elevation, with the exception of the first, third, and fifth, are very short. +The stratification is everywhere disturbed and intricate; nowhere have I seen +more numerous faults and dikes. The whole district, from the sea to the +Cordillera, is more or less metalliferous; and I heard of gold, silver, copper, +lead, mercury, and iron veins. The metamorphic action, even in the lower +strata, has certainly been far less here than in Central Chile. +</p> + +<h3>VALLEY OF THE DESPOBLADO.</h3> + +<p> +This great barren valley, which has already been alluded to, enters the main +valley of Copiapo a little above the town: it runs at first northerly, then +N.E., and more easterly into the Cordillera; I followed its dreary course to +the foot of the first main ridge. I will not give a detailed section, because +it would be essentially similar to that already given, and because the +stratification is exceedingly complicated. After leaving the plutonic hills +near the town, I met first, as in the main valley, with the gypseous formation, +having the same diversified character as before, and soon afterwards with +masses of porphyritic conglomerate, about one thousand feet in thickness. In +the lower part of this formation there were very thick beds composed of +fragments of claystone porphyries, both angular and rounded, with the smaller +ones partially blended together and the basis rendered porphyritic; these beds +separated distinct streams, from sixty to eighty feet in thickness, of +claystone lavas. Near Paipote, also, there was much true porphyritic +breccia-conglomerate: nevertheless, few of these masses were metamorphosed to +the same degree with the corresponding formation in Central Chile. I did not +meet in this valley with any true andesite, but only with imperfect andesitic +porphyry, including large crystals of hornblende: numerous as have been the +varieties of intrusive porphyries already mentioned, there were here mountains +composed of a new kind, having a compact, smooth, cream-coloured basis, +including only a few crystals of feldspar, and mottled with dendritic spots of +oxide of iron. There were also some mountains of a porphyry with a brick-red +basis, containing irregular, often lens-shaped, patches of compact feldspar, +and crystals of feldspar, which latter to my surprise I find to be orthite. +</p> + +<p> +At the foot of the first ridge of the main Cordillera, in the ravine of +Maricongo, and at an elevation which, from the extreme coldness and appearance +of the vegetation, I estimated at about ten thousand feet, I found beds of +white sandstone and of limestone including the Pecten Dufreynoyi, Terebratula +aenigma, and some Gryphites. This ridge throws the water on the one hand into +the Pacific, and on the other, as I was informed, into a great gravel-covered, +basin-like plain, including a salt- lake, and without any drainage-exit. In +crossing the Cordillera by this Pass, it is said that three principal ridges +must be traversed, instead of two, or only one as in Central Chile. +</p> + +<p> +The crest of this first main ridge and the surrounding mountains, with the +exception of a few lofty pinnacles, are capped by a great thickness of a +horizontally stratified, tufaceous deposit. The lowest bed is of a pale purple +colour, hard, fine-grained, and full of broken crystals of feldspar and scales +of mica. The middle bed is coarser, and less hard, and hence weathers into very +sharp pinnacles; it includes very small fragments of granite, and innumerable +ones of all sizes of grey vesicular trachyte, some of which were distinctly +rounded. The uppermost bed is about two hundred feet in thickness, of a darker +colour and apparently hard: but I had not time to ascend to it. These three +horizontal beds may be seen for the distance of many leagues, especially +westward or in the direction of the Pacific, capping the summits of the +mountains, and standing on the opposite sides of the immense valleys at exactly +corresponding heights. If united they would form a plain, inclined very +slightly towards the Pacific; the beds become thinner in this direction, and +the tuff (judging from one point to which I ascended, some way down the valley) +finer-grained and of less specific gravity, though still compact and sonorous +under the hammer. The gently inclined, almost horizontal stratification, the +presence of some rounded pebbles, and the compactness of the lowest bed, though +rendering it probable, would not have convinced me that this mass had been of +subaqueous origin, for it is known that volcanic ashes falling on land and +moistened by rain often become hard and stratified; but beds thus originating, +and owing their consolidation to atmospheric moisture, would have covered +almost equally every neighbouring summit, high and low, and would not have left +those above a certain exact level absolutely bare; this circumstance seems to +me to prove that the volcanic ejections were arrested at their present, widely +extended, equable level, and there consolidated by some other means than simple +atmospheric moisture; and this no doubt must have been a sheet of water. A lake +at this great height, and without a barrier on any one side, is out of the +question; consequently we must conclude that the tufaceous matter was anciently +deposited beneath the sea. It was certainly deposited before the excavation of +the valleys, or at least before their final enlargement (I have endeavoured to +show in my “Journal” etc. (2nd edition) page 355, that this arid +valley was left by the retreating sea, as the land slowly rose, in the state in +which we now see it.); and I may add, that Mr. Lambert, a gentleman well +acquainted with this country, informs me, that in ascending the ravine of +Santandres (which branches off from the Despoblado) he met with streams of lava +and much erupted matter capping all the hills of granite and porphyry, with the +exception of some projecting points; he also remarked that the valleys had been +excavated subsequently to these eruptions. +</p> + +<p> +This volcanic formation, which I am informed by Mr. Lambert extends far +northward, is of interest, as typifying what has taken place on a grander scale +on the corresponding western side of the Cordillera of Peru. Under another +point of view, however, it possesses a far higher interest, as confirming that +conclusion drawn from the structure of the fringes of stratified shingle which +are prolonged from the plains at the foot of the Cordillera far up the +valleys,—namely, that this great range has been elevated in mass to a +height of between eight and nine thousand feet (I may here mention that on the +south side of the main valley of Copiapo, near Potrero Seco, the mountains are +capped by a thick mass of horizontally stratified shingle, at a height which I +estimated at between fifteen hundred and two thousand feet above the bed of the +valley. This shingle, I believe, forms the edge of a wide plain, which +stretches southwards between two mountain ranges.); and now, judging from this +tufaceous deposit, we may conclude that the horizontal elevation has been in +the district of Copiapo about ten thousand feet. +</p> + +<h3>(FIGURE 24.)</h3> + +<p> +In the valley of the Despoblado, the stratification, as before remarked has +been much disturbed, and in some points to a greater degree than I have +anywhere else seen. I will give two cases: a very thick mass of thinly +stratified red sandstone, including beds of conglomerate, has been crushed +together (as represented in Figure 24) into a yoke or urn-formed trough, so +that the strata on both sides have been folded inwards: on the right hand the +properly underlying porphyritic claystone conglomerate is seen overlying the +sandstone, but it soon becomes vertical, and then is inclined towards the +trough, so that the beds radiate like the spokes of a wheel: on the left hand, +the inverted porphyritic conglomerate also assumes a dip towards the trough, +not gradually, as on the right hand, but by means of a vertical fault and +synclinal break; and a little still further on towards the left, there is a +second great oblique fault (both shown by the arrow- lines), with the strata +dipping to a directly opposite point; these mountains are intersected by +infinitely numerous dikes, some of which can be seen to rise from hummocks of +greenstone, and can be traced for thousands of feet. In the second case, two +low ridges trend together and unite at the head of a little wedge-shaped +valley: throughout the right- hand ridge, the strata dip at 45 degrees to the +east; in the left-hand ridge, we have the very same strata and at first with +exactly the same dip; but in following this ridge up the valley, the strata are +seen very regularly to become more and more inclined until they stand vertical, +they then gradually fall over (the basset edges forming symmetrical serpentine +lines along the crest), till at the very head of the valley they are reversed +at an angle of 45 degrees: so that at this point the beds have been turned +through an angle of 135 degrees; and here there is a kind of anticlinal axis, +with the strata on both sides dipping to opposite points at an angle of 45 +degrees, but those on the left hand upside down. +</p> + +<h3>ON THE ERUPTIVE SOURCES OF THE PORPHYRITIC CLAYSTONE AND GREENSTONE LAVAS. +</h3> + +<p> +In Central Chile, from the extreme metamorphic action, it is in most parts +difficult to distinguish between the streams of porphyritic lava and the +porphyritic breccia-conglomerate, but here, at Copiapo, they are generally +perfectly distinct, and in the Despoblado, I saw for the first time, two great +strata of purple claystone porphyry, after having been for a considerable space +closely united together, one above the other, become separated by a mass of +fragmentary matter, and then both thin out;—the lower one more rapidly +than the upper and greater stream. Considering the number and thickness of the +streams of porphyritic lava, and the great thickness of the beds of +breccia-conglomerate, there can be little doubt that the sources of eruption +must originally have been numerous: nevertheless, it is now most difficult even +to conjecture the precise point of any one of the ancient submarine craters. I +have repeatedly observed mountains of porphyries, more or less distinctly +stratified towards their summits or on their flanks, without a trace of +stratification in their central and basal parts: in most cases, I believe this +is simply due either to the obliterating effects of metamorphic action, or to +such parts having been mainly formed of intrusive porphyries, or to both causes +conjoined; in some instances, however, it appeared to me very probable that the +great central unstratified masses of porphyry were the now partially denuded +nuclei of the old submarine volcanoes, and that the stratified parts marked the +points whence the streams flowed. In one case alone, and it was in this Valley +of the Despoblado, I was able actually to trace a thick stratum of purplish +porphyry, which for a space of some miles conformably overlay the usual +alternating beds of breccia-conglomerates and claystone lavas, until it became +united with, and blended into, a mountainous mass of various unstratified +porphyries. +</p> + +<p> +The difficulty of tracing the streams of porphyries to their ancient and +doubtless numerous eruptive sources, may be partly explained by the very +general disturbance which the Cordillera in most parts has suffered; but I +strongly suspect that there is a more specific cause, namely, THAT THE ORIGINAL +POINTS OF ERUPTION TEND TO BECOME THE POINTS OF INJECTION. This in itself does +not seem improbable; for where the earth’s crust has once yielded, it +would be liable to yield again, though the liquified intrusive matter might not +be any longer enabled to reach the submarine surface and flow as lava. I have +been led to this conclusion, from having so frequently observed that, where +part of an unstratified mountain-mass resembled in mineralogical character the +adjoining streams or strata, there were several other kinds of intrusive +porphyries and andesitic rocks injected into the same point. As these intrusive +mountain-masses form most of the axes-lines in the Cordillera, whether +anticlinal, uniclinal, or synclinal, and as the main valleys have generally +been hollowed out along these lines, the intrusive masses have generally +suffered much denudation. Hence they are apt to stand in some degree isolated, +and to be situated at the points where the valleys abruptly bend, or where the +main tributaries enter. On this view of there being a tendency in the old +points of eruption to become the points of subsequent injection and +disturbance, and consequently of denudation, it ceases to be surprising that +the streams of lava in the porphyritic claystone conglomerate formation, and in +other analogous cases, should most rarely be traceable to their actual sources. +</p> + +<h3>IQUIQUE, SOUTHERN PERU.</h3> + +<p> +Differently from what we have seen throughout Chile, the coast here is formed +not by the granitic series, but by an escarpment of the porphyritic +conglomerate formation, between two and three thousand feet in height. (The +lowest point, where the road crosses the coast-escarpment, is 1,900 feet by the +barometer above the level of the sea.) I had time only for a very short +examination; the chief part of the escarpment appears to be composed of various +reddish and purple, sometimes laminated, porphyries, resembling those of Chile; +and I saw some of the porphyritic breccia-conglomerate; the stratification +appeared but little inclined. The uppermost part, judging from the rocks near +the famous silver mine of Huantajaya, consists of laminated, impure, +argillaceous, purplish-grey limestone, associated, I believe, with some purple +sandstone. (Mr. Bollaert has described “Geological Proceedings” +volume 2 page 598, a singular mass of stratified detritus, gravel, and sand, +eighty-one yards in thickness, overlying the limestone, and abounding with +loose masses of silver ore. The miners believe that they can attribute these +masses to their proper veins.) In the limestone shells are found: the three +following species were given me:— +</p> + +<p> +Lucina Americana, E. Forbes. Terebratula inca, E. Forbes. Terebratula aenigma, +D’Orbigny. +</p> + +<p> +This latter species we have seen associated with the fossils of which lists +have been given in this chapter, in two places in the valley of Coquimbo, and +in the ravine of Maricongo at Copiapo. Considering this fact, and the +superposition of these beds on the porphyritic conglomerate formation; and, as +we shall immediately see, from their containing much gypsum, and from their +otherwise close general resemblance in mineralogical nature with the strata +described in the valley of Copiapo, I have little doubt that these +fossiliferous beds of Iquique belong to the great cretaceo-oolitic formation of +Northern Chile. Iquique is situated seven degrees latitude north of Copiapo; +and I may here mention, that an Ammonites, nov. species, and an Astarte, nov. +species, were given me from the Cerro Pasco, about ten degrees of latitude +north of Iquique, and M. D’Orbigny thinks that they probably indicate a +Neocomian formation. Again, fifteen degrees of latitude northward, in Colombia, +there is a grand fossiliferous deposit, now well known from the labours of Von +Buch, Lea, d’Orbigny, and Forbes, which belongs to the earlier stages of +the cretaceous system. Hence, bearing in mind the character of the few fossils +from Tierra del Fuego, there is some evidence that a great portion of the +stratified deposits of the whole vast range of the South American Cordillera +belongs to about the same geological epoch. +</p> + +<p> +Proceeding from the coast escarpment inwards, I crossed, in a space of about +thirty miles, an elevated undulatory district, with the beds dipping in various +directions. The rocks are of many kinds,—white laminated, sometimes +siliceous sandstone,—purple and red sandstone, sometimes so highly +calcareous as to have a crystalline fracture,—argillaceous +limestone,—black calcareous slate-rock, like that so often described at +Copiapo and other places,—thinly laminated, fine-grained, greenish, +indurated, sedimentary, fusible rocks, approaching in character to the so- +called pseudo-honestone of Chile, including thin contemporaneous veins of +gypsum,—and lastly, much calcareous, laminated porcelain jasper, of a +green colour, with red spots, and of extremely easy fusibility: I noticed one +conformable stratum of a freckled-brown, feldspathic lava. I may here mention +that I heard of great beds of gypsum in the Cordillera. The only novel point in +this formation, is the presence of innumerable thin layers of rock-salt, +alternating with the laminated and hard, but sometimes earthy, yellowish, or +bright red and ferruginous sandstones. The thickest layer of salt was only two +inches, and it thinned out at both ends. On one of these saliferous masses I +noticed a stratum about twelve feet thick, of dark-brown, hard brecciated, +easily fusible rock, containing grains of quartz and of black oxide of iron, +together with numerous imperfect fragments of shells. The problem of the origin +of salt is so obscure, that every fact, even geographical position, is worth +recording. (It is well known that stratified salt is found in several places on +the shores of Peru. The island of San Lorenzo, off Lima, is composed of a pile +of thin strata, about eight hundred feet in thickness, composed of yellowish +and purplish, hard siliceous, or earthy sandstones, alternating with thin +layers of shale, which in places passes into a greenish, semi-porcellanic, +fusible rock. There are some thin beds of reddish mudstone, and soft +ferruginous rotten-stones, with layers of gypsum. In nearly all these +varieties, especially in the softer sandstones, there are numerous thin seams +of rock-salt: I was informed that one layer has been found two inches in +thickness. The manner in which the minutest fissures of the dislocated beds +have been penetrated by the salt, apparently by subsequent infiltration, is +very curious. On the south side of the island, layers of coal and of impure +limestone have been discovered. Hence we here have salt, gypsum, and coal +associated together. The strata include veins of quartz, carbonate of lime, and +iron pyrites; they have been dislocated by an injected mass of greenish-brown +feldspathic trap. Not only is salt abundant on the extreme western limits of +the district between the Cordillera and the Pacific, but, according to Helms, +it is found in the outlying low hills on the eastern flank of the Cordillera. +These facts appear to me opposed to the theory, that rock-salt is due to the +sinking of water, charged with salt, in mediterranean spaces of the ocean. The +general character of the geology of these countries would rather lead to the +opinion, that its origin is in some way connected with volcanic heat at the +bottom of the sea: see on this subject Sir R. Murchison “Anniversary +Address to the Geological Society” 1843 page 65.) With the exception of +these saliferous beds, most of the rocks as already remarked, present a +striking general resemblance with the upper parts of the gypseous or +cretaceo-oolitic formation of Chile. +</p> + +<h3>METALLIFEROUS VEINS.</h3> + +<p> +I have only a few remarks to make on this subject: in nine mining districts, +some of them of considerable extent, which I visited in CENTRAL Chile, I found +the PRINCIPAL veins running from between [N. and N.W.] to [S. and S.E.] (These +mining districts are Yaquil near Nancagua, where the direction of the chief +veins, to which only in all cases I refer, is north and south; in the Uspallata +range, the prevailing line is N.N.W. and S.S.E.; in the C. de Prado, it is +N.N.W. and S.S.E.; near Illapel, it is N. by W. and S. by E.; at Los Hornos the +direction varies from between [N. and N.W.] to [S. and S.E.]; at the C. de los +Hornos (further northward), it is N.N.W. and S.S.E.; at Panuncillo, it is +N.N.W. and S.S.E.; and, lastly, at Arqueros, the direction is N.W. and S.E.): +in some other places, however, their courses appeared quite irregular, as is +said to be generally the case in the whole valley of Copiapo: at Tambillos, +south of Coquimbo, I saw one large copper vein extending east and west. It is +worthy of notice, that the foliation of the gneiss and mica-slate, where such +rocks occur, certainly tend to run like the metalliferous veins, though often +irregularly, in a direction a little westward of north. At Yaquil, I observed +that the principal auriferous veins ran nearly parallel to the grain or +imperfect cleavage of the surrounding GRANITIC rocks. With respect to the +distribution of the different metals, copper, gold, and iron are generally +associated together, and are most frequently found (but with many exceptions, +as we shall presently see) in the rocks of the lower series, between the +Cordillera and the Pacific, namely, in granite, syenite, altered feldspathic +clay-slate, gneiss, and as near Guasco mica-schist. The copper-ores consist of +sulphurets, oxides, and carbonates, sometimes with laminae of native metal: I +was assured that in some cases (as at Panuncillo S.E. of Coquimbo), the upper +part of the same vein contains oxides, and the lower part sulphurets of copper. +(The same fact has been observed by Mr. Taylor in Cuba: “London +Philosophical Journal” volume 11 page 21.) Gold occurs in its native +form; it is believed that, in many cases, the upper part of the vein is the +most productive part: this fact probably is connected with the abundance of +this metal in the stratified detritus of Chile, which must have been chiefly +derived from the degradation of the upper portions of the rocks. These +superficial beds of well-rounded gravel and sand, containing gold, appeared to +me to have been formed under the sea close to the beach, during the slow +elevation of the land: Schmidtmeyer remarks that in Chile gold is sought for in +shelving banks at the height of some feet on the sides of the streams, and not +in their beds, as would have been the case had this metal been deposited by +common alluvial action. (“Travels in Chile” page 29.) Very +frequently the copper-ores, including some gold, are associated with abundant +micaceous specular iron. Gold is often found in iron-pyrites: at two gold mines +at Yaquil (near Nancagua), I was informed by the proprietor that in one the +gold was always associated with copper-pyrites, and in the other with +iron-pyrites: in this latter case, it is said that if the vein ceases to +contain iron-pyrites, it is yet worth while to continue the search, but if the +iron-pyrites, when it reappears, is not auriferous, it is better at once to +give up working the vein. Although I believe copper and gold are most +frequently found in the lower granitic and metamorphic schistose series, yet +these metals occur both in the porphyritic conglomerate formation (as on the +flanks of the Bell of Quillota and at Jajuel), and in the superincumbent +strata. At Jajuel I was informed that the copper-ore, with some gold, is found +only in the greenstones and altered feldspathic clay-slate, which alternate +with the purple porphyritic conglomerate. Several gold veins and some of +copper- ore are worked in several parts of the Uspallata range, both in the +metamorphosed strata, which have been shown to have been of probably subsequent +origin to the Neocomian or gypseous formation of the main Cordillera, and in +the intrusive andesitic rocks of that range. At Los Hornos (N.E. of Illapel), +likewise, there are numerous veins of copper- pyrites and of gold, both in the +strata of the gypseous formation and in the injected hills of andesite and +various porphyries. +</p> + +<p> +Silver, in the form of a chloride, sulphuret, or an amalgam, or in its native +state, and associated with lead and other metals, and at Arqueros with pure +native copper, occurs chiefly in the upper great gypseous or cretaceo-oolitic +formation which forms probably the richest mass in Chile. We may instance the +mining districts of Arqueros near Coquimbo, and of nearly the whole valley of +Copiapo, and of Iquique (where the principal veins run N.E. by E. and S.W. by +W.), in Peru. Hence comes Molina’s remark, that silver is born in the +cold and solitary deserts of the Upper Cordillera. There are, however, +exceptions to this rule: at Paral (S.E. of Coquimbo) silver is found in the +porphyritic conglomerate formation; as I suspect is likewise the case at S. +Pedro de Nolasko in the Peuquenes Pass. Rich argentiferous lead is found in the +clay-slate of the Uspallata range; and I saw an old silver-mine in a hill of +syenite at the foot of the Bell of Quillota: I was also assured that silver has +been found in the andesitic and porphyritic region between the town of Copiapo +and the Pacific. I have stated in a previous part of this chapter, that in two +neighbouring mines at Arqueros the veins in one were productive when they +traversed the singular green sedimentary beds, and unproductive when crossing +the reddish beds; whereas at the other mine exactly the reverse takes place; I +have also described the singular and rare case of numerous particles of native +silver and of the chloride being disseminated in the green rock at the distance +of a yard from the vein. Mercury occurs with silver both at Arqueros and at +Copiapo: at the base of C. de los Hornos (S.E. of Coquimbo, a different place +from Los Hornos, before mentioned) I saw in a syenitic rock numerous quartzose +veins, containing a little cinnabar in nests: there were here other parallel +veins of copper and of a ferrugino-auriferous ore. I believe tin has never been +found in Chile. +</p> + +<p> +From information given me by Mr. Nixon of Yaquil (At the Durazno mine, the gold +is associated with copper-pyrites, and the veins contain large prisms of +plumbago. Crystallised carbonate of lime is one of the commonest minerals in +the matrix of the Chilean veins.), and by others, it appears that in Chile +those veins are generally most permanently productive, which, consisting of +various minerals (sometimes differing but slightly from the surrounding rocks), +include parallel strings RICH in metals; such a vein is called a veta real. +More commonly the mines are worked only where one, two, or more thin veins or +strings running in a different direction, intersect a POOR “veta +real:” it is unanimously believed that at such points of intersection +(cruceros), the quantity of metal is much greater than that contained in other +parts of the intersecting veins. In some cruceros or points of intersection, +the metals extend even beyond the walls of the main, broad, stony vein. It is +said that the greater the angle of intersection, the greater the produce; and +that nearly parallel strings attract each other; in the Uspallata range, I +observed that numerous thin auri-ferruginous veins repeatedly ran into knots, +and then branched out again. I have already described the remarkable manner in +which rocks of the Uspallata range are indurated and blackened (as if by a +blast of gunpowder) to a considerable distance from the metallic veins. +</p> + +<p> +Finally, I may observe, that the presence of metallic veins seems obviously +connected with the presence of intrusive rocks, and with the degree of +metamorphic action which the different districts of Chile have undergone. (Sir +R. Murchison and his fellow travellers have given some striking facts on this +subject in their account of the Ural Mountains (“Geological +Proceedings” volume 3 page 748.) Such metamorphosed areas are generally +accompanied by numerous dikes and injected masses of andesite and various +porphyries: I have in several places traced the metalliferous veins from the +intrusive masses into the encasing strata. Knowing that the porphyritic +conglomerate formation consists of alternate streams of submarine lavas and of +the debris of anciently erupted rocks, and that the strata of the upper +gypseous formation sometimes include submarine lavas, and are composed of +tuffs, mudstones, and mineral substances, probably due to volcanic +exhalations,—the richness of these strata is highly remarkable when +compared with the erupted beds, often of submarine origin, but NOT +METAMORPHOSED, which compose the numerous islands in the Pacific, Indian, and +Atlantic Oceans; for in these islands metals are entirely absent, and their +nature even unknown to the aborigines. +</p> + +<p> +A SUMMARY OF THE GEOLOGICAL HISTORY OF THE CHILEAN CORDILLERA, AND OF THE +SOUTHERN PARTS OF SOUTH AMERICA. +</p> + +<p> +We have seen that the shores of the Pacific, for a space of 1,200 miles from +Tres Montes to Copiapo, and I believe for a very much greater distance, are +composed, with the exception of the tertiary basins, of metamorphic schists, +plutonic rocks, and more or less altered clay-slate. On the floor of the ocean +thus constituted, vast streams of various purplish claystone and greenstone +porphyries were poured forth, together with great alternating piles of angular +and rounded fragments of similar rocks ejected from the submarine craters. From +the compactness of the streams and fragments, it is probable that, with the +exception of some districts in Northern Chile, the eruptions took place in +profoundly deep water. The orifices of eruption appear to have been studded +over a breadth, with some outliers, of from fifty to one hundred miles: and +closely enough together, both north and south, and east and west, for the +ejected matter to form a continuous mass, which in Central Chile is more than a +mile in thickness. I traced this mould-like mass, for only 450 miles; but +judging from what I saw at Iquique, from specimens, and from published +accounts, it appears to have a manifold greater length. In the basal parts of +the series, and especially towards the flanks of the range, mud, since +converted into a feldspathic slaty rock, and sometimes into greenstone, was +occasionally deposited between the beds of erupted matter: with this exception +the uniformity of the porphyritic rocks is very remarkable. +</p> + +<p> +At the period when the claystone and greenstone porphyries nearly or quite +ceased being erupted, that great pile of strata which, from often abounding +with gypsum, I have generally called the gypseous formation was deposited, and +feldspathic lavas, together with other singular volcanic rocks, were +occasionally poured forth: I am far from pretending that any distinct line of +demarcation can be drawn between this formation and the underlying porphyries +and porphyritic conglomerate, but in a mass of such great thickness, and +between beds of such widely different mineralogical nature, some division was +necessary. At about the commencement of the gypseous period, the bottom of the +sea here seems first to have been peopled by shells, not many in kind, but +abounding in individuals. At the P. del Inca the fossils are embedded near the +base of the formation; in the Peuquenes range, at different levels, halfway up, +and even higher in the series; hence, in these sections, the whole pile of +strata belongs to the same period: the same remark is applicable to the beds at +Copiapo, which attain a thickness of between seven and eight thousand feet. The +fossil shells in the Cordillera of Central Chile, in the opinion of all the +palaeontologists who have examined them, belong to the earlier stages of the +cretaceous system; whilst in Northern Chile there is a most singular mixture of +cretaceous and oolitic forms: from the geological relations, however, of these +two districts, I cannot but think that they all belong to nearly the same +epoch, which I have provisionally called cretaceo-oolitic. +</p> + +<p> +The strata in this formation, composed of black calcareous shaly-rocks of red +and white, and sometimes siliceous sandstone, of coarse conglomerates, +limestones, tuffs, dark mudstones, and those singular fine-grained rocks which +I have called pseudo-honestones, vast beds of gypsum, and many other jaspery +and scarcely describable varieties, vary and replace each other in short +horizontal distances, to an extent, I believe, unequalled even in any tertiary +basin. Most of these substances are easily fusible, and have apparently been +derived either from volcanoes still in quiet action, or from the attrition of +volcanic products. If we picture to ourselves the bottom of the sea, rendered +uneven in an extreme degree, with numerous craters, some few occasionally in +eruption, but the greater number in the state of solfataras, discharging +calcareous, siliceous, ferruginous matters, and gypsum or sulphuric acid to an +amount surpassing, perhaps, even the existing sulphureous volcanoes of Java +(Von Buch’s “Description Physique des Iles Canaries” page +428.), we shall probably understand the circumstances under which this singular +pile of varying strata was accumulated. The shells appear to have lived at the +quiescent periods when only limestone or calcareo-argillaceous matter was +depositing. From Dr. Gillies’ account, this gypseous or cretaceo-oolitic +formation extends as far south as the Pass of Planchon, and I followed it +northward at intervals for 500 miles: judging from the character of the beds +with the Terebratula aenigma, at Iquique, it extends from four to five hundred +miles further: and perhaps even for ten degrees of latitude north of Iquique to +the Cerro Pasco, not far from Lima: again, we know that a cretaceous formation, +abounding with fossils, is largely developed north of the equator, in Colombia: +in Tierra del Fuego, at about this same period, a wide district of clay-slate +was deposited, which in its mineralogical characters and external features, +might be compared to the Silurian regions of North Wales. The gypseous +formation, like that of the porphyritic breccia- conglomerate on which it +rests, is of inconsiderable breadth; though of greater breadth in Northern than +in Central Chile. +</p> + +<p> +As the fossil shells in this formation are covered, in the Peuquenes ridge, by +a great thickness of strata; at the Puente del Inca, by at least five thousand +feet; at Coquimbo, though the superposition there is less plainly seen, by +about six thousand feet; and at Copiapo, certainly by five or six thousand, and +probably by seven thousand feet (the same species there recurring in the upper +and lower parts of the series), we may feel confident that the bottom of the +sea subsided during this cretaceo-oolitic period, so as to allow of the +accumulation of the superincumbent submarine strata. This conclusion is +confirmed by, or perhaps rather explains, the presence of the many beds at many +levels of coarse conglomerate, the well- rounded pebbles in which we cannot +believe were transported in very deep water. Even the underlying porphyries at +Copiapo. with their highly amygdaloidal surfaces, do not appear to have flowed +under great pressure. The great sinking movement thus plainly indicated, must +have extended in a north and south line for at least four hundred miles, and +probably was co- extensive with the gypseous formation. +</p> + +<p> +The beds of conglomerate just referred to, and the extraordinarily numerous +silicified trunks of fir-trees at Los Hornos, perhaps at Coquimbo and at two +distant points in the valley of Copiapo, indicate that land existed at this +period in the neighbourhood. This land, or islands, in the northern part of the +district of Copiapo, must have been almost exclusively composed, judging from +the nature of the pebbles of granite: in the southern parts of Copiapo, it must +have been mainly formed of claystone porphyries, with some mica-schist, and +with much sandstone and jaspery rocks exactly like the rocks in the gypseous +formation, and no doubt belonging to its basal series. In several other places +also, during the accumulation of the gypseous formation, its basal parts and +the underlying porphyritic conglomerate must likewise have been already +partially upheaved and exposed to wear and tear; near the Puente del Inca and +at Coquimbo, there must have existed masses of mica-schist or some such rock, +whence were derived the many small pebbles of opaque quartz. It follows from +these facts, that in some parts of the Cordillera the upper beds of the +gypseous formation must lie unconformably on the lower beds; and the whole +gypseous formation, in parts, unconformably on the porphyritic conglomerate; +although I saw no such cases, yet in many places the gypseous formation is +entirely absent; and this, although no doubt generally caused by quite +subsequent denudation, may in others be due to the underlying porphyritic +conglomerate having been locally upheaved before the deposition of the gypseous +strata, and thus having become the source of the pebbles of porphyry embedded +in them. In the porphyritic conglomerate formation, in its lower and middle +parts, there is very rarely any evidence, with the exception of the small +quartz pebbles at Jajuel near Aconcagua, and of the single pebble of granite at +Copiapo, of the existence of neighbouring land: in the upper parts, however, +and especially in the district of Copiapo, the number of thoroughly +well-rounded pebbles of compact porphyries make me believe, that, as during the +prolonged accumulation of the gypseous formation the lower beds had already +been locally upheaved and exposed to wear and tear, so it was with the +porphyritic conglomerate. Hence in following thus far the geological history of +the Cordillera, it may be inferred that the bed of a deep and open, or nearly +open, ocean was filled up by porphyritic eruptions, aided probably by some +general and some local elevations, to that comparatively shallow level at which +the cretaceo- oolitic shells first lived. At this period, the submarine craters +yielded at intervals a prodigious supply of gypsum and other mineral +exhalations, and occasionally, in certain places poured forth lavas, chiefly of +a feldspathic nature: at this period, islands clothed with fir-trees and +composed of porphyries, primary rocks, and the lower gypseous strata had +already been locally upheaved, and exposed to the action of the +waves;—the general movement, however, at this time having been over a +very wide area, one of slow subsidence, prolonged till the bed of the sea sank +several thousand feet. +</p> + +<p> +In Central Chile, after the deposition of a great thickness of the gypseous +strata, and after their upheaval, by which the Cumbre and adjoining ranges were +formed, a vast pile of tufaceous matter and submarine lava was accumulated, +where the Uspallata chain now stands; also after the deposition and upheaval of +the equivalent gypseous strata of the Peuquenes range, the great thick mass of +conglomerate in the valley of Tenuyan was accumulated: during the deposition of +the Uspallata strata, we know absolutely, from the buried vertical trees, that +there was a subsidence of some thousand feet; and we may infer from the nature +of the conglomerate in the valley of Tenuyan, that a similar and perhaps +contemporaneous movement there took place. We have, then, evidence of a second +great period of subsidence; and, as in the case of the subsidence which +accompanied the accumulation of the cretaceo-oolitic strata, so this latter +subsidence appears to have been complicated by alternate or local elevatory +movement— for the vertical trees, buried in the midst of the Uspallata +strata, must have grown on dry land, formed by the upheaval of the lower +submarine beds. Presently I shall have to recapitulate the facts, showing that +at a still later period, namely, at nearly the commencement of the old tertiary +deposits of Patagonia and of Chile, the continent stood at nearly its present +level, and then, for the third time, slowly subsided to the amount of several +hundred feet, and was afterwards slowly re-uplifted to its present level. +</p> + +<p> +The highest peaks of the Cordillera appear to consist of active or more +commonly dormant volcanoes,—such as Tupungato, Maypu, and Aconcagua, +which latter stands 23,000 feet above the level of the sea, and many others. +The next highest peaks are formed of the gypseous and porphyritic strata, +thrown into vertical or highly inclined positions. Besides the elevation thus +gained by angular displacements, I infer, without any hesitation—from the +stratified gravel-fringes which gently slope up the valleys of the Cordillera +from the gravel-capped plains at their base, which latter are connected with +the plains, still covered with recent shells on the coast— that this +great range has been upheaved in mass by a slow movement, to an amount of at +least 8,000 feet. In the Despoblado Valley, north of Copiapo, the horizontal +elevation, judging from the compact, stratified tufaceous deposit, capping the +distant mountains at corresponding heights, was about ten thousand feet. It is +very possible, or rather probable, that this elevation in mass may not have +been strictly horizontal, but more energetic under the Cordillera, than towards +the coast on either side; nevertheless, movements of this kind may be +conveniently distinguished from those by which strata have been abruptly broken +and upturned. When viewing the Cordillera, before having read Mr. +Hopkins’s profound “Researches on Physical Geology,” the +conviction was impressed on me, that the angular dislocations, however violent, +were quite subordinate in importance to the great upward movement in mass, and +that they had been caused by the edges of the wide fissures, which necessarily +resulted from the tension of the elevated area, having yielded to the inward +rush of fluidified rock, and having thus been upturned. +</p> + +<p> +The ridges formed by the angularly upheaved strata are seldom of great length: +in the central parts of the Cordillera they are generally parallel to each +other, and run in north and south lines; but towards the flanks they often +extend more or less obliquely. The angular displacement has been much more +violent in the central than in the exterior MAIN lines; but it has likewise +been violent in some of the MINOR lines on the extreme flanks. The violence has +been very unequal on the same short lines; the crust having apparently tended +to yield on certain points along the lines of fissures. These points, I have +endeavoured to show, were probably first foci of eruption, and afterwards of +injected masses of porphyry and andesite. (Sir R. Murchison and his companions +state “Geological Proceedings” volume 3 page 747, that no true +granite appears in the higher Ural Mountains; but that syenitic +greenstone—a rock closely analogous to our andesite—is far the most +abundant of the intrusive masses.) The close similarity of the andesitic +granites and porphyries, throughout Chile, Tierra del Fuego, and even in Peru, +is very remarkable. The prevalence of feldspar cleaving like albite, is common +not only to the andesites, but (as I infer from the high authority of Professor +G. Rose, as well as from my own measurements) to the various claystone and +greenstone porphyries, and to the trachytic lavas of the Cordillera. The +andesitic rocks have in most cases been the last injected ones, and they +probably form a continuous dome under this great range: they stand in intimate +relationship with the modern lavas; and they seem to have been the immediate +agent in metamorphosing the porphyritic conglomerate formation, and often +likewise the gypseous strata, to the extraordinary extent to which they have +suffered. +</p> + +<p> +With respect to the age at which the several parallel ridges composing the +Cordillera were upthrown, I have little evidence. Many of them may have been +contemporaneously elevated and injected in the same manner as in volcanic +archipelagoes lavas are contemporaneously ejected on the parallel lines of +fissure. (“Volcanic Islands” etc.) But the pebbles apparently +derived from the wear and tear of the porphyritic conglomerate formation, which +are occasionally present in the upper parts of this same formation, and are +often present in the gypseous formation, together with the pebbles from the +basal parts of the latter formation in its upper strata, render it almost +certain that portions, we may infer ridges, of these two formations were +successively upheaved. In the case of the gigantic Portillo range, we may feel +almost certain that a preexisting granitic line was upraised (not by a single +blow, as shown by the highly inclined basaltic streams in the valley on its +eastern flank) at a period long subsequent to the upheavement of the parallel +Peuquenes range. (I have endeavoured to show in my “Journal” 2nd +edition page 321, that the singular fact of the river, which drains the valley +between these two ranges, passing through the Portillo and higher line, is +explained by its slow and subsequent elevation. There are many analogous cases +in the drainage of rivers: see “Edinburgh New Philosophical +Journal” volume 28 pages 33 and 44.) Again, subsequently to the +upheavement of the Cumbre chain, that of Uspallata was formed and elevated; and +afterwards, I may add, in the plain of Uspallata, beds of sand and gravel were +violently upthrown. The manner in which the various kinds of porphyries and +andesites have been injected one into the other, and in which the infinitely +numerous dikes of various composition intersect each other, plainly show that +the stratified crust has been stretched and yielded many times over the same +points. With respect to the age of the axes of elevation between the Pacific +and the Cordillera, I know little: but there are some lines which +must—namely, those running north and south in Chiloe, those eight or nine +east and west, parallel, far-extended, most symmetrical uniclinal lines at P. +Rumena, and the short N.W.-S.E. and N.E.- S.W. lines at Concepcion—have +been upheaved long after the formation of the Cordillera. Even during the +earthquake of 1835, when the linear north and south islet of St. Mary was +uplifted several feet above the surrounding area, we perhaps see one feeble +step in the formation of a subordinate mountain-axis. In some cases, moreover, +for instance, near the baths of Cauquenes, I was forcibly struck with the small +size of the breaches cut through the exterior mountain-ranges, compared with +the size of the same valleys higher up where entering the Cordillera; and this +circumstance appeared to me scarcely explicable, except on the idea of the +exterior lines having been subsequently upthrown, and therefore having been +exposed to a less amount of denudation. From the manner in which the fringes of +gravel are prolonged in unbroken slopes up the valleys of the Cordillera, I +infer that most of the greater dislocations took place during the earlier parts +of the great elevation in mass: I have, however, elsewhere given a case, and M. +de Tschudi has given another, of a ridge thrown up in Peru across the bed of a +river, and consequently after the final elevation of the country above the +level of the sea. (“Reise in Peru” Band 2 s.8: Author’s +“Journal” 2nd edition page 359.) +</p> + +<p> +Ascending to the older tertiary formations, I will not again recapitulate the +remarks already given at the end of the Fifth Chapter,—on their great +extent, especially along the shores of the Atlantic—on their antiquity, +perhaps corresponding with that of the eocene deposits of Europe,—on the +almost entire dissimilarity, though the formations are apparently +contemporaneous, of the fossils from the eastern and western coasts, as is +likewise the case, even in a still more marked degree, with the shells now +living in these opposite though approximate seas,—on the climate of this +period not having been more tropical than what might have been expected from +the latitudes of the places under which the deposits occur; a circumstance +rendered well worthy of notice, from the contrast with what is known to have +been the case during the older tertiary periods of Europe, and likewise from +the fact of the southern hemisphere having suffered at a much later period, +apparently at the same time with the northern hemisphere, a colder or more +equable temperature, as shown by the zones formerly affected by ice-action. Nor +will I recapitulate the proofs of the bottom of the sea, both on the eastern +and western coast, having subsided seven or eight hundred feet during this +tertiary period; the movement having apparently been co-extensive, or nearly +co-extensive, with the deposits of this age. Nor will I again give the facts +and reasoning on which the proposition was founded, that when the bed of the +sea is either stationary or rising, circumstances are far less favourable than +when its level is sinking, to the accumulation of conchiferous deposits of +sufficient thickness, extension, and hardness to resist, when upheaved, the +ordinary vast amount of denudation. We have seen that the highly remarkable +fact of the absence of any EXTENSIVE formations containing recent shells, +either on the eastern or western coasts of the continent,—though these +coasts now abound with living mollusca,—though they are, and apparently +have always been, as favourable for the deposition of sediment as they were +when the tertiary formations were copiously deposited,—and though they +have been upheaved to an amount quite sufficient to bring up strata from the +depths the most fertile for animal life—can be explained in accordance +with the above proposition. As a deduction, it was also attempted to be shown, +first, that the want of close sequence in the fossils of successive formations, +and of successive stages in the same formation, would follow from the +improbability of the same area continuing slowly to subside from one whole +period to another, or even during a single entire period; and secondly, that +certain epochs having been favourable at distant points, in the same quarter of +the world for the synchronous accumulation of fossiliferous strata, would +follow from movements of subsidence having apparently, like those of elevation, +contemporaneously affected very large areas. +</p> + +<p> +There is another point which deserves some notice, namely, the analogy between +the upper parts of the Patagonian tertiary formation, as well as of the upper +possibly contemporaneous beds at Chiloe and Concepcion, with the great gypseous +formation of Cordillera; for in both formations, the rocks, in their fusible +nature, in their containing gypsum, and in many other characters, show a +connection, either intimate or remote, with volcanic action; and as the strata +in both were accumulated during subsidence, it appears at first natural to +connect this sinking movement with a state of high activity in the neighbouring +volcanoes. During the cretaceo-oolitic period this certainly appears to have +been the case at the Puente del Inca, judging from the number of intercalated +lava-streams in the lower 3,000 feet of strata; but generally, the volcanic +orifices seem at this time to have existed as submarine solfataras, and were +certainly quiescent compared with their state during the accumulation of the +porphyritic conglomerate formation. During the deposition of the tertiary +strata we know that at S. Cruz, deluges of basaltic lava were poured forth; but +as these lie in the upper part of the series, it is possible that the +subsidence may at that time have ceased: at Chiloe, I was unable to ascertain +to what part of the series the pile of lavas belonged. The Uspallata tuffs and +great streams of submarine lavas, were probably intermediate in age between the +cretaceo- oolitic and older tertiary formations, and we know from the buried +trees that there was a great subsidence during their accumulation; but even in +this case, the subsidence may not have been strictly contemporaneous with the +great volcanic eruptions, for we must believe in at least one intercalated +period of elevation, during which the ground was upraised on which the now +buried trees grew. I have been led to make these remarks, and to throw some +doubt on the strict contemporaneousness of high volcanic activity and movements +of subsidence, from the conviction impressed on my mind by the study of coral +formations, that these two actions do not generally go on +synchronously;—on the contrary, that in volcanic districts, subsidence +ceases as soon as the orifices burst forth into renewed action, and only +recommences when they again have become dormant. (“The Structure and +Distribution of Coral Reefs.”) +</p> + +<p> +At a later period, the Pampean mud, of estuary origin, was deposited over a +wide area,—in one district conformably on the underlying old tertiary +strata, and in another district unconformably on them, after their upheaval and +denudation. During and before the accumulation, however, of these old tertiary +strata, and, therefore, at a very remote period, sediment, strikingly +resembling that of the Pampas, was deposited; showing during how long a time in +this case the same agencies were at work in the same area. The deposition of +the Pampean estuary mud was accompanied, at least in the southern parts of the +Pampas, by an elevatory movement, so that the M. Hermoso beds probably were +accumulated after the upheaval of those round the S. Ventana; and those at P. +Alta after the upheaval of the M. Hermoso strata; but there is some reason to +suspect that one period of subsidence intervened, during which mud was +deposited over the coarse sand of the Barrancas de S. Gregorio, and on the +higher parts of Banda Oriental. The mammiferous animals characteristic of this +formation, many of which differ as much from the present inhabitants of South +America, as do the eocene mammals of Europe from the present ones of that +quarter of the globe, certainly co-existed at B. Blanca with twenty species of +mollusca, one balanus, and two corals, all now living in the adjoining sea: +this is likewise the case in Patagonia with the Macrauchenia, which co-existed +with eight shells, still the commonest kinds on that coast. I will not repeat +what I have elsewhere said, on the place of habitation, food, wide range, and +extinction of the numerous gigantic mammifers, which at this late period +inhabited the two Americas. +</p> + +<p> +The nature and grouping of the shells embedded in the old tertiary formations +of Patagonia and Chile show us, that the continent at that period must have +stood only a few fathoms below its present level, and that afterwards it +subsided over a wide area, seven or eight hundred feet. The manner in which it +has since been rebrought up to its actual level, was described in detail in the +First and Second Chapters. It was there shown that recent shells are found on +the shores of the Atlantic, from Tierra del Fuego northward for a space of at +least 1,180 nautical miles, and at the height of about 100 feet in La Plata, +and of 400 feet in Patagonia. The elevatory movements on this side of the +continent have been slow; and the coast of Patagonia, up to the height in one +part of 950 feet and in another of 1,200 feet, is modelled into eight great, +step-like, gravel-capped plains, extending for hundreds of miles with the same +heights; this fact shows that the periods of denudation (which, judging from +the amount of matter removed, must have been long continued) and of elevation +were synchronous over surprisingly great lengths of coasts. On the shores of +the Pacific, upraised shells of recent species, generally, though not always, +in the same proportional numbers as in the adjoining sea, have actually been +found over a north and south space of 2,075 miles, and there is reason to +believe that they occur over a space of 2,480 miles. The elevation on this +western side of the continent has not been equable; at Valparaiso, within the +period during which upraised shells have remained undecayed on the surface, it +has been 1,300 feet, whilst at Coquimbo, 200 miles northward, it has been +within this same period only 252 feet. At Lima, the land has been uplifted at +least 80 feet since Indian man inhabited that district; but the level within +historical times apparently has subsided. At Coquimbo, in a height of 364 feet, +the elevation has been interrupted by five periods of comparative rest. At +several places the land has been lately, or still is, rising both insensibly +and by sudden starts of a few feet during earthquake-shocks; this shows that +these two kinds of upward movement are intimately connected together. For a +space of 775 miles, upraised recent shells are found on the two opposite sides +of the continent; and in the southern half of this space, it may be safely +inferred from the slope of the land up to the Cordillera, and from the shells +found in the central part of Tierra del Fuego, and high up the River Santa +Cruz, that the entire breadth of the continent has been uplifted. From the +general occurrence on both coasts of successive lines of escarpments, of +sand-dunes and marks of erosion, we must conclude that the elevatory movement +has been normally interrupted by periods, when the land either was stationary, +or when it rose at so slow a rate as not to resist the average denuding power +of the waves, or when it subsided. In the case of the present high sea-cliffs +of Patagonia and in other analogous instances, we have seen that the difficulty +in understanding how strata can be removed at those depths under the sea, at +which the currents and oscillations of the water are depositing a smooth +surface of mud, sand, and sifted pebbles, leads to the suspicion that the +formation or denudation of such cliffs has been accompanied by a sinking +movement. +</p> + +<p> +In South America, everything has taken place on a grand scale, and all +geological phenomena are still in active operation. We know how violent at the +present day the earthquakes are, we have seen how great an area is now rising, +and the plains of tertiary origin are of vast dimensions; an almost straight +line can be drawn from Tierra del Fuego for 1,600 miles northward, and probably +for a much greater distance, which shall intersect no formation older than the +Patagonian deposits; so equable has been the upheaval of the beds, that +throughout this long line, not a fault in the stratification or abrupt +dislocation was anywhere observable. Looking to the basal, metamorphic, and +plutonic rocks of the continent, the areas formed of them are likewise vast; +and their planes of cleavage and foliation strike over surprisingly great +spaces in uniform directions. The Cordillera, with its pinnacles here and there +rising upwards of twenty thousand feet above the level of the sea, ranges in an +unbroken line from Tierra del Fuego, apparently to the Arctic circle. This +grand range has suffered both the most violent dislocations, and slow, though +grand, upward and downward movements in mass; I know not whether the spectacle +of its immense valleys, with mountain-masses of once liquified and intrusive +rocks now bared and intersected, or whether the view of those plains, composed +of shingle and sediment hence derived, which stretch to the borders of the +Atlantic Ocean, is best adapted to excite our astonishment at the amount of +wear and tear which these mountains have undergone. +</p> + +<p> +The Cordillera from Tierra del Fuego to Mexico, is penetrated by volcanic +orifices, and those now in action are connected in great trains. The intimate +relation between their recent eruptions and the slow elevation of the continent +in mass, appears to me highly important, for no explanation of the one +phenomenon can be considered as satisfactory which is not applicable to the +other. (On the Connection of certain Volcanic Phenomena in South America: +“Geological Transactions” volume 5 page 609.) The permanence of the +volcanic action on this chain of mountains is, also, a striking fact; first, we +have the deluges of submarine lavas alternating with the porphyritic +conglomerate strata, then occasionally feldspathic streams and abundant mineral +exhalations during the gypseous or cretaceo- oolitic period: then the eruptions +of the Uspallata range, and at an ancient but unknown period, when the sea came +up to the eastern foot of the Cordillera, streams of basaltic lava at the foot +of the Portillo range; then the old tertiary eruptions; and lastly, there are +here and there amongst the mountains, much worn and apparently very ancient +volcanic formations without any craters; there are, also, craters quite +extinct, and others in the condition of solfataras, and others occasionally or +habitually in fierce action. Hence it would appear that the Cordillera has +been, probably with some quiescent periods, a source of volcanic matter from an +epoch anterior to our cretaceo-oolitic formation to the present day; and now +the earthquakes, daily recurrent on some part of the western coast, give little +hope that the subterranean energy is expended. +</p> + +<p> +Recurring to the evidence by which it was shown that some at least of the +parallel ridges, which together compose the Cordillera, were successively and +slowly upthrown at widely different periods; and that the whole range certainly +once, and almost certainly twice, subsided some thousand feet, and being then +brought up by a slow movement in mass, again, during the old tertiary +formations, subsided several hundred feet, and again was brought up to its +present level by a slow and often interrupted movement; we see how opposed is +this complicated history of changes slowly effected, to the views of those +geologists who believe that this great mountain-chain was formed in late times +by a single blow. I have endeavoured elsewhere to show, that the excessively +disturbed condition of the strata in the Cordillera, so far from indicating +single periods of extreme violence, presents insuperable difficulties, except +on the admission that the masses of once liquified rocks of the axes were +repeatedly injected with intervals sufficiently long for their successive +cooling and consolidation. (“Geological Transactions” volume 5 page +626.) Finally, if we look to the analogies drawn from the changes now in +progress in the earth’s crust, whether to the manner in which volcanic +matter is erupted, or to the manner in which the land is historically known to +have risen and sunk: or again, if we look to the vast amount of denudation +which every part of the Cordillera has obviously suffered, the changes through +which it has been brought into its present condition, will appear neither to +have been too slowly effected, nor to have been too complicated. +</p> + +<h3>NOTE.</h3> + +<p> +As, both in France and England, translations of a passage in Professor +Ehrenberg’s Memoir, often referred to in the Fourth Chapter of this +volume, have appeared, implying that Professor Ehrenberg believes, from the +character of the infusoria, that the Pampean formation was deposited by a +sea-debacle rushing over the land, I may state, on the authority of a letter to +me, that these translations are incorrect. The following is the passage in +question:— +</p> + +<p> +“Durch Beachtung der mikroscopischen Formen hat sich nun feststellen +lassen, das die Mastodonten-Lager am La Plata und die Knochen-Lager am Monte +Hermoso, who wie die der Riesen-Gurtelthiere in den Dunenhugeln bei Bahia +Blanca, beides in Patagonien, unveranderte brakische Susswasserbildungen sind, +die einst wohl sammtlich zum obersten Fluthgebiethe des Meeres im tieferen +Festlande gehorten.”—“Monatsberichten der konigl. Akad. +etc.” zu Berlin vom April 1845. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="link2HCH0009"></a> +INDEX.</h2> + +<p> +Abich, on a new variety of feldspar. +</p> + +<p> +Abrolhos islands. +</p> + +<p> +Absence of recent formations on the S. American coasts. +</p> + +<p> +Aguerros on elevation of Imperial. +</p> + +<p> +Albite, constituent mineral in andesite. —in rocks of Tierra del Fuego. +—in porphyries. —crystals of, with orthite. +</p> + +<p> +Alison, Mr., on elevation of Valparaiso. +</p> + +<p> +Alumina, sulphate of. +</p> + +<p> +Ammonites from Concepcion. +</p> + +<p> +Amolanas, Las. +</p> + +<p> +Amygdaloid, curious varieties of. +</p> + +<p> +Amygdaloids of the Uspallata range. —of Copiapo. +</p> + +<p> +Andesite of Chile. —in the valley of Maypu. —of the Cumbre pass. +—of the Uspallata range. —of Los Hornos. —of Copiapo. +</p> + +<p> +Anhydrite, concretions of. +</p> + +<p> +Araucaria, silicified wood of. Arica, elevation of. +</p> + +<p> +Arqueros, mines of. +</p> + +<p> +Ascension, gypsum deposited on. —laminated volcanic rocks of. +</p> + +<p> +Augite in fragments, in gneiss. —with albite, in lava. +</p> + +<p> +Austin, Mr. R.A.C., on bent cleavage lamina. +</p> + +<p> +Austin, Captain, on sea-bottom. +</p> + +<p> +Australia, foliated rocks of. +</p> + +<p> +Azara labiata, beds of, at San Pedro. +</p> + +<p> +Baculites vagina. +</p> + +<p> +Bahia Blanca, elevation of. —formations near. —character of living +shells of. +</p> + +<p> +Bahia (Brazil), elevation near. —crystalline rocks of. +</p> + +<p> +Ballard, M., on the precipitation of sulphate of soda. +</p> + +<p> +Banda Oriental, tertiary formations of. —crystalline rocks of. +</p> + +<p> +Barnacles above sea-level. —adhering to upraised shells. +</p> + +<p> +Basalt of S. Cruz. —streams of, in the Portillo range. —in the +Uspallata range. +</p> + +<p> +Basin chains of Chile. +</p> + +<p> +Beagle Channel. +</p> + +<p> +Beaumont, Elie de, on inclination of lava-streams. —on viscid +quartz-rocks. +</p> + +<p> +Beech-tree, leaves of fossil. +</p> + +<p> +Beechey, Captain, on sea-bottom. +</p> + +<p> +Belcher, Lieutenant, on elevated shells from Concepcion. +</p> + +<p> +Bella Vista, plain of. +</p> + +<p> +Benza, Dr., on decomposed granite. +</p> + +<p> +Bettington, Mr., on quadrupeds transported by rivers. +</p> + +<p> +Blake, Mr., on the decay of elevated shells near Iquique. —on nitrate of +soda. +</p> + +<p> +Bole. +</p> + +<p> +Bollaert, Mr., on mines of Iquique. +</p> + +<p> +Bones, silicified. —fossil, fresh condition of. +</p> + +<p> +Bottom of sea off Patagonia. +</p> + +<p> +Bougainville, on elevation of the Falkland islands. +</p> + +<p> +Boulder formation of S. Cruz. —of Falkland islands. —anterior to +certain extinct quadrupeds. —of Tierra del Fuego. +</p> + +<p> +Boulders in the Cordillera. —transported by earthquake-waves. —in +fine-grained tertiary deposits. +</p> + +<p> +Brande, Mr., on a mineral spring. +</p> + +<p> +Bravais, M., on elevation of Scandinavia. +</p> + +<p> +Brazil, elevation of. —crystalline rocks of. +</p> + +<p> +Broderip, Mr., on elevated shells from Concepcion. +</p> + +<p> +Brown, Mr. R., on silicified wood of Uspallata range. +</p> + +<p> +Brown, on silicified wood. +</p> + +<p> +Bucalema, elevated shells near. +</p> + +<p> +Buch, Von, on cleavage. —on cretaceous fossils of the Cordillera. +—on the sulphureous volcanoes of Java. +</p> + +<p> +Buenos Ayres. +</p> + +<p> +Burchell, Mr., on elevated shells of Brazil. +</p> + +<p> +Byron, on elevated shells. +</p> + +<p> +Cachapual, boulders in valley of. +</p> + +<p> +Caldcleugh, Mr., on elevation of Coquimbo. —on rocks of the Portillo +range. +</p> + +<p> +Callao, elevation near. —old town of. +</p> + +<p> +Cape of Good Hope, metamorphic rocks of. +</p> + +<p> +Carcharias megalodon. +</p> + +<p> +Carpenter, Dr., on microscopic organisms. +</p> + +<p> +Castro (Chiloe), beds near. +</p> + +<p> +Cauquenes Baths, boulders near. —pebbles in porphyry near. +—volcanic formation near. —stratification near. +</p> + +<p> +Caves above sea-level. +</p> + +<p> +Cervus pumilus, fossil-horns of. +</p> + +<p> +Chevalier, M., on elevation near Lima. +</p> + +<p> +Chile, structure of country between the Cordillera and the Pacific. +—tertiary formations of. —crystalline rocks in. —central, +geology of. —northern, geology of. +</p> + +<p> +Chiloe, gravel on coast. —elevation of. —tertiary formation of. +—crystalline rocks of. +</p> + +<p> +Chlorite-schist, near M. Video. +</p> + +<p> +Chonos archipelago, tertiary formations of. —crystalline rocks of. +</p> + +<p> +Chupat, Rio, scoriae transported by. +</p> + +<p> +Claro, Rio, fossiliferous beds of. +</p> + +<p> +Clay-shale of Los Hornos. +</p> + +<p> +Clay-slate, formation of, Tierra del Fuego. —of Concepcion. +—feldspathic, of Chile. — —of the Uspallata range. +—black siliceous, band of, in porphyritic formations of Chile. +</p> + +<p> +Claystone porphyry, formation of, in Chile. —origin of. —eruptive +sources of. +</p> + +<p> +Cleavage, definition of. —at Bahia. —Rio de Janeiro. +—Maldonado. —Monte Video. —S. Guitru-gueyu. —Falkland +I. —Tierra del Fuego. —Chonos I. —Chiloe. —Concepcion. +—Chile. —discussion on. +</p> + +<p> +Cleavage-laminae superficially bent. +</p> + +<p> +Cliffs, formation of. +</p> + +<p> +Climate, late changes in. —of Chile during tertiary period. +</p> + +<p> +Coal of Concepcion. —S. Lorenzo. +</p> + +<p> +Coast-denudation of St. Helena. +</p> + +<p> +Cobija, elevation of. +</p> + +<p> +Colombia, cretaceous formation of. +</p> + +<p> +Colonia del Sacramiento, elevation of. —Pampean formation near Colorado, +Rio, gravel of. —sand-dunes of. —Pampean formation near. +</p> + +<p> +Combarbala. +</p> + +<p> +Concepcion, elevation of. —deposits of. —crystalline rocks of. +</p> + +<p> +Conchalee, gravel-terraces of. +</p> + +<p> +Concretions of gypsum, at Iquique. —in sandstone at S. Cruz. —in +tufaceous tuff of Chiloe. —in gneiss. —in claystone-porphyry at +Port Desire. —in gneiss at Valparaiso. —in metamorphic rocks. +—of anhydrite. —relations of, to veins. +</p> + +<p> +Conglomerate claystone of Chile. —of Tenuyan. —of the Cumbre Pass. +—of Rio Claro. —of Copiapo. +</p> + +<p> +Cook, Captain, on form of sea-bottom. +</p> + +<p> +Copiapo, elevation of. —tertiary formations of. —secondary +formations of. +</p> + +<p> +Copper, sulphate of. —native, at Arqueros. —mines of, at +Panuncillo. —veins, distribution of. +</p> + +<p> +Coquimbo, elevation and terraces of. —tertiary formations of. +—secondary formations of. +</p> + +<p> +Corallines living on pebbles. +</p> + +<p> +Cordillera, valleys bordered by gravel fringes. —basal strata of. +—fossils of. —elevation of. —gypseous formations of. +—claystone-porphyries of. —andesitic rocks of. —volcanoes of. +</p> + +<p> +Coste, M., on elevation of Lemus. +</p> + +<p> +Coy inlet, tertiary formation of. +</p> + +<p> +Crassatella Lyellii. +</p> + +<p> +Cruickshanks, Mr., on elevation near Lima. +</p> + +<p> +Crystals of feldspar, gradual formation of, at Port Desire. +</p> + +<p> +Cumbre, Pass of, in Cordillera. +</p> + +<p> +Cuming, Mr., on habits of the Mesodesma. —on range of living shells on +west coast. +</p> + +<p> +Dana, Mr., on foliated rocks. —on amygdaloids. +</p> + +<p> +Darwin, Mount. +</p> + +<p> +D’Aubuisson, on concretions. —on foliated rocks. Decay, gradual, of +upraised shells. +</p> + +<p> +Decomposition of granite rocks. +</p> + +<p> +De la Beche, Sir H., his theoretical researches in geology. —on the +action of salt on calcareous rocks. —on bent cleavage-laminae. +</p> + +<p> +Denudation on coast of Patagonia. —great powers of. —of the +Portillo range. +</p> + +<p> +Deposits, saline. +</p> + +<p> +Despoblado, valley of. +</p> + +<p> +Detritus, nature of, in Cordillera. +</p> + +<p> +Devonshire, bent cleavage in. +</p> + +<p> +Dikes, in gneiss of Brazil. —near Rio de Janeiro. —pseudo, at Port +Desire. —in Tierra del Fuego. —in Chonos archipelago, containing +quartz. —near Concepcion, with quartz. —granitic-porphyritic, at +Valparaiso. —rarely vesicular in Cordillera. —absent in the central +ridges of the Portillo pass. —of the Portillo range, with grains of +quartz. —intersecting each other often. —numerous at Copiapo. +</p> + +<p> +Domeyko, M., on the silver mines of Coquimbo. on the fossils of Coquimbo. +</p> + +<p> +D’Orbigny, M. A., on upraised shells of Monte Video. —on elevated +shells at St. Pedro. —on elevated shells near B. Ayres. —on +elevation of S. Blas. —on the sudden elevation of La Plata. —on +elevated shells near Cobija. —on elevated shells near Arica. —on +the climate of Peru. —on salt deposits of Cobija. —on crystals of +gypsum in salt-lakes. —on absence of gypsum in the Pampean formation. +—on fossil remains from Bahia Blanca. —on fossil remains from the +banks of the Parana. —on the geology of St. Fe. —on the age of +Pampean formation. —on the Mastodon Andium. —on the geology of the +Rio Negro. —on the character of the Patagonian fossils. —on fossils +from Concepcion. — —from Coquimbo. — —from Payta. +—on fossil tertiary shells of Chile. —on cretaceous fossils of +Tierra del Fuego. — —from the Cordillera of Chile. +</p> + +<p> +Earth, marine origin of. +</p> + +<p> +Earthenware, fossil. +</p> + +<p> +Earthquake, effect of, at S. Maria. —elevation during, at Lemus. +—of 1822, at Valparaiso. —effects of, in shattering surface. +—fissures made by. —probable effects on cleavage. +</p> + +<p> +Earthquakes in Pampas. +</p> + +<p> +Earthquake-waves, power of, in throwing up shells. —effects of, near +Lima. —power of, in transporting boulders. +</p> + +<p> +Edmonston, Mr., on depths at which shells live at Valparaiso. +</p> + +<p> +Ehrenberg, Professor, on infusoria in the Pampean formation. —on +infusoria in the Patagonian formation. +</p> + +<p> +Elevation of La Plata. —Brazil. —Bahia Blanca. —San Blas. +—Patagonia. —Tierra del Fuego. —Falkland islands. +—Pampas. —Chonos archipelago. —Chiloe. —Chile. +—Valparaiso. —Coquimbo. —Guasco. —Iquique. +—Cobija. —Lima. —sudden, at S. Maria. — —at +Lemus. —insensible, at Chiloe. — —at Valparaiso. — +—at Coquimbo. —axes of, at Chiloe. — —at P. Rumena. +—at Concepcion. —unfavourable for the accumulation of permanent +deposits. —lines of, parallel to cleavage and foliation. —lines of, +oblique to foliation. —areas of, causing lines of elevation and cleavage. +—lines of, in the Cordillera. —slow, in the Portillo range. +—two periods of, in Cordillera of Central Chile. —of the Uspallata +range. —two periods of, in Cumbre Pass. —horizontal, in the +Cordillera of Copiapo. —axes of, coincident with volcanic orifices. +—of the Cordillera, summary on. +</p> + +<p> +Elliott, Captain, on human remains. +</p> + +<p> +Ensenada, elevated shells of. +</p> + +<p> +Entre Rios, geology of. +</p> + +<p> +Equus curvidens. +</p> + +<p> +Epidote in Tierra del Fuego. —in gneiss. —frequent in Chile. +—in the Uspallata range. —in porphyry of Coquimbo. +</p> + +<p> +Erman, M., on andesite. Escarpments, recent, of Patagonia. +</p> + +<p> +Extinction of fossil mammifers. +</p> + +<p> +Falkland islands, elevation of. —pebbles on coast. —geology of. +</p> + +<p> +Falkner, on saline incrustations. +</p> + +<p> +Faults, great, in Cordillera. +</p> + +<p> +Feldspar, earthy, metamorphosis of, at Port Desire. —albitic. +—crystals of, with albite. —orthitic, in conglomerate of Tenuyan. +—in granite of Portillo range. —in porphyries in the Cumbre Pass. +</p> + +<p> +Feuillee on sea-level at Coquimbo. +</p> + +<p> +Fissures, relations of, to concretions. —upfilled, at Port Desire. +—in clay-slate. +</p> + +<p> +Fitton, Dr., on the geology of Tierra del Fuego. +</p> + +<p> +Fitzroy, Captain, on the elevation of the Falkland islands. —on the +elevation of Concepcion. +</p> + +<p> +Foliation, definition of. —of rocks at Bahia. —Rio de Janeiro. +—Maldonado. —Monte Video. —S. Guitru-gueyu. —Falkland +I. —Tierra del Fuego. —Chonos archipelago. —Chiloe. +—Concepcion. —Chile. —discussion on. +</p> + +<p> +Forbes, Professor E., on cretaceous fossils of Concepcion. —on cretaceous +fossils and subsidence in Cumbre Pass. —on fossils from Guasco. — +—from Coquimbo. — —from Copiapo. —on depths at which +shells live. +</p> + +<p> +Formation, Pampean. — —area of. — —estuary origin. +—tertiary of Entre Rios. —of Banda Oriental. —volcanic, in +Banda Oriental. —of Patagonia. —summary on. —tertiary of +Tierra del Fuego. — —of the Chonos archipelago. — —of +Chiloe. — —of Chile. — —of Concepcion. — +—of Navidad. — —of Coquimbo. — —of Peru. — +—subsidence during. —volcanic, of Tres Montes. — —of +Chiloe. — —old, near Maldonado. — —with laminar +structure. — —ancient, in Tierra del Fuego. —recent, absent +on S. American coast. —metamorphic, of claystone-porphyry of Patagonia. +—foliation of. —plutonic, with laminar structure. +—palaeozoic, of the Falkland I. —claystone, at Concepcion. +—Jurassic, of Cordillera. —Neocomian, of the Portillo Pass. +—volcanic, of Cumbre Pass. —gypseous, of Los Hornos. — +—of Coquimbo. — —of Guasco. — —of Copiapo. +— —of Iquique. —cretaceo-oolitic, of Coquimbo. — +—of Guasco. — —of Copiapo. — —of Iquique. +</p> + +<p> +Fossils, Neocomian, of Portillo Pass. — —of Cumbre Pass. +—secondary, of Coquimbo. — —of Guasco. — —of +Copiapo. — —of Iquique. —palaeozoic, from the Falklands. +</p> + +<p> +Fragments of hornblende-rock in gneiss. —of gneiss in gneiss. +</p> + +<p> +Freyer, Lieutenant, on elevated shells of Arica. +</p> + +<p> +Frezier on sea-level at Coquimbo. +</p> + +<p> +Galapagos archipelago, pseudo-dikes of. +</p> + +<p> +Gallegos, Port, tertiary formation of. +</p> + +<p> +Garnets in gneiss. —in mica-slate. —at Panuncillo. +</p> + +<p> +Gardichaud, M., on granites of Brazil. +</p> + +<p> +Gay, M., on elevated shells. —on boulders in the Cordillera. —on +fossils from Cordillera of Coquimbo. +</p> + +<p> +Gill, Mr., on brickwork transported by an earthquake-wave. +</p> + +<p> +Gillies, Dr., on heights in the Cordillera. —on extension of the Portillo +range. +</p> + +<p> +Glen Roy, parallel roads of. —sloping terraces of. +</p> + +<p> +Gneiss, near Bahia. —of Rio de Janeiro. —decomposition of. +</p> + +<p> +Gold, distribution of. +</p> + +<p> +Gorodona, formations near. Granite, axis of oblique, to foliation. +—andesitic. —of Portillo range. —veins of, quartzose. +—pebble of, in porphyritic conglomerate. —conglomerate. +</p> + +<p> +Grauwacke of Uspallata range. +</p> + +<p> +Gravel at bottom of sea. —formation of, in Patagonia. —means of +transportation of. —strata of, inclined. +</p> + +<p> +Gravel-terraces in Cordillera. +</p> + +<p> +Greenough, Mr., on quartz veins. +</p> + +<p> +Greenstone, resulting from metamorphose hornblende-rock. —of Tierra del +Fuego. —on the summit of the Campana of Quillota. —porphyry. +—relation of, to clay-slate. +</p> + +<p> +Gryphaea orientalis. +</p> + +<p> +Guasco, elevation of. —secondary formation of. +</p> + +<p> +Guitru-gueyu, Sierra. +</p> + +<p> +Guyana, gneissic rocks of. +</p> + +<p> +Gypsum, nodules of, in gravel at Rio Negro. —deposited from sea-water. +—deposits of, at Iquique. —crystals of, in salt lakes. —in +Pampean formation. —in tertiary formation of Patagonia. —great +formation of, in the Portillo Pass. — —in the Cumbre Pass. — +—near Los Hornos. — —at Coquimbo. — —at Copiapo. +— —near Iquique. —of San Lorenzo. +</p> + +<p> +Hall, Captain, on terraces at Coquimbo. +</p> + +<p> +Hamilton, Mr., on elevation near Tacna. +</p> + +<p> +Harlan, Dr., on human remains. +</p> + +<p> +Hayes, Mr. A., on nitrate of soda. +</p> + +<p> +Henslow, Professor, on concretions. +</p> + +<p> +Herbert, Captain, on valleys in the Himalaya. +</p> + +<p> +Herradura Bay, elevated shells of. —tertiary formations of. +</p> + +<p> +Himalaya, valleys in. +</p> + +<p> +Hippurites Chilensis. +</p> + +<p> +Hitchcock, Professor, on dikes. +</p> + +<p> +Honestones, pseudo, of Coquimbo. —of Copiapo. +</p> + +<p> +Hooker, Dr. J.D., on fossil beech-leaves. +</p> + +<p> +Hopkins, Mr., on axes of elevation oblique to foliation. —on origin of +lines of elevation. +</p> + +<p> +Hornblende-rock, fragments of, in gneiss. +</p> + +<p> +Hornblende-schist, near M. Video. +</p> + +<p> +Hornos, Los, section near. +</p> + +<p> +Hornstone, dike of. +</p> + +<p> +Horse, fossil tooth of. +</p> + +<p> +Huafo island. —subsidence at. +</p> + +<p> +Huantajaya, mines of. +</p> + +<p> +Humboldt, on saline incrustations. —on foliations of gneiss. —on +concretions in gneiss. +</p> + +<p> +Icebergs, action on cleavage. +</p> + +<p> +Illapel, section near. +</p> + +<p> +Imperial, beds of shells near. +</p> + +<p> +Incrustations, saline. +</p> + +<p> +Infusoria in Pampean formation. —in Patagonian formation. +</p> + +<p> +Iodine, salts of. +</p> + +<p> +Iquique, elevation of. —saliferous deposits of. —cretaceo-oolitic +formation of. +</p> + +<p> +Iron, oxide of, in lavas. —in sedimentary beds. —tendency in, to +produce hollow concretions. —sulphate of. +</p> + +<p> +Isabelle, M., on volcanic rocks of Banda Oriental. +</p> + +<p> +Joints in clay-slate. +</p> + +<p> +Jukes, Mr., on cleavage in Newfoundland. +</p> + +<p> +Kamtschatka, andesite of. +</p> + +<p> +Kane, Dr., on the production of carbonate of soda. +</p> + +<p> +King George’s sound, calcareous beds of. +</p> + +<p> +Lakes, origin of. —fresh-water, near salt lakes. +</p> + +<p> +Lava, basaltic, of S. Cruz. —claystone-porphyry, at Chiloe. — +—ancient submarine. —basaltic, of the Portillo range. +—feldspathic, of the Cumbre Pass. —submarine, of the Uspallata +range. —basaltic, of the Uspallata range. —submarine, of Coquimbo. +—of Copiapo. +</p> + +<p> +Lemus island. +</p> + +<p> +Lemuy islet. +</p> + +<p> +Lignite of Chiloe. —of Concepcion. +</p> + +<p> +Lima, elevation of. +</p> + +<p> +Lime, muriate of. +</p> + +<p> +Limestone of Cumbre Pass. —of Coquimbo. —of Copiapo. +</p> + +<p> +Lund and Clausen on remains of caves in Brazil. +</p> + +<p> +Lund, M., on granites of Brazil. +</p> + +<p> +Lyell, M., on upraised shells retaining their colours. —on terraces at +Coquimbo. —on elevation near Lima. —on fossil horse’s tooth. +—on the boulder-formation being anterior to the extinction of North +American mammifers. —on quadrupeds washed down by floods. —on age +of American fossil mammifers. —on changes of climate. —on +denudation. —on foliation. +</p> + +<p> +MacCulloch, Dr., on concretions. —on beds of marble. +</p> + +<p> +Maclaren, Mr., letter to, on coral-formations. +</p> + +<p> +Macrauchenia Patachonica. +</p> + +<p> +Madeira, subsidence of. +</p> + +<p> +Magellan, Strait, elevation near, of. +</p> + +<p> +Magnesia, sulphate of, in veins. +</p> + +<p> +Malcolmson, Dr., on trees carried out to sea. +</p> + +<p> +Maldonado, elevation of. —Pampean formation of. —crystalline rocks +of. +</p> + +<p> +Mammalia, fossil, of Bahia Blanca. — —near St. Fe. — +—of Banda Oriental. — —of St. Julian. — —at Port +Gallegos. —washed down by floods. —number of remains of, and range +of, in Pampas. +</p> + +<p> +Man, skeletons of (Brazil). —remains of, near Lima. —Indian, +antiquity of. +</p> + +<p> +Marble, beds of. +</p> + +<p> +Maricongo, ravine of. +</p> + +<p> +Marsden, on elevation of Sumatra. +</p> + +<p> +Mastodon Andium, remains of. —range of. +</p> + +<p> +Maypu, Rio, mouth of, with upraised shells. —gravel fringes of. +—debouchement from the Cordillera. +</p> + +<p> +Megalonyx, range of. +</p> + +<p> +Megatherium, range of. +</p> + +<p> +Miers, Mr., on elevated shells. —on the height of the Uspallata plain. +</p> + +<p> +Minas, Las. +</p> + +<p> +Mocha Island, elevation of. —tertiary form of. —subsidence at. +</p> + +<p> +Molina, on a great flood. +</p> + +<p> +Monte Hermoso, elevation of. —fossils of. +</p> + +<p> +Monte Video, elevation of. —Pampean formation of. —crystalline +rocks of. +</p> + +<p> +Morris and Sharpe, Messrs., on the palaeozoic fossils of the Falklands. +</p> + +<p> +Mud, Pampean. —long deposited on the same area. +</p> + +<p> +Murchison, Sir R., on cleavage. —on waves transporting gravel. —on +origin of salt formations. —on the relations of metalliferous veins and +intrusive rocks. —on the absence of granite in the Ural. +</p> + +<p> +Nautilus d’Orbignyanus. +</p> + +<p> +Navidad, tertiary formations of, subsidence of. +</p> + +<p> +Negro, Rio, pumice of pebbles of. —gravel of. —salt lakes of. +—tertiary strata of. +</p> + +<p> +North America, fossil remains of. +</p> + +<p> +North Wales, sloping terraces absent in. —bent cleavage of. +</p> + +<p> +Neuvo Gulf, plains of. —tertiary formation of. +</p> + +<p> +Owen, Professor, on fossil mammiferous remains. +</p> + +<p> +Palmer, Mr., on transportation of gravel. +</p> + +<p> +Pampas, elevation of. —earthquakes of. —formation of. +—localities in which fossil mammifers have been found. +</p> + +<p> +Panuncillo, mines of. +</p> + +<p> +Parana, Rio, on saline incrustations. —Pampean formations near. —on +the S. Tandil. +</p> + +<p> +Parish, Sir W., on elevated shells near Buenos Ayres. —on earthquakes in +the Pampas. —on fresh-water near salt lakes. —on origin of Pampean +formation. +</p> + +<p> +Patagonia, elevation and plains of. —denudation of. +—gravel-formation of. —sea-cliffs of. —subsidence during +tertiary period. —crystalline rocks of. +</p> + +<p> +Payta, tertiary formations of. +</p> + +<p> +Pebbles of pumice. —decrease in size on the coast of Patagonia. +—means of transportation. —encrusted with living corallines. +—distribution of, at the eastern foot of Cordillera. —dispersal of, +in the Pampas. —zoned with colour. +</p> + +<p> +Pentland, Mr., on heights in the Cordillera. —on fossils of the +Cordillera. +</p> + +<p> +Pernambuco. +</p> + +<p> +Peru, tertiary formations of. +</p> + +<p> +Peuquenes, Pass of, in the Cordillera. —ridge of. +</p> + +<p> +Pholas, elevated shells of. +</p> + +<p> +Pitchstone of Chiloe. —of Port Desire. —near Cauquenes. +—layers of, in the Uspallata range. —of Los Hornos. —of +Coquimbo. +</p> + +<p> +Plains of Patagonia. —of Chiloe. —of Chile. —of Uspallata. +—on eastern foot of Cordillera. —of Iquique. +</p> + +<p> +Plata, La, elevation of. —tertiary formation of. —crystalline rocks +of. +</p> + +<p> +Playfair, Professor, on the transportation of gravel. +</p> + +<p> +Pluclaro, axis of. +</p> + +<p> +Pondicherry, fossils of. +</p> + +<p> +Porcelain rocks of Port Desire. —of the Uspallata range. +</p> + +<p> +Porphyry, pebbles of, strewed over Patagonia. +</p> + +<p> +Porphyry, claystone, of Chiloe, — —of Patagonia. — —of +Chile. —greenstone, of Chile. —doubly columnar. —claystone, +rare, on the eastern side of the Portillo Pass. —brick-red and orthitic, +of Cumbre Pass. —intrusive, repeatedly injected. —claystone of the +Uspallata range. — —of Copiapo. — —eruptive sources of. +</p> + +<p> +Port Desire, elevation and plains of. —tertiary formation of. +—porphyries of. +</p> + +<p> +Portillo Pass in the Cordillera. +</p> + +<p> +Portillo chain. —compared with that of the Uspallata. +</p> + +<p> +Prefil or sea-wall of Valparaiso. +</p> + +<p> +Puente del Inca, section of. +</p> + +<p> +Pumice, pebbles of. —conglomerate of R. Negro. —hills of, in the +Cordillera. +</p> + +<p> +Punta Alta, elevation of. —beds of. +</p> + +<p> +Quartz-rock of the S. Ventana. —C. Blanco. —Falkland islands. +—Portillo range. —viscidity of. —veins of, near Monte Video. +— —in dike of greenstone. —grains of, in mica slate. — +—in dikes. —veins of, relations to cleavage. +</p> + +<p> +Quillota, Campana of. +</p> + +<p> +Quintero, elevation of. +</p> + +<p> +Quiriquina, elevation of. —deposits of. +</p> + +<p> +Rancagua, plain of. +</p> + +<p> +Rapel, R. elevation near. +</p> + +<p> +Reeks, Mr. T., his analysis of decomposed shells. —his analysis of salts. +</p> + +<p> +Remains, human. +</p> + +<p> +Rio de Janeiro, elevation near. —crystalline rocks of. +</p> + +<p> +Rivers, small power of transporting pebbles. —small power of, in forming +valleys. —drainage of, in the Cordillera. +</p> + +<p> +Roads, parallel, of Glen Roy. +</p> + +<p> +Rocks, volcanic, of Banda Oriental. —Tres Montes. —Chiloe. +—Tierra del Fuego. —with laminar structure. +</p> + +<p> +Rodents, fossil, remains of. +</p> + +<p> +Rogers, Professor, address to Association of American Geologists. +</p> + +<p> +Rose, Professor G., on sulphate of iron at Copiapo. +</p> + +<p> +S. Blas, elevation of. +</p> + +<p> +S. Cruz, elevation and plains of. —valley of. —nature of gravel in +valley of. —boulder formation of. —tertiary formation of. +—subsidence at. +</p> + +<p> +S. Fe Bajada, formations of. +</p> + +<p> +S. George’s bay, plains of. +</p> + +<p> +S. Helena island, sea-cliffs, and subsidence of. +</p> + +<p> +S. Josef, elevation of. —tertiary formation of. +</p> + +<p> +S. Juan, elevation near. +</p> + +<p> +S. Julian, elevation and plains of. —salt lake of. —earthy deposit +with mammiferous remains. —tertiary formations of. —subsidence at. +</p> + +<p> +S. Lorenzo, elevation of. —old salt formation of. +</p> + +<p> +S. Mary, island of, elevation of. +</p> + +<p> +S. Pedro, elevation of. +</p> + +<p> +Salado, R., elevated shells of. —Pampean formation of. +</p> + +<p> +Salines. +</p> + +<p> +Salt, with upraised shell. —lakes of. —purity of, in salt lakes. +—deliquescent, necessary for the preservation of meat. —ancient +formation of, at Iquique. — —at S. Lorenzo. —strata of, +origin of. +</p> + +<p> +Salts, superficial deposits of. +</p> + +<p> +Sand-dunes of the Uruguay. —of the Pampas. —near Bahia Blanca. +—of the Colorado. —of S. Cruz. —of Arica. +</p> + +<p> +Sarmiento, Mount. +</p> + +<p> +Schmidtmeyer on auriferous detritus. +</p> + +<p> +Schomburghk, Sir R., on sea-bottom. —on the rocks of Guyana. +</p> + +<p> +Scotland, sloping terraces of. +</p> + +<p> +Sea, nature of bottom of, off Patagonia. —power of, in forming valleys. +</p> + +<p> +Sea cliffs, formation of. +</p> + +<p> +Seale, Mr., model of St. Helena. +</p> + +<p> +Sebastian Bay, tertiary formation of. +</p> + +<p> +Sedgwick, Professor, on cleavage. +</p> + +<p> +Serpentine of Copiapo. +</p> + +<p> +Serpulae, on upraised rocks. +</p> + +<p> +Shale-rock, of the Portillo Pass. —of Copiapo. +</p> + +<p> +Shells, upraised state of, in Patagonia. —elevated, too small for human +food. —transported far inland, for food. —upraised, proportional +numbers varying. — —gradual decay of. — —absent on high +plains of Chile. — —near Bahia Blanca. —preserved in +concretions. —living and fossil range of, on west coast. —living, +different on the east and west coast. +</p> + +<p> +Shingle of Patagonia. +</p> + +<p> +Siau, M., on sea-bottom. +</p> + +<p> +Silver mines of Arqueros. —of Chanuncillo. —of Iquique. +—distribution of. +</p> + +<p> +Slip, great, at S. Cruz. +</p> + +<p> +Smith, Mr., of Jordan Hill, on upraised shells retaining their colours. +—on Madeira. —on elevated seaweed. —on inclined gravel beds. +</p> + +<p> +Soda, nitrate of. —sulphate of, near Bahia Blanca. —carbonate of. +</p> + +<p> +Soundings off Patagonia. —in Tierra del Fuego. +</p> + +<p> +Spirifers. +</p> + +<p> +Spix and Martius on Brazil. Sprengel on the production of carbonate of soda. +</p> + +<p> +Springs, mineral, in the Cumbre Pass. +</p> + +<p> +Stratification of sandstone in metamorphic rocks. —of clay-slate in +Tierra del Fuego. —of the Cordillera of Central Chile. —little +disturbed in Cumbre Pass. —disturbance of, near Copiapo. +</p> + +<p> +Streams of lava at S. Cruz, inclination of. —in the Portillo range. +</p> + +<p> +String of cotton with fossil-shells. +</p> + +<p> +Struthiolaria ornata. +</p> + +<p> +Studer, M., on metamorphic rocks. +</p> + +<p> +Subsidence during formation of sea-cliffs. —near Lima. —probable, +during Pampean formation. —necessary for the accumulation of permanent +deposits. —during the tertiary formations of Chile and Patagonia. +—probable during the Neocomian formation of the Portillo Pass. +—probable during the formation of conglomerate of Tenuyan. —during +the Neocomian formation of the Cumbre Pass. —of the Uspallata range. +—great, at Copiapo. — —during the formation of the +Cordillera. +</p> + +<p> +Sulphur, volcanic exhalations of. +</p> + +<p> +Sumatra, promontories of. +</p> + +<p> +A Summary on the recent elevatory movements. —on the Pampean formation. +—on the tertiary formations of Patagonia and Chile. —on the Chilean +Cordillera. —on the cretaceo-oolitic formation. —on the subsidences +of the Cordillera. —on the elevation of the Cordillera. +</p> + +<p> +Tacna, elevation of. +</p> + +<p> +Tampico, elevated shells near. +</p> + +<p> +Tandil, crystalline rocks of. +</p> + +<p> +Tapalguen, Pampean formation of. —crystalline rocks of. +</p> + +<p> +Taylor, Mr., on copper veins of Cuba. +</p> + +<p> +Temperature of Chile during the tertiary period. +</p> + +<p> +Tension, lines of, origin of, axes of elevation and of cleavage. +</p> + +<p> +Tenuy Point, singular section of. +</p> + +<p> +Tenuyan, valley of. +</p> + +<p> +Terraces of the valley of S. Cruz. —of equable heights throughout +Patagonia. —of Patagonia, formation of. —of Chiloe. —at +Conchalee. —of Coquimbo. —not horizontal at Coquimbo. —of +Guasco. —of S. Lorenzo. —of gravel within the Cordillera. +</p> + +<p> +Theories on the origin of the Pampean formation. +</p> + +<p> +Tierra Amarilla. +</p> + +<p> +Tierra del Fuego, form of sea-bottom. —tertiary formations of. +—clay-slate formation of. —cretaceous formation of. +—crystalline rocks of. —cleavage of clay-slate. +</p> + +<p> +Tosca rock. +</p> + +<p> +Trachyte of Chiloe. —of Port Desire. —in the Cordillera. +</p> + +<p> +Traditions of promontories having been islands. —on changes of level near +Lima. +</p> + +<p> +Trees buried in plain of Iquique. —silicified, vertical, of the Uspallata +range. +</p> + +<p> +Tres Montes, elevation of. —volcanic rocks of. +</p> + +<p> +Trigonocelia insolita. +</p> + +<p> +Tristan Arroyo, elevated shells of. +</p> + +<p> +Tschudi, Mr., on subsidence near Lima. +</p> + +<p> +Tuff, calcareous, at Coquimbo. —on basin-plain near St. Jago. +—structure of, in Pampas. —origin of, in Pampas. —pumiceous, +of R. Negro. —Nuevo Gulf. —Port Desire. —S. Cruz. +—Patagonia, summary on Chiloe. —formation of, in Portillo chain. +—great deposit of, at Copiapo. +</p> + +<p> +Tuffs, volcanic, metamorphic, of Uspallata. —of Coquimbo. +</p> + +<p> +Ulloa, on rain in Peru. —on elevation near Lima. +</p> + +<p> +Uruguay, Rio, elevation of country near. +</p> + +<p> +Uspallata, plain of. —pass of. —range of. —concluding remarks +on. +</p> + +<p> +Valdivia, tertiary beds of. —mica-slate of. +</p> + +<p> +Valley of S. Cruz, structure of. —Coquimbo. —Guasco, structure of. +—Copiapo, structure of. —S. Cruz, tertiary formations of. +—Coquimbo, geology of. —Guasco, secondary formations of. +—Copiapo, secondary formations of. —Despoblado. +</p> + +<p> +Valleys in the Cordillera bordered by gravel fringes. —formation of. +—in the Cordillera. +</p> + +<p> +Valparaiso, elevation of. —gneiss of. +</p> + +<p> +Vein of quartz near Monte Video. —in mica-slate. —relations of, to +cleavage. —in a trap dike. —of granite, quartzose. +—remarkable, in gneiss, near Valparaiso. +</p> + +<p> +Veins, relations of, to concretions. —metalliferous, of the Uspallata +range. —metalliferous, discussion on. +</p> + +<p> +Venezuela, gneissic rocks of. +</p> + +<p> +Ventana, Sierra, Pampean formation near. —quartz-rock of. +</p> + +<p> +Villa Vincencio Pass. +</p> + +<p> +Volcan, Rio, mouth of. —fossils of. +</p> + +<p> +Volcanoes of the Cordillera. —absent, except near bodies of water. +—ancient submarine, in Cordillera. —action of, in relation to +changes of level. —long action of, in the Cordillera. +</p> + +<p> +Wafer on elevated shells. +</p> + +<p> +Waves caused by earthquakes, power of, in transporting boulders. —power +of, in throwing up shells. +</p> + +<p> +Weaver, Mr., on elevated shells. +</p> + +<p> +White, Martin., on sea-bottom. +</p> + +<p> +Wood, silicified, of Entre Rios. —S. Cruz. —Chiloe. +—Uspallata range. —Los Hornos. —Copiapo. +</p> + +<p> +Yeso, Rio, and plain of. +</p> + +<p> +Ypun Island, tertiary formation of. +</p> + +<p> +Zeagonite. +</p> + +</div><!--end chapter--> + +<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA ***</div> +<div style='text-align:left'> + +<div style='display:block; margin:1em 0'> +Updated editions will replace the previous one—the old editions will +be renamed. +</div> + +<div style='display:block; margin:1em 0'> +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the United +States without permission and without paying copyright +royalties. 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FOR PUBLIC DOMAIN ETEXTS*Ver.06/12/01*END* +[Portions of this header are copyright (C) 2001 by Michael S. Hart +and may be reprinted only when these Etexts are free of all fees.] +[Project Gutenberg is a TradeMark and may not be used in any sales +of Project Gutenberg Etexts or other materials be they hardware or +software or any other related product without express permission.] + + + + + +Etext Prepared by Sue Asscher asschers@dingoblue.net.au +Urangan, 17 June, 2001 + + + + + +GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA + +by CHARLES DARWIN + + + + +EDITORIAL NOTE. + +Although in some respects more technical in their subjects and style than +Darwin's "Journal," the books here reprinted will never lose their value +and interest for the originality of the observations they contain. Many +parts of them are admirably adapted for giving an insight into problems +regarding the structure and changes of the earth's surface, and in fact +they form a charming introduction to physical geology and physiography in +their application to special domains. The books themselves cannot be +obtained for many times the price of the present volume, and both the +general reader, who desires to know more of Darwin's work, and the student +of geology, who naturally wishes to know how a master mind reasoned on most +important geological subjects, will be glad of the opportunity of +possessing them in a convenient and cheap form. + +The three introductions, which my friend Professor Judd has kindly +furnished, give critical and historical information which makes this +edition of special value. + +G.T.B. + + + + +PLATE I. GEOLOGICAL SECTIONS THROUGH THE CORDILLERAS. + +SECTION 1/1. SECTION OF THE PEUQUENES OR PORTILLO PASS OF THE CORDILLERA. + +SECTION 1/2. SECTION OF THE CUMBRE OR USPALLATA PASS. + +SECTION 1/3. SECTION OF THE VALLEY OF COPIAPO TO THE BASE OF THE MAIN +CORDILLERA. + + +PLATE II. MAP OF SOUTHERN PORTION OF SOUTH AMERICA. + + + +TABLE OF CONTENTS. + + +CRITICAL INTRODUCTION. + + +CHAPTER I.--ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA. + +Upraised shells of La Plata.--Bahia Blanca, Sand-dunes and Pumice-pebbles.- +-Step-formed plains of Patagonia, with upraised shells.--Terrace-bounded +valley of Santa Cruz, formerly a sea-strait.--Upraised shells of Tierra del +Fuego.--Length and breadth of the elevated area.--Equability of the +movements, as shown by the similar heights of the plains.--Slowness of the +elevatory process.--Mode of formation of the step-formed plains.--Summary.- +-Great shingle formation of Patagonia; its extent, origin, and +distribution.--Formation of sea-cliffs. + + +CHAPTER II.--ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA. + +Chonos Archipelago.--Chiloe, recent and gradual elevation of, traditions of +the inhabitants on this subject.--Concepcion, earthquake and elevation of.- +-VALPARAISO, great elevation of, upraised shells, earth or marine origin, +gradual rise of the land within the historical period.--COQUIMBO, elevation +of, in recent times; terraces of marine origin, their inclination, their +escarpments not horizontal.--Guasco, gravel terraces of.--Copiapo.--PERU.-- +Upraised shells of Cobija, Iquique, and Arica.--Lima, shell-beds and sea- +beach on San Lorenzo.--Human remains, fossil earthenware, earthquake +debacle, recent subsidence.--On the decay of upraised shells.--General +summary. + + +CHAPTER III.--ON THE PLAINS AND VALLEYS OF CHILE:--SALIFEROUS SUPERFICIAL +DEPOSITS. + +Basin-like plains of Chile; their drainage, their marine origin.--Marks of +sea-action on the eastern flanks of the Cordillera.--Sloping terrace-like +fringes of stratified shingle within the valleys of the Cordillera; their +marine origin.--Boulders in the valley of Cachapual.--Horizontal elevation +of the Cordillera.--Formation of valleys.--Boulders moved by earthquake- +waves.--Saline superficial deposits.--Bed of nitrate of soda at Iquique.-- +Saline incrustations.--Salt-lakes of La Plata and Patagonia; purity of the +salt; its origin. + + +CHAPTER IV.--ON THE FORMATIONS OF THE PAMPAS. + +Mineralogical constitution.--Microscopical structure.--Buenos Ayres, shells +embedded in tosca-rock.--Buenos Ayres to the Colorado.--S. Ventana.--Bahia +Blanca; M. Hermoso, bones and infusoria of; P. Alta, shells, bones, and +infusoria of; co-existence of the recent shells and extinct mammifers.-- +Buenos Ayres to St. Fe.--Skeletons of Mastodon.--Infusoria.--Inferior +marine tertiary strata, their age.--Horse's tooth. BANDA ORIENTAL.-- +Superficial Pampean formation.--Inferior tertiary strata, variation of, +connected with volcanic action; Macrauchenia Patachonica at S. Julian in +Patagonia, age of, subsequent to living mollusca and to the erratic block +period. SUMMARY.--Area of Pampean formation.--Theories of origin.--Source +of sediment.--Estuary origin.--Contemporaneous with existing mollusca.-- +Relations to underlying tertiary strata. Ancient deposit of estuary +origin.--Elevation and successive deposition of the Pampean formation.-- +Number and state of the remains of mammifers; their habitation, food, +extinction, and range.--Conclusion.--Supplement on the thickness of the +Pampean formation.--Localities in Pampas at which mammiferous remains have +been found. + + +CHAPTER V.--ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE. + +Rio Negro.--S. Josef.--Port Desire, white pumiceous mudstone with +infusoria.--Port S. Julian.--Santa Cruz, basaltic lava of.--P. Gallegos.-- +Eastern Tierra del Fuego; leaves of extinct beech-trees.--Summary on the +Patagonian tertiary formations.--Tertiary formations of the Western Coast.- +-Chonos and Chiloe groups, volcanic rocks of.--Concepcion.--Navidad.-- +Coquimbo.--Summary.--Age of the tertiary formations.--Lines of elevation.-- +Silicified wood.--Comparative ranges of the extinct and living mollusca on +the West Coast of S. America.--Climate of the tertiary period.--On the +causes of the absence of recent conchiferous deposits on the coasts of +South America.--On the contemporaneous deposition and preservation of +sedimentary formations. + + +CHAPTER VI.--PLUTONIC AND METAMORPHIC ROCKS:--CLEAVAGE AND FOLIATION. +Brazil, Bahia, gneiss with disjointed metamorphosed dikes.--Strike of +foliation.--Rio de Janeiro, gneiss-granite, embedded fragment in, +decomposition of.--La Plata, metamorphic and old volcanic rocks of.--S. +Ventana.--Claystone porphyry formation of Patagonia; singular metamorphic +rocks; pseudo-dikes.--Falkland Islands, palaeozoic fossils of.--Tierra del +Fuego, clay-slate formation, cretaceous fossils of; cleavage and foliation; +form of land.--Chonos Archipelago, mica-schists, foliation disturbed by +granitic axis; dikes.--Chiloe.--Concepcion, dikes, successive formation +of.--Central and Northern Chile.--Concluding remarks on cleavage and +foliation.--Their close analogy and similar origin.--Stratification of +metamorphic schists.--Foliation of intrusive rocks.--Relation of cleavage +and foliation to the lines of tension during metamorphosis. + + +CHAPTER VII.--CENTRAL CHILE:--STRUCTURE OF THE CORDILLERA. + +Central Chile.--Basal formations of the Cordillera.--Origin of the +porphyritic clay-stone conglomerate.--Andesite.--Volcanic rocks.--Section +of the Cordillera by the Peuquenes or Portillo Pass.--Great gypseous +formation.--Peuquenes line; thickness of strata, fossils of.--Portillo +line.--Conglomerate, orthitic granite, mica-schist, volcanic rocks of.-- +Concluding remarks on the denudation and elevation of the Portillo line.-- +Section by the Cumbre, or Uspallata Pass.--Porphyries.--Gypseous strata.-- +Section near the Puente del Inca; fossils of.--Great subsidence.--Intrusive +porphyries.--Plain of Uspallata.--Section of the Uspallata chain.-- +Structure and nature of the strata.--Silicified vertical trees.--Great +subsidence.--Granitic rocks of axis.--Concluding remarks on the Uspallata +range; origin subsequent to that of the main Cordillera; two periods of +subsidence; comparison with the Portillo chain. + + +CHAPTER VIII.--NORTHERN CHILE.--CONCLUSION. + +Section from Illapel to Combarbala; gypseous formation with silicified +wood.--Panuncillo.--Coquimbo; mines of Arqueros; section up valley; +fossils.--Guasco, fossils of.--Copiapo, section up valley; Las Amolanas, +silicified wood.--Conglomerates, nature of former land, fossils, thickness +of strata, great subsidence.--Valley of Despoblado, fossils, tufaceous +deposit, complicated dislocations of.--Relations between ancient orifices +of eruption and subsequent axes of injection.--Iquique, Peru, fossils of, +salt-deposits.--Metalliferous veins.--Summary on the porphyritic +conglomerate and gypseous formations.--Great subsidence with partial +elevations during the cretaceo-oolitic period.--On the elevation and +structure of the Cordillera.--Recapitulation on the tertiary series.-- +Relation between movements of subsidence and volcanic action.--Pampean +formation.--Recent elevatory movements.--Long-continued volcanic action in +the Cordillera.--Conclusion. + + + +INDEX. + + + + +GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA + +BY + +CHARLES DARWIN. + + + + +CRITICAL INTRODUCTION. + +Of the remarkable "trilogy" constituted by Darwin's writings which deal +with the geology of the "Beagle," the member which has perhaps attracted +least attention, up to the present time is that which treats of the geology +of South America. The actual writing of this book appears to have occupied +Darwin a shorter period than either of the other volumes of the series; his +diary records that the work was accomplished within ten months, namely, +between July 1844 and April 1845; but the book was not actually issued till +late in the year following, the preface bearing the date "September 1846." +Altogether, as Darwin informs us in his "Autobiography," the geological +books "consumed four and a half years' steady work," most of the remainder +of the ten years that elapsed between the return of the "Beagle," and the +completion of his geological books being, it is sad to relate, "lost +through illness!" + +Concerning the "Geological Observations on South America," Darwin wrote to +his friend Lyell, as follows:--"My volume will be about 240 pages, +dreadfully dull, yet much condensed. I think whenever you have time to look +through it, you will think the collection of facts on the elevation of the +land and on the formation of terraces pretty good." + +"Much condensed" is the verdict that everyone must endorse, on rising from +the perusal of this remarkable book; but by no means "dull." The three and +a half years from April 1832 to September 1835, were spent by Darwin in +South America, and were devoted to continuous scientific work; the problems +he dealt with were either purely geological or those which constitute the +borderland between the geological and biological sciences. It is impossible +to read the journal which he kept during this time without being impressed +by the conviction that it contains all the germs of thought which +afterwards developed into the "Origin of Species." But it is equally +evident that after his return to England, biological speculations gradually +began to exercise a more exclusive sway over Darwin's mind, and tended to +dispossess geology, which during the actual period of the voyage certainly +engrossed most of his time and attention. The wonderful series of +observations made during those three and a half years in South America +could scarcely be done justice to, in the 240 pages devoted to their +exposition. That he executed the work of preparing the book on South +America in somewhat the manner of a task, is shown by many references in +his letters. Writing to Sir Joseph Hooker in 1845, he says, "I hope this +next summer to finish my South American Geology, then to get out a little +Zoology, and HURRAH FOR MY SPECIES WORK!" + +It would seem that the feeling of disappointment, which Darwin so often +experienced in comparing a book when completed, with the observations and +speculations which had inspired it, was more keenly felt in the case of his +volume on South America than any other. To one friend he writes, "I have of +late been slaving extra hard, to the great discomfiture of wretched +digestive organs, at South America, and thank all the fates, I have done +three-fourths of it. Writing plain English grows with me more and more +difficult, and never attainable. As for your pretending that you will read +anything so dull as my pure geological descriptions, lay not such a +flattering unction on my soul, for it is incredible." To another friend he +writes, "You do not know what you threaten when you propose to read it--it +is purely geological. I said to my brother, 'You will of course read it,' +and his answer was, 'Upon my life, I would sooner even buy it.'" + +In spite of these disparaging remarks, however, we are strongly inclined to +believe that this book, despised by its author, and neglected by his +contemporaries, will in the end be admitted to be one of Darwin's chief +titles to fame. It is, perhaps, an unfortunate circumstance that the great +success which he attained in biology by the publication of the "Origin of +Species" has, to some extent, overshadowed the fact that Darwin's claims as +a geologist, are of the very highest order. It is not too much to say that, +had Darwin not been a geologist, the "Origin of Species" could never have +been written by him. But apart from those geological questions, which have +an important bearing on biological thought and speculation, such as the +proofs of imperfection in the geological record, the relations of the later +tertiary faunas to the recent ones in the same areas, and the apparent +intermingling of types belonging to distant geological epochs, when we +study the palaeontology of remote districts,--there are other purely +geological problems, upon which the contributions made by Darwin are of the +very highest value. I believe that the verdict of the historians of science +will be that if Darwin had not taken a foremost place among the biologists +of this century, his position as a geologist would have been an almost +equally commanding one. + +But in the case of Darwin's principal geological work--that relating to the +origin of the crystalline schists,--geologists were not at the time +prepared to receive his revolutionary teachings. The influence of powerful +authority was long exercised, indeed, to stifle his teaching, and only now, +when this unfortunate opposition has disappeared, is the true nature and +importance of Darwin's purely geological work beginning to be recognised. + +The two first chapters of the "Geological Observations on South America," +deal with the proofs which exist of great, but frequently interrupted, +movements of elevation during very recent geological times. In connection +with this subject, Darwin's particular attention was directed to the +relations between the great earthquakes of South America--of some of which +he had impressive experience--and the permanent changes of elevation which +were taking place. He was much struck by the rapidity with which the +evidence of such great earth movements is frequently obliterated; and +especially with the remarkable way in which the action of rain-water, +percolating through deposits on the earth's surface, removes all traces of +shells and other calcareous organisms. It was these considerations which +were the parents of the generalisation that a palaeontological record can +only be preserved during those periods in which long-continued slow +subsidence is going on. This in turn, led to the still wider and more +suggestive conclusion that the geological record as a whole is, and never +can be more than, a series of more or less isolated fragments. The +recognition of this important fact constitutes the keystone to any theory +of evolution which seeks to find a basis in the actual study of the types +of life that have formerly inhabited our globe. + +In his third chapter, Darwin gives a number of interesting facts, collected +during his visits to the plains and valleys of Chili, which bear on the +question of the origin of saliferous deposits--the accumulation of salt, +gypsum, and nitrate of soda. This is a problem that has excited much +discussion among geologists, and which, in spite of many valuable +observations, still remains to a great extent very obscure. Among the +important considerations insisted upon by Darwin is that relating to the +absence of marine shells in beds associated with such deposits. He justly +argues that if the strata were formed in shallow waters, and then exposed +by upheaval to subaerial action, all shells and other calcareous organisms +would be removed by solution. + +Following Lyell's method, Darwin proceeds from the study of deposits now +being accumulated on the earth's surface, to those which have been formed +during the more recent periods of the geological history. + +His account of the great Pampean formation, with its wonderful mammalian +remains--Mastodon, Toxodon, Scelidotherium, Macrauchenia, Megatherium, +Megalonyx, Mylodon, and Glyptodon--this full of interest. His discovery of +the remains of a true Equus afforded a remarkable confirmation of the fact- +-already made out in North America--that species of horse had existed and +become extinct in the New World, before their introduction by the Spaniards +in the sixteenth century. Fully perceiving the importance of the microscope +in studying the nature and origin of such deposits as those of the Pampas, +Darwin submitted many of his specimens both to Dr. Carpenter in this +country, and to Professor Ehrenberg in Berlin. Many very important notes on +the microscopic organisms contained in the formation will be found +scattered through the chapter. + +Darwin's study of the older tertiary formations, with their abundant +shells, and their relics of vegetable life buried under great sheets of +basalt, led him to consider carefully the question of climate during these +earlier periods. In opposition to prevalent views on this subject, Darwin +points out that his observations are opposed to the conclusion that a +higher temperature prevailed universally over the globe during early +geological periods. He argues that "the causes which gave to the older +tertiary productions of the quite temperate zones of Europe a tropical +character, WERE OF A LOCAL CHARACTER AND DID NOT AFFECT THE WHOLE GLOBE." +In this, as in many similar instances, we see the beneficial influence of +extensive travel in freeing Darwin's mind from prevailing prejudices. It +was this widening of experience which rendered him so especially qualified +to deal with the great problem of the origin of species, and in doing so to +emancipate himself from ideas which were received with unquestioning faith +by geologists whose studies had been circumscribed within the limits of +Western Europe. + +In the Cordilleras of Northern and Central Chili, Darwin, when studying +still older formations, clearly recognised that they contain an admixture +of the forms of life, which in Europe are distinctive of the Cretaceous and +Jurassic periods respectively. He was thus led to conclude that the +classification of geological periods, which fairly well expresses the facts +that had been discovered in the areas where the science was first studied, +is no longer capable of being applied when we come to the study of widely +distant regions. This important conclusion led up to the further +generalisation that each great geological period has exhibited a +geographical distribution of the forms of animal and vegetable life, +comparable to that which prevails in the existing fauna and flora. To those +who are familiar with the extent to which the doctrine of universal +formations has affected geological thought and speculation, both long +before and since the time that Darwin wrote, the importance of this new +standpoint to which he was able to attain will be sufficiently apparent. +Like the idea of the extreme imperfection of the Geological Record, the +doctrine of LOCAL geological formations is found permeating and moulding +all the palaeontological reasonings of his great work. + +In one of Darwin's letters, written while he was in South America, there is +a passage we have already quoted, in which he expresses his inability to +decide between the rival claims upon his attention of "the old crystalline +group of rocks," and "the softer fossiliferous beds" respectively. The +sixth chapter of the work before us, entitled "Plutonic and Metamorphic +Rocks--Cleavage and Foliation," contains a brief summary of a series of +observations and reasonings upon these crystalline rocks, which are, we +believe, calculated to effect a revolution in geological science, and-- +though their value and importance have long been overlooked--are likely to +entitle Darwin in the future to a position among geologists, scarcely, if +at all, inferior to that which he already occupies among biologists. + +Darwin's studies of the great rock-masses of the Andes convinced him of the +close relations between the granitic or Plutonic rocks, and those which +were undoubtedly poured forth as lavas. Upon his return, he set to work, +with the aid of Professor Miller, to make a careful study of the minerals +composing the granites and those which occur in the lavas, and he was able +to show that in all essential respects they are identical. He was further +able to prove that there is a complete gradation between the highly +crystalline or granitic rock-masses, and those containing more or less +glassy matter between their crystals, which constitute ordinary lavas. The +importance of this conclusion will be realised when we remember that it was +then the common creed of geologists--and still continues to be so on the +Continent--that all highly crystalline rocks are of great geological +antiquity, and that the igneous ejections which have taken place since the +beginning of the tertiary periods differ essentially, in their composition, +their structure, and their mode of occurrence, from those which have made +their appearance at earlier periods of the world's history. + +Very completely have the conclusions of Darwin upon these subjects been +justified by recent researches. In England, the United States, and Italy, +examples of the gradual passage of rocks of truly granitic structure into +ordinary lavas have been described, and the reality of the transition has +been demonstrated by the most careful studies with the microscope. Recent +researches carried on in South America by Professor Stelzner, have also +shown the existence of a class of highly crystalline rocks--the +"Andengranites"--which combine in themselves many of the characteristics +which were once thought to be distinctive of the so-called Plutonic and +volcanic rocks. No one familiar with recent geological literature--even in +Germany and France, where the old views concerning the distinction of +igneous products of different ages have been most stoutly maintained--can +fail to recognise the fact that the principles contended for by Darwin bid +fair at no distant period to win universal acceptance among geologists all +over the globe. + +Still more important are the conclusions at which Darwin arrived with +respect to the origin of the schists and gneisses which cover so large an +area in South America. + +Carefully noting, by the aid of his compass and clinometer, at every point +which he visited, the direction and amount of inclination of the parallel +divisions in these rocks, he was led to a very important generalisation-- +namely, that over very wide areas the direction (strike) of the planes of +cleavage in slates, and of foliation in schists and gneisses, remained +constant, though the amount of their inclination (dip) often varied within +wide limits. Further than this it appeared that there was always a close +correspondence between the strike of the cleavage and foliation and the +direction of the great axes along which elevation had taken place in the +district. + +In Tierra del Fuego, Darwin found striking evidence that the cleavage +intersecting great masses of slate-rocks was quite independent of their +original stratification, and could often, indeed, be seen cutting across it +at right angles. He was also able to verify Sedgwick's observation that, in +some slates, glossy surfaces on the planes of cleavage arise from the +development of new minerals, chlorite, epidote or mica, and that in this +way a complete graduation from slates to true schists may be traced. + +Darwin further showed that in highly schistose rocks, the folia bend around +and encircle any foreign bodies in the mass, and that in some cases they +exhibit the most tortuous forms and complicated puckerings. He clearly saw +that in all cases the forces by which these striking phenomena must have +been produced were persistent over wide areas, and were connected with the +great movements by which the rocks had been upheaved and folded. + +That the distinct folia of quartz, feldspar, mica, and other minerals +composing the metamorphic schists could not have been separately deposited +as sediment was strongly insisted upon by Darwin; and in doing so he +opposed the view generally prevalent among geologists at that time. He was +thus driven to the conclusion that foliation, like cleavage, is not an +original, but a superinduced structure in rock-masses, and that it is the +result of re-crystallisation, under the controlling influence of great +pressure, of the materials of which the rock was composed. + +In studying the lavas of Ascension, as we have already seen, Darwin was led +to recognise the circumstance that, when igneous rocks are subjected to +great differential movements during the period of their consolidation, they +acquire a foliated structure, closely analogous to that of the crystalline +schists. Like his predecessor in this field of inquiry, Mr. Poulett Scrope, +Charles Darwin seems to have been greatly impressed by these facts, and he +argued from them that the rocks exhibiting the foliated structure must have +been in a state of plasticity, like that of a cooling mass of lava. At that +time the suggestive experiments of Tresca, Daubree, and others, showing +that solid masses under the influence of enormous pressure become actually +plastic, had not been published. Had Darwin been aware of these facts he +would have seen that it was not necessary to assume a state of imperfect +solidity in rock-masses in order to account for their having yielded to +pressure and tension, and, in doing so, acquiring the new characters which +distinguish the crystalline schists. + +The views put forward by Darwin on the origin of the crystalline schists +found an able advocate in Mr. Daniel Sharpe, who in 1852 and 1854 published +two papers, dealing with the geology of the Scottish Highlands and of the +Alps respectively, in which he showed that the principles arrived at by +Darwin when studying the South American rocks afford a complete explanation +of the structure of the two districts in question. + +But, on the other hand, the conclusions of Darwin and Sharpe were met with +the strongest opposition by Sir Roderick Murchison and Dr. A. Geikie, who +in 1861 read a paper before the Geological Society "On the Coincidence +between Stratification and Foliation in the Crystalline Rocks of the +Scottish Highlands," in which they insisted that their observations in +Scotland tended to entirely disprove the conclusions of Darwin that +foliation in rocks is a secondary structure, and entirely independent of +the original stratification of the rock-masses. + +Now it is a most significant circumstance that, no sooner did the officers +of the Geological Survey commence the careful and detailed study of the +Scottish Highlands than they found themselves compelled to make a formal +retraction of the views which had been put forward by Murchison and Geikie +in opposition to the conclusions of Darwin. The officers of the Geological +Survey have completely abandoned the view that the foliation of the +Highland rocks has been determined by their original stratification, and +admit that the structure is the result of the profound movements to which +the rocks have been subjected. The same conclusions have recently been +supported by observations made in many different districts--among which we +may especially refer to those of Dr. H. Reusch in Norway, and those of Dr. +J. Lehmann in Saxony. At the present time the arguments so clearly stated +by Darwin in the work before us, have, after enduring opposition or neglect +for a whole generation, begun to "triumph all along the line," and we may +look forward confidently to the near future, when his claim to be regarded +as one of the greatest of geological discoverers shall be fully vindicated. + +JOHN W. JUDD. + + +CHAPTER I. ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA. + +Upraised shells of La Plata. +Bahia Blanca, Sand-dunes and Pumice-pebbles. +Step-formed plains of Patagonia, with upraised Shells. +Terrace-bounded Valley of Santa Cruz, formerly a Sea-strait. +Upraised shells of Tierra del Fuego. +Length and breadth of the elevated area. +Equability of the movements, as shown by the similar heights of the plains. +Slowness of the elevatory process. +Mode of formation of the step-formed plains. +Summary. +Great Shingle Formation of Patagonia; its extent, origin, and distribution. +Formation of sea-cliffs. + +In the following Volume, which treats of the geology of South America, and +almost exclusively of the parts southward of the Tropic of Capricorn, I +have arranged the chapters according to the age of the deposits, +occasionally departing from this order, for the sake of geographical +simplicity. + +The elevation of the land within the recent period, and the modifications +of its surface through the action of the sea (to which subjects I paid +particular attention) will be first discussed; I will then pass on to the +tertiary deposits, and afterwards to the older rocks. Only those districts +and sections will be described in detail which appear to me to deserve some +particular attention; and I will, at the end of each chapter, give a +summary of the results. We will commence with the proofs of the upheaval of +the eastern coast of the continent, from the Rio Plata southward; and, in +the Second Chapter, follow up the same subject along the shores of Chile +and Peru. + +On the northern bank of the great estuary of the Rio Plata, near Maldonado, +I found at the head of a lake, sometimes brackish but generally containing +fresh water, a bed of muddy clay, six feet in thickness, with numerous +shells of species still existing in the Plata, namely, the Azara labiata, +d'Orbigny, fragments of Mytilus eduliformis, d'Orbigny, Paludestrina +Isabellei, d'Orbigny, and the Solen Caribaeus, Lam., which last was +embedded vertically in the position in which it had lived. These shells lie +at the height of only two feet above the lake, nor would they have been +worth mentioning, except in connection with analogous facts. + +At Monte Video, I noticed near the town, and along the base of the mount, +beds of a living Mytilus, raised some feet above the surface of the Plata: +in a similar bed, at a height from thirteen to sixteen feet, M. Isabelle +collected eight species, which, according to M. d'Orbigny, now live at the +mouth of the estuary. ("Voyage dans l'Amerique Merid.: Part. Geolog." page +21.) At Colonia del Sacramiento, further westward, I observed at the height +of about fifteen feet above the river, there of quite fresh water, a small +bed of the same Mytilus, which lives in brackish water at Monte Video. Near +the mouth of Uruguay, and for at least thirty-five miles northward, there +are at intervals large sandy tracts, extending several miles from the banks +of the river, but not raised much above its level, abounding with small +bivalves, which occur in such numbers that at the Agraciado they are sifted +and burnt for lime. Those which I examined near the A. S. Juan were much +worn: they consisted of Mactra Isabellei, d'Orbigny, mingled with few of +Venus sinuosa, Lam., both inhabiting, as I am informed by M. d'Orbigny, +brackish water at the mouth of the Plata, nearly or quite as salt as the +open sea. The loose sand, in which these shells are packed, is heaped into +low, straight, long lines of dunes, like those left by the sea at the head +of many bays. M. d'Orbigny has described an analogous phenomenon on a +greater scale, near San Pedro on the river Parana, where he found widely +extended beds and hillocks of sand, with vast numbers of the Azara labiata, +at the height of nearly 100 feet (English) above the surface of that river. +(Ibid page 43.) The Azara inhabits brackish water, and is not known to be +found nearer to San Pedro than Buenos Ayres, distant above a hundred miles +in a straight line. Nearer Buenos Ayres, on the road from that place to San +Isidro, there are extensive beds, as I am informed by Sir Woodbine Parish, +of the Azara labiata, lying at about forty feet above the level of the +river, and distant between two and three miles from it. ("Buenos Ayres" +etc. by Sir Woodbine Parish page 168.) These shells are always found on the +highest banks in the district: they are embedded in a stratified earthy +mass, precisely like that of the great Pampean deposit hereafter to be +described. In one collection of these shells, there were some valves of the +Venus sinuosa, Lam., the same species found with the Mactra on the banks of +the Uruguay. South of Buenos Ayres, near Ensenada, there are other beds of +the Azara, some of which seem to have been embedded in yellowish, +calcareous, semi-crystalline matter; and Sir W. Parish has given me from +the banks of the Arroyo del Tristan, situated in this same neighbourhood, +at the distance of about a league from the Plata, a specimen of a pale- +reddish, calcereo-argillaceous stone (precisely like parts of the Pampean +deposit the importance of which fact will be referred to in a succeeding +chapter), abounding with shells of an Azara, much worn, but which in +general form and appearance closely resemble, and are probably identical +with, the A. labiata. Besides these shells, cellular, highly crystalline +rock, formed of the casts of small bivalves, is found near Ensenada; and +likewise beds of sea-shells, which from their appearance appear to have +lain on the surface. Sir W. Parish has given me some of these shells, and +M. d'Orbigny pronounces them to be:-- + +1. Buccinanops globulosum, d'Orbigny. + +2. Olivancillaria auricularia, d'Orbigny. + +3. Venus flexuosa, Lam. + +4. Cytheraea (imperfect). + +5. Mactra Isabellei, d'Orbigny. + +6. Ostrea pulchella, d'Orbigny. + +Besides these, Sir W. Parish procured ("Buenos Ayres" etc. by Sir W. Parish +page 168.) (as named by Mr. G.B. Sowerby) the following shells:-- + +7. Voluta colocynthis. + +8. Voluta angulata. + +9. Buccinum (not spec.?). + +All these species (with, perhaps, the exception of the last) are recent, +and live on the South American coast. These shell-beds extend from one +league to six leagues from the Plata, and must lie many feet above its +level. I heard, also, of beds of shells on the Somborombon, and on the Rio +Salado, at which latter place, as M. d'Orbigny informs me, the Mactra +Isabellei and Venus sinuosa are found. + +During the elevation of the Provinces of La Plata, the waters of the +ancient estuary have but little affected (with the exception of the sand- +hills on the banks of the Parana and Uruguay) the outline of the land. M. +Parchappe, however, has described groups of sand dunes scattered over the +wide extent of the Pampas southward of Buenos Ayres (D'Orbigny "Voyage +Geolog." page 44.), which M. d'Orbigny attributes with much probability to +the action of the sea, before the plains were raised above its level. +(Before proceeding to the districts southward of La Plata, it may be worth +while just to state, that there is some evidence that the coast of Brazil +has participated in a small amount of elevation. Mr. Burchell informs me, +that he collected at Santos (latitude 24 degrees S.) oyster-shells, +apparently recent, some miles from the shore, and quite above the tidal +action. Westward of Rio de Janeiro, Captain Elliot is asserted (see Harlan +"Med. and Phys. Res." page 35 and Dr. Meigs in "Transactions of the +American Philosophical Society"), to have found human bones, encrusted with +sea-shells, between fifteen and twenty feet above the level of the sea. +Between Rio de Janeiro and Cape Frio I crossed sandy tracts abounding with +sea-shells, at a distance of a league from the coast; but whether these +tracts have been formed by upheaval, or through the mere accumulation of +drift sand, I am not prepared to assert. At Bahia (latitude 13 degrees S.), +in some parts near the coast, there are traces of sea-action at the height +of about twenty feet above its present level; there are also, in many +parts, remnants of beds of sandstone and conglomerate with numerous recent +shells, raised a little above the sea-level. I may add, that at the head of +Bahia Bay there is a formation, about forty feet in thickness, containing +tertiary shells apparently of fresh-water origin, now washed by the sea and +encrusted with Balini; this appears to indicate a small amount of +subsidence subsequent to its deposition. At Pernambuco (latitude 8 degrees +S.), in the alluvial or tertiary cliffs, surrounding the low land on which +the city stands, I looked in vain for organic remains, or other evidence of +changes in level.) + +SOUTHWARD OF THE PLATA. + +The coast as far as Bahia Blanca (in latitude 39 degrees S.) is formed +either of a horizontal range of cliffs, or of immense accumulations of +sand-dunes. Within Bahia Blanca, a small piece of tableland, about twenty +feet above high-water mark, called Punta Alta, is formed of strata of +cemented gravel and of red earthy mud, abounding with shells (with others +lying loose on the surface), and the bones of extinct mammifers. These +shells, twenty in number, together with a Balanus and two corals, are all +recent species, still inhabiting the neighbouring seas. They will be +enumerated in the Fourth Chapter, when describing the Pampean formation; +five of them are identical with the upraised ones from near Buenos Ayres. +The northern shore of Bahia Blanca is, in main part, formed of immense +sand-dunes, resting on gravel with recent shells, and ranging in lines +parallel to the shore. These ranges are separated from each other by flat +spaces, composed of stiff impure red clay, in which, at the distance of +about two miles from the coast, I found by digging a few minute fragments +of sea-shells. The sand-dunes extend several miles inland, and stand on a +plain, which slopes up to a height of between one hundred and two hundred +feet. Numerous, small, well-rounded pebbles of pumice lie scattered both on +the plain and sand-hillocks: at Monte Hermoso, on the flat summit of a +cliff, I found many of them at a height of 120 feet (angular measurement) +above the level of the sea. These pumice pebbles, no doubt, were originally +brought down from the Cordillera by the rivers which cross the continent, +in the same way as the river Negro anciently brought down, and still brings +down, pumice, and as the river Chupat brings down scoriae: when once +delivered at the mouth of a river, they would naturally have travelled +along the coasts, and been cast up during the elevation of the land, at +different heights. The origin of the argillaceous flats, which separate the +parallel ranges of sand-dunes, seems due to the tides here having a +tendency (as I believe they have on most shoal, protected coasts) to throw +up a bar parallel to the shore, and at some distance from it; this bar +gradually becomes larger, affording a base for the accumulation of sand- +dunes, and the shallow space within then becomes silted up with mud. The +repetition of this process, without any elevation of the land, would form a +level plain traversed by parallel lines of sand-hillocks; during a slow +elevation of the land, the hillocks would rest on a gently inclined +surface, like that on the northern shore of Bahia Blanca. I did not observe +any shells in this neighbourhood at a greater height than twenty feet; and +therefore the age of the sea-drifted pebbles of pumice, now standing at the +height of 120 feet, must remain uncertain. + +The main plain surrounding Bahia Blanca I estimated at from two hundred to +three hundred feet; it insensibly rises towards the distant Sierra Ventana. +There are in this neighbourhood some other and lower plains, but they do +not abut one at the foot of the other, in the manner hereafter to be +described, so characteristic of Patagonia. The plain on which the +settlement stands is crossed by many low sand-dunes, abounding with the +minute shells of the Paludestrina australis, d'Orbigny, which now lives in +the bay. This low plain is bounded to the south, at the Cabeza del Buey, by +the cliff-formed margin of a wide plain of the Pampean formation, which I +estimated at sixty feet in height. On the summit of this cliff there is a +range of high sand-dunes extending several miles in an east and west line. + +Southward of Bahia Blanca, the river Colorado flows between two plains, +apparently from thirty to forty feet in height. Of these plains, the +southern one slopes up to the foot of the great sandstone plateau of the +Rio Negro; and the northern one against an escarpment of the Pampean +deposit; so that the Colorado flows in a valley fifty miles in width, +between the upper escarpments. I state this, because on the low plain at +the foot of the northern escarpment, I crossed an immense accumulation of +high sand-dunes, estimated by the Gauchos at no less than eight miles in +breadth. These dunes range westward from the coast, which is twenty miles +distant, to far inland, in lines parallel to the valley; they are separated +from each other by argillaceous flats, precisely like those on the northern +shore of Bahia Blanca. At present there is no source whence this immense +accumulation of sand could proceed; but if, as I believe, the upper +escarpments once formed the shores of an estuary, in that case the +sandstone formation of the river Negro would have afforded an inexhaustible +supply of sand, which would naturally have accumulated on the northern +shore, as on every part of the coast open to the south winds between Bahia +Blanca and Buenos Ayres. + +At San Blas (40 degrees 40' S.) a little south of the mouth of the +Colorado, M. d'Orbigny found fourteen species of existing shells (six of +them identical with those from Bahia Blanca), embedded in their natural +positions. ("Voyage" etc. page 54.) From the zone of depth which these +shells are known to inhabit, they must have been uplifted thirty-two feet. +He also found, at from fifteen to twenty feet above this bed, the remains +of an ancient beach. + +Ten miles southward, but 120 miles to the west, at Port S. Antonio, the +Officers employed on the Survey assured me that they saw many old sea- +shells strewed on the surface of the ground, similar to those found on +other parts of the coast of Patagonia. At San Josef, ninety miles south in +nearly the same longitude, I found, above the gravel, which caps an old +tertiary formation, an irregular bed and hillock of sand, several feet in +thickness, abounding with shells of Patella deaurita, Mytilus Magellanicus, +the latter retaining much of its colour; Fusus Magellanicus (and a variety +of the same), and a large Balanus (probably B. Tulipa), all now found on +this coast: I estimated this bed at from eighty to one hundred feet above +the level of the sea. To the westward of this bay, there is a plain +estimated at between two hundred and three hundred feet in height: this +plain seems, from many measurements, to be a continuation of the sandstone +platform of the river Negro. The next place southward, where I landed, was +at Port Desire, 340 miles distant; but from the intermediate districts I +received, through the kindness of the Officers of the Survey, especially +from Lieutenant Stokes and Mr. King, many specimens and sketches, quite +sufficient to show the general uniformity of the whole line of coast. I may +here state, that the whole of Patagonia consists of a tertiary formation, +resting on and sometimes surrounding hills of porphyry and quartz: the +surface is worn into many wide valleys and into level step-formed plains, +rising one above another, all capped by irregular beds of gravel, chiefly +composed of porphyritic rocks. This gravel formation will be separately +described at the end of the chapter. + +My object in giving the following measurements of the plains, as taken by +the Officers of the Survey, is, as will hereafter be seen, to show the +remarkable equability of the recent elevatory movements. Round the southern +parts of Nuevo Gulf, as far as the River Chupat (seventy miles southward of +San Josef), there appear to be several plains, of which the best defined +are here represented. + +(In the following Diagrams: +1. Baseline is Level of sea. +2. Scale is 1/20 of inch to 100 feet vertical. +3. Height is shown in feet thus: +An. M. always stands for angular or trigonometrical measurement. +Ba. M. always stands for barometrical measurement. +Est. always stands for estimation by the Officers of the Survey. + +DIAGRAM 1. SECTION OF STEP-FORMED PLAINS SOUTH OF NUEVO GULF. + +From East (sea level) to West (high): +Terrace 1. 80 Est. +Terrace 2. 200-220 An. M. +Terrace 3. 350 An. M.) + +The upper plain is here well defined (called Table Hills); its edge forms a +cliff or line of escarpment many miles in length, projecting over a lower +plain. The lowest plain corresponds with that at San Josef with the recent +shells on its surface. Between this lowest and the uppermost plain, there +is probably more than one step-formed terrace: several measurements show +the existence of the intermediate one of the height given in Diagram 1. + +(DIAGRAM 2. SECTION OF PLAINS IN THE BAY OF ST. GEORGE. + +From East (sea level) to West (high): +Terrace 1. 250 An. M. +Terrace 2. 330 An. M. +Terrace 3. 580 An. M. +Terraces 4, 5 and 6 not measured. +Terrace 7. 1,200 Est.) + +Near the north headland of the great Bay of St. George (100 miles south of +the Chupat), two well-marked plains of 250 and 330 feet were measured: +these are said to sweep round a great part of the Bay. At its south +headland, 120 miles distant from the north headland, the 250 feet plain was +again measured. In the middle of the bay, a higher plain was found at two +neighbouring places (Tilli Roads and C. Marques) to be 580 feet in height. +Above this plain, towards the interior, Mr. Stokes informs me that there +were several other step-formed plains, the highest of which was estimated +at 1,200 feet, and was seen ranging at apparently the same height for 150 +miles northward. All these plains have been worn into great valleys and +much denuded. The section in Diagram 3 is illustrative of the general +structure of the great Bay of St. George. At the south headland of the Bay +of St. George (near C. Three Points) the 250 plain is very extensive. + +(DIAGRAM 3. SECTION OF PLAINS AT PORT DESIRE. + +From East (sea level) to West (high): +Terrace 1. 100 Est. +Terrace 2. 245-255 Ba. M. Shells on surface. +Terrace 3. 330 Ba. M. Shells on surface. +Terrace 4. Not measured.) + +At Port Desire (forty miles southward) I made several measurements with the +barometer of a plain, which extends along the north side of the port and +along the open coast, and which varies from 245 to 255 feet in height: this +plain abuts against the foot of a higher plain of 330 feet, which extends +also far northward along the coast, and likewise into the interior. In the +distance a higher inland platform was seen, of which I do not know the +height. In three separate places, I observed the cliff of the 245-255 feet +plain, fringed by a terrace or narrow plain estimated at about one hundred +feet in height. These plains are represented in the section Diagram 3. + +In many places, even at the distance of three and four miles from the +coast, I found on the gravel-capped surface of the 245-255 feet, and of the +330 feet plain, shells of Mytilus Magellanicus, M. edulis, Patella +deaurita, and another Patella, too much worn to be identified, but +apparently similar to one found abundantly adhering to the leaves of the +kelp. These species are the commonest now living on this coast. The shells +all appeared very old; the blue of the mussels was much faded; and only +traces of colour could be perceived in the Patellas, of which the outer +surfaces were scaling off. They lay scattered on the smooth surface of the +gravel, but abounded most in certain patches, especially at the heads of +the smaller valleys: they generally contained sand in their insides; and I +presume that they have been washed by alluvial action out of thin sandy +layers, traces of which may sometimes be seen covering the gravel. The +several plains have very level surfaces; but all are scooped out by +numerous broad, winding, flat-bottomed valleys, in which, judging from the +bushes, streams never flow. These remarks on the state of the shells, and +on the nature of the plains, apply to the following cases, so need not be +repeated. + +(DIAGRAM 4. SECTION OF PLAINS AT PORT S. JULIAN. + +From East (sea level) to West (high): +Terrace 1. Shells on surface. 90 Est. +Terrace 2. 430 An. M. +Terrace 3. 560 An. M. +Terrace 4. 950 An. M.) + +Southward of Port Desire, the plains have been greatly denuded, with only +small pieces of tableland marking their former extension. But opposite Bird +Island, two considerable step-formed plains were measured, and found +respectively to be 350 and 590 feet in height. This latter plain extends +along the coast close to Port St. Julian (110 miles south of Port Desire); +see Diagram 4. + +The lowest plain was estimated at ninety feet: it is remarkable from the +usual gravel-bed being deeply worn into hollows, which are filled up with, +as well as the general surface covered by, sandy and reddish earthy matter: +in one of the hollows thus filled up, the skeleton of the Macrauchenia +Patachonica, as will hereafter be described, was embedded. On the surface +and in the upper parts of this earthy mass, there were numerous shells of +Mytilus Magellanicus and M. edulis, Patella deaurita, and fragments of +other species. This plain is tolerably level, but not extensive; it forms a +promontory seven or eight miles long, and three or four wide. The upper +plains in Diagram 4 were measured by the Officers of the Survey; they were +all capped by thick beds of gravel, and were all more or less denuded; the +950 plain consists merely of separate, truncated, gravel-capped hills, two +of which, by measurement, were found to differ only three feet. The 430 +feet plain extends, apparently with hardly a break, to near the northern +entrance of the Rio Santa Cruz (fifty miles to the south); but it was there +found to be only 330 feet in height. + +(DIAGRAM 5. SECTION OF PLAINS AT THE MOUTH OF THE RIO SANTA CRUZ. + +From East (sea level) to West (high): +Terrace 1. (sloping) 355 Ba. M. Shells on surface. 463 Ba. M. +Terrace 2. 710 An. M. +Terrace 3. 840 An. M.) + +On the southern side of the mouth of the Santa Cruz we have Diagram 5, +which I am able to give with more detail than in the foregoing cases. + +The plain marked 355 feet (as ascertained by the barometer and by angular +measurement) is a continuation of the above-mentioned 330 feet plain: it +extends in a N.W. direction along the southern shores of the estuary. It is +capped by gravel, which in most parts is covered by a thin bed of sandy +earth, and is scooped out by many flat-bottomed valleys. It appears to the +eye quite level, but in proceeding in a S.S.W. course, towards an +escarpment distant about six miles, and likewise ranging across the country +in a N.W. line, it was found to rise at first insensibly, and then for the +last half-mile, sensibly, close up to the base of the escarpment: at this +point it was 463 feet in height, showing a rise of 108 feet in the six +miles. On this 355-463 feet plain, I found several shells of Mytilus +Magellanicus and of a Mytilus, which Mr. Sowerby informs me is yet unnamed, +though well-known as recent on this coast; Patella deaurita; Fusus, I +believe, Magellanicus, but the specimen has been lost; and at the distance +of four miles from the coast, at the height of about four hundred feet, +there were fragments of the same Patella and of a Voluta (apparently V. +ancilla) partially embedded in the superficial sandy earth. All these +shells had the same ancient appearance with those from the foregoing +localities. As the tides along this part of the coast rise at the Syzygal +period forty feet, and therefore form a well-marked beach-line, I +particularly looked out for ridges in crossing this plain, which, as we +have seen, rises 108 feet in about six miles, but I could not see any +traces of such. The next highest plain is 710 feet above the sea; it is +very narrow, but level, and is capped with gravel; it abuts to the foot of +the 840 feet plain. This summit-plain extends as far as the eye can range, +both inland along the southern side of the valley of the Santa Cruz, and +southward along the Atlantic. + +THE VALLEY OF THE R. SANTA CRUZ. + +This valley runs in an east and west direction to the Cordillera, a +distance of about one hundred and sixty miles. It cuts through the great +Patagonian tertiary formation, including, in the upper half of the valley, +immense streams of basaltic lava, which as well as the softer beds, are +capped by gravel; and this gravel, high up the river, is associated with a +vast boulder formation. (I have described this formation in a paper in the +"Geological Transactions" volume 6 page 415.) In ascending the valley, the +plain which at the mouth on the southern side is 355 feet high, is seen to +trend towards the corresponding plain on the northern side, so that their +escarpments appear like the shores of a former estuary, larger than the +existing one: the escarpments, also, of the 840 feet summit-plain (with a +corresponding northern one, which is met with some way up the valley), +appear like the shores of a still larger estuary. Farther up the valley, +the sides are bounded throughout its entire length by level, gravel-capped +terraces, rising above each other in steps. The width between the upper +escarpments is on an average between seven and ten miles; in one spot, +however, where cutting through the basaltic lava, it was only one mile and +a half. Between the escarpments of the second highest terrace the average +width is about four or five miles. The bottom of the valley, at the +distance of 110 miles from its mouth, begins sensibly to expand, and soon +forms a considerable plain, 440 feet above the level of the sea, through +which the river flows in a gut from twenty to forty feet in depth. I here +found, at a point 140 miles from the Atlantic, and seventy miles from the +nearest creek of the Pacific, at the height of 410 feet, a very old and +worn shell of Patella deaurita. Lower down the valley, 105 miles from the +Atlantic (longitude 71 degrees W.), and at an elevation of about 300 feet, +I also found, in the bed of the river, two much worn and broken shells of +the Voluta ancilla, still retaining traces of their colours; and one of the +Patella deaurita. It appeared that these shells had been washed from the +banks into the river; considering the distance from the sea, the desert and +absolutely unfrequented character of the country, and the very ancient +appearance of the shells (exactly like those found on the plains nearer the +coast), there is, I think, no cause to suspect that they could have been +brought here by Indians. + +The plain at the head of the valley is tolerably level, but water-worn, and +with many sand-dunes on it like those on a sea-coast. At the highest point +to which we ascended, it was sixteen miles wide in a north and south line; +and forty-five miles in length in an east and west line. It is bordered by +the escarpments, one above the other, of two plains, which diverge as they +approach the Cordillera, and consequently resemble, at two levels, the +shores of great bays facing the mountains; and these mountains are breached +in front of the lower plain by a remarkable gap. The valley, therefore, of +the Santa Cruz consists of a straight broad cut, about ninety miles in +length, bordered by gravel-capped terraces and plains, the escarpments of +which at both ends diverge or expand, one over the other, after the manner +of the shores of great bays. Bearing in mind this peculiar form of the +land--the sand-dunes on the plain at the head of the valley--the gap in the +Cordillera, in front of it--the presence in two places of very ancient +shells of existing species--and lastly, the circumstance of the 355-453 +feet plain, with the numerous marine remains on its surface, sweeping from +the Atlantic coast, far up the valley, I think we must admit, that within +the recent period, the course of the Santa Cruz formed a sea-strait +intersecting the continent. At this period, the southern part of South +America consisted of an archipelago of islands 360 miles in a north and +south line. We shall presently see, that two other straits also, since +closed, then cut through Tierra del Fuego; I may add, that one of them must +at that time have expanded at the foot of the Cordillera into a great bay +(now Otway Water) like that which formerly covered the 440 feet plain at +the head of the Santa Cruz. + +(DIAGRAM 6. NORTH AND SOUTH SECTION ACROSS THE TERRACES BOUNDING THE VALLEY +OF THE RIVER SANTA CRUZ, HIGH UP ITS COURSE. + +The height of each terrace, above the level of the river (furthest to +nearest to the river) in feet: + +A, north and south: 1,122 +B, north and south: 869 +C, north and south: 639 +D, north: not measured. D, north? (suggest south): 185 +E: 20 +Bed of River. + +Vertical scale 1/20 of inch to 100 feet; but terrace E, being only twenty +feet above the river, has necessarily been raised. The horizontal distances +much contracted; the distance from the edge of A North to A South being on +an average from seven to ten miles.) +I have said that the valley in its whole course is bordered by gravel- +capped plains. The section (Diagram 6), supposed to be drawn in a north and +south line across the valley, can scarcely be considered as more than +illustrative; for during our hurried ascent it was impossible to measure +all the plains at any one place. At a point nearly midway between the +Cordillera and the Atlantic, I found the plain (A north) 1,122 feet above +the river; all the lower plains on this side were here united into one +great broken cliff: at a point sixteen miles lower down the stream, I found +by measurement and estimation that B (north) was 869 above the river: very +near to where A (north) was measured, C (north) was 639 above the same +level: the terrace D (north) was nowhere measured: the lowest E (north) was +in many places about twenty feet above the river. These plains or terraces +were best developed where the valley was widest; the whole five, like +gigantic steps, occurred together only at a few points. The lower terraces +are less continuous than the higher ones, and appear to be entirely lost in +the upper third of the valley. Terrace C (south), however was traced +continuously for a great distance. The terrace B (north), at a point fifty- +five miles from the mouth of the river, was four miles in width; higher up +the valley this terrace (or at least the second highest one, for I could +not always trace it continuously) was about eight miles wide. This second +plain was generally wider than the lower ones--as indeed follows from the +valley from A (north) to A (south) being generally nearly double the width +of from B (north) to B (south). Low down the valley, the summit-plain A +(south) is continuous with the 840 feet plain on the coast, but it is soon +lost or unites with the escarpment of B (south). The corresponding plain A +(north), on the north side of the valley, appears to range continuously +from the Cordillera to the head of the present estuary of the Santa Cruz, +where it trends northward towards Port St. Julian. Near the Cordillera the +summit-plain on both sides of the valley is between 3,200 and 3,300 feet in +height; at 100 miles from the Atlantic, it is 1,416 feet, and on the coast +840 feet, all above the sea-beach; so that in a distance of 100 miles the +plain rises 576 feet, and much more rapidly near to the Cordillera. The +lower terraces B and C also appear to rise as they run up the valley; thus +D (north), measured at two points twenty-four miles apart, was found to +have risen 185 feet. From several reasons I suspect, that this gradual +inclination of the plains up the valley, has been chiefly caused by the +elevation of the continent in mass, having been the greater the nearer to +the Cordillera. + +All the terraces are capped with well-rounded gravel, which rests either on +the denuded and sometimes furrowed surface of the soft tertiary deposits, +or on the basaltic lava. The difference in height between some of the lower +steps or terraces seems to be entirely owing to a difference in the +thickness of the capping gravel. Furrows and inequalities in the gravel, +where such occur, are filled up and smoothed over with sandy earth. The +pebbles, especially on the higher plains, are often whitewashed, and even +cemented together by a white aluminous substance, and I occasionally found +this to be the case with the gravel on the terrace D. I could not perceive +any trace of a similar deposition on the pebbles now thrown up by the +river, and therefore I do not think that terrace D was river-formed. As the +terrace E generally stands about twenty feet above the bed of the river, my +first impression was to doubt whether even this lowest one could have been +so formed; but it should always be borne in mind, that the horizontal +upheaval of a district, by increasing the total descent of the streams, +will always tend to increase, first near the sea-coast and then further and +further up the valley, their corroding and deepening powers: so that an +alluvial plain, formed almost on a level with a stream, will, after an +elevation of this kind, in time be cut through, and left standing at a +height never again to be reached by the water. With respect to the three +upper terraces of the Santa Cruz, I think there can be no doubt, that they +were modelled by the sea, when the valley was occupied by a strait, in the +same manner (hereafter to be discussed) as the greater step-formed, shell- +strewed plains along the coast of Patagonia. + +To return to the shores of the Atlantic: the 840 feet plain, at the mouth +of the Santa Cruz, is seen extending horizontally far to the south; and I +am informed by the Officers of the Survey, that bending round the head of +Coy Inlet (sixty-five miles southward), it trends inland. Outliers of +apparently the same height are seen forty miles farther south, inland of +the river Gallegos; and a plain comes down to Cape Gregory (thirty-five +miles southward), in the Strait of Magellan, which was estimated at between +eight hundred and one thousand feet in height, and which, rising towards +the interior, is capped by the boulder formation. South of the Strait of +Magellan, there are large outlying masses of apparently the same great +tableland, extending at intervals along the eastern coast of Tierra del +Fuego: at two places here, 110 miles a part, this plain was found to be 950 +and 970 feet in height. + +From Coy Inlet, where the high summit-plain trends inland, a plain +estimated at 350 feet in height, extends for forty miles to the river +Gallegos. From this point to the Strait of Magellan, and on each side of +that Strait, the country has been much denuded and is less level. It +consists chiefly of the boulder formation, which rises to a height of +between one hundred and fifty and two hundred and fifty feet, and is often +capped by beds of gravel. At N.S. Gracia, on the north side of the Inner +Narrows of the Strait of Magellan, I found on the summit of a cliff, 160 +feet in height, shells of existing Patellae and Mytili, scattered on the +surface and partially embedded in earth. On the eastern coast, also, of +Tierra del Fuego, in latitude 53 degrees 20' south, I found many Mytili on +some level land, estimated at 200 feet in height. Anterior to the elevation +attested by these shells, it is evident by the present form of the land, +and by the distribution of the great erratic boulders on the surface, that +two sea-channels connected the Strait of Magellan both with Sebastian Bay +and with Otway Water. ("Geological Transactions" volume 6 page 419.) + +CONCLUDING REMARKS ON THE RECENT ELEVATION OF THE SOUTH-EASTERN COASTS OF +AMERICA, AND ON THE ACTION OF THE SEA ON THE LAND. + +Upraised shells of species, still existing as the commonest kinds in the +adjoining sea, occur, as we have seen, at heights of between a few feet and +410 feet, at intervals from latitude 33 degrees 40' to 53 degrees 20' +south. This is a distance of 1,180 geographical miles--about equal from +London to the North Cape of Sweden. As the boulder formation extends with +nearly the same height 150 miles south of 53 degrees 20', the most southern +point where I landed and found upraised shells; and as the level Pampas +ranges many hundred miles northward of the point, where M. d'Orbigny found +at the height of 100 feet beds of the Azara, the space in a north and south +line, which has been uplifted within the recent period, must have been much +above the 1,180 miles. By the term "recent," I refer only to that period +within which the now living mollusca were called into existence; for it +will be seen in the Fourth Chapter, that both at Bahia Blanca and P. S. +Julian, the mammiferous quadrupeds which co-existed with these shells +belong to extinct species. I have said that the upraised shells were found +only at intervals on this line of coast, but this in all probability may be +attributed to my not having landed at the intermediate points; for wherever +I did land, with the exception of the river Negro, shells were found: +moreover, the shells are strewed on plains or terraces, which, as we shall +immediately see, extend for great distances with a uniform height. I +ascended the higher plains only in a few places, owing to the distance at +which their escarpments generally range from the coast, so that I am far +from knowing that 410 feet is the maximum of elevation of these upraised +remains. The shells are those now most abundant in a living state in the +adjoining sea. (Captain King "Voyages of 'Adventure' and 'Beagle'" volume 1 +pages 6 and 133.) All of them have an ancient appearance; but some, +especially the mussels, although lying fully exposed to the weather, retain +to a considerable extent their colours: this circumstance appears at first +surprising, but it is now known that the colouring principle of the Mytilus +is so enduring, that it is preserved when the shell itself is completely +disintegrated. (See Mr. Lyell "Proofs of a Gradual Rising in Sweden" in the +"Philosophical Transactions" 1835 page 1. See also Mr. Smith of Jordan Hill +in the "Edinburgh New Philosophical Journal" volume 25 page 393.) Most of +the shells are broken; I nowhere found two valves united; the fragments are +not rounded, at least in none of the specimens which I brought home. + +With respect to the breadth of the upraised area in an east and west line, +we know from the shells found at the Inner Narrows of the Strait of +Magellan, that the entire width of the plain, although there very narrow, +has been elevated. It is probable that in this southernmost part of the +continent, the movement has extended under the sea far eastward; for at the +Falkland Islands, though I could not find any shells, the bones of whales +have been noticed by several competent observers, lying on the land at a +considerable distance from the sea, and at the height of some hundred feet +above it. ("Voyages of the 'Adventure' and 'Beagle'" volume 2 page 227. And +Bougainville's "Voyage" tome 1 page 112.) Moreover, we know that in Tierra +del Fuego the boulder formation has been uplifted within the recent period, +and a similar formation occurs on the north-western shores (Byron Sound) of +these islands. (I owe this fact to the kindness of Captain Sulivan, R.N., a +highly competent observer. I mention it more especially, as in my Paper +(page 427) on the Boulder Formation, I have, after having examined the +northern and middle parts of the eastern island, said that the formation +was here wholly absent.) The distance from this point to the Cordillera of +Tierra del Fuego, is 360 miles, which we may take as the probable width of +the recently upraised area. In the latitude of the R. Santa Cruz, we know +from the shells found at the mouth and head, and in the middle of the +valley, that the entire width (about 160 miles) of the surface eastward of +the Cordillera has been upraised. From the slope of the plains, as shown by +the course of the rivers, for several degrees northward of the Santa Cruz, +it is probable that the elevation attested by the shells on the coast has +likewise extended to the Cordillera. When, however, we look as far +northward as the provinces of La Plata, this conclusion would be very +hazardous; not only is the distance from Maldonado (where I found upraised +shells) to the Cordillera great, namely, 760 miles, but at the head of the +estuary of the Plata, a N.N.E. and S.S.W. range of tertiary volcanic rocks +has been observed (This volcanic formation will be described in Chapter IV. +It is not improbable that the height of the upraised shells at the head of +the estuary of the Plata, being greater than at Bahia Blanca or at San +Blas, may be owing to the upheaval of these latter places having been +connected with the distant line of the Cordillera, whilst that of the +provinces of La Plata was in connection with the adjoining tertiary +volcanic axis.), which may well indicate an axis of elevation quite +distinct from that of the Andes. Moreover, in the centre of the Pampas in +the chain of Cordova, severe earthquakes have been felt (See Sir W. +Parish's work on "La Plata" page 242. For a notice of an earthquake which +drained a lake near Cordova, see also Temple's "Travels in Peru." Sir W. +Parish informs me, that a town between Salta and Tucuman (north of Cordova) +was formerly utterly overthrown by an earthquake.); whereas at Mendoza, at +the eastern foot of the Cordillera, only gentle oscillations, transmitted +from the shores of the Pacific, have ever been experienced. Hence the +elevation of the Pampas may be due to several distinct axes of movement; +and we cannot judge, from the upraised shells round the estuary of the +Plata, of the breadth of the area uplifted within the recent period. + +Not only has the above specified long range of coast been elevated within +the recent period, but I think it may be safely inferred from the +similarity in height of the gravel-capped plains at distant points, that +there has been a remarkable degree of equability in the elevatory process. +I may premise, that when I measured the plains, it was simply to ascertain +the heights at which shells occurred; afterwards, comparing these +measurements with some of those made during the Survey, I was struck with +their uniformity, and accordingly tabulated all those which represented the +summit-edges of plains. The extension of the 330 to 355 feet plain is very +striking, being found over a space of 500 geographical miles in a north and +south line. A table (Table 1) of the measurements is given below. The +angular measurements and all the estimations (in feet) are by the Officers +of the Survey; the barometrical ones by myself:-- + +TABLE 1. + +Gallegos River to Coy Inlet (partly angular partly estimation) 350 +South Side of Santa Cruz (angular and barometric) 355 +North Side of Santa Cruz (angular and barometric) 330 +Bird Island, plain opposite to (angular) 350 +Port Desire, plain extending far along coast (barometric) 330 +St. George's Bay, north promontory (angular) 330 +Table Land, south of New Bay (angular) 350 + +A plain, varying from 245 to 255 feet, seems to extend with much uniformity +from Port Desire to the north of St. George's Bay, a distance of 170 miles; +and some approximate measurements (in feet), also given in Table 2 below, +indicate the much greater extension of 780 miles:-- + +TABLE 2. + +Coy Inlet, south of (partly angular and partly estimation) 200 to 300 +Port Desire (barometric) 245 to 255 +C. Blanco (angular) 250 +North Promontory of St. George's Bay (angular) 250 +South of New Bay (angular) 200 to 220 +North of S. Josef (estimation) 200 to 300 +Plain of Rio Negro (angular) 200 to 220 +Bahia Blanca (estimation) 200 to 300 + +The extension, moreover, of the 560 to 580, and of the 80 to 100 feet, +plains is remarkable, though somewhat less obvious than in the former +cases. Bearing in mind that I have not picked these measurements out of a +series, but have used all those which represented the edges of plains, I +think it scarcely possible that these coincidences in height should be +accidental. We must therefore conclude that the action, whatever it may +have been, by which these plains have been modelled into their present +forms, has been singularly uniform. + +These plains or great terraces, of which three and four often rise like +steps one behind the other, are formed by the denudation of the old +Patagonian tertiary beds, and by the deposition on their surfaces of a mass +of well-rounded gravel, varying, near the coast, from ten to thirty-five +feet in thickness, but increasing in thickness towards the interior. The +gravel is often capped by a thin irregular bed of sandy earth. The plains +slope up, though seldom sensibly to the eye, from the summit edge of one +escarpment to the foot of the next highest one. Within a distance of 150 +miles, between Santa Cruz to Port Desire, where the plains are particularly +well developed, there are at least seven stages or steps, one above the +other. On the three lower ones, namely, those of 100 feet, 250 feet, and +350 feet in height, existing littoral shells are abundantly strewed, either +on the surface, or partially embedded in the superficial sandy earth. By +whatever action these three lower plains have been modelled, so undoubtedly +have all the higher ones, up to a height of 950 feet at S. Julian, and of +1,200 feet (by estimation) along St. George's Bay. I think it will not be +disputed, considering the presence of the upraised marine shells, that the +sea has been the active power during stages of some kind in the elevatory +process. + +We will now briefly consider this subject: if we look at the existing +coast-line, the evidence of the great denuding power of the sea is very +distinct; for, from Cape St. Diego, in latitude 54 degrees 30' to the mouth +of the Rio Negro, in latitude 31 degrees (a length of more than eight +hundred miles), the shore is formed, with singularly few exceptions, of +bold and naked cliffs: in many places the cliffs are high; thus, south of +the Santa Cruz, they are between eight and nine hundred feet in height, +with their horizontal strata abruptly cut off, showing the immense mass of +matter which has been removed. Nearly this whole line of coast consists of +a series of greater or lesser curves, the horns of which, and likewise +certain straight projecting portions, are formed of hard rocks; hence the +concave parts are evidently the effect and the measure of the denuding +action on the softer strata. At the foot of all the cliffs, the sea shoals +very gradually far outwards; and the bottom, for a space of some miles, +everywhere consists of gravel. I carefully examined the bed of the sea off +the Santa Cruz, and found that its inclination was exactly the same, both +in amount and in its peculiar curvature, with that of the 355 feet plain at +this same place. If, therefore, the coast, with the bed of the adjoining +sea, were now suddenly elevated one or two hundred feet, an inland line of +cliffs, that is an escarpment, would be formed, with a gravel-capped plain +at its foot gently sloping to the sea, and having an inclination like that +of the existing 355 feet plain. From the denuding tendency of the sea, this +newly formed plain would in time be eaten back into a cliff: and +repetitions of this elevatory and denuding process would produce a series +of gravel-capped sloping terraces, rising one above another, like those +fronting the shores of Patagonia. + +The chief difficulty (for there are other inconsiderable ones) on this +view, is the fact,--as far as I can trust two continuous lines of soundings +carefully taken between Santa Cruz and the Falkland Islands, and several +scattered observations on this and other coasts,--that the pebbles at the +bottom of the sea QUICKLY and REGULARLY decrease in size with the +increasing depth and distance from the shore, whereas in the gravel on the +sloping plains, no such decrease in size was perceptible. + +Table 3 below gives the average result of many soundings off the Santa +Cruz:-- +TABLE 3. + +Under two miles from the shore, many of the pebbles were of large size, +mingled with some small ones. + +Column 1. Distance in miles from the shore. + +Column 2. Depth in fathoms. + +Column 3. Size of Pebbles. + +1. 2. 3. + +3 to 4 11 to 12 As large as walnuts; mingled in every case with + some smaller ones. + +6 to 7 17 to 19 As large as hazel-nuts. + +10 to 11 23 to 25 From three- to four-tenths of an inch in diameter. + +12 30 to 40 Two-tenths of an inch. + +22 to 150 45 to 65 One-tenth of an inch, to the finest sand. + +I particularly attended to the size of the pebbles on the 355 feet Santa +Cruz plain, and I noticed that on the summit-edge of the present sea cliffs +many were as large as half a man's head; and in crossing from these cliffs +to the foot of the next highest escarpment, a distance of six miles, I +could not observe any increase in their size. We shall presently see that +the theory of a slow and almost insensible rise of the land, will explain +all the facts connected with the gravel-capped terraces, better than the +theory of sudden elevations of from one to two hundred feet. + +M. d'Orbigny has argued, from the upraised shells at San Blas being +embedded in the positions in which they lived, and from the valves of the +Azara labiata high on the banks of the Parana being united and unrolled, +that the elevation of Northern Patagonia and of La Plata must have been +sudden; for he thinks, if it had been gradual, these shells would all have +been rolled on successive beach-lines. But in PROTECTED bays, such as in +that of Bahia Blanca, wherever the sea is accumulating extensive mud-banks, +or where the winds quietly heap up sand-dunes, beds of shells might +assuredly be preserved buried in the positions in which they had lived, +even whilst the land retained the same level; any, the smallest, amount of +elevation would directly aid in their preservation. I saw a multitude of +spots in Bahia Blanca where this might have been effected; and at Maldonado +it almost certainly has been effected. In speaking of the elevation of the +land having been slow, I do not wish to exclude the small starts which +accompany earthquakes, as on the coast of Chile; and by such movements beds +of shells might easily be uplifted, even in positions exposed to a heavy +surf, without undergoing any attrition: for instance, in 1835, a rocky flat +off the island of Santa Maria was at one blow upheaved above high-water +mark, and was left covered with gaping and putrefying mussel-shells, still +attached to the bed on which they had lived. If M. d'Orbigny had been aware +of the many long parallel lines of sand-hillocks, with infinitely numerous +shells of the Mactra and Venus, at a low level near the Uruguay; if he had +seen at Bahia Blanca the immense sand-dunes, with water-worn pebbles of +pumice, ranging in parallel lines, one behind the other, up a height of at +least 120 feet; if he had seen the sand-dunes, with the countless +Paludestrinas, on the low plain near the Fort at this place, and that long +line on the edge of the cliff, sixty feet higher up; if he had crossed that +long and great belt of parallel sand-dunes, eight miles in width, standing +at the height of from forty to fifty feet above the Colorado, where sand +could not now collect,--I cannot believe he would have thought that the +elevation of this great district had been sudden. Certainly the sand-dunes +(especially when abounding with shells), which stand in ranges at so many +different levels, must all have required long time for their accumulation; +and hence I do not doubt that the last 100 feet of elevation of La Plata +and Northern Patagonia has been exceedingly slow. + +If we extend this conclusion to Central and Southern Patagonia, the +inclination of the successively rising gravel-capped plains can be +explained quite as well, as by the more obvious view already given of a few +comparatively great and sudden elevations; in either case we must admit +long periods of rest, during which the sea ate deeply into the land. Let us +suppose the present coast to rise at a nearly equable, slow rate, yet +sufficiently quick to prevent the waves quite removing each part as soon as +brought up; in this case every portion of the present bed of the sea will +successively form a beach-line, and from being exposed to a like action +will be similarly affected. It cannot matter to what height the tides rise, +even if to forty feet as at Santa Cruz, for they will act with equal force +and in like manner on each successive line. Hence there is no difficulty in +the fact of the 355 feet plain at Santa Cruz sloping up 108 feet to the +foot of the next highest escarpment, and yet having no marks of any one +particular beach-line on it; for the whole surface on this view has been a +beach. I cannot pretend to follow out the precise action of the tidal-waves +during a rise of the land, slow, yet sufficiently quick to prevent or check +denudation: but if it be analogous to what takes place on protected parts +of the present coast, where gravel is now accumulating in large quantities, +an inclined surface, thickly capped by well-rounded pebbles of about the +same size, would be ultimately left. (On the eastern side of Chiloe, which +island we shall see in the next chapter is now rising, I observed that all +the beaches and extensive tidal-flats were formed of shingle.) On the +gravel now accumulating, the waves, aided by the wind, sometimes throw up a +thin covering of sand, together with the common coast-shells. Shells thus +cast up by gales, would, during an elevatory period, never again be touched +by the sea. Hence, on this view of a slow and gradual rising of the land, +interrupted by periods of rest and denudation, we can understand the +pebbles being of about the same size over the entire width of the step-like +plains,--the occasional thin covering of sandy earth,--and the presence of +broken, unrolled fragments of those shells, which now live exclusively near +the coast. + +SUMMARY OF RESULTS. + +It may be concluded that the coast on this side of the continent, for a +space of at least 1,180 miles, has been elevated to a height of 100 feet in +La Plata, and of 400 feet in Southern Patagonia, within the period of +existing shells, but not of existing mammifers. That in La Plata the +elevation has been very slowly effected: that in Patagonia the movement may +have been by considerable starts, but much more probably slow and quiet. In +either case, there have been long intervening periods of comparative rest, +during which the sea corroded deeply, as it is still corroding, into the +land. (I say COMPARATIVE and not ABSOLUTE rest, because the sea acts, as we +have seen, with great denuding power on this whole line of coast; and +therefore, during an elevation of the land, if excessively slow (and of +course during a subsidence of the land), it is quite possible that lines of +cliff might be formed.) That the periods of denudation and elevation were +contemporaneous and equable over great spaces of coast, as shown by the +equable heights of the plains; that there have been at least eight periods +of denudation, and that the land, up to a height of from 950 to 1,200 feet, +has been similarly modelled and affected: that the area elevated, in the +southernmost part of the continent, extended in breadth to the Cordillera, +and probably seaward to the Falkland Islands; that northward, in La Plata, +the breadth is unknown, there having been probably more than one axis of +elevation; and finally, that, anterior to the elevation attested by these +upraised shells, the land was divided by a Strait where the River Santa +Cruz now flows, and that further southward there were other sea-straits, +since closed. I may add, that at Santa Cruz, in latitude 50 degrees S., the +plains have been uplifted at least 1,400 feet, since the period when +gigantic boulders were transported between sixty and seventy miles from +their parent rock, on floating icebergs. + +Lastly, considering the great upward movements which this long line of +coast has undergone, and the proximity of its southern half to the volcanic +axis of the Cordillera, it is highly remarkable that in the many fine +sections exposed in the Pampean, Patagonian tertiary, and Boulder +formations, I nowhere observed the smallest fault or abrupt curvature in +the strata. + +GRAVEL FORMATION OF PATAGONIA. + +I will here describe in more detail than has been as yet incidentally done, +the nature, origin, and extent of the great shingle covering of Patagonia: +but I do not mean to affirm that all of this shingle, especially that on +the higher plains, belongs to the recent period. A thin bed of sandy earth, +with small pebbles of various porphyries and of quartz, covering a low +plain on the north side of the Rio Colorado, is the extreme northern limit +of this formation. These little pebbles have probably been derived from the +denudation of a more regular bed of gravel, capping the old tertiary +sandstone plateau of the Rio Negro. The gravel-bed near the Rio Negro is, +on an average, about ten or twelve feet in thickness; and the pebbles are +larger than on the northern side of the Colorado, being from one or two +inches in diameter, and composed chiefly of rather dark-tinted porphyries. +Amongst them I here first noticed a variety often to be referred to, +namely, a peculiar gallstone-yellow siliceous porphyry, frequently, but not +invariably, containing grains of quartz. The pebbles are embedded in a +white, gritty, calcareous matrix, very like mortar, sometimes merely +coating with a whitewash the separate stones, and sometimes forming the +greater part of the mass. In one place I saw in the gravel concretionary +nodules (not rounded) of crystallised gypsum, some as large as a man's +head. I traced this bed for forty-five miles inland, and was assured that +it extended far into the interior. As the surface of the calcareo- +argillaceous plain of Pampean formation, on the northern side of the wide +valley of the Colorado, stands at about the same height with the mortar- +like cemented gravel capping the sandstone on the southern side, it is +probable, considering the apparent equability of the subterranean movements +along this side of America, that this gravel of the Rio Negro and the upper +beds of the Pampean formation northward of the Colorado, are of nearly +contemporaneous origin, and that the calcareous matter has been derived +from the same source. + +Southward of the Rio Negro, the cliffs along the great bay of S. Antonio +are capped with gravel: at San Josef, I found that the pebbles closely +resembled those on the plain of the Rio Negro, but that they were not +cemented by calcareous matter. Between San Josef and Port Desire, I was +assured by the Officers of the Survey that the whole face of the country is +coated with gravel. At Port Desire and over a space of twenty-five miles +inland, on the three step-formed plains and in the valleys, I everywhere +passed over gravel which, where thickest, was between thirty and forty +feet. Here, as in other parts of Patagonia, the gravel, or its sandy +covering, was, as we have seen, often strewed with recent marine shells. +The sandy covering sometimes fills up furrows in the gravel, as does the +gravel in the underlying tertiary formations. The pebbles are frequently +whitewashed and even cemented together by a peculiar, white, friable, +aluminous, fusible substance, which I believe is decomposed feldspar. At +Port Desire, the gravel rested sometimes on the basal formation of +porphyry, and sometimes on the upper or the lower denuded tertiary strata. +It is remarkable that most of the porphyritic pebbles differ from those +varieties of porphyry which occur here abundantly in situ. The peculiar +gallstone-yellow variety was common, but less numerous than at Port S. +Julian, where it formed nearly one-third of the mass of the gravel; the +remaining part there consisting of pale grey and greenish porphyries with +many crystals of feldspar. At Port S. Julian, I ascended one of the flat- +topped hills, the denuded remnant of the highest plain, and found it, at +the height of 950 feet, capped with the usual bed of gravel. + +Near the mouth of the Santa Cruz, the bed of gravel on the 355 feet plain +is from twenty to about thirty-five feet in thickness. The pebbles vary +from minute ones to the size of a hen's egg, and even to that of half a +man's head; they consist of paler varieties of porphyry than those found +further northward, and there are fewer of the gallstone-yellow kind; +pebbles of compact black clay-slate were here first observed. The gravel, +as we have seen, covers the step-formed plains at the mouth, head, and on +the sides of the great valley of the Santa Cruz. At a distance of 110 miles +from the coast, the plain has risen to the height of 1,416 feet above the +sea; and the gravel, with the associated great boulder formation, has +attained a thickness of 212 feet. The plain, apparently with its usual +gravel covering, slopes up to the foot of the Cordillera to the height of +between 3,200 and 3,300 feet. In ascending the valley, the gravel gradually +becomes entirely altered in character: high up, we have pebbles of +crystalline feldspathic rocks, compact clay-slate, quartzose schists, and +pale-coloured porphyries; these rocks, judging both from the gigantic +boulders in the surface and from some small pebbles embedded beneath 700 +feet in thickness of the old tertiary strata, are the prevailing kinds in +this part of the Cordillera; pebbles of basalt from the neighbouring +streams of basaltic lava are also numerous; there are few or none of the +reddish or of the gallstone-yellow porphyries so common near the coast. +Hence the pebbles on the 350 feet plain at the mouth of the Santa Cruz +cannot have been derived (with the exception of those of compact clay- +slate, which, however, may equally well have come from the south) from the +Cordillera in this latitude; but probably, in chief part, from farther +north. + +Southward of the Santa Cruz, the gravel may be seen continuously capping +the great 840 feet plain: at the Rio Gallegos, where this plain is +succeeded by a lower one, there is, as I am informed by Captain Sulivan, an +irregular covering of gravel from ten to twelve feet in thickness over the +whole country. The district on each side of the Strait of Magellan is +covered up either with gravel or the boulder formation: it was interesting +to observe the marked difference between the perfectly rounded state of the +pebbles in the great shingle formation of Patagonia, and the more or less +angular fragments in the boulder formation. The pebbles and fragments near +the Strait of Magellan nearly all belong to rocks known to occur in Fuegia. +I was therefore much surprised in dredging south of the Strait to find, in +latitude 54 degrees 10' south, many pebbles of the gallstone-yellow +siliceous porphyry; I procured others from a great depth off Staten Island, +and others were brought me from the western extremity of the Falkland +Islands. (At my request, Mr. Kent collected for me a bag of pebbles from +the beach of White Rock harbour, in the northern part of the sound, between +the two Falkland Islands. Out of these well-rounded pebbles, varying in +size from a walnut to a hen's egg, with some larger, thirty-eight evidently +belonged to the rocks of these islands; twenty-six were similar to the +pebbles of porphyry found on the Patagonian plains, which rocks do not +exist in situ in the Falklands; one pebble belonged to the peculiar yellow +siliceous porphyry; thirty were of doubtful origin.) The distribution of +the pebbles of this peculiar porphyry, which I venture to affirm is not +found in situ either in Fuegia, the Falkland Islands, or on the coast of +Patagonia, is very remarkable, for they are found over a space of 840 miles +in a north and south line, and at the Falklands, 300 miles eastward of the +coast of Patagonia. Their occurrence in Fuegia and the Falklands may, +however, perhaps be due to the same ice-agency by which the boulders have +been there transported. + +We have seen that porphyritic pebbles of a small size are first met with on +the northern side of the Rio Colorado, the bed becoming well developed near +the Rio Negro: from this latter point I have every reason to believe that +the gravel extends uninterruptedly over the plains and valleys of Patagonia +for at least 630 nautical miles southward to the Rio Gallegos. From the +slope of the plains, from the nature of the pebbles, from their extension +at the Rio Negro far into the interior, and at the Santa Cruz close up to +the Cordillera, I think it highly probable that the whole breadth of +Patagonia is thus covered. If so, the average width of the bed must be +about two hundred miles. Near the coast the gravel is generally from ten to +thirty feet in thickness; and as in the valley of Santa Cruz it attains, at +some distance from the Cordillera, a thickness of 214 feet, we may, I +think, safely assume its average thickness over the whole area of 630 by +200 miles, at fifty feet! + +The transportal and origin of this vast bed of pebbles is an interesting +problem. From the manner in which they cap the step-formed plains, worn by +the sea within the period of existing shells, their deposition, at least on +the plains up to a height of 400 feet, must have been a recent geological +event. From the form of the continent, we may feel sure that they have come +from the westward, probably, in chief part from the Cordillera, but, +perhaps, partly from unknown rocky ridges in the central districts of +Patagonia. That the pebbles have not been transported by rivers, from the +interior towards the coast, we may conclude from the fewness and smallness +of the streams of Patagonia: moreover, in the case of the one great and +rapid river of Santa Cruz, we have good evidence that its transporting +power is very trifling. This river is from two to three hundred yards in +width, about seventeen feet deep in its middle, and runs with a singular +degree of uniformity five knots an hour, with no lakes and scarcely any +still reaches: nevertheless, to give one instance of its small transporting +power, upon careful examination, pebbles of compact basalt could not be +found in the bed of the river at a greater distance than ten miles below +the point where the stream rushes over the debris of the great basaltic +cliffs forming its shore: fragments of the CELLULAR varieties have been +washed down twice or thrice as far. That the pebbles in Central and +Northern Patagonia have not been transported by ice-agency, as seems to +have been the case to a considerable extent farther south, and likewise in +the northern hemisphere, we may conclude, from the absence of all angular +fragments in the gravel, and from the complete contrast in many other +respects between the shingle and neighbouring boulder formation. + +Looking to the gravel on any one of the step-formed plains, I cannot doubt, +from the several reasons assigned in this chapter, that it has been spread +out and leveled by the long-continued action of the sea, probably during +the slow rise of the land. The smooth and perfectly rounded condition of +the innumerable pebbles alone would prove long-continued action. But how +the whole mass of shingle on the coast-plains has been transported from the +mountains of the interior, is another and more difficult question. The +following considerations, however, show that the sea by its ordinary action +has considerable power in distributing pebbles. Table 3 above shows how +very uniformly and gradually the pebbles decrease in size with the +gradually seaward increasing depth and distance. (I may mention, that at +the distance of 150 miles from the Patagonian shore I carefully examined +the minute rounded particles in the sand, and found them to be fusible like +the porphyries of the great shingle bed. I could even distinguish particles +of the gallstone-yellow porphyry. It was interesting to notice how +gradually the particles of white quartz increased, as we approached the +Falkland Islands, which are thus constituted. In the whole line of +soundings between these islands and the coast of Patagonia dead or living +organic remains were most rare. On the relations between the depth of water +and the nature of the bottom, see Martin White on "Soundings in the +Channel" pages 4, 6, 175; also Captain Beechey's "Voyage to the Pacific" +chapter 18.) A series of this kind irresistibly leads to the conclusion, +that the sea has the power of sifting and distributing the loose matter on +its bottom. According to Martin White, the bed of the British Channel is +disturbed during gales at depths of sixty-three and sixty-seven fathoms, +and at thirty fathoms, shingle and fragments of shells are often deposited, +afterwards to be carried away again. ("Soundings in the Channel" pages 4, +166. M. Siau states ("Edinburgh New Philosophical Journal" volume 31 page +246), that he found the sediment, at a depth of 188 metres, arranged in +ripples of different degrees of fineness. There are some excellent +discussions on this and allied subjects in Sir H. De la Beche's +"Theoretical Researches.") Groundswells, which are believed to be caused by +distant gales, seem especially to affect the bottom: at such times, +according to Sir R. Schomburgk, the sea to a great distance round the West +Indian Islands, at depths from five to fifteen fathoms, becomes +discoloured, and even the anchors of vessels have been moved. ("Journal of +Royal Geographical Society" volume 5 page 25. It appears from Mr. Scott +Russell's investigations (see Mr. Murchison's "Anniversary Address +Geological Society" 1843 page 40), that in waves of translation the motion +of the particles of water is nearly as great at the bottom as at the top.) +There are, however, some difficulties in understanding how the sea can +transport pebbles lying at the bottom, for, from experiments instituted on +the power of running water, it would appear that the currents of the sea +have not sufficient velocity to move stones of even moderate size: +moreover, I have repeatedly found in the most exposed situations that the +pebbles which lie at the bottom are encrusted with full-grown living +corallines, furnished with the most delicate, yet unbroken spines: for +instance, in ten fathoms water off the mouth of the Santa Cruz, many +pebbles, under half an inch in diameter, were thus coated with Flustracean +zoophytes. (A pebble, one and a half inch square and half an inch thick, +was given me, dredged up from twenty-seven fathoms depth off the western +end of the Falkland Islands, where the sea is remarkably stormy, and +subject to violent tides. This pebble was encrusted on all sides by a +delicate living coralline. I have seen many pebbles from depths between +forty and seventy fathoms thus encrusted; one from the latter depth off +Cape Horn.) Hence we must conclude that these pebbles are not often +violently disturbed: it should, however, be borne in mind that the growth +of corallines is rapid. The view, propounded by Professor Playfair, will, I +believe, explain this apparent difficulty,--namely, that from the +undulations of the sea TENDING to lift up and down pebbles or other loose +bodies at the bottom, such are liable, when thus quite or partially raised, +to be moved even by a very small force, a little onwards. We can thus +understand how oceanic or tidal currents of no great strength, or that +recoil movement of the bottom-water near the land, called by sailors the +"undertow" (which I presume must extend out seaward as far as the BREAKING +waves impel the surface-water towards the beach), may gain the power during +storms of sifting and distributing pebbles even of considerable size, and +yet without so violently disturbing them as to injure the encrusting +corallines. (I may take this opportunity of remarking on a singular, but +very common character in the form of the bottom, in the creeks which deeply +penetrate the western shores of Tierra del Fuego; namely, that they are +almost invariably much shallower close to the open sea at their mouths than +inland. Thus, Cook, in entering Christmas Sound, first had soundings in +thirty-seven fathoms, then in fifty, then in sixty, and a little farther in +no bottom with 170 fathoms. The sealers are so familiar with this fact, +that they always look out for anchorage near the entrances of the creeks. +See, also, on this subject, the "Voyages of the 'Adventure' and 'Beagle'" +volume 1 page 375 and "Appendix" page 313. This Shoalness of the sea- +channels near their entrances probably results from the quantity of +sediment formed by the wear and tear of the outer rocks exposed to the full +force of the open sea. I have no doubt that many lakes, for instance in +Scotland, which are very deep within, and are separated from the sea +apparently only by a tract of detritus, were originally sea-channels with +banks of this nature near their mouths, which have since been upheaved.) + +The sea acts in another and distinct manner in the distribution of pebbles, +namely by the waves on the beach. Mr. Palmer, in his excellent memoir on +this subject, has shown that vast masses of shingle travel with surprising +quickness along lines of coast, according to the direction with which the +waves break on the beach and that this is determined by the prevailing +direction of the winds. ("Philosophical Transactions" 1834 page 576.) This +agency must be powerful in mingling together and disseminating pebbles +derived from different sources: we may, perhaps, thus understand the wide +distribution of the gallstone-yellow porphyry; and likewise, perhaps, the +great difference in the nature of the pebbles at the mouth of the Santa +Cruz from those in the same latitude at the head of the valley. + +I will not pretend to assign to these several and complicated agencies +their shares in the distribution of the Patagonian shingle: but from the +several considerations given in this chapter, and I may add, from the +frequency of a capping of gravel on tertiary deposits in all parts of the +world, as I have myself observed and seen stated in the works of various +authors, I cannot doubt that the power of widely dispersing gravel is an +ordinary contingent on the action of the sea; and that even in the case of +the great Patagonian shingle-bed we have no occasion to call in the aid of +debacles. I at one time imagined that perhaps an immense accumulation of +shingle had originally been collected at the foot of the Cordillera; and +that this accumulation, when upraised above the level of the sea, had been +eaten into and partially spread out (as off the present line of coast); and +that the newly-spread out bed had in its turn been upraised, eaten into, +and re-spread out; and so onwards, until the shingle, which was first +accumulated in great thickness at the foot of the Cordillera, had reached +in thinner beds its present extension. By whatever means the gravel +formation of Patagonia may have been distributed, the vastness of its area, +its thickness, its superficial position, its recent origin, and the great +degree of similarity in the nature of its pebbles, all appear to me well +deserving the attention of geologists, in relation to the origin of the +widely-spread beds of conglomerate belonging to past epochs. + +FORMATION OF CLIFFS. + +(DIAGRAM 7.--SECTION OF COAST-CLIFFS AND BOTTOM OF SEA, OFF THE ISLAND OF +ST. HELENA. + +Height in feet above sea level. + +Depths in fathoms. + +Vertical and horizontal scale, two inches to a nautical mile. The point +marked 1,600 feet is at the foot of High Knoll; point marked 510 feet is on +the edge of Ladder Hill. The strata consist of basaltic streams. + +Section left to right: + +Height at the foot of High Knoll: 1,600 at top of strata. + +Height on the edge of Ladder Hill: 510 at top of strata. + +Bottom at coast rocky only to a depth of five or six fathoms. + +30 fathoms: bottom mud and sand. + +100 fathoms sloping more sharply to 250 fathoms.) + +When viewing the sea-worn cliffs of Patagonia, in some parts between eight +hundred and nine hundred feet in height, and formed of horizontal tertiary +strata, which must once have extended far seaward--or again, when viewing +the lofty cliffs round many volcanic islands, in which the gentle +inclination of the lava-streams indicates the former extension of the land, +a difficulty often occurred to me, namely, how the strata could possibly +have been removed by the action of the sea at a considerable depth beneath +its surface. The section in Diagram 7, which represents the general form of +the land on the northern and leeward side of St. Helena (taken from Mr. +Seale's large model and various measurements), and of the bottom of the +adjoining sea (taken chiefly from Captain Austin's survey and some old +charts), will show the nature of this difficulty. + +If, as seems probable, the basaltic streams were originally prolonged with +nearly their present inclination, they must, as shown by the dotted line in +the section, once have extended at least to a point, now covered by the sea +to a depth of nearly thirty fathoms: but I have every reason to believe +they extended considerably further, for the inclination of the streams is +less near the coast than further inland. It should also be observed, that +other sections on the coast of this island would have given far more +striking results, but I had not the exact measurements; thus, on the +windward side, the cliffs are about two thousand feet in height and the +cut-off lava streams very gently inclined, and the bottom of the sea has +nearly a similar slope all round the island. How, then, has all the hard +basaltic rock, which once extended beneath the surface of the sea, been +worn away? According to Captain Austin, the bottom is uneven and rocky only +to that very small distance from the beach within which the depth is from +five to six fathoms; outside this line, to a depth of about one hundred +fathoms, the bottom is smooth, gently inclined, and formed of mud and sand; +outside the one hundred fathoms, it plunges suddenly into unfathomable +depths, as is so very commonly the case on all coasts where sediment is +accumulating. At greater depths than the five or six fathoms, it seems +impossible, under existing circumstances, that the sea can both have worn +away hard rock, in parts to a thickness of at least 150 feet, and have +deposited a smooth bed of fine sediment. Now, if we had any reason to +suppose that St. Helena had, during a long period, gone on slowly +subsiding, every difficulty would be removed: for looking at the diagram, +and imagining a fresh amount of subsidence, we can see that the waves would +then act on the coast-cliffs with fresh and unimpaired vigour, whilst the +rocky ledge near the beach would be carried down to that depth, at which +sand and mud would be deposited on its bare and uneven surface: after the +formation near the shore of a new rocky shoal, fresh subsidence would carry +it down and allow it to be smoothly covered up. But in the case of the many +cliff-bounded islands, for instance in some of the Canary Islands and of +Madeira, round which the inclination of the strata shows that the land once +extended far into the depths of the sea, where there is no apparent means +of hard rock being worn away--are we to suppose that all these islands have +slowly subsided? Madeira, I may remark, has, according to Mr. Smith of +Jordan Hill, subsided. Are we to extend this conclusion to the high, cliff- +bound, horizontally stratified shores of Patagonia, off which, though the +water is not deep even at the distance of several miles, yet the smooth +bottom of pebbles gradually decreasing in size with the increasing depth, +and derived from a foreign source, seem to declare that the sea is now a +depositing and not a corroding agent? I am much inclined to suspect, that +we shall hereafter find in all such cases, that the land with the adjoining +bed of the sea has in truth subsided: the time will, I believe, come, when +geologists will consider it as improbable, that the land should have +retained the same level during a whole geological period, as that the +atmosphere should have remained absolutely calm during an entire season. + + +CHAPTER II. ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA. + +Chonos Archipelago. +Chiloe, recent and gradual elevation of, traditions of the inhabitants on +this subject. +Concepcion, earthquake and elevation of. +VALPARAISO, great elevation of, upraised shells, earth of marine origin, +gradual rise of the land within the historical period. +COQUIMBO, elevation of, in recent times; terraces of marine origin, their +inclination, their escarpments not horizontal. +Guasco, gravel terraces of. +Copiapo. +PERU. +Upraised shells of Cobija, Iquique, and Arica. +Lima, shell-beds and sea-beach on San Lorenzo, human remains, fossil +earthenware, earthquake debacle, recent subsidence. +On the decay of upraised shells. +General summary. + +Commencing at the south and proceeding northward, the first place at which +I landed, was at Cape Tres Montes, in latitude 46 degrees 35'. Here, on the +shores of Christmas Cove, I observed in several places a beach of pebbles +with recent shells, about twenty feet above high-water mark. Southward of +Tres Montes (between latitude 47 and 48 degrees), Byron remarks, "We +thought it very strange, that upon the summits of the highest hills were +found beds of shells, a foot or two thick." ("Narrative of the Loss of the +'Wager'.") In the Chonos Archipelago, the island of Lemus (latitude 44 +degrees 30') was, according to M. Coste, suddenly elevated eight feet, +during the earthquake of 1829: he adds, "Des roches jadis toujours +couvertes par la mer, restant aujourd'hui constamment decouvertes." +("Comptes Rendus" October 1838 page 706.) In other parts of this +archipelago, I observed two terraces of gravel, abutting to the foot of +each other: at Lowe's Harbour (43 degrees 48'), under a great mass of the +boulder formation, about three hundred feet in thickness, I found a layer +of sand, with numerous comminuted fragments of sea-shells, having a fresh +aspect, but too small to be identified. + +THE ISLAND OF CHILOE. + +The evidence of recent elevation is here more satisfactory. The bay of San +Carlos is in most parts bounded by precipitous cliffs from about ten to +forty feet in height, their bases being separated from the present line of +tidal action by a talus, a few feet in height, covered with vegetation. In +one sheltered creek (west of P. Arena), instead of a loose talus, there was +a bare sloping bank of tertiary mudstone, perforated, above the line of the +highest tides, by numerous shells of a Pholas now common in the harbour. +The upper extremities of these shells, standing upright in their holes with +grass growing out of them, were abraded about a quarter of an inch, to the +same level with the surrounding worn strata. In other parts, I observed (as +at Pudeto) a great beach, formed of comminuted shells, twenty feet above +the present shore. In other parts again, there were small caves worn into +the foot of the low cliffs, and protected from the waves by the talus with +its vegetation: one such cave, which I examined, had its mouth about twenty +feet, and its bottom, which was filled with sand containing fragments of +shells and legs of crabs, from eight to ten feet above high-water mark. +From these several facts, and from the appearance of the upraised shells, I +inferred that the elevation had been quite recent; and on inquiring from +Mr. Williams, the Portmaster, he told me he was convinced that the land had +risen, or the sea fallen, four feet within the last four years. During this +period, there had been one severe earthquake, but no particular change of +level was then observed; from the habits of the people who all keep boats +in the protected creeks, it is absolutely impossible that a rise of four +feet could have taken place suddenly and been unperceived. Mr. Williams +believes that the change has been quite gradual. Without the elevatory +movement continues at a quick rate, there can be no doubt that the sea will +soon destroy the talus of earth at the foot of the cliffs round the bay, +and will then reach its former lateral extension, but not of course its +former level: some of the inhabitants assured me that one such talus, with +a footpath on it, was even already sensibly decreasing in width. + +I received several accounts of beds of shells, existing at considerable +heights in the inland parts of Chiloe; and to one of these, near Catiman, I +was guided by a countryman. Here, on the south side of the peninsula of +Lacuy, there was an immense bed of the Venus costellata and of an oyster, +lying on the summit-edge of a piece of tableland, 350 feet (by the +barometer) above the level of the sea. The shells were closely packed +together, embedded in and covered by a very black, damp, peaty mould, two +or three feet in thickness, out of which a forest of great trees was +growing. Considering the nature and dampness of this peaty soil, it is +surprising that the fine ridges on the outside of the Venus are perfectly +preserved, though all the shells have a blackened appearance. I did not +doubt that the black soil, which when dry, cakes hard, was entirely of +terrestrial origin, but on examining it under the microscope, I found many +very minute rounded fragments of shells, amongst which I could distinguish +bits of Serpulae and mussels. The Venus costellata, and the Ostrea (O. +edulis, according to Captain King) are now the commonest shells in the +adjoining bays. In a bed of shells, a few feet below the 350 feet bed, I +found a horn of the little Cervus humilis, which now inhabits Chiloe. + +The eastern or inland side of Chiloe, with its many adjacent islets, +consists of tertiary and boulder deposits, worn into irregular plains +capped by gravel. Near Castro, and for ten miles southward, and on the +islet of Lemuy, I found the surface of the ground to a height of between +twenty and thirty feet above high-water mark, and in several places +apparently up to fifty feet, thickly coated by much comminuted shells, +chiefly of the Venus costellata and Mytilus Chiloensis; the species now +most abundant on this line of coast. As the inhabitants carry immense +numbers of these shells inland, the continuity of the bed at the same +height was often the only means of recognising its natural origin. Near +Castro, on each side of the creek and rivulet of the Gamboa, three distinct +terraces are seen: the lowest was estimated at about one hundred and fifty +feet in height, and the highest at about five hundred feet, with the +country irregularly rising behind it; obscure traces, also, of these same +terraces could be seen along other parts of the coast. There can be no +doubt that their three escarpments record pauses in the elevation of the +island. I may remark that several promontories have the word Huapi, which +signifies in the Indian tongue, island, appended to them, such as +Huapilinao, Huapilacuy, Caucahuapi, etc.; and these, according to Indian +traditions, once existed as islands. In the same manner the term Pulo in +Sumatra is appended to the names of promontories, traditionally said to +have been islands (Marsden's "Sumatra" page 31.); in Sumatra, as in Chiloe, +there are upraised recent shells. The Bay of Carelmapu, on the mainland +north of Chiloe, according to Aguerros, was in 1643 a good harbour +("Descripcion Hist. de la Provincia de Chiloe" page 78. From the account +given by the old Spanish writers, it would appear that several other +harbours, between this point and Concepcion, were formerly much deeper than +they now are.); it is now quite useless, except for boats. + +VALDIVIA. + +I did not observe here any distinct proofs of recent elevation; but in a +bed of very soft sandstone, forming a fringe-like plain, about sixty feet +in height, round the hills of mica-slate, there are shells of Mytilus, +Crepidula, Solen, Novaculina, and Cytheraea, too imperfect to be +specifically recognised. At Imperial, seventy miles north of Valdivia, +Aguerros states that there are large beds of shells, at a considerable +distance from the coast, which are burnt for lime. (Ibid page 25.) The +island of Mocha, lying a little north of Imperial, was uplifted two feet, +during the earthquake of 1835. ("Voyages of 'Adventure' and 'Beagle'" +volume 2 page 415.) + +CONCEPCION. + +I cannot add anything to the excellent account by Captain Fitzroy of the +elevation of the land at this place, which accompanied the earthquake of +1835. (Ibid volume 2 page 412 et seq. In volume 5 page 601 of the +"Geological Transactions" I have given an account of the remarkable +volcanic phenomena, which accompanied this earthquake. These phenomena +appear to me to prove that the action, by which large tracts of land are +uplifted, and by which volcanic eruptions are produced, is in every respect +identical.) I will only recall to the recollection of geologists, that the +southern end of the island of St. Mary was uplifted eight feet, the central +part nine, and the northern end ten feet; and the whole island more than +the surrounding districts. Great beds of mussels, patellae, and chitons +still adhering to the rocks were upraised above high-water mark; and some +acres of a rocky flat, which was formerly always covered by the sea, was +left standing dry, and exhaled an offensive smell, from the many attached +and putrefying shells. It appears from the researches of Captain Fitzroy +that both the island of St. Mary and Concepcion (which was uplifted only +four or five feet) in the course of some weeks subsided, and lost part of +their first elevation. I will only add as a lesson of caution, that round +the sandy shores of the great Bay of Concepcion, it was most difficult, +owing to the obliterating effects of the great accompanying wave, to +recognise any distinct evidence of this considerable upheaval; one spot +must be excepted, where there was a detached rock which before the +earthquake had always been covered by the sea, but afterwards was left +uncovered. + +On the island of Quiriquina (in the Bay of Concepcion), I found, at an +estimated height of four hundred feet, extensive layers of shells, mostly +comminuted, but some perfectly preserved and closely packed in black +vegetable mould; they consisted of Concholepas, Fissurella, Mytilus, +Trochus, and Balanus. Some of these layers of shells rested on a thick bed +of bright-red, dry, friable earth, capping the surface of the tertiary +sandstone, and extending, as I observed whilst sailing along the coast, for +150 miles southward: at Valparaiso, we shall presently see that a similar +red earthy mass, though quite like terrestrial mould, is really in chief +part of recent marine origin. On the flanks of this island of Quiriquina, +at a less height than the 400 feet, there were spaces several feet square, +thickly strewed with fragments of similar shells. During a subsequent visit +of the "Beagle" to Concepcion, Mr. Kent, the assistant-surgeon, was so kind +as to make for me some measurements with the barometer: he found many +marine remains along the shores of the whole bay, at a height of about +twenty feet; and from the hill of Sentinella behind Talcahuano, at the +height of 160 feet, he collected numerous shells, packed together close +beneath the surface in black earth, consisting of two species of Mytilus, +two of Crepidula, one of Concholepas, of Fissurella, Venus, Mactra, Turbo, +Monoceros, and the Balanus psittacus. These shells were bleached, and +within some of the Balani other Balani were growing, showing that they must +have long lain dead in the sea. The above species I compared with living +ones from the bay, and found them identical; but having since lost the +specimens, I cannot give their names: this is of little importance, as Mr. +Broderip has examined a similar collection, made during Captain Beechey's +expedition, and ascertained that they consisted of ten recent species, +associated with fragments of Echini, crabs, and Flustrae; some of these +remains were estimated by Lieutenant Belcher to lie at the height of nearly +a thousand feet above the level of the sea. ("Zoology of Captain Beechey's +Voyage" page 162.) In some places round the bay, Mr. Kent observed that +there were beds formed exclusively of the Mytilus Chiloensis: this species +now lives in parts never uncovered by the tides. At considerable heights, +Mr. Kent found only a few shells; but from the summit of one hill, 625 feet +high, he brought me specimens of the Concholepas, Mytilus Chiloensis, and a +Turbo. These shells were softer and more brittle than those from the height +of 164 feet; and these latter had obviously a much more ancient appearance +than the same species from the height of only twenty feet. + +COAST NORTH OF CONCEPCION. + +The first point examined was at the mouth of the Rapel (160 miles north of +Concepcion and sixty miles south of Valparaiso), where I observed a few +shells at the height of 100 feet, and some barnacles adhering to the rocks +three or four feet above the highest tides: M. Gay found here recent shells +at the distance of two leagues from the shore. ("Annales des Scienc. Nat." +Avril 1833.) Inland there are some wide, gravel-capped plains, intersected +by many broad, flat-bottomed valleys (now carrying insignificant +streamlets), with their sides cut into successive wall-like escarpments, +rising one above another, and in many places, according to M. Gay, worn +into caves. The one cave (C. del Obispo) which I examined, resembled those +formed on many sea-coasts, with its bottom filled with shingle. These +inland plains, instead of sloping towards the coast, are inclined in an +opposite direction towards the Cordillera, like the successively rising +terraces on the inland or eastern side of Chiloe: some points of granite, +which project through the plains near the coast, no doubt once formed a +chain of outlying islands, on the inland shores of which the plains were +accumulated. At Bucalemu, a few miles northward of the Rapel, I observed at +the foot, and on the summit-edge of a plain, ten miles from the coast, many +recent shells, mostly comminuted, but some perfect. There were, also, many +at the bottom of the great valley of the Maypu. At San Antonio, shells are +said to be collected and burnt for lime. At the bottom of a great ravine +(Quebrada Onda, on the road to Casa Blanca), at the distance of several +miles from the coast, I noticed a considerable bed, composed exclusively of +Mesodesma donaciforme, Desh., lying on a bed of muddy sand: this shell now +lives associated together in great numbers, on tidal-flats on the coast of +Chile. + +VALPARAISO. + +During two successive years I carefully examined, part of the time in +company with Mr. Alison, into all the facts connected with the recent +elevation of this neighbourhood. In very many parts a beach of broken +shells, about fourteen or fifteen feet above high-water mark, may be +observed; and at this level the coast-rocks, where precipitous, are +corroded in a band. At one spot, Mr. Alison, by removing some birds' dung, +found at this same level barnacles adhering to the rocks. For several miles +southward of the bay, almost every flat little headland, between the +heights of 60 and 230 feet (measured by the barometer), is smoothly coated +by a thick mass of comminuted shells, of the same species, and apparently +in the same proportional numbers with those existing in the adjoining sea. +The Concholepas is much the most abundant, and the best preserved shell; +but I extracted perfectly preserved specimens of the Fissurella biradiata, +a Trochus and Balanus (both well-known, but according to Mr. Sowerby yet +unnamed) and parts of the Mytilus Chiloensis. Most of these shells, as well +as an encrusting Nullipora, partially retain their colour; but they are +brittle, and often stained red from the underlying brecciated mass of +primary rocks; some are packed together, either in black or reddish moulds; +some lie loose on the bare rocky surfaces. The total number of these shells +is immense; they are less numerous, though still far from rare, up a height +of 1,000 feet above the sea. On the summit of a hill, measured 557 feet, +there was a small horizontal band of comminuted shells, of which MANY +consisted (and likewise from lesser heights) of very young and small +specimens of the still living Concholepas, Trochus, Patellae, Crepidulae, +and of Mytilus Magellanicus (?) (Mr. Cuming informs me that he does not +think this species identical with, though closely resembling, the true M. +Magellanicus of the southern and eastern coast of South America; it lives +abundantly on the coast of Chile.): several of these shells were under a +quarter of an inch in their greatest diameter. My attention was called to +this circumstance by a native fisherman, whom I took to look at these +shell-beds; and he ridiculed the notion of such small shells having been +brought up for food; nor could some of the species have adhered when alive +to other larger shells. On another hill, some miles distant, and 648 feet +high, I found shells of the Concholepas and Trochus, perfect, though very +old, with fragments of Mytilus Chiloensis, all embedded in reddish-brown +mould: I also found these same species, with fragments of an Echinus and of +Balanus psittacus, on a hill 1,000 feet high. Above this height, shells +became very rare, though on a hill 1,300 feet high (Measured by the +barometer: the highest point in the range behind Valparaiso I found to be +1,626 feet above the level of the sea.), I collected the Concholepas, +Trochus, Fissurella, and a Patella. At these greater heights the shells are +almost invariably embedded in mould, and sometimes are exposed only by +tearing up bushes. These shells obviously had a very much more ancient +appearance than those from the lesser heights; the apices of the Trochi +were often worn down; the little holes made by burrowing animals were +greatly enlarged; and the Concholepas was often perforated quite through, +owing to the inner plates of shell having scaled off. + +Many of these shells, as I have said, were packed in, and were quite filled +with, blackish or reddish-brown earth, resting on the granitic detritus. I +did not doubt until lately that this mould was of purely terrestrial +origin, when with a microscope examining some of it from the inside of a +Concholepas from the height of about one hundred feet, I found that it was +in considerable part composed of minute fragments of the spines, mouth- +bones, and shells of Echini, and of minute fragments, of chiefly very young +Patellae, Mytili, and other species. I found similar microscopical +fragments in earth filling up the central orifices of some large +Fissurellae. This earth when crushed emits a sickly smell, precisely like +that from garden-mould mixed with guano. The earth accidentally preserved +within the shells, from the greater heights, has the same general +appearance, but it is a little redder; it emits the same smell when rubbed, +but I was unable to detect with certainty any marine remains in it. This +earth resembles in general appearance, as before remarked, that capping the +rocks of Quiriquina in the Bay of Concepcion, on which beds of sea-shells +lay. I have, also, shown that the black, peaty soil, in which the shells at +the height of 350 feet at Chiloe were packed, contained many minute +fragments of marine animals. These facts appear to me interesting, as they +show that soils, which would naturally be considered of purely terrestrial +nature, may owe their origin in chief part to the sea. + +Being well aware from what I have seen at Chiloe and in Tierra del Fuego, +that vast quantities of shells are carried, during successive ages, far +inland, where the inhabitants chiefly subsist on these productions, I am +bound to state that at greater heights than 557 feet, where the number of +very young and small shells proved that they had not been carried up for +food, the only evidence of the shells having been naturally left by the +sea, consists in their invariable and uniform appearance of extreme +antiquity--in the distance of some of the places from the coast, in others +being inaccessible from the nearest part of the beach, and in the absence +of fresh water for men to drink--in the shells NOT LYING IN HEAPS,--and, +lastly, in the close similarity of the soil in which they are embedded, to +that which lower down can be unequivocally shown to be in great part formed +from the debris of the sea animals. (In the "Proceedings of the Geological +Society" volume 2 page 446, I have given a brief account of the upraised +shells on the coast of Chile, and have there stated that the proofs of +elevation are not satisfactory above the height of 230 feet. I had at that +time unfortunately overlooked a separate page written during my second +visit to Valparaiso, describing the shells now in my possession from the +557 feet hill; I had not then unpacked my collections, and had not +reconsidered the obvious appearance of greater antiquity of the shells from +the greater heights, nor had I at that time discovered the marine origin of +the earth in which many of the shells are packed. Considering these facts, +I do not now feel a shadow of doubt that the shells, at the height of 1,300 +feet, have been upraised by natural causes into their present position.) + +With respect to the position in which the shells lie, I was repeatedly +struck here, at Concepcion, and at other places, with the frequency of +their occurrence on the summits and edges either of separate hills, or of +little flat headlands often terminating precipitously over the sea. The +several above-enumerated species of mollusca, which are found strewed on +the surface of the land from a few feet above the level of the sea up to +the height of 1,300 feet, all now live either on the beach, or at only a +few fathoms' depth: Mr. Edmondston, in a letter to Professor E. Forbes, +states that in dredging in the Bay of Valparaiso, he found the common +species of Concholepas, Fissurella, Trochus, Monoceros, Chitons, etc., +living in abundance from the beach to a depth of seven fathoms; and dead +shells occurred only a few fathoms deeper. The common Turritella cingulata +was dredged up living at even from ten to fifteen fathoms; but this is a +species which I did not find here amongst the upraised shells. Considering +this fact of the species being all littoral or sub-littoral, considering +their occurrence at various heights, their vast numbers, and their +generally comminuted state, there can be little doubt that they were left +on successive beach-lines during a gradual elevation of the land. The +presence, however, of so many whole and perfectly preserved shells appears +at first a difficulty on this view, considering that the coast is exposed +to the full force of an open ocean: but we may suppose, either that these +shells were thrown during gales on flat ledges of rock just above the level +of high-water mark, and that during the elevation of the land they are +never again touched by the waves, or, that during earthquakes, such as +those of 1822, 1835, and 1837, rocky reefs covered with marine-animals were +it one blow uplifted above the future reach of the sea. This latter +explanation is, perhaps, the most probable one with respect to the beds at +Concepcion entirely composed of the Mytilus Chiloensis, a species which +lives below the lowest tides; and likewise with respect to the great beds +occurring both north and south of Valparaiso, of the Mesodesma +donaciforme,--a shell which, as I am informed by Mr. Cuming, inhabits +sandbanks at the level of the lowest tides. But even in the case of shells +having the habits of this Mytilus and Mesodesma, beds of them, wherever the +sea gently throws up sand or mud, and thus protects its own accumulations, +might be upraised by the slowest movement, and yet remain undisturbed by +the waves of each new beach-line. + +It is worthy of remark, that nowhere near Valparaiso above the height of +twenty feet, or rarely of fifty feet, I saw any lines of erosion on the +solid rocks, or any beds of pebbles; this, I believe, may be accounted for +by the disintegrating tendency of most of the rocks in this neighbourhood. +Nor is the land here modelled into terraces: Mr. Alison, however, informs +me, that on both sides of one narrow ravine, at the height of 300 feet +above the sea, he found a succession of rather indistinct step-formed +beaches, composed of broken shells, which together covered a space of about +eighty feet vertical. + +I can add nothing to the accounts already published of the elevation of the +land at Valparaiso, which accompanied the earthquake of 1822 (Dr. Meyen +"Reise um Erde" Th. 1 s. 221, found in 1831 seaweed and other bodies still +adhering to some rocks which during the shock of 1822 were lifted above the +sea.): but I heard it confidently asserted, that a sentinel on duty, +immediately after the shock, saw a part of a fort, which previously was not +within the line of his vision, and this would indicate that the uplifting +was not horizontal: it would even appear from some facts collected by Mr. +Alison, that only the eastern half of the bay was then elevated. Through +the kindness of this same gentleman, I am able to give an interesting +account of the changes of level, which have supervened here within +historical periods: about the year 1680 a long sea-wall (or Prefil) was +built, of which only a few fragments now remain; up to the year 1817, the +sea often broke over it, and washed the houses on the opposite side of the +road (where the prison now stands); and even in 1819, Mr. J. Martin +remembers walking at the foot of this wall, and being often obliged to +climb over it to escape the waves. There now stands (1834) on the seaward +side of this wall, and between it and the beach, in one part a single row +of houses, and in another part two rows with a street between them. This +great extension of the beach in so short a time cannot be attributed simply +to the accumulation of detritus; for a resident engineer measured for me +the height between the lowest part of the wall visible, and the present +beach-line at spring-tides, and the difference was eleven feet six inches. +The church of S. Augustin is believed to have been built in 1614, and there +is a tradition that the sea formerly flowed very near it; by levelling, its +foundations were found to stand nineteen feet six inches above the highest +beach-line; so that we see in a period of 220 years, the elevation cannot +have been as much as nineteen feet six inches. From the facts given with +respect to the sea-wall, and from the testimony of the elder inhabitants, +it appears certain that the change in level began to be manifest about the +year 1817. The only sudden elevation of which there is any record occurred +in 1822, and this seems to have been less than three feet. Since that year, +I was assured by several competent observers, that part of an old wreck, +which is firmly embedded near the beach, has sensibly emerged; hence here, +as at Chiloe, a slow rise of the land appears to be now in progress. It +seems highly probable that the rocks which are corroded in a band at the +height of fourteen feet above the sea were acted on during the period, when +by tradition the base of S. Augustin church, now nineteen feet six inches +above the highest water-mark, was occasionally washed by the waves. + +VALPARAISO TO COQUIMBO. + +For the first seventy-five miles north of Valparaiso I followed the coast- +road, and throughout this space I observed innumerable masses of upraised +shells. About Quintero there are immense accumulations (worked for lime) of +the Mesodesma donaciforme, packed in sandy earth; they abound chiefly about +fifteen feet above high-water, but shells are here found, according to Mr. +Miers, to a height of 500 feet, and at a distance of three leagues from the +coast ("Travels in Chile" volume 1 pages 395, 458. I received several +similar accounts from the inhabitants, and was assured that there are many +shells on the plain of Casa Blanca, between Valparaiso and Santiago, at the +height of 800 feet.): I here noticed barnacles adhering to the rocks three +or four feet above the highest tides. In the neighbourhood of Plazilla and +Catapilco, at heights of between two hundred and three hundred feet, the +number of comminuted shells, with some perfect ones, especially of the +Mesodesma, packed in layers, was truly immense: the land at Plazilla had +evidently existed as a bay, with abrupt rocky masses rising out of it, +precisely like the islets in the broken bays now indenting this coast. On +both sides of the rivers Ligua, Longotomo, Guachen, and Quilimari, there +are plains of gravel about two hundred feet in height, in many parts +absolutely covered with shells. Close to Conchalee, a gravel-plain is +fronted by a lower and similar plain about sixty feet in height, and this +again is separated from the beach by a wide tract of low land: the surfaces +of all three plains or terraces were strewed with vast numbers of the +Concholepas, Mesodesma, an existing Venus, and other still existing +littoral shells. The two upper terraces closely resemble in miniature the +plains of Patagonia; and like them are furrowed by dry, flat-bottomed, +winding valleys. Northward of this place I turned inward; and therefore +found no more shells: but the valleys of Chuapa, Illapel, and Limari, are +bounded by gravel-capped plains, often including a lower terrace within. +These plains send bay-like arms between and into the surrounding hills; and +they are continuously united with other extensive gravel-capped plains, +separating the coast mountain-ranges from the Cordillera. + +COQUIMBO. + +A narrow fringe-like plain, gently inclined towards the sea, here extends +for eleven miles along the coast, with arms stretching up between the +coast-mountains, and likewise up the valley of Coquimbo: at its southern +extremity it is directly connected with the plain of Limari, out of which +hills abruptly rise like islets, and other hills project like headlands on +a coast. The surface of the fringe-like plain appears level, but differs +insensibly in height, and greatly in composition, in different parts. + +At the mouth of the valley of Coquimbo, the surface consists wholly of +gravel, and stands from 300 to 350 feet above the level of the sea, being +about one hundred feet higher than in other parts. In these other and lower +parts the superficial beds consist of calcareous matter, and rest on +ancient tertiary deposits hereafter to be described. The uppermost +calcareous layer is cream-coloured, compact, smooth-fractured, sub- +stalactiform, and contains some sand, earthy matter, and recent shells. It +lies on, and sends wedge-like veins into, a much more friable, calcareous, +tuff-like variety; and both rest on a mass about twenty feet in thickness, +formed of fragments of recent shells, with a few whole ones, and with small +pebbles firmly cemented together. (In many respects this upper hard, and +the underlying more friable, varieties, resemble the great superficial beds +at King George's Sound in Australia, which I have described in my +"Geological Observations on Volcanic Islands." There could be little doubt +that the upper layers there have been hardened by the action of rain on the +friable, calcareous matter, and that the whole mass has originated in the +decay of minutely comminuted sea-shells and corals.) This latter rock is +called by the inhabitants losa, and is used for building: in many parts it +is divided into strata, which dip at an angle of ten degrees seaward, and +appear as if they had originally been heaped in successive layers (as may +be seen on coral-reefs) on a steep beach. This stone is remarkable from +being in parts entirely formed of empty, pellucid capsules or cells of +calcareous matter, of the size of small seeds: a series of specimens +unequivocally showed that all these capsules once contained minute rounded +fragments of shells which have since been gradually dissolved by water +percolating through the mass. (I have incidentally described this rock in +the above work on Volcanic Islands.) + +The shells embedded in the calcareous beds forming the surface of this +fringe-like plain, at the height of from 200 to 250 feet above the sea, +consist of:-- + +1. Venus opaca. +2. Mulinia Byronensis. +3. Pecten purpuratus. +4. Mesodesma donaciforme. +5. Turritella cingulata. +6. Monoceros costatum. +7. Concholepas Peruviana. +8. Trochus (common Valparaiso species). +9. Calyptraea Byronensis. + +Although these species are all recent, and are all found in the +neighbouring sea, yet I was particularly struck with the difference in the +proportional numbers of the several species, and of those now cast up on +the present beach. I found only one specimen of the Concholepas, and the +Pecten was very rare, though both these shells are now the commonest kinds, +with the exception, perhaps, of the Calyptraea radians, of which I did not +find one in the calcareous beds. I will not pretend to determine how far +this difference in the proportional numbers depends on the age of the +deposit, and how far on the difference in nature between the present sandy +beaches and the calcareous bottom, on which the embedded shells must have +lived. + +(DIAGRAM 8.--SECTION OF PLAIN OF COQUIMBO. + +Section through Plain B-B and Ravine A. + +Surface of plain 252 feet above sea. + +A. Stratified sand, with recent shells in same proportions as on the beach, +half filling up a ravine. + +B. Surface of plain, with scattered shells in nearly same proportions as on +the beach. + +C. Upper calcareous bed, and D. Lower calcareous sandy bed (Losa), both +with recent shells, but not in same proportions as on the beach. + +E. Upper ferrugino-sandy old tertiary stratum, and F. Lower old tertiary +stratum, both with all, or nearly all, extinct shells.) + +On the bare surface of the calcareous plain, or in a thin covering of sand, +there were lying, at a height from 200 to 252 feet, many recent shells, +which had a much fresher appearance than the embedded ones: fragments of +the Concholepas, and of the common Mytilus, still retaining a tinge of its +colour, were numerous, and altogether there was manifestly a closer +approach in proportional numbers to those now lying on the beach. In a mass +of stratified, slightly agglutinated sand, which in some places covers up +the lower half of the seaward escarpment of the plain, the included shells +appeared to be in exactly the same proportional numbers with those on the +beach. On one side of a steep-sided ravine, cutting through the plain +behind Herradura Bay, I observed a narrow strip of stratified sand, +containing similar shells in similar proportional numbers; a section of the +ravine is represented in Diagram 8, which serves also to show the general +composition of the plain. I mention this case of the ravine chiefly because +without the evidence of the marine shells in the sand, any one would have +supposed that it had been hollowed out by simple alluvial action. + +The escarpment of the fringe-like plain, which stretches for eleven miles +along the coast, is in some parts fronted by two or three narrow, step- +formed terraces, one of which at Herradura Bay expands into a small plain. +Its surface was there formed of gravel, cemented together by calcareous +matter; and out of it I extracted the following recent shells, which are in +a more perfect condition than those from the upper plain:-- + +1. Calyptraea radians. +2. Turritella cingulata. +3. Oliva Peruviana. +4. Murex labiosus, var. +5. Nassa (identical with a living species). +6. Solen Dombeiana. +7. Pecten purpuratus. +8. Venus Chilensis. +9. Amphidesma rugulosum. The small irregular wrinkles of the posterior part +of this shell are rather stronger than in the recent specimens of this +species from Coquimbo. (G.B. Sowerby.) +10. Balanus (identical with living species). + +On the syenitic ridge, which forms the southern boundary of Herradura Bay +and Plain, I found the Concholepas and Turritella cingulata (mostly in +fragments), at the height of 242 feet above the sea. I could not have told +that these shells had not formerly been brought up by man, if I had not +found one very small mass of them cemented together in a friable calcareous +tuff. I mention this fact more particularly, because I carefully looked, in +many apparently favourable spots, at lesser heights on the side of this +ridge, and could not find even the smallest fragment of a shell. This is +only one instance out of many, proving that the absence of sea-shells on +the surface, though in many respects inexplicable, is an argument of very +little weight in opposition to other evidence on the recent elevation of +the land. The highest point in this neighbourhood at which I found upraised +shells of existing species was on an inland calcareous plain, at the height +of 252 feet above the sea. + +It would appear from Mr. Caldcleugh's researches, that a rise has taken +place here within the last century and a half ("Proceedings of the +Geological Society" volume 2 page 446.); and as no sudden change of level +has been observed during the not very severe earthquakes, which have +occasionally occurred here, the rising has probably been slow, like that +now, or quite lately, in progress at Chiloe and at Valparaiso: there are +three well-known rocks, called the Pelicans, which in 1710, according to +Feuillee, were a fleur d'eau, but now are said to stand twelve feet above +low-water mark: the spring-tides rise here only five feet. There is another +rock, now nine feet above high-water mark, which in the time of Frezier and +Feuillee rose only five or six feet out of water. Mr. Caldcleugh, I may +add, also shows (and I received similar accounts) that there has been a +considerable decrease in the soundings during the last twelve years in the +Bays of Coquimbo, Concepcion, Valparaiso, and Guasco; but as in these cases +it is nearly impossible to distinguish between the accumulation of sediment +and the upheavement of the bottom, I have not entered into any details. + +VALLEY OF COQUIMBO. + +(FIGURE 9. EAST AND WEST SECTION THROUGH THE TERRACES AT COQUIMBO, WHERE +THEY DEBOUCH FROM THE VALLEY, AND FRONT THE SEA. + +Vertical scale 1/10 of inch to 100 feet: horizontal scale much contracted. + +Height of terrace in feet from east (high) to west (low): +Terrace F. 364 +Terrace E. 302 +Terrace D. shown dotted, height not given. +Terrace C. 120 +Terrace B. 70 +Terrace A. 25 sloping down to level of sea at Town of Coquimbo.) + +The narrow coast-plain sends, as before stated, an arm, or more correctly a +fringe, on both sides, but chiefly on the southern side, several miles up +the valley. These fringes are worn into steps or terraces, which present a +most remarkable appearance, and have been compared (though not very +correctly) by Captain Basil Hall, to the parallel roads of Glen Roy in +Scotland: their origin has been ably discussed by Mr. Lyell. ("Principles +of Geology" 1st edition volume 3 page 131.) The first section which I will +give (Figure 9), is not drawn across the valley, but in an east and west +line at its mouth, where the step-formed terraces debouch and present their +very gently inclined surfaces towards the Pacific. + +The bottom plain (A) is about a mile in width, and rises quite insensibly +from the beach to a height of twenty-five feet at the foot of the next +plain; it is sandy, and abundantly strewed with shells. + +Plain or terrace B is of small extent, and is almost concealed by the +houses of the town, as is likewise the escarpment of terrace C. On both +sides of a ravine, two miles south of the town, there are two little +terraces, one above the other, evidently corresponding with B and C; and on +them marine remains of the species already enumerated were plentiful. +Terrace E is very narrow, but quite distinct and level; a little southward +of the town there were traces of a terrace D intermediate between E and C. +Terrace F is part of the fringe-like plain, which stretches for the eleven +miles along the coast; it is here composed of shingle, and is 100 feet +higher than where composed of calcareous matter. This greater height is +obviously due to the quantity of shingle, which at some former period has +been brought down the great valley of Coquimbo. + +Considering the many shells strewed over the terraces A, B, and C, and a +few miles southward on the calcareous plain, which is continuously united +with the upper step-like plain F, there cannot, I apprehend, be any doubt, +that these six terraces have been formed by the action of the sea; and that +their five escarpments mark so many periods of comparative rest in the +elevatory movement, during which the sea wore into the land. The elevation +between these periods may have been sudden and on AN AVERAGE not more than +seventy-two feet each time, or it may have been gradual and insensibly +slow. From the shells on the three lower terraces, and on the upper one, +and I may add on the three gravel-capped terraces at Conchalee, being all +littoral and sub-littoral species, and from the analogical facts given at +Valparaiso, and lastly from the evidence of a slow rising lately or still +in progress here, it appears to me far more probable that the movement has +been slow. The existence of these successive escarpments, or old cliff- +lines, is in another respect highly instructive, for they show periods of +comparative rest in the elevatory movement, and of denudation, which would +never even have been suspected from a close examination of many miles of +coast southward of Coquimbo. + +(FIGURE 10. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF COQUIMBO. + +From north F (high) through E?, D, C, B, A (low), B?, C, D?, E, F (high). + +Vertical scale 1/10 of inch to 100 feet: horizontal scale much contracted. + +Terraces marked with ? do not occur on that side of the valley, and are +introduced only to make the diagram more intelligible. A river and bottom- +plain of valley C, E, and F, on the south side of valley, are respectively, +197, 377, and 420 feet above the level of the sea. + +AA. The bottom of the valley, believed to be 100 feet above the sea: it is +continuously united with the lowest plain A of Figure 9. + +B. This terrace higher up the valley expands considerably; seaward it is +soon lost, its escarpment being united with that of C: it is not developed +at all on the south side of the valley. + +C. This terrace, like the last, is considerably expanded higher up the +valley. These two terraces apparently correspond with B and C of Figure 9. + +D is not well developed in the line of this section; but seaward it expands +into a plain: it is not present on the south side of the valley; but it is +met with, as stated under the former section, a little south of the town. + +E is well developed on the south side, but absent on the north side of the +valley: though not continuously united with E of Figure 9, it apparently +corresponds with it. + +F. This is the surface-plain, and is continuously united with that which +stretches like a fringe along the coast. In ascending the valley it +gradually becomes narrower, and is at last, at the distance of about ten +miles from the sea, reduced to a row of flat-topped patches on the sides of +the mountains. None of the lower terraces extend so far up the valley.) + +We come now to the terraces on the opposite sides of the east and west +valley of Coquimbo: the section in Figure 10 is taken in a north and south +line across the valley at a point about three miles from the sea. The +valley measured from the edges of the escarpments of the upper plain FF is +about a mile in width; but from the bases of the bounding mountains it is +from three to four miles wide. The terraces marked with an interrogative do +not exist on that side of the valley, but are introduced merely to render +the diagram more intelligible. + +These five terraces are formed of shingle and sand; three of them, as +marked by Captain B. Hall (namely, B, C, and F), are much more conspicuous +than the others. From the marine remains copiously strewed at the mouth of +the valley on the lower terraces, and southward of the town on the upper +one, they are, as before remarked, undoubtedly of marine origin; but within +the valley, and this fact well deserves notice, at a distance of from only +a mile and a half to three or four miles from the sea, I could not find +even a fragment of a shell. + +ON THE INCLINATION OF THE TERRACES OF COQUIMBO, AND ON THE UPPER AND BASAL +EDGES OF THEIR ESCARPMENTS NOT BEING HORIZONTAL. + +The surfaces of these terraces slope in a slight degree, as shown by the +sections in Figures 9 and 10 taken conjointly, both towards the centre of +the valley, and seawards towards its mouth. This double or diagonal +inclination, which is not the same in the several terraces, is, as we shall +immediately see, of simple explanation. There are, however, some other +points which at first appear by no means obvious,--namely, first, that each +terrace, taken in its whole breadth from the summit-edge of one escarpment +to the base of that above it, and followed up the valley, is not +horizontal; nor have the several terraces, when followed up the valley, all +the same inclination; thus I found the terraces C, E, and F, measured at a +point about two miles from the mouth of the valley, stood severally between +fifty-six to seventy-seven feet higher than at the mouth. Again, if we look +to any one line of cliff or escarpment, neither its summit-edge nor its +base is horizontal. On the theory of the terraces having been formed during +a slow and equable rise of the land, with as many intervals of rest as +there are escarpments, it appears at first very surprising that horizontal +lines of some kind should not have been left on the land. + +The direction of the diagonal inclination in the different terraces being +different,--in some being directed more towards the middle of the valley, +in others more towards its mouth,--naturally follows on the view of each +terrace, being an accumulation of successive beach-lines round bays, which +must have been of different forms and sizes when the land stood at +different levels: for if we look to the actual beach of a narrow creek, its +slope is directed towards the middle; whereas, in an open bay, or slight +concavity on a coast, the slope is towards the mouth, that is, almost +directly seaward; hence as a bay alters in form and size, so will the +direction of the inclination of its successive beaches become changed. + +(FIGURE 11. DIAGRAM OF A BAY IN A DISTRICT WHICH HAS BEGUN SLOWLY RISING) + +If it were possible to trace any one of the many beach-lines, composing +each sloping terrace, it would of course be horizontal; but the only lines +of demarcation are the summit and basal edges of the escarpments. Now the +summit-edge of one of these escarpments marks the furthest line or point to +which the sea has cut into a mass of gravel sloping seaward; and as the sea +will generally have greater power at the mouth than at the protected head +of the bay, so will the escarpment at the mouth be cut deeper into the +land, and its summit-edge be higher; consequently it will not be +horizontal. With respect to the basal or lower edges of the escarpments, +from picturing in one's mind ancient bays ENTIRELY surrounded at successive +periods by cliff-formed shores, one's first impression is that they at +least necessarily must be horizontal, if the elevation has been horizontal. +But here is a fallacy: for after the sea has, during a cessation of the +elevation, worn cliffs all round the shores of a bay, when the movement +recommences, and especially if it recommences slowly, it might well happen +that, at the exposed mouth of the bay, the waves might continue for some +time wearing into the land, whilst in the protected and upper parts +successive beach-lines might be accumulating in a sloping surface or +terrace at the foot of the cliffs which had been lately reached: hence, +supposing the whole line of escarpment to be finally uplifted above the +reach of the sea, its basal line or foot near the mouth will run at a lower +level than in the upper and protected parts of the bay; consequently this +basal line will not be horizontal. And it has already been shown that the +summit-edges of each escarpment will generally be higher near the mouth +(from the seaward sloping land being there most exposed and cut into) than +near the head of the bay; therefore the total height of the escarpments +will be greatest near the mouth; and further up the old bay or valley they +will on both sides generally thin out and die away: I have observed this +thinning out of the successive escarpment at other places besides Coquimbo; +and for a long time I was quite unable to understand its meaning. The rude +diagram in Figure 11 will perhaps render what I mean more intelligible; it +represents a bay in a district which has begun slowly rising. Before the +movement commenced, it is supposed that the waves had been enabled to eat +into the land and form cliffs, as far up, but with gradually diminishing +power, as the points AA: after the movement had commenced and gone on for a +little time, the sea is supposed still to have retained the power, at the +exposed mouth of the bay, of cutting down and into the land as it slowly +emerged; but in the upper parts of the bay it is supposed soon to have lost +this power, owing to the more protected situation and to the quantity of +detritus brought down by the river; consequently low land was there +accumulated. As this low land was formed during a slow elevatory movement, +its surface will gently slope upwards from the beach on all sides. Now, let +us imagine the bay, not to make the diagram more complicated, suddenly +converted into a valley: the basal line of the cliffs will of course be +horizontal, as far as the beach is now seen extending in the diagram; but +in the upper part of the valley, this line will be higher, the level of the +district having been raised whilst the low land was accumulating at the +foot of the inland cliffs. If, instead of the bay in the diagram being +suddenly converted into a valley, we suppose with much more probability it +to be upraised slowly, then the waves in the upper parts of the bay will +continue very gradually to fail to reach the cliffs, which are now in the +diagram represented as washed by the sea, and which, consequently, will be +left standing higher and higher above its level; whilst at the still +exposed mouth, it might well happen that the waves might be enabled to cut +deeper and deeper, both down and into the cliffs, as the land slowly rose. + +The greater or lesser destroying power of the waves at the mouths of +successive bays, comparatively with this same power in their upper and +protected parts, will vary as the bays become changed in form and size, and +therefore at different levels, at their mouths and heads, more or less of +the surfaces between the escarpments (that is, the accumulated beach-lines +or terraces) will be left undestroyed: from what has gone before we can see +that, according as the elevatory movements after each cessation recommence +more or less slowly, according to the amount of detritus delivered by the +river at the heads of the successive bays, and according to the degree of +protection afforded by their altered forms, so will a greater or less +extent of terrace be accumulated in the upper part, to which there will be +no surface at a corresponding level at the mouth: hence we can perceive why +no one terrace, taken in its whole breadth and followed up the valley, is +horizontal, though each separate beach-line must have been so; and why the +inclination of the several terraces, both transversely, and longitudinally +up the valley, is not alike. + +I have entered into this case in some detail, for I was long perplexed (and +others have felt the same difficulty) in understanding how, on the idea of +an equable elevation with the sea at intervals eating into the land, it +came that neither the terraces nor the upper nor lower edges of the +escarpments were horizontal. Along lines of coast, even of great lengths, +such as that of Patagonia, if they are nearly uniformly exposed, the +corroding power of the waves will be checked and conquered by the elevatory +movement, as often as it recommences, at about the same period; and hence +the terraces, or accumulated beach-lines, will commence being formed at +nearly the same levels: at each succeeding period of rest, they will, also, +be eaten into at nearly the same rate, and consequently there will be a +much closer coincidence in their levels and inclinations, than in the +terraces and escarpments formed round bays with their different parts very +differently exposed to the action of the sea. It is only where the waves +are enabled, after a long lapse of time, slowly to corrode hard rocks, or +to throw up, owing to the supply of sediment being small and to the surface +being steeply inclined, a narrow beach or mound, that we can expect, as at +Glen Roy in Scotland ("Philosophical Transactions" 1839 page 39.), a +distinct line marking an old sea-level, and which will be strictly +horizontal, if the subsequent elevatory movements have been so: for in +these cases no discernible effects will be produced, except during the long +intervening periods of rest; whereas in the case of step-formed coasts, +such as those described in this and the preceding chapter, the terraces +themselves are accumulated during the slow elevatory process, the +accumulation commencing sooner in protected than in exposed situations, and +sooner where there is copious supply of detritus than where there is +little; on the other hand, the steps or escarpments are formed during the +stationary periods, and are more deeply cut down and into the coast-land in +exposed than in protected situations;--the cutting action, moreover, being +prolonged in the most exposed parts, both during the beginning and ending, +if slow, of the upward movement. + +Although in the foregoing discussion I have assumed the elevation to have +been horizontal, it may be suspected, from the considerable seaward slope +of the terraces, both up the valley of S. Cruz and up that of Coquimbo, +that the rising has been greater inland than nearer the coast. There is +reason to believe (Mr. Place in the "Quarterly Journal of Science" 1824 +volume 17 page 42.), from the effects produced on the water-course of a +mill during the earthquake of 1822 in Chile, that the upheaval one mile +inland was nearly double, namely, between five and seven feet, to what it +was on the Pacific. We know, also, from the admirable researches of M. +Bravais, that in Scandinavia the ancient sea-beaches gently slope from the +interior mountain-ranges towards the coast, and that they are not parallel +one to the other ("Voyages de la Comm. du Nord" etc. also "Comptes Rendus" +October 1842.), showing that the proportional difference in the amount of +elevation on the coast and in the interior, varied at different periods. + +COQUIMBO TO GUASCO. + +In this distance of ninety miles, I found in almost every part marine +shells up to a height of apparently from two hundred to three hundred feet. +The desert plain near Choros is thus covered; it is bounded by the +escarpment of a higher plain, consisting of pale-coloured, earthy, +calcareous stone, like that of Coquimbo, with the same recent shells +embedded in it. In the valley of Chaneral, a similar bed occurs in which, +differently from that of Coquimbo, I observed many shells of the +Concholepas: near Guasco the same calcareous bed is likewise met with. + +In the valley of Guasco, the step-formed terraces of gravel are displaced +in a more striking manner than at any other point. I followed the valley +for thirty-seven miles (as reckoned by the inhabitants) from the coast to +Ballenar; in nearly the whole of this distance, five grand terraces, +running at corresponding heights on both sides of the broad valley, are +more conspicuous than the three best-developed ones at Coquimbo. They give +to the landscape the most singular and formal aspect; and when the clouds +hung low, hiding the neighbouring mountains, the valley resembled in the +most striking manner that of Santa Cruz. The whole thickness of these +terraces or plains seems composed of gravel, rather firmly aggregated +together, with occasional parting seams of clay: the pebbles on the upper +plain are often whitewashed with an aluminous substance, as in Patagonia. +Near the coast I observed many sea-shells on the lower plains. At Freyrina +(twelve miles up the valley), there are six terraces beside the bottom- +surface of the valley: the two lower ones are here only from two hundred to +three hundred yards in width, but higher up the valley they expand into +plains; the third terrace is generally narrow; the fourth I saw only in one +place, but there it was distinct for the length of a mile; the fifth is +very broad; the sixth is the summit-plain, which expands inland into a +great basin. Not having a barometer with me, I did not ascertain the height +of these plains, but they appeared considerably higher than those at +Coquimbo. Their width varies much, sometimes being very broad, and +sometimes contracting into mere fringes of separate flat-topped +projections, and then quite disappearing: at the one spot, where the fourth +terrace was visible, the whole six terraces were cut off for a short space +by one single bold escarpment. Near Ballenar (thirty-seven miles from the +mouth of the river), the valley between the summit-edges of the highest +escarpments is several miles in width, and the five terraces on both sides +are broadly developed: the highest cannot be less than six hundred feet +above the bed of the river, which itself must, I conceive, be some hundred +feet above the sea. + +A north and south section across the valley in this part is represented in +Figure 12. + +(FIGURE 12. NORTH AND SOUTH SECTION ACROSS THE VALLEY OF GUASCO, AND OF A +PLAIN NORTH OF IT. + +From left (north, high) to right (south, high) through plains B and A and +the River of Guasco at the Town of Ballenar.) + +On the northern side of the valley the summit-plain of gravel, A, has two +escarpments, one facing the valley, and the other a great basin-like plain, +B, which stretches for several leagues northward. This narrow plain, A, +with the double escarpment, evidently once formed a spit or promontory of +gravel, projecting into and dividing two great bays, and subsequently was +worn on both sides into steep cliffs. Whether the several escarpments in +this valley were formed during the same stationary periods with those of +Coquimbo, I will not pretend to conjecture; but if so the intervening and +subsequent elevatory movements must have been here much more energetic, for +these plains certainly stand at a much higher level than do those of +Coquimbo. + +COPIAPO. + +From Guasco to Copiapo, I followed the road near the foot of the +Cordillera, and therefore saw no upraised remains. At the mouth, however, +of the valley of Copiapo there is a plain, estimated by Meyen ("Reise um +die Erde" th. 1 s. 372 et seq.) between fifty and seventy feet in height, +of which the upper part consists chiefly of gravel, abounding with recent +shells, chiefly of the Concholepas, Venus Dombeyi, and Calyptraea +trochiformis. A little inland, on a plain estimated by myself at nearly +three hundred feet, the upper stratum was formed of broken shells and sand +cemented by white calcareous matter, and abounding with embedded recent +shells, of which the Mulinia Byronensis and Pecten purpuratus were the most +numerous. The lower plain stretches for some miles southward, and for an +unknown distance northward, but not far up the valley; its seaward face, +according to Meyen, is worn into caves above the level of the present +beach. The valley of Copiapo is much less steeply inclined and less direct +in its course than any other valley which I saw in Chile; and its bottom +does not generally consist of gravel: there are no step-formed terraces in +it, except at one spot near the mouth of the great lateral valley of the +Despoblado where there are only two, one above the other: lower down the +valley, in one place I observed that the solid rock had been cut into the +shape of a beach, and was smoothed over with shingle. + +Northward of Copiapo, in latitude 26 degrees S., the old voyager Wafer +found immense numbers of sea-shells some miles from the coast. (Burnett's +"Collection of Voyages" volume 4 page 193.) At Cobija (latitude 22 degrees +34') M. d'Orbigny observed beds of gravel and broken shells, containing ten +species of recent shells; he also found, on projecting points of porphyry, +at a height of 300 feet, shells of Concholepas, Chiton, Calyptraea, +Fissurella, and Patella, still attached to the spots on which they had +lived. M. d'Orbigny argues from this fact, that the elevation must have +been great and sudden ("Voyage, Part Geolog." page 94. M. d'Orbigny (page +98), in summing up, says: "S'il est certain (as he believes) que tous les +terrains en pente, compris entre la mer et les montagnes sont l'ancien +rivage de la mer, on doit supposer, pour l'ensemble, un exhaussement que ce +ne serait pas moindre de deux cent metres; il faudrait supposer encore que +ce soulevement n'a point ete graduel;...mais qu'il resulterait d'une seule +et meme cause fortuite," etc. Now, on this view, when the sea was forming +the beach at the foot of the mountains, many shells of Concholepas, Chiton, +Calyptraea, Fissurella, and Patella (which are known to live close to the +beach), were attached to rocks at a depth of 300 feet, and at a depth of +600 feet several of these same shells were accumulating in great numbers in +horizontal beds. From what I have myself seen in dredging, I believe this +to be improbable in the highest degree, if not impossible; and I think +everyone who has read Professor E. Forbes's excellent researches on the +subject, will without hesitation agree in this conclusion.): to me it +appears far more probable that the movement was gradual, with small starts +as during the earthquakes of 1822 and 1835, by which whole beds of shells +attached to the rocks were lifted above the subsequent reach of the waves. +M. d'Orbigny also found rolled pebbles extending up the mountain to a +height of at least six hundred feet. At Iquique (latitude 20 degrees 12' +S.), in a great accumulation of sand, at a height estimated between one +hundred and fifty and two hundred feet, I observed many large sea-shells +which I thought could not have been blown up by the wind to that height. +Mr. J.H. Blake has lately described these shells: he states that "inland +toward the mountains they form a compact uniform bed, scarcely a trace of +the original shells being discernible; but as we approach the shore, the +forms become gradually more distinct till we meet with the living shells on +the coast." ("Silliman's American Journal of Science" volume 44 page 2.) +This interesting observation, showing by the gradual decay of the shells +how slowly and gradually the coast must have been uplifted, we shall +presently see fully confirmed at Lima. At Arica (latitude 18 degrees 28'), +M. d'Orbigny found a great range of sand-dunes, fourteen leagues in length, +stretching towards Tacna, including recent shells and bones of Cetacea, and +reaching up to a height of 300 feet above the sea. ("Voyage" etc. page +101.) Lieutenant Freyer has given some more precise facts: he states (In a +letter to Mr. Lyell "Geological Proceedings" volume 2 page 179.) that the +Morro of Arica is about four hundred feet high; it is worn into obscure +terraces, on the bare rock of which he found Balini and Milleporae +adhering. At the height of between twenty and thirty feet the shells and +corals were in a quite fresh state, but at fifty feet they were much +abraded; there were, however, traces of organic remains at greater heights. +On the road from Tacna to Arequipa, between Loquimbo and Moquegua, Mr. M. +Hamilton found numerous recent sea shells in sand, at a considerable +distance from the sea. ("Edinburgh New Philosophical Journal" volume 30 +page 155.) + +LIMA. + +Northward of Arica, I know nothing of the coast for about a space of five +degrees of latitude; but near Callao, the port of Lima, there is abundant +and very curious evidence of the elevation of the land. The island of San +Lorenzo is upwards of one thousand feet high; the basset edges of the +strata composing the lower part are worn into three obscure, narrow, +sloping steps or ledges, which can be seen only when standing on them: they +probably resemble those described by Lieutenant Freyer at Arica. The +surface of the lower ledge, which extends from a low cliff overhanging the +sea to the foot of the next upper escarpment, is covered by an enormous +accumulation of recent shells. (M. Chevalier, in the "Voyage of the +'Bonite'" observed these shells; but his specimens were lost.--"L'Institut" +1838 page 151.) The bed is level, and in some parts more than two feet in +thickness; I traced it over a space of one mile in length, and heard of it +in other places: the uppermost part is eighty-five feet by the barometer +above high-water mark. The shells are packed together, but not stratified: +they are mingled with earth and stones, and are generally covered by a few +inches of detritus; they rest on a mass of nearly angular fragments of the +underlying sandstone, sometimes cemented together by common salt. I +collected eighteen species of shells of all ages and sizes. Several of the +univalves had evidently long lain dead at the bottom of the sea, for their +INSIDES were incrusted with Balani and Serpulae. All, according to Mr. G.B. +Sowerby, are recent species: they consist of:-- + +1. Mytilus Magellanicus: same as that found at Valparaiso, and there stated +to be probably distinct from the true M. Magellanicus of the east coast. + +2. Venus costellata, Sowerby "Zoological Proceedings." + +3. Pecten purpuratus, Lam. + +4. Chama, probably echinulata, Brod. + +5. Calyptraea Byronensis, Gray. + +6. Calyptraea radians (Trochus, Lam.) + +7. Fissurella affinis, Gray. + +8. Fissurella biradiata, Trembly. + +9. Purpura chocolatta, Duclos. + +10. Purpura Peruviana, Gray. + +11. Purpura labiata, Gray. + +12. Purpura buxea (Murex, Brod.). + +13. Concholepas Peruviana. + +14. Nassa, related to reticulata. + +15. Triton rudis, Brod. + +16. Trochus, not yet described, but well-known and very common. + +17 and 18. Balanus, two species, both common on the coast. + +These upraised shells appear to be nearly in the same proportional numbers- +-with the exception of the Crepidulae being more numerous--with those on +the existing beach. The state of preservation of the different species +differed much; but most of them were much corroded, brittle, and bleached: +the upper and lower surfaces of the Concholepas had generally quite scaled +off: some of the Trochi and Fissurellae still partially retain their +colours. It is remarkable that these shells, taken all together, have fully +as ancient an appearance, although the extremely arid climate appears +highly favourable for their preservation, as those from 1,300 feet at +Valparaiso, and certainly a more ancient appearance than those from five to +six hundred feet from Valparaiso and Concepcion; at which places I have +seen grass and other vegetables actually growing out of the shells. Many of +the univalves here at San Lorenzo were filled with, and united together by, +pure salt, probably left by the evaporation of the sea-spray, as the land +slowly emerged. (The underlying sandstone contains true layers of salt; so +that the salt may possibly have come from the beds in the higher parts of +the island; but I think more probably from the sea-spray. It is generally +asserted that rain never falls on the coast of Peru; but this is not quite +accurate; for, on several days, during our visit, the so-called Peruvian +dew fell in sufficient quantity to make the streets muddy, and it would +certainly have washed so deliquescent a substance as salt into the soil. I +state this because M. d'Orbigny, in discussing an analogous subject, +supposes that I had forgotten that it never rains on this whole line of +coast. See Ulloa's "Voyage" volume 2 English Translation page 67 for an +account of the muddy streets of Lima, and on the continuance of the mists +during the whole winter. Rain, also, falls at rare intervals even in the +driest districts, as, for instance, during forty days, in 1726, at Chocope +(7 degrees 46'); this rain entirely ruined ("Ulloa" etc. page 18) the mud +houses of the inhabitants.) On the highest parts of the ledge, small +fragments of the shells were mingled with, and evidently in process of +reduction into, a yellowish-white, soft, calcareous powder, tasting +strongly of salt, and in some places as fine as prepared medicinal chalk. + +FOSSIL-REMAINS OF HUMAN ART. + +In the midst of these shells on San Lorenzo, I found light corallines, the +horny ovule-cases of Mollusca, roots of seaweed (Mr. Smith of Jordan Hill +found pieces of seaweed in an upraised pleistocene deposit in Scotland. See +his admirable Paper in the "Edinburgh New Philosophical Journal" volume 25 +page 384.), bones of birds, the heads of Indian corn and other vegetable +matter, a piece of woven rushes, and another of nearly decayed COTTON +string. I extracted these remains by digging a hole, on a level spot; and +they had all indisputably been embedded with the shells. I compared the +plaited rush, the COTTON string, and Indian corn, at the house of an +antiquary, with similar objects, taken from the Huacas or burial-grounds of +the ancient Peruvians, and they were undistinguishable; it should be +observed that the Peruvians used string only of cotton. The small quantity +of sand or gravel with the shells, the absence of large stones, the width +and thickness of the bed, and the time requisite for a ledge to be cut into +the sandstone, all show that these remains were not thrown high up by an +earthquake-wave: on the other hand, these facts, together with the number +of dead shells, and of floating objects, both marine and terrestrial, both +natural and human, render it almost certain that they were accumulated on a +true beach, since upraised eighty-five feet, and upraised this much since +INDIAN MAN INHABITED PERU. The elevation may have been, either by several +small sudden starts, or quite gradual; in this latter case the unrolled +shells having been thrown up during gales beyond the reach of the waves +which afterwards broke on the slowly emerging land. I have made these +remarks, chiefly because I was at first surprised at the complete +difference in nature, between this broad, smooth, upraised bed of shells, +and the present shingle-beach at the foot of the low sandstone-cliffs; but +a beach formed, when the sea is cutting into the land, as is shown now to +be the case by the low bare sandstone-cliffs, ought not to be compared with +a beach accumulated on a gently inclined rocky surface, at a period when +the sea (probably owing to the elevatory movement in process) was not able +to eat into the land. With respect to the mass of nearly angular, salt- +cemented fragments of sandstone, which lie under the shells, and which are +so unlike the materials of an ordinary sea-beach; I think it probable after +having seen the remarkable effects of the earthquake of 1835 (I have +described this in my "Journal of Researches" page 303 2nd edition.), in +absolutely shattering as if by gunpowder the SURFACE of the primary rocks +near Concepcion, that a smooth bare surface of stone was left by the sea +covered by the shelly mass, and that afterwards when upraised, it was +superficially shattered by the severe shocks so often experienced here. + +The very low land surrounding the town of Callao, is to the south joined by +an obscure escarpment to a higher plain (south of Bella Vista), which +stretches along the coast for a length of about eight miles. This plain +appears to the eye quite level; but the sea-cliffs show that its height +varies (as far as I could estimate) from seventy to one hundred and twenty +feet. It is composed of thin, sometimes waving, beds of clay, often of +bright red and yellow colours, of layers of impure sand, and in one part +with a great stratified mass of granitic pebbles. These beds are capped by +a remarkable mass, varying from two to six feet in thickness, of reddish +loam or mud, containing many scattered and broken fragments of recent +marine shells, sometimes though rarely single large round pebble, more +frequently short irregular layers of fine gravel, and very many pieces of +red coarse earthenware, which from their curvatures must once have formed +parts of large vessels. The earthenware is of Indian manufacture; and I +found exactly similar pieces accidentally included within the bricks, of +which the neighbouring ancient Peruvian burial-mounds are built. These +fragments abounded in such numbers in certain spots, that it appeared as if +waggon-loads of earthenware had been smashed to pieces. The broken sea- +shells and pottery are strewed both on the surface, and throughout the +whole thickness of this upper loamy mass. I found them wherever I examined +the cliffs, for a space of between two and three miles, and for half a mile +inland; and there can be little doubt that this same bed extends with a +smooth surface several miles further over the entire plain. Besides the +little included irregular layers of small pebbles, there are occasionally +very obscure traces of stratification. + +At one of the highest parts of the cliff, estimated 120 feet above the sea, +where a little ravine came down, there were two sections, at right angles +to each other, of the floor of a shed or building. In both sections or +faces, two rows, one over the other, of large round stones could be +distinctly seen; they were packed close together on an artificial layer of +sand two inches thick, which had been placed on the natural clay-beds; the +round stones were covered by three feet in thickness of the loam with +broken sea-shells and pottery. Hence, before this widely spread-out bed of +loam was deposited, it is certain that the plain was inhabited; and it is +probable, from the broken vessels being so much more abundant in certain +spots than in others, and from the underlying clay being fitted for their +manufacture, that the kilns stood here. + +The smoothness and wide extent of the plain, the bulk of matter deposited, +and the obscure traces of stratification seem to indicate that the loam was +deposited under water; on the other hand, the presence of sea-shells, their +broken state, the pebbles of various sizes, and the artificial floor of +round stones, almost prove that it must have originated in a rush of water +from the sea over the land. The height of the plain, namely, 120 feet, +renders it improbable that an earthquake-wave, vast as some have here been, +could have broken over the surface at its present level; but when the land +stood eighty-five feet lower, at the period when the shells were thrown up +on the ledge at S. Lorenzo, and when as we know man inhabited this +district, such an event might well have occurred; and if we may further +suppose, that the plain was at that time converted into a temporary lake, +as actually occurred, during the earthquakes of 1713 and 1746, in the case +of the low land round Callao owing to its being encircled by a high +shingle-beach, all the appearances above described will be perfectly +explained. I must add, that at a lower level near the point where the +present low land round Callao joins the higher plain, there are appearances +of two distinct deposits both apparently formed by debacles: in the upper +one, a horse's tooth and a dog's jaw were embedded; so that both must have +been formed after the settlement of the Spaniards: according to Acosta, the +earthquake-wave of 1586 rose eighty-four feet. + +The inhabitants of Callao do not believe, as far as I could ascertain, that +any change in level is now in progress. The great fragments of brickwork, +which it is asserted can be seen at the bottom of the sea, and which have +been adduced as a proof of a late subsidence, are, as I am informed by Mr. +Gill, a resident engineer, loose fragments; this is probable, for I found +on the beach, and not near the remains of any building, masses of +brickwork, three and four feet square, which had been washed into their +present places, and smoothed over with shingle during the earthquake of +1746. The spit of land, on which the ruins of OLD Callao stand, is so +extremely low and narrow, that it is improbable in the highest degree that +a town should have been founded on it in its present state; and I have +lately heard that M. Tschudi has come to the conclusion, from a comparison +of old with modern charts, that the coast both south and north of Callao +has subsided. (I am indebted for this fact to Dr. E. Dieffenbach. I may add +that there is a tradition, that the islands of San Lorenzo and Fronton were +once joined, and that the channel between San Lorenzo and the mainland, now +above two miles in width, was so narrow that cattle used to swim over.) I +have shown that the island of San Lorenzo has been upraised eighty-five +feet since the Peruvians inhabited this country; and whatever may have been +the amount of recent subsidence, by so much more must the elevation have +exceeded the eighty-five feet. In several places in this neighbourhood, +marks of sea-action have been observed: Ulloa gives a detailed account of +such appearances at a point five leagues northward of Callao: Mr. +Cruikshank found near Lima successive lines of sea-cliffs, with rounded +blocks at their bases, at a height of 700 feet above the present level of +the sea. ("Observaciones sobre el Clima del Lima" par Dr. H. Unanue page +4.--Ulloa's "Voyage" volume 2 English Translation page 97.--For Mr. +Cruikshank's observations, see Mr. Lyell's "Principles of Geology" 1st +edition volume 3 page 130.) +ON THE DECAY OF UPRAISED SEA-SHELLS. + +I have stated that many of the shells on the lower inclined ledge or +terrace of San Lorenzo are corroded in a peculiar manner, and that they +have a much more ancient appearance than the same species at considerably +greater heights on the coast of Chile. I have, also, stated that these +shells in the upper part of the ledge, at the height of eighty-five feet +above the sea, are falling, and in some parts are quite changed into a +fine, soft, saline, calcareous powder. The finest part of this powder has +been analysed for me, at the request of Sir H. De la Beche, by the kindness +of Mr. Trenham Reeks of the Museum of Economic Geology; it consists of +carbonate of lime in abundance, of sulphate and muriate of lime, and of +muriate and sulphate of soda. The carbonate of lime is obviously derived +from the shells; and common salt is so abundant in parts of the bed, that, +as before remarked, the univalves are often filled with it. The sulphate of +lime may have been derived, as has probably the common salt, from the +evaporation of the sea-spray, during the emergence of the land; for +sulphate of lime is now copiously deposited from the spray on the shores of +Ascension. (See "Volcanic Islands" etc. by the Author.) The other saline +bodies may perhaps have been partially thus derived, but chiefly, as I +conclude from the following facts, through a different means. + +On most parts of the second ledge or old sea-beach, at a height of 170 +feet, there is a layer of white powder of variable thickness, as much in +some parts as two inches, lying on the angular, salt-cemented fragments of +sandstone and under about four inches of earth, which powder, from its +close resemblance in nature to the upper and most decayed parts of the +shelly mass, I can hardly doubt originally existed as a bed of shells, now +much collapsed and quite disintegrated. I could not discover with the +microscope a trace of organic structure in it; but its chemical +constituents, according to Mr. Reeks, are the same as in the powder +extracted from amongst the decaying shells on the lower ledge, with the +marked exception that the carbonate of lime is present in only very small +quantity. On the third and highest ledge, I observed some of this powder in +a similar position, and likewise occasionally in small patches at +considerably greater heights near the summit of the island. At Iquique, +where the whole face of the country is covered by a highly saliferous +alluvium, and where the climate is extremely dry, we have seen that, +according to Mr. Blake, the shells which are perfect near the beach become, +in ascending, gradually less and less perfect, until scarcely a trace of +their original structure can be discovered. It is known that carbonate of +lime and common salt left in a mass together, and slightly moistened, +partially decompose each other (I am informed by Dr. Kane, through Mr. +Reeks, that a manufactory was established on this principle in France, but +failed from the small quantity of carbonate of soda produced. Sprengel +"Gardeners' Chronicle" 1845 page 157, states, that salt and carbonate of +lime are liable to mutual decomposition in the soil. Sir H. De la Beche +informs me, that calcareous rocks washed by the spray of the sea, are often +corroded in a peculiar manner; see also on this latter subject "Gardeners' +Chronicle" page 675 1844.): now we have at San Lorenzo and at Iquique, in +the shells and salt packed together, and occasionally moistened by the so- +called Peruvian dew, the proper elements for this action. We can thus +understand the peculiar corroded appearance of the shells on San Lorenzo, +and the great decrease of quantity in the carbonate of lime in the powder +on the upper ledge. There is, however, a great difficulty on this view, for +the resultant salts should be carbonate of soda and muriate of lime; the +latter is present, but not the carbonate of soda. Hence I am led to the +perhaps unauthorised conjecture (which I shall hereafter have to refer to) +that the carbonate of soda, by some unexplained means, becomes converted +into a sulphate. + +If the above remarks be just, we are led to the very unexpected conclusion, +that a dry climate, by leaving the salt from the sea-spray undissolved, is +much less favourable to the preservation of upraised shells than a humid +climate. However this may be, it is interesting to know the manner in which +masses of shells, gradually upraised above the sea-level, decay and finally +disappear. + +SUMMARY ON THE RECENT ELEVATION OF THE WEST COAST OF SOUTH AMERICA. + +We have seen that upraised marine remains occur at intervals, and in some +parts almost continuously, from latitude 45 degrees 35' to 12 degrees S., +along the shores of the Pacific. This is a distance, in a north and south +line, of 2,075 geographical miles. From Byron's observations, the elevation +has no doubt extended sixty miles further south; and from the similarity in +the form of the country near Lima, it has probably extended many leagues +further north. (I may take this opportunity of stating that in a MS. in the +Geological Society by Mr. Weaver, it is stated that beds of oysters and +other recent shells are found thirty feet above the level of the sea, in +many parts of Tampico, in the Gulf of Mexico.) Along this great line of +coast, besides the organic remains, there are in very many parts, marks of +erosion, caves, ancient beaches, sand-dunes, and successive terraces of +gravel, all above the present level of the sea. From the steepness of the +land on this side of the continent, shells have rarely been found at +greater distances inland than from two to three leagues; but the marks of +sea-action are evident farther from the coast; for instance, in the valley +of Guasco, at a distance of between thirty and forty miles. Judging from +the upraised shells alone, the elevation in Chiloe has been 350 feet, at +Concepcion certainly 625 feet; and by estimation 1,000 feet; at Valparaiso +1,300 feet; at Coquimbo 252 feet; northward of this place, sea-shells have +not, I believe, been found above 300 feet; and at Lima they were falling +into decay (hastened probably by the salt) at 85 feet. Not only has this +amount of elevation taken place within the period of existing Mollusca and +Cirripedes; but their proportional numbers in the neighbouring sea have in +most cases remained the same. Near Lima, however, a small change in this +respect between the living and the upraised was observed: at Coquimbo this +was more evident, all the shells being existing species, but with those +embedded in the uppermost calcareous plain not approximating so closely in +proportional numbers, as do those that lie loose on its surface at the +height of 252 feet, and still less closely than those which are strewed on +the lower plains, which latter are identical in proportional numbers with +those now cast up on the beach. From this circumstance, and from not +finding, upon careful examination, near Coquimbo any shells at a greater +height than 252 feet, I believe that the recent elevation there has been +much less than at Valparaiso, where it has been 1,300 feet, and I may add, +than at Concepcion. This considerable inequality in the amount of elevation +at Coquimbo and Valparaiso, places only 200 miles apart, is not improbable, +considering, first, the difference in the force and number of the shocks +now yearly affecting different parts of this coast; and, secondly, the fact +of single areas, such as that of the province of Concepcion, having been +uplifted very unequally during the same earthquake. It would, in most +cases, be very hazardous to infer an inequality of elevation, from shells +being found on the surface or in superficial beds at different heights; for +we do not know on what their rate of decay depends; and at Coquimbo one +instance out of many has been given, of a promontory, which, from the +occurrence of one very small collection of lime-cemented shells, has +indisputably been elevated 242 feet, and yet on which, not even a fragment +of shell could be found on careful examination between this height and the +beach, although many sites appeared very favourable for the preservation of +organic remains: the absence, also, of shells on the gravel-terraces a +short distance up the valley of Coquimbo, though abundant on the +corresponding terraces at its mouth, should be borne in mind. + +There are other epochs, besides that of the existence of recent Mollusca, +by which to judge of the changes of level on this coast. At Lima, as we +have just seen, the elevation has been at least eighty-five feet, within +the Indo-human period; and since the arrival of the Spaniards in 1530, +there has apparently been a sinking of the surface. At Valparaiso, in the +course of 220 years, the rise must have been less than nineteen feet; but +it has been as much as from ten to eleven feet in the seventeen years +subsequently to 1817, and of this rise only a part can be attributed to the +earthquake of 1822, the remainder having been insensible and apparently +still, in 1834, in progress. At Chiloe the elevation has been gradual, and +about four feet during four years. At Coquimbo, also, it has been gradual, +and in the course of 150 years has amounted to several feet. The sudden +small upheavals, accompanied by earthquakes, as in 1822 at Valparaiso, in +1835 at Concepcion, and in 1837 in the Chonos Archipelago, are familiar to +most geologists, but the gradual rising of the coast of Chile has been +hardly noticed; it is, however, very important, as connecting together +these two orders of events. + +The rise of Lima, having been eighty-five feet within the period of man, is +the more surprising if we refer to the eastern coast of the continent, for +at Port S. Julian, in Patagonia, there is good evidence (as we shall +hereafter see) that when the land stood ninety feet lower, the +Macrauchenia, a mammiferous beast, was alive; and at Bahia Blanca, when it +stood only a few feet lower than it now does, many gigantic quadrupeds +ranged over the adjoining country. But the coast of Patagonia is some way +distant from the Cordillera, and the movement at Bahia Blanca is perhaps +noways connected with this great range, but rather with the tertiary +volcanic rocks of Banda Oriental, and therefore the elevation at these +places may have been infinitely slower than on the coast of Peru. All such +speculations, however, must be vague, for as we know with certainty that +the elevation of the whole coast of Patagonia has been interrupted by many +and long pauses, who will pretend to say that, in such cases, many and long +periods of subsidence may not also have been intercalated? + +In many parts of the coast of Chile and Peru there are marks of the action +of the sea at successive heights on the land, showing that the elevation +has been interrupted by periods of comparative rest in the upward movement, +and of denudation in the action of the sea. These are plainest at Chiloe, +where, in a height of about five hundred feet, there are three +escarpments,--at Coquimbo, where in a height of 364 feet, there are five,-- +at Guasco, where there are six, of which five may perhaps correspond with +those at Coquimbo, but if so, the subsequent and intervening elevatory +movements have been here much more energetic,--at Lima, where, in a height +of about 250 feet there are three terraces, and others, as it is asserted, +at considerably greater heights. The almost entire absence of ancient marks +of sea-action at defined levels along considerable spaces of coast, as near +Valparaiso and Concepcion, is highly instructive, for as it is improbable +that the elevation at these places alone should have been continuous, we +must attribute the absence of such marks to the nature and form of the +coast-rocks. Seeing over how many hundred miles of the coast of Patagonia, +and on how many places on the shores of the Pacific, the elevatory process +has been interrupted by periods of comparative rest, we may conclude, +conjointly with the evidence drawn from other quarters of the world, that +the elevation of the land is generally an intermittent action. From the +quantity of matter removed in the formation of the escarpments, especially +of those of Patagonia, it appears that the periods of rest in the movement, +and of denudation of the land, have generally been very long. In Patagonia, +we have seen that the elevation has been equable, and the periods of +denudation synchronous over very wide spaces of coast; on the shores of the +Pacific, owing to the terraces chiefly occurring in the valleys, we have +not equal means of judging on this point; and the very different heights of +the upraised shells at Coquimbo, Valparaiso, and Concepcion seem directly +opposed to such a conclusion. + +Whether on this side of the continent the elevation, between the periods of +comparative rest when the escarpments were formed, has been by small sudden +starts, such as those accompanying recent earthquakes, or, as is most +probable, by such starts conjointly with a gradual upward movement, or by +great and sudden upheavals, I have no direct evidence. But as on the +eastern coast, I was led to think, from the analogy of the last hundred +feet of elevation in La Plata, and from the nearly equal size of the +pebbles over the entire width of the terraces, and from the upraised shells +being all littoral species, that the elevation had been gradual; so do I on +this western coast, from the analogy of the movements now in progress, and +from the vast numbers of shells now living exclusively on or close to the +beach, which are strewed over the whole surface of the land up to very +considerable heights, conclude, that the movement here also has been slow +and gradual, aided probably by small occasional starts. We know at least +that at Coquimbo, where five escarpments occur in a height of 364 feet, the +successive elevations, if they have been sudden, cannot have been very +great. It has, I think, been shown that the occasional preservation of +shells, unrolled and unbroken, is not improbable even during a quite +gradual rising of the land; and their preservation, if the movement has +been aided by small starts, is quite conformable with what actually takes +place during recent earthquakes. + +Judging from the present action of the sea, along the shores of the +Pacific, on the deposits of its own accumulation, the present time seems in +most places to be one of comparative rest in the elevatory movement, and of +denudation of the land. Undoubtedly this is the case along the whole great +length of Patagonia. At Chiloe, however, we have seen that a narrow sloping +fringe, covered with vegetation, separates the present sea-beach from a +line of low cliffs, which the waves lately reached; here, then, the land is +gaining in breadth and height, and the present period is not one of rest in +the elevation and of contingent denudation; but if the rising be not +prolonged at a quick rate, there is every probability that the sea will +soon regain its former horizontal limits. I observed similar low sloping +fringes on several parts of the coast, both northward of Valparaiso and +near Coquimbo; but at this latter place, from the change in form which the +coast has undergone since the old escarpments were worn, it may be doubted +whether the sea, acting for any length of time at its present level, would +eat into the land; for it now rather tends to throw up great masses of +sand. It is from facts such as these that I have generally used the term +COMPARATIVE rest, as applied to the elevation of the land; the rest or +cessation in the movement being comparative both with what has preceded it +and followed it, and with the sea's power of corrosion at each spot and at +each level. Near Lima, the cliff-formed shores of San Lorenzo, and on the +mainland south of Callao, show that the sea is gaining on the land; and as +we have here some evidence that its surface has lately subsided or is still +sinking, the periods of comparative rest in the elevation and of contingent +denudation, may probably in many cases include periods of subsidence. It is +only, as was shown in detail when discussing the terraces of Coquimbo, when +the sea with difficulty and after a long lapse of time has either corroded +a narrow ledge into solid rock, or has heaped up on a steep surface a +NARROW mound of detritus, that we can confidently assert that the land at +that level and at that period long remained absolutely stationary. In the +case of terraces formed of gravel or sand, although the elevation may have +been strictly horizontal, it may well happen that no one level beach-line +may be traceable, and that neither the terraces themselves nor the summit +nor basal edges of their escarpments may be horizontal. + +Finally, comparing the extent of the elevated area, as deduced from the +upraised recent organic remains, on the two sides of the continent, we have +seen that on the Atlantic, shells have been found at intervals from Eastern +Tierra del Fuego for 1,180 miles northward, and on the Pacific for a space +of 2,075 miles. For a length of 775 miles, they occur in the same latitudes +on both sides of the continent. Without taking this circumstance into +consideration, it is probable from the reasons assigned in the last +chapter, that the entire breadth of the continent in Central Patagonia has +been uplifted in mass; but from other reasons there given, it would be +hazardous to extend this conclusion to La Plata. From the continent being +narrow in the southern-most parts of Patagonia, and from the shells found +at the Inner Narrows of the Strait of Magellan, and likewise far up the +valley of the Santa Cruz, it is probable that the southern part of the +western coast, which was not visited by me, has been elevated within the +period of recent Mollusca: if so, the shores of the Pacific have been +continuously, recently, and in a geological sense synchronously upraised, +from Lima for a length of 2,480 nautical miles southward,--a distance equal +to that from the Red Sea to the North Cape of Scandinavia! + + +CHAPTER III. ON THE PLAINS AND VALLEYS OF CHILE:--SALIFEROUS SUPERFICIAL +DEPOSITS. + +Basin-like plains of Chile; their drainage, their marine origin. +Marks of sea-action on the eastern flanks of the Cordillera. +Sloping terrace-like fringes of stratified shingle within the valleys of +the Cordillera; their marine origin. +Boulders in the valley of Cachapual. +Horizontal elevation of the Cordillera. +Formation of valleys. +Boulders moved by earthquake-waves. +Saline superficial deposits. +Bed of nitrate of soda at Iquique. +Saline incrustations. +Salt-lakes of La Plata and Patagonia; purity of the salt; its origin. + +The space between the Cordillera and the coast of Chile is on a rude +average from eighty to above one hundred miles in width; it is formed, +either of an almost continuous mass of mountains, or more commonly of +several nearly parallel ranges, separated by plains; in the more southern +parts of this province the mountains are quite subordinate to the plains; +in the northern part the mountains predominate. + +The basin-like plains at the foot of the Cordillera are in several respects +remarkable; that on which the capital of Chile stands is fifteen miles in +width, in an east and west line, and of much greater length in a north and +south line; it stands 1,750 feet above the sea; its surface appears smooth, +but really falls and rises in wide gentle undulations, the hollows +corresponding with the main valleys of the Cordillera: the striking manner +in which it abruptly comes up to the foot of this great range has been +remarked by every author since the time of Molina. (This plain is partially +separated into two basins by a range of hills; the southern half, according +to Meyen ("Reise um Erde" Th. 1 s. 274), falls in height, by an abrupt +step, of between fifteen and twenty feet.) Near the Cordillera it is +composed of a stratified mass of pebbles of all sizes, occasionally +including rounded boulders: near its western boundary, it consists of +reddish sandy clay, containing some pebbles and numerous fragments of +pumice, and sometimes passes into pure sand or into volcanic ashes. At +Podaguel, on this western side of the plain, beds of sand are capped by a +calcareous tuff, the uppermost layers being generally hard and +substalagmitic, and the lower ones white and friable, both together +precisely resembling the beds at Coquimbo, which contain recent marine +shells. Abrupt, but rounded, hummocks of rock rise out of this plain: those +of Sta. Lucia and S. Cristoval are formed of greenstone-porphyry almost +entirely denuded of its original covering of porphyritic claystone breccia; +on their summits, many fragments of rock (some of them kinds not found in +situ) are coated and united together by a white, friable, calcareous tuff, +like that found at Podaguel. When this matter was deposited on the summit +of S. Cristoval, the water must have stood 946 feet above the surface of +the surrounding plain. (Or 2,690 feet above the sea, as measured +barometrically by Mr. Eck. This tuff appears to the eye nearly pure; but +when placed in acid it leaves a considerable residue of sand and broken +crystals, apparently of feldspar. Dr. Meyen ("Reise" Th. 1 s. 269) says he +found a similar substance on the neighbouring hill of Dominico (and I found +it also on the Cerro Blanco), and he attributes it to the weathering of the +stone. In some places which I examined, its bulk put this view of its +origin quite out of the question; and I should much doubt whether the +decomposition of a porphyry would, in any case, leave a crust chiefly +composed of carbonate of lime. The white crust, which is commonly seen on +weathered feldspathic rocks, does not appear to contain any free carbonate +of lime.) + +To the south this basin-like plain contracts, and rising scarcely +perceptibly with a smooth surface, passes through a remarkable level gap in +the mountains, forming a true land-strait, and called the Angostura. It +then immediately expands into a second basin-formed plain: this again to +the south contracts into another land-strait, and expands into a third +basin, which, however, falls suddenly in level about forty feet. This third +basin, to the south, likewise contracts into a strait, and then again opens +into the great plain of San Fernando, stretching so far south that the +snowy peaks of the distant Cordillera are seen rising above its horizon as +above the sea. These plains, near the Cordillera, are generally formed of a +thick stratified mass of shingle (The plain of San Fernando has, according +to MM. Meyen and Gay "Reise" etc. Th. 1 ss. 295 and 298, near the +Cordillera, an upper step-formed plain of clay, on the surface of which +they found numerous blocks of rocks, from two to three feet long, either +lying single or piled in heaps, but all arranged in nearly straight +lines.); in other parts, of a red sandy clay, often with an admixture of +pumiceous matter. Although these basins are connected together like a +necklace, in a north and south line, by smooth land-straits, the streams +which drain them do not all flow north and south, but mostly westward, +through breaches worn in the bounding mountains; and in the case of the +second basin, or that of Rancagua, there are two distinct breaches. Each +basin, moreover, is not drained singly; thus, to give the most striking +instance, but not the only one, in proceeding southward over the plain of +Rancagua, we first find the water flowing northward to and through the +northern land-strait; then, without crossing any marked ridge or watershed, +we see it flowing south-westward towards the northern one of the two +breaches in the western mountainous boundary; and lastly, again without any +ridge, it flows towards the southern breach in these same mountains. Hence +the surface of this one basin-like plain, appearing to the eye so level, +has been modelled with great nicety, so that the drainage, without any +conspicuous watersheds, is directed towards three openings in the +encircling mountains. ((It appears from Captain Herbert's account of the +Diluvium of the Himalaya, "Gleanings of Science" Calcutta volume 2 page +164, that precisely similar remarks apply to the drainage of the plains or +valleys between those great mountains.) The streams flowing from the +southern basin-like plains, after passing through the breaches to the west, +unite and form the river Rapel, which enters the Pacific near Navidad. I +followed the southernmost branch of this river, and found that the basin or +plain of San Fernando is continuously and smoothly united with those +plains, which were described in the Second Chapter, as being worn near the +coast into successive cave-eaten escarpments, and still nearer to the +coast, as being strewed with upraised recent marine remains. + +I might have given descriptions of numerous other plains of the same +general form, some at the foot of the Cordillera, some near the coast, and +some halfway between these points. I will allude only to one other, namely, +the plain of Uspallata, lying on the eastern or opposite side of the +Cordillera, between that great range and the parallel lower range of +Uspallata. According to Miers, its surface is 6,000 feet above the level of +the sea: it is from ten to fifteen miles in width, and is said to extend +with an unbroken surface for 180 miles northwards: it is drained by two +rivers passing through breaches in the mountains to the east. On the banks +of the River Mendoza it is seen to be composed of a great accumulation of +stratified shingle, estimated at 400 feet in thickness. In general +appearance, and in numerous points of structure, this plain closely +resembles those of Chile. + +The origin and manner of formation of the thick beds of gravel, sandy clay, +volcanic detritus, and calcareous tuff, composing these basin-like plains, +is very important; because, as we shall presently show, they send arms or +fringes far up the main valleys of the Cordillera. Many of the inhabitants +believe that these plains were once occupied by lakes, suddenly drained; +but I conceive that the number of the separate breaches at nearly the same +level in the mountains surrounding them quite precludes this idea. Had not +such distinguished naturalists as MM. Meyen and Gay stated their belief +that these deposits were left by great debacles rushing down from the +Cordillera, I should not have noticed a view, which appears to me from many +reasons improbable in the highest degree--namely, from the vast +accumulation of WELL-ROUNDED PEBBLES--their frequent stratification with +layers of sand--the overlying beds of calcareous tuff--this same substance +coating and uniting the fragments of rock on the hummocks in the plain of +Santiago--and lastly even from the worn, rounded, and much denuded state of +these hummocks, and of the headlands which project from the surrounding +mountains. On the other hand, these several circumstances, as well as the +continuous union of the basins at the foot of the Cordillera, with the +great plain of the Rio Rapel which still retains the marks of sea-action at +various levels, and their general similarity in form and composition with +the many plains near the coast, which are either similarly marked or are +strewed with upraised marine remains, fully convince me that the mountains +bounding these basin-plains were breached, their islet-like projecting +rocks worn, and the loose stratified detritus forming their now level +surfaces deposited, by the sea, as the land slowly emerged. It is hardly +possible to state too strongly the perfect resemblance in outline between +these basin-like, long, and narrow plains of Chile (especially when in the +early morning the mists hanging low represented water), and the creeks and +fiords now intersecting the southern and western shores of the continent. +We can on this view of the sea, when the land stood lower, having long and +tranquilly occupied the spaces between the mountain-ranges, understand how +the boundaries of the separate basins were breached in more than one place; +for we see that this is the general character of the inland bays and +channels of Tierra del Fuego; we there, also, see in the sawing action of +the tides, which flow with great force in the cross channels, a power +sufficient to keep the breaches open as the land emerged. We can further +see that the waves would naturally leave the smooth bottom of each great +bay or channel, as it became slowly converted into land, gently inclined to +as many points as there were mouths, through which the sea finally +retreated, thus forming so many watersheds, without any marked ridges, on a +nearly level surface. The absence of marine remains in these high inland +plains cannot be properly adduced as an objection to their marine origin: +for we may conclude, from shells not being found in the great shingle beds +of Patagonia, though copiously strewed on their surfaces, and from many +other analogous facts, that such deposits are eminently unfavourable for +the embedment of such remains; and with respect to shells not being found +strewed on the surface of these basin-like plains, it was shown in the last +chapter that remains thus exposed in time decay and disappear. + +(FIGURE 13. SECTION OF THE PLAIN AT THE EASTERN FOOT OF THE CHILEAN +CORDILLERA. + +From Cordillera (left) through Talus-plain and Level surface, 2,700 feet +above sea, to Gravel terraces (right).) + +I observed some appearances on the plains at the eastern and opposite foot +of the Cordillera which are worth notice, as showing that the sea there +long acted at nearly the same level as on the basin-plains of Chile. The +mountains on this eastern side are exceedingly abrupt; they rise out of a +smooth, talus-like, very gentle, slope, from five to ten miles in width (as +represented in Figure 13), entirely composed of perfectly rounded pebbles, +often white-washed with an aluminous substance like decomposed feldspar. +This sloping plain or talus blends into a perfectly flat space a few miles +in width, composed of reddish impure clay, with small calcareous +concretions as in the Pampean deposit,--of fine white sand with small +pebbles in layers,--and of the above-mentioned white aluminous earth, all +interstratified together. This flat space runs as far as Mendoza, thirty +miles northward, and stands probably at about the same height, namely, +2,700 feet (Pentland and Miers) above the sea. To the east it is bounded by +an escarpment, eighty feet in height, running for many miles north and +south, and composed of perfectly round pebbles, and loose, white-washed, or +embedded in the aluminous earth: behind this escarpment there is a second +and similar one of gravel. Northward of Mendoza, these escarpments become +broken and quite obliterated; and it does not appear that they ever +enclosed a lake-like area: I conclude, therefore, that they were formed by +the sea, when it reached the foot of the Cordillera, like the similar +escarpments occurring at so many points on the coasts of Chile and +Patagonia. + +The talus-like plain slopes up with a smooth surface into the great dry +valleys of the Cordillera. On each hand of the Portillo valley, the +mountains are formed of red granite, mica-slate, and basalt, which all have +suffered a truly astonishing amount of denudation; the gravel in the +valley, as well as on the talus-like plain in front of it, is composed of +these rocks; but at the mouth of the valley, in the middle (height probably +about three thousand five hundred feet above the sea), a few small isolated +hillocks of several varieties of porphyry project, round which, on all +sides, smooth and often white-washed pebbles of these same porphyries, to +the exclusion of all others, extend to a circumscribed distance. Now, it is +difficult to conceive any other agency, except the quiet and long-continued +action of the sea on these hillocks, which could have rounded and +whitewashed the fragments of porphyry, and caused them to radiate from such +small and quite insignificant centres, in the midst of that vast stream of +stones which has descended from the main Cordillera. + +SLOPING TERRACES OF GRAVEL IN THE VALLEYS OF THE CORDILLERA. + +(FIGURE 14. GROUND-PLAN OF A BIFURCATING VALLEY IN THE CORDILLERA, bordered +by smooth, sloping gravel-fringes (AA), worn along the course of the river +into cliffs.) + +All the main valleys on both flanks of the Chilean Cordillera have formerly +had, or still have, their bottoms filled up to a considerable thickness by +a mass of rudely stratified shingle. In Central Chile the greater part of +this mass has been removed by the torrents; cliff-bounded fringes, more or +less continuous, being left at corresponding heights on both sides of the +valleys. These fringes, or as they may be called terraces, have a smooth +surface, and as the valleys rise, they gently rise with them: hence they +are easily irrigated, and afford great facilities for the construction of +the roads. From their uniformity, they give a remarkable character to the +scenery of these grand, wild, broken valleys. In width, the fringes vary +much, sometimes being only broad enough for the roads, and sometimes +expanding into narrow plains. Their surfaces, besides gently rising up the +valley, are slightly inclined towards its centre in such a manner as to +show that the whole bottom must once have been filled up with a smooth and +slightly concave mass, as still are the dry unfurrowed valleys of Northern +Chile. Where two valleys unite into one, these terraces are particularly +well exhibited, as is represented in Figure 14. The thickness of the gravel +forming these fringes, on a rude average, may be said to vary from thirty +to sixty or eighty feet; but near the mouths of the valleys it was in +several places from two to three hundred feet. The amount of matter removed +by the torrents has been immense; yet in the lower parts of the valleys the +terraces have seldom been entirely worn away on either side, nor has the +solid underlying rock been reached: higher up the valleys, the terraces +have frequently been removed on one or the other side, and sometimes on +both sides; but in this latter case they reappear after a short interval on +the line, which they would have held had they been unbroken. Where the +solid rock has been reached, it has been cut into deep and narrow gorges. +Still higher up the valleys, the terraces gradually become more and more +broken, narrower, and less thick, until, at a height of from seven to nine +thousand feet, they become lost, and blended with the piles of fallen +detritus. + +I carefully examined in many places the state of the gravel, and almost +everywhere found the pebbles equally and perfectly rounded, occasionally +with great blocks of rock, and generally distinctly stratified, often with +parting seams of sand. The pebbles were sometimes coated with a white +aluminous, and less frequently with a calcareous, crust. At great heights +up the valleys the pebbles become less rounded; and as the terraces become +obliterated, the whole mass passes into the nature of ordinary detritus. I +was repeatedly struck with the great difference between this detritus high +up the valleys, and the gravel of the terraces low down, namely, in the +greater number of the quite angular fragments in the detritus,--in the +unequal degree to which the other fragments have been rounded,--in the +quantity of associated earth,--in the absence of stratification,--and in +the irregularity of the upper surfaces. This difference was likewise well +shown at points low down the valleys, where precipitous ravines, cutting +through mountains of highly coloured rock, have thrown down wide, fan- +shaped accumulations of detritus on the terraces: in such cases, the line +of separation between the detritus and the terrace could be pointed out to +within an inch or two; the detritus consisting entirely of angular and only +partially rounded fragments of the adjoining coloured rocks; the stratified +shingle (as I ascertained by close inspection, especially in one case, in +the valley of the River Mendoza) containing only a small proportion of +these fragments, and those few well rounded. + +I particularly attended to the appearance of the terraces where the valleys +made abrupt and considerable bends, but I could perceive no difference in +their structure: they followed the bends with their usual nearly equable +inclination. I observed, also, in several valleys, that wherever large +blocks of any rock became numerous, either on the surface of the terrace or +embedded in it, this rock soon appeared higher up in situ: thus I have +noticed blocks of porphyry, of andesitic syenite, of porphyry and of +syenite, alternately becoming numerous, and in each case succeeded by +mountains thus constituted. There is, however, one remarkable exception to +this rule; for along the valley of the Cachapual, M. Gay found numerous +large blocks of white granite, which does not occur in the neighbourhood. I +observed these blocks, as well as others of andesitic syenite (not +occurring here in situ), near the baths of Cauquenes at a height of between +two and three hundred feet above the river, and therefore quite above the +terrace or fringe which borders that river; some miles up the valleys there +were other blocks at about the same height. I also noticed, at a less +height, just above the terrace, blocks of porphyries (apparently not found +in the immediately impending mountains), arranged in rude lines, as on a +sea-beach. All these blocks were rounded, and though large, not gigantic, +like the true erratic boulders of Patagonia and Fuegia. M. Gay states that +the granite does not occur in situ within a distance of twenty leagues +("Annales des Science Nat. " 1 series tome 28. M. Gay, as I was informed, +penetrated the Cordillera by the great oblique valley of Los Cupressos, and +not by the most direct line.); I suspect, for several reasons, that it will +ultimately be found at a much less distance, though certainly not in the +immediate neighbourhood. The boulders found by MM. Meyen and Gay on the +upper plain of San Fernando (mentioned in a previous note) probably belong +to this same class of phenomena. + +These fringes of stratified gravel occur along all the great valleys of the +Cordillera, as well as along their main branches; they are strikingly +developed in the valleys of the Maypu, Mendoza, Aconcagua, Cachapual, and +according to Meyen, in the Tinguirica. ("Reise" etc. Th. 1 s. 302.) In the +valleys, however, of Northern Chile, and in some on the eastern flank of +the Cordillera, as in the Portillo Valley, where streams have never flowed, +or are quite insignificant in volume, the presence of a mass of stratified +gravel can be inferred only from the smooth slightly concave form of the +bottom. One naturally seeks for some explanation of so general and striking +a phenomenon; that the matter forming the fringes along the valleys, or +still filling up their entire beds, has not fallen from the adjoining +mountains like common detritus, is evident from the complete contrast in +every respect between the gravel and the piles of detritus, whether seen +high up the valleys on their sides, or low down in front of the more +precipitous ravines; that the matter has not been deposited by debacles, +even if we could believe in debacles having rushed down EVERY valley, and +all their branches, eastward and westward from the central pinnacles of the +Cordillera, we must admit from the following reasons,--from the distinct +stratification of the mass,--its smooth upper surface,--the well-rounded +and sometimes encrusted state of the pebbles, so different from the loose +debris on the mountains,--and especially from the terraces preserving their +uniform inclination round the most abrupt bends. To suppose that as the +land now stands, the rivers deposited the shingle along the course of every +valley, and all their main branches, appears to me preposterous, seeing +that these same rivers not only are now removing and have removed much of +this deposit, but are everywhere tending to cut deep and narrow gorges in +the hard underlying rocks. + +I have stated that these fringes of gravel, the origin of which are +inexplicable on the notion of debacles or of ordinary alluvial action, are +directly continuous with the similarly-composed basin-like plains at the +foot of the Cordillera, which, from the several reasons before assigned, I +cannot doubt were modelled by the agency of the sea. Now if we suppose that +the sea formerly occupied the valleys of the Chilean Cordillera, in +precisely the same manner as it now does in the more southern parts of the +continent, where deep winding creeks penetrate into the very heart of, and +in the case of Obstruction Sound quite through, this great range; and if we +suppose that the mountains were upraised in the same slow manner as the +eastern and western coasts have been upraised within the recent period, +then the origin and formation of these sloping, terrace-like fringes of +gravel can be simply explained. For every part of the bottom of each valley +will, on this view, have long stood at the head of a sea creek, into which +the then existing torrents will have delivered fragments of rocks, where, +by the action of the tides, they will have been rolled, sometimes +encrusted, rudely stratified, and the whole surface levelled by the +blending together of the successive beach lines. (Sloping terraces of +precisely similar structure have been described by me "Philosophical +Transactions" 1839 page 58, in the valleys of Lochaber in Scotland, where, +at higher levels, the parallel roads of Glen Roy show the marks of the long +and quiet residence of the sea. I have no doubt that these sloping terraces +would have been present in the valleys of most of the European ranges, had +not every trace of them, and all wrecks of sea-action, been swept away by +the glaciers which have since occupied them. I have shown that this is the +case with the mountains ("London and Edinburgh Philosophical Journal" +volume 21 page 187) of North Wales.) As the land rose, the torrents in +every valley will have tended to have removed the matter which just before +had been arrested on, or near, the beach-lines; the torrents, also, having +continued to gain in force by the continued elevation increasing their +total descent from their sources to the sea. This slow rising of the +Cordillera, which explains so well the otherwise inexplicable origin and +structure of the terraces, judging from all known analogies, will probably +have been interrupted by many periods of rest; but we ought not to expect +to find any evidence of these periods in the structure of the gravel- +terraces: for, as the waves at the heads of deep creeks have little erosive +power, so the only effect of the sea having long remained at the same level +will be that the upper parts of the creeks will have become filled up at +such periods to the level of the water with gravel and sand; and that +afterwards the rivers will have thrown down on the filled-up parts a talus +of similar matter, of which the inclination (as at the head of a partially +filled-up lake) will have been determined by the supply of detritus, and +the force of the stream. (I have attempted to explain this process in a +more detailed manner, in a letter to Mr. Maclaren, published in the +"Edinburgh New Philosophical Journal" volume 35 page 288.) Hence, after the +final conversion of the creeks into valleys, almost the only difference in +the terraces at those points at which the sea stood long, will be a +somewhat more gentle inclination, with river-worn instead of sea-worn +detritus on the surface. + +I know of only one difficulty on the foregoing view, namely, the far- +transported blocks of rock high on the sides of the valley of the +Cachapual: I will not attempt any explanation of this phenomenon, but I may +state my belief that a mountain-ridge near the Baths of Cauquenes has been +upraised long subsequently to all the other ranges in the neighbourhood, +and that when this was effected the whole face of the country must have +been greatly altered. In the course of ages, moreover, in this and other +valleys, events may have occurred like, but even on a grander scale than, +that described by Molina, when a slip during the earthquake of 1762 banked +up for ten days the great River Lontue, which then bursting its barrier +"inundated the whole country," and doubtless transported many great +fragments of rock. ("Compendio de la Hist." etc. etc. tome 1 page 30. M. +Brongniart, in his report on M. Gay's labours "Annales des Sciences" 1833, +considers that the boulders in the Cachapual belong to the same class with +the erratic boulders of Europe. As the blocks which I saw are not gigantic, +and especially as they are not angular, and as they have not been +transported fairly across low spaces or wide valleys, I am unwilling to +class them with those which, both in the northern and southern hemisphere +"Geological Transactions" volume 6 page 415, have been transported by ice. +It is to be hoped that when M. Gay's long-continued and admirable labours +in Chile are published, more light will be thrown on this subject. However, +the boulders may have been primarily transported; the final position of +those of porphyry, which have been described as arranged at the foot of the +mountain in rude lines, I cannot doubt, has been due to the action of waves +on a beach. The valley of the Cachapual, in the part where the boulders +occur, bursts through the high ridge of Cauquenes, which runs parallel to, +but at some distance from, the Cordillera. This ridge has been subjected to +excessive violence; trachytic lava has burst from it, and hot springs yet +flow at its base. Seeing the enormous amount of denudation of solid rock in +the upper and much broader parts of this valley where it enters the +Cordillera, and seeing to what extent the ridge of Cauquenes now protects +the great range, I could not help believing (as alluded to in the text) +that this ridge with its trachytic eruptions had been thrown up at a much +later period than the Cordillera. If this has been the case, the boulders, +after having been transported to a low level by the torrents (which exhibit +in every valley proofs of their power of moving great fragments), may have +been raised up to their present height, with the land on which they +rested.) Finally, notwithstanding this one case of difficulty, I cannot +entertain any doubt, that these terrace-like fringes, which are +continuously united with the basin-shaped plains at the foot of the +Cordillera, have been formed by the arrestment of river-borne detritus at +successive levels, in the same manner as we see now taking place at the +heads of all those many, deep, winding fiords intersecting the southern +coasts. To my mind, this has been one of the most important conclusions to +which my observations on the geology of South America have led me; for we +thus learn that one of the grandest and most symmetrical mountain-chains in +the world, with its several parallel lines, has been together uplifted in +mass between seven and nine thousand feet, in the same gradual manner as +have the eastern and western coasts within the recent period. (I do not +wish to affirm that all the lines have been uplifted quite equally; slight +differences in the elevation would leave no perceptible effect on the +terraces. It may, however, be inferred, perhaps with one exception, that +since the period when the sea occupied these valleys, the several ranges +have not been dislocated by GREAT and ABRUPT faults or upheavals; for if +such had occurred, the terraces of gravel at these points would not have +been continuous. The one exception is at the lower end of a plain in the +Valle del Yeso (a branch of the Maypu), where, at a great height, the +terraces and valley appear to have been broken through by a line of +upheaval, of which the evidence is plain in the adjoining mountains; this +dislocation, perhaps, occurred AFTER THE ELEVATION of this part of the +valley above the level of the sea. The valley here is almost blocked up by +a pile about one thousand feet in thickness, formed, as far as I could +judge, from three sides, entirely, or at least in chief part, of gravel and +detritus. On the south side, the river has cut quite through this mass; on +the northern side, and on the very summit, deep ravines, parallel to the +line of the valley, are worn, as if the drainage from the valley above had +passed by these two lines before following its present course.) + +FORMATION OF VALLEYS. + +The bulk of solid rock which has been removed in the lower parts of the +valleys of the Cordillera has been enormous. It is only by reflecting on +such cases as that of the gravel beds of Patagonia, covering so many +thousand square leagues of surface, and which, if heaped into a ridge, +would form a mountain-range almost equal to the Cordillera, that the amount +of denudation becomes credible. The valleys within this range often follow +anticlinal but rarely synclinal lines; that is, the strata on the two sides +more often dip from the line of valley than towards it. On the flanks of +the range, the valleys most frequently run neither along anticlinal nor +synclinal axes, but along lines of flexure or faults: that is, the strata +on both sides dip in the same direction, but with different, though often +only slightly different, inclinations. As most of the nearly parallel +ridges which together form the Cordillera run approximately north and +south, the east and west valleys cross them in zig-zag lines, bursting +through the points where the strata have been least inclined. No doubt the +greater part of the denudation was affected at the periods when tidal- +creeks occupied the valleys, and when the outer flanks of the mountains +were exposed to the full force of an open ocean. I have already alluded to +the power of the tidal action in the channels connecting great bays; and I +may here mention that one of the surveying vessels in a channel of this +kind, though under sail, was whirled round and round by the force of the +current. We shall hereafter see, that of the two main ridges forming the +Chilean Cordillera, the eastern and loftiest one owes the greater part of +its ANGULAR upheaval to a period subsequent to the elevation of the western +ridge; and it is likewise probable that many of the other parallel ridges +have been angularly upheaved at different periods; consequently many parts +of the surfaces of these mountains must formerly have been exposed to the +full force of the waves, which, if the Cordillera were now sunk into the +sea, would be protected by parallel chains of islands. The torrents in the +valleys certainly have great power in wearing the rocks; as could be told +by the dull rattling sound of the many fragments night and day hurrying +downwards; and as was attested by the vast size of certain fragments, which +I was assured had been carried onwards during floods; yet we have seen in +the lower parts of the valleys, that the torrents have seldom removed all +the sea-checked shingle forming the terraces, and have had time since the +last elevation in mass only to cut in the underlying rocks, gorges, deep +and narrow, but quite insignificant in dimensions compared with the entire +width and depth of the valleys. + +Along the shores of the Pacific, I never ceased during my many and long +excursions to feel astonished at seeing every valley, ravine, and even +little inequality of surface, both in the hard granitic and soft tertiary +districts, retaining the exact outline, which they had when the sea left +their surfaces coated with organic remains. When these remains shall have +decayed, there will be scarcely any difference in appearance between this +line of coast-land and most other countries, which we are accustomed to +believe have assumed their present features chiefly through the agency of +the weather and fresh-water streams. In the old granitic districts, no +doubt it would be rash to attribute all the modifications of outline +exclusively to the sea-action; for who can say how often this lately +submerged coast may not previously have existed as land, worn by running +streams and washed by rain? This source of doubt, however, does not apply +to the districts superficially formed of the modern tertiary deposits. The +valleys worn by the sea, through the softer formations, both on the +Atlantic and Pacific sides of the continent, are generally broad, winding, +and flat-bottomed: the only district of this nature now penetrated by arms +of the sea, is the island of Chiloe. + +Finally, the conclusion at which I have arrived, with respect to the +relative powers of rain and sea water on the land, is, that the latter is +far the most efficient agent, and that its chief tendency is to widen the +valleys; whilst torrents and rivers tend to deepen them, and to remove the +wreck of the sea's destroying action. As the waves have more power, the +more open and exposed the space may be, so will they always tend to widen +more and more the mouths of valleys compared with their upper parts: hence, +doubtless, it is, that most valleys expand at their mouths,--that part, at +which the rivers flowing in them, generally have the least wearing power. + +When reflecting on the action of the sea on the land at former levels, the +effect of the great waves, which generally accompany earthquakes, must not +be overlooked: few years pass without a severe earthquake occurring on some +part of the west coast of South America; and the waves thus caused have +great power. At Concepcion, after the shock of 1835, I saw large slabs of +sandstone, one of which was six feet long, three in breadth, and two in +thickness, thrown high up on the beach; and from the nature of the marine +animals still adhering to it, it must have been torn up from a considerable +depth. On the other hand, at Callao, the recoil-wave of the earthquake of +1746 carried great masses of brickwork, between three and four feet square, +some way out seaward. During the course of ages, the effect thus produced +at each successive level, cannot have been small; and in some of the +tertiary deposits on this line of coast, I observed great boulders of +granite and other neighbouring rocks, embedded in fine sedimentary layers, +the transportal of which, except by the means of earthquake-waves, always +appeared to me inexplicable. + +SUPERFICIAL SALINE DEPOSITS. + +This subject may be here conveniently treated of: I will begin with the +most interesting case, namely, the superficial saline beds near Iquique in +Peru. The porphyritic mountains on the coast rise abruptly to a height of +between one thousand nine hundred and three thousand feet: between their +summits and an inland plain, on which the celebrated deposit of nitrate of +soda lies, there is a high undulatory district, covered by a remarkable +superficial saliferous crust, chiefly composed of common salt, either in +white, hard, opaque nodules, or mingled with sand, in this latter case +forming a compact sandstone. This saliferous superficial crust extends from +the edge of the coast-escarpment, over the whole face of the country; but +never attains, as I am assured by Mr. Bollaert (long resident here) any +great thickness. Although a very slight shower falls only at intervals of +many years, yet small funnel-shaped cavities show that the salt has been in +some parts dissolved. (It is singular how slowly, according to the +observations of M. Cordier on the salt-mountain of Cardona in Spain "Ann. +des Mines, Translation of Geolog. Mem." by De la Beche page 60, salt is +dissolved, where the amount of rain is supposed to be as much as 31.4 of an +inch in the year. It is calculated that only five feet in thickness is +dissolved in the course of a century.) In several places I saw large +patches of sand, quite moist, owing to the quantity of muriate of lime (as +ascertained by Mr. T. Reeks) contained in them. From the compact salt- +cemented sand being either red, purplish, or yellow, according to the +colour of the rocky strata on which it rested, I imagined that this +substance had probably been derived through common alluvial action from the +layers of salt which occur interstratified in the surrounding mountains +("Journal of Researches" page 444 first edition.): but from the interesting +details given by M. d'Orbigny, and from finding on a fresh examination of +this agglomerated sand, that it is not irregularly cemented, but consists +of thin layers of sand of different tints of colour, alternating with +excessively fine parallel layers of salt, I conclude that it is not of +alluvial origin. M. d'Orbigny observed analogous saline beds extending from +Cobija for five degrees of latitude northward, and at heights varying from +six hundred to nine hundred feet ("Voyage" etc. page 102. M. d'Orbigny +found this deposit intersected, in many places, by deep ravines, in which +there was no salt. Streams must once, though historically unknown, have +flowed in them; and M. d'Orbigny argues from the presence of undissolved +salt over the whole surrounding country, that the streams must have arisen +from rain or snow having fallen, not in the adjoining country, but on the +now arid Cordillera. I may remark, that from having observed ruins of +Indian buildings in absolutely sterile parts of the Chilian Cordillera +("Journal" 2nd edition page 357), I am led to believe that the climate, at +a time when Indian man inhabited this part of the continent, was in some +slight degree more humid than it is at present.): from finding recent sea- +shells strewed on these saliferous beds, and under them, great well-rounded +blocks, exactly like those on the existing beach, he believes that the +salt, which is invariably superficial, has been left by the evaporation of +the sea-water. This same conclusion must, I now believe, be extended to the +superficial saliferous beds of Iquique, though they stand about three +thousand feet above the level of the sea. + +Associated with the salt in the superficial beds, there are numerous, thin, +horizontal layers of impure, dirty-white, friable, gypseous and calcareous +tuffs. The gypseous beds are very remarkable, from abounding with, so as +sometimes to be almost composed of, irregular concretions, from the size of +an egg to that of a man's head, of very hard, compact, heavy gypsum, in the +form of anhydrite. This gypsum contains some foreign particles of stone; it +is stained, judging from its action with borax, with iron, and it exhales a +strong aluminous odour. The surfaces of the concretions are marked by +sharp, radiating, or bifurcating ridges, as if they had been (but not +really) corroded: internally they are penetrated by branching veins (like +those of calcareous spar in the septaria of the London clay) of pure white +anhydrite. These veins might naturally have been thought to have been +formed by subsequent infiltration, had not each little embedded fragment of +rock been likewise edged in a very remarkable manner by a narrow border of +the same white anhydrite: this shows that the veins must have been formed +by a process of segregation, and not of infiltration. Some of the little +included and CRACKED fragments of foreign rock are penetrated by the +anhydrite, and portions have evidently been thus mechanically displaced: at +St. Helena, I observed that calcareous matter, deposited by rain water, +also had the power to separate small fragments of rock from the larger +masses. ("Volcanic Islands" etc. page 87.) I believe the superficial +gypseous deposit is widely extended: I received specimens of it from +Pisagua, forty miles north of Iquique, and likewise from Arica, where it +coats a layer of pure salt. M. d'Orbigny found at Cobija a bed of clay, +lying above a mass of upraised recent shells, which was saturated with +sulphate of soda, and included thin layers of fibrous gypsum. ("Voyage +Geolog." etc. page 95.) These widely extended, superficial, beds of salt +and gypsum, appear to me an interesting geological phenomenon, which could +be presented only under a very dry climate. + +The plain or basin, on the borders of which the famous bed of nitrate of +soda lies, is situated at the distance of about thirty miles from the sea, +being separated from it by the saliferous district just described. It +stands at a height of 3,300 feet; its surface is level, and some leagues in +width; it extends forty miles northward, and has a total length (as I was +informed by Mr. Belford Wilson, the Consul-General at Lima) of 420 miles. +In a well near the works, thirty-six yards in depth, sand, earth, and a +little gravel were found: in another well, near Almonte, fifty yards deep, +the whole consisted, according to Mr. Blake, of clay, including a layer of +sand two feet thick, which rested on fine gravel, and this on coarse +gravel, with large rounded fragments of rock. (See an admirable paper +"Geological and Miscellaneous Notices of Tarapaca" in "Silliman's American +Journal" volume 44 page 1.) In many parts of this now utterly desert plain, +rushes and large prostrate trees in a hardened state, apparently Mimosas, +are found buried, at a depth from three to six feet; according to Mr. +Blake, they have all fallen to the south-west. The bed of nitrate of soda +is said to extend for forty to fifty leagues along the western margin of +the plain, but is not found in its central parts: it is from two to three +feet in thickness, and is so hard that it is generally blasted with +gunpowder; it slopes gently upwards from the edge of the plain to between +ten and thirty feet above its level. It rests on sand in which, it is said, +vegetable remains and broken shells have been found; shells have also been +found, according to Mr. Blake, both on and in the nitrate of soda. It is +covered by a superficial mass of sand, containing nodules of common salt, +and, as I was assured by a miner, much soft gypseous matter, precisely like +that in the superficial crust already described: certainly this crust, with +its characteristic concretions of anhydrite, comes close down to the edge +of the plain. + +The nitrate of soda varies in purity in different parts, and often contains +nodules of common salt. According to Mr. Blake, the proportion of nitrate +of soda varies from 20 to 75 per cent. An analysis by Mr. A. Hayes, of an +average specimen, gave:-- + +Nitrate of Soda.... 64.98 +Sulphate of Soda.... 3.00 +Chloride of Soda... 28.69 +Iodic Salts......... 0.63 +Shells and Marl..... 2.60 + 99.90 + +The "mother-water" at some of the refineries is very rich in iodic salts, +and is supposed to contain much muriate of lime. ("Literary Gazette" 1841 +page 475.) In an unrefined specimen brought home by myself, Mr. T. Reeks +has ascertained that the muriate of lime is very abundant. With respect to +the origin of this saline mass, from the manner in which the gently +inclined, compact bed follows for so many miles the sinuous margin of the +plain, there can be no doubt that it was deposited from a sheet of water: +from the fragments of embedded shells, from the abundant iodic salts, from +the superficial saliferous crust occurring at a higher level and being +probably of marine origin, and from the plain resembling in form those of +Chile and that of Uspallata, there can be little doubt that this sheet of +water was, at least originally, connected with the sea. (From an official +document, shown me by Mr. Belford Wilson, it appears that the first export +of nitrate of soda to Europe was in July 1830, on French account, in a +British ship:-- + +In year, the entire export was in Quintals. + 1830............................ 17,300 + 1831............................ 40,885 + 1832............................ 51,400 + 1833............................ 91,335 + 1834........................... 149,538 +The Spanish quintal nearly equals 100 English pounds.) + +THIN, SUPERFICIAL, SALINE INCRUSTATIONS. + +These saline incrustations are common in many parts of America: Humboldt +met with them on the tableland of Mexico, and the Jesuit Falkner and other +authors state that they occur at intervals over the vast plains extending +from the mouth of the Plata to Rioja and Catamarca. (Azara "Travels" volume +1 page 55, considers that the Parana is the eastern boundary of the +saliferous region; but I heard of "salitrales" in the Province of Entre +Rios.) Hence it is that during droughts, most of the streams in the Pampas +are saline. I nowhere met with these incrustations so abundantly as near +Bahia Blanca: square miles of the mud-flats, which near that place are +raised only a few feet above the sea, just enough to protect them from +being overflowed, appear, after dry weather, whiter than the ground after +the thickest hoar-frost. After rain the salts disappear, and every puddle +of water becomes highly saline; as the surface dries, the capillary action +draws the moisture up pieces of broken earth, dead sticks, and tufts of +grass, where the salt effloresces. The incrustation, where thickest, does +not exceed a quarter of an inch. M. Parchappe has analysed it (M. d'Orbigny +"Voyage" etc. Part. Hist. tome 1 page 664.); and finds that the specimens +collected at the extreme head of the low plain, near the River Manuello, +consist of 93 per cent of sulphate of soda, and 7 of common salt; whilst +the specimens taken close to the coast contain only 63 per cent of the +sulphate, and 37 of the muriate of soda. This remarkable fact, together +with our knowledge that the whole of this low muddy plain has been covered +by the sea within the recent period, must lead to the suspicion that the +common salt, by some unknown process, becomes in time changed into the +sulphate. Friable, calcareous matter is here abundant, and the case of the +apparent double decomposition of the shells and salt on San Lorenzo, should +not be forgotten. + +The saline incrustations, near Bahia Blanca, are not confined to, though +most abundant on, the low muddy flats; for I noticed some on a calcareous +plain between thirty and forty feet above the sea, and even a little occurs +in still higher valleys. Low alluvial tracts in the valleys of the Rivers +Negro and Colorado are also encrusted, and in the latter valley such spaces +appeared to be occasionally overflowed by the river. I observed saline +incrustations in some of the valleys of Southern Patagonia. At Port Desire +a low, flat, muddy valley was thickly incrusted by salts, which on analysis +by Mr. T. Reeks, are found to consist of a mixture of sulphate and muriate +of soda, with carbonate of lime and earthy matter. On the western side of +the continent, the southern coasts are much too humid for this phenomenon; +but in Northern Chile I again met with similar incrustations. On the +hardened mud, in parts of the broad, flat-bottomed valley of Copiapo, the +saline matter encrusts the ground to the thickness of some inches: +specimens, sent by Mr. Bingley to Apothecaries' Hall for analysis, were +said to consist of carbonate and sulphate of soda. Much sulphate of soda is +found in the desert of Atacama. In all parts of South America, the saline +incrustations occur most frequently on low damp surfaces of mud, where the +climate is rather dry; and these low surfaces have, in almost every case, +been upraised above the level of the sea, within the recent period. + +SALT-LAKES OF PATAGONIA AND LA PLATA. + +Salinas, or natural salt-lakes, occur in various formations on the eastern +side of the continent,--in the argillaceo-calcareous deposit of the Pampas, +in the sandstone of the Rio Negro, where they are very numerous, in the +pumiceous and other beds of the Patagonian tertiary formation, and in small +primary districts in the midst of this latter formation. Port S. Julian is +the most southerly point (latitude 49 degrees to 50 degrees) at which +salinas are known to occur. (According to Azara "Travels" volume 1 page 56, +there are salt-lakes as far north as Chaco (latitude 25 degrees), on the +banks of the Vermejo. The salt-lakes of Siberia appear (Pallas "Travels" +English Translation volume 1 page 284) to occur in very similar depressions +to those of Patagonia.) The depressions, in which these salt-lakes lie, are +from a few feet to sixty metres, as asserted by M. d'Orbigny, below the +surface of the surrounding plains ("Voyage Geolog." page 63.); and, +according to this same author, near the Rio Negro they all trend, either in +the N.E. and S.W. or in E. and W. lines, coincident with the general slope +of the plain. These depressions in the plain generally have one side lower +than the others, but there are no outlets for drainage. Under a less dry +climate, an outlet would soon have been formed, and the salt washed away. +The salinas occur at different elevations above the sea; they are often +several leagues in diameter; they are generally very shallow, but there is +a deep one in a quartz-rock formation near C. Blanco. In the wet season, +the whole, or a part, of the salt is dissolved, being redeposited during +the succeeding dry season. At this period the appearance of the snow-white +expanse of salt crystallised in great cubes, is very striking. In a large +salina, northward of the Rio Negro, the salt at the bottom, during the +whole year, is between two and three feet in thickness. + +The salt rests almost always on a thick bed of black muddy sand, which is +fetid, probably from the decay of the burrowing worms inhabiting it. +(Professor Ehrenberg examined some of this muddy sand, but was unable to +find in it any infusoria.) In a salina, situated about fifteen miles above +the town of El Carmen on the Rio Negro, and three or four miles from the +banks of that river, I observed that this black mud rested on gravel with a +calcareous matrix, similar to that spread over the whole surrounding +plains: at Port S. Julian the mud, also, rested on the gravel: hence the +depressions must have been formed anteriorly to, or contemporaneously with, +the spreading out of the gravel. I was informed that one small salina +occurs in an alluvial plain within the valley of the Rio Negro, and +therefore its origin must be subsequent to the excavation of that valley. +When I visited the salina, fifteen miles above the town, the salt was +beginning to crystallise, and on the muddy bottom there were lying many +crystals, generally placed crossways of sulphate of soda (as ascertained by +Mr. Reeks), and embedded in the mud, numerous crystals of sulphate of lime, +from one to three inches in length: M. d'Orbigny states that some of these +crystals are acicular and more than even nine inches in length ("Voyage +Geolog." page 64.); others are macled and of great purity: those I found +all contained some sand in their centres. As the black and fetid sand +overlies the gravel, and that overlies the regular tertiary strata, I think +there can be no doubt that these remarkable crystals of sulphate of lime +have been deposited from the waters of the lake. The inhabitants call the +crystals of selenite, the padre del sal, and those of the sulphate of soda, +the madre del sal; they assured me that both are found under the same +circumstances in several of the neighbouring salinas; and that the sulphate +of soda is annually dissolved, and is always crystallised before the common +salt on the muddy bottom. (This is what might have been expected; for M. +Ballard asserts "Acad. des Sciences" October 7, 1844, that sulphate of soda +is precipitated from solution more readily from water containing muriate of +soda in excess, than from pure water.) The association of gypsum and salt +in this case, as well as in the superficial deposits of Iquique, appears to +me interesting, considering how generally these substances are associated +in the older stratified formations. + +Mr. Reeks has analysed for me some of the salt from the salina near the Rio +Negro; he finds it composed entirely of chloride of sodium, with the +exception of 0.26 of sulphate of lime and of 0.22 of earthy matter: there +are no traces of iodic salts. Some salt from the salina Chiquitos, in the +Pampean formation, is equally pure. It is a singular fact, that the salt +from these salinas does not serve so well for preserving meat, as sea-salt +from the Cape de Verde Islands; and a merchant at Buenos Ayres told me that +he considered it as 50 per cent less valuable. The purity of the Patagonian +salt, or absence from it of those other saline bodies found in all sea- +water, is the only assignable cause for this inferiority; a conclusion +which is supported by the fact lately ascertained, that those salts answer +best for preserving cheese which contain most of the deliquescent +chlorides. ("Horticultural and Agricultural Gazette" 1845 page 93.) (It +would probably well answer for the merchants of Buenos Ayres (considering +the great consumption there of salt for preserving meat) to import the +deliquescent chlorides to mix with the salt from the salinas: I may call +attention to the fact, that at Iquique, a large quantity of muriate of +lime, left in the MOTHER-WATER during the refinement of the nitrate of +soda, is annually thrown away.) + +With respect to the origin of the salt in the salinas, the foregoing +analysis seems opposed to the view entertained by M. d'Orbigny and others, +and which seems so probable considering the recent elevation of this line +of coast, namely, that it is due to the evaporation of sea-water and to the +drainage from the surrounding strata impregnated with sea-salt. I was +informed (I know not whether accurately) that on the northern side of the +salina on the Rio Negro, there is a small brine spring which flows at all +times of the year: if this be so, the salt in this case at least, probably +is of subterranean origin. It at first appears very singular that fresh +water can often be procured in wells, and is sometimes found in small +lakes, quite close to these salinas. (Sir W. Parish states "Buenos Ayres" +etc. pages 122 and 170, that this is the case near the great salinas +westward of the S. Ventana. I have seen similar statements in an ancient +MS. Journal lately published by S. Angelis. At Iquique, where the surface +is so thickly encrusted with saline matter, I tasted water only slightly +brackish, procured in a well thirty-six yards deep; but here one feels less +surprise at its presence, as pure water might percolate under ground from +the not very distant Cordillera.) I am not aware that this fact bears +particularly on the origin of the salt; but perhaps it is rather opposed to +the view of the salt having been washed out of the surrounding superficial +strata, but not to its having been the residue of sea-water, left in +depressions as the land was slowly elevated. + + +CHAPTER IV. ON THE FORMATIONS OF THE PAMPAS. + +Mineralogical constitution. +Microscopical structure. +Buenos Ayres, shells embedded in tosca-rock. +Buenos Ayres to the Colorado. +San Ventana. +Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta, shells, bones, +and infusoria of; co-existence of the recent shells and extinct mammifers. +Buenos Ayres to Santa Fe. +Skeletons of Mastodon. +Infusoria. +Inferior marine tertiary strata, their age. +Horse's tooth. +BANDA ORIENTAL. +Superficial Pampean formation. +Inferior tertiary strata, variation of, connected with volcanic action; +Macrauchenia Patachonica at San Julian in Patagonia, age of, subsequent to +living mollusca and to the erratic block period. +SUMMARY. +Area of Pampean formation. +Theories of origin. +Source of sediment. +Estuary origin. +Contemporaneous with existing mollusca. +Relations to underlying tertiary strata. +Ancient deposit of estuary origin. +Elevation and successive deposition of the Pampean formation. +Number and state of the remains of mammifers; their habitation, food, +extinction, and range. +Conclusion. +Localities in Pampas at which mammiferous remains have been found. + +The Pampean formation is highly interesting from its vast extent, its +disputed origin, and from the number of extinct gigantic mammifers embedded +in it. It has upon the whole a very uniform character: consisting of a more +or less dull reddish, slightly indurated, argillaceous earth or mud, often, +but not always, including in horizontal lines concretions of marl, and +frequently passing into a compact marly rock. The mud, wherever I examined +it, even close to the concretions, did not contain any carbonate of lime. +The concretions are generally nodular, sometimes rough externally, +sometimes stalactiformed; they are of a compact structure, but often +penetrated (as well as the mud) by hair-like serpentine cavities, and +occasionally with irregular fissures in their centres, lined with minute +crystals of carbonate of lime; they are of white, brown, or pale pinkish +tints, often marked by black dendritic manganese or iron; they are either +darker or lighter tinted than the surrounding mass; they contain much +carbonate of lime, but exhale a strong aluminous odour, and leave, when +dissolved in acids, a large but varying residue, of which the greater part +consists of sand. These concretions often unite into irregular strata; and +over very large tracts of country, the entire mass consists of a hard, but +generally cavernous marly rock: some of the varieties might be called +calcareous tuffs. + +Dr. Carpenter has kindly examined under the microscope, sliced and polished +specimens of these concretions, and of the solid marl-rock, collected in +various places between the Colorado and Santa Fe Bajada. In the greater +number, Dr. Carpenter finds that the whole substance presents a tolerably +uniform amorphous character, but with traces of incipient crystalline +metamorphosis; in other specimens he finds microscopically minute rounded +concretions of an amorphous substance (resembling in size those in oolitic +rocks, but not having a concentric structure), united by a cement which is +often crystalline. In some, Dr. Carpenter can perceive distinct traces of +shells, corals, Polythalamia, and rarely of spongoid bodies. For the sake +of comparison, I sent Dr. Carpenter specimens of the calcareous rock, +formed chiefly of fragments of recent shells, from Coquimbo in Chile: in +one of these specimens, Dr. Carpenter finds, besides the larger fragments, +microscopical particles of shells, and a varying quantity of opaque +amorphous matter; in another specimen from the same bed, he finds the whole +composed of the amorphous matter, with layers showing indications of an +incipient crystalline metamorphosis: hence these latter specimens, both in +external appearance and in microscopical structure, closely resemble those +of the Pampas. Dr. Carpenter informs me that it is well known that chemical +precipitation throws down carbonate of lime in the opaque amorphous state; +and he is inclined to believe that the long-continued attrition of a +calcareous body in a state of crystalline or semi-crystalline aggregation +(as, for instance, in the ordinary shells of Mollusca, which, when sliced, +are transparent) may yield the same result. From the intimate relations +between all the Coquimbo specimens, I can hardly doubt that the amorphous +carbonate of lime in them has resulted from the attrition and decay of the +larger fragments of shell: whether the amorphous matter in the marly rocks +of the Pampas has likewise thus originated, it would be hazardous to +conjecture. + +For convenience' sake, I will call the marly rock by the name given to it +by the inhabitants, namely, Tosca-rock; and the reddish argillaceous earth, +Pampean mud. This latter substance, I may mention, has been examined for me +by Professor Ehrenberg, and the result of his examination will be given +under the proper localities. + +I will commence my descriptions at a central spot, namely, at Buenos Ayres, +and thence proceed first southward to the extreme limit of the deposit, and +afterwards northward. The plain on which Buenos Ayres stands is from thirty +to forty feet in height. The Pampean mud is here of a rather pale colour, +and includes small nearly white nodules, and other irregular strata of an +unusually arenaceous variety of tosca-rock. In a well at the depth of +seventy feet, according to Ignatio Nunez, much tosca-rock was met with, and +at several points, at one hundred feet deep, beds of sand have been found. +I have already given a list of the recent marine and estuary shells found +in many parts on the surface near Buenos Ayres, as far as three or four +leagues from the Plata. Specimens from near Ensenada, given me by Sir W. +Parish, where the rock is quarried just beneath the surface of the plain, +consist of broken bivalves, cemented by and converted into white +crystalline carbonate of lime. I have already alluded, in the first +chapter, to a specimen (also given me by Sir W. Parish) from the A. del +Tristan, in which shells, resembling in every respect the Azara labiata, +d'Orbigny, as far as their worn condition permits of comparison, are +embedded in a reddish, softish, somewhat arenaceous marly rock: after +careful comparison, with the aid of a microscope and acids, I can perceive +no difference between the basis of this rock and the specimens collected by +me in many parts of the Pampas. I have also stated, on the authority of Sir +W. Parish, that northward of Buenos Ayres, on the highest parts of the +plain, about forty feet above the Plata, and two or three miles from it, +numerous shells of the Azara labiata (and I believe of Venus sinuosa) occur +embedded in a stratified earthy mass, including small marly concretions, +and said to be precisely like the great Pampean deposit. Hence we may +conclude that the mud of the Pampas continued to be deposited to within the +period of this existing estuary shell. Although this formation is of such +immense extent, I know of no other instance of the presence of shells in +it. + +BUENOS AYRES TO THE RIO COLORADO. + +With the exception of a few metamorphic ridges, the country between these +two points, a distance of 400 geographical miles, belongs to the Pampean +formation, and in the southern part is generally formed of the harder and +more calcareous varieties. I will briefly describe my route: about twenty- +five miles S.S.W. of the capital, in a well forty yards in depth, the upper +part, and, as I was assured, the entire thickness, was formed of dark red +Pampean mud without concretions. North of the River Salado, there are many +lakes; and on the banks of one (near the Guardia) there was a little cliff +similarly composed, but including many nodular and stalactiform +concretions: I found here a large piece of tessellated armour, like that of +the Glyptodon, and many fragments of bones. The cliffs on the Salado +consist of pale-coloured Pampean mud, including and passing into great +masses of tosca-rock: here a skeleton of the Megatherium and the bones of +other extinct quadrupeds (see the list at the end of this chapter) were +found. Large quantities of crystallised gypsum (of which specimens were +given me) occur in the cliffs of this river; and likewise (as I was assured +by Mr. Lumb) in the Pampean mud on the River Chuelo, seven leagues from +Buenos Ayres: I mention this because M. d'Orbigny lays some stress on the +supposed absence of this mineral in the Pampean formation. + +Southward of the Salado the country is low and swampy, with tosca-rock +appearing at long intervals at the surface. On the banks, however, of the +Tapalguen (sixty miles south of the Salado) there is a large extent of +tosca-rock, some highly compact and even semi-crystalline, overlying pale +Pampean mud with the usual concretions. Thirty miles further south, the +small quartz-ridge of Tapalguen is fringed on its northern and southern +flank, by little, narrow, flat-topped hills of tosca-rock, which stand +higher than the surrounding plain. Between this ridge and the Sierra of +Guitru-gueyu, a distance of sixty miles, the country is swampy, with the +tosca-rock appearing only in four or five spots: this sierra, precisely +like that of Tapalguen, is bordered by horizontal, often cliff-bounded, +little hills of tosca-rock, higher than the surrounding plain. Here, also, +a new appearance was presented in some extensive and level banks of +alluvium or detritus of the neighbouring metamorphic rocks; but I neglected +to observe whether it was stratified or not. Between Guitru-gueyu and the +Sierra Ventana, I crossed a dry plain of tosca-rock higher than the country +hitherto passed over, and with small pieces of denuded tableland of the +same formation, standing still higher. + +The marly or calcareous beds not only come up nearly horizontally to the +northern and southern foot of the great quartzose mountains of the Sierra +Ventana, but interfold between the parallel ranges. The superficial beds +(for I nowhere obtained sections more than twenty feet deep) retain, even +close to the mountains, their usual character: the uppermost layer, +however, in one place included pebbles of quartz, and rested on a mass of +detritus of the same rock. At the very foot of the mountains, there were +some few piles of quartz and tosca-rock detritus, including land-shells; +but at the distance of only half a mile from these lofty, jagged, and +battered mountains, I could not, to my great surprise, find on the +boundless surface of the calcareous plain even a single pebble. Quartz- +pebbles, however, of considerable size have at some period been transported +to a distance of between forty and fifty miles to the shores of Bahia +Blanca. (Schmidtmeyer "Travels in Chile" page 150, states that he first +noticed on the Pampas, very small bits of red granite, when fifty miles +distant from the southern extremity of the mountains of Cordova, which +project on the plain, like a reef into the sea.) + +The highest peak of the St. Ventana is, by Captain Fitzroy's measurement, +3,340 feet, and the calcareous plain at its foot (from observations taken +by some Spanish officers) 840 feet above the sea-level. ("La Plata" etc. by +Sir W. Parish page 146.) On the flanks of the mountains, at a height of +three hundred or four hundred feet above the plain, there were a few small +patches of conglomerate and breccia, firmly cemented by ferruginous matter +to the abrupt and battered face of the quartz--traces being thus exhibited +of ancient sea-action. The high plain round this range sinks quite +insensibly to the eye on all sides, except to the north, where its surface +is broken into low cliffs. Round the Sierras Tapalguen, Guitru-gueyu, and +between the latter and the Ventana we have seen (and shall hereafter see +round some hills in Banda Oriental), that the tosca-rock forms low, flat- +topped, cliff-bounded hills, higher than the surrounding plains of similar +composition. From the horizontal stratification and from the appearance of +the broken cliffs, the greater height of the Pampean formation round these +primary hills ought not to be altogether or in chief part attributed to +these several points having been uplifted more energetically than the +surrounding country, but to the argillaceo-calcareous mud having collected +round them, when they existed as islets or submarine rocks, at a greater +height, than at the bottom of the adjoining open sea;--the cliffs having +been subsequently worn during the elevation of the whole country in mass. + +Southward of the Ventana, the plain extends farther than the eye can range; +its surface is not very level, having slight depressions with no drainage +exits; it is generally covered by a few feet in thickness of sandy earth; +and in some places, according to M. Parchappe, by beds of clay two yards +thick. (M. d'Orbigny "Voyage" Part Geolog. pages 47, 48.) On the banks of +the Sauce, four leagues S.E. of the Ventana, there is an imperfect section +about two hundred feet in height, displaying in the upper part tosca-rock +and in the lower part red Pampean mud. At the settlement of Bahia Blanca, +the uppermost plain is composed of very compact, stratified tosca-rock, +containing rounded grains of quartz distinguishable by the naked eye: the +lower plain, on which the fortress stands, is described by M. Parchappe as +composed of solid tosca-rock (Ibid.); but the sections which I examined +appeared more like a redeposited mass of this rock, with small pebbles and +fragments of quartz. I shall immediately return to the important sections +on the shores of Bahia Blanca. Twenty miles southward of this place, there +is a remarkable ridge extending W. by N. and E. by S., formed of small, +separate, flat-topped, steep-sided hills, rising between one hundred and +two hundred feet above the Pampean plain at its southern base, which plain +is a little lower than that to the north. The uppermost stratum in this +ridge consists of pale, highly calcareous, compact tosca-rock, resting (as +seen in one place) on reddish Pampean mud, and this again on a paler kind: +at the foot of the ridge, there is a well in reddish clay or mud. I have +seen no other instance of a chain of hills belonging to the Pampean +formation; and as the strata show no signs of disturbance, and as the +direction of the ridge is the same with that common to all the metamorphic +lines in this whole area, I suspect that the Pampean sediment has in this +instance been accumulated on and over a ridge of hard rocks, instead of, as +in the case of the above-mentioned Sierras, round their submarine flanks. +South of this little chain of tosca-rock, a plain of Pampean mud declines +towards the banks of the Colorado: in the middle a well has been dug in red +Pampean mud, covered by two feet of white, softish, highly calcareous +tosca-rock, over which lies sand with small pebbles three feet in +thickness--the first appearance of that vast shingle formation described in +the First Chapter. In the first section after crossing the Colorado, an old +tertiary formation, namely, the Rio Negro sandstone (to be described in the +next chapter), is met with: but from the accounts given me by the Gauchos, +I believe that at the mouth of the Colorado the Pampean formation extends a +little further southwards. + +BAHIA BLANCA. + +To return to the shores of this bay. At Monte Hermoso there is a good +section, about one hundred feet in height, of four distinct strata, +appearing to the eye horizontal, but thickening a little towards the N.W. +The uppermost bed, about twenty feet in thickness, consists of obliquely +laminated, soft sandstone, including many pebbles of quartz, and falling at +the surface into loose sand. The second bed, only six inches thick, is a +hard, dark-coloured sandstone. The third bed is pale-coloured Pampean mud; +and the fourth is of the same nature, but darker coloured, including in its +lower part horizontal layers and lines of concretions of not very compact +pinkish tosca-rock. The bottom of the sea, I may remark, to a distance of +several miles from the shore, and to a depth of between sixty and one +hundred feet, was found by the anchors to be composed of tosca-rock and +reddish Pampean mud. Professor Ehrenberg has examined for me specimens of +the two lower beds, and finds in them three Polygastrica and six +Phytolitharia. + +(The following list is given in the "Monatsberichten der konig. Akad. zu +Berlin" April 1845:-- +POLYGASTRICA. +Fragilaria rhabdosoma. +Gallionella distans. +Pinnularia? + +PHYTOLITHARIA. +Lithodontium Bursa. +Lithodontium furcatum. +Lithostylidium exesum. +Lithostylidium rude. +Lithostylidium Serra. +Spongolithis Fustis?) + +Of these, only one (Spongolithis Fustis?) is a marine form; five of them +are identical with microscopical structures of brackish-water origin, +hereafter to be mentioned, which form a central point in the Pampean +formation. In these two beds, especially in the lower one, bones of extinct +mammifers, some embedded in their proper relative positions and others +single, are very numerous in a small extent of the cliffs. These remains +consist of, first, the head of Ctenomys antiquus, allied to the living +Ctenomys Braziliensis; secondly, a fragment of the remains of a rodent; +thirdly, molar teeth and other bones of a large rodent, closely allied to, +but distinct from, the existing species of Hydrochoerus, and therefore +probably an inhabitant of fresh water; fourth and fifthly, portions of +vertebrae, limbs, ribs, and other bones of two rodents; sixthly, bones of +the extremities of some great megatheroid quadruped. (See "Fossil Mammalia" +page 109 by Professor Owen, in the "Zoology of the Voyage of the 'Beagle';" +and Catalogue page 36 of Fossil Remains in Museum of Royal College of +Surgeons.) The number of the remains of rodents gives to this collection a +peculiar character, compared with those found in any other locality. All +these bones are compact and heavy; many of them are stained red, with their +surfaces polished; some of the smaller ones are as black as jet. + +Monte Hermoso is between fifty and sixty miles distant in a S.E. line from +the Ventana, with the intermediate country gently rising towards it, and +all consisting of the Pampean formation. What relation, then, do these +beds, at the level of the sea and under it, bear to those on the flanks of +the Ventana, at the height of 840 feet, and on the flanks of the other +neighbouring sierras, which, from the reasons already assigned, do not +appear to owe their greater height to unequal elevation? When the tosca- +rock was accumulating round the Ventana, and when, with the exception of a +few small rugged primary islands, the whole wide surrounding plains must +have been under water, were the strata at Monte Hermoso depositing at the +bottom of a great open sea, between eight hundred and one thousand feet in +depth? I much doubt this; for if so, the almost perfect carcasses of the +several small rodents, the remains of which are so very numerous in so +limited a space, must have been drifted to this spot from the distance of +many hundred miles. It appears to me far more probable, that during the +Pampean period this whole area had commenced slowly rising (and in the +cliffs, at several different heights we have proofs of the land having been +exposed to sea-action at several levels), and that tracts of land had thus +been formed of Pampean sediment round the Ventana and the other primary +ranges, on which the several rodents and other quadrupeds lived, and that a +stream (in which perhaps the extinct aquatic Hydrochoerus lived) drifted +their bodies into the adjoining sea, into which the Pampean mud continued +to be poured from the north. As the land continued to rise, it appears that +this source of sediment was cut off; and in its place sand and pebbles were +borne down by stronger currents, and conformably deposited over the Pampean +strata. + +(FIGURE 15. SECTION OF BEDS WITH RECENT SHELLS AND EXTINCT MAMMIFERS, AT +PUNTA ALTA IN BAHIA BLANCA. (Showing beds from bottom to top: A, B, C, D.)) + +Punta Alta is situated about thirty miles higher up on the northern side of +this same bay: it consists of a small plain, between twenty and thirty feet +in height, cut off on the shore by a line of low cliffs about a mile in +length, represented in Figure 15 with its vertical scale necessarily +exaggerated. The lower bed (A) is more extensive than the upper ones; it +consists of stratified gravel or conglomerate, cemented by calcareo- +arenaceous matter, and is divided by curvilinear layers of pinkish marl, of +which some are precisely like tosca-rock, and some more sandy. The beds are +curvilinear, owing to the action of currents, and dip in different +directions; they include an extraordinary number of bones of gigantic +mammifers and many shells. The pebbles are of considerable size, and are of +hard sandstone, and of quartz, like that of the Ventana: there are also a +few well-rounded masses of tosca-rock. + +The second bed B is about fifteen feet in thickness, but towards both +extremities of the cliff (not included in the diagram) it either thins out +and dies away, or passes insensibly into an overlying bed of gravel. It +consists of red, tough clayey mud, with minute linear cavities; it is +marked with faint horizontal shades of colour; it includes a few pebbles, +and rarely a minute particle of shell: in one spot, the dermal armour and a +few bones of a Dasypoid quadruped were embedded in it: it fills up furrows +in the underlying gravel. With the exception of the few pebbles and +particles of shells, this bed resembles the true Pampean mud; but it still +more closely resembles the clayey flats (mentioned in the First Chapter) +separating the successively rising parallel ranges of sand-dunes. + +The bed C is of stratified gravel, like the lowest one; it fills up furrows +in the underlying red mud, and is sometimes interstratified with it, and +sometimes insensibly passes into it; as the red mud thins out, this upper +gravel thickens. Shells are more numerous in it than in the lower gravel; +but the bones, though some are still present, are less numerous. In one +part, however, where this gravel and the red mud passed into each other, I +found several bones and a tolerably perfect head of the Megatherium. Some +of the large Volutas, though embedded in the gravel-bed C, were filled with +the red mud, including great numbers of the little recent Paludestrina +australis. These three lower beds are covered by an unconformable mantle D +of stratified sandy earth, including many pebbles of quartz, pumice and +phonolite, land and sea-shells. + +M. d'Orbigny has been so obliging as to name for me the twenty species of +Mollusca embedded in the two gravel beds: they consist of:-- + +1. Volutella angulata, d'Orbigny, "Voyage" Mollusq. and Pal. +2. Voluta Braziliana, Sol +3. Olicancilleria Braziliensis d'Orbigny. +4. Olicancilleria auricularia, d'Orbigny. +5. Olivina puelchana, d'Orbigny. +6. Buccinanops cochlidium, d'Orbigny. +7. Buccinanops globulosum, d'Orbigny. +8. Colombella sertulariarum, d'Orbigny. +9. Trochus Patagonicus, and var. of ditto, d'Orbigny. +10. Paludestrina Australis, d'Orbigny. +11. Fissurella Patagonica, d'Orbigny. +12. Crepidula muricata, Lam. +13. Venus purpurata, Lam. +14. Venus rostrata, Phillippi. +15. Mytilus Darwinianus, d'Orbigny. +16. Nucula semiornata, d'Orbigny. +17. Cardita Patagonica, d'Orbigny. +18. Corbula Patagonica (?), d'Orbigny. +19. Pecten tethuelchus, d'Orbigny. +20. Ostrea puelchana, d'Orbigny. +21. A living species of Balanus. +22 and 23. An Astrae and encrusting Flustra, apparently identical with +species now living in the bay. + +All these shells now live on this coast, and most of them in this same bay. +I was also struck with the fact, that the proportional numbers of the +different kinds appeared to be the same with those now cast up on the +beach: in both cases specimens of Voluta, Crepidula, Venus, and Trochus are +the most abundant. Four or five of the species are the same with the +upraised shells on the Pampas near Buenos Ayres. All the specimens have a +very ancient and bleached appearance, and do not emit, when heated, an +animal odour: some of them are changed throughout into a white, soft, +fibrous substance; others have the space between the external walls, either +hollow, or filled up with crystalline carbonate of lime. (A Bulinus, +mentioned in the Introduction to the "Fossil Mammalia" in the "Zoology of +the Voyage of the 'Beagle'" has so much fresher an appearance, than the +marine species, that I suspect it must have fallen amongst the others, and +been collected by mistake.) + +The remains of the extinct mammiferous animals, from the two gravel beds +have been described by Professor Owen in the "Zoology of the Voyage of the +'Beagle':" they consist of, 1st, one nearly perfect head and three +fragments of heads of the Megatherium Cuvierii; 2nd, a lower jaw of +Megalonyx Jeffersonii; 3rd, lower jaw of Mylodon Darwinii; 4th, fragments +of a head of some gigantic Edental quadruped; 5th, an almost entire +skeleton of the great Scelidotherium leptocephalum, with most of the bones, +including the head, vertebrae, ribs, some of the extremities to the claw- +bone, and even, as remarked by Professor Owen, the knee-cap, all nearly in +their proper relative positions; 6th, fragments of the jaw and a separate +tooth of a Toxodon, belonging either to T. Platensis, or to a second +species lately discovered near Buenos Ayres; 7th, a tooth of Equus +curvidens; 8th, tooth of a Pachyderm, closely allied to Palaeotherium, of +which parts of the head have been lately sent from Buenos Ayres to the +British Museum; in all probability this pachyderm is identical with the +Macrauchenia Patagonica from Port S. Julian, hereafter to be referred to. +Lastly, and 9thly, in a cliff of the red clayey bed B, there was a double +piece, about three feet long and two wide, of the bony armour of a large +Dasypoid quadruped, with the two sides pressed nearly close together: as +the cliff is now rapidly washing away, this fossil probably was lately much +more perfect; from between its doubled-up sides, I extracted the middle and +ungual phalanges, united together, of one of the feet, and likewise a +separate phalanx: hence one or more of the limbs must have been attached to +the dermal case, when it was embedded. Besides these several remains in a +distinguishable condition, there were very many single bones: the greater +number were embedded in a space 200 yards square. The preponderance of the +Edental quadrupeds is remarkable; as is, in contrast with the beds of Monte +Hermoso, the absence of Rodents. Most of the bones are now in a soft and +friable condition, and, like the shells, do not emit when burnt an animal +odour. The decayed state of the bones may be partly owing to their late +exposure to the air and tidal-waves. Barnacles, Serpulae, and corallines +are attached to many of the bones, but I neglected to observe whether these +might not have grown on them since being exposed to the present tidal +action (After having packed up my specimens at Bahia Blanca, this point +occurred to me, and I noted it; but forgot it on my return, until the +remains had been cleaned and oiled: my attention has been lately called to +the subject by some remarks by M. d'Orbigny.); but I believe that some of +the barnacles must have grown on the Scelidotherium, soon after being +deposited, and before being WHOLLY covered up by the gravel. Besides the +remains in the condition here described, I found one single fragment of +bone very much rolled, and as black as jet, so as perfectly to resemble +some of the remains from Monte Hermoso. + +Very many of the bones had been broken, abraded, and rolled, before being +embedded. Others, even some of those included in the coarsest parts of the +the now hard conglomerate, still retain all their minutest prominences +perfectly preserved; so that I conclude that they probably were protected +by skin, flesh, or ligaments, whilst being covered up. In the case of the +Scelidotherium, it is quite certain that the whole skeleton was held +together by its ligaments, when deposited in the gravel in which I found +it. Some cervical vertebrae and a humerus of corresponding size lay so +close together, as did some ribs and the bones of a leg, that I thought +that they must originally have belonged to two skeletons, and not have been +washed in single; but as remains were here very numerous, I will not lay +much stress on these two cases. We have just seen that the armour of the +Dasypoid quadruped was certainly embedded together with some of the bones +of the feet. + +Professor Ehrenberg has examined for me specimens of the finer matter from +in contact with these mammiferous remains: he finds in them two +Polygastrica, decidedly marine forms; and six Phytolitharia, of which one +is probably marine, and the others either of fresh-water or terrestrial +origin. ("Monatsberichten der Akad. zu Berlin" April 1845. The list +consists of:-- + +POLYGASTRICA. +Gallionella sulcata. +Stauroptera aspera? fragm. + +PHYTOLITHARIA. +Lithasteriscus tuberculatus. +Lithostylidium Clepsammidium. +Lithostylidium quadratum. +Lithostylidium rude. +Lithostylidium unidentatum. +Spongolithis acicularis.) + +Only one of these eight microscopical bodies is common to the nine from +Monte Hermoso: but five of them are in common with those from the Pampean +mud on the banks of the Parana. The presence of any fresh-water infusoria, +considering the aridity of the surrounding country, is here remarkable: the +most probable explanation appears to be, that these microscopical organisms +were washed out of the adjoining great Pampean formation during its +denudation, and afterwards redeposited. + +We will now see what conclusions may be drawn from the facts above +detailed. It is certain that the gravel-beds and intermediate red mud were +deposited within the period, when existing species of Mollusca held to each +other nearly the same relative proportions as they do on the present coast. +These beds, from the number of littoral species, must have been accumulated +in shallow water; but not, judging from the stratification of the gravel +and the layers of marl, on a beach. From the manner in which the red clay +fills up furrows in the underlying gravel, and is in some parts itself +furrowed by the overlying gravel, whilst in other parts it either +insensibly passes into, or alternates with, this upper gravel, we may infer +several local changes in the currents, perhaps caused by slight changes, up +or down, in the level of the land. By the elevation of these beds, to which +period the alluvial mantle with pumice-pebbles, land and sea-shells +belongs, the plain of Punta Alta, from twenty to thirty feet in height, was +formed. In this neighbourhood there are other and higher sea-formed plains +and lines of cliffs in the Pampean formation worn by the denuding action of +the waves at different levels. Hence we can easily understand the presence +of rounded masses of tosca-rock in this lowest plain; and likewise, as the +cliffs at Monte Hermoso with their mammiferous remains stand at a higher +level, the presence of the one much-rolled fragment of bone which was as +black as jet: possibly some few of the other much-rolled bones may have +been similarly derived, though I saw only the one fragment, in the same +condition with those from Monte Hermoso. M. d'Orbigny has suggested that +all these mammiferous remains may have been washed out of the Pampean +formation, and afterwards redeposited together with the recent shells. +("Voyage" Part. Geolog. page 49.) Undoubtedly it is a marvellous fact that +these numerous gigantic quadrupeds, belonging, with the exception of the +Equus curvidens, to seven extinct genera, and one, namely, the Toxodon, not +falling into any existing family, should have co-existed with Mollusca, all +of which are still living species; but analogous facts have been observed +in North America and in Europe. In the first place, it should not be +overlooked, that most of the co-embedded shells have a more ancient and +altered appearance than the bones. In the second place, is it probable that +numerous bones not hardened by silex or any other mineral, could have +retained their delicate prominences and surfaces perfect if they had been +washed out of one deposit, and re-embedded in another:--this later deposit +being formed of large, hard pebbles, arranged by the action of currents or +breakers in shallow water into variously curved and inclined layers? The +bones which are now in so perfect a state of preservation, must, I +conceive, have been fresh and sound when embedded, and probably were +protected by skin, flesh, or ligaments. The skeleton of the Scelidotherium +indisputably was deposited entire: shall we say that when held together by +its matrix it was washed out of an old gravel-bed (totally unlike in +character to the Pampean formation), and re-embedded in another gravel-bed, +composed (I speak after careful comparison) of exactly the same kind of +pebbles, in the same kind of cement? I will lay no stress on the two cases +of several ribs and bones of the extremities having APPARENTLY been +embedded in their proper relative position: but will any one be so bold as +to affirm that it is possible, that a piece of the thin tessellated armour +of a Dasypoid quadruped, at least three feet long and two in width, and now +so tender that I was unable with the utmost care to extract a fragment more +than two or three inches square, could have been washed out of one bed, and +re-embedded in another, together with some of the small bones of the feet, +without having been dashed into atoms? We must then wholly reject M. +d'Orbigny's supposition, and admit as certain, that the Scelidotherium and +the large Dasypoid quadruped, and as highly probable, that the Toxodon, +Megatherium, etc., some of the bones of which are perfectly preserved, were +embedded for the first time, and in a fresh condition, in the strata in +which they were found entombed. These gigantic quadrupeds, therefore, +though belonging to extinct genera and families, coexisted with the twenty +above-enumerated Mollusca, the barnacle and two corals, still living on +this coast. From the rolled fragment of black bone, and from the plain of +Punta Alta being lower than that of Monte Hermoso, I conclude that the +coarse sub-littoral deposits of Punta Alta, are of subsequent origin to the +Pampean mud of Monte Hermoso; and the beds at this latter place, as we have +seen, are probably of subsequent origin to the high tosca-plain round the +Sierra Ventana: we shall, however, return, at the end of this chapter, to +the consideration of these several stages in the great Pampean formation. + +BUENOS AYRES TO ST. FE BAJADA, IN ENTRE RIOS. + +For some distance northward of Buenos Ayres, the escarpment of the Pampean +formation does not approach very near to the Plata, and it is concealed by +vegetation: but in sections on the banks of the Rios Luxan, Areco, and +Arrecifes, I observed both pale and dark reddish Pampean mud, with small, +whitish concretions of tosca; at all these places mammiferous remains have +been found. In the cliffs on the Parana, at San Nicolas, the Pampean mud +contains but little tosca; here M. d'Orbigny found the remains of two +rodents (Ctenomys Bonariensis and Kerodon antiquus) and the jaw of a Canis: +when on the river I could clearly distinguish in this fine line of cliffs, +"horizontal lines of variation both in tint and compactness." (I quote +these words from my note-book, as written down on the spot, on account of +the general absence of stratification in the Pampean formation having been +insisted on by M. d'Orbigny as a proof of the diluvial origin of this great +deposit.) The plain northward of this point is very level, but with some +depressions and lakes; I estimated its height at from forty to sixty feet +above the Parana. At the A. Medio the bright red Pampean mud contains +scarcely any tosca-rock; whilst at a short distance the stream of the +Pabon, forms a cascade, about twenty feet in height, over a cavernous mass +of two varieties of tosca-rock; of which one is very compact and semi- +crystalline, with seams of crystallised carbonate of lime: similar compact +varieties are met with on the Salidillo and Seco. The absolute identity (I +speak after a comparison of my specimens) between some of these varieties, +and those from Tapalguen, and from the ridge south of Bahia Blanca, a +distance of 400 miles of latitude, is very striking. + +At Rosario there is but little tosca-rock: near this place I first noticed +at the edge of the river traces of an underlying formation, which, twenty- +five miles higher up in the estancia of Gorodona, consists of a pale +yellowish clay, abounding with concretionary cylinders of a ferruginous +sandstone. This bed, which is probably the equivalent of the older tertiary +marine strata, immediately to be described in Entre Rios, only just rises +above the level of the Parana when low. The rest of the cliff at Gorodona, +is formed of red Pampean mud, with, in the lower part, many concretions of +tosca, some stalacti-formed, and with only a few in the upper part: at the +height of six feet above the river, two gigantic skeletons of the Mastodon +Andium were here embedded; their bones were scattered a few feet apart, but +many of them still held their proper relative positions: they were much +decayed and as soft as cheese, so that even one of the great molar teeth +fell into pieces in my hand. We here see that the Pampean deposit contains +mammiferous remains close to its base. On the banks of the Carcarana, a few +miles distant, the lowest bed visible was pale Pampean mud, with masses of +tosca-rock, in one of which I found a much decayed tooth of the Mastodon: +above this bed, there was a thin layer almost composed of small concretions +of white tosca, out of which I extracted a well preserved, but slightly +broken tooth of Toxodon Platensis: above this there was an unusual bed of +very soft impure sandstone. In this neighbourhood I noticed many single +embedded bones, and I heard of others having been found in so perfect a +state that they were long used as gate-posts: the Jesuit Falkner found here +the dermal armour of some gigantic Edental quadruped. + +In some of the red mud scraped from a tooth of one of the Mastodons at +Gorodona, Professor Ehrenberg finds seven Polygastrica and thirteen +Phytolitharia, all of them, I believe, with two exceptions, already known +species. ("Monatsberichten der konig. Akad. zu Berlin" April 1845. The list +consists of:-- + +POLYGASTRICA. +Campylodiscus clypeus. +Coscinodiscus subtilis. +Coscinodiscus al. sp. +Eunotia. +Gallionella granulata. +Himantidium gracile. +Pinnularia borealis.) + +Of these twenty, the preponderating number are of fresh-water origin; only +two species of Coscinodiscus and a Spongolithis show the direct influence +of the sea; therefore Professor Ehrenberg arrives at the important +conclusion that the deposit must have been of brackish-water origin. Of the +thirteen Phytolitharia, nine are met with in the two deposits in Bahia +Blanca, where there is evidence from two other species of Polygastrica that +the beds were accumulated in brackish water. The traces of coral, sponges, +and Polythalamia, found by Dr. Carpenter in the tosca-rock (of which I must +observe the greater number of specimens were from the upper beds in the +southern parts of the formation), apparently show a more purely marine +origin. + +At ST. FE BAJADA, in Entre Rios, the cliffs, estimated at between sixty and +seventy feet in height, expose an interesting section: the lower half +consists of tertiary strata with marine shells, and the upper half of the +Pampean formation. The lowest bed is an obliquely laminated, blackish, +indurated mud, with distinct traces of vegetable remains. (M. d'Orbigny +"Voyage" Part. Geolog. page 37, has given a detailed description of this +section, but as he does not mention this lowest bed, it may have been +concealed when he was there by the river. There is a considerable +discrepancy between his description and mine, which I can only account for +by the beds themselves varying considerably in short distances.) Above this +there is a thick bed of yellowish sandy clay, with much crystallised gypsum +and many shells of Ostreae, Pectens, and Arcae: above this there generally +comes an arenaceous crystalline limestone, but there is sometimes +interposed a bed, about twelve feet thick, of dark green, soapy clay, +weathering into small angular fragments. The limestone, where purest, is +white, highly crystalline, and full of cavities: it includes small pebbles +of quartz, broken shells, teeth of sharks, and sometimes, as I was +informed, large bones: it often contains so much sand as to pass into a +calcareous sandstone, and in such parts the great Ostrea Patagonica chiefly +abounds. (Captain Sulivan, R.N., has given me a specimen of this shell, +which he found in the cliffs at Point Cerrito, between twenty and thirty +miles above the Bajada.) In the upper part, the limestone alternates with +layers of fine white sand. The shells included in these beds have been +named for me by M. d'Orbigny: they consist of:-- + +1. Ostrea Patagonica, d'Orbigny, "Voyage" Part. Pal. +2. Ostrea Alvarezii, d'Orbigny, "Voyage" Part. Pal. +3. Pecten Paranensis, d'Orbigny, "Voyage" Part. Pal. +4. Pecten Darwinianus, d'Orbigny, "Voyage" Part. Pal. +5. Venus Munsterii, d'Orbigny, "Voyage" Pal. +6. Arca Bonplandiana, d'Orbigny, "Voyage" Pal. +7. Cardium Platense, d'Orbigny, "Voyage" Pal. +8. Tellina, probably nov. species, but too imperfect for description. + +PHYTOLITHARIA. + +Lithasteriscus tuberculatus. +Lithodontium bursa. +Lithodontium furcatum. +Lithodontium rostratum. +Lithostylidium Amphiodon. +Lithostylidium Clepsammidium. +Lithostylidium Hamus. +Lithostylidium polyedrum. +Lithostylidium quadratum. +Lithostylidium rude. +Lithostylidium Serra. +Lithostylidium unidentatum. +Spongolithis Fustis. + +These species are all extinct: the six first were found by M. d'Orbigny and +myself in the formations of the Rio Negro, S. Josef, and other parts of +Patagonia; and therefore, as first observed by M. d'Orbigny, these beds +certainly belong to the great Patagonian formation, which will be described +in the ensuing chapter, and which we shall see must be considered as a very +ancient tertiary one. North of the Bajada, M. d'Orbigny found, in beds +which he considers as lying beneath the strata here described, remains of a +Toxodon, which he has named as a distinct species from the T. Platensis of +the Pampean formation. Much silicified wood is found on the banks of the +Parana (and likewise on the Uruguay), and I was informed that they come out +of these lower beds; four specimens collected by myself are dicotyledonous. + +The upper half of the cliff, to a thickness of about thirty feet, consists +of Pampean mud, of which the lower part is pale-coloured, and the upper +part of a brighter red, with some irregular layers of an arenaceous variety +of tosca, and a few small concretions of the ordinary kind. Close above the +marine limestone, there is a thin stratum with a concretionary outline of +white hard tosca-rock or marl, which may be considered either as the +uppermost bed of the inferior deposits, or the lowest of the Pampean +formation; at one time I considered this bed as marking a passage between +the two formations: but I have since become convinced that I was deceived +on this point. In the section on the Parana, I did not find any mammiferous +remains; but at two miles distance on the A. Tapas (a tributary of the +Conchitas), they were extremely numerous in a low cliff of red Pampean mud +with small concretions, precisely like the upper bed on the Parana. Most of +the bones were solitary and much decayed; but I saw the dermal armour of a +gigantic Edental quadruped, forming a caldron-like hollow, four or five +feet in diameter, out of which, as I was informed, the almost entire +skeleton had been lately removed. I found single teeth of the Mastodon +Andium, Toxodon Platensis, and Equus curvidens, near to each other. As this +latter tooth approaches closely to that of the common horse, I paid +particular attention to its true embedment, for I did not at that time know +that there was a similar tooth hidden in the matrix with the other +mammiferous remains from Punta Alta. It is an interesting circumstance, +that Professor Owen finds that the teeth of this horse approach more +closely in their peculiar curvature to a fossil specimen brought by Mr. +Lyell from North America, than to those of any other species of Equus. +(Lyell "Travels in North America" volume 1 page 164 and "Proceedings of +Geological Society" volume 4 page 39.) + +The underlying marine tertiary strata extend over a wide area: I was +assured that they can be traced in ravines in an east and west line across +Entre Rios to the Uruguay, a distance of about 135 miles. In a S.E. +direction I heard of their existence at the head of the R. Nankay; and at +P. Gorda in Banda Oriental, a distance of 170 miles, I found the same +limestone, containing the same fossil shells, lying at about the same level +above the river as at St. Fe. In a southerly direction, these beds sink in +height, for at another P. Gorda in Entre Rios, the limestone is seen at a +much less height; and there can be little doubt that the yellowish sandy +clay, on a level with the river, between the Carcarana and S. Nicholas, +belongs to this same formation; as perhaps do the beds of sand at Buenos +Ayres, which lie at the bottom of the Pampean formation, about sixty feet +beneath the surface of the Plata. The southerly declination of these beds +may perhaps be due, not to unequal elevation, but to the original form of +the bottom of the sea, sloping from land situated to the north; for that +land existed at no great distance, we have evidence in the vegetable +remains in the lowest bed at St. Fe; and in the silicified wood and in the +bones of Toxodon Paranensis, found (according to M. d'Orbigny) in still +lower strata. + +BANDA ORIENTAL. + +This province lies on the northern side of the Plata, and eastward of the +Uruguay: it has a gentle undulatory surface, with a basis of primary rocks; +and is in most parts covered up with an unstratified mass, of no great +thickness, of reddish Pampean mud. In the eastern half, near Maldonado, +this deposit is more arenaceous than in the Pampas, it contains many though +small concretions of marl or tosca-rock, and others of highly ferruginous +sandstone; in one section, only a few yards in depth, it rested on +stratified sand. Near Monte Video this deposit in some spots appears to be +of greater thickness; and the remains of the Glyptodon and other extinct +mammifers have been found in it. In the long line of cliffs, between fifty +and sixty feet in height, called the Barrancas de S. Gregorio, which extend +westward of the Rio S. Lucia, the lower half is formed of coarse sand of +quartz and feldspar without mica, like that now cast up on the beach near +Maldonado; and the upper half of Pampean mud, varying in colour and +containing honeycombed veins of soft calcareous matter and small +concretions of tosca-rock arranged in lines, and likewise a few pebbles of +quartz. This deposit fills up hollows and furrows in the underlying sand; +appearing as if water charged with mud had invaded a sandy beach. These +cliffs extend far westward, and at a distance of sixty miles, near Colonia +del Sacramiento, I found the Pampean deposit resting in some places on this +sand, and in others on the primary rocks: between the sand and the reddish +mud, there appeared to be interposed, but the section was not a very good +one, a thin bed of shells of an existing Mytilus, still partially retaining +their colour. The Pampean formation in Banda Oriental might readily be +mistaken for an alluvial deposit: compared with that of the Pampas, it is +often more sandy, and contains small fragments of quartz; the concretions +are much smaller, and there are no extensive masses of tosca-rock. + +In the extreme western parts of this province, between the Uruguay and a +line drawn from Colonia to the R. Perdido (a tributary of the R. Negro), +the formations are far more complicated. Besides primary rocks, we meet +with extensive tracts and many flat-topped, horizontally stratified, cliff- +bounded, isolated hills of tertiary strata, varying extraordinarily in +mineralogical nature, some identical with the old marine beds of St. Fe +Bajada, and some with those of the much more recent Pampean formation. +There are, also, extensive LOW tracts of country covered with a deposit +containing mammiferous remains, precisely like that just described in the +more eastern parts of the province. Although from the smooth and unbroken +state of the country, I never obtained a section of this latter deposit +close to the foot of the higher tertiary hills, yet I have not the least +doubt that it is of quite subsequent origin; having been deposited after +the sea had worn the tertiary strata into the cliff-bounded hills. This +later formation, which is certainly the equivalent of that of the Pampas, +is well seen in the valleys in the estancia of Berquelo, near Mercedes; it +here consists of reddish earth, full of rounded grains of quartz, and with +some small concretions of tosca-rock arranged in horizontal lines, so as +perfectly to resemble, except in containing a little calcareous matter, the +formation in the eastern parts of Banda Oriental, in Entre Rios, and at +other places: in this estancia the skeleton of a great Edental quadruped +was found. In the valley of the Sarandis, at the distance of only a few +miles, this deposit has a somewhat different character, being whiter, +softer, finer-grained, and full of little cavities, and consequently of +little specific gravity; nor does it contain any concretions or calcareous +matter: I here procured a head, which when first discovered must have been +quite perfect, of the Toxodon Platensis, another of a Mylodon (This head +was at first considered by Professor Owen (in the "Zoology of the +'Beagle's' Voyage") as belonging to a distinct genus, namely, +Glossotherium.), perhaps M. Darwinii, and a large piece of dermal armour, +differing from that of the Glyptodon clavipes. These bones are remarkable +from their extraordinarily fresh appearance; when held over a lamp of +spirits of wine, they give out a strong odour and burn with a small flame; +Mr. T. Reeks has been so kind as to analyse some of the fragments, and he +finds that they contain about 7 per cent of animal matter, and 8 per cent +of water. (Liebig "Chemistry of Agriculture" page 194 states that fresh dry +bones contain from 32 to 33 per cent of dry gelatine. See also Dr. Daubeny, +in "Edinburgh New Philosophical Journal" volume 37 page 293.) + +The older tertiary strata, forming the higher isolated hills and extensive +tracts of country, vary, as I have said, extraordinarily in composition: +within the distance of a few miles, I sometimes passed over crystalline +limestone with agate, calcareous tuffs, and marly rocks, all passing into +each other,--red and pale mud with concretions of tosca-rock, quite like +the Pampean formation,--calcareous conglomerates and sandstones,--bright +red sandstones passing either into red conglomerate, or into white +sandstone,--hard siliceous sandstones, jaspery and chalcedonic rocks, and +numerous other subordinate varieties. I was unable to mark out the +relations of all these strata, and will describe only a few distinct +sections:--in the cliffs between P. Gorda on the Uruguay and the A. de +Vivoras, the upper bed is crystalline cellular limestone often passing into +calcareous sandstone, with impressions of some of the same shells as at St. +Fe Bajada; at P. Gorda, this limestone is interstratified with and rests +on, white sand, which covers a bed about thirty feet thick of pale-coloured +clay, with many shells of the great Ostrea Patagonica (In my "Journal" page +171 1st edition, I have hastily and inaccurately stated that the Pampean +mud, which is found over the eastern part of B. Oriental, lies OVER the +limestone at P. Gorda; I should have said that there was reason to infer +that it was a subsequent or superior deposit.): beneath this, in the +vertical cliff, nearly on a level with the river, there is a bed of red mud +absolutely like the Pampean deposit, with numerous often large concretions +of perfectly characterised white, compact tosca-rock. At the mouth of the +Vivoras, the river flows over a pale cavernous tosca-rock, quite like that +in the Pampas, and this APPEARED to underlie the crystalline limestone; but +the section was not unequivocal like that at P. Gorda. These beds now form +only a narrow and much denuded strip of land; but they must once have +extended much further; for on the next stream, south of the S. Juan, +Captain Sulivan, R.N., found a little cliff, only just above the surface of +the river, with numerous shells of the Venus Munsterii, D'Orbigny,--one of +the species occurring at St. Fe, and of which there are casts at P. Gorda: +the line of cliffs of the subsequently deposited true Pampean mud, extend +from Colonia to within half a mile of this spot, and no doubt once covered +up this denuded marine stratum. Again at Colonia, a Frenchman found, in +digging the foundations of a house, a great mass of the Ostrea Patagonica +(of which I saw many fragments), packed together just beneath the surface, +and directly superimposed on the gneiss. These sections are important: M. +d'Orbigny is unwilling to believe that beds of the same nature with the +Pampean formation ever underlie the ancient marine tertiary strata; and I +was as much surprised at it as he could have been; but the vertical cliff +at P. Gorda allowed of no mistake, and I must be permitted to affirm, that +after having examined the country from the Colorado to St. Fe Bajada, I +could not be deceived in the mineralogical character of the Pampean +deposit. + +Moreover, in a precipitous part of the ravine of Las Bocas, a red sandstone +is distinctly seen to overlie a thick bed of pale mud, also quite like the +Pampean formation, abounding with concretions of true tosca-rock. This +sandstone extends over many miles of country: it is as red as the brightest +volcanic scoriae; it sometimes passes into a coarse red conglomerate +composed of the underlying primary rocks; and often passes into a soft +white sandstone with red streaks. At the Calera de los Huerfanos, only a +quarter of a mile south of where I first met with the red sandstone, the +crystalline white limestone is quarried: as this bed is the uppermost, and +as it often passes into calcareous sandstone, interstratified with pure +sand; and as the red sandstone likewise passes into soft white sandstone, +and is also the uppermost bed, I believe that these two beds, though so +different, are equivalents. A few leagues southward of these two places, on +each side of the low primary range of S. Juan, there are some flat-topped, +cliff-bounded, separate little hills, very similar to those fringing the +primary ranges in the great plain south of Buenos Ayres: they are composed- +-1st, of calcareous tuff with many particles of quartz, sometimes passing +into a coarse conglomerate; 2nd, of a stone undistinguishable on the +closest inspection from the compacter varieties of tosca-rock; and 3rd, of +semi-crystalline limestone, including nodules of agate: these three +varieties pass insensibly into each other, and as they form the uppermost +stratum in this district, I believe that they, also, are the equivalents of +the pure crystalline limestone, and of the red and white sandstones and +conglomerates. + +Between these points and Mercedes on the Rio Negro, there are scarcely any +good sections, the road passing over limestone, tosca-rock, calcareous and +bright red sandstones, and near the source of the San Salvador over a wide +extent of jaspery rocks, with much milky agate, like that in the limestone +near San Juan. In the estancia of Berquelo, the separate, flat-topped, +cliff-bounded hills are rather higher than in the other parts of the +country; they range in a N.E. and S.W. direction; their uppermost beds +consist of the same bright red sandstone, passing sometimes into a +conglomerate, and in the lower part into soft white sandstone, and even +into loose sand: beneath this sandstone, I saw in two places layers of +calcareous and marly rocks, and in one place red Pampean-like earth; at the +base of these sections, there was a hard, stratified, white sandstone, with +chalcedonic layers. Near Mercedes, beds of the same nature and apparently +of the same age, are associated with compact, white, crystalline limestone, +including much botryoidal agate, and singular masses, like porcelain, but +really composed of a calcareo-siliceous paste. In sinking wells in this +district the chalcedonic strata seem to be the lowest. Beds, such as there +described, occur over the whole of this neighbourhood; but twenty miles +further up the R. Negro, in the cliffs of Perika, which are about fifty +feet in height, the upper bed is a prettily variegated chalcedony, mingled +with a pure white tallowy limestone; beneath this there is a conglomerate +of quartz and granite; beneath this many sandstones, some highly +calcareous; and the whole lower two-thirds of the cliff consists of earthy +calcareous beds of various degrees of purity, with one layer of reddish +Pampean-like mud. + +When examining the agates, the chalcedonic and jaspery rocks, some of the +limestones, and even the bright red sandstones, I was forcibly struck with +their resemblance to deposits formed in the neighbourhood of volcanic +action. I now find that M. Isabelle, in his "Voyage a Buenos Ayres," has +described closely similar beds on Itaquy and Ibicuy (which enter the +Uruguay some way north of the R. Negro) and these beds include fragments of +red decomposed true scoriae hardened by zeolite, and of black retinite: we +have then here good evidence of volcanic action during our tertiary period. +Still further north, near S. Anna, where the Parana makes a remarkable +bend, M. Bonpland found some singular amygdaloidal rocks, which perhaps may +belong to this same epoch. (M. d'Orbigny "Voyage" Part. Geolog. page 29) I +may remark that, judging from the size and well-rounded condition of the +blocks of rock in the above-described conglomerates, masses of primary +formation probably existed at this tertiary period above water: there is, +also, according to M. Isabelle, much conglomerate further north, at Salto. + +From whatever source and through whatever means the great Pampean formation +originated, we here have, I must repeat, unequivocal evidence of a similar +action at a period before that of the deposition of the marine tertiary +strata with extinct shells, at Santa Fe and P. Gorda. During also the +deposition of these strata, we have in the intercalated layers of red +Pampean-like mud and tosca-rock, and in the passage near S. Juan of the +semi-crystalline limestones with agate into tosca undistinguishable from +that of the Pampas, evidence of the same action, though continued only at +intervals and in a feeble manner. We have further seen that in this +district, at a period not only subsequent to the deposition of the tertiary +strata, but to their upheavement and most extensive denudation, true +Pampean mud with its usual characters and including mammiferous remains, +was deposited round and between the hills or islets formed of these +tertiary strata, and over the whole eastern and low primary districts of +Banda Oriental. + +EARTHY MASS, WITH EXTINCT MAMMIFEROUS REMAINS, OVER THE PORPHYRITIC GRAVEL +AT S. JULIAN, LATITUDE 49 DEGREES 14' S., IN PATAGONIA. + +(FIGURE 16. SECTION OF THE LOWEST PLAIN AT PORT S. JULIAN. + +(Section through beds from top to bottom: A, B, C, D, E, F.) + +AA. Superficial bed of reddish earth, with the remains of the Macrauchenia, +and with recent sea-shells on the surface. + +B. Gravel of porphyritic rocks. + +C. and D. Pumiceous mudstone.--Ancient tertiary formation. + +E. and F. Sandstone and argillaceous beds.--Ancient tertiary formation.) + +This case, though not coming strictly under the Pampean formation, may be +conveniently given here. On the south side of the harbour, there is a +nearly level plain (mentioned in the First Chapter) about seven miles long, +and three or four miles wide, estimated at ninety feet in height, and +bordered by perpendicular cliffs, of which a section is represented in +Figure 16. + +The lower old tertiary strata (to be described in the next chapter) are +covered by the usual gravel bed; and this by an irregular earthy, sometimes +sandy mass, seldom more than two or three feet in thickness, except where +it fills up furrows or gullies worn not only through the underlying gravel, +but even through the upper tertiary beds. This earthy mass is of a pale +reddish colour, like the less pure varieties of Pampean mud in Banda +Oriental; it includes small calcareous concretions, like those of tosca- +rock but more arenaceous, and other concretions of a greenish, indurated +argillaceous substance: a few pebbles, also, from the underlying gravel-bed +are also included in it, and these being occasionally arranged in +horizontal lines, show that the mass is of sub-aqueous origin. On the +surface and embedded in the superficial parts, there are numerous shells, +partially retaining their colours, of three or four of the now commonest +littoral species. Near the bottom of one deep furrow (represented in Figure +16), filled up with this earthy deposit, I found a large part of the +skeleton of the Macrauchenia Patachonica--a gigantic and most extraordinary +pachyderm, allied, according to Professor Owen, to the Palaeotherium, but +with affinities to the Ruminants, especially to the American division of +the Camelidae. Several of the vertebrae in a chain, and nearly all the +bones of one of the limbs, even to the smallest bones of the foot, were +embedded in their proper relative positions: hence the skeleton was +certainly united by its flesh or ligaments, when enveloped in the mud. This +earthy mass, with its concretions and mammiferous remains, filling up +furrows in the underlying gravel, certainly presents a very striking +resemblance to some of the sections (for instance, at P. Alta in B. Blanca, +or at the Barrancas de S. Gregorio) in the Pampean formation; but I must +believe that this resemblance is only accidental. I suspect that the mud +which at the present day is accumulating in deep and narrow gullies at the +head of the harbour, would, after elevation, present a very similar +appearance. The southernmost part of the true Pampean formation, namely, on +the Colorado, lies 560 miles of latitude north of this point. (In the +succeeding chapter I shall have to refer to a great deposit of extinct +mammiferous remains, lately discovered by Captain Sulivan, R.N., at a point +still further south, namely, at the R. Gallegos; their age must at present +remain doubtful.) + +With respect to the age of the Macrauchenia, the shells on the surface +prove that the mass in which the skeleton was enveloped has been elevated +above the sea within the recent period: I did not see any of the shells +embedded at a sufficient depth to assure me (though it be highly probable) +that the whole thickness of the mass was contemporaneous with these +INDIVIDUAL SPECIMENS. That the Macrauchenia lived subsequently to the +spreading out of the gravel on this plain is certain; and that this gravel, +at the height of ninety feet, was spread out long after the existence of +recent shells, is scarcely less certain. For, it was shown in the First +Chapter, that this line of coast has been upheaved with remarkable +equability, and that over a vast space both north and south of S. Julian, +recent species of shells are strewed on (or embedded in) the surface of the +250 feet plain, and of the 350 feet plain up to a height of 400 feet. These +wide step-formed plains have been formed by the denuding action of the +coast-waves on the old tertiary strata; and therefore, when the surface of +the 350 feet plain, with the shells on it, first rose above the level of +the sea, the 250 feet plain did not exist, and its formation, as well as +the spreading out of the gravel on its summit, must have taken place +subsequently. So also the denudation and the gravel-covering of the 90 feet +plain must have taken place subsequently to the elevation of the 250 feet +plain, on which recent shells are also strewed. Hence there cannot be any +doubt that the Macrauchenia, which certainly was entombed in a fresh state, +and which must have been alive after the spreading out of the gravel on the +90 feet plain, existed, not only subsequently to the upraised shells on the +surface of the 250 feet plain, but also to those on the 350 to 400 feet +plain: these shells, eight in number (namely, three species of Mytilus, two +of Patella, one Fusus, Voluta, and Balanus), are undoubtedly recent +species, and are the commonest kinds now living on this coast. At Punta +Alta in B. Blanca, I remarked how marvellous it was, that the Toxodon, a +mammifer so unlike to all known genera, should have co-existed with twenty- +three still living marine animals; and now we find that the Macrauchenia, a +quadruped only a little less anomalous than the Toxodon, also co-existed +with eight other still existing Mollusca: it should, moreover, be borne in +mind, that a tooth of a pachydermatous animal was found with the other +remains at Punta Alta, which Professor Owen thinks almost certainly +belonged to the Macrauchenia. + +Mr. Lyell has arrived at a highly important conclusion with respect to the +age of the North American extinct mammifers (many of which are closely +allied to, and even identical with, those of the Pampean formation), +namely, that they lived subsequently to the period when erratic boulders +were transported by the agency of floating ice in temperate latitudes. +("Geological Proceedings" volume 4 page 36.) Now in the valley of the Santa +Cruz, only fifty miles of latitude south of the spot where the Macrauchenia +was entombed, vast numbers of gigantic, angular boulders, which must have +been transported from the Cordillera on icebergs, lie strewed on the plain, +at the height of 1,400 feet above the level of the sea. In ascending to +this level, several step-formed plains must be crossed, all of which have +necessarily required long time for their formation; hence the lowest or +ninety feet plain, with its superficial bed containing the remains of the +Macrauchenia, must have been formed very long subsequently to the period +when the 1,400 feet plain was beneath the sea, and boulders were dropped on +it from floating masses of ice. (It must not be inferred from these +remarks, that the ice-action ceased in South America at this comparatively +ancient period; for in Tierra del Fuego boulders were probably transported +contemporaneously with, if not subsequently to, the formation of the ninety +feet plain at S. Julian, and at other parts of the coast of Patagonia.) Mr. +Lyell's conclusion, therefore, is thus far confirmed in the southern +hemisphere; and it is the more important, as one is naturally tempted to +admit so simple an explanation, that it was the ice-period that caused the +extinction of the numerous great mammifers which so lately swarmed over the +two Americas. + +SUMMARY AND CONCLUDING REMARKS ON THE PAMPEAN FORMATION. + +One of its most striking features is its great extent; I passed +continuously over it from the Colorado to St. Fe Bajada, a distance of 500 +geographical miles; and M. d'Orbigny traced it for 250 miles further north. +In the latitude of the Plata, I examined this formation at intervals over +an east and west line of 300 miles from Maldonado to the R. Carcarana; and +M. d'Orbigny believes it extends 100 miles further inland: from Mr. +Caldcleugh's travels, however, I should have thought that it had extended, +south of the Cordovese range, to near Mendoza, and I may add that I heard +of great bones having been found high up the R. Quinto. Hence the area of +the Pampean formation, as remarked by M. d'Orbigny, is probably at least +equal to that of France, and perhaps twice or thrice as great. In a basin, +surrounded by gravel-cliff (at a height of nearly three thousand feet), +south of Mendoza, there is, as described in the Third Chapter, a deposit +very like the Pampean, interstratified with other matter; and again at S. +Julian's, in Patagonia, 560 miles south of the Colorado, a small irregular +bed of a nearly similar nature contains, as we have just seen, mammiferous +remains. In the provinces of Moxos and Chiquitos (1,000 miles northward of +the Pampas), and in Bolivia, at a height of 4,000 metres, M. d'Orbigny has +described similar deposits, which he believes to have been formed by the +same agency contemporaneously with the Pampean formation. Considering the +immense distances between these several points, and their different +heights, it appears to me infinitely more probable, that this similarity +has resulted not from contemporaneousness of origin, but from the +similarity of the rocky framework of the continent: it is known that in +Brazil an immense area consists of gneissic rocks, and we shall hereafter +see, over how great a length the plutonic rocks of the Cordillera, the +overlying purple porphyries, and the trachytic ejections, are almost +identical in nature. + +Three theories on the origin of the Pampean formation have been +propounded:--First, that of a great debacle by M. d'Orbigny; this seems +founded chiefly on the absence of stratification, and on the number of +embedded remains of terrestrial quadrupeds. Although the Pampean formation +(like so many argillaceous deposits) is not divided into distinct and +separate strata, yet we have seen that in one good section it was striped +with horizontal zones of colour, and that in several specified places the +upper and lower parts differed, not only considerably in colour, but +greatly in constitution. In the southern part of the Pampas the upper mass +(to a certain extent stratified) generally consists of hard tosca-rock, and +the lower part of red Pampean mud, often itself divided into two or more +masses, varying in colour and in the quantity of included calcareous +matter. In Western Banda Oriental, beds of a similar nature, but of a +greater age, conformably underlie and are intercalated with the regularly +stratified tertiary formation. As a general rule, the marly concretions are +arranged in horizontal lines, sometimes united into irregular strata: +surely, if the mud had been tumultuously deposited in mass, the included +calcareous matter would have segregated itself irregularly, and not into +nodules arranged in horizontal lines, one above the other and often far +apart: this arrangement appears to me to prove that mud, differing slightly +in composition, was successively and quietly deposited. On the theory of a +debacle, a prodigious amount of mud, without a single pebble, is supposed +to have been borne over the wide surface of the Pampas, when under water: +on the other hand, over the whole of Patagonia, the same or another debacle +is supposed to have borne nothing but gravel,--the gravel and the fine mud +in the neighbourhood of the Rios Negro and Colorado having been borne to an +equal distance from the Cordillera, or imagined line of disturbance: +assuredly directly opposite effects ought not to be attributed to the same +agency. Where, again, could a mass of fine sediment, charged with +calcareous matter in a fit state for chemical segregation, and in quantity +sufficient to cover an area at least 750 miles long, and 400 miles broad, +to a depth of from twenty to thirty feet to a hundred feet, have been +accumulated, ready to be transported by the supposed debacle? To my mind it +is little short of demonstration, that a great lapse of time was necessary +for the production and deposition of the enormous amount of mudlike matter +forming the Pampas; nor should I have noticed the theory of a debacle, had +it not been adduced by a naturalist so eminent as M. d'Orbigny. + +A second theory, first suggested, I believe, by Sir W. Parish, is that the +Pampean formation was thrown down on low and marshy plains by the rivers of +this country before they assumed their present courses. The appearance and +composition of the deposit, the manner in which it slopes up and round the +primary ranges, the nature of the underlying marine beds, the estuary and +sea-shells on the surface, the overlying sandstone beds at M. Hermoso, are +all quite opposed to this view. Nor do I believe that there is a single +instance of a skeleton of one of the extinct mammifers having been found in +an upright position, as if it had been mired. + +The third theory, of the truth of which I cannot entertain the smallest +doubt, is that the Pampean formation was slowly accumulated at the mouth of +the former estuary of the Plata and in the sea adjoining it. I have come to +this conclusion from the reasons assigned against the two foregoing +theories, and from simple geographical considerations. From the numerous +shells of the Azara labiata lying loose on the surface of the plains, and +near Buenos Ayres embedded in the tosca-rock, we know that this formation +not only was formerly covered by, but that the uppermost parts were +deposited in, the brackish water of the ancient La Plata. Southward and +seaward of Buenos Ayres, the plains were upheaved from under water +inhabited by true marine shells. We further know from Professor Ehrenberg's +examination of the twenty microscopical organisms in the mud round the +tooth of the Mastodon high up the course of the Parana, that the bottom- +most part of this formation was of brackish-water origin. A similar +conclusion must be extended to the beds of like composition, at the level +of the sea and under it, at M. Hermoso in Bahia Blanca. Dr. Carpenter finds +that the harder varieties of tosca-rock, collected chiefly to the south, +contain marine spongoid bodies, minute fragments of shells, corals, and +Polythalamia; these perhaps may have been drifted inwards by the tides, +from the more open parts of the sea. The absence of shells, throughout this +deposit, with the exception of the uppermost layers near Buenos Ayres, is a +remarkable fact: can it be explained by the brackish condition of the +water, or by the deep mud at the bottom? I have stated that both the +reddish mud and the concretions of tosca-rock are often penetrated by +minute, linear cavities, such as frequently may be observed in fresh-water +calcareous deposits:--were they produced by the burrowing of small worms? +Only on this view of the Pampean formation having been of estuary origin, +can the extraordinary numbers (presently to be alluded to) of the embedded +mammiferous remains be explained. (It is almost superfluous to give the +numerous cases (for instance, in Sumatra; Lyell "Principles" volume 3 page +325 sixth edition, of the carcasses of animals having been washed out to +sea by swollen rivers; but I may refer to a recent account by Mr. +Bettington "Asiatic Society" 1845 June 21st, of oxen, deer, and bears being +carried into the Gulf of Cambray; see also the account in my "Journal" 2nd +edition page 133, of the numbers of animals drowned in the Plata during the +great, often recurrent, droughts.) + +With respect to the first origin of the reddish mud, I will only remark, +that the enormous area of Brazil consists in chief part of gneissic and +other granitic rocks, which have suffered decomposition, and been converted +into a red, gritty, argillaceous mass, to a greater depth than in any other +country which I have seen. The mixture of rounded grains, and even of small +fragments and pebbles of quartz, in the Pampean mud of Banda Oriental, is +evidently due to the neighbouring and underlying primary rocks. The estuary +mud was drifted during the Pampean period in a much more southerly course, +owing probably to the east and west primary ridges south of the Plata not +having been then elevated, than the mud of the Plata at present is; for it +was formerly deposited as far south as the Colorado. The quantity of +calcareous matter in this formation, especially in those large districts +where the whole mass passes into tosca-rock, is very great: I have already +remarked on the close resemblance in external and microscopical appearance, +between this tosca-rock and the strata at Coquimbo, which have certainly +resulted from the decay and attrition of recent shells: I dare not, +however, extend this conclusion to the calcareous rocks of the Pampas, more +especially as the underlying tertiary strata in western Banda Oriental show +that at that period there was a copious emission of carbonate of lime, in +connection with volcanic action. (I may add, that there are nearly similar +superficial calcareous beds at King George's Sound in Australia; and these +undoubtedly have been formed by the disintegration of marine remains see +"Volcanic Islands" etc. page 144. There is, however, something very +remarkable in the frequency of superficial, thin beds of earthy calcareous +matter, in districts where the surrounding rocks are not calcareous. Major +Charters, in a Paper read before the Geographical Society April 13, 1840 +and abstracted in the "Athenaeum" page 317, states that this is the case in +parts of Mexico, and that he has observed similar appearances in many parts +of South Africa. The circumstance of the uppermost stratum round the ragged +Sierra Ventana, consisting of calcareous or marly matter, without any +covering of alluvial matter, strikes me as very singular, in whatever +manner we view the deposition and elevation of the Pampean formation.) + +The Pampean formation, judging from its similar composition, and from the +apparent absolute specific identity of some of its mammiferous remains, and +from the generic resemblance of others, belongs over its vast area-- +throughout Banda Oriental, Entre Rios, and the wide extent of the Pampas as +far south as the Colorado,--to the same geological epoch. The mammiferous +remains occur at all depths from the top to the bottom of the deposit; and +I may add that nowhere in the Pampas is there any appearance of much +superficial denudation: some bones which I found near the Guardia del Monte +were embedded close to the surface; and this appears to have been the case +with many of those discovered in Banda Oriental: on the Matanzas, twenty +miles south of Buenos Ayres, a Glyptodon was embedded five feet beneath the +surface; numerous remains were found by S. Muniz, near Luxan, at an average +depth of eighteen feet; in Buenos Ayres a skeleton was disinterred at sixty +feet depth, and on the Parana I have described two skeletons of the +Mastodon only five or six feet above the very base of the deposit. With +respect to the age of this formation, as judged of by the ordinary standard +of the existence of Mollusca, the only evidence within the limits of the +true Pampas which is at all trustworthy, is afforded by the still living +Azara labiata being embedded in tosca-rock near Buenos Ayres. At Punta +Alta, however, we have seen that several of the extinct mammifers, most +characteristic of the Pampean formation, co-existed with twenty species of +Mollusca, a barnacle and two corals, all still living on this same coast;-- +for when we remember that the shells have a more ancient appearance than +the bones; that many of the bones, though embedded in a coarse +conglomerate, are perfectly preserved; that almost all the parts of the +skeleton of the Scelidotherium, even to the knee-cap, were lying in their +proper relative positions; and that a large piece of the fragile dermal +armour of a Dasypoid quadruped, connected with some of the bones of the +foot, had been entombed in a condition allowing the two sides to be doubled +together, it must assuredly be admitted that these mammiferous remains were +embedded in a fresh state, and therefore that the living animals co-existed +with the co-embedded shells. Moreover, the Macrauchenia Patachonica (of +which, according to Professor Owen, remains also occur in the Pampas of +Buenos Ayres, and at Punta Alta) has been shown by satisfactory evidence of +another kind, to have lived on the plains of Patagonia long after the +period when the adjoining sea was first tenanted by its present commonest +molluscous animals. We must, therefore, conclude that the Pampean formation +belongs, in the ordinary geological sense of the word, to the Recent +Period. (M. d'Orbigny believes "Voyage" Part. Geolog. page 81, that this +formation, though "tres voisine de la notre, est neanmoins de beaucoup +anterieure a notre creation.") + +At St. Fe Bajada, the Pampean estuary formation, with its mammiferous +remains, conformably overlies the marine tertiary strata, which (as first +shown by M. d'Orbigny) are contemporaneous with those of Patagonia, and +which, as we shall hereafter see, belong to a very ancient tertiary stage. +When examining the junction between these two formations, I thought that +the concretionary layer of marl marked a passage between the marine and +estuary stages. M. d'Orbigny disputes this view (as given in my "Journal"), +and I admit that it is erroneous, though in some degree excusable, from +their conformability and from both abounding with calcareous matter. It +would, indeed, have been a great anomaly if there had been a true passage +between a deposit contemporaneous with existing species of mollusca, and +one in which all the mollusca appear to be extinct. Northward of Santa Fe, +M. d'Orbigny met with ferruginous sandstones, marly rocks, and other beds, +which he considers as a distinct and lower formation; but the evidence that +they are not parts of the same with an altered mineralogical character, +does not appear to me quite satisfactory. + +In Western Banda Oriental, while the marine tertiary strata were +accumulating, there were volcanic eruptions, much silex and lime were +precipitated from solution, coarse conglomerates were formed, being derived +probably from adjoining land, and layers of red mud and marly rocks, like +those of the Pampean formation, were occasionally deposited. The true +Pampean deposit, with mammiferous remains, instead of as at Santa Fe +overlying conformably the tertiary strata, is here seen at a lower level +folding round and between the flat-topped, cliff-bounded hills, formed by a +upheaval and denudation of these same tertiary strata. The upheaval, having +occurred here earlier than at Santa Fe, may be naturally accounted for by +the contemporaneous volcanic action. At the Barrancas de S. Gregorio, the +Pampean deposit, as we have seen, overlies and fills up furrows in coarse +sand, precisely like that now accumulating on the shores near the mouth of +the Plata. I can hardly believe that this loose and coarse sand is +contemporaneous with the old tertiary and often crystalline strata of the +more western parts of the province; and am induced to suspect that it is of +subsequent origin. If that section near Colonia could be implicitly +trusted, in which, at a height of only fifteen feet above the Plata, a bed +of fresh-looking mussels, of an existing littoral species, appeared to lie +between the sand and the Pampean mud, I should conclude that Banda Oriental +must have stood, when the coarse sand was accumulating, at only a little +below its present level, and had then subsided, allowing the estuary +Pampean mud to cover far and wide its surface up to a height of some +hundred feet; and that after this subsidence the province had been uplifted +to its present level. + +In Western Banda Oriental, we know, from two unequivocal sections that +there is a mass, absolutely undistinguishable from the true Pampean +deposit, beneath the old tertiary strata. This inferior mass must be very +much more ancient than the upper deposit with its mammiferous remains, for +it lies beneath the tertiary strata in which all the shells are extinct. +Nevertheless, the lower and upper masses, as well as some intermediate +layers, are so similar in mineralogical character, that I cannot doubt that +they are all of estuary origin, and have been derived from the same great +source. At first it appeared to me extremely improbable, that mud of the +same nature should have been deposited on nearly the same spot, during an +immense lapse of time, namely, from a period equivalent perhaps to the +Eocene of Europe to that of the Pampean formation. But as, at the very +commencement of the Pampean period, if not at a still earlier period, the +Sierra Ventana formed a boundary to the south,--the Cordillera or the +plains in front of them to the west,--the whole province of Corrientes +probably to the north, for, according to M. d'Orbigny, it is not covered by +the Pampean deposit,--and Brazil, as known by the remains in the caves, to +the north-east; and as again, during the older tertiary period, land +already existed in Western Banda Oriental and near St. Fe Bajada, as may be +inferred from the vegetable debris, from the quantities of silicified wood, +and from the remains of a Toxodon found, according to M. d'Orbigny, in +still lower strata, we may conclude, that at this ancient period a great +expanse of water was surrounded by the same rocky framework which now +bounds the plains of Pampean formation. This having been the case, the +circumstance of sediment of the same nature having been deposited in the +same area during an immense lapse of time, though highly remarkable, does +not appear incredible. + +The elevation of the Pampas, at least of the southern parts, has been slow +and interrupted by several periods of rest, as may be inferred from the +plains, cliffs, and lines of sand-dunes (with shells and pumice-pebbles) +standing at different heights. I believe, also, that the Pampean mud +continued to be deposited, after parts of this formation had already been +elevated, in the same manner as mud would continue to be deposited in the +estuary of the Plata, if the mud-banks on its shores were now uplifted and +changed into plains: I believe in this from the improbability of so many +skeletons and bones having been accumulated at one spot, where M. Hermoso +now stands, at a depth of between eight hundred and one thousand feet, and +at a vast distance from any land except small rocky islets,--as must have +been the case, if the high tosca-plain round the Ventana and adjoining +Sierras, had not been already uplifted and converted into land, supporting +mammiferous animals. At Punta Alta we have good evidence that the gravel- +strata, which certainly belong to the true Pampean period, were accumulated +after the elevation in that neighbourhood of the main part of the Pampean +deposit, whence the rounded masses of tosca-rock were derived, and that +rolled fragment of black bone in the same peculiar condition with the +remains at Monte Hermoso. + +The number of the mammiferous remains embedded in the Pampas is, as I have +remarked, wonderful: it should be borne in mind that they have almost +exclusively been found in the cliffs and steep banks of rivers, and that, +until lately, they excited no attention amongst the inhabitants: I am +firmly convinced that a deep trench could not be cut in any line across the +Pampas, without intersecting the remains of some quadruped. It is difficult +to form an opinion in what part of the Pampas they are most numerous; in a +limited spot they could not well have been more numerous than they were at +P. Alta; the number, however, lately found by Senor F. Muniz, near Luxan, +in a central spot in the Pampas, is extraordinarily great: at the end of +this chapter I will give a list of all the localities at which I have heard +of remains having been discovered. Very frequently the remains consist of +almost perfect skeletons; but there are, also, numerous single bones, as +for instance at St. Fe. Their state of preservation varies much, even when +embedded near each other: I saw none others so perfectly preserved as the +heads of the Toxodon and Mylodon from the white soft earthy bed on the +Sarandis in Banda Oriental. It is remarkable that in two limited sections I +found no less than five teeth separately embedded, and I heard of teeth +having been similarly found in other parts: may we suppose that the +skeletons or heads were for a long time gently drifted by currents over the +soft muddy bottom, and that the teeth occasionally, here and there, dropped +out? + +It may be naturally asked, where did these numerous animals live? From the +remarkable discoveries of MM. Lund and Clausen, it appears that some of the +species found in the Pampas inhabited the highlands of Brazil: the Mastodon +Andium is embedded at great heights in the Cordillera from north of the +equator to at least as far south as Tarija (Humboldt states that the +Mastodon has been discovered in New Granada: it has been found in Quito. +When at Lima, I saw a tooth of a Mastodon in the possession of Don M. +Rivero, found at Playa Chica on the Maranon, near the Guallaga. Every one +has heard of the numerous remains of Mastodon in Bolivia.); and as there is +no higher land, there can be little doubt that this Mastodon must have +lived on the plains and valleys of that great range. These countries, +however, appear too far distant for the habitation of the individuals +entombed in the Pampas: we must probably look to nearer points, for +instance to the province of Corrientes, which, as already remarked, is said +not to be covered by the Pampean formation, and may therefore, at the +period of its deposition, have existed as dry land. I have already given my +reasons for believing that the animals embedded at M. Hermoso and at P. +Alta in Bahia Blanca, lived on adjoining land, formed of parts of the +already elevated Pampean deposit. With respect to the food of these many +great extinct quadrupeds, I will not repeat the facts given in my "Journal" +(second edition page 85), showing that there is no correlation between the +luxuriance of the vegetation of a country and the size of its mammiferous +inhabitants. I do not doubt that large animals could now exist, as far as +the amount, not kind, of vegetation is concerned, on the sterile plains of +Bahia Blanca and of the R. Negro, as well as on the equally, if not more +sterile plains of Southern Africa. The climate, however, may perhaps have +somewhat deteriorated since the mammifers embedded at Bahia Blanca lived +there; for we must not infer, from the continued existence of the same +shells on the present coasts, that there has been no change in climate; for +several of these shells now range northward along the shores of Brazil, +where the most luxuriant vegetation flourishes under a tropical +temperature. With respect to the extinction, which at first fills the mind +with astonishment, of the many great and small mammifers of this period, I +may also refer to the work above cited (second edition page 173), in which +I have endeavoured to show, that however unable we may be to explain the +precise cause, we ought not properly to feel more surprised at a species +becoming extinct than at one being rare; and yet we are accustomed to view +the rarity of any particular species as an ordinary event, not requiring +any extraordinary agency. + +The several mammifers embedded in the Pampean formation, which mostly +belong to extinct genera, and some even to extinct families or orders, and +which differ nearly, if not quite, as much as do the Eocene mammifers of +Europe from living quadrupeds having existed contemporaneously with +mollusca, all still inhabiting the adjoining sea, is certainly a most +striking fact. It is, however, far from being an isolated one; for, during +the late tertiary deposits of Britain, an elephant, rhinoceros, and +hippopotamus co-existed with many recent land and fresh-water shells; and +in North America, we have the best evidence that a mastodon, elephant, +megatherium, megalonyx, mylodon, an extinct horse and ox, likewise co- +existed with numerous land, fresh-water, and marine recent shells. (Many +original observations, and a summary on this subject, are given in Mr. +Lyell's paper in the "Geological Proceedings" volume 4 page 3 and in his +"Travels in North America" volume 1 page 164 and volume 2 page 60. For the +European analogous cases see Mr. Lyell's "Principles of Geology" 6th +edition volume 1 page 37.) The enumeration of these extinct North American +animals naturally leads me to refer to the former closer relation of the +mammiferous inhabitants of the two Americas, which I have discussed in my +"Journal," and likewise to the vast extent of country over which some of +them ranged: thus the same species of the Megatherium, Megalonyx, Equus (as +far as the state of their remains permits of identification), extended from +the Southern United States of North America to Bahia Blanca, in latitude 39 +degrees S., on the coast of Patagonia. The fact of these animals having +inhabited tropical and temperate regions, does not appear to me any great +difficulty, seeing that at the Cape of Good Hope several quadrupeds, such +as the elephant and hippopotamus, range from the equator to latitude 35 +degrees south. The case of the Mastodon Andium is one of more difficulty, +for it is found from latitude 36 degrees S., over, as I have reason to +believe, nearly the whole of Brazil, and up the Cordillera to regions +which, according to M. d'Orbigny, border on perpetual snow, and which are +almost destitute of vegetation: undoubtedly the climate of the Cordillera +must have been different when the mastodon inhabited it; but we should not +forget the case of the Siberian mammoth and rhinoceros, as showing how +severe a climate the larger pachydermata can endure; nor overlook the fact +of the guanaco ranging at the present day over the hot low deserts of Peru, +the lofty pinnacles of the Cordillera, and the damp forest-clad land of +Southern Tierra del Fuego; the puma, also, is found from the equator to the +Strait of Magellan, and I have seen its footsteps only a little below the +limits of perpetual snow in the Cordillera of Chile. + +At the period, so recent in a geological sense, when these extinct +mammifers existed, the two Americas must have swarmed with quadrupeds, many +of them of gigantic size; for, besides those more particularly referred to +in this chapter, we must include in this same period those wonderfully +numerous remains, some few of them specifically, and others generically +related to those of the Pampas, discovered by MM. Lund and Clausen in the +caves of Brazil. Finally, the facts here given show how cautious we ought +to be in judging of the antiquity of a formation from even a great amount +of difference between the extinct and living species in any one class of +animals;--we ought even to be cautious in accepting the general +proposition, that change in organic forms and lapse of time are at all, +necessarily, correlatives. + +... + +LOCALITIES WITHIN THE REGION OF THE PAMPAS WHERE GREAT BONES HAVE BEEN +FOUND. + +The following list, which includes every account which I have hitherto met +with of the discovery of fossil mammiferous remains in the Pampas, may be +hereafter useful to a geologist investigating this region, and it tends to +show their extraordinary abundance. I heard of and saw many fossils, the +original position of which I could not ascertain; and I received many +statements too vague to be here inserted. Beginning to the south:--we have +the two stations in Bahia Blanca, described in this chapter, where at P. +Alta, the Megatherium, Megalonyx, Scelidotherium, Mylodon, Holophractus (or +an allied genus), Toxodon, Macrauchenia, and an Equus were collected; and +at M. Hermoso a Ctenomys, Hydrochaerus, some other rodents and the bones of +a great megatheroid quadruped. Close north-east of the S. Tapalguen, we +have the Rios 'Huesos' (i.e. "bones"), which probably takes its name from +large fossil bones. Near Villa Nuevo, and at Las Averias, not far from the +Salado, three nearly perfect skeletons, one of the Megatherium, one of the +Glyptodon clavipes, and one of some great Dasypoid quadruped, were found by +the agent of Sir W. Parish (see his work "Buenos Ayres" etc. page 171). I +have seen the tooth of a Mastodon from the Salado; a little northward of +this river, on the borders of a lake near the G. del Monte, I saw many +bones, and one large piece of dermal armour; higher up the Salado, there is +a place called Monte "Huesos." On the Matanzas, about twenty miles south of +Buenos Ayres, the skeleton (vide page 178 of "Buenos Ayres" etc. by Sir W. +Parish) of a Glyptodon was found about five feet beneath the surface; here +also (see Catalogue of Royal College of Surgeons) remains of Glyptodon +clavipes, G. ornatus, and G. reticulatus were found. Signor Angelis, in a +letter which I have seen, refers to some great remains found in Buenos +Ayres, at a depth of twenty varas from the surface. Seven leagues north of +this city the same author found the skeletons of Mylodon robustus and +Glyptodon ornatus. From this neighbourhood he has lately sent to the +British Museum the following fossils:--Remains of three or four individuals +of Megatherium; of three species of Glyptodon; of three individuals of the +Mastodon Andium; of Macrauchenia; of a second species of Toxodon, different +from T. Platensis; and lastly, of the Machairodus, a wonderful large +carnivorous animal. M. d'Orbigny has lately received from the Recolate +"Voyage" Pal. page 144), near Buenos Ayres, a tooth of Toxodon Platensis. + +Proceeding northward, along the west bank of the Parana, we come to the Rio +Luxan, where two skeletons of the Megatherium have been found; and lately, +within eight leagues of the town of Luxan, Dr. F. X. Muniz has collected +("British Packet" Buenos Ayres September 25, 1841), from an average depth +of eighteen feet, very numerous remains, of no less than, as he believes, +nine distinct species of mammifers. At Areco, large bones have been found, +which are believed, by the inhabitants, to have been changed from small +bones, by the water of the river! At Arrecifes, the Glyptodon, sent to the +College of Surgeons, was found; and I have seen two teeth of a Mastodon +from this quarter. At S. Nicolas, M. d'Orbigny found remains of a Canis, +Ctenomys, and Kerodon; and M. Isabelle ("Voyage" page 332) refers to a +gigantic Armadillo found there. At S. Carlos, I heard of great bones. A +little below the mouth of the Carcarana, the two skeletons of Mastodon were +found; on the banks of this river, near S. Miguel, I found teeth of the +Mastodon and Toxodon; and "Falkner" (page 55) describes the osseous armour +of some great animal; I heard of many other bones in this neighbourhood. I +have seen, I may add, in the possession of Mr. Caldcleugh, the tooth of a +Mastodon Andium, said to have been found in Paraguay; I may here also refer +to a statement in this gentleman's travels (volume 1 page 48), of a great +skeleton having been found in the province of Bolivia in Brazil, on the R. +de las Contas. The furthest point westward in the Pampas, at which I have +HEARD of fossil bones, was high up on the banks of R. Quinto. + +In Entre Rios, besides the remains of the Mastodon, Toxodon, Equus, and a +great Dasypoid quadruped near St. Fe Bajada, I received an account of bones +having been found a little S.E. of P. Gorda (on the Parana), and of an +entire skeleton at Matanzas, on the Arroyo del Animal. + +In Banda Oriental, besides the remains of the Toxodon, Mylodon, and two +skeletons of great animals with osseous armour (distinct from that of the +Glyptodon), found on the Arroyos Sarandis and Berquelo, M. Isabelle +("Voyage" page 322) says, many bones have been found near the R. Negro, and +on the R. Arapey, an affluent of the Paraguay, in latitude 30 degrees 40 +minutes south. I heard of bones near the source of the A. Vivoras. I saw +the remains of a Dasypoid quadruped from the Arroyo Seco, close to M. +Video; and M. d'Orbigny refers ("Voyage" Geolog. page 24), to another found +on the Pedernal, an affluent of the St. Lucia; and Signor Angelis, in a +letter, states that a third skeleton of this family has been found, near +Canelones. I saw a tooth of the Mastodon from Talas, another affluent of +the St. Lucia. The most eastern point at which I heard of great bones +having been found, was at Solis Grande, between M. Video and Maldonado. + + +CHAPTER V. ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE. + +Rio Negro. +S. Josef. +Port Desire, white pumiceous mudstone with Infusoria. +Port S. Julian. +Santa Cruz, basaltic lava of. +P. Gallegos. +Eastern Tierra del Fuego; leaves of extinct beech-trees. +Summary on the Patagonian tertiary formations. +Tertiary formations of the Western Coast. +Chonos and Chiloe groups, volcanic rocks of. +Concepcion. +Navidad. +Coquimbo. +Summary. +Age of the tertiary formations. +Lines of elevation. +Silicified wood. +Comparative ranges of the extinct and living mollusca on the West Coast of +S. America. +Climate of the tertiary period. +On the causes of the absence of recent conchiferous deposits on the coast +of S. America. +On the contemporaneous deposition and preservation of sedimentary +formations. + +RIO NEGRO. + +I can add little to the details given by M. d'Orbigny on the sandstone +formation of this district. ("Voyage" Part Geolog. pages 57-65.) The cliffs +to the south of the river are about two hundred feet in height, and are +composed of sandstone of various tints and degrees of hardness. One layer, +which thinned out at both ends, consisted of earthy matter, of a pale +reddish colour, with some gypsum, and very like (I speak after comparison +of the specimens brought home) Pampean mud: above this was a layer of +compact marly rock with dendritic manganese. Many blocks of a conglomerate +of pumice-pebbles embedded in hard sandstone were strewed at the foot of +the cliff, and had evidently fallen from above. A few miles N.E. of the +town, I found, low down in the sandstone, a bed, a few inches in thickness, +of a white, friable, harsh-feeling sediment, which adheres to the tongue, +is of easy fusibility, and of little specific gravity; examined under the +microscope, it is seen to be pumiceous tuff, formed of broken transparent +crystals. In the cliffs south of the river, there is, also, a thin layer of +nearly similar nature, but finer grained, and not so white; it might easily +have been mistaken for a calcareous tuff, but it contains no lime: this +substance precisely resembles a most widely extended and thick formation in +Southern Patagonia, hereafter to be described, and which is remarkable for +being partially formed of infusoria. These beds, conjointly with the +conglomerate of pumice, are interesting, as showing the nature of the +volcanic action in the Cordillera during this old tertiary period. + +In a bed at the base of the southern cliffs, M. d'Orbigny found two extinct +fresh-water shells, namely, a Unio and Chilina. This bed rested on one with +bones of an extinct rodent, namely, the Megamys Patagoniensis; and this +again on another with extinct marine shells. The species found by M. +d'Orbigny in different parts of this formation consist of:-- + +1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe, and whole +coast of Patagonia). +2. Ostrea Ferrarisi, d'Orbigny, "Voyage, Pal." +3. Ostrea Alvarezii, d'Orbigny, "Voyage, Pal." (also at St. Fe, and S. +Josef). +4. Pecten Patagoniensis, d'Orbigny, "Voyage, Pal." +5. Venus Munsterii, d'Orbigny, "Voyage, Pal." (also at St. Fe). +6. Arca Bonplandiana, d'Orbigny, "Voyage, Pal." (also at St. Fe). + +According to M. d'Orbigny, the sandstone extends westward along the coast +as far as Port S. Antonio, and up the R. Negro far into the interior: +northward I traced it to the southern side of the Rio Colorado, where it +forms a low denuded plain. This formation, though contemporaneous with that +of the rest of Patagonia, is quite different in mineralogical composition, +being connected with it only by the one thin white layer: this difference +may be reasonably attributed to the sediment brought down in ancient times +by the Rio Negro; by which agency, also, we can understand the presence of +the fresh-water shells, and of the bones of land animals. Judging from the +identity of four of the above shells, this formation is contemporaneous (as +remarked by M. d'Orbigny) with that under the Pampean deposit in Entre Rios +and in Banda Oriental. The gravel capping the sandstone plain, with its +calcareous cement and nodules of gypsum, is probably, from the reasons +given in the First Chapter, contemporaneous with the uppermost beds of the +Pampean formation on the upper plain north of the Colorado. + +SAN JOSEF. + +My examination here was very short: the cliffs are about a hundred feet +high; the lower third consists of yellowish-brown, soft, slightly +calcareous, muddy sandstone, parts of which when struck emit a fetid smell. +In this bed the great Ostraea Patagonica, often marked with dendritic +manganese and small coral-lines, were extraordinarily numerous. I found +here the following shells:-- + +1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe and whole +coast of Patagonia). +2. Ostrea Alvarezii, d'Orbigny, "Voyage, Pal." (also at St. Fe and R. +Negro). +3. Pecten Paranensis, d'Orbigny, "Voyage, Pal." (also at St. Fe, S. Julian, +and Port Desire). +4. Pecten Darwinianus, d'Orbigny, "Voyage, Pal." (also at St. Fe). +5. Pecten actinodes, G.B. Sowerby. +6. Terebratula Patagonica, G.B. Sowerby (also S. Julian). +7. Casts of a Turritella. + +The four first of these species occur at St. Fe in Entre Rios, and the two +first in the sandstone of the Rio Negro. Above this fossiliferous mass, +there is a stratum of very fine-grained, pale brown mudstone, including +numerous laminae of selenite. All the strata appear horizontal, but when +followed by the eye for a long distance, they are seen to have a small +easterly dip. On the surface we have the porphyritic gravel, and on it sand +with recent shells. + +NUEVO GULF. + +From specimens and notes given me by Lieutenant Stokes, it appears that the +lower bed consists of soft muddy sandstone, like that of S. Josef, with +many imperfect shells, including the Pecten Paranensis, d'Orbigny, casts of +a Turritella and Scutella. On this there are two strata of the pale brown +mudstone, also like that of S. Josef, separated by a darker-coloured, more +argillaceous variety, including the Ostrea Patagonica. Professor Ehrenberg +has examined this mudstone for me: he finds in it three already known +microscopic organisms, enveloped in a fine-grained pumiceous tuff, which I +shall have immediately to describe in detail. Specimens brought to me from +the uppermost bed, north of the Rio Chupat, consist of this same substance, +but of a whiter colour. + +Tertiary strata, such as here described, appear to extend along the whole +coast between Rio Chupat and Port Desire, except where interrupted by the +underlying claystone porphyry, and by some metamorphic rocks; these hard +rocks, I may add, are found at intervals over a space of about five degrees +of latitude, from Point Union to a point between Port S. Julian and S. +Cruz, and will be described in the ensuing chapter. Many gigantic specimens +of the Ostraea Patagonica were collected in the Gulf of St. George. + +PORT DESIRE. + +A good section of the lowest fossiliferous mass, about forty feet in +thickness, resting on claystone porphyry, is exhibited a few miles south of +the harbour. The shells sufficiently perfect to be recognised consist of:-- + +1. Ostrea Patagonica, d'Orbigny, (also at St. Fe, and whole coast of +Patagonia). +2. Pecten Paranensis, d'Orbigny, "Voyage, Pal." (also at St. Fe, S. Josef, +S. Julian). +3. Pecten centralis, G.B. Sowerby (also at S. Julian and S. Cruz). +4. Cucullaea alta, G.B. Sowerby (also at S. Cruz). +5. Nucula ornata, G.B. Sowerby. +6. Turritella Patagonica, G.B. Sowerby. + +The fossiliferous strata, when not denuded, are conformably covered by a +considerable thickness of the fine-grained pumiceous mudstone, divided into +two masses: the lower half is very fine-grained, slightly unctuous, and so +compact as to break with a semi-conchoidal fracture, though yielding to the +nail; it includes laminae of selenite: the upper half precisely resembles +the one layer at the Rio Negro, and with the exception of being whiter, the +upper beds at San Josef and Nuevo Gulf. In neither mass is there any trace +to the naked eye of organic forms. Taking the entire deposit, it is +generally quite white, or yellowish, or feebly tinted with green; it is +either almost friable under the finger, or as hard as chalk; it is of easy +fusibility, of little specific gravity, is not harsh to the touch, adheres +to the tongue, and when breathed on exhales a strong aluminous odour; it +sometimes contains a very little calcareous matter, and traces (besides the +included laminae) of gypsum. Under the microscope, according to Professor +Ehrenberg, it consists of minute, triturated, cellular, glassy fragments of +pumice, with some broken crystals. ("Monatsberichten de konig. Akad. zu +Berlin" vom April 1845.) In the minute glassy fragments, Professor +Ehrenberg recognises organic structures, which have been affected by +volcanic heat: in the specimens from this place, and from Port S. Julian, +he finds sixteen Polygastrica and twelve Phytolitharia. Of these organisms, +seven are new forms, the others being previously known: all are of marine, +and chiefly of oceanic, origin. This deposit to the naked eye resembles the +crust which often appears on weathered surfaces of feldspathic rocks; it +likewise resembles those beds of earthy feldspathic matter, sometimes +interstratified with porphyritic rocks, as is the case in this very +district with the underlying purple claystone porphyry. From examining +specimens under a common microscope, and comparing them with other +specimens undoubtedly of volcanic origin, I had come to the same conclusion +with Professor Ehrenberg, namely, that this great deposit, in its first +origin, is of volcanic nature. + +PORT S. JULIAN. + +(FIGURE 17. SECTION OF THE STRATA EXHIBITED IN THE CLIFFS OF THE NINETY +FEET PLAIN AT PORT S. JULIAN. + +(Section through beds from top to bottom: A, B, C, D, E, F.)) + +On the south side of the harbour, Figure 17 gives the nature of the beds +seen in the cliffs of the ninety feet plain. Beginning at the top:-- + +1st, the earthy mass (AA), including the remains of the Macrauchenia, with +recent shells on the surface. + +Second, the porphyritic shingle (B), which in its lower part is +interstratified (owing, I believe, to redisposition during denudation) with +the white pumiceous mudstone. + +Third, this white mudstone, about twenty feet in thickness, and divided +into two varieties (C and D), both closely resembling the lower, fine- +grained, more unctuous and compact kind at Port Desire; and, as at that +place, including much selenite. + +Fourth, a fossiliferous mass, divided into three main beds, of which the +uppermost is thin, and consists of ferruginous sandstone, with many shells +of the great oyster and Pecten Paranensis; the middle bed (E) is a +yellowish earthy sandstone abounding with Scutellae; and the lowest bed (F) +is an indurated, greenish, sandy clay, including large concretions of +calcareous sandstone, many shells of the great oyster, and in parts almost +made up of fragments of Balanidae. Out of these three beds, I procured the +following twelve species, of which the two first were exceedingly numerous +in individuals, as were the Terebratulae and Turritellae in certain +layers:-- + +1. Ostrea Patagonica, d'Orbigny, "Voyage, Pal." (also at St. Fe, and whole +coast of Patagonia). +2. Pecten Paranensis, d'Orbigny, "Voyage, Pal." (St. Fe, S. Josef, Port +Desire). +3. Pecten centralis, G.B. Sowerby (also at Port Desire and S. Cruz). +4. Pecten geminatus, G.B. Sowerby. +5. Terebratula Patagonica, G.B. Sowerby (also S. Josef). +6. Struthiolaria ornata, G.B. Sowerby (also S. Cruz). +7. Fusus Patagonicus, G.B. Sowerby. +8. Fusus Noachinus, G.B. Sowerby. +9. Scalaria rugulosa, G.B. Sowerby. +10. Turritella ambulacrum, G.B. Sowerby (also S. Cruz). +11. Pyrula, cast of, like P. ventricosa of Sowerby, Tank Cat. +12. Balanus varians, G.B. Sowerby. +13. Scutella, differing from the species from Nuevo Gulf. + +At the head of the inner harbour of Port S. Julian, the fossiliferous mass +is not displayed, and the sea-cliffs from the water's edge to a height of +between one and two hundred feet are formed of the white pumiceous +mudstone, which here includes innumerable, far-extended, sometimes +horizontal, sometimes inclined or vertical laminae of transparent gypsum, +often about an inch in thickness. Further inland, with the exception of the +superficial gravel, the whole thickness of the truncated hills, which +represent a formerly continuous plain 950 feet in height, appears to be +formed of this white mudstone: here and there, however, at various heights, +thin earthy layers, containing the great oyster, Pecten Paranensis and +Turritella ambulacrum, are interstratified; thus showing that the whole +mass belongs to the same epoch. I nowhere found even a fragment of a shell +actually in the white deposit, and only a single cast of a Turritella. Out +of the eighteen microscopic organisms discovered by Ehrenberg in the +specimens from this place, ten are common to the same deposit at Port +Desire. I may add that specimens of this white mudstone, with the same +identical characters were brought me from two points,--one twenty miles +north of S. Julian, where a wide gravel-capped plain, 350 feet in height, +is thus composed; and the other forty miles south of S. Julian, where, on +the old charts, the cliffs are marked as "Chalk Hills." + +SANTA CRUZ. + +The gravel-capped cliffs at the mouth of the river are 355 feet in height: +the lower part, to a thickness of fifty or sixty feet, consists of a more +or less hardened, darkish, muddy, or argillaceous sandstone (like the +lowest bed of Port Desire), containing very many shells, some silicified +and some converted into yellow calcareous spar. The great oyster is here +numerous in layers; the Trigonocelia and Turritella are also very numerous: +it is remarkable that the Pecten Paranensis, so common in all other parts +of the coast, is here absent: the shells consist of:-- + +1. Ostrea Patagonica, d'Orbigny; "Voyage Pal." (also at St. Fe and whole +coast of Patagonia). +2. Pecten centralis, G.B. Sowerby (also P. Desire and S. Julian). +3. Venus meridionalis of G.B. Sowerby. +4. Crassatella Lyellii, G.B. Sowerby. +5. Cardium puelchum, G.B. Sowerby. +6. Cardita Patagonica, G.B. Sowerby. +7. Mactra rugata, G.B. Sowerby. +8. Mactra Darwinii, G.B. Sowerby. +9. Cucullaea alta, G.B. Sowerby (also P. Desire). +10. Trigonocelia insolita, G.B. Sowerby. +11. Nucula (?) glabra, G.B. Sowerby. +12. Crepidula gregaria, G.B. Sowerby. +13. Voluta alta, G.B. Sowerby. +14. Trochus collaris, G.B. Sowerby. +15. Natica solida (?), G.B. Sowerby +16. Struthiolaria ornata, G.B. Sowerby (also P. Desire). +17. Turritella ambulacrum, G.B. Sowerby (also P. S. Julian). + Imperfect fragments of the genera Byssoarca, Artemis, and Fusus. + +The upper part of the cliff is generally divided into three great strata, +differing slightly in composition, but essentially resembling the pumiceous +mudstone of the places farther north; the deposit, however, here is more +arenaceous, of greater specific gravity, and not so white: it is interlaced +with numerous thin veins, partially or quite filled with transverse fibres +of gypsum; these fibres were too short to reach across the vein, have their +extremities curved or bent: in the same veins with the gypsum, and likewise +in separate veins as well as in little nests, there is much powdery +sulphate of magnesia (as ascertained by Mr. Reeks) in an uncompressed form: +I believe that this salt has not heretofore been found in veins. Of the +three beds, the central one is the most compact, and more like ordinary +sandstone: it includes numerous flattened spherical concretions, often +united like a necklace, composed of hard calcareous sandstone, containing a +few shells: some of these concretions were four feet in diameter, and in a +horizontal line nine feet apart, showing that the calcareous matter must +have been drawn to the centres of attraction, from a distance of four feet +and a half on both sides. In the upper and lower finer-grained strata, +there were other concretions of a grey colour, containing calcareous +matter, and so fine-grained and compact, as almost to resemble porcelain- +rock: I have seen exactly similar concretions in a volcanic tufaceous bed +in Chiloe. Although in this upper fine-grained strata, organic remains were +very rare, yet I noticed a few of the great oyster; and in one included +soft ferruginous layer, there were some specimens of the Cucullaea alta +(found at Port Desire in the lower fossiliferous mass) and of the Mactra +rugata, which latter shell has been partially converted into gypsum. + +(FIGURE 18. SECTION OF THE PLAINS OF PATAGONIA, ON THE BANKS OF THE S. +CRUZ. + +(Section through strata (from top to bottom)): +Surface of plain with erratic boulders; 1,146 feet above the sea. +a. Gravel and boulders, 212 feet thick. +b. Basaltic lava, 322 feet thick. +c, d and e. Sedimentary layers, bed of small pebbles and talus +respectively, total 592 feet thick. +River of S. Cruz; here 280 feet above sea.) + +In ascending the valley of the S. Cruz, the upper strata of the coast- +cliffs are prolonged, with nearly the same characters, for fifty miles: at +about this point, they begin in the most gradual and scarcely perceptible +manner, to be banded with white lines; and after ascending ten miles +farther, we meet with distinct thin layers of whitish, greenish, and +yellowish fine-grained, fusible sediments. At eighty miles from the coast, +in a cliff thus composed, there were a few layers of ferruginous sandstone, +and of an argillaceous sandstone with concretions of marl like those in the +Pampas. (At this spot, for a space of three-quarters of a mile along the +north side of the river, and for a width of half a mile, there has been a +great slip, which has formed hills between sixty and seventy feet in +height, and has tilted the strata into highly inclined and even vertical +positions. The strata generally dipped at an angle of 45 degrees towards +the cliff from which they had slided. I have observed in slips, both on a +small and large scale, that this inward dip is very general. Is it due to +the hydrostatic pressure of water percolating with difficulty through the +strata acting with greater force at the base of the mass than against the +upper part?) At one hundred miles from the coast, that is at a central +point between the Atlantic and the Cordillera, we have the section in +Figure 18. + +The upper half of the sedimentary mass, under the basaltic lava, consists +of innumerable zones of perfectly white bright green, yellowish and +brownish, fine-grained, sometimes incoherent, sedimentary matter. The +white, pumiceous, trachytic tuff-like varieties are of rather greater +specific gravity than the pumiceous mudstone on the coast to the north; +some of the layers, especially the browner ones, are coarser, so that the +broken crystals are distinguishable with a weak lens. The layers vary in +character in short distances. With the exception of a few of the Ostrea +Patagonica, which appeared to have rolled down from the cliff above, no +organic remains were found. The chief difference between these layers taken +as a whole, and the upper beds both at the mouth of the river and on the +coast northward, seems to lie in the occasional presence of more colouring +matter, and in the supply having been intermittent; these characters, as we +have seen, very gradually disappear in descending the valley, and this fact +may perhaps be accounted for by the currents of a more open sea having +blended together the sediment from a distant and intermittent source. + +The coloured layers in the foregoing section rest on a mass, apparently of +great thickness (but much hidden by the talus), of soft sandstone, almost +composed of minute pebbles, from one-tenth to two-tenths of an inch in +diameter, of the rocks (with the entire exception of the basaltic lava) +composing the great boulders on the surface of the plain, and probably +composing the neighbouring Cordillera. Five miles higher up the valley, and +again thirty miles higher up (that is twenty miles from the nearest range +of the Cordillera), the lower plain included within the upper escarpments, +is formed, as seen on the banks of the river, of a nearly similar but +finer-grained, more earthy, laminated sandstone, alternating with +argillaceous beds, and containing numerous moderately sized pebbles of the +same rocks, and some shells of the great Ostrea Patagonica. (I found at +both places, but not in situ, quantities of coniferous and ordinary +dicotyledonous silicified wood, which was examined for me by Mr. R. Brown.) +As most of these shells had been rolled before being here embedded, their +presence does not prove that the sandstone belongs to the great Patagonian +tertiary formation, for they might have been redeposited in it, when the +valley existed as a sea-strait; but as amongst the pebbles there were none +of basalt, although the cliffs on both sides of the valley are composed of +this rock, I believe that the sandstone does belong to this formation. At +the highest point to which we ascended, twenty miles distant from the +nearest slope of the Cordillera, I could see the horizontally zoned white +beds, stretching under the black basaltic lava, close up to the mountains; +so that the valley of the S. Cruz gives a fair idea of the constitution of +the whole width of Patagonia. + +BASALTIC LAVA OF THE S. CRUZ. + +This formation is first met with sixty-seven miles from the mouth of the +river; thence it extends uninterruptedly, generally but not exclusively on +the northern side of the valley, close up to the Cordillera. The basalt is +generally black and fine-grained, but sometimes grey and laminated; it +contains some olivine, and high up the valley much glassy feldspar, where, +also, it is often amygdaloidal; it is never highly vesicular, except on the +sides of rents and on the upper and lower, spherically laminated surfaces. +It is often columnar; and in one place I saw magnificent columns, each face +twelve feet in width, with their interstices filled up with calcareous +tuff. The streams rest conformably on the white sedimentary beds, but I +nowhere saw the actual junction; nor did I anywhere see the white beds +actually superimposed on the lava; but some way up the valley at the foot +of the uppermost escarpments, they must be thus superimposed. Moreover, at +the lowest point down the valley, where the streams thin out and terminate +in irregular projections, the spaces or intervals between these projections +are filled up to the level of the now denuded and gravel-capped surfaces of +the plains, with the white-zoned sedimentary beds; proving that this matter +continued to be deposited after the streams had flowed. Hence we may +conclude that the basalt is contemporaneous with the upper parts of the +great tertiary formation. + +The lava where first met with is 130 feet in thickness: it there consists +of two, three, or perhaps more streams, divided from each other by +vesicular spheroids like those on the surface. From the streams having, as +it appears, extended to different distances, the terminal points are of +unequal heights. Generally the surface of the basalt is smooth them in one +part high up the valley, it was so uneven and hummocky, that until I +afterwards saw the streams extending continuously on both sides of the +valley up to a height of about three thousand feet close to the Cordillera, +I thought that the craters of eruption were probably close at hand. This +hummocky surface I believe to have been caused by the crossing and heaping +up of different streams. In one place, there were several rounded ridges +about twenty feet in height, some of them as broad as high, and some +broader, which certainly had been formed whilst the lava was fluid, for in +transverse sections each ridge was seen to be concentrically laminated, and +to be composed of imperfect columns radiating from common centres, like the +spokes of wheels. + +The basaltic mass where first met with is, as I have said, 130 feet in +thickness, and, thirty-five miles higher up the valley, it increases to 322 +feet. In the first fourteen and a half miles of this distance, the upper +surface of the lava, judging from three measurements taken above the level +of the river (of which the apparently very uniform inclination has been +calculated from its total height at a point 135 miles from the mouth), +slopes towards the Atlantic at an angle of only 0 degrees 7 minutes twenty +seconds: this must be considered only as an approximate measurement, but it +cannot be far wrong. Taking the whole thirty-five miles, the upper surface +slopes at an angle of 0 degrees 10 minutes 53 seconds; but this result is +of no value in showing the inclination of any one stream, for halfway +between the two points of measurement, the surface suddenly rises between +one hundred and two hundred feet, apparently caused by some of the +uppermost streams having extended thus far and no farther. From the +measurement made at these two points, thirty-five miles apart, the mean +inclination of the sedimentary beds, over which the lava has flowed, is NOW +(after elevation from under the sea) only 0 degrees 7 minutes 52 seconds: +for the sake of comparison, it may be mentioned that the bottom of the +present sea in a line from the mouth of the S. Cruz to the Falkland +Islands, from a depth of seventeen fathoms to a depth of eighty-five +fathoms, declines at an angle of 0 degrees 1 minute 22 seconds; between the +beach and the depth of seventeen fathoms, the slope is greater. From a +point about half-way up the valley, the basaltic mass rises more abruptly +towards the foot of the Cordillera, namely, from a height of 1,204 feet, to +about 3,000 feet above the sea. + +This great deluge of lava is worthy, in its dimensions, of the great +continent to which it belongs. The aggregate streams have flowed from the +Cordillera to a distance (unparalleled, I believe, in any case yet known) +of about one hundred geographical miles. Near their furthest extremity +their total thickness is 130 feet, which increase thirty-five miles farther +inland, as we have just seen, to 322 feet. The least inclination given by +M. E. de Beaumont of the upper surface of a lava-stream, namely 0 degrees +30 minutes, is that of the great subaerial eruption in 1783 from Skaptar +Jukul in Iceland; and M. E. de Beaumont shows that it must have flowed down +a mean inclination of less than 0 degrees 20 minutes. ("Memoires pour +servir" etc. pages 178 and 217.) But we now see that under the pressure of +the sea, successive streams have flowed over a smooth bottom with a mean +inclination of not more than 0 degrees 7 minutes 52 seconds; and that the +upper surface of the terminal portion (over a space of fourteen and a half +miles) has an inclination of not more than 0 degrees 7 minutes 20 seconds. +If the elevation of Patagonia has been greater nearer the Cordillera than +near the Atlantic (as is probable), then these angles are now all too +large. I must repeat, that although the foregoing measurements, which were +all carefully taken with the barometer, may not be absolutely correct, they +cannot be widely erroneous. + +Southward of the S. Cruz, the cliffs of the 840 feet plain extend to Coy +Inlet, and owing to the naked patches of the white sediment, they are said +on the charts to be "like the coast of Kent." At Coy Inlet the high plain +trends inland, leaving flat-topped outliers. At Port Gallegos (latitude 51 +degrees 35 minutes, and ninety miles south of S. Cruz), I am informed by +Captain Sulivan, R.N., that there is a gravel-capped plain from two to +three hundred feet in height, formed of numerous strata, some fine-grained +and pale-coloured, like the upper beds at the mouth of the S. Cruz, others +rather dark and coarser, so as to resemble gritstones or tuffs; these +latter include rather large fragments of apparently decomposed volcanic +rocks; there are, also, included layers of gravel. This formation is highly +remarkable, from abounding with mammiferous remains, which have not as yet +been examined by Professor Owen, but which include some large, but mostly +small, species of Pachydermata, Edentata, and Rodentia. From the appearance +of the pale-coloured, fine-grained beds, I was inclined to believe that +they corresponded with the upper beds of the S. Cruz; but Professor +Ehrenberg, who has examined some of the specimens, informs me that the +included microscopical organisms are wholly different, being fresh and +brackish-water forms. Hence the two to three hundred feet plain at Port +Gallegos is of unknown age, but probably of subsequent origin to the great +Patagonian tertiary formation. + +EASTERN TIERRA DEL FUEGO. + +Judging from the height, the general appearance, and the white colour of +the patches visible on the hill sides, the uppermost plain, both on the +north and western side of the Strait of Magellan, and along the eastern +coast of Tierra del Fuego as far south as near Port St. Polycarp, probably +belongs to the great Patagonian tertiary formation, These higher table- +ranges are fringed by low, irregular, extensive plains, belonging to the +boulder formation (Described in the "Geological Transactions" volume 6 page +415.), and composed of coarse unstratified masses, sometimes associated (as +north of C. Virgin's) with fine, laminated, muddy sandstones. The cliffs in +Sebastian Bay are 200 feet in height, and are composed of fine sandstones, +often in curvilinear layers, including hard concretions of calcareous +sandstone, and layers of gravel. In these beds there are fragments of wood, +legs of crabs, barnacles encrusted with corallines still partially +retaining their colour, imperfect fragments of a Pholas distinct from any +known species, and of a Venus, approaching very closely to, but slightly +different in form from, the V. lenticularis, a species living on the coast +of Chile. Leaves of trees are numerous between the laminae of the muddy +sandstone; they belong, as I am informed by Dr. J.D. Hooker, to three +species of deciduous beech, different from the two species which compose +the great proportion of trees in this forest-clad land. ("Botany of the +Antarctic Voyage" page 212.) From these facts it is difficult to +conjecture, whether we here see the basal part of the great Patagonian +formation, or some later deposit. + +SUMMARY ON THE PATAGONIAN TERTIARY FORMATION. + +Four out of the seven fossil shells, from St. Fe in Entre Rios, were found +by M. d'Orbigny in the sandstone of the Rio Negro, and by me at San Josef. +Three out of the six from San Josef are identical with those from Port +Desire and S. Julian, which two places have together fifteen species, out +of which three are common to both. Santa Cruz has seventeen species, out of +which five are common to Port Desire and S. Julian. Considering the +difference in latitude between these several places, and the small number +of species altogether collected, namely thirty-six, I conceive the above +proportional number of species in common, is sufficient to show that the +lower fossiliferous mass belongs nearly, I do not say absolutely, to the +same epoch. What this epoch may be, compared with the European tertiary +stages, M. d'Orbigny will not pretend to determine. The thirty-six species +(including those collected by myself and by M. d'Orbigny) are all extinct, +or at least unknown; but it should be borne in mind, that the present coast +consists of shingle, and that no one, I believe, has dredged here for +shells; hence it is not improbable that some of the species may hereafter +be found living. Some few of the species are closely related with existing +ones; this is especially the case, according to M. d'Orbigny and Mr. +Sowerby, with the Fusus Patagonicus; and, according to Mr. Sowerby, with +the Pyrula, the Venus meridionalis, the Crepidula gregaria, and the +Turritella ambulacrum, and T. Patagonica. At least three of the genera, +namely, Cucullaea, Crassatella, and (as determined by Mr. Sowerby) +Struthiolaria, are not found in this quarter of the world; and Trigonocelia +is extinct. The evidence taken altogether indicates that this great +tertiary formation is of considerable antiquity; but when treating of the +Chilean beds, I shall have to refer again to this subject. + +The white pumiceous mudstone, with its abundant gypsum, belongs to the same +general epoch with the underlying fossiliferous mass, as may be inferred +from the shells included in the intercalated layers at Nuevo Gulf, S. +Julian, and S. Cruz. Out of the twenty-seven marine microscopic structures +found by Professor Ehrenberg in the specimens from S. Julian and Port +Desire, ten are common to these two places: the three found at Nuevo Gulf +are distinct. I have minutely described this deposit, from its remarkable +characters and its wide extension. From Coy Inlet to Port Desire, a +distance of 230 miles, it is certainly continuous; and I have reason to +believe that it likewise extends to the Rio Chupat, Nuevo Gulf, and San +Josef, a distance of 570 miles: we have, also, seen that a single layer +occurs at the Rio Negro. At Port S. Julian it is from eight to nine hundred +feet in thickness; and at S. Cruz it extends, with a slightly altered +character, up to the Cordillera. From its microscopic structure, and from +its analogy with other formations in volcanic districts, it must be +considered as originally of volcanic origin: it may have been formed by the +long-continued attrition of vast quantities of pumice, or judging from the +manner in which the mass becomes, in ascending the valley of S. Cruz, +divided into variously coloured layers, from the long-continued eruption of +clouds of fine ashes. In either case, we must conclude, that the southern +volcanic orifices of the Cordillera, now in a dormant state, were at about +this period over a wide space, and for a great length of time, in action. +We have evidence of this fact, in the latitude of the Rio Negro, in the +sandstone-conglomerate with pumice, and demonstrative proof of it, at S. +Cruz, in the vast deluges of basaltic lava: at this same tertiary period, +also, there is distinct evidence of volcanic action in Western Banda +Oriental. + +The Patagonian tertiary formation extends continuously, judging from +fossils alone, from S. Cruz to near the Rio Colorado, a distance of above +six hundred miles, and reappears over a wide area in Entre Rios and Banda +Oriental, making a total distance of 1,100 miles; but this formation +undoubtedly extends (though no fossils were collected) far south of the S. +Cruz, and, according to M. d'Orbigny, 120 miles north of St. Fe. At S. Cruz +we have seen that it extends across the continent; being on the coast about +eight hundred feet in thickness (and rather more at S. Julian), and rising +with the contemporaneous lava-streams to a height of about three thousand +feet at the base of the Cordillera. It rests, wherever any underlying +formation can be seen, on plutonic and metamorphic rocks. Including the +newer Pampean deposit, and those strata in Eastern Tierra del Fuego of +doubtful age, as well as the boulder formation, we have a line of more than +twenty-seven degrees of latitude, equal to that from the Straits of +Gibraltar to the south of Iceland, continuously composed of tertiary +formations. Throughout this great space the land has been upraised, without +the strata having been in a single instance, as far as my means of +observation went, unequally tilted or dislocated by a fault. + +TERTIARY FORMATIONS ON THE WEST COAST. + +CHONOS ARCHIPELAGO. + +The numerous islands of this group, with the exception of Lemus, Ypun, +consist of metamorphic schists; these two islands are formed of softish +grey and brown, fusible, often laminated sandstones, containing a few +pebbles, fragments of black lignite, and numerous mammillated concretions +of hard calcareous sandstone. Out of these concretions at Ypun (latitude 40 +degrees 30 minutes S.), I extracted the four following extinct species of +shells:-- + +1. Turritella suturalis, G.B. Sowerby (also Navidad). +2. Sigaretus subglobosus, G.B. Sowerby (also Navidad). +3. Cytheraea (?) sulculosa (?), G.B. Sowerby (also Chiloe and Huafo?). +4. Voluta, fragments of. + +In the northern parts of this group there are some cliffs of gravel and of +the boulder formation. In the southern part (at P. Andres in Tres Montes), +there is a volcanic formation, probably of tertiary origin. The lavas +attain a thickness of from two to three hundred feet; they are extremely +variable in colour and nature, being compact, or brecciated, or cellular, +or amygdaloidal with zeolite, agate and bole, or porphyritic with glassy +albitic feldspar. There is also much imperfect rubbly pitchstone, with the +interstices charged with powdery carbonate of lime apparently of +contemporaneous origin. These lavas are conformably associated with strata +of breccia and of brown tuff containing lignite. The whole mass has been +broken up and tilted at an angle of 45 degrees, by a series of great +volcanic dikes, one of which was thirty yards in breadth. This volcanic +formation resembles one, presently to be described, in Chiloe. + +HUAFO. + +This island lies between the Chonos and Chiloe groups: it is about eight +hundred feet high, and perhaps has a nucleus of metamorphic rocks. The +strata which I examined consisted of fine-grained muddy sandstones, with +fragments of lignite and concretions of calcareous sandstone. I collected +the following extinct shells, of which the Turritella was in great +numbers:-- + +1. Bulla cosmophila, G.B. Sowerby. +2. Pleurotoma subaequalis, G.B. Sowerby. +3. Fusus cleryanus, d'Orbigny, "Voyage Pal." (also at Coquimbo). +4. Triton leucostomoides, G.B. Sowerby. +5. Turritella Chilensis, G.B. Sowerby (also Mocha). +6. Venus, probably a distinct species, but very imperfect. +7. Cytheraea (?) sulculosa (?), probably a distinct species, but very +imperfect. +8. Dentalium majus, G.B. Sowerby. + +CHILOE. + +This fine island is about one hundred miles in length. The entire southern +part, and the whole western coast, consists of mica-schist, which likewise +is seen in the ravines of the interior. The central mountains rise to a +height of 3,000 feet, and are said to be partly formed of granite and +greenstone: there are two small volcanic districts. The eastern coast, and +large parts of the northern extremity of the island are composed of gravel, +the boulder formation, and underlying horizontal strata. The latter are +well displayed for twenty miles north and south of Castro; they vary in +character from common sandstone to fine-grained, laminated mudstones: all +the specimens which I examined are easily fusible, and some of the beds +might be called volcanic grit-stones. These latter strata are perhaps +related to a mass of columnar trachyte which occurs behind Castro. The +sandstone occasionally includes pebbles, and many fragments and layers of +lignite; of the latter, some are apparently formed of wood and others of +leaves: one layer on the N.W. side of Lemuy is nearly two feet in +thickness. There is also much silicified wood, both common dicotyledonous +and coniferous: a section of one specimen in the direction of the medullary +rays has, as I am informed by Mr. R. Brown, the discs in a double row +placed alternately, and not opposite as in the true Araucaria. I found +marine remains only in one spot, in some concretions of hard calcareous +sandstone: in several other districts I have observed that organic remains +were exclusively confined to such concretions; are we to account for this +fact, by the supposition that the shells lived only at these points, or is +it not more probable that their remains were preserved only where +concretions were formed? The shells here are in a bad state, they consist +of:-- + +1. Tellinides (?) oblonga, G.B. Sowerby (a solenella in M. d'Orbigny's +opinion). +2. Natica striolata, G.B. Sowerby. +3. Natica (?) pumila, G.B. Sowerby. +4. Cytheraea (?) sulculosa, G.B. Sowerby (also Ypun and Huafo?). + +At the northern extremity of the island, near S. Carlos, there is a large +volcanic formation, between five and seven hundred feet in thickness. The +commonest lava is blackish-grey or brown, either vesicular, or amygdaloidal +with calcareous spar and bole: most even of the darkest varieties fuse into +a pale-coloured glass. The next commonest variety is a rubbly, rarely well +characterised pitchstone (fusing into a white glass) which passes in the +most irregular manner into stony grey lavas. This pitchstone, as well as +some purple claystone porphyry, certainly flowed in the form of streams. +These various lavas often pass, at a considerable depth from the surface, +in the most abrupt and singular manner into wacke. Great masses of the +solid rock are brecciated, and it was generally impossible to discover +whether the recementing process had been an igneous or aqueous action. (In +a cliff of the hardest fragmentary mass, I found several tortuous, vertical +veins, varying in thickness from a few tenths of an inch to one inch and a +half, of a substance which I have not seen described. It is glossy, and of +a brown colour; it is thinly laminated, with the laminae transparent and +elastic; it is a little harder than calcareous spar; it is infusible under +the blowpipe, sometimes decrepitates, gives out water, curls up, blackens, +and becomes magnetic. Borax easily dissolves a considerable quantity of it, +and gives a glass tinged with green. I have no idea what its true nature +is. On first seeing it, I mistook it for lignite!) The beds are obscurely +separated from each other; they are sometimes parted by seams of tuff and +layers of pebbles. In one place they rested on, and in another place were +capped by, tuffs and girt-stones, apparently of submarine origin. + +The neighbouring peninsula of Lacuy is almost wholly formed of tufaceous +deposits, connected probably in their origin with the volcanic hills just +described. The tuffs are pale-coloured, alternating with laminated +mudstones and sandstones (all easily fusible), and passing sometimes into +fine-grained white beds strikingly resembling the great upper infusorial +deposit of Patagonia, and sometimes into brecciolas with pieces of pumice +in the last stage of decay; these again pass into ordinary coarse breccias +and conglomerates of hard rocks. Within very short distances, some of the +finer tuffs often passed into each other in a peculiar manner, namely, by +irregular polygonal concretions of one variety increasing so much and so +suddenly in size, that the second variety, instead of any longer forming +the entire mass, was left merely in thin veins between the concretions. In +a straight line of cliffs, at Point Tenuy, I examined the following +remarkable section (Figure 19):-- + +(FIGURE 19.) + +On the left hand, the lower part (AA) consists of regular, alternating +strata of brown tuffs and greenish laminated mudstone, gently inclined to +the right, and conformably covered by a mass (B left) of a white, tufaceous +and brecciolated deposit. On the right hand, the whole cliff (BB right) +consists of the same white tufaceous matter, which on this side presents +scarcely a trace of stratification, but to the left becomes very gradually +and rather indistinctly divided into strata quite conformable with the +underlying beds (AA): moreover, a few hundred yards further to the left, +where the surface has been less denuded, the tufaceous strata (B left) are +conformably covered by another set of strata, like the underlying ones (AA) +of this section. In the middle of the diagram, the beds (AA) are seen to be +abruptly cut off, and to abut against the tufaceous non-stratified mass; +but the line of junction has been accidentally not represented steep +enough, for I particularly noticed that before the beds had been tilted to +the right, this line must have been nearly vertical. It appears that a +current of water cut for itself a deep and steep submarine channel, and at +the same time or afterwards filled it up with the tufaceous and +brecciolated matter, and spread the same over the surrounding submarine +beds; the matter becoming stratified in these more distant and less +troubled parts, and being moreover subsequently covered up by other strata +(like AA) not shown in the diagram. It is singular that three of the beds +(of AA) are prolonged in their proper direction, as represented, beyond the +line of junction into the white tufaceous matter: the prolonged portions of +two of the beds are rounded; in the third, the terminal fragment has been +pushed upwards: how these beds could have been left thus prolonged, I will +not pretend to explain. In another section on the opposite side of a +promontory, there was at the foot of this same line of junction, that is at +the bottom of the old submarine channel, a pile of fragments of the strata +(AA), with their interstices filled up with white tufaceous matter: this is +exactly what might have been anticipated under such circumstances. + +(FIGURE 20. GROUND PLAN SHOWING THE RELATION BETWEEN VEINS AND +CONCRETIONARY ZONES IN A MASS OF TUFF.) + +The various tufaceous and other beds at this northern end of Chiloe +probably belong to about the same age with those near Castro, and they +contain, as there, many fragments of black lignite and of silicified and +pyritous wood, often embedded close together. They also contain many and +singular concretions: some are of hard calcareous sandstone, in which it +would appear that broken volcanic crystals and scales of mica have been +better preserved (as in the case of the organic remains near Castro) than +in the surrounding mass. Other concretions in the white brecciola are of a +hard, ferruginous, yet fusible, nature; they are as round as cannon-balls, +and vary from two or three inches to two feet in diameter; their insides +generally consist either of fine, scarcely coherent volcanic sand (The +frequent tendency in iron to form hollow concretions or shell containing +incoherent matter is singular; D'Aubuisson ("Traite de Geogn." tome 1 page +318) remarks on this circumstance.), or of an argillaceous tuff; in this +latter case, the external crust was quite thin and hard. Some of these +spherical balls were encircled in the line of their equators, by a +necklace-like row of smaller concretions. Again there were other +concretions, irregularly formed, and composed of a hard, compact, ash- +coloured stone, with an almost porcelainous fracture, adhesive to the +tongue, and without any calcareous matter. These beds are, also, interlaced +by many veins, containing gypsum, ferruginous matter, calcareous spar, and +agate. It was here seen with remarkable distinctness, how intimately +concretionary action and the production of fissures and veins are related +together. Figure 20 is an accurate representation of a horizontal space of +tuff, about four feet long by two and a half in width: the double lines +represent the fissures partially filled with oxide of iron and agate: the +curvilinear lines show the course of the innumerable, concentric, +concretionary zones of different shades of colour and of coarseness in the +particles of tuff. The symmetry and complexity of the arrangement gave the +surface an elegant appearance. It may be seen how obviously the fissures +determine (or have been determined by) the shape, sometimes of the whole +concretion, and sometimes only of its central parts. The fissures also +determine the curvatures of the long undulating zones of concretionary +action. From the varying composition of the veins and concretions, the +amount of chemical action which the mass has undergone is surprisingly +great; and it would likewise appear from the difference in size in the +particles of the concretionary zones, that the mass, also, has been +subjected to internal mechanical movements. + +In the peninsula of Lacuy, the strata over a width of four miles have been +upheaved by three distinct, and some other indistinct, lines of elevation, +ranging within a point of north and south. One line, about two hundred feet +in height, is regularly anticlinal, with the strata dipping away on both +sides, at an angle of 15 degrees, from a central "valley of elevation," +about three hundred yards in width. A second narrow steep ridge, only sixty +feet high, is uniclinal, the strata throughout dipping westward; those on +both flanks being inclined at an angle of from ten to fifteen degrees; +whilst those on the ridge dip in the same direction at an angle of between +thirty and forty degrees. This ridge, traced northwards, dies away; and the +beds at its terminal point, instead of dipping westward, are inclined 12 +degrees to the north. This case interested me, as being the first in which +I found in South America, formations perhaps of tertiary origin, broken by +lines of elevation. + +VALDIVIA: ISLAND OF MOCHA. + +The formations of Chiloe seem to extend with nearly the same character to +Valdivia, and for some leagues northward of it: the underlying rocks are +micaceous schists, and are covered up with sandstone and other sedimentary +beds, including, as I was assured, in many places layers of lignite. I did +not land on Mocha (latitude 38 degrees 20 minutes), but Mr. Stokes brought +me specimens of the grey, fine-grained, slightly calcareous sandstone, +precisely like that of Huafo, containing lignite and numerous Turritellae. +The island is flat topped, 1,240 feet in height, and appears like an +outlier of the sedimentary beds on the mainland. The few shells collected +consist of:-- + +1. Turritella Chilensis, G.B. Sowerby (also at Huafo). +2. Fusus, very imperfect, somewhat resembling F. subreflexus of Navidad, +but probably different. +3. Venus, fragments of. + +CONCEPCION. + +Sailing northward from Valdivia, the coast-cliffs are seen, first to assume +near the R. Tolten, and thence for 150 miles northward, to be continued +with the same mineralogical characters, immediately to be described at +Concepcion. I heard in many places of beds of lignite, some of it fine and +glossy, and likewise of silicified wood; near the Tolten the cliffs are +low, but they soon rise in height; and the horizontal strata are prolonged, +with a nearly level surface, until coming to a more lofty tract between +points Rumena and Lavapie. Here the beds have been broken up by at least +eight or nine parallel lines of elevation, ranging E. or E.N.E. and W. or +W.S.W. These lines can be followed with the eye many miles into the +interior; they are all uniclinal, the strata in each dipping to a point +between S. and S.S.E. with an inclination in the central lines of about +forty degrees, and in the outer ones of under twenty degrees. This band of +symmetrically troubled country is about eight miles in width. + +The island of Quiriquina, in the Bay of Concepcion, is formed of various +soft and often ferruginous sandstones, with bands of pebbles, and with the +lower strata sometimes passing into a conglomerate resting on the +underlying metamorphic schists. These beds include subordinate layers of +greenish impure clay, soft micaceous and calcareous sandstones, and reddish +friable earthy matter with white specks like decomposed crystals of +feldspar; they include, also, hard concretions, fragments of shells, +lignite, and silicified wood. In the upper part they pass into white, soft +sediments and brecciolas, very like those described at Chiloe; as indeed is +the whole formation. At Lirguen and other places on the eastern side of the +bay, there are good sections of the lower sandstones, which are generally +ferruginous, but which vary in character, and even pass into an +argillaceous nature; they contain hard concretions, fragments of lignite, +silicified wood, and pebbles (of the same rocks with the pebbles in the +sandstones of Quiriquina), and they alternate with numerous, often very +thin layers of imperfect coal, generally of little specific gravity. The +main bed here is three feet thick; and only the coal of this one bed has a +glossy fracture. Another irregular, curvilinear bed of brown, compact +lignite, is remarkable for being included in a mass of coarse gravel. These +imperfect coals, when placed in a heap, ignite spontaneously. The cliffs on +this side of the bay, as well as on the island of Quiriquina, are capped +with red friable earth, which, as stated in the Second Chapter, is of +recent formation. The stratification in this neighbourhood is generally +horizontal; but near Lirguen the beds dip N.W. at an angle of 23 degrees; +near Concepcion they are also inclined: at the northern end of Quiriquina +they have been tilted at an angle of 30 degrees, and at the southern end at +angles varying from 15 degrees to 40 degrees: these dislocations must have +taken place under the sea. + +A collection of shells, from the island of Quiriquina, has been described +by M. d'Orbigny: they are all extinct, and from their generic character, M. +d'Orbigny inferred that they were of tertiary origin: they consist of:-- + +1. Scalaria Chilensis, d'Orbigny, "Voyage, Part Pal." +2. Natica Araucana, d'Orbigny, "Voyage, Part Pal." +3. Natica australis, d'Orbigny, "Voyage, Part Pal." +4. Fusus difficilis, d'Orbigny, "Voyage, Part Pal." +5. Pyrula longirostra, d'Orbigny, "Voyage, Part Pal." +6. Pleurotoma Araucana, d'Orbigny, "Voyage, Part Pal." +7. Cardium auca, d'Orbigny, "Voyage, Part Pal." +8. Cardium acuticostatum, d'Orbigny, "Voyage, Part Pal." +9. Venus auca, d'Orbigny, "Voyage, Part Pal." +10. Mactra cecileana, d'Orbigny, "Voyage, Part Pal." +11. Mactra Araucana, d'Orbigny, "Voyage, Part Pal." +12. Arca Araucana, d'Orbigny, "Voyage, Part Pal." +13. Nucula Largillierti, d'Orbigny, "Voyage, Part Pal." +14. Trigonia Hanetiana, d'Orbigny, "Voyage, Part Pal." + +During a second visit of the "Beagle" to Concepcion, Mr. Kent collected for +me some silicified wood and shells out of the concretions in the sandstone +from Tome, situated a short distance north of Lirguen. They consist of:-- + +1. Natica australis, d'Orbigny, "Voyage, Part Pal." +2. Mactra Araucana, d'Orbigny, "Voyage, Part Pal." +3. Trigonia Hanetiana, d'Orbigny, "Voyage, Part Pal." +4. Pecten, fragments of, probably two species, but too imperfect for +description. +5. Baculites vagina, E. Forbes. +6. Nautilus d'Orbignyanus, E. Forbes. + +Besides these shells, Captain Belcher found here an Ammonite, nearly three +feet in diameter, and so heavy that he could not bring it away; fragments +are deposited at Haslar Hospital: he also found the silicified vertebrae of +some very large animal. ("Zoology of Captain Beechey's Voyage" page 163.) +From the identity in mineralogical nature of the rocks, and from Captain +Belcher's minute description of the coast between Lirguen and Tome, the +fossiliferous concretions at this latter place certainly belong to the same +formation with the beds examined by myself at Lirguen; and these again are +undoubtedly the same with the strata of Quiriquina; moreover; the three +first of the shells from Tome, though associated in the same concretions +with the Baculite, are identical with the species from Quiriquina. Hence +all the sandstone and lignitiferous beds in this neighbourhood certainly +belong to the same formation. Although the generic character of the +Quiriquina fossils naturally led M. d'Orbigny to conceive that they were of +tertiary origin, yet as we now find them associated with the Baculites +vagina and with an Ammonite, we must, in the opinion of M. d'Orbigny, and +if we are guided by the analogy of the northern hemisphere, rank them in +the Cretaceous system. Moreover, the Baculites vagina, which is in a +tolerable state of preservation, appears to Professor E. Forbes certainly +to be identical with a species, so named by him, from Pondicherry in India; +where it is associated with numerous decidedly cretaceous species, which +approach most nearly to Lower Greensand or Neocomian forms: this fact, +considering the vast distance between Chile and India, is truly surprising. +Again, the Nautilus d'Orbignyanus, as far as its imperfect state allows of +comparison, resembles, as I am informed by Professor Forbes, both in its +general form and in that of its chambers, two species from the Upper +Greensand. It may be added that every one of the above-named genera from +Quiriquina, which have an apparently tertiary character, are found in the +Pondicherry strata. There are, however, some difficulties on this view of +the formations at Concepcion being cretaceous, which I shall afterwards +allude to; and I will here only state that the Cardium auca is found also +at Coquimbo, the beds at which place, there can be no doubt, are tertiary. + +NAVIDAD. (I was guided to this locality by the Report on M. Gay's +"Geological Researches" in the "Annales des Scienc. Nat." 1st series tome +28.) + +The Concepcion formation extends some distance northward, but how far I +know not; for the next point at which I landed was at Navidad, 160 miles +north of Concepcion, and 60 miles south of Valparaiso. The cliffs here are +about eight hundred feet in height: they consist, wherever I could examine +them, of fine-grained, yellowish, earthy sandstones, with ferruginous +veins, and with concretions of hard calcareous sandstone. In one part, +there were many pebbles of the common metamorphic porphyries of the +Cordillera: and near the base of the cliff, I observed a single rounded +boulder of greenstone, nearly a yard in diameter. I traced this sandstone +formation beneath the superficial covering of gravel, for some distance +inland: the strata are slightly inclined from the sea towards the +Cordillera, which apparently has been caused by their having been +accumulated against or round outlying masses of granite, of which some +points project near the coast. The sandstone contains fragments of wood, +either in the state of lignite or partially silicified, sharks' teeth, and +shells in great abundance, both high up and low down the sea-cliffs. +Pectunculus and Oliva were most numerous in individuals, and next to them +Turritella and Fusus. I collected in a short time, though suffering from +illness, the following thirty-one species, all of which are extinct, and +several of the genera do not now range (as we shall hereafter show) nearly +so far south:-- + +1. Gastridium cepa, G.B. Sowerby. +2. Monoceros, fragments of, considered by M. d'Orbigny as a new species. +3. Voluta alta, G.B. Sowerby (considered by M. d'Orbigny as distinct from +the V. alta of Santa Cruz). +4. Voluta triplicata, G.B. Sowerby. +5. Oliva dimidiata, G.B. Sowerby. +6. Pleurotoma discors, G.B. Sowerby. +7. Pleurotoma turbinelloides, G.B. Sowerby. +8. Fusus subreflexus, G.B. Sowerby. +9. Fusus pyruliformis, G.B. Sowerby. +10. Fusus, allied to F. regularis (considered by M. d'Orbigny as a distinct +species). +11. Turritella suturalis, G.B. Sowerby. +12. Turritella Patagonica, G.B. Sowerby (fragments of). +13. Trochus laevis, G.B. Sowerby. +14. Trochus collaris, G.B. Sowerby (considered by M. d'Orbigny as the young +of the T. laevis). +15. Cassis monilifer, G.B. Sowerby. +16. Pyrula distans, G.B. Sowerby. +17. Triton verruculosus, G.B. Sowerby. +18. Sigaretus subglobosus, G.B. Sowerby. +19. Natica solida, G.B. Sowerby. (It is doubtful whether the Natica solida +of S. Cruz is the same species with this.) +20. Terebra undulifera, G.B. Sowerby. +21. Terebra costellata, G.B. Sowerby. +22. Bulla (fragments of). +23. Dentalium giganteum, do. +24. Dentalium sulcosum, do. +25. Corbis (?) laevigata, do. +26. Cardium multiradiatum, do. +27. Venus meridionalis, do. +28. Pectunculus dispar, (?) Desh. (considered by M. d'Orbigny as a distinct +species). +29, 30. Cytheraea and Mactra, fragments of (considered by M. d'Orbigny as +new species). +31. Pecten, fragments of. + +COQUIMBO. + +(FIGURE 21. SECTION OF THE TERTIARY FORMATION AT COQUIMBO. + +From Level of Sea to Surface of plain, 252 feet above sea, through levels +F, E, D and C: + +F.--Lower sandstone, with concretions and silicified bones, with fossil +shells, all, or nearly all, extinct. + +E.--Upper ferruginous sandstone, with numerous Balani, with fossil shells, +all, or nearly all, extinct. + +C and D.--Calcareous beds with recent shells. + +A.--Stratified sand in a ravine, also with recent shells.) + +For more than two hundred miles northward of Navidad, the coast consists of +plutonic and metamorphic rocks, with the exception of some quite +insignificant superficial beds of recent origin. At Tonguay, twenty-five +miles south of Coquimbo, tertiary beds recommence. I have already minutely +described in the Second Chapter, the step-formed plains of Coquimbo, and +the upper calcareous beds (from twenty to thirty feet in thickness) +containing shells of recent species, but in different proportions from +those on the beach. There remains to be described only the underlying +ancient tertiary beds, represented in Figure 21 by the letters F and E:-- + +I obtained good sections of bed F only in Herradura Bay: it consists of +soft whitish sandstone, with ferruginous veins, some pebbles of granite, +and concretionary layers of hard calcareous sandstone. These concretions +are remarkable from the great number of large silicified bones, apparently +of cetaceous animals, which they contain; and likewise of a shark's teeth, +closely resembling those of the Carcharias megalodon. Shells of the +following species, of which the gigantic Oyster and Perna are the most +conspicuous, are numerously embedded in the concretions:-- + +1. Bulla ambigua, d'Orbigny "Voyage" Pal. +2. Monoceros Blainvillii, d'Orbigny "Voyage" Pal. +3. Cardium auca, d'Orbigny "Voyage" Pal. +4. Panopaea Coquimbensis, d'Orbigny "Voyage" Pal. +5. Perna Gaudichaudi, d'Orbigny "Voyage" Pal. +6. Artemis ponderosa; Mr. Sowerby can find no distinguishing character +between this fossil and the recent A. ponderosa; it is certainly an +Artemis, as shown by the pallial impression. +7. Ostrea Patagonica (?); Mr. Sowerby can point out no distinguishing +character between this species and that so eminently characteristic of the +great Patagonian formation; but he will not pretend to affirm that they are +identical. +8. Fragments of a Venus and Natica. + +The cliffs on one side of Herradura Bay are capped by a mass of stratified +shingle, containing a little calcareous matter, and I did not doubt that it +belonged to the same recent formation with the gravel on the surrounding +plains, also cemented by calcareous matter, until to my surprise, I found +in the midst of it, a single thin layer almost entirely composed of the +above gigantic oyster. + +At a little distance inland, I obtained several sections of the bed E, +which, though different in appearance from the lower bed F, belongs to the +same formation: it consists of a highly ferruginous sandy mass, almost +composed, like the lowest bed at Port S. Julian, of fragments of Balanidae; +it includes some pebbles, and layers of yellowish-brown mudstone. The +embedded shells consist of:-- + +1. Monoceros Blainvillii, d'Orbigny "Voyage" Pal. +2. Monoceros ambiguus, G.B. Sowerby. +3. Anomia alternans, G.B. Sowerby. +4. Pecten rudis, G.B. Sowerby. +5. Perna Gaudichaudi, d'Orbigny "Voyage" Pal. +6. Ostrea Patagonica (?), d'Orbigny "Voyage" Pal. +7. Ostrea, small species, in imperfect state; it appeared to me like a +small kind now living in, but very rare in the bay. +8. Mytilus Chiloensis; Mr. Sowerby can find no distinguishing character +between this fossil, as far as its not very perfect condition allows of +comparison, and the recent species. +9. Balanus Coquimbensis, G.B. Sowerby. +10. Balanus psittacus? King. This appears to Mr. Sowerby and myself +identical with a very large and common species now living on the coast. + +The uppermost layers of this ferrugino-sandy mass are conformably covered +by, and impregnated to the depth of several inches with, the calcareous +matter of the bed D called losa: hence I at one time imagined that there +was a gradual passage between them; but as all the species are recent in +the bed D, whilst the most characteristic shells of the uppermost layers of +E are the extinct Perna, Pecten, and Monoceros, I agree with M. d'Orbigny, +that this view is erroneous, and that there is only a mineralogical passage +between them, and no gradual transition in the nature of their organic +remains. Besides the fourteen species enumerated from these two lower beds, +M. d'Orbigny has described ten other species given to him from this +locality; namely:-- + +1. Fusus Cleryanus, d'Orbigny "Voyage" Pal. +2. Fusus petitianus, d'Orbigny "Voyage" Pal. +3. Venus hanetiana, d'Orbigny "Voyage" Pal. +4. Venus incerta (?) d'Orbigny "Voyage" Pal. +5. Venus Cleryana, d'Orbigny "Voyage" Pal. +6. Venus petitiana, d'Orbigny "Voyage" Pal. +7. Venus Chilensis, d'Orbigny "Voyage" Pal. +8. Solecurtus hanetianus, d'Orbigny "Voyage" Pal. +9. Mactra auca, d'Orbigny "Voyage" Pal. +10. Oliva serena, d'Orbigny "Voyage" Pal. + +Of these twenty-four shells, all are extinct, except, according to Mr. +Sowerby, the Artemis ponderosa, Mytilus Chiloensis, and probably the great +Balanus. + +COQUIMBO TO COPIAPO. + +A few miles north of Coquimbo, I met with the ferruginous, balaniferous +mass E with many silicified bones; I was informed that these silicified +bones occur also at Tonguay, south of Coquimbo: their number is certainly +remarkable, and they seem to take the place of the silicified wood, so +common on the coast-formations of Southern Chile. In the valley of +Chaneral, I again saw this same formation, capped with the recent +calcareous beds. I here left the coast, and did not see any more of the +tertiary formations, until descending to the sea at Copiapo: here in one +place I found variously coloured layers of sand and soft sandstone, with +seams of gypsum, and in another place, a comminuted shelly mass, with +layers of rotten-stone and seams of gypsum, including many of the extinct +gigantic oyster: beds with these oysters are said to occur at English +Harbour, a few miles north of Copiapo. + +COAST OF PERU. + +With the exception of deposits containing recent shells and of quite +insignificant dimensions, no tertiary formations have been observed on this +coast, for a space of twenty-two degrees of latitude north of Copiapo, +until coming to Payta, where there is said to be a considerable calcareous +deposit: a few fossils have been described by M. d'Orbigny from this place, +namely:-- + +1. Rostellaria Gaudichaudi, d'Orbigny "Voyage" Pal. +2. Pectunculus Paytensis, d'Orbigny "Voyage" Pal. +3. Venus petitiana, d'Orbigny "Voyage" Pal. +4. Ostrea Patagonica? This great oyster (of which specimens have been given +me) cannot be distinguished by Mr. Sowerby from some of the varieties from +Patagonia; though it would be hazardous to assert it is the same with that +species, or with that from Coquimbo. + +CONCLUDING REMARKS. + +The formations described in this chapter, have, in the case of Chiloe and +probably in that of Concepcion and Navidad, apparently been accumulated in +troughs formed by submarine ridges extending parallel to the ancient shores +of the continent; in the case of the islands of Mocha and Huafo it is +highly probable, and in that of Ypun and Lemus almost certain, that they +were accumulated round isolated rocky centres or nuclei, in the same manner +as mud and sand are now collecting round the outlying islets and reefs in +the West Indian Archipelago. Hence, I may remark, it does not follow that +the outlying tertiary masses of Mocha and Huafo were ever continuously +united at the same level with the formations on the mainland, though they +may have been of contemporaneous origin, and been subsequently upraised to +the same height. In the more northern parts of Chile, the tertiary strata +seem to have been separately accumulated in bays, now forming the mouths of +valleys. + +The relation between these several deposits on the shores of the Pacific, +is not nearly so clear as in the case of the tertiary formations on the +Atlantic. Judging from the form and height of the land (evidence which I +feel sure is here much more trustworthy than it can ever be in such broken +continents as that of Europe), from the identity of mineralogical +composition, from the presence of fragments of lignite and of silicified +wood, and from the intercalated layers of imperfect coal, I must believe +that the coast-formations from Central Chiloe to Concepcion, a distance of +400 miles, are of the same age: from nearly similar reasons, I suspect that +the beds of Mocha, Huafo, and Ypun, belong also to the same period. The +commonest shell in Mocha and Huafo is the same species of Turritella; and I +believe the same Cytheraea is found on the islands of Huafo, Chiloe, and +Ypun; but with these trifling exceptions, the few organic remains found at +these places are distinct. The numerous shells from Navidad, with the +exception of two, namely, the Sigaretus and Turritella found at Ypun, are +likewise distinct from those found in any other part of this coast. +Coquimbo has Cardium auca in common with Concepcion, and Fusus Cleryanus +with Huafo; I may add, that Coquimbo has Venus petitiana, and a gigantic +oyster (said by M. d'Orbigny also to be found a little south of Concepcion) +in common with Payta, though this latter place is situated twenty-two +degrees northward of latitude 27 degrees, to which point the Coquimbo +formation extends. + +From these facts, and from the generic resemblance of the fossils from the +different localities, I cannot avoid the suspicion that they all belong to +nearly the same epoch, which epoch, as we shall immediately see, must be a +very ancient tertiary one. But as the Baculite, especially considering its +apparent identity with the Cretaceous Pondicherry species, and the presence +of an Ammonite, and the resemblance of the Nautilus to two upper greensand +species, together afford very strong evidence that the formation of +Concepcion is a Secondary one; I will, in my remarks on the fossils from +the other localities, put on one side those from Concepcion and from +Eastern Chiloe, which, whatever their age may be, appear to me to belong to +one group. I must, however, again call attention to the fact that the +Cardium auca is found both at Concepcion and in the undoubtedly tertiary +strata of Coquimbo: nor should the possibility be overlooked, that as +Trigonia, though known in the northern hemisphere only as a Secondary +genus, has living representatives in the Australian seas, so a Baculite, +Ammonite, and Trigonia may have survived in this remote part of the +southern ocean to a somewhat later period than to the north of the equator. + +Before passing in review the fossils from the other localities, there are +two points, with respect to the formations between Concepcion and Chiloe, +which deserve some notice. First, that though the strata are generally +horizontal, they have been upheaved in Chiloe in a set of parallel +anticlinal and uniclinal lines ranging north and south,--in the district +near P. Rumena by eight or nine far-extended, most symmetrical, uniclinal +lines ranging nearly east and west,--and in the neighbourhood of Concepcion +by less regular single lines, directed both N.E. and S.W., and N.W. and +S.E. This fact is of some interest, as showing that within a period which +cannot be considered as very ancient in relation to the history of the +continent, the strata between the Cordillera and the Pacific have been +broken up in the same variously directed manner as have the old plutonic +and metamorphic rocks in this same district. The second point is, that the +sandstone between Concepcion and Southern Chiloe is everywhere +lignitiferous, and includes much silicified wood; whereas the formations in +Northern Chile do not include beds of lignite or coal, and in place of the +fragments of silicified wood there are silicified bones. Now, at the +present day, from Cape Horn to near Concepcion, the land is entirely +concealed by forests, which thin out at Concepcion, and in Central and +Northern Chile entirely disappear. This coincidence in the distribution of +the fossil wood and the living forests may be quite accidental; but I +incline to take a different view of it; for, as the difference in climate, +on which the presence of forests depends, is here obviously in chief part +due to the form of the land, and as the Cordillera undoubtedly existed when +the lignitiferous beds were accumulating, I conceive it is not improbable +that the climate, during the lignitiferous period, varied on different +parts of the coast in a somewhat similar manner as it now does. Looking to +an earlier epoch, when the strata of the Cordillera were depositing, there +were islands which even in the latitude of Northern Chile, where now all is +irreclaimably desert, supported large coniferous forests. + +TABLE 4. + +Column 1. Genera, with living and tertiary species on the west coast of +South America. (M. d'Orbigny states that the genus Natica is not found on +the coast of Chile; but Mr. Cuming found it at Valparaiso. Scalaria was +found at Valparaiso; Arca, at Iquique, in latitude 20, by Mr. Cuming; Arca, +also, was found by Captain King, at Juan Fernandez, in latitude 33 degrees +30'S.) + +Column 2. Latitudes, in which found fossil on the coasts of Chile and Peru. +(In degrees and minutes.) + +Column 3. Southernmost latitude, in which found living on the west coast of +South America. (In degrees and minutes.) + +Bulla : 30 to 43 30 : 12 near Lima. + +Cassis : 34 : 1 37. + +Pyrula : 34 (and 36 30 at Concepcion) : 5 Payta. + +Fusus : 30 and 43 30 : 23 Mexillones; reappears at the St. of Magellan. + +Pleurotoma : 34 to 43 30 : 2 18 St. Elena. + +Terebra : 34 : 5 Payta. + +Sigaretus : 34 to 44 30 : 12 Lima. + +Anomia : 30 : 7 48. + +Perna : 30 : 1 23 Xixappa. + +Cardium : 30 to 34 (and 36 30 at Concepcion) : 5 Payta. + +Artemis : 30 : 5 Payta. + +Voluta : 34 to 44 30 : Mr. Cuming does not know of any species living on +the west coast, between the equator and latitude 43 south; from this +latitude a species is found as far south as Tierra del Fuego. + +Seventy-nine species of fossil shells, in a tolerably recognisable +condition, from the coast of Chile and Peru, are described in this volume, +and in the Palaeontological part of M. d'Orbigny's "Voyage": if we put on +one side the twenty species exclusively found at Concepcion and Chiloe, +fifty-nine species from Navidad and the other specified localities remain. +Of these fifty-nine species only an Artemis, a Mytilus and Balanus, all +from Coquimbo, are (in the opinion of Mr. Sowerby, but not in that of M. +d'Orbigny) identical with living shells; and it would certainly require a +better series of specimens to render this conclusion certain. Only the +Turritella Chilensis from Huafo and Mocha, the T. Patagonica and Venus +meridionalis from Navidad, come very near to recent South American shells, +namely, the two Turritellas to T. cingulata, and the Venus to V. exalbida: +some few other species come rather less near; and some few resemble forms +in the older European tertiary deposits: none of the species resemble +secondary forms. Hence I conceive there can be no doubt that these +formations are tertiary,--a point necessary to consider, after the case of +Concepcion. The fifty-nine species belong to thirty-two genera; of these, +Gastridium is extinct, and three or four of the genera (viz. Panopaea, +Rostellaria, Corbis (?), and I believe Solecurtus) are not now found on the +west coast of South America. Fifteen of the genera have on this coast +living representatives in about the same latitudes with the fossil species; +but twelve genera now range very differently to what they formerly did. The +idea of Table 4, in which the difference between the extension in latitude +of the fossil and existing species is shown, is taken from M. d'Orbigny's +work; but the range of the living shells is given on the authority of Mr. +Cuming, whose long-continued researches on the conchology of South America +are well-known. + +When we consider that very few, if any, of the fifty-nine fossil shells are +identical with, or make any close approach to, living species; when we +consider that some of the genera do not now exist on the west coast of +South America, and that no less than twelve genera out of the thirty-two +formerly ranged very differently from the existing species of the same +genera, we must admit that these deposits are of considerable antiquity, +and that they probably verge on the commencement of the tertiary era. May +we not venture to believe, that they are of nearly contemporaneous origin +with the Eocene formations of the northern hemisphere? + +Comparing the fossil remains from the coast of Chile (leaving out, as +before, Concepcion and Chiloe) with those from Patagonia, we may conclude, +from their generic resemblance, and from the small number of the species +which from either coast approach closely to living forms, that the +formations of both belong to nearly the same epoch; and this is the opinion +of M. D'Orbigny. Had not a single fossil shell been common to the two +coasts, it could not have been argued that the formations belonged to +different ages; for Messrs. Cuming and Hinds have found, on the comparison +of nearly two thousand living species from the opposite sides of South +America, only one in common, namely, the Purpura lapillus from both sides +of the Isthmus of Panama: even the shells collected by myself amongst the +Chonos Islands and on the coast of Patagonia, are dissimilar, and we must +descend to the apex of the continent, to Tierra del Fuego, to find these +two great conchological provinces united into one. Hence it is remarkable +that four or five of the fossil shells from Navidad, namely, Voluta alta, +Turritella Patagonica, Trochus collaris, Venus meridionalis, perhaps Natica +solida, and perhaps the large oyster from Coquimbo, are considered by Mr. +Sowerby as identical with species from Santa Cruz and P. Desire. M. +d'Orbigny, however, admits the perfect identity only of the Trochus. + +ON THE TEMPERATURE OF THE TERTIARY PERIOD. + +As the number of the fossil species and genera from the western and eastern +coasts is considerable, it will be interesting to consider the probable +nature of the climate under which they lived. We will first take the case +of Navidad, in latitude 34 degrees, where thirty-one species were +collected, and which, as we shall presently see, must have inhabited +shallow water, and therefore will necessarily well exhibit the effects of +temperature. Referring to Table 4 we find that the existing species of the +genera Cassis, Pyrula, Pleurotoma, Terebra, and Sigaretus, which are +generally (though by no means invariably) characteristic of warmer +latitudes, do not at the present day range nearly so far south on this line +of coast as the fossil species formerly did. Including Coquimbo, we have +Perna in the same predicament. The first impression from this fact is, that +the climate must formerly have been warmer than it now is; but we must be +very cautious in admitting this, for Cardium, Bulla, and Fusus (and, if we +include Coquimbo, Anomia and Artemis) likewise formerly ranged farther +south than they now do; and as these genera are far from being +characteristic of hot climates, their former greater southern range may +well have been owing to causes quite distinct from climate: Voluta, again, +though generally so tropical a genus, is at present confined on the west +coast to colder or more southern latitudes than it was during the tertiary +period. The Trochus collaris, moreover, and, as we have just seen according +to Mr. Sowerby, two or three other species, formerly ranged from Navidad as +far south as Santa Cruz in latitude 50 degrees. If, instead of comparing +the fossils of Navidad, as we have hitherto done, with the shells now +living on the west coast of South America, we compare them with those found +in other parts of the world, under nearly similar latitudes; for instance, +in the southern parts of the Mediterranean or of Australia, there is no +evidence that the sea off Navidad was formerly hotter than what might have +been expected from its latitude, even if it was somewhat warmer than it now +is when cooled by the great southern polar current. Several of the most +tropical genera have no representative fossils at Navidad; and there are +only single species of Cassis, Pyrula, and Sigaretus, two of Pleurotoma and +two of Terebra, but none of these species are of conspicuous size. In +Patagonia, there is even still less evidence in the character of the +fossils, of the climate having been formerly warmer. (It may be worth while +to mention that the shells living at the present day on this eastern side +of South America, in latitude 40 degrees, have perhaps a more tropical +character than those in corresponding latitudes on the shores of Europe: +for at Bahia Blanca and S. Blas, there are two fine species of Voluta and +four of Oliva.) As from the various reasons already assigned, there can be +little doubt that the formations of Patagonia and at least of Navidad and +Coquimbo in Chile, are the equivalents of an ancient stage in the tertiary +formations of the northern hemisphere, the conclusion that the climate of +the southern seas at this period was not hotter than what might have been +expected from the latitude of each place, appears to me highly important; +for we must believe, in accordance with the views of Mr. Lyell, that the +causes which gave to the older tertiary productions of the quite temperate +zones of Europe a tropical character, WERE OF A LOCAL CHARACTER AND DID NOT +AFFECT THE ENTIRE GLOBE. On the other hand, I have endeavoured to show, in +the "Geological Transactions," that, at a much later period, Europe and +North and South America were nearly contemporaneously subjected to ice- +action, and consequently to a colder, or at least more equable, climate +than that now characteristic of the same latitudes. + +ON THE ABSENCE OF EXTENSIVE MODERN CONCHIFEROUS DEPOSITS IN SOUTH AMERICA; +AND ON THE CONTEMPORANEOUSNESS OF THE OLDER TERTIARY DEPOSITS AT DISTANT +POINTS BEING DUE TO CONTEMPORANEOUS MOVEMENTS OF SUBSIDENCE. + +Knowing from the researches of Professor E. Forbes, that molluscous animals +chiefly abound within a depth of 100 fathoms and under, and bearing in mind +how many thousand miles of both coasts of South America have been upraised +within the recent period by a slow, long-continued, intermittent movement,- +-seeing the diversity in nature of the shores and the number of shells now +living on them,--seeing also that the sea off Patagonia and off many parts +of Chile, was during the tertiary period highly favourable to the +accumulation of sediment,--the absence of extensive deposits including +recent shells over these vast spaces of coast is highly remarkable. The +conchiferous calcareous beds at Coquimbo, and at a few isolated points +northward, offer the most marked exception to this statement; for these +beds are from twenty to thirty feet in thickness, and they stretch for some +miles along shore, attaining, however, only a very trifling breadth. At +Valdivia there is some sandstone with imperfect casts of shells, which +POSSIBLY may belong to the recent period: parts of the boulder formation +and the shingle-beds on the lower plains of Patagonia probably belong to +this same period, but neither are fossiliferous: it also so happens that +the great Pampean formation does not include, with the exception of the +Azara, any mollusca. There cannot be the smallest doubt that the upraised +shells along the shores of the Atlantic and Pacific, whether lying on the +bare surface, or embedded in mould or in sand-hillocks, will in the course +of ages be destroyed by alluvial action: this probably will be the case +even with the calcareous beds of Coquimbo, so liable to dissolution by +rain-water. If we take into consideration the probability of oscillations +of level and the consequent action of the tidal-waves at different heights, +their destruction will appear almost certain. Looking to an epoch as far +distant in futurity as we now are from the past Miocene period, there seems +to me scarcely a chance, under existing conditions, of the numerous shells +now living in those zones of depths most fertile in life, and found +exclusively on the western and south-eastern coasts of South America, being +preserved to this imaginary distant epoch. A whole conchological series +will in time be swept away, with no memorials of their existence preserved +in the earth's crust. + +Can any light be thrown on this remarkable absence of recent conchiferous +deposits on these coasts, on which, at an ancient tertiary epoch, strata +abounding with organic remains were extensively accumulated? I think there +can, namely, by considering the conditions necessary for the preservation +of a formation to a distant age. Looking to the enormous amount of +denudation which on all sides of us has been effected,--as evidenced by the +lofty cliffs cutting off on so many coasts horizontal and once far-extended +strata of no great antiquity (as in the case of Patagonia),--as evidenced +by the level surface of the ground on both sides of great faults and +dislocations,--by inland lines of escarpments, by outliers, and numberless +other facts, and by that argument of high generality advanced by Mr. Lyell, +namely, that every SEDIMENTARY formation, whatever its thickness may be, +and over however many hundred square miles it may extend, is the result and +the measure of an equal amount of wear and tear of pre-existing formations; +considering these facts, we must conclude that, as an ordinary rule, a +formation to resist such vast destroying powers, and to last to a distant +epoch, must be of wide extent, and either in itself, or together with +superincumbent strata, be of great thickness. In this discussion, we are +considering only formations containing the remains of marine animals, +which, as before mentioned, live, with some exceptions within (most of them +much within) depths of 100 fathoms. How, then, can a thick and widely +extended formation be accumulated, which shall include such organic +remains? First, let us take the case of the bed of the sea long remaining +at a stationary level: under these circumstances it is evident that +CONCHIFEROUS strata can accumulate only to the same thickness with the +depth at which the shells can live; on gently inclined coasts alone can +they accumulate to any considerable width; and from the want of +superincumbent pressure, it is probable that the sedimentary matter will +seldom be much consolidated: such formations have no very good chance, when +in the course of time they are upraised, of long resisting the powers of +denudation. The chance will be less if the submarine surface, instead of +having remained stationary, shall have gone on slowly rising during the +deposition of the strata, for in this case their total thickness must be +less, and each part, before being consolidated or thickly covered up by +superincumbent matter, will have had successively to pass through the +ordeal of the beach; and on most coasts, the waves on the beach tend to +wear down and disperse every object exposed to their action. Now, both on +the south-eastern and western shores of South America, we have had clear +proofs that the land has been slowly rising, and in the long lines of lofty +cliffs, we have seen that the tendency of the sea is almost everywhere to +eat into the land. Considering these facts, it ceases, I think, to be +surprising, that extensive recent conchiferous deposits are entirely absent +on the southern and western shores of America. + +Let us take the one remaining case, of the bed of the sea slowly subsiding +during a length of time, whilst sediment has gone on being deposited. It is +evident that strata might thus accumulate to any thickness, each stratum +being deposited in shallow water, and consequently abounding with those +shells which cannot live at great depths: the pressure, also, I may +observe, of each fresh bed would aid in consolidating all the lower ones. +Even on a rather steep coast, though such must ever be unfavourable to +widely extended deposits, the formations would always tend to increase in +breadth from the water encroaching on the land. Hence we may admit that +periods of slow subsidence will commonly be most favourable to the +accumulation of CONCHIFEROUS deposits, of sufficient thickness, extension, +and hardness, to resist the average powers of denudation. + +We have seen that at an ancient tertiary epoch, fossiliferous deposits were +extensively deposited on the coasts of South America; and it is a very +interesting fact, that there is evidence that these ancient tertiary beds +were deposited during a period of subsidence. Thus, at Navidad, the strata +are about eight hundred feet in thickness, and the fossil shells are +abundant both at the level of the sea and some way up the cliffs; having +sent a list of these fossils to Professor E. Forbes, he thinks they must +have lived in water between one and ten fathoms in depth: hence the bottom +of the sea on which these shells once lived must have subsided at least 700 +feet to allow of the superincumbent matter being deposited. I must here +remark, that, as all these and the following fossil shells are extinct +species, Professor Forbes necessarily judges of the depths at which they +lived only from their generic character, and from the analogical +distribution of shells in the northern hemisphere; but there is no just +cause from this to doubt the general results. At Huafo the strata are about +the same thickness, namely, 800 feet, and Professor Forbes thinks the +fossils found there cannot have lived at a greater depth than fifty +fathoms, or 300 feet. These two points, namely, Navidad and Huafo, are 570 +miles apart, but nearly halfway between them lies Mocha, an island 1,200 +feet in height, apparently formed of tertiary strata up to its level +summit, and with many shells, including the same Turritella with that found +at Huafo, embedded close to the level of the sea. In Patagonia, shells are +numerous at Santa Cruz, at the foot of the 350 feet plain, which has +certainly been formed by the denudation of the 840 feet plain, and +therefore was originally covered by strata that number of feet in +thickness, and these shells, according to Professor Forbes, probably lived +at a depth of between seven and fifteen fathoms: at Port S. Julian, sixty +miles to the north, shells are numerous at the foot of the ninety feet +plain (formed by the denudation of the 950 feet plain), and likewise +occasionally at the height of several hundred feet in the upper strata; +these shells must have lived in water somewhere between five and fifty +fathoms in depth. Although in other parts of Patagonia I have no direct +evidence of shoal-water shells having been buried under a great thickness +of superincumbent submarine strata, yet it should be borne in mind that the +lower fossiliferous strata with several of the same species of Mollusca, +the upper tufaceous beds, and the high summit-plain, stretch for a +considerable distance southward, and for hundreds of miles northward; +seeing this uniformity of structure, I conceive it may be fairly concluded +that the subsidence by which the shells at Santa Cruz and S. Julian were +carried down and covered up, was not confined to these two points, but was +co-extensive with a considerable portion of the Patagonian tertiary +formation. In a succeeding chapter it will be seen, that we are led to a +similar conclusion with respect to the secondary fossiliferous strata of +the Cordillera, namely, that they also were deposited during a long- +continued and great period of subsidence. +From the foregoing reasoning, and from the facts just given, I think we +must admit the probability of the following proposition: namely, that when +the bed of the sea is either stationary or rising, circumstances are far +less favourable, than when the level is sinking, to the accumulation of +CONCHIFEROUS deposits of sufficient thickness and extension to resist, when +upheaved, the average vast amount of denudation. This result appears to me, +in several respects, very interesting: every one is at first inclined to +believe that at innumerable points, wherever there is a supply of sediment, +fossiliferous strata are now forming, which at some future distant epoch +will be upheaved and preserved; but on the views above given, we must +conclude that this is far from being the case; on the contrary, we require +(1st), a long-continued supply of sediment; (2nd), an extensive shallow +area; and (3rd), that this area shall slowly subside to a great depth, so +as to admit the accumulation of a widely extended thick mass of +superincumbent strata. In how few parts of the world, probably, do these +conditions at the present day concur! We can thus, also, understand the +general want of that close sequence in fossiliferous formations which we +might theoretically have anticipated; for, without we suppose a subsiding +movement to go on at the same spot during an enormous period, from one +geological era to another, and during the whole of this period sediment to +accumulate at the proper rate, so that the depth should not become too +great for the continued existence of molluscous animals, it is scarcely +possible that there should be a perfect sequence at the same spot in the +fossil shells of the two geological formations. (Professor H.D. Rogers, in +his excellent address to the Association of American Geologists +("Silliman's Journal" volume 47 page 277) makes the following remark: "I +question if we are at all aware how COMPLETELY the whole history of all +departed time lies indelibly recorded with the amplest minuteness of detail +in the successive sediments of the globe, how effectually, in other words, +every period of time HAS WRITTEN ITS OWN HISTORY, carefully preserving +every created form and every trace of action." I think the correctness of +such remarks is more than doubtful, even if we except (as I suppose he +would) all those numerous organic forms which contain no hard parts.) So +far from a very long-continued subsidence being probable, many facts lead +to the belief that the earth's surface oscillates up and down; and we have +seen that during the elevatory movements there is but a small chance of +DURABLE fossiliferous deposits accumulating. + +Lastly, these same considerations appear to throw some light on the fact +that certain periods appear to have been favourable to the deposition, or +at least to the preservation, of contemporaneous formations at very distant +points. We have seen that in South America an enormous area has been rising +within the recent period; and in other quarters of the globe immense spaces +appear to have risen contemporaneously. From my examination of the coral- +reefs of the great oceans, I have been led to conclude that the bed of the +sea has gone on slowly sinking within the present era, over truly vast +areas: this, indeed, is in itself probable, from the simple fact of the +rising areas having been so large. In South America we have distinct +evidence that at nearly the same tertiary period, the bed of the sea off +parts of the coast of Chile and off Patagonia was sinking, though these +regions are very remote from each other. If, then, it holds good, as a +general rule, that in the same quarter of the globe the earth's crust tends +to sink and rise contemporaneously over vast spaces, we can at once see, +that we have at distant points, at the same period, those very conditions +which appear to be requisite for the accumulation of fossiliferous masses +of sufficient extension, thickness, and hardness, to resist denudation, and +consequently to last unto an epoch distant in futurity. (Professor Forbes +has some admirable remarks on this subject, in his "Report on the Shells of +the Aegean Sea." In a letter to Mr. Maclaren ("Edinburgh New Philosophical +Journal" January 1843), I partially entered into this discussion, and +endeavoured to show that it was highly improbable, that upraised atolls or +barrier-reefs, though of great thickness, should, owing to their small +extension or breadth, be preserved to a distant future period.) + + +CHAPTER VI. PLUTONIC AND METAMORPHIC ROCKS:--CLEAVAGE AND FOLIATION. + +Brazil, Bahia, gneiss with disjointed metamorphosed dikes. +Strike of foliation. +Rio de Janeiro, gneiss-granite, embedded fragment in, decomposition of. +La Plata, metamorphic and old volcanic rocks of. +S. Ventana. +Claystone porphyry formation of Patagonia; singular metamorphic rocks; +pseudo-dikes. +Falkland Islands, Palaeozoic fossils of. +Tierra del Fuego, clay-slate formation, cretaceous fossils of; cleavage and +foliation; form of land. +Chonos Archipelago, mica-schists, foliation disturbed by granitic axis; +dikes. +Chiloe. +Concepcion, dikes, successive formation of. +Central and Northern Chile. +Concluding remarks on cleavage and foliation. +Their close analogy and similar origin. +Stratification of metamorphic schists. +Foliation of intrusive rocks. +Relation of cleavage and foliation to the lines of tension during +metamorphosis. + +The metamorphic and plutonic formations of the several districts visited by +the "Beagle" will be here chiefly treated of, but only such cases as appear +to me new, or of some special interest, will be described in detail; at the +end of the chapter I will sum up all the facts on cleavage and foliation,-- +to which I particularly attended. + +BAHIA, BRAZIL: latitude 13 degrees south. + +The prevailing rock is gneiss, often passing, by the disappearance of the +quartz and mica, and by the feldspar losing its red colour, into a +brilliantly grey primitive greenstone. Not unfrequently quartz and +hornblende are arranged in layers in almost amorphous feldspar. There is +some fine-grained syenitic granite, orbicularly marked by ferruginous +lines, and weathering into vertical, cylindrical holes, almost touching +each other. In the gneiss, concretions of granular feldspar and others of +garnets with mica occur. The gneiss is traversed by numerous dikes composed +of black, finely crystallised, hornblendic rock, containing a little glassy +feldspar and sometimes mica, and varying in thickness from mere threads to +ten feet: these threads, which are often curvilinear, could sometimes be +traced running into the larger dikes. One of these dikes was remarkable +from having been in two or three places laterally disjointed, with unbroken +gneiss interposed between the broken ends, and in one part with a portion +of the gneiss driven, apparently whilst in a softened state, into its side +or wall. In several neighbouring places, the gneiss included angular, well- +defined, sometimes bent, masses of hornblende rock, quite like, except in +being more perfectly crystallised, that forming the dikes, and, at least in +one instance, containing (as determined by Professor Miller) augite as well +as hornblende. In one or two cases these angular masses, though now quite +separate from each other by the solid gneiss, had, from their exact +correspondence in size and shape, evidently once been united; hence I +cannot doubt that most or all of the fragments have been derived from the +breaking up of the dikes, of which we see the first stage in the above- +mentioned laterally disjointed one. The gneiss close to the fragments +generally contained many large crystals of hornblende, which are entirely +absent or rare in other parts: its folia or laminae were gently bent round +the fragments, in the same manner as they sometimes are round concretions. +Hence the gneiss has certainly been softened, its composition modified, and +its folia arranged, subsequently to the breaking up of the dikes, these +latter also having been at the same time bent and softened. (Professor +Hitchcock "Geology of Massachusetts" volume 2 page 673, gives a closely +similar case of a greenstone dike in syenite.) + +I must here take the opportunity of premising, that by the term CLEAVAGE I +imply those planes of division which render a rock, appearing to the eye +quite or nearly homogeneous, fissile. By the term FOLIATION, I refer to the +layers or plates of different mineralogical nature of which most +metamorphic schists are composed; there are, also, often included in such +masses, alternating, homogeneous, fissile layers or folia, and in this case +the rock is both foliated and has a cleavage. By STRATIFICATION, as applied +to these formations, I mean those alternate, parallel, large masses of +different composition, which are themselves frequently either foliated or +fissile,--such as the alternating so-called strata of mica-slate, gneiss, +glossy clay-slate, and marble. + +The folia of the gneiss within a few miles round Bahia generally strike +irregularly, and are often curvilinear, dipping in all directions at +various angles: but where best defined, they extended most frequently in a +N.E. by N. (or East 50 degrees N.) and S.W. by S. line, corresponding +nearly with the coast-line northwards of the bay. I may add that Mr. +Gardner found in several parts of the province of Ceara, which lies between +four and five hundred miles north of Bahia, gneiss with the folia extending +E. 45 degrees N.; and in Guyana according to Sir R. Schomburgk, the same +rock strikes E. 57 degrees N. Again, Humboldt describes the gneiss-granite +over an immense area in Venezuela and even in Colombia, as striking E. 50 +degrees N., and dipping to the N.W. at an angle of fifty degrees. (Gardner +"Geological Section of the British Association" 1840. For Sir R. +Schomburgk's observations see "Geographical Journal" 1842 page 190. See +also Humboldt's discussion on Loxodrism in the "Personal Narrative.") Hence +all the observations hitherto made tend to show that the gneissic rocks +over the whole of this part of the continent have their folia extending +generally within almost a point of the compass of the same direction. (I +landed at only one place north of Bahia, namely, at Pernambuco. I found +there only soft, horizontally stratified matter, formed from disintegrated +granitic rocks, and some yellowish impure limestone, probably of a tertiary +epoch. I have described a most singular natural bar of hard sandstone, +which protects the harbour, in the 19th volume 1841 page 258 of the "London +and Edinburgh Philosophical Magazine." + +ABROLHOS ISLETS, Latitude 18 degrees S. off the coast of Brazil. + +Although not strictly in place, I do not know where I can more conveniently +describe this little group of small islands. The lowest bed is a sandstone +with ferruginous veins; it weathers into an extraordinary honeycombed mass; +above it there is a dark-coloured argillaceous shale; above this a coarser +sandstone--making a total thickness of about sixty feet; and lastly, above +these sedimentary beds, there is a fine conformable mass of greenstone, in +some parts having a columnar structure. All the strata, as well as the +surface of the land, dip at an angle of about 12 degrees to N. by W. Some +of the islets are composed entirely of the sedimentary, others of the +trappean rocks, generally, however, with the sandstone, cropping out on the +southern shores.) + +RIO DE JANEIRO. + +This whole district is almost exclusively formed of gneiss, abounding with +garnets, and porphyritic with large crystals, even three and four inches in +length, of orthoclase feldspar: in these crystals mica and garnets are +often enclosed. At the western base of the Corcovado, there is some +ferruginous carious quartz-rock; and in the Tijeuka range, much fine- +grained granite. I observed boulders of greenstone in several places; and +on the islet of Villegagnon, and likewise on the coast some miles +northward, two large trappean dikes. The porphyritic gneiss, or gneiss- +granite as it has been called by Humboldt, is only so far foliated that the +constituent minerals are arranged with a certain degree of regularity, and +may be said to have a "GRAIN," but they are not separated into distinct +folia or laminae. There are, however, several other varieties of gneiss +regularly foliated, and alternating with each other in so-called strata. +The stratification and foliation of the ordinary gneisses, and the +foliation or "grain" of the gneiss-granite, are parallel to each other, and +generally strike within a point of N.E. and S.W. dipping at a high angle +(between 50 and 60 degrees) generally to S.E.: so that here again we meet +with the strike so prevalent over the more northern parts of this +continent. The mountains of gneiss-granite are to a remarkable degree +abruptly conical, which seems caused by the rock tending to exfoliate in +thick, conically concentric layers: these peaks resemble in shape those of +phonolite and other injected rocks on volcanic islands; nor is the grain or +foliation (as we shall afterwards see) any difficulty on the idea of the +gneiss-granite having been an intrusive rather than a metamorphic +formation. The lines of mountains, but not always each separate hill, range +nearly in the same direction with the foliation and so-called +stratification, but rather more easterly. + +(FIGURE 22. FRAGMENT OF GNEISS EMBEDDED IN ANOTHER VARIETY OF THE SAME +ROCK.) + +On a bare gently inclined surface of the porphyritic gneiss in Botofogo +Bay, I observed the appearance represented in Figure 22. +A fragment seven yards long and two in width, with angular and distinctly +defined edges, composed of a peculiar variety of gneiss with dark layers of +mica and garnets, is surrounded on all sides by the ordinary gneiss- +granite; both having been dislocated by a granitic vein. The folia in the +fragment and in the surrounding rock strike in the same N.N.E. and S.S.W. +line; but in the fragment they are vertical, whereas in the gneiss-granite +they dip at a small angle, as shown by the arrows, to S.S.E. This fragment, +considering its great size, its solitary position, and its foliated +structure parallel to that of the surrounding rock, is, as far as I know, a +unique case: and I will not attempt any explanation of its origin. + +The numerous travellers in this country, have all been greatly surprised at +the depth to which the gneiss and other granitic rocks, as well as the +talcose slates of the interior, have been decomposed. (Spix and Martius +have collected in an Appendix to their "Travels," the largest body of facts +on this subject. See also some remarks by M. Lund in his communications to +the Academy at Copenhagen; and others by M. Gaudichaud in Freycinet +"Voyage.") Near Rio, every mineral except the quartz has been completely +softened, in some places to a depth little less than one hundred feet. (Dr. +Benza describes granitic rock, "Madras Journal of Literature" etc. October +183? page 246), in the Neelgherries, decomposed to a depth of forty feet.) +The minerals retain their positions in folia ranging in the usual +direction; and fractured quartz veins may be traced from the solid rock, +running for some distance into the softened, mottled, highly coloured, +argillaceous mass. It is said that these decomposed rocks abound with gems +of various kinds, often in a fractured state, owing, as some have supposed, +to the collapse of geodes, and that they contain gold and diamonds. At Rio, +it appeared to me that the gneiss had been softened before the excavation +(no doubt by the sea) of the existing, broad, flat-bottomed valleys; for +the depth of decomposition did not appear at all conformable with the +present undulations of the surface. The porphyritic gneiss, where now +exposed to the air, seems to withstand decomposition remarkably well; and I +could see no signs of any tendency to the production of argillaceous masses +like those here described. I was also struck with the fact, that where a +bare surface of this rock sloped into one of the quiet bays, there were no +marks of erosion at the level of the water, and the parts both beneath and +above it preserved a uniform curve. At Bahia, the gneiss rocks are +similarly decomposed, with the upper parts insensibly losing their +foliation, and passing, without any distinct line of separation, into a +bright red argillaceous earth, including partially rounded fragments of +quartz and granite. From this circumstance, and from the rocks appearing to +have suffered decomposition before the excavation of the valleys, I suspect +that here, as at Rio, the decomposition took place under the sea. The +subject appeared to me a curious one, and would probably well repay careful +examination by an able mineralogist. + +THE NORTHERN PROVINCES OF LA PLATA. + +According to some observations communicated to me by Mr. Fox, the coast +from Rio de Janeiro to the mouth of the Plata seems everywhere to be +granitic, with a few trappean dikes. At Port Alegre, near the boundary of +Brazil, there are porphyries and diorites. (M. Isabelle "Voyage a Buenos +Ayres" page 479.) At the mouth of the Plata, I examined the country for +twenty-five miles west, and for about seventy miles north of Maldonado: +near this town, there is some common gneiss, and much, in all parts of the +country, of a coarse-grained mixture of quartz and reddish feldspar, often, +however, assuming a little dark-green imperfect hornblende, and then +immediately becoming foliated. The abrupt hillocks thus composed, as well +as the highly inclined folia of the common varieties of gneiss, strike +N.N.E. or a little more easterly, and S.S.W. Clay-slate is occasionally met +with, and near the L. del Potrero, there is white marble, rendered fissile +from the presence of hornblende, mica, and asbestus; the cleavage of these +rocks and their stratification, that is the alternating masses thus +composed, strike N.N.E. and S.S.W. like the foliated gneisses, and have an +almost vertical dip. The Sierra Larga, a low range five miles west of +Maldonado, consists of quartzite, often ferruginous, having an arenaceous +feel, and divided into excessively thin, almost vertical laminae or folia +by microscopically minute scales, apparently of mica, and striking in the +usual N.N.E. and S.S.W. direction. The range itself is formed of one +principal line with some subordinate ones; and it extends with remarkable +uniformity far northward (it is said even to the confines of Brazil), in +the same line with the vertically ribboned quartz rock of which it is +composed. The S. de Las Animas is the highest range in the country; I +estimated it at 1,000 feet; it runs north and south, and is formed of +feldspathic porphyry; near its base there is a N.N.W. and S.S.E. ridge of a +conglomerate in a highly porphyritic basis. + +Northward of Maldonado, and south of Las Minas, there is an E. and W. hilly +band of country, some miles in width, formed of siliceous clay-slate, with +some quartz, rock, and limestone, having a tortuous irregular cleavage, +generally ranging east and west. E. and S.E. of Las Minas there is a +confused district of imperfect gneiss and laminated quartz, with the hills +ranging in various directions, but with each separate hill generally +running in the same line with the folia of the rocks of which it is +composed: this confusion appears to have been caused by the intersection of +the [E. and W.] and [N.N.E. and S.S.W.] strikes. Northward of Las Minas, +the more regular northerly ranges predominate: from this place to near +Polanco, we meet with the coarse-grained mixture of quartz and feldspar, +often with the imperfect hornblende, and then becoming foliated in a N. and +S. line--with imperfect clay-slate, including laminae of red crystallised +feldspar--with white or black marble, sometimes containing asbestus and +crystals of gypsum--with quartz-rock--with syenite--and lastly, with much +granite. The marble and granite alternate repeatedly in apparently vertical +masses: some miles northward of the Polanco, a wide district is said to be +entirely composed of marble. It is remarkable, how rare mica is in the +whole range of country north and westward of Maldonado. Throughout this +district, the cleavage of the clay-slate and marble--the foliation of the +gneiss and the quartz--the stratification or alternating masses of these +several rocks--and the range of the hills, all coincide in direction; and +although the country is only hilly, the planes of division are almost +everywhere very highly inclined or vertical. + +Some ancient submarine volcanic rocks are worth mentioning, from their +rarity on this eastern side of the continent. In the valley of the Tapas +(fifty or sixty miles N. of Maldonado) there is a tract three or four miles +in length, composed of various trappean rocks with glassy feldspar--of +apparently metamorphosed grit-stones--of purplish amygdaloids with large +kernels of carbonate of lime (Near the Pan de Azucar there is some greenish +porphyry, in one place amygdaloidal with agate.)--and much of a harshish +rock with glassy feldspar intermediate in character between claystone +porphyry and trachyte. This latter rock was in one spot remarkable from +being full of drusy cavities, lined with quartz crystals, and arranged in +planes, dipping at an angle of 50 degrees to the east, and striking +parallel to the foliation of an adjoining hill composed of the common +mixture of quartz, feldspar, and imperfect hornblende: this fact perhaps +indicates that these volcanic rocks have been metamorphosed, and their +constituent parts rearranged, at the same time and according to the same +laws, with the granitic and metamorphic formations of this whole region. In +the valley of the Marmaraya, a few miles south of the Tapas, a band of +trappean and amygdaloidal rock is interposed between a hill of granite and +an extensive surrounding formation of red conglomerate, which (like that at +the foot of the S. Animas) has its basis porphyritic with crystals of +feldspar, and which hence has certainly suffered metamorphosis. + +MONTE VIDEO. + +The rocks here consist of several varieties of gneiss, with the feldspar +often yellowish, granular and imperfectly crystallised, alternating with, +and passing insensibly into, beds, from a few yards to nearly a mile in +thickness, of fine or coarse grained, dark-green hornblendic slate; this +again often passing into chloritic schist. These passages seem chiefly due +to changes in the mica, and its replacement by other minerals. At Rat +Island I examined a mass of chloritic schist, only a few yards square, +irregularly surrounded on all sides by the gneiss, and intricately +penetrated by many curvilinear veins of quartz, which gradually BLEND into +the gneiss: the cleavage of the chloritic schist and the foliation of the +gneiss were exactly parallel. Eastward of the city there is much fine- +grained, dark-coloured gneiss, almost assuming the character of hornblende- +slate, which alternates in thin laminae with laminae of quartz, the whole +mass being transversely intersected by numerous large veins of quartz: I +particularly observed that these veins were absolutely continuous with the +alternating laminae of quartz. In this case and at Rat Island, the passage +of the gneiss into imperfect hornblendic or into chloritic slate, seemed to +be connected with the segregation of the veins of quartz. (Mr. Greenough +page 78 "Critical Examination" etc., observes that quartz in mica-slate +sometimes appears in beds and sometimes in veins. Von Buch also in his +"Travels in Norway" page 236, remarks on alternating laminae of quartz and +hornblende-slate replacing mica-schist.) + +The Mount, a hill believed to be 450 feet in height, from which the place +takes its name, is much the highest land in this neighbourhood: it consists +of hornblendic slate, which (except on the eastern and disturbed base) has +an east and west nearly vertical cleavage; the longer axis of the hill also +ranges in this same line. Near the summit the hornblende-slate gradually +becomes more and more coarsely crystallised, and less plainly laminated, +until it passes into a heavy, sonorous greenstone, with a slaty conchoidal +fracture; the laminae on the north and south sides near the summit dip +inwards, as if this upper part had expanded or bulged outwards. This +greenstone must, I conceive, be considered as metamorphosed hornblende- +slate. The Cerrito, the next highest, but much less elevated point, is +almost similarly composed. In the more western parts of the province, +besides gneiss, there is quartz-rock, syenite, and granite; and at Colla, I +heard of marble. + +Near M. Video, the space which I more accurately examined was about fifteen +miles in an east and west line, and here I found the foliation of the +gneiss and the cleavage of the slates generally well developed, and +extending parallel to the alternating strata composed of the gneiss, +hornblendic and chloritic schists. These planes of division all range +within one point of east and west, frequently east by south and west by +north; their dip is generally almost vertical, and scarcely anywhere under +45 degrees: this fact, considering how slightly undulatory the surface of +the country is, deserves attention. Westward of M. Video, towards the +Uruguay, wherever the gneiss is exposed, the highly inclined folia are seen +striking in the same direction; I must except one spot where the strike was +N.W. by W. The little Sierra de S. Juan, formed of gneiss and laminated +quartz, must also be excepted, for it ranges between [N. to N.E.] and [S. +to S.W.] and seems to belong to the same system with the hills in the +Maldonado district. Finally, we have seen that, for many miles northward of +Maldonado and for twenty-five miles westward of it, as far as the S. de las +Animas, the foliation, cleavage, so-called stratification and lines of +hills, all range N.N.E. and S.S.W., which is nearly coincident with the +adjoining coast of the Atlantic. Westward of the S. de las Animas, as far +as even the Uruguay, the foliation, cleavage, and stratification (but not +lines of hills, for there are no defined ones) all range about E. by S. and +W. by N., which is nearly coincident with the direction of the northern +shore of the Plata; in the confused country near Las Minas, where these two +great systems appear to intersect each other, the cleavage, foliation, and +stratification run in various directions, but generally coincide with the +line of each separate hill. + +SOUTHERN LA PLATA. + +The first ridge, south of the Plata, which projects through the Pampean +formation, is the Sierra Tapalguen and Vulcan, situated 200 miles southward +of the district just described. This ridge is only a few hundred feet in +height, and runs from C. Corrientes in a W.N.W. line for at least 150 miles +into the interior: at Tapalguen, it is composed of unstratified granular +quartz, remarkable from forming tabular masses and small plains, surrounded +by precipitous cliffs: other parts of the range are said to consist of +granite: and marble is found at the S. Tinta. It appears from M. +Parchappe's observations, that at Tandil there is a range of quartzose +gneiss, very like the rocks of the S. Larga near Maldonado, running in the +same N.N.E. and S.S.W. direction; so that the framework of the country here +is very similar to that on the northern shore of the Plata. (M. d'Orbigny's +"Voyage" Part. Geolog. page 46. I have given a short account of the +peculiar forms of the quartz hills of Tapalguen, so unusual in a +metamorphic formation, in my "Journal of Researches" 2nd edition page 116.) + +The Sierra Guitru-gueyu is situated sixty miles south of the S. Tapalguen: +it consists of numerous parallel, sometimes blended together ridges, about +twenty-three miles in width, and five hundred feet in height above the +plain, and extending in a N.W. and S.E. direction. Skirting round the +extreme S.E. termination, I ascended only a few points, which were composed +of a fine-grained gneiss, almost composed of feldspar with a little mica, +and passing in the upper parts of the hills into a rather compact purplish +clay-slate. The cleavage was nearly vertical, striking in a N.W. by W. and +S.E. by E. line, nearly, though not quite, coincident with the direction of +the parallel ridges. + +The Sierra Ventana lies close south of that of Guitru-gueyu; it is +remarkable from attaining a height, very unusual on this side of the +continent, of 3,340 feet. It consists up to its summit, of quartz, +generally pure and white, but sometimes reddish, and divided into thick +laminae or strata: in one part there is a little glossy clay-slate with a +tortuous cleavage. The thick layers of quartz strike in a W. 30 degrees N. +line, dipping southerly at an angle of 45 degrees and upwards. The +principal line of mountains, with some quite subordinate parallel ridges, +range about W. 45 degrees N.: but at their S.E. termination, only W. 25 +degrees N. This Sierra is said to extend between twenty and thirty leagues +into the interior. + +PATAGONIA. + +With the exception perhaps of the hill of S. Antonio (600 feet high) in the +Gulf of S. Matias, which has never been visited by a geologist, crystalline +rocks are not met with on the coast of Patagonia for a space of 380 miles +south of the S. Ventana. At this point (latitude 43 degrees 50 minutes), at +Points Union and Tombo, plutonic rocks are said to appear, and are found, +at rather wide intervals, beneath the Patagonian tertiary formation for a +space of about three hundred miles southward, to near Bird Island, in +latitude 48 degrees 56 minutes. Judging from specimens kindly collected for +me by Mr. Stokes, the prevailing rock at Ports St. Elena, Camerones, +Malaspina, and as far south as the Paps of Pineda, is a purplish-pink or +brownish claystone porphyry, sometimes laminated, sometimes slightly +vesicular, with crystals of opaque feldspar and with a few grains of +quartz; hence these porphyries resemble those immediately to be described +at Port Desire, and likewise a series which I have seen from P. Alegre on +the southern confines of Brazil. This porphyritic formation further +resembles in a singularly close manner the lowest stratified formation of +the Cordillera of Chile, which, as we shall hereafter see, has a vast +range, and attains a great thickness. At the bottom of the Gulf of St. +George, only tertiary deposits appear to be present. At Cape Blanco, there +is quartz rock, very like that of the Falkland Islands, and some hard, blue +siliceous clay-slate. + +At Port Desire there is an extensive formation of the claystone porphyry, +stretching at least twenty-five miles into the interior: it has been +denuded and deeply worn into gullies before being covered up by the +tertiary deposits, through which it here and there projects in hills; those +north of the bay being 440 feet in height. The strata have in several +places been tilted at small angles, generally either to N.N.W. or S.S.E. By +gradual passages and alternations, the porphyries change incessantly in +nature. I will describe only some of the principal mineralogical changes, +which are highly instructive, and which I carefully examined. The +prevailing rock has a compact purplish base, with crystals of earthy or +opaque feldspar, and often with grains of quartz. There are other +varieties, with an almost truly trachytic base, full of little angular +vesicles and crystals of glassy feldspar; and there are beds of black +perfect pitchstone, as well as of a concretionary imperfect variety. On a +casual inspection, the whole series would be thought to be of the same +plutonic or volcanic nature with the trachytic varieties and pitchstone; +but this is far from being the case, as much of the porphyry is certainly +of metamorphic origin. Besides the true porphyries, there are many beds of +earthy, quite white or yellowish, friable, easily fusible matter, +resembling chalk, which under the microscope is seen to consist of minute +broken crystals, and which, as remarked in a former chapter, singularly +resembles the upper tufaceous beds of the Patagonian tertiary formation. +This earthy substance often becomes coarser, and contains minute rounded +fragments of porphyries and rounded grains of quartz, and in one case so +many of the latter as to resemble a common sandstone. These beds are +sometimes marked with true lines of aqueous deposition, separating +particles of different degrees of coarseness; in other cases there are +parallel ferruginous lines not of true deposition, as shown by the +arrangement of the particles, though singularly resembling them. The more +indurated varieties often include many small and some larger angular +cavities, which appear due to the removal of earthy matter: some varieties +contain mica. All these earthy and generally white stones insensibly pass +into more indurated sonorous varieties, breaking with a conchoidal +fracture, yet of small specific gravity; many of these latter varieties +assume a pale purple tint, being singularly banded and veined with +different shades, and often become plainly porphyritic with crystals of +feldspar. The formation of these crystals could be most clearly traced by +minute angular and often partially hollow patches of earthy matter, first +assuming a FIBROUS STRUCTURE, then passing into opaque imperfectly shaped +crystals, and lastly, into perfect glassy crystals. When these crystals +have appeared, and when the basis has become compact, the rock in many +places could not be distinguished from a true claystone porphyry without a +trace of mechanical structure. + +In some parts, these earthy or tufaceous beds pass into jaspery and into +beautifully mottled and banded porcelain rocks, which break into splinters, +translucent at their edges, hard enough to scratch glass, and fusible into +white transparent beads: grains of quartz included in the porcelainous +varieties can be seen melting into the surrounding paste. In other parts, +the earthy or tufaceous beds either insensibly pass into, or alternate +with, breccias composed of large and small fragments of various purplish +porphyries, with the matrix generally porphyritic: these breccias, though +their subaqueous origin is in many places shown both by the arrangement of +their smaller particles and by an oblique or current lamination, also pass +into porphyries, in which every trace of mechanical origin and +stratification has been obliterated. + +Some highly porphyritic though coarse-grained masses, evidently of +sedimentary origin, and divided into thin layers, differing from each other +chiefly in the number of embedded grains of quartz, interested me much from +the peculiar manner in which here and there some of the layers terminated +in abrupt points, quite unlike those produced by a layer of sediment +naturally thinning out, and apparently the result of a subsequent process +of metamorphic aggregation. In another common variety of a finer texture, +the aggregating process had gone further, for the whole mass consisted of +quite short, parallel, often slightly curved layers or patches, of whitish +or reddish finely granulo-crystalline feldspathic matter, generally +terminating at both ends in blunt points; these layers or patches further +tended to pass into wedge or almond-shaped little masses, and these finally +into true crystals of feldspar, with their centres often slightly drusy. +The series was so perfect that I could not doubt that these large crystals, +which had their longer axes placed parallel to each other, had primarily +originated in the metamorphosis and aggregation of alternating layers of +tuff; and hence their parallel position must be attributed (unexpected +though the conclusion may be), not to laws of chemical action, but to the +original planes of deposition. I am tempted briefly to describe three other +singular allied varieties of rock; the first without examination would have +passed for a stratified porphyritic breccia, but all the included angular +fragments consisted of a border of pinkish crystalline feldspathic matter, +surrounding a dark translucent siliceous centre, in which grains of quartz +not quite blended into the paste could be distinguished: this uniformity in +the nature of the fragments shows that they are not of mechanical, but of +concretionary origin, having resulted perhaps from the self-breaking up and +aggregation of layers of indurated tuff containing numerous grains of +quartz,--into which, indeed, the whole mass in one part passed. The second +variety is a reddish non-porphyritic claystone, quite full of spherical +cavities, about half an inch in diameter, each lined with a collapsed crust +formed of crystals of quartz. The third variety also consists of a pale +purple non-porphyritic claystone, almost wholly formed of concretionary +balls, obscurely arranged in layers, of a less compact and paler coloured +claystone; each ball being on one side partly hollow and lined with +crystals of quartz. + +PSEUDO-DIKES. + +Some miles up the harbour, in a line of cliffs formed of slightly +metamorphosed tufaceous and porphyritic claystone beds, I observed three +vertical dikes, so closely resembling in general appearance ordinary +volcanic dikes, that I did not doubt, until closely examining their +composition, that they had been injected from below. The first is straight, +with parallel sides, and about four feet wide; it consists of whitish, +indurated tufaceous matter, precisely like some of the beds intersected by +it. The second dike is more remarkable; it is slightly tortuous, about +eighteen inches thick, and can be traced for a considerable distance along +the beach; it is of a purplish-red or brown colour, and is formed chiefly +of ROUNDED grains of quartz, with broken crystals of earthy feldspar, +scales of black mica, and minute fragments of claystone porphyry, all +firmly united together in a hard sparing base. The structure of this dike +shows obviously that it is of mechanical and sedimentary origin; yet it +thinned out upwards, and did not cut through the uppermost strata in the +cliffs. This fact at first appears to indicate that the matter could not +have been washed in from above (Upfilled fissures are known to occur both +in volcanic and in ordinary sedimentary formations. At the Galapagos +Archipelago "Volcanic Islands" etc., there are some striking examples of +pseudo-dikes composed of hard tuff.); but if we reflect on the suction +which would result from a deep-seated fissure being formed, we may admit +that if the fissure were in any part open to the surface, mud and water +might well be drawn into it along its whole course. The third dike +consisted of a hard, rough, white rock, almost composed of broken crystals +of glassy feldspar, with numerous scales of black mica, cemented in a +scanty base; there was little in the appearance of this rock, to preclude +the idea of its having been a true injected feldspathic dike. The matter +composing these three pseudo-dikes, especially the second one, appears to +have suffered, like the surrounding strata, a certain degree of metamorphic +action; and this has much aided the deceptive appearance. At Bahia, in +Brazil, we have seen that a true injected hornblendic dike, not only has +suffered metamorphosis, but has been dislocated and even diffused in the +surrounding gneiss, under the form of separate crystals and of fragments. + +FALKLAND ISLANDS. + +I have described these islands in a paper published in the third volume of +the "Geological Journal." The mountain-ridges consist of quartz, and the +lower country of clay-slate and sandstone, the latter containing Palaeozoic +fossils. These fossils have been separately described by Messrs. Morris and +Sharpe: some of them resemble Silurian, and others Devonian forms. In the +eastern part of the group the several parallel ridges of quartz extend in a +west and east line; but further westward the line becomes W.N.W. and +E.S.E., and even still more northerly. The cleavage-planes of the clay- +slate are highly inclined, generally at an angle of above 50 degrees, and +often vertical; they strike almost invariably in the same direction with +the quartz ranges. The outline of the indented shores of the two main +islands, and the relative positions of the smaller islets, accord with the +strike both of the main axes of elevation and of the cleavage of the clay- +slate. + +TIERRA DEL FUEGO. + +My notes on the geology of this country are copious, but as they are +unimportant, and as fossils were found only in one district, a brief sketch +will be here sufficient. The east coast from the S. of Magellan (where the +boulder formation is largely developed) to St. Polycarp's Bay is formed of +horizontal tertiary strata, bounded some way towards the interior by a +broad mountainous band of clay-slate. This great clay-slate formation +extends from St. Le Maire westward for 140 miles, along both sides of the +Beagle Channel to near its bifurcation. South of this channel, it forms all +Navarin Island, and the eastern half of Hoste Island and of Hardy +Peninsula; north of the Beagle Channel it extends in a north-west line on +both sides of Admiralty Sound to Brunswick Peninsula in the St. of +Magellan, and I have reason to believe, stretches far up the eastern side +of the Cordillera. The western and broken side of Tierra del Fuego towards +the Pacific is formed of metamorphic schists, granite and various trappean +rocks: the line of separation between the crystalline and clay-slate +formations can generally be distinguished, as remarked by Captain King, by +the parallelism in the clay-slate districts of the shores and channels, +ranging in a line between [W. 20 degrees to 40 degrees N.] and [E. 20 +degrees to 40 degrees S.]. ("Geographical Journal" volume 1 page 155.) + +The clay-slate is generally fissile, sometimes siliceous or ferruginous, +with veins of quartz and calcareous spar; it often assumes, especially on +the loftier mountains, an altered feldspathic character, passing into +feldspathic porphyry: occasionally it is associated with breccia and +grauwacke. At Good Success Bay, there is a little intercalated black +crystalline limestone. At Port Famine much of the clay-slate is calcareous, +and passes either into a mudstone or into grauwacke, including odd-shaped +concretions of dark argillaceous limestone. Here alone, on the shore a few +miles north of Port Famine, and on the summit of Mount Tarn (2,600 feet +high), I found organic remains; they consist of:-- + +1. Ancyloceras simplex, d'Orbigny "Pal Franc" Mount Tarn. +2. Fusus (in imperfect state), d'Orbigny "Pal Franc" Mount Tarn. +3. Natica, d'Orbigny "Pal Franc" Mount Tarn. +4. Pentacrimus, d'Orbigny "Pal Franc" Mount Tarn. +5. Lucina excentrica, G.B. Sowerby, Port Famine. +6. Venus (in imperfect state), G.B. Sowerby, Port Famine. +7. Turbinolia (?), G.B. Sowerby, Port Famine. +8. Hamites elatior, G.B. Sowerby, Port Famine. + +M. d'Orbigny states that MM. Hombron and Grange found in this neighbourhood +an Ancyloceras, perhaps A. simplex, an Ammonite, a Plicatula and Modiola. +("Voyage" Part Geolog. page 242.) M. d'Orbigny believes from the general +character of these fossils, and from the Ancyloceras being identical (as +far as its imperfect condition allows of comparison) with the A. simplex of +Europe, that the formation belongs to an early stage of the Cretaceous +system. Professor E. Forbes, judging only from my specimens, concurs in the +probability of this conclusion. The Hamites elatior of the above list, of +which a description has been given by Mr. Sowerby, and which is remarkable +from its large size, has not been seen either by M. d'Orbigny or Professor +E. Forbes, as, since my return to England, the specimens have been lost. +The great clay-slate formation of Tierra del Fuego being cretaceous, is +certainly a very interesting fact,--whether we consider the appearance of +the country, which, without the evidence afforded by the fossils, would +form the analogy of most known districts, probably have been considered as +belonging to the Palaeozoic series,--or whether we view it as showing that +the age of this terminal portion of the great axis of South America, is the +same (as will hereafter be seen) with the Cordillera of Chile and Peru. + +The clay-slate in many parts of Tierra del Fuego, is broken by dikes and by +great masses of greenstone, often highly hornblendic (In a greenstone-dike +in the Magdalen Channel, the feldspar cleaved with the angle of albite. +This dike was crossed, as well as the surrounding slate, by a large vein of +quartz, a circumstance of unusual occurrence.): almost all the small islets +within the clay-slate districts are thus composed. The slate near the dikes +generally becomes paler-coloured, harder, less fissile, of a feldspathic +nature, and passes into a porphyry or greenstone: in one case, however, it +became more fissile, of a red colour, and contained minute scales of mica, +which were absent in the unaltered rock. On the east side of Ponsonby Sound +some dikes composed of a pale sonorous feldspathic rock, porphyritic with a +little feldspar, were remarkable from their number,--there being within the +space of a mile at least one hundred,--from their nearly equalling in bulk +the intermediate slate,--and more especially from the excessive fineness +(like the finest inlaid carpentry) and perfect parallelism of their +junctions with the almost vertical laminae of clay-slate. I was unable to +persuade myself that these great parallel masses had been injected, until I +found one dike which abruptly thinned out to half its thickness, and had +one of its walls jagged, with fragments of the slate embedded in it. + +In Southern Tierra del Fuego, the clay-slate towards its S.W. boundary, +becomes much altered and feldspathic. Thus on Wollaston Island slate and +grauwacke can be distinctly traced passing into feldspathic rocks and +greenstones, including iron pyrites and epidote, but still retaining traces +of cleavage with the usual strike and dip. One such metamorphosed mass was +traversed by large vein-like masses of a beautiful mixture (as ascertained +by Professor Miller) of green epidote, garnets, and white calcareous spar. +On the northern point of this same island, there were various ancient +submarine volcanic rocks, consisting of amygdaloids with dark bole and +agate,--of basalt with decomposed olivine--of compact lava with glassy +feldspar,--and of a coarse conglomerate of red scoriae, parts being +amygdaloidal with carbonate of lime. The southern part of Wollaston Island +and the whole of Hermite and Horn Islands, seem formed of cones of +greenstone; the outlying islets of Il Defenso and D. Raminez are said to +consist of porphyritic lava. (Determined by Professor Jameson. Weddell's +"Voyage" page 169.) In crossing Hardy Peninsula, the slate still retaining +traces of its usual cleavage, passes into columnar feldspathic rocks, which +are succeeded by an irregular tract of trappean and basaltic rocks, +containing glassy feldspar and much iron pyrites: there is, also, some +harsh red claystone porphyry, and an almost true trachyte, with needles of +hornblende, and in one spot a curious slaty rock divided into quadrangular +columns, having a base almost like trachyte, with drusy cavities lined by +crystals, too imperfect, according to Professor Miller, to be measured, but +resembling Zeagonite. (See Mr. Brooke's Paper in the "London Philosophical +Magazine" volume 10. This mineral occurs in an ancient volcanic rock near +Rome.) In the midst of these singular rocks, no doubt of ancient submarine +volcanic origin, a high hill of feldspathic clay-slate projected, retaining +its usual cleavage. Near this point, there was a small hillock, having the +aspect of granite, but formed of white albite, brilliant crystals of +hornblende (both ascertained by the reflecting goniometer) and mica; but +with no quartz. No recent volcanic district has been observed in any part +of Tierra del Fuego. + +Five miles west of the bifurcation of the Beagle Channel, the slate- +formation, instead of becoming, as in the more southern parts of Tierra del +Fuego, feldspathic, and associated with trappean or old volcanic rocks, +passes by alternations into a great underlying mass of fine gneiss and +glossy clay-slate, which at no great distance is succeeded by a grand +formation of mica-slate containing garnets. The folia of these metamorphic +schists strike parallel to the cleavage-planes of the clay-slate, which +have a very uniform direction over the whole of this part of the country: +the folia, however, are undulatory and tortuous, whilst the cleavage- +laminae of the slate are straight. These schists compose the chief +mountain-chain of Southern Tierra del Fuego, ranging along the north side +of the northern arm of the Beagle Channel, in a short W.N.W. and E.S.E. +line, with two points (Mounts Sarmiento and Darwin) rising to heights of +6,800 and 6,900 feet. On the south-western side of this northern arm of the +Beagle Channel, the clay-slate is seen with its STRATA dipping from the +great chain, so that the metamorphic schists here form a ridge bordered on +each side by clay-slate. Further north, however, to the west of this great +range, there is no clay-slate, but only gneiss, mica, and hornblendic +slates, resting on great barren hills of true granite, and forming a tract +about sixty miles in width. Again, westward of these rocks, the outermost +islands are of trappean formation, which, from information obtained during +the voyages of the "Adventure" and "Beagle," seem, together with granite, +chiefly to prevail along the western coast as far north as the entrance of +the St. of Magellan (See the Paper by Captain King in the "Geographical +Journal"; also a Letter to Dr. Fitton in "Geological Proceedings" volume 1 +page 29; also some observations by Captain Fitzroy "Voyages" volume 1 page +375. I am indebted also to Mr. Lyell for a series of specimens collected by +Lieutenant Graves.): a little more inland, on the eastern side of Clarence +Island and S. Desolation, granite, greenstone, mica-slate, and gneiss +appear to predominate. I am tempted to believe, that where the clay-slate +has been metamorphosed at great depths beneath the surface, gneiss, mica- +slate, and other allied rocks have been formed, but where the action has +taken place nearer the surface, feldspathic porphyries, greenstones, etc., +have resulted, often accompanied by submarine volcanic eruptions. + +Only one other rock, met with in both arms of the Beagle Channel, deserves +any notice, namely a granulo-crystalline mixture of white albite, black +hornblende (ascertained by measurement of the crystals, and confirmed by +Professor Miller), and more or less of brown mica, but without any quartz. +This rock occurs in large masses, closely resembling in external form +granite or syenite: in the southern arm of the Channel, one such mass +underlies the mica-slate, on which clay-slate was superimposed: this +peculiar plutonic rock which, as we have seen, occurs also in Hardy +Peninsula, is interesting, from its perfect similarity with that (hereafter +often to be referred to under the name of andesite) forming the great +injected axes of the Cordillera of Chile. + +The stratification of the clay-slate is generally very obscure, whereas the +cleavage is remarkably well defined: to begin with the extreme eastern +parts of Tierra del Fuego; the cleavage-planes near the St. of Le Maire +strike either W. and E. or W.S.W. and E.N.E., and are highly inclined; the +form of the land, including Staten Island, indicates that the axes of +elevation have run in this same line, though I was unable to distinguish +the planes of stratification. Proceeding westward, I accurately examined +the cleavage of the clay-slate on the northern, eastern, and western sides +(thirty-five miles apart) of Navarin Island, and everywhere found the +laminae ranging with extreme regularity, W.N.W. and E.S.E., seldom varying +more than one point of the compass from this direction. (The clay-slate in +this island was in many places crossed by parallel smooth joints. Out of +five cases, the angle of intersection between the strike of these joints +and that of the cleavage-laminae was in two cases 45 degrees and in two +others 79 degrees.) Both on the east and west coasts, I crossed at right +angles the cleavage-planes for a space of about eight miles, and found them +dipping at an angle of between 45 degrees and 90 degrees, generally to +S.S.W., sometimes to N.N.E., and often quite vertically. The S.S.W. dip was +occasionally succeeded abruptly by a N.N.E. dip, and this by a vertical +cleavage, or again by the S.S.W. dip; as in a lofty cliff on the eastern +end of the island the laminae of slate were seen to be folded into very +large steep curves, ranging in the usual W.N.W. line, I suspect that the +varying and opposite dips may possibly be accounted for by the cleavage- +laminae, though to the eye appearing straight, being parts of large abrupt +curves, with their summits cut off and worn down. + +In several places I was particularly struck with the fact, that the fine +laminae of the clay-slate, where cutting straight through the bands of +stratification, and therefore indisputably true cleavage-planes, differed +slightly in their greyish and greenish tints of colour, in compactness, and +in some of the laminae having a rather more jaspery appearance than others. +I have not seen this fact recorded, and it appears to me important, for it +shows that the same cause which has produced the highly fissile structure, +has altered in a slight degree the mineralogical character of the rock in +the same planes. The bands of stratification, just alluded to, can be +distinguished in many places, especially in Navarin Island, but only on the +weathered surfaces of the slate; they consist of slightly undulatory zones +of different shades of colour and of thicknesses, and resemble the marks +(more closely than anything else to which I can compare them) left on the +inside of a vessel by the draining away of some dirty slightly agitated +liquid: no difference in composition, corresponding with these zones, could +be seen in freshly fractured surfaces. In the more level parts of Navarin +Island, these bands of stratification were nearly horizontal; but on the +flanks of the mountains they were inclined from them, but in no instance +that I saw at a very high angle. There can, I think, be no doubt that these +zones, which appear only on the weathered surfaces, are the last vestiges +of the original planes of stratification, now almost obliterated by the +highly fissile and altered structure which the mass has assumed. + +The clay-slate cleaves in the same W.N.W. and E.S.E. direction, as on +Navarin Island, on both sides of the Beagle Channel, on the eastern side of +Hoste Island, on the N.E. side of Hardy Peninsula, and on the northern +point of Wollaston Island; although in these two latter localities the +cleavage has been much obscured by the metamorphosed and feldspathic +condition of the slate. Within the area of these several islands, including +Navarin Island, the direction of the stratification and of the mountain- +chains is very obscure; though the mountains in several places appeared to +range in the same W.N.W. line with the cleavage: the outline of the coast, +however, does not correspond with this line. Near the bifurcation of the +Beagle Channel, where the underlying metamorphic schists are first seen, +they are foliated (with some irregularities), in this same W.N.W. line, and +parallel, as before stated, to the main mountain-axis of this part of the +country. Westward of this main range, the metamorphic schists are foliated, +though less plainly, in the same direction, which is likewise common to the +zone of old erupted trappean rocks, forming the outermost islets. Hence the +area, over which the cleavage of the slate and the foliation of the +metamorphic schists extends with an average W.N.W. and E.S.E. strike, is +about forty miles in a north and south line, and ninety miles in an east +and west line. + +Further northward, near Port Famine, the stratification of the clay-slate +and of the associated rocks, is well defined, and there alone the cleavage +and strata-planes are parallel. A little north of this port there is an +anticlinal axis ranging N.W. (or a little more westerly) and S.E.: south of +the port, as far as Admiralty Sound and Gabriel Channel, the outline of the +land clearly indicates the existence of several lines of elevation in this +same N.W. direction, which, I may add, is so uniform in the western half of +the St. of Magellan, that, as Captain King has remarked, "a parallel ruler +placed on the map upon the projecting points of the south shore, and +extended across the strait, will also touch the headlands on the opposite +coast." ("Geographical Journal" volume 1 page 170.) It would appear, from +Captain King's observations, that over all this area the cleavage extends +in the same line. Deep-water channels, however, in all parts of Tierra del +Fuego have burst through the trammels both of stratification and cleavage; +most of them may have been formed during the elevation of the land by long- +continued erosion, but others, for instance the Beagle Channel, which +stretches like a narrow canal for 120 miles obliquely through the +mountains, can hardly have thus originated. + +Finally, we have seen that in the extreme eastern point of Tierra del +Fuego, the cleavage and coast-lines extend W. and E. and even W.S.W. and +E.N.E.: over a large area westward, the cleavage, the main range of +mountains, and some subordinate ranges, but not the outlines of the coast, +strike W.N.W., and E.S.E.: in the central and western parts of the St. of +Magellan, the stratification, the mountain-ranges, the outlines of the +coast, and the cleavage all strike nearly N.W. and S.E. North of the +strait, the outline of the coast, and the mountains on the mainland, run +nearly north and south. Hence we see, at this southern point of the +continent, how gradually the Cordillera bend, from their north and south +course of so many thousand miles in length, into an E. and even E.N.E. +direction. + +WEST COAST, FROM THE SOUTHERN CHONOS ISLANDS TO NORTHERN CHILE. + +The first place at which we landed north of the St. of Magellan was near +Cape Tres Montes, in latitude 47 degrees S. Between this point and the +Northern Chonos Islands, a distance of 200 miles, the "Beagle" visited +several points, and specimens were collected for me from the intermediate +spaces by Lieutenant Stokes. The predominant rock is mica-slate, with thick +folia of quartz, very frequently alternating with and passing into a +chloritic, or into a black, glossy, often striated, slightly anthracitic +schist, which soils paper, and becomes white under a great heat, and then +fuses. Thin layers of feldspar, swelling at intervals into well +crystallised kernels, are sometimes included in these black schists; and I +observed one mass of the ordinary black variety insensibly lose its fissile +structure, and pass into a singular mixture of chlorite, epidote, feldspar, +and mica. Great veins of quartz are numerous in the mica-schists; wherever +these occur the folia are much convoluted. In the southern part of the +Peninsula of Tres Montes, a compact altered feldspathic rock with crystals +of feldspar and grains of quartz is the commonest variety; this rock +exhibits occasionally traces of an original brecciated structure, and often +presents (like the altered state of Tierra del Fuego) traces of cleavage- +planes, which strike in the same direction with the folia of mica-schist +further northward. (The peculiar, abruptly conical form of the hills in +this neighbourhood, would have led any one at first to have supposed that +they had been formed of injected or intrusive rocks. At Inchemo Island, a +similar rock gradually becomes granulo-crystalline and acquires scales of +mica; and this variety at S. Estevan becomes highly laminated, and though +still exhibiting some rounded grains of quartz, passes into the black, +glossy, slightly anthracitic schist, which, as we have seen, repeatedly +alternates with and passes into the micaceous and chloritic schists. Hence +all the rocks on this line of coast belong to one series, and insensibly +vary from an altered feldspathic clay-slate into largely foliated, true +mica-schist. + +The cleavage of the homogeneous schists, the foliation of those composed of +more or less distinct minerals in layers, and the planes of alternation of +the different varieties or so-called stratification, are all parallel, and +preserve over this 200 miles of coast a remarkable degree of uniformity in +direction. At the northern end of the group, at Low's Harbour, the well- +defined folia of mica-schist everywhere ranged within eight degrees (or +less than one point of the compass) of N. 19 degrees W. and S. 19 degrees +E.; and even the point of dip varied very little, being always directed to +the west and generally at an angle of forty degrees; I should mention that +I had here good opportunities of observation, for I followed the naked rock +on the beach, transversely to the strike, for a distance of four miles and +a half, and all the way attended to the dip. Along the outer islands for +100 miles south of Low's Harbour, Lieutenant Stokes, during his boat- +survey, kindly observed for me the strike of the foliation, and he assures +me that it was invariably northerly, and the dip with one single exception +to the west. Further south at Vallenar Bay, the strike was almost +universally N. 25 degrees W. and the dip, generally at an angle of about 40 +degrees to W. 25 degrees S., but in some places almost vertical. Still +farther south, in the neighbourhood of the harbours of Anna Pink, S. +Estevan and S. Andres, and (judging from a distance) along the southern +part of Tres Montes, the foliation and cleavage extended in a line between +[N. 11 degrees to 22 degrees W.] and [S. 11 degrees to 22 degrees E.]; and +the planes dipped generally westerly, but often easterly, at angles varying +from a gentle inclination to vertical. At A. Pink's Harbour, where the +schists generally dipped easterly, wherever the angle became very high, the +strike changed from N. 11 degrees W. to even as much as N. 45 degrees W.: +in an analogous manner at Vallenar Bay, where the dip was westerly (viz. on +an average directed to W. 25 degrees S.), as soon as the angle became very +high, the planes struck in a line more than 25 degrees west of north. The +average result from all the observations on this 200 miles of coast, is a +strike of N. 19 degrees W. and S. 19 degrees E.: considering that in each +specified place my examination extended over an area of several miles, and +that Lieutenant Stokes' observations apply to a length of 100 miles, I +think this remarkable uniformity is pretty well established. The +prevalence, throughout the northern half of this line of coast, of a dip in +one direction, that is to the west, instead of being sometimes west and +sometimes east, is, judging from what I have elsewhere seen, an unusual +circumstance. In Brazil, La Plata, the Falkland Islands, and Tierra del +Fuego, there is generally an obvious relation between the axis of +elevation, the outline of the coast, and the strike of the cleavage or +foliation: in the Chonos Archipelago, however, neither the minor details of +the coast-line, nor the chain of the Cordillera, nor the subordinate +transverse mountain-axes, accord with the strike of the foliation and +cleavage: the seaward face of the numerous islands composing this +Archipelago, and apparently the line of the Cordillera, range N. 11 degrees +E., whereas, as we have just seen, the average strike of the foliation is +N. 19 degrees W. + +There is one interesting exception to the uniformity in the strike of the +foliation. At the northern point of Tres Montes (latitude 45 degrees 52 +minutes) a bold chain of granite, between two and three thousand feet in +height, runs from the coast far into the interior, in an E.S.E. line, or +more strictly E. 28 degrees S. and W. 28 degrees N. (In the distance, other +mountains could be seen apparently ranging N.N.E. and S.S.W. at right +angles to this one. I may add, that not far from Vallenar Bay there is a +fine range, apparently of granite, which has burst through the mica-slate +in a N.E. by E. and S.W. by S. line.) In a bay, at the northern foot of +this range, there are a few islets of mica-slate, with the folia in some +parts horizontal, but mostly inclined at an average angle of 20 degrees to +the north. On the northern steep flank of the range, there are a few +patches (some quite isolated, and not larger than half a-crown!) of the +mica-schist, foliated with the same northerly dip. On the broad summit, as +far as the southern crest, there is much mica-slate, in some places even +400 feet in thickness, with the folia all dipping north, at angles varying +from 5 degrees to 20 degrees, but sometimes mounting up to 30 degrees. The +southern flank consists of bare granite. The mica-slate is penetrated by +small veins of granite, branching from the main body. (The granite within +these veins, as well as generally at the junction with the mica-slate, is +more quartzose than elsewhere. The granite, I may add, is traversed by +dikes running for a very great length in the line of the mountains; they +are composed of a somewhat laminated eurite, containing crystals of +feldspar, hornblende, and octagons of quartz.) Leaving out of view the +prevalent strike of the folia in other parts of this Archipelago, it might +have been expected that they would have dipped N. 28 degrees E., that is +directly from the ridge, and, considering its abruptness, at a high +inclination; but the real dip, as we have just seen, both at the foot and +on the northern flank, and over the entire summit, is at a small angle, and +directed nearly due north. From these considerations it occurred to me, +that perhaps we here had the novel and curious case of already inclined +laminae obliquely tilted at a subsequent period by the granitic axis. Mr. +Hopkins, so well known from his mathematical investigations, has most +kindly calculated the problem: the proposition sent was,--Take a district +composed of laminae, dipping at an angle of 40 degrees to W. 19 degrees S., +and let an axis of elevation traverse it in an E. 28 degrees S. line, what +will the position of the laminae be on the northern flank after a tilt, we +will first suppose, of 45 degrees? Mr. Hopkins informs me, that the angle +of the dip will be 28 degrees 31 minutes, and its direction to north 30 +degrees 33 minutes west. (On the south side of the axis (where, however, I +did not see any mica-slate) the dip of the folia would be at an angle of 77 +degrees 55 minutes, directed to west 35 degrees 33 minutes south. Hence the +two points of dip on the opposite sides of the range, instead of being as +in ordinary cases directly opposed to each other at an angle of 180 +degrees, would here be only 86 degrees 50 minutes apart.) By varying the +supposed angle of the tilt, our previously inclined folia can be thrown +into any angle between 26 degrees, which is the least possible angle, and +90 degrees; but if a small inclination be thus given to them, their point +of dip will depart far from the north, and therefore not accord with the +actual position of the folia of mica-schist on our granitic range. Hence it +appears very difficult, without varying considerably the elements of the +problem, thus to explain the anomalous strike and dip of the foliated mica- +schist, especially in those parts, namely, at the base of the range, where +the folia are almost horizontal. Mr. Hopkins, however, adds, that great +irregularities and lateral thrusts might be expected in every great line of +elevation, and that these would account for considerable deviations from +the calculated results: considering that the granitic axis, as shown by the +veins, has indisputably been injected after the perfect formation of the +mica-slate, and considering the uniformity of the strike of the folia +throughout the rest of the Archipelago, I cannot but still think that their +anomalous position at this one point is someway directly and mechanically +related to the intrusion of this W.N.W. and E.S.E. mountain-chain of +granite. + +Dikes are frequent in the metamorphic schists of the Chonos Islands, and +seem feebly to represent that great band of trappean and ancient volcanic +rocks on the south-western coast of Tierra del Fuego. At S. Andres I +observed in the space of half-a-mile, seven broad, parallel dikes, composed +of three varieties of trap, running in a N.W. and S.E. line, parallel to +the neighbouring mountain-ranges of altered clay-slate; but they must be of +long subsequent origin to these mountains; for they intersected the +volcanic formation described in the last chapter. North of Tres Montes, I +noticed three dikes differing from each other in composition, one of them +having a euritic base including large octagons of quartz; these dikes, as +well as several of porphyritic greenstone at Vallenar Bay, extended N.E. +and S.W., nearly at right angles to the foliation of the schists, but in +the line of their joints. At Low's Harbour, however, a set of great +parallel dikes, one ninety yards and another sixty yards in width, have +been guided by the foliation of the mica-schist, and hence are inclined +westward at an angle of 45 degrees: these dikes are formed of various +porphyritic traps, some of which are remarkable from containing numerous +rounded grains of quartz. A porphyritic trap of this latter kind, passed in +one of the dikes into a most curious hornstone, perfectly white, with a +waxy fracture and pellucid edges, fusible, and containing many grains of +quartz and specks of iron pyrites. In the ninety-yard dike several large, +apparently now quite isolated, fragments of mica-slate were embedded: but +as their foliation was exactly parallel to that of the surrounding solid +rock, no doubt these new separate fragments originally formed wedge-shaped +depending portions of a continuous vault or crust, once extending over the +dike, but since worn down and denuded. + +CHILOE, VALDIVIA, CONCEPCION. + +In Chiloe, a great formation of mica-schist strikingly resembles that of +the Chonos Islands. For a space of eleven miles on the S.E. coast, the +folia were very distinct, though slightly convoluted, and ranged within a +point of N.N.W. and S.S.E., dipping either E.N.E. or more commonly W.S.W., +at an average angle of 22 degrees (in one spot, however, at 60 degrees), +and therefore decidedly at a lesser inclination than amongst the Chonos +Islands. On the west and north-western shores, the foliation was often +obscure, though, where best defined, it ranged within a point of N. by W. +and S. by E., dipping either easterly or westerly, at varying and generally +very small angles. Hence, from the southern part of Tres Montes to the +northern end of Chiloe, a distance of 300 miles, we have closely allied +rocks with their folia striking on an average in the same direction, namely +between N. 11 degrees and 22 degrees W. Again, at Valdivia, we meet with +the same mica-schist, exhibiting nearly the same mineralogical passages as +in the Chonos Archipelago, often, however, becoming more ferruginous, and +containing so much feldspar as to pass into gneiss. The folia were +generally well defined; but nowhere else in South America did I see them +varying so much in direction: this seemed chiefly caused by their forming +parts, as I could sometimes distinctly trace, of large flat curves: +nevertheless, both near the settlement and towards the interior, a N.W. and +S.E. strike seemed more frequent than any other direction; the angle of the +dip was generally small. At Concepcion, a highly glossy clay-slate had its +cleavage often slightly curvilinear, and inclined, seldom at a high angle, +towards various points of the compass: but here, as at Valdivia, a N.W. and +S.E. strike seemed to be the most frequent one. +((FIGURE 23.) I observed in some parts that the tops of the laminae of the +clay-slate (b in Figure 23) under the superficial detritus and soil (a) +were bent, sometimes without being broken, as represented in Figure 23, +which is copied from one given by Sir H. De la Beche (page 42 "Geological +Manual") of an exactly similar phenomenon in Devonshire. Mr. R.A.C. Austen, +also, in his excellent paper on S.E. Devon ("Geological Transactions" +volume 6 page 437), has described this phenomenon; he attributes it to the +action of frosts, but at the same time doubts whether the frosts of the +present day penetrate to a sufficient depth. As it is known that +earthquakes particularly affect the surface of the ground, it occurred to +me that this appearance might perhaps be due, at least at Concepcion, to +their frequent occurrence; the superficial layers of detritus being either +jerked in one direction, or, where the surface was inclined, pushed a +little downwards during each strong vibration. In North Wales I have seen a +somewhat analogous but less regular appearance, though on a greater scale +("London Philosophical Magazine" volume 21 page 184), and produced by a +quite different cause, namely, by the stranding of great icebergs; this +latter appearance has also been observed in N. America.) + +In certain spots large quartz veins were numerous, and near them, the +cleavage, as was the case with the foliation of the schists in the Chonos +Archipelago, became extremely tortuous. + +At the northern end of Quiriquina Island, in the Bay of Concepcion, at +least eight rudely parallel dikes, which have been guided to a certain +extent by the cleavage of the slate, occur within the space of a quarter of +a mile. They vary much in composition, resembling in many respects the +dikes at Low's Harbour: the greater number consist of feldspathic +porphyries, sometimes containing grains of quartz: one, however, was black +and brilliant, like an augitic rock, but really formed of feldspar; others +of a feldspathic nature were perfectly white, with either an earthy or +crystalline fracture, and including grains and regular octagons of quartz; +these white varieties passed into ordinary greenstones. Although, both here +and at Low's Harbour, the nature of the rock varied considerably in the +same dike, yet I cannot but think that at these two places and in other +parts of the Chonos group, where the dikes, though close to each other and +running parallel, are of different composition, that they must have been +formed at different periods. In the case of Quiriquina this is a rather +interesting conclusion, for these eight parallel dikes cut through the +metamorphic schists in a N.W. and S.E. line, and since their injection the +overlying cretaceous or tertiary strata have been tilted (whilst still +under the sea) from a N.W. by N. and S.E. by S. line; and again, during the +great earthquake of February 1835, the ground in this neighbourhood was +fissured in N.W. and S.E. lines; and from the manner in which buildings +were thrown down, it was evident that the surface undulated in this same +direction. ("Geological Transactions" volume 6 pages 602 and 617. "Journal +of Researches" 2nd edition page 307.) + +CENTRAL AND NORTHERN CHILE. + +Northward of Concepcion, as far as Copiapo, the shores of the Pacific +consist, with the exception of some small tertiary basins, of gneiss, mica- +schist, altered clay-slate, granite, greenstone and syenite: hence the +coast from Tres Montes to Copiapo, a distance of 1,200 miles, and I have +reason to believe for a much greater space, is almost similarly +constituted. + +Near Valparaiso the prevailing rock is gneiss, generally including much +hornblende: concretionary balls formed of feldspar, hornblende and mica, +from two or three feet in diameter, are in very many places conformably +enfolded by the foliated gneiss: veins of quartz and feldspar, including +black schorl and well-crystallised epidote, are numerous. Epidote likewise +occurs in the gneiss in thin layers, parallel to the foliation of the mass. +One large vein of a coarse granitic character was remarkable from in one +part quite changing its character, and insensibly passing into a blackish +porphyry, including acicular crystals of glassy feldspar and of hornblende: +I have never seen any other such case. (Humboldt "Personal Narrative" +volume 4 page 60, has described with much surprise, concretionary balls, +with concentric divisions, composed of partially vitreous feldspar, +hornblende, and garnets, included within great veins of gneiss, which cut +across the mica-slate near Venezuela.) + +I shall in the few following remarks on the rocks of Chile allude +exclusively to their foliation and cleavage. In the gneiss round Valparaiso +the strike of the foliation is very variable, but I think about N. by W. +and S. by E. is the commonest direction; this likewise holds good with the +cleavage of the altered feldspathic clay-slates, occasionally met with on +the coast for ninety miles north of Valparaiso. Some feldspathic slate, +alternating with strata of claystone porphyry in the Bell of Quillota and +at Jajuel, and therefore, perhaps, belonging to a later period than the +metamorphic schists on the coast, cleaved in this same direction. In the +Eastern Cordillera, in the Portillo Pass, there is a grand mass of mica- +slate, foliated in a north and south line, and with a high westerly dip: in +the Uspallata range, clay-slate and grauwacke have a highly inclined, +nearly north and south cleavage, though in some parts the strike is +irregular: in the main or Cumbre range, the direction of the cleavage in +the feldspathic clay-slate is N.W. and S.E. + +Between Coquimbo and Guasco there are two considerable formations of mica- +slate, in one of which the rock passed sometimes into common clay-slate and +sometimes into a glossy black variety, very like that in the Chonos +Archipelago. The folia and cleavage of these rocks ranged between [N. and +N.W. by N.] and [S. and S.W. by S.]. Near the Port of Guasco several +varieties of altered clay-slate have a quite irregular cleavage. Between +Guasco and Copiapo, there are some siliceous and talcaceous slates cleaving +in a north and south line, with an easterly dip of between 60 and 70 +degrees: high up, also, the main valley of Copiapo, there is mica-slate +with a high easterly dip. In the whole space between Valparaiso and Copiapo +an easterly dip is much more common than an opposite or westerly one. + +CONCLUDING REMARKS ON CLEAVAGE AND FOLIATION. + +In this southern part of the southern hemisphere, we have seen that the +cleavage-laminae range over wide areas with remarkable uniformity, cutting +straight through the planes of stratification, but yet being parallel in +strike to the main axes of elevation, and generally to the outlines of the +coast. (In my paper on the Falkland Islands "Geological Journal" volume 3 +page 267, I have given a curious case on the authority of Captain Sulivan, +R.N., of much folded beds of clay-slate, in some of which the cleavage is +perpendicular to the horizon, and in others it is perpendicular to each +curvature or fold of the bed: this appears a new case.) The dip, however, +is as variable, both in angle and in direction (that is, sometimes being +inclined to the one side and sometimes to the directly opposite side), as +the strike is uniform. In all these respects there is a close agreement +with the facts given by Professor Sedgwick in his celebrated memoir in the +"Geological Transactions," and by Sir R.I. Murchison in his various +excellent discussions on this subject. The Falkland Islands, and more +especially Tierra del Fuego, offer striking instances of the lines of +cleavage, the principle axes of elevation, and the outlines of the coast, +gradually changing together their courses. The direction which prevails +throughout Tierra del Fuego and the Falkland Islands, namely, from west +with some northing to east with some southing, is also common to the +several ridges in Northern Patagonia and in the western parts of Banda +Oriental: in this latter province, in the Sierra Tapalguen, and in the +Western Falkland Island, the W. by N., or W.N.W. and E.S.E., ridges, are +crossed at right angles by others ranging N.N.E. and S.S.W. + +The fact of the cleavage-laminae in the clay-slate of Tierra del Fuego, +where seen cutting straight through the planes of stratification, and where +consequently there could be no doubt about their nature, differing slightly +in colour, texture, and hardness, appears to me very interesting. In a +thick mass of laminated, feldspathic and altered clay-slate, interposed +between two great strata of porphyritic conglomerate in Central Chile, and +where there could be but little doubt about the bedding, I observed similar +slight differences in composition, and likewise some distinct thin layers +of epidote, parallel to the highly inclined cleavage of the mass. Again, I +incidentally noticed in North Wales, where glaciers had passed over the +truncated edges of the highly inclined laminae of clay-slate, that the +surface, though smooth, was worn into small parallel undulations, caused by +the competent laminae being of slightly different degrees of hardness. +("London Philosophical Magazine" volume 21 page 182.) With reference to the +slates of North Wales, Professor Sedgwick describes the planes of cleavage, +as "coated over with chlorite and semi-crystalline matter, which not only +merely define the planes in question, but strike in parallel flakes through +the whole mass of the rock." ("Geological Transactions" volume 3 page 471.) +In some of those glossy and hard varieties of clay-slate, which may often +be seen passing into mica-schist, it has appeared to me that the cleavage- +planes were formed of excessively thin, generally slighted convoluted, +folia, composed of microscopically minute scales of mica. From these +several facts, and more especially from the case of the clay-slate in +Tierra del Fuego, it must, I think, be concluded, that the same power which +has impressed on the slate its fissile structure or cleavage has tended to +modify its mineralogical character in parallel planes. + +Let us now turn to the foliation of the metamorphic schists, a subject +which has been much less attended to. As in the case of cleavage-laminae, +the folia preserve over very large areas a uniform strike: thus Humboldt +found for a distance of 300 miles in Venezuela, and indeed over a much +larger space, gneiss, granite, mica, and clay-slate, striking very +uniformly N.E. and S.W., and dipping at an angle of between 60 and 70 +degrees to N.W. ("Personal Narrative" volume 6 page 59 et seq.); it would +even appear from the facts given in this chapter, that the metamorphic +rocks throughout the north-eastern part of South America are generally +foliated within two points of N.E. and S.W. Over the eastern parts of Banda +Oriental, the foliation strikes with a high inclination, very uniformly +N.N.E. to S.S.W., and over the western parts, in a W. by N. and E. by S. +line. For a space of 300 miles on the shores of the Chonos and Chiloe +Islands, we have seen that the foliation seldom deviates more than a point +of the compass from a N. 19 degrees W. and S. 19 degrees E. strike. As in +the case of cleavage, the angle of the dip in foliated rocks is generally +high but variable, and alternates from one side of the line of strike to +the other side, sometimes being vertical: in the Northern Chonos Islands, +however, the folia are inclined almost always to the west; in nearly the +same manner, the cleavage-laminae in Southern Tierra del Fuego certainly +dip much more frequently to S.S.W. than to the opposite point. In Eastern +Banda Oriental, in parts of Brazil, and in some other districts, the +foliation runs in the same direction with the mountain-ranges and adjoining +coast-lines: amongst the Chonos Islands, however, this coincidence fails, +and I have given my reasons for suspecting that one granitic axis has burst +through and tilted the already inclined folia of mica-schist: in the case +of cleavage, the coincidence between its strike and that of the main +stratification seems sometimes to fail. (Cases are given by Mr. Jukes in +his "Geology of Newfoundland" page 130.) Foliation and cleavage resemble +each other in the planes winding round concretions, and in becoming +tortuous where veins of quartz abound. (I have seen in Brazil and Chile +concretions thus enfolded by foliated gneiss; and Macculloch "Highlands" +volume 1 page 64, has described a similar case. For analogous cases in +clay-slate, see Professor Henslow's Memoir in "Cambridge Philosophical +Transactions" volume 1 page 379, and Macculloch's "Classification of Rocks" +page 351. With respect to both foliation and cleavage becoming tortuous +where quartz-veins abound, I have seen instances near Monte Video, at +Concepcion, and in the Chonos Islands. See also Mr. Greenough's "Critical +Examination" page 78.) On the flanks of the mountains both in Tierra del +Fuego and in other countries, I have observed that the cleavage-planes +frequently dip at a high angle inwards; and this was long ago observed by +Von Buch to be the case in Norway: this fact is perhaps analogous to the +folded, fan-like or radiating structure in the metamorphic schists of the +Alps, in which the folia in the central crests are vertical and on the two +flanks inclined inwards. (Studer in "Edinburgh New Philosophical Journal" +volume 23 page 144.) Where masses of fissile and foliated rocks alternate +together, the cleavage and foliation, in all cases which I have seen, are +parallel. Where in one district the rocks are fissile, and in another +adjoining district they are foliated, the planes of cleavage and foliation +are likewise generally parallel: this is the case with the feldspathic +homogeneous slates in the southern part of the Chonos group, compared with +the fine foliated mica-schists of the northern part; so again the clay- +slate of the whole eastern side of Tierra del Fuego cleaves in exactly the +same line with the foliated gneiss and mica-slate of the western coast; +other analogous instances might have been adduced. (I have given a case in +Australia. See my "Volcanic Islands.") + +With respect to the origin of the folia of quartz, mica, feldspar, and +other minerals composing the metamorphic schists, Professor Sedgwick, Mr. +Lyell, and most authors believe, that the constituent parts of each layer +were separately deposited as sediment, and then metamorphosed. This view, +in the majority of cases, I believe to be quite untenable. In those not +uncommon instances, where a mass of clay-slate, in approaching granite, +gradually passes into gneiss, we clearly see that folia of distinct +minerals can originate through the metamorphosis of a homogeneous fissile +rock. (I have described in "Volcanic Islands" a good instance of such a +passage at the Cape of Good Hope.) The deposition, it may be remarked, of +numberless alternations of pure quartz, and of the elements of mica or +feldspar does not appear a probable event. (See some excellent remarks on +this subject, in D'Aubuisson's "Traite de Geog." tome 1 page 297. Also some +remarks by Mr. Dana in "Silliman's American Journal" volume 45 page 108.) +In those districts in which the metamorphic schists are foliated in planes +parallel to the cleavage of the rocks in an adjoining district, are we to +believe that the folia are due to sedimentary layers, whilst the cleavage- +laminae, though parallel, have no relation whatever to such planes of +deposition? On this view, how can we reconcile the vastness of the areas +over which the strike of the foliation is uniform, with what we see in +disturbed districts composed of true strata: and especially, how can we +understand the high and even vertical dip throughout many wide districts, +which are not mountainous, and throughout some, as in Western Banda +Oriental, which are not even hilly? Are we to admit that in the northern +part of the Chonos Archipelago, mica-slate was first accumulated in +parallel horizontal folia to a thickness of about four geographical miles, +and then upturned at an angle of forty degrees; whilst, in the southern +part of this same Archipelago, the cleavage-laminae of closely allied +rocks, which none would imagine had ever been horizontal, dip at nearly the +same angle, to nearly the same point? + +Seeing, then, that foliated schists indisputably are sometimes produced by +the metamorphosis of homogeneous fissile rocks; seeing that foliation and +cleavage are so closely analogous in the several above-enumerated respects; +seeing that some fissile and almost homogeneous rocks show incipient +mineralogical changes along the planes of their cleavage, and that other +rocks with a fissile structure alternate with, and pass into varieties with +a foliated structure, I cannot doubt that in most cases foliation and +cleavage are parts of the same process: in cleavage there being only an +incipient separation of the constituent minerals; in foliation a much more +complete separation and crystallisation. + +The fact often referred to in this chapter, of the foliation and the so- +called strata in the metamorphic series,--that is, the alternating masses +of different varieties of gneiss, mica-schist, and hornblende-slate, etc.,- +-being parallel to each other, at first appears quite opposed to the view, +that the folia have no relation to the planes of original deposition. Where +the so-called beds are not very thick and of widely different mineralogical +composition from each other, I do not think that there is any difficulty in +supposing that they have originated in an analogous manner with the +separate folia. We should bear in mind what thick strata, in ordinary +sedimentary masses, have obviously been formed by a concretionary process. +In a pile of volcanic rocks on the Island of Ascension, there are strata, +differing quite as much in appearance as the ordinary varieties of the +metamorphic schists, which undoubtedly have been produced, not by +successive flowings of lava, but by internal molecular changes. Near Monte +Video, where the stratification, as it would be called, of the metamorphic +series is, in most parts, particularly well developed, being as usual, +parallel to the foliation, we have seen that a mass of chloritic schist, +netted with quartz-veins, is entangled in gneiss, in such a manner as to +show that it had certainly originated in some process of segregation: +again, in another spot, the gneiss tended to pass into hornblendic schist +by alternating with layers of quartz; but these layers of quartz almost +certainly had never been separately deposited, for they were absolutely +continuous with the numerous intersecting veins of quartz. I have never had +an opportunity of tracing for any distance, along the line both of strike +and of dip, the so-called beds in the metamorphic schists, but I strongly +suspect that they would not be found to extend with the same character, +very far in the line either of their dip or strike. Hence I am led to +believe, that most of the so-called beds are of the nature of complex +folia, and have not been separately deposited. Of course, this view cannot +be extended to THICK masses included in the metamorphic series, which are +of totally different composition from the adjoining schists, and which are +far extended, as is sometimes the case with quartz and marble; these must +generally be of the nature of true strata. (Macculloch "Classification of +Rocks" page 364, states that primary limestones are often found in +irregular masses or great nodules, "which can scarcely be said to possess a +stratified shape!") Such strata, however, will almost always strike in the +same direction with the folia, owing to the axes of elevation being in most +countries parallel to the strike of the foliation; but they will generally +dip at a different angle from that of the foliation; and the angle of the +foliation in itself almost always varies much: hence, in crossing a +metamorphosed schistose district, it would require especial attention to +discriminate between true strata of deposition and complex foliated masses. +The mere presence of true strata in the midst of a set of metamorphic +schists, is no argument that the foliation is of sedimentary origin, +without it be further shown in each case, that the folia not only strike, +but dip throughout in parallel planes with those of the true +stratification. + +As in some cases it appears that where a fissile rock has been exposed to +partial metamorphic action, for instance from the irruption of granite, the +foliation has supervened on the already existing cleavage-planes; so +perhaps in some instances, the foliation of a rock may have been determined +by the original planes of deposition or of oblique current-laminae: I have, +however, myself, never seen such a case, and I must maintain that in most +extensive metamorphic areas, the foliation is the extreme result of that +process, of which cleavage is the first effect. That foliation may arise +without any previous structural arrangement in the mass, we may infer from +injected, and therefore once liquified, rocks, both of volcanic and +plutonic origin, sometimes having a "grain" (as expressed by Professor +Sedgwick), and sometimes being composed of distinct folia or laminae of +different compositions. In my work on "Volcanic Islands," I have given +several instances of this structure in volcanic rocks, and it is not +uncommonly seen in plutonic masses--thus, in the Cordillera of Chile, there +are gigantic mountain-like masses of red granite, which have been injected +whilst liquified, and which, nevertheless, display in parts a decidedly +laminar structure. (As remarked in a former part of this chapter, I suspect +that the boldly conical mountains of gneiss-granite, near Rio de Janeiro, +in which the constituent minerals are arranged in parallel planes, are of +intrusive origin. We must not, however, forget the lesson of caution taught +by the curious claystone porphyries of Port Desire, in which we have seen +that the breaking up and aggregation of a thinly stratified tufaceous mass, +has yielded a rock semi-porphyritic with crystals of feldspar, arranged in +the planes of original deposition.) + +Finally, we have seen that the planes of cleavage and of foliation, that +is, of the incipient process and of the final result, generally strike +parallel to the principal axes of elevation, and to the outline of the +land: the strike of the axes of elevation (that is, of the lines of +fissures with the strata on their edges upturned), according to the +reasoning of Mr. Hopkins, is determined by the form of the area undergoing +changes of level, and the consequent direction of the lines of tension and +fissure. Now, in that remarkable pile of volcanic rocks at Ascension, which +has several times been alluded to (and in some other cases), I have +endeavoured to show, that the lamination of the several varieties, and +their alternations, have been caused by the moving mass, just before its +final consolidation, having been subjected (as in a glacier) to planes of +different tension; this difference in the tension affecting the crystalline +and concretionary processes. (In "Volcanic Islands.") One of the varieties +of rock thus produced at Ascension, at first sight, singularly resembles a +fine-grained gneiss; it consists of quite straight and parallel zones of +excessive tenuity, of more or less coloured crystallised feldspar, of +distinct crystals of quartz, diopside, and oxide of iron. These +considerations, notwithstanding the experiments made by Mr. Fox, showing +the influence of electrical currents in producing a structure like that of +cleavage, and notwithstanding the apparently inexplicable variation, both +in the inclination of the cleavage-laminae and in their dipping first to +one side and then to the other side of the line of strike, lead me to +suspect that the planes of cleavage and foliation are intimately connected +with the planes of different tension, to which the area was long subjected, +AFTER the main fissures or axes of upheavement had been formed, but BEFORE +the final consolidation of the mass and the total cessation of all +molecular movement. + + +CHAPTER VII. CENTRAL CHILE:--STRUCTURE OF THE CORDILLERA. + +Central Chile. +Basal formations of the Cordillera. +Origin of the porphyritic clay-stone conglomerate. +Andesite. +Volcanic rocks. +Section of the Cordillera by the Peuquenes are Portillo Pass. +Great gypseous formation. +Peuquenes line; thickness of strata, fossils of. +Portillo line. +Conglomerate, orthitic granite, mica-schist, volcanic rocks of. +Concluding remarks on the denudation and elevation of the Portillo line. +Section by the Cumbre, or Uspallata Pass. +Porphyries. +Gypseous strata. +Section near the Puente del Inca; fossils of. +Great subsidence. +Intrusive porphyries. +Plain of Uspallata. +Section of the Uspallata chain. +Structure and nature of the strata. +Silicified vertical trees. +Great subsidence. +Granitic rocks of axis. +Concluding remarks on the Uspallata range; origin subsequent to that of the +main Cordillera; two periods of subsidence; comparison with the Portillo +chain. + +The district between the Cordillera and the Pacific, on a rude average, is +from about eighty to one hundred miles in width. It is crossed by many +chains of mountains, of which the principal ones, in the latitude of +Valparaiso and southward of it, range nearly north and south; but in the +more northern parts of the province, they run in almost every possible +direction. Near the Pacific, the mountain-ranges are generally formed of +syenite or granite, and or of an allied euritic porphyry; in the low +country, besides these granitic rocks and greenstone, and much gneiss, +there are, especially northward of Valparaiso, some considerable districts +of true clay-slate with quartz veins, passing into a feldspathic and +porphyritic slate; there is also some grauwacke and quartzose and jaspery +rocks, the latter occasionally assuming the character of the basis of +claystone porphyry: trap-dikes are numerous. Nearer the Cordillera the +ranges (such as those of S. Fernando, the Prado (Meyen "Reise um Erde" th. +1 s. 235.), and Aconcagua) are formed partly of granitic rocks, and partly +of purple porphyritic conglomerates, claystone porphyry, greenstone +porphyry, and other rocks, such as we shall immediately see, form the basal +strata of the main Cordillera. In the more northern parts of Chile, this +porphyritic series extends over large tracts of country far from the +Cordillera; and even in Central Chile such occasionally occur in outlying +positions. + +I will describe the Campana of Quillota, which stands only fifteen miles +from the Pacific, as an instance of one of these outlying masses. This hill +is conspicuous from rising to the height of 6,400 feet: its summit shows a +nucleus, uncovered for a height of 800 feet, of fine greenstone, including +epidote and octahedral magnetic iron ore; its flanks are formed of great +strata of porphyritic claystone conglomerate associated with various true +porphyries and amygdaloids, alternating with thick masses of a highly +feldspathic, sometimes porphyritic, pale-coloured slaty rock, with its +cleavage-laminae dipping inwards at a high angle. At the base of the hill +there are syenites, a granular mixture of quartz and feldspar, and harsh +quartzose rocks, all belonging to the basal metamorphic series. I may +observe that at the foot of several hills of this class, where the +porphyries are first seen (as near S. Fernando, the Prado, Las Vacas, +etc.), similar harsh quartzose rocks and granular mixtures of quartz and +feldspar occur, as if the more fusible constituent parts of the granitic +series had been drawn off to form the overlying porphyries. + +In Central Chile, the flanks of the main Cordillera, into which I +penetrated by four different valleys, generally consist of distinctly +stratified rocks. The strata are inclined at angles varying from sometimes +even under ten, to twenty degrees, very rarely exceeding forty degrees: in +some, however, of the quite small, exterior, spur-like ridges, the +inclination was not unfrequently greater. The dip of the strata in the main +outer lines was usually outwards or from the Cordillera, but in Northern +Chile frequently inwards,--that is, their basset-edges fronted the Pacific. +Dikes occur in extraordinary numbers. In the great, central, loftiest +ridges, the strata, as we shall presently see, are almost always highly +inclined and often vertical. Before giving a detailed account of my two +sections across the Cordillera, it will, I think, be convenient to describe +the basal strata as seen, often to a thickness of four or five thousand +feet, on the flanks of the outer lines. + +BASAL STRATA OF THE CORDILLERA. + +The prevailing rock is a purplish or greenish, porphyritic claystone +conglomerate. The embedded fragments vary in size from mere particles to +blocks as much as six or eight inches (rarely more) in diameter; in many +places, where the fragments were minute, the signs of aqueous deposition +were unequivocally distinct; where they were large, such evidence could +rarely be detected. The basis is generally porphyritic with perfect +crystals of feldspar, and resembles that of a true injected claystone +porphyry: often, however, it has a mechanical or sedimentary aspect, and +sometimes (as at Jajuel) is jaspery. The included fragments are either +angular, or partially or quite rounded (Some of the rounded fragments in +the porphyritic conglomerate near the Baths of Cauquenes, were marked with +radii and concentric zones of different shades of colour: any one who did +not know that pebbles, for instance flint pebbles from the chalk, are +sometimes zoned concentrically with their worn and rounded surfaces, might +have been led to infer, that these balls of porphyry were not true pebbles, +but had originated in concretionary action.); in some parts the rounded, in +others the angular fragments prevail, and usually both kinds are mixed +together: hence the word BRECCIA ought strictly to be appended to the term +PORPHYRITIC CONGLOMERATE. The fragments consist of many varieties of +claystone porphyry, usually of nearly the same colour with the surrounding +basis, namely, purplish-reddish, brownish, mottled or bright green; +occasionally fragments of a laminated, pale-coloured, feldspathic rock, +like altered clay-slate are included; as are sometimes grains of quartz, +but only in one instance in Central Chile (namely, at the mines of Jajuel) +a few pebbles of quartz. I nowhere observed mica in this formation, and +rarely hornblende; where the latter mineral did occur, I was generally in +doubt whether the mass really belonged to this formation, or was of +intrusive origin. Calcareous spar occasionally occurs in small cavities; +and nests and layers of epidote are common. In some few places in the +finer-grained varieties (for instance, at Quillota), there were short, +interrupted layers of earthy feldspar, which could be traced, exactly as at +Port Desire, passing into large crystals of feldspar: I doubt, however, +whether in this instance the layers had ever been separately deposited as +tufaceous sediment. + +All the varieties of porphyritic conglomerates and breccias pass into each +other, and by innumerable gradations into porphyries no longer retaining +the least trace of mechanical origin: the transition appears to have been +effected much more easily in the finer-grained, than in the coarser-grained +varieties. In one instance, near Cauquenes, I noticed that a porphyritic +conglomerate assumed a spheroidal structure, and tended to become columnar. +Besides the porphyritic conglomerates and the perfectly characterised +porphyries, of metamorphic origin, there are other porphyries, which, +though differing not at all or only slightly in composition, certainly have +had a different origin: these consist of pink or purple claystone +porphyries, sometimes including grains of quartz,--of greenstone porphyry, +and of other dusky rocks, all generally porphyritic with fine, large, +tabular, opaque crystals, often placed crosswise, of feldspar cleaving like +albite (judging from several measurements), and often amygdaloidal with +silex, agate, carbonate of lime, green and brown bole. (This bole is a very +common mineral in the amygdaloidal rocks; it is generally of a greenish- +brown colour, with a radiating structure; externally it is black with an +almost metallic lustre, but often coated by a bright green film. It is soft +and can be scratched by a quill; under the blowpipe swells greatly and +becomes scaly, then fuses easily into a black magnetic bead. This substance +is evidently similar to that which often occurs in submarine volcanic +rocks. An examination of some very curious specimens of a fine porphyry +(from Jajuel) leads me to suspect that some of these amygdaloidal balls, +instead of having been deposited in pre-existing air-vesicles, are of +concretionary origin; for in these specimens, some of the pea-shaped little +masses (often externally marked with minute pits) are formed of a mixture +of green earth with stony matter, like the basis of the porphyry, including +minute imperfect crystals of feldspar; and these pea-shaped little masses +are themselves amygdaloidal with minute spheres of the green earth, each +enveloped by a film of white, apparently feldspathic, earthy matter: so +that the porphyry is doubly amygdaloidal. It should not, however, be +overlooked, that all the strata here have undergone metamorphic action, +which may have caused crystals of feldspar to appear, and other changes to +be effected, in the originally simple amygdaloidal balls. Mr. J.D. Dana, in +an excellent paper on Trap-rocks "Edinburgh New Philosophical Journal" +volume 41 page 198, has argued with great force, that all amygdaloidal +minerals have been deposited by aqueous infiltration. I may take this +opportunity of alluding to a curious case, described in my work on +"Volcanic Islands," of an amygdaloid with many of its cells only half +filled up with a mesotypic mineral. M. Rose has described an amygdaloid, +brought by Dr. Meyen "Reise um Erde" Th. 1. s. 316, from Chile, as +consisting of crystallised quartz, with crystals of stilbite within, and +lined externally by green earth.) These several porphyritic and +amygdaloidal varieties never show any signs of passing into masses of +sedimentary origin: they occur both in great and small intrusive masses, +and likewise in strata alternating with those of the porphyritic +conglomerate, and with the planes of junction often quite distinct, yet not +seldom blended together. In some of these intrusive masses, the porphyries +exhibit, more or less plainly, a brecciated structure, like that often seen +in volcanic masses. These brecciated porphyries could generally be +distinguished at once from the metamorphosed, porphyritic breccia- +conglomerates, by all the fragments being angular and being formed of the +same variety, and by the absence of every trace of aqueous deposition. One +of the porphyries above specified, namely, the greenstone porphyry with +large tabular crystals of albite, is particularly abundant, and in some +parts of the Cordillera (as near St. Jago) seemed more common even than the +purplish porphyritic conglomerate. Numerous dikes likewise consist of this +greenstone porphyry; others are formed of various fine-grained trappean +rocks; but very few of claystone porphyry: I saw no true basaltic dikes. + +In several places in the lower part of the series, but not everywhere, +thick masses of a highly feldspathic, often porphyritic, slaty rock occur +interstratified with the porphyritic conglomerate; I believe in one or two +cases blackish limestone has been found in a similar position. The +feldspathic rock is of a pale grey or greenish colour; it is easily +fusible; where porphyritic, the crystals of feldspar are generally small +and vitreous: it is distinctly laminated, and sometimes includes parallel +layers of epidote (This mineral is extremely common in all the formations +of Chile; in the gneiss near Valparaiso and in the granitic veins crossing +it, in the injected greenstone crowning the C. of Quillota, in some +granitic porphyries, in the porphyritic conglomerate, and in the +feldspathic clay-slates.); the lamination appears to be distinct from +stratification. Occasionally this rock is somewhat curious; and at one +spot, namely, at the C. of Quillota, it had a brecciated structure. Near +the mines of Jajuel, in a thick stratum of this feldspathic, porphyritic +slate, there was a layer of hard, blackish, siliceous, infusible, compact +clay-slate, such as I saw nowhere else; at the same place I was able to +follow for a considerable distance the junction between the slate and the +conformably underlying porphyritic conglomerate, and they certainly passed +gradually into each other. Wherever these slaty feldspathic rocks abound, +greenstone seems common; at the C. of Quillota a bed of well-crystallised +greenstone lay conformably in the midst of the feldspathic slate, with the +upper and lower junctions passing insensibly into it. From this point, and +from the frequently porphyritic condition of the slate, I should perhaps +have considered this rock as an erupted one (like certain laminated +feldspathic lavas in the trachytic series), had I not seen in Tierra del +Fuego how readily true clay-slate becomes feldspathic and porphyritic, and +had I not seen at Jajuel the included layer of black, siliceous clay-slate, +which no one could have thought of igneous origin. The gentle passage of +the feldspathic slate, at Jajuel, into the porphyritic conglomerate, which +is certainly of aqueous origin, should also be taken in account. + +The alternating strata of porphyries and porphyritic conglomerate, and with +the occasionally included beds of feldspathic slate, together make a grand +formation; in several places within the Cordillera, I estimated its +thickness at from six to seven thousand feet. It extends for many hundred +miles, forming the western flank of the Chilean Cordillera; and even at +Iquique in Peru, 850 miles north of the southernmost point examined by me +in Chile, the coast-escarpment which rises to a height of between two and +three thousand feet is thus composed. In several parts of Northern Chile +this formation extends much further towards the Pacific, over the granitic +and metamorphic lower rocks, than it does in Central Chile; but the main +Cordillera may be considered as its central line, and its breadth in an +east and west direction is never great. At first the origin of this thick, +massive, long but narrow formation, appeared to me very anomalous: whence +were derived, and how were dispersed the innumerable fragments, often of +large size, sometimes angular and sometimes rounded, and almost invariably +composed of porphyritic rocks? Seeing that the interstratified porphyries +are never vesicular and often not even amygdaloidal, we must conclude that +the pile was formed in deep water; how then came so many fragments to be +well rounded and so many to remain angular, sometimes the two kinds being +equally mingled, sometimes one and sometimes the other preponderating? That +the claystone, greenstone, and other porphyries and amygdaloids, which lie +CONFORMABLY between the beds of conglomerate, are ancient submarine lavas, +I think there can be no doubt; and I believe we must look to the craters +whence these streams were erupted, as the source of the breccia- +conglomerate; after the great explosion, we may fairly imagine that the +water in the heated and scarcely quiescent crater would remain for a +considerable time sufficiently agitated to triturate and round the loose +fragments, few or many in number, would be shot forth at the next eruption, +associated with few or many angular fragments, according to the strength of +the explosion. (This certainly seems to have taken place in some recent +volcanic archipelagos, as at the Galapagos, where numerous craters are +exclusively formed of tuff and fragments of lava.) The porphyritic +conglomerate being purple or reddish, even when alternating with dusty- +coloured or bright green porphyries and amygdaloids, is probably an +analogous circumstance to the scoriae of the blackish basalts being often +bright red. The ancient submarine orifices whence the porphyries and their +fragments were ejected having been arranged in a band, like most still +active volcanoes, accounts for the thickness, the narrowness, and linear +extension of this formation. + +This whole great pile of rock has suffered much metamorphic action, as is +very obvious in the gradual formation and appearance of the crystals of +albitic feldspar and of epidote--in the bending together of the fragments-- +in the appearance of a laminated structure in the feldspathic slate--and, +lastly, in the disappearance of the planes of stratification, which could +sometimes be seen on the same mountain quite distinct in the upper part, +less and less plain on the flanks, and quite obliterated at the base. +Partly owing to this metamorphic action, and partly to the close +relationship in origin, I have seen fragments of porphyries--taken from a +metamorphosed conglomerate--from a neighbouring stream of lava--from the +nucleus or centre (as it appeared to me) of the whole submarine volcano-- +and lastly from an intrusive mass of quite subsequent origin, all of which +were absolutely undistinguishable in external characters. + +One other rock, of plutonic origin, and highly important in the history of +the Cordillera, from having been injected in most of the great axes of +elevation, and from having apparently been instrumental in metamorphosing +the superincumbent strata, may be conveniently described in this +preliminary discussion. It has been called by some authors ANDESITE: it +mainly consists of well-crystallised white albite (as determined with the +goniometer in numerous specimens both by Professor Miller and myself), of +less perfectly crystallised green hornblende, often associated with much +mica, with chlorite and epidote, and occasionally with a few grains of +quartz: in one instance in Northern Chile, I found crystals of orthitic or +potash feldspar, mingled with those of albite. (I here, and elsewhere, call +by this name, those feldspathic minerals which cleave like albite: but it +now appears ("Edinburgh New Philosophical Journal" volume 24 page 181) that +Abich has analysed a mineral from the Cordillera, associated with +hornblende and quartz (probably the same rock with that here under +discussion), which cleaves like albite, but which is a new and distinct +kind, called by him ANDESINE. It is allied to leucite, with the greater +proportion of its potash replaced by lime and soda. This mineral seems +scarcely distinguishable from albite, except by analysis.) Where the mica +and quartz are abundant, the rock cannot be distinguished from granite; and +it may be called andesitic granite. Where these two minerals are quite +absent, and when, as often then happens, the crystals of albite are +imperfect and blend together, the rock may be called andesitic porphyry, +which bears nearly the same relation to andesitic granite that euritic +porphyry does to common granite. These andesitic rocks form mountain masses +of a white colour, which, in their general outline and appearance--in their +joints--in their occasionally including dark-coloured, angular fragments, +apparently of some pre-existing rock--and in the great dikes branching from +them into the superincumbent strata, manifest a close and striking +resemblance to masses of common granite and syenite: I never, however, saw +in these andesitic rocks, those granitic veins of segregation which are so +common in true granites. We have seen that andesite occurs in three places +in Tierra del Fuego; in Chile, from S. Fernando to Copiapo, a distance of +450 miles, I found it under most of the axes of elevation; in a collection +of specimens from the Cordillera of Lima in Peru, I immediately recognised +it; and Erman states that it occurs in Eastern Kamtschatka. ("Geographical +Journal" volume 9 page 510.) From its wide range, and from the important +part it has played in the history of the Cordillera, I think this rock has +well deserved its distinct name of Andesite. + +The few still active volcanoes in Chile are confined to the central and +loftiest ranges of the Cordillera; and volcanic matter, such as appears to +have been of subaerial eruption, is everywhere rare. According to Meyen, +there is a hill of pumice high up the valley of the Maypu, and likewise a +trachytic formation at Colina, a village situated north of St. Jago. +("Reise um Erde" Th. 1 ss. 338 and 362.) Close to this latter city, there +are two hills formed of a pale feldspathic porphyry, remarkable from being +doubly columnar, great cylindrical columns being subdivided into smaller +four- or five-sided ones; and a third hillock (Cerro Blanco) is formed of a +fragmentary mass of rock, which I believed to be of volcanic origin, +intermediate in character between the above feldspathic porphyry and common +trachyte, and containing needles of hornblende and granular oxide of iron. +Near the Baths of Cauquenes, between two short parallel lines of elevation, +where they are intersected by the valley, there is a small, though distinct +volcanic district; the rock is a dark grey (andesitic) trachyte, which +fuses into a greenish-grey bead, and is formed of long crystals of +fractured glassy albite (judging from one measurement) mingled with well- +formed crystals, often twin, of augite. The whole mass is vesicular, but +the surface is darker coloured and much more vesicular than any other part. +This trachyte forms a cliff-bounded, horizontal, narrow strip on the steep +southern side of the valley, at the height of four or five hundred feet +above the river-bed; judging from an apparently corresponding line of cliff +on the northern side, the valley must once have been filled up to this +height by a field of lava. On the summit of a lofty mountain some leagues +higher up this same valley of the Cachapual, I found columnar pitchstone +porphyritic with feldspar; I do not suppose this rock to be of volcanic +origin, and only mention it here, from its being intersected by masses and +dikes of a VESICULAR rock, approaching in character to trachyte; in no +other part of Chile did I observe vesicular or amygdaloidal dikes, though +these are so common in ordinary volcanic districts. + +PASSAGE OF THE ANDES BY THE PORTILLO OR PEQUENES PASS. + +Although I crossed the Cordillera only once by this pass, and only once by +that of the Cumbre or Uspallata (presently to be described), riding slowly +and halting occasionally to ascend the mountains, there are many +circumstances favourable to obtaining a more faithful sketch of their +structure than would at first be thought possible from so short an +examination. The mountains are steep and absolutely bare of vegetation; the +atmosphere is resplendently clear; the stratification distinct; and the +rocks brightly and variously coloured: some of the natural sections might +be truly compared for distinctness to those coloured ones in geological +works. Considering how little is known of the structure of this gigantic +range, to which I particularly attended, most travellers having collected +only specimens of the rocks, I think my sketch-sections, though necessarily +imperfect, possess some interest. Section 1/1 in Plate 1 which I will now +describe in detail, is on a horizontal scale of a third of an inch to a +nautical mile, and on a vertical scale of one inch to a mile (or 6,000 +feet). The width of the range (excluding a few outlying hillocks), from the +plain on which St. Jago the capital of Chile stands, to the Pampas, is +sixty miles, as far as I can judge from the maps, which differ from each +other and are all EXCEEDINGLY imperfect. The St. Jago plain at the mouth of +the Maypu, I estimate from adjoining known points at 2,300 feet, and the +Pampas at 3,500 feet, both above the level of the sea. The height of the +Pequenes line, according to Dr. Gillies, is 13,210 feet ("Journal of +Natural and Geographical Science" August 1830.); and that of the Portillo +line (both in the gaps where the road crosses them) is 14,345 feet; the +lowest part of the intermediate valley of Tenuyan is 7,530 feet--all above +the level of the sea. + +The Cordillera here, and indeed I believe throughout Chile, consist of +several parallel, anticlinal and uniclinal mountain-lines, ranging north, +or north with a little westing, and south. Some exterior and much lower +ridges often vary considerably from this course, projecting like oblique +spurs from the main ranges: in the district towards the Pacific, the +mountains, as before remarked, extend in various directions, even east and +west. In the main exterior lines, the strata, as also before remarked, are +seldom inclined at a high angle; but in the central lofty ridges they are +almost always highly inclined, broken by many great faults, and often +vertical. As far as I could judge, few of the ranges are of great length: +and in the central parts of the Cordillera, I was frequently able to follow +with my eye a ridge gradually becoming higher and higher, as the +stratification increased in inclination, from one end where its height was +trifling and its strata gently inclined to the other end where vertical +strata formed snow-clad pinnacles. Even outside the main Cordillera, near +the baths of Cauquenes, I observed one such case, where a north and south +ridge had its strata in the valley inclined at 37 degrees, and less than a +mile south of it at 67 degrees: another parallel and similarly inclined +ridge rose at the distance of about five miles, into a lofty mountain with +absolutely vertical strata. Within the Cordillera, the height of the ridges +and the inclination of the strata often became doubled and trebled in much +shorter distances than five miles; this peculiar form of upheaval probably +indicates that the stratified crust was thin, and hence yielded to the +underlying intrusive masses unequally, at certain points on the lines of +fissure. + +The valleys, by which the Cordillera are drained, follow the anticlinal or +rarely synclinal troughs, which deviate most from the usual north and south +course; or still more commonly those lines of faults or of unequal +curvature (that is, lines with the strata on both hands dipping in the same +direction, but at a somewhat different angle) which deviate most from a +northerly course. Occasionally the torrents run for some distance in the +north and south valleys, and then recover their eastern or western course +by bursting through the ranges at those points where the strata have been +least inclined and the height consequently is less. Hence the valleys, +along which the roads run, are generally zigzag; and, in drawing an east +and west section, it is necessary to contract greatly that which is +actually seen on the road. + +Commencing at the western end of Section 1/1 where the R. Maypu debouches +on the plain of St. Jago, we immediately enter on the porphyritic +conglomerate formation, and in the midst of it find some hummocks [A] of +granite and syenite, which probably (for I neglected to collect specimens) +belong to the andesitic class. These are succeeded by some rugged hills [B] +of dark-green, crystalline, feldspathic and in some parts slaty rocks, +which I believe belong to the altered clay-slate formation. From this +point, great mountains of purplish and greenish, generally thinly +stratified, highly porphyritic conglomerates, including many strata of +amygdaloidal and greenstone porphyries, extend up the valley to the +junction of the rivers Yeso and Volcan. As the valley here runs in a very +southerly course, the width of the porphyritic conglomerate formation is +quite conjectural; and from the same cause, I was unable to make out much +about the stratification. In most of the exterior mountains the dip was +gentle and directed inwards; and at only one spot I observed an inclination +as high as 50 degrees. Near the junction of the R. Colorado with the main +stream, there is a hill of whitish, brecciated, partially decomposed +feldspathic porphyry, having a volcanic aspect but not being really of that +nature: at Tolla, however, in this valley, Dr. Meyen met with a hill of +pumice containing mica. ("Reise um Erde" Th.1 ss. 338, 341.) At the +junction of the Yeso and Volcan [D] there is an extensive mass, in white +conical hillocks, of andesite, containing some mica, and passing either +into andesitic granite, or into a spotted, semi-granular mixture of albitic +(?) feldspar and hornblende: in the midst of this formation Dr. Meyen found +true trachyte. The andesite is covered by strata of dark-coloured, +crystalline, obscurely porphyritic rocks, and above them by the ordinary +porphyritic conglomerates,--the strata all dipping away at a small angle +from the underlying mass. The surrounding lofty mountains appear to be +entirely composed of the porphyritic conglomerate, and I estimated its +thickness here at between six and seven thousand feet. +Beyond the junction of the Yeso and Volcan, the porphyritic strata appear +to dip towards the hillocks of andesite at an angle of 40 degrees; but at +some distant points on the same ridge they are bent up and vertical. +Following the valley of the Yeso, trending N.E. (and therefore still +unfavourable for our transverse section), the same porphyritic conglomerate +formation is prolonged to near the Cuestadel Indio, situated at the western +end of the basin (like a drained lake) of Yeso. Some way before arriving at +this point, distant lofty pinnacles capped by coloured strata belonging to +the great gypseous formation could first be seen. From the summit of the +Cuesta, looking southward, there is a magnificent sectional view of a +mountain-mass, at least 2,000 feet in thickness [E], of fine andesite +granite (containing much black mica, a little chlorite and quartz), which +sends great white dikes far into the superincumbent, dark-coloured, +porphyritic conglomerates. At the line of junction the two formations are +wonderfully interlaced together: in the lower part of the porphyritic +conglomerate, the stratification has been quite obliterated, whilst in the +upper part it is very distinct, the beds composing the crests of the +surrounding mountains being inclined at angles of between 70 and 80 +degrees, and some being even vertical. On the northern side of the valley, +there is a great corresponding mass of andesitic granite, which is encased +by porphyritic conglomerate, dipping both on the western and eastern sides, +at about 80 degrees to west, but on the eastern side with the tips of the +strata bent in such a manner, as to render it probable that the whole mass +has been on that side thrown over and inverted. + +In the valley basin of the Yeso, which I estimated at 7,000 feet above the +level of the sea, we first reach at [F] the gypseous formation. Its +thickness is very great. It consists in most parts of snow-white, hard, +compact gypsum, which breaks with a saccharine fracture, having translucent +edges; under the blowpipe gives out much vapour; it frequently includes +nests and exceedingly thin layers of crystallised, blackish carbonate of +lime. Large, irregularly shaped concretions (externally still exhibiting +lines of aqueous deposition) of blackish-grey, but sometimes white, +coarsely and brilliantly crystallised, hard anhydrite, abound within the +common gypsum. Hillocks, formed of the hardest and purest varieties of the +white gypsum, stand up above the surrounding parts, and have their surfaces +cracked and marked, just like newly baked bread. There is much pale brown, +soft argillaceous gypsum; and there were some intercalated green beds which +I had not time to reach. I saw only one fragment of selenite or transparent +gypsum, and that perhaps may have come from some subsequently formed vein. +From the mineralogical characters here given, it is probable that these +gypseous beds have undergone some metamorphic action. The strata are much +hidden by detritus, but they appeared in most parts to be highly inclined; +and in an adjoining lofty pinnacle they could be distinctly seen bending +up, and becoming vertical, conformably with the underlying porphyritic +conglomerate. In very many parts of the great mountain-face [F], composed +of thin gypseous beds, there were innumerable masses, irregularly shaped +and not like dikes, yet with well-defined edges, of an imperfectly +granular, pale greenish, or yellowish-white rock, essentially composed of +feldspar, with a little chlorite or hornblende, epidote, iron-pyrites, and +ferruginous powder: I believe that these curious trappean masses have been +injected from the not far distant mountain-mass [E] of andesite whilst +still fluid, and that owing to the softness of the gypseous strata they +have not acquired the ordinary forms of dikes. Subsequently to the +injection of these feldspathic rocks, a great dislocation has taken place; +and the much shattered gypseous strata here overlie a hillock [G], composed +of vertical strata of impure limestone and of black highly calcareous shale +including threads of gypsum: these rocks, as we shall presently see, belong +to the upper parts of the gypseous series, and hence must here have been +thrown down by a vast fault. + +Proceeding up the valley-basin of the Yeso, and taking our section +sometimes on one hand and sometimes on the other, we come to a great hill +of stratified porphyritic conglomerate [H] dipping at 45 degrees to the +west; and a few hundred yards farther on, we have a bed between three or +four hundred feet thick of gypsum [I] dipping eastward at a very high +angle: here then we have a fault and anticlinal axis. On the opposite side +of the valley, a vertical mass of red conglomerate, conformably underlying +the gypsum, appears gradually to lose its stratification and passes into a +mountain of porphyry. The gypsum [I] is covered by a bed [K], at least +1,000 feet in thickness, of a purplish-red, compact, heavy, fine-grained +sandstone or mudstone, which fuses easily into a white enamel, and is seen +under a lens to contain triturated crystals. This is succeeded by a bed +[L], 1,000 feet thick (I believe I understate the thickness) of gypsum, +exactly like the beds before described; and this again is capped by another +great bed [M] of purplish-red sandstone. All these strata dip eastward; but +the inclination becomes less and less, as we leave the first and almost +vertical bed [I] of gypsum. + +Leaving the basin-plain of Yeso, the road rapidly ascends, passing by +mountains composed of the gypseous and associated beds, with their +stratification greatly disturbed and therefore not easily intelligible: +hence this part of the section has been left uncoloured. Shortly before +reaching the great Pequenes ridge, the lowest stratum visible [N] is a red +sandstone or mudstone, capped by a vast thickness of black, compact, +calcareous, shaly rock [O], which has been thrown into four lofty, though +small ridges: looking northward, the strata in these ridges are seen +gradually to rise in inclination, becoming in some distant pinnacles +absolutely vertical. + +The ridge of Pequenes, which divides the waters flowing into the Pacific +and Atlantic Oceans, extends in a nearly N.N.W. and S.S.E. line; its strata +dip eastward at an angle of between 30 and 45 degrees, but in the higher +peaks bending up and becoming almost vertical. Where the road crosses this +range, the height is 13,210 feet above the sea-level, and I estimated the +neighbouring pinnacles at from fourteen to fifteen thousand feet. The +lowest stratum visible in this ridge is a red stratified sandstone [P]; on +it are superimposed two great masses [Q and S] of black, hard, compact, +even having a conchoidal fracture, calcareous, more or less laminated +shale, passing into limestone: this rock contains organic remains, +presently to be enumerated. The compacter varieties fuse easily in a white +glass; and this I may add is a very general character with all the +sedimentary beds in the Cordillera: although this rock when broken is +generally quite black, it everywhere weathers into an ash-grey tint. +Between these two great masses [Q and S], a bed [R] of gypsum is +interposed, about three hundred feet in thickness, and having the same +characters as heretofore described. I estimated the total thickness of +these three beds [Q, R, S] at nearly three thousand feet; and to this must +be added, as will be immediately seen, a great overlying mass of red +sandstone. + +In descending the eastern slope of this great central range, the strata, +which in the upper part dip eastward at about an angle of 40 degrees, +become more and more curved, till they are nearly vertical; and a little +further onwards there is seen on the further side of a ravine, a thick mass +of strata of bright red sandstone [T], with their upper extremities +slightly curved, showing that they were once conformably prolonged over the +beds [S]: on the southern and opposite side of the road, this red sandstone +and the underlying black shaly rocks stand vertical, and in actual +juxtaposition. Continuing to descend, we come to a synclinal valley filled +with rubbish, beyond which we have the red sandstone [T2] corresponding +with [T], and now dipping, as is seen both north and south of the road, at +45 degrees to the west; and under it, the beds [S2, R2, Q2, and I believe +P2] in corresponding order and of similar composition, with those on the +western flank of the Pequenes range, but dipping westward. Close to the +synclinal valley the dip of these strata is 45 degrees, but at the eastern +or farther end of the series it increases to 60 degrees. Here the great +gypseous formation abruptly terminates, and is succeeded eastward by a pile +of more modern strata. Considering how violently these central ranges have +been dislocated, and how very numerous dikes are in the exterior and lower +parts of the Cordillera, it is remarkable that I did not here notice a +single dike. The prevailing rock in this neighbourhood is the black, +calcareous, compact shale, whilst in the valley-basin of the Yeso the +purplish red sandstone or mudstone predominates,--both being associated +with gypseous strata of exactly the same nature. It would be very difficult +to ascertain the relative superposition of these several masses, for we +shall afterwards see in the Cumbre Pass that the gypseous and intercalated +beds are lens-shaped, and that they thin out, even where very thick, and +disappear in short horizontal distances: it is quite possible that the +black shales and red sandstones may be contemporaneous, but it is more +probable that the former compose the uppermost parts of the series. + +The fossils above alluded to in the black calcareous shales are few in +number, and are in an imperfect condition; they consist, as named for me by +M. d'Orbigny, of:-- + +1. Ammonite, indeterminable, near to A. recticostatus, d'Orbigny, "Pal. +Franc." (Neocomian formation). +2. Gryphaea, near to G. Couloni (Neocomian formations of France and +Neufchatel). +3. Natica, indeterminable. +4. Cyprina rostrata, d'Orbigny, "Pal. Franc." (Neocomian formation). +5. Rostellaria angulosa (?), d'Orbigny, "Pal. de l'Amer. Mer." +6. Terebratula (?). + +Some of the fragments of Ammonites were as thick as a man's arm: the +Gryphaea is much the most abundant shell. These fossils M. d'Orbigny +considers as belonging to the Neocomian stage of the Cretaceous system. Dr. +Meyen, who ascended the valley of the Rio Volcan, a branch of the Yeso, +found a nearly similar, but apparently more calcareous formation, with much +gypsum, and no doubt the equivalent of that here described ("Reise um Erde" +etc. Th. 1 s. 355.): the beds were vertical, and were prolonged up to the +limits of perpetual snow; at the height of 9,000 feet above the sea, they +abounded with fossils, consisting, according to Von Buch ("Descript. Phys. +des Iles Canaries" page 471.), of:-- + +1. Exogyra (Gryphaea) Couloni, absolutely identical with specimens from the +Jura and South of France. +2. Trigonia costata, identical with those found in the upper Jurassic beds +at Hildesheim. +3. Pecten striatus, identical with those found in the upper Jurassic beds +at Hildesheim. +4. Cucullaea, corresponding in form to C. longirostris, so frequent in the +upper Jurassic beds of Westphalia. +5. Ammonites resembling A. biplex. + +Von Buch concludes that this formation is intermediate between the +limestone of the Jura and the chalk, and that it is analogous with the +uppermost Jurassic beds forming the plains of Switzerland. Hence M. +D'Orbigny and Von Buch, under different terms, compare these fossils to +those from the same late stage in the secondary formations of Europe. + +Some of the fossils which I collected were found a good way down the +western slope of the main ridge, and hence must originally have been +covered up by a great thickness of the black shaly rock, independently of +the now denuded, thick, overlying masses of red sandstone. I neglected at +the time to estimate how many hundred or rather thousand feet thick the +superincumbent strata must have been: and I will not now attempt to do so. +This, however, would have been a highly interesting point, as indicative of +a great amount of subsidence, of which we shall hereafter find in other +parts of the Cordillera analogous evidence during this same period. The +altitude of the Peuquenes Range, considering its not great antiquity, is +very remarkable; many of the fossils were embedded at the height of 13,210 +feet, and the same beds are prolonged up to at least from fourteen to +fifteen thousand feet above the level of the sea. + +THE PORTILLO OR EASTERN CHAIN. + +The valley of Tenuyan, separating the Peuquenes and Portillo lines, is, as +estimated by Dr. Gillies and myself, about twenty miles in width; the +lowest part, where the road crosses the river, being 7,500 feet above the +sea-level. The pass on the Portillo line is 14,365 feet high (1,100 feet +higher than that on the Peuquenes), and the neighbouring pinnacles must, I +conceive, rise to nearly 16,000 feet above the sea. The river draining the +intermediate valley of Tenuyan, passes through the Portillo line. To return +to our section:--shortly after leaving the lower beds [P2] of the gypseous +formation, we come to grand masses of a coarse, red conglomerate [V], +totally unlike any strata hitherto seen in the Cordillera. This +conglomerate is distinctly stratified, some of the beds being well defined +by the greater size of the pebbles: the cement is calcareous and sometimes +crystalline, though the mass shows no signs of having been metamorphosed. +The included pebbles are either perfectly or only partially rounded: they +consist of purplish sandstones, of various porphyries, of brownish +limestone, of black calcareous, compact shale precisely like that in situ +in the Peuquenes range, and CONTAINING SOME OF THE SAME FOSSIL SHELLS; also +very many pebbles of quartz, some of micaceous schist, and numerous, +broken, rounded crystals of a reddish orthitic or potash feldspar (as +determined by Professor Miller), and these from their size must have been +derived from a coarse-grained rock, probably granite. From this feldspar +being orthitic, and even from its external appearance, I venture positively +to affirm that it has not been derived from the rocks of the western +ranges; but, on the other hand, it may well have come, together with the +quartz and metamorphic schists, from the eastern or Portillo line, for this +line mainly consists of coarse orthitic granite. The pebbles of the +fossiliferous slate and of the purple sandstone, certainly have been +derived from the Peuquenes or western ranges. + +The road crosses the valley of Tenuyan in a nearly east and west line, and +for several miles we have on both hands the conglomerate, everywhere +dipping west and forming separate great mountains. The strata, where first +met with, after leaving the gypseous formation, are inclined westward at an +angle of only 20 degrees, which further on increases to about 45 degrees. +The gypseous strata, as we have seen, are also inclined westward: hence, +when looking from the eastern side of the valley towards the Peuquenes +range, a most deceptive appearance is presented, as if the newer beds of +conglomerate dipped directly under the much older beds of the gypseous +formation. In the middle of the valley, a bold mountain of unstratified +lilac-coloured porphyry (with crystals of hornblende) projects; and further +on, a little south of the road, there is another mountain, with its strata +inclined at a small angle eastwards, which in its general aspect and +colour, resembles the porphyritic conglomerate formation, so rare on this +side of the Peuquenes line and so grandly developed throughout the western +ranges. + +The conglomerate is of great thickness: I do not suppose that the strata +forming the separate mountain-masses [V,V,V] have ever been prolonged over +each other, but that one mass has been broken up by several, distinct, +parallel, uniclinal lines of elevation. Judging therefore of the thickness +of the conglomerate, as seen in the separate mountain-masses, I estimated +it at least from one thousand five hundred to two thousand feet. The lower +beds rest conformably on some singularly coloured, soft strata [W], which I +could not reach to examine; and these again rest conformably on a thick +mass of micaceous, thinly laminated, siliceous sandstone [X], associated +with a little black clay-slate. These lower beds are traversed by several +dikes of decomposing porphyry. The laminated sandstone is directly +superimposed on the vast masses of granite [Y,Y] which mainly compose the +Portillo range. The line of junction between this latter rock, which is of +a bright red colour, and the whitish sandstone was beautifully distinct; +the sandstone being penetrated by numerous, great, tortuous dikes branching +from the granite, and having been converted into a granular quartz rock +(singularly like that of the Falkland Islands), containing specks of an +ochrey powder, and black crystalline atoms, apparently of imperfect mica. +The quartzose strata in one spot were folded into a regular dome. + +The granite which composes the magnificent bare pinnacles and the steep +western flank of the Portillo chain, is of a brick-red colour, coarsely +crystallised, and composed of orthitic or potash feldspar, quartz, and +imperfect mica in small quantity, sometimes passing into chlorite. These +minerals occasionally assume a laminar or foliated arrangement. The fact of +the feldspar being orthitic in this range, is very remarkable, considering +how rare, or rather, as I believe, entirely absent, this mineral is +throughout the western ranges, in which soda-feldspar, or at least a +variety cleaving like albite, is so extremely abundant. In one spot on the +western flank, and on the eastern flank near Los Manantiales and near the +crest, I noticed some great masses of a whitish granite, parts of it fine- +grained, and parts containing large crystals of feldspar; I neglected to +collect specimens, so I do not know whether this feldspar is also orthitic, +though I am inclined to think so from its general appearance. I saw also +some syenite and one mass which resembled andesite, but of which I likewise +neglected to collect specimens. From the manner in which the whitish +granites formed separate mountain-masses in the midst of the brick-red +variety, and from one such mass near the crest being traversed by numerous +veins of flesh-coloured and greenish eurite (into which I occasionally +observed the brick-red granite insensibly passing), I conclude that the +white granites probably belong to an older formation, almost overwhelmed +and penetrated by the red granite. + +On the crest I saw also, at a short distance, some coloured stratified +beds, apparently like those [W] at the western base, but was prevented +examining them by a snowstorm: Mr. Caldcleugh, however, collected here +specimens of ribboned jasper, magnesian limestone, and other minerals. +("Travels" etc. volume 1 page 308.) A little way down the eastern slope a +few fragments of quartz and mica-slate are met with; but the great +formation of this latter rock [Z], which covers up much of the eastern +flank and base of the Portillo range, cannot be conveniently examined until +much lower down at a place called Mal Paso. The mica-schist here consists +of thick layers of quartz, with intervening folia of finely-scaly mica, +often passing into a substance like black glossy clay-slate: in one spot, +the layers of the quartz having disappeared, the whole mass became +converted into glossy clay-slate. Where the folia were best defined, they +were inclined at a high angle westward, that is, towards the range. The +line of junction between the dark mica-slate and the coarse red granite was +most clearly distinguishable from a vast distance: the granite sent many +small veins into the mica-slate, and included some angular fragments of it. +As the sandstone on the western base has been converted by the red granite +into a granular quartz-rock, so this great formation of mica-schist may +possibly have been metamorphosed at the same time and by the same means; +but I think it more probable, considering its more perfect metamorphic +character and its well-pronounced foliation, that it belongs to an anterior +epoch, connected with the white granites: I am the more inclined to this +view, from having found at the foot of the range the mica-schist +surrounding a hummock [Y2], exclusively composed of white granite. Near Los +Arenales, the mountains on all sides are composed of the mica-slate; and +looking backwards from this point up to the bare gigantic peaks above, the +view was eminently interesting. The colours of the red granite and the +black mica-slate are so distinct, that with a bright light these rocks +could be readily distinguished even from the Pampas, at a level of at least +9,000 feet below. The red granite, from being divided by parallel joints, +has weathered into sharp pinnacles, on some of which, even on some of the +loftiest, little caps of mica-schist could be clearly seen: here and there +isolated patches of this rock adhered to the mountain-flanks, and these +often corresponded in height and position on the opposite sides of the +immense valleys. Lower down the schist prevailed more and more, with only a +few quite small points of granite projecting through. Looking at the entire +eastern face of the Portillo range, the red colour far exceeds in area the +black; yet it was scarcely possible to doubt that the granite had once been +almost wholly encased by the mica-schist. + +At Los Arenales, low down on the eastern flank, the mica-slate is traversed +by several closely adjoining, broad dikes, parallel to each other and to +the foliation of the schist. The dikes are formed of three different +varieties of rock, of which a pale brown feldspathic porphyry with grains +of quartz was much the most abundant. These dikes with their granules of +quartz, as well as the mica-schist itself, strikingly resemble the rocks of +the Chonos Archipelago. At a height of about twelve hundred feet above the +dikes, and perhaps connected with them, there is a range of cliffs formed +of successive lava-streams [AA], between three and four hundred feet in +thickness, and in places finely columnar. The lava consists of dark- +greyish, harsh rocks, intermediate in character between trachyte and +basalt, containing glassy feldspar, olivine, and a little mica, and +sometimes amygdaloidal with zeolite: the basis is either quite compact, or +crenulated with air-vesicles arranged in laminae. The streams are separated +from each other by beds of fragmentary brown scoriae, firmly cemented +together, and including a few well-rounded pebbles of lava. From their +general appearance, I suspect that these lava-streams flowed at an ancient +period under the pressure of the sea, when the Atlantic covered the Pampas +and washed the eastern foot of the Cordillera. (This conclusion might, +perhaps, even have been anticipated, from the general rarity of volcanic +action, except near the sea or large bodies of water. Conformably with this +rule, at the present day, there are no active volcanoes on this eastern +side of the Cordillera; nor are severe earthquakes experienced here.) On +the opposite and northern side of the valley there is another line of lava- +cliffs at a corresponding height; the valley between being of considerable +breadth, and as nearly as I could estimate 1,500 feet in depth. This field +of lava is confined on both sides by the mountains of mica-schist, and +slopes down rapidly but irregularly to the edge of the Pampas, where, +having a thickness of about two hundred feet, it terminates against a +little range of claystone porphyry. The valley in this lower part expands +into a bay-like, gentle slope, bordered by the cliffs of lava, which must +certainly once have extended across this wide expanse. The inclination of +the streams from Los Arenales to the mouth of the valley is so great, that +at the time (though ignorant of M. Elie de Beaumont's researches on the +extremely small slope over which lava can flow, and yet retain a compact +structure and considerable thickness) I concluded that they must +subsequently to their flowing have been upheaved and tilted from the +mountains; of this conclusion I can now entertain not the smallest doubt. + +At the mouth of the valley, within the cliffs of the above lava-field, +there are remnants, in the form of separate small hillocks and of lines of +low cliffs, of a considerable deposit of compact white tuff (quarried for +filtering-stones), composed of broken pumice, volcanic crystals, scales of +mica, and fragments of lava. This mass has suffered much denudation; and +the hard mica-schist has been deeply worn, since the period of its +deposition; and this period must have been subsequent to the denudation of +the basaltic lava-streams, as attested by their encircling cliffs standing +at a higher level. At the present day, under the existing arid climate, +ages might roll past without a square yard of rock of any kind being +denuded, except perhaps in the rarely moistened drainage-channel of the +valley. Must we then look back to that ancient period, when the waves of +the sea beat against the eastern foot of the Cordillera, for a power +sufficient to denude extensively, though superficially, this tufaceous +deposit, soft although it be? + +There remains only to mention some little water-worn hillocks [BB], a few +hundred feet in height, and mere mole-hills compared with the gigantic +mountains behind them, which rise out of the sloping, shingle-covered +margin of the Pampas. The first little range is composed of a brecciated +purple porphyritic claystone, with obscurely marked strata dipping at 70 +degrees to the S.W.; the other ranges consist of--a pale-coloured +feldspathic porphyry,--a purple claystone porphyry with grains of quartz,-- +and a rock almost exclusively composed of brick-red crystals of feldspar. +These outermost small lines of elevation extend in a N.W. by W. and S.E. by +S. direction. + +CONCLUDING REMARKS ON THE PORTILLO RANGE. + +When on the Pampas and looking southward, and whilst travelling northward, +I could see for very many leagues the red granite and dark mica-schist +forming the crest and eastern flank of the Portillo line. This great range, +according to Dr. Gillies, can be traced with little interruption for 140 +miles southward to the R. Diamante, where it unites with the western +ranges: northward, according to this same author, it terminates where the +R. Mendoza debouches from the mountains; but a little further north in the +eastern part of the Cumbre section, there are, as we shall hereafter see, +some mountain-masses of a brick-red porphyry, the last injected amidst many +other porphyries, and having so close an analogy with the coarse red +granite of the Portillo line, that I am tempted to believe that they belong +to the same axis of injection; if so, the Portillo line is at least 200 +miles in length. Its height, even in the lowest gap in the road, is 14,365 +feet, and some of the pinnacles apparently attain an elevation of about +16,000 feet above the sea. The geological history of this grand chain +appears to me eminently interesting. We may safely conclude, that at a +former period the valley of Tenuyan existed as an arm of the sea, about +twenty-miles in width, bordered on one hand by a ridge or chain of islets +of the black calcareous shales and purple sandstones of the gypseous +formation; and on the other hand, by a ridge or chain of islets composed of +mica-slate, white granite, and perhaps to a partial extent of red granite. +These two chains, whilst thus bordering the old sea-channel, must have been +exposed for a vast lapse of time to alluvial and littoral action, during +which the rocks were shattered, the fragments rounded, and the strata of +conglomerate accumulated to a thickness of at least fifteen hundred or two +thousand feet. The red orthitic granite now forms, as we have seen, the +main part of the Portillo chain: it is injected in dikes not only into the +mica-schist and white granites, but into the laminated sandstone, which it +has metamorphosed, and which it has thrown off, together with the +conformably overlying coloured beds and stratified conglomerate, at an +angle of forty-five degrees. To have thrown off so vast a pile of strata at +this angle, is a proof that the main part of the red granite (whether or +not portions, as perhaps is probable, previously existed) was injected in a +liquified state after the accumulation both of the laminated sandstone and +of the conglomerate; this conglomerate, we know, was accumulated, not only +after the deposition of the fossiliferous strata of the Peuquenes line, but +after their elevation and long-continued denudation: and these +fossiliferous strata belong to the early part of the Cretaceous system. +Late, therefore, in a geological sense, as must be the age of the main part +of the red granite, I can conceive nothing more impressive than the eastern +view of this great range, as forcing the mind to grapple with the idea of +the thousands of thousands of years requisite for the denudation of the +strata which originally encased it,--for that the fluidified granite was +once encased, its mineralogical composition and structure, and the bold +conical shape of the mountain-masses, yield sufficient evidence. Of the +encasing strata we see the last vestiges in the coloured beds on the crest, +in the little caps of mica-schist on some of the loftiest pinnacles, and in +the isolated patches of this same rock at corresponding heights on the now +bare and steep flanks. + +The lava-streams at the eastern foot of the Portillo are interesting, not +so much from the great denudation which they have suffered at a +comparatively late period as from the evidence they afford by their +inclination taken conjointly with their thickness and compactness, that +after the great range had assumed its present general outline, it continued +to rise as an axis of elevation. The plains extending from the base of the +Cordillera to the Atlantic show that the continent has been upraised in +mass to a height of 3,500 feet, and probably to a much greater height, for +the smooth shingle-covered margin of the Pampas is prolonged in a gentle +unbroken slope far up many of the great valleys. Nor let it be assumed that +the Peuquenes and Portillo ranges have undergone only movements of +elevation; for we shall hereafter see, that the bottom of the sea subsided +several thousand feet during the deposition of strata, occupying the same +relative place in the Cordillera, with those of the Peuquenes ridge; +moreover, we shall see from the unequivocal evidence of buried upright +trees, that at a somewhat later period, during the formation of the +Uspallata chain, which corresponds geographically with that of the +Portillo, there was another subsidence of many thousand feet: here, indeed, +in the valley of Tenuyan, the accumulation of the coarse stratified +conglomerate to a thickness of fifteen hundred or two thousand feet, offers +strong presumptive evidence of subsidence; for all existing analogies lead +to the belief that large pebbles can be transported only in shallow water, +liable to be affected by currents and movements of undulation--and if so, +the shallow bed of the sea on which the pebbles were first deposited must +necessarily have sunk to allow of the accumulation of the superincumbent +strata. What a history of changes of level, and of wear and tear, all since +the age of the latter secondary formations of Europe, does the structure of +this one great mountain-chain reveal! + +PASSAGE OF THE ANDES BY THE CUMBRE OR USPALLATA PASS. + +This Pass crosses the Andes about sixty miles north of that just described: +the section given in Plate 1, Section 1/2, is on the same scale as before, +namely, at one-third of an inch to a mile in distance, and one inch to a +mile (or 6,000 feet) in height. Like the last section, it is a mere sketch, +and cannot pretend to accuracy, though made under favourable circumstances. +We will commence as before, with the western half, of which the main range +bears the name of the Cumbre (that is the Ridge), and corresponds to the +Peuquenes line in the former section; as does the Uspallata range, though +on a much smaller scale, to that of the Portillo. Near the point where the +river Aconcagua debouches on the basin plain of the same name, at a height +of about two thousand three hundred feet above the sea, we meet with the +usual purple and greenish porphyritic claystone conglomerate. Beds of this +nature, alternating with numerous compact and amygdaloidal porphyries, +which have flowed as submarine lavas, and associated with great mountain- +masses of various, injected, non-stratified porphyries, are prolonged the +whole distance up to the Cumbre or central ridge. One of the commonest +stratified porphyries is of a green colour, highly amygdaloidal with the +various minerals described in the preliminary discussion, and including +fine tabular crystals of albite. The mountain-range north (often with a +little westing) and south. The stratification, wherever I could clearly +distinguish it, was inclined westward or towards the Pacific, and, except +near the Cumbre, seldom at angles above 25 degrees. Only at one spot on +this western side, on a lofty pinnacle not far from the Cumbre, I saw +strata apparently belonging to the gypseous formation, and conformably +capping a pile of stratified porphyries. Hence, both in composition and in +stratification, the structure of the mountains on this western side of the +divortium aquarum, is far more simple than in the corresponding part of the +Peuquenes section. In the porphyritic claystone conglomerate, the +mechanical structure and the planes of stratification have generally been +much obscured and even quite obliterated towards the base of the series, +whilst in the upper parts, near the summits of the mountains, both are +distinctly displayed. In these upper portions the porphyries are generally +lighter coloured. In three places [X, Y, Z] masses of andesite are exposed: +at [Y], this rock contained some quartz, but the greater part consisted of +andesitic porphyry, with only a few well-developed crystals of albite, and +forming a great white mass, having the external aspect of granite, capped +by much dark unstratified porphyry. In many parts of the mountains, there +are dikes of a green colour, and other white ones, which latter probably +spring from underlying masses of andesite. + +The Cumbre, where the road crosses it, is, according to Mr. Pentland, +12,454 feet above the sea; and the neighbouring peaks, composed of dark +purple and whitish porphyries, some obscurely stratified with a westerly +dip, and others without a trace of stratification, must exceed 13,000 feet +in height. Descending the eastern slope of the Cumbre, the structure +becomes very complicated, and generally differs on the two sides of the +east and west line of road and section. First we come to a great mass [A] +of nearly vertical, singularly contorted strata, composed of highly compact +red sandstones, and of often calcareous conglomerates, and penetrated by +green, yellow, and reddish dikes; but I shall presently have an opportunity +of describing in some detail an analogous pile of strata. These vertical +beds are abruptly succeeded by others [B], of apparently nearly the same +nature but more metamorphosed, alternating with porphyries and limestones; +these dip for a short space westward, but there has been here an +extraordinary dislocation, which, on the north side of the road, appears to +have determined the excavation of the north and south valley of the R. de +las Cuevas. On this northern side of the road, the strata [B] are prolonged +till they come in close contact with a jagged lofty mountain [D] of dark- +coloured, unstratified, intrusive porphyry, where the beds have been more +highly inclined and still more metamorphosed. This mountain of porphyry +seems to form a short axis of elevation, for south of the road in its line +there is a hill [C] of porphyritic conglomerate with absolutely vertical +strata. + +We now come to the gypseous formation: I will first describe the structure +of the several mountains, and then give in one section a detailed account +of the nature of the rocks. On the north side of the road, which here runs +in an east and west valley, the mountain of porphyry [D] is succeeded by a +hill [E] formed of the upper gypseous strata tilted, at an angle of between +70 and 80 degrees to the west, by a uniclinal axis of elevation which does +not run parallel to the other neighbouring ranges, and which is of short +length; for on the south side of the valley its prolongation is marked only +by a small flexure in a pile of strata inclined by a quite separate axis. A +little further on the north and south valley of Horcones enters at right +angles our line of section; its western side is bounded by a hill of +gypseous strata [F] dipping westward at about 45 degrees, and its eastern +side by a mountain of similar strata [G] inclined westward at 70 degrees, +and superimposed by an oblique fault on another mass of the same strata +[H], also inclined westward, but at an angle of about 30 degrees: the +complicated relation of these three masses [F, G, H] is explained by the +structure of a great mountain-range lying some way to the north, in which a +regular anticlinal axis (represented in the section by dotted lines) is +seen, with the strata on its eastern side again bending up and forming a +distinct uniclinal axis, of which the beds marked [H] form the lower part. +This great uniclinal line is intersected, near the Puente del Inca, by the +valley along which the road runs, and the strata composing it will be +immediately described. On the south side of the road, in the space +corresponding with the mountains [E, F, and G], the strata everywhere dip +westward generally at an angle of 30 degrees, occasionally mounting up to +45 degrees, but not in an unbroken line, for there are several vertical +faults, forming separate uniclinal masses, all dipping in the same +direction,--a form of elevation common in the Cordillera. We thus see that +within a narrow space, the gypseous strata have been upheaved and crushed +together by a great uniclinal, anticlinal, and one lesser uniclinal line +[E] of elevation; and that between these three lines and the Cumbre, in the +sandstones, conglomerates and porphyritic formation, there have been at +least two or three other great elevatory axes. + +The uniclinal axis [I] intersected near the Puente del Inca (of which the +strata at [H] form a part) ranges N. by W. and S. by E., forming a chain of +mountains, apparently little inferior in height to the Cumbre: the strata, +as we have seen, dip at an average angle of 30 degrees to the west. (At +this place, there are some hot and cold springs, the warmest having a +temperature, according to Lieutenant Brand "Travels," page 240, of 91 +degrees; they emit much gas. According to Mr. Brande, of the Royal +Institution, ten cubical inches contain forty-five grains of solid matter, +consisting chiefly of salt, gypsum, carbonate of lime, and oxide of iron. +The water is charged with carbonic acid and sulphuretted hydrogen. These +springs deposit much tufa in the form of spherical balls. They burst forth, +as do those of Cauquenes, and probably those of Villa Vicencio, on a line +of elevation.) The flanks of the mountains are here quite bare and steep, +affording an excellent section; so that I was able to inspect the strata to +a thickness of about 4,000 feet, and could clearly distinguish their +general nature for 1,000 feet higher, making a total thickness of 5,000 +feet, to which must be added about 1,000 feet of the inferior strata seen a +little lower down the valley, I will describe this one section in detail, +beginning at the bottom. + +1st. The lowest mass is the altered clay-slate described in the preliminary +discussion, and which in this line of section was here first met with. +Lower down the valley, at the R. de las Vacas, I had a better opportunity +of examining it; it is there in some parts well characterised, having a +distinct, nearly vertical, tortuous cleavage, ranging N.W. and S.E., and +intersected by quartz veins: in most parts, however, it is crystalline and +feldspathic, and passes into a true greenstone often including grains of +quartz. The clay-slate, in its upper half, is frequently brecciated, the +embedded angular fragments being of nearly the same nature with the paste. + +2nd. Several strata of purplish porphyritic conglomerate, of no very great +thickness, rest conformably upon the feldspathic slate. A thick bed of +fine, purple, claystone porphyry, obscurely brecciated (but not of +metamorphosed sedimentary origin), and capped by porphyritic conglomerate, +was the lowest bed actually examined in this section at the Puente del +Inca. + +3rd. A stratum, eighty feet thick, of hard and very compact impure whitish +limestone, weathering bright red, with included layers brecciated and re- +cemented. Obscure marks of shell are distinguishable in it. + +4th. A red, quartzose, fine-grained conglomerate, with grains of quartz, +and with patches of white earthy feldspar, apparently due to some process +of concretionary crystalline action; this bed is more compact and +metamorphosed than any of the overlying conglomerates. + +5th. A whitish cherty limestone, with nodules of bluish argillaceous +limestone. + +6th. A white conglomerate, with many particles of quartz, almost blending +into the paste. + +7th. Highly siliceous, fine-grained white sandstone. + +8th and 9th. Red and white beds not examined. + +10th. Yellow, fine-grained, thinly stratified, magnesian (judging from its +slow dissolution in acids) limestone: it includes some white quartz +pebbles, and little cavities, lined with calcareous spar, some retaining +the form of shells. + +11th. A bed between twenty and thirty feet thick, quite conformable with +the underlying ones, composed of a hard basis, tinged lilac-grey +porphyritic with NUMEROUS crystals of whitish feldspar, with black mica and +little spots of soft ferruginous matter: evidently a submarine lava. + +12th. Yellow magnesian limestone, as before, part-stained purple. + +13th. A most singular rock; basis purplish grey, obscurely crystalline, +easily fusible into a dark green glass, not hard, thickly speckled with +crystals more or less perfect of white carbonate of lime, of red hydrous +oxide of iron, of a white and transparent mineral like analcime, and of a +green opaque mineral like soap-stone; the basis is moreover amygdaloidal +with many spherical balls of white crystallised carbonate of lime, of which +some are coated with the red oxide of iron. I have no doubt, from the +examination of a superincumbent stratum (19), that this is a submarine +lava; though in Northern Chile, some of the metamorphosed sedimentary beds +are almost as crystalline, and of as varied composition. + +14th. Red sandstone, passing in the upper part into a coarse, hard, red +conglomerate, 300 feet thick, having a calcareous cement, and including +grains of quartz and broken crystals of feldspar; basis infusible; the +pebbles consist of dull purplish porphyries, with some of quartz, from the +size of a nut to a man's head. This is the coarsest conglomerate in this +part of the Cordillera: in the middle there was a white layer not examined. + +15th. Grand thick bed, of a very hard, yellowish-white rock, with a +crystalline feldspathic base, including large crystals of white feldspar, +many little cavities mostly full of soft ferruginous matter, and numerous +hexagonal plates of black mica. The upper part of this great bed is +slightly cellular; the lower part compact: the thickness varied a little in +different parts. Manifestly a submarine lava; and is allied to bed 11. + +16th and 17th. Dull purplish, calcareous, fine-grained, compact sandstones, +which pass into coarse white conglomerates with numerous particles of +quartz. + +18th. Several alternations of red conglomerate, purplish sandstone, and +submarine lava, like that singular rock forming bed 13. + +19th. A very heavy, compact, greenish-black stone, with a fine-grained +obviously crystalline basis, containing a few specks of white calcareous +spar, many specks of the crystallised hydrous red oxide of iron, and some +specks of a green mineral; there are veins and nests filled with epidote: +certainly a submarine lava. + +20th. Many thin strata of compact, fine-grained, pale purple sandstone. + +21st. Gypsum in a nearly pure state, about three hundred feet in thickness: +this bed, in its concretions of anhydrite and layers of small blackish +crystals of carbonate of lime, exactly resembles the great gypseous beds in +the Peuquenes range. + +22nd. Pale purple and reddish sandstone, as in bed 20: about three hundred +feet in thickness. + +23rd. A thick mass composed of layers, often as thin as paper and +convoluted, of pure gypsum with others very impure, of a purplish colour. + +24th. Pure gypsum, thick mass. + +25th. Red sandstones, of great thickness. + +26th. Pure gypsum, of great thickness. + +27th. Alternating layers of pure and impure gypsum, of great thickness. + +I was not able to ascend to these few last great strata, which compose the +neighbouring loftiest pinnacles. The thickness, from the lowest to the +uppermost bed of gypsum, cannot be less than 2,000 feet: the beds beneath I +estimated at 3,000 feet, and this does not include either the lower parts +of the porphyritic conglomerate, or the altered clay-slate; I conceive the +total thickness must be about six thousand feet. I distinctly observed that +not only the gypsum, but the alternating sandstones and conglomerates were +lens-shaped, and repeatedly thinned out and replaced each other: thus in +the distance of about a mile, a bed 300 feet thick of sandstone between two +beds of gypsum, thinned out to nothing and disappeared. The lower part of +this section differs remarkably,--in the much greater diversity of its +mineralogical composition,--in the abundance of calcareous matter,--in the +greater coarseness of some of the conglomerates,--and in the numerous +particles and well-rounded pebbles, sometimes of large size, of quartz,-- +from any other section hitherto described in Chile. From these +peculiarities and from the lens-form of the strata, it is probable that +this great pile of strata was accumulated on a shallow and very uneven +bottom, near some pre-existing land formed of various porphyries and +quartz-rock. The formation of porphyritic claystone conglomerate does not +in this section attain nearly its ordinary thickness; this may be PARTLY +attributed to the metamorphic action having been here much less energetic +than usual, though the lower beds have been affected to a certain degree. +If it had been as energetic as in most other parts of Chile, many of the +beds of sandstone and conglomerate, containing rounded masses of porphyry, +would doubtless have been converted into porphyritic conglomerate; and +these would have alternated with, and even blended into, crystalline and +porphyritic strata without a trace of mechanical structure,--namely, into +those which, in the present state of the section, we see are unquestionably +submarine lavas. + +The beds of gypsum, together with the red alternating sandstones and +conglomerates, present so perfect and curious a resemblance with those seen +in our former section in the basin-valley of Yeso, that I cannot doubt the +identity of the two formations: I may add, that a little westward of the P. +del Inca, a mass of gypsum passed into a fine-grained, hard, brown +sandstone, which contained some layers of black, calcareous, compact, shaly +rock, precisely like that seen in such vast masses on the Peuquenes range. + +Near the Puente del Inca, numerous fragments of limestone, containing some +fossil remains, were scattered on the ground: these fragments so perfectly +resemble the limestone of bed No. 3, in which I saw impressions of shells, +that I have no doubt they have fallen from it. The yellow magnesian +limestone of bed No. 10, which also includes traces of shells, has a +different appearance. These fossils (as named by M. d'Orbigny) consist of:- +- + +Gryphaea, near to G. Couloni (Neocomian formation). +Arca, perhaps A. Gabrielis, d'Orbigny, "Pal. Franc." (Neocomian formation). + +Mr. Pentland made a collection of shells from this same spot, and Von Buch +considers them as consisting of:-- + +Trigonia, resembling in form T. costata. +Pholadomya, like one found by M. Dufresnoy near Alencon. +Isocardi excentrica, Voltz., identical with that from the Jura. +("Description Phys. des Iles Can." page 472.) + +Two of these shells, namely, the Gryphaea and Trigonia, appear to be +identical with species collected by Meyen and myself on the Peuquenes +range; and in the opinion of Von Buch and M. d'Orbigny, the two formations +belong to the same age. I must here add, that Professor E. Forbes, who has +examined my specimens from this place and from the Peuquenes range, has +likewise a strong impression that they indicate the Cretaceous period, and +probably an early epoch in it: so that all the palaeontologists who have +seen these fossils nearly coincide in opinion regarding their age. The +limestone, however, with these fossils here lies at the very base of the +formation, just above the porphyritic conglomerate, and certainly several +thousand feet lower in the series, than the equivalent, fossiliferous, +black, shaly rocks high up on the Peuquenes range. + +It is well worthy of remark that these shells, or at least those of which I +saw impressions in the limestone (bed No. 3), must have been covered up, on +the LEAST computation, by 4,000 feet of strata: now we know from Professor +E. Forbes's researches, that the sea at greater depths than 600 feet +becomes exceedingly barren of organic beings,--a result quite in accordance +with what little I have seen of deep-sea soundings. Hence, after this +limestone with its shells was deposited, the bottom of the sea where the +main line of the Cordillera now stands, must have subsided some thousand +feet to allow of the deposition of the superincumbent submarine strata. +Without supposing a movement of this kind, it would, moreover, be +impossible to understand the accumulation of the several lower strata of +COARSE, well-rounded conglomerates, which it is scarcely possible to +believe were spread out in profoundly deep water, and which, especially +those containing pebbles of quartz, could hardly have been rounded in +submarine craters and afterwards ejected from them, as I believe to have +been the case with much of the porphyritic conglomerate formation. I may +add that, in Professor Forbes's opinion, the above-enumerated species of +mollusca probably did not live at a much greater depth than twenty fathoms, +that is only 120 feet. + +To return to our section down the valley; standing on the great N. by W. +and S. by E. uniclinal axis of the Puente del Inca, of which a section has +just been given, and looking north-east, greater tabular masses of gypseous +formation (KK) could be seen in the distance, very slightly inclined +towards the east. Lower down the valley, the mountains are almost +exclusively composed of porphyries, many of them of intrusive origin and +non-stratified, others stratified, but with the stratification seldom +distinguishable except in the upper parts. Disregarding local disturbances, +the beds are either horizontal or inclined at a small angle eastwards: +hence, when standing on the plain of Uspallata and looking to the west or +backwards, the Cordillera appear composed of huge, square, nearly +horizontal, tabular masses: so wide a space, with such lofty mountains so +equably elevated, is rarely met with within the Cordillera. In this line of +section, the interval between the Puente del Inca and the neighbourhood of +the Cumbre, includes all the chief axes of dislocation. + +The altered clay-slate formation, already described, is seen in several +parts of the valley as far down as Las Vacas, underlying the porphyritic +conglomerate. At the Casa de Pujios [L], there is a hummock of (andesitic?) +granite; and the stratification of the surrounding mountains here changes +from W. by S. to S.W. Again, near the R. Vacas there is a larger formation +of (andesitic?) granite [M], which sends a meshwork of veins into the +superincumbent clay-slate, and which locally throws off the strata, on one +side to N.W. and on the other to S.E. but not at a high angle: at the +junction, the clay-slate is altered into fine-grained greenstone. This +granitic axis is intersected by a green dike, which I mention, because I do +not remember having elsewhere seen dikes in this lowest and latest +intrusive rock. From the R. Vacas to the plain of Uspallata, the valley +runs N.E., so that I have had to contract my section; it runs exclusively +through porphyritic rocks. As far as the Pass of Jaula, the claystone +conglomerate formation, in most parts highly porphyritic, and crossed by +numerous dikes of greenstone porphyry, attains a great thickness: there is +also much intrusive porphyry. From the Jaula to the plain, the +stratification has been in most places obliterated, except near the tops of +some of the mountains; and the metamorphic action has been extremely great. +In this space, the number and bulk of the intrusive masses of differently +coloured porphyries, injected one into another and intersected by dikes, is +truly extraordinary. I saw one mountain of whitish porphyry, from which two +huge dikes, thinning out, branched DOWNWARDS into an adjoining blackish +porphyry. Another hill of white porphyry, which had burst through dark- +coloured strata, was itself injected by a purple, brecciated, and +recemented porphyry, both being crossed by a green dike, and both having +been upheaved and injected by a granitic dome. One brick-red porphyry, +which above the Jaula forms an isolated mass in the midst of the +porphyritic conglomerate formation, and lower down the valley a magnificent +group of peaked mountains, differs remarkably from all the other +porphyries. It consists of a red feldspathic base, including some rather +large crystals of red feldspar, numerous large angular grains of quartz, +and little bits of a soft green mineral answering in most of its characters +to soapstone. The crystals of red feldspar resemble in external appearance +those of orthite, though, from being partially decomposed, I was unable to +measure them; and they certainly are quite unlike the variety, so +abundantly met with in almost all the other rocks of this line of section, +and which, wherever I tried it, cleaved like albite. This brick-red +porphyry appears to have burst through all the other porphyries, and +numerous red dikes traversing the neighbouring mountains have proceeded +from it: in some few places, however, it was intersected by white dikes. +From this posteriority of intrusive origin,--from the close general +resemblance between this red porphyry and the red granite of the Portillo +line, the only difference being that the feldspar here is less perfectly +granular, and that soapstone replaces the mica, which is there imperfect +and passes into chlorite,--and from the Portillo line a little southward of +this point appearing to blend (according to Dr. Gillies) into the western +ranges,--I am strongly urged to believe (as formerly remarked) that the +grand mountain-masses composed of this brick-red porphyry belong to the +same axis of injection with the granite of the Portillo line; if so, the +injection of this porphyry probably took place, as long subsequently to the +several axes of elevation in the gypseous formation near the Cumbre, as the +injection of the Portillo granite has been shown to have been subsequent to +the elevation of the gypseous strata composing the Peuquenes range; and +this interval, we have seen, must have been a very long one. + +The Plain of Uspallata has been briefly described in Chapter 3; it +resembles the basin-plains of Chile; it is ten or fifteen miles wide, and +is said to extend for 180 miles northward; its surface is nearly six +thousand feet above the sea; it is composed, to a thickness of some hundred +feet of loosely aggregated, stratified shingle, which is prolonged with a +gently sloping surface up the valleys in the mountains on both sides. One +section in this plain [Z] is interesting, from the unusual circumstance of +alternating layers of almost loose red and white sand with lines of pebbles +(from the size of a nut to that of an apple), and beds of gravel, being +inclined at an angle of 45 degrees, and in some spots even at a higher +angle. (I find that Mr. Smith of Jordan Hill has described ("Edinburgh New +Philosophical Journal" volume 25 page 392) beds of sand and gravel, near +Edinburgh, tilted at an angle of 60 degrees, and dislocated by miniature +faults.) These beds are dislocated by small faults: and are capped by a +thick mass of horizontally stratified gravel, evidently of subaqueous +origin. Having been accustomed to observe the irregularities of beds +accumulated under currents, I feel sure that the inclination here has not +been thus produced. The pebbles consist chiefly of the brick-red porphyry +just described and of white granite, both probably derived from the ranges +to the west, and of altered clay-slate and of certain porphyries, +apparently belonging to the rocks of the Uspallata chain. This plain +corresponds geographically with the valley of Tenuyan between the Portillo +and Peuquenes ranges; but in that valley the shingle, which likewise has +been derived both from the eastern and western ranges, has been cemented +into a hard conglomerate, and has been throughout tilted at a considerable +inclination; the gravel there apparently attains a much greater thickness, +and is probably of higher antiquity. + +THE USPALLATA RANGE. + +The road by the Villa Vicencio Pass does not strike directly across the +range, but runs for some leagues northward along its western base: and I +must briefly describe the rocks here seen, before continuing with the +coloured east and west section. At the mouth of the valley of Canota, and +at several points northwards, there is an extensive formation of a glossy +and harsh, and of a feldspathic clay-slate, including strata of grauwacke, +and having a tortuous, nearly vertical cleavage, traversed by numerous +metalliferous veins and others of quartz. The clay-slate is in many parts +capped by a thick mass of fragments of the same rock, firmly recemented; +and both together have been injected and broken up by very numerous +hillocks, ranging north and south, of lilac, white, dark and salmon- +coloured porphyries: one steep, now denuded, hillock of porphyry had its +face as distinctly impressed with the angles of a fragmentary mass of the +slate, with some of the points still remaining embedded, as sealing-wax +could be by a seal. At the mouth of this same valley of Canota, in a fine +escarpment having the strata dipping from 50 to 60 degrees to the N.E. +(Nearly opposite to this escarpment, there is another corresponding one, +with the strata dipping not to the exactly opposite point, or S.W., but to +S.S.W.: consequently the two escarpments trend towards each other, and some +miles southward they become actually united: this is a form of elevation +which I have not elsewhere seen.), the clay-slate formation is seen to be +covered by--(1st) a purple, claystone porphyry resting unconformably in +some parts on the solid slate, and in others on a thick fragmentary mass; +(2nd), a conformable stratum of compact blackish rock, having a spheroidal +structure, full of minute acicular crystals of glassy feldspar, with red +spots of oxide of iron; (3rd), a great stratum of purplish-red claystone +porphyry, abounding with crystals of opaque feldspar, and laminated with +thin, parallel, often short, layers, and likewise with great irregular +patches of white, earthy, semi-crystalline feldspar; this rock (which I +noticed in other neighbouring places) perfectly resembles a curious variety +described at Port Desire, and occasionally occurs in the great porphyritic +conglomerate formation of Chile; (4th), a thin stratum of greenish white, +indurated tuff, fusible and containing broken crystals and particles of +porphyries; (5th), a grand mass, imperfectly columnar and divided into +three parallel and closely joined strata, of cream-coloured claystone +porphyry; (6th), a thick stratum of lilac-coloured porphyry, which I could +see was capped by another bed of the cream-coloured variety; I was unable +to examine the still higher parts of the escarpment. These conformably +stratified porphyries, though none are either vesicular are amygdaloidal, +have evidently flowed as submarine lavas: some of them are separated from +each other by seams of indurated tuff, which, however, are quite +insignificant in thickness compared with the porphyries. This whole pile +resembles, but not very closely, some of the less brecciated parts of the +great porphyritic conglomerate formation of Chile; but it does not probably +belong to the same age, as the porphyries here rest unconformably on the +altered feldspathic clay-slate, whereas the porphyritic conglomerate +formation alternates with and rests conformably on it. These porphyries, +moreover, with the exception of the one blackish stratum, and of the one +indurated, white tufaceous bed, differ from the beds composing the +Uspallata range in the line of the Villa Vicencio Pass. + +I will now give, first, a sketch of the structure of the range, as +represented in the section, and will then describe its composition and +interesting history. At its western foot, a hillock [N] is seen to rise out +of the plain, with its strata dipping at 70 degrees to the west, fronted by +strata [O] inclined at 45 degrees to the east, thus forming a little north +and south anticlinal axis. Some other little hillocks of similar +composition, with their strata highly inclined, range N.E. and S.W., +obliquely to the main Uspallata line. The cause of these dislocations, +which, though on a small scale, have been violent and complicated, is seen +to lie in hummocks of lilac, purple and red porphyries, which have been +injected in a liquified state through and into the underlying clay-slate +formation. Several dykes were exposed here, but in no other part, that I +saw of this range. As the strata consist of black, white, greenish and +brown-coloured rocks, and as the intrusive porphyries are so brightly +tinted, a most extraordinary view was presented, like a coloured geological +drawing. On the gently inclined main western slope [PP], above the little +anticlinal ridges just mentioned, the strata dip at an average angle of 25 +degrees to the west; the inclination in some places being only 19 degrees, +in some few others as much as 45 degrees. The masses having these different +inclinations, are separated from each other by parallel vertical faults [as +represented at Pa], often giving rise to separate, parallel, uniclinal +ridges. The summit of the main range is broad and undulatory, with the +stratification undulatory and irregular: in a few places granitic and +porphyritic masses [Q] protrude, which, from the small effect they have +locally produced in deranging the strata, probably form the upper points of +a regular, great underlying dome. These denuded granitic points, I +estimated at about nine thousand feet in height above the sea. On the +eastern slope, the strata in the upper part are regularly inclined at about +25 degrees to the east, so that the summit of this chain, neglecting small +irregularities, forms a broad anticlinal axis. Lower down, however, near +Los Hornillos [R], there is a well-marked synclinal axis, beyond which the +strata are inclined at nearly the same angle, namely from 20 to 30 degrees, +inwards or westward. Owing to the amount of denudation which this chain has +suffered, the outline of the gently inclined eastern flank scarcely offers +the slightest indication of this synclinal axis. The stratified beds, which +we have hitherto followed across the range, a little further down are seen +to lie, I believe unconformably, on a broad mountainous band of clay-slate +and grauwacke. The strata and laminae of this latter formation, on the +extreme eastern flank, are generally nearly vertical; further inwards they +become inclined from 45 to 80 degrees to the west: near Villa Vicencio [S] +there is apparently an anticlinal axis, but the structure of this outer +part of the clay-slate formation is so obscure, that I have not marked the +planes of stratification in the section. On the margin of the Pampas, some +low, much dislocated spurs of this same formation, project in a north- +easterly line, in the same oblique manner as do the ridges on the western +foot, and as is so frequently the case with those at the base of the main +Cordillera. + +I will now describe the nature of the beds, beginning at the base on the +eastern side. First, for the clay-slate formation: the slate is generally +hard and bluish, with the laminae coated by minute micaceous scales; it +alternates many times with a coarse-grained, greenish grauwacke, containing +rounded fragments of quartz and bits of slate in a slightly calcareous +basis. The slate in the upper part generally becomes purplish, and the +cleavage so irregular that the whole consists of mere splinters. Transverse +veins of quartz are numerous. At the Calera, some leagues distant, there is +a dark crystalline limestone, apparently included in this formation. With +the exception of the grauwacke being here more abundant, and the clay-slate +less altered, this formation closely resembles that unconformably +underlying the porphyries at the western foot of this same range; and +likewise that alternating with the porphyritic conglomerate in the main +Cordillera. This formation is a considerable one, and extends several +leagues southward to near Mendoza: the mountains composed of it rise to a +height of about two thousand feet above the edge of the Pampas, or about +seven thousand feet above the sea. (I infer this from the height of V. +Vicencio, which was ascertained by Mr. Miers to be 5,328 feet above the +sea.) + +Secondly: the most usual bed on the clay-slate is a coarse, white, slightly +calcareous conglomerate, of no great thickness, including broken crystals +of feldspar, grains of quartz, and numerous pebbles of brecciated claystone +porphyry, but without any pebbles of the underlying clay-slate. I nowhere +saw the actual junction between this bed and the clay-slate, though I spent +a whole day in endeavouring to discover their relations. In some places I +distinctly saw the white conglomerate and overlying beds inclined at from +25 to 30 degrees to the west, and at the bottom of the same mountain, the +clay-slate and grauwacke inclined to the same point, but at an angle from +70 to 80 degrees: in one instance, the clay-slate dipped not only at a +different angle, but to a different point from the overlying formation. In +these cases the two formations certainly appeared quite unconformable: +moreover, I found in the clay-slate one great, vertical, dike-like fissure, +filled up with an indurated whitish tuff, quite similar to some of the +upper beds presently to be described; and this shows that the clay-slate +must have been consolidated and dislocated before their deposition. On the +other hand, the stratification of the slate and grauwacke, in some cases +gradually and entirely disappeared in approaching the overlying white +conglomerate; in other cases the stratification of the two formations +became strictly conformable; and again in other cases, there was some +tolerably well characterised clay-slate lying above the conglomerate. (The +coarse, mechanical structure of many grauwackes has always appeared to me a +difficulty; for the texture of the associated clay-slate and the nature of +the embedded organic remains where present, indicate that the whole has +been a deep-water deposit. Whence have the sometimes included angular +fragments of clay-slate, and the rounded masses of quartz and other rocks, +been derived? Many deep-water limestones, it is well known, have been +brecciated, and then firmly recemented.) The most probable conclusion +appears to be, that after the clay-slate formation had been dislocated and +tilted, but whilst under the sea, a fresh and more recent deposition of +clay-slate took place, on which the white conglomerate was conformably +deposited, with here and there a thin intercalated bed of clay-slate. On +this view the white conglomerates and the presently to be described tuffs +and lavas are really unconformable to the main part of the clay-slate; and +this, as we have seen, certainly is the case with the clay-stone lavas in +the valley of Canota, at the western and opposite base of the range. + +Thirdly: on the white conglomerate, strata several hundred feet in +thickness are superimposed, varying much in nature in short distances: the +commonest variety is a white, much indurated tuff, sometimes slightly +calcareous, with ferruginous spots and water-lines, often passing into +whitish or purplish compact, fine-grained grit or sandstones; other +varieties become semi-porcellanic, and tinted faint green or blue; others +pass into an indurated shale: most of these varieties are easily fusible. + +Fourthly: a bed, about one hundred feet thick of a compact, partially +columnar, pale-grey, feldspathic lava, stained with iron, including very +numerous crystals of opaque feldspar, and with some crystallised and +disseminated calcareous matter. The tufaceous stratum on which this +feldspathic lava rests is much hardened, stained purple, and has a +spherico-concretionary structure; it here contains a good many pebbles of +claystone porphyry. + +Fifthly: thin beds, 400 feet in thickness, varying much in nature, +consisting of white and ferruginous tuffs, in some parts having a +concretionary structure, in others containing rounded grains and a few +pebbles of quartz; also passing into hard gritstones and into greenish +mudstones: there is, also, much of a bluish-grey and green semi-porcellanic +stone. + +Sixthly: a volcanic stratum, 250 feet in thickness, of so varying a nature +that I do not believe a score of specimens would show all the varieties; +much is highly amygdaloidal, much compact; there are greenish, blackish, +purplish, and grey varieties, rarely including crystals of green augite and +minute acicular ones of feldspar, but often crystals and amygdaloidal +masses of white, red, and black carbonate of lime. Some of the blackish +varieties of this rock have a conchoidal fracture and resemble basalt; +others have an irregular fracture. Some of the grey and purplish varieties +are thickly speckled with green earth and with white crystalline carbonate +of lime; others are largely amygdaloidal with green earth and calcareous +spar. Again, other earthy varieties, of greenish, purplish and grey tints, +contain much iron, and are almost half composed of amygdaloidal balls of +dark brown bole, of a whitish indurated feldspathic matter, of bright green +earth, of agate, and of black and white crystallised carbonate of lime. All +these varieties are easily fusible. Viewed from a distance, the line of +junction with the underlying semi-porcellanic strata was distinct; but when +examined closely, it was impossible to point out within a foot where the +lava ended and where the sedimentary mass began: the rock at the time of +junction was in most places hard, of a bright green colour, and abounded +with irregular amygdaloidal masses of ferruginous and pure calcareous spar, +and of agate. + +Seventhly: strata, eighty feet in thickness, of various indurated tuffs, as +before; many of the varieties have a fine basis including rather coarse +extraneous particles; some of them are compact and semi-porcellanic, and +include vegetable impressions. + +Eighthly: a bed, about fifty feet thick, of greenish-grey, compact, +feldspathic lava, with numerous small crystals of opaque feldspar, black +augite, and oxide of iron. The junction with the bed on which it rested, +was ill defined; balls and masses of the feldspathic rock being enclosed in +much altered tuff. + +Ninthly: indurated tuffs, as before. + +Tenthly: a conformable layer, less than two feet in thickness, of +pitchstone, generally brecciated, and traversed by veins of agate and of +carbonate of lime: parts are composed of apparently concretionary fragments +of a more perfect variety, arranged in horizontal lines in a less perfectly +characterised variety. I have much difficulty in believing that this thin +layer of pitchstone flowed as lava. + +Eleventhly: sedimentary and tufaceous beds as before, passing into +sandstone, including some conglomerate: the pebbles in the latter are of +claystone porphyry, well rounded, and some as large as cricket-balls. + +Twelfthly: a bed of compact, sonorous, feldspathic lava, like that of bed +No. 8, divided by numerous joints into large angular blocks. + +Thirteenthly: sedimentary beds as before. + +Fourteenthly: a thick bed of greenish or greyish black, compact basalt +(fusing into a black enamel), with small crystals, occasionally +distinguishable, of feldspar and augite: the junction with the underlying +sedimentary bed, differently from that in most of the foregoing streams, +here was quite distinct:--the lava and tufaceous matter preserving their +perfect characters within two inches of each other. This rock closely +resembles certain parts of that varied and singular lava-stream No. 6; it +likewise resembles, as we shall immediately see, many of the great upper +beds on the western flank and on the summit of this range. + +The pile of strata here described attains a great thickness; and above the +last-mentioned volcanic stratum, there were several other great tufaceous +beds alternating with submarine lavas, which I had not time to examine; but +a corresponding series, several thousand feet in thickness, is well +exhibited on the crest and western flank of the range. Most of the lava- +streams on the western side are of a jet-black colour and basaltic nature; +they are either compact and fine-grained, including minute crystals of +augite and feldspar, or they are coarse-grained and abound with rather +large coppery-brown crystals of an augitic mineral. (Very easily fusible +into a jet-black bead, attracted by the magnet: the crystals are too much +tarnished to be measured by the goniometer.) Another variety was of a dull- +red colour, having a claystone brecciated basis, including specks of oxide +of iron and of calcareous spar, and amygdaloidal with green earth: there +were apparently several other varieties. These submarine lavas often +exhibit a spheroidal, and sometimes an imperfect columnar structure: their +upper junctions are much more clearly defined than their lower junctions; +but the latter are not so much blended into the underlying sedimentary beds +as is the case in the eastern flank. On the crest and western flank of the +range, the streams, viewed as a whole, are mostly basaltic; whilst those on +the eastern side, which stand lower in the series, are, as we have seen, +mostly feldspathic. + +The sedimentary strata alternating with the lavas on the crest and western +side, are of an almost infinitely varying nature; but a large proportion of +them closely resemble those already described on the eastern flank: there +are white and brown, indurated, easily fusible tuffs,--some passing into +pale blue and green semi-porcellanic rocks,--others into brownish and +purplish sandstones and gritstones, often including grains of quartz,-- +others into mudstone containing broken crystals and particles of rock, and +occasionally single large pebbles. There was one stratum of a bright red, +coarse, volcanic gritstone; another of conglomerate; another of a black, +indurated, carbonaceous shale marked with imperfect vegetable impressions; +this latter bed, which was thin, rested on a submarine lava, and followed +all the considerable inequalities of its upper surface. Mr. Miers states +that coal has been found in this range. Lastly, there was a bed (like No. +10 on the eastern flank) evidently of sedimentary origin, and remarkable +from closely approaching in character to an imperfect pitchstone, and from +including extremely thin layers of perfect pitchstone, as well as nodules +and irregular fragments (but not resembling extraneous fragments) of this +same rock arranged in horizontal lines: I conceive that this bed, which is +only a few feet in thickness, must have assumed its present state through +metamorphic and concretionary action. Most of these sedimentary strata are +much indurated, and no doubt have been partially metamorphosed: many of +them are extraordinarily heavy and compact; others have agate and +crystalline carbonate of lime disseminated throughout them. Some of the +beds exhibit a singular concretionary arrangement, with the curves +determined by the lines of fissure. There are many veins of agate and +calcareous spar, and innumerable ones of iron and other metals, which have +blackened and curiously affected the strata to considerable distances on +both sides. + +Many of these tufaceous beds resemble, with the exception of being more +indurated, the upper beds of the Great Patagonian tertiary formation, +especially those variously coloured layers high up the River Santa Cruz, +and in a remarkable degree the tufaceous formation at the northern end of +Chiloe. I was so much struck with this resemblance, that I particularly +looked out for silicified wood, and found it under the following +extraordinary circumstances. High up on this western flank, at a height +estimated at 7,000 feet above the sea, in a broken escarpment of thin +strata, composed of compact green gritstone passing into a fine mudstone, +and alternating with layers of coarser, brownish, very heavy mudstone, +including broken crystals and particles of rock almost blended together, I +counted the stumps of fifty-two trees. (For the information of any future +traveller, I will describe the spot in detail. Proceeding eastward from the +Agua del Zorro, and afterwards leaving on the north side of the road a +rancho attached to some old goldmines, you pass through a gully with low +but steep rocks on each hand: the road then bends, and the ascent becomes +steeper. A few hundred yards farther on, a stone's throw on the south side +of the road, the white calcareous stumps may be seen. The spot is about +half a mile east of the Agua del Zorro.) They projected between two and +five feet above the ground, and stood at exactly right angles to the +strata, which were here inclined at an angle of about 25 degrees to the +west. Eleven of these trees were silicified and well preserved; Mr. R. +Brown has been so kind as to examine the wood when sliced and polished; he +says it is coniferous, partaking of the characters of the Araucarian tribe, +with some curious points of affinity with the Yew. The bark round the +trunks must have been circularly furrowed with irregular lines, for the +mudstone round them is thus plainly marked. One cast consisted of dark +argillaceous limestone; and forty of them of coarsely crystallised +carbonate of lime, with cavities lined by quartz crystals: these latter +white calcareous columns do not retain any internal structure, but their +external form plainly shows their origin. All the stumps have nearly the +same diameter, varying from one foot to eighteen inches; some of them stand +within a yard of each other; they are grouped in a clump within a space of +about sixty yards across, with a few scattered round at the distance of 150 +yards. They all stand at about the same level. The longest stump stood +seven feet out of the ground: the roots, if they are still preserved, are +buried and concealed. No one layer of the mudstone appeared much darker +than the others, as if it had formerly existed as soil, nor could this be +expected, for the same agents which replaced with silex and lime the wood +of the trees, would naturally have removed all vegetable matter from the +soil. Besides the fifty-two upright trees, there were a few fragments, like +broken branches, horizontally embedded. The surrounding strata are crossed +by veins of carbonate of lime, agate, and oxide of iron; and a poor gold +vein has been worked not far from the trees. + +The green and brown mudstone beds including the trees, are conformably +covered by much indurated, compact, white or ferruginous tuffs, which pass +upwards into a fine-grained, purplish sedimentary rock: these strata, +which, together, are from four to five hundred feet in thickness, rest on a +thick bed of submarine lava, and are conformably covered by another great +mass of fine-grained basalt, which I estimated at 1,000 feet in thickness, +and which probably has been formed by more than one stream. (This rock is +quite black, and fuses into a black bead, attracted strongly by the magnet; +it breaks with a conchoidal fracture; the included crystals of augite are +distinguishable by the naked eye, but are not perfect enough to be +measured: there are many minute acicular crystals of glassy feldspar.) +Above this mass I could clearly distinguish five conformable alternations, +each several hundred feet in thickness, of stratified sedimentary rocks and +lavas, such as have been previously described. Certainly the upright trees +have been buried under several thousand feet in thickness of matter, +accumulated under the sea. As the trees obviously must once have grown on +dry land, what an enormous amount of subsidence is thus indicated! +Nevertheless, had it not been for the trees there was no appearance which +would have led any one even to have conjectured that these strata had +subsided. As the land, moreover, on which the trees grew, is formed of +subaqueous deposits, of nearly if not quite equal thickness with the +superincumbent strata, and as these deposits are regularly stratified and +fine-grained, not like the matter thrown up on a sea-beach, a previous +upward movement, aided no doubt by the great accumulation of lavas and +sediment, is also indicated. (At first I imagined, that the strata with the +trees might have been accumulated in a lake: but this seems highly +improbable; for, first, a very deep lake was necessary to receive the +matter below the trees, then it must have been drained for their growth, +and afterwards re-formed and made profoundly deep, so as to receive a +subsequent accumulation of matter SEVERAL THOUSAND feet in thickness. And +all this must have taken place necessarily before the formation of the +Uspallata range, and therefore on the margin of the wide level expanse of +the Pampas! Hence I conclude, that it is infinitely more probable that the +strata were accumulated under the sea: the vast amount of denudation, +moreover, which this range has suffered, as shown by the wide valleys, by +the exposure of the very trees and by other appearances, could have been +effected, I conceive, only by the long-continued action of the sea; and +this shows that the range was either upheaved from under the sea, or +subsequently let down into it. From the natural manner in which the stumps +(fifty-two in number) are GROUPED IN A CLUMP, and from their all standing +vertically to the strata, it is superfluous to speculate on the chance of +the trees having been drifted from adjoining land, and deposited upright: I +may, however, mention that the late Dr. Malcolmson assured me, that he once +met in the Indian Ocean, fifty miles from land, several cocoa-nut trees +floating upright, owing to their roots being loaded with earth.) + +In nearly the middle of the range, there are some hills [Q], before alluded +to, formed of a kind of granite externally resembling andesite, and +consisting of a white, imperfectly granular, feldspathic basis, including +some perfect crystals apparently of albite (but I was unable to measure +them), much black mica, epidote in veins, and very little or no quartz. +Numerous small veins branch from this rock into the surrounding strata; and +it is a singular fact that these veins, though composed of the same kind of +feldspar and small scales of mica as in the solid rock, abound with +innumerable minute ROUNDED grains of quartz: in the veins or dikes also, +branching from the great granitic axis in the peninsula of Tres Montes, I +observed that quartz was more abundant in them than in the main rock: I +have heard of other analogous cases: can we account for this fact, by the +long-continued vicinity of quartz when cooling, and by its having been thus +more easily sucked into fissures than the other constituent minerals of +granite? (See a paper by M. Elie de Beaumont, "Soc. Philomath." May 1839 +"L'Institut." 1839 page 161.) The strata encasing the flanks of these +granitic or andesite masses, and forming a thick cap on one of their +summits, appear originally to have been of the same tufaceous nature with +the beds already described, but they are now changed into porcellanic, +jaspery, and crystalline rocks, and into others of a white colour with a +harsh texture, and having a siliceous aspect, though really of a +feldspathic nature and fusible. Both the granitic intrusive masses and the +encasing strata are penetrated by innumerable metallic veins, mostly +ferruginous and auriferous, but some containing copper-pyrites and a few +silver: near the veins, the rocks are blackened as if blasted by gunpowder. +The strata are only slightly dislocated close round these hills, and hence, +perhaps, it may be inferred that the granitic masses form only the +projecting points of a broad continuous axis-dome, which has given to the +upper parts of this range its anticlinal structure. + +CONCLUDING REMARKS ON THE USPALLATA RANGE. + +I will not attempt to estimate the total thickness of the pile of strata +forming this range, but it must amount to many thousand feet. The +sedimentary and tufaceous beds have throughout a general similarity, though +with infinite variations. The submarine lavas in the lower part of the +series are mostly feldspathic, whilst in the upper part, on the summit and +western flank, they are mostly basaltic. We are thus reminded of the +relative position in most recent volcanic districts of the trachytic and +basaltic lavas,--the latter from their greater weight having sunk to a +lower level in the earth's crust, and having consequently been erupted at a +later period over the lighter and upper lavas of the trachytic series. (See +on this subject, "Volcanic Islands" etc. by the Author.) Both the basaltic +and feldspathic submarine streams are very compact; none being vesicular, +and only a few amygdaloidal: the effects which some of them, especially +those low in the series, have produced on the tufaceous beds over which +they have flowed is highly curious. Independently of this local metamorphic +action, all the strata undoubtedly display an indurated and altered +character; and all the rocks of this range--the lavas, the alternating +sediments, the intrusive granite and porphyries, and the underlying clay- +slate--are intersected by metalliferous veins. The lava-strata can often be +seen extending for great distances, conformably with the under and +overlying beds; and it was obvious that they thickened towards the west. +Hence the points of eruption must have been situated westward of the +present range, in the direction of the main Cordillera: as, however, the +flanks of the Cordillera are entirely composed of various porphyries, +chiefly claystone and greenstone, some intrusive, and others belonging to +the porphyritic conglomerate formation, but all quite unlike these +submarine lava-streams, we must in all probability look to the plain of +Uspallata for the now deeply buried points of eruption. + +Comparing our section of the Uspallata range with that of the Cumbre, we +see, with the exception of the underlying clay-slate, and perhaps of the +intrusive rocks of the axes, a striking dissimilarity in the strata +composing them. The great porphyritic conglomerate formation has not +extended as far as this range; nor have we here any of the gypseous strata, +the magnesian and other limestones, the red sandstones, the siliceous beds +with pebbles of quartz, and comparatively little of the conglomerates, all +of which form such vast masses over the basal series in the main +Cordillera. On the other hand, in the Cordillera, we do not find those +endless varieties of indurated tuffs, with their numerous veins and +concretionary arrangement, and those grit and mud stones, and singular +semi-porcellanic rocks, so abundant in the Uspallata range. The submarine +lavas, also, differ considerably; the feldspathic streams of the Cordillera +contain much mica, which is absent in those of the Uspallata range: in this +latter range we have seen on how grand a scale, basaltic lava has been +poured forth, of which there is not a trace in the Cordillera. This +dissimilarity is the more striking, considering that these two parallel +chains are separated by a plain only between ten and fifteen miles in +width; and that the Uspallata lavas, as well as no doubt the alternating +tufaceous beds, have proceeded from the west, from points apparently +between the two ranges. To imagine that these two piles of strata were +contemporaneously deposited in two closely adjoining, very deep, submarine +areas, separated from each other by a lofty ridge, where a plain now +extends, would be a gratuitous hypothesis. And had they been +contemporaneously deposited, without any such dividing ridge, surely some +of the gypseous and other sedimentary matter forming such immensely thick +masses in the Cordillera, would have extended this short distance +eastwards; and surely some of the Uspallata tuffs and basalts also +accumulated to so great a thickness, would have extended a little westward. +Hence I conclude, that it is far from probable that these two series are +not contemporaneous; but that the strata of one of the chains were +deposited, and even the chain itself uplifted, before the formation of the +other:--which chain, then, is the oldest? Considering that in the Uspallata +range the lowest strata on the western flank lie unconformably on the clay- +slate, as probably is the case with those on the eastern flank, whereas in +the Cordillera all the overlying strata lie conformably on this formation:- +-considering that in the Uspallata range some of the beds, both low down +and high up in the series, are marked with vegetable impressions, showing +the continued existence of neighbouring land;--considering the close +general resemblance between the deposits of this range and those of +tertiary origin in several parts of the continent;--and lastly, even +considering the lesser height and outlying position of the Uspallata +range,--I conclude that the strata composing it are in all probability of +subsequent origin, and that they were accumulated at a period when a deep +sea studded with submarine volcanoes washed the eastern base of the already +partially elevated Cordillera. + +This conclusion is of much importance, for we have seen that in the +Cordillera, during the deposition of the Neocomian strata, the bed of the +sea must have subsided many thousand feet: we now learn that at a later +period an adjoining area first received a great accumulation of strata, and +was upheaved into land on which coniferous trees grew, and that this area +then subsided several thousand feet to receive the superincumbent submarine +strata, afterwards being broken up, denuded, and elevated in mass to its +present height. I am strengthened in this conclusion of there having been +two distinct, great periods of subsidence, by reflecting on the thick mass +of coarse stratified conglomerate in the valley of Tenuyan, between the +Peuquenes and Portillo lines; for the accumulation of this mass seems to +me, as previously remarked, almost necessarily to have required a prolonged +subsidence; and this subsidence, from the pebbles in the conglomerate +having been to a great extent derived from the gypseous or Neocomian strata +of the Peuquenes line, we know must have been quite distinct from, and +subsequent to, that sinking movement which probably accompanied the +deposition of the Peuquenes strata, and which certainly accompanied the +deposition of the equivalent beds near the Puente del Inca, in this line of +section. + +The Uspallata chain corresponds in geographical position, though on a small +scale, with the Portillo line; and its clay-slate formation is probably the +equivalent of the mica-schist of the Portillo, there metamorphosed by the +old white granites and syenites. The coloured beds under the conglomerate +in the valley of Tenuyan, of which traces are seen on the crest of the +Portillo, and even the conglomerate itself, may perhaps be synchronous with +the tufaceous beds and submarine lavas of the Uspallata range; an open sea +and volcanic action in the latter case, and a confined channel between two +bordering chains of islets in the former case, having been sufficient to +account for the mineralogical dissimilarity of the two series. From this +correspondence between the Uspallata and Portillo ranges, perhaps in age +and certainly in geographical position, one is tempted to consider the one +range as the prolongation of the other; but their axes are formed of +totally different intrusive rocks; and we have traced the apparent +continuation of the red granite of the Portillo in the red porphyries +diverging into the main Cordillera. Whether the axis of the Uspallata range +was injected before, or as perhaps is more probable, after that of the +Portillo line, I will not pretend to decide; but it is well to remember +that the highly inclined lava-streams on the eastern flank of the Portillo +line, prove that its angular upheavement was not a single and sudden event; +and therefore that the anticlinal elevation of the Uspallata range may have +been contemporaneous with some of the later angular movements by which the +gigantic Portillo range gained its present height above the adjoining +plain. + + +CHAPTER VIII. NORTHERN CHILE. CONCLUSION. + +Section from Illapel to Combarbala; gypseous formation with silicified +wood. +Panuncillo. +Coquimbo; mines of Arqueros; section up valley; fossils. +Guasco, fossils of. +Copiapo, section up valley; Las Amolanas, silicified wood. +Conglomerates, nature of former land, fossils, thickness of strata, great +subsidence. +Valley of Despoblado, fossils, tufaceous deposit, complicated dislocations +of. +Relations between ancient orifices of eruption and subsequent axes of +injection. +Iquique, Peru, fossils of, salt-deposits. +Metalliferous veins. +Summary on the porphyritic conglomerate and gypseous formations. +Great subsidence with partial elevations during the cretaceo-oolitic +period. +On the elevation and structure of the Cordillera. +Recapitulation on the tertiary series. +Relation between movements of subsidence and volcanic action. +Pampean formation. +Recent elevatory movements. +Long-continued volcanic action in the Cordillera. +Conclusion. + +VALPARAISO TO COQUIMBO. + +I have already described the general nature of the rocks in the low country +north of Valparaiso, consisting of granites, syenites, greenstones, and +altered feldspathic clay-slate. Near Coquimbo there is much hornblendic +rock and various dusky-coloured porphyries. I will describe only one +section in this district, namely, from near Illapel in a N.E. line to the +mines of Los Hornos, and thence in a north by east direction to Combarbala, +at the foot of the main Cordillera. + +Near Illapel, after passing for some distance over granite, andesite, and +andesitic porphyry, we come to a greenish stratified feldspathic rock, +which I believe is altered clay-slate, conformably capped by porphyries and +porphyritic conglomerate of great thickness, dipping at an average angle of +20 degrees to N.E. by N. The uppermost beds consist of conglomerates and +sandstone only a little metamorphosed, and conformably covered by a +gypseous formation of very great thickness, but much denuded. This gypseous +formation, where first met with, lies in a broad valley or basin, a little +southward of the mines of Los Hornos: the lower half alone contains gypsum, +not in great masses as in the Cordillera, but in innumerable thin layers, +seldom more than an inch or two in thickness. The gypsum is either opaque +or transparent, and is associated with carbonate of lime. The layers +alternate with numerous varying ones of a calcareous clay-shale (with +strong aluminous odour, adhering to the tongue, easily fusible into a pale +green glass), more or less indurated, either earthy and cream-coloured, or +greenish and hard. The more indurated varieties have a compact, +homogeneous, almost crystalline fracture, and contain granules of +crystallised oxide of iron. Some of the varieties almost resemble +honestones. There is also a little black, hardly fusible, siliceo- +calcareous clay-slate, like some of the varieties alternating with gypsum +on the Peuquenes range. + +The upper half of this gypseous formation is mainly formed of the same +calcareous clay-shale rock, but without any gypsum, and varying extremely +in nature: it passes from a soft, coarse, earthy, ferruginous state, +including particles of quartz, into compact claystones with crystallised +oxide of iron,--into porcellanic layers, alternating with seams of +calcareous matter,--and into green porcelain-jasper, excessively hard, but +easily fusible. Strata of this nature alternate with much black and brown +siliceo-calcareous slate, remarkable from the wonderful number of huge +embedded logs of silicified wood. This wood, according to Mr. R. Brown, is +(judging from several specimens) all coniferous. Some of the layers of the +black siliceous slate contained irregular angular fragments of imperfect +pitchstone, which I believe, as in the Uspallata range, has originated in a +metamorphic process. There was one bed of a marly tufaceous nature, and of +little specific gravity. Veins of agate and calcareous spar are numerous. +The whole of this gypseous formation, especially the upper half, has been +injected, metamorphosed, and locally contorted by numerous hillocks of +intrusive porphyries crowded together in an extraordinary manner. These +hillocks consist of purple claystone and of various other porphyries, and +of much white feldspathic greenstone passing into andesite; this latter +variety included in one case crystals of orthitic and albitic feldspar +touching each other, and others of hornblende, chlorite, and epidote. The +strata surrounding these intrusive hillocks at the mines of Los Hornos, are +intersected by many veins of copper-pyrites, associated with much micaceous +iron-ore, and by some of gold: in the neighbourhood of these veins the +rocks are blackened and much altered. The gypsum near the intrusive masses +is always opaque. One of these hillocks of porphyry was capped by some +stratified porphyritic conglomerate, which must have been brought up from +below, through the whole immense thickness of the overlying gypseous +formation. The lower beds of the gypseous formation resemble the +corresponding and probably contemporaneous strata of the main Cordillera; +whilst the upper beds in several respects resemble those of the Uspallata +chain, and possibly may be contemporaneous with them; for I have +endeavoured to show that the Uspallata beds were accumulated subsequently +to the gypseous or Neocomian formations of the Cordillera. + +This pile of strata dips at an angle of about 20 degrees to N.E. by N., +close up to the foot of the Cuesta de Los Hornos, a crooked range of +mountains formed of intrusive rocks of the same nature with the above +described hillocks. Only in one or two places, on this south-eastern side +of the range, I noticed a narrow fringe of the upper gypseous strata +brushed up and inclined south-eastward from it. On its north-eastern flank, +and likewise on a few of the summits, the stratified porphyritic +conglomerate is inclined N.E.: so that, if we disregard the very narrow +anticlinal fringe of gypseous strata at its S.E. foot, this range forms a +second uniclinal axis of elevation. Proceeding in a north-by-east direction +to the village of Combarbala, we come to a third escarpment of the +porphyritic conglomerate, dipping eastwards, and forming the outer range of +the main Cordillera. The lower beds were here more jaspery than usual, and +they included some white cherty strata and red sandstones, alternating with +purple claystone porphyry. Higher up in the Cordillera there appeared to be +a line of andesitic rocks; and beyond them, a fourth escarpment of the +porphyritic conglomerate, again dipping eastwards or inwards. The overlying +gypseous strata, if they ever existed here, have been entirely removed. + +COPPER MINES OF PANUNCILLO. + +From Combarbala to Coquimbo, I traversed the country in a zigzag direction, +crossing and recrossing the porphyritic conglomerate and finding in the +granitic districts an unusual number of mountain-masses composed of various +intrusive, porphyritic rocks, many of them andesitic. One common variety +was greenish-black, with large crystals of blackish albite. At Panuncillo a +short N.N.W. and S.S.E. ridge, with a nucleus formed of greenstone and of a +slate-coloured porphyry including crystals of glassy feldspar, deserves +notice, from the very singular nature of the almost vertical strata +composing it. These consist chiefly of a finer and coarser granular +mixture, not very compact, of white carbonate of lime, of protoxide of iron +and of yellowish garnets (ascertained by Professor Miller), each grain +being an almost perfect crystal. Some of the varieties consist exclusively +of granules of the calcareous spar; and some contain grains of copper ore, +and, I believe, of quartz. These strata alternate with a bluish, compact, +fusible, feldspathic rock. Much of the above granular mixture has, also, a +pseudo-brecciated structure, in which fragments are obscurely arranged in +planes parallel to those of the stratification, and are conspicuous on the +weathered surfaces. The fragments are angular or rounded, small or large, +and consist of bluish or reddish compact feldspathic matter, in which a few +acicular crystals of feldspar can sometimes be seen. The fragments often +blend at their edges into the surrounding granular mass, and seem due to a +kind of concretionary action. + +These singular rocks are traversed by many copper veins, and appear to rest +conformably on the granular mixture (in parts as fine-grained as a +sandstone) of quartz, mica, hornblende, and feldspar; and this on fine- +grained, common gneiss; and this on a laminated mass, composed of pinkish +ORTHITIC feldspar, including a few specks of hornblende; and lastly, this +on granite, which together with andesitic rocks, form the surrounding +district. + +COQUIMBO: MINING DISTRICT OF ARQUEROS. + +At Coquimbo the porphyritic conglomerate formation approaches nearer to the +Pacific than in any other part of Chile visited by me, being separated from +the coast by a tract only a few miles broad of the usual plutonic rocks, +with the addition of a porphyry having a red euritic base. In proceeding to +the mines of Arqueros, the strata of porphyritic conglomerate are at first +nearly horizontal, an unusual circumstance, and afterwards they dip gently +to S.S.E. After having ascended to a considerable height, we come to an +undulatory district in which the famous silver mines are situated; my +examination was chiefly confined to those of S. Rosa. Most of the rocks in +this district are stratified, dipping in various directions, and many of +them are of so singular a nature, that at the risk of being tedious I must +briefly describe them. The commonest variety is a dull-red, compact, finely +brecciated stone, containing much iron and innumerable white crystallised +particles of carbonate of lime, and minute extraneous fragments. Another +variety is almost equally common near S. Rosa; it has a bright green, +scanty basis, including distinct crystals and patches of white carbonate of +lime, and grains of red, semi-micaceous oxide of iron; in parts the basis +becomes dark green, and assumes an obscure crystalline arrangement, and +occasionally in parts it becomes soft and slightly translucent like +soapstone. These red and green rocks are often quite distinct, and often +pass into each other; the passage being sometimes affected by a fine +brecciated structure, particles of the red and green matter being mingled +together. Some of the varieties appear gradually to become porphyritic with +feldspar; and all of them are easily fusible into pale or dark-coloured +beads, strongly attracted by the magnet. I should perhaps have mistaken +several of these stratified rocks for submarine lavas, like some of those +described at the Puente del Inca, had I not examined, a few leagues +eastward of this point, a fine series of analogous but less metamorphosed, +sedimentary beds belonging to the gypseous formation, and probably derived +from a volcanic source. + +This formation is intersected by numerous metalliferous veins, running, +though irregularly, N.W. and S.E., and generally at right angles to the +many dikes. The veins consist of native silver, of muriate of silver, an +amalgam of silver, cobalt, antimony, and arsenic, generally embedded in +sulphate of barytes. (See the Report on M. Domeyko's account of those +mines, in the "Comptes Rendus" tome 14 page 560.) I was assured by Mr. +Lambert, that native copper without a trace of silver has been found in the +same vein with native silver without a trace of copper. At the mines of +Aristeas, the silver veins are said to be unproductive as soon as they pass +into the green strata, whereas at S. Rosa, only two or three miles distant, +the reverse happens; and at the time of my visit, the miners were working +through a red stratum, in the hope of the vein becoming productive in the +underlying green sedimentary mass. I have a specimen of one of these green +rocks, with the usual granules of white calcareous spar and red oxide of +iron, abounding with disseminated particles of glittering native and +muriate of silver, yet taken at the distance of one yard from any vein,--a +circumstance, as I was assured, of very rare occurrence. + +SECTION EASTWARD, UP THE VALLEY OF COQUIMBO. + +After passing for a few miles over the coast granitic series, we come to +the porphyritic conglomerate, with its usual characters, and with some of +the beds distinctly displaying their mechanical origin. The strata, where +first met with, are, as before stated, only slightly inclined; but near the +Hacienda of Pluclaro, we come to an anticlinal axis, with the beds much +dislocated and shifted by a great fault, of which not a trace is externally +seen in the outline of the hill. I believe that this anticlinal axis can be +traced northwards, into the district of Arqueros, where a conspicuous hill +called Cerro Blanco, formed of a harsh, cream-coloured euritic rock, +including a few crystals of reddish feldspar, and associated with some +purplish claystone porphyry, seems to fall on a line of elevation. In +descending from the Arqueros district, I crossed on the northern border of +the valley, strata inclined eastward from the Pluclaro axis: on the +porphyritic conglomerate there rested a mass, some hundred feet thick, of +brown argillaceous limestone, in parts crystalline, and in parts almost +composed of Hippurites Chilensis, d'Orbigny; above this came a black +calcareous shale, and on it a red conglomerate. In the brown limestone, +with the Hippurites, there was an impression of a Pecten and a coral, and +great numbers of a large Gryphaea, very like, and, according to Professor +E. Forbes, probably identical with G. Orientalis, Forbes MS.,--a cretaceous +species (probably upper greensand) from Verdachellum, in Southern India. +These fossils seem to occupy nearly the same position with those at the +Puente del Inca,--namely, at the top of the porphyritic conglomerate, and +at the base of the gypseous formation. + +A little above the Hacienda of Pluclaro, I made a detour on the northern +side of the valley, to examine the superincumbent gypseous strata, which I +estimated at 6,000 feet in thickness. The uppermost beds of the porphyritic +conglomerate, on which the gypseous strata conformably rest, are variously +coloured, with one very singular and beautiful stratum composed of purple +pebbles of various kinds of porphyry, embedded in white calcareous spar, +including cavities lined with bright-green crystallised epidote. The whole +pile of strata belonging to both formations is inclined, apparently from +the above-mentioned axis of Pluclaro, at an angle of between 20 and 30 +degrees to the east. I will here give a section of the principal beds met +with in crossing the entire thickness of the gypseous strata. + +Firstly: above the porphyritic conglomerate formation, there is a fine- +grained, red, crystalline sandstone. + +Secondly: a thick mass of smooth-grained, calcareo-aluminous, shaly rock, +often marked with dendritic manganese, and having, where most compact, the +external appearance of honestone. It is easily fusible. I shall for the +future, for convenience' sake, call this variety pseudo-honestone. Some of +the varieties are quite black when freshly broken, but all weather into a +yellowish-ash coloured, soft, earthy substance, precisely as is the case +with the compact shaly rocks of the Peuquenes range. This stratum is of the +same general nature with many of the beds near Los Hornos in the Illapel +section. In this second bed, or in the underlying red sandstone (for the +surface was partially concealed by detritus), there was a thick mass of +gypsum, having the same mineralogical characters with the great beds +described in our sections across the Cordillera. + +Thirdly: a thick stratum of fine-grained, red, sedimentary matter, easily +fusible into a white glass, like the basis of claystone porphyry; but in +parts jaspery, in parts brecciated, and including crystalline specks of +carbonate of lime. In some of the jaspery layers, and in some of the black +siliceous slaty bands, there were irregular seams of imperfect pitchstone, +undoubtedly of metamorphic origin, and other seams of brown, crystalline +limestone. Here, also, were masses, externally resembling ill-preserved +silicified wood. + +Fourthly and fifthly: calcareous pseudo-honestone; and a thick stratum +concealed by detritus. + +Sixthly: a thinly stratified mass of bright green, compact, smooth-grained, +calcareo-argillaceous stone, easily fusible, and emitting a strong +aluminous odour: the whole has a highly angulo-concretionary structure; and +it resembles, to a certain extent, some of the upper tufaceo-infusorial +deposits of the Patagonian tertiary formation. It is in its nature allied +to our pseudo-honestone, and it includes well characterised layers of that +variety; and other layers of a pale green, harder, and brecciated variety; +and others of red sedimentary matter, like that of bed Three. Some pebbles +of porphyries are embedded in the upper part. + +Seventhly: red sedimentary matter or sandstone like that of bed One, +several hundred feet in thickness, and including jaspery layers, often +having a finely brecciated structure. + +Eighthly: white, much indurated, almost crystalline tuff, several hundred +feet in thickness, including rounded grains of quartz and particles of +green matter like that of bed Six. Parts pass into a very pale green, semi- +porcellanic stone. + +Ninthly: red or brown coarse conglomerate, three or four hundred feet +thick, formed chiefly of pebbles of porphyries, with volcanic particles, in +an arenaceous, non-calcareous, fusible basis: the upper two feet are +arenaceous without any pebbles. + +Tenthly: the last and uppermost stratum here exhibited, is a compact, +slate-coloured porphyry, with numerous elongated crystals of glassy +feldspar, from one hundred and fifty to two hundred feet in thickness; it +lies strictly conformably on the underlying conglomerate, and is +undoubtedly a submarine lava. + +This great pile of strata has been broken up in several places by intrusive +hillocks of purple claystone porphyry, and by dikes of porphyritic +greenstone: it is said that a few poor metalliferous veins have been +discovered here. From the fusible nature and general appearance of the +finer-grained strata, they probably owe their origin (like the allied beds +of the Uspallata range, and of the Upper Patagonian tertiary formations), +to gentle volcanic eruptions, and to the abrasion of volcanic rocks. +Comparing these beds with those in the mining district of Arqueros, we see +at both places rocks easily fusible, of the same peculiar bright green and +red colours, containing calcareous matter, often having a finely brecciated +structure, often passing into each other, and often alternating together: +hence I cannot doubt that the only difference between them, lies in the +Arqueros beds having been more metamorphosed (in conformity with their more +dislocated and injected condition), and consequently in the calcareous +matter, oxide of iron and green colouring matter, having been segregated +under a more crystalline form. + +The strata are inclined, as before stated, from 20 to 30 degrees eastward, +towards an irregular north and south chain of andesitic porphyry and of +porphyritic greenstone, where they are abruptly cut off. In the valley of +Coquimbo, near to the H. of Gualliguaca, similar plutonic rocks are met +with, apparently a southern prolongation of the above chain; and eastward +of it we have an escarpment of the porphyritic conglomerate, with the +strata inclined at a small angle eastward, which makes the third +escarpment, including that nearest the coast. Proceeding up the valley we +come to another north and south line of granite, andesite, and blackish +porphyry, which seem to lie in an irregular trough of the porphyritic +conglomerate. Again, on the south side of the R. Claro, there are some +irregular granitic hills, which have thrown off the strata of porphyritic +conglomerate to the N.W. by W.; but the stratification here has been much +disturbed. I did not proceed any farther up the valley, and this point is +about two-thirds of the distance between the Pacific and the main +Cordillera. + +I will describe only one other section, namely, on the north side of the R. +Claro, which is interesting from containing fossils: the strata are much +dislocated by faults and dikes, and are inclined to the north, towards a +mountain of andesite and porphyry, into which they appear to become almost +blended. As the beds approach this mountain, their inclination increases up +to an angle of 70 degrees, and in the upper part, the rocks become highly +metamorphosed. The lowest bed visible in this section, is a purplish hard +sandstone. Secondly, a bed two or three hundred feet thick, of a white +siliceous sandstone, with a calcareous cement, containing seams of slaty +sandstone, and of hard yellowish-brown (dolomitic?) limestone; numerous, +well-rounded, little pebbles of quartz are included in the sandstone. +Thirdly, a dark coloured limestone with some quartz pebbles, from fifty to +sixty feet in thickness, containing numerous silicified shells, presently +to be enumerated. Fourthly, very compact, calcareous, jaspery sandstone, +passing into (fifthly) a great bed, several hundred feet thick, of +conglomerate, composed of pebbles of white, red, and purple porphyries, of +sandstone and quartz, cemented by calcareous matter. I observed that some +of the finer parts of this conglomerate were much indurated within a foot +of a dike eight feet in width, and were rendered of a paler colour with the +calcareous matter segregated into white crystallised particles; some parts +were stained green from the colouring matter of the dike. Sixthly, a thick +mass, obscurely stratified, of a red sedimentary stone or sandstone, full +of crystalline calcareous matter, imperfect crystals of oxide of iron, and +I believe of feldspar, and therefore closely resembling some of the highly +metamorphosed beds at Arqueros: this bed was capped by, and appeared to +pass in its upper part into, rocks similarly coloured, containing +calcareous matter, and abounding with minute crystals, mostly elongated and +glassy, of reddish albite. Seventhly, a conformable stratum of fine reddish +porphyry with large crystals of (albitic?) feldspar; probably a submarine +lava. Eighthly, another conformable bed of green porphyry, with specks of +green earth and cream-coloured crystals of feldspar. I believe that there +are other superincumbent crystalline strata and submarine lavas, but I had +not time to examine them. + +The upper beds in this section probably correspond with parts of the great +gypseous formation; and the lower beds of red sandstone conglomerate and +fossiliferous limestone no doubt are the equivalents of the Hippurite +stratum, seen in descending from Arqueros to Pluclaro, which there lies +conformably upon the porphyritic conglomerate formation. The fossils found +in the third bed, consist of:-- + +Pecten Dufreynoyi, d'Orbigny, "Voyage, Part Pal." +This species, which occurs here in vast numbers, according to M. D'Orbigny, +resembles certain cretaceous forms. + +Ostrea hemispherica, d'Orbigny, "Voyage" etc. + +Also resembles, according to the same author, cretaceous forms. + +Terebratula aenigma, d'Orbigny, "Voyage" etc. (Pl. 22 Figures 10-12.) + +Is allied, according to M. d'Orbigny, to T. concinna from the Forest +Marble. A series of this species, collected in several localities hereafter +to be referred to, has been laid before Professor Forbes; and he informs me +that many of the specimens are almost undistinguishable from our oolitic T. +tetraedra, and that the varieties amongst them are such as are found in +that variable species. Generally speaking, the American specimens of T. +aenigma may be distinguished from the British T. tetraedra, by the surface +having the ribs sharp and well-defined to the beak, whilst in the British +species they become obsolete and smoothed down; but this difference is not +constant. Professor Forbes adds, that, possibly, internal characters may +exist, which would distinguish the American species from its European +allies. + +Spirifer linguiferoides, E. Forbes. + +Professor Forbes states that this species is very near to S. linguifera of +Phillips (a carboniferous limestone fossil), but probably distinct. M. +d'Orbigny considers it as perhaps indicating the Jurassic period. + +Ammonites, imperfect impression of. + +M. Domeyko has sent to France a collection of fossils, which, I presume, +from the description given, must have come from the neighbourhood of +Arqueros; they consist of:-- + +Pecten Dufreynoyi, d'Orbigny, "Voyage" Part Pal. +Ostrea hemispherica, d'Orbigny, "Voyage" Part Pal. +Turritella Andii, d'Orbigny, "Voyage" Part Pal. (Pleurotomaria Humboldtii +of Von Buch). +Hippurites Chilensis, d'Orbigny, "Voyage" Part Pal. + +The specimens of this Hippurite, as well as those I collected in my descent +from Arqueros, are very imperfect; but in M. d'Orbigny's opinion they +resemble, as does the Turritella Andii, cretaceous (upper greensand) forms. + +Nautilus Domeykus, d'Orbigny, "Voyage" Part Pal. +Terebratula aenigma, d'Orbigny, "Voyage" Part Pal. +Terebratula ignaciana, d'Orbigny, "Voyage" Part Pal. + +This latter species was found by M. Domeyko in the same block of limestone +with the T. aenigma. According to M. d'Orbigny, it comes near to T. +ornithocephala from the Lias. A series of this species collected at Guasco, +has been examined by Professor E. Forbes, and he states that it is +difficult to distinguish between some of the specimens and the T. hastata +from the mountain limestone; and that it is equally difficult to draw a +line between them and some Marlstone Terebratulae. Without a knowledge of +the internal structure, it is impossible at present to decide on their +identity with analogous European forms. + +The remarks given on the several foregoing shells, show that, in M. +d'Orbigny's opinion, the Pecten, Ostrea, Turritella, and Hippurite indicate +the cretaceous period; and the Gryphaea appears to Professor Forbes to be +identical with a species, associated in Southern India with unquestionably +cretaceous forms. On the other hand, the two Terebratulae and the Spirifer +point, in the opinion both of M. d'Orbigny and Professor Forbes, to the +oolitic series. Hence M. d'Orbigny, not having himself examined this +country, has concluded that there are here two distinct formations; but the +Spirifer and T. aenigma were certainly included in the same bed with the +Pecten and Ostrea, whence I extracted them; and the geologist M. Domeyko +sent home the two Terebratulae with the other-named shells, from the same +locality, without specifying that they came from different beds. Again, as +we shall presently see, in a collection of shells given me from Guasco, the +same species, and others presenting analogous differences, are mingled +together, and are in the same condition; and lastly, in three places in the +valley of Copiapo, I found some of these same species similarly grouped. +Hence there cannot be any doubt, highly curious though the fact be, that +these several fossils, namely, the Hippurites, Gryphaea, Ostrea, Pecten, +Turritella, Nautilus, two Terebratulae, and Spirifer all belong to the same +formation, which would appear to form a passage between the oolitic and +cretaceous systems of Europe. Although aware how unusual the term must +sound, I shall, for convenience' sake, call this formation cretaceo- +oolitic. Comparing the sections in this valley of Coquimbo with those in +the Cordillera described in the last chapter, and bearing in mind the +character of the beds in the intermediate district of Los Hornos, there is +certainly a close general mineralogical resemblance between them, both in +the underlying porphyritic conglomerate, and in the overlying gypseous +formation. Considering this resemblance, and that the fossils from the +Puente del Inca at the base of the gypseous formation, and throughout the +greater part of its entire thickness on the Peuquenes range, indicate the +Neocomian period,--that is, the dawn of the cretaceous system, or, as some +have believed, a passage between this latter and the oolitic series--I +conclude that probably the gypseous and associated beds in all the sections +hitherto described, belong to the same great formation, which I have +denominated--cretaceo-oolitic. I may add, before leaving Coquimbo, that M. +Gay found in the neighbouring Cordillera, at the height of 14,000 feet +above the sea, a fossiliferous formation, including a Trigonia and +Pholadomya (D'Orbigny "Voyage" Part Geolog. page 242.);--both of which +genera occur at the Puente del Inca. + +COQUIMBO TO GUASCO. + +The rocks near the coast, and some way inland, do not differ from those +described northwards of Valparaiso: we have much greenstone, syenite, +feldspathic and jaspery slate, and grauwackes having a basis like that of +claystone; there are some large tracts of granite, in which the constituent +minerals are sometimes arranged in folia, thus composing an imperfect +gneiss. There are two large districts of mica-schists, passing into glossy +clay-slate, and resembling the great formation in the Chonos Archipelago. +In the valley of Guasco, an escarpment of porphyritic conglomerate is first +seen high up the valley, about two leagues eastward of the town of +Ballenar. I heard of a great gypseous formation in the Cordillera; and a +collection of shells made there was given me. These shells are all in the +same condition, and appear to have come from the same bed: they consist +of:-- + +Turritella Andii, d'Orbigny, "Voyage" Part Pal. +Pecten Dufreynoyi, d'Orbigny, "Voyage" Part Pal. +Terebatula ignaciana, d'Orbigny, "Voyage" Part Pal. + +The relations of these species have been given under the head of Coquimbo. + +Terebratula aenigma, d'Orbigny, "Voyage" Part Pal. + +This shell M. d'Orbigny does not consider identical with his T. aenigma, +but near to T. obsoleta. Professor Forbes thinks that it is certainly a +variety of T. aenigma: we shall meet with this variety again at Copiapo. + +Spirifer Chilensis, E. Forbes. + +Professor Forbes remarks that this fossil resembles several carboniferous +limestone Spirifers; and that it is also related to some liassic species, +as S. Wolcotii. + +If these shells had been examined independently of the other collections, +they would probably have been considered, from the characters of the two +Terebratulae, and from the Spirifer, as oolitic; but considering that the +first species, and according to Professor Forbes, the four first, are +identical with those from Coquimbo, the two formations no doubt are the +same, and may, as I have said, be provisionally called cretaceo-oolitic. + +VALLEY OF COPIAPO. + +The journey from Guasco to Copiapo, owing to the utterly desert nature of +the country, was necessarily so hurried, that I do not consider my notes +worth giving. In the valley of Copiapo some of the sections are very +interesting. From the sea to the town of Copiapo, a distance estimated at +thirty miles, the mountains are composed of greenstone, granite, andesite, +and blackish porphyry, together with some dusky-green feldspathic rocks, +which I believe to be altered clay-slate: these mountains are crossed by +many brown-coloured dikes, running north and south. Above the town, the +main valley runs in a south-east and even more southerly course towards the +Cordillera, where it is divided into three great ravines, by the northern +one of which, called Jolquera, I penetrated for a short distance. The +section, Section 1/3 in Plate 1, gives an eye-sketch of the structure and +composition of the mountains on both sides of this valley: a straight east +and west line from the town to the Cordillera is perhaps not more than +thirty miles, but along the valley the distance is much greater. Wherever +the valley trended very southerly, I have endeavoured to contract the +section into its true proportion. This valley, I may add, rises much more +gently than any other valley which I saw in Chile. + +To commence with our section, for a short distance above the town we have +hills of the granitic series, together with some of that rock [A], which I +suspect to be altered clay-slate, but which Professor G. Rose, judging from +specimens collected by Meyen at P. Negro, states is serpentine passing into +greenstone. We then come suddenly to the great gypseous formation [B], +without having passed over, differently from, in all the sections hitherto +described, any of the porphyritic conglomerate. The strata are at first +either horizontal or gently inclined westward; then highly inclined in +various directions, and contorted by underlying masses of intrusive rocks; +and lastly, they have a regular eastward dip, and form a tolerably well +pronounced north and south line of hills. This formation consists of thin +strata, with innumerable alternations, of black, calcareous slate-rock, of +calcareo-aluminous stones like those at Coquimbo, which I have called +pseudo-honestones of green jaspery layers, and of pale-purplish, +calcareous, soft rotten-stone, including seams and veins of gypsum. These +strata are conformably overlaid by a great thickness of thinly stratified, +compact limestone with included crystals of carbonate of lime. At a place +called Tierra Amarilla, at the foot of a mountain thus composed there is a +broad vein, or perhaps stratum, of a beautiful and curious crystallised +mixture, composed, according to Professor G. Rose, of sulphate of iron +under two forms, and of the sulphates of copper and alumina (Meyen's +"Reise" etc. Th. 1, s. 394.): the section is so obscure that I could not +make out whether this vein or stratum occurred in the gypseous formation, +or more probably in some underlying masses [A], which I believe are altered +clay-slate. + +SECOND AXIS OF ELEVATION. + +After the gypseous masses [B], we come to a line of hills of unstratified +porphyry [C], which on their eastern side blend into strata of great +thickness of porphyritic conglomerate, dipping eastward. This latter +formation, however, here has not been nearly so much metamorphosed as in +most parts of Central Chile; it is composed of beds of true purple +claystone porphyry, repeatedly alternating with thick beds of purplish-red +conglomerate with the well-rounded, large pebbles of various porphyries, +not blended together. + +THIRD AXIS OF ELEVATION. + +Near the ravine of Los Hornitos, there is a well-marked line of elevation, +extending for many miles in a N.N.E. and S.S.W. direction, with the strata +dipping in most parts (as in the second axis) only in one direction, +namely, eastward at an average angle of between 30 and 40 degrees. Close to +the mouth of the valley, however, there is, as represented in the section, +a steep and high mountain [D], composed of various green and brown +intrusive porphyries enveloped with strata, apparently belonging to the +upper parts of the porphyritic conglomerate, and dipping both eastward and +westward. I will describe the section seen on the eastern side of this +mountain [D], beginning at the base with the lowest bed visible in the +porphyritic conglomerate, and proceeding upwards through the gypseous +formation. Bed 1 consists of reddish and brownish porphyry varying in +character, and in many parts highly amygdaloidal with carbonate of lime, +and with bright green and brown bole. Its upper surface is throughout +clearly defined, but the lower surface is in most parts indistinct, and +towards the summit of the mountain [D] quite blended into the intrusive +porphyries. Bed 2, a pale lilac, hard but not heavy stone, slightly +laminated, including small extraneous fragments, and imperfect as well as +some perfect and glassy crystals of feldspar; from one hundred and fifty to +two hundred feet in thickness. When examining it in situ, I thought it was +certainly a true porphyry, but my specimens now lead me to suspect that it +possibly may be a metamorphosed tuff. From its colour it could be traced +for a long distance, overlying in one part, quite conformably to the +porphyry of bed 1, and in another not distant part, a very thick mass of +conglomerate, composed of pebbles of a porphyry chiefly like that of bed 1: +this fact shows how the nature of the bottom formerly varied in short +horizontal distances. Bed 3, white, much indurated tuff, containing minute +pebbles, broken crystals, and scales of mica, varies much in thickness. +This bed is remarkable from containing many globular and pear-shaped, +externally rusty balls, from the size of an apple to a man's head, of very +tough, slate-coloured porphyry, with imperfect crystals of feldspar: in +shape these balls do not resemble pebbles, AND I BELIEVE THAT THEY ARE +SUBAQUEOUS VOLCANIC BOMBS; they differ from SUBAERIAL bombs only in not +being vesicular. Bed 4; a dull purplish-red, hard conglomerate, with +crystallised particles and veins of carbonate of lime, from three hundred +to four hundred feet in thickness. The pebbles are of claystone porphyries +of many varieties; they are tolerably well rounded, and vary in size from a +large apple to a man's head. This bed includes three layers of coarse, +black, calcareous, somewhat slaty rock: the upper part passes into a +compact red sandstone. + +In a formation so highly variable in mineralogical nature, any division not +founded on fossil remains, must be extremely arbitrary: nevertheless, the +beds below the last conglomerate may, in accordance with all the sections +hitherto described, be considered as belonging to the porphyritic +conglomerate, and those above it to the gypseous formation, marked [E] in +the section. The part of the valley in which the following beds are seen is +near Potrero Seco. Bed 5, compact, fine-grained, pale greenish-grey, non- +calcareous, indurated mudstone, easily fusible into a pale green and white +glass. Bed 6, purplish, coarse-grained, hard sandstone, with broken +crystals of feldspar and crystallised particles of carbonate of lime; it +possesses a slightly nodular structure. Bed 7, blackish-grey, much +indurated, calcareous mudstone, with extraneous particles of unequal size; +the whole being in parts finely brecciated. In this mass there is a +stratum, twenty feet in thickness, of impure gypsum. Bed 8, a greenish +mudstone, with several layers of gypsum. Bed 9, a highly indurated, easily +fusible, white tuff, thickly mottled with ferruginous matter, and including +some white semi-porcellanic layers, which are interlaced with ferruginous +veins. This stone closely resembles some of the commonest varieties in the +Uspallata chain. Bed 10, a thick bed of rather bright green, indurated +mudstone or tuff, with a concretionary nodular structure so strongly +developed that the whole mass consists of balls. I will not attempt to +estimate the thickness of the strata in the gypseous formation hitherto +described, but it must certainly be very many hundred feet. Bed 11 is at +least 800 feet in thickness: it consists of thin layers of whitish, +greenish, or more commonly brown, fine-grained, indurated tuffs, which +crumble into angular fragments: some of the layers are semi-porcellanic, +many of them highly ferruginous, and some are almost composed of carbonate +of lime and iron with drusy cavities lined with quartzf-crystals. Bed 12, +dull purplish or greenish or dark-grey, very compact and much indurated +mudstone: estimated at 1,500 feet in thickness: in some parts this rock +assumes the character of an imperfect coarse clay-slate; but viewed under a +lens, the basis always has a mottled appearance, with the edges of the +minute component particles blending together. Parts are calcareous, and +there are numerous veins of highly crystalline carbonate of lime charged +with iron. The mass has a nodular structure, and is divided by only a few +planes of stratification: there are, however, two layers, each about +eighteen inches thick, of a dark brown, finer-grained stone, having a +conchoidal, semi-porcellanic fracture, which can be followed with the eye +for some miles across the country. + +I believe this last great bed is covered by other nearly similar +alternations; but the section is here obscured by a tilt from the next +porphyritic chain, presently to be described. I have given this section in +detail, as being illustrative of the general character of the mountains in +this neighbourhood; but it must not be supposed that any one stratum long +preserves the same character. At a distance of between only two and three +miles the green mudstones and white indurated tuffs are to a great extent +replaced by red sandstone and black calcareous shaly rocks, alternating +together. The white indurated tuff, bed 11, here contains little or no +gypsum, whereas on the northern and opposite side of the valley, it is of +much greater thickness and abounds with layers of gypsum, some of them +alternating with thin seams of crystalline carbonate of lime. The +uppermost, dark-coloured, hard mudstone, bed 12, is in this neighbourhood +the most constant stratum. The whole series differs to a considerable +extent, especially in its upper part, from that met with at [BB], in the +lower part of the valley; nevertheless, I do not doubt that they are +equivalents. + +FOURTH AXIS OF ELEVATION (VALLEY OF COPIAPO). + +This axis is formed of a chain of mountains [F], of which the central +masses (near La Punta) consist of andesite containing green hornblende and +coppery mica, and the outer masses of greenish and black porphyries, +together with some fine lilac-coloured claystone porphyry; all these +porphyries being injected and broken up by small hummocks of andesite. The +central great mass of this latter rock, is covered on the eastern side by a +black, fine-grained, highly micaceous slate, which, together with the +succeeding mountains of porphyry, are traversed by numerous white dikes, +branching from the andesite, and some of them extending in straight lines, +to a distance of at least two miles. The mountains of porphyry eastward of +the micaceous schist soon, but gradually, assume (as observed in so many +other cases) a stratified structure, and can then be recognised as a part +of the porphyritic conglomerate formation. These strata [G] are inclined at +a high angle to the S.E., and form a mass from fifteen hundred to two +thousand feet in thickness. The gypseous masses to the west already +described, dip directly towards this axis, with the strata only in a few +places (one of which is represented in the section) thrown from it: hence +this fourth axis is mainly uniclinal towards the S.E., and just like our +third axis, only locally anticlinal. + +The above strata of porphyritic conglomerate [G] with their south-eastward +dip, come abruptly up against beds of the gypseous formation [H], which are +gently, but irregularly, inclined westward: so that there is here a +synclinal axis and great fault. Further up the valley, here running nearly +north and south, the gypseous formation is prolonged for some distance; but +the stratification is unintelligible, the whole being broken up by faults, +dikes, and metalliferous veins. The strata consist chiefly of red +calcareous sandstones, with numerous veins in the place of layers, of +gypsum; the sandstone is associated with some black calcareous slate-rock, +and with green pseudo-honestones, passing into porcelain-jasper. Still +further up the valley, near Las Amolanas [I], the gypseous strata become +more regular, dipping at an angle of between 30 and 40 degrees to W.S.W., +and conformably overlying, near the mouth of the ravine of Jolquera, strata +[K] of porphyritic conglomerate. The whole series has been tilted by a +partially concealed axis [L], of granite, andesite, and a granitic mixture +of white feldspar, quartz, and oxide of iron. + +FIFTH AXIS OF ELEVATION (VALLEY OF COPIAPO, NEAR LOS AMOLANAS). + +I will describe in some detail the beds [I] seen here, which, as just +stated, dip to W.S.W., at an angle of from 30 to 40 degrees. I had not time +to examine the underlying porphyritic conglomerate, of which the lowest +beds, as seen at the mouth of the Jolquera, are highly compact, with +crystals of red oxide of iron; and I am not prepared to say whether they +are chiefly of volcanic or metamorphic origin. On these beds there rests a +coarse purplish conglomerate, very little metamorphosed, composed of +pebbles of porphyry, but remarkable from containing one pebble of granite;- +-of which fact no instance has occurred in the sections hitherto described. +Above this conglomerate, there is a black siliceous claystone, and above it +numerous alternations of dark-purplish and green porphyries, which may be +considered as the uppermost limit of the porphyritic conglomerate +formation. + +Above these porphyries comes a coarse, arenaceous conglomerate, the lower +half white and the upper half of a pink colour, composed chiefly of pebbles +of various porphyries, but with some of red sandstone and jaspery rocks. In +some of the more arenaceous parts of the conglomerate, there was an oblique +or current lamination; a circumstance which I did not elsewhere observe. +Above this conglomerate, there is a vast thickness of thinly stratified, +pale-yellowish, siliceous sandstone, passing into a granular quartz-rock, +used for grindstones (hence the name of the place Las Amolanas), and +certainly belonging to the gypseous formation, as does probably the +immediately underlying conglomerate. In this yellowish sandstone there are +layers of white and pale-red siliceous conglomerate; other layers with +small, well-rounded pebbles of white quartz, like the bed at the R. Claro +at Coquimbo; others of a greenish, fine-grained, less siliceous stone, +somewhat resembling the pseudo-honestones lower down the valley; and +lastly, others of a black calcareous shale-rock. In one of the layers of +conglomerate, there was embedded a fragment of mica-slate, of which this is +the first instance; hence perhaps, it is from a formation of mica-slate, +that the numerous small pebbles of quartz, both here and at Coquimbo, have +been derived. Not only does the siliceous sandstone include layers of the +black, thinly stratified, not fissile, calcareous shale-rock, but in one +place the whole mass, especially the upper part, was, in a marvellously +short horizontal distance, after frequent alternations, replaced by it. +When this occurred, a mountain-mass, several thousand feet in thickness was +thus composed; the black calcareous shale-rock, however, always included +some layers of the pale-yellowish siliceous sandstone, of the red +conglomerate, and of the greenish jaspery and pseudo-honestone varieties. +It likewise included three or four widely separated layers of a brown +limestone, abounding with shells immediately to be described. This pile of +strata was in parts traversed by many veins of gypsum. The calcareous +shale-rock, though when freshly broken quite black, weathers into an ash- +colour: in which respect and in general appearance, it perfectly resembles +those great fossiliferous beds of the Peuquenes range, alternating with +gypsum and red sandstone, described in the last chapter. + +The shells out of the layers of brown limestone, included in the black +calcareous shale-rock, which latter, as just stated, replaces the white +siliceous sandstone, consist of:-- + +Pecten Dufreynoyi, d'Orbigny, "Voyage" Part Pal. +Turritella Andii, d'Orbigny, "Voyage" Part Pal. + +Astarte Darwinii, E. Forbes. +Gryphaea Darwinii, E. Forbes. + +An intermediate form between G. gigantea and G. incurva. + +Gryphaea nov. spec.?, E. Forbes. +Perna Americana, E. Forbes. +Avicula, nov. spec. + +Considered by Mr. G.B. Sowerby as the A. echinata, by M. d'Orbigny as +certainly a new and distinct species, having a Jurassic aspect. The +specimen has been unfortunately lost. + +Terebratula aenigma, d'Orbigny, (var. of do. E. Forbes.) + +This is the same variety, with that from Guasco, considered by M. D'Orbigny +to be a distinct species from his T. aenigma, and related to T. obsoleta. + +Plagiostoma and Ammonites, fragments of. + +The lower layers of the limestone contained thousands of the Gryphaea; and +the upper ones as many of the Turritella, with the Gryphaea (nov. species) +and Serpulae adhering to them; in all the layers, the Terebratula and +fragments of the Pecten were included. It was evident, from the manner in +which species were grouped together, that they had lived where now +embedded. Before making any further remarks, I may state, that higher up +this same valley we shall again meet with a similar association of shells; +and in the great Despoblado Valley, which branches off near the town from +that of Copiapo, the Pecten Dufreynoyi, some Gryphites (I believe G. +Darwinii), and the TRUE Terebratula aenigma of d'Orbigny were found +together in an equivalent formation, as will be hereafter seen. A specimen +also, I may add, of the true T. aenigma, was given me from the +neighbourhood of the famous silver mines of Chanuncillo, a little south of +the valley of the Copiapo, and these mines, from their position, I have no +doubt, lie within the great gypseous formation: the rocks close to one of +the silver veins, judging from fragments shown me, resemble those singular +metamorphosed deposits from the mining district of Arqueros near Coquimbo. + +I will reiterate the evidence on the association of these several shells in +the several localities. + +COQUIMBO. + +In the same bed, Rio Claro: +Pecten Dufreynoyi. +Ostrea hemispherica. +Terebratula aenigma. +Spirifer linguiferoides. + +Same bed, near Arqueros: +Hippurites Chilensis. +Gryphaea orientalis. + +Collected by M. Domeyko from the same locality, apparently near Arqueros: +Terebratula aenigma and Terebratula ignaciana, in same block of limestone: +Pecten Dufreynoyi. +Ostrea hemispherica. +Hippurites Chilensis. +Turritella Andii. +Nautilus Domeykus. + +GUASCO. + +In a collection from the Cordillera, given me: the specimens all in the +same condition: +Pecten Dufreynoyi. +Turritella Andii. +Terebratula ignaciana. +Terebratula aenigma, var. +Spirifer Chilensis. + +COPIAPO. + +Mingled together in alternating beds in the main valley of Copiapo near Las +Amolanas, and likewise higher up the valley: +Pecten Dufreynoyi. +Turritella Andii. +Terebratula aenigma, var. as at Guasco. +Astarte Darwinii. +Gryphaea Darwinii. +Gryphaea nov. species? +Perna Americana. +Avicula, nov. species. + +Main valley of Copiapo, apparently same formation with that of Amolanas: +Terebratula aenigma (true). + +In the same bed, high up the great lateral valley of the Despoblado, in the +ravine of Maricongo: +Terebratula aenigma (true). +Pecten Dufreynoyi. +Gryphaea Darwinii? + +Considering this table, I think it is impossible to doubt that all these +fossils belong to the same formation. If, however, the species from Las +Amolanas, in the Valley of Copiapo, had, as in the case of those from +Guasco, been separately examined, they would probably have been ranked as +oolitic; for, although no Spirifers were found here, all the other species, +with the exception of the Pecten, Turritella, and Astarte, have a more +ancient aspect than cretaceous forms. On the other hand, taking into +account the evidence derived from the cretaceous character of these three +shells, and of the Hippurites, Gryphaea orientalis, and Ostrea, from +Coquimbo, we are driven back to the provisional name already used of +cretaceo-oolitic. From geological evidence, I believe this formation to be +the equivalent of the Neocomian beds of the Cordillera of Central Chile. + +To return to our section near Las Amolanas:--Above the yellow siliceous +sandstone, or the equivalent calcareous slate-rock, with its bands of +fossil-shells, according as the one or other prevails, there is a pile of +strata, which cannot be less than from two to three thousand feet in +thickness, in main part composed of a coarse, bright red conglomerate, with +many intercalated beds of red sandstone, and some of green and other +coloured porcelain-jaspery layers. The included pebbles are well-rounded, +varying from the size of an egg to that of a cricket-ball, with a few +larger; and they consist chiefly of porphyries. The basis of the +conglomerate, as well as some of the alternating thin beds, are formed of a +red, rather harsh, easily fusible sandstone, with crystalline calcareous +particles. This whole great pile is remarkable from the thousands of huge, +embedded, silicified trunks of trees, one of which was eight feet long, and +another eighteen feet in circumference: how marvellous it is, that every +vessel in so thick a mass of wood should have been converted into silex! I +brought home many specimens, and all of them, according to Mr. R. Brown, +present a coniferous structure. + +Above this great conglomerate, we have from two to three hundred feet in +thickness of red sandstone; and above this, a stratum of black calcareous +slate-rock, like that which alternates with and replaces the underlying +yellowish-white, siliceous sandstone. Close to the junction between this +upper black slate-rock and the upper red sandstone, I found the Gryphaea +Darwinii, the Turritella Andii, and vast numbers of a bivalve, too +imperfect to be recognised. Hence we see that, as far as the evidence of +these two shells serves--and the Turritella is an eminently characteristic +species--the whole thickness of this vast pile of strata belongs to the +same age. Again, above the last-mentioned upper red sandstone, there were +several alternations of the black, calcareous slate-rock; but I was unable +to ascend to them. All these uppermost strata, like the lower ones, vary +extremely in character in short horizontal distances. The gypseous +formation, as here seen, has a coarser, more mechanical texture, and +contains much more siliceous matter than the corresponding beds lower down +the valley. Its total thickness, together with the upper beds of the +porphyritic conglomerate, I estimated at least at 8,000 feet; and only a +small portion of the porphyritic conglomerate, which on the eastern flank +of the fourth axis of elevation appeared to be from fifteen hundred to two +thousand feet thick, is here included. As corroborative of the great +thickness of the gypseous formation, I may mention that in the Despoblado +Valley (which branches from the main valley a little above the town of +Copiapo) I found a corresponding pile of red and white sandstones, and of +dark, calcareous, semi-jaspery mudstones, rising from a nearly level +surface and thrown into an absolutely vertical position; so that, by +pacing, I ascertained their thickness to be nearly two thousand seven +hundred feet; taking this as a standard of comparison, I estimated the +thickness of the strata ABOVE the porphyritic conglomerate at 7,000 feet. + +The fossils before enumerated, from the limestone-layers in the whitish +siliceous sandstone, are now covered, on the least computation, by strata +from 5,000 to 6,000 feet in thickness. Professor E. Forbes thinks that +these shells probably lived at a depth of from about 30 to 40 fathoms, that +is from 180 to 240 feet; anyhow, it is impossible that they could have +lived at the depth of from 5,000 to 6,000 feet. Hence in this case, as in +that of the Puente del Inca, we may safely conclude that the bottom of the +sea on which the shells lived, subsided, so as to receive the +superincumbent submarine strata: and this subsidence must have taken place +during the existence of these shells; for, as I have shown, some of them +occur high up as well as low down in the series. That the bottom of the sea +subsided, is in harmony with the presence of the layers of coarse, well- +rounded pebbles included throughout this whole pile of strata, as well as +of the great upper mass of conglomerate from 2,000 to 3,000 feet thick; for +coarse gravel could hardly have been formed or spread out at the profound +depths indicated by the thickness of the strata. The subsidence, also, must +have been slow to have allowed of this often-recurrent spreading out of the +pebbles. Moreover, we shall presently see that the surfaces of some of the +streams of porphyritic lava beneath the gypseous formation, are so highly +amygdaloidal that it is scarcely possible to believe that they flowed under +the vast pressure of a deep ocean. The conclusion of a great subsidence +during the existence of these cretaceo-oolitic fossils, may, I believe, be +extended to the district of Coquimbo, although owing to the fossiliferous +beds there not being directly covered by the upper gypseous strata, which +in the section north of the valley are about 6,000 feet in thickness, I did +not there insist on this conclusion. + +The pebbles in the above conglomerates, both in the upper and lower beds, +are all well rounded, and, though chiefly composed of various porphyries, +there are some of red sandstone and of a jaspery stone, both like the rocks +intercalated in layers in this same gypseous formation; there was one +pebble of mica-slate and some of quartz, together with many particles of +quartz. In these respects there is a wide difference between the gypseous +conglomerates and those of the porphyritic-conglomerate formation, in which +latter, angular and rounded fragments, almost exclusively composed of +porphyries, are mingled together, and which, as already often remarked, +probably were ejected from craters deep under the sea. From these facts I +conclude, that during the formation of the conglomerates, land existed in +the neighbourhood, on the shores of which the innumerable pebbles were +rounded and thence dispersed, and on which the coniferous forests +flourished--for it is improbable that so many thousand logs of wood should +have drifted from any great distance. This land, probably islands, must +have been mainly formed of porphyries, with some mica-slate, whence the +quartz was derived, and with some red sandstone and jaspery rocks. This +latter fact is important, as it shows that in this district, even +previously to the deposition of the lower gypseous or cretaceo-oolitic +beds, strata of an analogous nature had elsewhere, no doubt in the more +central ranges of the Cordillera, been elevated; thus recalling to our +minds the relations of the Cumbre and Uspallata chains. Having already +referred to the great lateral valley of the Despoblado, I may mention that +above the 2,700 feet of red and white sandstone and dark mudstone, there is +a vast mass of coarse, hard, red conglomerate, some thousand feet in +thickness, which contains much silicified wood, and evidently corresponds +with the great upper conglomerate at Las Amolanas: here, however, the +conglomerate consists almost exclusively of pebbles of granite, and of +disintegrated crystals of reddish feldspar and quartz firmly recemented +together. In this case, we may conclude that the land whence the pebbles +were derived, and on which the now silicified trees once flourished, was +formed of granite. + +The mountains near Las Amolanas, composed of the cretaceo-oolitic strata, +are interlaced with dikes like a spider's web, to an extent which I have +never seen equalled, except in the denuded interior of a volcanic crater: +north and south lines, however, predominate. These dikes are composed of +green, white, and blackish rocks, all porphyritic with feldspar, and often +with large crystals of hornblende. The white varieties approach closely in +character to andesite, which composes as we have seen, the injected axes of +so many of the lines of elevation. Some of the green varieties are finely +laminated, parallel to the walls of the dikes. + +SIXTH AXIS OF ELEVATION (VALLEY OF COPIAPO). + +This axis consists of a broad mountainous mass [O] of andesite, composed of +albite, brown mica, and chlorite, passing into andesitic granite, with +quartz: on its western side it has thrown off, at a considerable angle, a +thick mass of stratified porphyries, including much epidote [NN], and +remarkable only from being divided into very thin beds, as highly +amygdaloidal on their surfaces as subaerial lava-streams are often +vesicular. This porphyritic formation is conformably covered, as seen some +way up the ravine of Jolquera, by a mere remnant of the lower part of the +cretaceo-oolitic formation [MM], which in one part encases, as represented +in the coloured section, the foot of the andesitic axis [L], of the already +described fifth line, and in another part entirely conceals it: in this +latter case, the gypseous or cretaceo-oolitic strata falsely appeared to +dip under the porphyritic conglomerate of the fifth axis. The lowest bed of +the gypseous formation, as seen here [M], is of yellowish siliceous +sandstone, precisely like that of Amolanas, interlaced in parts with veins +of gypsum, and including layers of the black, calcareous, non-fissile +slate-rock: the Turritella Andii, Pecten Dufreynoyi, Terebratula aenigma, +var., and some Gryphites were embedded in these layers. The sandstone +varies in thickness from only twenty to eighty feet; and this variation is +caused by the inequalities in the upper surface of an underlying stream of +purple claystone porphyry. Hence the above fossils here lie at the very +base of the gypseous or cretaceo-oolitic formation, and hence they were +probably once covered up by strata about seven thousand feet in thickness: +it is, however, possible, though from the nature of all the other sections +in this district not probable, that the porphyritic claystone lava may in +this case have invaded a higher level in the series. Above the sandstone +there is a considerable mass of much indurated, purplish-black, calcareous +claystone, allied in nature to the often-mentioned black calcareous slate- +rock. +Eastward of the broad andesitic axis of this sixth line, and penetrated by +many dikes from it, there is a great formation [P] of mica-schist, with its +usual variations, and passing in one part into a ferruginous quartz-rock. +The folia are curved and highly inclined, generally dipping eastward. It is +probable that this mica-schist is an old formation, connected with the +granitic rocks and metamorphic schists near the coast; and that the one +fragment of mica-slate, and the pebbles of quartz low down in the gypseous +formation at Las Amolanas, have been derived from it. The mica-schist is +succeeded by stratified porphyritic conglomerate [Q] of great thickness, +dipping eastward with a high inclination: I have included this latter +mountain-mass in the same anticlinal axis with the porphyritic streams +[NN]; but I am far from sure that the two masses may not have been +independently upheaved. + +SEVENTH AXIS OF ELEVATION. + +Proceeding up the ravine, we come to another mass [R] of andesite; and +beyond this, we again have a very thick, stratified porphyritic formation +[S], dipping at a small angle eastward, and forming the basal part of the +main Cordillera. I did not ascend the ravine any higher; but here, near +Castano, I examined several sections, of which I will not give the details, +only observing, that the porphyritic beds, or submarine lavas, preponderate +greatly in bulk over the alternating sedimentary layers, which have been +but little metamorphosed: these latter consist of fine-grained red tuffs +and of whitish volcanic grit-stones, together with much of a singular, +compact rock, having an almost crystalline basis, finely brecciated with +red and green fragments, and occasionally including a few large pebbles. +The porphyritic lavas are highly amygdaloidal, both on their upper and +lower surfaces; they consist chiefly of claystone porphyry, but with one +common variety, like some of the streams at the Puente del Inca, having a +grey mottled basis, abounding with crystals of red hydrous oxide of iron, +green ones apparently of epidote, and a few glassy ones of feldspar. This +pile of strata differs considerably from the basal strata of the Cordillera +in Central Chile, and may possibly belong to the upper and gypseous series: +I saw, however, in the bed of the valley, one fragment of porphyritic +breccia-conglomerate, exactly like those great masses met with in the more +southern parts of Chile. + +Finally, I must observe, that though I have described between the town of +Copiapo and the western flank of the main Cordillera seven or eight axes of +elevation, extending nearly north and south, it must not be supposed that +they all run continuously for great distances. As was stated to be the case +in our sections across the Cordillera of Central Chile, so here most of the +lines of elevation, with the exception of the first, third, and fifth, are +very short. The stratification is everywhere disturbed and intricate; +nowhere have I seen more numerous faults and dikes. The whole district, +from the sea to the Cordillera, is more or less metalliferous; and I heard +of gold, silver, copper, lead, mercury, and iron veins. The metamorphic +action, even in the lower strata, has certainly been far less here than in +Central Chile. + +VALLEY OF THE DESPOBLADO. + +This great barren valley, which has already been alluded to, enters the +main valley of Copiapo a little above the town: it runs at first northerly, +then N.E., and more easterly into the Cordillera; I followed its dreary +course to the foot of the first main ridge. I will not give a detailed +section, because it would be essentially similar to that already given, and +because the stratification is exceedingly complicated. After leaving the +plutonic hills near the town, I met first, as in the main valley, with the +gypseous formation, having the same diversified character as before, and +soon afterwards with masses of porphyritic conglomerate, about one thousand +feet in thickness. In the lower part of this formation there were very +thick beds composed of fragments of claystone porphyries, both angular and +rounded, with the smaller ones partially blended together and the basis +rendered porphyritic; these beds separated distinct streams, from sixty to +eighty feet in thickness, of claystone lavas. Near Paipote, also, there was +much true porphyritic breccia-conglomerate: nevertheless, few of these +masses were metamorphosed to the same degree with the corresponding +formation in Central Chile. I did not meet in this valley with any true +andesite, but only with imperfect andesitic porphyry, including large +crystals of hornblende: numerous as have been the varieties of intrusive +porphyries already mentioned, there were here mountains composed of a new +kind, having a compact, smooth, cream-coloured basis, including only a few +crystals of feldspar, and mottled with dendritic spots of oxide of iron. +There were also some mountains of a porphyry with a brick-red basis, +containing irregular, often lens-shaped, patches of compact feldspar, and +crystals of feldspar, which latter to my surprise I find to be orthite. + +At the foot of the first ridge of the main Cordillera, in the ravine of +Maricongo, and at an elevation which, from the extreme coldness and +appearance of the vegetation, I estimated at about ten thousand feet, I +found beds of white sandstone and of limestone including the Pecten +Dufreynoyi, Terebratula aenigma, and some Gryphites. This ridge throws the +water on the one hand into the Pacific, and on the other, as I was +informed, into a great gravel-covered, basin-like plain, including a salt- +lake, and without any drainage-exit. In crossing the Cordillera by this +Pass, it is said that three principal ridges must be traversed, instead of +two, or only one as in Central Chile. + +The crest of this first main ridge and the surrounding mountains, with the +exception of a few lofty pinnacles, are capped by a great thickness of a +horizontally stratified, tufaceous deposit. The lowest bed is of a pale +purple colour, hard, fine-grained, and full of broken crystals of feldspar +and scales of mica. The middle bed is coarser, and less hard, and hence +weathers into very sharp pinnacles; it includes very small fragments of +granite, and innumerable ones of all sizes of grey vesicular trachyte, some +of which were distinctly rounded. The uppermost bed is about two hundred +feet in thickness, of a darker colour and apparently hard: but I had not +time to ascend to it. These three horizontal beds may be seen for the +distance of many leagues, especially westward or in the direction of the +Pacific, capping the summits of the mountains, and standing on the opposite +sides of the immense valleys at exactly corresponding heights. If united +they would form a plain, inclined very slightly towards the Pacific; the +beds become thinner in this direction, and the tuff (judging from one point +to which I ascended, some way down the valley) finer-grained and of less +specific gravity, though still compact and sonorous under the hammer. The +gently inclined, almost horizontal stratification, the presence of some +rounded pebbles, and the compactness of the lowest bed, though rendering it +probable, would not have convinced me that this mass had been of subaqueous +origin, for it is known that volcanic ashes falling on land and moistened +by rain often become hard and stratified; but beds thus originating, and +owing their consolidation to atmospheric moisture, would have covered +almost equally every neighbouring summit, high and low, and would not have +left those above a certain exact level absolutely bare; this circumstance +seems to me to prove that the volcanic ejections were arrested at their +present, widely extended, equable level, and there consolidated by some +other means than simple atmospheric moisture; and this no doubt must have +been a sheet of water. A lake at this great height, and without a barrier +on any one side, is out of the question; consequently we must conclude that +the tufaceous matter was anciently deposited beneath the sea. It was +certainly deposited before the excavation of the valleys, or at least +before their final enlargement (I have endeavoured to show in my "Journal" +etc. (2nd edition) page 355, that this arid valley was left by the +retreating sea, as the land slowly rose, in the state in which we now see +it.); and I may add, that Mr. Lambert, a gentleman well acquainted with +this country, informs me, that in ascending the ravine of Santandres (which +branches off from the Despoblado) he met with streams of lava and much +erupted matter capping all the hills of granite and porphyry, with the +exception of some projecting points; he also remarked that the valleys had +been excavated subsequently to these eruptions. + +This volcanic formation, which I am informed by Mr. Lambert extends far +northward, is of interest, as typifying what has taken place on a grander +scale on the corresponding western side of the Cordillera of Peru. Under +another point of view, however, it possesses a far higher interest, as +confirming that conclusion drawn from the structure of the fringes of +stratified shingle which are prolonged from the plains at the foot of the +Cordillera far up the valleys,--namely, that this great range has been +elevated in mass to a height of between eight and nine thousand feet (I may +here mention that on the south side of the main valley of Copiapo, near +Potrero Seco, the mountains are capped by a thick mass of horizontally +stratified shingle, at a height which I estimated at between fifteen +hundred and two thousand feet above the bed of the valley. This shingle, I +believe, forms the edge of a wide plain, which stretches southwards between +two mountain ranges.); and now, judging from this tufaceous deposit, we may +conclude that the horizontal elevation has been in the district of Copiapo +about ten thousand feet. + +(FIGURE 24.) + +In the valley of the Despoblado, the stratification, as before remarked has +been much disturbed, and in some points to a greater degree than I have +anywhere else seen. I will give two cases: a very thick mass of thinly +stratified red sandstone, including beds of conglomerate, has been crushed +together (as represented in Figure 24) into a yoke or urn-formed trough, so +that the strata on both sides have been folded inwards: on the right hand +the properly underlying porphyritic claystone conglomerate is seen +overlying the sandstone, but it soon becomes vertical, and then is inclined +towards the trough, so that the beds radiate like the spokes of a wheel: on +the left hand, the inverted porphyritic conglomerate also assumes a dip +towards the trough, not gradually, as on the right hand, but by means of a +vertical fault and synclinal break; and a little still further on towards +the left, there is a second great oblique fault (both shown by the arrow- +lines), with the strata dipping to a directly opposite point; these +mountains are intersected by infinitely numerous dikes, some of which can +be seen to rise from hummocks of greenstone, and can be traced for +thousands of feet. In the second case, two low ridges trend together and +unite at the head of a little wedge-shaped valley: throughout the right- +hand ridge, the strata dip at 45 degrees to the east; in the left-hand +ridge, we have the very same strata and at first with exactly the same dip; +but in following this ridge up the valley, the strata are seen very +regularly to become more and more inclined until they stand vertical, they +then gradually fall over (the basset edges forming symmetrical serpentine +lines along the crest), till at the very head of the valley they are +reversed at an angle of 45 degrees: so that at this point the beds have +been turned through an angle of 135 degrees; and here there is a kind of +anticlinal axis, with the strata on both sides dipping to opposite points +at an angle of 45 degrees, but those on the left hand upside down. + +ON THE ERUPTIVE SOURCES OF THE PORPHYRITIC CLAYSTONE AND GREENSTONE LAVAS. + +In Central Chile, from the extreme metamorphic action, it is in most parts +difficult to distinguish between the streams of porphyritic lava and the +porphyritic breccia-conglomerate, but here, at Copiapo, they are generally +perfectly distinct, and in the Despoblado, I saw for the first time, two +great strata of purple claystone porphyry, after having been for a +considerable space closely united together, one above the other, become +separated by a mass of fragmentary matter, and then both thin out;--the +lower one more rapidly than the upper and greater stream. Considering the +number and thickness of the streams of porphyritic lava, and the great +thickness of the beds of breccia-conglomerate, there can be little doubt +that the sources of eruption must originally have been numerous: +nevertheless, it is now most difficult even to conjecture the precise point +of any one of the ancient submarine craters. I have repeatedly observed +mountains of porphyries, more or less distinctly stratified towards their +summits or on their flanks, without a trace of stratification in their +central and basal parts: in most cases, I believe this is simply due either +to the obliterating effects of metamorphic action, or to such parts having +been mainly formed of intrusive porphyries, or to both causes conjoined; in +some instances, however, it appeared to me very probable that the great +central unstratified masses of porphyry were the now partially denuded +nuclei of the old submarine volcanoes, and that the stratified parts marked +the points whence the streams flowed. In one case alone, and it was in this +Valley of the Despoblado, I was able actually to trace a thick stratum of +purplish porphyry, which for a space of some miles conformably overlay the +usual alternating beds of breccia-conglomerates and claystone lavas, until +it became united with, and blended into, a mountainous mass of various +unstratified porphyries. + +The difficulty of tracing the streams of porphyries to their ancient and +doubtless numerous eruptive sources, may be partly explained by the very +general disturbance which the Cordillera in most parts has suffered; but I +strongly suspect that there is a more specific cause, namely, THAT THE +ORIGINAL POINTS OF ERUPTION TEND TO BECOME THE POINTS OF INJECTION. This in +itself does not seem improbable; for where the earth's crust has once +yielded, it would be liable to yield again, though the liquified intrusive +matter might not be any longer enabled to reach the submarine surface and +flow as lava. I have been led to this conclusion, from having so frequently +observed that, where part of an unstratified mountain-mass resembled in +mineralogical character the adjoining streams or strata, there were several +other kinds of intrusive porphyries and andesitic rocks injected into the +same point. As these intrusive mountain-masses form most of the axes-lines +in the Cordillera, whether anticlinal, uniclinal, or synclinal, and as the +main valleys have generally been hollowed out along these lines, the +intrusive masses have generally suffered much denudation. Hence they are +apt to stand in some degree isolated, and to be situated at the points +where the valleys abruptly bend, or where the main tributaries enter. On +this view of there being a tendency in the old points of eruption to become +the points of subsequent injection and disturbance, and consequently of +denudation, it ceases to be surprising that the streams of lava in the +porphyritic claystone conglomerate formation, and in other analogous cases, +should most rarely be traceable to their actual sources. + +IQUIQUE, SOUTHERN PERU. + +Differently from what we have seen throughout Chile, the coast here is +formed not by the granitic series, but by an escarpment of the porphyritic +conglomerate formation, between two and three thousand feet in height. (The +lowest point, where the road crosses the coast-escarpment, is 1,900 feet by +the barometer above the level of the sea.) I had time only for a very short +examination; the chief part of the escarpment appears to be composed of +various reddish and purple, sometimes laminated, porphyries, resembling +those of Chile; and I saw some of the porphyritic breccia-conglomerate; the +stratification appeared but little inclined. The uppermost part, judging +from the rocks near the famous silver mine of Huantajaya, consists of +laminated, impure, argillaceous, purplish-grey limestone, associated, I +believe, with some purple sandstone. (Mr. Bollaert has described +"Geological Proceedings" volume 2 page 598, a singular mass of stratified +detritus, gravel, and sand, eighty-one yards in thickness, overlying the +limestone, and abounding with loose masses of silver ore. The miners +believe that they can attribute these masses to their proper veins.) In the +limestone shells are found: the three following species were given me:-- + +Lucina Americana, E. Forbes. +Terebratula inca, E. Forbes. +Terebratula aenigma, D'Orbigny. + +This latter species we have seen associated with the fossils of which lists +have been given in this chapter, in two places in the valley of Coquimbo, +and in the ravine of Maricongo at Copiapo. Considering this fact, and the +superposition of these beds on the porphyritic conglomerate formation; and, +as we shall immediately see, from their containing much gypsum, and from +their otherwise close general resemblance in mineralogical nature with the +strata described in the valley of Copiapo, I have little doubt that these +fossiliferous beds of Iquique belong to the great cretaceo-oolitic +formation of Northern Chile. Iquique is situated seven degrees latitude +north of Copiapo; and I may here mention, that an Ammonites, nov. species, +and an Astarte, nov. species, were given me from the Cerro Pasco, about ten +degrees of latitude north of Iquique, and M. D'Orbigny thinks that they +probably indicate a Neocomian formation. Again, fifteen degrees of latitude +northward, in Colombia, there is a grand fossiliferous deposit, now well +known from the labours of Von Buch, Lea, d'Orbigny, and Forbes, which +belongs to the earlier stages of the cretaceous system. Hence, bearing in +mind the character of the few fossils from Tierra del Fuego, there is some +evidence that a great portion of the stratified deposits of the whole vast +range of the South American Cordillera belongs to about the same geological +epoch. + +Proceeding from the coast escarpment inwards, I crossed, in a space of +about thirty miles, an elevated undulatory district, with the beds dipping +in various directions. The rocks are of many kinds,--white laminated, +sometimes siliceous sandstone,--purple and red sandstone, sometimes so +highly calcareous as to have a crystalline fracture,--argillaceous +limestone,--black calcareous slate-rock, like that so often described at +Copiapo and other places,--thinly laminated, fine-grained, greenish, +indurated, sedimentary, fusible rocks, approaching in character to the so- +called pseudo-honestone of Chile, including thin contemporaneous veins of +gypsum,--and lastly, much calcareous, laminated porcelain jasper, of a +green colour, with red spots, and of extremely easy fusibility: I noticed +one conformable stratum of a freckled-brown, feldspathic lava. I may here +mention that I heard of great beds of gypsum in the Cordillera. The only +novel point in this formation, is the presence of innumerable thin layers +of rock-salt, alternating with the laminated and hard, but sometimes +earthy, yellowish, or bright red and ferruginous sandstones. The thickest +layer of salt was only two inches, and it thinned out at both ends. On one +of these saliferous masses I noticed a stratum about twelve feet thick, of +dark-brown, hard brecciated, easily fusible rock, containing grains of +quartz and of black oxide of iron, together with numerous imperfect +fragments of shells. The problem of the origin of salt is so obscure, that +every fact, even geographical position, is worth recording. (It is well +known that stratified salt is found in several places on the shores of +Peru. The island of San Lorenzo, off Lima, is composed of a pile of thin +strata, about eight hundred feet in thickness, composed of yellowish and +purplish, hard siliceous, or earthy sandstones, alternating with thin +layers of shale, which in places passes into a greenish, semi-porcellanic, +fusible rock. There are some thin beds of reddish mudstone, and soft +ferruginous rotten-stones, with layers of gypsum. In nearly all these +varieties, especially in the softer sandstones, there are numerous thin +seams of rock-salt: I was informed that one layer has been found two inches +in thickness. The manner in which the minutest fissures of the dislocated +beds have been penetrated by the salt, apparently by subsequent +infiltration, is very curious. On the south side of the island, layers of +coal and of impure limestone have been discovered. Hence we here have salt, +gypsum, and coal associated together. The strata include veins of quartz, +carbonate of lime, and iron pyrites; they have been dislocated by an +injected mass of greenish-brown feldspathic trap. Not only is salt abundant +on the extreme western limits of the district between the Cordillera and +the Pacific, but, according to Helms, it is found in the outlying low hills +on the eastern flank of the Cordillera. These facts appear to me opposed to +the theory, that rock-salt is due to the sinking of water, charged with +salt, in mediterranean spaces of the ocean. The general character of the +geology of these countries would rather lead to the opinion, that its +origin is in some way connected with volcanic heat at the bottom of the +sea: see on this subject Sir R. Murchison "Anniversary Address to the +Geological Society" 1843 page 65.) With the exception of these saliferous +beds, most of the rocks as already remarked, present a striking general +resemblance with the upper parts of the gypseous or cretaceo-oolitic +formation of Chile. + +METALLIFEROUS VEINS. + +I have only a few remarks to make on this subject: in nine mining +districts, some of them of considerable extent, which I visited in CENTRAL +Chile, I found the PRINCIPAL veins running from between [N. and N.W.] to +[S. and S.E.] (These mining districts are Yaquil near Nancagua, where the +direction of the chief veins, to which only in all cases I refer, is north +and south; in the Uspallata range, the prevailing line is N.N.W. and +S.S.E.; in the C. de Prado, it is N.N.W. and S.S.E.; near Illapel, it is N. +by W. and S. by E.; at Los Hornos the direction varies from between [N. and +N.W.] to [S. and S.E.]; at the C. de los Hornos (further northward), it is +N.N.W. and S.S.E.; at Panuncillo, it is N.N.W. and S.S.E.; and, lastly, at +Arqueros, the direction is N.W. and S.E.): in some other places, however, +their courses appeared quite irregular, as is said to be generally the case +in the whole valley of Copiapo: at Tambillos, south of Coquimbo, I saw one +large copper vein extending east and west. It is worthy of notice, that the +foliation of the gneiss and mica-slate, where such rocks occur, certainly +tend to run like the metalliferous veins, though often irregularly, in a +direction a little westward of north. At Yaquil, I observed that the +principal auriferous veins ran nearly parallel to the grain or imperfect +cleavage of the surrounding GRANITIC rocks. With respect to the +distribution of the different metals, copper, gold, and iron are generally +associated together, and are most frequently found (but with many +exceptions, as we shall presently see) in the rocks of the lower series, +between the Cordillera and the Pacific, namely, in granite, syenite, +altered feldspathic clay-slate, gneiss, and as near Guasco mica-schist. The +copper-ores consist of sulphurets, oxides, and carbonates, sometimes with +laminae of native metal: I was assured that in some cases (as at Panuncillo +S.E. of Coquimbo), the upper part of the same vein contains oxides, and the +lower part sulphurets of copper. (The same fact has been observed by Mr. +Taylor in Cuba: "London Philosophical Journal" volume 11 page 21.) Gold +occurs in its native form; it is believed that, in many cases, the upper +part of the vein is the most productive part: this fact probably is +connected with the abundance of this metal in the stratified detritus of +Chile, which must have been chiefly derived from the degradation of the +upper portions of the rocks. These superficial beds of well-rounded gravel +and sand, containing gold, appeared to me to have been formed under the sea +close to the beach, during the slow elevation of the land: Schmidtmeyer +remarks that in Chile gold is sought for in shelving banks at the height of +some feet on the sides of the streams, and not in their beds, as would have +been the case had this metal been deposited by common alluvial action. +("Travels in Chile" page 29.) Very frequently the copper-ores, including +some gold, are associated with abundant micaceous specular iron. Gold is +often found in iron-pyrites: at two gold mines at Yaquil (near Nancagua), I +was informed by the proprietor that in one the gold was always associated +with copper-pyrites, and in the other with iron-pyrites: in this latter +case, it is said that if the vein ceases to contain iron-pyrites, it is yet +worth while to continue the search, but if the iron-pyrites, when it +reappears, is not auriferous, it is better at once to give up working the +vein. Although I believe copper and gold are most frequently found in the +lower granitic and metamorphic schistose series, yet these metals occur +both in the porphyritic conglomerate formation (as on the flanks of the +Bell of Quillota and at Jajuel), and in the superincumbent strata. At +Jajuel I was informed that the copper-ore, with some gold, is found only in +the greenstones and altered feldspathic clay-slate, which alternate with +the purple porphyritic conglomerate. Several gold veins and some of copper- +ore are worked in several parts of the Uspallata range, both in the +metamorphosed strata, which have been shown to have been of probably +subsequent origin to the Neocomian or gypseous formation of the main +Cordillera, and in the intrusive andesitic rocks of that range. At Los +Hornos (N.E. of Illapel), likewise, there are numerous veins of copper- +pyrites and of gold, both in the strata of the gypseous formation and in +the injected hills of andesite and various porphyries. + +Silver, in the form of a chloride, sulphuret, or an amalgam, or in its +native state, and associated with lead and other metals, and at Arqueros +with pure native copper, occurs chiefly in the upper great gypseous or +cretaceo-oolitic formation which forms probably the richest mass in Chile. +We may instance the mining districts of Arqueros near Coquimbo, and of +nearly the whole valley of Copiapo, and of Iquique (where the principal +veins run N.E. by E. and S.W. by W.), in Peru. Hence comes Molina's remark, +that silver is born in the cold and solitary deserts of the Upper +Cordillera. There are, however, exceptions to this rule: at Paral (S.E. of +Coquimbo) silver is found in the porphyritic conglomerate formation; as I +suspect is likewise the case at S. Pedro de Nolasko in the Peuquenes Pass. +Rich argentiferous lead is found in the clay-slate of the Uspallata range; +and I saw an old silver-mine in a hill of syenite at the foot of the Bell +of Quillota: I was also assured that silver has been found in the andesitic +and porphyritic region between the town of Copiapo and the Pacific. I have +stated in a previous part of this chapter, that in two neighbouring mines +at Arqueros the veins in one were productive when they traversed the +singular green sedimentary beds, and unproductive when crossing the reddish +beds; whereas at the other mine exactly the reverse takes place; I have +also described the singular and rare case of numerous particles of native +silver and of the chloride being disseminated in the green rock at the +distance of a yard from the vein. Mercury occurs with silver both at +Arqueros and at Copiapo: at the base of C. de los Hornos (S.E. of Coquimbo, +a different place from Los Hornos, before mentioned) I saw in a syenitic +rock numerous quartzose veins, containing a little cinnabar in nests: there +were here other parallel veins of copper and of a ferrugino-auriferous ore. +I believe tin has never been found in Chile. + +From information given me by Mr. Nixon of Yaquil (At the Durazno mine, the +gold is associated with copper-pyrites, and the veins contain large prisms +of plumbago. Crystallised carbonate of lime is one of the commonest +minerals in the matrix of the Chilean veins.), and by others, it appears +that in Chile those veins are generally most permanently productive, which, +consisting of various minerals (sometimes differing but slightly from the +surrounding rocks), include parallel strings RICH in metals; such a vein is +called a veta real. More commonly the mines are worked only where one, two, +or more thin veins or strings running in a different direction, intersect a +POOR "veta real:" it is unanimously believed that at such points of +intersection (cruceros), the quantity of metal is much greater than that +contained in other parts of the intersecting veins. In some cruceros or +points of intersection, the metals extend even beyond the walls of the +main, broad, stony vein. It is said that the greater the angle of +intersection, the greater the produce; and that nearly parallel strings +attract each other; in the Uspallata range, I observed that numerous thin +auri-ferruginous veins repeatedly ran into knots, and then branched out +again. I have already described the remarkable manner in which rocks of the +Uspallata range are indurated and blackened (as if by a blast of gunpowder) +to a considerable distance from the metallic veins. + +Finally, I may observe, that the presence of metallic veins seems obviously +connected with the presence of intrusive rocks, and with the degree of +metamorphic action which the different districts of Chile have undergone. +(Sir R. Murchison and his fellow travellers have given some striking facts +on this subject in their account of the Ural Mountains ("Geological +Proceedings" volume 3 page 748.) Such metamorphosed areas are generally +accompanied by numerous dikes and injected masses of andesite and various +porphyries: I have in several places traced the metalliferous veins from +the intrusive masses into the encasing strata. Knowing that the porphyritic +conglomerate formation consists of alternate streams of submarine lavas and +of the debris of anciently erupted rocks, and that the strata of the upper +gypseous formation sometimes include submarine lavas, and are composed of +tuffs, mudstones, and mineral substances, probably due to volcanic +exhalations,--the richness of these strata is highly remarkable when +compared with the erupted beds, often of submarine origin, but NOT +METAMORPHOSED, which compose the numerous islands in the Pacific, Indian, +and Atlantic Oceans; for in these islands metals are entirely absent, and +their nature even unknown to the aborigines. + +SUMMARY OF THE GEOLOGICAL HISTORY OF THE CHILEAN CORDILLERA, AND OF THE +SOUTHERN PARTS OF SOUTH AMERICA. + +We have seen that the shores of the Pacific, for a space of 1,200 miles +from Tres Montes to Copiapo, and I believe for a very much greater +distance, are composed, with the exception of the tertiary basins, of +metamorphic schists, plutonic rocks, and more or less altered clay-slate. +On the floor of the ocean thus constituted, vast streams of various +purplish claystone and greenstone porphyries were poured forth, together +with great alternating piles of angular and rounded fragments of similar +rocks ejected from the submarine craters. From the compactness of the +streams and fragments, it is probable that, with the exception of some +districts in Northern Chile, the eruptions took place in profoundly deep +water. The orifices of eruption appear to have been studded over a breadth, +with some outliers, of from fifty to one hundred miles: and closely enough +together, both north and south, and east and west, for the ejected matter +to form a continuous mass, which in Central Chile is more than a mile in +thickness. I traced this mould-like mass, for only 450 miles; but judging +from what I saw at Iquique, from specimens, and from published accounts, it +appears to have a manifold greater length. In the basal parts of the +series, and especially towards the flanks of the range, mud, since +converted into a feldspathic slaty rock, and sometimes into greenstone, was +occasionally deposited between the beds of erupted matter: with this +exception the uniformity of the porphyritic rocks is very remarkable. + +At the period when the claystone and greenstone porphyries nearly or quite +ceased being erupted, that great pile of strata which, from often abounding +with gypsum, I have generally called the gypseous formation was deposited, +and feldspathic lavas, together with other singular volcanic rocks, were +occasionally poured forth: I am far from pretending that any distinct line +of demarcation can be drawn between this formation and the underlying +porphyries and porphyritic conglomerate, but in a mass of such great +thickness, and between beds of such widely different mineralogical nature, +some division was necessary. At about the commencement of the gypseous +period, the bottom of the sea here seems first to have been peopled by +shells, not many in kind, but abounding in individuals. At the P. del Inca +the fossils are embedded near the base of the formation; in the Peuquenes +range, at different levels, halfway up, and even higher in the series; +hence, in these sections, the whole pile of strata belongs to the same +period: the same remark is applicable to the beds at Copiapo, which attain +a thickness of between seven and eight thousand feet. The fossil shells in +the Cordillera of Central Chile, in the opinion of all the palaeontologists +who have examined them, belong to the earlier stages of the cretaceous +system; whilst in Northern Chile there is a most singular mixture of +cretaceous and oolitic forms: from the geological relations, however, of +these two districts, I cannot but think that they all belong to nearly the +same epoch, which I have provisionally called cretaceo-oolitic. + +The strata in this formation, composed of black calcareous shaly-rocks of +red and white, and sometimes siliceous sandstone, of coarse conglomerates, +limestones, tuffs, dark mudstones, and those singular fine-grained rocks +which I have called pseudo-honestones, vast beds of gypsum, and many other +jaspery and scarcely describable varieties, vary and replace each other in +short horizontal distances, to an extent, I believe, unequalled even in any +tertiary basin. Most of these substances are easily fusible, and have +apparently been derived either from volcanoes still in quiet action, or +from the attrition of volcanic products. If we picture to ourselves the +bottom of the sea, rendered uneven in an extreme degree, with numerous +craters, some few occasionally in eruption, but the greater number in the +state of solfataras, discharging calcareous, siliceous, ferruginous +matters, and gypsum or sulphuric acid to an amount surpassing, perhaps, +even the existing sulphureous volcanoes of Java (Von Buch's "Description +Physique des Iles Canaries" page 428.), we shall probably understand the +circumstances under which this singular pile of varying strata was +accumulated. The shells appear to have lived at the quiescent periods when +only limestone or calcareo-argillaceous matter was depositing. From Dr. +Gillies' account, this gypseous or cretaceo-oolitic formation extends as +far south as the Pass of Planchon, and I followed it northward at intervals +for 500 miles: judging from the character of the beds with the Terebratula +aenigma, at Iquique, it extends from four to five hundred miles further: +and perhaps even for ten degrees of latitude north of Iquique to the Cerro +Pasco, not far from Lima: again, we know that a cretaceous formation, +abounding with fossils, is largely developed north of the equator, in +Colombia: in Tierra del Fuego, at about this same period, a wide district +of clay-slate was deposited, which in its mineralogical characters and +external features, might be compared to the Silurian regions of North +Wales. The gypseous formation, like that of the porphyritic breccia- +conglomerate on which it rests, is of inconsiderable breadth; though of +greater breadth in Northern than in Central Chile. + +As the fossil shells in this formation are covered, in the Peuquenes ridge, +by a great thickness of strata; at the Puente del Inca, by at least five +thousand feet; at Coquimbo, though the superposition there is less plainly +seen, by about six thousand feet; and at Copiapo, certainly by five or six +thousand, and probably by seven thousand feet (the same species there +recurring in the upper and lower parts of the series), we may feel +confident that the bottom of the sea subsided during this cretaceo-oolitic +period, so as to allow of the accumulation of the superincumbent submarine +strata. This conclusion is confirmed by, or perhaps rather explains, the +presence of the many beds at many levels of coarse conglomerate, the well- +rounded pebbles in which we cannot believe were transported in very deep +water. Even the underlying porphyries at Copiapo. with their highly +amygdaloidal surfaces, do not appear to have flowed under great pressure. +The great sinking movement thus plainly indicated, must have extended in a +north and south line for at least four hundred miles, and probably was co- +extensive with the gypseous formation. + +The beds of conglomerate just referred to, and the extraordinarily numerous +silicified trunks of fir-trees at Los Hornos, perhaps at Coquimbo and at +two distant points in the valley of Copiapo, indicate that land existed at +this period in the neighbourhood. This land, or islands, in the northern +part of the district of Copiapo, must have been almost exclusively +composed, judging from the nature of the pebbles of granite: in the +southern parts of Copiapo, it must have been mainly formed of claystone +porphyries, with some mica-schist, and with much sandstone and jaspery +rocks exactly like the rocks in the gypseous formation, and no doubt +belonging to its basal series. In several other places also, during the +accumulation of the gypseous formation, its basal parts and the underlying +porphyritic conglomerate must likewise have been already partially upheaved +and exposed to wear and tear; near the Puente del Inca and at Coquimbo, +there must have existed masses of mica-schist or some such rock, whence +were derived the many small pebbles of opaque quartz. It follows from these +facts, that in some parts of the Cordillera the upper beds of the gypseous +formation must lie unconformably on the lower beds; and the whole gypseous +formation, in parts, unconformably on the porphyritic conglomerate; +although I saw no such cases, yet in many places the gypseous formation is +entirely absent; and this, although no doubt generally caused by quite +subsequent denudation, may in others be due to the underlying porphyritic +conglomerate having been locally upheaved before the deposition of the +gypseous strata, and thus having become the source of the pebbles of +porphyry embedded in them. In the porphyritic conglomerate formation, in +its lower and middle parts, there is very rarely any evidence, with the +exception of the small quartz pebbles at Jajuel near Aconcagua, and of the +single pebble of granite at Copiapo, of the existence of neighbouring land: +in the upper parts, however, and especially in the district of Copiapo, the +number of thoroughly well-rounded pebbles of compact porphyries make me +believe, that, as during the prolonged accumulation of the gypseous +formation the lower beds had already been locally upheaved and exposed to +wear and tear, so it was with the porphyritic conglomerate. Hence in +following thus far the geological history of the Cordillera, it may be +inferred that the bed of a deep and open, or nearly open, ocean was filled +up by porphyritic eruptions, aided probably by some general and some local +elevations, to that comparatively shallow level at which the cretaceo- +oolitic shells first lived. At this period, the submarine craters yielded +at intervals a prodigious supply of gypsum and other mineral exhalations, +and occasionally, in certain places poured forth lavas, chiefly of a +feldspathic nature: at this period, islands clothed with fir-trees and +composed of porphyries, primary rocks, and the lower gypseous strata had +already been locally upheaved, and exposed to the action of the waves;--the +general movement, however, at this time having been over a very wide area, +one of slow subsidence, prolonged till the bed of the sea sank several +thousand feet. + +In Central Chile, after the deposition of a great thickness of the gypseous +strata, and after their upheaval, by which the Cumbre and adjoining ranges +were formed, a vast pile of tufaceous matter and submarine lava was +accumulated, where the Uspallata chain now stands; also after the +deposition and upheaval of the equivalent gypseous strata of the Peuquenes +range, the great thick mass of conglomerate in the valley of Tenuyan was +accumulated: during the deposition of the Uspallata strata, we know +absolutely, from the buried vertical trees, that there was a subsidence of +some thousand feet; and we may infer from the nature of the conglomerate in +the valley of Tenuyan, that a similar and perhaps contemporaneous movement +there took place. We have, then, evidence of a second great period of +subsidence; and, as in the case of the subsidence which accompanied the +accumulation of the cretaceo-oolitic strata, so this latter subsidence +appears to have been complicated by alternate or local elevatory movement-- +for the vertical trees, buried in the midst of the Uspallata strata, must +have grown on dry land, formed by the upheaval of the lower submarine beds. +Presently I shall have to recapitulate the facts, showing that at a still +later period, namely, at nearly the commencement of the old tertiary +deposits of Patagonia and of Chile, the continent stood at nearly its +present level, and then, for the third time, slowly subsided to the amount +of several hundred feet, and was afterwards slowly re-uplifted to its +present level. + +The highest peaks of the Cordillera appear to consist of active or more +commonly dormant volcanoes,--such as Tupungato, Maypu, and Aconcagua, which +latter stands 23,000 feet above the level of the sea, and many others. The +next highest peaks are formed of the gypseous and porphyritic strata, +thrown into vertical or highly inclined positions. Besides the elevation +thus gained by angular displacements, I infer, without any hesitation--from +the stratified gravel-fringes which gently slope up the valleys of the +Cordillera from the gravel-capped plains at their base, which latter are +connected with the plains, still covered with recent shells on the coast-- +that this great range has been upheaved in mass by a slow movement, to an +amount of at least 8,000 feet. In the Despoblado Valley, north of Copiapo, +the horizontal elevation, judging from the compact, stratified tufaceous +deposit, capping the distant mountains at corresponding heights, was about +ten thousand feet. It is very possible, or rather probable, that this +elevation in mass may not have been strictly horizontal, but more energetic +under the Cordillera, than towards the coast on either side; nevertheless, +movements of this kind may be conveniently distinguished from those by +which strata have been abruptly broken and upturned. When viewing the +Cordillera, before having read Mr. Hopkins's profound "Researches on +Physical Geology," the conviction was impressed on me, that the angular +dislocations, however violent, were quite subordinate in importance to the +great upward movement in mass, and that they had been caused by the edges +of the wide fissures, which necessarily resulted from the tension of the +elevated area, having yielded to the inward rush of fluidified rock, and +having thus been upturned. + +The ridges formed by the angularly upheaved strata are seldom of great +length: in the central parts of the Cordillera they are generally parallel +to each other, and run in north and south lines; but towards the flanks +they often extend more or less obliquely. The angular displacement has been +much more violent in the central than in the exterior MAIN lines; but it +has likewise been violent in some of the MINOR lines on the extreme flanks. +The violence has been very unequal on the same short lines; the crust +having apparently tended to yield on certain points along the lines of +fissures. These points, I have endeavoured to show, were probably first +foci of eruption, and afterwards of injected masses of porphyry and +andesite. (Sir R. Murchison and his companions state "Geological +Proceedings" volume 3 page 747, that no true granite appears in the higher +Ural Mountains; but that syenitic greenstone--a rock closely analogous to +our andesite--is far the most abundant of the intrusive masses.) The close +similarity of the andesitic granites and porphyries, throughout Chile, +Tierra del Fuego, and even in Peru, is very remarkable. The prevalence of +feldspar cleaving like albite, is common not only to the andesites, but (as +I infer from the high authority of Professor G. Rose, as well as from my +own measurements) to the various claystone and greenstone porphyries, and +to the trachytic lavas of the Cordillera. The andesitic rocks have in most +cases been the last injected ones, and they probably form a continuous dome +under this great range: they stand in intimate relationship with the modern +lavas; and they seem to have been the immediate agent in metamorphosing the +porphyritic conglomerate formation, and often likewise the gypseous strata, +to the extraordinary extent to which they have suffered. + +With respect to the age at which the several parallel ridges composing the +Cordillera were upthrown, I have little evidence. Many of them may have +been contemporaneously elevated and injected in the same manner as in +volcanic archipelagoes lavas are contemporaneously ejected on the parallel +lines of fissure. ("Volcanic Islands" etc.) But the pebbles apparently +derived from the wear and tear of the porphyritic conglomerate formation, +which are occasionally present in the upper parts of this same formation, +and are often present in the gypseous formation, together with the pebbles +from the basal parts of the latter formation in its upper strata, render it +almost certain that portions, we may infer ridges, of these two formations +were successively upheaved. In the case of the gigantic Portillo range, we +may feel almost certain that a preexisting granitic line was upraised (not +by a single blow, as shown by the highly inclined basaltic streams in the +valley on its eastern flank) at a period long subsequent to the upheavement +of the parallel Peuquenes range. (I have endeavoured to show in my +"Journal" 2nd edition page 321, that the singular fact of the river, which +drains the valley between these two ranges, passing through the Portillo +and higher line, is explained by its slow and subsequent elevation. There +are many analogous cases in the drainage of rivers: see "Edinburgh New +Philosophical Journal" volume 28 pages 33 and 44.) Again, subsequently to +the upheavement of the Cumbre chain, that of Uspallata was formed and +elevated; and afterwards, I may add, in the plain of Uspallata, beds of +sand and gravel were violently upthrown. The manner in which the various +kinds of porphyries and andesites have been injected one into the other, +and in which the infinitely numerous dikes of various composition intersect +each other, plainly show that the stratified crust has been stretched and +yielded many times over the same points. With respect to the age of the +axes of elevation between the Pacific and the Cordillera, I know little: +but there are some lines which must--namely, those running north and south +in Chiloe, those eight or nine east and west, parallel, far-extended, most +symmetrical uniclinal lines at P. Rumena, and the short N.W.-S.E. and N.E.- +S.W. lines at Concepcion--have been upheaved long after the formation of +the Cordillera. Even during the earthquake of 1835, when the linear north +and south islet of St. Mary was uplifted several feet above the surrounding +area, we perhaps see one feeble step in the formation of a subordinate +mountain-axis. In some cases, moreover, for instance, near the baths of +Cauquenes, I was forcibly struck with the small size of the breaches cut +through the exterior mountain-ranges, compared with the size of the same +valleys higher up where entering the Cordillera; and this circumstance +appeared to me scarcely explicable, except on the idea of the exterior +lines having been subsequently upthrown, and therefore having been exposed +to a less amount of denudation. From the manner in which the fringes of +gravel are prolonged in unbroken slopes up the valleys of the Cordillera, I +infer that most of the greater dislocations took place during the earlier +parts of the great elevation in mass: I have, however, elsewhere given a +case, and M. de Tschudi has given another, of a ridge thrown up in Peru +across the bed of a river, and consequently after the final elevation of +the country above the level of the sea. ("Reise in Peru" Band 2 s.8: +Author's "Journal" 2nd edition page 359.) + +Ascending to the older tertiary formations, I will not again recapitulate +the remarks already given at the end of the Fifth Chapter,--on their great +extent, especially along the shores of the Atlantic--on their antiquity, +perhaps corresponding with that of the eocene deposits of Europe,--on the +almost entire dissimilarity, though the formations are apparently +contemporaneous, of the fossils from the eastern and western coasts, as is +likewise the case, even in a still more marked degree, with the shells now +living in these opposite though approximate seas,--on the climate of this +period not having been more tropical than what might have been expected +from the latitudes of the places under which the deposits occur; a +circumstance rendered well worthy of notice, from the contrast with what is +known to have been the case during the older tertiary periods of Europe, +and likewise from the fact of the southern hemisphere having suffered at a +much later period, apparently at the same time with the northern +hemisphere, a colder or more equable temperature, as shown by the zones +formerly affected by ice-action. Nor will I recapitulate the proofs of the +bottom of the sea, both on the eastern and western coast, having subsided +seven or eight hundred feet during this tertiary period; the movement +having apparently been co-extensive, or nearly co-extensive, with the +deposits of this age. Nor will I again give the facts and reasoning on +which the proposition was founded, that when the bed of the sea is either +stationary or rising, circumstances are far less favourable than when its +level is sinking, to the accumulation of conchiferous deposits of +sufficient thickness, extension, and hardness to resist, when upheaved, the +ordinary vast amount of denudation. We have seen that the highly remarkable +fact of the absence of any EXTENSIVE formations containing recent shells, +either on the eastern or western coasts of the continent,--though these +coasts now abound with living mollusca,--though they are, and apparently +have always been, as favourable for the deposition of sediment as they were +when the tertiary formations were copiously deposited,--and though they +have been upheaved to an amount quite sufficient to bring up strata from +the depths the most fertile for animal life--can be explained in accordance +with the above proposition. As a deduction, it was also attempted to be +shown, first, that the want of close sequence in the fossils of successive +formations, and of successive stages in the same formation, would follow +from the improbability of the same area continuing slowly to subside from +one whole period to another, or even during a single entire period; and +secondly, that certain epochs having been favourable at distant points, in +the same quarter of the world for the synchronous accumulation of +fossiliferous strata, would follow from movements of subsidence having +apparently, like those of elevation, contemporaneously affected very large +areas. + +There is another point which deserves some notice, namely, the analogy +between the upper parts of the Patagonian tertiary formation, as well as of +the upper possibly contemporaneous beds at Chiloe and Concepcion, with the +great gypseous formation of Cordillera; for in both formations, the rocks, +in their fusible nature, in their containing gypsum, and in many other +characters, show a connection, either intimate or remote, with volcanic +action; and as the strata in both were accumulated during subsidence, it +appears at first natural to connect this sinking movement with a state of +high activity in the neighbouring volcanoes. During the cretaceo-oolitic +period this certainly appears to have been the case at the Puente del Inca, +judging from the number of intercalated lava-streams in the lower 3,000 +feet of strata; but generally, the volcanic orifices seem at this time to +have existed as submarine solfataras, and were certainly quiescent compared +with their state during the accumulation of the porphyritic conglomerate +formation. During the deposition of the tertiary strata we know that at S. +Cruz, deluges of basaltic lava were poured forth; but as these lie in the +upper part of the series, it is possible that the subsidence may at that +time have ceased: at Chiloe, I was unable to ascertain to what part of the +series the pile of lavas belonged. The Uspallata tuffs and great streams of +submarine lavas, were probably intermediate in age between the cretaceo- +oolitic and older tertiary formations, and we know from the buried trees +that there was a great subsidence during their accumulation; but even in +this case, the subsidence may not have been strictly contemporaneous with +the great volcanic eruptions, for we must believe in at least one +intercalated period of elevation, during which the ground was upraised on +which the now buried trees grew. I have been led to make these remarks, and +to throw some doubt on the strict contemporaneousness of high volcanic +activity and movements of subsidence, from the conviction impressed on my +mind by the study of coral formations, that these two actions do not +generally go on synchronously;--on the contrary, that in volcanic +districts, subsidence ceases as soon as the orifices burst forth into +renewed action, and only recommences when they again have become dormant. +("The Structure and Distribution of Coral Reefs.") + +At a later period, the Pampean mud, of estuary origin, was deposited over a +wide area,--in one district conformably on the underlying old tertiary +strata, and in another district unconformably on them, after their upheaval +and denudation. During and before the accumulation, however, of these old +tertiary strata, and, therefore, at a very remote period, sediment, +strikingly resembling that of the Pampas, was deposited; showing during how +long a time in this case the same agencies were at work in the same area. +The deposition of the Pampean estuary mud was accompanied, at least in the +southern parts of the Pampas, by an elevatory movement, so that the M. +Hermoso beds probably were accumulated after the upheaval of those round +the S. Ventana; and those at P. Alta after the upheaval of the M. Hermoso +strata; but there is some reason to suspect that one period of subsidence +intervened, during which mud was deposited over the coarse sand of the +Barrancas de S. Gregorio, and on the higher parts of Banda Oriental. The +mammiferous animals characteristic of this formation, many of which differ +as much from the present inhabitants of South America, as do the eocene +mammals of Europe from the present ones of that quarter of the globe, +certainly co-existed at B. Blanca with twenty species of mollusca, one +balanus, and two corals, all now living in the adjoining sea: this is +likewise the case in Patagonia with the Macrauchenia, which co-existed with +eight shells, still the commonest kinds on that coast. I will not repeat +what I have elsewhere said, on the place of habitation, food, wide range, +and extinction of the numerous gigantic mammifers, which at this late +period inhabited the two Americas. + +The nature and grouping of the shells embedded in the old tertiary +formations of Patagonia and Chile show us, that the continent at that +period must have stood only a few fathoms below its present level, and that +afterwards it subsided over a wide area, seven or eight hundred feet. The +manner in which it has since been rebrought up to its actual level, was +described in detail in the First and Second Chapters. It was there shown +that recent shells are found on the shores of the Atlantic, from Tierra del +Fuego northward for a space of at least 1,180 nautical miles, and at the +height of about 100 feet in La Plata, and of 400 feet in Patagonia. The +elevatory movements on this side of the continent have been slow; and the +coast of Patagonia, up to the height in one part of 950 feet and in another +of 1,200 feet, is modelled into eight great, step-like, gravel-capped +plains, extending for hundreds of miles with the same heights; this fact +shows that the periods of denudation (which, judging from the amount of +matter removed, must have been long continued) and of elevation were +synchronous over surprisingly great lengths of coasts. On the shores of the +Pacific, upraised shells of recent species, generally, though not always, +in the same proportional numbers as in the adjoining sea, have actually +been found over a north and south space of 2,075 miles, and there is reason +to believe that they occur over a space of 2,480 miles. The elevation on +this western side of the continent has not been equable; at Valparaiso, +within the period during which upraised shells have remained undecayed on +the surface, it has been 1,300 feet, whilst at Coquimbo, 200 miles +northward, it has been within this same period only 252 feet. At Lima, the +land has been uplifted at least 80 feet since Indian man inhabited that +district; but the level within historical times apparently has subsided. At +Coquimbo, in a height of 364 feet, the elevation has been interrupted by +five periods of comparative rest. At several places the land has been +lately, or still is, rising both insensibly and by sudden starts of a few +feet during earthquake-shocks; this shows that these two kinds of upward +movement are intimately connected together. For a space of 775 miles, +upraised recent shells are found on the two opposite sides of the +continent; and in the southern half of this space, it may be safely +inferred from the slope of the land up to the Cordillera, and from the +shells found in the central part of Tierra del Fuego, and high up the River +Santa Cruz, that the entire breadth of the continent has been uplifted. +From the general occurrence on both coasts of successive lines of +escarpments, of sand-dunes and marks of erosion, we must conclude that the +elevatory movement has been normally interrupted by periods, when the land +either was stationary, or when it rose at so slow a rate as not to resist +the average denuding power of the waves, or when it subsided. In the case +of the present high sea-cliffs of Patagonia and in other analogous +instances, we have seen that the difficulty in understanding how strata can +be removed at those depths under the sea, at which the currents and +oscillations of the water are depositing a smooth surface of mud, sand, and +sifted pebbles, leads to the suspicion that the formation or denudation of +such cliffs has been accompanied by a sinking movement. + +In South America, everything has taken place on a grand scale, and all +geological phenomena are still in active operation. We know how violent at +the present day the earthquakes are, we have seen how great an area is now +rising, and the plains of tertiary origin are of vast dimensions; an almost +straight line can be drawn from Tierra del Fuego for 1,600 miles northward, +and probably for a much greater distance, which shall intersect no +formation older than the Patagonian deposits; so equable has been the +upheaval of the beds, that throughout this long line, not a fault in the +stratification or abrupt dislocation was anywhere observable. Looking to +the basal, metamorphic, and plutonic rocks of the continent, the areas +formed of them are likewise vast; and their planes of cleavage and +foliation strike over surprisingly great spaces in uniform directions. The +Cordillera, with its pinnacles here and there rising upwards of twenty +thousand feet above the level of the sea, ranges in an unbroken line from +Tierra del Fuego, apparently to the Arctic circle. This grand range has +suffered both the most violent dislocations, and slow, though grand, upward +and downward movements in mass; I know not whether the spectacle of its +immense valleys, with mountain-masses of once liquified and intrusive rocks +now bared and intersected, or whether the view of those plains, composed of +shingle and sediment hence derived, which stretch to the borders of the +Atlantic Ocean, is best adapted to excite our astonishment at the amount of +wear and tear which these mountains have undergone. + +The Cordillera from Tierra del Fuego to Mexico, is penetrated by volcanic +orifices, and those now in action are connected in great trains. The +intimate relation between their recent eruptions and the slow elevation of +the continent in mass, appears to me highly important, for no explanation +of the one phenomenon can be considered as satisfactory which is not +applicable to the other. (On the Connection of certain Volcanic Phenomena +in South America: "Geological Transactions" volume 5 page 609.) The +permanence of the volcanic action on this chain of mountains is, also, a +striking fact; first, we have the deluges of submarine lavas alternating +with the porphyritic conglomerate strata, then occasionally feldspathic +streams and abundant mineral exhalations during the gypseous or cretaceo- +oolitic period: then the eruptions of the Uspallata range, and at an +ancient but unknown period, when the sea came up to the eastern foot of the +Cordillera, streams of basaltic lava at the foot of the Portillo range; +then the old tertiary eruptions; and lastly, there are here and there +amongst the mountains, much worn and apparently very ancient volcanic +formations without any craters; there are, also, craters quite extinct, and +others in the condition of solfataras, and others occasionally or +habitually in fierce action. Hence it would appear that the Cordillera has +been, probably with some quiescent periods, a source of volcanic matter +from an epoch anterior to our cretaceo-oolitic formation to the present +day; and now the earthquakes, daily recurrent on some part of the western +coast, give little hope that the subterranean energy is expended. + +Recurring to the evidence by which it was shown that some at least of the +parallel ridges, which together compose the Cordillera, were successively +and slowly upthrown at widely different periods; and that the whole range +certainly once, and almost certainly twice, subsided some thousand feet, +and being then brought up by a slow movement in mass, again, during the old +tertiary formations, subsided several hundred feet, and again was brought +up to its present level by a slow and often interrupted movement; we see +how opposed is this complicated history of changes slowly effected, to the +views of those geologists who believe that this great mountain-chain was +formed in late times by a single blow. I have endeavoured elsewhere to +show, that the excessively disturbed condition of the strata in the +Cordillera, so far from indicating single periods of extreme violence, +presents insuperable difficulties, except on the admission that the masses +of once liquified rocks of the axes were repeatedly injected with intervals +sufficiently long for their successive cooling and consolidation. +("Geological Transactions" volume 5 page 626.) Finally, if we look to the +analogies drawn from the changes now in progress in the earth's crust, +whether to the manner in which volcanic matter is erupted, or to the manner +in which the land is historically known to have risen and sunk: or again, +if we look to the vast amount of denudation which every part of the +Cordillera has obviously suffered, the changes through which it has been +brought into its present condition, will appear neither to have been too +slowly effected, nor to have been too complicated. + +NOTE. + +As, both in France and England, translations of a passage in Professor +Ehrenberg's Memoir, often referred to in the Fourth Chapter of this volume, +have appeared, implying that Professor Ehrenberg believes, from the +character of the infusoria, that the Pampean formation was deposited by a +sea-debacle rushing over the land, I may state, on the authority of a +letter to me, that these translations are incorrect. The following is the +passage in question:-- + +"Durch Beachtung der mikroscopischen Formen hat sich nun feststellen +lassen, das die Mastodonten-Lager am La Plata und die Knochen-Lager am +Monte Hermoso, who wie die der Riesen-Gurtelthiere in den Dunenhugeln bei +Bahia Blanca, beides in Patagonien, unveranderte brakische +Susswasserbildungen sind, die einst wohl sammtlich zum obersten +Fluthgebiethe des Meeres im tieferen Festlande gehorten."--"Monatsberichten +der konigl. Akad. etc." zu Berlin vom April 1845. + + +INDEX. + +Abich, on a new variety of feldspar. + +Abrolhos islands. + +Absence of recent formations on the S. American coasts. + +Aguerros on elevation of Imperial. + +Albite, constituent mineral in andesite. +--in rocks of Tierra del Fuego. +--in porphyries. +--crystals of, with orthite. + +Alison, Mr., on elevation of Valparaiso. + +Alumina, sulphate of. + +Ammonites from Concepcion. + +Amolanas, Las. + +Amygdaloid, curious varieties of. + +Amygdaloids of the Uspallata range. +--of Copiapo. + +Andesite of Chile. +--in the valley of Maypu. +--of the Cumbre pass. +--of the Uspallata range. +--of Los Hornos. +--of Copiapo. + +Anhydrite, concretions of. + +Araucaria, silicified wood of. +Arica, elevation of. + +Arqueros, mines of. + +Ascension, gypsum deposited on. +--laminated volcanic rocks of. + +Augite in fragments, in gneiss. +--with albite, in lava. + +Austin, Mr. R.A.C., on bent cleavage lamina. + +Austin, Captain, on sea-bottom. + +Australia, foliated rocks of. + +Azara labiata, beds of, at San Pedro. + +Baculites vagina. + +Bahia Blanca, elevation of. +--formations near. +--character of living shells of. + +Bahia (Brazil), elevation near. +--crystalline rocks of. + +Ballard, M., on the precipitation of sulphate of soda. + +Banda Oriental, tertiary formations of. +--crystalline rocks of. + +Barnacles above sea-level. +--adhering to upraised shells. + +Basalt of S. Cruz. +--streams of, in the Portillo range. +--in the Uspallata range. + +Basin chains of Chile. + +Beagle Channel. + +Beaumont, Elie de, on inclination of lava-streams. +--on viscid quartz-rocks. + +Beech-tree, leaves of fossil. + +Beechey, Captain, on sea-bottom. + +Belcher, Lieutenant, on elevated shells from Concepcion. + +Bella Vista, plain of. + +Benza, Dr., on decomposed granite. + +Bettington, Mr., on quadrupeds transported by rivers. + +Blake, Mr., on the decay of elevated shells near Iquique. +--on nitrate of soda. + +Bole. + +Bollaert, Mr., on mines of Iquique. + +Bones, silicified. +--fossil, fresh condition of. + +Bottom of sea off Patagonia. + +Bougainville, on elevation of the Falkland islands. + +Boulder formation of S. Cruz. +--of Falkland islands. +--anterior to certain extinct quadrupeds. +--of Tierra del Fuego. + +Boulders in the Cordillera. +--transported by earthquake-waves. +--in fine-grained tertiary deposits. + +Brande, Mr., on a mineral spring. + +Bravais, M., on elevation of Scandinavia. + +Brazil, elevation of. +--crystalline rocks of. + +Broderip, Mr., on elevated shells from Concepcion. + +Brown, Mr. R., on silicified wood of Uspallata range. + +Brown, on silicified wood. + +Bucalema, elevated shells near. + +Buch, Von, on cleavage. +--on cretaceous fossils of the Cordillera. +--on the sulphureous volcanoes of Java. + +Buenos Ayres. + +Burchell, Mr., on elevated shells of Brazil. + +Byron, on elevated shells. + +Cachapual, boulders in valley of. + +Caldcleugh, Mr., on elevation of Coquimbo. +--on rocks of the Portillo range. + +Callao, elevation near. +--old town of. + +Cape of Good Hope, metamorphic rocks of. + +Carcharias megalodon. + +Carpenter, Dr., on microscopic organisms. + +Castro (Chiloe), beds near. + +Cauquenes Baths, boulders near. +--pebbles in porphyry near. +--volcanic formation near. +--stratification near. + +Caves above sea-level. + +Cervus pumilus, fossil-horns of. + +Chevalier, M., on elevation near Lima. + +Chile, structure of country between the Cordillera and the Pacific. +--tertiary formations of. +--crystalline rocks in. +--central, geology of. +--northern, geology of. + +Chiloe, gravel on coast. +--elevation of. +--tertiary formation of. +--crystalline rocks of. + +Chlorite-schist, near M. Video. + +Chonos archipelago, tertiary formations of. +--crystalline rocks of. + +Chupat, Rio, scoriae transported by. + +Claro, Rio, fossiliferous beds of. + +Clay-shale of Los Hornos. + +Clay-slate, formation of, Tierra del Fuego. +--of Concepcion. +--feldspathic, of Chile. +-- --of the Uspallata range. +--black siliceous, band of, in porphyritic formations of Chile. + +Claystone porphyry, formation of, in Chile. +--origin of. +--eruptive sources of. + +Cleavage, definition of. +--at Bahia. +--Rio de Janeiro. +--Maldonado. +--Monte Video. +--S. Guitru-gueyu. +--Falkland I. +--Tierra del Fuego. +--Chonos I. +--Chiloe. +--Concepcion. +--Chile. +--discussion on. + +Cleavage-laminae superficially bent. + +Cliffs, formation of. + +Climate, late changes in. +--of Chile during tertiary period. + +Coal of Concepcion. +--S. Lorenzo. + +Coast-denudation of St. Helena. + +Cobija, elevation of. + +Colombia, cretaceous formation of. + +Colonia del Sacramiento, elevation of. +--Pampean formation near Colorado, Rio, gravel of. +--sand-dunes of. +--Pampean formation near. + +Combarbala. + +Concepcion, elevation of. +--deposits of. +--crystalline rocks of. + +Conchalee, gravel-terraces of. + +Concretions of gypsum, at Iquique. +--in sandstone at S. Cruz. +--in tufaceous tuff of Chiloe. +--in gneiss. +--in claystone-porphyry at Port Desire. +--in gneiss at Valparaiso. +--in metamorphic rocks. +--of anhydrite. +--relations of, to veins. + +Conglomerate claystone of Chile. +--of Tenuyan. +--of the Cumbre Pass. +--of Rio Claro. +--of Copiapo. + +Cook, Captain, on form of sea-bottom. + +Copiapo, elevation of. +--tertiary formations of. +--secondary formations of. + +Copper, sulphate of. +--native, at Arqueros. +--mines of, at Panuncillo. +--veins, distribution of. + +Coquimbo, elevation and terraces of. +--tertiary formations of. +--secondary formations of. + +Corallines living on pebbles. + +Cordillera, valleys bordered by gravel fringes. +--basal strata of. +--fossils of. +--elevation of. +--gypseous formations of. +--claystone-porphyries of. +--andesitic rocks of. +--volcanoes of. + +Coste, M., on elevation of Lemus. + +Coy inlet, tertiary formation of. + +Crassatella Lyellii. + +Cruickshanks, Mr., on elevation near Lima. + +Crystals of feldspar, gradual formation of, at Port Desire. + +Cumbre, Pass of, in Cordillera. + +Cuming, Mr., on habits of the Mesodesma. +--on range of living shells on west coast. + +Dana, Mr., on foliated rocks. +--on amygdaloids. + +Darwin, Mount. + +D'Aubuisson, on concretions. +--on foliated rocks. +Decay, gradual, of upraised shells. + +Decomposition of granite rocks. + +De la Beche, Sir H., his theoretical researches in geology. +--on the action of salt on calcareous rocks. +--on bent cleavage-laminae. + +Denudation on coast of Patagonia. +--great powers of. +--of the Portillo range. + +Deposits, saline. + +Despoblado, valley of. + +Detritus, nature of, in Cordillera. + +Devonshire, bent cleavage in. + +Dikes, in gneiss of Brazil. +--near Rio de Janeiro. +--pseudo, at Port Desire. +--in Tierra del Fuego. +--in Chonos archipelago, containing quartz. +--near Concepcion, with quartz. +--granitic-porphyritic, at Valparaiso. +--rarely vesicular in Cordillera. +--absent in the central ridges of the Portillo pass. +--of the Portillo range, with grains of quartz. +--intersecting each other often. +--numerous at Copiapo. + +Domeyko, M., on the silver mines of Coquimbo. +on the fossils of Coquimbo. + +D'Orbigny, M. A., on upraised shells of Monte Video. +--on elevated shells at St. Pedro. +--on elevated shells near B. Ayres. +--on elevation of S. Blas. +--on the sudden elevation of La Plata. +--on elevated shells near Cobija. +--on elevated shells near Arica. +--on the climate of Peru. +--on salt deposits of Cobija. +--on crystals of gypsum in salt-lakes. +--on absence of gypsum in the Pampean formation. +--on fossil remains from Bahia Blanca. +--on fossil remains from the banks of the Parana. +--on the geology of St. Fe. +--on the age of Pampean formation. +--on the Mastodon Andium. +--on the geology of the Rio Negro. +--on the character of the Patagonian fossils. +--on fossils from Concepcion. +-- --from Coquimbo. +-- --from Payta. +--on fossil tertiary shells of Chile. +--on cretaceous fossils of Tierra del Fuego. +-- --from the Cordillera of Chile. + +Earth, marine origin of. + +Earthenware, fossil. + +Earthquake, effect of, at S. Maria. +--elevation during, at Lemus. +--of 1822, at Valparaiso. +--effects of, in shattering surface. +--fissures made by. +--probable effects on cleavage. + +Earthquakes in Pampas. + +Earthquake-waves, power of, in throwing up shells. +--effects of, near Lima. +--power of, in transporting boulders. + +Edmonston, Mr., on depths at which shells live at Valparaiso. + +Ehrenberg, Professor, on infusoria in the Pampean formation. +--on infusoria in the Patagonian formation. + +Elevation of La Plata. +--Brazil. +--Bahia Blanca. +--San Blas. +--Patagonia. +--Tierra del Fuego. +--Falkland islands. +--Pampas. +--Chonos archipelago. +--Chiloe. +--Chile. +--Valparaiso. +--Coquimbo. +--Guasco. +--Iquique. +--Cobija. +--Lima. +--sudden, at S. Maria. +-- --at Lemus. +--insensible, at Chiloe. +-- --at Valparaiso. +-- --at Coquimbo. +--axes of, at Chiloe. +-- --at P. Rumena. +--at Concepcion. +--unfavourable for the accumulation of permanent deposits. +--lines of, parallel to cleavage and foliation. +--lines of, oblique to foliation. +--areas of, causing lines of elevation and cleavage. +--lines of, in the Cordillera. +--slow, in the Portillo range. +--two periods of, in Cordillera of Central Chile. +--of the Uspallata range. +--two periods of, in Cumbre Pass. +--horizontal, in the Cordillera of Copiapo. +--axes of, coincident with volcanic orifices. +--of the Cordillera, summary on. + +Elliott, Captain, on human remains. + +Ensenada, elevated shells of. + +Entre Rios, geology of. + +Equus curvidens. + +Epidote in Tierra del Fuego. +--in gneiss. +--frequent in Chile. +--in the Uspallata range. +--in porphyry of Coquimbo. + +Erman, M., on andesite. +Escarpments, recent, of Patagonia. + +Extinction of fossil mammifers. + +Falkland islands, elevation of. +--pebbles on coast. +--geology of. + +Falkner, on saline incrustations. + +Faults, great, in Cordillera. + +Feldspar, earthy, metamorphosis of, at Port Desire. +--albitic. +--crystals of, with albite. +--orthitic, in conglomerate of Tenuyan. +--in granite of Portillo range. +--in porphyries in the Cumbre Pass. + +Feuillee on sea-level at Coquimbo. + +Fissures, relations of, to concretions. +--upfilled, at Port Desire. +--in clay-slate. + +Fitton, Dr., on the geology of Tierra del Fuego. + +Fitzroy, Captain, on the elevation of the Falkland islands. +--on the elevation of Concepcion. + +Foliation, definition of. +--of rocks at Bahia. +--Rio de Janeiro. +--Maldonado. +--Monte Video. +--S. Guitru-gueyu. +--Falkland I. +--Tierra del Fuego. +--Chonos archipelago. +--Chiloe. +--Concepcion. +--Chile. +--discussion on. + +Forbes, Professor E., on cretaceous fossils of Concepcion. +--on cretaceous fossils and subsidence in Cumbre Pass. +--on fossils from Guasco. +-- --from Coquimbo. +-- --from Copiapo. +--on depths at which shells live. + +Formation, Pampean. +-- --area of. +-- --estuary origin. +--tertiary of Entre Rios. +--of Banda Oriental. +--volcanic, in Banda Oriental. +--of Patagonia. +--summary on. +--tertiary of Tierra del Fuego. +-- --of the Chonos archipelago. +-- --of Chiloe. +-- --of Chile. +-- --of Concepcion. +-- --of Navidad. +-- --of Coquimbo. +-- --of Peru. +-- --subsidence during. +--volcanic, of Tres Montes. +-- --of Chiloe. +-- --old, near Maldonado. +-- --with laminar structure. +-- --ancient, in Tierra del Fuego. +--recent, absent on S. American coast. +--metamorphic, of claystone-porphyry of Patagonia. +--foliation of. +--plutonic, with laminar structure. +--palaeozoic, of the Falkland I. +--claystone, at Concepcion. +--Jurassic, of Cordillera. +--Neocomian, of the Portillo Pass. +--volcanic, of Cumbre Pass. +--gypseous, of Los Hornos. +-- --of Coquimbo. +-- --of Guasco. +-- --of Copiapo. +-- --of Iquique. +--cretaceo-oolitic, of Coquimbo. +-- --of Guasco. +-- --of Copiapo. +-- --of Iquique. + +Fossils, Neocomian, of Portillo Pass. +-- --of Cumbre Pass. +--secondary, of Coquimbo. +-- --of Guasco. +-- --of Copiapo. +-- --of Iquique. +--palaeozoic, from the Falklands. + +Fragments of hornblende-rock in gneiss. +--of gneiss in gneiss. + +Freyer, Lieutenant, on elevated shells of Arica. + +Frezier on sea-level at Coquimbo. + +Galapagos archipelago, pseudo-dikes of. + +Gallegos, Port, tertiary formation of. + +Garnets in gneiss. +--in mica-slate. +--at Panuncillo. + +Gardichaud, M., on granites of Brazil. + +Gay, M., on elevated shells. +--on boulders in the Cordillera. +--on fossils from Cordillera of Coquimbo. + +Gill, Mr., on brickwork transported by an earthquake-wave. + +Gillies, Dr., on heights in the Cordillera. +--on extension of the Portillo range. + +Glen Roy, parallel roads of. +--sloping terraces of. + +Gneiss, near Bahia. +--of Rio de Janeiro. +--decomposition of. + +Gold, distribution of. + +Gorodona, formations near. +Granite, axis of oblique, to foliation. +--andesitic. +--of Portillo range. +--veins of, quartzose. +--pebble of, in porphyritic conglomerate. +--conglomerate. + +Grauwacke of Uspallata range. + +Gravel at bottom of sea. +--formation of, in Patagonia. +--means of transportation of. +--strata of, inclined. + +Gravel-terraces in Cordillera. + +Greenough, Mr., on quartz veins. + +Greenstone, resulting from metamorphose hornblende-rock. +--of Tierra del Fuego. +--on the summit of the Campana of Quillota. +--porphyry. +--relation of, to clay-slate. + +Gryphaea orientalis. + +Guasco, elevation of. +--secondary formation of. + +Guitru-gueyu, Sierra. + +Guyana, gneissic rocks of. + +Gypsum, nodules of, in gravel at Rio Negro. +--deposited from sea-water. +--deposits of, at Iquique. +--crystals of, in salt lakes. +--in Pampean formation. +--in tertiary formation of Patagonia. +--great formation of, in the Portillo Pass. +-- --in the Cumbre Pass. +-- --near Los Hornos. +-- --at Coquimbo. +-- --at Copiapo. +-- --near Iquique. +--of San Lorenzo. + +Hall, Captain, on terraces at Coquimbo. + +Hamilton, Mr., on elevation near Tacna. + +Harlan, Dr., on human remains. + +Hayes, Mr. A., on nitrate of soda. + +Henslow, Professor, on concretions. + +Herbert, Captain, on valleys in the Himalaya. + +Herradura Bay, elevated shells of. +--tertiary formations of. + +Himalaya, valleys in. + +Hippurites Chilensis. + +Hitchcock, Professor, on dikes. + +Honestones, pseudo, of Coquimbo. +--of Copiapo. + +Hooker, Dr. J.D., on fossil beech-leaves. + +Hopkins, Mr., on axes of elevation oblique to foliation. +--on origin of lines of elevation. + +Hornblende-rock, fragments of, in gneiss. + +Hornblende-schist, near M. Video. + +Hornos, Los, section near. + +Hornstone, dike of. + +Horse, fossil tooth of. + +Huafo island. +--subsidence at. + +Huantajaya, mines of. + +Humboldt, on saline incrustations. +--on foliations of gneiss. +--on concretions in gneiss. + +Icebergs, action on cleavage. + +Illapel, section near. + +Imperial, beds of shells near. + +Incrustations, saline. + +Infusoria in Pampean formation. +--in Patagonian formation. + +Iodine, salts of. + +Iquique, elevation of. +--saliferous deposits of. +--cretaceo-oolitic formation of. + +Iron, oxide of, in lavas. +--in sedimentary beds. +--tendency in, to produce hollow concretions. +--sulphate of. + +Isabelle, M., on volcanic rocks of Banda Oriental. + +Joints in clay-slate. + +Jukes, Mr., on cleavage in Newfoundland. + +Kamtschatka, andesite of. + +Kane, Dr., on the production of carbonate of soda. + +King George's sound, calcareous beds of. + +Lakes, origin of. +--fresh-water, near salt lakes. + +Lava, basaltic, of S. Cruz. +--claystone-porphyry, at Chiloe. +-- --ancient submarine. +--basaltic, of the Portillo range. +--feldspathic, of the Cumbre Pass. +--submarine, of the Uspallata range. +--basaltic, of the Uspallata range. +--submarine, of Coquimbo. +--of Copiapo. + +Lemus island. + +Lemuy islet. + +Lignite of Chiloe. +--of Concepcion. + +Lima, elevation of. + +Lime, muriate of. + +Limestone of Cumbre Pass. +--of Coquimbo. +--of Copiapo. + +Lund and Clausen on remains of caves in Brazil. + +Lund, M., on granites of Brazil. + +Lyell, M., on upraised shells retaining their colours. +--on terraces at Coquimbo. +--on elevation near Lima. +--on fossil horse's tooth. +--on the boulder-formation being anterior to the extinction of North +American mammifers. +--on quadrupeds washed down by floods. +--on age of American fossil mammifers. +--on changes of climate. +--on denudation. +--on foliation. + +MacCulloch, Dr., on concretions. +--on beds of marble. + +Maclaren, Mr., letter to, on coral-formations. + +Macrauchenia Patachonica. + +Madeira, subsidence of. + +Magellan, Strait, elevation near, of. + +Magnesia, sulphate of, in veins. + +Malcolmson, Dr., on trees carried out to sea. + +Maldonado, elevation of. +--Pampean formation of. +--crystalline rocks of. + +Mammalia, fossil, of Bahia Blanca. +-- --near St. Fe. +-- --of Banda Oriental. +-- --of St. Julian. +-- --at Port Gallegos. +--washed down by floods. +--number of remains of, and range of, in Pampas. + +Man, skeletons of (Brazil). +--remains of, near Lima. +--Indian, antiquity of. + +Marble, beds of. + +Maricongo, ravine of. + +Marsden, on elevation of Sumatra. + +Mastodon Andium, remains of. +--range of. + +Maypu, Rio, mouth of, with upraised shells. +--gravel fringes of. +--debouchement from the Cordillera. + +Megalonyx, range of. + +Megatherium, range of. + +Miers, Mr., on elevated shells. +--on the height of the Uspallata plain. + +Minas, Las. + +Mocha Island, elevation of. +--tertiary form of. +--subsidence at. + +Molina, on a great flood. + +Monte Hermoso, elevation of. +--fossils of. + +Monte Video, elevation of. +--Pampean formation of. +--crystalline rocks of. + +Morris and Sharpe, Messrs., on the palaeozoic fossils of the Falklands. + +Mud, Pampean. +--long deposited on the same area. + +Murchison, Sir R., on cleavage. +--on waves transporting gravel. +--on origin of salt formations. +--on the relations of metalliferous veins and intrusive rocks. +--on the absence of granite in the Ural. + +Nautilus d'Orbignyanus. + +Navidad, tertiary formations of, subsidence of. + +Negro, Rio, pumice of pebbles of. +--gravel of. +--salt lakes of. +--tertiary strata of. + +North America, fossil remains of. + +North Wales, sloping terraces absent in. +--bent cleavage of. + +Neuvo Gulf, plains of. +--tertiary formation of. + +Owen, Professor, on fossil mammiferous remains. + +Palmer, Mr., on transportation of gravel. + +Pampas, elevation of. +--earthquakes of. +--formation of. +--localities in which fossil mammifers have been found. + +Panuncillo, mines of. + +Parana, Rio, on saline incrustations. +--Pampean formations near. +--on the S. Tandil. + +Parish, Sir W., on elevated shells near Buenos Ayres. +--on earthquakes in the Pampas. +--on fresh-water near salt lakes. +--on origin of Pampean formation. + +Patagonia, elevation and plains of. +--denudation of. +--gravel-formation of. +--sea-cliffs of. +--subsidence during tertiary period. +--crystalline rocks of. + +Payta, tertiary formations of. + +Pebbles of pumice. +--decrease in size on the coast of Patagonia. +--means of transportation. +--encrusted with living corallines. +--distribution of, at the eastern foot of Cordillera. +--dispersal of, in the Pampas. +--zoned with colour. + +Pentland, Mr., on heights in the Cordillera. +--on fossils of the Cordillera. + +Pernambuco. + +Peru, tertiary formations of. + +Peuquenes, Pass of, in the Cordillera. +--ridge of. + +Pholas, elevated shells of. + +Pitchstone of Chiloe. +--of Port Desire. +--near Cauquenes. +--layers of, in the Uspallata range. +--of Los Hornos. +--of Coquimbo. + +Plains of Patagonia. +--of Chiloe. +--of Chile. +--of Uspallata. +--on eastern foot of Cordillera. +--of Iquique. + +Plata, La, elevation of. +--tertiary formation of. +--crystalline rocks of. + +Playfair, Professor, on the transportation of gravel. + +Pluclaro, axis of. + +Pondicherry, fossils of. + +Porcelain rocks of Port Desire. +--of the Uspallata range. + +Porphyry, pebbles of, strewed over Patagonia. + +Porphyry, claystone, of Chiloe, +-- --of Patagonia. +-- --of Chile. +--greenstone, of Chile. +--doubly columnar. +--claystone, rare, on the eastern side of the Portillo Pass. +--brick-red and orthitic, of Cumbre Pass. +--intrusive, repeatedly injected. +--claystone of the Uspallata range. +-- --of Copiapo. +-- --eruptive sources of. + +Port Desire, elevation and plains of. +--tertiary formation of. +--porphyries of. + +Portillo Pass in the Cordillera. + +Portillo chain. +--compared with that of the Uspallata. + +Prefil or sea-wall of Valparaiso. + +Puente del Inca, section of. + +Pumice, pebbles of. +--conglomerate of R. Negro. +--hills of, in the Cordillera. + +Punta Alta, elevation of. +--beds of. + +Quartz-rock of the S. Ventana. +--C. Blanco. +--Falkland islands. +--Portillo range. +--viscidity of. +--veins of, near Monte Video. +-- --in dike of greenstone. +--grains of, in mica slate. +-- --in dikes. +--veins of, relations to cleavage. + +Quillota, Campana of. + +Quintero, elevation of. + +Quiriquina, elevation of. +--deposits of. + +Rancagua, plain of. + +Rapel, R. elevation near. + +Reeks, Mr. T., his analysis of decomposed shells. +--his analysis of salts. + +Remains, human. + +Rio de Janeiro, elevation near. +--crystalline rocks of. + +Rivers, small power of transporting pebbles. +--small power of, in forming valleys. +--drainage of, in the Cordillera. + +Roads, parallel, of Glen Roy. + +Rocks, volcanic, of Banda Oriental. +--Tres Montes. +--Chiloe. +--Tierra del Fuego. +--with laminar structure. + +Rodents, fossil, remains of. + +Rogers, Professor, address to Association of American Geologists. + +Rose, Professor G., on sulphate of iron at Copiapo. + +S. Blas, elevation of. + +S. Cruz, elevation and plains of. +--valley of. +--nature of gravel in valley of. +--boulder formation of. +--tertiary formation of. +--subsidence at. + +S. Fe Bajada, formations of. + +S. George's bay, plains of. + +S. Helena island, sea-cliffs, and subsidence of. + +S. Josef, elevation of. +--tertiary formation of. + +S. Juan, elevation near. + +S. Julian, elevation and plains of. +--salt lake of. +--earthy deposit with mammiferous remains. +--tertiary formations of. +--subsidence at. + +S. Lorenzo, elevation of. +--old salt formation of. + +S. Mary, island of, elevation of. + +S. Pedro, elevation of. + +Salado, R., elevated shells of. +--Pampean formation of. + +Salines. + +Salt, with upraised shell. +--lakes of. +--purity of, in salt lakes. +--deliquescent, necessary for the preservation of meat. +--ancient formation of, at Iquique. +-- --at S. Lorenzo. +--strata of, origin of. + +Salts, superficial deposits of. + +Sand-dunes of the Uruguay. +--of the Pampas. +--near Bahia Blanca. +--of the Colorado. +--of S. Cruz. +--of Arica. + +Sarmiento, Mount. + +Schmidtmeyer on auriferous detritus. + +Schomburghk, Sir R., on sea-bottom. +--on the rocks of Guyana. + +Scotland, sloping terraces of. + +Sea, nature of bottom of, off Patagonia. +--power of, in forming valleys. + +Sea cliffs, formation of. + +Seale, Mr., model of St. Helena. + +Sebastian Bay, tertiary formation of. + +Sedgwick, Professor, on cleavage. + +Serpentine of Copiapo. + +Serpulae, on upraised rocks. + +Shale-rock, of the Portillo Pass. +--of Copiapo. + +Shells, upraised state of, in Patagonia. +--elevated, too small for human food. +--transported far inland, for food. +--upraised, proportional numbers varying. +-- --gradual decay of. +-- --absent on high plains of Chile. +-- --near Bahia Blanca. +--preserved in concretions. +--living and fossil range of, on west coast. +--living, different on the east and west coast. + +Shingle of Patagonia. + +Siau, M., on sea-bottom. + +Silver mines of Arqueros. +--of Chanuncillo. +--of Iquique. +--distribution of. + +Slip, great, at S. Cruz. + +Smith, Mr., of Jordan Hill, on upraised shells retaining their colours. +--on Madeira. +--on elevated seaweed. +--on inclined gravel beds. + +Soda, nitrate of. +--sulphate of, near Bahia Blanca. +--carbonate of. + +Soundings off Patagonia. +--in Tierra del Fuego. + +Spirifers. + +Spix and Martius on Brazil. +Sprengel on the production of carbonate of soda. + +Springs, mineral, in the Cumbre Pass. + +Stratification of sandstone in metamorphic rocks. +--of clay-slate in Tierra del Fuego. +--of the Cordillera of Central Chile. +--little disturbed in Cumbre Pass. +--disturbance of, near Copiapo. + +Streams of lava at S. Cruz, inclination of. +--in the Portillo range. + +String of cotton with fossil-shells. + +Struthiolaria ornata. + +Studer, M., on metamorphic rocks. + +Subsidence during formation of sea-cliffs. +--near Lima. +--probable, during Pampean formation. +--necessary for the accumulation of permanent deposits. +--during the tertiary formations of Chile and Patagonia. +--probable during the Neocomian formation of the Portillo Pass. +--probable during the formation of conglomerate of Tenuyan. +--during the Neocomian formation of the Cumbre Pass. +--of the Uspallata range. +--great, at Copiapo. +-- --during the formation of the Cordillera. + +Sulphur, volcanic exhalations of. + +Sumatra, promontories of. + +Summary on the recent elevatory movements. +--on the Pampean formation. +--on the tertiary formations of Patagonia and Chile. +--on the Chilean Cordillera. +--on the cretaceo-oolitic formation. +--on the subsidences of the Cordillera. +--on the elevation of the Cordillera. + +Tacna, elevation of. + +Tampico, elevated shells near. + +Tandil, crystalline rocks of. + +Tapalguen, Pampean formation of. +--crystalline rocks of. + +Taylor, Mr., on copper veins of Cuba. + +Temperature of Chile during the tertiary period. + +Tension, lines of, origin of, axes of elevation and of cleavage. + +Tenuy Point, singular section of. + +Tenuyan, valley of. + +Terraces of the valley of S. Cruz. +--of equable heights throughout Patagonia. +--of Patagonia, formation of. +--of Chiloe. +--at Conchalee. +--of Coquimbo. +--not horizontal at Coquimbo. +--of Guasco. +--of S. Lorenzo. +--of gravel within the Cordillera. + +Theories on the origin of the Pampean formation. + +Tierra Amarilla. + +Tierra del Fuego, form of sea-bottom. +--tertiary formations of. +--clay-slate formation of. +--cretaceous formation of. +--crystalline rocks of. +--cleavage of clay-slate. + +Tosca rock. + +Trachyte of Chiloe. +--of Port Desire. +--in the Cordillera. + +Traditions of promontories having been islands. +--on changes of level near Lima. + +Trees buried in plain of Iquique. +--silicified, vertical, of the Uspallata range. + +Tres Montes, elevation of. +--volcanic rocks of. + +Trigonocelia insolita. + +Tristan Arroyo, elevated shells of. + +Tschudi, Mr., on subsidence near Lima. + +Tuff, calcareous, at Coquimbo. +--on basin-plain near St. Jago. +--structure of, in Pampas. +--origin of, in Pampas. +--pumiceous, of R. Negro. +--Nuevo Gulf. +--Port Desire. +--S. Cruz. +--Patagonia, summary on Chiloe. +--formation of, in Portillo chain. +--great deposit of, at Copiapo. + +Tuffs, volcanic, metamorphic, of Uspallata. +--of Coquimbo. + +Ulloa, on rain in Peru. +--on elevation near Lima. + +Uruguay, Rio, elevation of country near. + +Uspallata, plain of. +--pass of. +--range of. +--concluding remarks on. + +Valdivia, tertiary beds of. +--mica-slate of. + +Valley of S. Cruz, structure of. +--Coquimbo. +--Guasco, structure of. +--Copiapo, structure of. +--S. Cruz, tertiary formations of. +--Coquimbo, geology of. +--Guasco, secondary formations of. +--Copiapo, secondary formations of. +--Despoblado. + +Valleys in the Cordillera bordered by gravel fringes. +--formation of. +--in the Cordillera. + +Valparaiso, elevation of. +--gneiss of. + +Vein of quartz near Monte Video. +--in mica-slate. +--relations of, to cleavage. +--in a trap dike. +--of granite, quartzose. +--remarkable, in gneiss, near Valparaiso. + +Veins, relations of, to concretions. +--metalliferous, of the Uspallata range. +--metalliferous, discussion on. + +Venezuela, gneissic rocks of. + +Ventana, Sierra, Pampean formation near. +--quartz-rock of. + +Villa Vincencio Pass. + +Volcan, Rio, mouth of. +--fossils of. + +Volcanoes of the Cordillera. +--absent, except near bodies of water. +--ancient submarine, in Cordillera. +--action of, in relation to changes of level. +--long action of, in the Cordillera. + +Wafer on elevated shells. + +Waves caused by earthquakes, power of, in transporting boulders. +--power of, in throwing up shells. + +Weaver, Mr., on elevated shells. + +White, Martin., on sea-bottom. + +Wood, silicified, of Entre Rios. +--S. Cruz. +--Chiloe. +--Uspallata range. +--Los Hornos. +--Copiapo. + +Yeso, Rio, and plain of. + +Ypun Island, tertiary formation of. + +Zeagonite. + + + + + +End of Project Gutenberg Etext South American Geology, by Charles Darwin + diff --git a/old/3620.zip b/old/3620.zip Binary files differnew file mode 100644 index 0000000..6a714e4 --- /dev/null +++ b/old/3620.zip |