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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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