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+The Project Gutenberg EBook of Coral Reefs, Volcanic Islands, South American Geology, 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'll have
+to check the laws of the country where you are located before using this ebook.
+
+Title: Coral Reefs, Volcanic Islands, South American Geology
+also:
+Title: The Structure and Distribution of Coral-Reefs, Geological
+Observations on Volcanic Islands, and Geological Observations on
+South America.
+
+Author: Charles Darwin
+
+Release Date: May, 2003 [Etext #4022]
+[Most recently updated: May 23, 2020]
+
+Language: English
+
+Character set encoding: UTF-8
+
+*** START OF THIS PROJECT GUTENBERG EBOOK CORAL REEFS, VOLCANIC ISLANDS, SOUTH AMERICAN GEOLOGY ***
+
+
+
+Produced by Sue Asscher
+
+
+
+
+Coral Reefs, Volcanic Islands, South American Geology
+
+by Charles Darwin
+
+
+
+
+EDITORIAL NOTE
+
+
+Although in some respects more technical in their subjects and style
+than Darwin’s “Journal,” the books here reprinted will never lose their
+value and interest for the originality of the observations they
+contain. Many parts of them are admirably adapted for giving an insight
+into problems regarding the structure and changes of the earth’s
+surface, and in fact they form a charming introduction to physical
+geology and physiography in their application to special domains. The
+books themselves cannot be obtained for many times the price of the
+present volume, and both the general reader, who desires to know more
+of Darwin’s work, and the student of geology, who naturally wishes to
+know how a master mind reasoned on most important geological subjects,
+will be glad of the opportunity of possessing them in a convenient and
+cheap form.
+
+The three introductions, which my friend Professor Judd has kindly
+furnished, give critical and historical information which makes this
+edition of special value.
+
+G.T.B.
+
+
+
+
+CONTENTS
+
+THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.
+
+CRITICAL INTRODUCTION
+
+INTRODUCTION
+
+Chapter I—ATOLLS OR LAGOON-ISLANDS.
+
+_Section I_—DESCRIPTION OF KEELING ATOLL.
+Corals on the outer margin.—Zone of Nulliporæ.—Exterior
+reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari
+and Holuthuriæ subsisting on corals.—Changes in the condition of the
+reefs and islets.—Probable subsidence of the atoll.—Future state of the
+lagoon.
+
+_Section II_—GENERAL DESCRIPTION OF ATOLLS. General form and size of
+atolls, their reefs and islets.—External slope.—Zone of
+Nulliporæ.—Conglomerate.—Depth of lagoons.—Sediment.—Reefs submerged
+wholly or in part.—Breaches in the reef.—Ledge-formed shores round
+certain lagoons.—Conversion of lagoons into land.
+
+_Section III_—ATOLLS OF THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK.
+Maldiva Archipelago.—Ring-formed reefs, marginal and central.—Great
+depths in the lagoons of the southern atolls.—Reefs in the lagoons all
+rising to the surface.—Position of islets and breaches in the reefs,
+with respect to the prevalent winds and action of the
+waves.—Destruction of islets.—Connection in the position and submarine
+foundation of distinct atolls.—The apparent disseverment of large
+atolls.—The Great Chagos Bank.—Its submerged condition and
+extraordinary structure.
+
+Chapter II—BARRIER REEFS.
+
+Closely resemble in general form and structure atoll-reefs.—Width and
+depth of the lagoon-channels.—Breaches through the reef in front of
+valleys, and generally on the leeward side.—Checks to the filling up of
+the lagoon-channels.—Size and constitution of the encircled
+islands.—Number of islands within the same reef.—Barrier-reefs of New
+Caledonia and Australia.—Position of the reef relative to the slope of
+the adjoining land.—Probable great thickness of barrier-reefs.
+
+Chapter III—FRINGING OR SHORE-REEFS.
+
+Reefs of Mauritius.—Shallow channel within the reef.—Its slow filling
+up.—Currents of water formed within it.—Upraised reefs.—Narrow
+fringing-reefs in deep seas.—Reefs on the coast of E. Africa and of
+Brazil.—Fringing-reefs in very shallow seas, round banks of sediment
+and on worn-down islands.—Fringing-reefs affected by currents of the
+sea.—Coral coating the bottom of the sea, but not forming reefs.
+
+Chapter IV—ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.
+
+_Section I_—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
+FAVOURABLE TO THEIR INCREASE.
+
+_Section II_—ON THE RATE OF GROWTH OF CORAL-REEFS.
+
+_Section III_—ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE.
+
+Chapter V—THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
+CORAL-REEFS.
+
+The atolls of the larger archipelagoes are not formed on submerged
+craters, or on banks of sediment.—Immense areas interspersed with
+atolls.—Recent changes in their state.—The origin of barrier-reefs and
+of atolls.—Their relative forms.—The step-formed ledges and walls round
+the shores of some lagoons.—The ring-formed reefs of the Maldiva
+atolls.—The submerged condition of parts or of the whole of some
+annular reefs.—The disseverment of large atolls.—The union of atolls by
+linear reefs.—The Great Chagos Bank.—Objections, from the area and
+amount of subsidence required by the theory, considered.—The probable
+composition of the lower parts of atolls.
+
+Chapter VI—ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
+THEORY OF THEIR FORMATION.
+
+Description of the coloured map.—Proximity of atolls and barrier-
+reefs.—Relation in form and position of atolls with ordinary
+islands.—Direct evidence of subsidence difficult to be detected.—Proofs
+of recent elevation where fringing-reefs occur.—Oscillations of
+level.—Absence of active volcanoes in the areas of
+subsidence.—Immensity of the areas which have been elevated and have
+subsided.—Their relation to the present distribution of the land.—Areas
+of subsidence elongated, their intersection and alternation with those
+of elevation.—Amount and slow rate of the subsidence.—Recapitulation.
+
+Appendix
+
+Containing a detailed description of the reefs and islands in Plate
+III.
+
+GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.
+
+CRITICAL INTRODUCTION
+
+Chapter I—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.
+
+Rocks of the lowest series.—A calcareous sedimentary deposit, with
+recent shells, altered by the contact of superincumbent lava, its
+horizontality and extent.—Subsequent volcanic eruptions, associated
+with calcareous matter in an earthy and fibrous form, and often
+enclosed within the separate cells of the scoriæ.—Ancient and
+obliterated orifices of eruption of small size.—Difficulty of tracing
+over a bare plain recent streams of lava.—Inland hills of more ancient
+volcanic rock.—Decomposed olivine in large masses.—Feldspathic rocks
+beneath the upper crystalline basaltic strata.—Uniform structure and
+form of the more ancient volcanic hills.—Form of the valleys near the
+coast.—Conglomerate now forming on the sea beach.
+
+Chapter II—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.
+
+FERNANDO NORONHA.—Precipitous hill of phonolite. TERCEIRA.—Trachytic
+rocks: their singular decomposition by steam of high temperature.
+TAHITI.—Passage from wacke into trap; singular volcanic rock with the
+vesicles half-filled with mesotype. MAURITIUS.—Proofs of its recent
+elevation.—Structure of its more ancient mountains; similarity with St.
+Jago. ST. PAUL’S ROCKS.—Not of volcanic origin.—Their singular
+mineralogical composition.
+
+Chapter III—ASCENSION.
+
+Basaltic lavas.—Numerous craters truncated on the same side.—Singular
+structure of volcanic bombs.—Aeriform explosions.—Ejected granite
+fragments.—Trachytic rocks.—Singular veins.—Jasper, its manner of
+formation.—Concretions in pumiceous tuff.—Calcareous deposits and
+frondescent incrustations on the coast.—Remarkable laminated beds,
+alternating with, and passing into obsidian.—Origin of
+obsidian.—Lamination of volcanic rocks.
+
+Chapter IV—ST. HELENA.
+
+Lavas of the feldspathic, basaltic, and submarine series.—Section of
+Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and Prosperous
+Bays.—Basaltic ring.—Central crateriform ridge, with an internal ledge
+and a parapet.—Cones of phonolite.—Superficial beds of calcareous
+sandstone.—Extinct land-shells.—Beds of detritus.—Elevation of the
+land.—Denudation.—Craters of elevation.
+
+Chapter V—GALAPAGOS ARCHIPELAGO.
+
+Chatham Island.—Craters composed of a peculiar kind of tuff.—Small
+basaltic craters, with hollows at their bases.—Albemarle Island; fluid
+lavas, their composition.—Craters of tuff; inclination of their
+exterior diverging strata, and structure of their interior converging
+strata.—James Island, segment of a small basaltic crater; fluidity and
+composition of its lava-streams, and of its ejected
+fragments.—Concluding remarks on the craters of tuff, and on the
+breached condition of their southern sides.—Mineralogical composition
+of the rocks of the archipelago.—Elevation of the land.—Direction of
+the fissures of eruption.
+
+Chapter VI—TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.
+
+The sinking of crystals in fluid lava.—Specific gravity of the
+constituent parts of trachyte and of basalt, and their consequent
+separation.—Obsidian.—Apparent non-separation of the elements of
+plutonic rocks.—Origin of trap-dikes in the plutonic
+series.—Distribution of volcanic islands; their prevalence in the great
+oceans.—They are generally arranged in lines.—The central volcanoes of
+Von Buch doubtful.—Volcanic islands bordering continents.—Antiquity of
+volcanic islands, and their elevation in mass.—Eruptions on parallel
+lines of fissure within the same geological period.
+
+Chapter VII—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.
+
+New South Wales.—Sandstone formation.—Embedded pseudo-fragments of
+shale.—Stratification.—Current-cleavage.—Great valleys.—Van Diemen’s
+Land.—Palæozoic formation.—Newer formation with volcanic
+rocks.—Travertin with leaves of extinct plants.—Elevation of the
+land.—New Zealand.—King George’s Sound.—Superficial ferruginous
+beds.—Superficial calcareous deposits, with casts of branches; its
+origin from drifted particles of shells and corals.—Their extent.—Cape
+of Good Hope.—Junction of the granite and clay-slate.—Sandstone
+formation.
+
+GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA.
+
+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, palæozoic fossils
+of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of;
+cleavage and foliation; form of land.—Chonos Archipelago, mica-schists,
+foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes,
+successive formation of.—Central and Northern Chile.—Concluding remarks
+on cleavage and foliation.—Their close analogy and similar
+origin.—Stratification of metamorphic schists.—Foliation of intrusive
+rocks.—Relation of cleavage and foliation to the lines of tension
+during metamorphosis.
+
+Chapter VII—CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.
+
+Central Chile.—Basal formations of the Cordillera.—Origin of the
+porphyritic clay-stone conglomerate.—Andesite.—Volcanic rocks.—Section
+of the Cordillera by the Peuquenes or Portillo Pass.—Great gypseous
+formation.—Peuquenes line; thickness of strata, fossils of.—Portillo
+line.—Conglomerate, orthitic granite, mica-schist, volcanic rocks
+of.—Concluding remarks on the denudation and elevation of the Portillo
+line.—Section by the Cumbre, or Uspallata Pass.—Porphyries.—Gypseous
+strata.—Section near the Puente del Inca; fossils of.—Great
+subsidence.—Intrusive porphyries.—Plain of Uspallata.—Section of the
+Uspallata chain.—Structure and nature of the strata.—Silicified
+vertical trees.—Great subsidence.—Granitic rocks of axis.—Concluding
+remarks on the Uspallata range; origin subsequent to that of the main
+Cordillera; two periods of subsidence; comparison with the Portillo
+chain.
+
+Chapter VIII—NORTHERN CHILE.—CONCLUSION.
+
+Section from Illapel to Combarbala; gypseous formation with silicified
+wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley;
+fossils.—Guasco, fossils of.—Copiapo, section up valley; Las Amolanas,
+silicified wood.—Conglomerates, nature of former land, fossils,
+thickness of strata, great subsidence.—Valley of Despoblado, fossils,
+tufaceous deposit, complicated dislocations of.—Relations between
+ancient orifices of eruption and subsequent axes of injection.—Iquique,
+Peru, fossils of, salt-deposits.—Metalliferous veins.—Summary on the
+porphyritic conglomerate and gypseous formations.—Great subsidence with
+partial elevations during the cretaceo-oolitic period.—On the elevation
+and structure of the Cordillera.—Recapitulation on the tertiary
+series.—Relation between movements of subsidence and volcanic
+action.—Pampean formation.—Recent elevatory movements.—Long-continued
+volcanic action in the Cordillera.—Conclusion.
+
+Index to “Coral-Reefs”
+
+Index to “Volcanic Islands”
+
+Index to “South American Observations”
+
+
+
+
+THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.
+
+
+
+
+CRITICAL INTRODUCTION
+
+
+A scientific discovery is the outcome of an interesting process of
+evolution in the mind of its author. When we are able to detect the
+germs of thought in which such a discovery has originated, and to trace
+the successive stages of the reasoning by which the crude idea has
+developed into an epoch-making book, we have the materials for
+reconstructing an important chapter of scientific history. Such a
+contribution to the story of the “making of science” may be furnished
+in respect to Darwin’s famous theory of coral-reefs, and the clearly
+reasoned treatise in which it was first fully set forth.
+
+The subject of corals and coral-reefs is one concerning which much
+popular misconception has always prevailed. The misleading comparison
+of coral-rock with the combs of bees and the nests of wasps is perhaps
+responsible for much of this misunderstanding; one writer has indeed
+described a coral-reef as being “built by fishes by means of their
+teeth.” Scarcely less misleading, however, are the references we so
+frequently meet with, both in prose and verse, to the “skill,”
+“industry,” and “perseverance” of the “coral-insect” in “building” his
+“home.” As well might we praise men for their cleverness in making
+their own skeletons, and laud their assiduity in filling churchyards
+with the same. The polyps and other organisms, whose remains accumulate
+to form a coral-reef, simply live and perform their natural functions,
+and then die, leaving behind them, in the natural course of events, the
+hard calcareous portions of their structures to add to the growing
+reef.
+
+While the forms of coral-reefs and coral-islands are sometimes very
+remarkable and worthy of attentive study, there is no ground, it need
+scarcely be added, for the suggestion that they afford proofs of design
+on the part of the living builders, or that, in the
+words of Flinders, they constitute breastworks, defending the workshops
+from whence “infant colonies might be safely sent forth.”
+
+It was not till the beginning of the present century that travellers
+like Beechey, Chamisso, Quoy and Gaimard, Moresby, Nelson, and others,
+began to collect accurate details concerning the forms and structure of
+coral-masses, and to make such observations on the habits of
+reef-forming polyps, as might serve as a basis for safe reasoning
+concerning the origin of coral-reefs and islands. In the second volume
+of Lyell’s “Principles of Geology,” published in 1832, the final
+chapter gives an admirable summary of all that was then known on the
+subject. At that time, the ring-form of the atolls was almost
+universally regarded as a proof that they had grown up on submerged
+volcanic craters; and Lyell gave his powerful support to that theory.
+
+Charles Darwin was never tired of acknowledging his indebtedness to
+Lyell. In dedicating to his friend the second edition of his
+“Naturalist’s Voyage Round the World,” Darwin writes that he does so
+“with grateful pleasure, as an acknowledgment that the chief part of
+whatever scientific merit this journal and the other works of the
+author may possess, has been derived from studying the well-known and
+admirable ‘Principles of Geology.’”
+
+The second volume of Lyell’s “Principles” appeared after Darwin had
+left England; but it was doubtless sent on to him without delay by his
+faithful friend and correspondent, Professor Henslow. It appears to
+have reached Darwin at a most opportune moment, while, in fact, he was
+studying the striking evidences of slow and long-continued, but often
+interrupted movement on the west coast of South America. Darwin’s acute
+mind could not fail to detect the weakness of the then prevalent theory
+concerning the origin of the ring-shaped atolls—and the difficulty
+which he found in accepting the volcanic theory, as an explanation of
+the phenomena of coral-reefs, is well set forth in his book.
+
+In an interesting fragment of autobiography, Darwin has given us a very
+clear account of the way in which the leading idea of the theory of
+coral-reefs originated in his mind; he writes, “No other work of mine
+was begun in so deductive a spirit as this, for the whole theory was
+thought out on the west coast of South America, before I had seen a
+true coral-reef. I had therefore only to verify and extend my views by
+a careful examination of living reefs. But it should be observed that I
+had during the two previous years been incessantly attending to the
+effects on the
+shores of South America of the intermittent elevation of the land,
+together with the denudation and deposition of sediment. This
+necessarily led me to reflect much on the effects of subsidence, and it
+was easy to replace in imagination the continued deposition of sediment
+by the upward growth of corals. To do this was to form my theory of the
+formation of barrier-reefs and atolls.”
+
+On her homeward voyage, the _Beagle_ visited Tahiti, Australia, and
+some of the coral-islands in the Indian Ocean, and Darwin had an
+opportunity of testing and verifying the conclusion at which he had
+arrived by studying the statements of other observers.
+
+I well recollect a remarkable conversation I had with Darwin, shortly
+after the death of Lyell. With characteristic modesty, he told me that
+he never fully realised the importance of his theory of coral-reefs
+till he had an opportunity of discussing it with Lyell, shortly after
+the return of the _Beagle_. Lyell, on receiving from the lips of its
+author a sketch of the new theory, was so overcome with delight that he
+danced about and threw himself into the wildest contortions, as was his
+manner when excessively pleased. He wrote shortly afterwards to Darwin
+as follows:—“I could think of nothing for days after your lesson on
+coral-reefs, but of the tops of submerged continents. It is all true,
+but do not flatter yourself that you will be believed till you are
+growing bald like me, with hard work and vexation at the incredulity of
+the world.” On May 24th, 1837, Lyell wrote to Sir John Herschel as
+follows:—“I am very full of Darwin’s new theory of coral-islands, and
+have urged Whewell to make him read it at our next meeting. I must give
+up my volcanic crater forever, though it cost me a pang at first, for
+it accounted for so much.” Dr. Whewell was president of the Geological
+Society at the time, and on May 31st, 1837, Darwin read a paper
+entitled “On Certain Areas of Elevation and Subsidence in the Pacific
+and Indian oceans, as deduced from the Study of Coral Formations,” an
+abstract of which appeared in the second volume of the Society’s
+proceedings.
+
+It was about this time that Darwin, having settled himself in lodgings
+at Great Marlborough Street, commenced the writing of his book on
+“Coral-Reefs.” Many delays from ill-health and the interruption of
+other work, caused the progress to be slow, and his journal speaks of
+“recommencing” the subject in February 1839, shortly after his
+marriage, and again in October of the same year. In July 1841, he
+states that he began once more “after more than thirteen month’s
+interval,” and the last proof-sheet of
+the book was not corrected till May 6th, 1842. Darwin writes in his
+autobiography, “This book, though a small one, cost me twenty months of
+hard work, as I had to read every work on the islands of the Pacific,
+and to consult many charts.” The task of elaborating and writing out
+his books was, with Darwin, always a very slow and laborious one; but
+it is clear that in accomplishing the work now under consideration,
+there was a long and constant struggle with the lethargy and weakness
+resulting from the sad condition of his health at that time.
+
+Lyell’s anticipation that the theory of coral-reefs would be slow in
+meeting with general acceptance was certainly not justified by the
+actual facts. On the contrary the new book was at once received with
+general assent among both geologists and zoologists, and even attracted
+a considerable amount of attention from the general public.
+
+It was not long before the coral-reef theory of Darwin found an able
+exponent and sturdy champion in the person of the great American
+naturalist, Professor James D. Dana. Two years after the return of the
+_Beagle_ to England, the ships of the United States Exploring
+Expedition set sail upon their four years’ cruise, under the command of
+Captain Wilkes, and Dana was a member of the scientific staff. When, in
+1839, the expedition arrived at Sydney, a newspaper paragraph was found
+which gave the American naturalist the first intimation of Darwin’s new
+theory of the origin of atolls and barrier-reefs. Writing in 1872, Dana
+describes the effect produced on his mind by reading this passage:—“The
+paragraph threw a flood of light over the subject, and called forth
+feelings of peculiar satisfaction, and of gratefulness to Mr. Darwin,
+which still come up afresh whenever the subject of coral islands is
+mentioned. The Gambier Islands in the Paumotus, which gave him the key
+to the theory, I had not seen; but on reaching the Feejees, six months
+later, in 1840, I found there similar facts on a still grander scale
+and of a more diversified character, so that I was afterward enabled to
+speak of his theory as established with more positiveness than he
+himself, in his philosophic caution, had been ready to adopt. His work
+on coral-reefs appeared in 1842, when my report on the subject was
+already in manuscript. It showed that the conclusions on other points,
+which we had independently reached, were for the most part the same.
+The principal points of difference relate to the reason for the absence
+of corals from some coasts, and the evidence therefrom as to changes of
+level, and the distribution of the oceanic regions of
+elevation and subsidence—topics which a wide range of travel over the
+Pacific brought directly and constantly to my attention.”
+
+Among the Reports of the United States Exploring Expedition, two
+important works from the pen of Professor Dana made their
+appearance;—one on “Zoophytes,” which treats at length on “Corals and
+Coral-Animals,” and the other on “Coral-Reefs and Islands.” In 1872,
+Dana prepared a work of a more popular character in which some of the
+chief results of his studies are described; it bore the title of
+“Corals and Coral-Islands.” Of this work, new and enlarged editions
+appeared in 1874 and 1890 in America, while two editions were published
+in this country in 1872 and 1875. In all these works their author,
+while maintaining an independent judgment on certain matters of detail,
+warmly defends the views of Darwin on all points essential to the
+theory.
+
+Another able exponent and illustrator of the theory of coral-reefs was
+found in Professor J. B. Jukes, who accompanied H.M.S. _Fly_, as
+naturalist, during the survey of the Great Barrier-Reef—in the years
+1842 to 1846. Jukes, who was a man of great acuteness as well as
+independence of mind, concludes his account of the great Australian
+reefs with the following words:—“After seeing much of the Great
+Barrier-Reefs, and reflecting much upon them, and trying if it were
+possible by any means to evade the conclusions to which Mr. Darwin has
+come, I cannot help adding that his hypothesis is perfectly
+satisfactory to my mind, and rises beyond a mere hypothesis into the
+true theory of coral-reefs.”
+
+As the result of the clear exposition of the subject by Darwin, Lyell,
+Dana, and Jukes, the theory of coral-reefs had, by the middle of the
+present century, commanded the almost universal assent of both
+biologists and geologists. In 1859 Baron von Richthofen brought forward
+new facts in its support, by showing that the existence of the thick
+masses of dolomitic limestone in the Tyrol could be best accounted for
+if they were regarded as of coralline origin and as being formed during
+a period of long continued subsidence. The same views were maintained
+by Professor Mojsisovics in his “Dolomit-riffe von Südtirol und
+Venetien,” which appeared in 1879.
+
+The first serious note of dissent to the generally accepted theory was
+heard in 1863, when a distinguished German naturalist, Dr. Karl Semper,
+declared that his study of the Pelew Islands showed that uninterrupted
+subsidence could not have been going on in that region. Dr. Semper’s
+objections were very carefully
+considered by Mr. Darwin, and a reply to them appeared in the second
+and revised edition of his “Coral-Reefs,” which was published in 1874.
+With characteristic frankness and freedom from prejudice, Darwin
+admitted that the facts brought forward by Dr. Semper proved that in
+certain specified cases, subsidence could not have played the chief
+part in originating the peculiar forms of the coral-islands. But while
+making this admission, he firmly maintained that exceptional cases,
+like those described in the Pelew Islands, were not sufficient to
+invalidate the theory of subsidence as applied to the widely spread
+atolls, encircling reefs, and barrier-reefs of the Pacific and Indian
+Oceans. It is worthy of note that to the end of his life Darwin
+maintained a friendly correspondence with Semper concerning the points
+on which they were at issue.
+
+After the appearance of Semper’s work, Dr. J. J. Rein published an
+account of the Bermudas, in which he opposed the interpretation of the
+structure of the islands given by Nelson and other authors, and
+maintained that the facts observed in them are opposed to the views of
+Darwin. Although, so far as I am aware, Darwin had no opportunity of
+studying and considering these particular objections, it may be
+mentioned that two American geologists have since carefully re-examined
+the district—Professor W. N. Rice in 1884 and Professor A. Heilprin in
+1889—and they have independently arrived at the conclusion that Dr.
+Rein’s objections cannot be maintained.
+
+The most serious opposition to Darwin’s coral-reef theory, however, was
+that which developed itself after the return of H.M.S. _Challenger_
+from her famous voyage. Mr. John Murray, one of the staff of
+naturalists on board that vessel, propounded a new theory of
+coral-reefs, and maintained that the view that they were formed by
+subsidence was one that was no longer tenable; these objections have
+been supported by Professor Alexander Agassiz in the United States, and
+by Dr. A. Geikie, and Dr. H. B. Guppy in this country.
+
+Although Mr. Darwin did not live to bring out a third edition of his
+“Coral-Reefs,” I know from several conversations with him that he had
+given the most patient and thoughtful consideration to Mr. Murray’s
+paper on the subject. He admitted to me that had he known, when he
+wrote his work, of the abundant deposition of the remains of calcareous
+organisms on the sea floor, he might have regarded this cause as
+sufficient in a few cases to raise the summits of submerged volcanoes
+or other mountains
+to a level at which reef-forming corals can commence to flourish. But
+he did not think that the admission that under certain favourable
+conditions, atolls might be thus formed without subsidence,
+necessitated an abandonment of his theory in the case of the
+innumerable examples of the kind which stud the Indian and Pacific
+Oceans.
+
+A letter written by Darwin to Professor Alexander Agassiz in May 1881
+shows exactly the attitude which careful consideration of the subject
+led him to maintain towards the theory propounded by Mr. Murray:—“You
+will have seen,” he writes, “Mr. Murray’s views on the formation of
+atolls and barrier-reefs. Before publishing my book, I thought long
+over the same view, but only as far as ordinary marine organisms are
+concerned, for at that time little was known of the multitude of minute
+oceanic organisms. I rejected this view, as from the few dredgings made
+in the _Beagle_, in the south temperate regions, I concluded that
+shells, the smaller corals, etc., decayed and were dissolved when not
+protected by the deposition of sediment, and sediment could not
+accumulate in the open ocean. Certainly, shells, etc., were in several
+cases completely rotten, and crumbled into mud between my fingers; but
+you will know whether this is in any degree common. I have expressly
+said that a bank at the proper depth would give rise to an atoll, which
+could not be distinguished from one formed during subsidence. I can,
+however, hardly believe in the existence of as many banks (there having
+been no subsidence) as there are atolls in the great oceans, within a
+reasonable depth, on which minute oceanic organisms could have
+accumulated to the depth of many hundred feet.”
+
+Darwin’s concluding words in the same letter written within a year of
+his death, are a striking proof of the candour and openness of mind
+which he preserved so well to the end, in this as in other
+controversies.
+
+“If I am wrong, the sooner I am knocked on the head and annihilated so
+much the better. It still seems to me a marvellous thing that there
+should not have been much, and long-continued, subsidence in the beds
+of the great oceans. I wish some doubly rich millionaire would take it
+into his head to have borings made in some of the Pacific and Indian
+atolls, and bring home cores for slicing from a depth of 500 or 600
+feet.”
+
+It is noteworthy that the objections to Darwin’s theory have for the
+most part proceeded from zoologists, while those who have fully
+appreciated the geological aspect of the question, have been
+the staunchest supporters of the theory of subsidence. The desirability
+of such boring operations in atolls has been insisted upon by several
+geologists, and it may be hoped that before many years have passed
+away, Darwin’s hopes may be realised, either with or without the
+intervention of the “doubly rich millionaire.”
+
+Three years after the death of Darwin, the veteran Professor Dana
+re-entered the lists and contributed a powerful defence of the theory
+of subsidence in the form of a reply to an essay written by the ablest
+exponent of the anti-Darwinian views on this subject, Dr. A. Geikie.
+While pointing out that the Darwinian position had been to a great
+extent misunderstood by its opponents, he showed that the rival theory
+presented even greater difficulties than those which it professed to
+remove.
+
+During the last five years, the whole question of the origin of
+coral-reefs and islands has been re-opened, and a controversy has
+arisen, into which, unfortunately, acrimonious elements have been very
+unnecessarily introduced. Those who desire it, will find clear and
+impartial statements of the varied and often mutually destructive views
+put forward by different authors, in three works which have made their
+appearance within the last year,—“The Bermuda Islands,” by Professor
+Angelo Heilprin; “Corals and Coral-Islands,” new edition by Professor
+J. D. Dana; and the third edition of Darwin’s “Coral-Reefs,” with Notes
+and Appendix by Professor T. G. Bonney.
+
+Most readers will, I think, rise from the perusal of these works with
+the conviction that, while on certain points of detail it is clear
+that, through the want of knowledge concerning the action of marine
+organisms in the open ocean, Darwin was betrayed into some grave
+errors, yet the main foundations of his argument have not been
+seriously impaired by the new facts observed in the deep-sea
+researches, or by the severe criticism to which his theory has been
+subjected during the last ten years. On the other hand, I think it will
+appear that much misapprehension has been exhibited by some of Darwin’s
+critics, as to what his views and arguments really were; so that the
+reprint and wide circulation of the book in its original form is
+greatly to be desired, and cannot but be attended with advantage to all
+those who will have the fairness to acquaint themselves with Darwin’s
+views at first hand, before attempting to reply to them.
+
+JOHN W. JUDD.
+
+
+CORAL-REEFS
+
+
+INTRODUCTION
+
+The object of this volume is to describe from my own observation and
+the works of others, the principal kinds of coral-reefs, more
+especially those occurring in the open ocean, and to explain the origin
+of their peculiar forms. I do not here treat of the polypifers, which
+construct these vast works, except so far as relates to their
+distribution, and to the conditions favourable to their vigorous
+growth. Without any distinct intention to classify coral-reefs, most
+voyagers have spoken of them under the following heads:
+“lagoon-islands,” or “atolls,” “barrier” or “encircling reefs,” and
+“fringing” or “shore-reefs.” The lagoon-islands have received much the
+most attention; and it is not surprising, for every one must be struck
+with astonishment, when he first beholds one of these vast rings of
+coral-rock, often many leagues in diameter, here and there surmounted
+by a low verdant island with dazzling white shores, bathed on the
+outside by the foaming breakers of the ocean, and on the inside
+surrounding a calm expanse of water, which from reflection, is of a
+bright but pale green colour. The naturalist will feel this
+astonishment more deeply after having examined the soft and almost
+gelatinous bodies of these apparently insignificant creatures, and when
+he knows that the solid reef increases only on the outer edge, which
+day and night is lashed by the breakers of an ocean never at rest. Well
+did François Pyrard de Laval, in the year 1605, exclaim, “C’est une
+mérueille de voir chacun de ces atollons, enuironné d’un grand banc de
+pierre tout autour, n’y ayant point d’artifice humain.” The
+accompanying sketch of Whitsunday island, in the South Pacific, taken
+from Captain Beechey’s admirable “Voyage,” although excellent of its
+kind, gives but a faint idea of the singular aspect of one of these
+lagoon-islands.
+
+Whitsunday Island is of small size, and the whole circle has been
+converted into land, which is a comparatively rare circumstance. As the
+reef of a lagoon-island generally supports many separate small islands,
+the word “island,” applied to the whole, is often the cause of
+confusion; hence I have invariably used in this volume the term
+“atoll,” which is the name given to these circular groups of
+coral-islets by their
+inhabitants in the Indian Ocean, and is synonymous with “lagoon-
+island.”
+
+[Illustration: Whitsunday Island]
+
+Barrier-reefs, when encircling small islands, have been comparatively
+little noticed by voyagers; but they well deserve attention. In their
+structure they are little less marvellous than atolls, and they give a
+singular and most picturesque character to the scenery of the islands
+they surround. In the accompanying sketch, taken from the “Voyage of
+the _Coquille_,” the reef is seen from within, from one of the high
+peaks of the island of Bolabola.[1] Here, as in Whitsunday Island, the
+whole of that part of the reef which is visible is converted into land.
+This is a circumstance of rare occurrence; more usually a snow-white
+line of great breakers, with here and there an islet crowned by
+cocoa-nut trees, separates the smooth waters of the lagoon-like channel
+from the waves of the open sea. The barrier-reefs of Australia and of
+New Caledonia, owing to their enormous dimensions, have excited much
+attention: in structure and form they resemble those encircling many of
+the smaller islands in the Pacific Ocean.
+
+ [1] I have taken the liberty of simplifying the foreground, and
+ leaving out a mountainous island in the far distance.
+
+
+[Illustration: Island of Bolabola]
+
+With respect to fringing, or shore-reefs, there is little in their
+structure which needs explanation; and their name expresses their
+comparatively
+small extension. They differ from barrier-reefs in not lying so far
+from the shore, and in not having within a broad channel of deep water.
+Reefs also occur around submerged banks of sediment and of worn-down
+rock; and others are scattered quite irregularly where the sea is very
+shallow; these in most respects are allied to those of the fringing
+class, but they are of comparatively little interest.
+
+I have given a separate chapter to each of the above classes, and have
+described some one reef or island, on which I possessed most
+information, as typical; and have afterwards compared it with others of
+a like kind. Although this classification is useful from being obvious,
+and from including most of the coral-reefs existing in the open sea, it
+admits of a more fundamental division into barrier and atoll-formed
+reefs on the one hand, where there is a great apparent difficulty with
+respect to the foundation on which they must first have grown; and into
+fringing-reefs on the other, where, owing to the nature of the slope of
+the adjoining land, there is no such difficulty. The two blue tints and
+the red colour[2] on the map (Plate III), represent this main division,
+as explained in the beginning of the last chapter. In the Appendix,
+every existing coral-reef, except some on the coast of Brazil not
+included in the map, is briefly described in geographical order, as far
+as I possessed information; and any particular spot may be found by
+consulting the Index.
+
+Several theories have been advanced to explain the origin of atolls or
+lagoon-islands, but scarcely one to account for barrier-reefs. From the
+limited depths at which reef-building polypifers can flourish, taken
+into consideration with certain other circumstances, we are compelled
+to conclude, as it will be seen, that both in atolls and barrier-reefs,
+the foundation on which the coral was primarily attached, has subsided;
+and that during this downward movement, the reefs have grown upwards.
+This conclusion, it will be further seen, explains most satisfactorily
+the outline and general form of atolls and barrier-reefs, and likewise
+certain peculiarities in their structure. The distribution, also, of
+the different kinds of coral-reefs, and their position with relation to
+the areas of recent elevation, and to the points subject to volcanic
+eruptions, fully accord with this theory of their origin.[3]
+
+ [2] Replaced by numbers in this edition.
+
+
+ [3] A brief account of my views on coral formations, now published in
+ my Journal of Researches, was read May 31st, 1837, before the
+ Geological Society, and an abstract has appeared in the Proceedings.
+
+
+In the several original surveys, from which the small plans on this
+plate have been reduced, the coral-reefs are engraved in very different
+styles. For the sake of uniformity, I have adopted the style used in
+the charts of the Chagos Archipelago, published by the East Indian
+Company, from the survey by Captain Moresby and Lieutenant Powell. The
+surface of the reef, which dries at low water, is represented by a
+surface with small crosses: the coral-islets on the reef are marked by
+small linear spaces, on which a few cocoa-nut trees, out of all
+proportion too large, have been introduced for the sake of clearness.
+The entire _annular reef_, which when surrounding an open expanse of
+water, forms an “atoll,” and when surrounding one or more high islands,
+forms an encircling “barrier-reef,” has a nearly uniform structure. The
+reefs in some of the original surveys are represented merely by a
+single line with crosses, so that their breadth is not given; I have
+had such reefs engraved of the width usually attained by coral-reefs. I
+have not thought it worth while to introduce all those small and very
+numerous reefs, which occur within the lagoons of most atolls and
+within the lagoon-channels of most barrier-reefs, and which stand
+either isolated, or are attached to the shores of the reef or land. At
+Peros Banhos none of the lagoon-reefs rise to the surface of the water;
+a few of them have been introduced, and are marked by plain dotted
+circles. A few of the deepest soundings are laid down within each reef;
+they are in fathoms, of six English feet.
+
+_Plate I_—Map showing the resemblance in form between barrier
+coral-reefs surrounding mountainous islands, and atolls or lagoon
+islands.
+
+
+[Illustration: Map showing the resemblance in form.]
+
+Fig. 1—VANIKORO, situated in the western part of the South Pacific;
+taken from the survey by Captain D’Urville in the _Astrolabe_; the
+soundings on the southern side of the island, namely, from thirty to
+forty fathoms, are given from the voyage of the Chev. Dillon; the other
+soundings are laid down from the survey by D’Urville; height of the
+summit of the island is 3,032 feet. The principal small detached reefs
+within the lagoon-channel have in this instance been represented. The
+southern shore of the island is narrowly fringed by a reef: if the
+engraver had carried this reef entirely round both islands, this figure
+would have served (by leaving out in imagination the barrier-reef) as a
+good specimen of an abruptly-sided island, surrounded by a reef of the
+fringing class.
+
+Fig. 2—HOGOLEU, or ROUG, in the Caroline Archipelago; taken from the
+“Atlas of the Voyage of the _Astrolabe,_” compiled from the surveys of
+Captains Duperrey and D’Urville; the depth of the immense lagoon-like
+space within the reef is not known.
+
+Fig. 3—RAIATEA, in the Society Archipelago; from the map given in the
+quarto edition of “Cook’s First Voyage;” it is probably not accurate.
+
+Fig. 4—BOW, or HEYOU ATOLL (or lagoon-island), in the Low Archipelago,
+from the survey by Captain Beechey, R.N.; the lagoon is choked up with
+reefs, but the average greatest depth of about twenty fathoms, is given
+from the published account of the voyage.
+
+Fig. 5—BOLABOLA, in the Society Archipelago, from the survey of Captain
+Duperrey in the _ Coquille_: the soundings in this and the following
+figures have been altered from French feet to English fathoms; height
+of highest point of the island 4,026 feet.
+
+[Illustration: Map showing the resemblance in form.]
+
+Fig. 6.—MAURUA, in the Society Archipelago; from the survey by Captain
+Duperrey in the _ Coquille_: height of land about eight hundred feet.
+
+Fig. 7—POUYNIPÈTE, or SENIAVINE, in the Caroline Archipelago; from the
+survey by Admiral Lutké.
+
+Fig. 8—GAMBIER ISLANDS, in the southern part of the Low Archipelago;
+from the survey by Captain Beechey; height of highest island, 1,246
+feet; the islands are surrounded by extensive and irregular reefs; the
+reef on the southern side is submerged.
+
+Fig. 9—PEROS BANHOS ATOLL (or lagoon-island), in the Chagos group in
+the Indian Ocean; from the survey by Captain Moresby and Lieutenant
+Powell; not nearly all the small submerged reefs in the lagoon are
+represented; the annular reef on the southern side is submerged.
+
+Fig. 10—KEELING, or COCOS ATOLL (or lagoon-island), in the Indian
+Ocean; from the survey by Captain Fitzroy; the lagoon south of the
+dotted line is very shallow, and is left almost bare at low water; the
+part north of the line is choked up with irregular reefs. The annular
+reef on the north-west side is broken, and blends into a shoal
+sandbank, on which the sea breaks.
+
+
+
+
+Chapter I ATOLLS OR LAGOON-ISLANDS
+
+_Section I_—KEELING ATOLL
+
+Corals on the outer margin.—Zone of Nulliporæ.—Exterior
+reef.—Islets.—Coral-conglomerate.—Lagoon.—Calcareous sediment.—Scari
+and Holuthuriæ subsisting on corals.—Changes in the condition of the
+reefs and islets.—Probable subsidence of the atoll.—Future state of the
+lagoon.
+
+KEELING or COCOS atoll is situated in the Indian Ocean, in 12° 5′ S.,
+and longitude 90° 55′ E.: a reduced chart of it was made from the
+survey of Captain Fitzroy and the Officers of H.M.S. _Beagle_, is given
+in Plate I, Fig. 10. The greatest width of this atoll is nine miles and
+a half. Its structure is in most respects characteristic of the class
+to which it belongs, with the exception of the shallowness of the
+lagoon. The accompanying woodcut represents a vertical section,
+supposed to be drawn at low water from the outer coast across one of
+the low islets (one being taken of average dimensions) to within the
+lagoon.
+
+[Illustration: Vertical section of one of the low islets.]
+
+A.—Level of the sea at low water: where the letter A is placed, the
+depth is twenty-five fathoms, and the distance rather more than one
+hundred and fifty yards from the edge of the reef.
+B.—Outer edge of that flat part of the reef, which dries at low water:
+the edge either consists of a convex mound, as represented, or of
+rugged points, like those a little farther seaward, beneath the water.
+ C.—A flat of coral-rock, covered at high water.
+ D.—A low projecting ledge of brecciated coral-rock washed by the waves
+ at high water.
+ E.—A slope of loose fragments, reached by the sea only during gales:
+ the upper part, which is from six to twelve feet high, is clothed with
+ vegetation. The surface of the islet gently slopes to the lagoon.
+ F.—Level of the lagoon at low water.
+
+
+The section is true to the scale in a horizontal line, but it could not
+be made so in a vertical one, as the average greatest height of the
+land is only between six and twelve feet above high-water mark.
+I will describe the section, commencing with the outer margin. I must
+first observe that the reef-building polypifers, not being tidal
+animals, require to be constantly submerged or washed by the breakers.
+I was assured by Mr. Liesk, a very intelligent resident on these
+islands, as well as by some chiefs at Tahiti (Otaheite), that an
+exposure to the rays of the sun for a very short time invariably causes
+their destruction. Hence it is possible only under the most favourable
+circumstances, afforded by an unusually low tide and smooth water, to
+reach the outer margin, where the coral is alive. I succeeded only
+twice in gaining this part, and found it almost entirely composed of a
+living Porites, which forms great irregularly rounded masses (like
+those of an Astræa, but larger) from four to eight feet broad, and
+little less in thickness. These mounds are separated from each other by
+narrow crooked channels, about six feet deep, most of which intersect
+the line of reef at right angles. On the furthest mound, which I was
+able to reach by the aid of a leaping-pole, and over which the sea
+broke with some violence, although the day was quite calm and the tide
+low, the polypifers in the uppermost cells were all dead, but between
+three and four inches lower down on its side they were living, and
+formed a projecting border round the upper and dead surface. The coral
+being thus checked in its upward growth, extends laterally, and hence
+most of the masses, especially those a little further inwards, had
+broad flat dead summits. On the other hand I could see, during the
+recoil of the breakers, that a few yards further seaward, the whole
+convex surface of the Porites was alive; so that the point where we
+were standing was almost on the exact upward and shoreward limit of
+existence of those corals which form the outer margin of the reef. We
+shall presently see that there are other organic productions, fitted to
+bear a somewhat longer exposure to the air and sun.
+
+Next, but much inferior in importance to the Porites, is the _
+Millepora complanata._[1]
+
+ [1] This Millepora (Palmipora of Blainville), as well as the _M.
+ alcicornis_, possesses the singular property of stinging the skin
+ where it is delicate, as on the face and arm.
+
+It grows in thick vertical plates, intersecting each other at various
+angles, and forms an exceedingly strong honeycombed mass, which
+generally affects a circular form, the marginal plates alone being
+alive. Between these plates and in the protected crevices on the reef,
+a multitude of branching zoophytes and other productions flourish, but
+the Porites and Millepora alone seem able to resist the fury of the
+breakers on its upper and outer edge: at the depth of a few fathoms
+other kinds of stony corals live. Mr. Liesk, who was intimately
+acquainted with every part of this reef, and likewise with that of
+North Keeling atoll, assured me that these corals invariably compose
+the outer margin. The lagoon is inhabited by quite a distinct set of
+corals, generally brittle and thinly branched; but a Porites,
+apparently of the same species with that on the outside, is found
+there, although it does not seem to thrive, and certainly does not
+attain the thousandth part in bulk of the masses opposed to the
+breakers.
+
+
+The woodcut shows the form of the bottom off the reef: the water
+deepens for a space between one and two hundred yards wide, very
+gradually to twenty-five fathoms (A in section), beyond which the sides
+plunge into the unfathomable ocean at an angle of 45°.[2] To the depth
+of ten or twelve fathoms the bottom is exceedingly rugged, and seems
+formed of great masses of living coral, similar to those on the margin.
+The arming of the lead here invariably came up quite clean, but deeply
+indented, and chains and anchors which were lowered, in the hopes of
+tearing up the coral, were broken. Many small fragments, however, of _
+Millepora alcicornis_ were brought up; and on the arming from an
+eight-fathom cast, there was a perfect impression of an Astræa,
+apparently alive. I examined the rolled fragments cast on the beach
+during gales, in order further to ascertain what corals grew outside
+the reef. The fragments consisted of many kinds, of which the Porites
+already mentioned and a Madrepora, apparently the _M. corymbosa_, were
+the most abundant. As I searched in vain in the hollows on the reef and
+in the lagoon, for a living specimen of this Madrepore, I conclude that
+it is confined to a zone outside, and beneath the surface, where it
+must be very abundant. Fragments of the _Millepora alcicornis_ and of
+an Astræa were also numerous; the former is found, but not in
+proportionate numbers, in the hollows on the reef; but the Astræa I did
+not see living. Hence we may infer, that these are the kinds of coral
+which form the rugged sloping surface (represented in the woodcut by an
+uneven line), round and beneath the external margin. Between twelve and
+twenty fathoms the arming came up an equal number of times smoothed
+with sand, and indented with coral: an anchor and lead were lost at the
+respective depths of thirteen and sixteen fathoms. Out of twenty-five
+soundings taken at a greater depth than twenty fathoms, every one
+showed that the bottom was covered with sand; whereas, at a less depth
+than twelve fathoms, every sounding showed that it was exceedingly
+rugged, and free from all extraneous particles. Two soundings were
+obtained at the depth of 360 fathoms, and several between two hundred
+and three hundred fathoms. The sand brought up from these depths
+consisted of finely triturated fragments of stony zoophytes, but not,
+as far as I could distinguish, of a particle of any lamelliform genus:
+fragments of shells were rare.
+
+ [2] The soundings from which this section is laid down were taken with
+ great care by Captain Fitzroy himself. He used a bell-shaped lead,
+ having a diameter of four inches, and the armings each time were cut
+ off and brought on board for me to examine. The arming is a
+ preparation of tallow, placed in the concavity at the bottom of the
+ lead. Sand, and even small fragments of rock, will adhere to it; and
+ if the bottom be of rock it brings up an exact impression of its
+ surface.
+
+At a distance of 2,200 yards from the breakers, Captain Fitzroy found
+no bottom with a line of 7,200 feet in length; hence the submarine
+slope of this coral formation is steeper than that of any volcanic
+cone. Off the mouth of the lagoon, and likewise off the northern point
+of the atoll, where the currents act violently, the inclination, owing
+to the accumulation of sediment, is less. As the arming of the lead
+from
+all the greater depths showed a smooth sandy bottom, I at first
+concluded that the whole consisted of a vast conical pile of calcareous
+sand, but the sudden increase of depth at some points, and the
+circumstance of the line having been cut, as if rubbed, when between
+five hundred and six hundred fathoms were out, indicate the probable
+existence of submarine cliffs.
+
+On the margin of the reef, close within the line where the upper
+surface of the Porites and of the Millepora is dead, three species of
+Nullipora flourish. One grows in thin sheets, like a lichen on old
+trees; the second in stony knobs, as thick as a man’s finger, radiating
+from a common centre; and the third, which is less common, in a
+moss-like reticulation of thin, but perfectly rigid branches.[3] The
+three species occur either separately or mingled together; and they
+form by their successive growth a layer two or three feet in thickness,
+which in some cases is hard, but where formed of the lichen-like kind,
+readily yields an impression to the hammer: the surface is of a reddish
+colour. These Nulliporæ, although able to exist above the limit of true
+corals, seem to require to be bathed during the greater part of each
+tide by breaking water, for they are not found in any abundance in the
+protected hollows on the back part of the reef, where they might be
+immersed either during the whole or an equal proportional time of each
+tide. It is remarkable that organic productions of such extreme
+simplicity, for the Nulliporæ undoubtedly belong to one of the lowest
+classes of the vegetable kingdom, should be limited to a zone so
+peculiarly circumstanced. Hence the layer composed by their growth
+merely fringes the reef for a space of about twenty yards in width,
+either under the form of separate mammillated projections, where the
+outer masses of coral are separate, or, more commonly, where the corals
+are united into a solid margin, as a continuous smooth convex mound (B
+in woodcut), like an artificial breakwater. Both the mound and
+mammillated projections stand about three feet higher than any other
+part of the reef, by which term I do not include the islets, formed by
+the accumulation of rolled fragments. We shall hereafter see that other
+coral reefs are protected by a similar thick growth of Nulliporæ on the
+outer margin, the part most exposed to the breakers, and this must
+effectually aid in preserving it from being worn down.
+
+ [3] This last species is of a beautiful bright peach-blossom colour.
+ Its branches are about as thick as crow-quills; they are slightly
+ flattened and knobbed at the extremities. The extremities only are
+ alive and brightly coloured. The two other species are of a dirty
+ purplish-white. The second species is extremely hard; its short
+ knob-like branches are cylindrical, and do not grow thicker at their
+ extremities.
+
+The woodcut represents a section across one of the islets on the reef,
+but if all that part which is above the level of C were removed, the
+section would be that of a simple reef, as it occurs where no islet has
+been formed. It is this reef which essentially forms the atoll. It is a
+ring, enclosing the lagoon on all sides except at the northern end,
+where there are two open spaces, through one of which ships can enter.
+The reef varies in width from two hundred and fifty to five
+hundred yards, its surface is level, or very slightly inclined towards
+the lagoon, and at high tide the sea breaks entirely over it: the water
+at low tide thrown by the breakers on the reef, is carried by the many
+narrow and shoal gullies or channels on its surface, into the lagoon: a
+return stream sets out of the lagoon through the main entrance. The
+most frequent coral in the hollows on the reef is _Pocillopora
+verrucosa_, which grows in short sinuous plates, or branches, and when
+alive is of a beautiful pale lake-red: a Madrepora, closely allied or
+identical with _M. pocillifera_, is also common. As soon as an islet is
+formed, and the waves are prevented breaking entirely over the reef,
+the channels and hollows in it become filled up with cemented
+fragments, and its surface is converted into a hard smooth floor (C of
+woodcut), like an artificial one of freestone. This flat surface varies
+in width from one hundred to two hundred, or even three hundred yards,
+and is strewed with a few large fragments of coral torn up during
+gales: it is uncovered only at low water. I could with difficulty, and
+only by the aid of a chisel, procure chips of rock from its surface,
+and therefore could not ascertain how much of it is formed by the
+aggregation of detritus, and how much by the outward growth of mounds
+of corals, similar to those now living on the margin. Nothing can be
+more singular than the appearance at low tide of this “flat” of naked
+stone, especially where it is externally bounded by the smooth convex
+mound of Nulliporæ, appearing like a breakwater built to resist the
+waves, which are constantly throwing over it sheets of foaming water.
+The characteristic appearance of this “flat” is shown in the foregoing
+woodcut of Whitsunday atoll.
+
+The islets on the reef are first formed between two hundred and three
+hundred yards from its outer edge, through the accumulation of a pile
+of fragments, thrown together by some unusually strong gale. Their
+ordinary width is under a quarter of a mile, and their length varies
+from a few yards to several miles. Those on the south-east and windward
+side of the atoll, increase solely by the addition of fragments on
+their outer side; hence the loose blocks of coral, of which their
+surface is composed, as well as the shells mingled with them, almost
+exclusively consist of those kinds which live on the outer coast. The
+highest part of the islets (excepting hillocks of blown sand, some of
+which are thirty feet high), is close to the outer beach (E of the
+woodcut), and averages from six to ten feet above ordinary high-water
+mark. From the outer beach the surface slopes gently to the shores of
+the lagoon, which no doubt has been caused by the breakers the further
+they have rolled over the reef, having had less power to throw up
+fragments. The little waves of the lagoon heap up sand and fragments of
+thinly-branched corals on the inner side of the islets on the leeward
+side of the atoll; and these islets are broader than those to windward,
+some being even eight hundred yards in width; but the land thus added
+is very low. The fragments beneath the surface are cemented into a
+solid mass, which is exposed as a ledge (D of the woodcut), projecting
+some yards in front of the outer shore and from two to four feet high.
+This ledge is just reached by the waves at ordinary high-water:
+it extends in front of all the islets, and everywhere has a water-worn
+and scooped appearance. The fragments of coral which are occasionally
+cast on the “flat” are during gales of unusual violence swept together
+on the beach, where the waves each day at high-water tend to remove and
+gradually wear them down; but the lower fragments having become firmly
+cemented together by the percolation of calcareous matter, resist the
+daily tides longer, and hence project as a ledge. The cemented mass is
+generally of a white colour, but in some few parts reddish from
+ferruginous matter; it is very hard, and is sonorous under the hammer;
+it is obscurely divided by seams, dipping at a small angle seaward; it
+consists of fragments of the corals which grow on the outer margin,
+some quite and others partially rounded, some small and others between
+two and three feet across; and of masses of previously formed
+conglomerate, torn up, rounded, and re-cemented; or it consists of a
+calcareous sandstone, entirely composed of rounded particles, generally
+almost blended together, of shells, corals, the spines of echini, and
+other such organic bodies; rocks, of this latter kind, occur on many
+shores, where there are no coral reefs. The structure of the coral in
+the conglomerate has generally been much obscured by the infiltration
+of spathose calcareous matter; and I collected a very interesting
+series, beginning with fragments of unaltered coral, and ending with
+others, where it was impossible to discover with the naked eye any
+trace of organic structure. In some specimens I was unable, even with
+the aid of a lens, and by wetting them, to distinguish the boundaries
+of the altered coral and spathose limestone. Many even of the blocks of
+coral lying loose on the beach, had their central parts altered and
+infiltrated.
+
+The lagoon alone remains to be described; it is much shallower than
+that of most atolls of considerable size. The southern part is almost
+filled up with banks of mud and fields of coral, both dead and alive,
+but there are considerable spaces, between three and four fathoms, and
+smaller basins, from eight to ten fathoms deep. Probably about half its
+area consists of sediment, and half of coral-reefs. The corals
+composing these reefs have a very different aspect from those on the
+outside; they are very numerous in kind, and most of them are thinly
+branched. Meandrina, however, lives in the lagoon, and great rounded
+masses of this coral are numerous, lying quite or almost loose on the
+bottom. The other commonest kinds consist of three closely allied
+species of true Madrepora in thin branches; of _Seriatapora subulata_;
+two species of Porites[4] with cylindrical branches, one of which forms
+circular clumps, with the exterior branches only alive; and lastly, a
+coral something like an Explanaria, but with stars on both surfaces,
+growing in thin, brittle, stony, foliaceous expansions, especially in
+the deeper basins of the lagoon. The reefs on which
+these corals grow are very irregular in form, are full of cavities, and
+have not a solid flat surface of dead rock, like that surrounding the
+lagoon; nor can they be nearly so hard, for the inhabitants made with
+crowbars a channel of considerable length through these reefs, in which
+a schooner, built on the S.E. islet, was floated out. It is a very
+interesting circumstance, pointed out to us by Mr. Liesk, that this
+channel, although made less than ten years before our visit, was then,
+as we saw, almost choked up with living coral, so that fresh
+excavations would be absolutely necessary to allow another vessel to
+pass through it.
+
+ [4] This Porites has somewhat the habit of _P. clavaria_, but the
+ branches are not knobbed at their ends. When alive it is of a yellow
+ colour, but after having been washed in fresh water and placed to dry,
+ a jet-black slimy substance exuded from the entire surface, so that
+ the specimen now appears as if it had been dipped in ink.
+
+The sediment from the deepest parts in the lagoon, when wet, appeared
+chalky, but when dry, like very fine sand. Large soft banks of similar,
+but even finer grained mud, occur on the S.E. shore of the lagoon,
+affording a thick growth of a Fucus, on which turtle feed: this mud,
+although discoloured by vegetable matter, appears from its entire
+solution in acids to be purely calcareous. I have seen in the Museum of
+the Geological Society, a similar but more remarkable substance,
+brought by Lieutenant Nelson from the reefs of Bermuda, which, when
+shown to several experienced geologists, was mistaken by them for true
+chalk. On the outside of the reef much sediment must be formed by the
+action of the surf on the rolled fragments of coral; but in the calm
+waters of the lagoon, this can take place only in a small degree. There
+are, however, other and unexpected agents at work here: large shoals of
+two species of Scarus, one inhabiting the surf outside the reef and the
+other the lagoon, subsist entirely, as I was assured by Mr. Liesk, the
+intelligent resident before referred to, by browsing on the living
+polypifers. I opened several of these fish, which are very numerous and
+of considerable size, and I found their intestines distended by small
+pieces of coral, and finely ground calcareous matter. This must daily
+pass from them as the finest sediment; much also must be produced by
+the infinitely numerous vermiform and molluscous animals, which make
+cavities in almost every block of coral. Dr. J. Allan, of Forres, who
+has enjoyed the best means of observation, informs me in a letter that
+the Holothuriæ (a family of Radiata) subsist on living coral; and the
+singular structure of bone within the anterior extremity of their
+bodies, certainly appears well adapted for this purpose. The number of
+the species of Holothuria, and of the individuals which swarm on every
+part of these coral-reefs, is extraordinarily great; and many shiploads
+are annually freighted, as is well-known, for China with the trepang,
+which is a species of this genus. The amount of coral yearly consumed,
+and ground down into the finest mud, by these several creatures, and
+probably by many other kinds, must be immense. These facts are,
+however, of more importance in another point of view, as showing us
+that there are living checks to the growth of coral-reefs, and that the
+almost universal law of “consumed and be consumed,” holds good even
+with the polypifers forming those massive bulwarks, which are able to
+withstand the force of the open ocean.
+
+Considering that Keeling atoll, like other coral formations, has been
+entirely formed by the growth of organic beings, and the accumulation
+of their detritus, one is naturally led to inquire how long it has
+continued, and how long it is likely to continue, in its present state.
+Mr. Liesk informed me that he had seen an old chart in which the
+present long island on the S.E. side was divided by several channels
+into as many islets; and he assures me that the channels can still be
+distinguished by the smaller size of the trees on them. On several
+islets, also, I observed that only young cocoa-nut trees were growing
+on the extremities; and that older and taller trees rose in regular
+succession behind them; which shows that these islets have very lately
+increased in length. In the upper and south-eastern part of the lagoon,
+I was much surprised by finding an irregular field of at least a mile
+square of branching corals, still upright, but entirely dead. They
+consisted of the species already mentioned; they were of a brown
+colour, and so rotten, that in trying to stand on them I sank halfway
+up the leg, as if through decayed brushwood. The tops of the branches
+were barely covered by water at the time of lowest tide. Several facts
+having led me to disbelieve in any elevation of the whole atoll, I was
+at first unable to imagine what cause could have killed so large a
+field of coral. Upon reflection, however, it appeared to me that the
+closing up of the above-mentioned channels would be a sufficient cause;
+for before this, a strong breeze by forcing water through them into the
+head of the lagoon, would tend to raise its level. But now this cannot
+happen, and the inhabitants observe that the tide rises to a less
+height, during a high S.E. wind, at the head than at the mouth of the
+lagoon. The corals, which, under the former condition of things, had
+attained the utmost possible limit of upward growth, would thus
+occasionally be exposed for a short time to the sun, and be killed.
+
+Besides the increase of dry land, indicated by the foregoing facts, the
+exterior solid reef appears to have grown outwards. On the western side
+of the atoll, the “flat” lying between the margin of the reef and the
+beach, is very wide; and in front of the regular beach with its
+conglomerate basis, there is, in most parts, a bed of sand and loose
+fragments with trees growing out of it, which apparently is not reached
+even by the spray at high water. It is evident some change has taken
+place since the waves formed the inner beach; that they formerly beat
+against it with violence was evident, from a remarkably thick and
+water-worn point of conglomerate at one spot, now protected by
+vegetation and a bank of sand; that they beat against it in the same
+peculiar manner in which the swell from windward now obliquely curls
+round the margin of the reef, was evident from the conglomerate having
+been worn into a point projecting from the beach in a similarly oblique
+manner. This retreat in the line of action of the breakers might
+result, either from the surface of the reef in front of the islets
+having been submerged at one time, and afterward having grown upwards,
+or from the mounds of coral on the margin having continued to grow
+outwards. That an outward growth of this part is in process, can hardly
+be doubted from the fact already mentioned of the mounds of Porites
+with their summits apparently lately killed, and their sides only three
+or four inches lower down thickened by a fresh layer of living coral.
+But there is a difficulty on this supposition which I must not pass
+over. If the
+whole, or a large part of the “flat,” had been formed by the outward
+growth of the margin, each successive margin would naturally have been
+coated by the Nulliporæ, and so much of the surface would have been of
+equal height with the existing zone of living Nulliporæ: this is not
+the case, as may be seen in the woodcut. It is, however, evident from
+the abraded state of the “flat,” with its original inequalities filled
+up, that its surface has been much modified; and it is possible that
+the hinder portions of the zone of Nulliporæ, perishing as the reef
+grows outwards, might be worn down by the surf. If this has not taken
+place, the reef can in no part have increased outwards in breadth since
+its formation, or at least since the Nulliporæ formed the convex mound
+on its margin; for the zone thus formed, and which stands between two
+and three feet above the other parts of the reef, is nowhere much above
+twenty yards in width.
+
+Thus far we have considered facts, which indicate, with more or less
+probability, the increase of the atoll in its different parts: there
+are others having an opposite tendency. On the south-east side,
+Lieutenant Sulivan, to whose kindness I am indebted for many
+interesting observations, found the conglomerate projecting on the reef
+nearly fifty yards in front of the beach: we may infer from what we see
+in all other parts of the atoll, that the conglomerate was not
+originally so much exposed, but formed the base of an islet, the front
+and upper part of which has since been swept away. The degree to which
+the conglomerate, round nearly the whole atoll, has been scooped,
+broken up, and the fragments cast on the beach, is certainly very
+surprising, even on the view that it is the office of occasional gales
+to pile up fragments, and of the daily tides to wear them away. On the
+western side, also, of the atoll, where I have described a bed of sand
+and fragments with trees growing out of it, in front of an old beach,
+it struck both Lieutenant Sulivan and myself, from the manner in which
+the trees were being washed down, that the surf had lately recommenced
+an attack on this line of coast. Appearances indicating a slight
+encroachment of the water on the land, are plainer within the lagoon: I
+noticed in several places, both on its windward and leeward shores, old
+cocoa-nut trees falling with their roots undermined, and the rotten
+stumps of others on the beach, where the inhabitants assured us the
+cocoa-nut could not now grow. Captain Fitzroy pointed out to me, near
+the settlement, the foundation posts of a shed, now washed by every
+tide, but which the inhabitants stated, had seven years before stood
+above high watermark. In the calm waters of the lagoon, directly
+connected with a great, and therefore stable ocean, it seems very
+improbable that a change in the currents, sufficiently great to cause
+the water to eat into the land on all sides, should have taken place
+within a limited period. From these considerations I inferred, that
+probably the atoll had lately subsided to a small amount; and this
+inference was strengthened by the circumstance, that in 1834, two years
+before our visit, the island had been shaken by a severe earthquake,
+and by two slighter ones during the ten previous years. If, during
+these subterranean disturbances, the atoll did subside, the downward
+movement must have been very small, as we must
+conclude from the fields of dead coral still lipping the surface of the
+lagoon, and from the breakers on the western shore not having yet
+regained the line of their former action. The subsidence must, also,
+have been preceded by a long period of rest, during which the islets
+extended to their present size, and the living margin of the reef grew
+either upwards, or as I believe outwards, to its present distance from
+the beach.
+
+Whether this view be correct or not, the above facts are worthy of
+attention, as showing how severe a struggle is in progress on these low
+coral formations between the two nicely balanced powers of land and
+water. With respect to the future state of Keeling atoll, if left
+undisturbed, we can see that the islets may still extend in length; but
+as they cannot resist the surf until broken by rolling over a wide
+space, their increase in breadth must depend on the increasing breadth
+of the reef; and this must be limited by the steepness of the submarine
+flanks, which can be added to only by sediment derived from the wear
+and tear of the coral. From the rapid growth of the coral in the
+channel cut for the schooner, and from the several agents at work in
+producing fine sediment, it might be thought that the lagoon would
+necessarily become quickly filled up. Some of this sediment, however,
+is transported into the open sea, as appears from the soundings off the
+mouth of the lagoon, instead of being deposited within it. The
+deposition, moreover, of sediment, checks the growth of coral-reefs, so
+that these two agencies cannot act together with full effect in filling
+it up. We know so little of the habits of the many different species of
+corals, which form the lagoon-reefs, that we have no more reasons for
+supposing that their whole surface would grow up as quickly as the
+coral did in the schooner-channel, than for supposing that the whole
+surface of a peat-moss would increase as quickly as parts are known to
+do in holes, where the peat has been cut away. These agencies,
+nevertheless, tend to fill up the lagoon; but in proportion as it
+becomes shallower, so must the polypifers be subject to many injurious
+agencies, such as impure water and loss of food. For instance, Mr.
+Liesk informed me, that some years before our visit unusually heavy
+rain killed nearly all the fish in the lagoon, and probably the same
+cause would likewise injure the corals. The reefs also, it must be
+remembered, cannot possibly rise above the level of the lowest
+spring-tide, so that the final conversion of the lagoon into land must
+be due to the accumulation of sediment; and in the midst of the clear
+water of the ocean, and with no surrounding high land, this process
+must be exceedingly slow.
+
+_Section II_—GENERAL DESCRIPTION OF ATOLLS.
+
+General form and size of atolls, their reefs and islets.—External
+slope.—Zone of Nulliporæ.—Conglomerate.—Depth of
+lagoons.—Sediment.—Reefs submerged wholly or in part.—Breaches in the
+reef.—Ledge-formed shores round certain lagoons.—Conversion of lagoons
+into land.
+
+I will here give a sketch of the general form and structure of the many
+atolls and atoll-formed reefs which occur in the Pacific and Indian
+Oceans, comparing them with Keeling atoll. The Maldiva atolls
+and the Great Chagos Bank differ in so many respects, that I shall
+devote to them, besides occasional references, a third section of this
+chapter. Keeling atoll may be considered as of moderate dimensions and
+of regular form. Of the thirty-two islands surveyed by Captain Beechey
+in the Low Archipelago, the longest was found to be thirty miles, and
+the shortest less than a mile; but Vliegen atoll, situated in another
+part of the same group, appears to be sixty miles long and twenty
+broad. Most of the atolls in this group are of an elongated form; thus
+Bow Island is thirty miles in length, and on an average only six in
+width (See Fig. 4, Plate I), and Clermont Tonnere has nearly the same
+proportions. In the Marshall Archipelago (the Ralick and Radack group
+of Kotzebue) several of the atolls are more than thirty miles in
+length, and Rimsky Korsacoff is fifty-four long, and twenty wide, at
+the broadest part of its irregular outline. Most of the atolls in the
+Maldiva Archipelago are of great size, one of them (which, however,
+bears a double name) measured in a medial and slightly curved line, is
+no less than eighty-eight geographical miles long, its greatest width
+being under twenty, and its least only nine and a half miles. Some
+atolls have spurs projecting from them; and in the Marshall group there
+are atolls united together by linear reefs, for instance Menchikoff
+Island (See Fig. 3, Plate II), which is sixty miles in length, and
+consists of three loops tied together. In far the greater number of
+cases an atoll consists of a simple elongated ring, with its outline
+moderately regular.
+
+The average width of the annular wreath may be taken as about a quarter
+of a mile. Captain Beechey[5] says that in the atolls of the Low
+Archipelago it exceeded in no instance half a mile. The description
+given of the structure and proportional dimensions of the reef and
+islets of Keeling atoll, appears to apply perfectly to nearly all the
+atolls in the Pacific and Indian Oceans. The islets are first formed
+some way back either on the projecting points of the reef, especially
+if its form be angular, or on the sides of the main entrances into the
+lagoon—that is in both cases, on points where the breakers can act
+during gales of wind in somewhat different directions, so that the
+matter thrown up from one side may accumulate against that before
+thrown up from another. In Lutké’s chart of the Caroline atolls, we see
+many instances of the former case; and the occurrence of islets, as if
+placed for beacons, on the points where there is a gateway or breach
+through the reef, has been noticed by several authors. There are some
+atoll-formed reefs, rising to the surface of the sea and partly dry at
+low water, on which from some cause islets have never been formed; and
+there are others on which they have been formed, but have subsequently
+been worn away. In atolls of small dimensions the islets frequently
+become united into a single horse-shoe or ring-formed strip; but Diego
+Garcia, although an atoll of considerable size, being thirteen miles
+and a half in length, has its lagoon entirely surrounded, except at the
+northern end, by a belt of land, on an average a third of a mile in
+width. To show how small the total area of the annular reef and the
+land is in islands of this class,
+I may quote a remark from the voyage of Lutké, namely, that if the
+forty-three rings, or atolls, in the Caroline Archipelago, were put one
+within another, and over a steeple in the centre of St. Petersburg, the
+whole world would not cover that city and its suburbs.
+
+ [5] Beechey’s “Voyage to the Pacific and Beering’s Straits,” chapter
+ viii.
+
+
+The form of the bottom off Keeling atoll, which gradually slopes to
+about twenty fathoms at the distance of between one and two hundred
+yards from the edge of the reef, and then plunges at an angle of 45°
+into unfathomable depths, is exactly the same[6] with that of the
+sections of the atolls in the Low Archipelago given by Captain Beechey.
+The nature, however, of the bottom seems to differ, for this officer[7]
+informs me that all the soundings, even the deepest, were on coral, but
+he does not know whether dead or alive. The slope round Christmas atoll
+(Lat. 1° 4′ N., 157° 45′ W.), described by Cook,[8] is considerably
+less, at about half a mile from the edge of the reef, the average depth
+was about fourteen fathoms on a fine sandy bottom, and at a mile, only
+between twenty and forty fathoms. It has no doubt been owing to this
+gentle slope, that the strip of land surrounding its lagoon, has
+increased in one part to the extraordinary width of three miles; it is
+formed of successive ridges of broken shells and corals, like those on
+the beach. I know of no other instance of such width in the reef of an
+atoll; but Mr. F. D. Bennett informs me that the inclination of the
+bottom round Caroline atoll in the Pacific, is like that off Christmas
+Island, very gentle. Off the Maldiva and Chagos atolls, the inclination
+is much more abrupt; thus at Heawandoo Pholo, Lieutenant Powell[9]
+found fifty and sixty fathoms close to the edge of the reef, and at 300
+yards distance there was no bottom with a 300-yard line. Captain
+Moresby informs me, that at 100 fathoms from the mouth of the lagoon of
+Diego Garcia, he found no bottom with 150 fathoms; this is the more
+remarkable, as the slope is generally less abrupt in front of channels
+through a reef, owing to the accumulation of sediment. At Egmont
+Island, also, at 150 fathoms from the reef, soundings were struck with
+150 fathoms. Lastly, at Cardoo atoll, only sixty yards from the reef,
+no bottom was obtained, as I am informed by Captain Moresby, with a
+line of 200 fathoms! The currents run with great force round these
+atolls, and where they are strongest, the inclination appears to be
+most abrupt. I am informed by the same authority, that wherever
+soundings were obtained off these islands, the bottom was invariably
+sandy: nor was there any reason to suspect the existence of submarine
+cliffs, as there was at Keeling Island.[10] Here then occurs a
+difficulty; can sand accumulate on a slope, which, in some cases,
+appears to exceed fifty-five degrees? It must be observed, that I speak
+of slopes where soundings were obtained, and not of such cases, as that
+of Cardoo, where the nature of the bottom is unknown, and where its
+inclination must be nearly vertical. M. Elie de Beaumont[11] has
+argued, and there is no higher authority on this subject, from the
+inclination at which snow slides down in avalanches, that a bed of sand
+or mud cannot be formed at a greater angle than thirty degrees.
+Considering the number of soundings on sand, obtained round the Maldiva
+and Chagos atolls, which appears to indicate a greater angle, and the
+extreme abruptness of the sand-banks in the West Indies, as will be
+mentioned in the Appendix, I must conclude that the adhesive property
+of wet sand counteracts its gravity, in a much greater ratio than has
+been allowed for by M. Elie de Beaumont. From the facility with which
+calcareous sand becomes agglutinated, it is not necessary to suppose
+that the bed of loose sand is thick.
+
+ [6] The form of the bottom round the Marshall atolls in the Northern
+ Pacific is probably similar: Kotzebue (“First Voyage,” vol. ii, p. 16)
+ says: “We had at a small distance from the reef, forty fathoms depth,
+ which increased a little further so much that we could find no
+ bottom.”
+
+
+ [7] I must be permitted to express my obligation to Captain Beechey,
+ for the very kind manner in which he has given me information on
+ several points, and to own the great assistance I have derived from
+ his excellent published work.
+
+
+ [8] Cook’s “Third Voyage,” vol. ii, chap. 10.
+
+
+ [9] This fact is taken from a MS. account of these groups lent me by
+ Captain Moresby. See also Captain Moresby’s paper on the Maldiva
+ atolls in the _Geographical Journal_, vol. v, p. 401.
+
+
+ [10] Off some of the islands in the Low Archipelago the bottom appears
+ to descend by ledges. Off Elizabeth Island, which, however, consists
+ of raised coral, Captain Beechey (page 45, 4to ed.) describes three
+ ledges: the first had an easy slope from the beach to a distance of
+ about fifty yards: the second extended two hundred yards with
+ twenty-five fathoms on it, and then ended abruptly, like the first;
+ and immediately beyond this there was no bottom with two hundred
+ fathoms.
+
+
+ [11] “Memoires pour servir à une description Geolog. de France,” tome
+ iv, p. 216.
+
+
+Captain Beechey has observed, that the submarine slope is much less at
+the extremities of the more elongated atolls in the Low Archipelago,
+than at their sides; in speaking of Ducie’s Island he says[12] the
+buttress, as it may be called, which “has the most powerful enemy (the
+S.W. swell) to oppose, is carried out much further, and with less
+abruptness than the other.” In some cases, the less inclination of a
+certain part of the external slope, for instance of the northern
+extremities of the two Keeling atolls, is caused by a prevailing
+current which there accumulates a bed of sand. Where the water is
+perfectly tranquil, as within a lagoon, the reefs generally grow up
+perpendicularly, and sometimes even overhang their bases; on the other
+hand, on the leeward side of Mauritius, where the water is generally
+tranquil, although not invariably so, the reef is very gently inclined.
+Hence it appears that the exterior angle varies much; nevertheless in
+the close similarity in form between the sections of Keeling atoll and
+of the atolls in the Low Archipelago, in the general steepness of the
+reefs of the Maldiva and Chagos atolls, and in the perpendicularity of
+those rising out of water always tranquil, we may discern the effects
+of uniform laws; but from the complex action of the surf and currents,
+on the growing powers of the coral and on the deposition of sediment,
+we can by no means follow out all the results.
+
+ [12] Beechey’s “Voyage,” 4to ed., p. 44.
+
+
+Where islets have been formed on the reef, that part which I have
+sometimes called the “flat” and which is partly dry at low water,
+appears similar in every atoll. In the Marshall group in the North
+Pacific, it may be inferred from Chamisso’s description, that the reef,
+where islets have not been formed on it, slopes gently from the
+external margin to the shores of the lagoon; Flinders states that the
+Australian barrier has a similar inclination inwards, and I have no
+doubt it is of general occurrence, although, according to Ehrenberg,
+the reefs of the Red Sea offer an exception. Chamisso observes that
+“the red colour of the reef (at the Marshall atolls) under the breakers
+is caused by a Nullipora, which covers the stone _wherever the waves
+beat_; and, under favourable circumstances, assumes a stalactical
+form,”—a description perfectly applicable to the margin of Keeling
+atoll.[13] Although Chamisso does not state that the masses of
+Nulliporæ form points or a mound, higher than the flat, yet I believe
+that this is the case; for Kotzebue,[14] in another part, speaks of the
+rocks on the edge of the reef “as visible for about two feet at low
+water,” and these rocks we may feel quite certain are not formed of
+true coral,[15] Whether a smooth convex mound of Nulliporæ, like that
+which appears as if artificially constructed to protect the margin of
+Keeling Island, is of frequent occurrence round atolls, I know not; but
+we shall presently meet with it, under precisely the same form, on the
+outer edge of the “barrier-reefs” which encircle the Society Islands.
+
+ [13] Kotzebue’s “First Voyage,” vol. iii, p. 142. Near Porto Praya, in
+ the Cape de Verde Islands, some basaltic rocks, lashed by no
+ inconsiderable surf, were completely enveloped with a layer of
+ Nulliporæ. The entire surface over many square inches, was coloured of
+ a peach-blossomed red; the layer, however, was of no greater thickness
+ than paper. Another kind, in the form of projecting knobs, grew in the
+ same situation. These Nulliporæ are closely related to those described
+ on the coral-reefs, but I believe are of different species.
+
+
+ [14] Kotzebue’s “First Voyage,” vol. ii, p. 16. Lieutenant Nelson, in
+ his excellent memoir in the Geological Transactions (vol. ii, p. 105),
+ alludes to the rocky points mentioned by Kotzebue, and infers that
+ they consist of Serpulæ, which compose incrusting masses on the reefs
+ of Bermudas, as they likewise do on a sandstone bar off the coast of
+ Brazil (which I have described in _London Phil. Journal,_ October
+ 1841). These masses of Serpulæ hold the same position, relatively to
+ the action of the sea, with the Nulliporæ on the coral-reefs in the
+ Indian and Pacific Oceans.
+
+
+ [15] Captain Moresby, in his valuable paper “on the Northern atolls of
+ Maldivas” (_Geographical Journal_, vol. v), says that the edges of the
+ reefs there stand above water at low spring-tides.
+
+
+There appears to be scarcely a feature in the structure of Keeling
+reef, which is not of common, if not of universal occurrence, in other
+atolls. Thus Chamisso describes[16] a layer of coarse conglomerate,
+outside the islets round the Marshall atolls which “appears on its
+upper surface uneven and eaten away.” From drawings, with appended
+remarks, of Diego Garcia in the Chagos group and of several of the
+Maldiva atolls, shown me by Captain Moresby,[17] it is evident that
+their outer coasts are subject to the same round of decay and
+renovation as those of Keeling atoll. From the description of the
+atolls in the Low Archipelago, given in Captain Beechey’s “Voyage,” it
+is not apparent that any conglomerate coral-rock was there observed.
+
+ [16] Kotzebue’s “First Voyage,” vol. iii, p. 144.
+
+
+ [17] See also Moresby on the Northern atolls of the Maldivas,
+ _Geographical Journal_, vol v, p. 400.
+
+
+The lagoon in Keeling atoll is shallow; in the atolls of the Low
+Archipelago the depth varies from 20 to 38 fathoms, and in the Marshall
+Group, according to Chamisso, from 30 to 35; in the Caroline atolls it
+is only a little less. Within the Maldiva atolls there are large spaces
+with 45 fathoms, and some soundings are laid down of 49 fathoms. The
+greater part of the bottom in most lagoons, is formed of sediment;
+large spaces have exactly the same depth, or the depth varies so
+insensibly, that it is evident that no other means, excepting aqueous
+deposition, could have leveled the surface so equally. In the Maldiva
+atolls this is very conspicuous, and likewise in some of the Caroline
+and Marshall Islands. In the former large spaces consist of sand and
+_soft clay_; and Kotzebue speaks of clay having been found within one
+of the Marshall atolls. No doubt this clay is calcareous mud, similar
+to that at Keeling Island, and to that at Bermuda already referred to,
+as undistinguishable from disintegrated chalk, and which Lieutenant
+Nelson says is called there pipe-clay.[18]
+
+ [18] I may here observe that on the coast of Brazil, where there is
+ much coral, the soundings near the land are described by Admiral
+ Roussin, in the _Pilote du Brésil_, as siliceous sand, mingled with
+ much finely comminuted particles of shells and coral. Further in the
+ offing, for a space of 1,300 miles along the coast, from the Abrolhos
+ Islands to Maranham, the bottom in many places is composed of “tuf
+ blanc, mêlé ou formé de madrépores broyés.” This white substance,
+ probably, is analogous to that which occurs within the above-mentioned
+ lagoons; it is sometimes, according to Roussin, firm, and he compares
+ it to mortar.
+
+
+Where the waves act with unequal force on the two sides of an atoll,
+the islets appear to be first formed, and are generally of greater
+continuity on the more exposed shore. The islets, also, which are
+placed to leeward, are in most parts of the Pacific liable to be
+occasionally swept entirely away by gales, equalling hurricanes in
+violence, which blow in an opposite direction to the ordinary
+trade-wind. The absence of the islets on the leeward side of atolls, or
+when present their lesser dimensions compared with those to windward,
+is a comparatively unimportant fact; but in several instances the reef
+itself on the leeward side, retaining its usual defined outline, does
+not rise to the surface by several fathoms. This is the case with the
+southern side of Peros Banhos (Plate 1, Fig. 9) in the Chagos group,
+with Mourileu atoll,[19] in the Caroline Archipelago, and with the
+barrier-reef (Plate I, Fig. 8) of the Gambier Islands. I allude to the
+latter reef, although belonging to
+another class, because Captain Beechey was first led by it to observe
+the peculiarity in the question. At Peros Banhos the submerged part is
+nine miles in length, and lies at an average depth of about five
+fathoms; its surface is nearly level, and consists of hard stone, with
+a thin covering of loose sand. There is scarcely any living coral on
+it, even on the outer margin, as I have been particularly assured by
+Captain Moresby; it is, in fact, a wall of dead coral-rock, having the
+same width and transverse section with the reef in its ordinary state,
+of which it is a continuous portion. The living and perfect parts
+terminate abruptly, and abut on the submerged portions, in the same
+manner as on the sides of an ordinary passage through the reef. The
+reef to leeward in other cases is nearly or quite obliterated, and one
+side of the lagoon is left open; for instance, at Oulleay (Caroline
+Archipelago), where a crescent-formed reef is fronted by an irregular
+bank, on which the other half of the annular reef probably once stood.
+At Namonouito, in the same Archipelago, both these modifications of the
+reef concur; it consists of a great flat bank, with from twenty to
+twenty-five fathoms water on it; for a length of more than forty miles
+on its southern side it is open and without any reef, whilst on the
+other sides it is bounded by a reef, in parts rising to the surface and
+perfectly characterised, in parts lying some fathoms submerged. In the
+Chagos group there are annular reefs, entirely submerged, which have
+the same structure as the submerged and defined portions just
+described. The Speaker’s Bank offers an excellent example of this
+structure; its central expanse, which is about twenty-two fathoms deep,
+is twenty-four miles across; the external rim is of the usual width of
+annular reefs, and is well-defined; it lies between six and eight
+fathoms beneath the surface, and at the same depth there are scattered
+knolls in the lagoon. Captain Moresby believes the rim consists of dead
+rock, thinly covered with sand, and he is certain this is the case with
+the external rim of the Great Chagos Bank, which is also essentially a
+submerged atoll. In both these cases, as in the submerged portion of
+the reef at Peros Banhos, Captain Moresby feels sure that the quantity
+of living coral, even on the outer edge overhanging the deep-sea water,
+is quite insignificant. Lastly, in several parts of the Pacific and
+Indian Oceans there are banks, lying at greater depths than in the
+cases just mentioned, of the same form and size with the neighbouring
+atolls, but with their atoll-like structure wholly obliterated. It
+appears from the survey of Freycinet, that there are banks of this kind
+in the Caroline Archipelago, and, as is reported, in the Low
+Archipelago. When we discuss the origin of the different classes of
+coral formations, we shall see that the submerged state of the whole of
+some atoll-formed reefs, and of portions of others, generally but not
+invariably on the leeward side, and the existence of more deeply
+submerged banks now possessing little or no signs of their original
+atoll-like structure, are probably the effects of a uniform
+cause,—namely, the death of the coral, during the subsidence of the
+area, in which the atolls or banks are situated.
+
+ [19] Frederick Lutké’s “Voyage autour du Monde,” vol. ii, p. 291. See
+ also his account of Namonouito, at pp. 97 and 105, and the chart of
+ Oulleay in the Atlas.
+
+
+There is seldom, with the exception of the Maldiva atolls, more than
+two or three channels, and generally only one leading into the lagoon,
+of sufficient depth for a ship to enter. in small atolls, there is
+usually not even one. Where there is deep water, for instance above
+twenty fathoms, in the middle of the lagoon, the channels through the
+reef are seldom as deep as the centre,—it may be said that the rim only
+of the saucer-shaped hollow forming the lagoon is notched. Mr.
+Lyell[20] has observed that the growth of the coral would tend to
+obstruct all the channels through a reef, except those kept open by
+discharging the water, which during high tide and the greater part of
+each ebb is thrown over its circumference. Several facts indicate that
+a considerable quantity of sediment is likewise discharged through
+these channels; and Captain Moresby informs me that he has observed,
+during the change of the monsoon, the sea discoloured to a distance off
+the entrances into the Maldiva and Chagos atolls. This, probably, would
+check the growth of the coral in them, far more effectually than a mere
+current of water. In the many small atolls without any channel, these
+causes have not prevented the entire ring attaining the surface. The
+channels, like the submerged and effaced parts of the reef, very
+generally though not invariably occur on the leeward side of the atoll,
+or on that side, according to Beechey,[21] which, from running in the
+same direction with the prevalent wind, is not fully exposed to it.
+Passages between the islets on the reef, through which boats can pass
+at high water, must not be confounded with ship-channels, by which the
+annular reef itself is breached. The passages between the islets occur,
+of course, on the windward as well as on the leeward side; but they are
+more frequent and broader to leeward, owing to the lesser dimensions of
+the islets on that side.
+
+ [20] “Principles of Geology,” vol. iii, p. 289.
+
+
+ [21] Beechey’s “Voyage,” 4to ed., vol. i, p. 189.
+
+
+At Keeling atoll the shores of the lagoon shelve gradually, where the
+bottom is of sediment, and irregularly or abruptly where there are
+coral-reefs; but this is by no means the universal structure in other
+atolls. Chamisso,[22] speaking in general terms of the lagoons in the
+Marshall atolls, says the lead generally sinks “from a depth of two or
+three fathoms to twenty or twenty-four, and you may pursue a line in
+which on one side of the boat you may see the bottom, and on the other
+the azure-blue deep water.” The shores of the lagoon-like channel
+within the barrier-reef at Vanikoro have a similar structure. Captain
+Beechey has described a modification of this structure (and he believes
+it is not uncommon) in two atolls in the Low Archipelago, in which the
+shores of the lagoon descend by a few, broad, slightly inclined ledges
+or steps: thus at Matilda atoll,[23] the great exterior reef, the
+surface of which is gently inclined towards and beneath the surface of
+the lagoon, ends abruptly in a little cliff three fathoms deep; at its
+foot, a ledge forty yards wide extends, shelving gently inwards
+like the surface-reef, and terminated by a second little cliff five
+fathoms deep; beyond this, the bottom of the lagoon slopes to twenty
+fathoms, which is the average depth of its centre. These ledges seem to
+be formed of coral-rock; and Captain Beechey says that the lead often
+descended several fathoms through holes in them. In some atolls, all
+the coral reefs or knolls in the lagoon come to the surface at low
+water; in other cases of rarer occurrence, all lie at nearly the same
+depth beneath it, but most frequently they are quite irregular,—some
+with perpendicular, some with sloping sides,—some rising to the
+surface, and others lying at all intermediate depths from the bottom
+upwards. I cannot, therefore, suppose that the union of such reefs
+could produce even one uniformly sloping ledge, and much less two or
+three, one beneath the other, and each terminated by an abrupt wall. At
+Matilda Island, which offers the best example of the step-like
+structure, Captain Beechey observes that the coral-knolls within the
+lagoon are quite irregular in their height. We shall hereafter see that
+the theory which accounts for the ordinary form of atolls, apparently
+includes this occasional peculiarity in their structure.
+
+ [22] Kotzebue’s “First Voyage,” vol. iii, p. 142.
+
+
+ [23] Beechey’s “Voyage,” 4to ed., vol. i, p. 160. At Whitsunday Island
+ the bottom of the lagoon slopes gradually towards the centre, and then
+ deepens suddenly, the edge of the bank being nearly perpendicular.
+ This bank is formed of coral and dead shells.
+
+
+In the midst of a group of atolls, there sometimes occur small, flat,
+very low islands of coral formation, which probably once included a
+lagoon, since filled up with sediment and coral-reefs. Captain Beechey
+entertains no doubt that this has been the case with the two small
+islands, which alone of thirty-one surveyed by him in the Low
+Archipelago, did not contain lagoons. Romanzoff Island (in lat. 15 deg
+S.) is described by Chamisso[24] as formed by a dam of madreporitic
+rock inclosing a flat space, thinly covered with trees, into which the
+sea on the leeward side occasionally breaks. North Keeling atoll
+appears to be in a rather less forward stage of conversion into land;
+it consists of a horse-shoe shaped strip of land surrounding a muddy
+flat, one mile in its longest axis, which is covered by the sea only at
+high water. When describing South Keeling atoll, I endeavoured to show
+how slow the final process of filling up a lagoon must be;
+nevertheless, as all causes do tend to produce this effect, it is very
+remarkable that not one instance, as I believe, is known of a
+moderately sized lagoon being filled up even to the low water-line at
+spring-tides, much less of such a one being converted into land. It is,
+likewise, in some degree remarkable, how few atolls, except small ones,
+are surrounded by a single linear strip of land, formed by the union of
+separate islets. We cannot suppose that the many atolls in the Pacific
+and Indian Oceans all have had a late origin, and yet should they
+remain at their present level, subjected only to the action of the sea
+and to the growing powers of the coral, during as many centuries as
+must have elapsed since any of the earlier tertiary epochs, it cannot,
+I think, be doubted that their lagoons and the islets on their reef,
+would present a totally different appearance from what they now do.
+This consideration leads to the suspicion that some renovating agency
+(namely subsidence) comes into play at intervals, and perpetuates their
+original structure.
+
+ [24] Kotzebue’s “First Voyage,” vol. iii, p. 221.
+
+_Section III_—ATOLLS OF THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK
+
+Maldiva Archipelago.—Ring-formed reefs, marginal and central.—Great
+depths in the lagoons of the southern atolls.—Reefs in the lagoons all
+rising to the surface.—Position of islets and breaches in the reefs,
+with respect to the prevalent winds and action of the
+waves.—Destruction of islets.—Connection in the position and submarine
+foundation of distinct atolls.—The apparent disseverment of large
+atolls.—The Great Chagos Bank.—Its submerged condition and
+extraordinary structure.
+
+Although occasional references have been made to the Maldiva atolls,
+and to the banks in the Chagos group, some points of their structure
+deserve further consideration. My description is derived from an
+examination of the admirable charts lately published from the survey of
+Captain Moresby and Lieutenant Powell, and more especially from
+information which Captain Moresby has communicated to me in the kindest
+manner.
+
+The Maldiva Archipelago is 470 miles in length, with an average breadth
+of about 50 miles. The form and dimensions of the atolls, and their
+singular position in a double line, may be seen, but not well, in the
+greatly reduced chart (Fig. 6) in  Plate II. The dimensions of the
+longest atoll in the group (called by the double name of
+Milla-dou-Madou and Tilla-dou-Matte) have already been given; it is 88
+miles in a medial and slightly curved line, and is less than 20 miles
+in its broadest part. Suadiva, also, is a noble atoll, being 44 miles
+across in one direction, and 34 in another, and the great included
+expanse of water has a depth of between 250 and 300 feet. The smaller
+atolls in this group differ in no respect from ordinary ones; but the
+larger ones are remarkable from being breached by numerous deep-water
+channels leading into the lagoon; for instance, there are 42 channels,
+through which a ship could enter the lagoon of Suadiva. In the three
+southern large atolls, the separate portions of reef between these
+channels have the ordinary structure, and are linear; but in the other
+atolls, especially the more northern ones, these portions are
+ring-formed, like miniature atolls. Other ring-formed reefs rise out of
+the lagoons, in the place of those irregular ones which ordinarily
+occur there. In the reduction of the chart of Mahlos Mahdoo (Plate II,
+Fig. 4), it was not found easy to define the islets and the little
+lagoons within each reef, so that the ring-formed structure is very
+imperfectly shown; in the large published charts of Tilla-dou-Matte,
+the appearance of these rings, from standing further apart from each
+other, is very remarkable. The rings on the margin are generally
+elongated; many of them are three, and some even five miles, in
+diameter; those within the lagoon are usually smaller, few being more
+than two miles across, and the greater number rather less than one. The
+depth of the little lagoon within these small annular reefs is
+generally from five to seven fathoms, but occasionally more; and in Ari
+atoll many of the central ones are twelve, and some even more than
+twelve fathoms deep. These rings rise abruptly from the platform or
+bank, on which they are placed; their outer margin is
+invariably bordered by living coral[25] within which there is a flat
+surface of coral rock; of this flat, sand and fragments have in many
+cases accumulated and been converted into islets, clothed with
+vegetation. I can, in fact, point out no essential difference between
+these little ring-formed reefs (which, however, are larger, and contain
+deeper lagoons than many true atolls that stand in the open sea), and
+the most perfectly characterised atolls, excepting that the ring-formed
+reefs are based on a shallow foundation, instead of on the floor of the
+open sea, and that instead of being scattered irregularly, they are
+grouped closely together on one large platform, with the marginal rings
+arranged in a rudely formed circle.
+
+ [25] Captain Moresby informs me that _Millepora complanata_ is one of
+ the commonest kinds on the outer margin, as it is at Keeling atoll.
+
+The perfect series which can be traced from portions of simple linear
+reef, to others including long linear lagoons, and from these again to
+oval or almost circular rings, renders it probable that the latter are
+merely modifications of the linear or normal state. It is conformable
+with this view, that the ring-formed reefs on the margin, even where
+most perfect and standing furthest apart, generally have their longest
+axes directed in the line which the reef would have held, if the atoll
+had been bounded by an ordinary wall. We may also infer that the
+central ring-formed reefs are modifications of those irregular ones,
+which are found in the lagoons of all common atolls. It appears from
+the charts on a large scale, that the ring-like structure is contingent
+on the marginal channels or breaches being wide; and, consequently, on
+the whole interior of the atoll being freely exposed to the waters of
+the open sea. When the channels are narrow or few in number, although
+the lagoon be of great size and depth (as in Suadiva), there are no
+ring-formed reefs; where the channels are somewhat broader, the
+marginal portions of reef, and especially those close to the larger
+channels, are ring-formed, but the central ones are not so; where they
+are broadest, almost every reef throughout the atoll is more or less
+perfectly ring-formed. Although their presence is thus contingent on
+the openness of the marginal channels, the theory of their formation,
+as we shall hereafter see, is included in that of the parent atolls, of
+which they form the separate portions.
+
+The lagoons of all the atolls in the southern part of the Archipelago
+are from ten to twenty fathoms deeper than those in the northern part.
+This is well exemplified in the case of Addoo, the southernmost atoll
+in the group, for although only nine miles in its longest diameter, it
+has a depth of thirty-nine fathoms, whereas all the other small atolls
+have comparatively shallow lagoons; I can assign no adequate cause for
+this difference in depth. In the central and deepest part of the
+lagoons, the bottom consists, as I am informed by Captain Moresby, of
+stiff clay (probably a calcareous mud); nearer the border it consists
+of sand, and in the channels through the reef, of hard sand-banks,
+sandstone, conglomerate rubble, and a little live coral. Close outside
+the reef and the line joining its detached portions (where intersected
+by many channels), the bottom is sandy, and it slopes abruptly into
+unfathomable depths.
+In most lagoons the depth is considerably greater in the centre than in
+the channels; but in Tilla-dou-Matte, where the marginal ring-formed
+reefs stand far apart, the same depth is carried across the entire
+atoll, from the deep-water line on one side to that on the other. I
+cannot refrain from once again remarking on the singularity of these
+atolls,—a great sandy and generally concave disc rises abruptly from
+the unfathomable ocean, with its central expanse studded and its border
+symmetrically fringed with oval basins of coral-rock, just lipping the
+surface of the sea, sometimes clothed with vegetation, and each
+containing a little lake of clear water!
+
+In the southern Maldiva atolls, of which there are nine large ones, all
+the small reefs within the lagoons come to the surface, and are dry at
+low water spring-tides; hence in navigating them, there is no danger
+from submarine banks. This circumstance is very remarkable, as within
+some atolls, for instance those of the neighbouring Chagos group, not a
+single reef comes to the surface, and in most other cases a few only
+do, and the rest lie at all intermediate depths from the bottom
+upwards. When treating of the growth of coral I shall again refer to
+this subject.
+
+Although in the neighbourhood of the Maldiva Archipelago the winds,
+during the monsoons, blow during nearly an equal time from opposite
+quarters, and although, as I am informed by Captain Moresby, the
+westerly winds are the strongest, yet the islets are almost all placed
+on the eastern side of the northern atolls, and on the south-eastern
+side of the southern atolls. That the formation of the islets is due to
+detritus thrown up from the outside, as in the ordinary manner, and not
+from the interior of the lagoons, may, I think be safely inferred from
+several considerations, which it is hardly worth while to detail. As
+the easterly winds are not the strongest, their action probably is
+aided by some prevailing swell or current.
+
+In groups of atolls, exposed to a trade-wind, the ship-channels into
+the lagoons are almost invariably situated on the leeward or less
+exposed side of the reef, and the reef itself is sometimes either
+wanting there, or is submerged. A strictly analogous, but different
+fact, may be observed at the Maldiva atolls—namely, that where two
+atolls stand in front of each other, the breaches in the reef are the
+most numerous on their near, and therefore less exposed, sides. Thus on
+the near sides of Ari and the two Nillandoo atolls, which face S. Mãle,
+Phaleedoo, and Moloque atolls, there are seventy-three deep-water
+channels, and only twenty-five on their outer sides; on the near side
+of the three latter named atolls there are fifty-six openings, and only
+thirty-seven on their outsides. It is scarcely possible to attribute
+this difference to any other cause than the somewhat different action
+of the sea on the two sides, which would ensue from the protection
+afforded by the two rows of atolls to each other. I may here remark
+that in most cases, the conditions favourable to the greater
+accumulation of fragments on the reef and to its more perfect
+continuity on one side of the atoll than on the other, have concurred,
+but this has not been the case with the Maldivas; for we have seen that
+the islets are placed on the eastern or south-eastern
+sides, whilst the breaches in the reef occur indifferently on any side,
+where protected by an opposite atoll. The reef being more continuous on
+the outer and more exposed sides of those atolls which stand near each
+other, accords with the fact, that the reef of the southern atolls is
+more continuous than that of the northern ones; for the former, as I am
+informed by Captain Moresby, are more constantly exposed than the
+northern atolls to a heavy surf.
+
+The date of the first formation of some of the islets in this
+Archipelago is known to the inhabitants; on the other hand, several
+islets, and even some of those which are believed to be very old, are
+now fast wearing away. The work of destruction has, in some instances,
+been completed in ten years. Captain Moresby found on one water-washed
+reef the marks of wells and graves, which were excavated when it
+supported an islet. In South Nillandoo atoll, the natives say that
+three of the islets were formerly larger: in North Nillandoo there is
+one now being washed away; and in this latter atoll Lieutenant Prentice
+found a reef, about six hundred yards in diameter, which the natives
+positively affirmed was lately an island covered with cocoa-nut trees.
+It is now only partially dry at low water spring-tides, and is (in
+Lieutenant Prentice’s words) “entirely covered with live coral and
+madrepore.” In the northern part, also, of the Maldiva Archipelago and
+in the Chagos group, it is known that some of the islets are
+disappearing. The natives attribute these effects to variations in the
+currents of the sea. For my own part I cannot avoid suspecting that
+there must be some further cause, which gives rise to such a cycle of
+change in the action of the currents of the great and open ocean.
+
+Several of the atolls in this Archipelago are so related to each other
+in form and position, that at the first glance one is led to suspect
+that they have originated in the disseverment of a single one. Mãle
+consists of three perfectly characterised atolls, of which the shape
+and relative position are such, that a line drawn closely round all
+three, gives a symmetrical figure; to see this clearly, a larger chart
+is required than that of the Archipelago in Plate II; the channel
+separating the two northern Male atolls is only little more than a mile
+wide, and no bottom was found in it with 100 fathoms. Powell’s Island
+is situated at the distance of two miles and a half off the northern
+end of Mahlos Mahdoo (see Fig. 4, Plate II), at the exact point where
+the two sides of the latter, if prolonged, would meet; no bottom,
+however, was found in the channel with 200 fathoms; in the wider
+channel between Horsburgh atoll and the southern end of Mahlos Mahdoo,
+no bottom was found with 250 fathoms. In these and similar cases, the
+relation consists only in the form and position of the atolls. But in
+the channel between the two Nillandoo atolls, although three miles and
+a quarter wide, soundings were struck at the depth of 200 fathoms; the
+channel between Ross and Ari atolls is four miles wide, and only 150
+fathoms deep. Here then we have, besides the relation of form, a
+submarine connection. The fact of soundings having been obtained
+between two separate and perfectly characterised atolls is in itself
+interesting, as it has never, I believe, been effected in any of the
+many
+other groups of atolls in the Pacific and Indian seas. In continuing to
+trace the connection of adjoining atolls, if a hasty glance be taken at
+the chart (Fig. 4, Plate II) of Mahlos Mahdoo, and the line of
+unfathomable water be followed, no one will hesitate to consider it as
+one atoll. But a second look will show that it is divided by a
+bifurcating channel, of which the northern arm is about one mile and
+three-quarters in width, with an average depth of 125 fathoms, and the
+southern one three-quarters of a mile wide, and rather less deep. These
+channels resemble in the slope of their sides and general form, those
+which separate atolls in every respect distinct; and the northern arm
+is wider than that dividing two of the Mãle atolls. The ring-formed
+reefs on the sides of this bifurcating channel are elongated, so that
+the northern and southern portions of Mahlos Mahdoo may claim, as far
+as their external outline is concerned, to be considered as distinct
+and perfect atolls. But the intermediate portion, lying in the fork of
+the channel, is bordered by reefs less perfect than those which
+surround any other atoll in the group of equally small dimensions.
+Mahlos Mahdoo, therefore, is in every respect in so intermediate a
+condition, that it may be considered either as a single atoll nearly
+dissevered into three portions, or as three atolls almost perfect and
+intimately connected. This is an instance of a very early stage of the
+apparent disseverment of an atoll, but a still earlier one in many
+respects is exhibited at Tilla-dou-Matte. In one part of this atoll,
+the ring-formed reefs stand so far apart from each other, that the
+inhabitants have given different names to the northern and southern
+halves; nearly all the rings, moreover, are so perfect and stand so
+separate, and the space from which they rise is so level and unlike a
+true lagoon, that we can easily imagine the conversion of this one
+great atoll, not into two or three portions, but into a whole group of
+miniature atolls. A perfect series such as we have here traced,
+impresses the mind with an idea of actual change; and it will hereafter
+be seen, that the theory of subsidence, with the upward growth of the
+coral, modified by accidents of probable occurrence, will account for
+the occasional disseverment of large atolls.
+
+_Plate II_—Great Chagos Bank, New Caledonia, Menchikoff Atoll, etc.
+
+
+[Illustration: Great Chagos Bank]
+
+Fig. 1.—GREAT CHAGOS BANK, in the Indian Ocean; taken from the survey
+by Captain Moresby and Lieutenant Powell; the parts which are shaded,
+with the exception of two or three islets on the western and northern
+sides, do not rise to the surface, but are submerged from four to ten
+fathoms; the banks bounded by the dotted lines lie from fifteen to
+twenty fathoms beneath the surface, and are formed of sand; the central
+space is of mud, and from thirty to fifty fathoms deep.
+
+
+Fig. 2.—A vertical section, on the same scale, in an eastern and
+western line across the Great Chagos Bank, given for the sake of
+exhibiting more clearly its structure.
+
+
+[Illustration: New Caledonia, Menchikoff Atoll, etc.]
+
+Fig. 3.—Menchikoff Atoll (or lagoon-island), in the Marshall
+Archipelago, Northern Pacific Ocean; from Krusenstern’s “Atlas of the
+Pacific;” originally surveyed by Captain Hagemeister; the depth within
+the lagoons is unknown.
+
+
+Fig. 4.—MAHLOS MAHDOO ATOLL, together with Horsburgh atoll, in the
+Maldiva Archipelago; from the survey by Captain Moresby and Lieutenant
+Powell; the white spaces in the middle of the separate small reefs,
+both on the margin and in the middle part, are meant to represent
+little lagoons; but it was found not possible to distinguish them
+clearly from the small islets, which have been formed on these same
+small reefs; many of the smaller reefs could not be introduced; the
+nautical mark (dot over a dash) over the figures 250 and 200, between
+Mahlos Mahdoo and Horsburgh atoll and Powell’s island, signifies that
+soundings were not obtained at these depths.
+
+
+Fig. 5.—NEW CALEDONIA, in the western part of the Pacific; from
+Krusenstern’s “Atlas,” compiled from several surveys; I have slightly
+altered the northern point of the reef, in accordance with the “Atlas
+of the Voyage of the _Astrolabe_.” In Krusenstern’s “Atlas,” the reef
+is represented by a single line with crosses; I have for the sake of
+uniformity added an interior line.
+
+
+Fig. 6.—MALDIVA ARCHIPELAGO, in the Indian Ocean; from the survey by
+Captain Moresby and Lieutenant Powell.
+
+
+The Great Chagos bank alone remains to be described. In the Chagos
+group there are some ordinary atolls, some annular reefs rising to the
+surface but without any islets on them, and some atoll-formed banks,
+either quite submerged, or nearly so. Of the latter, the Great Chagos
+Bank is much the largest, and differs in its structure from the others:
+a plan of it is given in Plate II, Fig. 1, in which, for the sake of
+clearness, I have had the parts under ten fathoms deep finely shaded:
+an east and west vertical section is given in Fig. 2, in which the
+vertical scale has been necessarily exaggerated. Its longest axis is
+ninety nautical miles, and another line drawn at right angles to the
+first, across the broadest part, is seventy. The central part consists
+of a level muddy flat, between forty and fifty fathoms deep, which is
+surrounded on all sides, with the exception of some breaches, by the
+steep edges of a set of banks, rudely arranged in a circle. These banks
+consist of sand, with a very little live coral; they vary in breadth
+from five to twelve miles, and on an average lie about sixteen fathoms
+beneath the surface;
+they are bordered by the steep edges of a third narrow and upper bank,
+which forms the rim to the whole. This rim is about a mile in width,
+and with the exception of two or three spots where islets have been
+formed, is submerged between five and ten fathoms. It consists of
+smooth hard rock, covered with a thin layer of sand, but with scarcely
+any live coral; it is steep on both sides, and outwards slopes abruptly
+into unfathomable depths. At the distance of less than half a mile from
+one part, no bottom was found with 190 fathoms; and off another point,
+at a somewhat greater distance, there was none with 210 fathoms. Small
+steep-sided banks or knolls, covered with luxuriantly growing coral,
+rise from the interior expanse to the same level with the external rim,
+which, as we have seen, is formed only of dead rock. It is impossible
+to look at the plan (Fig. 1, Plate II), although reduced to so small a
+scale, without at once perceiving that the Great Chagos Bank is, in the
+words of Captain Moresby,[26] “nothing more than a half-drowned atoll.”
+But of what great dimensions, and of how extraordinary an internal
+structure? We shall hereafter have to consider both the cause of its
+submerged condition, a state common to other banks in the group, and
+the origin of the singular submarine terraces, which bound the central
+expanse: these, I think, it can be shown, have resulted from a cause
+analogous to that which has produced the bifurcating channel across
+Mahlos Mahdoo.
+
+ [26] This officer has had the kindness to lend me an excellent MS.
+ account of the Chagos Islands; from this paper, from the published
+ charts, and from verbal information communicated to me by Captain
+ Moresby, the above account of the Great Chagos Bank is taken.
+
+
+
+
+Chapter II BARRIER REEFS
+
+
+Closely resemble in general form and structure atoll-reefs.—Width and
+depth of the lagoon-channels.—Breaches through the reef in front of
+valleys, and generally on the leeward side.—Checks to the filling up of
+the lagoon-channels.—Size and constitution of the encircled
+islands.—Number of islands within the same reef.—Barrier-reefs of New
+Caledonia and Australia.—Position of the reef relative to the slope of
+the adjoining land.—Probable great thickness of barrier-reefs.
+
+The term “barrier” has been generally applied to that vast reef which
+fronts the N.E. shore of Australia, and by most voyagers likewise to
+that on the western coast of New Caledonia. At one time I thought it
+convenient thus to restrict the term, but as these reefs are similar in
+structure, and in position relatively to the land, to those, which,
+like a wall with a deep moat within, encircle many smaller islands, I
+have classed them together. The reef, also, on the west coast of New
+Caledonia, circling round the extremities of the island, is an
+intermediate form between a small encircling reef and the Australian
+barrier, which stretches for a thousand miles in nearly a straight
+line.
+
+The geographer Balbi has in effect described those barrier-reefs, which
+encircle moderately sized islands, by calling them atolls with high
+land rising from within their central expanse. The general resemblance
+between the reefs of the barrier and atoll classes may be seen in the
+small, but accurately reduced charts on Plate I,[1] and this
+resemblance can be further shown to extend to every part of the
+structure. Beginning with the outside of the reef; many scattered
+soundings off Gambier, Oualan, and some other encircled islands, show
+that close to the breakers there exists a narrow shelving margin,
+beyond which the ocean becomes suddenly unfathomable; but off the west
+coast of New Caledonia, Captain Kent[2] found no bottom with 150
+fathoms, at two ships’ length from the reef; so that the slope here
+must be nearly as precipitous as off the Maldiva atolls.
+
+ [1] The authorities from which these charts have been reduced,
+ together with some remarks on them are given in a separately appended
+ page, descriptive of the Plates.
+
+
+ [2] Dalrymple, “Hydrog. Mem.” vol. iii.
+
+I can give little information regarding the kinds of corals which live
+on the outer margin. When I visited the reef at Tahiti, although it was
+low water, the surf was too violent for me to see the living masses;
+but, according to what I heard from some intelligent native chiefs,
+they resemble in their rounded and branchless forms, those on the
+margin of Keeling atoll. The extreme verge of the reef, which was
+visible between the breaking waves at low water, consisted of a
+rounded, convex, artificial-like breakwater, entirely coated with
+Nulliporæ, and absolutely similar to that which I have described at
+Keeling atoll. From what I heard when at Tahiti, and from the writings
+of the Revs. W. Ellis and J. Williams, I conclude that this peculiar
+structure is common to most of the encircled islands of the Society
+Archipelago. The reef within this mound or breakwater, has an extremely
+irregular surface, even more so than between the islets on the reef of
+Keeling atoll, with which alone (as there are no islets on the reef of
+Tahiti) it can properly be compared. At Tahiti, the reef is very
+irregular in width; but round many other encircled islands, for
+instance, Vanikoro or Gambier Islands (Figs 1 and 8, Plate I), it is
+quite as regular, and of the same average width, as in true atolls.
+Most barrier-reefs on the inner side slope irregularly into the
+lagoon-channel (as the space of deep water separating the reef from the
+included land may be called), but at Vanikoro the reef slopes only for
+a short distance, and then terminates abruptly in a submarine wall,
+forty feet high,—a structure absolutely similar to that described by
+Chamisso in the Marshall atolls.
+
+In the Society Archipelago, Ellis[3] states, that the reefs generally
+lie at the distance of from one to one and a half miles, and,
+occasionally, even at more than three miles, from the shore. The
+central mountains are generally bordered by a fringe of flat, and often
+marshy, alluvial
+land, from one to four miles in width. This fringe consists of
+coral-sand and detritus thrown up from the lagoon-channel, and of soil
+washed down from the hills; it is an encroachment on the channel,
+analogous to that low and inner part of the islets in many atolls which
+is formed by the accumulation of matter from the lagoon. At Hogoleu
+(Fig. 2, Plate I), in the Caroline Archipelago,[4] the reef on the
+south side is no less than twenty miles; on the east side, five; and on
+the north side, fourteen miles from the encircled high islands.
+
+ [3] Consult, on this and other points, the “Polynesian Researches,” by
+ the Rev. W. Ellis, an admirable work, full of curious information.
+
+
+ [4] See “Hydrographical Mem.” and the “Atlas of the Voyage of the
+ _Astrolabe_,” by Captain Dumont D’Urville, p. 428.
+
+
+The lagoon channels may be compared in every respect with true lagoons.
+In some cases they are open, with a level bottom of fine sand; in
+others they are choked up with reefs of delicately branched corals,
+which have the same general character as those within the Keeling
+atoll. These internal reefs either stand separately, or more commonly
+skirt the shores of the included high islands. The depth of the
+lagoon-channel round the Society Islands varies from two or three to
+thirty fathoms; in Cook’s[5] chart of Ulieta, however, there is one
+sounding laid down of forty-eight fathoms; at Vanikoro there are
+several of fifty-four and one of fifty-six and a half fathoms
+(English), a depth which even exceeds by a little that of the interior
+of the great Maldiva atolls. Some barrier-reefs have very few islets on
+them; whilst others are surmounted by numerous ones; and those round
+part of Bolabola (Plate I, Fig. 5) form a single linear strip. The
+islets first appear either on the angles of the reef, or on the sides
+of the breaches through it, and are generally most numerous on the
+windward side. The reef to leeward retaining its usual width, sometimes
+lies submerged several fathoms beneath the surface; I have already
+mentioned Gambier Island as an instance of this structure. Submerged
+reefs, having a less defined outline, dead, and covered with sand, have
+been observed (see Appendix) off some parts of Huaheine and Tahiti. The
+reef is more frequently breached to leeward than to windward; thus I
+find in Krusenstern’s “Memoir on the Pacific,” that there are passages
+through the encircling reef on the leeward side of each of the seven
+Society Islands, which possess ship-harbours; but that there are
+openings to windward through the reef of only three of them. The
+breaches in the reef are seldom as deep as the interior lagoon-like
+channel; they generally occur in front of the main valleys, a
+circumstance which can be accounted for, as will be seen in the fourth
+chapter, without much difficulty. The breaches being situated in front
+of the valleys, which descend indifferently on all sides, explains
+their more frequent occurrence through the windward side of
+barrier-reefs than through the windward side of atolls,—for in atolls
+there is no included land to influence the position of the breaches.
+
+ [5] See the chart in vol. i of Hawkesworth’s 4to ed. of “Cook’s First
+ Voyage.”
+
+It is remarkable, that the lagoon-channels round mountainous islands
+have not in every instance been long ago filled up with coral and
+sediment; but it is more easily accounted for than appears at first
+sight. In cases like that of Hogoleu and the Gambier Islands, where a
+few
+small peaks rise out of a great lagoon, the conditions scarcely differ
+from those of an atoll, and I have already shown, at some length, that
+the filling up of a true lagoon must be an extremely slow process.
+Where the channel is narrow, the agency, which on unprotected coasts is
+most productive of sediment, namely the force of the breakers, is here
+entirely excluded, and the reef being breached in the front of the main
+valleys, much of the finer mud from the rivers must be transported into
+the open sea. As a current is formed by the water thrown over the edge
+of atoll-formed reefs, which carries sediment with it through the
+deep-water breaches, the same thing probably takes place in
+barrier-reefs, and this would greatly aid in preventing the
+lagoon-channel from being filled up. The low alluvial border, however,
+at the foot of the encircled mountains, shows that the work of filling
+up is in progress; and at Maura (Plate I, Fig. 6), in the Society
+group, it has been almost effected, so that there remains only one
+harbour for small craft.
+
+If we look at a set of charts of barrier-reefs, and leave out in
+imagination the encircled land, we shall find that, besides the many
+points already noticed of resemblance, or rather of identity in
+structure with atolls, there is a close general agreement in form,
+average dimensions, and grouping. Encircling barrier-reefs, like
+atolls, are generally elongated, with an irregularly rounded, though
+sometimes angular outline. There are atolls of all sizes, from less
+than two miles in diameter to sixty miles (excluding Tilla-dou-Matte,
+as it consists of a number of almost independent atoll-formed reefs);
+and there are encircling barrier-reefs from three miles and a half to
+forty-six miles in diameter,—Turtle Island being an instance of the
+former, and Hogoleu of the latter. At Tahiti the encircled island is
+thirty-six miles in its longest axis, whilst at Maurua it is only a
+little more than two miles. It will be shown, in the last chapter in
+this volume, that there is the strictest resemblance in the grouping of
+atolls and of common islands, and consequently there must be the same
+resemblance in the grouping of atolls and of encircling barrier-reefs.
+
+The islands lying within reefs of this class, are of very various
+heights. Tahiti[6] is 7,000 feet; Maurua about 800; Aitutaki 360, and
+Manouai only 50. The geological nature of the included land varies: in
+most cases it is of ancient volcanic origin, owing apparently to the
+fact that islands of this nature are most frequent within all great
+seas; some, however, are of madreporitic limestone, and others of
+primary formation, of which latter kind New Caledonia offers the best
+example. The central land consists either of one island, or of several:
+thus, in the Society group, Eimeo stands by itself; while Taha and
+Raiatea (Fig. 3, Plate I), both moderately large islands of nearly
+equal size, are included in one reef. Within the reef of the Gambier
+group there are four large and some smaller islands (Fig. 8, Plate I);
+within that of
+Hogoleu (Fig. 2, Plate I) nearly a dozen small islands are scattered
+over the expanse of one vast lagoon.
+
+ [6] The height of Tahiti is given from Captain Beechey; Maurua from
+ Mr. F. D. Bennett (_Geograph. Journ._ vol. viii, p. 220); Aitutaki
+ from measurements made on board the _Beagle_; and Manouai or Harvey
+ Island, from an estimate by the Rev. J. Williams. The two latter
+ islands, however, are not in some respects well characterised examples
+ of the encircled class.
+
+After the details now given, it may be asserted that there is not one
+point of essential difference between encircling barrier-reefs and
+atolls: the latter enclose a simple sheet of water, the former encircle
+an expanse with one or more islands rising from it. I was much struck
+with this fact, when viewing, from the heights of Tahiti, the distant
+island of Eimeo standing within smooth water, and encircled by a ring
+of snow-white breakers. Remove the central land, and an annular reef
+like that of an atoll in an early stage of its formation is left;
+remove it from Bolabola, and there remains a circle of linear
+coral-islets, crowned with tall cocoa-nut trees, like one of the many
+atolls scattered over the Pacific and Indian Oceans.
+
+The barrier-reefs of Australia and of New Caledonia deserve a separate
+notice from their great dimensions. The reef on the west coast of New
+Caledonia (Fig. 5, Plate II) is 400 miles in length; and for a length
+of many leagues it seldom approaches within eight miles of the shore;
+and near the southern end of the island, the space between the reef and
+the land is sixteen miles in width. The Australian barrier extends,
+with a few interruptions, for nearly a thousand miles; its average
+distance from the land is between twenty and thirty miles; and in some
+parts from fifty to seventy. The great arm of the sea thus included, is
+from ten to twenty-five fathoms deep, with a sandy bottom; but towards
+the southern end, where the reef is further from the shore, the depth
+gradually increases to forty, and in some parts to more than sixty
+fathoms. Flinders[7] has described the surface of this reef as
+consisting of a hard white agglomerate of different kinds of coral,
+with rough projecting points. The outer edge is the highest part; it is
+traversed by narrow gullies, and at rare intervals is breached by
+ship-channels. The sea close outside is profoundly deep; but, in front
+of the main breaches, soundings can sometimes be obtained. Some low
+islets have been formed on the reef.
+
+ [7] Flinders’ “Voyage to Terra Australis,” vol. ii, p. 88.
+
+
+There is one important point in the structure of barrier-reefs which
+must here be considered. The accompanying diagrams represent north and
+south vertical sections, taken through the highest points of Vanikoro,
+Gambier, and Maurua Islands, and through their encircling reefs. The
+scale both in the horizontal and vertical direction is the same,
+namely, a quarter of an inch to a nautical mile. The height and width
+of these islands is known; and I have attempted to represent the form
+of the land from the shading of the hills in the large published
+charts. It has long been remarked, even from the time of Dampier, that
+considerable degree of relation subsists between the inclination of
+that part of the land which is beneath water and that above it; hence
+the dotted line in the three sections, probably, does not widely differ
+in inclination from the actual submarine prolongation of the land. If
+we now look at the outer edge of the reef (AA), and bear in mind that
+the plummet on the right hand represents a depth of 1,200 feet, we must
+conclude that the vertical thickness of these barrier coral-reefs is
+very great.
+
+
+[Illustration: Vertical thickness of Vanikoro, Gambier and Maurua.]
+
+1. VANIKORO, from the “Atlas of the Voyage of the _Astrolabe_,” by D.
+D’Urville.
+2. GAMBIER ISLAND, from Beechey.
+3. MAURUA, from the “Atlas of the Voyage of the _ Coquille_,” by
+Duperrey.
+
+The horizontal line is the level of the sea, from which on the right
+hand a plummet descends, representing a depth of 200 fathoms, or 1,200
+feet. The vertical shading shows the section of the land, and the
+horizontal shading that of the encircling barrier-reef: from the
+smallness of the scale, the lagoon-channel could not be represented.
+AA.—Outer edge of the coral-reefs, where the sea breaks.
+BB.—The shore of the encircled islands.
+
+
+I must observe that if the sections had been taken in any other
+direction across these islands, or across other encircled islands,[8]
+the result would have been the same. In the succeeding chapter it will
+be shown that reef-building polypifers cannot flourish at great
+depths,—for instance, it is highly improbable that they could exist at
+a quarter of the depth represented by the plummet on the right hand of
+the woodcut. Here there is a great _apparent_ difficulty—how were the
+basal parts of these barrier-reef formed? It will, perhaps, occur to
+some, that the actual reefs formed of coral are not of great thickness,
+but that before their first growth, the coasts of these encircled
+islands were deeply eaten into, and a broad but shallow submarine ledge
+thus left, on the edge of which the coral grew; but if this had been
+the case, the shore would have been invariably bounded by lofty cliffs,
+and not have sloped down to the lagoon-channel, as it does in many
+instances. On this view,[9] moreover, the cause of the reef springing
+up at such a great distance from the land, leaving a deep and broad
+moat within, remains altogether unexplained. A supposition of the same
+nature,
+and appearing at first more probable is, that the reefs sprung up from
+banks of sediment, which had accumulated round the shore previously to
+the growth of the coral; but the extension of a bank to the same
+distance round an unbroken coast, and in front of those deep arms of
+the sea (as in Raiatea, see Plate II, Fig. 3) which penetrate nearly to
+the heart of some encircled islands, is exceedingly improbable. And
+why, again, should the reef spring up, in some cases steep on both
+sides like a wall, at a distance of two, three or more miles from the
+shore, leaving a channel often between two hundred and three hundred
+feet deep, and rising from a depth which we have reason to believe is
+destructive to the growth of coral? An admission of this nature cannot
+possibly be made. The existence, also, of the deep channel, utterly
+precludes the idea of the reef having grown outwards, on a foundation
+slowly formed on its outside, by the accumulation of sediment and coral
+detritus. Nor, again, can it be asserted, that the reef-building corals
+will not grow, excepting at a great distance from the land; for, as we
+shall soon see, there is a whole class of reefs, which take their name
+from growing closely attached (especially where the sea is deep) to the
+beach. At New Caledonia (see Plate II, Fig. 5) the reefs which run in
+front of the west coast are prolonged in the same line 150 miles beyond
+the northern extremity of the island, and this shows that some
+explanation, quite different from any of those just suggested, is
+required. The continuation of the reefs on each side of the submarine
+prolongation of New Caledonia, is an exceedingly interesting fact, if
+this part formerly existed as the northern extremity of the island, and
+before the attachment of the coral had been worn down by the action of
+the sea, or if it originally existed at its present height, with or
+without beds of sediment on each flank, how can we possibly account for
+the reefs, not growing on the crest of this submarine portion, but
+fronting its sides, in the same line with the reefs which front the
+shores of the lofty island? We shall hereafter see, that there is one,
+and I believe only one, solution of this difficulty.
+
+ [8] In the fifth chapter an east and west section across the Island of
+ Bolabola and its barrier-reefs is given, for the sake of illustrating
+ another point. The unbroken line in it (woodcut No. 5) is the section
+ referred to. The scale is .57 of an inch to a mile; it is taken from
+ the “Atlas of the Voyage of the _Coquille_,” by Duperrey. The depth of
+ the lagoon-channel is exaggerated.
+
+
+ [9] The Rev. D. Tyerman and Mr. Bennett (“Journal of Voyage and
+ Travels,” vol. i, p. 215) have briefly suggested this explanation of
+ the origin of the encircling reefs of the Society Islands.
+
+
+One other supposition to account for the position of encircling
+barrier-reefs remains, but it is almost too preposterous to be
+mentioned; namely, that they rest on enormous submarine craters,
+surrounding the included islands. When the size, height, and form of
+the islands in the Society group are considered, together with the fact
+that all are thus encircled, such a notion will be rejected by almost
+every one. New Caledonia, moreover, besides its size, is composed of
+primitive formations, as are some of the Comoro Islands;[10] and
+Aitutaki consists of calcareous rock. We must, therefore, reject these
+several explanations, and conclude that the vertical thickness of
+barrier-reefs, from their outer edges to the foundation on which they
+rest (from AA in the section to the dotted lines) is really great; but
+in this, there is no difficulty, for it is not necessary to suppose
+that the coral has sprung up from an immense depth, as will be evident
+when the theory of the upward growth of coral-reefs, during the slow
+subsidence of their foundation, is discussed.
+
+ [10] I have been informed that this is the case by Dr. Allan of
+ Forres, who has visited this group.
+
+
+
+
+Chapter III FRINGING OR SHORE-REEFS
+
+
+Reefs of Mauritius.—Shallow channel within the reef.—Its slow filling
+up.—Currents of water formed within it.—Upraised reefs.—Narrow
+fringing-reefs in deep seas.—Reefs on the coast of East Africa and of
+Brazil.—Fringing-reefs in very shallow seas, round banks of sediment
+and on worn-down islands.—Fringing-reefs affected by currents of the
+sea.—Coral coating the bottom of the sea, but not forming reefs.
+
+Fringing-reefs, or, as they have been called by some voyagers,
+shore-reefs, whether skirting an island or part of a continent, might
+at first be thought to differ little, except in generally being of less
+breadth, from barrier-reefs. As far as the superficies of the actual
+reef is concerned this is the case; but the absence of an interior
+deep-water channel, and the close relation in their horizontal
+extension with the probable slope beneath the sea of the adjoining
+land, present essential points of difference.
+
+The reefs which fringe the island of Mauritius offer a good example of
+this class. They extend round its whole circumference, with the
+exception of two or three parts,[1] where the coast is almost
+precipitous, and where, if as is probable the bottom of the sea has a
+similar inclination, the coral would have no foundation on which to
+become attached. A similar fact may sometimes be observed even in reefs
+of the barrier class, which follow much less closely the outline of the
+adjoining land; as, for instance, on the south-east and precipitous
+side of Tahiti, where the encircling reef is interrupted. On the
+western side of the Mauritius, which was the only part I visited, the
+reef generally lies at the distance of about half a mile from the
+shore; but in some parts it is distant from one to two, and even three
+miles. But even in this last case, as the coast-land is gently inclined
+from the foot of the mountains to the sea-beach, and as the soundings
+outside the reef indicate an equally gentle slope beneath the water,
+there is no reason for supposing that the basis of the reef, formed by
+the prolongation of the strata of the island, lies at a greater depth
+than that at which the polypifers could begin constructing the reef.
+Some allowance, however, must be made for the outward extension of the
+corals on a foundation of sand and detritus, formed from their own
+wear, which would give to the reef a somewhat greater vertical
+thickness, than would otherwise be possible.
+
+ [1] This fact is stated on the authority of the Officier du Roi, in
+ his extremely interesting “Voyage à l’Isle de France,” undertaken in
+ 1768. According to Captain Carmichael (Hooker’s _Bot. Misc._ vol. ii,
+ p. 316) on one part of the coast there is a space for sixteen miles
+ without a reef.
+
+The outer edge of the reef on the western or leeward side of the island
+is tolerably well defined, and is a little higher than any other part.
+It chiefly consists of large strongly branched corals, of the genus
+Madrepora, which also form a sloping bed some way out to sea: the
+kinds of coral growing in this part will be described in the ensuing
+chapter. Between the outer margin and the beach, there is a flat space
+with a sandy bottom and a few tufts of living coral; in some parts it
+is so shallow, that people, by avoiding the deeper holes and gullies,
+can wade across it at low water; in other parts it is deeper, seldom
+however exceeding ten or twelve feet, so that it offers a safe coasting
+channel for boats. On the eastern and windward side of the island,
+which is exposed to a heavy surf, the reef was described to me as
+having a hard smooth surface, very slightly inclined inwards, just
+covered at low-water, and traversed by gullies; it appears to be quite
+similar in structure to the reefs of the barrier and atoll classes.
+
+The reef of Mauritius, in front of every river and streamlet, is
+breached by a straight passage: at Grand Port, however, there is a
+channel like that within a barrier-reef; it extends parallel to the
+shore for four miles, and has an average depth of ten or twelve
+fathoms; its presence may probably be accounted for by two rivers which
+enter at each end of the channel, and bend towards each other. The fact
+of reefs of the fringing class being always breached in front of
+streams, even of those which are dry during the greater part of the
+year, will be explained, when the conditions unfavourable to the growth
+of coral are considered. Low coral-islets, like those on barrier-reefs
+and atolls, are seldom formed on reefs of this class, owing apparently
+in some cases to their narrowness, and in others to the gentle slope of
+the reef outside not yielding many fragments to the breakers. On the
+windward side, however, of the Mauritius, two or three small islets
+have been formed.
+
+It appears, as will be shown in the ensuing chapter, that the action of
+the surf is favourable to the vigorous growth of the stronger corals,
+and that sand or sediment, if agitated by the waves, is injurious to
+them. Hence it is probable that a reef on a shelving shore, like that
+of Mauritius, would at first grow up, not attached to the actual beach,
+but at some little distance from it; and the corals on the outer margin
+would be the most vigorous. A shallow channel would thus be formed
+within the reef, and as the breakers are prevented acting on the shores
+of the island, and as they do not ordinarily tear up many fragments
+from the outside, and as every streamlet has its bed prolonged in a
+straight line through the reef, this channel could be filled up only
+very slowly with sediment. But a beach of sand and of fragments of the
+smaller kinds of coral seems, in the case of Mauritius, to be slowly
+encroaching on the shallow channel. On many shelving and sandy coasts,
+the breakers tend to form a bar of sand a little way from the beach,
+with a slight increase of depth within it; for instance, Captain
+Grey[2] states that the west coast of Australia, in latitude 24°, is
+fronted by a sand bar about two hundred yards in width, on which there
+is only two feet of water; but within it the depth increases to two
+fathoms. Similar bars, more or less perfect, occur on other coasts. In
+these cases I suspect that the shallow channel (which no doubt during
+storms is occasionally obliterated) is scooped out by the flowing away
+of the
+water thrown beyond the line, on which the waves break with the
+greatest force. At Pernambuco a bar of hard sandstone,[3] which has the
+same external form and height as a coral-reef, extends nearly parallel
+to the coast; within this bar currents, apparently caused by the water
+thrown over it during the greater part of each tide, run strongly, and
+are wearing away its inner wall. From these facts it can hardly be
+doubted, that within most fringing-reefs, especially within those lying
+some distance from the land, a return stream must carry away the water
+thrown over the outer edge; and the current thus produced, would tend
+to prevent the channel being filled up with sediment, and might even
+deepen it under certain circumstances. To this latter belief I am led,
+by finding that channels are almost universally present within the
+fringing-reefs of those islands which have undergone recent elevatory
+movements; and this could hardly have been the case, if the conversion
+of the very shallow channel into land had not been counteracted to a
+certain extent.
+
+ [2] Captain Grey’s “Journal of Two Expeditions,” vol. i, p. 369.
+
+
+ [3] I have described this singular structure in the _Lond. and Edin.
+ Phil. Mag.,_ October 1841.
+
+
+A fringing-reef, if elevated in a perfect condition above the level of
+the sea, ought to present the singular appearance of a broad dry moat
+within a low mound. The author[4] of an interesting pedestrian tour
+round the Mauritius, seems to have met with a structure of this kind:
+he says “J’observai que là, où la mer étale, indépendamment des rescifs
+du large, il y à terre _une espèce d’effoncement_ ou chemin couvert
+naturel. On y pourrait mettre du canon,” etc. In another place he adds,
+“Avant de passer le Cap, on remarque un gros banc de corail élevé de
+plus de quinze pieds: c’est une espèce de rescif, que la mer abandonné,
+il regne au pied une longue flaque d’eau, dont on pourrait faire un
+bassin pour de petits vaisseaux.” But the margin of the reef, although
+the highest and most perfect part, from being most exposed to the surf,
+would generally during a slow rise of the land be either partially or
+entirely worn down to that level, at which corals could renew their
+growth on its upper edge. On some parts of the coast-land of Mauritius
+there are little hillocks of coral-rock, which are either the last
+remnants of a continuous reef, or of low islets formed on it. I
+observed that two such hillocks between Tamarin Bay and the Great Black
+River; they were nearly twenty feet high, about two hundred yards from
+the present beach, and about thirty feet above its level. They rose
+abruptly from a smooth surface, strewed with worn fragments of coral.
+They consisted in their lower part of hard calcareous sandstone, and in
+their upper of great blocks of several species of Astræa and Madrepora,
+loosely aggregated; they were divided into irregular beds, dipping
+seaward, in one hillock at an angle of 8°, and in the other at 18°. I
+suspect that the superficial parts of the reefs, which have been
+upraised together with the islands they fringe, have generally been
+much more modified by the wearing action of the sea, than those of
+Mauritius.
+
+ [4] “Voyage à l’Isle de France, par un Officier du Roi,” part i, pp.
+ 192, 200.
+
+
+Many islands[5] are fringed by reefs quite similar to those of
+Mauritius; but on coasts where the sea deepens very suddenly the reefs
+are much narrower, and their limited extension seems evidently to
+depend on the high inclination of the submarine slope; a relation,
+which, as we have seen, does not exist in reefs of the barrier class.
+The fringing-reefs on steep coasts are frequently not more than from
+fifty to one hundred yards in width; they have a nearly smooth, hard
+surface, scarcely uncovered at low water, and without any interior
+shoal channel, like that within those fringing-reefs, which lie at a
+greater distance from the land. The fragments torn up during gales from
+the outer margin are thrown over the reef on the shores of the island.
+I may give as instances, Wateeo, where the reef is described by Cook as
+being a hundred yards wide; and Mauti and Elizabeth[6] Islands, where
+it is only fifty yards in width: the sea round these islands is very
+deep.
+
+ [5] I may give Cuba, as another instance; Mr. Taylor (_Loudon’s Mag.
+ of Nat. Hist.,_ vol. ix, p. 449) has described a reef several miles in
+ length between Gibara and Vjaro, which extends parallel to the shore
+ at the distance of between half and the third part of a mile, and
+ encloses a space of shallow water, with a sandy bottom and tufts of
+ coral. Outside the edge of the reef, which is formed of great
+ branching corals, the depth is six and seven fathoms. This coast has
+ been upheaved at no very distant geological period.
+
+
+ [6] Mauti is described by Lord Byron in the voyage of H.M.S. _
+ Blonde_, and Elizabeth Island by Captain Beechey.
+
+
+Fringing-reefs, like barrier-reefs, both surround islands, and front
+the shores of continents. In the charts of the eastern coast of Africa,
+by Captain Owen, many extensive fringing-reefs are laid down; thus, for
+a space of nearly forty miles, from latitude 1° 15′ to 1° 45′ S., a
+reef fringes the shore at an average distance of rather more than one
+mile, and therefore at a greater distance than is usual in reefs of
+this class; but as the coast-land is not lofty, and as the bottom
+shoals very gradually (the depth being only from eight to fourteen
+fathoms at a mile and a half outside the reef), its extension thus far
+from the land offers no difficulty. The external margin of this reef is
+described, as formed of projecting points, within which there is a
+space, from six to twelve feet deep, with patches of living coral on
+it. At Mukdeesha (lat. 2° 1′ N.) “the port is formed,” it is said,[7]
+“by a long reef extending eastward, four or five miles, within which
+there is a narrow channel, with ten to twelve feet of water at low
+spring-tides;” it lies at the distance of a quarter of a mile from the
+shore. Again, in the plan of Mombas (lat. 4° S.), a reef extends for
+thirty-six miles, at the distance of from half a mile to one mile and a
+quarter from the shore; within it, there is a channel navigable “for
+canoes and small craft,” between six and fifteen feet deep: outside the
+reef the depth is about thirty fathoms at the distance of nearly half a
+mile. Part of this reef is very symmetrical, and has a uniform breadth
+of two hundred yards.
+
+ [7] Owen’s “Africa,” vol. i, p. 357, from which work the foregoing
+ facts are likewise taken.
+
+
+The coast of Brazil is in many parts fringed by reefs. Of these, some
+are not of coral formation; for instance, those near Bahia and in front
+of Pernambuco; but a few miles south of this latter city, the reef
+follows[8] so closely every turn of the shore, that I can hardly doubt
+it is of coral; it runs at the distance of three-quarters of a mile
+from the land, and within it the depth is from ten to fifteen feet. I
+was assured by an intelligent pilot that at Ports Frances and Maceio,
+the outer part of the reef consists of living coral, and the inner of a
+white stone, full of large irregular cavities, communicating with the
+sea. The bottom of the sea off the coast of Brazil shoals gradually to
+between thirty and forty fathoms, at the distance of between nine and
+ten leagues from the land.
+
+ [8] See Baron Roussin’s “Pilote du Brésil,” and accompanying
+ hydrographical memoir.
+
+From the description now given, we must conclude that the dimensions
+and structure of fringing-reefs depend entirely on the greater or less
+inclination of the submarine slope, conjoined with the fact that
+reef-building polypifers can exist only at limited depths. It follows
+from this, that where the sea is very shallow, as in the Persian Gulf
+and in parts of the East Indian Archipelago, the reefs lose their
+fringing character, and appear as separate and irregularly scattered
+patches, often of considerable area. From the more vigorous growth of
+the coral on the outside, and from the conditions being less favourable
+in several respects within, such reefs are generally higher and more
+perfect in their marginal than in their central parts; hence these
+reefs sometimes assume (and this circumstance ought not to be
+overlooked) the appearance of atolls; but they differ from atolls in
+their central expanse being much less deep, in their form being less
+defined, and in being based on a shallow foundation. But when in a deep
+sea reefs fringe banks of sediment, which have accumulated beneath the
+surface, round either islands or submerged rocks, they are
+distinguished with difficulty on the one hand from encircling
+barrier-reefs, and on the other from atolls. In the West Indies there
+are reefs, which I should probably have arranged under both these
+classes, had not the existence of large and level banks, lying a little
+beneath the surface, ready to serve as the basis for the attachment of
+coral, been occasionally brought into view by the entire or partial
+absence of reefs on them, and had not the formation of such banks,
+through the accumulation of sediment now in progress, been sufficiently
+evident. Fringing-reefs sometimes coat, and thus protect the
+foundations of islands, which have been worn down by the surf to the
+level of the sea. According to Ehrenberg, this has been extensively the
+case with the islands in the Red Sea, which formerly ranged parallel to
+the shores of the mainland, with deep water within them: hence the
+reefs now coating their bases are situated relatively to the land like
+barrier-reefs, although not belonging to that class; but there are, as
+I believe, in the Red Sea some true barrier-reefs. The reefs of this
+sea and of the West Indies will be described in the Appendix. In some
+cases, fringing-reefs appear to be considerably modified in outline by
+the course of the prevailing currents. Dr. J. Allan informs me that on
+the east coast of Madagascar almost every headland and low point of
+sand has a coral-reef extending from it in a S.W. and N.E. line,
+parallel
+to the currents on that shore. I should think the influence of the
+currents chiefly consisted in causing an extension, in a certain
+direction, of a proper foundation for the attachment of the coral.
+Round many intertropical islands, for instance the Abrolhos on the
+coast of Brazil surveyed by Captain Fitzroy, and, as I am informed by
+Mr. Cuming, round the Philippines, the bottom of the sea is entirely
+coated by irregular masses of coral, which although often of large
+size, do not reach the surface and form proper reefs. This must be
+owing, either to insufficient growth, or to the absence of those kinds
+of corals which can withstand the breaking of the waves.
+
+The three classes, atoll-formed, barrier, and fringing-reefs, together
+with the modifications just described of the latter, include all the
+most remarkable coral formations anywhere existing. At the commencement
+of the last chapter in the volume, where I detail the principles on
+which the map (Plate III) is coloured, the exceptional cases will be
+enumerated.
+
+
+
+
+Chapter IV ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS
+
+
+In this chapter I will give all the facts which I have collected,
+relating to the distribution of coral-reefs,—to the conditions
+favourable to their increase,—to the rate of their growth,—and to the
+depth at which they are formed.
+
+These subjects have an important bearing on the theory of the origin of
+the different classes of coral-reefs.
+
+_Section I_—ON THE DISTRIBUTION OF CORAL-REEFS, AND ON THE CONDITIONS
+FAVOURABLE TO THEIR INCREASE
+
+With regard to the limits of latitude, over which coral-reefs extend, I
+have nothing new to add. The Bermuda Islands, in 32° 15′ N., is the
+point furthest removed from the equator, in which they appear to exist;
+and it has been suggested that their extension so far northward in this
+instance is owing to the warmth of the Gulf Stream. In the Pacific, the
+Loo Choo Islands, in latitude 27° N., have reefs on their shores, and
+there is an atoll in 28° 30′, situated N.W. of the Sandwich
+Archipelago. In the Red Sea there are coral-reefs in latitude 30°. In
+the southern hemisphere coral-reefs do not extend so far from the
+equatorial sea. In the Southern Pacific there are only a few reefs
+beyond the line of the tropics, but Houtmans Abrolhos, on the western
+shores of Australia in latitude 29° S., are of coral formation.
+
+The proximity of volcanic land, owing to the lime generally evolved
+from it, has been thought to be favourable to the increase of
+coral-reefs.
+There is, however, not much foundation for this view; for nowhere are
+coral-reefs more extensive than on the shores of New Caledonia, and of
+north-eastern Australia, which consist of primary formations; and in
+the largest groups of atolls, namely the Maldiva, Chagos, Marshall,
+Gilbert, and Low Archipelagoes, there is no volcanic or other kind of
+rock, excepting that formed of coral.
+
+The entire absence of coral-reefs in certain large areas within the
+tropical seas, is a remarkable fact. Thus no coral-reefs were observed,
+during the surveying voyages of the _Beagle_ and her tender on the west
+coast of South America south of the equator, or round the Galapagos
+Islands. It appears, also, that there are none[1] north of the equator;
+Mr. Lloyd, who surveyed the Isthmus of Panama, remarked to me, that
+although he had seen corals living in the Bay of Panama, yet he had
+never observed any reefs formed by them. I at first attributed this
+absence of reefs on the coasts of Peru and of the Galapagos Islands,[2]
+to the coldness of the currents from the south, but the Gulf of Panama
+is one of the hottest pelagic districts in the world.[3] In the central
+parts of the Pacific there are islands entirely free from reefs; in
+some few of these cases I have thought that this was owing to recent
+volcanic action; but the existence of reefs round the greater part of
+Hawaii, one of the Sandwich Islands, shows that recent volcanic action
+does not necessarily prevent their growth.
+
+ [1] I have been informed that this is the case, by Lieutenant Ryder,
+ R.N., and others who have had ample opportunities for observation.
+
+
+ [2] The mean temperature of the surface sea from observations made by
+ the direction of Captain Fitzroy on the shores of the Galapagos
+ Islands, between the 16th of September and the 20th of October, 1835,
+ was 68° Fahr. The lowest temperature observed was 58.5° at the
+ south-west end of Albemarle Island; and on the west coast of this
+ island, it was several times 62° and 63°. The mean temperature of the
+ sea in the Low Archipelago of atolls, and near Tahiti, from similar
+ observations made on board the _Beagle_, was (although further from
+ the equator) 77.5°, the lowest any day being 76.5°. Therefore we have
+ here a difference of 9.5° in mean temperature, and 18° in extremes; a
+ difference doubtless quite sufficient to affect the distribution of
+ organic beings in the two areas.
+
+
+ [3] Humboldt’s “Personal Narrative,” vol. vii, p. 434.
+
+In the last chapter I stated that the bottom of the sea round some
+islands is thickly coated with living corals, which nevertheless do not
+form reefs, either from insufficient growth, or from the species not
+being adapted to contend with the breaking waves.
+
+I have been assured by several people, that there are no coral-reefs on
+the west coast of Africa,[4] or round the islands in the Gulf of
+Guinea. This perhaps may be attributed, in part, to the sediment
+brought down by the many rivers debouching on that coast, and to the
+extensive mud-banks,
+which line great part of it. But the islands of St. Helena, Ascension,
+the Cape Verdes, St. Paul’s, and Fernando Noronha, are, also, entirely
+without reefs, although they lie far out at sea, are composed of the
+same ancient volcanic rocks, and have the same general form, with those
+islands in the Pacific, the shores of which are surrounded by gigantic
+walls of coral-rock. With the exception of Bermuda, there is not a
+single coral-reef in the central expanse of the Atlantic Ocean. It
+will, perhaps, be suggested that the quantity of carbonate of lime in
+different parts of the sea, may regulate the presence of reefs. But
+this cannot be the case, for at Ascension, the waves charged to excess
+precipitate a thick layer of calcareous matter on the tidal rocks; and
+at St. Jago, in the Cape Verdes, carbonate of lime not only is abundant
+on the shores, but it forms the chief part of some upraised
+post-tertiary strata. The apparently capricious distribution,
+therefore, of coral-reefs, cannot be explained by any of these obvious
+causes; but as the study of the terrestrial and better known half of
+the world must convince every one that no station capable of supporting
+life is lost,—nay more, that there is a struggle for each station,
+between the different orders of nature,—we may conclude that in those
+parts of the intertropical sea, in which there are no coral-reefs,
+there are other organic bodies supplying the place of the reef-building
+polypifers. It has been shown in the chapter on Keeling atoll that
+there are some species of large fish, and the whole tribe of Holothuriæ
+which prey on the tenderer parts of the corals. On the other hand, the
+polypifers in their turn must prey on some other organic beings; the
+decrease of which from any cause would cause a proportionate
+destruction of the living coral. The relations, therefore, which
+determine the formation of reefs on any shore, by the vigorous growth
+of the efficient kinds of coral, must be very complex, and with our
+imperfect knowledge quite inexplicable. From these considerations, we
+may infer that changes in the condition of the sea, not obvious to our
+senses, might destroy all the coral-reefs in one area, and cause them
+to appear in another: thus, the Pacific or Indian Ocean might become as
+barren of coral-reefs as the Atlantic now is, without our being able to
+assign any adequate cause for such a change.
+
+ [4] It might be concluded, from a paper by Captain Owen (_Geograph.
+ Journ._, vol. ii, p. 89), that the reefs off Cape St. Anne and the
+ Sherboro’ Islands were of coral, although the author states that they
+ are not purely coralline. But I have been assured by Lieutenant
+ Holland, R.N., that these reefs are not of coral, or at least that
+ they do not at all resemble those in the West Indies.
+
+
+It has been a question with some naturalists, which part of a reef is
+most favourable to the growth of coral. The great mounds of living
+Porites and of Millepora round Keeling atoll occur exclusively on the
+extreme verge of the reef, which is washed by a constant succession of
+breakers; and living coral nowhere else forms solid masses. At the
+Marshall islands the larger kinds of coral (chiefly species of Astræa,
+a genus closely allied to Porites) “which form rocks measuring several
+fathoms in thickness,” prefer, according to Chamisso,[5] the most
+violent surf. I have stated that the outer margin of the Maldiva atolls
+consists of living corals (some of which, if not all, are of the same
+species with those at Keeling atoll), and here the surf is so
+tremendous, that even large ships have been thrown, by a single heave
+of the sea, high and dry on the reef, all on board thus escaping with
+their lives.
+
+
+Ehrenberg[6] remarks, that in the Red Sea the strongest corals live on
+the outer reefs, and appear to love the surf; he adds, that the more
+branched kinds abound a little way within, but that even these in still
+more protected places, become smaller. Many other facts having a
+similar tendency might be adduced.[7] It has, however, been doubted by
+MM. Quoy and Gaimard, whether any kind of coral can even withstand,
+much less flourish in, the breakers of an open sea:[8] they affirm that
+the saxigenous lithophytes flourish only where the water is tranquil,
+and the heat intense. This statement has passed from one geological
+work to another; nevertheless, the protection of the whole reef
+undoubtedly is due to those kinds of coral, which cannot exist in the
+situations thought by these naturalists to be most favourable to them.
+For should the outer and living margin perish, of any one of the many
+low coral-islands, round which a line of great breakers is incessantly
+foaming, the whole, it is scarcely possible to doubt, would be washed
+away and destroyed, in less than half a century. But the vital energies
+of the corals conquer the mechanical power of the waves; and the large
+fragments of reef torn up by every storm, are replaced by the slow but
+steady growth of the innumerable polypifers, which form the living zone
+on its outer edge.
+
+ [5] Kotzebue’s “First Voyage” (Eng. Trans.), vol. iii, pp. 142, 143,
+ 331.
+
+
+ [6] Ehrenberg, “Über die Natur und Bildung der Corallen Bänke im
+ rothen Meere,” p. 49.
+
+
+ [7] In the West Indies, as I am informed by Captain Bird Allen, R.N.,
+ it is the common belief of those, who are best acquainted with the
+ reefs, that the coral flourishes most, where freely exposed to the
+ swell of the open sea.
+
+
+ [8] “Annales des Sciences Naturelles,” tome vi, pp. 276, 278.—“Là où
+ les ondes sont agitées, les Lytophytés ne peuvent travailler, parce
+ qu’elles détruiraient leurs fragiles édifices,” etc.
+
+
+From these facts, it is certain, that the strongest and most massive
+corals flourish, where most exposed. The less perfect state of the reef
+of most atolls on the leeward and less exposed side, compared with its
+state to windward; and the analogous case of the greater number of
+breaches on the near sides of those atolls in the Maldiva Archipelago,
+which afford some protection to each other, are obviously explained by
+this circumstance. If the question had been, under what conditions the
+greater number of species of coral, not regarding their bulk and
+strength, were developed, I should answer,—probably in the situations
+described by MM. Quoy and Gaimard, where the water is tranquil and the
+heat intense. The total number of species of coral in the
+circumtropical seas must be very great: in the Red Sea alone, 120
+kinds, according to Ehrenberg,[9] have been observed.
+
+ [9] Ehrenberg, “Über die Natur,” etc., p. 46.
+
+
+The same author has observed that the recoil of the sea from a steep
+shore is injurious to the growth of coral, although waves breaking over
+a bank are not so. Ehrenberg also states, that where there is much
+sediment, placed so as to be liable to be moved by the waves there is
+little or no coral; and a collection of living specimens placed by him
+on a sandy shore died in the course of a few days.[10] An experiment,
+however, will presently be related in which some large masses of living
+coral increased rapidly in size, after having been secured by stakes on
+a sandbank. That loose sediment should be injurious to the living
+polypifers, appears, at first sight, probable; and accordingly, in
+sounding off Keeling atoll, and (as will hereafter be shown) off
+Mauritius, the arming of the lead invariably came up clean, where the
+coral was growing vigorously. This same circumstance has probably given
+rise to a strange belief, which, according to Captain Owen,[11] is
+general amongst the inhabitants of the Maldiva atolls, namely that
+corals have roots, and therefore that if merely broken down to the
+surface, they grow up again; but, if rooted out, they are permanently
+destroyed. By this means the inhabitants keep their harbours clear; and
+thus the French Governor of St. Mary’s in Madagascar, “cleared out and
+made a beautiful little port at that place.” For it is probable that
+sand would accumulate in the hollows formed by tearing out the corals,
+but not on the broken and projecting stumps, and therefore, in the
+former case, the fresh growth of the coral might be thus prevented.
+
+ [10] _Ibid_., p. 49.
+
+
+ [11] Captain Owen on the Geography of the Maldiva Islands, _Geograph.
+ Journal_, vol. ii, p. 88.
+
+
+In the last chapter I remarked that fringing-reefs are almost
+universally breached, where streams enter the sea.[12] Most authors
+have attributed this fact to the injurious effects of the fresh water,
+even where it enters the sea only in small quantity, and during a part
+of the year. No doubt brackish water would prevent or retard the growth
+of coral; but I believe that the mud and sand which is deposited, even
+by rivulets when flooded, is a much more efficient check. The reef on
+each side of the channel leading into Port Louis at Mauritius, ends
+abruptly in a wall, at the foot of which I sounded and found a bed of
+thick mud. This steepness of the sides appears to be a general
+character in such breaches. Cook,[13] speaking of one at Raiatea, says,
+“like all the rest, it is very steep on both sides.” Now, if it were
+the fresh water mingling with the salt which prevented the growth of
+coral, the reef certainly would not terminate abruptly, but as the
+polypifers nearest the impure stream would grow less vigorously than
+those farther off, so would the reef gradually thin away. On the other
+hand, the sediment brought down from the land would only prevent the
+growth of the coral in the line of its deposition, but would not check
+it on the side, so that the reefs might increase till they overhung the
+bed of the channel. The breaches are much fewer in number, and front
+only the larger valleys in reefs of the encircling barrier class. They
+probably are kept open in the same manner as those into the lagoon of
+an atoll, namely, by the
+force of the currents and the drifting outwards of fine sediment. Their
+position in front of valleys, although often separated from the land by
+deep water lagoon-channels, which it might be thought would entirely
+remove the injurious effects both of the fresh water and the sediment,
+will receive a simple explanation when we discuss the origin of
+barrier-reefs.
+
+ [12] Lieutenant Wellstead and others have remarked that this is the
+ case in the Red Sea; Dr. Rüppell (“Reise in Abyss.” Band. i, p. 142)
+ says that there are pear-shaped harbours in the upraised coral-coast,
+ into which periodical streams enter. From this circumstance, I
+ presume, we must infer that before the upheaval of the strata now
+ forming the coast-land, fresh water and sediment entered the sea at
+ these points; and the coral being thus prevented growing, the
+ pear-shaped harbours were produced.
+
+
+ [13] Cook’s “First Voyage,” vol. ii, p. 271 (Hawkesworth’s edit.)
+
+
+In the vegetable kingdom every different station has its peculiar group
+of plants, and similar relations appear to prevail with corals. We have
+already described the great difference between the corals within the
+lagoon of an atoll and those on its outer margin. The corals, also, on
+the margin of Keeling Island occurred in zones; thus the _Porites_ and
+_Millepora complanata_ grow to a large size only where they are washed
+by a heavy sea, and are killed by a short exposure to the air; whereas,
+three species of Nullipora also live amidst the breakers, but are able
+to survive uncovered for a part of each tide; at greater depths, a
+strong Madrepora and _Millepora alcicornis_ are the commonest kinds,
+the former appearing to be confined to this part, beneath the zone of
+massive corals, minute encrusting corallines and other organic bodies
+live. If we compare the external margin of the reef at Keeling atoll
+with that on the leeward side of Mauritius, which are very differently
+circumstanced, we shall find a corresponding difference in the
+appearance of the corals. At the latter place, the genus Madrepora is
+preponderant over every other kind, and beneath the zone of massive
+corals there are large beds of Seriatopora. There is also a marked
+difference, according to Captain Moresby,[14]between the great
+branching corals of the Red Sea, and those on the reefs of the Maldiva
+atolls.
+
+ [14] Captain Moresby on the Northern Maldiva atolls, _Geograph.
+ Journal_, vol. v, p. 401.
+
+
+These facts, which in themselves are deserving of notice, bear,
+perhaps, not very remotely, on a remarkable circumstance which has been
+pointed out to me by Captain Moresby, namely, that with very few
+exceptions, none of the coral-knolls within the lagoons of Peros
+Banhos, Diego Garcia, and the Great Chagos Bank (all situated in the
+Chagos group), rise to the surface of the water; whereas all those,
+with equally few exceptions, within Solomon and Egmont atolls in the
+same group, and likewise within the large southern Maldiva atolls,
+reach the surface. I make these statements, after having examined the
+charts of each atoll. In the lagoon of Peros Banhos, which is nearly
+twenty miles across, there is only one single reef which rises to the
+surface; in Diego Garcia there are seven, but several of these lie
+close to the margin of the lagoon, and need scarcely have been
+reckoned; in the Great Chagos Bank there is not one. On the other hand,
+in the lagoons of some of the great southern Maldiva atolls, although
+thickly studded with reefs, every one without exception rises to the
+surface; and on an average there are less than two submerged reefs in
+each atoll; in the northern atolls, however, the submerged lagoon-reefs
+are not quite so rare. The submerged reefs in the Chagos atolls
+generally have from one to seven fathoms water on them, but some have
+from seven to ten. Most of them are small with
+very steep sides;[15] at Peros Banhos they rise from a depth of about
+thirty fathoms, and some of them in the Great Chagos Bank from above
+forty fathoms; they are covered, Captain Moresby informs me, with
+living and healthy coral, two and three feet high, consisting of
+several species. Why then have not these lagoon-reefs reached the
+surface, like the innumerable ones in the atolls above named? If we
+attempt to assign any difference in their external conditions, as the
+cause of this diversity, we are at once baffled. The lagoon of Diego
+Garcia is not deep, and is almost wholly surrounded by its reef; Peros
+Banhos is very deep, much larger, with many wide passages communicating
+with the open sea. On the other hand, of those atolls, in which all or
+nearly all the lagoon-reefs have reached the surface, some are small,
+others large, some shallow, others deep, some well-enclosed, and others
+open.
+
+ [15] Some of these statements were not communicated to me verbally by
+ Captain Moresby, but are taken from the MS. account before alluded to,
+ of the Chagos Group.
+
+Captain Moresby informs me that he has seen a French chart of Diego
+Garcia made eighty years before his survey, and apparently very
+accurate; and from it he infers, that during this interval there has
+not been the smallest change in the depth on any of the knolls within
+the lagoon. It is also known that during the last fifty-one years, the
+eastern channel into the lagoon has neither become narrower, nor
+decreased in depth; and as there are numerous small knolls of living
+coral within it, some change might have been anticipated. Moreover, as
+the whole reef round the lagoon of this atoll has been converted into
+land—an unparalleled case, I believe, in an atoll of such large
+size,—and as the strip of land is for considerable spaces more than
+half a mile wide—also a very unusual circumstance,—we have the best
+possible evidence, that Diego Garcia has remained at its present level
+for a very long period. With this fact, and with the knowledge that no
+sensible change has taken place during eighty years in the
+coral-knolls, and considering that every single reef has reached the
+surface in other atolls, which do not present the smallest appearance
+of being older than Diego Garcia and Peros Banhos, and which are placed
+under the same external conditions with them, one is led to conclude
+that these submerged reefs, although covered with luxuriant coral, have
+no tendency to grow upwards, and that they would remain at their
+present levels for an almost indefinite period.
+
+From the number of these knolls, from their position, size, and form,
+many of them being only one or two hundred yards across, with a rounded
+outline, and precipitous sides,—it is indisputable that they have been
+formed by the growth of coral; and this makes the case much more
+remarkable. In Peros Banhos and in the Great Chagos Bank, some of these
+almost columnar masses are 200 feet high, and their summits lie only
+from two to eight fathoms beneath the surface; therefore, a small
+proportional amount more of growth would cause them to attain the
+surface, like those numerous knolls, which rise from an equally great
+depth within the Maldiva atolls. We can hardly suppose that time has
+been wanting for the upward growth of
+the coral, whilst in Diego Garcia, the broad annular strip of land,
+formed by the continued accumulation of detritus, shows how long this
+atoll has remained at its present level. We must look to some other
+cause than the rate of growth; and I suspect it will be found in the
+reefs being formed of different species of corals, adapted to live at
+different depths. The Great Chagos Bank is situated in the centre of
+the Chagos Group, and the Pitt and Speaker Banks at its two extreme
+points. These banks resemble atolls, except in their external rim being
+about eight fathoms submerged, and in being formed of dead rock, with
+very little living coral on it: a portion nine miles long of the
+annular reef of Peros Banhos atoll is in the same condition. These
+facts, as will hereafter be shown, render it very probable that the
+whole group at some former period subsided seven or eight fathoms; and
+that the coral perished on the outer margin of those atolls which are
+now submerged, but that it continued alive, and grew up to the surface
+on those which are now perfect. If these atolls did subside, and if
+from the suddenness of the movement or from any other cause, those
+corals which are better adapted to live at a certain depth than at the
+surface, once got possession of the knolls, supplanting the former
+occupants, they would exert little or no tendency to grow upwards. To
+illustrate this, I may observe, that if the corals of the upper zone on
+the outer edge of Keeling atoll were to perish, it is improbable that
+those of the lower zone would grow to the surface, and thus become
+exposed to conditions for which they do not appear to be adapted. The
+conjecture, that the corals on the submerged knolls within the Chagos
+atolls have analogous habits with those of the lower zone outside
+Keeling atoll, receives some support from a remark by Captain Moresby,
+namely, that they have a different appearance from those on the reefs
+in the Maldiva atolls, which, as we have seen, all rise to the surface:
+he compares the kind of difference to that of the vegetation under
+different climates. I have entered at considerable length into this
+case, although unable to throw much light on it, in order to show that
+an equal tendency to upward growth ought not to be attributed to all
+coral-reefs,—to those situated at different depths,—to those forming
+the ring of an atoll or those on the knolls within a lagoon,—to those
+in one area and those in another. The inference, therefore, that one
+reef could not grow up to the surface within a given time, because
+another, not known to be covered with the same species of corals, and
+not known to be placed under conditions exactly the same, has not
+within the same time reached the surface, is unsound.
+
+_Section II_—ON THE RATE OF GROWTH OF CORAL-REEFS
+
+The remark made at the close of the last section, naturally leads to
+this division of our subject, which has not, I think, hitherto been
+considered under a right point of view. Ehrenberg[16] has stated, that
+in
+the Red Sea, the corals only coat other rocks in a layer from one to
+two feet in thickness, or at most to a fathom and a half; and he
+disbelieves that, in any case, they form, by their own proper growth,
+great masses, stratum over stratum. A nearly similar observation has
+been made by MM. Quoy and Gaimard,[17] with respect to the thickness of
+some upraised beds of coral, which they examined at Timor and some
+other places. Ehrenberg[18] saw certain large massive corals in the Red
+Sea, which he imagines to be of such vast antiquity, that they might
+have been beheld by Pharaoh; and according to Mr. Lyell[19] there are
+certain corals at Bermuda, which are known by tradition, to have been
+living for centuries. To show how slowly coral-reefs grow upwards,
+Captain Beechey[20] has adduced the case of the Dolphin Reef off
+Tahiti, which has remained at the same depth beneath the surface,
+namely about two fathoms and a half, for a period of sixty-seven years.
+There are reefs in the Red Sea, which certainly do not appear[21] to
+have increased in dimensions during the last half-century, and from the
+comparison of old charts with recent surveys, probably not during the
+last two hundred years. These, and other similar facts, have so
+strongly impressed many with the belief of the extreme slowness of the
+growth of corals, that they have even doubted the possibility of
+islands in the great oceans having been formed by their agency. Others,
+again, who have not been overwhelmed by this difficulty, have admitted
+that it would require thousands, and tens of thousands of years, to
+form a mass, even of inconsiderable thickness; but the subject has not,
+I believe, been viewed in the proper light.
+
+ [16] Ehrenberg, as before cited, pp. 39, 46, and 50.
+
+
+ [17] “Annales des Sciences Nat.” tom. vi, p. 28.
+
+
+ [18] Ehrenberg, _ut sup._, p. 42.
+
+
+ [19] Lyell’s “Principles of Geology,” book iii, ch. xviii.
+
+
+ [20] Beechey’s “Voyage to the Pacific,” ch. viii.
+
+
+ [21] Ehrenberg, _ut sup._, p. 43.
+
+That masses of considerable thickness have been formed by the growth of
+coral, may be inferred with certainty from the following facts. In the
+deep lagoons of Peros Banhos and of the Great Chagos Bank, there are,
+as already described, small steep-sided knolls covered with living
+coral. There are similar knolls in the southern Maldiva atolls, some of
+which, as Captain Moresby assures me, are less than a hundred yards in
+diameter, and rise to the surface from a depth of between two hundred
+and fifty and three hundred feet. Considering their number, form, and
+position, it would be preposterous to suppose that they are based on
+pinnacles of any rock, not of coral formation; or that sediment could
+have been heaped up into such small and steep isolated cones. As no
+kind of living coral grows above the height of a few feet, we are
+compelled to suppose that these knolls have been formed by the
+successive growth and death of many individuals,—first one being broken
+off or killed by some accident, and then another, and one set of
+species being replaced by another set with different habits, as the
+reef rose nearer the surface, or as other changes supervened. The
+spaces between the corals would become filled up with fragments and
+sand, and such matter would probably soon be consolidated, for we learn
+from
+Lieutenant Nelson,[22] that at Bermuda a process of this kind takes
+place beneath water, without the aid of evaporation. In reefs, also, of
+the barrier class, we may feel sure, as I have shown, that masses of
+great thickness have been formed by the growth of the coral; in the
+case of Vanikoro, judging only from the depth of the moat between the
+land and the reef, the wall of coral-rock must be at least three
+hundred feet in vertical thickness.
+
+ [22] “Geological Transactions,” vol. v, p. 113.
+
+
+It is unfortunate that the upraised coral-islands in the Pacific have
+not been examined by a geologist. The cliffs of Elizabeth Island, in
+the Low Archipelago, are eighty feet high, and appear, from Captain
+Beechey’s description, to consist of a homogeneous coral-rock. From the
+isolated position of this island, we may safely infer that it is an
+upraised atoll, and therefore that it has been formed by masses of
+coral, grown together. Savage Island seems, from the description of the
+younger Forster,[23] to have a similar structure, and its shores are
+about forty feet high: some of the Cook Islands also appear[24] to be
+similarly composed. Captain Belcher, R.N., in a letter which Captain
+Beaufort showed me at the admiralty, speaking of Bow atoll, says, “I
+have succeeded in boring forty-five feet through coral-sand, when the
+auger became jammed by the falling in of the surrounding _ creamy_
+matter.” On one of the Maldiva atolls, Captain Moresby bored to a depth
+of twenty-six feet, when his auger also broke: he has had the kindness
+to give me the matter brought up; it is perfectly white, and like
+finely triturated coral-rock.
+
+ [23] Forster’s “Voyage round the World with Cook,” vol. ii, pp. 163,
+ 167.
+
+
+ [24] Williams’s “Narrative of Missionary Enterprise,” p. 30.
+
+
+In my description of Keeling atoll, I have given some facts, which show
+that the reef probably has grown outwards; and I have found, just
+within the outer margin, the great mounds of Porites and of Millepora,
+with their summits lately killed, and their sides subsequently
+thickened by the growth of the coral: a layer, also, of Nullipora had
+already coated the dead surface. As the external slope of the reef is
+the same round the whole of this atoll, and round many other atolls,
+the angle of inclination must result from an adaption between the
+growing powers of the coral, and the force of the breakers, and their
+action on the loose sediment. The reef, therefore, could not increase
+outwards, without a nearly equal addition to every part of the slope,
+so that the original inclination might be preserved, and this would
+require a large amount of sediment, all derived from the wear of corals
+and shells, to be added to the lower part. Moreover, at Keeling atoll,
+and probably in many other cases, the different kinds of corals would
+have to encroach on each other; thus the Nulliporæ cannot increase
+outwards without encroaching on the Porites and _ Millepora
+complanata,_ as is now taking place; nor these latter without
+encroaching on the strongly branched Madreporet, the _ Millepora
+alcicornis,_ and some Astræas; nor these again without a foundation
+being formed for them within the requisite depth, by the accumulation
+of sediment. How slow, then, must be the ordinary lateral or outward
+growth of such reefs. But off
+Christmas atoll, where the sea is much more shallow than is usual, we
+have good reason to believe that, within a period not very remote, the
+reef has increased considerably in width. The land has the
+extraordinary breadth of three miles; it consists of parallel ridges of
+shells and broken corals, which furnish “an incontestable proof,” as
+observed by Cook,[25] “that the island has been produced by accessions
+from the sea, and is in a state of increase.” The land is fronted by a
+coral-reef, and from the manner in which islets are known to be formed,
+we may feel confident that the reef was not three miles wide, when the
+first, or most backward ridge, was thrown up; and, therefore, we must
+conclude that the reef has grown outwards during the accumulation of
+the successive ridges. Here then, a wall of coral-rock of very
+considerable breadth has been formed by the outward growth of the
+living margin, within a period during which ridges of shells and
+corals, lying on the bare surface, have not decayed. There can be
+little doubt, from the account given by Captain Beechey, that Matilda
+atoll, in the Low Archipelago, has been converted in the space of
+thirty-four years, from being, as described by the crew of a wrecked
+whaling vessel, a “reef of rocks” into a lagoon-island, fourteen miles
+in length, with “one of its sides covered nearly the whole way with
+high trees.”[26] The islets, also, on Keeling atoll, it has been shown,
+have increased in length, and since the construction of an old chart,
+several of them have become united into one long islet; but in this
+case, and in that of Matilda atoll, we have no proof, and can only
+infer as probable, that the reef, that is the foundation of the islets,
+has increased as well as the islets themselves.
+
+ [25] Cook’s “Third Voyage,” book III, ch. x.
+
+
+ [26] Beechey’s “Voyage to the Pacific,” ch. vii and viii.
+
+
+After these considerations, I attach little importance, as indicating
+the ordinary and still less the possible rate of _ outward_ growth of
+coral-reefs, to the fact that certain reefs in the Red Sea have not
+increased during a long interval of time; or to other such cases, as
+that of Ouluthy atoll in the Caroline group, where every islet,
+described a thousand years before by Cantova was found in the same
+state by Lutké,[27]—without it could be shown that, in these cases, the
+conditions were favourable to the vigorous and unopposed growth of the
+corals living in the different zones of depth, and that a proper basis
+for the extent of the reef was present. The former conditions must
+depend on many contingencies, and in the deep oceans where coral
+formations most abound, a basis within the requisite depth can rarely
+be present.
+
+ [27] F. Lutké’s “Voyage autour du Monde.” In the group Elato, however,
+ it appears that what is now the islet Falipi, is called in Cantova’s
+ Chart, the Banc de Falipi. It is not stated whether this has been
+ caused by the growth of coral, or by the accumulation of sand.
+
+Nor do I attach any importance to the fact of certain submerged reefs,
+as those off Tahiti, or those within Diego Garcia not now being nearer
+the surface than they were many years ago, as an indication of the rate
+under favourable circumstances of the _upward_ growth of reefs; after
+it has been shown, that all the reefs have grown to the surface in some
+of the Chagos atolls, but that in neighbouring atolls which appear to
+be of equal antiquity and to be exposed to the same external
+conditions, every reef remains submerged; for we are almost driven to
+attribute this to a difference, not in the rate of growth, but in the
+habits of the corals in the two cases.
+
+In an old-standing reef, the corals, which are so different in kind on
+different parts of it, are probably all adapted to the stations they
+occupy, and hold their places, like other organic beings, by a struggle
+one with another, and with external nature; hence we may infer that
+their growth would generally be slow, except under peculiarly
+favourable circumstances. Almost the only natural condition, allowing a
+quick upward growth of the whole surface of a reef, would be a slow
+subsidence of the area in which it stood; if, for instance, Keeling
+atoll were to subside two or three feet, can we doubt that the
+projecting margin of live coral, about half an inch in thickness, which
+surrounds the dead upper surfaces of the mounds of Porites, would in
+this case form a concentric layer over them, and the reef thus increase
+upwards, instead of, as at present, outwards? The Nulliporæ are now
+encroaching on the Porites and Millepora, but in this case might we not
+confidently expect that the latter would, in their turn, encroach on
+the Nulliporæ? After a subsidence of this kind, the sea would gain on
+the islets, and the great fields of dead but upright corals in the
+lagoon, would be covered by a sheet of clear water; and might we not
+then expect that these reefs would rise to the surface, as they
+anciently did when the lagoon was less confined by islets, and as they
+did within a period of ten years in the schooner-channel, cut by the
+inhabitants? In one of the Maldiva atolls, a reef, which within a very
+few years existed as an islet bearing cocoa-nut trees, was found by
+Lieutenant Prentice “_entirely covered with live coral and Madrepore._”
+The natives believe that the islet was washed away by a change in the
+currents, but if, instead of this, it had quietly subsided, surely
+every part of the island which offered a solid foundation, would in a
+like manner have become coated with living coral.
+
+Through steps such as these, any thickness of rock, composed of a
+singular intermixture of various kinds of corals, shells, and
+calcareous sediment, might be formed; but without subsidence, the
+thickness would necessarily be determined by the depth at which the
+reef-building polypifers can exist. If it be asked, at what rate in
+years I suppose a reef of coral favourably circumstanced could grow up
+from a given depth; I should answer, that we have no precise evidence
+on this point, and comparatively little concern with it. We see, in
+innumerable points over wide areas, that the rate has been sufficient,
+either to bring up the reefs from various depths to the surface, or, as
+is more probable, to keep them at the surface, during progressive
+subsidences; and this is a much more important standard of comparison
+than any cycle of years.
+
+It may, however, be inferred from the following facts, that the rate
+in years under favourable circumstances would be very far from slow.
+Dr. Allan, of Forres, has, in his MS. Thesis deposited in the library
+of the Edinburgh University (extracts from which I owe to the kindness
+of Dr. Malcolmson), the following account of some experiments, which he
+tried during his travels in the years 1830 to 1832 on the east coast of
+Madagascar. “To ascertain the rise and progress of the coral-family,
+and fix the number of species met with at Foul Point (latitude 17° 40′)
+twenty species of coral were taken off the reef and planted apart on a
+sand-bank _three feet deep at low water._ Each portion weighed ten
+pounds, and was kept in its place by stakes. Similar quantities were
+placed in a clump and secured as the rest. This was done in December
+1830. In July following, each detached mass was nearly level with the
+sea at low water, quite immovable, and several feet long, stretching as
+the parent reef, with the coast current from north to south. The masses
+accumulated in a clump were found equally increased, but some of the
+species in such unequal ratios, as to be growing over each other.” The
+loss of Dr. Allan’s magnificent collection by shipwreck, unfortunately
+prevents its being known to what genera these corals belonged; but from
+the numbers experimented on, it is certain that all the more
+conspicuous kinds must have been included. Dr. Allan informs me, in a
+letter, that he believes it was a Madrepora, which grew most
+vigorously. One may be permitted to suspect that the level of the sea
+might possibly have been somewhat different at the two stated periods;
+nevertheless, it is quite evident that the growth of the ten-pound
+masses, during the six or seven months, at the end of which they were
+found immovably fixed[28] and several feet in length, must have been
+very great. The fact of the different kinds of coral, when placed in
+one clump, having increased in extremely unequal ratios, is very
+interesting, as it shows the manner in which a reef, supporting many
+species of coral, would probably be affected by a change in the
+external conditions favouring one kind more than another. The growth of
+the masses of coral in N. and S. lines parallel to the prevailing
+currents, whether due to the drifting of sediment or to the simple
+movement of the water, is, also, a very interesting circumstance.
+
+ [28] It is stated by De la Beche (“Geological Manual,” p. 143), on the
+ authority of Mr. Lloyd, who surveyed the Isthmus of Panama, that some
+ specimens of Polypifers, placed by him in a sheltered pool of water,
+ were found in the course of a few days firmly fixed by the secretion
+ of a stony matter, to the bottom.
+
+
+A fact, communicated to me by Lieutenant Wellstead, I.N., in some
+degree corroborates the result of Dr. Allan’s experiments: it is, that
+in the Persian Gulf a ship had her copper bottom encrusted in the
+course of twenty months with a layer of coral, _two feet_ in thickness,
+which it required great force to remove, when the vessel was docked: it
+was not ascertained to what order this coral belonged. The case of the
+schooner-channel choked up with coral in an interval of less than ten
+years, in the lagoon of Keeling atoll, should be here borne
+in mind. We may also infer, from the trouble which the inhabitants of
+the Maldiva atolls take to root out, as they express it, the
+coral-knolls from their harbours, that their growth can hardly be very
+slow.[29]
+
+ [29] Mr. Stutchbury (_West of England Journal_, No. I, p. 50.) has
+ described a specimen of Agaricia, “weighing 2 lbs. 9 oz., which
+ surrounds a species of oyster, whose age could not be more than two
+ years, and yet is completely enveloped by this dense coral.” I presume
+ that the oyster was living when the specimen was procured; otherwise
+ the fact tells nothing. Mr. Stutchbury also mentions an anchor, which
+ had become entirely encrusted with coral in fifty years; other cases,
+ however, are recorded of anchors which have long remained amidst
+ coral-reefs without having become coated. The anchor of the _Beagle_,
+ in 1832, after having been down exactly one month at Rio de Janeiro,
+ was so thickly coated by two species of Tubularia, that large spaces
+ of the iron were entirely concealed; the tufts of this horny zoophyte
+ were between two and three inches in length. It has been attempted to
+ compute, but I believe erroneously, the rate of growth of a reef, from
+ the fact mentioned by Captain Beechey, of the _ Chama gigas_ being
+ embedded in coral-rock. But it should be remembered, that some species
+ of this genus invariably live, both whilst young and old, in cavities,
+ which the animal has the power of enlarging with its growth. I saw
+ many of these shells thus embedded in the outer “flat” of Keeling
+ atoll, which is composed of dead rock; and therefore the cavities in
+ this case had no relation whatever with the growth of coral. M.
+ Lesson, also, speaking of this shell (Partie Zoolog. “Voyage de la
+ _Coquille_”), has remarked, “que constamment ses valves étaient
+ engagés complétement dans la masse des Madrepores.”
+
+From the facts given in this section, it may be concluded, first, that
+considerable thicknesses of rock have certainly been formed within the
+present geological area by the growth of coral and the accumulation of
+its detritus; and, secondly, that the increase of individual corals and
+of reefs, both outwards or horizontally and upwards or vertically,
+under the peculiar conditions favourable to such increase, is not slow,
+when referred either to the standard of the average oscillations of
+level in the earth’s crust, or to the more precise but less important
+one of a cycle of years.
+
+_Section III_—ON THE DEPTHS AT WHICH REEF-BUILDING POLYPIFERS CAN LIVE
+
+I have already described in detail, which might have appeared trivial,
+the nature of the bottom of the sea immediately surrounding Keeling
+atoll; and I will now describe with almost equal care the soundings off
+the fringing-reefs of Mauritius. I have preferred this arrangement, for
+the sake of grouping together facts of a similar nature. I sounded with
+the wide bell-shaped lead which Captain Fitzroy used at Keeling Island,
+but my examination of the bottom was confined to a few miles of coast
+(between Port Louis and Tomb Bay) on the leeward side of the island.
+The edge of the reef is formed of great shapeless masses
+of branching Madrepores, which chiefly consist of two
+species,—apparently _M. corymbosa_ and _ pocillifera_,—mingled with a
+few other kinds of coral. These masses are separated from each other by
+the most irregular gullies and cavities, into which the lead sinks many
+feet. Outside this irregular border of Madrepores, the water deepens
+gradually to twenty fathoms, which depth generally is found at the
+distance of from half to three-quarters of a mile from the reef. A
+little further out the depth is thirty fathoms, and thence the bank
+slopes rapidly into the depths of the ocean. This inclination is very
+gentle compared with that outside Keeling and other atolls, but
+compared with most coasts it is steep. The water was so clear outside
+the reef, that I could distinguish every object forming the rugged
+bottom. In this part, and to a depth of eight fathoms, I sounded
+repeatedly, and at each cast pounded the bottom with the broad lead,
+nevertheless the arming invariably came up perfectly clean, but deeply
+indented. From eight to fifteen fathoms a little calcareous sand was
+occasionally brought up, but more frequently the arming was simply
+indented. In all this space the two Madrepores above mentioned, and two
+species of Astræa, with rather large[30] stars, seemed the commonest
+kinds; and it must be noticed that twice at the depth of fifteen
+fathoms, the arming was marked with a clean impression of an Astræa.
+Besides these lithophytes, some fragments of the _Millepora
+alcicornis,_ which occurs in the same relative position at Keeling
+Island, were brought up; and in the deeper parts there were large beds
+of a Seriatopora, different from _S. subulata_, but closely allied to
+it. On the beach within the reef, the rolled fragments consisted
+chiefly of the corals just mentioned, and of a massive Porites, like
+that at Keeling atoll, of a Meandrina, _ Pocillopora verrucosa_, and of
+numerous fragments of Nullipora. From fifteen to twenty fathoms the
+bottom was, with few exceptions, either formed of sand, or thickly
+covered with Seriatopora: this delicate coral seems to form at these
+depths extensive beds unmingled with any other kind. At twenty fathoms,
+one sounding brought up a fragment of Madrepora apparently _M.
+pocillifera_, and I believe it is the same species (for I neglected to
+bring specimens from both stations) which mainly forms the upper margin
+of the reef; if so, it grows in depths varying from
+0 to 20 fathoms. Between 20 and 23 fathoms I obtained several
+soundings, and they all showed a sandy bottom, with one exception at 30
+fathoms, when the arming came up scooped out, as if by the margin of a
+large Caryophyllia. Beyond 33 fathoms I sounded only once; and from 86
+fathoms, at the distance of one mile and a third from the edge of the
+reef, the arming brought up calcareous sand with a pebble of volcanic
+rock. The circumstance of the arming having invariably come up quite
+clean, when sounding within a certain number of fathoms off the reefs
+of Mauritius and Keeling atoll (eight fathoms in the former case, and
+twelve in the latter) and of its having always come up (with one
+exception) smoothed and covered with sand, when the depth exceeded
+twenty fathoms, probably indicates a criterion, by which the limits of
+the vigorous growth of coral might in all cases be readily ascertained.
+I do not, however, suppose that if a vast number of soundings were
+obtained round these islands, the limit above assigned would be found
+never to vary, but I conceive the facts are sufficient to show, that
+the exceptions would be few. The circumstance of a _gradual_ change, in
+the two cases, from a field of clean coral to a smooth sandy bottom, is
+far more important in indicating the depth at which the larger kinds of
+coral flourish than almost any number of separate observations on the
+depth, at which certain species have been dredged up. For we can
+understand the gradation, only as a prolonged struggle against
+unfavourable conditions. If a person were to find the soil clothed with
+turf on the banks of a stream of water, but on going to some distance
+on one side of it, he observed the blades of grass growing thinner and
+thinner, with intervening patches of sand, until he entered a desert of
+sand, he would safely conclude, especially if changes of the same kind
+were noticed in other places, that the presence of the water was
+absolutely necessary to the formation of a thick bed of turf: so may we
+conclude, with the same feeling of certainty, that thick beds of coral
+are formed only at small depths beneath the surface of the sea.
+
+ [30] Since the preceding pages were printed off, I have received from
+ Mr. Lyell a very interesting pamphlet, entitled “Remarks upon Coral
+ Formations,” etc., by J. Couthouy, Boston, United States, 1842. There
+ is a statement (p. 6), on the authority of the Rev. J. Williams,
+ corroborating the remarks made by Ehrenberg and Lyell (p. 71 of this
+ volume), on the antiquity of certain individual corals in the Red Sea
+ and at Bermuda; namely, that at Upolu, one of the Navigator Islands,
+ “particular clumps of coral are known to the fishermen by name,
+ derived from either some particular configuration or tradition
+ attached to them, and handed down from time immemorial.” With respect
+ to the thickness of masses of coral-rock, it clearly appears, from the
+ descriptions given by Mr. Couthouy (pp. 34, 58) that Mangaia and
+ Aurora Islands are upraised atolls, composed of coral rock: the level
+ summit of the former is about three hundred feet, and that of Aurora
+ Island is two hundred feet above the sea-level.
+
+
+I have endeavoured to collect every fact, which might either invalidate
+or corroborate this conclusion. Captain Moresby, whose opportunities
+for observation during his survey of the Maldiva and Chagos
+Archipelagoes have been unrivalled, informs me, that the upper part or
+zone of the steep-sided reefs, on the inner and outer coasts of the
+atolls in both groups, invariably consists of coral, and the lower
+parts of sand. At seven or eight fathoms depth, the bottom is formed,
+as could be seen through the clear water, of great living masses of
+coral, which at about ten fathoms generally stand some way apart from
+each other, with patches of white sand between them, and at a little
+greater depth these patches become united into a smooth steep slope,
+without any coral. Captain Moresby, also, informs me in support of his
+statement, that he found only decayed coral on the Padua Bank (northern
+part of the Laccadive group) which has an average depth between
+twenty-five and thirty-five fathoms, but that on some other banks in
+the same group with only ten or twelve fathoms water on them (for
+instance, the Tillacapeni bank), the coral was living.
+With regard to the coral-reefs in the Red Sea, Ehrenberg has the
+following passage:—“The living corals do not descend there into great
+depths. On the edges of islets and near reefs, where the depth was
+small, very many lived; but we found no more even at six fathoms. The
+pearl-fishers at Yemen and Massaua asserted that there was no coral
+near the pearl-banks at nine fathoms depth, but only sand. We were not
+able to institute any more special researches.”[31] I am, however,
+assured both by Captain Moresby and Lieutenant Wellstead, that in the
+more northern parts of the Red Sea, there are extensive beds of living
+coral at a depth of twenty-five fathoms, in which the anchors of their
+vessels were frequently entangled. Captain Moresby attributes the less
+depth, at which the corals are able to live in the places mentioned by
+Ehrenberg, to the greater quantity of sediment there; and the
+situations, where they were flourishing at the depth of twenty-five
+fathoms, were protected, and the water was extraordinarily limpid. On
+the leeward side of Mauritius where I found the coral growing at a
+somewhat greater depth than at Keeling atoll, the sea, owing apparently
+to its tranquil state, was likewise very clear. Within the lagoons of
+some of the Marshall atolls, where the water can be but little
+agitated, there are, according to Kotzebue, living beds of coral in
+twenty-five fathoms. From these facts, and considering the manner in
+which the beds of clean coral off Mauritius, Keeling Island, the
+Maldiva and Chagos atolls, graduated into a sandy slope, it appears
+very probable that the depth, at which reef-building polypifers can
+exist, is partly determined by the extent of inclined surface, which
+the currents of the sea and the recoiling waves have the power to keep
+free from sediment.
+
+ [31] Ehrenberg, “Über die Natur,” etc., p. 50.
+
+
+MM. Quoy and Gaimard[32] believe that the growth of coral is confined
+within very limited depths; and they state that they never found any
+fragment of an Astræa (the genus they consider most efficient in
+forming reefs) at a depth above twenty-five or thirty feet. But we have
+seen that in several places the bottom of the sea is paved with massive
+corals at more than twice this depth; and at fifteen fathoms (or twice
+this depth) off the reefs of Mauritius, the arming was marked with the
+distinct impression of a living Astræa. _Millepora alcicornis_ lives in
+from 0 to 12 fathoms, and the genera Madrepora and Seriatopora from 0
+to 20 fathoms. Captain Moresby has given me a specimen of _Sideropora
+scabra_ (Porites of Lamarck) brought up alive from 17 fathoms. Mr.
+Couthouy[33] states that he has dredged up on the Bahama banks
+considerable masses of Meandrina from 16 fathoms, and he has seen this
+coral growing in 20 fathoms. A Caryophyllia, half an inch in diameter,
+was dredged up alive from 80 fathoms off Juan Fernandez (latitude 33°
+S.) by Captain P. P. King:[34] this is the most remarkable fact with
+which I am acquainted, showing the depth at which a genus of
+corals often found on reefs, can exist.[35] We ought, however, to feel
+less
+surprise at this fact, as Caryophyllia alone of the lamelliform genera,
+ranges far beyond the tropics; it is found in Zetland[36] in Lat. 60°
+N. in deep water, and I procured a small species from Tierra del Fuego
+in Lat. 53° S. Captain Beechey informs me, that branches of pink and
+yellow coral were frequently brought up from between twenty and
+twenty-five fathoms off the Low atolls; and Lieutenant Stokes, writing
+to me from the N.W. coast of Australia, says that a strongly branched
+coral was procured there from thirty fathoms; unfortunately it is not
+known to what genera these corals belong.
+
+ [32] “Annales des Sci. Nat.” tom. vi.
+
+
+ [33] “Remarks on Coral Formations,” p. 12.
+
+
+ [34] I am indebted to Mr. Stokes for having kindly communicated this
+ fact to me, together with much other valuable information.
+
+
+ [35] I will record in the form of a note all the facts that I have
+ been able to collect on the depths, both within and without the
+ tropics, at which those corals and corallines can live, which there is
+ no reason to suppose ever materially aid in the construction of a
+ reef.
+    Ellis (“Nat. Hist. of Coralline,” p. 96) states that Ombellularia
+    was procured in latitude 79° N. _ sticking_ to a _line_ from the
+    depth of 236 fathoms; hence this coral either must have been
+    floating loose, or was entangled in stray line at the bottom. Off
+    Keeling atoll a compound Ascidia (Sigillina) was brought up from 39
+    fathoms, and a piece of sponge, apparently living, from 70, and a
+    fragment of Nullipora also apparently living from 92 fathoms. At a
+    greater depth than 90 fathoms off this coral island, the bottom was
+    thickly strewed with joints of Halimeda and small fragments of
+    other Nulliporæ, but all dead. Captain B. Allen, R.N., informs me
+    that in the survey of the West Indies it was noticed that between
+    the depth of 10 and 200 fathoms, the sounding lead very generally
+    came up coated with the dead joints of a Halimeda, of which he
+    showed me specimens. Off Pernambuco, in Brazil, in about twelve
+    fathoms, the bottom was covered with fragments dead and alive of a
+    dull red Nullipora, and I infer from Roussin’s chart, that a bottom
+    of this kind extends over a wide area. On the beach, within the
+    coral-reefs of Mauritius, vast quantities of fragments of Nulliporæ
+    were piled up. From these facts it appears, that these simply
+    organized bodies are amongst the most abundant productions of the
+    sea.
+
+
+ [36] Fleming’s “British Animals,” genus Caryophyllia.
+
+Name of Zoophyte	Depth in
+Fathoms	Country and
+S. Latitude	Authority Sertularia	40	Cape Horn 66°	(Where
+none is given, the observation is my own.) Cellaria	Ditto	Ditto
+Cellaria. A minute scarlet encrusted species, found
+living	190	Keeling Atoll 12° Cellaria. An allied, small stony
+sub-generic form	48	S. Cruz River 50° A coral allied to
+Vincularia, with eight rows of cells	40	Cape Horn Tubulipora,
+near to T. patima	Ditto	Ditto Ditto	94	East Chiloe 43°
+Cellepora, several species, and allied sub-generic forms	40	Cape
+Horn Ditto	40 and 57	Chonos Arch. 45° Ditto	48	S. Cruz 50°
+Eschara	30	Tierra del Fuego 53° Ditto	48	S. Cruz R. 50°
+Retepora	40	Cape Horn Ditto	100	C. Good Hope 34°	Quoy
+and Gaimard, _Ann. Scien. Nat.,_ t. vi, p. 284. Millepora, a strong
+coral with cylindrical branches, of a pink colour, abut two inches
+high, resembling in the form of its orifices M. aspera of Lamarck	94
+and 30	E. Chiloe 43°
+Tierra del Fuego 53° Coralium	120	Barbary 33° N.	Peyssonel in
+paper read to Royal society May 1752. Antipathes	16	Chonos 45°
+Gorgonia (or an allied form)	160	Abrolhos on the coast of Brazil
+18°	Capt. Beechey informed me of this fact in a letter.
+
+Although the limit of depth, at which each particular kind of coral
+ceases to exist, is far from being accurately known; yet when we bear
+in mind the manner in which the clumps of coral gradually became
+infrequent at about the same depth, and wholly disappeared at a greater
+depth than twenty fathoms, on the slope round Keeling atoll, on the
+leeward side of the Mauritius, and at rather less depth, both without
+and within the atolls of the Maldiva and Chagos Archipelagoes; and when
+we know that the reefs round these islands do not differ from other
+coral formations in their form and structure, we may, I think, conclude
+that in ordinary cases, reef- building polypifers do not flourish at
+greater depths than between twenty and thirty fathoms.
+
+It has been argued[37] that reefs may possibly rise from very great
+depths through the means of small corals, first making a platform for
+the growth of the stronger kinds. This, however, is an arbitrary
+supposition: it is not always remembered, that in such cases there is
+an antagonist power in action, namely, the decay of organic bodies,
+when not protected by a covering of sediment, or by their own rapid
+growth. We have, moreover, no right to calculate on unlimited time for
+the accumulation of small organic bodies into great masses. Every fact
+in geology proclaims that neither the land, nor the bed of the sea
+retain for indefinite periods the same level. As well might it be
+imagined that the British Seas would in time become choked up with beds
+of oysters, or that the numerous small corallines off the inhospitable
+shores of Tierra del Fuego would in time form a solid and extensive
+coral-reef.
+
+ [37] _Journal of the Royal Geographical Society,_ 1831, p. 218.
+
+
+
+
+Chapter V THEORY OF THE FORMATION OF THE DIFFERENT CLASSES OF
+CORAL-REEFS
+
+
+The atolls of the larger archipelagoes are not formed on submerged
+craters, or on banks of sediment.—Immense areas interspersed with
+atolls.—Their subsidence.—The effects of storms and earthquakes on
+atolls.—Recent changes in their state.—The origin of barrier-reefs and
+of atolls.—Their relative forms.—The step-formed ledges and walls round
+the shores of some lagoons.—The ring-formed reefs of the Maldiva
+atolls.—The submerged condition of parts or of the whole of some
+annular reefs.—The disseverment of large atolls.—The union of atolls by
+linear reefs.—The Great Chagos Bank.—Objections from the area and
+amount of subsidence required by the theory, considered.—The probable
+composition of the lower parts of atolls.
+
+The naturalists who have visited the Pacific, seem to have had their
+attention riveted by the lagoon-islands, or atolls,—those singular
+rings of coral-land which rise abruptly out of the unfathomable
+ocean—and have passed over, almost unnoticed, the scarcely less
+wonderful encircling barrier-reefs. The theory most generally received
+on the formation of atolls, is that they are based on submarine
+craters; but where can we find a crater of the shape of Bow atoll,
+which is five times as long as it is broad (Plate I, Fig. 4); or like
+that of Menchikoff Island (Plate II, Fig. 3), with its three loops,
+together sixty miles in length; or like Rimsky Korsacoff, narrow,
+crooked, and fifty-four miles long; or like the northern Maldiva
+atolls, made up of numerous ring-formed reefs, placed on the margin of
+a disc,—one of which discs is eighty-eight miles in length, and only
+from ten to twenty in breadth? It is, also, not a little improbable,
+that there should have existed as many craters of immense size crowded
+together beneath the sea, as there are now in some parts atolls. But
+this theory lies under a greater difficulty, as will be evident, when
+we consider on what foundations the atolls of the larger archipelagoes
+rest: nevertheless, if the rim of a crater afforded a basis at the
+proper depth, I am far from denying that a reef like a perfectly
+characterised atoll might not be formed; some such, perhaps, now exist;
+but I cannot believe in the possibility of the greater number having
+thus originated.
+
+An earlier and better theory was proposed by Chamisso;[1] he supposes
+that as the more massive kinds of corals prefer the surf, the outer
+portions, in a reef rising from a submarine basis, would first reach
+the surface and consequently form a ring. But on this view it must be
+assumed, that in every case the basis consists of a flat bank; for if
+it were conically formed, like a mountainous mass, we can see no reason
+why the coral should spring up from the flanks, instead of from the
+central and highest parts: considering the number of the atolls in the
+Pacific and Indian Oceans, this assumption is very improbable. As the
+lagoons of atolls are sometimes even more than forty fathoms deep, it
+must, also, be assumed on this view, that at a depth at which the waves
+do not break, the coral grows more vigorously on the edges of a bank
+than on its central part; and this is an assumption without any
+evidence in support of it. I remarked, in the third chapter, that a
+reef, growing on a detached bank, would tend to assume an atoll-like
+structure; if, therefore, corals were to grow up from a bank, with a
+level surface some fathoms submerged, having steep sides and being
+situated in a deep sea, a reef not to be distinguished from an atoll,
+might be formed: I believe some such exist in the West Indies. But a
+difficulty of the same kind with that affecting the crater theory,
+runners, as we shall presently see, this view inapplicable to the
+greater number of atolls.
+
+ [1] Kotzebue’s “First Voyage,” vol. iii, p. 331.
+
+No theory worthy of notice has been advanced to account for those
+barrier-reefs, which encircle islands of moderate dimensions. The great
+reef which fronts the coast of Australia has been supposed, but without
+any special facts, to rest on the edge of a submarine precipice,
+extending parallel to the shore. The origin of the third class or of
+fringing-reefs presents, I believe, scarcely any difficulty, and is
+simply consequent on the polypifers not growing up from great depths,
+and their not flourishing close to gently shelving beaches where the
+water is often turbid.
+
+What cause, then, has given to atolls and barrier-reefs their
+characteristic forms? Let us see whether an important deduction will
+not follow from the consideration of these two circumstances, first,
+the reef-building corals flourishing only at limited depths; and
+secondly, the vastness of the areas interspersed with coral-reefs and
+coral-islets, none of which rise to a greater height above the level of
+the sea, than that attained by matter thrown up by the waves and winds.
+I do not make this latter statement vaguely; I have carefully sought
+for descriptions of every island in the intertropical seas; and my task
+has been in some degree abridged by a map of the Pacific, corrected in
+1834 by MM. D’Urville and Lottin, in which the low islands are
+distinguished from the high ones (even from those much less than a
+hundred feet in height) by being written without a capital letter; I
+have detected a few errors in this map, respecting the height of some
+of the islands, which will be noticed in the Appendix, where I treat of
+coral formations in geographical order. To the Appendix, also, I must
+refer for a more particular account of the data on which the statements
+on the next page are grounded. I have ascertained, and chiefly from the
+writings of Cook, Kotzebue, Bellinghausen, Duperrey, Beechey, and
+Lutké, regarding the Pacific; and from Moresby[2] with respect to the
+Indian Ocean, that in the following cases the term “low island”
+strictly means land of the height commonly attained by matter thrown up
+by the winds and the
+waves of an open sea. If we draw a line (the plan I have always
+adopted) joining the external atolls of that part of the Low
+Archipelago in which the islands are numerous, the figure will be a
+pointed ellipse (reaching from Hood to Lazaref Island), of which the
+longer axis is 840 geographical miles, and the shorter 420 miles; in
+this space[3] none of the innumerable islets united into great rings
+rise above the stated level. The Gilbert group is very narrow, and 300
+miles in length. In a prolonged line from this group, at the distance
+of 240 miles, is the Marshall Archipelago, the figure of which is an
+irregular square, one end being broader than the other; its length is
+520 miles, with an average width of 240; these two groups together are
+1,040 miles in length, and all their islets are low. Between the
+southern end of the Gilbert and the northern end of Low Archipelago,
+the ocean is thinly strewed with islands, all of which, as far as I
+have been able to ascertain, are low; so that from nearly the southern
+end of the Low Archipelago, to the northern end of the Marshall
+Archipelago, there is a narrow band of ocean, more than 4,000 miles in
+length, containing a great number of islands, all of which are low. In
+the western part of the Caroline Archipelago, there is a space of 480
+miles in length, and about 100 broad, thinly interspersed with low
+islands. Lastly, in the Indian Ocean, the archipelago of the Maldivas
+is 470 miles in length, and 60 in breadth; that of the Laccadives is
+150 by 100 miles; as there is a low island between these two groups,
+they may be considered as one group of 1,000 miles in length. To this
+may be added the Chagos group of low islands, situated 280 miles
+distant, in a line prolonged from the southern extremity of the
+Maldivas. This group, including the submerged banks, is 170 miles in
+length and 80 in breadth. So striking is the uniformity in direction of
+these three archipelagoes, all the islands of which are low, that
+Captain Moresby, in one of his papers, speaks of them as parts of one
+great chain, nearly 1,500 miles long. I am, then, fully justified in
+repeating, that enormous spaces, both in the Pacific and Indian Oceans,
+are interspersed with islands, of which not one rises above that
+height, to which the waves and winds in an open sea can heap up matter.
+On what foundations, then, have these reefs and islets of coral been
+constructed? A foundation must originally have been present beneath
+each atoll at that limited depth, which is indispensable for the first
+growth of the reef-building polypifers. A conjecture will perhaps be
+hazarded, that the requisite bases might have been afforded by the
+accumulation of great banks of sediment, which owing to the action of
+superficial currents (aided possibly by the undulatory movement of the
+sea) did not quite reach the surface,—as actually appears to have been
+the case in some parts of the West Indian Sea. But in the form and
+disposition of the groups of atolls, there is nothing to countenance
+this notion; and the assumption without any proof, that a number of
+immense piles of sediment have been heaped on the floor of the great
+Pacific and Indian Oceans, in their central parts far remote from land,
+and where the dark blue colour of the limpid water bespeaks its purity,
+cannot for one moment be admitted.
+
+ [2] See also Captain Owen’s and Lieutenant Wood’s papers in the
+ _Geographical Journal_, on the Maldiva and Laccadive Archipelagoes.
+ These officers particularly refer to the lowness of the islets; but I
+ chiefly ground my assertion respecting these two groups, and the
+ Chagos group, from information communicated to me by Captain Moresby.
+
+
+ [3] I find from Mr. Couthouy’s pamphlet (p. 58) that Aurora Island is
+ about two hundred feet in height; it consists of coral-rock, and seems
+ to have been formed by the elevation of an atoll. It lies north-east
+ of Tahiti, close without the line bounding the space coloured dark
+ blue in the map appended to this volume. Honden Island, which is
+ situated in the extreme north-west part of the Low Archipelago,
+ according to measurements made on board the _Beagle_, whilst sailing
+ by, is 114 feet from the _summit of the trees_ to the water’s edge.
+ This island appeared to resemble the other atolls of the group.
+
+
+The many widely-scattered atolls must, therefore, rest on rocky bases.
+But we cannot believe that the broad summit of a mountain lies buried
+at the depth of a few fathoms beneath every atoll, and nevertheless
+throughout the immense areas above-named, with not one point of rock
+projecting above the level of the sea; for we may judge with some
+accuracy of mountains beneath the sea, by those on the land; and where
+can we find a single chain several hundred miles in length and of
+considerable breadth, much less several such chains, with their many
+broad summits attaining the same height, within from 120 to 180 feet?
+If the data be thought insufficient, on which I have grounded my
+belief, respecting the depth at which the reef-building polypifers can
+exist, and it be assumed that they can flourish at a depth of even one
+hundred fathoms, yet the weight of the above argument is but little
+diminished, for it is almost equally improbable, that as many submarine
+mountains, as there are low islands in the several great and widely
+separated areas above specified, should all rise within six hundred
+feet of the surface of the sea and not one above it, as that they
+should be of the same height within the smaller limit of one or two
+hundred feet. So highly improbable is this supposition, that we are
+compelled to believe, that the bases of the many atolls did never at
+any one period all lie submerged within the depth of a few fathoms
+beneath the surface, but that they were brought into the requisite
+position or level, some at one period and some at another, through
+movements in the earth’s crust. But this could not have been effected
+by elevation, for the belief that points so numerous and so widely
+separated were successively uplifted to a certain level, but that not
+one point was raised above that level, is quite as improbable as the
+former supposition, and indeed differs little from it. It will probably
+occur to those who have read Ehrenberg’s account of the Reefs of the
+Red Sea, that many points in these great areas may have been elevated,
+but that as soon as raised, the protuberant parts were cut off by the
+destroying action of the waves: a moment’s reflection, however, on the
+basin-like form of the atolls, will show that this is impossible; for
+the upheaval and subsequent abrasion of an island would leave a flat
+disc, which might become coated with coral, but not a deeply concave
+surface; moreover, we should expect to see, in some parts at least, the
+rock of the foundation brought to the surface. If, then, the
+foundations of the many atolls were not uplifted into the requisite
+position, they must of necessity have subsided into it; and this at
+once solves every difficulty,[4]
+for we may safely infer, from the facts given in the last chapter, that
+during a gradual subsidence the corals would be favourably
+circumstanced for building up their solid frame works and reaching the
+surface, as island after island slowly disappeared. Thus areas of
+immense extent in the central and most profound parts of the great
+oceans, might become interspersed with coral-islets, none of which
+would rise to a greater height than that attained by detritus heaped up
+by the sea, and nevertheless they might all have been formed by corals,
+which absolutely required for their growth a solid foundation within a
+few fathoms of the surface.
+
+ [4] The additional difficulty on the crater hypothesis before alluded
+ to, will now be evident; for on this view the volcanic action must be
+ supposed to have formed within the areas specified a vast number of
+ craters, all rising within a few fathoms of the surface, and not one
+ above it. The supposition that the craters were at different times
+ upraised above the surface, and were there abraded by the surf and
+ subsequently coated by corals, is subject to nearly the same
+ objections with those given above in this paragraph; but I consider it
+ superfluous to detail all the arguments opposed to such a notion.
+ Chamisso’s theory, from assuming the existence of so many banks, all
+ lying at the proper depth beneath the water, is also vitally
+ defective. The same observation applies to an hypothesis of Lieutenant
+ Nelson’s (“Geolog. Trans.” vol. v, p. 122), who supposes that the
+ ring-formed structure is caused by a greater number of germs of corals
+ becoming attached to the declivity, than to the central plateau of a
+ submarine bank: it likewise applies to the notion formerly entertained
+ (Forster’s “Observ.,” p. 151), that lagoon-islands owe their peculiar
+ form to the instinctive tendencies of the polypifers. According to
+ this latter view, the corals on the outer margin of the reef
+ instinctively expose themselves to the surf in order to afford
+ protection to corals living in the lagoon, which belong to other
+ genera, and to other families!
+
+
+It would be out of place here to do more than allude to the many facts,
+showing that the supposition of a gradual subsidence over large areas
+is by no means improbable. We have the clearest proof that a movement
+of this kind is possible, in the upright trees buried under the strata
+many thousand feet in thickness; we have also every reason for
+believing that there are now large areas gradually sinking, in the same
+manner as others are rising. And when we consider how many parts of the
+surface of the globe have been elevated within recent geological
+periods, we must admit that there have been subsidences on a
+corresponding scale, for otherwise the whole globe would have swollen.
+It is very remarkable that Mr. Lyell,[5] even in the first edition of
+his “Principles of Geology,” inferred that the amount of subsidence in
+the Pacific must have exceeded that of elevation, from the area of land
+being very small relatively to the agents there tending to form it,
+namely, the growth of coral and volcanic action. But it will be asked,
+are there any direct proofs of a subsiding movement in those areas, in
+which subsidence will explain a phenomenon otherwise inexplicable?
+This, however, can hardly be expected, for it must ever be most
+difficult, excepting in countries long civilised, to detect a movement,
+the tendency of which is to conceal the part affected. In barbarous and
+semi-civilised
+nations how long might not a slow movement, even of elevation such as
+that now affecting Scandinavia, have escaped attention!
+
+ [5] “Principles of Geology,” sixth edition, vol. iii, p. 386.
+
+
+Mr. Williams[6] insists strongly that the traditions of the natives,
+which he has taken much pains in collecting, do not indicate the
+appearance of any new islands: but on the theory of a gradual
+subsidence, all that would be apparent would be, the water sometimes
+encroaching slowly on the land, and the land again recovering by the
+accumulation of detritus its former extent, and perhaps sometimes the
+conversion of an atoll with coral islets on it, into a bare or into a
+sunken annular reef. Such changes would naturally take place at the
+periods when the sea rose above its usual limits, during a gale of more
+than ordinary strength; and the effects of the two causes would be
+hardly distinguishable. In Kotzebue’s “Voyage” there are accounts of
+islands, both in the Caroline and Marshall Archipelagoes, which have
+been partly washed away during hurricanes; and Kadu, the native who was
+on board one of the Russian vessels, said “he saw the sea at Radack
+rise to the feet of the cocoa-nut trees; but it was conjured in
+time.”[7] A storm lately entirely swept away two of the Caroline
+islands, and converted them into shoals; it partly, also, destroyed two
+other islands.[8] According to a tradition which was communicated to
+Captain Fitzroy, it is believed in the Low Archipelago, that the
+arrival of the first ship caused a great inundation, which destroyed
+many lives. Mr. Stutchbury relates, that in 1825, the western side of
+Chain Atoll, in the same group, was completely devastated by a
+hurricane, and not less than 300 lives lost: “in this instance it was
+evident, even to the natives, that the hurricane alone was not
+sufficient to account for the violent agitation of the ocean.”[9] That
+considerable changes have taken place recently in some of the atolls in
+the Low Archipelago, appears certain from the case already given of
+Matilda Island: with respect to Whitsunday and Gloucester Islands in
+this same group, we must either attribute great inaccuracy to their
+discoverer, the famous circumnavigator Wallis, or believe that they
+have undergone a considerable change in the period of fifty-nine years,
+between his voyage and that of Captain Beechey’s. Whitsunday Island is
+described by Wallis as “about four miles long, and three wide,” now it
+is only one mile and a half long. The appearance of Gloucester Island,
+in Captain Beechey’s words,[10] “has been accurately described by its
+discoverer, but its present form and extent differ materially.”
+Blenheim reef, in the Chagos group, consists of a water-washed annular
+reef, thirteen miles in circumference, surrounding a lagoon ten fathoms
+deep: on its surface there were a few worn patches of conglomerate
+coral-rock, of about the size of hovels; and these Captain Moresby
+considered as being, without doubt, the last remnants of islets; so
+that here an atoll has been converted into an atoll-formed reef. The
+inhabitants of the Maldiva Archipelago, as long ago as 1605, declared,
+“that the high tides and violent currents were diminishing the number
+of the islands:”[11] and I have already shown, on the authority of
+Captain Moresby, that the work of destruction is still in progress; but
+that on the other hand the first formation of some islets is known to
+the present inhabitants. In such cases, it would be exceedingly
+difficult to detect a gradual subsidence of the foundation, on which
+these mutable structures rest.
+
+ [6] Williams’s “Narrative of Missionary Enterprise,” p. 31.
+
+
+ [7] Kotzebue’s “First Voyage,” vol. iii, p. 168.
+
+
+ [8] M. Desmoulins in “Comptes Rendus,” 1840, p. 837.
+
+
+ [9] _West of England Journal_, No. I, p. 35.
+
+
+ [10] Beechey’s “Voyage to the Pacific,” chap. vii, and Wallis’s
+ “Voyage in the _Dolphin_,” chap. iv.
+
+
+ [11] See an extract from Pyrard’s Voyage in Captain Owen’s paper on
+ the Maldiva Archipelago, in the _Geographical Journal_, vol. ii, p.
+ 84.
+
+
+Some of the archipelagoes of low coral-islands are subject to
+earthquakes: Captain Moresby informs me that they are frequent, though
+not very strong, in the Chagos group, which occupies a very central
+position in the Indian Ocean, and is far from any land not of coral
+formation. One of the islands in this group was formerly covered by a
+bed of mould, which, after an earthquake, disappeared, and was believed
+by the residents to have been washed by the rain through the broken
+masses of underlying rock; the island was thus rendered unproductive.
+Chamisso[12] states, that earthquakes are felt in the Marshall atolls,
+which are far from any high land, and likewise in the islands of the
+Caroline Archipelago. On one of the latter, namely Oulleay atoll,
+Admiral Lutké, as he had the kindness to inform me, observed several
+straight fissures about a foot in width, running for some hundred yards
+obliquely across the whole width of the reef. Fissures indicate a
+stretching of the earth’s crust, and, therefore, probably changes in
+its level; but these coral-islands, which have been shaken and
+fissured, certainly have not been elevated, and, therefore, probably
+they have subsided. In the chapter on Keeling atoll, I attempted to
+show by direct evidence, that the island underwent a movement of
+subsidence, during the earthquakes lately felt there.
+
+ [12] See Chamisso, in Kotzebue’s “First Voyage,” vol. iii, p. 182 and
+ 136.
+
+The facts stand thus;—there are many large tracts of ocean, without any
+high land, interspersed with reefs and islets, formed by the growth of
+those kinds of corals, which cannot live at great depths; and the
+existence of these reefs and low islets, in such numbers and at such
+distant points, is quite inexplicable, excepting on the theory, that
+the bases on which the reefs first became attached, slowly and
+successively sank beneath the level of the sea, whilst the corals
+continued to grow upwards. No positive facts are opposed to this view,
+and some general considerations render it probable. There is evidence
+of change in form, whether or not from subsidence, on some of these
+coral-islands; and there is evidence of subterranean disturbances
+beneath them. Will then the theory, to which we have thus been led,
+solve the curious problem,—what has given to each class of reef its
+peculiar form?
+
+Let us in imagination place within one of the subsiding areas, an
+island surrounded by a “fringing-reef,”—that kind, which alone offers
+no difficulty in the explanation of its origin. Let the unbroken lines,
+and the oblique shading in the woodcut (No. 4) represent a vertical
+section through such an island; and the horizontal shading will
+represent the section of the reef. Now, as the island sinks down,
+either a few feet at a time or quite insensibly, we may safely infer
+from what we know of the conditions favourable to the growth of coral,
+that the living masses bathed by the surf on the margin of the reef,
+will soon regain the surface. The water, however, will encroach, little
+by little, on the shore, the island becoming lower and smaller, and the
+space between the edge of the reef and the beach proportionately
+broader. A section of the reef and island in this state, after a
+subsidence of several hundred feet, is given by the dotted lines:
+coral-islets are supposed to have been formed on the new reef, and a
+ship is anchored in the lagoon-channel. This section is in every
+respect that of an encircling barrier-reef; it is, in fact, a section
+taken[13] east and west through the highest point of the encircled
+island of Bolabola; of which a plan is given in Plate I, Fig. 5. The
+same section is more clearly shown in the following woodcut (No. 5) by
+the unbroken lines. The width of the reef, and its slope, both on the
+outer and inner side, will have been determined by the growing powers
+of the coral, under the conditions (for instance the force of the
+breakers and of the currents) to which it has been exposed; and the
+lagoon-channel will be deeper or shallower, in proportion to the growth
+of the delicately branched corals within the reef, and to the
+accumulation of sediment, relatively, also, to the rate of subsidence
+and the length of the intervening stationary periods.
+
+ [13] The section has been made from the chart given in the “Atlas of
+ the Voyage of the _Coquille_.” The scale is .57 of an inch to a mile.
+ The height of the island, according to M. Lesson, is 4,026 feet. The
+ deepest part of the lagoon-channel is 162 feet; its depth is
+ exaggerated in the woodcut for the sake of clearness.
+
+
+[Illustration: Vertical section of an island of Bolabola.]
+
+AA—Outer edge of the reef at the level of the sea.
+BB—Shores of the island.
+A′A′—Outer edge of the reef, after its upward growth during a period of
+subsidence.
+CC—The lagoon-channel between the reef and the shores of the now
+encircled land.
+B′B′—The shores of the encircled island.
+
+N.B.—In this, and the following woodcut, the subsidence of the land
+could only be represented by an apparent rise in the level of the sea.
+
+It is evident in this section, that a line drawn perpendicularly down
+from the outer edge of the new reef to the foundation of solid rock,
+exceeds by as many feet as there have been feet of subsidence, that
+small limit of depth at which the effective polypifers can live—the
+corals having grown up, as the whole sank down, from a basis formed of
+other corals and their consolidated fragments. Thus the difficulty on
+this head, which before seemed so great, disappears.
+
+As the space between the reef and the subsiding shore continued to
+increase in breadth and depth, and as the injurious effects of the
+sediment and fresh water borne down from the land were consequently
+lessened, the greater number of the channels, with which the reef in
+its fringing state must have been breached, especially those which
+fronted the smaller streams, will have become choked up with the growth
+of coral: on the windward side of the reef, where the coral grows most
+vigorously, the breaches will probably have first been closed. In
+barrier-reefs, therefore, the breaches kept open by draining the tidal
+waters of the lagoon-channel, will generally be placed on the leeward
+side, and they will still face the mouths of the larger streams,
+although removed beyond the influence of their sediment and fresh
+water;—and this, it has been shown, is commonly the case.
+
+[Illustration: Vertical section of an island of Bolabola.]
+
+A′A′—Outer edges of the barrier-reef at the level of the sea. The
+cocoa-nut trees represent coral-islets formed on the reef.
+CC—The lagoon-channel.
+B′B′—The shores of the island, generally formed of low alluvial land
+and of coral detritus from the lagoon-channel.
+A″A″—The outer edges of the reef now forming an atoll.
+C′—The lagoon of the newly formed atoll. According to the scale, the
+depth of the lagoon and of the lagoon-channel is exaggerated.
+
+
+Referring to the diagram shown above, in which the newly formed
+barrier-reef is represented by unbroken lines, instead of by dots as in
+the former woodcut, let the work of subsidence go on, and the doubly
+pointed hill will form two small islands (or more, according to the
+number of the hills) included within one annular reef. Let the island
+continue subsiding, and the coral-reef will continue growing up on its
+own foundation, whilst the water gains inch by inch on the land, until
+the last and highest pinnacle is covered, and there remains a perfect
+atoll. A vertical section of this atoll is shown in the woodcut by the
+dotted lines;—a ship is anchored in its lagoon, but islets are not
+supposed yet to have been formed on the reef. The depth of the lagoon
+and the
+width and slope of the reef, will depend on the circumstances just
+referred to under barrier-reefs. Any further subsidence will produce no
+change in the atoll, except perhaps a diminution in its size, from the
+reef not growing vertically upwards; but should the currents of the sea
+act violently upon it, and should the corals perish on part or on the
+whole of its margin, changes would result during subsidence which will
+be presently noticed. I may here observe, that a bank either of rock or
+of hardened sediment, level with the surface of the sea, and fringed
+with living coral, would (if not so small as to allow the central space
+to be quickly filled up with detritus) by subsidence be converted
+immediately into an atoll, without passing, as in the case of a reef
+fringing the shore of an island, through the intermediate form of a
+barrier-reef. If such a bank lay a few fathoms submerged, the simple
+growth of the coral (as remarked in the third chapter) without the aid
+of subsidence, would produce a structure scarcely to be distinguished
+from a true atoll; for in all cases the corals on the outer margin of a
+reef, from having space and being freely exposed to the open sea, will
+grow vigorously and tend to form a continuous ring whilst the growth of
+the less massive kinds on the central expanse, will be checked by the
+sediment formed there, and by that washed inwards by the breakers; and
+as the space becomes shallower, their growth will, also, be checked by
+the impurities of the water, and probably by the small amount of food
+brought by the enfeebled currents, in proportion to the surface of
+living reefs studded with innumerable craving mouths: the subsidence of
+a reef based on a bank of this kind, would give depth to its central
+expanse or lagoon, steepness to its flanks, and through the free growth
+of the coral, symmetry to its outline:—I may here repeat that the
+larger groups of atolls in the Pacific and Indian Oceans cannot be
+supposed to be founded on banks of this nature.
+
+If, instead of the island in the diagram, the shore of a continent
+fringed by a reef had subsided, a great barrier-reef, like that on the
+north-east coast of Australia, would have necessarily resulted; and it
+would have been separated from the main land by a deep-water channel,
+broad in proportion to the amount of subsidence, and to the less or
+greater inclination of the neighbouring coast-line. The effect of the
+continued subsidence of a great barrier-reef of this kind, and its
+probable conversion into a chain of separate atolls, will be noticed,
+when we discuss the apparent progressive disseverment of the larger
+Maldiva atolls.
+
+We now are able to perceive that the close similarity in form,
+dimensions, structure, and relative position (which latter point will
+hereafter be more fully noticed) between fringing and encircling
+barrier-reefs, and between these latter and atolls, is the necessary
+result of the transformation, during subsidence of the one class into
+the other. On this view, the three classes of reefs ought to graduate
+into each other. Reefs having intermediate character between those of
+the fringing and barrier classes do exist; for instance, on the
+south-west coast of Madagascar, a reef extends for several miles,
+within which there is a broad channel from seven to eight fathoms deep,
+but the sea does not deepen abruptly outside the reef. Such cases,
+however, are open to
+some doubts, for an old fringing-reef, which had extended itself a
+little on a basis of its own formation, would hardly be distinguishable
+from a barrier-reef, produced by a small amount of subsidence, and with
+its lagoon-channel nearly filled up with sediment during a long
+stationary period. Between barrier-reefs, encircling either one lofty
+island or several small low ones, and atolls including a mere expanse
+of water, a striking series can be shown: in proof of this, I need only
+refer to the first plate in this volume, which speaks more plainly to
+the eye, than any description could to the ear. The authorities from
+which the charts have been engraved, together with some remarks on them
+and descriptive of the plates, are given above. At New Caledonia (Plate
+II, Fig. 5.) the barrier-reefs extend for 150 miles on each side of the
+submarine prolongation of the island; and at their northern extremity
+they appear broken up and converted into a vast atoll-formed reef,
+supporting a few low coral-islets: we may imagine that we here see the
+effects of subsidence actually in progress, the water always
+encroaching on the northern end of the island, towards which the
+mountains slope down, and the reefs steadily building up their massive
+fabrics in the lines of their ancient growth.
+
+We have as yet only considered the origin of barrier-reefs and atolls
+in their simplest form; but there remain some peculiarities in
+structure and some special cases, described in the two first chapters,
+to be accounted for by our theory. These consist—in the inclined ledge
+terminated by a wall, and sometimes succeeded by a second ledge with a
+wall, round the shores of certain lagoons and lagoon-channels; a
+structure which cannot, as I endeavoured to show, be explained by the
+simple growing powers of the corals,—in the ring or basin-like forms of
+the central reefs, as well as of the separate marginal portions of the
+northern Maldiva atolls,—in the submerged condition of the whole, or of
+parts of certain barrier and atoll-formed reefs; where only a part is
+submerged, this being generally to leeward,—in the apparent progressive
+disseverment of some of the Maldiva atolls,—in the existence of
+irregularly formed atolls, some being tied together by linear reefs,
+and others with spurs projecting from them,—and, lastly, in the
+structure and origin of the Great Chagos Bank.
+
+_Step-formed ledges round certain lagoons._—If we suppose an atoll to
+subside at an extremely slow rate, it is difficult to follow out the
+complex results. The living corals would grow up on the outer margin;
+and likewise probably in the gullies and deeper parts of the bare
+surface of the annular reef; the water would encroach on the islets,
+but the accumulation of fresh detritus might possibly prevent their
+entire submergence. After a subsidence of this very slow nature, the
+surface of the annular reef sloping gently into the lagoon, would
+probably become united with the irregular reefs and banks of sand,
+which line the shores of most lagoons. Should, however, the atoll be
+carried down by a more rapid movement, the whole surface of the annular
+reef, where there was a foundation of solid matter, would be favourably
+circumstanced for the fresh growth of coral; but as the corals grew
+upwards on its exterior margin, and the waves broke heavily on this
+part, the increase of the massive polypifers on the inner side would be
+checked from the want of water. Consequently, the exterior parts would
+first reach the surface, and the new annular reef thus formed on the
+old one, would have its summit inclined inwards, and be terminated by a
+subaqueous wall, formed by the upward growth of the coral (before being
+much checked), from the inner edge of the solid parts of the old reef.
+The inner portion of the new reef, from not having grown to the
+surface, would be covered by the waters of the lagoon. Should a
+subsidence of the same kind be repeated, the corals would again grow up
+in a wall, from all the solid parts of the resunken reef, and,
+therefore, not from within the sandy shores of the lagoon; and the
+inner part of the new annular reef would, from being as before checked
+in its upward growth, be of less height than the exterior parts, and
+therefore would not reach the surface of the lagoon. In this case the
+shores of the lagoon would be surrounded by two inclined ledges, one
+beneath the other, and both abruptly terminated by subaqueous
+cliffs.[14]
+
+ [14] According to Mr. Couthouy (p. 26) the external reef round many
+ atolls descends by a succession of ledges or terraces. He attempts, I
+ doubt whether successfully, to explain this structure somewhat in the
+ same manner as I have attempted, with respect to the internal ledges
+ round the lagoons of some atolls. More facts are wanted regarding the
+ nature both of the interior and exterior step-like ledges: are all the
+ ledges, or only the upper ones, covered with living coral? If they are
+ all covered, are the kinds different on the ledges according to the
+ depth? Do the interior and exterior ledges occur together in the same
+ atolls; if so, what is their total width, and is the intervening
+ surface-reef narrow, etc.?
+
+_The ring or basin-formed reefs of the northern Maldiva atolls._—I may
+first observe, that the reefs within the lagoons of atolls and within
+lagoon-channels, would, if favourably circumstanced, grow upwards
+during subsidence in the same manner as the annular rim; and,
+therefore, we might expect that such lagoon- reefs, when not surrounded
+and buried by an accumulation of sediment more rapid than the rate of
+subsidence, would rise abruptly from a greater depth than that at which
+the efficient polypifers can flourish: we see this well exemplified in
+the small abruptly-sided reefs, with which the deep lagoons of the
+Chagos and Southern Maldiva atolls are studded. With respect to the
+ring or basin-formed reefs of the Northern Maldiva atolls, it is
+evident, from the perfectly continuous series which exists that the
+marginal rings, although wider than the exterior or bounding reef of
+ordinary atolls, are only modified portions of such a reef; it is also
+evident that the central rings, although wider than the knolls or reefs
+which commonly occur in lagoons, occupy their place. The ring-like
+structure has been shown to be contingent on the breaches into the
+lagoon being broad and numerous, so that all the reefs which are bathed
+by the waters of the lagoon are placed under nearly the same conditions
+with the outer coast of an atoll standing in the open sea. Hence the
+exterior and living margins of these reefs must have been favourably
+circumstanced for growing outwards, and increasing beyond the usual
+breadth; and they must likewise have been favourably circumstanced for
+growing
+vigorously upwards, during the subsiding movements, to which by our
+theory the whole archipelago has been subjected; and subsidence with
+this upward growth of the margins would convert the central space of
+each little reef into a small lagoon. This, however, could only take
+place with those reefs, which had increased to a breadth sufficient to
+prevent their central spaces from being almost immediately filled up
+with the sand and detritus driven inwards from all sides: hence it is
+that few reefs, which are less than half a mile in diameter, even in
+the atolls where the basin-like structure is most strikingly exhibited,
+include lagoons. This remark, I may add, applies to all coral-reefs
+wherever found. The basin-formed reefs of the Maldiva Archipelago may,
+in fact, be briefly described, as small atolls formed during subsidence
+over the separate portions of large and broken atolls, in the same
+manner as these latter were formed over the barrier-reefs, which
+encircled the islands of a large archipelago now wholly submerged.
+
+_Submerged and dead reefs._—In the second section of the first chapter,
+I have shown that there are in the neighbourhood of atolls, some deeply
+submerged banks, with level surfaces; that there are others, less
+deeply but yet wholly submerged, having all the characters of perfect
+atolls, but consisting merely of dead coral-rock; that there are
+barrier-reefs and atolls with merely a portion of their reef, generally
+on the leeward side, submerged; and that such portions either retain
+their perfect outline, or they appear to be quite effaced, their former
+place being marked only by a bank, conforming in outline with that part
+of the reef which remains perfect. These several cases are, I believe,
+intimately related together, and can be explained by the same means.
+There, perhaps, exist some submerged reefs, covered with living coral
+and growing upwards, but to these I do not here refer. As we see that
+in those parts of the ocean, where coral-reefs are most abundant, one
+island is fringed and another neighbouring one is not fringed; as we
+see in the same archipelago, that all the reefs are more perfect in one
+part of it than in another, for instance, in the southern half compared
+with the northern half of the Maldiva Archipelago, and likewise on the
+outer coasts compared with the inner coasts of the atolls in this same
+group, which are placed in a double row; as we know that the existence
+of the innumerable polypifers forming a reef, depends on their
+sustenance, and that they are preyed on by other organic beings; and,
+lastly, as we know that some inorganic causes are highly injurious to
+the growth of coral, it cannot be expected that during the round of
+change to which earth, air, and water are exposed, the reef-building
+polypifers should keep alive for perpetuity in any one place; and still
+less can this be expected, during the progressive subsidences, perhaps
+at some periods more rapid than at others, to which by our theory these
+reefs and islands have been subjected and are liable. It is, then, not
+improbable that the corals should sometimes perish either on the whole
+or on part of a reef; if on part, the dead portion, after a small
+amount of subsidence, would still retain its proper outline and
+position beneath the water. After a more prolonged
+subsidence, it would probably form, owing to the accumulation of
+sediment, only the margin of a flat bank, marking the limits of the
+former lagoon. Such dead portions of reef would generally lie on the
+leeward side,[15] for the impure water and fine sediment would more
+easily flow out from the lagoon over this side of the reef, where the
+force of the breakers is less than to windward; and therefore the
+corals would be less vigorous on this side, and be less able to resist
+any destroying agent. It is likewise owing to this same cause, that
+reefs are more frequently breached to leeward by narrow channels,
+serving as by ship-channels, than to windward. If the corals perished
+entirely, or on the greater part of the circumference of an atoll, an
+atoll-shaped bank of dead rock, more or less entirely submerged, would
+be produced; and further subsidence, together with the accumulation of
+sediment, would often obliterate its atoll-like structure, and leave
+only a bank with a level surface.
+
+ [15] Mr. Lyell, in the first edition of his “Principles of Geology,”
+ offered a somewhat different explanation of this structure. He
+ supposes that there has been subsidence; but he was not aware that the
+ submerged portions of reef were in most cases, if not in all, dead;
+ and he attributes the difference in height in the two sides of most
+ atolls, chiefly to the greater accumulation of detritus to windward
+ than to leeward. But as matter is accumulated only on the backward
+ part of the reef, the front part would remain of the same height on
+ both sides. I may here observe that in most cases (for instance, at
+ Peros Banhos, the Gambier group and the Great Chagos Bank), and I
+ suspect in all cases, the dead and submerged portions do not blend or
+ slope into the living and perfect parts, but are separated from them
+ by an abrupt line. In some instances small patches of living reef rise
+ to the surface from the middle of the submerged and dead parts.
+
+In the Chagos group of atolls, within an area of 160 miles by 60, there
+are two atoll-formed banks of dead rock (besides another very imperfect
+one), entirely submerged; a third, with merely two or three very small
+pieces of living reef rising to the surface; and a fourth, namely,
+Peros Banhos (Plate I, Fig. 9), with a portion nine miles in length
+dead and submerged. As by our theory this area has subsided, and as
+there is nothing improbable in the death, either from changes in the
+state of the surrounding sea or from the subsidence being great or
+sudden, of the corals on the whole, or on portions of some of the
+atolls, the case of the Chagos group presents no difficulty. So far
+indeed are any of the above-mentioned cases of submerged reefs from
+being inexplicable, that their occurrence might have been anticipated
+on our theory, and as fresh atolls are supposed to be in progressive
+formation by the subsidence of encircling barrier-reefs, a weighty
+objection, namely that the number of atolls must be increasing
+infinitely, might even have been raised, if proofs of the occasional
+destruction and loss of atolls could not have been adduced.
+
+_The disseverment of the larger Maldiva atolls._—The apparent
+progressive disseverment in the Maldiva Archipelago of large atolls
+into smaller ones, is, in many respects, an important consideration,
+and requires an explanation. The graduated series which marks, as I
+believe, this process, can be observed only in the northern half of the
+group, where the atolls have exceedingly imperfect margins, consisting
+of detached basin-formed reefs. The currents of the sea flow across
+these atolls, as I am informed by Captain Moresby, with considerable
+force, and drift the sediment from side to side during the monsoons,
+transporting much of it seaward; yet the currents sweep with greater
+force round their flanks. It is historically known that these atolls
+have long existed in their present state; and we can believe, that even
+during a very slow subsidence they might thus remain, the central
+expanse being kept at nearly its original depth by the accumulation of
+sediment. But in the action of such nicely balanced forces during a
+progressive subsidence (like that, to which by our theory this
+archipelago has been subjected), it would be strange if the currents of
+the sea should never make a direct passage across some one of the
+atolls, through the many wide breaches in their margins. If this were
+once effected, a deep-water channel would soon be formed by the removal
+of the finer sediment, and the check to its further accumulation; and
+the sides of the channel would be worn into a slope like that on the
+outer coasts, which are exposed to the same force of the currents. In
+fact, a channel precisely like that bifurcating one which divides
+Mahlos Mahdoo (Plate II, Fig. 4), would almost necessarily be formed.
+The scattered reefs situated near the borders of the new ocean-channel,
+from being favourably placed for the growth of coral, would, by their
+extension, tend to produce fresh margins to the dissevered portions;
+such a tendency is very evident (as may be seen in the large published
+chart) in the elongated reefs on the borders of the two channels
+intersecting Mahlos Mahdoo. Such channels would become deeper with
+continued subsidence, and probably from the reefs not growing up
+perpendicularly, somewhat broader. In this case, and more especially if
+the channels had been formed originally of considerable breadth, the
+dissevered portions would become perfect and distinct atolls, like Ari
+and Ross atolls (Plate II, Fig. 6), or like the two Nillandoo atolls,
+which must be considered as distinct, although related in form and
+position, and separated from each other by channels, which though deep
+have been sounded. Further subsidence would render such channels
+unfathomable, and the dissevered portions would then resemble Phaleedoo
+and Moluque atolls, or Mahlos Mahdoo and Horsburgh atolls (Plate II,
+Fig. 4), which are related to each other in no respect except in
+proximity and position. Hence, on the theory of subsidence, the
+disseverment of large atolls, which have imperfect margins (for
+otherwise their disseverment would be scarcely possible), and which are
+exposed to strong currents, is far from being an improbable event; and
+the several stages, from close relation to entire isolation in the
+atolls of the Maldiva Archipelago, are readily explicable.
+
+We might go even further, and assert as not improbable, that the first
+formation of the Maldiva Archipelago was due to a barrier-reef, of
+nearly the same dimensions with that of New Caledonia (Plate II, Fig.
+5), for if, in imagination, we complete the subsidence of that great
+island, we might anticipate from the present broken condition of the
+northern portion of the reef, and from the almost entire absence of
+reefs on the eastern coast, that the barrier-reef after repeated
+subsidences, would become during its upward growth separated into
+distinct portions; and these portions would tend to assume an
+atoll-like structure, from the coral growing with vigour round their
+entire circumferences, when freely exposed to an open sea. As we have
+some large islands partly submerged with barrier-reefs marking their
+former limits, such as New Caledonia, so our theory makes it probable
+that there should be other large islands wholly submerged; and these,
+we may now infer, would be surmounted, not by one enormous atoll, but
+by several large elongated ones, like the atolls in the Maldiva group;
+and these again, during long periods of subsidence, would sometimes
+become dissevered into smaller atolls. I may add, that both in the
+Marshall and Caroline Archipelagoes, there are atolls standing close
+together, which have an evident relationship in form: we may suppose,
+in such cases, either that two or more encircled islands originally
+stood close together, and afforded bases for two or more atolls, or
+that one atoll has been dissevered. From the position, as well as form,
+of three atolls in the Caroline Archipelago (the Namourrek and Elato
+group), which are placed in an irregular circle, I am strongly tempted
+to believe that they have originated by the process of
+disseverment.[16]
+
+ [16] The same remark is, perhaps, applicable to the islands of Ollap,
+ Fanadik, and Tamatam in the Caroline Archipelago, of which charts are
+ given in the atlas of Duperrey’s voyage: a line drawn through the
+ linear reefs and lagoons of these three islands forms a semicircle.
+ Consult also, the atlas of Lutké’s voyage; and for the Marshall group
+ that of Kotzebue; for the Gilbert group consult the atlas of
+ Duperrey’s voyage. Most of the points here referred to may, however,
+ be seen in Krusenstern’s general Atlas of the Pacific.
+
+_Irregularly formed atolls._—In the Marshall group, Musquillo atoll
+consists of two loops united in one point; and Menchikoff atoll is
+formed of three loops, two of which (as may be seen in Fig. 3, Plate
+II) are connected by a mere ribbon-shaped reef, and the three together
+are sixty miles in length. In the Gilbert group some of the atolls have
+narrow strips of reef, like spurs, projecting from them. There occur
+also in parts of the open sea, a few linear and straight reefs,
+standing by themselves; and likewise some few reefs in the form of
+crescents, with their extremities more or less curled inwards. Now, the
+upward growth of a barrier-reef which fronted only one side of an
+island, or one side of an elongated island with its extremities (of
+which cases exist), would produce after the complete subsidence of the
+land, mere strips or crescent or hook-formed reefs: if the island thus
+partially fronted became divided during subsidence into two or more
+islands, these islands would be united together by linear reefs; and
+from the further growth of the coral along their shores together with
+subsidence, reefs of various forms might ultimately be produced, either
+atolls united together by linear reefs, or atolls with spurs projecting
+from them. Some, however, of the more simple forms above specified,
+might, as we have seen, be equally well produced by the coral perishing
+during
+subsidence on part of the circumference of an atoll, whilst on the
+other parts it continued to grow up till it reached the surface.
+
+_The Great Chagos Bank._—I have already shown that the submerged
+condition of the Great Chagos Bank (Plate II, Fig. 1, with its section
+Fig. 2), and of some other banks in the Chagos group, may in all
+probability be attributed to the coral having perished before or during
+the movements of subsidence, to which this whole area by our theory has
+been subjected. The external rim or upper ledge (shaded in the chart),
+consists of dead coral-rock thinly covered with sand; it lies at an
+average depth of between five and eight fathoms, and perfectly
+resembles in form the annular reef of an atoll. The banks of the second
+level, the boundaries of which are marked by dotted lines in the chart,
+lie from about fifteen to twenty fathoms beneath the surface; they are
+several miles broad, and terminate in a very steep slope round the
+central expanse. This central expanse I have already described, as
+consisting of a level muddy flat between thirty and forty fathoms deep.
+The banks of the second level, might at first sight be thought
+analogous to the internal step-like ledge of coral-rock which borders
+the lagoons of some atolls, but their much greater width, and their
+being formed of sand, are points of essential difference. On the
+eastern side of the atoll some of the banks are linear and parallel,
+resembling islets in a great river, and pointed directly towards a
+great breach on the opposite side of the atoll; these are best seen in
+the large published chart. I inferred from this circumstance, that
+strong currents sometimes set directly across this vast bank; and I
+have since heard from Captain Moresby that this is the case. I
+observed, also, that the channels or breaches through the rim, were all
+of the same depth as the central lagoon-like space into which they
+lead; whereas the channels into the other atolls of the Chagos group,
+and as I believe into most other large atolls, are not nearly as deep
+as their lagoons: for instance at Peros Banhos, the channels are only
+of the same depth, namely between ten and twenty fathoms, as the bottom
+of the lagoon for a space about a mile and a half in width round its
+shores, whilst the central expanse of the lagoon is from thirty-five to
+forty fathoms deep. Now, if an atoll during a gradual subsidence once
+became entirely submerged, like the Great Chagos Bank, and therefore no
+longer exposed to the surf, very little sediment could be formed from
+it; and consequently the channels leading into the lagoon from not
+being filled up with drifted sand and coral detritus, would continue
+increasing in depth, as the whole sank down. In this case, we might
+expect that the currents of the open sea, instead of any longer
+sweeping round the submarine flanks, would flow directly through the
+breaches across the lagoon, removing in their course the finer
+sediment, and preventing its further accumulation. We should then have
+the submerged reef forming an external and upper rim of rock, and
+beneath this portion of the sandy bottom of the old lagoon, intersected
+by deep-water channels or breaches, and thus formed into separate
+marginal banks; and these would be cut off by steep slopes, overhanging
+the central space, worn down by the passage of the oceanic currents.
+
+
+By these means, I have scarcely any doubt that the Great Chagos Bank
+has originated,—a structure which at first appeared to me far more
+anomalous than any I had met with. The process of formation is nearly
+the same with that, by which Mahlos Mahdoo had been trisected; but in
+the Chagos Bank the channels of the oceanic currents entering at
+several different quarters, have united in a central space.
+
+This great atoll-formed bank appears to be in an early stage of
+disseverment; should the work of subsidence go on, from the submerged
+and dead condition of the whole reef, and the imperfection of the
+south-east quarter a mere wreck would probably be left. The Pitt’s
+Bank, situated not far southward, appears to be precisely in this
+state; it consists of a moderately level, oblong bank of sand, lying
+from 10 to 20 fathoms beneath the surface, with two sides protected by
+a narrow ledge of rock which is submerged between 5 and 8 fathoms. A
+little further south, at about the same distance as the southern rim of
+the Great Chagos Bank is from the northern rim, there are two other
+small banks with from 10 to 20 fathoms on them; and not far eastward
+soundings were struck on a sandy bottom, with between 110 and 145
+fathoms. The northern portion with its ledge-like margin, closely
+resembles any one segment of the Great Chagos Bank, between two of the
+deep-water channels, and the scattered banks, southward appear to be
+the last wrecks of less perfect portions.
+
+I have examined with care the charts of the Indian and Pacific Oceans,
+and have now brought before the reader all the examples, which I have
+met with, of reefs differing from the type of the class to which they
+belong; and I think it has been satisfactorily shown, that they are all
+included in our theory, modified by occasional accidents which might
+have been anticipated as probable. In this course we have seen, that in
+the lapse of ages encircling barrier-reefs are occasionally converted
+into atolls, the name of atoll being properly applicable, at the moment
+when the last pinnacle of encircled land sinks beneath the surface of
+the sea. We have, also, seen that large atolls during the progressive
+subsidence of the areas in which they stand, sometimes become
+dissevered into smaller ones; at other times, the reef-building
+polypifers having entirely perished, atolls are converted into
+atoll-formed banks of dead rock; and these again through further
+subsidence and the accumulation of sediment modified by the force of
+the oceanic currents, pass into level banks with scarcely any
+distinguishing character. Thus may the history of an atoll be followed
+from its first origin, through the occasional accidents of its
+existence, to its destruction and final obliteration.
+
+_Objections to the theory of the formation of atolls and
+barrier-reefs._—The vast amount of subsidence, both horizontally or in
+area, and vertically or in depth, necessary to have submerged every
+mountain, even the highest, throughout the immense spaces of ocean
+interspersed with atolls, will probably strike most people as a
+formidable objection to my theory. But as continents, as large as the
+spaces supposed to have subsided, have been raised above the level of
+the sea,—as whole regions are now rising, for instance, in Scandinavia
+and South America,—and as
+no reason can be assigned, why subsidences should not have occurred in
+some parts of the earth’s crust on as great a scale both in extent and
+amount as those of elevation, objections of this nature strike me as of
+little force. The remarkable point is that movements to such an extent
+should have taken place within a period, during which the polypifers
+have continued adding matter on and above the same reefs. Another and
+less obvious objection to the theory will perhaps be advanced from the
+circumstance, of the lagoons within atolls and within barrier-reefs
+never having become in any one instance during prolonged subsidences of
+a greater depth than sixty fathoms, and seldom more than forty fathoms;
+but we already admit, if the theory be worth considering, that the rate
+of subsidence has not exceeded that of the upward growth of the coral
+on the exterior margin; we are, therefore, only further required to
+admit, that the subsidence has not exceeded in rate the filling up of
+the interior spaces by the growth of the corals living there, and by
+the accumulation of sediment. As this filling up must take place very
+slowly within barrier-reefs lying far from the land, and within atolls
+which are of large dimensions and which have open lagoons with very few
+reefs, we are led to conclude that the subsidence thus
+counter-balanced, must have been slow in an extraordinary degree; a
+conclusion which accords with our only means, namely, with what is
+known of the rate and manner of recent elevatory movements, of judging
+by analogy what is the probable rate of subsidence.
+
+In this chapter it has, I think, been shown, that the theory of
+subsidence, which we were compelled to receive from the necessity of
+giving to the corals, in certain large areas, foundations at the
+requisite depth, explains both the normal structure and the less
+regular forms of those two great classes of reefs, which have justly
+excited the astonishment of all persons who have sailed through the
+Pacific and Indian Oceans. But further to test the truth of the theory,
+a crowd of questions will occur to the reader: Do the different kinds
+of reefs, which have been produced by the same kind of movement,
+generally lie within the same areas? What is their relation of form and
+position,—for instance, do adjoining groups of atolls, and the separate
+atolls in these groups, bear the same relation to each other which
+islands do in common archipelagoes? Have we reason to believe, that
+where there are fringing-reefs, there has not lately been subsidence;
+or, for it is almost our only way of ascertaining this point, are there
+frequently proofs of recent elevation? Can we by this means account for
+the presence of certain classes of reefs in some large areas, and their
+entire absence in others? Do the areas which have subsided, as
+indicated by the presence of atolls and barrier-reefs, and the areas
+which have remained stationary or have been upraised, as shown by
+fringing-reefs, bear any determinate relation to each other; and are
+the dimensions of these areas such as harmonise with the greatness of
+the subterranean changes, which, it must be supposed, have lately taken
+place beneath them? Is there any connection between the movements thus
+indicated, and recent volcanic action? All these questions ought to
+receive answers in accordance with the theory; and if this can be
+satisfactorily shown, not only is the theory
+confirmed, but as deductions, the answers are in themselves important.
+Under this latter point of view, these questions will be chiefly
+considered in the following chapter.[17]
+
+ [17] I may take this opportunity of briefly considering the
+ appearances, which would probably be presented by a vertical and deep
+ section across a coral formation (referring chiefly to an atoll),
+ formed by the upward growth of coral during successive subsidences.
+ This is a subject worthy of attention, as a means of comparison with
+ ancient coral-strata. The circumferential parts would consist of
+ massive species, in a vertical position, with their interstices filled
+ up with detritus; but this would be the part most subject to
+ subsequent denudation and removal. It is useless to speculate how
+ large a portion of the exterior annular reef would consist of upright
+ coral, and how much of fragmentary rock, for this would depend on many
+ contingencies,—such as on the rate of subsidence, occasionally
+ allowing a fresh growth of coral to cover the whole surface, and on
+ the breakers having force sufficient to throw fragments over this same
+ space. The conglomerate which composes the base of the islets, would
+ (if not removed by denudation together with the exterior reef on which
+ it rests) be conspicuous from the size of the fragments,—the different
+ degrees in which they have been rounded,—the presence of fragments of
+ conglomerate torn up, rounded, and recemented,—and from the oblique
+ stratification. The corals which lived in the lagoon-reefs at each
+ successive level, would be preserved upright, and they would consist
+ of many kinds, generally much branched. In this part, however, a very
+ large proportion of the rock (and in some cases nearly all of it)
+ would be formed of sedimentary matter, either in an excessively fine,
+ or in a moderately coarse state, and with the particles almost blended
+ together. The conglomerate which was formed of rounded pieces of the
+ branched corals, on the shores of the lagoon, would differ from that
+ formed on the islets and derived from the outer coast; yet both might
+ have accumulated very near each other. I have seen a conglomerate
+ limestone from Devonshire like a conglomerate now forming on the
+ shores of the Maldiva atolls. The stratification taken as a whole,
+ would be horizontal; but the conglomerate beds resting on the exterior
+ reef, and the beds of sandstone on the shores of the lagoon (and no
+ doubt on the external flanks) would probably be divided (as at Keeling
+ atoll and at Mauritius) by numerous layers dipping at considerable
+ angles in different directions. The calcareous sandstone and
+ coral-rock would almost necessarily contain innumerable shells,
+ echini, and the bones of fish, turtle, and perhaps of birds; possibly,
+ also, the bones of small saurians, as these animals find their way to
+ the islands far remote from any continent. The large shells of some
+ species of Tridacna would be found vertically imbedded in the solid
+ rock, in the position in which they lived. We might expect also to
+ find a mixture of the remains of pelagic and littoral animals in the
+ strata formed in the lagoon, for pumice and the seeds of plants are
+ floated from distant countries into the lagoons of many atolls: on the
+ outer coast of Keeling atoll, near the mouth of the lagoon, the case
+ of a pelagic Pteropodous animal was brought up on the arming of the
+ sounding lead. All the loose blocks of coral on Keeling atoll were
+ burrowed by vermiform animals; and as every cavity, no doubt,
+ ultimately becomes filled with spathose limestone, slabs of the rock
+ taken from a considerable depth, would, if polished, probably exhibit
+ the excavations of such burrowing animals. The conglomerate and
+ fine-grained beds of coral-rock would be hard, sonorous, white and
+ composed of nearly pure calcareous matter; in some few parts, judging
+ from the specimens at Keeling atoll, they would probably contain a
+ small quantity of iron. Floating pumice and scoriæ, and occasionally
+ stones transported in the root of trees (see my “Journal of
+ Researches,” page 549) appear the only sources, through which foreign
+ matter is brought to coral-formations standing in the open ocean. The
+ area over which sediment is transported from coral-reefs must be
+ considerable: Captain Moresby informs me that during the change of
+ monsoons the sea is discoloured to a considerable distance off the
+ Maldiva and Chagos atolls. The sediment of fringing and barrier
+ coral-reefs must be mingled with the mud, which is brought down from
+ the land, and is transported seaward through the breaches, which occur
+ in front of almost every valley. If the atolls of the larger
+ archipelagoes were upraised, the bed of the ocean being converted into
+ land, they would form flat-topped mountains, varying in diameter from
+ a few miles (the smallest atolls being worn away) to sixty miles; and
+ from being horizontally stratified and of similar composition, they
+ would, as Mr. Lyell has remarked, falsely appear as if they had
+ originally been united into one vast continuous mass. Such great
+ strata of coral-rock would rarely be associated with erupted volcanic
+ matter, for this could only take place, as may be inferred from what
+ follows in the next chapter, when the area, in which they were
+ situated, commenced to rise, or at least ceased to subside. During the
+ enormous period necessary to effect an elevation of the kind just
+ alluded to, the surface would necessarily be denuded to a great
+ thickness; hence it is highly improbable that any fringing-reef, or
+ even any barrier-reef, at least of those encircling small islands,
+ would be preserved. From this same cause, the strata which were formed
+ within the lagoons of atolls and lagoon-channels of barrier-reefs, and
+ which must consist in a large part of sedimentary matter, would more
+ often be preserved to future ages, than the exterior solid reef,
+ composed of massive corals in an upright position; although it is on
+ this exterior part that the present existence and further growth of
+ atolls and barrier-reefs entirely depend.
+
+
+_ Plate III_—Map showing the distribution of coral-reefs and active
+volcanoes.
+
+
+[Illustration: Map showing distribution of coral-reefs and active
+volcanoes.]
+
+[Illustration: Map showing distribution of coral-reefs and active
+volcanoes.]
+
+[Illustration: Map showing distribution of coral-reefs and active
+volcanoes.]
+
+The principles, on which this map was coloured, are explained in the
+beginning of Chapter VI; and the authorities for each particular spot
+are detailed in the Appendix to _Coral Reefs._ The names not printed in
+upper case in the Index refer to the Appendix.
+
+
+
+
+Chapter VI ON THE DISTRIBUTION OF CORAL-REEFS WITH REFERENCE TO THE
+THEORY OF THEIR FORMATION
+
+
+Description of the coloured map.—Proximity of atolls and
+barrier-reefs.—Relation in form and position of atolls with ordinary
+islands.—Direct evidence of subsidence difficult to be detected.—Proofs
+of recent elevation where fringing-reefs occur.—Oscillations of
+level.—Absence of active volcanoes in the areas of
+subsidence.—Immensity of the areas which have been elevated and have
+subsided.—Their relation to the present distribution of the land.—Areas
+of subsidence elongated, their intersection and alternation with those
+of elevation.—Amount and slow rate of the subsidence.—Recapitulation.
+
+
+It will be convenient to give here a short account of the appended map
+(Plate III):[1] a fuller one, with the data for colouring each spot, is
+reserved
+for the Appendix; and every place there referred to may be found in the
+Index. A larger chart would have been desirable; but, small as the
+adjoined one is, it is the result of many months’ labour. I have
+consulted, as far as I was able, every original voyage and map; and the
+colours were first laid down on charts on a larger scale. The same blue
+colour, with merely a difference in the depth of tint, is used for
+atolls or lagoon-islands, and barrier-reefs, for we have seen, that as
+far as the actual coral-formation is concerned, they have no
+distinguishing character. Fringing-reefs have been coloured red, for
+between them on the one hand, and barrier-reefs and atolls on the
+other, there is an important distinction with respect to the depth
+beneath the surface, at which we are compelled to believe their
+foundations lie. The two distinct colours, therefore, mark two great
+types of structure.
+
+ [1] Inasmuch as the coloured map would have proved too costly to be
+ given in this series, the indications of colour have been replaced by
+ numbers referring to the dotted groups of reefs, etc. The author’s
+ original wording, however, is retained in full, as it will be easy to
+ refer to the map by the numbers, and thus the flow of the narrative is
+ undisturbed.
+
+The _dark blue colour_ [represented by (3) in our plate] represents
+atolls and submerged annular reefs, with deep water in their centres. I
+have coloured as atolls, a few low and small coral-islands, without
+lagoons; but this has been done only when it clearly appeared that they
+originally contained lagoons, since filled up with sediment: when there
+were not good grounds for this belief, they have been left uncoloured.
+
+The _pale blue colour_ [represented by (2)] represents barrier-reefs.
+The most obvious character of reefs of this class is the broad and
+deep-water moat within the reef: but this, like the lagoons of small
+atolls, is liable to become filled up with detritus and with reefs of
+delicately branched corals: when, therefore, a reef round the entire
+circumference of an island extends very far into a profoundly deep sea,
+so that it can hardly be confounded with a fringing-reef which must
+rest on a foundation of rock within a small depth, it has been coloured
+pale blue, although it does not include a deep-water moat: but this has
+only been done rarely, and each case is distinctly mentioned in the
+Appendix.
+
+The _red colour_ (4) represents reefs fringing the land quite closely
+where the sea is deep, and where the bottom is gently inclined
+extending to a moderate distance from it, but not having a deep-water
+moat or lagoon-like space parallel to the shore. It must be remembered
+that fringing-reefs are frequently _breached_ in front of rivers and
+valleys by deepish channels, where mud has been deposited. A space of
+thirty miles in width has been coloured round or in front of the reefs
+of each class, in order that the colours might be conspicuous on the
+appended map, which is reduced to so small a scale.
+
+The _vermillion spots_, and streaks (1) represent volcanoes now in
+action, or historically known to have been so. They are chiefly laid
+down from Von Buch’s work on the Canary Islands; and my reasons for
+making a few alterations are given in the note below.[2]
+
+ [2] I have also made considerable use of the geological part of
+ Berghaus’ “Physical Atlas.” Beginning at the eastern side of the
+ Pacific, I have added to the number of the volcanoes in the southern
+ part of the Cordillera, and have coloured Juan Fernandez according to
+ observations collected during the voyage of the _Beagle_ (“Geol.
+ Trans.,” vol. v, p. 601). I have added a volcano to Albemarle Island,
+ one of the Galapagos Archipelago (the author’s “Journal of
+ Researches,” p. 457). In the Sandwich group there are no active
+ volcanoes, except at Hawaii; but the Rev. W. Ellis informs me, there
+ are streams of lava apparently modern on Maui, having a very recent
+ appearance, which can be traced to the craters whence they flowed. The
+ same gentleman informs me, that there is no reason to believe that any
+ active volcano exists in the Society Archipelago; nor are there any
+ known in the Samoa or Navigator group, although some of the streams of
+ lava and craters there appear recent. In the Friendly group, the Rev.
+ J. Williams says (“Narrative of Missionary Enterprise,” p. 29) that
+ Toofoa and Proby Islands are active volcanoes. I infer from Hamilton’s
+ “Voyage in the _Pandora_” (p. 95), that Proby Island is synonymous
+ with Onouafou, but I have not ventured to colour it. There can be no
+ doubt respecting Toofoa, and Captain Edwards (Von Buch, p. 386) found
+ the lava of recent eruption at Amargura still smoking. Berghaus marks
+ four active volcanoes actually within the Friendly group; but I do not
+ know on what authority: I may mention that Maurelle describes Latte as
+ having a burnt-up appearance: I have marked only Toofoa and Amargura.
+ South of the New Hebrides lies Matthews Rock, which is drawn and
+ described as an active crater in the “Voyage of the _Astrolabe_.”
+ Between it and the volcano on the eastern side of New Zealand, lies
+ Brimstone Island, which from the high temperature of the water in the
+ crater, may be ranked as active (Berghaus “Vorbemerk,” II Lief. S.
+ 56). Malte Brun, vol. xii, p. 231, says that there is a volcano near
+ port St. Vincent in New Caledonia. I believe this to be an error,
+ arising from a smoke seen on the _opposite_ coast by Cook (“Second
+ Voyage,” vol. ii, p. 23) which smoke went out at night. The Mariana
+ Islands, especially the northern ones, contain many craters (see
+ Freycinet’s “Hydrog. Descript.”) which are not active. Von Buch,
+ however, states (p. 462) on the authority of La Peyrouse, that there
+ are no less than seven volcanoes between these islands and Japan.
+ Gemelli Creri (Churchill’s “Collect.” vol. iv, p. 458), says there are
+ two active volcanoes in latitude 23° 30′, and in latitude 24°: but I
+ have not coloured them. From the statements in Beechey’s “Voyage” (p.
+ 518, 4to edit.) I have coloured one in the northern part of the Bonin
+ group. M. S. Julien has clearly made out from Chinese manuscripts not
+ very ancient (“Comptes Rendus,” 1840, p. 832), that there are two
+ active volcanoes on the eastern side of Formosa. In Torres Straits, on
+ Cap Island (9° 48′ S., 142° 39′ E.) a volcano was seen burning with
+ great violence in 1793 by Captain Bampton (see Introduction to
+ Flinders’ “Voyage,” p. 41). Mr. M’Clelland (Report of Committee for
+ investigating Coal in India, p. 39) has shown that the volcanic band
+ passing through Barren Island must be extended northwards. It appears
+ by an old chart, that Cheduba was once an active volcano (see also _
+ Silliman’s North American Journal_, vol. xxxviii, p. 385). In
+ Berghaus’ “Phys. Atlas,” 1840, No. 7 of Geological Part, a volcano on
+ the coast of Pondicherry is said to have burst forth in 1757.
+ Ordinaire (“Hist. Nat. des Volcans,” p. 218) says that there is one at
+ the mouth of the Persian Gulf, but I have not coloured it, as he gives
+ no particulars. A volcano in Amsterdam, or St. Paul’s, in the southern
+ part of the Indian Ocean, has been seen (_Naut. Mag._ 1838, p. 842) in
+ action. Dr. J. Allan, of Forres, informs me in a letter, that when he
+ was at Joanna, he saw at night flames apparently volcanic, issuing
+ from the chief Comoro Island, and that the Arabs assured him that they
+ were volcanic, adding that the volcano burned more during the wet
+ season. I have marked this as a volcano, though with some hesitation,
+ on account of the possibility of the flame arising from gaseous
+ sources.
+
+
+The uncoloured coasts consist, first and chiefly, of those, where there
+are no coral-reefs, or such small portions as to be quite
+insignificant. Secondly, of those coasts where there are reefs, but
+where the sea is very shallow, for in this case the reefs generally lie
+far from the land, and become very irregular, in their forms: where
+they have not become irregular, they have been coloured. thirdly, if I
+had the means of ascertaining the fact, I should not colour a reef
+merely coating the edges of a submarine crater, or of a level submerged
+bank; for such superficial
+formations differ essentially, even when not in external appearance,
+from reefs whose foundations as well as superficies have been wholly
+formed by the growth of coral. Fourthly, in the Red Sea, and within
+some parts of the East Indian Archipelago (if the imperfect charts of
+the latter can be trusted), there are many scattered reefs, of small
+size, represented in the chart by mere dots, which rise out of deep
+water: these cannot be arranged under either of the three classes: in
+the Red Sea, however, some of these little reefs, from their position,
+seem once to have formed parts of a continuous barrier. There exist,
+also, scattered in the open ocean, some linear and irregularly formed
+strips of coral-reef, which, as shown in the last chapter, are probably
+allied in their origin to atolls; but as they do not belong to that
+class, they have not been coloured; they are very few in number and of
+insignificant dimensions. Lastly, some reefs are left uncoloured from
+the want of information respecting them, and some because they are of
+an intermediate structure between the barrier and fringing classes. The
+value of the map is lessened, in proportion to the number of reefs
+which I have been obliged to leave uncoloured, although, in a
+theoretical point of view, few of them present any great difficulty:
+but their number is not very great, as will be found by comparing the
+map with the statements in the Appendix. I have experienced more
+difficulty in colouring fringing-reefs than in colouring barrier-reefs,
+as the former, from their much less dimensions, have less attracted the
+attention of navigators. As I have had to seek my information from all
+kinds of sources, and often from indirect ones, I do not venture to
+hope that the map is free from many errors. Nevertheless, I trust it
+will give an approximately correct view of the general distribution of
+the coral-reefs over the whole world (with the exception of some
+fringing-reefs on the coast of Brazil, not included within the limits
+of the map), and of their arrangement into the three great classes,
+which, though necessarily very imperfect from the nature of the objects
+classified, have been adopted by most voyagers. I may further remark,
+that the dark blue colour represents land entirely composed of
+coral-rock; the pale blue, land with a wide and thick border of
+coral-rock; and the red, a mere narrow fringe of coral-rock.
+
+Looking now at the map under the theoretical point of view indicated in
+the last chapter, the two blue tints signify that the foundations of
+the reefs thus coloured have subsided to a considerable amount, at a
+slower rate than that of the upward growth of the corals, and that
+probably in many cases they are still subsiding. The red signifies that
+the shores which support fringing-reefs have not subsided (at least to
+any considerable
+amount, for the effects of a subsidence on a small scale would in no
+case be distinguishable); but that they have remained nearly stationary
+since the period when they first became fringed by reefs; or that they
+are now rising or have been upraised, with new lines of reefs
+successively formed on them: these latter alternatives are obviously
+implied, as newly formed lines of shore, after elevations of the land,
+would be in the same state with respect to the growth of
+fringing-reefs, as stationary coasts. If during the prolonged
+subsidence of a shore, coral-reefs grew for the first time on it, or if
+an old barrier-reef were destroyed and submerged, and new reefs became
+attached to the land, these would necessarily at first belong to the
+fringing class, and, therefore, be coloured red, although the coast was
+sinking: but I have no reason to believe, that from this source of
+error, any coast has been coloured wrongly with respect to movement
+indicated. Well characterised atolls and encircling barrier-reefs,
+where several occur in a group, or a single barrier-reef if of large
+dimensions, leave scarcely any doubt on the mind respecting the
+movement by which they have been produced; and even a small amount of
+subsequent elevation is soon betrayed. The evidence from a single atoll
+or a single encircling barrier-reef, must be received with some
+caution, for the former may possibly be based upon a submerged crater
+or bank, and the latter on a submerged margin of sediment, or of
+worn-down rock. From these remarks we may with greater certainty infer
+that the spaces, especially the larger ones, tinted blue in the map,
+have subsided, than that the red spaces have remained stationary, or
+have been upraised.
+
+_On the grouping of the different classes of reefs._—Having made these
+preliminary remarks, I will consider first how far the grouping of the
+different kinds of coral-islands and reefs is corroborative of the
+truth of the theory. A glance at the map shows that the reefs, coloured
+blue and red, produced under widely different conditions, are not
+indiscriminately mixed together. Atolls and barrier-reefs, on the other
+hand, as may be seen by the two blue tints, generally lie near each
+other; and this would be the natural result of both having been
+produced during the subsidence of the areas in which they stand. Thus,
+the largest group of encircled islands is that of the Society
+Archipelago; and these islands are surrounded by atolls, and only
+separated by a narrow space from the large group of Low atolls. In the
+midst of the Caroline atolls, there are three fine encircled islands.
+The northern point of the barrier-reef of New Caledonia seems itself,
+as before remarked, to form a complete large atoll. The great
+Australian barrier is described as including both atolls and small
+encircled islands. Captain King[3] mentions many atoll-formed and
+encircling coral-reefs, some of which lie within the barrier, and
+others may be said (for instance between lat. 16° and 13°) to form part
+of it. Flinders[4] has described an atoll-formed reef in lat. 10°,
+seven miles long and from one to three broad, resembling a boot in
+shape, with apparently very deep water
+within. Eight miles westward of this, and forming part of the barrier,
+lie the Murray Islands, which are high and are encircled. In the
+Corallian Sea, between the two great barriers of Australia and New
+Caledonia, there are many low islets and coral-reefs, some of which are
+annular, or horse-shoe shaped. Observing the smallness of the scale of
+the map, the parallels of latitude being nine hundred miles apart, we
+see that none of the large groups of reefs and islands supposed to have
+been produced by long-continued subsidence, lie near extensive lines of
+coast coloured red, which are supposed to have remained stationary
+since the growth of their reefs, or to have been upraised and new lines
+of reefs formed on them. Where the red and blue circles do occur near
+each other, I am able, in several instances, to show that there have
+been oscillations of level, subsidence having preceded the elevation of
+the red spots; and elevation having preceded the subsidence of the blue
+spots: and in this case the juxtaposition of reefs belonging to the two
+great types of structure is little surprising. We may, therefore,
+conclude that the proximity in the same areas of the two classes of
+reefs, which owe their origin to the subsidence of the earth’s crust,
+and their separation from those formed during its stationary or
+uprising condition, holds good to the full extent, which might have
+been anticipated by our theory.
+
+ [3] Sailing directions, appended to vol. ii of his “Surveying Voyage
+ to Australia.”
+
+
+ [4] “Voyage to Terra Australis,” vol. ii, p. 336.
+
+As groups of atolls have originated in the upward growth, at each fresh
+sinking of the land, of those reefs which primarily fringed the shores
+of one great island, or of several smaller ones; so we might expect
+that these rings of coral-rock, like so many rude outline charts, will
+still retain some traces of the general form, or at least general
+range, of the land, round which they were first modelled. That this is
+the case with the atolls in the Southern Pacific as far as their range
+is concerned, seems highly probable, when we observe that the three
+principal groups are directed in north-west and south-east lines, and
+that nearly all the land in the S. Pacific ranges in this same
+direction; namely, N. Western Australia, New Caledonia, the northern
+half of New Zealand, the New Hebrides, Saloman, Navigator, Society,
+Marquesas, and Austral archipelagoes: in the Northern Pacific, the
+Caroline atolls abut against the north-west line of the Marshall
+atolls, much in the same manner as the east and west line of islands
+from Ceram to New Britain do on New Ireland: in the Indian Ocean the
+Laccadive and Maldiva atolls extend nearly parallel to the western and
+mountainous coast of India. In most respects, there is a perfect
+resemblance with ordinary islands in the grouping of atolls and in
+their form: thus the outline of all the larger groups is elongated; and
+the greater number of the individual atolls are elongated in the same
+direction with the group, in which they stand. The Chagos group is less
+elongated than is usual with other groups, and the individual atolls in
+it are likewise but little elongated; this is strikingly seen by
+comparing them with the neighbouring Maldiva atolls. In the Marshall
+and Maldiva archipelagoes, the atolls are ranged in two parallel lines,
+like the mountains in a great double mountain-chain. Some of the
+atolls, in the larger archipelagoes, stand so near to each other, and
+have such an evident relationship in
+form, that they compose little sub-groups: in the Caroline Archipelago,
+one such sub-group consists of Pouynipete, a lofty island encircled by
+a barrier-reef, and separated by a channel only four miles and a half
+wide from Andeema atoll, with a second atoll a little further off. In
+all these respects an examination of a series of charts will show how
+perfectly groups of atolls resemble groups of common islands.
+
+_On the direct evidence of the blue spaces in the map having subsided
+during the upward growth of the reefs so coloured, and of the red
+spaces having remained stationary, or having been upraised._—With
+respect to subsidence, I have shown in the last chapter, that we cannot
+expect to obtain in countries inhabited only by semi-civilised races,
+demonstrative proofs of a movement, which invariably tends to conceal
+its own evidence. But on the coral-islands supposed to have been
+produced by subsidence, we have proofs of changes in their external
+appearance—of a round of decay and renovation—of the last vestiges of
+land on some—of its first commencement on others: we hear of storms
+desolating them to the astonishment of their inhabitants: we know by
+the great fissures with which some of them are traversed, and by the
+earthquakes felt under others, that subterranean disturbances of some
+kind are in progress. These facts, if not directly connected with
+subsidence, as I believe they are, at least show how difficult it would
+be to discover proofs of such movement by ordinary means. At Keeling
+atoll, however, I have described some appearances, which seem directly
+to show that subsidence did take place there during the late
+earthquakes. Vanikoro, according to Chevalier Dillon,[5] is often
+violently shaken by earthquakes, and there, the unusual depth of the
+channel between the shore and the reef,—the almost entire absence of
+islets on the reef,—its wall-like structure on the inner side, and the
+small quantity of low alluvial land at the foot of the mountains, all
+seem to show that this island has not remained long at its present
+level, with the lagoon-channel subjected to the accumulation of
+sediment, and the reef to the wear and tear of the breakers. At the
+Society Archipelago, on the other hand, where a slight tremor is only
+rarely felt, the shoaliness of the lagoon-channels round some of the
+islands, the number of islets formed on the reefs of others, and the
+broad belt of low land at the foot of the mountains, indicate that,
+although there must have been great subsidence to have produced the
+barrier-reefs, there has since elapsed a long stationary period.[6]
+
+ [5] See Captain Dillon’s “Voyage in search of La Peyrouse.” M. Cordier
+ in his “Report on the Voyage of the ‘Astrolabe’” (p. cxi, vol. i),
+ speaking of Vanikoro, says the shores are surrounded by reefs of
+ madrepore, “qu’on assure etre de formation tout-a-fait moderne.” I
+ have in vain endeavoured to learn some further particulars about this
+ remarkable passage. I may here add, that according to our theory, the
+ island of Pouynipete (Plate I, Fig. 7), in the Caroline Archipelago,
+ being encircled by a barrier-reef, must have subsided. In the _ New S.
+ Wales Lit. Advert._ February 1835 (which I have seen through the
+ favour of Dr. Lloghtsky), there is an account of this island
+ (subsequently confirmed by Mr. Campbell), in which it is said, “At the
+ N.E. end, at a place called Tamen, there are ruins of a town, _now
+ only_ accessible by boats, the waves _reaching to the steps of the
+ houses._” Judging from this passage, one would be tempted to conclude
+ that the island must have subsided, since these houses were built. I
+ may, also, here append a statement in Malte Brun (vol. ix, p. 775,
+ given without any authority), that the sea gains in an extraordinary
+ manner on the coast of Cochin China, which lies in front and near the
+ subsiding coral-reefs in the China Sea: as the coast is granitic, and
+ not alluvial, it is scarcely possible that the encroachment of the sea
+ can be owing to the washing away of the land; and if so, it must be
+ due to subsidence.
+
+
+ [6] Mr. Couthouy states (“Remarks,” p. 44) that at Tahiti and Eimeo
+ the space between the reef and the shore has been nearly filled up by
+ the extension of those coral-reefs, which within most barrier-reefs
+ merely fringe the land. From this circumstance, he arrives at the same
+ conclusion as I have done, that the Society Islands since their
+ subsidence, have remained stationary during a long period; but he
+ further believes that they have recently commenced rising, as well as
+ the whole area of the Low Archipelago. He does not give any detailed
+ proofs regarding the elevation of the Society Islands, but I shall
+ refer to this subject in another part of this chapter. Before making
+ some further comments, I may observe how satisfactory it is to me, to
+ find Mr. Couthouy affirming, that “having personally examined a large
+ number of coral-islands, and also residing eight months among the
+ volcanic class, having shore and partially encircling reefs, I may be
+ permitted to state that my own observations have impressed a
+ conviction of the correctness of the theory of Mr. Darwin.”
+    This gentleman believes, that subsequently to the subsidence by
+    which the atolls in the Low Archipelago were produced, the whole
+    area has been elevated to the amount of a few feet; this would
+    indeed be a remarkable fact; but as far as I am able to judge, the
+    grounds of his conclusion are not sufficiently strong. He states
+    that he found in almost every atoll which he visited, the shores of
+    the lagoon raised from eighteen to thirty inches above the
+    sea-level, and containing imbedded Tridacnæ and corals standing as
+    they grew; some of the corals were dead in their upper parts, but
+    below a certain line they continued to flourish. In the lagoons,
+    also, he frequently met with clusters of Madrepore, with their
+    extremities standing from one inch to a foot above the surface of
+    the water. Now, these appearances are exactly what I should have
+    expected, without any subsequent elevation having taken place; and
+    I think Mr. Couthouy has not borne in mind the indisputable fact,
+    that corals, when constantly bathed by the surf, can exist at a
+    higher level than in quite tranquil water, as in a lagoon. As long,
+    therefore, as the waves continued at low water to break entirely
+    over parts of the annular reef of an atoll, submerged to a small
+    depth, the corals and shells attached on these parts might continue
+    living at a level above the smooth surface of the lagoon, into
+    which the waves rolled; but as soon as the outer edge of the reef
+    grew up to its utmost possible height, or if the reef were very
+    broad nearly to that height, the force of the breakers would be
+    checked, and the corals and shells on the inner parts near the
+    lagoon would occasionally be left dry, and thus be partially or
+    wholly destroyed. Even in atolls, which have not lately subsided,
+    if the outer margin of the reef continued to increase in breadth
+    seaward (each fresh zone of corals rising to the same vertical
+    height as at Keeling atoll), the line where the waves broke most
+    heavily would advance outwards, and therefore the corals, which
+    when living near the margin, were washed by the breaking waves
+    during the whole of each tide, would cease being so, and would
+    therefore be left on the backward part of the reef standing exposed
+    and dead. The case of the madrepores in the lagoons with the tops
+    of their branches exposed, seems to be an analogous fact, to the
+    great fields of dead but upright corals in the lagoon of Keeling
+    atoll; a condition of things which I have endeavoured to show, has
+    resulted from the lagoon having become more and more enclosed and
+    choked up with reefs, so that during high winds, the rising of the
+    tide (as observed by the inhabitants) is checked, and the corals,
+    which had formerly grown to the greatest possible height, are
+    occasionally exposed, and thus are killed: and this is a condition
+    of things, towards which almost every atoll in the intervals of its
+    subsidence must be tending. Or if we look to the state of an atoll
+    directly after a subsidence of some fathoms, the waves would roll
+    heavily over the entire circumference of the reef, and the surface
+    of the lagoon would, like the ocean, never be quite at rest, and
+    therefore the corals in the lagoon, from being constantly laved by
+    the rippling water, might extend their branches to a little greater
+    height than they could, when the lagoon became enclosed and
+    protected. Christmas atoll (2° N. lat.) which has a very shallow
+    lagoon, and differs in several respects from most atolls, possibly
+    may have been elevated recently; but its highest part appears
+    (Couthouy, p. 46) to be only ten feet above the sea-level. The
+    facts of a second class, adduced by Mr. Couthouy, in support of the
+    alleged recent elevation of the Low Archipelago, are not all
+    (especially those referring to a shelf of rock) quite intelligible
+    to me; he believes that certain enormous fragments of rock on the
+    reef, must have been moved into their present position, when the
+    reef was at a lower level; but here again the force of the breakers
+    on any inner point of the reef being diminished by its outward
+    growth without any change in its level, has not, I think, been
+    borne in mind. We should, also, not overlook the occasional agency
+    of waves caused by earthquakes and hurricanes. Mr. Couthouy further
+    argues, that since these great fragments were deposited and fixed
+    on the reef, they have been elevated; he infers this from the
+    greatest amount of erosion not being near their bases, where they
+    are unceasingly washed by the reflux of the tides, but at some
+    height on their sides, near the line of high-water mark, as shown
+    in an accompanying diagram. My former remark again applies here,
+    with this further observation, that as the waves have to roll over
+    a wide space of reef before they reach the fragments, their force
+    must be greatly increased with the increasing depth of water as the
+    tide rises, and therefore I should have expected that the chief
+    line of present erosion would have coincided with the line of
+    high-water mark; and if the reef had grown outwards, that there
+    would have been lines of erosion at greater heights. The
+    conclusion, to which I am finally led by the interesting
+    observations of Mr. Couthouy is, that the atolls in the Low
+    Archipelago have, like the Society Islands, remained at a
+    stationary level for a long period: and this probably is the
+    ordinary course of events, subsidence supervening after long
+    intervals of rest.
+
+
+Turning now to the red colour; as on our map, the areas which have sunk
+slowly downwards to great depths are many and large, we might naturally
+have been led to conjecture, that with such great changes of level in
+progress, the coasts which have been fringed probably for ages (for we
+have no reason to believe that coral-reefs are of short duration),
+would not have remained all this time stationary, but would frequently
+have undergone movements of elevation. This supposition, we shall
+immediately see, holds good to a remarkable extent; and although a
+stationary condition of the land can hardly ever be open to proof, from
+the evidence being only negative, we are, in some degree, enabled to
+ascertain the correctness of the parts coloured red on the map, by the
+direct testimony of upraised organic remains of a modern date. Before
+going into the details on this head (printed in small type), I may
+mention, that when reading a memoir on coral formations by MM. Quoy and
+Gaimard[7] I was astonished to find, for I knew that they had crossed
+both the Pacific and Indian Oceans, that their descriptions were
+applicable only to reefs of the fringing class; but my astonishment
+ended satisfactorily, when I discovered that, by a strange chance, all
+the islands which these eminent naturalists had visited, though several
+in number, namely, the Mauritius, Timor, New Guinea, the Mariana, and
+Sandwich Archipelagoes, could be shown by their own statements to have
+been elevated within a recent geological era.
+
+ [7] “Annales des Sciences Nat.” tom. vi, p. 279, etc.
+
+
+In the eastern half of the Pacific, the _ Sandwich Islands_ are all
+fringed, and almost every naturalist who has visited them, has remarked
+on the abundance of elevated corals and shells, apparently identical
+with living species. The Rev. W. Ellis informs me, that he has noticed
+round several parts of Hawaii, beds of coral-detritus, about twenty
+feet above the level of the sea, and where the coast is low they extend
+far inland. Upraised coral-rock forms a considerable part of the
+borders of Oahu; and at Elizabeth Island[8] it composes three strata,
+each about ten feet thick. Nihau, which forms the northern, as Hawaii
+does the southern end of the group (350 miles in length), likewise
+seems to consist of coral and volcanic rocks. Mr. Couthouy[9] has
+lately described with interesting details, several upraised beaches,
+ancient reefs with their surfaces perfectly preserved, and beds of
+recent shells and corals, at the islands of Maui, Morokai, Oahu, and
+Tauai (or Kauai) in this group. Mr. Pierce, an intelligent resident at
+Oahu, is convinced, from changes which have taken place within his
+memory, during the last sixteen years, “that the elevation is at
+present going forward at a very perceptible rate.” The natives at Kauai
+state that the land is there gaining rapidly on the sea, and Mr.
+Couthouy has no doubt, from the nature of the strata, that this has
+been effected by an elevation of the land.
+
+ [8] “Zoology of Captain Beechey’s Voyage,” p. 176. See also MM. Quoy
+ and Gaimard in “Annales de Scien. Nat.” tom. vi.
+
+
+ [9] “Remarks on Coral Formations,” p. 51.
+
+
+In the southern part of the Low Archipelago, Elizabeth Island is
+described by Captain Beechey,[10] as being quite flat, and about eighty
+feet in height; it is entirely composed of dead corals, forming a
+honeycombed, but compact rock. In cases like this, of an island having
+exactly the appearance, which the elevation of any one of the smaller
+surrounding atolls with a shallow lagoon would present, one is led to
+conclude (with little better reason, however, than the improbability of
+such small and low fabrics lasting, for an immense period, exposed to
+the many destroying agents of nature), that the elevation has taken
+place at an epoch not geologically remote. When merely the surface of
+an island of ordinary formation is strewed with marine
+bodies, and that continuously, or nearly so, from the beach to a
+certain height, and not above that height, it is exceedingly improbable
+that such organic remains, although they may not have been specially
+examined, should belong to any ancient period. It is necessary to bear
+these remarks in mind, in considering the evidence of the elevatory
+movements in the Pacific and Indian Oceans, as it does not often rest
+on specific determinations, and therefore should be received with
+caution. Six of the _Cook and Austral Islands_ (S.W. of the Society
+group), are fringed; of these, five were described to me by the Rev. J.
+Williams, as formed of coral-rock, associated with some basalt in
+Mangaia), and the sixth as lofty and basaltic. Mangaia is nearly three
+hundred feet high, with a level summit; and according to Mr. S.
+Wilson[11] it is an upraised reef; “and there are in the central
+hollow, formerly the bed of the lagoon, many scattered patches of
+coral-rock, some of them raised to a height of forty feet.” These
+knolls of coral-rock were evidently once separate reefs in the lagoon
+of an atoll. Mr. Martens, at Sydney, informed me that this island is
+surrounded by a terrace-like plain at about the height of a hundred
+feet, which probably marks a pause in its elevation. From these facts
+we may infer, perhaps, that the Cook and Austral Islands have been
+upheaved at a period probably not very remote.
+
+ [10] Beechey’s “Voyage in the Pacific,” p. 46, 4to ed.
+
+
+ [11] Couthouy’s “Remarks,” p. 34.
+
+
+_Savage Island_ (S.E. of the Friendly group), is about forty feet in
+height. Forster[12] describes the plants as already growing out of the
+dead, but still upright and spreading trees of coral; and the younger
+Forster[13] believes that an ancient lagoon is now represented by a
+central plain; here we cannot doubt that the elevatory forces have
+recently acted. The same conclusion may be extended, though with
+somewhat less certainty, to the islands of the _friendly group_, which
+have been well described in the second and third voyages of Cook. The
+surface of Tongatabou is low and level, but with some parts a hundred
+feet high; the whole consists of coral-rock, “which yet shows the
+cavities and irregularities worn into it by the action of the
+tides.”[14] On Eoua the same appearances were noticed at an elevation
+of between two hundred and three hundred feet. Vavao, also, at the
+opposite or northern end of the group, consists, according to the Rev.
+J. Williams, of coral-rock. Tongatabou, with its northern extensive
+reefs, resembles either an upraised atoll with one half originally
+imperfect, or one unequally elevated; and Anamouka, an atoll equally
+elevated. This latter island contains[15] in its centre a salt-water
+lake, about a mile-and-a-half in diameter, without any communication
+with the sea, and around it the land rises gradually like a bank; the
+highest part is only between twenty and thirty feet; but on this part,
+as well as on the rest of the land (which, as Cook observes, rises
+above the height of true lagoon-islands), coral-rock, like that on the
+beach, was found. In the _Navigator Archipelago_, Mr. Couthouy[16]
+found on Manua many and very large fragments of coral at the height of
+eighty feet, “on a steep hill-side, rising half a mile inland from a
+low sandy plain abounding in marine remains.” The fragments were
+embedded in a mixture of decomposed lava and sand. It is not stated
+whether they were accompanied by shells, or whether the corals
+resembled recent species; as these
+remains were embedded they possibly may belong to a remote epoch; but I
+presume this was not the opinion of Mr. Couthouy. Earthquakes are very
+frequent in this archipelago.
+
+ [12] “Observations made during Voyage round the World,” p. 147.
+
+
+ [13] “Voyage,” vol. ii, p. 163.
+
+
+ [14] Cook’s “Third Voyage” (4to ed.), vol. i, p. 314.
+
+
+ [15] _Ibid_., vol. i, p. 235.
+
+
+ [16] “Remarks on Coral-Formations,” p. 50.
+
+
+Still proceeding westward we come to the _New Hebrides_; on these
+islands, Mr. G. Bennett (author of “Wanderings in New South Wales”),
+informs me he found much coral at a great altitude, which he considered
+of recent origin. Respecting _Santa Cruz_, and the _Solomon
+Archipelago_, I have no information; but at New Ireland, which forms
+the northern point of the latter chain, both Labillardiere and Lesson
+have described large beds of an apparently very modern madreporitic
+rock, with the form of the corals little altered. The latter author[17]
+states that this formation composes a newer line of coast, modelled
+round an ancient one. There only remains to be described in the
+Pacific, that curved line of fringed islands, of which the _ Marianas_
+form the main part. Of these Guam, Rota, Tiniam, Saypan, and some
+islets farther north, are described by Quoy and Gaimard,[18] and
+Chamisso,[19] as chiefly composed of madreporitic limestone, which
+attains a considerable elevation, and is in several cases worn into
+successively rising cliffs: the two former naturalists seem to have
+compared the corals and shells with the existing ones, and state that
+they are of recent species. _Fais_, which lies in the prolonged line of
+the Marianas, is the only island in this part of the sea which is
+fringed; it is ninety feet high, and consists entirely of madreporitic
+rock.[20]
+
+ [17] “Voyage de la _Coquille_,” Part. Zoolog.
+
+
+ [18] Freycinet’s “Voyage autour du Monde.” See also the
+ “Hydrographical Memoir,” p. 215.
+
+
+ [19] Kotzebue’s “First Voyage.”
+
+
+ [20] Lutké’s “Voyage,” vol. ii, p. 304.
+
+
+In the _East Indian Archipelago_, many authors have recorded proofs of
+recent elevation. M. Lesson[21] states, that near Port Dory, on the
+north coast of New Guinea, the shores are flanked, to the height of 150
+feet, by madreporitic strata of modern date. He mentions similar
+formations at Waigiou, Amboina, Bourou, Ceram, Sonda, and Timor: at
+this latter place, MM. Quoy and Gaimard[22] have likewise described the
+primitive rocks, as coated to a considerable height with coral. Some
+small islets eastward of Timor are said in Kolff’s “Voyage,”[23] to
+resemble small coral islets upraised some feet above the sea. Dr.
+Malcolmson informs me that Dr. Hardie found in JAVA an extensive
+formation, containing an abundance of shells, of which the greater part
+appear to be of existing species. Dr. Jack[24] has described some
+upraised shells and corals, apparently recent, on Pulo Nias off
+_Sumatra_; and Marsden relates in his history of this great island,
+that the names of many promontories, show that they were originally
+islands. On part of the west
+coast of _Borneo_ and at the _Sooloo Islands_, the form of the land,
+the nature of the soil, and the water-washed rocks, present
+appearances[25] (although it is doubtful whether such vague evidence is
+worthy of mention), of having recently been covered by the sea; and the
+inhabitants of the Sooloo Islands believe that this has been the case.
+Mr. Cuming, who has lately investigated, with so much success, the
+natural history of the _Philippines_, found near Cabagan, in Luzon,
+about fifty feet above the level of the R. Cagayan, and seventy miles
+from its mouth, a large bed of fossil shells: these, he informs me, are
+of the same species with those now existing on the shores of the
+neighbouring islands. From the accounts given us by Captain Basil Hall
+and Captain Beechey[26] of the lines of inland reefs, and walls of
+coral-rock worn into caves, above the present reach of the waves, at
+the _Loo Choo_ Islands, there can be little doubt that they have been
+upraised at no very remote period.
+
+ [21] Partie Zoolog., “Voyage de la _Coquille_.”
+
+
+ [22] “Ann. des Scien. Nat.” tom. vi, p. 281.
+
+
+ [23] Translated by Windsor Earl, chapters vi, vii.
+
+
+ [24] “Geolog. Transact.” 2nd series, vol. i, p. 403. On the Peninsula
+ of Malacca, in front of Pinang, 5° 30′ N., Dr. Ward collected some
+ shells, which Dr. Malcolmson informs me, although not compared with
+ existing species, had a recent appearance. Dr. Ward describes in this
+ neighbourhood (“Trans. Asiat. Soc.” vol. xviii, part ii, p. 166) a
+ single water-worn rock, with a conglomerate of sea-shells at its base,
+ situated six miles inland, which, according to the traditions of the
+ natives, was once surrounded by the sea. Captain Low has also
+ described (_Ibid_., part i, p. 131) mounds of shells lying two miles
+ inland on this line of coast.
+
+
+ [25] “Notices of the East Indian Arch.” Singapore, 1828, p. 6, and
+ Append., p. 43.
+
+
+ [26] Captain B. Hall, “Voyage to Loo Choo,” Append., pp. xxi and xxv.
+ Captain Beechey’s “Voyage,” p. 496.
+
+
+Dr. Davy[27] describes the northern province of _Ceylon_ as being very
+low, and consisting of a limestone with shells and corals of very
+recent origin; he adds, that it does not admit of a doubt that the sea
+has retired from this district even within the memory of man. There is
+also some reason for believing that the western shores of India, north
+of Ceylon, have been upraised within the recent period.[28] _Mauritius_
+has certainly been upraised within the recent period, as I have stated
+in the chapter on fringing-reefs. The northern extremity of
+_Madagascar_ is described by Captain Owen[29] as formed of madreporitic
+rock, as likewise are the shores and outlying islands along an immense
+space of _ Eastern Africa_, from a little north of the equator for nine
+hundred miles southward. Nothing can be more vague than the expression
+“madreporitic rock;” but at the same time it is, I think, scarcely
+possible to look at the chart of the linear islets, which rise to a
+greater height than can be accounted for by the growth of coral, in
+front of the coast, from the equator to 2° S., without feeling
+convinced that a line of fringing-reefs has been elevated at a period
+so recent, that no great changes have since taken place on the surface
+of this part of the globe. Some, also, of the
+higher islands of madreporitic rock on this coast, for instance Pemba,
+have very singular forms, which seem to show the combined effect of the
+growth of coral round submerged banks, and their subsequent upheaval.
+Dr. Allan informs me that he never observed any elevated organic
+remains on the _Seychelles_, which come under our fringed class.
+
+ [27] “Travels in Ceylon,” p. 13. This madreporitic formation is
+ mentioned by M. Cordier in his report to the Institute (May 4th,
+ 1839), on the voyage of the _Chevrette_, as one of immense extent, and
+ belonging to the latest tertiary period.
+
+
+ [28] Dr. Benza, in his “Journey through the N. Circars” (the _ Madras
+ Lit. and Scient. Journ._ vol. v.) has described a formation with
+ recent fresh-water and marine shells, occurring at the distance of
+ three or four miles from the present shore. Dr. Benza, in conversation
+ with me, attributed their position to a rise of the land. Dr.
+ Malcolmson, however (and there cannot be a higher authority on the
+ geology of India) informs me that he suspects that these beds may have
+ been formed by the mere action of the waves and currents accumulating
+ sediment. From analogy I should much incline to Dr. Benza’s opinion.
+
+
+ [29] Owen’s “Africa,” vol. ii, p. 37, for Madagascar; and for S.
+ Africa, vol. i, pp. 412 and 426. Lieutenant Boteler’s narrative
+ contains fuller particulars regarding the coral-rock, vol. i, p. 174,
+ and vol. ii, pp. 41 and 54. See also Ruschenberger’s “Voyage round the
+ World,” vol. i, p. 60.
+
+
+The nature of the formations round the shores of the _Red Sea_, as
+described by several authors, shows that the whole of this large area
+has been elevated within a very recent tertiary epoch. A part of this
+space in the appended map, is coloured blue, indicating the presence of
+barrier-reefs: on which circumstance I shall presently make some
+remarks. Rüppell[30] states that the tertiary formation, of which he
+has examined the organic remains, forms a fringe along the shores with
+a uniform height of from thirty and forty feet from the mouth of the
+Gulf of Suez to about latitude 26°; but that south of 26°, the beds
+attain only the height of from twelve to fifteen feet. This, however,
+can hardly be quite accurate; although possibly there may be a decrease
+in the elevation of the shores in the middle parts of the Red Sea, for
+Dr. Malcolmson (as he informs me) collected from the cliffs of Camaran
+Island (lat. 15° 30′ S.) shells and corals, apparently recent, at a
+height between thirty and forty feet; and Mr. Salt (“Travels in
+Abyssinia”) describes a similar formation a little southward on the
+opposite shore at Amphila. Moreover, near the mouth of the Gulf of
+Suez, although on the coast opposite to that on which Dr. Rüppell says
+that the modern beds attain a height of only thirty to forty feet, Mr.
+Burton[31] found a deposit replete with existing species of shells, at
+the height of 200 feet. In an admirable series of drawings by Captain
+Moresby, I could see how continuously the cliff-bounded low plains of
+this formation extended with a nearly equable height, both on the
+eastern and western shores. The southern coast of Arabia seems to have
+been subjected to the same elevatory movement, for Dr. Malcolmson found
+at Sahar low cliffs containing shells and corals, apparently of recent
+species.
+
+ [30] Rüppell, “Reise in Abyssinien,” Band i., s. 141.
+
+
+ [31] Lyell’s “Principles of Geology,” 5th ed., vol. iv, p. 25.
+
+
+The _Persian Gulf_ abounds with coral-reefs; but as it is difficult to
+distinguish them from sand-banks in this shallow sea, I have coloured
+only some near the mouth; towards the head of the gulf Mr.
+Ainsworth[32] says that the land is worn into terraces, and that the
+beds contain organic remains of existing forms. The _West Indian
+Archipelago_ of “fringed” islands, alone remains to be mentioned;
+evidence of an elevation within a late tertiary epoch of nearly the
+whole of this great area, may be found in the works of almost all the
+naturalists who have visited it. I will give some of the principal
+references in a note.[33]
+
+ [32] Ainsworth’s “Assyria and Babylon,” p. 217.
+
+
+ [33] On Florida and the north shores of the Gulf of Mexico, Rogers’
+ “Report to Brit. Assoc.” vol. iii, p. 14.—On the shores of Mexico,
+ Humboldt, “Polit. Essay on New Spain,” vol. i, p. 62. (I have also
+ some corroborative facts with respect to the shores of
+ Mexico.)—Honduras and the Antilles, Lyell’s “Principles,” 5th ed.,
+ vol. iv, p. 22.—Santa Cruz and Barbadoes, Prof. Hovey, “Silliman’s
+ Journal”, vol. xxxv, p. 74.—St. Domingo, Courrojolles, “Journ de
+ Phys.” tom. liv., p. 106.—Bahamas, “United Service Journal”, No. lxxi,
+ pp. 218 and 224. Jamaica, De la Beche, “Geol. Man.” p. 142.—Cuba,
+ Taylor in “Lond. and Edin. Mag.” vol. xi, p. 17. Dr. Daubeny also, at
+ a meeting of the Geolog. Soc., orally described some very modern beds
+ lying on the N.W. parts of Cuba. I might have added many other less
+ important references.
+
+
+It is very remarkable on reviewing these details, to observe in how
+many instances fringing-reefs round the shores, have coincided with the
+existence on the land of upraised organic remains, which seem, from
+evidence more or less satisfactory, to belong to a late tertiary
+period. It may, however, be objected, that similar proofs of elevation,
+perhaps, occur on the coasts coloured blue in our map: but this
+certainly is not the case with the few following and doubtful
+exceptions.
+
+The entire area of the Red Sea appears to have been upraised within a
+modern period; nevertheless I have been compelled (though on
+unsatisfactory evidence, as given in the Appendix) to class the reefs
+in the middle part, as barrier-reefs; should, however, the statements
+prove accurate to the less height of the tertiary bed in this middle
+part, compared with the northern and southern districts, we might well
+suspect that it had subsided subsequently to the general elevation by
+which the whole area has been upraised. Several authors[34] have stated
+that they have observed shells and corals high up on the mountains of
+the Society Islands,—a group encircled by barrier-reefs, and,
+therefore, supposed to have subsided: at Tahiti Mr. Stutchbury found on
+the apex of one of the highest mountains, between 5,000 and 7,000 feet
+above the level of the sea, “a distinct and regular stratum of
+semi-fossil coral.” At Tahiti, however, other naturalists, as well as
+myself, have searched in vain at a low level near the coast, for
+upraised shells or masses of coral-reef, where if present they could
+hardly have been overlooked. From this fact, I concluded that probably
+the organic remains strewed high up on the surface of the land, had
+originally been embedded in the volcanic strata, and had subsequently
+been washed out by the rain. I have since heard from the Rev. W. Ellis,
+that the remains which he met with, were (as he believes)
+interstratified with an argillaceous tuff; this likewise was the case
+with the shells observed by the Rev. D. Tyerman at Huaheine. These
+remains have not been specifically examined; they may, therefore, and
+especially the stratum observed by Mr. Stutchbury at an immense height,
+be contemporaneous with the first formation of the Society Islands, and
+be of any degree of antiquity; or they may have been deposited at some
+subsequent, but probably not very recent, period of elevation; for if
+the period had been recent, the entire surface of the coast land of
+these islands, where the reefs are so extensive, would have been coated
+with upraised coral,
+which certainly is not the case. Two of the Harvey, or Cook Islands,
+namely, Aitutaki and Manouai, are encircled by reefs, which extend so
+far from the land, that I have coloured them blue, although with much
+hesitation, as the space within the reef is shallow, and the outline of
+the land is not abrupt. These two islands consist of coral-rock; but I
+have no evidence of their recent elevation, besides, the improbability
+of Mangaia, a fringed island in the same group (but distant 170 miles),
+having retained its nearly perfect atoll-like structure, during any
+immense lapse of time after its upheaval. The Red Sea, therefore, is
+the only area in which we have clear proofs of the recent elevation of
+a district, which, by our theory (although the barrier-reefs are there
+not well characterised), has lately subsided. But we have no reason to
+be surprised at oscillation, of level of this kind having occasionally
+taken place. There can be scarcely any doubt that Savage, Aurora,[35]
+and Mangaia Islands, and several of the islands in the Friendly group,
+existed originally as atolls, and these have undoubtedly since been
+upraised to some height above the level of the sea; so that by our
+theory, there has here, also, been an oscillation of level,—elevation
+having succeeded subsidence, instead of, as in the middle part of the
+Red Sea and at the Harvey Islands, subsidence having probably succeeded
+recent elevation.
+
+ [34] Ellis, in his “Polynesian Researches,” was the first to call
+ attention to these remains (vol. i, p. 38), and the tradition of the
+ natives concerning them. See also Williams, “Nar. of Missionary
+ Enterprise,” p. 21; also Tyerman and G. Bennett, “Journal of Voyage,”
+ vol. i, p. 213; also Mr. Couthouy’s “Remarks,” p. 51; but this
+ principal fact, namely, that there is a mass of upraised coral on the
+ narrow peninsula of Tiarubu, is from hearsay evidence; also Mr.
+ Stutchbury, _West of England Journal_, No. i, p. 54. There is a
+ passage in Von Zach, “Corres. Astronom.” vol. x, p. 266, inferring an
+ uprising at Tahiti, from a footpath now used, which was formerly
+ impassable; but I particularly inquired from several native chiefs,
+ whether they knew of any change of this kind, and they were unanimous
+ in giving me an answer in the negative.
+
+
+ [35] Aurora Island is described by Mr. Couthouy (“Remarks,” p. 58); it
+ lies 120 miles north-east of Tahiti; it is not coloured in the
+ appended map, because it does not appear to be fringed by living
+ reefs. Mr. Couthouy describes its summit as “presenting a broad
+ table-land which declines a few feet towards the centre, where we may
+ suppose the lagoon to have been placed.” It is about two hundred feet
+ in height, and consists of reef-rock and conglomerate, with existing
+ species of coral embedded in it. The island has been elevated at two
+ successive periods; the cliffs being marked halfway up with a
+ horizontal water-worn line of deep excavations. Aurora Island seems
+ closely to resemble in structure Elizabeth Island, at the southern end
+ of the Low Archipelago.
+
+It is an interesting fact, that Fais, which, from its composition,
+form, height, and situation at the western end of the Caroline
+Archipelago, one is strongly induced to believe existed before its
+upheaval as an atoll, lies exactly in the prolongation of the curved
+line of the Mariana group, which we know to be a line of recent
+elevation. I may add, that Elizabeth Island, in the southern part of
+the Low Archipelago, which seems to have had the same kind of origin as
+Fais, lies near Pitcairn Island, the only one in this part of the ocean
+which is high, and at the same time not surrounded by an encircling
+barrier-reef.
+
+_On the absence of active volcanoes in the areas of subsidence, and on
+their frequent presence in the areas of elevation._—Before making some
+concluding remarks on the relations of the spaces coloured blue and
+red, it will be convenient to consider the position on our map of the
+volcanoes historically known to have been in action. It is impossible
+not to be struck, first with the absence of volcanoes in the great
+areas of subsidence tinted pale and dark blue,—namely, in the central
+parts of the Indian Ocean, in the China Sea, in the sea between the
+barriers
+of Australia and New Caledonia, in the Caroline, Marshall, Gilbert, and
+Low Archipelagoes; and, secondly, with the coincidence of the principal
+volcanic chains with the parts coloured red, which indicates the
+presence of fringing-reefs; and, as we have just seen, the presence in
+most cases of upraised organic remains of a modern date. I may here
+remark that the reefs were all coloured before the volcanoes were added
+to the map, or indeed before I knew of the existence of several of
+them.
+
+The volcano in Torres Strait, at the northern point of Australia, is
+that which lies nearest to a large subsiding area, although situated
+125 miles within the outer margin of the actual barrier-reef. The Great
+Comoro Island, which probably contains a volcano, is only twenty miles
+distant from the barrier-reef of Mohila; Ambil volcano, in the
+Philippines, is distant only a little more than sixty miles from the
+atoll-formed Appoo reef: and there are two other volcanoes in the map
+within ninety miles of circles coloured blue. These few cases, which
+thus offer partial exceptions to the rule, of volcanoes being placed
+remote from the areas of subsidence, lie either near single and
+isolated atolls, or near small groups of encircled islands; and these
+by our theory can have, in few instances, subsided to the same amount
+in depth or area, as groups of atolls. There is not one active volcano
+within several hundred miles of an archipelago, or even a small group
+of atolls. It is, therefore, a striking fact that in the Friendly
+Archipelago, which owes its origin to the elevation of a group of
+atolls, two volcanoes, and, perhaps, others are known to be in action:
+on the other hand, on several of the encircled islands in the Pacific,
+supposed by our theory to have subsided, there are old craters and
+streams of lava, which show the effects of past and ancient eruptions.
+In these cases, it would appear as if the volcanoes had come into
+action, and had become extinguished on the same spots, according as the
+elevating or subsiding movements prevailed.
+
+There are some other coasts on the map, where volcanoes in a state of
+action concur with proofs of recent elevation, besides those coloured
+red from being fringed by coral-reefs. Thus I hope to show in a future
+volume, that nearly the whole line of the west coast of South America,
+which forms the greatest volcanic chain in the world, from near the
+equator for a space of between 2,000 and 3,000 miles southward, has
+undergone an upward movement during a late geological period. The
+islands on the north-western shores of the Pacific, which form the
+second greatest volcanic chain, are very imperfectly known; but Luzon,
+in the Philippines, and the Loo Choo Islands, have been recently
+elevated; and at Kamtschatka[36] there are extensive tertiary beds of
+modern date. Evidence of the same nature, but not very satisfactory,
+may be detected in Northern New Zealand where there are two volcanoes.
+The co-existence in other parts of the world of active volcanoes, with
+upraised beds of a modern tertiary origin, will occur to
+every geologist.[37] Nevertheless, until it could be shown that
+volcanoes were inactive, or did not exist in subsiding areas, the
+conclusion that their distribution depended on the nature of the
+subterranean movements in progress, would have been hazardous. But now,
+viewing the appended map, it may, I think, be considered as almost
+established, that volcanoes are often (not necessarily always) present
+in those areas where the subterranean motive power has lately forced,
+or is now forcing outwards, the crust of the earth, but that they are
+invariably absent in those, where the surface has lately subsided or is
+still subsiding.[38]
+
+ [36] At Sedanka, in latitude 58° N. (Von Buch’s “Descrip. des Isles
+ Canaries,” p. 455). In a forthcoming part, I shall give the evidence
+ referred to with respect to the elevation of New Zealand.
+
+
+ [37] During the subterranean disturbances which took place in Chile,
+ in 1835, I have shown (“Geolog. Trans.” 2nd Ser., vol. v, p. 606) that
+ at the same moment that a large district was upraised, volcanic matter
+ burst forth at widely separated points, through both new and old
+ vents.
+
+
+ [38] We may infer from this rule, that in any old deposit, which
+ contains interstratified beds of erupted matter, there was at the
+ period, and in the area of its formation, a _tendency_ to an upward
+ movement in the earth’s surface, and certainly no movement of
+ subsidence.
+
+
+_On the relations of the areas of subsidence and elevation._—The
+immense surfaces on the map, which, both by our theory and by the plain
+evidence of upraised marine remains, have undergone a change of level
+either downwards or upwards during a late period, is a most remarkable
+fact. The existence of continents shows that the areas have been
+immense which at some period have been upraised; in South America we
+may feel sure, and on the north-western shores of the Indian Ocean we
+may suspect, that this rising is either now actually in progress, or
+has taken place quite recently. By our theory, we may conclude that the
+areas are likewise immense which have lately subsided, or, judging from
+the earthquakes occasionally felt and from other appearances, are now
+subsiding. The smallness of the scale of our map should not be
+overlooked: each of the squares on it contains (not allowing for the
+curvature of the earth) 810,000 square miles. Look at the space of
+ocean from near the southern end of the Low Archipelago to the northern
+end of the Marshall Archipelago, a length of 4,500 miles, in which, as
+far as is known, every island, except Aurora which lies just without
+the Low Archipelago, is atoll-formed. The eastern and western
+boundaries of our map are continents, and they are rising areas: the
+central spaces of the great Indian and Pacific Oceans, are mostly
+subsiding; between them, north of Australia, lies the most broken land
+on the globe, and there the rising parts are surrounded and penetrated
+by areas of subsidence,[39] so that the prevailing movements now in
+progress, seem to accord with the actual states of surface of the great
+divisions of the world.
+
+ [39] I suspect that the Arru and Timor-laut Islands present an
+ included small area of subsidence, like that of the China Sea, but I
+ have not ventured to colour them from my imperfect information, as
+ given in the Appendix.
+
+The blue spaces on the map are nearly all elongated; but it does not
+necessarily follow from this (a caution, for which I am indebted to Mr.
+Lyell), that the areas of subsidence were likewise elongated; for
+the subsidence of a long, narrow space of the bed of the ocean,
+including in it a transverse chain of mountains, surmounted by atolls,
+would only be marked on the map by a transverse blue band. But where a
+chain of atolls and barrier-reefs lies in an elongated area, between
+spaces coloured red, which therefore have remained stationary or have
+been upraised, this must have resulted either from the area of
+subsidence having originally been elongated (owing to some tendency in
+the earth’s crust thus to subside), or from the subsiding area having
+originally been of an irregular figure, or as broad as long, and having
+since been narrowed by the elevation of neighbouring districts. Thus
+the areas, which subsided during the formation of the great north and
+south lines of atolls in the Indian Ocean,—of the east and west line of
+the Caroline atolls,—and of the north-west and south-east line of the
+barrier-reefs of New Caledonia and Louisiade, must have originally been
+elongated, or if not so, they must have since been made elongated by
+elevations, which we know to belong to a recent period.
+
+I infer from Mr. Hopkins’ researches,[40] that for the formation of a
+long chain of mountains, with few lateral spurs, an area elongated in
+the same direction with the chain, must have been subjected to an
+elevatory movement. Mountain-chains, however, when already formed,
+although running in very different directions, it seems[41] may be
+raised together by a widely-acting force: so, perhaps, mountain-chains
+may subside together. Hence, we cannot tell, whether the Caroline and
+Marshall Archipelagoes, two groups of atolls running in different
+directions and meeting each other, have been formed by the subsidence
+of two areas, or of one large area, including two distinct lines of
+mountains. We have, however, in the southern prolongation of the
+Mariana Islands, probable evidence of a line of recent elevation having
+intersected one of recent subsidence. A view of the map will show that,
+generally, there is a tendency to alternation in the parallel areas
+undergoing opposite kinds of movement; as if the sinking of one area
+balanced the rising of another.
+
+ [40] “Researches in Physical Geology,” Transact. Cambridge Phil. Soc.,
+ vol. vi, part i.
+
+
+ [41] For instance in S. America from lat. 34°, for very many degrees
+ southward there are upraised beds containing recent species of shells,
+ on both the Atlantic and Pacific side of the continent, and from the
+ gradual ascent of the land, although with very unequal slopes, on both
+ sides towards the Cordillera, I think it can hardly be doubted that
+ the entire width has been upraised in mass within the recent period.
+ In this case the two W.N.W. and E.S.E. mountain-lines, namely the
+ Sierra Ventana and the S. Tapalguen, and the great north and south
+ line of the Cordillera have been together raised. In the West Indies
+ the N. and S. line of the Eastern Antilles, and the E. and W. line of
+ Jamaica, appear both to have been upraised within the latest
+ geological period.
+
+The existence in many parts of the world of high table-land, proves
+that large surfaces have been upraised in mass to considerable heights
+above the level of the ocean; although the highest points in almost
+every country consist of upturned strata, or erupted matter: and from
+the immense spaces scattered with atolls, which indicate that land
+originally existed there, although not one pinnacle now remains above
+the level of the sea, we may conclude that wide areas have subsided to
+an amount, sufficient to bury not only any formerly existing
+table-land, but even the heights formed by fractured strata, and
+erupted matter. The effects produced on the land by the later elevatory
+movements, namely, successively rising cliffs, lines of erosion, and
+beds of literal shells and pebbles, all requiring time for their
+production, prove that these movements have been very slow; we can,
+however, infer this with safety, only with respect to the few last
+hundred feet of rise. But with reference to the whole vast amount of
+subsidence, necessary to have produced the many atolls widely scattered
+over immense spaces, it has already been shown (and it is, perhaps, the
+most interesting conclusion in this volume), that the movements must
+either have been uniform and exceedingly slow, or have been effected by
+small steps, separated from each other by long intervals of time,
+during which the reef-constructing polypifers were able to bring up
+their solid frameworks to the surface. We have little means of judging
+whether many considerable oscillations of level have generally occurred
+during the elevation of large tracts; but we know, from clear
+geological evidence, that this has frequently taken place; and we have
+seen on our map, that some of the same islands have both subsided and
+been upraised. I conclude, however, that most of the large blue spaces,
+have subsided without many and great elevatory oscillations, because
+only a few upraised atolls have been observed: the supposition that
+such elevations have taken place, but that the upraised parts have been
+worn down by the surf, and thus have escaped observation, is overruled
+by the very considerable depth of the lagoons of all the larger atolls;
+for this could not have been the case, if they had suffered repeated
+elevations and abrasion. From the comparative observations made in
+these latter pages, we may finally conclude, that the subterranean
+changes which have caused some large areas to rise, and others to
+subside, have acted in a very similar manner.
+
+_Recapitulation._—In the three first chapters, the principal kinds of
+coral-reefs were described in detail, and they were found to differ
+little, as far as relates to the actual surface of the reef. An atoll
+differs from an encircling barrier-reef only in the absence of land
+within its central expanse; and a barrier-reef differs from a
+fringing-reef, in being placed at a much greater distance from the land
+with reference to the probable inclination of its submarine foundation,
+and in the presence of a deep-water lagoon-like space or moat within
+the reef. In the fourth chapter the growing powers of the
+reef-constructing polypifers were discussed; and it was shown, that
+they cannot flourish beneath a very limited depth. In accordance with
+this limit, there is no difficulty respecting the foundations on which
+fringing-reefs are based; whereas, with barrier-reefs and atolls, there
+is a great apparent difficulty on this head; in barrier-reefs from the
+improbability of the rock of the coast or of banks of sediment
+extending, in every instance, so far seaward within the required
+depth;—and in atolls, from the immensity of the
+spaces over which they are interspersed, and the apparent necessity for
+believing that they are all supported on mountain-summits, which
+although rising very near to the surface-level of the sea, in no one
+instance emerge above it. To escape this latter most improbable
+admission, which implies the existence of submarine chains of mountains
+of almost the same height, extending over areas of many thousand square
+miles, there is but one alternative; namely, the prolonged subsidence
+of the foundations, on which the atolls were primarily based, together
+with the upward growth of the reef-constructing corals. On this view
+every difficulty vanishes; fringing reefs are thus converted into
+barrier-reefs; and barrier-reefs, when encircling islands, are thus
+converted into atolls, the instant the last pinnacle of land sinks
+beneath the surface of the ocean.
+
+Thus the ordinary forms and certain peculiarities in the structure of
+atolls and barrier-reefs can be explained;—namely, the wall-like
+structure on their inner sides, the basin or ring-like shape both of
+the marginal and central reefs in the Maldiva atolls—the union of some
+atolls as if by a ribbon—the apparent disseverment of others—and the
+occurrence, in atolls as well as in barrier-reefs, of portions of reef,
+and of the whole of some reefs, in a dead and submerged state, but
+retaining the outline of living reefs. Thus can be explained the
+existence of breaches through barrier-reefs in front of valleys, though
+separated from them by a wide space of deep water; thus, also, the
+ordinary outline of groups of atolls and the relative forms of the
+separate atolls one to another; thus can be explained the proximity of
+the two kinds of reefs formed during subsidence, and their separation
+from the spaces where fringing-reefs abound. On searching for other
+evidence of the movements supposed by our theory, we find marks of
+change in atolls and in barrier-reefs, and of subterranean disturbances
+under them; but from the nature of things, it is scarcely possible to
+detect any direct proofs of subsidence, although some appearances are
+strongly in favour of it. On the fringed coasts, however, the presence
+of upraised marine bodies of a recent epoch, plainly show, that these
+coasts, instead of having remained stationary, which is all that can be
+directly inferred from our theory, have generally been elevated.
+
+Finally, when the two great types of structure, namely barrier-reefs
+and atolls on the one hand, and fringing-reefs on the other, were laid
+down in colours on our map, a magnificent and harmonious picture of the
+movements, which the crust of the earth has within a late period
+undergone, is presented to us. We there see vast areas rising, with
+volcanic matter every now and then bursting forth through the vents or
+fissures with which they are traversed. We see other wide spaces slowly
+sinking without any volcanic outburst, and we may feel sure, that this
+sinking must have been immense in amount as well as in area, thus to
+have buried over the broad face of the ocean every one of those
+mountains, above which atolls now stand like monuments, marking the
+place of their former existence. Reflecting how powerful an agent with
+respect to denudation, and consequently to the nature and thickness of
+the deposits in accumulation, the sea must ever be, when acting
+for prolonged periods on the land, during either its slow emergence or
+subsidence; reflecting, also, on the final effects of these movements
+in the interchange of land and ocean-water on the climate of the earth,
+and on the distribution of organic beings, I may be permitted to hope,
+that the conclusions derived from the study of coral-formations,
+originally attempted merely to explain their peculiar forms, may be
+thought worthy of the attention of geologists.
+
+
+
+
+APPENDIX.
+CONTAINING A DETAILED DESCRIPTION OF THE REEFS AND ISLANDS IN PLATE
+III.
+
+
+In the beginning of the last chapter I stated the principles on which
+the map is coloured. There only remains to be said, that it is an exact
+copy of one by M. C. Gressier, published by the Dépôt Général de la
+Marine, in 1835. The names have been altered into English, and the
+longitude has been reduced to that of Greenwich. The colours were first
+laid down on accurate charts, on a large scale. The data, on which the
+volcanoes historically known to have been in action, have been marked
+with vermillion, were given in a note to the last chapter. I will
+commence my description on the eastern side of the map, and will
+describe each group of islands consecutively, proceeding westward
+across the Pacific and Indian Oceans, but ending with the West Indies.
+
+The WESTERN SHORES OF AMERICA appear to be entirely without
+coral-reefs; south of the equator the survey of the _Beagle_, and north
+of it, the published charts show that this is the case. Even in the Bay
+of _Panama_, where corals flourish, there are no true coral-reefs, as I
+have been informed by Mr. Lloyd. There are no coral-reefs in the
+_Galapagos_ Archipelago, as I know from personal inspection; and I
+believe there are none on the _ Cocos, Revilla-gigedo_, and other
+neighbouring islands. _ Clipperton_ rock, 10° N., 109° W., has lately
+been surveyed by Captain Belcher; in form it is like the crater of a
+volcano. From a drawing appended to the MS. plan in the Admiralty, it
+evidently is not an atoll. The eastern parts of the Pacific present an
+enormous area, without any islands, except _E_, and _Sala_, and _Gomez_
+Islands, which do not appear to be surrounded by reefs.
+
+The LOW ARCHIPELAGO.—This group consists of about eighty atolls: it
+will be quite superfluous to refer to descriptions of each. In
+D’Urville and Lottin’s chart, one island (_Wolchonsky_) is written with
+a capital letter, signifying, as explained in a former chapter, that it
+is a high island; but this must be a mistake, as the original chart by
+Bellinghausen shows that it is a true atoll. Captain Beechey says of
+the thirty-two groups which he examined (of the greater number of which
+I have seen beautiful MS. charts in the Admiralty), that twenty-nine
+now contain lagoons, and he believes the other three originally did.
+Bellinghausen (see an account of his Russian voyage, in the “Biblioth.
+des Voyages,” 1834, p. 443) says, that the seventeen islands which he
+discovered resembled each other in structure, and he has given charts
+on a large scale of all of them. Kotzebue has given plans of several;
+Cook and Bligh mention others; a few were seen during the voyage of the
+_Beagle_; and notices of other atolls are scattered through several
+publications. The _ Actæon_ group in this archipelago has lately been
+discovered (_Geograph. Journ._, vol. vii, p. 454); it consists of three
+small and low islets, one of which has a lagoon. Another lagoon-island
+has been discovered (_Naut. Mag._, 1839, p. 770), in 22° 4′ S., and
+136° 20′ W. Towards the S.E. part of the group, there are some islands
+of different formation: _ Elizabeth_ Island is described by Beechey (p.
+46, 4to ed.) as fringed by reefs, at the distance of between two and
+three hundred yards; coloured red. _Pitcairn_ Island, in the immediate
+neighbourhood, according to the same authority, has no reefs of any
+kind, although numerous pieces of coral are thrown up on the beach; the
+sea close to its shore is very deep (see “Zool. of Beechey’s Voyage,”
+p. 164); it is left uncoloured. _Gambier_ Islands (see Plate I Fig. 8),
+are encircled by a barrier-reef; the greatest depth within is
+thirty-eight fathoms; coloured pale blue. _Aurora_ Island, which lies
+N.E. of Tahiti close to the large space coloured dark blue in the map,
+has been already described in a note (), on the authority of Mr.
+Couthouy; it is an upraised atoll, but as it does not appear to be
+fringed by living reefs, it is left uncoloured.
+
+The SOCIETY Arch. is separated by a narrow space from the Low
+Archipelago; and in their parallel direction they manifest some
+relation to each other. I have already described the general character
+of the reefs of these fine encircled islands. In the “Atlas of the
+_Coquille’s_ Voyage” there is a good general chart of the group, and
+separate plans of some of the islands. _ Tahiti_, the largest island in
+the group, is almost surrounded, as seen in Cook’s chart, by a reef
+from half a mile to a mile and a half from the shore, with from ten to
+thirty fathoms within it. Some considerable submerged reefs lying
+parallel to the shore, with a broad and deep space within, have lately
+been discovered (_Naut. Mag._, 1836, p. 264) on the N.E. coast of the
+island, where none are laid down by Cook. At _Eimeo_ the reef “which
+like a ring surrounds it, is in some places one or two miles distant
+from the shore, in others united to the beach” (Ellis, “Polynesian
+Researches,” vol. i, p. 18, 12mo edition). Cook found deep water
+(twenty fathoms) in some of the harbours within the reef. Mr. Couthouy,
+however, states (“Remarks,” p. 45) that both at Tahiti and Eimeo, the
+space between the barrier-reef and the shore, has been almost filled
+up,—“a nearly continuous fringing-reef surrounding the island, and
+varying from a few yards to rather more than a mile in width, the
+lagoons merely forming canals between this and the sea-reef,” that is
+the barrier-reef. _Tapamanoa_ is surrounded by a reef at a considerable
+distance from the shore; from the island being small it is breached, as
+I am informed by the Rev. W.
+Ellis, only by a narrow and crooked boat channel. This is the lowest
+island in the group, its height probably not exceeding 500 feet. A
+little way north of Tahiti, the low coral-islets of _ Teturoa_ are
+situated; from the description of them given me by the Rev. J. Williams
+(the author of the “Narrative of Missionary Enterprise”), I should have
+thought they had formed a small atoll, and likewise from the
+description given by the Rev. D. Tyerman and G. Bennett (“Journal of
+Voyage and Travels,” vol. i, p. 183), who say that ten low coral-islets
+“are comprehended within one general reef, and separated from each
+other by interjacent lagoons;” but as Mr. Stutchbury (_West of England
+Journal_, vol. i, p. 54) describes it as consisting of a mere narrow
+ridge, I have left it uncoloured. _Maitea_, eastward of the group, is
+classed by Forster as a high encircled island; but from the account
+given by the Rev. D. Tyerman and G. Bennett (vol. i, p. 57) it appears
+to be an exceedingly abrupt cone, rising from the sea without any reef;
+I have left it uncoloured. It would be superfluous to describe the
+northern islands in this group, as they may be well seen in the chart
+accompanying the 4to edition of Cook’s “Voyages,” and in the “Atlas of
+the _Coquille’s_ Voyage.” _Maurua_ is the only one of the northern
+islands, in which the water within the reef is not deep, being only
+four and a half fathoms; but the great width of the reef, stretching
+three miles and a half southward of the land (which is represented in
+the drawing in the “Atlas of the _ Coquille’s_ Voyage” as descending
+abruptly to the water) shows, on the principle explained in the
+beginning of the last chapter, that it belongs to the barrier class. I
+may here mention, from information communicated to me by the Rev. W.
+Ellis, that on the N.E. side of _Huaheine_ there is a bank of sand,
+about a quarter of a mile wide, extending parallel to the shore, and
+separated from it by an extensive and deep lagoon; this bank of sand
+rests on coral-rock, and undoubtedly was originally a living reef.
+North of Bolabola lies the atoll of _Toubai_ (Motou-iti of the
+“_Coquille’s_ Atlas”) which is coloured dark blue; the other islands,
+surrounded by barrier-reefs, are pale blue; three of them are
+represented in Figs 3, 4, and 5, in Plate I. There are three low
+coral-groups lying a little E. of the Society Archipelago, and almost
+forming part of it, namely _ Bellinghausen_, which is said by Kotzebue
+(“Second Voyage,” vol. ii, p. 255), to be a lagoon-island; _Mopeha_,
+which, from Cook’s description (“Second Voyage,” book iii, chap. i), no
+doubt is an atoll; and the _Scilly_ Islands, which are said by Wallis
+(“Voyage,” chap. ix) to form a _group_ of _low_ islets and shoals, and,
+therefore, probably, they compose an atoll: the two former have been
+coloured blue, but not the latter.
+
+MENDANA OR MARQUESAS Group.—These islands are entirely without reefs,
+as may be seen in Krusenstern’s Atlas, making a remarkable contrast
+with the adjacent group of the Society Islands. Mr. F. D. Bennett has
+given some account of this group, in the seventh volume of the
+_Geograph. Journ._ He informs me that all the islands have the same
+general character, and that the water is very deep close to their
+shores. He visited three of them, namely, _Dominicana, Christiana,_ and
+_Roapoa_; their beaches are strewed with rounded masses of coral, and
+although no regular reefs exist, yet the shore is in many places lined
+by coral-rock, so that a boat grounds on this formation. Hence these
+islands ought probably to come within the class of fringed islands and
+be coloured red; but as I am determined to err on the cautious side, I
+have left them uncoloured.
+
+COOK or HARVEY and AUSTRAL ISLAND.—_Palmerston_ Island is minutely
+described as an atoll by Captain Cook during his voyage in 1774;
+coloured blue. _Aitutaki_ was partially surveyed by the _ Beagle_ (see
+map accompanying “Voyages of _Adventure_ and _Beagle_”); the land is
+hilly, sloping gently to the beach; the highest point is 360 feet; on
+the southern side the reef projects five miles from the land: off this
+point the _Beagle_ found no bottom with 270 fathoms: the reef is
+surmounted by many low coral-islets. Although within the reef the water
+is exceedingly shallow, not being more than a few feet deep, as I am
+informed by the Rev. J. Williams, nevertheless, from the great
+extension of this reef into a profoundly deep ocean, this island
+probably belongs, on the principle lately adverted to, to the barrier
+class, and I have coloured it pale blue; although with much
+hesitation.—_Manouai_ or _Harvey_ Island. The highest point is about
+fifty feet: the Rev. J. Williams informs me that the reef here,
+although it lies far from the shore, is less distant than at Aitutaki,
+but the water within the reef is rather deeper: I have also coloured
+this pale blue with many doubts.—Round _Mitiaro_ Island, as I am
+informed by Mr. Williams, the reef is attached to the shore; coloured
+red.—_Mauki_ or Maouti; the reef round this island (under the name of
+Parry Island, in the “Voyage of H.M.S. _ Blonde_,” p. 209) is described
+as a coral-flat, only fifty yards wide, and two feet under water. This
+statement has been corroborated by Mr. Williams, who calls the reef
+attached; coloured red.—_Aitu_, or Wateeo; a moderately elevated hilly
+island, like the others of this group. The reef is described in Cook’s
+“Voyage,” as attached to the shore, and about one hundred yards wide;
+coloured red.—_Fenoua-iti_; Cook describes this island as very low, not
+more than six or seven feet high (vol. i, book ii, chap. iii, 1777); in
+the chart published in the “_Coquille’s_ Atlas,” a reef is engraved
+close to the shore: this island is not mentioned in the list given by
+Mr. Williams (page 16) in the “Narrative of Missionary Enterprise;”
+nature doubtful. As it is so near Atiu, it has been unavoidably
+coloured red.—_Rarotonga_; Mr. Williams informs me that it is a lofty
+basaltic island with an attached reef; coloured red.—There are three
+islands, _Rourouti, Roxburgh_, and _Hull_, of which I have not been
+able to obtain any account, and have left them uncoloured. Hull Island,
+in the French chart, is written with small letters as being
+low.—_Mangaia_; height about three hundred feet; “the surrounding reef
+joins the shore” (Williams, “Narrative,” p. 18); coloured
+red.—_Rimetara_; Mr. Williams informs me that the reef is rather close
+to the shore; but, from information given me by Mr. Ellis, the reef
+does not appear to be quite so closely attached to it as in the
+foregoing cases: the island is about three hundred feet high (_Naut.
+Mag._, 1839, p. 738); coloured red.—_Rurutu_; Mr. Williams and Mr.
+Ellis inform me that this island has an attached reef; coloured red. It
+is described by Cook under the name of Oheteroa: he says it is not
+surrounded, like the neighbouring islands by a reef; he must have meant
+a distant reef.—_Toubouai_; in Cook’s chart (“Second Voyage,” vol. ii,
+p. 2) the reef is laid down in part one mile, and in part two miles
+from the shore. Mr. Ellis (“Polynes. Res.” vol. iii, p. 381) says the
+low land round the base of the island is very extensive; and this
+gentleman informs me that the water within the reef appears deep;
+coloured blue.—_Raivaivai_, or Vivitao; Mr. Williams informs me that
+the reef is here distant: Mr. Ellis, however, says that this is
+certainly not the case on one side of the island; and he believes that
+the water within the reef is not deep; hence I have left it
+uncoloured.—_Lancaster_ Reef, described in _ Naut. Mag._, 1833 (p.
+693), as an extensive crescent-formed coral-reef. I have not coloured
+it.—_Rapa_, or Oparree; from the accounts given of it by Ellis and
+Vancouver, there does not appear to be any reef.—_I. de Bass_ is an
+adjoining island, of which I cannot find any account.—_Kemin_ Island;
+Krusenstern seems hardly to know its position, and gives no further
+particulars.
+
+ISLANDS BETWEEN _the Low and Gilbert Archipelagoes._
+
+_Caroline_ Island (10° S., 150 deg W.) is described by Mr. F. D.
+Bennett (_Geograph. Journ._, vol. vii, p. 225) as containing a fine
+lagoon; coloured blue.—_Flint_ Island (11° S., 151° W.); Krusenstern
+believes that it is the same with Peregrino, which is described by
+Quiros (Burney’s “Chron. Hist.,” vol. ii, p. 283) as “a cluster of
+small islands connected by a reef, and forming a lagoon in the middle;”
+coloured blue.—_Wostock_ is an island a little more than half a mile in
+diameter, and apparently quite flat and low, and was discovered by
+Bellinghausen; it is situated a little west of Caroline Island, but it
+is not placed on the French charts; I have not coloured it, although I
+entertain little doubt from the chart of Bellinghausen, that it
+originally contained a small lagoon.—_Penrhyn_ Island (9° S., 158° W.);
+a plan of it in the “Atlas of the First Voyage” of Kotzebue, shows that
+it is an atoll; blue.—_Slarbuck_ Island (5° S., 156° W.) is described
+in Byron’s “Voyage in the _Blonde_” (p. 206) as formed of a flat
+coral-rock, with no trees; the height not given; not coloured.—_Malden_
+Island (4° S., 154° W.); in the same voyage (p. 205) this island is
+said to be of coral formation, and no part above forty feet high; I
+have not ventured to colour it, although, from being of
+coral-formation, it is probably fringed; in which case it should be
+red.—_Jarvis_, or _Bunker_ Island (0° 20′ S., 160° W.) is described by
+Mr. F. D. Bennett (_Geograph. Journ._, vol. vii, p. 227) as a narrow,
+low strip of coral-formation; not coloured.—_Brook_, is a small low
+island between the two latter; the position, and perhaps even the
+existence of it is doubtful; not coloured.—_Pescado_ and _Humphrey_
+Islands; I can find out nothing about these islands, except that the
+latter appears to be small and low; not coloured.—_Rearson_, or Grand
+Duke Alexander’s (10° S., 161° W.); an atoll, of which a plan is given
+by Bellinghausen; blue.—_Souvoroff_ Islands (13° S., 163° W.); Admiral
+Krusenstern, in the most obliging manner, obtained for me an account of
+these islands from Admiral Lazareff, who discovered them. They consist
+of five very low
+islands of coral-formation, two of which are connected by a reef, with
+deep water close to it. They do not surround a lagoon, but are so
+placed that a line drawn through them includes an oval space, part of
+which is shallow; these islets, therefore, probably once (as is the
+case with some of the islands in the Caroline Archipelago) formed a
+single atoll; but I have not coloured them.—_Danger_ Island (10° S.,
+166° W.); described as low by Commodore Byron, and more lately surveyed
+by Bellinghausen; it is a small atoll with three islets on it;
+blue.—_Clarence_ Island (9° S., 172° W.); discovered in the _Pandora_
+(G. Hamilton’s “Voyage,” p. 75): it is said, “in running along the
+land, we saw several canoes crossing the _lagoons_;” as this island is
+in the close vicinity of other low islands, and as it is said, that the
+natives make reservoirs of water in old cocoa-nut trees (which shows
+the nature of the land), I have no doubt it is an atoll, and have
+coloured it blue. _York_ Island (8° S., 172° W.) is described by
+Commodore Byron (chap. x of his “Voyage”) as an atoll; blue.—_Sydney_
+Island (4° S., 172° W.) is about three miles in diameter, with its
+interior occupied by a lagoon (Captain Tromelin, “Annal. Marit.” 1829,
+p. 297); blue.—_Phoenix_ Island (4° S., 171° W.) is nearly circular,
+low, sandy, not more than two miles in diameter, and very steep outside
+(Tromelin, “Annal. Marit.” 1829, p. 297); it may be inferred that this
+island originally contained a lagoon, but I have not coloured it.—_New
+Nantucket_ (0° 15′ N., 174° W.). From the French chart it must be a low
+island; I can find nothing more about it or about _Mary_ Island; both
+uncoloured.—_Gardner_ Island (5° S., 174° W.) from its position is
+certainly the same as _Kemin_ Island described (Krusenstern, p. 435,
+Appen. to Mem., published 1827) as having a lagoon in its centre; blue.
+
+ISLANDS SOUTH _of the Sandwich Archipelago._
+
+_Christmas_ Island (2° N., 157° W.). Captain Cook, in his “Third
+Voyage” (vol. ii, chap. x), has given a detailed account of this atoll.
+The breadth of the islets on the reef is unusually great, and the sea
+near it does not deepen so suddenly as is generally the case. It has
+more lately been visited by Mr. F. D. Bennett (_Geograph. Journ._, vol.
+vii, p. 226); and he assures me that it is low and of coral-formation:
+I particularly mention this, because it is engraved with a capital
+letter, signifying a high island, in D’Urville and Lottin’s chart. Mr.
+Couthouy, also, has given some account of it (“Remarks,” p. 46) from
+the Hawaiian “Spectator”; he believes it has lately undergone a small
+elevation, but his evidence does not appear to me satisfactory; the
+deepest part of the lagoon is said to be only ten feet; nevertheless, I
+have coloured it blue.—_Fanning_ Island (4° N., 158° W.) according to
+Captain Tromelin (“Ann. Maritim.,” 1829, p. 283), is an atoll: his
+account as observed by Krusenstern, differs from that given in
+Fanning’s “Voyage” (p. 224), which, however, is far from clear;
+coloured blue.—_Washington_ Island (4° N., 159° W.) is engraved as a
+low island in D’Urville’s chart, but is described by Fanning (p. 226)
+as having a much greater elevation than Fanning Island, and hence I
+presume it is not an atoll; not coloured.—_Palmyra_ Island (6° N., 162°
+W.) is an atoll divided into two parts (Krusenstern’s “Mem. Suppl.,” p.
+50, also Fanning’s “Voyage,” p. 233); blue.—_Smyth’s_ or Johnston’s
+Islands (17° N., 170° W.). Captain Smyth, R.N., has had the kindness to
+inform me that they consist of two very low, small islands, with a
+dangerous reef off the east end of them. Captain Smyth does not
+recollect whether these islets, together with the reef, surrounded a
+lagoon; uncoloured.
+
+SANDWICH ARCHIPELAGO.—_Hawaii_; in the chart in Freycinet’s “Atlas,”
+small portions of the coast are fringed by reefs; and in the
+accompanying “Hydrog. Memoir,” reefs are mentioned in several places,
+and the coral is said to injure the cables. On one side of the islet of
+Kohaihai there is a bank of sand and coral with five feet water on it,
+running parallel to the shore, and leaving a channel of about fifteen
+feet deep within. I have coloured this island red, but it is very much
+less perfectly fringed than others of the group.—_Maui_; in Freycinet’s
+chart of the anchorage of Raheina, two or three miles of coast are seen
+to be fringed; and in the “Hydrog. Memoir,” “banks of coral along
+shore” are spoken of. Mr. F. D. Bennett informs me that the reefs, on
+an average, extend about a quarter of a mile from the beach; the land
+is not very steep, and outside the reefs the sea does not become deep
+very suddenly; coloured red.—_Morotoi_, I presume, is fringed:
+Freycinet speaks of the breakers extending along the shore at a little
+distance from it. From the chart, I believe it is fringed; coloured
+red.—_Oahu_; Freycinet, in his “Hydrog. Memoir,” mentions some of the
+reefs. Mr. F. D. Bennett informs me that the shore is skirted for forty
+or fifty miles in length. There is even a harbour for ships formed by
+the reefs, but it is at the mouth of a valley; red.—_Atooi_, in La
+Peyrouse’s charts, is represented as fringed by a reef, in the same
+manner as Oahu and Morotoi; and this, as I have been informed by Mr.
+Ellis, on part at least of the shore, is of coral-formation: the reef
+does not leave a deep channel within; red.—_Oneehow_; Mr. Ellis
+believes that this island is also fringed by a coral-reef: considering
+its close proximity to the other islands, I have ventured to colour it
+red. I have in vain consulted the works of Cook, Vancouver, La
+Peyrouse, and Lisiansky, for any satisfactory account of the small
+islands and reefs, which lie scattered in a N.W. line prolonged from
+the Sandwich group, and hence have left them uncoloured, with one
+exception; for I am indebted to Mr. F. D. Bennett for informing me of
+an atoll-formed reef, in latitude 28° 22′, longitude 178° 30′ W., on
+which the _ Gledstanes_ was wrecked in 1837. It is apparently of large
+size, and extends in a N.W. and S.E. line: very few islets have been
+formed on it. The lagoon seems to be shallow; at least, the deepest
+part which was surveyed was only three fathoms. Mr. Couthouy
+(“Remarks,” p. 38) describes this island under the name of _ Ocean_
+island. Considerable doubts should be entertained regarding the nature
+of a reef of this kind, with a very shallow lagoon, and standing far
+from any other atoll, on account of the possibility of a crater or flat
+bank of rock lying at the proper depth beneath the surface of the
+water, thus affording a foundation for a ring-formed coral-reef. I
+have, however, thought myself compelled, from its large size and
+symmetrical outline, to colour it blue.
+
+SAMOA or NAVIGATOR GROUP.—Kotzebue, in his “Second Voyage,”
+contrasts the structure of these islands with many others in the
+Pacific, in not being furnished with harbours for ships, formed by
+distant coral-reefs. The Rev. J. Williams, however, informs me, that
+coral-reefs do occur in irregular patches on the shores of these
+islands; but that they do not form a continuous band, as round Mangaia,
+and other such perfect cases of fringed islands. From the charts
+accompanying La Peyrouse’s “Voyage,” it appears that the north shore of
+_Savaii, Maouna, Orosenga_, and _ Manua_, are fringed by reefs. La
+Peyrouse, speaking of Maouna (p. 126), says that the coral-reef
+surrounding its shores, almost touches the beach; and is breached in
+front of the little coves and streams, forming passages for canoes, and
+probably even for boats. Further on (p. 159), he extends the same
+observation to all the islands which he visited. Mr. Williams in his
+“Narrative,” speaks of a reef going round a small island attached to
+_Oyolava_, and returning again to it: all these islands have been
+coloured red.—A chart of _Rose_ Island, at the extreme west end of the
+group, is given by Freycinet, from which I should have thought that it
+had been an atoll; but according to Mr. Couthouy (“Remarks,” p. 43), it
+consists of a reef, only a league in circuit, surmounted by a very few
+low islets; the lagoon is very shallow, and is strewed with numerous
+large boulders of volcanic rock. This island, therefore, probably
+consists of a bank of rock, a few feet submerged, with the outer margin
+of its upper surface fringed with reefs; hence it cannot be properly
+classed with atolls, in which the foundations are always supposed to
+lie at a depth, greater than that at which the reef-constructing
+polypifers can live; not coloured.
+
+_Beveridge_ Reef, 20° S., 167° W., is described in the _Naut. Mag._
+(May 1833, p. 442) as ten miles long in a N. and S. line, and eight
+wide; “in the inside of the reef there appears deep water;” there is a
+passage near the S.W. corner: this therefore seems to be a submerged
+atoll, and is coloured blue.
+
+_Savage_ Island, 19° S., 170° W., has been described by Cook and
+Forster. The younger Forster (vol. ii, p. 163) says it is about forty
+feet high: he suspects that it contains a low plain, which formerly was
+the lagoon. The Rev. J. Williams informs me that the reef fringing its
+shores, resembles that round Mangaia; coloured red.
+
+FRIENDLY ARCHIPELAGO.—_Pylstaart_ Island. Judging from the chart in
+Freycinet’s “Atlas,” I should have supposed that it had been regularly
+fringed; but as nothing is said in the “Hydrog. Memoir” (or in the
+“Voyage” of Tasman, the discoverer) about coral-reefs, I have left it
+uncoloured.—_Tongatabou_: In the “Atlas of the Voyage of the
+_Astrolabe_,” the whole south side of the island is represented as
+narrowly fringed by the same reef which forms an extensive platform on
+the northern side. The origin of this latter reef, which might have
+been mistaken for a barrier-reef, has already been attempted to be
+explained, when giving the proofs of the recent elevation of this
+island.—In Cook’s charts the little outlying island also of _Eoaigee_,
+is represented as fringed; coloured red.—_Eoua._ I cannot make out from
+Captain Cook’s charts and descriptions, that this island has any reef,
+although the bottom of the neighbouring sea seems to be corally, and
+the island itself is formed of coral-rock.
+
+
+Forster, however, distinctly (“Observations,” p. 14) classes it with
+high islands having reefs, but it certainly is not encircled by a
+barrier-reef and the younger Forster (“Voyage,” vol. i, p. 426) says,
+that “a bed of coral-rocks surrounded the coast towards the
+landing-place.” I have therefore classed it with the fringed islands
+and coloured it red. The several islands lying N.W. of Tongatabou,
+namely _Anamouka, Komango, Kotou, Lefouga, Foa_, etc., are seen in
+Captain Cook’s chart to be fringed by reefs, in several of them are
+connected together. From the various statements in the first volume of
+Cook’s “Third Voyage,” and especially in the fourth and sixth chapters,
+it appears that these reefs are of coral-formation, and certainly do
+not belong to the barrier class; coloured red.—_Toufoa and Kao_,
+forming the western part of the group, according to Forster have no
+reefs; the former is an active volcano.—_Vavao._ There is a chart of
+this singularly formed island, by Espinoza: according to Mr. Williams
+it consists of coral-rock: the Chevalier Dillon informs me that it is
+not fringed; not coloured. Nor are the islands of _Latte_ and
+_Amargura_, for I have not seen plans on a large scale of them, and do
+not know whether they are fringed.
+
+_Niouha_, 16° S., 174° W., or _Keppel_ Island of Wallis, or _Cocos_
+Island. From a view and chart of this island given in Wallis’s “Voyage”
+(4to ed.) it is evidently encircled by a reef; coloured blue: it is
+however remarkable that _Boscawen_ Island, immediately adjoining, has
+no reef of any kind; uncoloured.
+
+_Wallis_ Island, 13° S., 176° W., a chart and view of this island in
+Wallis’s “Voyage” (4to ed.) shows that it is encircled. A view of it in
+the _Naut. Mag._, July 1833, p. 376, shows the same fact; blue.
+
+_Alloufatou_, or _Horn_ Island, _Onouafu_, or _ Proby_ Island, and
+_Hunter_ Islands, lie between the Navigator and Fidji groups. I can
+find no distinct accounts of them.
+
+FIDJI or VITI GROUP.—The best chart of the numerous islands of this
+group, will be found in the “Atlas of the _ Astrolabe’s_ Voyage.” From
+this, and from the description given in the “Hydrog. Memoir,”
+accompanying it, it appears that many of these islands are bold and
+mountainous, rising to the height of between 3,000 and 4,000 feet. Most
+of the islands are surrounded by reefs, lying far from the land, and
+outside of which the ocean appears very deep. The _Astrolabe_ sounded
+with ninety fathoms in several places about a mile from the reefs, and
+found no bottom. Although the depth within the reef is not laid down,
+it is evident from several expressions, that Captain D’Urville believes
+that ships could anchor within, if passages existed through the outer
+barriers. The Chevallier Dillon informs me that this is the case: hence
+I have coloured this group blue. In the S.E. part lies _ Batoa_, or
+_Turtle_ Island of Cook (“Second Voyage,” vol. ii, p. 23, and chart,
+4to ed.) surrounded by a coral-reef, “which in some places extends two
+miles from the shore;” within the reef the water appears to be deep,
+and outside it is unfathomable; coloured pale blue. At the distance of
+a few miles, Captain Cook (_Ibid_., p. 24) found a circular coral-reef,
+four or five leagues in circuit, with deep water within; “in short, the
+bank wants only a few little islets to make it exactly like one of the
+half-drowned isles so
+often mentioned,”—namely, atolls. South of Batoa, lies the high island
+of _Ono_, which appears in Bellinghausen’s “Atlas” to be encircled; as
+do some other small islands to the south; coloured pale blue; near Ono,
+there is an annular reef, quite similar to the one just described in
+the words of Captain Cook; coloured dark blue.
+
+_Rotoumah_, 13° S., 179° E.—From the chart in Duperrey’s “Atlas,” I
+thought this island was encircled, and had coloured it blue, but the
+Chevallier Dillon assures me that the reef is only a shore or fringing
+one; red.
+
+_Independence_ Island, 10° S., 179° E., is described by Mr. G. Bennett,
+(_United Service Journ._, 1831, part ii, p. 197) as a low island of
+coral-formation, it is small, and does not appear to contain a lagoon,
+although an opening through the reef is referred to. A lagoon probably
+once existed, and has since been filled up; left uncoloured.
+
+ELLICE GROUP.—_Oscar, Peyster_, and _Ellice_ Islands are figured in
+Arrowsmith’s “Chart of the Pacific” (corrected to 1832) as atolls, and
+are said to be very low; blue.—_Nederlandisch_ Island. I am greatly
+indebted to the kindness of Admiral Krusenstern, for sending me the
+original documents concerning this island. From the plans given by
+Captains Eeg and Khremtshenko, and from the detailed account given by
+the former, it appears that it is a narrow coral-island, about two
+miles long, containing a small lagoon. The sea is very deep close to
+the shore, which is fronted by sharp coral-rocks. Captain Eeg compares
+the lagoon with that of other coral-islands; and he distinctly says,
+the land is “very low.” I have therefore coloured it blue. Admiral
+Krusenstern (“Memoir on the Pacific,” Append., 1835) states that its
+shores are eighty feet high; this probably arose from the height of the
+cocoa-nut trees, with which it is covered, being mistaken for
+land.—_Gran Cocal_ is said in Krusenstern’s “Memoir,” to be low, and to
+be surrounded by a reef; it is small, and therefore probably once
+contained a lagoon; uncoloured.—_St. Augustin._ From a chart and view
+of it, given in the “Atlas of the _Coquille’s_ Voyage,” it appears to
+be a small atoll, with its lagoon partly filled up; coloured blue.
+
+GILBERT GROUP.—The chart of this group, given in the “Atlas of the
+_Coquille’s_ Voyage,” at once shows that it is composed of ten well
+characterised atolls. In D’Urville and Lottin’s chart, _Sydenham_ is
+written with a capital letter, signifying that it is high; but this
+certainly is not the case, for it is a perfectly characterised atoll,
+and a sketch, showing how low it is, is given in the “_Coquille’s_
+Atlas.” Some narrow strip-like reefs project from the southern side of
+_Drummond_ atoll, and render it irregular. The southern island of the
+group is called _Chase_ (in some charts, _ Rotches_); of this I can
+find no account, but Mr. F. D. Bennett discovered (_Geograph. Journ._,
+vol. vii, p. 229), a low extensive island in nearly the same latitude,
+about three degrees westward of the longitude assigned to Rotches, but
+very probably it is the same island. Mr. Bennett informs me that the
+man at the masthead reported an appearance of lagoon-water in the
+centre; and, therefore, considering its position, I have coloured it
+blue.—_Pitt_ Island, at the extreme northern point of the group, is
+left uncoloured, as its exact position and nature
+is not known.—_Byron_ Island, which lies a little to the eastward, does
+not appear to have been visited since Commodore Byron’s voyage, and it
+was then seen only from a distance of eighteen miles; it is said to be
+low; uncoloured.
+
+_Ocean, Pleasant_, and _Atlantic_ Islands all lie considerably to the
+west of the Gilbert group: I have been unable to find any distinct
+account of them. Ocean Island is written with small letters in the
+French chart, but in Krusenstern’s “Memoir” it is said to be high.
+
+MARSHALL GROUP.—We are well acquainted with this group from the
+excellent charts of the separate islands, made during the two voyages
+of Kotzebue: a reduced one of the whole group may be easily seen in
+Krusenstern’s “Atlas,” and in Kotzebue’s “Second Voyage.” The group
+consists (with the exception of two _little_ islands which probably
+have had their lagoon filled up) of a double row of twenty-three large
+and well-characterised atolls, from the examination of which Chamisso
+has given us his well-known account of coral-formations. I include
+_Gaspar Rico_, or _Cornwallis_ Island in this group, which is described
+by Chamisso (Kotzebue’s “First Voyage,” vol. iii, p. 179) “as a low
+sickle-formed group, with mould only on the windward side.” Gaspard
+Island is considered by some geographers as a distinct island lying
+N.E. of the group, but it is not entered in the chart by Krusenstern;
+left uncoloured. In the S.W. part of this group lies _Baring_ Island,
+of which little is known (see Krusenstern’s “Appendix,” 1835, p. 149).
+I have left it uncoloured; but _Boston_ Island I have coloured blue, as
+it is described (_Ibid_.) as consisting of fourteen small islands,
+which, no doubt, enclose a lagoon, as represented in a chart in the
+“‘Coquille’s’ Atlas.”—Two islands, _Aur Kawen_ and _Gaspar Rico_, are
+written in the French chart with capital letters; but this is an error,
+for from the account given by Chamisso in Kotzebue’s “First Voyage,”
+they are certainly low. The nature, position, and even existence, of
+the shoals and small islands north of the Marshall group, are doubtful.
+
+NEW HEBRIDES.—Any chart, on even a small scale, of these islands, will
+show that their shores are almost without reefs, presenting a
+remarkable contrast with those of New Caledonia on the one hand, and
+the Fidji group on the other. Nevertheless, I have been assured by Mr.
+G. Bennett, that coral grows vigorously on their shores; as indeed,
+will be further shown in some of the following notices. As, therefore,
+these islands are not encircled, and as coral grows vigorously on their
+shores, we might almost conclude, without further evidence, that they
+were fringed, and hence I have applied the red colour with rather
+greater freedom than in other instances.—_Matthew’s Rock_, an active
+volcano, some way south of the group (of which a plan is given in the
+“Atlas of the _Astrolabe’s_ Voyage”) does not appear to have reefs of
+any kind about it.—_Annatom_, the southernmost of the Hebrides; from a
+rough woodcut given in the _United Service Journal_ (1831, part iii, p.
+190), accompanying a paper by Mr. Bennett, it appears that the shore is
+fringed; coloured red.—_Tanna._ Forster, in his “Observations” (p. 22),
+says Tanna has on its shores coral-rock and madrepores; and the younger
+Forster, in his account (vol. ii, p. 269) speaking of the harbour
+says, the whole S.E. side consists of coral-reefs, which are overflowed
+at high-water; part of the southern shore in Cook’s chart is
+represented as fringed; coloured red.—_Immer_ is described (_United
+Service Journal,_ 1831, part iii, p. 192) by Mr. Bennett as being of
+moderate elevation, with cliffs appearing like sandstone: coral grows
+in patches on its shore, but I have not coloured it; and I mention
+these facts, because Immer might have been thought from Forster’s
+classification (“Observations,” p. 14), to have been a low island or
+even an atoll.—_Erromango_ Island; Cook (“Second Voyage,” vol. ii, p.
+45, 4to ed.) speaks of rocks everywhere _lining_ the coast, and the
+natives offered to haul his boat over the breakers to the sandy beach:
+Mr. Bennett, in a letter to the Editor of the _Singapore Chron._,
+alludes to the _reefs_ on its shores. It may, I think, be safely
+inferred from these passages that the shore is fringed in parts by
+coral-reefs; coloured red.—_Sandwich_ Island. The east coast is said
+(Cook’s “Second Voyage,” vol. ii, p. 41) to be low, and to be guarded
+by a chain of breakers. In the accompanying chart it is seen to be
+fringed by a reef; coloured red.—_Mallicollo._ Forster speaks of the
+reef-bounded shore: the reef is about thirty yards wide, and so shallow
+that a boat cannot pass over it. Forster also (“Observations,” p. 23)
+says, that the rocks of the sea-shore consist of madrepore. In the plan
+of Sandwich harbour, the headlands are represented as fringed; coloured
+red.—_Aurora_ and _Pentecost_ Islands, according to Bougainville,
+apparently have no reefs; nor has the large island of _S. Espiritu_,
+nor _Bligh_ Island or _Banks’_ Islands, which latter lie to the N.E. of
+the Hebrides. But in none of these cases, have I met with any detailed
+account of their shores, or seen plans on a large scale; and it will be
+evident, that a fringing-reef of only thirty or even a few hundred
+yards in width, is of so little importance to navigation, that it will
+seldom be noticed, excepting by chance; and hence I do not doubt that
+several of these islands, now left uncoloured, ought to be red.
+
+SANTA CRUZ GROUP.—_Vanikoro_ (Fig. 1, Plate I) offers a striking
+example of a barrier- reef: it was first described by the Chevalier
+Dillon, in his voyage, and was surveyed in the _Astrolabe_; coloured
+pale blue.—_Tikopia_ and _Fataka_ Islands appear, from the descriptions
+of Dillon and D’Urville, to have no reefs; _ Anouda_ is a low, flat
+island, surrounded by cliffs (“_Astrolabe_ Hydrog.” and Krusenstern,
+“Mem.” vol. ii, p. 432); these are uncoloured. _Toupoua_ (_Otooboa_ of
+Dillon) is stated by Captain Tromelin (“Annales Marit.” 1829, p. 289)
+to be almost entirely included in a reef, lying at the distance of two
+miles from the shore. There is a space of three miles without any reef,
+which, although indented with bays, offers no anchorage from the
+extreme depth of the water close to the shore: Captain Dillon also
+speaks of the reefs fronting this island; coloured blue.—_Santa-Cruz._
+I have carefully examined the works of Carteret, D’Entrecasteaux,
+Wilson, and Tromelin, and I cannot discover any mention of reefs on its
+shores; left uncoloured.—_Tinakoro_ is a constantly active volcano
+without reefs.—_Mendana Isles_ (mentioned by Dillon under the name of
+_Mammee_, etc.); said by Krusenstern to be low, and intertwined with
+reefs. I do not believe they include a lagoon; I have left them
+uncoloured.—_Duff’s_ Islands compose a small group
+directed in a N.W. and S.E. band; they are described by Wilson (p. 296,
+“Miss. Voy.” 4to ed.), as formed by bold-peaked land, with the islands
+surrounded by coral-reefs, extending about half a mile from the shore;
+at a distance of a mile from the reefs he found only seven fathoms. As
+I have no reason for supposing there is deep water within these reefs,
+I have coloured them red. _Kennedy_ Island, N.E. of Duff’s. I have been
+unable to find any account of it.
+
+NEW CALEDONIA.—The great barrier-reefs on the shores of this island
+have already been described (Fig. 5, Plate II). They have been visited
+by Labillardiere, Cook, and the northern point by D’Urville; this
+latter part so closely resembles an atoll that I have coloured it dark
+blue. The _Loyalty_ group is situated eastward of this island; from the
+chart and description given in the “Voyage of the _Astrolabe_,” they do
+not appear to have any reefs; north of this group, there are some
+extensive low reefs (called _Astrolabe_ and _Beaupré_,) which do not
+seem to be atoll-formed; these are left uncoloured.
+
+AUSTRALIAN BARRIER-REEF.—The limits of this great reef, which has
+already been described, have been coloured from the charts of Flinders
+and King. In the northern parts, an atoll-formed reef, lying outside
+the barrier, has been described by Bligh, and is coloured dark blue. In
+the space between Australia and New Caledonia, called by Flinders the
+Corallian Sea, there are numerous reefs. Of these, some are represented
+in Krusenstern’s “Atlas” as having an atoll-like structure; namely,
+_Bampton_ shoal, _Frederic, Vine_ or Horse-shoe, and _ Alert_ reefs;
+these have been coloured dark blue.
+
+LOUISIADE.—The dangerous reefs which front and surround the western,
+southern, and northern coasts of this so-called peninsula and
+archipelago, seem evidently to belong to the barrier class. The land is
+lofty, with a low fringe on the coast; the reefs are distant, and the
+sea outside them profoundly deep. Nearly all that is known of this
+group is derived from the labours of D’Entrecasteaux and Bougainville:
+the latter has represented one continuous reef ninety miles long,
+parallel to the shore, and in places as much as ten miles from it;
+coloured pale blue. A little distance northward we have the _Laughlan_
+Islands, the reefs round which are engraved in the “Atlas of the Voyage
+of the _Astrolabe_,” in the same manner as in the encircled islands of
+the Caroline Archipelago, the reef is, in parts, a mile and a half from
+the shore, to which it does not appear to be attached; coloured blue.
+At some little distance from the extremity of the Louisiade lies the
+_Wells_ reef, described in G. Hamilton’s “Voyage in H.M.S. _Pandora_”
+(p. 100): it is said, “We found we had got embayed in a double reef,
+which will soon be an island.” As this statement is only intelligible
+on the supposition of the reef being crescent or horse-shoe formed,
+like so many other submerged annular reefs, I have ventured to colour
+it blue.
+
+SOLOMON ARCHIPELAGO.—The chart in Krusenstern’s “Atlas” shows that
+these islands are not encircled, and as coral appears from the works of
+Surville, Bougainville, and Labillardiere, to grow on their shores,
+this circumstance, as in the case of the New Hebrides, is a presumption
+that they are fringed. I cannot find out anything from
+D’Entrecasteaux’s
+“Voyage,” regarding the southern islands of the group, so have left
+them uncoloured.—_Malayta_ Island in a rough MS. chart in the Admiralty
+has its northern shore fringed.—_Ysabel_ Island, the N.E. part of this
+island, in the same chart, is also fringed: Mendana, speaking (Burney,
+vol. i, p. 280) of an islet adjoining the northern coast, says it is
+surrounded by reefs; the shores, also of Port Praslin appear regularly
+fringed.—_Choiseul_ Island. In Bougainville’s “Chart of Choiseul Bay,”
+parts of the shores are fringed by coral-reefs.—_Bougainville_ Island.
+According to D’Entrecasteaux the western shore abounds with
+coral-reefs, and the smaller islands are said to be attached to the
+larger ones by reefs; all the before-mentioned islands have been
+coloured red.—_Bouka_ Islands. Captain Duperrey has kindly informed me
+in a letter that he passed close round the northern side of this island
+(of which a plan is given in his “Atlas of the _Coquille’s_ Voyage”),
+and that it was “garnie d’une bande de récifs à fleur d’eau adherentes
+au rivage;” and he infers, from the abundance of coral on the islands
+north and south of Bouka, that the reef probably is of coral; coloured
+red.
+
+Off the north coast of the Solomon Archipelago there are several small
+groups which are little known; they appear to be low, and of
+coral-formation; and some of them probably have an atoll-like
+structure; the Chevallier Dillon, however, informs me that this is not
+the case with the B. de _Candelaria.—Outong Java_, according to the
+Spanish navigator, Maurelle, is thus characterised; but this is the
+only one which I have ventured to colour blue.
+
+NEW IRELAND.—The shores of the S.W. point of this island and some
+adjoining islets, are fringed by reefs, as may be seen in the “Atlases
+of the Voyages of the _Coquille_ and _Astrolabe_.” M. Lesson observes
+that the reefs are open in front of each streamlet. The _Duke of
+York’s_ Island is also fringed; but with regard to the other parts of
+_New Ireland, New Hanover_, and the small islands lying northward, I
+have been unable to obtain any information. I will only add that no
+part of New Ireland appears to be fronted by distant reefs. I have
+coloured red only the above specified portions.
+
+NEW BRITAIN AND THE NORTHERN SHORE OF NEW GUINEA.—From the charts in
+the “Voyage of the _Astrolabe_,” and from the “Hydrog. Memoir,” it
+appears that these coasts are entirely without reefs, as are the
+_Schouten_ Islands, lying close to the northern shore of New Guinea.
+The western and south-western parts of New Guinea, will be treated of
+when we come to the islands of the East Indian Archipelago.
+
+ADMIRALTY GROUP.—From the accounts by Bougainville, Maurelle,
+D’Entrecasteaux, and the scattered notices collected by Horsburgh, it
+appears, that some of the many islands composing it, are high, with a
+bold outline; and others are very low, small and interlaced with reefs.
+All the high islands appear to be fronted by distant reefs rising
+abruptly from the sea, and within some of which there is reason to
+believe that the water is deep. I have therefore little doubt they are
+of the barrier class.—In the southern part of the group we have _
+Elizabeth_ Island, which is surrounded by a reef at the distance of a
+mile; and two miles eastward of it (Krusenstern, “Append.” 1835, p. 42)
+there is a little island
+containing a lagoon.—Near here, also lies _ Circular-reef_ (Horsburgh,
+“Direct.,” vol. i, p. 691, 4th ed.), “three or four miles in diameter
+having deep water inside with an opening at the N.N.W. part, and on the
+outside steep to.” I have from these data, coloured the group pale
+blue, and _ Circular-reef_ dark blue.—The _Anachorites, Echequier_, and
+_Hermites_, consist of innumerable low islands of coral-formation,
+which probably have atoll-like forms; but not being able to ascertain
+this, I have not coloured them, nor _Durour_ Island, which is described
+by Carteret as low.
+
+The CAROLINE ARCHIPELAGO is now well-known, chiefly from the
+hydrographical labours of Lutké; it contains about forty groups of
+atolls, and three encircled islands, two of which are engraved in Figs
+2 and 7, Plate I. Commencing with the eastern part; the encircling reef
+round _Ualen_ appears to be only about half a mile from the shore; but
+as the land is low and covered with mangroves (“Voyage autour du
+Monde,” par F. Lutké, vol. i, p. 339), the real margin has not probably
+been ascertained. The extreme depth in one of the harbours within the
+reef is thirty-three fathoms (see charts in “Atlas of _Coquille’s_
+Voyage”), and outside at half a mile distant from the reef, no bottom
+was obtained with two hundred and fifty fathoms. The reef is surmounted
+by many islets, and the lagoon-like channel within is mostly shallow,
+and appears to have been much encroached on by the low land surrounding
+the central mountains; these facts show that time has allowed much
+detritus to accumulate; coloured pale blue.—_Pouynipète_, or Seniavine.
+In the greater part of the circumference of this island, the reef is
+about one mile and three quarters distant; on the north side it is five
+miles off the included high islets. The reef is broken in several
+places; and just within it, the depth in one place is thirty fathoms,
+and in another, twenty-eight, beyond which, to all appearance, there
+was “un porte vaste et sur” (Lutké, vol. ii, p. 4); coloured pale
+blue.—_Hogoleu_ or _Roug_. This wonderful group contains at least
+sixty-two islands, and its reef is one hundred and thirty-five miles in
+circuit. Of the islands, only a few, about six or eight (see “Hydrog.
+Descrip.” p. 428, of the “Voyage of the _Astrolabe_,” and the large
+accompanying chart taken chiefly from that given by Duperrey) are high,
+and the rest are all small, low, and formed on the reef. The depth of
+the great interior lake has not been ascertained; but Captain D’Urville
+appears to have entertained no doubt about the possibility of taking in
+a frigate. The reef lies no less than fourteen miles distant from the
+northern coasts of the interior high islands, seven from their western
+sides, and twenty from the southern; the sea is deep outside. This
+island is a likeness on a grand scale to the Gambier group in the Low
+Archipelago. Of the groups of low[1] islands forming the chief part of
+the Caroline Archipelago, all those of larger size, have the true
+atoll-structure (as may be seen in the “Atlas” by Captain Lutké), and
+some even of the very small ones, as _ Macaskill_ and _Duperrey_, of
+which plans are given in the
+“Atlas of the _Coquille’s_ Voyage.” There are, however, some low small
+islands of coral-formation, namely _Ollap, Tamatam, Bigali, Satahoual_,
+which do not contain lagoons; but it is probable that lagoons
+originally existed, but have since filled up: Lutké (vol. ii, p. 304)
+seems to have thought that all the low islands, with only one
+exception, contained lagoons. From the sketches, and from the manner in
+which the margins of these islands are engraved in the “Atlas of the
+Voyage of the _Coquille_,” it might have been thought that they were
+not low; but by a comparison with the remarks of Lutké (vol. ii, p.
+107, regarding Bigali) and of Freycinet (“Hydrog. Memoir _ L’Uranie_
+Voyage,” p. 188, regarding Tamatam, Ollap, etc.), it will be seen that
+the artist must have represented the land incorrectly. The most
+southern island in the group, namely _ Piguiram_, is not coloured,
+because I have found no account of it. _Nougouor_, or _Monte Verdison_,
+which was not visited by Lutké, is described and figured by Mr. Bennett
+(_United Service Journal_, January 1832) as an atoll. All the
+above-mentioned islands have been coloured blue.
+
+ [1] In D’Urville and Lottin’s chart, Peserare is written with capital
+ letters; but this evidently is an error, for it is one of the low
+ islets on the reef of Namonouyto (see Lutké’s charts)—a regular atoll.
+
+WESTERN PART OF THE CAROLINE ARCHIPELAGO.—_Fais_ Island is ninety feet
+high, and is surrounded, as I have been informed by Admiral Lutké, by a
+narrow reef of living coral, of which the broadest part, as represented
+in the charts, is only 150 yards; coloured red.—_Philip_ Island., I
+believe, is low; but Hunter, in his “Historical Journal,” gives no
+clear account of it; uncoloured.—_Elivi_; from the manner in which the
+islets on the reefs are engraved, in the “Atlas of the _Astrolabe’s_
+Voyage,” I should have thought they were above the ordinary height, but
+Admiral Lutké assures me this is not the case: they form a regular
+atoll; coloured blue.—_Gouap_ (_Eap_ of Chamisso), is a high island
+with a reef (see chart in “Voyage of the _Astrolabe_”), more than a
+mile distant in most parts from the shore, and two miles in one part.
+Captain D’Urville thinks that there would be anchorage (“Hydrog.
+Descript. _Astrolabe_ Voyage,” p. 436) for ships within the reef, if a
+passage could be found; coloured pale blue.—_Goulou_, from the chart in
+the “_Astrolabe’s_ Atlas,” appears to be an atoll. D’Urville (“Hydrog.
+Descript.” p. 437) speaks of the low islets on the reef; coloured dark
+blue.
+
+PELEW ISLANDS.—Krusenstern speaks of some of the islands being
+mountainous; the reefs are distant from the shore, and there are spaces
+within them, and not opposite valleys, with from ten to fifteen
+fathoms. According to a MS. chart of the group by Lieutenant Elmer in
+the Admiralty, there is a large space within the reef with deepish
+water; although the high land does not hold a central position with
+respect to the reefs, as is generally the case, I have little doubt
+that the reefs of the Pelew Islands ought to be ranked with the barrier
+class, and I have coloured them pale blue. In Lieutenant Elmer’s chart
+there is a horseshoe-formed shoal, laid down thirteen miles N.W. of
+Pelew, with fifteen fathoms within the reef, and some dry banks on it;
+coloured dark blue.—_Spanish, Martires, Sanserot, Pulo Anna_ and
+_Mariere_ Islands are not coloured, because I know nothing about them,
+excepting that according to Krusenstern, the second, third, and fourth
+mentioned, are
+low, placed on coral-reefs, and therefore, perhaps, contain lagoons;
+but Pulo Mariere is a little higher.
+
+MARIANA ARCHIPELAGO, or LADRONES.—_Guahan._ Almost the whole of this
+island is fringed by reefs, which extend in most parts about a third of
+a mile from the land. Even where the reefs are most extensive, the
+water within them is shallow. In several parts there is a navigable
+channel for boats and canoes within the reefs. In Freycinet’s “Hydrog.
+Mem.” there is an account of these reefs, and in the “Atlas,” a map on
+a large scale; coloured red.—_Rota_. “L’ile est presque entièrement
+entourée des récifs” (p. 212, Freycinet’s “Hydrog. Mem.”). These reefs
+project about a quarter of a mile from the shore; coloured
+red.—_Tinian. The eastern_ coast is precipitous, and is without reefs;
+but the western side is fringed like the last island; coloured
+red.—_Saypan_. The N.E. coast, and likewise the western shores appear
+to be fringed; but there is a great, irregular, horn-like reef
+projecting far from this side; coloured red.—_Farallon de Medinilla_,
+appears so regularly and closely fringed in Freycinet’s charts, that I
+have ventured to colour it red, although nothing is said about reefs in
+the “Hydrographical Memoir.” The several islands which form the
+northern part of the group are volcanic (with the exception perhaps of
+Torres, which resembles in form the madreporitic island of Medinilla),
+and appear to be without reefs.—_Mangs_, however, is described (by
+Freycinet, p. 219, “Hydrog.”) from some Spanish charts, as formed of
+small islands placed “au milieu des nombreux récifs;” and as these
+reefs in the general chart of the group do not project so much as a
+mile; and as there is no appearance from a double line, of the
+existence of deep water within, I have ventured, although with much
+hesitation, to colour them red. Respecting _Folger_ and _ Marshall_
+Islands which lie some way east of the Marianas, I can find out
+nothing, excepting that they are probably low. Krusenstern says this of
+Marshall Island; and Folger Island is written with small letters in
+D’Urville’s chart; uncoloured.
+
+BONIN OR ARZOBISPO GROUP.—_Peel_ Island has been examined by Captain
+Beechey, to whose kindness I am much indebted for giving me information
+regarding it: “At Port Lloyd there is a great deal of coral; and the
+inner harbour is entirely formed by coral-reefs, which extend outside
+the port along the coast.” Captain Beechey, in another part of his
+letter to me, alludes to the reefs fringing the island in all
+directions; but at the same time it must be observed that the surf
+washes the volcanic rocks of the coast in the greater part of its
+circumference. I do not know whether the other islands of the
+Archipelago are fringed; I have coloured Peel Island red.—_Grampus_
+Island to the eastward, does not appear (Meare’s “Voyage,” p. 95) to
+have any reefs, nor does _ Rosario_ Island (from Lutké’s chart), which
+lies to the westward. Respecting the few other islands in this part of
+the sea, namely the _Sulphur_ Islands, with an active volcano, and
+those lying between Bonin and Japan (which are situated near the
+extreme limit in latitude, at which reefs are formed), I have not been
+able to find any clear account.
+
+WEST END OF NEW GUINEA.—_Port Dory._ From the charts in the “Voyage of
+the _Coquille_,” it would appear that the coast in this part
+is fringed by coral-reefs; M. Lesson, however, remarks that the coral
+is sickly; coloured red.—_Waigiou._ A considerable portion of the
+northern shores of these islands is seen in the charts (on a large
+scale) in Freycinet’s “Atlas” to be fringed by coral-reefs. Forrest (p.
+21, “Voyage to New Guinea”) alludes to the coral-reefs lining the heads
+of Piapis Bay; and Horsburgh (vol. ii, p. 599, 4th edit.), speaking of
+the islands in Dampier Strait, says “sharp coral-rocks line their
+shores;” coloured red.—In the sea north of these islands, we have
+_Guedes_ (or _ Freewill_, or _St. David’s_), which from the chart given
+in the 4to edit. of Carteret’s “Voyage,” must be an atoll. Krusenstern
+says the islets are very low; coloured blue.—_Carteret’s Shoals_, in 2°
+53′ N., are described as circular, with stony points showing all round,
+with deeper water in the middle; coloured blue.—_Aiou_; the plan of
+this group, given in the “Atlas of the Voyage of the _Astrolabe_,”
+shows that it is an atoll; and, from a chart in Forrest’s “Voyage,” it
+appears that there is twelve fathoms within the circular reef; coloured
+blue.—The S.W. coast of New Guinea appears to be low, muddy, and devoid
+of reefs. The _Arru, Timor-laut_, and _ Tenimber_ groups have lately
+been examined by Captain Kolff, the MS. translation of which, by Mr. W.
+Earl, I have been permitted to read, through the kindness of Captain
+Washington, R.N. These islands are mostly rather low, and are
+surrounded by distant reefs (the Ki Islands, however, are lofty, and,
+from Mr. Stanley’s survey, appear without reefs); the sea in some parts
+is shallow, in others profoundly deep (as near Larrat). From the
+imperfection of the published charts, I have been unable to decide to
+which class these reefs belong. From the distance to which they extend
+from the land, where the sea is very deep, I am strongly inclined to
+believe they ought to come within the barrier class, and be coloured
+blue; but I have been forced to leave them uncoloured.—The
+last-mentioned groups are connected with the east end of Ceram by a
+chain of small islands, of which the small groups of _Ceram-laut,
+Goram_ and _Keffing_ are surrounded by very extensive reefs, projecting
+into deep water, which, as in the last case, I strongly suspect belong
+to the barrier class; but I have not coloured them. From the south side
+of Keffing, the reefs project five miles (Windsor Earl’s “Sailing
+Direct. for the Arafura Sea,” p. 9).
+
+CERAM.—In various charts which I have examined, several parts of the
+coast are represented as fringed by reefs.—_Manipa_ Island, between
+Ceram and Bourou, in an old MS. chart in the Admiralty, is fringed by a
+very irregular reef, partly dry at low water, which I do not doubt is
+of coral-formation; both islands coloured red.—_Bourou_; parts of this
+island appear fringed by coral-reefs, namely, the eastern coast, as
+seen in Freycinet’s chart; and _Cajeli Bay_, which is said by Horsburgh
+(vol. ii, p. 630) to be lined by coral-reefs, that stretch out a little
+way, and have only a few feet water on them. In several charts,
+portions of the islands forming the AMBOINA GROUP are fringed by reefs;
+for instance, _Noessa, Harenca_, and _Ucaster_, in Freycinet’s charts.
+The above-mentioned islands have been coloured red, although the
+evidence is not very satisfactory.—North of Bourou the parallel line of
+the _ Xulla_ Isles extends: I have not been able to find out anything
+about them, excepting
+that Horsburgh (vol. ii, p. 543) says that the northern shore is
+surrounded by a reef at the distance of two or three miles;
+uncoloured.—_Mysol Group_; the Kanary Islands are said by Forrest
+(“Voyage,” p. 130) to be divided from each other by deep straits, and
+are lined with coral-rocks; coloured red.—_Guebe_, lying between
+Waigiou and Gilolo, is engraved as if fringed; and it is said by
+Freycinet, that all the soundings under five fathoms were on coral;
+coloured red.—_Gilolo_. In a chart published by Dalrymple, the numerous
+islands on the western, southern (_Batchian_ and the _Strait of
+Patientia_), and eastern sides appear fringed by narrow reefs; these
+reefs, I suppose, are of coral, for it is said in “Malte Brun” (vol.
+xii, p. 156), “Sur les côtes (of Batchian) comme _dans les plupart_ des
+iles de cet archipel, il y a de rocs de médrepores d’une beauté et
+d’une variété infimies.” Forrest, also (p. 50), says Seland, near
+Batchian, is a little island with reefs of coral; coloured red.—_Morty_
+Island (north of Gilolo). Horsburgh (vol. ii, p. 506) says the northern
+coast is lined by reefs, projecting one or two miles, and having no
+soundings close to them; I have left it uncoloured, although, as in
+some former cases, it ought probably to be pale blue.—_Celebes._ The
+western and northern coasts appear in the charts to be bold and without
+reefs. Near the extreme northern point, however, an islet in the
+_Straits of Limbe_, and parts of the adjoining shore, appear to be
+fringed: the east side of the bay of _Manado_, has deep water, and is
+fringed by sand and coral (“_Astrol._ Voyage,” Hydrog. Part, pp.
+453-4); this extreme point, therefore, I have coloured red.—Of the
+islands leading from this point to Magindanao, I have not been able to
+find any account, except of _ Serangani_, which appears surrounded by
+narrow reefs; and Forrest (“Voyage,” p. 164) speaks of coral on its
+shores; I have, therefore, coloured this island red. To the eastward of
+this chain lie several islands; of which I cannot find any account,
+except of _Karkalang_, which is said by Horsburgh (vol. ii, p. 504) to
+be lined by a dangerous reef, projecting several miles from the
+northern shore; not coloured.
+
+ISLANDS NEAR TIMOR.—The account of the following islands is taken from
+Captain D. Kolff’s “Voyage,” in 1825, translated by Mr. W. Earl, from
+the Dutch.—_Lette_ has “reefs extending along shore at the distance of
+half a mile from the land.”—_Moa_ has reefs on the S.W. part.—_Lakor_
+has a reef lining its shore; these islands are coloured red.—Still more
+eastward, _ Luan_ has, differently from the last-mentioned islands, an
+extensive reef; it is steep outside, and within there is a depth of
+twelve feet; from these facts, it is impossible to decide to which
+class this island belongs.—_Kissa_, off the point of Timor, has its
+“shore fronted by a reef, steep too on the outer side, over which small
+proahs can go at the time of high water;” coloured red.—_Timor_; most
+of the points, and some considerable spaces of the northern shore, are
+seen in Freycinet’s chart to be fringed by coral-reefs; and mention is
+made of them in the accompanying “Hydrog. Memoir;” coloured
+red.—_Savu_, S.E. of Timor, appears in Flinders’ chart to be fringed;
+but I have not coloured it, as I do not know that the reefs are of
+coral.—_Sandalwood_ Island has, according to Horsburgh (vol. ii, p.
+607), a reef on its southern shore, four miles distant from the land;
+as the neighbouring sea is deep,
+and generally bold, this probably is a barrier-reef, but I have not
+ventured to colour it.
+
+N.W. COAST OF AUSTRALIA.—It appears, in Captain King’s Sailing
+Directions (“Narrative of Survey,” vol. ii, pp. 325-369), that there
+are many extensive coral-reefs skirting, often at considerable
+distances, the N.W. shores, and encompassing the small adjoining
+islets. Deep water, in no instance, is represented in the charts
+between these reefs and the land; and, therefore, they probably belong
+to the fringing class. But as they extend far into the sea, which is
+generally shallow, even in places where the land seems to be somewhat
+precipitous; I have not coloured them. Houtman’s Abrolhos (lat. 28° S.
+on west coast) have lately been surveyed by Captain Wickham (as
+described in _Naut. Mag._ 1841, p. 511): they lie on the edge of a
+steeply shelving bank, which extends about thirty miles seaward, along
+the whole line of coast. The two southern reefs, or islands, enclose a
+lagoon-like space of water, varying in depth from five to fifteen
+fathoms, and in one spot with twenty-three fathoms. The greater part of
+the island has been formed on their inland sides, by the accumulation
+of fragments of coral; the seaward face consisting of nearly bare
+ledges of rock. Some of the specimens, brought home by Captain Wickham,
+contained fragments of marine shells, but others did not; and these
+closely resembled a formation at King George’s Sound, principally due
+to the action of the wind on calcareous dust, which I shall describe in
+a forthcoming part. From the extreme irregularity of these reefs with
+their lagoons, and from their position on a bank, the usual depth of
+which is only thirty fathoms, I have not ventured to class them with
+atolls, and hence have left them uncoloured.—_Rowley Shoals._ These lie
+some way from the N.W. coast of Australia: according to Captain King
+(“Narrative of Survey,” vol. i, p. 60), they are of coral-formation.
+They rise abruptly from the sea, and Captain King had no bottom with
+170 fathoms close to them. Three of them are crescent-shaped; they are
+mentioned by Mr. Lyell, on the authority of Captain King, with
+reference to the direction of their open sides. “A third oval reef of
+the same group is entirely submerged” (“Principles of Geology,” book
+iii, chap. xviii); coloured blue.—_Scott’s Reefs_, lying north of
+Rowley Shoals, are briefly described by Captain Wickham (_Naut. Mag._
+1841, p. 440): they appear to be of great size, of a circular form, and
+“with smooth water within, forming probably a lagoon of great extent.”
+There is a break on the western side, where there probably is an
+entrance: the water is very deep off these reefs; coloured blue.
+
+Proceeding westward along the great volcanic chain of the East Indian
+Archipelago, _Solor Strait_ is represented in a chart published by
+Dalrymple from a Dutch MS., as fringed; as are parts of _Flores_, of
+_Adenara_, and of _Solor._ Horsburgh speaks of coral growing on these
+shores; and therefore I have no doubt that the reefs are of coral, and
+accordingly have coloured them red. We hear from Horsburgh (vol. ii, p.
+602) that a coral-flat bounds the shores of _Sapy_ Bay. From the same
+authority it appears (p. 610) that reefs fringe the island of _
+Timor-Young_, on the N. shore of Sumbawa; and, likewise (p. 600),
+that _Bally_ town in _Lombock_, is fronted by a reef, stretching along
+the shore at a distance of a hundred fathoms, with channels through it
+for boats; these places, therefore, have been coloured red.—_Bally_
+Island. In a Dutch MS. chart on a large scale of Java, which was
+brought from that island by Dr. Horsfield, who had the kindness to show
+it me at the India House, its western, northern, and southern shores
+appear very regularly fringed by a reef (see also Horsburgh, vol. ii,
+p. 593); and as coral is found abundantly there, I have not the least
+doubt that the reef is of coral, and therefore have coloured it red.
+
+JAVA.—My information regarding the reefs of this great island is
+derived from the chart just mentioned. The greater part of _Maduara_ is
+represented in it as regularly fringed, and likewise portions of the
+coast of Java immediately south of it. Dr. Horsfield informs me that
+coral is very abundant near _Sourabaya._ The islets and parts of the N.
+coast of Java, west of _Point Buang_, or _Japara_, are fringed by
+reefs, said to be of coral. _Lubeck_, or _Bavian_ Islands, lying at
+some distance from the shore of Java, are regularly fringed by
+coral-reefs. _Carimon Java_ appears equally so, though it is not
+directly said that the reefs are of coral; there is a depth between
+thirty and forty fathoms round these islands. Parts of the shores of
+_Sunda Strait_, where the water is from forty to eighty fathoms deep,
+and the islets near _Batavia_ appear in several charts to be fringed.
+In the Dutch chart the southern shore, in the narrowest part of the
+island, is in two places fringed by reefs of coral. West of _
+Segorrowodee_ Bay, and the extreme S.E. and E. portions are likewise
+fringed by coral-reefs; all the above-mentioned places coloured red.
+
+_Macassar Strait_; the east coast of Borneo appears, in most parts,
+free from reefs, and where they occur, as on the east coast of
+_Pamaroong_, the sea is very shallow; hence no part is coloured. In
+_Macassar_ Strait itself, in about lat. 2° S., there are many small
+islands with coral-shoals projecting far from them. There are also (old
+charts by Dalrymple) numerous little flats of coral, not rising to the
+surface of the water, and shelving suddenly from five fathoms to no
+bottom with fifty fathoms; they do not appear to have a lagoon-like
+structure. There are similar coral-shoals a little farther south; and
+in lat. 4° 55′ there are two, which are engraved from modern surveys,
+in a manner which might represent an annular reef with deep water
+inside: Captain Moresby, however, who was formerly in this sea, doubts
+this fact, so that I have left them uncoloured: at the same time I may
+remark, that these two shoals make a nearer approach to the atoll-like
+structure than any other within the E. Indian Archipelago. Southward of
+these shoals there are other low islands and irregular coral-reefs; and
+in the space of sea, north of the great volcanic chain, from Timor to
+Java, we have also other islands, such as the _Postillions, Kalatoa,
+Tokan-Bessees_, etc., which are chiefly low, and are surrounded by very
+irregular and distant reefs. From the imperfect charts I have seen, I
+have not been able to decide whether they belong to the atoll or
+barrier-classes, or whether they merely fringe submarine banks, and
+gently sloping land. In the Bay of _Bonin_, between the two southern
+arms of Celebes, there are numerous coral-reefs;
+but none of them seem to have an atoll-like structure. I have,
+therefore, not coloured any of the islands in this part of the sea; I
+think it, however, exceedingly probable that some of them ought to be
+blue. I may add that there is a harbour on the S.E. coast of _Bouton_
+which, according to an old chart, is formed by a reef, parallel to the
+shore, with deep water within; and in the “Voyage of the _Coquille_,”
+some neighbouring islands are represented with reefs a good way
+distant, but I do not know whether with deep water within. I have not
+thought the evidence sufficient to permit me to colour them.
+
+SUMATRA.—Commencing with the west coast and outlying islands, _Engano
+Island_ is represented in the published chart as surrounded by a narrow
+reef, and Napier, in his “Sailing Directions,” speaks of the reef being
+of coral (also Horsburgh, vol. ii, p. 115); coloured red.—_Rat Island_
+(3° 51′ S.) is surrounded by reefs of coral, partly dry at low water,
+(Horsburgh, vol. ii, p. 96).—_Trieste Island_ (4° 2′ S.). The shore is
+represented in a chart which I saw at the India House, as fringed in
+such a manner, that I feel sure the fringe consists of coral; but as
+the island is so low, that the sea sometimes flows quite over it
+(Dampier, “Voyage,” vol. i, p. 474), I have not coloured it.—_Pulo
+Dooa_ (lat. 3°). In an old chart it is said there are chasms in the
+reefs round the island, admitting boats to the watering-place, and that
+the southern islet consists of a mass of sand and coral.—_Pulo Pisang_;
+Horsburgh (vol. ii, p. 86) says that the rocky coral-bank, which
+stretches about forty yards from the shore, is steep to all round: in a
+chart, also, which I have seen, the island is represented as regularly
+fringed.—_Pulo Mintao_ is lined with reefs on its west side (Horsburgh,
+vol. ii, p. 107).—_Pulo Baniak_; the same authority (vol. ii, p. 105),
+speaking of a part, says it is faced with coral-rocks.—_Minguin_ (3°
+36′ N.). A coral-reef fronts this place, and projects into the sea
+nearly a quarter of a mile (“Notices of the Indian Arch.” published at
+Singapore, p. 105).—_Pulo Brassa_ (5° 46′ N.). A reef surrounds it at a
+cable’s length (Horsburgh, vol. ii, p. 60). I have coloured all the
+above-specified points red. I may here add, that both Horsburgh and Mr.
+Moor (in the “Notices” just alluded to) frequently speak of the
+numerous reefs and banks of coral on the west coast of Sumatra; but
+these nowhere have the structure of a barrier-reef, and Marsden
+(“History of Sumatra”) states, that where the coast is flat, the
+fringing-reefs extend furthest from it. The northern and southern
+points, and the greater part of the east coast, are low, and faced with
+mud banks, and therefore without coral.
+
+NICOBAR ISLANDS.—The chart represents the islands of this group as
+fringed by reefs. With regard to _Great Nicobar_, Captain Moresby
+informs me, that it is fringed by reefs of coral, extending between two
+and three hundred yards from the shore. The _Northern Nicobars_ appear
+so regularly fringed in the published charts, that I have no doubt the
+reefs are of coral. This group, therefore, is coloured red.
+
+ANDAMAN ISLANDS.—From an examination of the MS. chart, on a large
+scale, of this island, by Captain Arch. Blair, in the Admiralty,
+several portions of the coast appear fringed; and as Horsburgh speaks
+of coral-reefs being numerous in the vicinity of these islands, I
+should have
+coloured them red, had not some expressions in a paper in the “Asiatic
+Researches” (vol. iv, p. 402) led me to doubt the existence of reefs;
+uncoloured.
+
+The coast of _Malacca, Tenasserim_ and the coasts northward, appear in
+the greater part to be low and muddy: where reefs occur, as in parts of
+_Malacca Straits_, and near _ Singapore_, they are of the fringing
+kind; but the water is so shoal, that I have not coloured them. In the
+sea, however, between Malacca and the west coast of Borneo, where there
+is a greater depth from forty to fifty fathoms, I have coloured red
+some of the groups, which are regularly fringed. The northern _Natunas_
+and the _Anambas_ Islands are represented in the charts on a large
+scale, published in the “Atlas of the Voyage of the _ Favourite_,” as
+fringed by reefs of coral, with very shoal water within
+them.—_Tumbelan_ and _Bunoa_ Islands (1° N.) are represented in the
+English charts as surrounded by a very regular fringe.—_St. Barbes_ (0°
+15′ N.) is said by Horsburgh (vol. ii, p. 279) to be fronted by a reef,
+over which boats can land only at high water.—The shore of _Borneo_ at
+_Tunjong Apee_ is also fronted by a reef, extending not far from the
+land (Horsburgh, vol. ii, p. 468). These places I have coloured red;
+although with some hesitation, as the water is shallow. I might perhaps
+have added _Pulo Leat_, in Gaspar Strait, _Lucepara_, and _Carimata_;
+but as the sea is confined and shallow, and the reefs not very regular,
+I have left them uncoloured.
+
+The water shoals gradually towards the whole west coast of _ Borneo_: I
+cannot make out that it has any reefs of coral. The islands, however,
+off the northern extremity, and near the S.W. end of _Palawan_, are
+fringed by very distant coral-reefs; thus the reefs in the case of
+_Balabac_ are no less than five miles from the land; but the sea, in
+the whole of this district, is so shallow, that the reefs might be
+expected to extend very far from the land. I have not, therefore,
+thought myself authorised to colour them. The N.E. point of Borneo,
+where the water is very shoal, is connected with Magindanao by a chain
+of islands called the _Sooloo Archipelago_, about which I have been
+able to obtain very little information; _Pangootaran_, although ten
+miles long, entirely consists of a bed of coral-rock (“Notices of E.
+Indian Arch.” p. 58): I believe from Horsburgh that the island is low;
+not coloured.—_Tahow Bank_, in some old charts, appears like a
+submerged atoll; not coloured. Forrest (“Voyage,” p. 21) states that
+one of the islands near Sooloo is surrounded by coral-rocks; but there
+is no distant reef. Near the S. end of _ Basselan_, some of the islets
+in the chart accompanying Forrest’s “Voyage,” appear fringed with
+reefs; hence I have coloured, though unwillingly, parts of the Sooloo
+group red. The sea between Sooloo and Palawan, near the shoal coast of
+Borneo, is interspersed with irregular reefs and shoal patches; not
+coloured: but in the northern part of this sea, there are two low
+islets, _ Cagayanes_ and _Cavilli_, surrounded by extensive
+coral-reefs; the breakers round the latter (Horsburgh, vol. ii, p. 513)
+extend five or six miles from a sandbank, which forms the only dry
+part; these breakers are steep to outside; there appears to be an
+opening through them on one side, with four or five fathoms within:
+from this description, I strongly suspect that Cavilli
+ought to be considered an atoll; but, as I have not seen any chart of
+it, on even a moderately large scale, I have not coloured it. The
+islets off the northern end of _Palawan_, are in the same case as those
+off the southern end, namely they are fringed by reefs, some way
+distant from the shore, but the water is exceedingly shallow;
+uncoloured. The western shore of Palawan will be treated of under the
+head of China Sea.
+
+PHILIPPINE ARCHIPELAGO.—A chart on a large scale of _Appoo Shoal_,
+which lies near the S.E. coast of Mindoro, has been executed by Captain
+D. Ross: it appears atoll-formed, but with rather an irregular outline;
+its diameter is about ten miles; there are two well-defined passages
+leading into the interior lagoon, which appears open; close outside the
+reef all round, there is no bottom with seventy fathoms; coloured
+blue.—_Mindoro_: the N.W. coast is represented in several charts, as
+fringed by a reef, and _Luban Island_ is said, by Horsburgh (vol. ii,
+p. 436), to be “lined by a reef.”—_Luzon_: Mr. Cuming, who has lately
+investigated with so much success the Natural History of the
+Philippines, informs me, that about three miles of the shore north of
+Point St. Jago, is fringed by a reef; as are (Horsburgh, vol. ii, p.
+437) the Three Friars off Silanguin Bay. Between Point Capones and
+Playa Honda, the coast is “lined by a coral-reef, stretching out nearly
+a mile in some places,” (Horsburgh); and Mr. Cuming visited some
+fringing-reefs on parts of this coast, namely, near Puebla, Iba, and
+Mansinglor. In the neighbourhood of Solon-solon Bay, the shore is lined
+(Horsburgh, ii, p. 439) by coral-reefs, stretching out a great way:
+there are also reefs about the islets off Solamague; and as I am
+informed by Mr. Cuming, near St. Catalina, and a little north of it.
+The same gentleman informs me there are reefs on the S.E. point of this
+island in front of Samar, extending from Malalabon to Bulusan. These
+appear to be the principal fringing-reefs on the coasts of Luzon; and
+they have all been coloured red. Mr. Cuming informs me that none of
+them have deep water within; although it appears from Horsburgh that
+some few extend to a considerable distance from the shore. Within the
+Philippine Archipelago, the shores of the islands do not appear to be
+commonly fringed, with the exception of the S. shore of _ Masbate_, and
+nearly the whole of _Bohol_; which are both coloured red. On the S.
+shore of _Magindanao_, Bunwoot Island is surrounded (according to
+Forrest, “Voyage,” p. 253), by a coral-reef, which in the chart appears
+one of the fringing class. With respect to the eastern coasts of the
+whole Archipelago, I have not been able to obtain any account.
+
+BABUYAN ISLANDS.—Horsburgh says (vol. ii, p. 442), coral-reefs line the
+shores of the harbour in Fuga; and the charts show there are other
+reefs about these islands. Camiguin has its shore in parts lined by
+coral-rock (Horsburgh, p. 443); about a mile off shore there is between
+thirty and thirty-five fathoms. The plan of Port San Pio Quinto shows
+that its shores are fringed with coral; coloured red.—BASHEE ISLANDS:
+Horsburgh, speaking of the southern part of the group (vol. ii, p. 445)
+says the shores of both islands are fortified by a reef, and through
+some of the gaps in it, the natives can pass in their
+boats in fine weather; the bottom near the land is coral-rock. From the
+published charts, it is evident that several of these islands are most
+regularly fringed; coloured red. The northern islands are left
+uncoloured, as I have been unable to find any account of them.—FORMOSA.
+The shores, especially the western one, seem chiefly composed of mud
+and sand, and I cannot make out that they are anywhere lined by reefs;
+except in a harbour (Horsburgh, vol. ii, p. 449) at the extreme
+northern point: hence, of course, the whole of this island is left
+uncoloured. The small adjoining islands are in the same case.—PATCHOW,
+OR MADJIKO-SIMA GROUPS. _Patchuson_ has been described by Captain
+Broughton (“Voy. to the N. Pacific,” p. 191); he says, the boats, with
+some difficulty, found a passage through the coral-reefs, which extend
+along the coast, nearly half a mile off it. The boats were well
+sheltered within the reef; but it does not appear that the water is
+deep there. Outside the reef the depth is very irregular, varying from
+five to fifty fathoms; the form of the land is not very abrupt;
+coloured red.—_Taypin-san_; from the description given (p. 195) by the
+same author, it appears that a very irregular reef extends, to the
+distance of several miles, from the southern island; but whether it
+encircles a space of deep water is not evident; nor, indeed, whether
+these outlying reefs are connected with those more immediately
+adjoining the land; left uncoloured. I may here just add that the shore
+of _Kumi_ (lying west of Patchow), has a narrow reef attached to it in
+the plan of it, in La Peyrouse’s “Atlas;” but it does not appear in the
+account of the voyage that it is of coral; uncoloured.—LOO CHOO. The
+greater part of the coast of this moderately hilly island, is skirted
+by reefs, which do not extend far from the shore, and which do not
+leave a channel of deep water within them, as may be seen in the charts
+accompanying Captain B. Hall’s voyage to Loo Choo (see also remarks in
+Appendix, pp. xxi. and xxv.). There are, however, some ports with deep
+water, formed by reefs in front of valleys, in the same manner as
+happens at Mauritius. Captain Beechey, in a letter to me, compares
+these reefs with those encircling the Society Islands; but there
+appears to me a marked difference between them, in the less distance at
+which the Loo Choo reefs lie from the land with relation to the
+probable submarine inclination, and in the absence of an interior deep
+water-moat or channel, parallel to the land. Hence, I have classed
+these reefs with fringing-reefs, and coloured them red.—PESCADORES
+(west of Formosa). Dampier (vol. i, p. 416), has compared the
+appearance of the land to the southern parts of England. The islands
+are interlaced with coral-reefs; but as the water is very shoal, and as
+spits of sand and gravel (Horsburgh, vol. ii, p. 450) extend far out
+from them, it is impossible to draw any inferences regarding the nature
+of the reefs.
+
+CHINA SEA.—Proceeding from north to south, we first meet the _Pratas
+Shoal_ (lat. 20° N.) which, according to Horsburgh (vol. ii, p. 335),
+is composed of coral, is of a circular form, and has a low islet on it.
+The reef is on a level with the water’s edge, and when the sea runs
+high, there are breakers mostly all round, “but the water within seems
+pretty deep in some places; although steep-to in most parts outside,
+there appear to be several parts where a ship might find anchorage
+outside the breakers;” coloured blue.—The _ Paracells_ have been
+accurately surveyed by Captain D. Ross, and charts on a large scale
+published: but few low islets have been formed on these shoals, and
+this seems to be a general circumstance in the China Sea; the sea close
+outside the reefs is very deep; several of them have a lagoon-like
+structure; or separate islets (_Prattle, Robert, Drummond_, etc.) are
+so arranged round a moderately shallow space, as to appear as if they
+had once formed one large atoll.—_Bombay Shoal_ (one of the Paracells)
+has the form of an annular reef, and is “apparently deep within;” it
+seems to have an entrance (Horsburgh, vol. ii, p. 332) on its west
+side; it is very steep outside.—_Discovery Shoal_, also is of an oval
+form, with a lagoon-like space within, and three openings leading into
+it, in which there is a depth from two to twenty fathoms. Outside, at
+the distance (Horsburgh, vol. ii, p. 333) of only twenty yards from the
+reef, soundings could not be obtained. The Paracells are coloured
+blue.—_Macclesfield Bank_: this is a coral-bank of great size, lying
+east of the Paracells; some parts of the bank are level, with a sandy
+bottom, but, generally, the depth is very irregular. It is intersected
+by deep cuts or channels. I am not able to perceive in the published
+charts (its limits, however, are not very accurately known) whether the
+central part is deeper, which I suspect is the case, as in the Great
+Chagos Bank, in the Indian Ocean; not coloured.—_Scarborough Shoal_:
+this coral-shoal is engraved with a double row of crosses, forming a
+circle, as if there was deep water within the reef: close outside there
+was no bottom, with a hundred fathoms; coloured blue.—The sea off the
+west coast of Palawan and the northern part of Borneo is strewed with
+shoals: _Swallow Shoal_, according to Horsburgh (vol. ii, p. 431) “is
+formed, _like most_ of the shoals hereabouts, of a belt of coral-rocks,
+“with a basin of deep water within.”—_Half-Moon Shoal_ has a similar
+structure; Captain D. Ross describes it, as a narrow belt of
+coral-rock, with a basin of deep water in the centre,” and deep sea
+close outside.—_Bombay Shoal_ appears (Horsburgh, vol. ii, p. 432) “to
+be a basin of smooth water surrounded by breakers.” These three shoals
+I have coloured blue.—The _Paraquas Shoals_ are of a circular form,
+with deep gaps running through them; not coloured.—A bank gradually
+shoaling to the depth of thirty fathoms, extends to a distance of about
+twenty miles from the northern part of _Borneo_, and to thirty miles
+from the northern part of _Palawan._ Near the land this bank appears
+tolerably free from danger, but a little further out it is thickly
+studded with coral-shoals, which do not generally rise quite to the
+surface; some of them are very steep-to, and others have a fringe of
+shoal-water round them. I should have thought that these shoals had
+level surfaces, had it not been for the statement made by Horsburgh
+“that most of the shoals hereabouts are formed of a belt of coral.”
+But, perhaps that expression was more particularly applied to the
+shoals further in the offing. If these reefs of coral have a
+lagoon-like structure, they should have been coloured blue, and they
+would have formed an imperfect barrier in front of Palawan and the
+northern part of Borneo. But, as the water
+is not very deep, these reefs may have grown up from inequalities on
+the bank: I have not coloured them.—The coast of _China, Tonquin_, and
+_Cochin-China_, forming the western boundary of the China Sea, appear
+to be without reefs: with regard to the two last-mentioned coasts, I
+speak after examining the charts on a large scale in the “Atlas of the
+Voyage of the _ Favourite_.”
+
+INDIAN OCEAN.—_South Keeling_ atoll has been specially described. Nine
+miles north of it lies North Keeling, a very small atoll, surveyed by
+the _ Beagle_, the lagoon of which is dry at low water.—_Christmas
+Island_, lying to the east, is a high island, without, as I have been
+informed by a person who passed it, any reefs at all.—CEYLON: a space
+about eighty miles in length of the south-western and southern shores
+of these islands has been described by Mr. Twynam (_Naut. Mag._ 1836,
+pp. 365 and 518); parts of this space appear to be very regularly
+fringed by coral-reefs, which extend from a quarter to half a mile from
+the shore. These reefs are in places breached, and afford safe
+anchorage for the small trading craft. Outside, the sea gradually
+deepens; there is forty fathoms about six miles off shore: this part I
+have coloured red. In the published charts of Ceylon there appear to be
+fringing-reefs in several parts of the south-eastern shores, which I
+have also coloured red.—At Venloos Bay the shore is likewise fringed.
+North of Trincomalee there are also reefs of the same kind. The sea off
+the northern part of Ceylon is exceedingly shallow; and therefore I
+have not coloured the reefs which fringe portions of its shores, and
+the adjoining islets, as well as the Indian promontory of _Madura._
+
+CHAGOS, MALDIVA, AND LACCADIVE ARCHIPELAGOES.—These three great groups
+which have already been often noticed, are now well-known from the
+admirable surveys of Captain Moresby and Lieutenant Powell. The
+published charts, which are worthy of the most attentive examination,
+at once show that the _Chagos_ and _Maldiva_ groups are entirely formed
+of great atolls, or lagoon-formed reefs, surmounted by islets. In the
+_Laccadive_ group, this structure is less evident; the islets are low,
+not exceeding the usual height of coral-formations (see Lieutenant
+Wood’s account, _Geograph. Journ._, vol. vi, p. 29), and most of the
+reefs are circular, as may be seen in the published charts; and within
+several of them, as I am informed by Captain Moresby, there is deepish
+water; these, therefore, have been coloured blue. Directly north, and
+almost forming part of this group, there is a long, narrow, slightly
+curved bank, rising out of the depths of the ocean, composed of sand,
+shells, and decayed coral, with from twenty-three to thirty fathoms on
+it. I have no doubt that it has had the same origin with the other
+Laccadive banks; but as it does not deepen towards the centre I have
+not coloured it. I might have referred to other authorities regarding
+these three archipelagoes; but after the publication of the charts by
+Captain Moresby, to whose personal kindness in giving me much
+information I am exceedingly indebted, it would have been superfluous.
+
+_Sahia de Malha_ bank consists of a series of narrow banks, with from
+eight to sixteen fathoms on them; they are arranged in a semicircular
+manner, round a space about forty fathoms deep, which slopes on the
+S.E. quarter to unfathomable depths; they are steep-to on both sides,
+but more especially on the ocean-side. Hence this bank closely
+resembles in structure, and I may add from Captain Moresby’s
+information in composition, the Pitt’s Bank in the Chagos group; and
+the Pitt’s Bank, must, after what has been shown of the Great Chagos
+Bank, be considered as a sunken, half-destroyed atoll; hence coloured
+blue.—_Cargados Carajos Bank._ Its southern portion consists of a
+large, curved, coral-shoal, with some low islets on its eastern edge,
+and likewise some on the western side, between which there is a depth
+of about twelve fathoms. Northward, a great bank extends. I cannot
+(probably owing to the want of perfect charts) refer this reef and bank
+to any class;—therefore not coloured.—_Ile de Sable_ is a little
+island, lying west of C. Carajos, only some toises in height (“Voyage
+of the _Favourite_,” vol. i, p. 130); it is surrounded by reefs; but
+its structure is unintelligible to me. There are some small banks north
+of it, of which I can find no clear account.—_Mauritius._ The reefs
+round this island have been described in the chapter on fringing-reefs;
+coloured red.—_Rodriguez._ The coral-reefs here are exceedingly
+extensive; in one part they project even five miles from the shore. As
+far as I can make out, there is no deep-water moat within them; and the
+sea outside does not deepen very suddenly. The outline, however, of the
+land appears to be (“Life of Sir J. Makintosh,” vol. ii, p. 165) hilly
+and rugged. I am unable to decide whether these reefs belong to the
+barrier class; as seems probable from their great extension, or to the
+fringing class; uncoloured.—_Bourbon._ The greater part of the shores
+of this island are without reefs; but Captain Carmichael (Hooker’s
+“Bot. Misc.”) states that a portion, fifteen miles in length, on the
+S.E. side, is imperfectly fringed with coral reefs: I have not thought
+this sufficient to colour the island.
+
+SEYCHELLES.—The rocky islands of primary formation, composing this
+group, rise from a very extensive and tolerably level bank, having a
+depth between twenty and forty fathoms. In Captain Owen’s chart, and in
+that in the “Atlas of the Voyage of the _Favourite_,” it appears that
+the east side of _Mahe_ and the adjoining islands of _St. Anne_ and _
+Cerf_, are regularly fringed by coral-reefs. A portion of the S.E. part
+of _Curieuse Island_, the N., and part of the S.W. shore of _Praslin
+Island_, and the whole west side of _Digue Island_, appear fringed.
+From a MS. account of these islands by Captain F. Moresby, in the
+Admiralty, it appears that _ Silhouette_ is also fringed; he states
+that all these islands are formed of granite and quartz, that they rise
+abruptly from the sea, and that “coral-reefs have grown round them, and
+project for some distance.” Dr. Allan, of Forres, who visited these
+islands, informs me that there is no deep water between the reefs and
+the shore. The above specified points have been coloured red. _
+Amirantes Islands_: The small islands of this neighbouring group,
+according to the MS. account of them by Captain F. Moresby, are
+situated on an extensive bank; they consist of the debris of corals and
+shells; are only about twenty feet in height, and are environed by
+reefs, some attached to the shore, and some rather distant from it.—I
+have taken great pains to procure plans and
+information regarding the several islands lying between S.E. and S.W.
+of the Amirantes, and the Seychelles; relying chiefly on Captain F.
+Moresby and Dr. Allan, it appears that the greater number,
+namely—_Platte, Alphonse, Coetivi, Galega, Providence, St. Pierre,
+Astova, Assomption_, and _ Glorioso_, are low, formed of sand or
+coral-rock, and irregularly shaped; they are situated on very extensive
+banks, and are connected with great coral-reefs. Galega is said by Dr.
+Allan, to be rather higher than the other islands; and St. Pierre is
+described by Captain F. Moresby, as being cavernous throughout, and as
+not consisting of either limestone or granite. These islands, as well
+as the Amirantes, certainly are not atoll-formed, and they differ as a
+group from every other group with which I am acquainted; I have not
+coloured them; but probably the reefs belong to the fringing class.
+Their formation is attributed, both by Dr. Allan and Captain F.
+Moresby, to the action of the currents, here exceedingly violent, on
+banks, which no doubt have had an independent geological origin. They
+resemble in many respects some islands and banks in the West Indies,
+which owe their origin to a similar agency, in conjunction with an
+elevation of the entire area. In close vicinity to the several islands,
+there are three others of an apparently different nature: first, _Juan
+de Nova_, which appears from some plans and accounts to be an atoll;
+but from others does not appear to be so; not coloured. Secondly
+_Cosmoledo_; “this group consists of a ring of coral, ten leagues in
+circumference, and a quarter of a mile broad in some places, enclosing
+a magnificent lagoon, into which there did not appear a single opening”
+(Horsburgh, vol. i, p. 151); coloured blue. Thirdly, _Aldabra_; it
+consists of three islets, about twenty-five feet in height, with red
+cliffs (Horsburgh, vol. i, p. 176) surrounding a very shallow basin or
+lagoon. The sea is profoundly deep close to the shore. Viewing this
+island in a chart, it would be thought an atoll; but the foregoing
+description shows that there is something different in its nature; Dr.
+Allan also states that it is cavernous, and that the coral-rock has a
+vitrified appearance. Is it an upheaved atoll, or the crater of a
+volcano?—uncoloured.
+
+COMORO GROUP.—_Mayotta_, according to Horsburgh (vol. i, p. 216, 4th
+ed.), is completely surrounded by a reef, which runs at the distance of
+three, four, and in some places even five miles from the land; in an
+old chart, published by Dalrymple, a depth in many places of thirty-six
+and thirty-eight fathoms is laid down within the reef. In the same
+chart, the space of open water within the reef in some parts is even
+more than three miles wide: the land is bold and peaked; this island,
+therefore, is encircled by a well-characterised barrier-reef, and is
+coloured pale blue.—_Johanna_; Horsburgh says (vol. i, p. 217) this
+island from the N.W. to the S.W. point, is bounded by a reef, at the
+distance of two miles from the shore; in some parts, however, the reef
+must be attached, since Lieutenant Boteler (“Narr.” vol. i, p. 161)
+describes a passage through it, within which there is room only for a
+few boats. Its height, as I am informed by Dr. Allan, is about 3,500
+feet; it is very precipitous, and is composed of granite, greenstone,
+and quartz; coloured blue.—_Mohilla_; on the S. side of this island
+there is
+anchorage, in from thirty to forty-five fathoms, between a reef and the
+shore (Horsburgh, vol. i, p. 214); in Captain Owen’s chart of
+Madagascar, this island is represented as encircled; coloured
+blue.—_Great Comoro Island_ is, as I am informed by Dr. Allan, about
+8,000 feet high, and apparently volcanic; it is not regularly
+encircled; but reefs of various shapes and dimensions, jut out from
+every headland on the W., S., and S.E. coasts, inside of which reefs
+there are channels, often parallel with the shore, with deep water. On
+the north-western coasts the reefs appear attached to the shores. The
+land near the coast is in some places bold, but generally speaking it
+is flat; Horsburgh says (vol. i, p. 214) the water is profoundly deep
+close to the _shore_, from which expression I presume some parts are
+without reefs. From this description I apprehend the reef belongs to
+the barrier class; but I have not coloured it, as most of the charts
+which I have seen, represent the reefs round it as very much less
+extensive than round the other islands in the group.
+
+MADAGASCAR.—My information is chiefly derived from the published charts
+by Captain Owen, and the accounts given by him and by Lieutenant
+Boteler. Commencing at the S.W. extremity of the island; towards the
+northern part of the _Star Bank_ (in lat. 25° S.) the coast for ten
+miles is fringed by a reef; coloured red. The shore immediately S. of
+_St. Augustine’s Bay_ appears fringed; but _Tullear_ Harbour, directly
+N. of it, is formed by a narrow reef ten miles long, extending parallel
+to the shore, with from four to ten fathoms within it. If this reef had
+been more extensive, it must have been classed as a barrier-reef; but
+as the line of coast falls inwards here, a submarine bank perhaps
+extends parallel to the shore, which has offered a foundation for the
+growth of the coral; I have left this part uncoloured. From lat. 22°
+16′ to 21° 37′, the shore is fringed by coral-reefs (see Lieutenant
+Boteler’s “Narrative,” vol. ii, p. 106), less than a mile in width, and
+with shallow water within. There are outlying coral-shoals in several
+parts of the offing, with about ten fathoms between them and the shore,
+and the depth of the sea one mile and a half seaward, is about thirty
+fathoms. The part above specified is engraved on a large scale; and as
+in the charts on rather a smaller scale the same fringe of reef extends
+as far as lat. 33° 15′; I have coloured the whole of this part of the
+coast red. The islands of _Juan de Nova_ (in lat. 17° S.) appear in the
+charts on a large scale to be fringed, but I have not been able to
+ascertain whether the reefs are of coral; uncoloured. The main part of
+the west coast appears to be low, with outlying sandbanks, which,
+Lieutenant Boteler (vol. ii, p. 106) says, “are faced on the edge of
+deep water by a line of sharp-pointed coral-rocks.” Nevertheless I have
+not coloured this part, as I cannot make out by the charts that the
+coast itself is fringed. The headlands of _Narrenda_ and _Passandava_
+Bays (14° 40′) and the islands in front of _Radama Harbour_ are
+represented in the plans as regularly fringed, and have accordingly
+been coloured red. With respect to the _East coast of Madagascar_, Dr.
+Allan informs me in a letter, that the whole line of coast, from
+_Tamatave_, in 18° 12′, to _C. Amber_, at the extreme northern point of
+the island, is bordered by coral-reefs. The land is low, uneven,
+and gradually rising from the coast. From Captain Owen’s charts, also,
+the existence of these reefs, which evidently belong to the fringing
+class, on some parts, namely N. of _British Sound_, and near _Ngoncy_,
+of the above line of coast might have been inferred. Lieutenant Boteler
+(vol. i, p. 155) speaks of “the reef surrounding the island of _St.
+Mary’s_ at a small distance from the shore.” In a previous chapter I
+have described, from the information of Dr. Allan, the manner in which
+the reefs extend in N.E. lines from the headlands on this coast, thus
+sometimes forming rather deep channels within them, this seems caused
+by the action of the currents, and the reefs spring up from the
+submarine prolongations of the sandy headlands. The above specified
+portion of the coast is coloured red. The remaining S.E. portions do
+not appear in any published chart to possess reefs of any kind; and the
+Rev. W. Ellis, whose means of information regarding this side of
+Madagascar have been extensive, informs me he believes there are none.
+
+EAST COAST OF AFRICA.—Proceeding from the northern part, the coast
+appears, for a considerable space, without reefs. My information, I may
+here observe, is derived from the survey by Captain Owen, together with
+his narrative; and that by Lieutenant Boteler. At _Mukdeesha_ (10° 1′
+N.) there is a coral-reef extending four or five miles along the shore
+(Owen’s “Narr.” vol. i, p. 357) which in the chart lies at the distance
+of a quarter of a mile from the shore, and has within it from six to
+ten feet water: this then is a fringing-reef, and is coloured red. From
+_ Juba_, a little S. of the equator, to _Lamoo_ (in 2° 20′ S.) “the
+coast and islands are formed of madrepore” (Owen’s “Narrative,” vol. i,
+p. 363). The chart of this part (entitled _ Dundas Islands_), presents
+an extraordinary appearance; the coast of the mainland is quite
+straight and it is fronted at the average distance of two miles, by
+exceedingly narrow, straight islets, fringed with reefs. Within the
+chain of islets, there are extensive tidal flats and muddy bays, into
+which many rivers enter; the depths of these spaces varies from one to
+four fathoms—the latter depth not being common, and about twelve feet
+the average. Outside the chain of islets, the sea, at the distance of a
+mile, varies in depth from eight to fifteen fathoms. Lieutenant Boteler
+(“Narr.,” vol. i, p. 369) describes the muddy bay of _Patta_, which
+seems to resemble other parts of this coast, as fronted by small,
+narrow, level islets formed of decomposing coral, the margin of which
+is seldom of greater height than twelve feet, overhanging the rocky
+surface from which the islets rise. Knowing that the islets are formed
+of coral, it is, I think, scarcely possible to view the coast, and not
+at once conclude that we here see a fringing-reef, which has been
+upraised a few feet: the unusual depth of from two to four fathoms
+within some of these islets, is probably due to muddy rivers having
+prevented the growth of coral near the shore. There is, however, one
+difficulty on this view, namely, that before the elevation took place,
+which converted the reef into a chain of islets, the water must
+apparently have been still deeper; on the other hand it may be supposed
+that the formation of a nearly perfect barrier in front, of so large an
+extent of coast, would cause the currents (especially in front of the
+rivers), to deepen their muddy beds. When describing in
+the chapter on fringing-reefs, those of Mauritius, I have given my
+reasons for believing that the shoal spaces within reefs of this kind,
+must, in many instances, have been deepened. However this may be, as
+several parts of this line of coast are undoubtedly fringed by living
+reefs, I have coloured it red.—_Maleenda_ (3° 20′ S.). In the plan of
+the harbour, the south headland appears fringed; and in Owen’s chart on
+a larger scale, the reefs are seen to extend nearly thirty miles
+southward; coloured red.—_Mombas_ (4° 5′ S.). The island which forms
+the harbour, “is surrounded by cliffs of madrepore, capable of being
+rendered almost impregnable” (Owen’s “Narr.,” vol. i, p. 412). The
+shore of the mainland N. and S. of the harbour, is most regularly
+fringed by a coral-reef at a distance from half a mile to one mile and
+a quarter from the land; within the reef the depth is from nine to
+fifteen feet; outside the reef the depth at rather less than half a
+mile is thirty fathoms. From the charts it appears that a space about
+thirty-six miles in length, is here fringed; coloured red.—_Pemba_ (5°
+S.) is an island of coral-formation, level, and about two hundred feet
+in height (Owen’s “Narr.,” vol. i, p. 425); it is thirty-five miles
+long, and is separated from the mainland by a deep sea. The outer coast
+is represented in the chart as regularly fringed; coloured red. The
+mainland in front of Pemba is likewise fringed; but there also appear
+to be some outlying reefs with deep water between them and the shore. I
+do not understand their structure, either from the charts or the
+description, therefore have not coloured them.—_Zanzibar_ resembles
+Pemba in most respects; its southern half on the western side and the
+neighbouring islets are fringed; coloured red. On the mainland, a
+little S. of Zanzibar, there are some banks parallel to the coast,
+which I should have thought had been formed of coral, had it not been
+said (Boteler’s “Narr.,” vol. ii, p. 39) that they were composed of
+sand; not coloured.—_Latham’s Bank_ is a small island, fringed by
+coral-reefs; but being only ten feet high, it has not been
+coloured.—_Monfeea_ is an island of the same character as Pemba; its
+outer shore is fringed, and its southern extremity is connected with
+Keelwa Point on the mainland by a chain of islands fringed by reefs;
+coloured red. The four last-mentioned islands resemble in many respects
+some of the islands in the Red Sea, which will presently be
+described.—_Keelwa._ In a plan of the shore, a space of twenty miles N.
+and S. of this place is fringed by reefs, apparently of coral: these
+reefs are prolonged still further southward in Owen’s general chart.
+The coast in the plans of the rivers _Lindy_ and _Monghow_ (9° 59′ and
+10° 7′ S.) has the same structure; coloured red.—_Querimba Islands_
+(from 10° 40′ to 13° S.). A chart on a large scale is given of these
+islands; they are low, and of coral-formation (Boteler’s “Narr.,” vol.
+ii, p. 54); and generally have extensive reefs projecting from them
+which are dry at low water, and which on the outside rise abruptly from
+a deep sea: on their insides they are separated from the continent by a
+channel, or rather a succession of bays, with an average depth of ten
+fathoms. The small headlands on the continent also have coral-banks
+attached to them; and the Querimba islands and banks are placed on the
+lines of prolongation of these headlands, and are
+separated from them by very shallow channels. It is evident that
+whatever cause, whether the drifting of sediment or subterranean
+movements, produced the headlands, likewise produced, as might have
+been expected, submarine prolongations to them; and these towards their
+outer extremities, have since afforded a favourable basis for the
+growth of coral-reefs, and subsequently for the formation of islets. As
+these reefs clearly belong to the fringing class, the Querimba islands
+have been coloured red.—_Monabila_ (13° 32′ S.). In the plan of this
+harbour, the headlands outside are fringed by reefs apparently of
+coral; coloured red.—_Mozambique_ (150° S.) The outer part of the
+island on which the city is built, and the neighbouring islands, are
+fringed by coral-reefs; coloured red. From the description given in
+Owen’s “Narr.” (vol. i, p. 162), the shore from _ Mozambique_ to
+_Delagoa Bay_ appears to be low and sandy; many of the shoals and
+islets off this line of coast are of coral-formation; but from their
+small size and lowness, it is not possible, from the charts, to know
+whether they are truly fringed. Hence this portion of coast is left
+uncoloured, as are likewise those parts more northward, of which no
+mention has been made in the foregoing pages from the want of
+information.
+
+PERSIAN GULF.—From the charts lately published on a large scale by the
+East India Company, it appears that several parts, especially the
+southern shores of this gulf, are fringed by coral-reefs; but as the
+water is very shallow, and as there are numerous sandbanks, which are
+difficult to distinguish on the chart from reefs, I have not coloured
+the upper part red. Towards the mouth, however, where the water is
+rather deeper, the islands of _Ormuz_ and _Larrack_, appear so
+regularly fringed, that I have coloured them red. There are certainly
+no atolls in the Persian Gulf. The shores of _ Immaum_, and of the
+promontory forming the southern headland of the Persian Gulf, seem to
+be without reefs. The whole S.W. part (except one or two small patches)
+of _Arabia Felix_, and the shores of _Socotra_ appear from the charts
+and memoir of Captain Haines (_Geograph. Journ._, 1839, p. 125) to be
+without any reefs. I believe there are no extensive coral-reefs on any
+part of the coasts of _India_, except on the low promontory of _Madura_
+(as already mentioned) in front of Ceylon.
+
+RED SEA.—My information is chiefly derived from the admirable charts
+published by the East India Company in 1836, from personal
+communication with Captain Moresby, one of the surveyors, and from the
+excellent memoir, “Über die Natur der Corallen-Bänken des Rothen
+Meeres,” by Ehrenberg. The plains immediately bordering the Red Sea
+seem chiefly to consist of a sedimentary formation of the newer
+tertiary period. The shore is, with the exception of a few parts,
+fringed by coral-reefs. The water is generally profoundly deep close to
+the shore; but this fact, which has attracted the attention of most
+voyagers, seems to have no necessary connection with the presence of
+reefs; for Captain Moresby particularly observed to me, that, in lat.
+24° 10′ on the eastern side, there is a piece of coast, with very deep
+water close to it, without any reefs, but not differing in other
+respects from the usual nature of the coast-line. The most remarkable
+feature in the Red Sea
+is the chain of submerged banks, reefs, and islands, lying some way
+from the shore, chiefly on the eastern side; the space within being
+deep enough to admit a safe navigation in small vessels. The banks are
+generally of an oval form, and some miles in width; but some of them
+are very long in proportion to their width. Captain Moresby informs me
+that any one, who had not made actual plans of them, would be apt to
+think that they were much more elongated than they really are. Many of
+them rise to the surface, but the greater number lie from five to
+thirty fathoms beneath it, with irregular soundings on them. They
+consist of sand and living coral; coral on most of them, according to
+Captain Moresby, covering the greater part of their surface. They
+extend parallel to the shore, and they are not unfrequently connected
+in their middle parts by short transverse banks with the mainland. The
+sea is generally profoundly deep quite close to them, as it is near
+most parts of the coast of the mainland; but this is not universally
+the case, for between lat. 15° and 17° the water deepens quite
+gradually from the banks, both on the eastern and western shores,
+towards the middle of the sea. Islands in many parts arise from these
+banks; they are low, flat-topped, and consist of the same horizontally
+stratified formation with that forming the plain-like margin of the
+mainland. Some of the smaller and lower islands consist of mere sand.
+Captain Moresby informs me, that small masses of rock, the remnants of
+islands, are left on many banks where there is now no dry land.
+Ehrenberg also asserts that most of the islets, even the lowest, have a
+flat abraded basis, composed of the same tertiary formation: he
+believes that as soon as the surf wears down the protuberant parts of a
+bank, just beneath the level of the sea, the surface becomes protected
+from further abrasion by the growth of coral, and he thus accounts for
+the existence of so many banks standing on a level with the surface of
+this sea. It appears that most of the islands are certainly decreasing
+in size.
+
+The form of the banks and islands is most singular in the part just
+referred to, namely, from lat. 15° to 17°, where the sea deepens quite
+gradually: the _Dhalac_ group, on the western coast, is surrounded by
+an intricate archipelago of islets and shoals; the main island is very
+irregularly shaped, and it includes a bay seven miles long, by four
+across, in which no bottom was found with 252 feet: there is only one
+entrance into this bay, half a mile wide, and with an island in front
+of it. The submerged banks on the eastern coast, within the same
+latitudes, round _ Farsan Island_, are, likewise, penetrated by many
+narrow creeks of deep water; one is twelve miles long, in the form of a
+hatchet, in which, close to its broad upper end, soundings were not
+struck with 360 feet, and its entrance is only half a mile wide: in
+another creek of the same nature, but even with a more irregular
+outline, there was no bottom with 480 feet. The island of Farsan,
+itself, has as singular a form as any of its surrounding banks. The
+bottom of the sea round the Dhalac and Farsan Islands consists chiefly
+of sand and agglutinated fragments, but, in the deep and narrow creeks,
+it consists of mud; the islands themselves consist of thin,
+horizontally stratified, modern tertiary
+beds, containing but little broken coral,[2] their shores are fringed
+by living coral-reefs.
+
+ [2] Rüppell, “Reise in Abyssinie,” Band. i, S. 247.
+
+
+From the account given by Rüppell[3] of the manner in which Dhalac has
+been rent by fissures, the opposite sides of which have been unequally
+elevated (in one instance to the amount of fifty feet), it seems
+probable that its irregular form, as well as probably that of Farsan,
+may have been partly caused by unequal elevations; but, considering the
+general form of the banks, and of the deep-water creeks, together with
+the composition of the land, I think their configuration is more
+probably due in great part to strong currents having drifted sediment
+over an uneven bottom: it is almost certain that their form cannot be
+attributed to the growth of coral. Whatever may have been the precise
+origin of the Dhalac and Farsan Archipelagoes, the greater number of
+the banks on the eastern side of the Red Sea seem to have originated
+through nearly similar means. I judge of this from their similarity in
+configuration (in proof of which I may instance a bank on the east
+coast in lat. 22°; and although it is true that the northern banks
+generally have a less complicated outline), and from their similarity
+in composition, as may be observed in their upraised portions. The
+depth within the banks northward of lat. 17°, is usually greater, and
+their outer sides shelve more abruptly (circumstances which seem to go
+together) than in the Dhalac and Farsan Archipelagoes; but this might
+easily have been caused by a difference in the action of the currents
+during their formation: moreover, the greater quantity of living coral,
+which, according to Captain Moresby, exists on the northern banks,
+would tend to give them steeper margins.
+
+ [3] _Ibid_., S. 245.
+
+From this account, brief and imperfect as it is, we can see that the
+great chain of banks on the eastern coast, and on the western side in
+the southern portion, differ greatly from true barrier-reefs wholly
+formed by the growth of coral. It is indeed the direct conclusion of
+Ehrenberg (“Über die,” etc., pp. 45 and 51), that they are connected in
+their origin quite secondarily with the growth of coral; and he remarks
+that the islands off the coast of Norway, if worn down level with the
+sea, and merely coated with living coral, would present a nearly
+similar appearance. I cannot, however, avoid suspecting, from
+information given me by Dr. Malcolmson and Captain Moresby, that
+Ehrenberg has rather under-rated the influence of corals, in some
+places at least, on the formation of the tertiary deposits of the Red
+Sea.
+
+_The west coast of the Red Sea between lat. 19° and 22°._—There are, in
+this space, reefs, which, if I had known nothing of those in other
+parts of the Red Sea, I should unhesitatingly have considered as
+barrier-reefs; and, after deliberation, I have come to the same
+conclusion. One of these reefs, in 20° 15′, is twenty miles long, less
+than a mile in width (but expanding at the northern end into a disc),
+slightly sinuous, and extending parallel to the mainland at the
+distance of five miles from it, with very deep water within; in one
+spot soundings were not obtained with 205 fathoms. Some leagues further
+south, there is another linear reef, very narrow, ten miles long, with
+other small portions
+of reef, north and south, almost connected with it; and within this
+line of reefs (as well as outside) the water is profoundly deep. There
+are also some small linear and sickle-formed reefs, lying a little way
+out at sea. All these reefs are covered, as I am informed by Captain
+Moresby, by living corals. Here, then, we have all the characters of
+reefs of the barrier class; and in some outlying reefs we have an
+approach to the structure of atolls. The source of my doubts about the
+classification of these reefs, arises from having observed in the
+Dhalac and Farsan groups the narrowness and straightness of several
+spits of sand and rock: one of these spits in the Dhalac group is
+nearly fifteen miles long, only two broad, and it is bordered on each
+side with deep water; so that, if worn down by the surf, and coated
+with living corals, it would form a reef nearly similar to those within
+the space under consideration. There is, also, in this space (lat. 21°)
+a peninsula, bordered by cliffs, with its extremity worn down to the
+level of the sea, and its basis fringed with reefs: in the line of
+prolongation of this peninsula, there lies the island of _ Macowa_
+(formed, according to Captain Moresby, of the usual tertiary deposit),
+and some smaller islands, large parts of which likewise appear to have
+been worn down, and are now coated with living corals. If the removal
+of the strata in these several cases had been more complete, the reefs
+thus formed would have nearly resembled those barrier-like ones now
+under discussion. Notwithstanding these facts, I cannot persuade myself
+that the many very small, isolated, and sickle-formed reefs and others,
+long, nearly straight, and very narrow, with the water unfathomably
+deep close round them, could possibly have been formed by corals merely
+coating banks of sediment, or the abraded surfaces of irregularly
+shaped islands. I feel compelled to believe that the foundations of
+these reefs have subsided, and that the corals, during their upward
+growth, have given to these reefs their present forms: I may remark
+that the subsidence of narrow and irregularly-shaped peninsulas and
+islands, such as those existing on the coasts of the Red Sea, would
+afford the requisite foundations for the reefs in question.
+
+_The west coast from lat. 22° to 24°._—This part of the coast (north of
+the space coloured blue on the map) is fronted by an irregularly
+shelving bank, from about ten to thirty fathoms deep; numerous little
+reefs, some of which have the most singular shapes, rise from this
+bank. It may be observed, respecting one of them, in lat. 23° 10′, that
+if the promontory in lat. 24° were worn down to the level of the sea,
+and coated with corals, a very similar and grotesquely formed reef
+would be produced. Many of the reefs on this part of the coast may thus
+have originated; but there are some sickle, and almost atoll-formed
+reefs lying in deep water off the promontory in lat. 24°, which lead me
+to suppose that all these reefs are more probably allied to the barrier
+or atoll classes. I have not, however, ventured to colour this portion
+of coast. _On the west coast from lat. 19° to 17°_ (south of space
+coloured blue on the map), there are many low islets of very small
+dimensions, not much elongated, and rising out of great depths at a
+distance from the coast; these cannot be classed either with atolls, or
+barrier- or fringing-reefs. I may here remark that the outlying reefs
+on the west
+coast, between lat. 19° and 24°, are the only ones in the Red Sea,
+which approach in structure to the true atolls of the Indian and
+Pacific Oceans, but they present only imperfect miniature likenesses of
+them.
+
+_Eastern coast._—I have felt the greatest doubt about colouring any
+portion of this coast, north of the fringing-reefs round the Farsan
+Islands in 16° 10′. There are many small outlying coral-reefs along the
+whole line of coast; but as the greater number rise from banks not very
+deeply submerged (the formation of which has been shown to be only
+secondarily connected with the growth of coral), their origin may be
+due simply to the growth of knolls of corals, from an irregular
+foundation situated within a limited depth. But between lat. 18° and
+20°, there are so many linear, elliptic, and extremely small reefs,
+rising abruptly out of profound depths, that the same reasons, which
+led me to colour blue a portion of the west coast, have induced me to
+do the same in this part. There exist some small outlying reefs rising
+from deep water, north of lat. 20° (the northern limit coloured blue),
+on the east coast; but as they are not very numerous and scarcely any
+of them linear, I have thought it right to leave them uncoloured.
+
+In the _southern parts_ of the Red Sea, considerable spaces of the
+mainland, and of some of the Dhalac islands, are skirted by reefs,
+which, as I am informed by Captain Moresby, are of living coral, and
+have all the characters of the fringing class. As in these latitudes,
+there are no outlying linear or sickle-formed reefs, rising out of
+unfathomable depths, I have coloured these parts of the coast red. On
+similar grounds, I have coloured red the _northern parts of the western
+coast_ (north of lat. 24° 30′), and likewise the shores of the chief
+part of the _Gulf of Suez._ In the _Gulf of Acaba_, as I am informed by
+Captain Moresby there are no coral-reefs, and the water is profoundly
+deep.
+
+WEST INDIES.—My information regarding the reefs of this area, is
+derived from various sources, and from an examination of numerous
+charts; especially of those lately executed during the survey under
+Captain Owen, R.N. I lay under particular obligation to Captain Bird
+Allen, R.N., one of the members of the late survey, for many personal
+communications on this subject. As in the case of the Red Sea, it is
+necessary to make some preliminary remarks on the submerged banks of
+the West Indies, which are in some degree connected with coral-reefs,
+and cause considerable doubts in their classification. That large
+accumulations of sediment are in progress on the West Indian shores,
+will be evident to any one who examines the charts of that sea,
+especially of the portion north of a line joining Yucutan and Florida.
+The area of deposition seems less intimately connected with the
+debouchement of the great rivers, than with the course of the
+sea-currents; as is evident from the vast extension of the banks from
+the promontories of Yucutan and Mosquito.
+
+Besides the coast-banks, there are many of various dimensions which
+stand quite isolated; these closely resemble each other, they lie from
+two or three to twenty or thirty fathoms under water, and are composed
+of sand, sometimes firmly agglutinated, with little or no coral; their
+surfaces are smooth and nearly level, shelving only to the amount of a
+few fathoms, very gradually all round towards their edges, where they
+plunge abruptly into the unfathomable sea. This steep inclination of
+their sides, which is likewise characteristic of the coast-banks, is
+very remarkable: I may give as an instance, the Misteriosa Bank, on the
+edges of which the soundings change in 250 fathoms horizontal distance,
+from 11 to 210 fathoms; off the northern point of the bank of Old
+Providence, in 200 fathoms horizontal distance, the change is from 19
+to 152 fathoms; off the Great Bahama Bank, in 160 fathoms horizontal
+distance, the inclination is in many places from 10 fathoms to no
+bottom with 190 fathoms. On coasts in all parts of the world, where
+sediment is accumulating, something of this kind may be observed; the
+banks shelve very gently far out to sea, and then terminate abruptly.
+The form and composition of the banks standing in the middle parts of
+the W. Indian Sea, clearly show that their origin must be chiefly
+attributed to the accumulation of sediment; and the only obvious
+explanation of their isolated position is the presence of a nucleus,
+round which the currents have collected fine drift matter. Any one who
+will compare the character of the bank surrounding the hilly island of
+Old Providence, with those banks in its neighbourhood which stand
+isolated, will scarcely doubt that they surround submerged mountains.
+We are led to the same conclusion by examining the bank called Thunder
+Knoll, which is separated from the Great Mosquito Bank by a channel
+only seven miles wide, and 145 fathoms deep. There cannot be any doubt
+that the Mosquito Bank has been formed by the accumulation of sediment
+round the promontory of the same name; and Thunder Knoll resembles the
+Mosquito Bank, in the state of its surface submerged twenty fathoms, in
+the inclinations of its sides, in composition, and in every other
+respect. I may observe, although the remark is here irrelevant, that
+geologists should be cautious in concluding that all the outlyers of
+any formation have once been connected together, for we here see that
+deposits, doubtless of exactly the same nature, may be deposited with
+large valley-like spaces between them.
+
+Linear strips of coral-reefs and small knolls project from many of the
+isolated, as well as coast-banks; sometimes they occur quite
+irregularly placed, as on the Mosquito Bank, but more generally they
+form crescents on the windward side, situated some little distance
+within the outer edge of the banks:—thus on the Serranilla Bank they
+form an interrupted chain which ranges between two and three miles
+within the windward margin: generally they occur, as on Roncador,
+Courtown, and Anegada Banks, nearer the line of deep water. Their
+occurrence on the windward side is conformable to the general rule, of
+the efficient kinds of corals flourishing best where most exposed; but
+their position some way within the line of deep water I cannot explain,
+without it be, that a depth somewhat less than that close to the outer
+margin of the banks, is most favourable to their growth. Where the
+corals have formed a nearly continuous rim, close to the windward edge
+of a bank some fathoms submerged, the reef closely resembles an atoll;
+but if the bank surrounds an island (as in the case of Old Providence),
+the reef resembles an encircling barrier-reef. I should undoubtedly
+have classed some of these fringed banks as imperfect atolls, or
+barrier-reefs, if the sedimentary nature of their foundations had not
+been evident from the presence of other neighbouring banks, of similar
+forms and of similar composition, but without the crescent-like
+marginal reef: in the third chapter, I observed that probably some
+atoll-like reefs did exist, which had originated in the manner here
+supposed.
+
+Proofs of elevation within recent tertiary periods abound, as referred
+to in the sixth chapter, over nearly the whole area of the West Indies.
+Hence it is easy to understand the origin of the low land on the
+coasts, where sediment is now accumulating; for instance on the
+northern part of Yucutan, and on the N.E. part of Mosquito, where the
+land is low, and where extensive banks appear to be in progressive
+formation. Hence, also, the origin of the Great Bahama Banks, which are
+bordered on their western and southern edges by very narrow, long,
+singularly shaped islands, formed of sand, shells, and coral-rock, and
+some of them about a hundred feet in height, is easily explained by the
+elevation of banks fringed on their windward (western and southern)
+sides by coral-reefs. On this view, however, we must suppose either
+that the chief part of the surfaces of the great Bahama sandbanks were
+all originally deeply submerged, and were brought up to their present
+level by the same elevatory action, which formed the linear islands; or
+that during the elevation of the banks, the superficial currents and
+swell of the waves continued wearing them down and keeping them at a
+nearly uniform level: the level is not quite uniform; for, in
+proceeding from the N.W. end of the Bahama group towards the S.E. end,
+the depth of the banks increases, and the area of land decreases, in a
+very gradual and remarkable manner. The latter view, namely, that these
+banks have been worn down by the currents and swell during their
+elevation, seems to me the most probable one. It is, also, I believe,
+applicable to many banks, situated in widely distant parts of the West
+Indian Sea, which are wholly submerged; for, on any other view, we must
+suppose, that the elevatory forces have acted with astonishing
+uniformity.
+
+The shores of the Gulf of Mexico, for the space of many hundred miles,
+is formed by a chain of lagoons, from one to twenty miles in breadth
+(“Columbian Navigator,” p. 178, etc.), containing either fresh or salt
+water, and separated from the sea by linear strips of sand. Great
+spaces of the shores of Southern Brazil,[4] and of the United States
+from Long Island (as observed by Professor Rogers) to Florida have the
+same character. Professor Rogers, in his “Report to the British
+Association” (vol. iii, p. 13), speculates on the origin of these low,
+sandy, linear islets; he states that the layers of which they are
+composed are too homogeneous, and contain too large a proportion of
+shells, to permit the common supposition of their formation being
+simply due to matter thrown up, where it now lies, by the surf: he
+considers these islands as upheaved bars or shoals, which were
+deposited in lines where opposed currents met. It is evident that these
+islands and spits of sand parallel to the coast, and separated from it
+by shallow lagoons, have no necessary connection with coral-formations.
+But in Southern Florida, from the accounts I have received from persons
+who have resided there, the upraised islands seem to be formed of
+strata, containing a good deal of coral, and they are extensively
+fringed by living reefs; the channels within these islands are in some
+places between two and three miles wide, and five or six fathoms deep,
+though generally[5] they are less in depth than width. After having
+seen how frequently banks of sediment in the West Indian Sea are
+fringed by reefs, we can readily conceive that bars of sediment might
+be greatly aided in their formation along a line of coast, by the
+growth of corals; and such bars would, in that case, have a deceptive
+resemblance with true barrier-reefs.
+
+ [4] In the _London and Edinburgh Philosophical Journal,_ 1841, p. 257,
+ I have described a singular bar of sandstone lying parallel to the
+ coast off Pernambuco in Brazil, which probably is an analogous
+ formation.
+
+
+ [5] In the ordinary sea-charts, no lagoons appear on the coast of
+ Florida, north of 26°; but Major Whiting (_Silliman’s Journal_, vol.
+ xxxv, p. 54) says that many are formed by sand thrown up along the
+ whole line of coast from St. Augustine’s to Jupiter Inlet.
+
+Having now endeavoured to remove some sources of doubt in classifying
+the reefs of the West Indies, I will give my authorities for colouring
+such portions of the coast as I have thought myself warranted in doing.
+Captain Bird Allen informs me, that most of the islands on the _Bahama
+Banks_ are fringed, especially on their windward sides, with living
+reefs; and hence I have coloured those, which are thus represented in
+Captain Owen’s late chart, red. The same officer informs me, that the
+islands along the southern part of _Florida_ are similarly fringed;
+coloured red. CUBA: Proceeding along the northern coast, at the
+distance of forty miles from the extreme S.E. point, the shores are
+fringed by reefs, which extend westward for a space of 160 miles, with
+only a few breaks. Parts of these reefs are represented in the plans of
+the harbours on this coast by Captain Owen; and an excellent
+description is given of them by Mr. Taylor (Loudon’s “Mag. of Nat.
+Hist.,” vol. ix, p. 449); he states that they enclosed a space called
+the “_baxo_,” from half to three-quarters of a mile in width, with a
+sandy bottom, and a little coral. In most parts people can wade, at low
+water, to the reef; but in some parts the depth is between two and
+three fathoms. Close outside the reef, the depth is between six and
+seven fathoms; these well-characterised fringing-reefs are coloured
+red. Westward of longitude 77° 30′, on the northern side of Cuba, a
+great bank commences, which extends along the coast for nearly four
+degrees of longitude. In the place of its commencement, in its
+structure, and in the “_cays_,” or low islands on its edge, there is a
+marked correspondence (as observed by Humboldt, “Pers. Narr.,” vol.
+vii, p. 88) between it and the Great Bahama and Sal Banks, which lie
+directly in front. Hence one is led to attribute the same origin to
+both these sets of banks; namely, the accumulation of sediment,
+conjoined with an elevatory movement, and the growth of
+coral on their outward edges; those parts which appear fringed by
+living reefs are coloured red. Westward of these banks, there is a
+portion of coast apparently without reefs, except in the harbours, the
+shores of which seem in the published plans to be fringed. The
+_Colorado Shoals_ (see Captain Owen’s charts), and the low land at the
+western end of Cuba, correspond as closely in relative position and
+structure to the banks at the extreme point of Florida, as the banks
+above described on the north side of Cuba, do to the Bahamas, the depth
+within the islets and reefs on the outer edge of the _Colorados_, is
+generally between two and three fathoms, increasing to twelve fathoms
+in the southern part, where the bank becomes nearly open, without
+islets or coral-reefs; the portions which are fringed are coloured red.
+The southern shore of Cuba is deeply concave, and the included space is
+filled up with mud and sandbanks, low islands and coral-reefs. Between
+the mountainous _Isle of Pines_ and the southern shore of Cuba, the
+general depth is only between two and three fathoms; and in this part
+small islands, formed of fragmentary rock and broken madrepores
+(Humboldt, “Pers. Narr.,” vol. vii, pp. 51, 86 to 90, 291, 309, 320),
+rise abruptly, and just reach the surface of the sea. From some
+expressions used in the “Columbian Navigator” (vol. i, pt ii, p. 94),
+it appears that considerable spaces along the outer coast of Southern
+Cuba are bounded by cliffs of coral-rock, formed probably by the
+upheaval of coral-reefs and sandbanks. The charts represent the
+southern part of the Isle of Pines as fringed by reefs, which the
+“Columb. Navig.” says extend some way from the coast, but have only
+from nine to twelve feet water on them; these are coloured red.—I have
+not been able to procure any detailed description of the large groups
+of banks and “cays” further eastward on the southern side of Cuba;
+within them there is a large expanse, with a muddy bottom, from eight
+to twelve fathoms deep; although some parts of this line of coast are
+represented in the general charts of the West Indies, as fringed, I
+have not thought it prudent to colour them. The remaining portion of
+the south coast of Cuba appears to be without coral-reefs.
+
+YUCUTAN.—The N.E. part of the promontory appears in Captain Owen’s
+charts to be fringed; coloured red. The eastern coast, from 20° to 18°
+is fringed. South of lat. 18°, there commences the most remarkable reef
+in the West Indies: it is about one hundred and thirty miles in length,
+ranging in a N. and S. line, at an average distance of fifteen miles
+from the coast. The islets on it are all low, as I have been informed
+by Captain B. Allen; the water deepens suddenly on the outside of the
+reef, but not more abruptly than off many of the sedimentary banks:
+within its southern extremity (off _Honduras_) the depth is twenty-five
+fathoms; but in the more northern parts, the depth soon increases to
+ten fathoms, and within the northernmost part, for a space of twenty
+miles, the depth is only from one to two fathoms. In most of these
+respects we have the characteristics of a barrier-reef; nevertheless,
+from observing, first, that the channel within the reef is a
+continuation of a great irregular bay, which penetrates the mainland to
+the depth of fifty miles; and secondly, that considerable spaces of
+this barrier-like reef are
+described in the charts (for instance, in lat. 16° 45′ and 16° 12′) as
+formed of pure sand; and thirdly, from knowing that sediment is
+accumulating in many parts of the West Indies in banks parallel to the
+shore; I have not ventured to colour this reef as a barrier, without
+further evidence that it has really been formed by the growth of
+corals, and that it is not merely in parts a spit of sand, and in other
+parts a worn down promontory, partially coated and fringed by reefs; I
+lean, however, to the probability of its being a barrier-reef, produced
+by subsidence. To add to my doubts, immediately on the outside of this
+barrier-like reef, _Turneffe, Lighthouse_, and _Glover_ reefs are
+situated, and these reefs have so completely the form of atolls, that
+if they had occurred in the Pacific, I should not have hesitated about
+colouring them blue. _Turneffe Reef_ seems almost entirely filled up
+with low mud islets; and the depth within the other two reefs is only
+from one to three fathoms. From this circumstance and from their
+similarity in form, structure, and relative position, both to the bank
+called _Northern Triangles_, on which there is an islet between seventy
+and eighty feet, and to _Cozumel_ Island, the level surface of which is
+likewise between seventy and eighty feet in height, I consider it more
+probable that the three foregoing banks are the worn down bases of
+upheaved shoals, fringed with corals, than that they are true atolls,
+wholly produced by the growth of coral during subsidence; left
+uncoloured.
+
+In front of the eastern _Mosquito_ coast, there are between lat. 12°
+and 16° some extensive banks (already mentioned, p. 148), with high
+islands rising from their centres; and there are other banks wholly
+submerged, both of which kinds of banks are bordered, near their
+windward margins, by crescent-shaped coral-reefs. But it can hardly be
+doubted, as was observed in the preliminary remarks, that these banks
+owe their origin, like the great bank extending from the Mosquito
+promontory, almost entirely to the accumulation of sediment, and not to
+the growth of corals; hence I have not coloured them.
+
+_Cayman Island:_ this island appears in the charts to be fringed; and
+Captain B. Allen informs me that the reefs extend about a mile from the
+shore, and have only from five to twelve feet water within them;
+coloured red.—_Jamaica:_ judging from the charts, about fifteen miles
+of the S.E. extremity, and about twice that length on the S.W.
+extremity, and some portions on the S. side near Kingston and Port
+Royal, are regularly fringed, and therefore are coloured red. From the
+plans of some harbours on the N. side of Jamaica, parts of the coast
+appear to be fringed; but as these are not represented in the charts of
+the whole island, I have not coloured them.—_St. Domingo:_ I have not
+been able to obtain sufficient information, either from plans of the
+harbours, or from general charts, to enable me to colour any part of
+the coast, except sixty miles from Port de Plata westward, which seems
+very regularly fringed; many other parts, however, of the coast are
+probably fringed, especially towards the eastern end of the
+island.—_Puerto Rico:_ considerable portions of the southern, western,
+and eastern coasts, and some parts of the northern coast, appear in the
+charts to be fringed; coloured red.—Some miles in length of the
+southern side of the Island of _St. Thomas_ is fringed; most of the
+_Virgin Gorda_ Islands, as I am informed by Mr. Schomburgk, are
+fringed; the shores of _Anegada_, as well as the bank on which it
+stands, are likewise fringed; these islands have been coloured red. The
+greater part of the southern side of _Santa Cruz_ appears in the Danish
+survey to be fringed (see also Prof. Hovey’s account of this island, in
+_Silliman’s Journal_, vol. xxxv, p. 74); the reefs extend along the
+shore for a considerable space, and project rather more than a mile;
+the depth within the reef is three fathoms; coloured red.—The _
+Antilles_, as remarked by Von Buch (“Descrip. Iles Canaries,” p. 494),
+may be divided into two linear groups, the western row being volcanic,
+and the eastern of modern calcareous origin; my information is very
+defective on the whole group. Of the eastern islands, _Barbuda_ and the
+western coasts of _Antigua_ and _Mariagalante_ appear to be fringed:
+this is also the case with _Barbadoes_, as I have been informed by a
+resident; these islands are coloured red. On the shores of the Western
+Antilles, of volcanic origin, very few coral-reefs appear to exist. The
+island of _Martinique_, of which there are beautifully executed French
+charts, on a very large scale, alone presents any appearance worthy of
+special notice. The south-western, southern, and eastern coasts,
+together forming about half the circumference of the island, are
+skirted by very irregular banks, projecting generally rather less than
+a mile from the shore, and lying from two to five fathoms submerged. In
+front of almost every valley, they are breached by narrow, crooked,
+steep-sided passages. The French engineers ascertained by boring, that
+these submerged banks consisted of madreporitic rocks, which were
+covered in many parts by thin layers of mud or sand. From this fact,
+and especially from the structure of the narrow breaches, I think there
+can be little doubt that these banks once formed living reefs, which
+fringed the shores of the island, and like other reefs probably reached
+the surface. From some of these submerged banks reefs of living coral
+rise abruptly, either in small detached patches, or in lines parallel
+to, but some way within the outer edges of the banks on which they are
+based. Besides the above banks which skirt the shores of the island,
+there is on the eastern side a range of linear banks, similarly
+constituted, twenty miles in length, extending parallel to the coast
+line, and separated from it by a space between two and four miles in
+width, and from five to fifteen fathoms in depth. From this range of
+detached banks, some linear reefs of living coral likewise rise
+abruptly; and if they had been of greater length (for they do not front
+more than a sixth part of the circumference of the island), they would
+necessarily from their position have been coloured as barrier-reefs; as
+the case stands they are left uncoloured. I suspect that after a small
+amount of subsidence, the corals were killed by sand and mud being
+deposited on them, and the reefs being thus prevented from growing
+upwards, the banks of madreporitic rock were left in their present
+submerged condition.
+
+THE BERMUDA ISLANDS have been carefully described by Lieutenant Nelson,
+in an excellent Memoir in the “Geological Transactions” (vol. v, part
+i, p. 103). In the form of the bank or reef, on one side of which
+the islands stand, there is a close general resemblance to an atoll;
+but in the following respects there is a considerable
+difference,—first, in the margin of the reef not forming (as I have
+been informed by Mr. Chaffers, R.N.) a flat, solid surface, laid bare
+at low water, and regularly bounding the internal space of shallow
+water or lagoon; secondly, in the border of gradually shoaling water,
+nearly a mile and a half in width, which surrounds the entire outside
+of the reef (as is laid down in Captain Hurd’s chart); and thirdly, in
+the size, height, and extraordinary form of the islands, which present
+little resemblance to the long, narrow, simple islets, seldom exceeding
+half a mile in breadth, which surmount the annular reefs of almost all
+the atolls in the Indian and Pacific Oceans. Moreover, there are
+evident proofs (Nelson, _ Ibid_., p. 118), that islands similar to the
+existing ones, formerly extended over other parts of the reef. It
+would, I believe, be difficult to find a true atoll with land exceeding
+thirty feet in height; whereas, Mr. Nelson estimates the highest point
+of the Bermuda Islands to be 260 feet; if, however, Mr. Nelson’s view,
+that the whole of the land consists of sand drifted by the winds, and
+agglutinated together, were proved correct, this difference would be
+immaterial; but, from his own account (p. 118), there occur in one
+place, five or six layers of red earth, interstratified with the
+ordinary calcareous rock, and including stones too heavy for the wind
+to have moved, without having at the same time utterly dispersed every
+grain of the accompanying drifted matter. Mr. Nelson attributes the
+origin of these several layers, with their embedded stones, to as many
+violent catastrophes; but further investigation in such cases has
+generally succeeded in explaining phenomena of this kind by ordinary
+and simpler means. Finally, I may remark, that these islands have a
+considerable resemblance in shape to Barbuda in the West Indies, and to
+Pemba on the eastern coast of Africa, which latter island is about two
+hundred feet in height, and consists of coral-rock. I believe that the
+Bermuda Islands, from being fringed by living reefs, ought to have been
+coloured red; but I have left them uncoloured, on account of their
+general resemblance in external form to a lagoon-island or atoll.
+
+
+
+
+GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.
+
+CRITICAL INTRODUCTION.
+
+
+The preparation of the series of works published under the general
+title “Geology of the Voyage of the _Beagle_” occupied a great part of
+Darwin’s time during the ten years that followed his return to England.
+The second volume of the series, entitled “Geological Observations on
+Volcanic Islands, with Brief Notices on the Geology of Australia and
+the Cape of Good Hope,” made its appearance in 1844. The materials for
+this volume were collected in part during the outward voyage, when the
+_Beagle_ called at St. Jago in the Cape de Verde Islands, and St.
+Paul’s Rocks, and at Fernando Noronha, but mainly during the homeward
+cruise; then it was that the Galapagos Islands were surveyed, the Low
+Archipelago passed through, and Tahiti visited; after making calls at
+the Bay of Islands, in New Zealand, and also at Sydney, Hobart Town and
+King George’s Sound in Australia, the _Beagle_ sailed across the Indian
+Ocean to the little group of the Keeling or Cocos Islands, which Darwin
+has rendered famous by his observations, and thence to Mauritius;
+calling at the Cape of Good Hope on her way, the ship then proceeded
+successively to St. Helena and Ascension, and revisited the Cape de
+Verde Islands before finally reaching England.
+
+Although Darwin was thus able to gratify his curiosity by visits to a
+great number of very interesting volcanic districts, the voyage opened
+for him with a bitter disappointment. He had been reading Humboldt’s
+“Personal Narrative” during his last year’s residence in Cambridge, and
+had copied out from it long passages about Teneriffe. He was actually
+making inquiries as to the best means of visiting that island, when the
+offer was made to him to accompany Captain Fitzroy in the _Beagle._ His
+friend Henslow too, on parting with him, had given him the advice to
+procure and read the recently published first volume of the
+“Principles of Geology,” though he warned him against accepting the
+views advocated by its author. During the time the _ Beagle_ was
+beating backwards and forwards when the voyage commenced, Darwin,
+although hardly ever able to leave his berth, was employing all the
+opportunities which the terrible sea-sickness left him, in studying
+Humboldt and Lyell. We may therefore form an idea of his feelings when,
+on the ship reaching Santa Cruz, and the Peak of Teneriffe making its
+appearance among the clouds, they were suddenly informed that an
+outbreak of cholera would prevent any landing!
+
+Ample compensation for this disappointment was found, however, when the
+ship reached Porta Praya in St. Jago, the largest of the Cape de Verde
+Islands. Here he spent three most delightful weeks, and really
+commenced his work as a geologist and naturalist. Writing to his father
+he says, “Geologising in a volcanic country is most delightful; besides
+the interest attached to itself, it leads you into most beautiful and
+retired spots. Nobody but a person fond of Natural History can imagine
+the pleasure of strolling under cocoa-nuts in a thicket of bananas and
+coffee-plants, and an endless number of wild flowers. And this island,
+that has given me so much instruction and delight, is reckoned the most
+uninteresting place that we perhaps shall touch at during our voyage.
+It certainly is generally very barren, but the valleys are more
+exquisitely beautiful, from the very contrast. It is utterly useless to
+say anything about the scenery; it would be as profitable to explain to
+a blind man colours, as to a person who has not been out of Europe, the
+total dissimilarity of a tropical view. Whenever I enjoy anything, I
+always look forward to writing it down, either in my log-book (which
+increases in bulk), or in a letter; so you must excuse raptures, and
+those raptures badly expressed. I find my collections are increasing
+wonderfully, and from Rio I think I shall be obliged to send a cargo
+home.”
+
+The indelible impression made on Darwin’s mind by this first visit to a
+volcanic island, is borne witness to by a remarkable passage in the
+“Autobiography” written by him in 1876. “The geology of St. Jago is
+very striking, yet simple; a stream of lava formerly flowed over the
+bed of the sea, formed of triturated recent shells and corals, which it
+has baked into a hard white rock. Since then the whole island has been
+upheaved. But the line of white rock revealed to me a new and important
+fact, namely that there had been afterwards subsidence round the
+craters which had
+since been in action, and had poured forth lava. It then first dawned
+on me that I might perhaps write a book on the geology of the various
+countries visited, and this made me thrill with delight. That was a
+memorable hour to me, and how distinctly I can call to mind the low
+cliff of lava beneath which I rested, with the sun glaring hot, a few
+strange desert plants growing near and with living corals in the tidal
+pools at my feet.”
+
+Only five years before, when listening to poor Professor Jameson’s
+lectures on the effete Wernerianism, which at that time did duty for
+geological teaching, Darwin had found them “incredibly dull,” and he
+declared that “the sole effect they produced on me was a determination
+never so long as I lived to read a book on Geology, or in any way to
+study the science.”
+
+What a contrast we find in the expressions which he makes use of in
+referring to Geological Science, in his letters written home from the
+_Beagle_! After alluding to the delight of collecting and studying
+marine animals, he exclaims, “But Geology carries the day!” Writing to
+Henslow he says, “I am quite charmed with Geology, but, like the wise
+animal between two bundles of hay, I do not know which to like best;
+the old crystalline group of rocks, or the softer and more
+fossiliferous beds.” And just as the long voyage is about to come to a
+close he again writes, “I find in Geology a never-failing interest; as
+it has been remarked, it creates the same grand ideas respecting this
+world which Astronomy does for the Universe.” In this passage Darwin
+doubtless refers to a remark of Sir John Herschel’s in his admirable
+“Preliminary Discourse on the Study of Natural Philosophy,”—a book
+which exercised a most remarkable and beneficial influence on the mind
+of the young naturalist.
+
+If there cannot be any doubt as to the strong predilection in Darwin’s
+mind for geological studies, both during and after the memorable
+voyage, there is equally little difficulty in perceiving the school of
+geological thought which, in spite of the warnings of Sedgwick and
+Henslow, had obtained complete ascendancy over his mind. He writes in
+1876: “The very first place which I examined, namely St. Jago in the
+Cape de Verde Islands, showed me clearly the wonderful superiority of
+Lyell’s manner of treating Geology, compared with that of any other
+author, whose works I had with me, or ever afterwards read.” And again,
+“The science of Geology is enormously indebted to Lyell—more so, as I
+believe, than to any other man who ever lived . . . I am proud to
+remember that the first place, namely, St. Jago, in the Cape de
+Verde Archipelago, in which I geologised, convinced me of the infinite
+superiority of Lyell’s views over those advocated in any other work
+known to me.”
+
+The passages I have cited will serve to show the spirit in which Darwin
+entered upon his geological studies, and the perusal of the following
+pages will furnish abundant proofs of the enthusiasm, acumen, and
+caution with which his researches were pursued.
+
+Large collections of rocks and minerals were made by Darwin during his
+researches, and sent home to Cambridge, to be kept under the care of
+his faithful friend Henslow. After visiting his relations and friends,
+Darwin’s first care on his return to England was to unpack and examine
+these collections. He accordingly, at the end of 1836, took lodgings
+for three months in Fitzwilliam Street, Cambridge, so as to be near
+Henslow; and in studying and determining his geological specimens
+received much valuable aid from the eminent crystallographer and
+mineralogist, Professor William Hallows Miller.
+
+The actual writing of the volume upon volcanic islands was not
+commenced till 1843, when Darwin had settled in the spot which became
+his home for the rest of his life—the famous house at Down, in Kent.
+Writing to his friend Mr. Fox, on March 28th, 1843, he says, “I am very
+slowly progressing with a volume, or rather pamphlet, on the volcanic
+islands which we visited: I manage only a couple of hours per day, and
+that not very regularly. It is uphill work writing books, which cost
+money in publishing, and which are not read even by geologists.”
+
+The work occupied Darwin during the whole of the year 1843, and was
+issued in the spring of the following year, the actual time engaged in
+preparing it being recorded in his diary as “from the summer of 1842 to
+January 1844;” but the author does not appear to have been by any means
+satisfied with the result when the book was finished. He wrote to
+Lyell, “You have pleased me much by saying that you intend looking
+through my ‘Volcanic Islands;’ it cost me eighteen months!!! and I have
+heard of very few who have read it. Now I shall feel, whatever little
+(and little it is) there is confirmatory of old work, or new, will work
+its effect and not be lost.” To Sir Joseph Hooker he wrote, “I have
+just finished a little volume on the volcanic islands which we visited.
+I do not know how far you care for dry simple geology, but I hope you
+will let me send you a copy.”
+
+Every geologist knows how full of interest and suggestiveness is
+this book of Darwin’s on volcanic islands. Probably the scant
+satisfaction which its author seemed to find in it may be traced to the
+effect of a contrast which he felt between the memory of glowing
+delights he had experienced when, hammer in hand, he roamed over new
+and interesting scenes, and the slow, laborious, and less congenial
+task of re-writing and arranging his notes in book-form.
+
+In 1874, in writing an account of the ancient volcanoes of the
+Hebrides, I had frequent occasion to quote Mr. Darwin’s observations on
+the Atlantic volcanoes, in illustration of the phenomena exhibited by
+the relics of still older volcanoes in our own islands. Darwin, in
+writing to his old friend Sir Charles Lyell upon the subject, says, “I
+was not a little pleased to see my volcanic book quoted, for I thought
+it was completely dead and forgotten.”
+
+Two years later the original publishers of this book and of that on
+South America proposed to re-issue them. Darwin at first hesitated, for
+he seemed to think there could be little of abiding interest in them;
+he consulted me upon the subject in one of the conversations which I
+used to have with him at that time, and I strongly urged upon him the
+reprint of the works. I was much gratified when he gave way upon the
+point, and consented to their appearing just as originally issued. In
+his preface he says, “Owing to the great progress which Geology has
+made in recent times, my views on some few points may be somewhat
+antiquated, but I have thought it best to leave them as they originally
+appeared.”
+
+It may be interesting to indicate, as briefly as possible, the chief
+geological problem upon which the publication of Darwin’s “Volcanic
+Islands” threw new and important light. The merit of the work consisted
+in supplying interesting observations, which in some cases have proved
+of crucial value in exploding prevalent fallacies; in calling attention
+to phenomena and considerations that had been quite overlooked by
+geologists, but have since exercised an important influence in moulding
+geological speculation; and lastly in showing the importance which
+attaches to small and seemingly insignificant causes, some of which
+afford a key to the explanation of very curious geological problems.
+
+Visiting as he did the districts in which Von Buch and others had found
+what they thought to be evidence of the truth of “Elevation-craters,”
+Darwin was able to show that the facts were capable of a totally
+different interpretation. The views originally put forward by the old
+German geologist and traveller, and almost
+universally accepted by his countrymen, had met with much support from
+Elie de Beaumont and Dufrenoy, the leaders of geological thought in
+France. They were, however, stoutly opposed by Scrope and Lyell in this
+country, and by Constant Prevost and Virlet on the other side of the
+channel. Darwin, in the work before us, shows how little ground there
+is for the assumption that the great ring-craters of the Atlantic
+islands have originated in gigantic blisters of the earth’s surface
+which, opening at the top, have given origin to the craters. Admitting
+the influence of the injection of lava into the structure of the
+volcanic cones, in increasing their bulk and elevation, he shows that,
+in the main, the volcanoes are built up by repeated ejections causing
+an accumulation of materials around the vent.
+
+While, however, agreeing on the whole with Scrope and Lyell, as to the
+explosive origin of ordinary volcanic craters, Darwin clearly saw that,
+in some cases, great craters might be formed or enlarged, by the
+subsidence of the floors after eruptions. The importance of this
+agency, to which too little attention has been directed by geologists,
+has recently been shown by Professor Dana, in his admirable work on
+Kilauea and the other great volcanoes of the Hawaiian Archipelago.
+
+The effects of subsidence at a volcanic centre in producing a downward
+dip of the strata around it, was first pointed out by Darwin, as the
+result of his earliest work in the Cape de Verde Islands. Striking
+illustrations of the same principle have since been pointed out by M.
+Robert and others in Iceland, by Mr. Heaphy in New Zealand, and by
+myself in the Western Isles of Scotland.
+
+Darwin again and again called attention to the evidence that volcanic
+vents exhibit relations to one another which can only be explained by
+assuming the existence of lines of fissure in the earth’s crust, along
+which the lavas have made their way to the surface. But he, at the same
+time, clearly saw that there was no evidence of the occurrence of great
+deluges of lava along such fissures; he showed how the most remarkable
+plateaux, composed of successive lava sheets, might be built up by
+repeated and moderate ejections from numerous isolated vents; and he
+expressly insists upon the rapidity with which the cinder-cones around
+the orifices of ejection and the evidences of successive outflows of
+lava would be obliterated by denudation.
+
+
+One of the most striking parts of the book is that in which he deals
+with the effects of denudation in producing “basal wrecks”
+or worn down stumps of volcanoes. He was enabled to examine a series of
+cases in which could be traced every gradation, from perfect volcanic
+cones down to the solidified plugs which had consolidated in the vents
+from which ejections had taken place. Darwin’s observations on these
+points have been of the greatest value and assistance to all who have
+essayed to study the effects of volcanic action during earlier periods
+of the earth’s history. Like Lyell, he was firmly persuaded of the
+continuity of geological history, and ever delighted in finding
+indications, in the present order of nature, that the phenomena of the
+past could be accounted for by means of causes which are still in
+operation. Lyell’s last work in the field was carried on about his home
+in Forfarshire, and only a few months before his death he wrote to
+Darwin: “All the work which I have done has confirmed me in the belief
+that the only difference between Palæozoic and recent volcanic rocks is
+no more than we must allow for, by the enormous time to which the
+products of the oldest volcanoes have been subjected to chemical
+changes.”
+
+Darwin was greatly impressed, as the result of his studies of volcanic
+phenomena, followed by an examination of the great granite-masses of
+the Andes, with the relations between the so-called Plutonic rocks and
+those of undoubtedly volcanic origin. It was indeed a fortunate
+circumstance, that after studying some excellent examples of recent
+volcanic rocks, he proceeded to examine in South America many fine
+illustrations of the older igneous rock-masses, and especially of the
+most highly crystalline types of the same, and then on his way home had
+opportunities of reviving the impression made upon him by the fresh and
+unaltered volcanic rocks. Some of the general considerations suggested
+by these observations were discussed in a paper read by him before the
+Geological Society, on March 7th, 1838, under the title “On the
+Connection of Certain Volcanic Phenomena, and On the Formation of
+Mountain-chains, and the Effect of Continental Elevations.” The exact
+bearing of these two classes of facts upon one another are more fully
+discussed in his book on South American geology.
+
+The proofs of recent elevation around many of the volcanic islands led
+Darwin to conclude that volcanic areas were, as a rule, regions in
+which upward movements were taking place, and he was naturally led to
+contrast them with the areas in which, as he showed, the occurrence of
+atolls, encircling reefs, and barrier-reefs afford indication of
+subsidence. In this way he was able to
+map out the oceanic areas in different zones, along which opposite
+kinds of movement were taking place. His conclusions on this subject
+were full of novelty and suggestiveness.
+
+Very clearly did Darwin recognise the importance of the fact that most
+of the oceanic islands appear to be of volcanic origin, though he was
+careful to point out the remarkable exceptions which somewhat
+invalidate the generalisation. In his “Origin of Species” he has
+elaborated the idea and suggested the theory of the permanence of
+ocean-basins, a suggestion which has been adopted and pushed farther by
+subsequent authors, than we think its originator would have approved.
+His caution and fairness of mind on this and similar speculative
+questions was well-known to all who were in the habit of discussing
+them with him.
+
+
+Some years before the voyage of the _Beagle,_ Mr. Poulett Scrope had
+pointed out the remarkable analogies that exist between certain igneous
+rocks of banded structure, as seen in the Ponza Islands, and the
+foliated crystalline schists. It does not appear that Darwin was
+acquainted with this remarkable memoir, but quite independently he
+called attention to the same phenomena when he came to study some very
+similar rocks which occur in the island of Ascension. Coming fresh from
+the study of the great masses of crystalline schist in the South
+American continent, he was struck by the circumstance that in the
+undoubtedly igneous rocks of Ascension we find a similar separation of
+the constituent minerals along parallel “folia.” These observations led
+Darwin to the same conclusion as that arrived at some time before by
+Scrope—namely that when crystallisation takes place in rock masses
+under the influence of great deforming stresses, a separation and
+parallel arrangement of the constituent minerals will result. This is a
+process which is now fully recognised as having been a potent factor in
+the production of the metamorphic rock, and has been called by more
+recent writers “dynamo-metamorphism.”
+
+In this, and in many similar discussions, in which exact mineralogical
+knowledge was required, it is remarkable how successful Darwin was in
+making out the true facts with regard to the rocks he studied by the
+simple aid of a penknife and pocket-lens, supplemented by a few
+chemical tests and the constant use of the blowpipe. Since his day, the
+method of study of rocks by thin sections under the microscope has been
+devised, and has become a most efficient aid in all petrographical
+inquiries. During the voyage of H.M.S. _Challenger,_ many of the
+islands studied by
+Darwin have been revisited and their rocks collected. The results of
+their study by one of the greatest masters of the science of
+micropetrography—Professor Renard of Brussels—have been recently
+published in one of the volumes of “Reports on the _Challenger_
+Expedition.” While much that is new and valuable has been contributed
+to geological science by these more recent investigations, and many
+changes have been made in nomenclature and other points of detail, it
+is interesting to find that all the chief facts described by Darwin and
+his friend Professor Miller have stood the test of time and further
+study, and remain as a monument of the acumen and accuracy in minute
+observation of these pioneers in geological research.
+
+JOHN W. JUDD.
+
+
+
+
+Chapter I ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.
+
+
+Rocks of the lowest series.—A calcareous sedimentary deposit, with
+recent shells, altered by the contact of superincumbent lava, its
+horizontality and extent.—Subsequent volcanic eruptions, associated
+with calcareous matter in an earthy and fibrous form, and often
+enclosed within the separate cells of the scoriæ.—Ancient and
+obliterated orifices of eruption of small size. Difficulty of tracing
+over a bare plain recent streams of lava.—Inland hills of more ancient
+volcanic rock.—Decomposed olivine in large masses. Feldspathic rocks
+beneath the upper crystalline basaltic strata. Uniform structure and
+form of the more ancient volcanic hills.—Form of the valleys near the
+coast. Conglomerate now forming on the sea beach.
+
+
+The island of St. Jago extends in a N.N.W. and S.S.E. direction, thirty
+miles in length by about twelve in breadth. My observations, made
+during two visits, were confined to the southern portion within the
+distance of a few leagues from Porto Praya. The country, viewed from
+the sea, presents a varied outline: smooth conical hills of a reddish
+colour (like Red Hill in Fig. 1[1]) and others less regular,
+flat-topped, and of a blackish colour (like A, B, C,) rise from
+successive, step-formed plains of lava. At a distance, a chain of
+mountains, many thousand feet in height, traverses the interior of the
+island. There is no active volcano in St. Jago, and only one in the
+group, namely at Fogo. The island since being inhabited has not
+suffered from destructive earthquakes.
+
+ [1] The outline of the coast, the position of the villages,
+ streamlets, and of most of the hills in this woodcut, are copied from
+ the chart made on board H.M.S. _Leven._ The square-topped hills (A, B,
+ C, etc.) are put in merely by eye, to illustrate my description.
+
+The lowest rocks exposed on the coast near Porto Praya, are highly
+crystalline and compact; they appear to be of ancient, submarine,
+volcanic origin; they are unconformably covered by a thin, irregular,
+calcareous deposit, abounding with shells of a late tertiary period;
+and this again is capped by a wide sheet of basaltic lava, which has
+flowed in successive streams from the interior of the island, between
+the square-topped hills marked A, B, C, etc. Still more recent streams
+of lava have been erupted from the scattered cones, such as Red and
+Signal Post Hills. The upper strata of the square-topped hills are
+intimately related in mineralogical composition, and in other respects,
+with the lowest series of the coast-rocks, with which they seem to be
+continuous.
+
+[Illustration: Part of St. Jago, one of the Cape de Verde islands.]
+
+Part of St. Jago, one of the Cape de Verde islands.
+
+_Mineralogical description of the rocks of the lowest series._—These
+rocks possess an extremely varying character; they consist of black,
+brown, and grey, compact, basaltic bases, with numerous crystals of
+augite, hornblende, olivine, mica, and sometimes glassy feldspar. A
+common variety is almost entirely composed of crystals of augite with
+olivine. Mica, it is known, seldom occurs where augite abounds; nor
+probably does the present case offer a real exception, for the mica (at
+least in my best characterised specimen, in which one nodule of this
+mineral is nearly half an inch in length) is as perfectly rounded as a
+pebble in a conglomerate, and evidently has not been crystallised in
+the base, in which it is now enclosed, but has proceeded from the
+fusion of some pre-existing rock. These compact lavas alternate with
+tuffs, amygdaloids, and wacke, and in some places with coarse
+conglomerate. Some of the argillaceous wackes are of a dark green
+colour, others, pale yellowish-green, and others nearly white; I was
+surprised to find that some of the latter varieties, even where
+whitest, fused into a jet black enamel, whilst some of the green
+varieties afforded only a pale gray bead. Numerous dikes, consisting
+chiefly of highly compact augitic rocks, and of gray amygdaloidal
+varieties, intersect the strata, which have in several places been
+dislocated with considerable violence, and thrown into highly inclined
+positions. One line of disturbance crosses the northern end of Quail
+Island (an islet in the Bay of Porto Praya), and can be followed to the
+mainland. These disturbances took place before the deposition of the
+recent sedimentary bed; and the
+surface, also, had previously been denuded to a great extent, as is
+shown by many truncated dikes.
+
+_Description of the calcareous deposit overlying the foregoing volcanic
+rocks._—This stratum is very conspicuous from its white colour, and
+from the extreme regularity with which it ranges in a horizontal line
+for some miles along the coast. Its average height above the sea,
+measured from the upper line of junction with the superincumbent
+basaltic lava, is about sixty feet; and its thickness, although varying
+much from the inequalities of the underlying formation, may be
+estimated at about twenty feet. It consists of quite white calcareous
+matter, partly composed of organic _débris_, and partly of a substance
+which may be aptly compared in appearance with mortar. Fragments of
+rock and pebbles are scattered throughout this bed, often forming,
+especially in the lower part, a conglomerate. Many of the fragments of
+rock are whitewashed with a thin coating of calcareous matter. At Quail
+Island, the calcareous deposit is replaced in its lowest part by a
+soft, brown, earthy tuff, full of Turritellæ; this is covered by a bed
+of pebbles, passing into sandstone, and mixed with fragments of echini,
+claws of crabs, and shells; the oyster-shells still adhering to the
+rock on which they grew. Numerous white balls appearing like pisolitic
+concretions, from the size of a walnut to that of an apple, are
+embedded in this deposit; they usually have a small pebble in their
+centres. Although so like concretions, a close examination convinced me
+that they were Nulliporæ, retaining their proper forms, but with their
+surfaces slightly abraded: these bodies (plants as they are now
+generally considered to be) exhibit under a microscope of ordinary
+power, no traces of organisation in their internal structure. Mr.
+George R. Sowerby has been so good as to examine the shells which I
+collected: there are fourteen species in a sufficiently perfect
+condition for their characters to be made out with some degree of
+certainty, and four which can be referred only to their genera. Of the
+fourteen shells, of which a list is given in the Appendix, eleven are
+recent species; one, though undescribed, is perhaps identical with a
+species which I found living in the harbour of Porto Praya; the two
+remaining species are unknown, and have been described by Mr. Sowerby.
+Until the shells of this Archipelago and of the neighbouring coasts are
+better known, it would be rash to assert that even these two latter
+shells are extinct. The number of species which certainly belong to
+existing kinds, although few in number, are sufficient to show that the
+deposit belongs to a late tertiary period. From its mineralogical
+character, from the number and size of the embedded fragments, and from
+the abundance of Patellæ, and other littoral shells, it is evident that
+the whole was accumulated in a shallow sea, near an ancient coast-line.
+
+_Effects produced by the flowing of the superincumbent basaltic lava
+over the calcareous deposit._—These effects are very curious. The
+calcareous matter is altered to the depth of about a foot beneath the
+line of junction; and a most perfect gradation can be traced, from
+loosely aggregated, small, particles of shells, corallines, and
+Nulliporæ, into a rock, in which not a trace of mechanical origin can
+be discovered,
+even with a microscope. Where the metamorphic change has been greatest,
+two varieties occur. The first is a hard, compact, white, fine-grained
+rock, striped with a few parallel lines of black volcanic particles,
+and resembling a sandstone, but which, upon close examination, is seen
+to be crystallised throughout, with the cleavages so perfect that they
+can be readily measured by the reflecting goniometer. In specimens,
+where the change has been less complete, when moistened and examined
+under a strong lens, the most interesting gradation can be traced, some
+of the rounded particles retaining their proper forms, and others
+insensibly melting into the granulo-crystalline paste. The weathered
+surface of this stone, as is so frequently the case with ordinary
+limestones, assumes a brick-red colour.
+
+The second metamorphosed variety is likewise a hard rock, but without
+any crystalline structure. It consists of a white, opaque, compact,
+calcareous stone, thickly mottled with rounded, though regular, spots
+of a soft, earthy, ochraceous substance. This earthy matter is of a
+pale yellowish-brown colour, and appears to be a mixture of carbonate
+of lime with iron; it effervesces with acids, is infusible, but
+blackens under the blowpipe, and becomes magnetic. The rounded form of
+the minute patches of earthy substance, and the steps in the progress
+of their perfect formation, which can be followed in a suit of
+specimens, clearly show that they are due either to some power of
+aggregation in the earthy particles amongst themselves, or more
+probably to a strong attraction between the atoms of the carbonate of
+line, and consequently to the segregation of the earthy extraneous
+matter. I was much interested by this fact, because I have often seen
+quartz rocks (for instance, in the Falkland Islands, and in the lower
+Silurian strata of the Stiper-stones in Shropshire), mottled in a
+precisely analogous manner, with little spots of a white, earthy
+substance (earthy feldspar?); and these rocks, there was good reason to
+suppose, had undergone the action of heat,—a view which thus receives
+confirmation. This spotted structure may possibly afford some
+indication in distinguishing those formations of quartz, which owe
+their present structure to igneous action, from those produced by the
+agency of water alone; a source of doubt, which I should think from my
+own experience, that most geologists, when examining arenaceo-quartzose
+districts must have experienced.
+
+The lowest and most scoriaceous part of the lava, in rolling over the
+sedimentary deposit at the bottom of the sea, has caught up large
+quantities of calcareous matter, which now forms a snow-white, highly
+crystalline basis to a breccia, including small pieces of black, glossy
+scoriæ. A little above this, where the lime is less abundant, and the
+lava more compact, numerous little balls, composed of spicula of
+calcareous spar, radiating from common centres, occupy the interstices.
+In one part of Quail Island, the lime has thus been crystallised by the
+heat of the superincumbent lava, where it is only thirteen feet in
+thickness; nor had the lava been originally thicker, and since reduced
+by degradation, as could be told from the degree of cellularity of its
+surface. I have already observed that the sea must have been shallow
+in which the calcareous deposit was accumulated. In this case,
+therefore, the carbonic acid gas has been retained under a pressure,
+insignificant compared with that (a column of water, 1,708 feet in
+height) originally supposed by Sir James Hall to be requisite for this
+end: but since his experiments, it has been discovered that pressure
+has less to do with the retention of carbonic acid gas, than the nature
+of the circumjacent atmosphere; and hence, as is stated to be the case
+by Mr. Faraday,[2] masses of limestone are sometimes fused and
+crystallised even in common limekilns. Carbonate of lime can be heated
+to almost any degree, according to Faraday, in an atmosphere of
+carbonic acid gas, without being decomposed; and Gay-Lussac found that
+fragments of limestone, placed in a tube and heated to a degree, not
+sufficient by itself to cause their decomposition, yet immediately
+evolved their carbonic acid, when a stream of common air or steam was
+passed over them: Gay-Lussac attributes this to the mechanical
+displacement of the nascent carbonic acid gas. The calcareous matter
+beneath the lava, and especially that forming the crystalline spicula
+between the interstices of the scoriæ, although heated in an atmosphere
+probably composed chiefly of steam, could not have been subjected to
+the effects of a passing stream; and hence it is, perhaps, that they
+have retained their carbonic acid, under a small amount of pressure.
+
+ [2] I am much indebted to Mr. E. W. Brayley in having given me the
+ following references to papers on this subject: Faraday in the
+ _Edinburgh New Philosophical Journal_, vol. xv, p. 398; Gay-Lussac in
+ _Annales de Chem. et Phys.,_ tome lxiii, p. 219, translated in the
+ _London and Edinburgh Philosophical Magazine,_ vol. x, p. 496.
+
+The fragments of scoriæ, embedded in the crystalline calcareous basis,
+are of a jet black colour, with a glossy fracture like pitchstone.
+Their surfaces, however, are coated with a layer of a reddish-orange,
+translucent substance, which can easily be scratched with a knife;
+hence they appear as if overlaid by a thin layer of rosin. Some of the
+smaller fragments are partially changed throughout into this substance:
+a change which appears quite different from ordinary decomposition. At
+the Galapagos Archipelago (as will be described in a future chapter),
+great beds are formed of volcanic ashes and particles of scoriæ, which
+have undergone a closely similar change.
+
+_The extent and horizontality of the calcareous stratum._—The upper
+line of surface of the calcareous stratum, which is so conspicuous from
+being quite white and so nearly horizontal, ranges for miles along the
+coast, at the height of about sixty feet above the sea. The sheet of
+basalt, by which it is capped, is on an average eighty feet in
+thickness. Westward of Porto Praya beyond Red Hill, the white stratum
+with the superincumbent basalt is covered up by more recent streams.
+Northward of Signal Post Hill, I could follow it with my eye, trending
+away for several miles along the sea cliffs. The distance thus observed
+is about seven miles; but I cannot doubt from its regularity that it
+extends much farther. In some ravines at right angles to the coast, it
+is seen gently dipping towards the sea, probably with the same
+inclination
+as when deposited round the ancient shores of the island. I found only
+one inland section, namely, at the base of the hill marked A, where, at
+the height of some hundred feet, this bed was exposed; it here rested
+on the usual compact augitic rock associated with wacke, and was
+covered by the widespread sheet of modern basaltic lava. Some
+exceptions occur to the horizontality of the white stratum: at Quail
+Island, its upper surface is only forty feet above the level of the
+sea; here also the capping of lava is only between twelve and fifteen
+feet in thickness; on the other hand, at the north-east side of Porto
+Praya harbour, the calcareous stratum, as well as the rock on which it
+rests, attain a height above the average level: the inequality of level
+in these two cases is not, as I believe, owing to unequal elevation,
+but to original irregularities at the bottom of the sea. Of this fact,
+at Quail Island, there was clear evidence in the calcareous deposit
+being in one part of much greater than the average thickness, and in
+another part being entirely absent; in this latter case, the modern
+basaltic lavas rested directly on those of more ancient origin.
+
+Fig. 2
+
+
+[Illustration: Signal Post Hill]
+
+SIGNAL POST HILL
+A—Ancient volcanic rocks.  B—Calcareous stratum. C—Upper balastic lava.
+
+Under Signal Post Hill, the white stratum dips into the sea in a
+remarkable manner. This hill is conical, 450 feet in height, and
+retains some traces of having had a crateriform structure; it is
+composed chiefly of matter erupted posteriorly to the elevation of the
+great basaltic plain, but partly of lava of apparently submarine origin
+and of considerable antiquity. The surrounding plain, as well as the
+eastern flank of this hill, has been worn into steep precipices,
+overhanging the sea. In these precipices, the white calcareous stratum
+may be seen, at the height of about seventy feet above the beach,
+running for some miles both northward and southward of the hill, in a
+line appearing to be perfectly horizontal; but for a space of a quarter
+of a mile directly under the hill, it dips into the sea and disappears.
+On the south side the dip is gradual, on the north side it is more
+abrupt, as is shown in Fig. 2. As neither the calcareous stratum, nor
+the superincumbent basaltic lava (as far as the latter can be
+distinguished from the more modern ejections), appears to thicken as it
+dips, I infer that these strata were not originally accumulated in a
+trough, the centre of which afterwards became a point of eruption; but
+that they have subsequently been disturbed and bent. We may suppose
+either that Signal Post Hill subsided after its elevation with the
+surrounding country, or that it never was uplifted to the same height
+with it. This latter seems to me the most probable alternative, for
+during the slow and equable elevation of this portion of the island,
+the subterranean motive
+power, from expending part of its force in repeatedly erupting volcanic
+matter from beneath this point, would, it is likely, have less force to
+uplift it. Something of the same kind seems to have occurred near Red
+Hill, for when tracing upwards the naked streams of lava from near
+Porto Praya towards the interior of the island, I was strongly induced
+to suspect, that since the lava had flowed, the slope of the land had
+been slightly modified, either by a small subsidence near Red Hill, or
+by that portion of the plain having been uplifted to a less height
+during the elevation of the whole area.
+
+_The basaltic lava, superincumbent on the calcareous deposit._—This
+lava is of a pale grey colour, fusing into a black enamel; its fracture
+is rather earthy and concretionary; it contains olivine in small
+grains. The central parts of the mass are compact, or at most
+crenulated with a few minute cavities, and are often columnar. At Quail
+Island this structure was assumed in a striking manner; the lava in one
+part being divided into horizontal laminæ, which became in another part
+split by vertical fissures into five-sided plates; and these again,
+being piled on each other, insensibly became soldered together, forming
+fine symmetrical columns. The lower surface of the lava is vesicular,
+but sometimes only to the thickness of a few inches; the upper surface,
+which is likewise vesicular, is divided into balls, frequently as much
+as three feet in diameter, made up of concentric layers. The mass is
+composed of more than one stream; its total thickness being, on an
+average, about eighty feet: the lower portion has certainly flowed
+beneath the sea, and probably likewise the upper portion. The chief
+part of this lava has flowed from the central districts, between the
+hills marked A, B, C, etc., in the woodcut-map. The surface of the
+country, near the coast, is level and barren; towards the interior, the
+land rises by successive terraces, of which four, when viewed from a
+distance, could be distinctly counted.
+
+_Volcanic eruptions subsequent to the elevation of the coastland; the
+ejected matter associated with earthy lime._—These recent lavas have
+proceeded from those scattered, conical, reddish-coloured hills, which
+rise abruptly from the plain-country near the coast. I ascended some of
+them, but will describe only one, namely, _Red Hill_, which may serve
+as a type of its class, and is remarkable in some especial respects.
+Its height is about six hundred feet; it is composed of bright red,
+highly scoriaceous rock of a basaltic nature; on one side of its summit
+there is a hollow, probably the last remnant of a crater. Several of
+the other hills of this class, judging from their external forms, are
+surmounted by much more perfect craters. When sailing along the coast,
+it was evident that a considerable body of lava had flowed from Red
+Hill, over a line of cliff about one hundred and twenty feet in height,
+into the sea: this line of cliff is continuous with that forming the
+coast, and bounding the plain on both sides of this hill; these
+streams, therefore, were erupted, after the formation of the
+coast-cliffs, from Red Hill, when it must have stood, as it now does,
+above the level of the sea. This conclusion accords with the highly
+scoriaceous condition of all the rock on it, appearing to be of
+subaerial formation: and this is
+important, as there are some beds of calcareous matter near its summit,
+which might, at a hasty glance, have been mistaken for a submarine
+deposit. These beds consist of white, earthy, carbonate of lime,
+extremely friable so as to be crushed with the least pressure; the most
+compact specimens not resisting the strength of the fingers. Some of
+the masses are as white as quicklime, and appear absolutely pure; but
+on examining them with a lens, minute particles of scoriæ can always be
+seen, and I could find none which, when dissolved in acids, did not
+leave a residue of this nature. It is, moreover, difficult to find a
+particle of the lime which does not change colour under the blowpipe,
+most of them even becoming glazed. The scoriaceous fragments and the
+calcareous matter are associated in the most irregular manner,
+sometimes in obscure beds, but more generally as a confused breccia,
+the lime in some parts and the scoriæ in others being most abundant.
+Sir H. De la Beche has been so kind as to have some of the purest
+specimens analysed, with a view to discover, considering their volcanic
+origin, whether they contained much magnesia; but only a small portion
+was found, such as is present in most limestones.
+
+Fragments of the scoriæ embedded in the calcareous mass, when broken,
+exhibit many of their cells lined and partly filled with a white,
+delicate, excessively fragile, moss-like, or rather conferva-like,
+reticulation of carbonate of lime. These fibres, examined under a lens
+of one-tenth of an inch focal distance, appear cylindrical; they are
+rather above one-thousandth of an inch in diameter; they are either
+simply branched, or more commonly united into an irregular mass of
+network, with the meshes of very unequal sizes and of unequal numbers
+of sides. Some of the fibres are thickly covered with extremely minute
+spicula, occasionally aggregated into little tuffs; and hence they have
+a hairy appearance. These spicula are of the same diameter throughout
+their length; they are easily detached, so that the object-glass of the
+microscope soon becomes scattered over with them. Within the cells of
+many fragments of the scoria, the lime exhibits this fibrous structure,
+but generally in a less perfect degree. These cells do not appear to be
+connected with one another. There can be no doubt, as will presently be
+shown, that the lime was erupted, mingled with the lava in its fluid
+state, and therefore I have thought it worth while to describe minutely
+this curious fibrous structure, of which I know nothing analogous. From
+the earthy condition of the fibres, this structure does not appear to
+be related to crystallisation.
+
+Other fragments of the scoriaceous rock from this hill, when broken,
+are often seen marked with short and irregular white streaks, which are
+owing to a row of separate cells being partly, or quite, filled with
+white calcareous powder. This structure immediately reminded me of the
+appearance in badly kneaded dough, of balls and drawn-out streaks of
+flour, which have remained unmixed with the paste; and I cannot doubt
+that small masses of the lime, in the same manner remaining unmixed
+with the fluid lava, have been drawn out when the whole was in motion.
+I carefully examined, by trituration and solution in acids, pieces of
+the scoriæ, taken from within half-an-inch of those
+cells which were filled with the calcareous powder, and they did not
+contain an atom of free lime. It is obvious that the lava and lime have
+on a large scale been very imperfectly mingled; and where small
+portions of the lime have been entangled within a piece of the viscid
+lava, the cause of their now occupying, in the form of a powder or of a
+fibrous reticulation, the vesicular cavities, is, I think, evidently
+due to the confined gases having most readily expanded at the points
+where the incoherent lime rendered the lava less adhesive.
+
+A mile eastward of the town of Praya, there is a steep-sided gorge,
+about one hundred and fifty yards in width, cutting through the
+basaltic plain and underlying beds, but since filled up by a stream of
+more modern lava. This lava is dark grey, and in most parts compact and
+rudely columnar; but at a little distance from the coast, it includes
+in an irregular manner a brecciated mass of red scoriæ mingled with a
+considerable quantity of white, friable, and in some parts, nearly pure
+earthy lime, like that on the summit of Red Hill. This lava, with its
+entangled lime, has certainly flowed in the form of a regular stream;
+and, judging from the shape of the gorge, towards which the drainage of
+the country (feeble though it now be) still is directed, and from the
+appearance of the bed of loose water-worn blocks with their interstices
+unfilled, like those in the bed of a torrent, on which the lava rests,
+we may conclude that the stream was of subaerial origin. I was unable
+to trace it to its source, but, from its direction, it seemed to have
+come from Signal Post Hill, distant one mile and a quarter, which, like
+Red Hill, has been a point of eruption subsequent to the elevation of
+the great basaltic plain. It accords with this view, that I found on
+Signal Post Hill, a mass of earthy, calcareous matter of the same
+nature, mingled with scoriæ. I may here observe that part of the
+calcareous matter forming the horizontal sedimentary bed, especially
+the finer matter with which the embedded fragments of rock are
+whitewashed, has probably been derived from similar volcanic eruptions,
+as well as from triturated organic remains: the underlying, ancient,
+crystalline rocks, also, are associated with much carbonate of lime,
+filling amygdaloidal cavities, and forming irregular masses, the nature
+of which latter I was unable to understand.
+
+Considering the abundance of earthy lime near the summit of Red Hill, a
+volcanic cone six hundred feet in height, of subaerial
+growth,—considering the intimate manner in which minute particles and
+large masses of scoriæ are embedded in the masses of nearly pure lime,
+and on the other hand, the manner in which small kernels and streaks of
+the calcareous powder are included in solid pieces of the
+scoriæ,—considering, also, the similar occurrence of lime and scoriæ
+within a stream of lava, also supposed, with good reason, to have been
+of modern subaerial origin, and to have flowed from a hill, where
+earthy lime also occurs: I think, considering these facts, there can be
+no doubt that the lime has been erupted, mingled with the molten lava.
+I am not aware that any similar case has been described: it appears to
+me an interesting one, inasmuch as most geologists must have speculated
+on the probable effects of a volcanic focus, bursting through
+deep-seated beds
+of different mineralogical composition. The great abundance of free
+silex in the trachytes of some countries (as described by Beudant in
+Hungary, and by P. Scrope in the Panza Islands), perhaps solves the
+inquiry with respect to deep-seated beds of quartz; and we probably
+here see it answered, where the volcanic action has invaded subjacent
+masses of limestone. One is naturally led to conjecture in what state
+the now earthy carbonate of lime existed, when ejected with the
+intensely heated lava: from the extreme cellularity of the scoriæ on
+Red Hill, the pressure cannot have been great, and as most volcanic
+eruptions are accompanied by the emission of large quantities of steam
+and other gases, we here have the most favourable conditions, according
+to the views at present entertained by chemists, for the expulsion of
+the carbonic acid.[3] Has the slow re-absorption of this gas, it may be
+asked, given to the lime in the cells of the lava, that peculiar
+fibrous structure, like that of an efflorescing salt? Finally, I may
+remark on the great contrast in appearance between this earthy lime,
+which must have been heated in a free atmosphere of steam and other
+gases, while the white, crystalline, calcareous spar, produced by a
+single thin sheet of lava (as at Quail Island) rolling over similar
+earthy lime and the _débris_ of organic remains, at the bottom of a
+shallow sea.
+
+ [3] Whilst deep beneath the surface, the carbonate of lime was, I
+ presume, in a fluid state. Hutton, it is known, thought that all
+ amygdaloids were produced by drops of molten limestone floating in the
+ trap, like oil in water: this no doubt is erroneous, but if the matter
+ forming the summit of Red Hill had been cooled under the pressure of a
+ moderately deep sea, or within the walls of a dike, we should, in all
+ probability, have had a trap rock associated with large masses of
+ compact, crystalline, calcareous spar, which, according to the views
+ entertained by many geologists, would have been wrongly attributed to
+ subsequent infiltration.
+
+
+_Signal Post Hill._—This hill has already been several times mentioned,
+especially with reference to the remarkable manner in which the white
+calcareous stratum, in other parts so horizontal (Fig. 2), dips under
+it into the sea. It has a broad summit, with obscure traces of a
+crateriform structure, and is composed of basaltic rocks,[4] some
+compact, others highly cellular with inclined beds of loose scoriæ, of
+which some are associated with earthy lime. Like Red Hill, it has been
+the source of eruptions, subsequently to the elevation of the
+surrounding basaltic plain; but unlike that hill, it has undergone
+considerable denudation, and has been the seat of volcanic action at a
+remote period, when beneath the sea. I judge of this latter
+circumstance from finding on its inland flank the last remnants of
+three small
+points of eruption. These points are composed of glossy scoriæ,
+cemented by crystalline calcareous spar, exactly like the great
+submarine calcareous deposit, where the heated lava has rolled over it:
+their demolished state can, I think, be explained only by the denuding
+action of the waves of the sea. I was guided to the first orifice by
+observing a sheet of lava, about two hundred yards square, with
+steepish sides, superimposed on the basaltic plain with no adjoining
+hillock, whence it could have been erupted; and the only trace of a
+crater which I was able to discover, consisted of some inclined beds of
+scoriæ at one of its corners. At the distance of fifty yards from a
+second level-topped patch of lava, but of much smaller size, I found an
+irregular circular group of masses of cemented, scoriaceous breccia,
+about six feet in height, which doubtless had once formed the point of
+eruption. The third orifice is now marked only by an irregular circle
+of cemented scoriæ, about four yards in diameter, and rising in its
+highest point scarcely three feet above the level of the plain, the
+surface of which, close all round, exhibits its usual appearance: here
+we have a horizontal basal section of a volcanic spiracle, which,
+together with all its ejected matter, has been almost totally
+obliterated.
+
+ [4] Of these, one common variety is remarkable for being full of small
+ fragments of a dark jasper-red earthy mineral, which, when examined
+ carefully, shows an indistinct cleavage; the little fragments are
+ elongated in form, are soft, are magnetic before and after being
+ heated, and fuse with difficulty into a dull enamel. This mineral is
+ evidently closely related to the oxides of iron, but I cannot
+ ascertain what it exactly is. The rock containing this mineral is
+ crenulated with small angular cavities, which are lined and filled
+ with yellowish crystals of carbonate of lime.
+
+
+The stream of lava, which fills the narrow gorge[5] eastward of the
+town of Praya, judging from its course, seems, as before remarked, to
+have come from Signal Post Hill, and to have flowed over the plain,
+after its elevation: the same observation applies to a stream (possibly
+part of the same one) capping the sea cliffs, a little eastward of the
+gorge. When I endeavoured to follow these streams over the stony level
+plain, which is almost destitute of soil and vegetation, I was much
+surprised to find, that although composed of hard basaltic matter, and
+not having been exposed to marine denudation, all distant traces of
+them soon became utterly lost. But I have since observed at the
+Galapagos Archipelago, that it is often impossible to follow even great
+deluges of quite recent lava across older streams, except by the size
+of the bushes growing on them, or by the comparative states of
+glossiness of their surfaces,—characters which a short lapse of time
+would be sufficient quite to obscure. I may remark, that in a level
+country, with a dry climate, and with the wind blowing always in one
+direction (as at the Cape de Verde Archipelago), the effects of
+atmospheric degradation are probably much greater than would at first
+be expected; for soil in this case accumulates only in a few protected
+hollows, and being blown in one direction, it is always travelling
+towards the sea in the form of the finest dust, leaving the surface of
+the rocks bare, and exposed to the full effects of renewed meteoric
+action.
+
+ [5] The sides of this gorge, where the upper basaltic stratum is
+ intersected, are almost perpendicular. The lava, which has since
+ filled it up, is attached to these sides, almost as firmly as a dike
+ is to its walls. In most cases, where a stream of lava has flowed down
+ a valley, it is bounded on each side by loose scoriaceous masses.
+
+_Inland hills of more ancient volcanic rocks._—These hills are laid
+down by eye, and marked as A, B, C, etc., in Map 1. They are related in
+mineralogical composition, and are probably directly
+continuous with the lowest rocks exposed on the coast. These hills,
+viewed from a distance, appear as if they had once formed part of an
+irregular tableland, and from their corresponding structure and
+composition this probably has been the case. They have flat, slightly
+inclined summits, and are, on an average, about six hundred feet in
+height; they present their steepest slope towards the interior of the
+island, from which point they radiate outwards, and are separated from
+each other by broad and deep valleys, through which the great streams
+of lava, forming the coast-plains, have descended. Their inner and
+steeper escarpments are ranged in an irregular curve, which rudely
+follows the line of the shore, two or three miles inland from it. I
+ascended a few of these hills, and from others, which I was able to
+examine with a telescope, I obtained specimens, through the kindness of
+Mr. Kent, the assistant-surgeon of the _Beagle_; although by these
+means I am acquainted with only a part of the range, five or six miles
+in length, yet I scarcely hesitate, from their uniform structure, to
+affirm that they are parts of one great formation, stretching round
+much of the circumference of the island.
+
+The upper and lower strata of these hills differ greatly in
+composition. The upper are basaltic, generally compact, but sometimes
+scoriaceous and amygdaloidal, with associated masses of wacke: where
+the basalt is compact, it is either fine-grained or very coarsely
+crystallised; in the latter case it passes into an augitic rock,
+containing much olivine; the olivine is either colourless, or of the
+usual yellow and dull reddish shades. On some of the hills, beds of
+calcareous matter, both in an earthy and in a crystalline form,
+including fragments of glossy scoriæ, are associated with the basaltic
+strata. These strata differ from the streams of basaltic lava forming
+the coast-plains, only in being more compact, and in the crystals of
+augite, and in the grains of olivine being of much greater
+size;—characters which, together with the appearance of the associated
+calcareous beds, induce me to believe that they are of submarine
+formation.
+
+Some considerable masses of wacke, which are associated with these
+basaltic strata, and which likewise occur in the basal series on the
+coast, especially at Quail Island, are curious. They consist of a pale
+yellowish-green argillaceous substance, of a crumbling texture when
+dry, but unctuous when moist: in its purest form, it is of a beautiful
+green tint, with translucent edges, and occasionally with obscure
+traces of an original cleavage. Under the blowpipe it fuses very
+readily into a dark grey, and sometimes even black bead, which is
+slightly magnetic. From these characters, I naturally thought that it
+was one of the pale species, decomposed, of the genus augite;—a
+conclusion supported by the unaltered rock being full of large separate
+crystals of black augite, and of balls and irregular streaks of dark
+grey augitic rock. As the basalt ordinarily consists of augite, and of
+olivine often tarnished and of a dull red colour, I was led to examine
+the stages of decomposition of this latter mineral, and I found, to my
+surprise, that I could trace a nearly perfect gradation from unaltered
+olivine to the green wacke. Part of the same grain under the blowpipe
+would in some instances
+behave like olivine, its colour being only slightly changed, and part
+would give a black magnetic bead. Hence I can have no doubt that the
+greenish wacke originally existed as olivine; but great chemical
+changes must have been effected during the act of decomposition thus to
+have altered a very hard, transparent, infusible mineral, into a soft,
+unctuous, easily melted, argillaceous substance.[6]
+
+ [6] D’Aubuisson “Traité de Géognosie” (tome ii, p. 569) mentions, on
+ the authority of M. Marcel de Serres, masses of green earth near
+ Montpellier, which are supposed to be due to the decomposition of
+ olivine. I do not, however, find, that the action of this mineral
+ under the blowpipe being entirely altered, as it becomes decomposed,
+ has been noticed; and the knowledge of this fact is important, as at
+ first it appears highly improbable that a hard, transparent,
+ refractory mineral should be changed into a soft, easily fused clay,
+ like this of St. Jago. I shall hereafter describe a green substance,
+ forming threads within the cells of some vesicular basaltic rocks in
+ Van Diemen’s Land, which behave under the blowpipe like the green
+ wacke of St. Jago; but its occurrence in cylindrical threads, shows it
+ cannot have resulted from the decomposition of olivine, a mineral
+ always existing in the form of grains or crystals.
+
+The basal strata of these hills, as well as some neighbouring,
+separate, bare, rounded hillocks, consist of compact, fine-grained,
+non-crystalline (or so slightly as scarcely to be perceptible),
+ferruginous, feldspathic rocks, and generally in a state of
+semi-decomposition. Their fracture is exceedingly irregular, and
+splintery; yet small fragments are often very tough. They contain much
+ferruginous matter, either in the form of minute grains with a metallic
+lustre, or of brown hair-like threads: the rock in this latter case
+assuming a pseudo-brecciated structure. These rocks sometimes contain
+mica and veins of agate. Their rusty brown or yellowish colour is
+partly due to the oxides of iron, but chiefly to innumerable,
+microscopically minute, black specks, which, when a fragment is heated,
+are easily fused, and evidently are either hornblende or augite. These
+rocks, therefore, although at first appearing like baked clay or some
+altered sedimentary deposit, contain all the essential ingredients of
+trachyte; from which they differ only in not being harsh, and in not
+containing crystals of glassy feldspar. As is so often the case with
+trachytic formation, no stratification is here apparent. A person would
+not readily believe that these rocks could have flowed as lava; yet at
+St. Helena there are well-characterised streams (as will be described
+in an ensuing chapter) of nearly similar composition. Amidst the
+hillocks composed of these rocks, I found in three places, smooth
+conical hills of phonolite, abounding with fine crystals of glassy
+feldspar, and with needles of hornblende. These cones of phonolite, I
+believe, bear the same relation to the surrounding feldspathic strata
+which some masses of coarsely crystallised augitic rock, in another
+part of the island, bear to the surrounding basalt, namely, that both
+have been injected. The rocks of a feldspathic nature being anterior in
+origin to the basaltic strata, which cap them, as well as to the
+basaltic streams of the coast-plains, accords with the usual order of
+succession of these two grand divisions of the volcanic series.
+
+
+The strata of most of these hills in the upper part, where alone the
+planes of division are distinguishable, are inclined at a small angle
+from the interior of the island towards the sea-coast. The inclination
+is not the same in each hill; in that marked A it is less than in B, D,
+or E; in C the strata are scarcely deflected from a horizontal plane,
+and in F (as far as I could judge without ascending it) they are
+slightly inclined in a reverse direction, that is, inwards and towards
+the centre of the island. Notwithstanding these differences of
+inclination, their correspondence in external form, and in the
+composition both of their upper and lower parts,—their relative
+position in one curved line, with their steepest sides turned
+inwards,—all seem to show that they originally formed parts of one
+platform; which platform, as before remarked, probably extended round a
+considerable portion of the circumference of the island. The upper
+strata certainly flowed as lava, and probably beneath the sea, as
+perhaps did the lower feldspathic masses: how then come these strata to
+hold their present position, and whence were they erupted?
+
+In the centre of the island[7] there are lofty mountains, but they are
+separated from the steep inland flanks of these hills by a wide space
+of lower country: the interior mountains, moreover, seem to have been
+the source of those great streams of basaltic lava which, contracting
+as they pass between the bases of the hills in question, expand into
+the coast-plains. Round the shores of St. Helena there is a rudely
+formed ring of basaltic rocks, and at Mauritius there are remnants of
+another such a ring round part, if not round the whole, of the island;
+here again the same question immediately occurs, how came these masses
+to hold their present position, and whence were they erupted? The same
+answer, whatever it may be, probably applies in these three cases; and
+in a future chapter we shall recur to this subject.
+
+ [7] I saw very little of the inland parts of the island. Near the
+ village of St. Domingo, there are magnificent cliffs of rather
+ coarsely crystallised basaltic lava. Following the little stream in
+ this valley, about a mile above the village, the base of the great
+ cliff was formed of a compact fine-grained basalt, conformably covered
+ by a bed of pebbles. Near Fuentes, I met with pap-formed hills of the
+ compact feldspathic series of rocks.
+
+_Valleys near the coast._—These are broad, very flat, and generally
+bounded by low cliff-formed sides. Portions of the basaltic plain are
+sometimes nearly or quite isolated by them; of which fact, the space on
+which the town of Praya stands offers an instance. The great valley
+west of the town has its bottom filled up to a depth of more than
+twenty feet by well-rounded pebbles, which in some parts are firmly
+cemented together by white calcareous matter. There can be no doubt,
+from the form of these valleys, that they were scooped out by the waves
+of the sea, during that equable elevation of the land, of which the
+horizontal calcareous deposit, with its existing species of marine
+remains, gives evidence. Considering how well shells have been
+preserved in this stratum, it is singular that I could not find even a
+single small fragment of shell in the conglomerate at the bottom of the
+valleys. The bed of pebbles in the valley west of the town is
+intersected by a second valley joining it as a tributary, but even this
+valley appears much too wide and flat-bottomed to have been formed by
+the small quantity of water, which falls only during one short wet
+season; for at other times of the year these valleys are absolutely
+dry.
+
+_Recent conglomerate._—On the shores of Quail Island, I found fragments
+of brick, bolts of iron, pebbles, and large fragments of basalt, united
+by a scanty base of impure calcareous matter into a firm conglomerate.
+To show how exceedingly firm this recent conglomerate is, I may
+mention, that I endeavoured with a heavy geological hammer to knock out
+a thick bolt of iron, which was embedded a little above low-water mark,
+but was quite unable to succeed.
+
+
+
+
+Chapter II FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.
+
+
+FERNANDO NORONHA.—Precipitous hill of phonolite. TERCEIRA.—Trachytic
+rocks: their singular decomposition by steam of high temperature.
+TAHITI.—Passage from wacke into trap; singular volcanic rock with the
+vesicles half-filled with mesotype. MAURITIUS.—Proofs of its recent
+elevation. Structure of its more ancient mountains; similarity with St.
+Jago. ST. PAUL’S ROCKS.—Not of volcanic origin. Their singular
+mineralogical composition.
+
+_Fernando Noronha._—During our short visit at this and the four
+following islands, I observed very little worthy of description.
+Fernando Noronha is situated in the Atlantic Ocean, in lat. 3° 50′ S.,
+and 230 miles distant from the coast of South America. It consists of
+several islets, together nine miles in length by three in breadth. The
+whole seems to be of volcanic origin; although there is no appearance
+of any crater, or of any one central eminence. The most remarkable
+feature is a hill 1,000 feet high, of which the upper 400 feet consist
+of a precipitous, singularly shaped pinnacle, formed of columnar
+phonolite, containing numerous crystals of glassy feldspar, and a few
+needles of hornblende. From the highest accessible point of this hill,
+I could distinguish in different parts of the group several other
+conical hills, apparently of the same nature. At St. Helena there are
+similar, great, conical, protuberant masses of phonolite, nearly one
+thousand feet in height, which have been formed by the injection of
+fluid feldspathic lava into yielding strata. If this hill has had, as
+is probable, a similar origin, denudation has been here effected on an
+enormous scale. Near the base of this hill, I observed beds of white
+tuff, intersected by numerous dikes, some of amygdaloidal basalt and
+others of trachyte; and beds of slaty phonolite with the planes of
+cleavage directed N.W. and S.E. Parts of this rock, where the crystals
+were scanty, closely resembled common clay-slate, altered by the
+contact of a trap-dike. The lamination of rocks, which undoubtedly have
+once been fluid, appears to me a subject well deserving attention. On
+the beach there were numerous
+fragments of compact basalt, of which rock a distant façade of columns
+seemed to be formed.
+
+_Terceira in the Azores._—The central parts of this island consist of
+irregularly rounded mountains of no great elevation, composed of
+trachyte, which closely resembles in general character the trachyte of
+Ascension, presently to be described. This formation is in many parts
+overlaid, in the usual order of superposition, by streams of basaltic
+lava, which near the coast compose nearly the whole surface. The course
+which these streams have followed from their craters, can often be
+followed by the eye. The town of Angra is overlooked by a crateriform
+hill (Mount Brazil), entirely built of thin strata of fine-grained,
+harsh, brown-coloured tuff. The upper beds are seen to overlap the
+basaltic streams on which the town stands. This hill is almost
+identical in structure and composition with numerous crateriformed
+hills in the Galapagos Archipelago.
+
+_Effects of steam on the trachytic rocks._—In the central part of the
+island there is a spot, where steam is constantly issuing in jets from
+the bottom of a small ravine-like hollow, which has no exit, and which
+abuts against a range of trachytic mountains. The steam is emitted from
+several irregular fissures: it is scentless, soon blackens iron, and is
+of much too high temperature to be endured by the hand. The manner in
+which the solid trachyte is changed on the borders of these orifices is
+curious: first, the base becomes earthy, with red freckles evidently
+due to the oxidation of particles of iron; then it becomes soft; and
+lastly, even the crystals of glassy feldspar yield to the dissolving
+agent. After the mass is converted into clay, the oxide of iron seems
+to be entirely removed from some parts, which are left perfectly white,
+whilst in other neighbouring parts, which are of the brightest red
+colour, it seems to be deposited in greater quantity; some other masses
+are marbled with two distinct colours. Portions of the white clay, now
+that they are dry, cannot be distinguished by the eye from the finest
+prepared chalk; and when placed between the teeth they feel equally
+soft-grained; the inhabitants use this substance for white-washing
+their houses. The cause of the iron being dissolved in one part, and
+close by being again deposited, is obscure; but the fact has been
+observed in several other places.[1] In some half-decayed specimens, I
+found small, globular aggregations of yellow hyalite, resembling
+gum-arabic, which no doubt had been deposited by the steam.
+
+ [1] Spallanzani, Dolomieu, and Hoffman have described similar cases in
+ the Italian volcanic islands. Dolomieu says the iron at the Panza
+ Islands is redeposited in the form of veins (p. 86 “Mémoire sur les
+ Isles Ponces”). These authors likewise believe that the steam deposits
+ silica: it is now experimentally known that vapour of a high
+ temperature is able to dissolve silica.
+
+As there is no escape for the rain-water, which trickles down the sides
+of the ravine-like hollow, whence the steam issues, it must all
+percolate downwards through the fissures at its bottom. Some of the
+inhabitants informed me that it was on record that flames (some
+luminous appearance?) had originally proceeded from these cracks,
+and that the flames had been succeeded by the steam; but I was not able
+to ascertain how long this was ago, or anything certain on the subject.
+When viewing the spot, I imagined that the injection of a large mass of
+rock. like the cone of phonolite at Fernando Noronha, in a semi-fluid
+state, by arching the surface might have caused a wedge-shaped hollow
+with cracks at the bottom, and that the rain-water percolating to the
+neighbourhood of the heated mass, would during many succeeding years be
+driven back in the form of steam.
+
+_Tahiti (Otaheite)._—I visited only a part of the north-western side of
+this island, and this part is entirely composed of volcanic rocks. Near
+the coast there are several varieties of basalt, some abounding with
+large crystals of augite and tarnished olivine, others compact and
+earthy,—some slightly vesicular, and others occasionally amygdaloidal.
+These rocks are generally much decomposed, and to my surprise, I found
+in several sections that it was impossible to distinguish, even
+approximately, the line of separation between the decayed lava and the
+alternating beds of tuff. Since the specimens have become dry, it is
+rather more easy to distinguish the decomposed igneous rocks from the
+sedimentary tuffs. This gradation in character between rocks having
+such widely different origins, may I think be explained by the yielding
+under pressure of the softened sides of the vesicular cavities, which
+in many volcanic rocks occupy a large proportion of their bulk. As the
+vesicles generally increase in size and number in the upper parts of a
+stream of lava, so would the effects of their compression increase; the
+yielding, moreover, of each lower vesicle must tend to disturb all the
+softened matter above it. Hence we might expect to trace a perfect
+gradation from an unaltered crystalline rock to one in which all the
+particles (although originally forming part of the same solid mass) had
+undergone mechanical displacement; and such particles could hardly be
+distinguished from others of similar composition, which had been
+deposited as sediment. As lavas are sometimes laminated in their upper
+parts even horizontal lines, appearing like those of aqueous
+deposition, could not in all cases be relied on as a criterion of
+sedimentary origin. From these considerations it is not surprising that
+formerly many geologists believed in real transitions from aqueous
+deposits, through wacke, into igneous traps.
+
+In the valley of Tia-auru, the commonest rocks are basalts with much
+olivine, and in some cases almost composed of large crystals of augite.
+I picked up some specimens, with much glassy feldspar, approaching in
+character to trachyte. There were also many large blocks of vesicular
+basalt, with the cavities beautifully lined with chabasie (?), and
+radiating bundles of mesotype. Some of these specimens presented a
+curious appearance, owing to a number of the vesicles being half filled
+up with a white, soft, earthy mesotypic mineral, which intumesced under
+the blowpipe in a remarkable manner. As the upper surfaces in all the
+half-filled cells are exactly parallel, it is evident that this
+substance has sunk to the bottom of each cell from its weight.
+Sometimes, however, it entirely fills the cells. Other cells are either
+quite
+filled, or lined, with small crystals, apparently of chabasie; these
+crystals, also, frequently line the upper half of the cells partly
+filled with the earthy mineral, as well as the upper surface of this
+substance itself, in which case the two minerals appear to blend into
+each other. I have never seen any other amygdaloid[2] with the cells
+half filled in the manner here described; and it is difficult to
+imagine the causes which determined the earthy mineral to sink from its
+gravity to the bottom of the cells, and the crystalline mineral to
+adhere in a coating of equal thickness round the sides of the cells.
+
+ [2] MacCulloch, however, has described and given a plate of (“Geolog.
+ Trans.” 1st series, vol. iv, p. 225) a trap rock, with cavities filled
+ up horizontally with quartz and chalcedony. The upper halves of these
+ cavities are often filled by layers, which follow each irregularity of
+ the surface, and by little depending stalactites of the same siliceous
+ substances.
+
+The basic strata on the sides of the valley are gently inclined
+seaward, and I nowhere observed any sign of disturbance; the strata are
+separated from each other by thick, compact beds of conglomerate, in
+which the fragments are large, some being rounded, but most angular.
+From the character of these beds, from the compact and crystalline
+condition of most of the lavas, and from the nature of the infiltrated
+minerals, I was led to conjecture that they had originally flowed
+beneath the sea. This conclusion agrees with the fact that the Rev. W.
+Ellis found marine remains at a considerable height, which he believes
+were interstratified with volcanic matter; as is likewise described to
+be the case by Messrs. Tyerman and Bennett at Huaheine, an island in
+this same archipelago. Mr. Stutchbury also discovered near the summit
+of one of the loftiest mountains of Tahiti, at the height of several
+thousand feet, a stratum of semi-fossil coral. None of these remains
+have been specifically examined. On the coast, where masses of
+coral-rock would have afforded the clearest evidence, I looked in vain
+for any signs of recent elevation. For references to the above
+authorities, and for more detailed reasons for not believing that
+Tahiti has been recently elevated, I must refer to the “Structure and
+Distribution of Coral-Reefs.”
+
+_Mauritius._—Approaching this island on the northern or north-western
+side, a curved chain of bold mountains, surmounted by rugged pinnacles,
+is seen to rise from a smooth border of cultivated land, which gently
+slopes down to the coast. At the first glance, one is tempted to
+believe that the sea lately reached the base of these mountains, and
+upon examination, this view, at least with respect to the inferior
+parts of the border, is found to be perfectly correct. Several
+authors[3] have described masses of upraised coral-rock round the
+greater part of the circumference of the island. Between Tamarin Bay
+and the Great Black River I observed, in company with Captain Lloyd,
+two hillocks of coral-rock, formed in their lower part of hard
+calcareous sandstone, and in their upper of great blocks, slightly
+aggregated, of Astræa and Madrepora, and of fragments of basalt; they
+were divided into beds dipping seaward, in one case at an angle of 8°,
+and in the other at 18°; they had a water-worn appearance, and they
+rose abruptly from a smooth surface, strewed with rolled débris of
+organic remains, to a height of about twenty feet. The Officier du Roi,
+in his most interesting tour in 1768 round the island, has described
+masses of upraised coral-rocks, still retaining that moat-like
+structure (see my “Coral Reefs”) which is characteristic of the living
+reefs. On the coast northward of Port Louis, I found the lava concealed
+for a considerable space inland by a conglomerate of corals and shells,
+like those on the beach, but in parts consolidated by red ferruginous
+matter. M. Bory St. Vincent has described similar calcareous beds over
+nearly the whole of the plain of Pamplemousses. Near Port Louis, when
+turning over some large stones, which lay in the bed of a stream at the
+head of a protected creek, and at the height of some yards above the
+level of spring tides, I found several shells of serpula still adhering
+to their under sides.
+
+ [3] Captain Carmichael, in Hooker’s “Bot. Misc.,” vol. ii, p. 301.
+ Captain Lloyd has lately, in the “Proceedings of the Geological
+ Society” (vol. iii, p. 317), described carefully some of these masses.
+ In the “Voyage à l’Isle de France, par un Officier du Roi,” many
+ interesting facts are given on this subject. Consult also “Voyage aux
+ Quatre Isles d’Afrique, par M. Bory St. Vincent.”
+
+The jagged mountains near Port Louis rise to a height of between two
+and three thousand feet; they consist of strata of basalt, obscurely
+separated from each other by firmly aggregated beds of fragmentary
+matter; and they are intersected by a few vertical dikes. The basalt in
+some parts abounds with large crystals of augite and olivine, and is
+generally compact. The interior of the island forms a plain, raised
+probably about a thousand feet above the level of the sea, and composed
+of streams of lava which have flowed round and between the rugged
+basaltic mountains. These more recent lavas are also basaltic, but less
+compact, and some of them abound with feldspar, so that they even fuse
+into a pale coloured glass. On the banks of the Great River, a section
+is exposed nearly five hundred feet deep, worn through numerous thin
+sheets of the lava of this series, which are separated from each other
+by beds of scoriæ. They seem to have been of subaerial formation, and
+to have flowed from several points of eruption on the central platform,
+of which the Piton du Milieu is said to be the principal one. There are
+also several volcanic cones, apparently of this modern period, round
+the circumference of the island, especially at the northern end, where
+they form separate islets.
+
+The mountains composed of the more compact and crystalline basalt, form
+the main skeleton of the island. M. Bailly[4] states that they all “se
+développent autour d’elle comme une ceinture d’immenses remparts,
+toutes affectant une pente plus ou moins enclinée vers le rivage de la
+mer; tandis, au contraire, que vers le centre de l’ile elles presentent
+une coupe abrupte, et souvent taillée à pic. Toutes ces montagnes sont
+formées de couches parallèles inclinées du centre de l’ile vers la
+mer.” These statements have been disputed, though not in detail, by M.
+Quoy, in the voyage of Freycinet. As far as my limited means of
+observation went, I found
+them perfectly correct.[5] The mountains on the N.W. side of the
+island, which I examined, namely, La Pouce, Peter Botts, Corps de
+Garde, Les Mamelles, and apparently another farther southward, have
+precisely the external shape and stratification described by M. Bailly.
+They form about a quarter of his girdle of ramparts. Although these
+mountains now stand quite detached, being separated from each other by
+breaches, even several miles in width, through which deluges of lava
+have flowed from the interior of the island; nevertheless, seeing their
+close general similarity, one must feel convinced that they originally
+formed parts of one continuous mass. Judging from the beautiful map of
+the Mauritius, published by the Admiralty from a French MS., there is a
+range of mountains (M. Bamboo) on the opposite side of the island,
+which correspond in height, relative position, and external form, with
+those just described. Whether the girdle was ever complete may well be
+doubted; but from M. Bailly’s statements, and my own observations, it
+may be safely concluded that mountains with precipitous inland flanks,
+and composed of strata dipping outwards, once extended round a
+considerable portion of the circumference of the island. The ring
+appears to have been oval and of vast size; its shorter axis, measured
+across from the inner sides of the mountains near Port Louis and those
+near Grand Port, being no less than thirteen geographical miles in
+length. M. Bailly boldly supposes that this enormous gulf, which has
+since been filled up to a great extent by streams of modern lava, was
+formed by the sinking in of the whole upper part of one great volcano.
+
+ [4] “Voyage aux Terres Australes,” tome i, p. 54.
+
+
+ [5] M. Lesson, in his account of this island, in the “Voyage of the
+ _Coquille_,” seems to follow M. Bailly’s views.
+
+It is singular in how many respects those portions of St. Jago and of
+Mauritius which I visited agree in their geological history. At both
+islands, mountains of similar external form, stratification, and (at
+least in their upper beds) composition, follow in a curved chain the
+coast-line. These mountains in each case appear originally to have
+formed parts of one continuous mass. The basaltic strata of which they
+are composed, from their compact and crystalline structure, seem, when
+contrasted with the neighbouring basaltic streams of subaerial
+formation, to have flowed beneath the pressure of the sea, and to have
+been subsequently elevated. We may suppose that the wide breaches
+between the mountains were in both cases worn by the waves, during
+their gradual elevation—of which process, within recent times, there is
+abundant evidence on the coast-land of both islands. At both, vast
+streams of more recent basaltic lavas have flowed from the interior of
+the island, round and between the ancient basaltic hills; at both,
+moreover, recent cones of eruption are scattered around the
+circumference of the island; but at neither have eruptions taken place
+within the period of history. As remarked in the last chapter, it is
+probable that these ancient basaltic mountains, which resemble (at
+least in many respects) the basal and disturbed remnants of two
+gigantic volcanoes, owe their present form, structure, and position, to
+the action of similar causes.
+
+
+_St. Paul’s Rocks._—This small island is situated in the Atlantic
+Ocean, nearly one degree north of the equator, and 540 miles distant
+from South America, in 29° 15′ west longitude. Its highest point is
+scarcely fifty feet above the level of the sea; its outline is
+irregular, and its entire circumference barely three-quarters of a
+mile. This little point of rock rises abruptly out of the ocean; and,
+except on its western side, soundings were not obtained, even at the
+short distance of a quarter of a mile from its shore. It is not of
+volcanic origin; and this circumstance, which is the most remarkable
+point in its history (as will hereafter be referred to), properly ought
+to exclude it from the present volume. It is composed of rocks, unlike
+any which I have met with, and which I cannot characterise by any name,
+and must therefore describe.
+
+The simplest, and one of the most abundant kinds, is a very compact,
+heavy, greenish-black rock, having an angular, irregular fracture, with
+some points just hard enough to scratch glass, and infusible. This
+variety passes into others of paler green tints, less hard, but with a
+more crystalline fracture, and translucent on their edges; and these
+are fusible into a green enamel. Several other varieties are chiefly
+characterised by containing innumerable threads of dark-green
+serpentine, and by having calcareous matter in their interstices. These
+rocks have an obscure, concretionary structure, and are full of
+variously coloured angular pseudo fragments. These angular pseudo
+fragments consist of the first-described dark green rock, of a brown
+softer kind, of serpentine, and of a yellowish harsh stone, which,
+perhaps, is related to serpentine rock. There are other vesicular,
+calcareo-ferruginous, soft stones. There is no distinct stratification,
+but parts are imperfectly laminated; and the whole abounds with
+innumerable veins, and vein-like masses, both small and large. Of these
+vein-like masses, some calcareous ones, which contain minute fragments
+of shells, are clearly of subsequent origin to the others.
+
+_A glossy incrustation._—Extensive portions of these rocks are coated
+by a layer of a glossy polished substance, with a pearly lustre and of
+a greyish white colour; it follows all the inequalities of the surface,
+to which it is firmly attached. When examined with a lens, it is found
+to consist of numerous exceedingly thin layers, their aggregate
+thickness being about the tenth of an inch. It is considerably harder
+than calcareous spar, but can be scratched with a knife; under the
+blowpipe it scales off, decrepitates, slightly blackens, emits a fetid
+odour, and becomes strongly alkaline: it does not effervesce in
+acids.[6] I presume this substance has been deposited by water draining
+from the birds’ dung, with which the rocks are covered. At Ascension,
+near a cavity in the rocks which was filled with a laminated mass of
+infiltrated birds’ dung, I found some irregularly formed, stalactitical
+masses of apparently the same nature. These masses, when broken, had an
+earthy texture; but on their outsides, and especially at their
+extremities, they were formed of a pearly substance, generally in
+little globules, like the
+enamel of teeth, but more translucent, and so hard as just to scratch
+plate-glass. This substance slightly blackens under the blowpipe, emits
+a bad smell, then becomes quite white, swelling a little, and fuses
+into a dull white enamel; it does not become alkaline; nor does it
+effervesce in acids. The whole mass had a collapsed appearance, as if
+in the formation of the hard glossy crust the whole had shrunk much. At
+the Abrolhos Islands on the coast of Brazil, where also there is much
+birds’ dung, I found a great quantity of a brown, arborescent substance
+adhering to some trap-rock. In its arborescent form, this substance
+singularly resembles some of the branched species of Nullipora. Under
+the blowpipe, it behaves like the specimens from Ascension; but it is
+less hard and glossy, and the surface has not the shrunk appearance.
+
+ [6] In my “Journal” I have described this substance; I then believed
+ that it was an impure phosphate of lime.
+
+
+
+
+Chapter III ASCENSION.
+
+
+Basaltic lavas.—Numerous craters truncated on the same side.—Singular
+structure of volcanic bombs.—Aeriform explosions.—Ejected granitic
+fragments.—Trachytic rocks.—Singular veins.—Jasper, its manner of
+formation.—Concretions in pumiceous tuff.—Calcareous deposits and
+frondescent incrustations on the coast.—Remarkable laminated beds,
+alternating with, and passing into, obsidian.—Origin of
+obsidian.—Lamination of volcanic rocks.
+
+
+This island is situated in the Atlantic Ocean, in lat. 8° S., long. 14°
+W. It has the form of an irregular triangle (see map below), each side
+being about six miles in length. Its highest point is 2,870 feet[1]
+above the level of the sea. The whole is volcanic, and, from the
+absence of proofs to the contrary, I believe of subaerial origin. The
+fundamental rock is everywhere of a pale colour, generally compact, and
+of a feldspathic nature. In the S.E. portion of the island, where the
+highest land is situated, well characterised trachyte, and other
+congenerous rocks of that varying here and there a hill or single point
+of rock (one of which near the sea-coast, north of the Fort, is only
+two or three yards across) of the trachyte still remaining exposed.
+
+ [1] _Geographical Journal_, vol. v, p. 243.
+
+
+Illustration: Island of Ascension PLATE IV.
+
+_Basaltic rocks._—The overlying basaltic lava is in some parts
+extremely vesicular, in others little so; it is of a black colour, but
+sometimes contains crystals of glassy feldspar, and seldom much
+olivine. These streams appear to have possessed singularly little
+fluidity; their side walls and lower ends being very steep, and even as
+much as between twenty and thirty feet in height. Their surface is
+extraordinarily rugged,
+and from a short distance appears as if studded with small craters.
+These projections consist of broad, irregularly conical, hillocks,
+traversed by fissures, and composed of the same unequally scoriaceous
+basalt with the surrounding streams, but having an obscure tendency to
+a columnar structure; they rise to a height between ten and thirty feet
+above the general surface, and have been formed, as I presume, by the
+heaping up of the viscid lava at points of greater resistance. At the
+base of several of these hillocks, and occasionally likewise on more
+level parts, solid ribs, composed of angulo-globular masses of basalt,
+resembling in size and outline arched sewers or gutters of brickwork,
+but not being hollow, project between two or three feet above the
+surface of the streams; what their origin may have been, I do not know.
+Many of the superficial fragments from these basaltic streams present
+singularly convoluted forms; and some specimens could hardly be
+distinguished from logs of dark-coloured wood without their bark.
+
+Many of the basaltic streams can be traced, either to points of
+eruption at the base of the great central mass of trachyte, or to
+separate, conical, red-coloured hills, which are scattered over the
+northern and western borders of the island. Standing on the central
+eminence, I counted between twenty and thirty of these cones of
+eruption. The greater number of them had their truncated summits cut
+off obliquely, and they all sloped towards the S.E., whence the
+trade-wind blows.[2] This structure no doubt has been caused by the
+ejected fragments and ashes being always blown, during eruptions, in
+greater quantity towards one side than towards the other. M. Moreau de
+Jonnes has made a similar observation with respect to the volcanic
+orifices in the West Indian Islands.
+
+ [2] M. Lesson in the “Zoology of the Voyage of the _Coquille_,” p. 490
+ has observed this fact. Mr. Hennah (“Geolog. Proceedings,” 1835, p.
+ 189) further remarks that the most extensive beds of ashes at
+ Ascension invariably occur on the leeward side of the island.
+
+
+_Volcanic bombs._—These occur in great numbers strewed on the ground,
+and some of them lie at considerable distances from any points of
+eruption. They vary in size from that of an apple to that of a man’s
+body; they are either spherical or pear-shaped, or with the hinder part
+(corresponding to the tail of a comet) irregular, studded with
+projecting points, and even concave. Their surfaces are rough, and
+fissured with branching cracks; their internal structure is either
+irregularly scoriaceous and compact, or it presents a symmetrical and
+very curious appearance. An irregular segment of a bomb of this latter
+kind, of which I found several, is accurately represented in figure No.
+3. Its size was about that of a man’s head. The whole interior is
+coarsely cellular; the cells averaging in diameter about the tenth of
+an inch; but nearer the outside they gradually decrease in size. This
+part is succeeded by a well-defined shell of compact lava, having a
+nearly uniform thickness of about the third of an inch; and the shell
+is overlaid by a somewhat thicker coating of finely cellular lava (the
+cells varying from the fiftieth to the hundredth of an inch in
+diameter), which forms the external surface: the line separating the
+shell of compact
+lava from the outer scoriaceous crust is distinctly defined. This
+structure is very simply explained, if we suppose a mass of viscid,
+scoriaceous matter, to be projected with a rapid, rotatory motion
+through the air; for whilst the external crust, from cooling, became
+solidified (in the state we now see it), the centrifugal force, by
+relieving the pressure in the interior parts of the bomb, would allow
+the heated vapours to expand their cells; but these being driven by the
+same force against the already-hardened crust, would become, the nearer
+they were to this part, smaller and smaller or less expanded, until
+they became packed into a solid, concentric shell. As we know that
+chips from a grindstone[3] can be flirted off, when made to revolve
+with sufficient velocity, we need not doubt that the centrifugal force
+would have power to modify the structure of a softened bomb, in the
+manner here supposed. Geologists have remarked, that the external form
+of a bomb at once bespeaks the history of its aerial course, and few
+now see that the internal structure can speak, with almost equal
+plainness, of its rotatory movement.
+
+ [3] Nichol’s “Architecture of the Heavens.”
+
+
+No. 3
+
+
+[Illustration: Fragment of a spherical volcanic bomb.]
+
+Fragment of a spherical volcanic bomb, with the inferior parts coarsely
+cellular, coated by a concentric layer of compact lava, and this again
+by a crust of finely cellular rock.
+
+
+No. 4
+
+
+[Illustration: Volcanic bomb of obsidian from Australia.]
+
+Volcanic bomb of obsidian from Australia. The figure at left gives a
+front view; the figure at right a side view of the same object.
+
+
+M. Bory St. Vincent[4] has described some balls of lava from the Isle
+of Bourbon, which have a closely similar structure. His explanation,
+however (if I understand it rightly), is very different from that which
+I have given; for he supposes that they have rolled, like snowballs,
+down the sides of the crater. M. Beudant,[5] also, has described some
+singular little balls of obsidian, never more than six or eight inches
+in diameter, which he found strewed on the surface of the ground: their
+form is always oval; sometimes they are much swollen in the middle, and
+even spindle-shaped: their surface is regularly marked with concentric
+ridges and furrows, all of which on the same ball are at right angles
+to one axis: their interior is compact and glassy. M. Beudant supposes
+that masses of lava, when soft, were shot into the air, with a rotatory
+movement round the same axis, and that the form and superficial ridges
+of the bombs were thus produced. Sir Thomas Mitchell has given me what
+at first appears to be the half of a much flattened oval ball of
+obsidian; it has a singular artificial-like appearance, which is well
+represented (of the natural size) in figure No. 4. It was found in its
+present state, on a great sandy plain between the rivers Darling and
+Murray, in Australia, and at the distance of several hundred miles from
+any known volcanic region. It seems to have been embedded in some
+reddish tufaceous matter; and may have been transported either by the
+aborigines or by natural means. The external saucer consists of compact
+obsidian, of a bottle-green colour, and is filled with finely cellular
+black lava, much less transparent and glassy than the obsidian. The
+external surface is marked with four or five not quite perfect ridges,
+which are represented rather too distinctly in figure No. 4. Here,
+then, we have the external structure described by M. Beudant, and the
+internal cellular condition of the bombs from Ascension. The lip of the
+saucer is slightly concave, exactly like the margin of a soup-plate,
+and its inner edge overlaps a little the central cellular lava. This
+structure is so symmetrical round the entire circumference, that one is
+forced to suppose that the bomb burst during its rotatory course,
+before being quite solidified, and that the lip and edges were thus
+slightly modified and turned inwards. It may be remarked that the
+superficial ridges are in planes, at right angles to an axis,
+transverse to the longer axis of the flattened oval: to explain this
+circumstance, we may suppose that when the bomb burst, the axis of
+rotation changed.
+
+ [4] “Voyage aux Quatre Isles d’Afrique” tome i, p. 222.
+
+
+ [5] “Voyage en Hongrie,” tome ii, p. 214.
+
+
+_Aeriform explosions._—The flanks of Green Mountain and the surrounding
+country are covered by a great mass, some hundred feet in thickness, of
+loose fragments. The lower beds generally consist of fine-grained,
+slightly consolidated tuffs,[6] and the upper beds of great
+loose fragments, with alternating finer beds.[7] One white ribbon-like
+layer of decomposed, pumiceous breccia, was curiously bent into deep
+unbroken curves, beneath each of the large fragments in the
+superincumbent stratum. From the relative position of these beds, I
+presume that a narrow-mouthed crater, standing nearly in the position
+of Green Mountain, like a great air-gun, shot forth, before its final
+extinction, this vast accumulation of loose matter. Subsequently to
+this event, considerable dislocations have taken place, and an oval
+circus has been formed by subsidence. This sunken space lies at the
+north-eastern foot of Green Mountain, and is well represented in Map 2.
+Its longer axis, which is connected with a N.E. and S.W. line of
+fissure, is three-fifths of a nautical mile in length; its sides are
+nearly perpendicular, except in one spot, and about four hundred feet
+in height; they consist, in the lower part, of a pale basalt with
+feldspar, and in the upper part, of the tuff and loose ejected
+fragments; the bottom is smooth and level, and under almost any other
+climate a deep lake would have been formed here. From the thickness of
+the bed of loose fragments, with which the surrounding country is
+covered, the amount of aeriform matter necessary for their projection
+must have been enormous; hence we may suppose it probable that after
+the explosions vast subterranean caverns were left, and that the
+falling in of the roof of one of these produced the hollow here
+described. At the Galapagos Archipelago, pits of a similar character,
+but of a much smaller size, frequently occur at the bases of small
+cones of eruption.
+
+ [6] Some of this peperino, or tuff, is sufficiently hard not to be
+ broken by the greatest force of the fingers.
+
+
+ [7] On the northern side of the Green Mountain a thin seam, about an
+ inch in thickness, of compact oxide of iron, extends over a
+ considerable area; it lies conformably in the lower part of the
+ stratified mass of ashes and fragments. This substance is of a
+ reddish-brown colour, with an almost metallic lustre; it is not
+ magnetic, but becomes so after having been heated under the blowpipe,
+ by which it is blackened and partly fused. This seam of compact stone,
+ by intercepting the little rain-water which falls on the island, gives
+ rise to a small dripping spring, first discovered by Dampier. It is
+ the only fresh water on the island, so that the possibility of its
+ being inhabited has entirely depended on the occurrence of this
+ ferruginous layer.
+
+
+_Ejected granitic fragments._—In the neighbourhood of Green Mountain,
+fragments of extraneous rock are not unfrequently found embedded in the
+midst of masses of scoriæ. Lieutenant Evans, to whose kindness I am
+indebted for much information, gave me several specimens, and I found
+others myself. They nearly all have a granitic structure, are brittle,
+harsh to the touch, and apparently of altered colours. _First_, a white
+syenite, streaked and mottled with red; it consists of
+well-crystallised feldspar, numerous grains of quartz, and brilliant,
+though small, crystals of hornblende. The feldspar and hornblende in
+this and the succeeding cases have been determined by the reflecting
+goniometer, and the quartz by its action under the blowpipe. The
+feldspar in these ejected fragments, like the glassy kind in the
+trachyte, is from its cleavage a potash-feldspar. _Secondly_, a
+brick-red mass of feldspar, quartz, and small dark patches of a decayed
+mineral; one minute particle of which I was able to ascertain, by its
+cleavage, to be hornblende.
+_Thirdly_, a mass of confusedly crystallised white feldspar, with
+little nests of a dark-coloured mineral, often carious, externally
+rounded, having a glossy fracture, but no distinct cleavage: from
+comparison with the second specimen, I have no doubt that it is fused
+hornblende. _Fourthly_, a rock, which at first appears a simple
+aggregation of distinct and large-sized crystals of dusty-coloured
+Labrador feldspar;[8] but in their interstices there is some white
+granular feldspar, abundant scales of mica, a little altered
+hornblende, and, as I believe, no quartz. I have described these
+fragments in detail, because it is rare[9] to find granitic rocks
+ejected from volcanoes with their _minerals unchanged_, as is the case
+with the first specimen, and partially with the second. One other large
+fragment, found in another spot, is deserving of notice; it is a
+conglomerate, containing small fragments of granitic, cellular, and
+jaspery rocks, and of hornstone porphyries, embedded in a base of
+wacke, threaded by numerous thin layers of a concretionary pitchstone
+passing into obsidian. These layers are parallel, slightly tortuous,
+and short; they thin out at their ends, and resemble in form the layers
+of quartz in gneiss. It is probable that these small embedded fragments
+were not separately ejected, but were entangled in a fluid volcanic
+rock, allied to obsidian; and we shall presently see that several
+varieties of this latter series of rock assume a laminated structure.
+
+ [8] Professor Miller has been so kind as to examine this mineral. He
+ obtained two good cleavages of 86° 30′ and 86° 50′. The mean of
+ several, which I made, was 86° 30′. Professor Miller states that these
+ crystals, when reduced to a fine powder, are soluble in hydrochloric
+ acid, leaving some undissolved silex behind; the addition of oxalate
+ of ammonia gives a copious precipitate of lime. He further remarks,
+ that according to Von Kobell, anorthite (a mineral occurring in the
+ ejected fragments at Mount Somma) is always white and transparent, so
+ that if this be the case, these crystals from Ascension must be
+ considered as Labrador feldspar. Professor Miller adds, that he has
+ seen an account, in Erdmann’s “Journal für tecnische Chemie,” of a
+ mineral ejected from a volcano which had the external characters of
+ Labrador feldspar, but differed in the analysis from that given by
+ mineralogists of this mineral: the author attributed this difference
+ to an error in the analysis of Labrador feldspar, which is very old.
+
+
+ [9] Daubeny, in his work on Volcanoes (p. 386), remarks that this is
+ the case; and Humboldt, in his “Personal Narrative,” vol. i, p. 236,
+ says “In general, the masses of known primitive rocks, I mean those
+ which perfectly resemble our granites, gneiss, and mica-slate, are
+ very rare in lavas: the substances we generally denote by the name of
+ granite, thrown out by Vesuvius, are mixtures of nepheline, mica, and
+ pyroxene.”
+
+
+_Trachytic series of rocks._—Those occupy the more elevated and
+central, and likewise the south-eastern, parts of the island. The
+trachyte is generally of a pale brown colour, stained with small darker
+patches; it contains broken and bent crystals of glassy feldspar,
+grains of specular iron, and black microscopical points, which latter,
+from being easily fused, and then becoming magnetic, I presume are
+hornblende. The greater number of the hills, however, are composed of a
+quite white, friable stone, appearing like a trachytic tuff. Obsidian,
+hornstone, and several kinds of laminated feldspathic rocks, are
+associated with the trachyte. There is no distinct stratification; nor
+could I distinguish a crateriform structure in any of the hills of this
+series. Considerable dislocations have taken place; and many fissures
+in these rocks are yet left open, or are only partially filled with
+loose fragments. Within the space,[10] mainly formed of trachyte, some
+basaltic streams have burst forth; and not far from the summit of Green
+Mountain, there is one stream of quite black, vesicular basalt,
+containing minute crystals of glassy feldspar, which have a rounded
+appearance.
+
+ [10] This space is nearly included by a line sweeping round Green
+ Mountain, and joining the hills, called the Weather Port Signal,
+ Holyhead, and that denominated (improperly in a geological sense) “the
+ Crater of an old volcano.”
+
+
+The soft white stone above mentioned is remarkable from its singular
+resemblance, when viewed in mass, to a sedimentary tuff: it was long
+before I could persuade myself that such was not its origin; and other
+geologists have been perplexed by closely similar formations in
+trachytic regions. In two cases, this white earthy stone formed
+isolated hills; in a third, it was associated with columnar and
+laminated trachyte; but I was unable to trace an actual junction. It
+contains numerous crystals of glassy feldspar and black microscopical
+specks, and is marked with small darker patches, exactly as in the
+surrounding trachyte. Its basis, however, when viewed under the
+microscope, is generally quite earthy; but sometimes it exhibits a
+decidedly crystalline structure. On the hill marked “Crater of an old
+volcano,” it passes into a pale greenish-grey variety, differing only
+in its colour, and in not being so earthy; the passage was in one case
+effected insensibly; in another, it was formed by numerous, rounded and
+angular, masses of the greenish variety, being embedded in the white
+variety;—in this latter case, the appearance was very much like that of
+a sedimentary deposit, torn up and abraded during the deposition of a
+subsequent stratum. Both these varieties are traversed by innumerable
+tortuous veins (presently to be described), which are totally unlike
+injected dikes, or indeed any other veins which I have ever seen. Both
+varieties include a few scattered fragments, large and small, of
+dark-coloured scoriaceous rocks, the cells of some of which are
+partially filled with the white earthy stone; they likewise include
+some huge blocks of a cellular porphyry.[11] These fragments project
+from the weathered surface, and perfectly resemble fragments embedded
+in a true sedimentary tuff. But as it is known that extraneous
+fragments of cellular rock are sometimes included in columnar trachyte,
+in phonolite,[12] and in other compact lavas, this circumstance is not
+any real argument for the sedimentary origin of the white earthy
+stone.[13] The insensible passage of the greenish variety
+into the white one, and likewise the more abrupt passage by fragments
+of the former being embedded in the latter, might result from slight
+differences in the composition of the same mass of molten stone, and
+from the abrading action of one such part still fluid on another part
+already solidified. The curiously formed veins have, I believe, been
+formed by siliceous matter being subsequently segregated. But my chief
+reason for believing that these soft earthy stones, with their
+extraneous fragments, are not of sedimentary origin, is the extreme
+improbability of crystals of feldspar, black microscopical specks, and
+small stains of a darker colour occurring in the same proportional
+numbers in an aqueous deposit, and in masses of solid trachyte.
+Moreover, as I have remarked, the microscope occasionally reveals a
+crystalline structure in the apparently earthy basis. On the other
+hand, the partial decomposition of such great masses of trachyte,
+forming whole mountains, is undoubtedly a circumstance of not easy
+explanation.
+
+ [11] The porphyry is dark coloured; it contains numerous, often
+ fractured, crystals of white opaque feldspar, also decomposing
+ crystals of oxide of iron; its vesicles include masses of delicate,
+ hair-like, crystals, apparently of analcime.
+
+
+ [12] D’Aubuisson, “Traité de Géognosie,” tome ii, p. 548.
+
+
+ [13] Dr. Daubeny (on Volcanoes, p. 180) seems to have been led to
+ believe that certain trachytic formations of Ischia and of the Puy de
+ Dôme, which closely resemble these of Ascension, were of sedimentary
+ origin, chiefly from the frequent presence in them “of scoriform
+ portions, different in colour from the matrix.” Dr. Daubeny adds, that
+ on the other hand, Brocchi, and other eminent geologists, have
+ considered these beds as earthy varieties of trachyte; he considers
+ the subject deserving of further attention.
+
+_Veins in the earthy trachytic masses._—These veins are extraordinarily
+numerous, intersecting in the most complicated manner both coloured
+varieties of the earthy trachyte: they are best seen on the flanks of
+the “Crater of the old volcano.” They contain crystals of glassy
+feldspar, black microscopical specks and little dark stains, precisely
+as in the surrounding rock; but the basis is very different, being
+exceedingly hard, compact, somewhat brittle, and of rather less easy
+fusibility. The veins vary much, and suddenly, from the tenth of an
+inch to one inch in thickness; they often thin out, not only on their
+edges, but in their central parts, thus leaving round, irregular
+apertures; their surfaces are rugged. They are inclined at every
+possible angle with the horizon, or are horizontal; they are generally
+curvilinear, and often interbranch one with another. From their
+hardness they withstand weathering, and projecting two or three feet
+above the ground, they occasionally extend some yards in length; these
+plate-like veins, when struck, emit a sound, almost like that of a
+drum, and they may be distinctly seen to vibrate; their fragments,
+which are strewed on the ground, clatter like pieces of iron when
+knocked against each other. They often assume the most singular forms;
+I saw a pedestal of the earthy trachyte, covered by a hemispherical
+portion of a vein, like a great umbrella, sufficiently large to shelter
+two persons. I have never met with, or seen described, any veins like
+these; but in form they resemble the ferruginous seams, due to some
+process of segregation, occurring not uncommonly in sandstones,—for
+instance, in the New Red sandstone of England. Numerous veins of jasper
+and of siliceous sinter, occurring on the summit of this same hill,
+show that there has been some abundant source of silica, and as these
+plate-like veins differ from the trachyte
+only in their greater hardness, brittleness, and less easy fusibility,
+it appears probable that their origin is due to the segregation or
+infiltration of siliceous matter, in the same manner as happens with
+the oxides of iron in many sedimentary rocks.
+
+_Siliceous sinter and jasper._—The siliceous sinter is either quite
+white, of little specific gravity, and with a somewhat pearly fracture,
+passing into pinkish pearl quartz; or it is yellowish white, with a
+harsh fracture, and it then contains an earthy powder in small
+cavities. Both varieties occur, either in large irregular masses in the
+altered trachyte, or in seams included in broad, vertical, tortuous,
+irregular veins of a compact, harsh stone of a dull red colour,
+appearing like a sandstone. This stone, however, is only altered
+trachyte; and a nearly similar variety, but often honeycombed,
+sometimes adheres to the projecting plate-like veins, described in the
+last paragraph. The jasper is of an ochre yellow or red colour; it
+occurs in large irregular masses, and sometimes in veins, both in the
+altered trachyte and in an associated mass of scoriaceous basalt. The
+cells of the scoriaceous basalt are lined or filled with fine,
+concentric layers of chalcedony, coated and studded with bright-red
+oxide of iron. In this rock, especially in the rather more compact
+parts, irregular angular patches of the red jasper are included, the
+edges of which insensibly blend into the surrounding mass; other
+patches occur having an intermediate character between perfect jasper
+and the ferruginous, decomposed, basaltic base. In these patches, and
+likewise in the large vein-like masses of jasper, there occur little
+rounded cavities, of exactly the same size and form with the air-cells,
+which in the scoriaceous basalt are filled and lined with layers of
+chalcedony. Small fragments of the jasper, examined under the
+microscope, seem to resemble the chalcedony with its colouring matter
+not separated into layers, but mingled in the siliceous paste, together
+with some impurities. I can understand these facts,—namely, the
+blending of the jasper into the semi-decomposed basalt,—its occurrence
+in angular patches, which clearly do not occupy pre-existing hollows in
+the rock,—and its containing little vesicles filled with chalcedony,
+like those in the scoriaceous lava,—only on the supposition that a
+fluid, probably the same fluid which deposited the chalcedony in the
+air-cells, removed in those parts where there were no cavities, the
+ingredients of the basaltic rock, and left in their place silica and
+iron, and thus produced the jasper. In some specimens of silicified
+wood, I have observed, that in the same manner as in the basalt, the
+solid parts were converted into a dark-coloured homogeneous stone,
+whereas the cavities formed by the larger sap-vessels (which may be
+compared with the air-vesicles in the basaltic lava) and other
+irregular hollows, apparently produced by decay, were filled with
+concentric layers of chalcedony; in this case, there can be little
+doubt that the same fluid deposited the homogeneous base and the
+chalcedonic layers. After these considerations, I cannot doubt but that
+the jasper of Ascension may be viewed as a volcanic rock silicified, in
+precisely the same sense as this term is applied to wood, when
+silicified; we are equally ignorant of the means by which every atom of
+wood, whilst in a perfect state, is
+removed and replaced by atoms of silica, as we are of the means by
+which the constituent parts of a volcanic rock could be thus acted
+on.[14] I was led to the careful examination of these rocks, and to the
+conclusion here given, from having heard the Rev. Professor Henslow
+express a similar opinion, regarding the origin in trap-rocks of many
+chalcedonies and agates. Siliceous deposits seem to be very general, if
+not of universal occurrence, in partially decomposed trachytic
+tuffs;[15] and as these hills, according to the view above given,
+consist of trachyte softened and altered in situ, the presence of free
+silica in this case may be added as one more instance to the list.
+
+ [14] Beudant (“Voyage en Hongrie,” tome iii, pp. 502, 504) describes
+ kidney-shaped masses of jasper-opal, which either blend into the
+ surrounding trachytic conglomerate, or are embedded in it like
+ chalk-flints; and he compares them with the fragments of opalised
+ wood, which are abundant in this same formation. Beudant, however,
+ appears to have viewed the process of their formation rather as one of
+ simple infiltration than of molecular exchange; but the presence of a
+ concretion, wholly different from the surrounding matter, if not
+ formed in a pre-existing hollow, clearly seems to me to require,
+ either a molecular or mechanical displacement of the atoms, which
+ occupied the space afterwards filled by it. The jasper-opal of Hungary
+ passes into chalcedony, and therefore in this case, as in that of
+ Ascension, jasper seems to be intimately related in origin with
+ chalcedony.
+
+
+ [15] Beudant (“Voyage Min.,” tome iii, p. 507) enumerates cases in
+ Hungary, Germany, Central France, Italy, Greece, and Mexico.
+
+_Concretions in pumiceous tuff._—The hill, marked in Map 2 “Crater of
+an old volcano,” has no claims to this appellation, which I could
+discover, except in being surmounted by a circular, very shallow,
+saucer-like summit, nearly half a mile in diameter. This hollow has
+been nearly filled up with many successive sheets of ashes and scoriæ,
+of different colours, and slightly consolidated. Each successive
+saucer-shaped layer crops out all round the margin, forming so many
+rings of various colours, and giving to the hill a fantastic
+appearance. The outer ring is broad, and of a white colour; hence it
+resembles a course round which horses have been exercised, and has
+received the name of the Devil’s Riding School, by which it is most
+generally known. These successive layers of ashes must have fallen over
+the whole surrounding country, but they have all been blown away except
+in this one hollow, in which probably moisture accumulated, either
+during an extraordinary year when rain fell, or during the storms often
+accompanying volcanic eruptions. One of the layers of a pinkish colour,
+and chiefly derived from small, decomposed fragments of pumice, is
+remarkable, from containing numerous concretions. These are generally
+spherical, from half an inch to three inches in diameter; but they are
+occasionally cylindrical, like those of iron-pyrites in the chalk of
+Europe. They consist of a very tough, compact, pale-brown stone, with a
+smooth and even fracture. They are divided into concentric layers by
+thin white partitions, resembling the external superficies; six or
+eight of such layers are distinctly defined near the outside; but those
+towards the inside generally become indistinct, and blend into a
+homogeneous
+mass. I presume that these concentric layers were formed by the
+shrinking of the concretion, as it became compact. The interior part is
+generally fissured by minute cracks or septaria, which are lined, both
+by black, metallic, and by other white and crystalline specks, the
+nature of which I was unable to ascertain. Some of the larger
+concretions consist of a mere spherical shell, filled with slightly
+consolidated ashes. The concretions contain a small proportion of
+carbonate of lime: a fragment placed under the blowpipe decrepitates,
+then whitens and fuses into a blebby enamel, but does not become
+caustic. The surrounding ashes do not contain any carbonate of lime;
+hence the concretions have probably been formed, as is so often the
+case, by the aggregation of this substance. I have not met with any
+account of similar concretions; and considering their great toughness
+and compactness, their occurrence in a bed, which probably has been
+subjected only to atmospheric moisture, is remarkable.
+
+_Formation of calcareous rocks on the sea-coast._—On several of the
+sea-beaches, there are immense accumulations of small, well-rounded
+particles of shells and corals, of white, yellowish, and pink colours,
+interspersed with a few volcanic particles. At the depth of a few feet,
+these are found cemented together into stone, of which the softer
+varieties are used for building; there are other varieties, both coarse
+and fine-grained, too hard for this purpose: and I saw one mass divided
+into even layers half an inch in thickness, which were so compact that
+when struck with a hammer they rang like flint. It is believed by the
+inhabitants, that the particles become united in the course of a single
+year. The union is effected by calcareous matter; and in the most
+compact varieties, each rounded particle of shell and volcanic rock can
+be distinctly seen to be enveloped in a husk of pellucid carbonate of
+lime. Extremely few perfect shells are embedded in these agglutinated
+masses; and I have examined even a large fragment under a microscope,
+without being able to discover the least vestige of striæ or other
+marks of external form: this shows how long each particle must have
+been rolled about, before its turn came to be embedded and
+cemented.[16] One of the most compact varieties, when placed in acid,
+was entirely dissolved, with the exception of some flocculent animal
+matter; its specific gravity was 2·63. The specific gravity of ordinary
+limestone varies from 2·6 to 2·75; pure Carrara marble was found by Sir
+H. De la Beche[17] to be 2·7. It is remarkable that these rocks of
+Ascension, formed close to the surface, should be nearly as compact as
+marble, which has undergone the action of heat and pressure in the
+plutonic regions.
+
+ [16] The eggs of the turtle being buried by the parent, sometimes
+ become enclosed in the solid rock. Mr. Lyell has given a figure
+ (“Principles of Geology,” book iii, ch. 17) of some eggs, containing
+ the bones of young turtles, found thus entombed.
+
+
+ [17] “Researches in Theoretical Geology,” p. 12.
+
+The great accumulation of loose calcareous particles, lying on the
+beach near the Settlement, commences in the month of October, moving
+towards the S.W., which, as I was informed by Lieutenant
+Evans, is caused by a change in the prevailing direction of the
+currents. At this period the tidal rocks, at the S.W. end of the beach,
+where the calcareous sand is accumulating, and round which the currents
+sweep, become gradually coated with a calcareous incrustation, half an
+inch in thickness. It is quite white, compact, with some parts slightly
+spathose, and is firmly attached to the rock. After a short time it
+gradually disappears, being either redissolved, when the water is less
+charged with lime, or more probably is mechanically abraded. Lieutenant
+Evans has observed these facts, during the six years he has resided at
+Ascension. The incrustation varies in thickness in different years: in
+1831 it was unusually thick. When I was there in July, there was no
+remnant of the incrustation; but on a point of basalt, from which the
+quarrymen had lately removed a mass of the calcareous freestone, the
+incrustation was perfectly preserved. Considering the position of the
+tidal-rocks, and the period at which they become coated, there can be
+no doubt that the movement and disturbance of the vast accumulation of
+calcareous particles, many of them being partially agglutinated
+together, cause the waves of the sea to be so highly charged with
+carbonate of lime, that they deposit it on the first objects against
+which they impinge. I have been informed by Lieutenant Holland, R.N.,
+that this incrustation is formed on many parts of the coast, on most of
+which, I believe, there are likewise great masses of comminuted shells.
+
+_A frondescent calcareous incrustation._—In many respects this is a
+singular deposit; it coats throughout the year the tidal volcanic
+rocks, that project from the beaches composed of broken shells. Its
+general appearance is well represented in Figure 5; but the fronds or
+discs, of which it is composed, are generally so closely crowded
+together as to touch. These fronds have their sinuous edges finely
+crenulated, and they project over their pedestals or supports; their
+upper surfaces are either slightly concave, or slightly convex; they
+are highly polished, and of a dark grey or jet black colour; their form
+is irregular, generally circular, and from the tenth of an inch to one
+inch and a half in diameter; their thickness, or amount of their
+projection from the rock on which they stand, varies much, about a
+quarter of an inch being perhaps most usual. The fronds occasionally
+become more and more convex, until they pass into botryoidal masses
+with their summits fissured; when in this state, they are glossy and of
+an intense black, so as to resemble some fused metallic substance. I
+have shown the incrustation, both in this latter and in its ordinary
+state to several geologists, but not one could conjecture its origin,
+except that perhaps it was of volcanic nature!
+
+No. 5
+
+
+[Illustration: An incrustration of calcareous and animal matter.]
+
+An incrustation of calcareous and animal matter, coating the
+tidal-rocks at Ascension.
+
+
+The substance forming the fronds has a very compact and often almost
+crystalline fracture; the edges being translucent, and hard enough
+easily to scratch calcareous spar. Under the blowpipe it immediately
+becomes white, and emits a strong animal odour, like that from fresh
+shells. It is chiefly composed of carbonate of lime; when placed in
+muriatic acid it froths much, leaving a residue of sulphate of lime,
+and of an oxide of iron, together with a black powder, which is not
+soluble in heated acids. This latter substance seems to be
+carbonaceous,
+and is evidently the colouring matter. The sulphate of lime is
+extraneous, and occurs in distinct, excessively minute, lamellar
+plates, studded on the surface of the fronds, and embedded between the
+fine layers of which they are composed; when a fragment is heated in
+the blowpipe, these lamellæ are immediately rendered visible. The
+original outline of the fronds may often be traced, either to a minute
+particle of shell fixed in a crevice of the rock, or to several
+cemented together; these first become deeply corroded, by the
+dissolving power of the waves, into sharp ridges, and then are coated
+with successive layers of the glossy, grey, calcareous incrustation.
+The inequalities of the primary support affect the outline of every
+successive layer, in the same manner as may often be seen in
+bezoar-stones, when an object like a nail forms the centre of
+aggregation. The crenulated edges, however, of the frond appear to be
+due to the corroding power of the surf on its own deposit, alternating
+with fresh depositions. On some smooth basaltic rocks on the coast of
+St. Jago, I found an exceedingly thin layer of brown calcareous matter,
+which under a lens presented a miniature likeness of the crenulated and
+polished fronds of Ascension; in this case a basis was not afforded by
+any projecting extraneous particles. Although the incrustation at
+Ascension is persistent throughout the year; yet from the abraded
+appearance of some parts, and from the fresh appearance of other parts,
+the whole seems to undergo a round of decay and renovation, due
+probably to changes in the form of the shifting beach, and consequently
+in the action of the breakers: hence probably it is, that the
+incrustation never acquires a great thickness. Considering the position
+of the encrusted rocks in the midst of the calcareous beach, together
+with its composition, I think there can be no doubt that its origin is
+due to the dissolution and subsequent deposition of the matter
+composing the rounded particles of shells and corals.[18] From this
+source
+it derives its animal matter, which is evidently the colouring
+principle. The nature of the deposit, in its incipient stage, can often
+be well seen upon a fragment of white shell, when jammed between two of
+the fronds; it then appears exactly like the thinnest wash of a pale
+grey varnish. Its darkness varies a little, but the jet blackness of
+some of the fronds and of the botryoidal masses seems due to the
+translucency of the successive grey layers. There is, however, this
+singular circumstance, that when deposited on the under side of ledges
+of rock or in fissures, it appears always to be of a pale, pearly grey
+colour, even when of considerable thickness: hence one is led to
+suppose, that an abundance of light is necessary to the development of
+the dark colour, in the same manner as seems to be the case with the
+upper and exposed surfaces of the shells of living mollusca, which are
+always dark, compared with their under surfaces and with the parts
+habitually covered by the mantle of the animal. In this
+circumstance,—in the immediate loss of colour and in the odour emitted
+under the blowpipe,—in the degree of hardness and translucency of the
+edges,—and in the beautiful polish of the surface,[19] rivalling when
+in a fresh state that of the finest Oliva, there is a striking analogy
+between this inorganic incrustation and the shells of living molluscous
+animals.[20] This appears to me to be an interesting physiological
+fact.[21]
+
+ [18] The selenite, as I have remarked is extraneous, and must have
+ been derived from the sea-water. It is an interesting circumstance
+ thus to find the waves of the ocean, sufficiently charged with
+ sulphate of lime, to deposit it on the rocks, against which they dash
+ every tide. Dr. Webster has described (“Voyage of the _Chanticleer,_”
+ vol. ii, p. 319) beds of gypsum and salt, as much as two feet in
+ thickness, left by the evaporation of the spray on the rocks on the
+ windward coast. Beautiful stalactites of selenite, resembling in form
+ those of carbonate of lime, are formed near these beds. Amorphous
+ masses of gypsum, also, occur in caverns in the interior of the
+ island; and at Cross Hill (an old crater) I saw a considerable
+ quantity of salt oozing from a pile of scoriæ. In these latter cases,
+ the salt and gypsum appear to be volcanic products.)
+
+
+ [19] From the fact described in my “Journal of Researches” of a
+ coating of oxide of iron, deposited by a streamlet on the rocks in its
+ bed (like a nearly similar coating at the great cataracts of the
+ Orinoco and Nile), becoming finely polished where the surf acts, I
+ presume that the surf in this instance, also, is the polishing agent.)
+
+
+ [20] In the section descriptive of St. Paul’s Rocks, I have described
+ a glossy, pearly substance, which coats the rocks, and an allied
+ stalactitical incrustation from Ascension, the crust of which
+ resembles the enamel of teeth, but is hard enough to scratch
+ plate-glass. Both these substances contain animal matter, and seem to
+ have been derived from water in filtering through birds’ dung.
+
+
+ [21] Mr. Horner and Sir David Brewster have described (“Philosophical
+ Transactions,” 1836, p. 65) a singular “artificial substance,
+ resembling shell.” It is deposited in fine, transparent, highly
+ polished, brown-coloured laminæ, possessing peculiar optical
+ properties, on the inside of a vessel, in which cloth, first prepared
+ with glue and then with lime, is made to revolve rapidly in water. It
+ is much softer, more transparent, and contains more animal matter,
+ than the natural incrustation at Ascension; but we here again see the
+ strong tendency which carbonate of lime and animal matter evince to
+ form a solid substance allied to shell.
+
+
+_Singular laminated beds alternating with and passing into
+obsidian._—These beds occur within the trachytic district, at the
+western base of Green Mountain, under which they dip at a high
+inclination. They are only partially exposed, being covered up by
+modern ejections; from this cause, I was unable to trace their junction
+with the trachyte, or to discover whether they had flowed as a stream
+of lava, or had been injected amidst the overlying strata. There are
+three principal beds of obsidian, of which the thickest forms the base
+of the section. The alternating stony layers appear to me eminently
+curious, and shall be first described, and afterwards their passage
+into the obsidian. They have an extremely diversified appearance; five
+principal varieties may be noticed, but these insensibly blend into
+each other by endless gradations.
+
+First.—A pale grey, irregularly and coarsely laminated,[22]
+harsh-feeling rock, resembling clay-slate which has been in contact
+with a trap-dike, and with a fracture of about the same degree of
+crystalline structure. This rock, as well as the following varieties,
+easily fuses into a pale glass. The greater part is honeycombed with
+irregular, angular, cavities, so that the whole has a curious
+appearance, and some fragments resemble in a remarkable manner
+silicified logs of decayed wood. This variety, especially where more
+compact, is often marked with thin whitish streaks, which are either
+straight or wrap round, one behind the other, the elongated carious
+hollows.
+
+ [22] This term is open to some misinterpretation, as it may be applied
+ both to rocks divided into laminæ of exactly the same composition, and
+ to layers firmly attached to each other, with no fissile tendency, but
+ composed of different minerals, or of different shades of colour. The
+ term “laminated,” in this chapter, is applied in these latter senses;
+ where a homogeneous rock splits, as in the former sense, in a given
+ direction, like clay-slate, I have used the term “fissile.”
+
+Secondly.—A bluish grey or pale brown, compact, heavy, homogeneous
+stone, with an angular, uneven, earthy fracture; viewed, however, under
+a lens of high power, the fracture is seen to be distinctly
+crystalline, and even separate minerals can be distinguished.
+
+Thirdly.—A stone of the same kind with the last, but streaked with
+numerous, parallel, slightly tortuous, white lines of the thickness of
+hairs. These white lines are more crystalline than the parts between
+them; and the stone splits along them: they frequently expand into
+exceedingly thin cavities, which are often only just perceptible with a
+lens. The matter forming the white lines becomes better crystallised in
+these cavities, and Professor Miller was fortunate enough, after
+several trials, to ascertain that the white crystals, which are the
+largest, were of quartz,[23] and that the minute green transparent
+needles were augite, or, as they would more generally be called,
+diopside: besides these crystals, there are some minute, dark specks
+without a trace of
+crystalline, and some fine, white, granular, crystalline matter which
+is probably feldspar. Minute fragments of this rock are easily fusible.
+
+ [23] Professor Miller informs me that the crystals which he measured
+ had the faces _P, z, m_ of the figure (147) given by Haidinger in his
+ Translation of Mohs; and he adds, that it is remarkable, that none of
+ them had the slightest trace of faces _r_ of the regular six-sided
+ prism.
+
+Fourthly.—A compact crystalline rock, banded in straight lines with
+innumerable layers of white and grey shades of colour, varying in width
+from the thirtieth to the two-hundredth of an inch; these layers seem
+to be composed chiefly of feldspar, and they contain numerous perfect
+crystals of glassy feldspar, which are placed lengthways; they are also
+thickly studded with microscopically minute, amorphous, black specks,
+which are placed in rows, either standing separately, or more
+frequently united, two or three or several together, into black lines,
+thinner than a hair. When a small fragment is heated in the blowpipe,
+the black specks are easily fused into black brilliant beads, which
+become magnetic,—characters that apply to no common mineral except
+hornblende or augite. With the black specks there are mingled some
+others of a red colour, which are magnetic before being heated, and no
+doubt are oxide of iron. Round two little cavities, in a specimen of
+this variety, I found the black specks aggregated into minute crystals,
+appearing like those of augite or hornblende, but too dull and small to
+be measured by the goniometer; in the specimen, also, I could
+distinguish amidst the crystalline feldspar, grains, which had the
+aspect of quartz. By trying with a parallel ruler, I found that the
+thin grey layers and the black hair-like lines were absolutely straight
+and parallel to each other. It is impossible to trace the gradation
+from the homogeneous grey rocks to these striped varieties, or indeed
+the character of the different layers in the same specimen, without
+feeling convinced that the more or less perfect whiteness of the
+crystalline feldspathic matter depends on the more or less perfect
+aggregation of diffused matter, into the black and red specks of
+hornblende and oxide of iron.
+
+Fifthly.—A compact heavy rock, not laminated, with an irregular,
+angular, highly crystalline, fracture; it abounds with distinct
+crystals of glassy feldspar, and the crystalline feldspathic base is
+mottled with a black mineral, which on the weathered surface is seen to
+be aggregated into small crystals, some perfect, but the greater number
+imperfect. I showed this specimen to an experienced geologist, and
+asked him what it was; he answered, as I think every one else would
+have done, that it was a primitive greenstone. The weathered surface,
+also, of the banded variety in figure No. 4, strikingly resembles a
+worn fragment of finely laminated gneiss.
+
+These five varieties, with many intermediate ones, pass and repass into
+each other. As the compact varieties are quite subordinate to the
+others, the whole may be considered as laminated or striped. The
+laminæ, to sum up their characteristics, are either quite straight, or
+slightly tortuous, or convoluted; they are all parallel to each other,
+and to the intercalating strata of obsidian; they are generally of
+extreme thinness; they consist either of an apparently homogeneous,
+compact rock, striped with different shades of grey and brown colours,
+or of crystalline feldspathic layers in a more or less perfect state of
+purity, and of different thicknesses, with distinct crystals of glassy
+feldspar
+placed lengthways, or of very thin layers chiefly composed of minute
+crystals of quartz and augite, or composed of black and red specks of
+an augitic mineral and of an oxide of iron, either not crystallised or
+imperfectly so. After having fully described the obsidian, I shall
+return to the subject of the lamination of rocks of the trachytic
+series.
+
+The passage of the foregoing beds into the strata of glassy obsidian is
+effected in several ways: first, angulo-modular masses of obsidian,
+both large and small, abruptly appear disseminated in a slaty, or in an
+amorphous, pale-coloured, feldspathic rock, with a somewhat pearly
+fracture. Secondly, small irregular nodules of the obsidian, either
+standing separately, or united into thin layers, seldom more than the
+tenth of an inch in thickness, alternate repeatedly with very thin
+layers of a feldspathic rock, which is striped with the finest parallel
+zones of colour, like an agate, and which sometimes passes into the
+nature of pitchstone; the interstices between the nodules of obsidian
+are generally filled by soft white matter, resembling pumiceous ashes.
+Thirdly, the whole substance of the bounding rock suddenly passes into
+an angulo-concretionary mass of obsidian. Such masses (as well as the
+small nodules) of obsidian are of a pale green colour, and are
+generally streaked with different shades of colour, parallel to the
+laminæ of the surrounding rock; they likewise generally contain minute
+white sphærulites, of which half is sometimes embedded in a zone of one
+shade of colour, and half in a zone of another shade. The obsidian
+assumes its jet black colour and perfectly conchoidal fracture, only
+when in large masses; but even in these, on careful examination and on
+holding the specimens in different lights, I could generally
+distinguish parallel streaks of different shades of darkness.
+
+No. 6
+
+
+[Illustration: Opaque brown sphærulites, drawn on an enlarged scale.]
+
+Opaque brown sphærulites, drawn on an enlarged scale. The upper ones
+are externally marked with parallel ridges. The internal radiating
+structure of the lower ones, is much too plainly represented.
+
+
+No. 7
+
+
+[Illustration: A layer, formed by the union of minute brown
+sphærulites.
+
+A layer, formed by the union of minute brown sphærulites, intersecting
+two other similar layers: the whole represented of nearly the natural
+size.
+
+One of the commonest transitional rocks deserves in several respects a
+further description. It is of a very complicated nature, and consists
+of numerous thin, slightly tortuous layers of a pale-coloured
+feldspathic stone, often passing into an imperfect pitchstone,
+alternating with layers formed of numberless little globules of two
+varieties of obsidian, and of two kinds of sphærulites, embedded in a
+soft or in a hard pearly base. The sphærulites are either white and
+translucent, or dark brown and opaque; the former are quite spherical,
+of small size, and distinctly radiated from their centre. The dark
+brown sphærulites are less perfectly round, and vary in diameter from
+the twentieth to the thirtieth of an inch; when broken they exhibit
+towards their centres, which are whitish, an obscure radiating
+structure; two of them when united sometimes have only one central
+point of radiation; there is occasionally a trace of or a hollow
+crevice in their centres. They stand either separately, or are united
+two or three or many together into irregular groups, or more commonly
+into layers, parallel to the stratification of the mass. This union in
+many cases is so perfect, that the two sides of the layer thus formed,
+are quite even; and these layers, as they become less brown and opaque,
+cannot be distinguished from the alternating layers of the
+pale-coloured feldspathic stone. The sphærulites, when not united, are
+generally compressed in the plane of the lamination of the mass; and in
+this same plane, they are often marked internally, by zones of
+different
+shades of colour, and externally by small ridges and furrows. In the
+upper part of figure No. 6, the sphærulites with the parallel ridges
+and furrows are represented on an enlarged scale, but they are not well
+executed; and in the lower part, their usual manner of grouping is
+shown. In another specimen, a thin layer formed of the brown
+sphærulites closely united together, intersects, as represented in
+figure No. 7, a layer of similar composition; and after running for a
+short space in a slightly curved line, again intersects it, and
+likewise a second layer lying a little way beneath that first
+intersected. The small nodules also of obsidian are sometimes
+externally marked with ridges and furrows, parallel to the lamination
+of the mass, but always less plainly than the sphærulites. These
+obsidian nodules are generally angular, with their edges blunted: they
+are often impressed with the form of the adjoining sphærulites, than
+which they are always larger; the separate nodules seldom appear to
+have drawn each other out by exerting a mutually attractive force. Had
+I not found in some cases, a distinct centre of attraction in these
+nodules of obsidian, I should have
+been led to have considered them as residuary matter, left during the
+formation of the pearlstone, in which they are embedded, and of the
+sphærulitic globules.
+
+The sphærulites and the little nodules of obsidian in these rocks so
+closely resemble, in general form and structure, concretions in
+sedimentary deposits, that one is at once tempted to attribute to them
+an analogous origin. They resemble ordinary concretions in the
+following respects: in their external form,—in the union of two or
+three, or of several, into an irregular mass, or into an even-sided
+layer,—in the occasional intersection of one such layer by another, as
+in the case of chalk-flints,—in the presence of two or three kinds of
+nodules, often close together, in the same basis,—in their fibrous,
+radiating structure, with occasional hollows in their centres,—in the
+co-existence of a laminary, concretionary, and radiating structure, as
+is so well developed in the concretions of magnesian limestone,
+described by Professor Sedgwick.[24] Concretions in sedimentary
+deposits, it is known, are due to the separation from the surrounding
+mass of the whole or part of some mineral substance, and its
+aggregation round certain points of attraction. Guided by this fact, I
+have endeavoured to discover whether obsidian and the sphærulites (to
+which may be added marekanite and pearlstone, both of them occurring in
+nodular concretions in the trachytic series) differ in their
+constituent parts, from the minerals generally composing trachytic
+rocks. It appears from three analyses, that obsidian contains on an
+average 76 per cent of silica; from one analysis, that sphærulites
+contain 79·12; from two, that marekanite contains 79·25; and from two
+other analyses, that pearlstone contains 75·62 of silica.[25] Now, the
+constituent parts of trachyte, as far as they can be distinguished
+consist of feldspar, containing 65·21 of silica; or of albite,
+containing 69·09; of hornblende, containing 55·27,[26] and of oxide of
+iron: so that the foregoing glassy concretionary substances all contain
+a larger proportion of silica than that occurring in ordinary
+feldspathic or trachytic rocks. D’Aubuisson,[27] also, has remarked on
+the large proportion of silica compared with alumina, in six analyses
+of obsidian and pearlstone given in Brongniart’s “Mineralogy.” Hence I
+conclude, that the foregoing concretions have been formed by a process
+of aggregation, strictly analogous to that which takes place in aqueous
+deposits, acting chiefly on the silica, but likewise on some of the
+other elements of the surrounding mass, and thus producing the
+different concretionary varieties. From the well-known effects of rapid
+cooling[28] in giving glassiness of
+texture, it is probably necessary that the entire mass, in cases like
+that of Ascension, should have cooled at a certain rate; but
+considering the repeated and complicated alterations of nodules and
+thin layers of a glassy texture with other layers quite stony or
+crystalline, all within the space of a few feet or even inches, it is
+hardly possible that they could have cooled at different rates, and
+thus have acquired their different textures.
+
+ [24] “Geological Transactions,” vol. 3, part i, p. 37.
+
+
+ [25] The foregoing analyses are taken from Beudant “Traité de
+ Minéralogie,” tome ii, p. 113; and one analysis of obsidian from
+ Phillips’s “Mineralogy.”
+
+
+ [26] These analyses are taken from Von Kobell’s “Grundzüge der
+ Mineralogie,” 1838.
+
+
+ [27] “Traité de Géogn.,” tome ii, p. 535.
+
+
+ [28] This is seen in the manufacture of common glass, and in Gregory
+ Watts’s experiments on molten trap; also on the natural surfaces of
+ lava-streams, and on the side-walls of dikes.
+
+
+The natural sphærulites in these rocks[29] very closely resemble those
+produced in glass, when slowly cooled. In some fine specimens of
+partially devitrified glass, in the possession of Mr. Stokes, the
+sphærulites are united into straight layers with even sides, parallel
+to each other, and to one of the outer surfaces, exactly as in the
+obsidian. These layers sometimes interbranch and form loops; but I did
+not see any case of actual intersection. They form the passage from the
+perfectly glassy portions, to those nearly homogeneous and stony, with
+only an obscure concretionary structure. In the same specimen, also,
+sphærulites differing slightly in colour and in structure, occur
+embedded close together. Considering these facts, it is some
+confirmation of the view above given of the concretionary origin of the
+obsidian and natural sphærulites, to find that M. Dartigues,[30] in his
+curious paper on this subject, attributes the production of sphærulites
+in glass, to the different ingredients obeying their own laws of
+attraction and becoming aggregated. He is led to believe that this
+takes place, from the difficulty in remelting sphærulitic glass,
+without the whole be first thoroughly pounded and mixed together; and
+likewise from the fact, that the change takes place most readily in
+glass composed of many ingredients. In confirmation of M. Dartigues’
+view, I may remark, that M. Fleuriau de Bellevue[31] found that the
+sphærulitic portions of devitrified glass were acted on both by nitric
+acid and under the blowpipe, in a different manner from the compact
+paste in which they were embedded.
+
+ [29] I do not know whether it is generally known, that bodies having
+ exactly the same appearance as sphærulites, sometimes occur in agates.
+ Mr. Robert Brown showed me in an agate, formed within a cavity in a
+ piece of silicified wood, some little specks, which were only just
+ visible to the naked eye: these specks, when placed by him under a
+ lens of high power, presented a beautiful appearance: they were
+ perfectly circular, and consisted of the finest fibres of a brown
+ colour, radiating with great exactness from a common centre. These
+ little radiating stars are occasionally intersected, and portions are
+ quite cut off by the fine, ribbon-like zones of colour in the agate.
+ In the obsidian of Ascension, the halves of a sphærulite often lie in
+ different zones of colour, but they are not cut off by them, as in the
+ agate.
+
+
+ [30] _Journal de Physique,_ tome 59 (1804), pp. 10, 12.
+
+
+ [31] _Idem,_ tome 60 (1805), p. 418.
+
+
+_Comparison of the obsidian beds and alternating strata of ascension,
+with those of other countries._—I have been struck with much surprise,
+how closely the excellent description of the obsidian rocks of Hungary,
+given by Beudant,[32] and that by Humboldt, of the same formation in
+Mexico and Peru,[33] and likewise the descriptions given by several
+authors[34] of the trachytic regions in the Italian islands, agree with
+my observations at Ascension. Many passages might have been transferred
+without alteration from the works of the above authors, and would have
+been applicable to this island. They all agree in the laminated and
+stratified character of the whole series; and Humboldt speaks of some
+of the beds of obsidian being ribboned like jasper.[35] They all agree
+in the nodular or concretionary character of the obsidian, and of the
+passage of these nodules into layers. They all refer to the repeated
+alterations, often in undulatory planes, of glassy, pearly, stony, and
+crystalline layers: the crystalline layers, however, seem to be much
+more perfectly developed at Ascension, than in the above-named
+countries. Humboldt compares some of the stony beds, when viewed from a
+distance, to strata of a schistose sandstone. Sphærulites are described
+as occurring abundantly in all cases; and they everywhere seem to mark
+the passage, from the perfectly glassy to the stony and crystalline
+beds. Beudant’s account[36] of his “perlite lithoide globulaire” in
+every, even the most trifling particular, might have been written for
+the little brown sphærulitic globules of the rocks of Ascension.
+
+ [32] “Voyage en Hongrie,” tome i, p. 330; tome ii, pp. 221 and 315;
+ tome iii, pp. 369, 371, 377, 381.
+
+
+ [33] “Essai Géognostique,” pp. 176, 326, 328.
+
+
+ [34] P. Scrope, in “Geological Transactions,” vol. ii (second series)
+ p. 195. Consult, also, Dolomieu’s “Voyage aux Isles Lipari,” and
+ D’Aubuisson, “Traité de Géogn.,” tome ii, p. 534.
+
+
+ [35] In Mr. Stokes’ fine collection of obsidians from Mexico, I
+ observe that the sphærulites are generally much larger than those of
+ Ascension; they are generally white, opaque, and are united into
+ distinct layers: there are many singular varieties, different from any
+ at Ascension. The obsidians are finely zoned, in quite straight or
+ curved lines, with exceedingly slight differences of tint, of
+ cellularity, and of more or less perfect degrees of glassiness.
+ Tracing some of the less perfectly glassy zones, they are seen to
+ become studded with minute white sphærulites, which become more and
+ more numerous, until at last they unite and form a distinct layer: on
+ the other hand, at Ascension, only the brown sphærulites unite and
+ form layers; the white ones always being irregularly disseminated.
+ Some specimens at the Geological Society, said to belong to an
+ obsidian formation from Mexico, have an earthy fracture, and are
+ divided in the finest parallel laminæ, by specks of a black mineral,
+ like the augitic or hornblendic specks in the rocks at Ascension.
+
+
+ [36] Beudant’s “Voyage,” tome iii, p. 373.
+
+
+From the close similarity in so many respects, between the obsidian
+formations of Hungary, Mexico, Peru, and of some of the Italian
+islands, with that of Ascension, I can hardly doubt that in all these
+cases, the obsidian and the sphærulites owe their origin to a
+concretionary aggregation of the silica, and of some of the other
+constituent elements, taking place whilst the liquified mass cooled at
+a certain required rate. It is, however, well-known, that in several
+places, obsidian has flowed in streams like lava; for instance, at
+Teneriffe, at the Lipari Islands, and at Iceland.[37] In these cases,
+the superficial parts are the most
+perfectly glassy, the obsidian passing at the depth of a few feet into
+an opaque stone. In an analysis by Vauquelin of a specimen of obsidian
+from Hecla, which probably flowed as lava, the proportion of silica is
+nearly the same as in the nodular or concretionary obsidian from
+Mexico. It would be interesting to ascertain, whether the opaque
+interior portions and the superficial glassy coating contained the same
+proportional constituent parts: we know from M. Dufrénoy[38] that the
+exterior and interior parts of the same stream of lava sometimes differ
+considerably in their composition. Even should the whole body of the
+stream of obsidian turn out to be similarly composed with nodular
+obsidian, it would only be necessary, in accordance with the foregoing
+facts, to suppose that lava in these instances had been erupted with
+its ingredients mixed in the same proportion, as in the concretionary
+obsidian.
+
+ [37] For Teneriffe, see von Buch, “Descript. des Isles Canaries,” pp.
+ 184 and 190; for the Lipari Islands, see Dolomieu’s “Voyage,” p. 34;
+ for Iceland, see Mackenzie’s “Travels,” p. 369.
+
+
+ [38] “Mémoires pour servir a une Descript. Géolog. de la France,” tome
+ iv, p. 371.
+
+_Lamination of volcanic rocks of the trachytic series._
+
+We have seen that, in several and widely distant countries, the strata
+alternating with beds of obsidian, are highly laminated. The nodules,
+also, both large and small, of the obsidian, are zoned with different
+shades of colour; and I have seen a specimen from Mexico in Mr. Stokes’
+collection, with its external surface weathered[39] into ridges and
+furrows, corresponding with the zones of different degrees of
+glassiness: Humboldt,[40] moreover, found on the Peak of Teneriffe, a
+stream of obsidian divided by very thin, alternating, layers of pumice.
+Many other lavas of the feldspathic series are laminated; thus, masses
+of common trachyte at Ascension are divided by fine earthy lines, along
+which the rock splits, separating thin layers of slightly different
+shades of colour; the greater number, also, of the embedded crystals of
+glassy feldspar are placed lengthways in the same direction. Mr. P.
+Scrope[41] has described a remarkable columnar trachyte in the Panza
+Islands, which seems to have been injected into an overlying mass of
+trachytic conglomerate: it is striped with zones, often of extreme
+tenuity, of different textures and colours; the harder and darker zones
+appearing to contain a larger proportion of silica. In another part of
+the island, there are layers of pearlstone and pitchstone, which in
+many respects resemble those of Ascension. The zones in the columnar
+trachyte are generally contorted; they extend uninterruptedly for a
+great length in a vertical direction, and apparently parallel to the
+walls of the dike-like mass. Von Buch[42] has described at Teneriffe, a
+stream of lava
+containing innumerable thin, plate-like crystals of feldspar, which are
+arranged like white threads, one behind the other, and which mostly
+follow the same direction. Dolomieu[43] also states, that the grey
+lavas of the modern cone of Vulcano, which have a vitreous texture, are
+streaked with parallel white lines: he further describes a solid
+pumice-stone which possesses a fissile structure, like that of certain
+micaceous schists. Phonolite, which I may observe is often, if not
+always, an injected rock, also, often has a fissile structure; this is
+generally due to the parallel position of the embedded crystals of
+feldspar, but sometimes, as at Fernando Noronha, seems to be nearly
+independent of their presence.[44] From these facts we see, that
+various rocks of the feldspathic series have either a laminated or
+fissile structure, and that it occurs both in masses which have
+injected into overlying strata, and in others which have flowed as
+streams of lava.
+
+ [39] MacCulloch states (“Classification of Rocks,” p. 531), that the
+ exposed surfaces of the pitchstone dikes in Arran are furrowed “with
+ undulating lines, resembling certain varieties of marbled paper, and
+ which evidently result from some corresponding difference of laminar
+ structure.”
+
+
+ [40] “Personal Narrative,” vol. i, p. 222.
+
+
+ [41] “Geological Transactions,” vol. ii (second series), p. 195.
+
+
+ [42] “Description des Iles Canaries,” p. 184.
+
+
+ [43] “Voyage aux Isles de Lipari,” pp. 35 and 85.
+
+
+ [44] In this case, and in that of the fissile pumice-stone, the
+ structure is very different from that in the foregoing cases, where
+ the laminæ consist of alternate layers of different composition or
+ texture. In some sedimentary formations, however, which apparently are
+ homogeneous and fissile, as in glossy clay-slate, there is reason to
+ believe, according to D’Aubuisson, that the laminæ are really due to
+ excessively thin, alternating, layers of mica.
+
+
+The laminæ of the beds, alternating with the obsidian at Ascension, dip
+at a high angle under the mountain, at the base of which they are
+situated; and they do not appear as if they had been inclined by
+violence. A high inclination is common to these beds in Mexico, Peru,
+and in some of the Italian islands:[45] on the other hand, in Hungary,
+the layers are horizontal; the laminæ, also, of some of the
+lava-streams above referred to, as far as I can understand the
+descriptions given of them, appear to be highly inclined or vertical. I
+doubt whether in any of these cases, the laminæ have been tilted into
+their present position; and in some instances, as in that of the
+trachyte described by Mr. Scrope, it is almost certain that they have
+been originally formed with a high inclination. In many of these cases,
+there is evidence that the mass of liquified rock has moved in the
+direction of the laminæ. At Ascension, many of the air-cells have a
+drawn out appearance, and are crossed by coarse semi-glassy fibres, in
+the direction of the laminæ; and some of the layers, separating the
+sphærulitic globules, have a scored appearance, as if produced by the
+grating of the globules. I have seen a specimen of zoned obsidian from
+Mexico, in Mr. Stokes’ collection, with the surfaces of the
+best-defined layers streaked or furrowed with parallel lines; and these
+lines or streaks precisely resembled those, produced on the surface of
+a mass of artificial glass by its having been poured out of a vessel.
+Humboldt, also, has described little cavities, which he compares to the
+tails of comets, behind sphærulites in laminated obsidian rocks from
+Mexico, and Mr. Scrope has
+described other cavities behind fragments embedded in his laminated
+trachyte, and which he supposes to have been produced during the
+movement of the mass.[46] From such facts, most authors have attributed
+the lamination of these volcanic rocks to their movement whilst
+liquified. Although it is easy to perceive, why each separate air-cell,
+or each fibre in pumice-stone,[47] should be drawn out in the direction
+of the moving mass; it is by no means at first obvious why such
+air-cells and fibres should be arranged by the movement, in the same
+planes, in laminæ absolutely straight and parallel to each other, and
+often of extreme tenuity; and still less obvious is it, why such layers
+should come to be of slightly different composition and of different
+textures.
+
+ [45] See Phillips’ “Mineralogy,” for the Italian Islands, p. 136. For
+ Mexico and Peru, see Humboldt’s “Essai Géognostique.” Mr. Edwards also
+ describes the high inclination of the obsidian rocks of the Cerro del
+ Navaja in Mexico in the _Proc. of the Geolog. Soc._ June 1838.
+
+
+ [46] “Geological Transactions,” vol. ii (second series), p. 200 etc.
+ These embedded fragments, in some instances, consist of the laminated
+ trachyte broken off and “enveloped in those parts, which still
+ remained liquid.” Beudant, also, frequently refers in his great work
+ on “Hungary” (tome iii, p. 386), to trachytic rocks, irregularly
+ spotted with fragments of the same varieties, which in other parts
+ form the parallel ribbons. In these cases, we must suppose, that after
+ part of the molten mass had assumed a laminated structure, a fresh
+ irruption of lava broke up the mass, and involved fragments, and that
+ subsequently the whole became relaminated.
+
+
+ [47] Dolomieu’s “Voyage,” p. 64.
+
+
+In endeavouring to make out the cause of the lamination of these
+igneous feldspathic rocks, let us return to the facts so minutely
+described at Ascension. We there see, that some of the thinnest layers
+are chiefly formed by numerous, exceedingly minute, though perfect,
+crystals of different minerals; that other layers are formed by the
+union of different kinds of concretionary globules, and that the layers
+thus formed, often cannot be distinguished from the ordinary
+feldspathic and pitchstone layers, composing a large portion of the
+entire mass. The fibrous radiating structure of the sphærulites seems,
+judging from many analogous cases, to connect the concretionary and
+crystalline forces: the separate crystals, also, of feldspar all lie in
+the same parallel planes.[48] These allied forces, therefore, have
+played an important part in the lamination of the mass, but they cannot
+be considered the primary force; for the several kinds of nodules, both
+the smallest and largest, are internally zoned with excessively fine
+shades of colour, parallel to the lamination of the whole; and many of
+them are, also, externally marked in the same direction with parallel
+ridges and furrows, which have not been produced by weathering.
+
+ [48] The formation, indeed, of a large crystal of any mineral in a
+ rock of mixed composition implies an aggregation of the requisite
+ atoms, allied to concretionary action. The cause of the crystals of
+ feldspar in these rocks of Ascension, being all placed lengthways, is
+ probably the same with that which elongates and flattens all the brown
+ sphærulitic globules (which behave like feldspar under the blowpipe)
+ in this same direction.
+
+Some of the finest streaks of colour in the stony layers, alternating
+with the obsidian, can be distinctly seen to be due to an incipient
+crystallisation of the constituent minerals. The extent to which the
+minerals have crystallised can, also, be distinctly seen to be
+connected
+with the greater or less size, and with the number, of the minute,
+flattened, crenulated air-cavities or fissures. Numerous facts, as in
+the case of geodes, and of cavities in silicified wood, in primary
+rocks, and in veins, show that crystallisation is much favoured by
+space. Hence, I conclude, that, if in a mass of cooling volcanic rock,
+any cause produced in parallel planes a number of minute fissures or
+zones of less tension (which from the pent-up vapours would often be
+expanded into crenulated air-cavities), the crystallisation of the
+constituent parts, and probably the formation of concretions, would be
+superinduced or much favoured in such planes; and thus, a laminated
+structure of the kind we are here considering would be generated.
+
+That some cause does produce parallel zones of less tension in volcanic
+rocks, during their consolidation, we must admit in the case of the
+thin alternate layers of obsidian and pumice described by Humboldt, and
+of the small, flattened, crenulated air-cells in the laminated rocks of
+Ascension; for on no other principle can we conceive why the confined
+vapours should through their expansion form air-cells or fibres in
+separate, parallel planes, instead of irregularly throughout the mass.
+In Mr. Stokes’ collection, I have seen a beautiful example of this
+structure, in a specimen of obsidian from Mexico, which is shaded and
+zoned, like the finest agate, with numerous, straight, parallel layers,
+more or less opaque and white, or almost perfectly glassy; the degree
+of opacity and glassiness depending on the number of microscopically
+minute, flattened air-cells; in this case, it is scarcely possible to
+doubt but that the mass, to which the fragment belonged, must have been
+subjected to some, probably prolonged, action, causing the tension
+slightly to vary in the successive planes.
+
+Several causes appear capable of producing zones of different tension,
+in masses semi-liquified by heat. In a fragment of devitrified glass, I
+have observed layers of sphærulites which appeared, from the manner in
+which they were abruptly bent, to have been produced by the simple
+contraction of the mass in the vessel, in which it cooled. In certain
+dikes on Mount Etna, described by M. Elie de Beaumont,[49] as bordered
+by alternating bands of scoriaceous and compact rock, one is led to
+suppose that the stretching movement of the surrounding strata, which
+originally produced the fissures, continued whilst the injected rock
+remained fluid. Guided, however, by Professor Forbes’[50] clear
+description of the zoned structure of glacier-ice, far the most
+probable explanation of the laminated structure of these feldspathic
+rocks appears to be, that they have been stretched whilst slowly
+flowing onwards in a pasty condition,[51] in precisely the same manner
+as Professor Forbes believes, that the ice of moving glaciers is
+stretched and fissured. In both cases,
+the zones may be compared to those in the finest agates; in both, they
+extend in the direction in which the mass has flowed, and those exposed
+on the surface are generally vertical: in the ice, the porous laminæ
+are rendered distinct by the subsequent congelation of infiltrated
+water, in the stony feldspathic lavas, by subsequent crystalline and
+concretionary action. The fragment of glassy obsidian in Mr. Stokes’
+collection, which is zoned with minute air-cells must strikingly
+resemble, judging from Professor Forbes’ descriptions, a fragment of
+the zoned ice; and if the rate of cooling and nature of the mass had
+been favourable to its crystallisation or to concretionary action, we
+should here have had the finest parallel zones of different composition
+and texture. In glaciers, the lines of porous ice and of minute
+crevices seem to be due to an incipient stretching, caused by the
+central parts of the frozen stream moving faster than the sides and
+bottom, which are retarded by friction: hence in glaciers of certain
+forms and towards the lower end of most glaciers, the zones become
+horizontal. May we venture to suppose that in the feldspathic lavas
+with horizontal laminæ, we see an analogous case? All geologists, who
+have examined trachytic regions, have come to the conclusion, that the
+lavas of this series have possessed an exceedingly imperfect fluidity;
+and as it is evident that only matter thus characterised would be
+subject to become fissured and to be formed into zones of different
+tensions, in the manner here supposed, we probably see the reason why
+augitic lavas, which appear generally to have possessed a high degree
+of fluidity, are not,[52] like the feldspathic lavas, divided into
+laminæ of different composition and texture. Moreover, in the augitic
+series, there never appears to be any tendency to concretionary action,
+which we have seen plays an important part in the lamination of rocks,
+of the trachytic series, or at least in rendering that structure
+apparent.
+
+ [49] “Mém. pour servir,” etc., tome iv, p. 131.
+
+
+ [50] _Edinburgh New Phil. Journal,_ 1842, p. 350.
+
+
+ [51] I presume that this is nearly the same explanation which Mr.
+ Scrope had in his mind, when he speaks (“Geolog. Transact.,” vol. ii,
+ second series, p. 228) of the ribboned structure of his trachytic
+ rocks, having arisen, from “a linear extension of the mass, while in a
+ state of imperfect liquidity, coupled with a concretionary process.”
+
+
+ [52] Basaltic lavas, and many other rocks, are not unfrequently
+ divided into thick laminæ or plates, of the same composition, which
+ are either straight or curved; these being crossed by vertical lines
+ of fissure, sometimes become united into columns. This structure seems
+ related, in its origin, to that by which many rocks, both igneous and
+ sedimentary, become traversed by parallel systems of fissures.
+
+Whatever may be thought of the explanation here advanced of the
+laminated structure of the rocks of the trachytic series, I venture to
+call the attention of geologists to the simple fact, that in a body of
+rock at Ascension, undoubtedly of volcanic origin, layers often of
+extreme tenuity, quite straight, and parallel to each other, have been
+produced;—some composed of distinct crystals of quartz and diopside,
+mingled with amorphous augitic specks and granular feldspar,—others
+entirely composed of these black augitic specks, with granules of oxide
+of iron,—and lastly, others formed of crystalline feldspar, in a more
+or less perfect state of purity, together with numerous crystals of
+feldspar, placed lengthways. At this island, there is reason to
+believe, and in some analogous cases, it is certainly known, that the
+laminæ have originally been formed with their present high inclination.
+Facts of this nature are manifestly of importance, with relation to the
+structural
+origin of that grand series of plutonic rocks, which like the volcanic
+have undergone the action of heat, and which consist of alternate
+layers of quartz, feldspar, mica and other minerals.
+
+
+
+
+Chapter IV ST. HELENA
+
+
+Lavas of the feldspathic, basaltic, and submarine series.—Section of
+Flagstaff Hill and of the Barn.—Dikes.—Turk’s Cap and Prosperous
+Bays.—Basaltic ring.—Central crateriform ridge, with an internal ledge
+and a parapet. Cones of phonolite. Superficial beds of calcareous
+sandstone.—Extinct land-shells.—Beds of detritus.—Elevation of the
+land.—Denudation.—Craters of elevation.
+
+
+The whole island is of volcanic origin; its circumference, according to
+Beatson,[1] is about twenty-eight miles. The central and largest part
+consists of rocks of a feldspathic nature, generally decomposed to an
+extraordinary degree; and when in this state, presenting a singular
+assemblage of alternating, red, purple, brown, yellow, and white, soft,
+argillaceous beds. From the shortness of our visit, I did not examine
+these beds with care; some of them, especially those of the white,
+yellow, and brown shades, originally existed as streams of lava, but
+the greater number were probably ejected in the form of scoriæ and
+ashes: other beds of a purple tint, porphyritic with crystal-shaped
+patches of a white, soft substance, which are now unctuous, and yield,
+like wax, a polished streak to the nail, seem once to have existed as
+solid claystone-porphyries: the red argillaceous beds generally have a
+brecciated structure, and no doubt have been formed by the
+decomposition of scoriæ. Several extensive streams, however, belonging
+to this series, retain their stony character; these are either of a
+blackish-green colour, with minute acicular crystals of feldspar, or of
+a very pale tint, and almost composed of minute, often scaly, crystals
+of feldspar, abounding with microscopical black specks; they are
+generally compact and laminated; others, however, of similar
+composition, are cellular and somewhat decomposed. None of these rocks
+contain large crystals of feldspar, or have the harsh fracture peculiar
+to trachyte. These feldspathic lavas and tuffs are the uppermost or
+those last erupted; innumerable dikes, however, and great masses of
+molten rock, have subsequently been injected into them. They converge,
+as they rise, towards the central curved ridge, of which one point
+attains the elevation of 2,700 feet. This ridge is the highest land in
+the island; and it once formed the northern rim of a great crater,
+whence the lavas of this series flowed: from its ruined condition, from
+the southern half having been removed, and from the violent dislocation
+which the whole island has undergone, its structure is rendered very
+obscure.
+
+ [1] Governor Beatson’s “Account of St. Helena.”
+
+
+_Basaltic series._—The margin of the island is formed by a rude circle
+of great, black, stratified, ramparts of basalt, dipping seaward, and
+worn into cliffs, which are often nearly perpendicular, and vary in
+height from a few hundred feet to two thousand. This circle, or rather
+horse-shoe shaped ring, is open to the south, and is breached by
+several other wide spaces. Its rim or summit generally projects little
+above the level of the adjoining inland country; and the more recent
+feldspathic lavas, sloping down from the central heights, generally
+abut against and overlap its inner margin; on the north-western side of
+the island, however, they appear (judging from a distance) to have
+flowed over and concealed portions of it. In some parts, where the
+basaltic ring has been breached, and the black ramparts stand detached,
+the feldspathic lavas have passed between them, and now overhang the
+sea-coast in lofty cliffs. The basaltic rocks are of a black colour and
+thinly stratified; they are generally highly vesicular, but
+occasionally compact; some of them contain numerous crystals of glassy
+feldspar and octahedrons of titaniferous iron; others abound with
+crystals of augite and grains of olivine. The vesicles are frequently
+lined with minute crystals (of chabasie?) and even become amygdaloidal
+with them. The streams are separated from each other by cindery matter,
+or by a bright red, friable, saliferous tuff, which is marked by
+successive lines like those of aqueous deposition; and sometimes it has
+an obscure, concretionary structure. The rocks of this basaltic series
+occur nowhere except near the coast. In most volcanic districts the
+trachytic lavas are of anterior origin to the basaltic; but here we
+see, that a great pile of rock, closely related in composition to the
+trachytic family, has been erupted subsequently to the basaltic strata:
+the number, however, of dikes, abounding with large crystals of augite,
+with which the feldspathic lavas have been injected, shows perhaps some
+tendency to a return to the more usual order of superposition.
+
+_Basal submarine lavas._—The lavas of this basal series lie immediately
+beneath both the basaltic and feldspathic rocks. According to Mr.
+Seale,[2] they may be seen at intervals on the sea-beach round the
+entire island. In the sections which I examined, their nature varied
+much; some of the strata abound with crystals of augite; others are of
+a brown colour, either laminated or in a rubbly condition; and many
+parts are highly amygdaloidal with calcareous matter. The successive
+sheets are either closely united together, or are separated from each
+other by beds of scoriaceous rock and of laminated tuff, frequently
+containing well-rounded fragments. The interstices of these beds are
+filled with gypsum and salt; the gypsum also sometimes occurring in
+thin layers. From the large quantity of these two substances, from the
+presence of rounded pebbles in the tuffs, and from the abundant
+amygdaloids, I cannot doubt that these basal volcanic strata flowed
+beneath the sea. This remark ought perhaps to be extended to a part of
+the superincumbent basaltic rocks; but on this point, I was not able to
+obtain clear evidence. The
+strata of the basal series, whenever I examined them, were intersected
+by an extraordinary number of dikes.
+
+ [2] “Geognosy of the Island of St. Helena.” Mr. Seale has constructed
+ a gigantic model of St. Helena, well worth visiting, which is now
+ deposited at Addiscombe College, in Surrey.
+
+_Flagstaff Hill and the Barn._—I will now describe some of the more
+remarkable sections, and will commence with these two hills, which form
+the principal external feature on the north-eastern side of the island.
+The square, angular outline, and black colour of the Barn, at once show
+that it belongs to the basaltic series; whilst the smooth, conical
+figure, and the varied bright tints of Flagstaff Hill, render it
+equally clear, that it is composed of the softened, feldspathic rocks.
+These two lofty hills are connected (as is shown in figure No. 8) by a
+sharp ridge, which is composed of the rubbly lavas of the basal series.
+The strata of this ridge dip westward, the inclination becoming less
+and less towards the Flagstaff; and the upper feldspathic strata of
+this hill can be seen, though with some difficulty, to dip conformably
+to the W.S.W. Close to the Barn, the strata of the ridge are nearly
+vertical, but are much obscured by innumerable dikes; under this hill,
+they probably change from being vertical into being inclined into an
+opposite direction; for the upper or basaltic strata, which are about
+eight hundred or one thousand feet in thickness, are inclined
+north-eastward, at an angle between thirty and forty degrees.
+
+No. 8
+
+
+[Illustration: Flagstaff Hill and the Barn.]
+
+The double lines represent the basaltic strata; the single, the basal
+submarine strata; the dotted, the upper feldspathic strata; the dikes
+are shaded transversely.
+
+
+This ridge, and likewise the Barn and Flagstaff Hills, are interlaced
+by dikes, many of which preserve a remarkable parallelism in a N.N.W.
+and S.S.E. direction. The dikes chiefly consist of a rock, porphyritic
+with large crystals of augite; others are formed of a fine-grained and
+brown-coloured trap. Most of these dikes are coated by a glossy
+layer,[3] from one to two-tenths of an inch in thickness, which, unlike
+true pitchstone, fuses into a black enamel; this layer is evidently
+analogous to the glossy superficial coating of many lava streams. The
+dikes can often be followed for great lengths both horizontally and
+vertically, and
+they seem to preserve a nearly uniform thickness:[4] Mr. Seale states,
+that one near the Barn, in a height of 1,260 feet, decreases in width
+only four inches,—from nine feet at the bottom, to eight feet and eight
+inches at the top. On the ridge, the dikes appear to have been guided
+in their course, to a considerable degree, by the alternating soft and
+hard strata: they are often firmly united to the harder strata, and
+they preserve their parallelism for such great lengths, that in very
+many instances it was impossible to conjecture, which of the beds were
+dikes, and which streams of lava. The dikes, though so numerous on this
+ridge, are even more numerous in the valleys a little south of it, and
+to a degree I never saw equalled anywhere else: in these valleys they
+extend in less regular lines, covering the ground with a network, like
+a spider’s web, and with some parts of the surface even appearing to
+consist wholly of dikes, interlaced by other dikes.
+
+ [3] This circumstance has been observed (Lyell, “Principles of
+ Geology,” vol. iv, chap. x, p. 9) in the dikes of the Atrio del
+ Cavallo, but apparently it is not of very common occurrence. Sir G.
+ Mackenzie, however, states (p. 372, “Travels in Iceland”) that all the
+ veins in Iceland have a “black vitreous coating on their sides.”
+ Captain Carmichael, speaking of the dikes in Tristan d’Acunha, a
+ volcanic island in the Southern Atlantic, says (“Linnæan
+ Transactions,” vol. xii, p. 485) that their sides, “where they come in
+ contact with the rocks, are invariably in a semi-vitrified state.”
+
+
+ [4] “Geognosy of the Island of St. Helena,” plate 5.
+
+
+From the complexity produced by the dikes, from the high inclination
+and anticlinal dip of the strata of the basal series, which are
+overlaid, at the opposite ends of the short ridge, by two great masses
+of different ages and of different composition, I am not surprised that
+this singular section has been misunderstood. It has even been supposed
+to form part of a crater; but so far is this from having been the case,
+that the summit of Flagstaff Hill once formed the lower extremity of a
+sheet of lava and ashes, which were erupted from the central,
+crateriform ridge. Judging from the slope of the contemporaneous
+streams in an adjoining and undisturbed part of the island, the strata
+of the Flagstaff Hill must have been upturned at least twelve hundred
+feet, and probably much more, for the great truncated dikes on its
+summit show that it has been largely denuded. The summit of this hill
+now nearly equals in height the crateriform ridge; and before having
+been denuded, it was probably higher than this ridge, from which it is
+separated by a broad and much lower tract of country; we here,
+therefore, see that the lower extremities of a set of lava-streams have
+been tilted up to as great a height as, or perhaps greater height than,
+the crater, down the flanks of which they originally flowed. I believe
+that dislocations on so grand a scale are extremely rare[5] in volcanic
+districts. The formation of such numbers of dikes in this part of the
+island shows that the surface must here have been stretched to a quite
+extraordinary degree: this stretching, on the ridge between Flagstaff
+and Barn Hills, probably took place subsequently (though perhaps
+immediately so) to the strata being tilted; for had the strata at that
+time extended horizontally, they would in all probability have been
+fissured and injected transversely, instead of in the planes of their
+stratification. Although the space between the Barn and Flagstaff Hill
+presents a distinct anticlinal line extending north and south, and
+though most of the dikes range with much regularity in the same line,
+nevertheless, at only a mile due south of the ridge the strata lie
+undisturbed. Hence the disturbing force seems to have acted under
+a point, rather than along a line. The manner in which it has acted, is
+probably explained by the structure of Little Stony-top, a mountain
+2,000 feet high, situated a few miles southward of the Barn; we there
+see, even from a distance, a dark-coloured, sharp, wedge of compact
+columnar rock, with the bright-coloured feldspathic strata, sloping
+away on each side from its uncovered apex. This wedge, from which it
+derives its name of Stony-top, consists of a body of rock, which has
+been injected whilst liquified into the overlying strata; and if we may
+suppose that a similar body of rock lies injected, beneath the ridge
+connecting the Barn and Flagstaff, the structure there exhibited would
+be explained.
+
+ [5] M. Constant Prevost (“Mém. de la Soc. Géolog.,” tome ii) observes
+ that “les produits volcaniques n’ont que localement et rarement même
+ dérangé le sol, à travers lequel ils se sont fait jour.”
+
+
+No. 9
+
+
+[Illustration: Prosperous Hill and The Barn.]
+
+The double lines represent the basaltic strata; the single, the basal
+submarine strata; the dotted, the upper feldspathic strata.
+
+_Turk’s Cap and Prosperous Bays._—Prosperous Hill is a great, black,
+precipitous mountain, situated two miles and a half south of the Barn,
+and composed, like it, of basaltic strata. These rest, in one part, on
+the brown-coloured, porphyritic beds of the basal series, and in
+another part, on a fissured mass of highly scoriaceous and amygdaloidal
+rock, which seems to have formed a small point of eruption beneath the
+sea, contemporaneously with the basal series. Prosperous Hill, like the
+Barn, is traversed by many dikes, of which the greater number range
+north and south, and its strata dip, at an angle of about 20 degrees,
+rather obliquely from the island towards the sea. The space between
+Prosperous Hill and the Barn, as represented in figure No. 9, consists
+of lofty cliffs, composed of the lavas of the upper or feldspathic
+series, which rest, though unconformably, on the basal submarine
+strata, as we have seen that they do at Flagstaff Hill. Differently,
+however, from in that hill, these upper strata are nearly horizontal,
+gently rising towards the interior of the island; and they are composed
+of greenish-black, or more commonly, pale brown, compact lavas, instead
+of softened and highly coloured matter. These brown-coloured, compact
+lavas, consist almost entirely of small glimmering scales, or of minute
+acicular crystals, of feldspar, placed close by the side of each other,
+and abounding with minute black specks, apparently of hornblende. The
+basaltic strata of Prosperous Hill project only a little above the
+level of the gently-sloping, feldspathic streams, which wind round and
+abut against their upturned edges. The inclination of the basaltic
+strata seems to be too great to have been caused by their having flowed
+down a slope, and they must have been tilted into their present
+position before the eruption of the feldspathic streams.
+
+_Basaltic ring._—Proceeding round the Island, the lavas of the upper
+series, southward of Prosperous Hill, overhang the sea in lofty
+precipices. Further on, the headland, called Great Stony-top, is
+composed, as I
+believe, of basalt; as is Long Range Point, on the inland side of which
+the coloured beds abut. On the southern side of the island, we see the
+basaltic strata of the South Barn, dipping obliquely seaward at a
+considerable angle; this headland, also, stands a little above the
+level of the more modern, feldspathic lavas. Further on, a large space
+of coast, on each side of Sandy Bay, has been much denuded, and there
+seems to be left only the basal wreck of the great, central crater. The
+basaltic strata reappear, with their seaward dip, at the foot of the
+hill, called Man-and-Horse; and thence they are continued along the
+whole north-western coast to Sugar-Loaf Hill, situated near to the
+Flagstaff; and they everywhere have the same seaward inclination, and
+rest, in some parts at least, on the lavas of the basal series. We thus
+see that the circumference of the island is formed by a much-broken
+ring, or rather, a horse-shoe, of basalt, open to the south, and
+interrupted on the eastern side by many wide breaches. The breadth of
+this marginal fringe on the north-western side, where alone it is at
+all perfect, appears to vary from a mile to a mile and a half. The
+basaltic strata, as well as those of the subjacent basal series, dip,
+with a moderate inclination, where they have not been subsequently
+disturbed, towards the sea. The more broken state of the basaltic ring
+round the eastern half, compared with the western half of the island,
+is evidently due to the much greater denuding power of the waves on the
+eastern or windward side, as is shown by the greater height of the
+cliffs on that side, than to leeward. Whether the margin of basalt was
+breached, before or after the eruption of the lavas of the upper
+series, is doubtful; but as separate portions of the basaltic ring
+appear to have been tilted before that event, and from other reasons,
+it is more probable, that some at least of the breaches were first
+formed. Reconstructing in imagination, as far as is possible, the ring
+of basalt, the internal space or hollow, which has since been filled up
+with the matter erupted from the great central crater, appears to have
+been of an oval figure, eight or nine miles in length by about four
+miles in breadth, and with its axis directed in a N.E. and S.W. line,
+coincident with the present longest axis of the island.
+
+_The central curved ridge._—This ridge consists, as before remarked, of
+grey feldspathic lavas, and of red, brecciated, argillaceous tuffs,
+like the beds of the upper coloured series. The grey lavas contain
+numerous, minute, black, easily fusible specks; and but very few large
+crystals of feldspar. They are generally much softened; with the
+exception of this character, and of being in many parts highly
+cellular, they are quite similar to those great sheets of lava which
+overhang the coast at Prosperous Bay. Considerable intervals of time
+appear to have elapsed, judging from the marks of denudation, between
+the formation of the successive beds, of which this ridge is composed.
+On the steep northern slope, I observed in several sections a much worn
+undulating surface of red tuff, covered by grey, decomposed,
+feldspathic lavas, with only a thin earthy layer interposed between
+them. In an adjoining part, I noticed a trap-dike, four feet wide, cut
+off and covered up by the feldspathic lava, as is represented in figure
+No. 9. The ridge ends on the eastern side in a hook, which is not
+represented clearly enough in any
+map which I have seen; towards the western end, it gradually slopes
+down and divides into several subordinate ridges. The best defined
+portion between Diana’s Peak and Nest Lodge, which supports the highest
+pinnacles in the island varying from 2,000 to 2,700 feet, is rather
+less than three miles long in a straight line. Throughout this space
+the ridge has a uniform appearance and structure; its curvature
+resembles that of the coast-line of a great bay, being made up of many
+smaller curves, all open to the south. The northern and outer side is
+supported by narrow ridges or buttresses, which slope down to the
+adjoining country. The inside is much steeper, and is almost
+precipitous; it is formed of the basset edges of the strata, which
+gently decline outwards. Along some parts of the inner side, a little
+way beneath the summit, a flat ledge extends, which imitates in outline
+the smaller curvatures of the crest. Ledges of this kind occur not
+unfrequently within volcanic craters, and their formation seems to be
+due to the sinking down of a level sheet of hardened lava, the edges of
+which remain (like the ice round a pool, from which the water has been
+drained) adhering to the sides.[6]
+
+ [6] A most remarkable instance of this structure is described in Ellis
+ “Polynesian Researches” (second edition), where an admirable drawing
+ is given of the successive ledges or terraces, on the borders of the
+ immense crater at Hawaii, in the Sandwich Islands.
+
+
+No. 10
+
+
+[Illustration: Dike]
+
+1—Grey feldspathic lava.
+2—A layer, one inch in thickness, of a reddish earthy matter.
+3—Brecciated, red, argillaceous tuff.
+
+
+In some parts, the ridge is surmounted by a wall or parapet,
+perpendicular on both sides. Near Diana’s Peak this wall is extremely
+narrow. At the Galapagos Archipelago I observed parapets, having a
+quite similar structure and appearance, surmounting several of the
+craters; one, which I more particularly examined, was composed of
+glossy, red scoriæ firmly cemented together; being externally
+perpendicular, and extending round nearly the whole circumference of
+the crater, it rendered it almost inaccessible. The Peak of Teneriffe
+and Cotopaxi, according to Humboldt, are similarly constructed; he
+states[7] that “at their summits a circular wall surrounds the crater,
+which wall, at a distance, has the appearance of a small cylinder
+placed on a truncated cone. On Cotopaxi[8] this peculiar structure is
+visible to the naked eye at more than two thousand toises’ distance;
+and no person has ever reached its crater. On the Peak of Teneriffe,
+the parapet is so high, that it would be impossible to reach the
+caldera, if on the eastern side there did not exist a breach.” The
+origin of these circular parapets is probably due to the heat or
+vapours from the crater, penetrating and hardening the sides to a
+nearly equal depth, and afterwards to the mountain being slowly acted
+on by the weather, which would leave the hardened part, projecting in
+the form of a cylinder or circular parapet.
+
+ [7] “Personal Narrative,” vol. i, p. 171.
+
+
+ [8] Humboldt’s “Picturesque Atlas,” folio, pl. 10.
+
+
+From the points of structure in the central ridge, now
+enumerated,—namely, from the convergence towards it of the beds of the
+upper series,—from the lavas there becoming highly cellular,—from the
+flat ledge, extending along its inner and precipitous side, like that
+within some still active craters,—from the parapet-like wall on its
+summit,—and lastly, from its peculiar curvature, unlike that of any
+common line of elevation, I cannot doubt that this curved ridge forms
+the last remnant of a great crater. In endeavouring, however, to trace
+its former outline, one is soon baffled; its western extremity
+gradually slopes down, and, branching into other ridges, extends to the
+sea-coast; the eastern end is more curved, but it is only a little
+better defined. Some appearances lead me to suppose that the southern
+wall of the crater joined the present ridge near Nest Lodge; in this
+case the crater must have been nearly three miles long, and about a
+mile and a half in breadth. Had the denudation of the ridge and the
+decomposition of its constituent rocks proceeded a few steps further,
+and had this ridge, like several other parts of the island, been broken
+up by great dikes and masses of injected matter, we should in vain have
+endeavoured to discover its true nature. Even now we have seen that at
+Flagstaff Hill the lower extremity and most distant portion of one
+sheet of the erupted matter has been upheaved to as great a height as
+the crater down which it flowed, and probably even to a greater height.
+It is interesting thus to trace the steps by which the structure of a
+volcanic district becomes obscured, and finally obliterated: so near to
+this last stage is St. Helena, that I believe no one has hitherto
+suspected that the central ridge or axis of the island is the last
+wreck of the crater, whence the most modern volcanic streams were
+poured forth.
+
+The great hollow space or valley southward of the central curved ridge,
+across which the half of the crater must once have extended, is formed
+of bare, water-worn hillocks and ridges of red, yellow, and brown
+rocks, mingled together in chaos-like confusion, interlaced by dikes,
+and without any regular stratification. The chief part consists of red
+decomposing scoriæ, associated with various kinds of tuff and yellow
+argillaceous beds, full of broken crystals, those of augite being
+particularly large. Here and there masses of highly cellular and
+amygdaloidal lavas protrude. From one of the ridges in the midst of the
+valley, a conical precipitous hill, called Lot, boldly stands up, and
+forms a most singular and conspicuous object. It is composed of
+phonolite, divided in one part into great curved laminæ, in another,
+into angular concretionary balls, and in a third part into outwardly
+radiating columns. At its base the strata of lava, tuff, and scoriæ,
+dip away on all sides;[9] the uncovered portion is 197 feet[10] in
+height, and its horizontal section gives an oval figure. The phonolite
+is of a greenish-grey
+colour, and is full of minute acicular crystals of feldspar; in most
+parts it has a conchoidal fracture, and is sonorous, yet it is
+crenulated with minute air-cavities. In a S.W. direction from Lot,
+there are some other remarkable columnar pinnacles, but of a less
+regular shape, namely, Lot’s Wife, and the Asses’ Ears, composed of
+allied kinds of rock. From their flattened shape, and their relative
+position to each other, they are evidently connected on the same line
+of fissure. It is, moreover, remarkable that this same N.E. and S.W.
+line, joining Lot and Lot’s Wife, if prolonged would intersect
+Flagstaff Hill, which, as before stated, is crossed by numerous dikes
+running in this direction, and which has a disturbed structure,
+rendering it probable that a great body of once fluid rock lies
+injected beneath it.
+
+ [9] Abich in his “Views of Vesuvius” (plate vi), has shown the manner
+ in which beds, under nearly similar circumstances, are tilted up. The
+ upper beds are more turned up than the lower; and he accounts for
+ this, by showing that the lava insinuates itself horizontally between
+ the lower beds.
+
+
+ [10] This height is given by Mr. Seale in his Geognosy of the island.
+ The height of the summit above the level of the sea is said to be
+ 1,444 feet.
+
+
+In this same great valley there are several other conical masses of
+injected rock (one, I observed, was composed of compact greenstone),
+some of which are not connected, as far as is apparent, with any line
+of dike; whilst others are obviously thus connected. Of these dikes,
+three or four great lines stretch across the valley in a N.E. and S.W.
+direction, parallel to that one connecting the Asses’ Ears, Lot’s Wife,
+and probably Lot. The number of these masses of injected rock is a
+remarkable feature in the geology of St. Helena. Besides those just
+mentioned, and the hypothetical one beneath Flagstaff Hill, there is
+Little Stony-top and others, as I have reason to believe, at the
+Man-and-Horse, and at High Hill. Most of these masses, if not all of
+them, have been injected subsequently to the last volcanic eruptions
+from the central crater. The formation of conical bosses of rock on
+lines of fissure, the walls of which are in most cases parallel, may
+probably be attributed to inequalities in the tension, causing small
+transverse fissures, and at these points of intersection the edges of
+the strata would naturally yield, and be easily turned upwards.
+Finally, I may remark, that hills of phonolite everywhere are apt[11]
+to assume singular and even grotesque shapes, like that of Lot: the
+peak at Fernando Noronha offers an instance; at St. Jago, however, the
+cones of phonolite, though tapering, have a regular form. Supposing, as
+seems probable, that all such hillocks or obelisks have originally been
+injected, whilst liquified, into a mould formed by yielding strata, as
+certainly has been the case with Lot, how are we to account for the
+frequent abruptness and singularity of their outlines, compared with
+similarly injected masses of greenstone and basalt? Can it be due to a
+less perfect degree of fluidity, which is generally supposed to be
+characteristic of the allied trachytic lavas?
+
+ [11] D’Aubuisson, in his “Traité de Géognosie” (tome ii, p. 540)
+ particularly remarks that this is the case.
+
+_Superficial deposits._—Soft calcareous sandstone occurs in extensive,
+though thin, superficial beds, both on the northern and southern shores
+of the island. It consists of very minute, equal-sized, rounded
+particles of shells, and other organic bodies, which partially retain
+their yellow, brown, and pink colours, and occasionally, though very
+rarely, present an obscure trace of their original external forms. I in
+vain endeavoured to find a single unrolled fragment of a shell. The
+colour of the particles
+is the most obvious character by which their origin can be recognised,
+the tints being affected (and an odour produced) by a moderate heat, in
+the same manner as in fresh shells. The particles are cemented
+together, and are mingled with some earthy matter: the purest masses,
+according to Beatson, contain 70 per cent of carbonate of lime. The
+beds, varying in thickness from two or three feet to fifteen feet, coat
+the surface of the ground; they generally lie on that side of the
+valley which is protected from the wind, and they occur at the height
+of several hundred feet above the level of the sea. Their position is
+the same which sand, if now drifted by the trade-wind, would occupy;
+and no doubt they thus originated, which explains the equal size and
+minuteness of the particles, and likewise the entire absence of whole
+shells, or even of moderately-sized fragments. It is remarkable that at
+the present day there are no shelly beaches on any part of the coast,
+whence calcareous dust could be drifted and winnowed; we must,
+therefore, look back to a former period when before the land was worn
+into the present great precipices, a shelving coast, like that of
+Ascension, was favourable to the accumulation of shelly detritus. Some
+of the beds of this limestone are between six hundred and seven hundred
+feet above the sea; but part of this height may possibly be due to an
+elevation of the land, subsequent to the accumulation of the calcareous
+sand.
+
+The percolation of rain-water has consolidated parts of these beds into
+a solid rock, and has formed masses of dark brown, stalagmitic
+limestone. At the Sugar-Loaf quarry, fragments of rock on the adjoining
+slopes[12] have been thickly coated by successive fine layers of
+calcareous matter. It is singular, that many of these pebbles have
+their entire surfaces coated, without any point of contact having been
+left uncovered; hence, these pebbles must have been lifted up by the
+slow deposition between them of the successive films of carbonate of
+lime. Masses of white, finely oolitic rock are attached to the outside
+of some of these coated pebbles. Von Buch has described a compact
+limestone at Lanzarote, which seems perfectly to resemble the
+stalagmitic deposition just mentioned: it coats pebbles, and in parts
+is finely oolitic: it forms a far-extended layer, from one inch to two
+or three feet in thickness, and it occurs at the height of 800 feet
+above the sea, but only on that side of the island exposed to the
+violent north-western winds. Von Buch remarks,[13] that it is not found
+in hollows, but only on the unbroken and inclined surfaces of the
+mountain. He believes, that it has been deposited by the spray which is
+borne over the whole island by these violent winds. It appears,
+however, to me much more probable that it has been formed, as at St.
+Helena, by the percolation of water through finely comminuted shells:
+for when sand is blown on
+a much-exposed coast, it always tends to accumulate on broad, even
+surfaces, which offer a uniform resistance to the winds. At the
+neighbouring island, moreover, of Feurteventura,[14] there is an earthy
+limestone, which, according to Von Buch, is quite similar to specimens
+which he has seen from St. Helena, and which he believes to have been
+formed by the drifting of shelly detritus.
+
+ [12] In the earthy detritus on several parts of this hill, irregular
+ masses of very impure, crystallised sulphate of lime occur. As this
+ substance is now being abundantly deposited by the surf at Ascension,
+ it is possible that these masses may thus have originated; but if so,
+ it must have been at a period when the land stood at a much lower
+ level. This earthy selenite is now found at a height of between six
+ hundred and seven hundred feet.
+
+
+ [13] “Description des Isles Canaries,” p. 293.
+
+
+ [14] _Idem,_ pp. 314 and 374.
+
+
+The upper beds of the limestone, at the above-mentioned quarry on the
+Sugar-Loaf Hill, are softer, finer-grained and less pure, than the
+lower beds. They abound with fragments of land-shells, and with some
+perfect ones; they contain, also, the bones of birds, and the large
+eggs,[15] apparently of water-fowl. It is probable that these upper
+beds remained long in an unconsolidated form, during which time, these
+terrestrial productions were embedded. Mr. G. R. Sowerby has kindly
+examined three species of land-shells, which I procured from this bed,
+and has described them in detail. One of them is a Succinea, identical
+with a species now living abundantly on the island; the two others,
+namely, _ Cochlogena fossilis_ and _Helix biplicata_, are not known in
+a recent state: the latter species was also found in another and
+different locality, associated with a species of Cochlogena which is
+undoubtedly extinct.
+
+ [15] Colonel Wilkes, in a catalogue presented with some specimens to
+ the Geological Society, states that as many as ten eggs were found by
+ one person. Dr. Buckland has remarked (“Geolog. Trans.,” vol. v, p.
+ 474) on these eggs.
+
+
+_Beds of extinct land-shells._—Land-shells, all of which appear to be
+species now extinct, occur embedded in earth, in several parts of the
+island. The greater number have been found at a considerable height on
+Flagstaff Hill. On the N.W. side of this hill, a rain-channel exposes a
+section of about twenty feet in thickness, of which the upper part
+consists of black vegetable mould, evidently washed down from the
+heights above, and the lower part of less black earth, abounding with
+young and old shells, and with their fragments: part of this earth is
+slightly consolidated by calcareous matter, apparently due to the
+partial decomposition of some of the shells. Mr. Seale, an intelligent
+resident, who first called attention to these shells, gave me a large
+collection from another locality, where the shells appear to have been
+embedded in very black earth. Mr. G. R. Sowerby has examined these
+shells, and has described them. There are seven species, namely, one
+Cochlogena, two species of the genus Cochlicopa, and four of Helix;
+none of these are known in a recent state, or have been found in any
+other country. The smaller species were picked out of the inside of the
+large shells of the _Cochlogena aurisvulpina._ This last-mentioned
+species is in many respects a very singular one; it was classed, even
+by Lamarck, in a marine genus, and having thus been mistaken for a
+sea-shell, and the smaller accompanying species having been overlooked,
+the exact localities where it was found have been measured, and the
+elevation of this island thus deduced! It is very remarkable that all
+the shells of this species found by me in one spot, form a distinct
+variety, as described by Mr. Sowerby, from those
+procured from another locality by Mr. Seale. As this Cochlogena is a
+large and conspicuous shell, I particularly inquired from several
+intelligent countrymen whether they had ever seen it alive; they all
+assured me that they had not, and they would not even believe that it
+was a land animal: Mr. Seale, moreover, who was a collector of shells
+all his life at St. Helena, never met with it alive. Possibly some of
+the smaller species may turn out to be yet living kinds; but, on the
+other hand, the two land-shells which are now living on the island in
+great numbers, do not occur embedded, as far as is yet known, with the
+extinct species. I have shown in my “Journal,”[16] that the extinction
+of these land-shells possibly may not be an ancient event; as a great
+change took place in the state of the island about one hundred and
+twenty years ago, when the old trees died, and were not replaced by
+young ones, these being destroyed by the goats and hogs, which had run
+wild in numbers, from the year 1502. Mr. Seale states, that on
+Flagstaff Hill, where we have seen that the embedded land-shells are
+especially numerous, traces are everywhere discoverable, which plainly
+indicate that it was once thickly clothed with trees; at present not
+even a bush grows there. The thick bed of black vegetable mould which
+covers the shell-bed, on the flanks of this hill, was probably washed
+down from the upper part, as soon as the trees perished, and the
+shelter afforded by them was lost.
+
+ [16] “Journal of Researches,” p. 582.
+
+
+_Elevation of the land._—Seeing that the lavas of the basal series,
+which are of submarine origin, are raised above the level of the sea,
+and at some places to the height of many hundred feet, I looked out for
+superficial signs of the elevation of the land. The bottoms of some of
+the gorges, which descend to the coast, are filled up to the depth of
+about a hundred feet, by rudely divided layers of sand, muddy clay, and
+fragmentary masses; in these beds, Mr. Seale has found the bones of the
+tropic-bird and of the albatross; the former now rarely, and the latter
+never visiting the island. From the difference between these layers,
+and the sloping piles of detritus which rest on them, I suspect that
+they were deposited, when the gorges stood beneath the sea. Mr. Seale,
+moreover, has shown that some of the fissure-like gorges[17] become,
+with a concave outline, gradually rather wider at the bottom than at
+the top; and this peculiar structure was probably caused by the wearing
+action of the sea, when it entered the lower part of these gorges. At
+greater heights, the evidence of the rise of the land is even less
+clear: nevertheless, in a bay-like depression on the table-land behind
+Prosperous Bay, at the height of about a thousand feet, there are
+flat-topped masses of rock, which it is scarcely conceivable, could
+have been insulated from the surrounding and similar strata, by any
+other agency than the denuding action of a sea-beach. Much denudation,
+indeed, has been effected at great elevations, which it would not be
+easy to explain by any other means: thus, the flat summit of the Barn,
+which is 2,000 feet high, presents, according to Mr. Seale, a perfect
+network of truncated dikes; on hills like the Flagstaff, formed of soft
+rock, we might suppose that the dikes had been worn down and cut off by
+meteoric agency, but we can hardly suppose this possible with the hard,
+basaltic strata of the Barn.
+
+ [17] A fissure-like gorge, near Stony-top, is said by Mr. Seale to be
+ 840 feet deep, and only 115 feet in width.
+
+
+_Coast denudation._—The enormous cliffs, in many parts between one and
+two thousand feet in height, with which this prison-like island is
+surrounded, with the exception of only a few places, where narrow
+valleys descend to the coast, is the most striking feature in its
+scenery. We have seen that portions of the basaltic ring, two or three
+miles in length by one or two miles in breadth, and from one to two
+thousand feet in height, have been wholly removed. There are, also,
+ledges and banks of rock, rising out of profoundly deep water, and
+distant from the present coast between three and four miles, which,
+according to Mr. Seale, can be traced to the shore, and are found to be
+the continuations of certain well-known great dikes. The swell of the
+Atlantic Ocean has obviously been the active power in forming these
+cliffs; and it is interesting to observe that the lesser, though still
+great, height of the cliffs on the leeward and partially protected side
+of the island (extending from the Sugar-Loaf Hill to South West Point),
+corresponds with the lesser degree of exposure. When reflecting on the
+comparatively low coasts of many volcanic islands, which also stand
+exposed in the open ocean, and are apparently of considerable
+antiquity, the mind recoils from an attempt to grasp the number of
+centuries of exposure, necessary to have ground into mud and to have
+dispersed the enormous cubic mass of hard rock which has been pared off
+the circumference of this island. The contrast in the superficial state
+of St. Helena, compared with the nearest island, namely, Ascension, is
+very striking. At Ascension, the surfaces of the lava-streams are
+glossy, as if just poured forth, their boundaries are well defined, and
+they can often be traced to perfect craters, whence they were erupted;
+in the course of many long walks, I did not observe a single dike; and
+the coast round nearly the entire circumference is low, and has been
+eaten back (though too much stress must not be placed on this fact, as
+the island may have been subsiding) into a little wall only from ten to
+thirty feet high. Yet during the 340 years, since Ascension has been
+known, not even the feeblest signs of volcanic action have been
+recorded.[18] On the other hand, at St. Helena, the course of no one
+stream of lava can be traced, either by the state of its boundaries or
+of its superficies; the mere wreck of one great crater is left; not the
+valleys only, but the surfaces of some of the highest hills, are
+interlaced by worn-down dikes, and, in many
+places, the denuded summits of great cones of injected rock stand
+exposed and naked; lastly, as we have seen, the entire circuit of the
+island has been deeply worn back into the grandest precipices.
+
+ [18] In the _Nautical Magazine_ for 1835, p. 642, and for 1838, p.
+ 361, and in the “Comptes Rendus,” April 1838, accounts are given of a
+ series of volcanic phenomena—earthquakes—troubled water—floating
+ scoriæ and columns of smoke—which have been observed at intervals
+ since the middle of the last century, in a space of open sea between
+ longitudes 20° and 22° west, about half a degree south of the equator.
+ These facts seem to show, that an island or an archipelago is in
+ process of formation in the middle of the Atlantic: a line joining St.
+ Helena and Ascension, prolonged, intersects this slowly nascent focus
+ of volcanic action.
+
+_Craters of Elevation._
+
+There is much resemblance in structure and in geological history
+between St. Helena, St. Jago, and Mauritius. All three islands are
+bounded (at least in the parts which I was able to examine) by a ring
+of basaltic mountains, now much broken, but evidently once continuous.
+These mountains have, or apparently once had, their escarpments steep
+towards the interior of the island, and their strata dip outwards. I
+was able to ascertain, only in a few cases, the inclination of the
+beds; nor was this easy, for the stratification was generally obscure,
+except when viewed from a distance. I feel, however, little doubt that,
+according to the researches of M. Elie de Beaumont, their average
+inclination is greater than that which they could have acquired,
+considering their thickness and compactness, by flowing down a sloping
+surface. At St. Helena, and at St. Jago, the basaltic strata rest on
+older and probably submarine beds of different composition. At all
+three islands, deluges of more recent lavas have flowed from the centre
+of the island, towards and between the basaltic mountains; and at St.
+Helena the central platform has been filled up by them. All three
+islands have been raised in mass. At Mauritius the sea, within a late
+geological period, must have reached to the foot of the basaltic
+mountains, as it now does at St. Helena; and at St. Jago it is cutting
+back the intermediate plain towards them. In these three islands, but
+especially at St. Jago and at Mauritius, when, standing on the summit
+of one of the old basaltic mountains, one looks in vain towards the
+centre of the island,—the point towards which the strata beneath one’s
+feet, and of the mountains on each side, rudely converge,—for a source
+whence these strata could have been erupted; but one sees only a vast
+hollow platform stretched beneath, or piles of matter of more recent
+origin.
+
+These basaltic mountains come, I presume, into the class of Craters of
+elevation: it is immaterial whether the rings were ever completely
+formed, for the portions which now exist have so uniform a structure,
+that, if they do not form fragments of true craters, they cannot be
+classed with ordinary lines of elevation. With respect to their origin,
+after having read the works of Mr. Lyell,[19] and of MM. C. Prevost and
+Virlet, I cannot believe that the great central hollows have been
+formed by a simple dome-shaped elevation, and the consequent arching of
+the strata. On the other hand, I have very great difficulty in
+admitting that these basaltic mountains are merely the basal fragments
+of great volcanoes, of which the summits have either been blown off, or
+more probably swallowed up by subsidence. These rings are, in some
+instances, so immense, as at St. Jago and at Mauritius, and their
+occurrence is so frequent, that I can hardly persuade myself to adopt
+this explanation. Moreover, I suspect that the following circumstances,
+from their frequent concurrence, are someway connected together,—a
+connection not implied in either of the above views: namely, first, the
+broken state of the ring; showing that the now detached portions have
+been exposed to great denudation, and in some cases, perhaps, rendering
+it probable that the ring never was entire; secondly, the great amount
+of matter erupted from the central area after or during the formation
+of the ring; and thirdly, the elevation of the district in mass. As far
+as relates to the inclination of the strata being greater than that
+which the basal fragments of ordinary volcanoes would naturally
+possess, I can readily believe that this inclination might have been
+slowly acquired by that amount of elevation, of which, according to M.
+Elie de Beaumont, the numerous upfilled fissures or dikes are the
+evidence and the measure,—a view equally novel and important, which we
+owe to the researches of that geologist on Mount Etna.
+
+ [19] “Principles of Geology” (fifth edit.), vol. ii, p. 171.
+
+
+A conjecture, including the above circumstances, occurred to me, when,—
+with my mind fully convinced, from the phenomena of 1835 in South
+America,[20] that the forces which eject matter from volcanic orifices
+and raise continents in mass are identical,—I viewed that part of the
+coast of St. Jago, where the horizontally upraised, calcareous stratum
+dips into the sea, directly beneath a cone of subsequently erupted
+lava. The conjecture is that, during the slow elevation of a volcanic
+district or island, in the centre of which one or more orifices
+continue open, and thus relieve the subterranean forces, the borders
+are elevated more than the central area; and that the portions thus
+upraised do not slope gently into the central, less elevated area, as
+does the calcareous stratum under the cone at St. Jago, and as does a
+large part of the circumference of Iceland,[21] but that they are
+separated from it by curved faults.
+We might expect, from what we see along ordinary faults, that the
+strata on the upraised side, already dipping outwards from their
+original formation as lava-streams, would be tilted from the line of
+fault, and thus have their inclination increased. According to this
+hypothesis, which I am tempted to extend only to some few cases, it is
+not probable that the ring would ever be formed quite perfect; and from
+the elevation being slow, the upraised portions would generally be
+exposed to much denudation, and hence the ring become broken; we might
+also expect to find occasional inequalities in the dip of the upraised
+masses, as is the case at St. Jago. By this hypothesis the elevation of
+the districts in mass, and the flowing of deluges of lava from the
+central platforms, are likewise connected together. On this view the
+marginal basaltic mountains of the three foregoing islands might still
+be considered as forming “Craters of elevation;” the kind of elevation
+implied having been slow, and the central hollow or platform having
+been formed, not by the arching of the surface, but simply by that part
+having been upraised to a less height.
+
+ [20] I have given a detailed account of these phenomena, in a paper
+ read before the Geological Society in March 1838. At the instant of
+ time, when an immense area was convulsed and a large tract elevated,
+ the districts immediately surrounding several of the great vents in
+ the Cordillera remained quiescent; the subterranean forces being
+ apparently relieved by the eruptions, which then recommenced with
+ great violence. An event of somewhat the same kind, but on an
+ infinitely smaller scale, appears to have taken place, according to
+ Abich (“Views of Vesuvius,” plates i and ix), within the great crater
+ of Vesuvius, where a platform on one side of a fissure was raised in
+ mass twenty feet, whilst on the other side, a train of small volcanoes
+ burst forth in eruption.
+
+
+ [21] It appears, from information communicated to me in the most
+ obliging manner by M. E. Robert, that the circumferential parts of
+ Iceland, which are composed of ancient basaltic strata alternating
+ with tuff, dip inland, thus forming a gigantic saucer. M. Robert found
+ that this was the case, with a few and quite local exceptions, for a
+ space of coast several hundred miles in length. I find this statement
+ corroborated, as far as regards one place, by Mackenzie in his
+ “Travels” (p. 377), and in another place by some MS. notes kindly lent
+ me by Dr. Holland. The coast is deeply indented by creeks, at the head
+ of which the land is generally low. M. Robert informs me, that the
+ inwardly dipping strata appear to extend as far as this line, and that
+ their inclination usually corresponds with the slope of the surface,
+ from the high coast-mountains to the low land at the head of these
+ creeks. In the section described by Sir G. Mackenzie, the dip is 120.
+ The interior parts of the island chiefly consist, as far as is known,
+ of recently erupted matter. The great size, however, of Iceland,
+ equalling the bulkiest part of England, ought perhaps to exclude it
+ from the class of islands we have been considering; but I cannot avoid
+ suspecting that if the coast-mountains, instead of gently sloping into
+ the less elevated central area, had been separated from it by
+ irregularly curved faults, the strata would have been tilted seaward,
+ and a “Crater of elevation,” like that of St. Jago or that of
+ Mauritius, but of much vaster dimensions, would have been formed. I
+ will only further remark, that the frequent occurrence of extensive
+ lakes at the foot of large volcanoes, and the frequent association of
+ volcanic and fresh-water strata, seem to indicate that the areas
+ around volcanoes are apt to be depressed beneath the level of the
+ adjoining country, either from having been less elevated, or from the
+ effects of subsidence.
+
+
+
+
+Chapter V GALAPAGOS ARCHIPELAGO.
+
+
+Chatham Island.—Craters composed of a peculiar kind of tuff.—Small
+basaltic craters, with hollows at their bases.—Albemarle Island, fluid
+lavas, their composition.—Craters of tuff, inclination of their
+exterior diverging strata, and structure of their interior converging
+strata.—James Island, segment of a small basaltic crater; fluidity and
+composition of its lava-streams, and of its ejected
+fragments.—Concluding remarks on the craters of tuff, and on the
+breached condition of their southern sides.—Mineralogical composition
+of the rocks of the archipelago.—Elevation of the land. Direction of
+the fissures of eruption.
+
+
+This archipelago is situated under the equator, at a distance of
+between five and six hundred miles from the west coast of South
+America. It consists of five principal islands, and of several small
+ones, which together are equal in area,[1] but not in extent of land,
+to Sicily, conjointly with the Ionian Islands. They are all volcanic:
+on two, craters have been seen in eruption, and on several of the other
+islands, streams of lava have a recent appearance. The larger islands
+are chiefly composed of solid rock, and they rise with a tame outline
+to a height of between one and four thousand feet. They are sometimes,
+but not generally, surmounted by one principal orifice. The craters
+vary in size from mere spiracles to huge caldrons several miles in
+circumference; they are extraordinarily numerous, so that I should
+think, if enumerated, they would be found to exceed two thousand; they
+are formed either of scoriæ and lava, or of a brown-coloured tuff; and
+these latter craters are in several respects remarkable. The whole
+group was surveyed by the officers of the _Beagle._ I visited myself
+four of the principal islands, and received specimens from all the
+others. Under the head of the different islands I will describe only
+that which appears to me deserving of attention.
+
+ [1] I exclude from this measurement, the small volcanic islands of
+ Culpepper and Wenman, lying seventy miles northward of the group.
+ Craters were visible on all the islands of the group, except on Towers
+ Island, which is one of the lowest; this island is, however, formed of
+ volcanic rocks.
+
+
+No. 11
+
+
+[Illustration: Galapagos Archipelago.]
+
+Galapagos Archipelago
+
+
+CHATHAM ISLAND. _Craters composed of a singular kind of tuff._—Towards
+the eastern end of this island there occur two craters composed of two
+kinds of tuff; one kind being friable, like slightly consolidated
+ashes; and the other compact, and of a different nature from anything
+which I have met with described. This latter substance, where it is
+best characterised, is of a yellowish-brown colour, translucent, and
+with a lustre somewhat resembling resin; it is brittle, with an
+angular, rough, and very irregular fracture, sometimes, however, being
+slightly granular, and even obscurely crystalline: it can readily be
+scratched with a knife, yet some points are hard enough just to mark
+common glass; it fuses with ease into a blackish-green glass. The mass
+contains numerous broken crystals of olivine and augite, and small
+particles of black and brown scoriæ; it is often traversed by thin
+seams of calcareous matter. It generally affects a nodular or
+concretionary structure. In a hand specimen, this substance would
+certainly be mistaken for a pale and peculiar variety of pitchstone;
+but when seen in mass its stratification, and the numerous layers of
+fragments of basalt, both angular and rounded, at once render its
+subaqueous origin evident. An examination of a series of specimens
+shows that this resin-like substance results from a chemical change on
+small particles of pale and dark-coloured scoriaceous rocks; and this
+change could be distinctly traced in different stages round the edges
+of even the same particle. The position near the coast of all the
+craters composed of this kind of tuff or peperino, and their breached
+condition, renders it probable that they were all formed when standing
+immersed in the sea; considering this circumstance, together with the
+remarkable absence of large beds of ashes in the whole archipelago, I
+think it highly probable that much the greater part of the tuff has
+originated from the trituration of fragments of the grey, basaltic
+lavas in the mouths of craters standing in the sea. It may be asked
+whether the heated water within these craters has produced this
+singular change in the small scoriaceous particles and given to them
+their translucent, resin-like fracture. Or has the associated lime
+played any part in this change? I ask these questions from having found
+at St. Jago, in the Cape de Verde Islands, that where a great stream of
+molten lava has flowed over a calcareous bottom into the sea, the
+outermost film, which in other parts resembles pitchstone, is changed,
+apparently by its contact with the carbonate of lime, into a resin-like
+substance, precisely like the best characterised specimens of the tuff
+from this archipelago.[2]
+
+ [2] The concretions containing lime, which I have described at
+ Ascension, as formed in a bed of ashes, present some degree of
+ resemblance to this substance, but they have not a resinous fracture.
+ At St. Helena, also, I found veins of a somewhat similar, compact, but
+ non-resinous substance, occurring in a bed of pumiceous ashes,
+ apparently free from calcareous matter: in neither of these cases
+ could heat have acted.
+
+
+To return to the two craters: one of them stands at the distance of a
+league from the coast, the intervening tract consisting of a calcareous
+tuff, apparently of submarine origin. This crater consists of a circle
+of hills some of which stand quite detached, but all have a very
+regular,
+quâ-quâ versal dip, at an inclination of between thirty and forty
+degrees. The lower beds, to the thickness of several hundred feet,
+consist of the resin-like stone, with embedded fragments of lava. The
+upper beds, which are between thirty and forty feet in thickness, are
+composed of a thinly stratified, fine-grained, harsh, friable,
+brown-coloured tuff, or peperino.[3] A central mass without any
+stratification, which must formerly have occupied the hollow of the
+crater, but is now attached only to a few of the circumferential hills,
+consists of a tuff, intermediate in character between that with a
+resin-like, and that with an earthy fracture. This mass contains white
+calcareous matter in small patches. The second crater (520 feet in
+height) must have existed until the eruption of a recent, great stream
+of lava, as a separate islet; a fine section, worn by the sea, shows a
+grand funnel-shaped mass of basalt, surrounded by steep, sloping flanks
+of tuff, having in parts an earthy, and in others a semi-resinous
+fracture. The tuff is traversed by several broad, vertical dikes, with
+smooth and parallel sides, which I did not doubt were formed of basalt,
+until I actually broke off fragments. These dikes, however, consist of
+tuff like that of the surrounding strata, but more compact, and with a
+smoother fracture; hence we must conclude, that fissures were formed
+and filled up with the finer mud or tuff from the crater, before its
+interior was occupied, as it now is, by a solidified pool of basalt.
+Other fissures have been subsequently formed, parallel to these
+singular dikes, and are merely filled with loose rubbish. The change
+from ordinary scoriaceous particles to the substance with a
+semi-resinous fracture, could be clearly followed in portions of the
+compact tuff of these dikes.
+
+ [3] Those geologists who restrict the term of “tuff” to ashes of a
+ white colour, resulting from the attrition of feldspathic lavas, would
+ call these brown-coloured strata “peperino.”
+
+
+No. 12
+
+
+[Illustration: The Kicker Rock.]
+
+The Kicker Rock, 400 feet high.
+
+At the distance of a few miles from these two craters, stands the
+Kicker Rock, or islet, remarkable from its singular form. It is
+unstratified, and is composed of compact tuff, in parts having the
+resin-like fracture. It is probable that this amorphous mass, like that
+similar mass in the case first described, once filled up the central
+hollow of a crater, and that its flanks, or sloping walls, have since
+been worn quite away by the sea, in which it stands exposed.
+
+_Small basaltic craters._—A bare, undulating tract, at the eastern end
+of Chatham Island, is remarkable from the number, proximity, and form
+of the small basaltic craters with which it is studded. They consist,
+either
+of a mere conical pile, or, but less commonly, of a circle, of black
+and red, glossy scoriæ, partially cemented together. They vary in
+diameter from thirty to one hundred and fifty yards, and rise from
+about fifty to one hundred feet above the level of the surrounding
+plain. From one small eminence, I counted sixty of these craters, all
+of which were within a third of a mile from each other, and many were
+much closer. I measured the distance between two very small craters,
+and found that it was only thirty yards from the summit-rim of one to
+the rim of the other. Small streams of black, basaltic lava, containing
+olivine and much glassy feldspar, have flowed from many, but not from
+all of these craters. The surfaces of the more recent streams were
+exceedingly rugged, and were crossed by great fissures; the older
+streams were only a little less rugged; and they were all blended and
+mingled together in complete confusion. The different growth, however,
+of the trees on the streams, often plainly marked their different ages.
+Had it not been for this latter character, the streams could in few
+cases have been distinguished; and, consequently, this wide undulatory
+tract might have (as probably many tracts have) been erroneously
+considered as formed by one great deluge of lava, instead of by a
+multitude of small streams, erupted from many small orifices.
+
+In several parts of this tract, and especially at the base of the small
+craters, there are circular pits, with perpendicular sides, from twenty
+to forty feet deep. At the foot of one small crater, there were three
+of these pits. They have probably been formed, by the falling in of the
+roofs of small caverns.[4] In other parts, there are mammiform
+hillocks, which resemble great bubbles of lava, with their summits
+fissured by irregular cracks, which appeared, upon entering them, to be
+very deep; lava has not flowed from these hillocks. There are, also,
+other very regular, mammiform hillocks, composed of stratified lava,
+and surmounted by circular, steep-sided hollows, which, I suppose have
+been formed by a body of gas, first, arching the strata into one of the
+bubble-like hillocks, and then, blowing off its summit. These several
+kinds of hillocks and pits, as well as the numerous, small, scoriaceous
+craters, all show that this tract has been penetrated, almost like a
+sieve, by the passage of heated vapours. The more regular hillocks
+could only have been heaved up, whilst the lava was in a softened
+state.[5]
+
+ [4] (M. Elie de Beaumont has described (“Mém. pour servir,” etc., tome
+ iv, p. 113) many “petits cirques d’eboulement” on Etna, of some of
+ which the origin is historically known.
+
+
+ [5] Sir G. Mackenzie (“Travels in Iceland,” pp. 389 to 392) has
+ described a plain of lava at the foot of Hecla, everywhere heaved up
+ into great bubbles or blisters. Sir George states that this cavernous
+ lava composes the uppermost stratum; and the same fact is affirmed by
+ Von Buch (“Descript. des Isles Canaries,” p. 159), with respect to the
+ basaltic stream near Rialejo, in Teneriffe. It appears singular that
+ it should be the upper streams that are chiefly cavernous, for one
+ sees no reason why the upper and lower should not have been equally
+ affected at different times;—have the inferior streams flowed beneath
+ the pressure of the sea, and thus been flattened, after the passage
+ through them, of bodies of gas?
+
+
+ALBEMARLE ISLAND.—This island consists of five, great, flat-topped
+craters, which, together with the one on the adjoining island of
+Narborough, singularly resemble each other, in form and height. The
+southern one is 4,700 feet high, two others are 3,720 feet, a third
+only 50 feet higher, and the remaining ones apparently of nearly the
+same height. Three of these are situated on one line, and their craters
+appear elongated in nearly the same direction. The northern crater,
+which is not the largest, was found by the triangulation to measure,
+externally, no less than three miles and one-eighth of a mile in
+diameter. Over the lips of these great, broad caldrons, and from little
+orifices near their summits, deluges of black lava have flowed down
+their naked sides.
+
+_Fluidity of different lavas._—Near Tagus or Banks’ Cove, I examined
+one of these great streams of lava, which is remarkable from the
+evidence of its former high degree of fluidity, especially when its
+composition is considered. Near the sea-coast this stream is several
+miles in width. It consists of a black, compact base, easily fusible
+into a black bead, with angular and not very numerous air-cells, and
+thickly studded with large, fractured crystals of glassy albite,[6]
+varying from the tenth of an inch to half an inch in diameter. This
+lava, although at first sight appearing eminently porphyritic, cannot
+properly be considered so, for the crystals have evidently been
+enveloped, rounded, and penetrated by the lava, like fragments of
+foreign rock in a trap-dike. This was very clear in some specimens of a
+similar lava, from Abingdon Island, in which the only difference was,
+that the vesicles were spherical and more numerous. The albite in these
+lavas is in a similar condition with the leucite of Vesuvius, and with
+the olivine, described by Von Buch,[7] as projecting in great balls
+from the basalt of Lanzarote. Besides the albite, this lava contains
+scattered grains of a green mineral, with no distinct cleavage, and
+closely resembling olivine;[8] but as it fuses easily into a green
+glass, it belongs probably to the augitic family: at James Island,
+however, a similar lava contained true olivine. I obtained specimens
+from the actual
+surface, and from a depth of four feet, but they differed in no
+respect. The high degree of fluidity of this lava-stream was at once
+evident, from its smooth and gently sloping surface, from the manner in
+which the main stream was divided by small inequalities into little
+rills, and especially from the manner in which its edges, far below its
+source, and where it must have been in some degree cooled, thinned out
+to almost nothing; the actual margin consisting of loose fragments, few
+of which were larger than a man’s head. The contrast between this
+margin, and the steep walls, above twenty feet high, bounding many of
+the basaltic streams at Ascension, is very remarkable. It has generally
+been supposed that lavas abounding with large crystals, and including
+angular vesicles,[9] have possessed little fluidity; but we see that
+the case has been very different at Albemarle Island. The degree of
+fluidity in different lavas, does not seem to correspond with any
+_apparent_ corresponding amount of difference in their composition: at
+Chatham Island, some streams, containing much glassy albite and some
+olivine, are so rugged, that they may be compared to a sea frozen
+during a storm; whilst the great stream at Albemarle Island is almost
+as smooth as a lake when ruffled by a breeze. At James Island, black
+basaltic lava, abounding with small grains of olivine, presents an
+intermediate degree of roughness; its surface being glossy, and the
+detached fragments resembling, in a very singular manner, folds of
+drapery, cables, and pieces of the bark of trees.[10]
+
+ [6] In the Cordillera of Chile, I have seen lava very closely
+ resembling this variety at the Galapagos Archipelago. It contained,
+ however, besides the albite, well-formed crystals of augite, and the
+ base (perhaps in consequence of the aggregation of the augitic
+ particles) was a shade lighter in colour. I may here remark, that in
+ all these cases, I call the feldspathic crystals, _albite_, from their
+ cleavage-planes (as measured by the reflecting goniometer)
+ corresponding with those of that mineral. As, however, other species
+ of this genus have lately been discovered to cleave in nearly the same
+ planes with albite, this determination must be considered as only
+ provisional. I examined the crystals in the lavas of many different
+ parts of the Galapagos group, and I found that none of them, with the
+ exception of some crystals from one part of James Island, cleaved in
+ the direction of orthite or potash-feldspar.
+
+
+ [7] “Description des Isles Canaries,” p. 295.
+
+
+ [8] Humboldt mentions that he mistook a green augitic mineral,
+ occurring in the volcanic rocks of the Cordillera of Quito, for
+ olivine.
+
+
+ [9] The irregular and angular form of the vesicles is probably caused
+ by the unequal yielding of a mass composed, in almost equal
+ proportion, of solid crystals and of a viscid base. It certainly seems
+ a general circumstance, as might have been expected, that in lava,
+ which has possessed a high degree of fluidity, _as well as an
+ even-sized grain_, the vesicles are internally smooth and spherical.
+
+
+ [10] A specimen of basaltic lava, with a few small broken crystals of
+ albite, given me by one of the officers, is perhaps worthy of
+ description. It consists of cylindrical ramifications, some of which
+ are only the twentieth of an inch in diameter, and are drawn out into
+ the sharpest points. The mass has not been formed like a stalactite,
+ for the points terminate both upwards and downwards. Globules, only
+ the fortieth of an inch in diameter, have dropped from some of the
+ points, and adhere to the adjoining branches. The lava is vesicular,
+ but the vesicles never reach the surface of the branches, which are
+ smooth and glossy. As it is generally supposed that vesicles are
+ always elongated in the direction of the movement of the fluid mass, I
+ may observe, that in these cylindrical branches, which vary from a
+ quarter to only the twentieth of an inch in diameter, every air-cell
+ is spherical.
+
+
+_Craters of tuff._—About a mile southward of Banks’ Cove, there is a
+fine elliptic crater, about five hundred feet in depth, and
+three-quarters of a mile in diameter. Its bottom is occupied by a lake
+of brine, out of which some little crateriform hills of tuff rise. The
+lower beds are formed of compact tuff, appearing like a subaqueous
+deposit; whilst the upper beds, round the entire circumference, consist
+of a harsh, friable tuff, of little specific gravity, but often
+containing fragments of rock in layers. This upper tuff contains
+numerous pisolitic balls, about the size of small bullets, which differ
+from the surrounding matter, only in being slightly
+harder and finer grained. The beds dip away very regularly on all
+sides, at angles varying, as I found by measurement, from twenty-five
+to thirty degrees. The external surface of the crater slopes at a
+nearly similar inclination, and is formed by slightly convex ribs, like
+those on the shell of a pecten or scallop, which become broader as they
+extend from the mouth of the crater to its base. These ribs are
+generally from eight to twenty feet in breadth, but sometimes they are
+as much as forty feet broad; and they resemble old, plastered, much
+flattened vaults, with the plaster scaling off in plates: they are
+separated from each other by gullies, deepened by alluvial action. At
+their upper and narrow ends, near the mouth of the crater, these ribs
+often consist of real hollow passages, like, but rather smaller than,
+those often formed by the cooling of the crust of a lava-stream, whilst
+the inner parts have flowed onward;—of which structure I saw many
+examples at Chatham Island. There can be no doubt but that these hollow
+ribs or vaults have been formed in a similar manner, namely, by the
+setting or hardening of a superficial crust on streams of mud, which
+have flowed down from the upper part of the crater. In another part of
+this same crater, I saw open concave gutters between one and two feet
+wide, which appear to have been formed by the hardening of the lower
+surface of a mud stream, instead of, as in the former case, of the
+upper surface. From these facts I think it is certain that the tuff
+must have flowed as mud.[11] This mud may have been formed either
+within the crater, or from ashes deposited on its upper parts, and
+afterwards washed down by torrents of rain. The former method, in most
+of the cases, appears the more probable one; at James Island, however,
+some beds of the friable kind of tuff extend so continuously over an
+uneven surface, that probably they were formed by the falling of
+showers of ashes.
+
+ [11] This conclusion is of some interest, because M. Dufrenoy (“Mém.
+ pour servir,” tome iv, p. 274) has argued from strata of tuff,
+ apparently of similar composition with that here described, being
+ inclined at angles between 18° and 20°, that Monte Nuevo and some
+ other craters of Southern Italy have been formed by upheaval. From the
+ facts given above, of the vaulted character of the separate rills, and
+ from the tuff not extending in horizontal sheets round these
+ crateriform hills, no one will suppose that the strata have here been
+ produced by elevation; and yet we see that their inclination is above
+ 20°, and often as much as 30°. The consolidated strata also, of the
+ internal talus, as will be immediately seen, dips at an angle of above
+ 30°.
+
+Within this same crater, strata of coarse tuff, chiefly composed of
+fragments of lava, abut, like a consolidated talus, against the inside
+walls. They rise to a height of between one hundred and one hundred and
+fifty feet above the surface of the internal brine-lake; they dip
+inwards, and are inclined at an angle varying from thirty to thirty-six
+degrees. They appear to have been formed beneath water, probably at a
+period when the sea occupied the hollow of the crater. I was surprised
+to observe that beds having this great inclination did not, as far as
+they could be followed, thicken towards their lower extremities.
+
+_Banks’ Cove._—This harbour occupies part of the interior of a
+shattered crater of tuff larger than that last described. All the tuff
+is
+compact, and includes numerous fragments of lava; it appears like a
+subaqueous deposit. The most remarkable feature in this crater is the
+great development of strata converging inwards, as in the last case, at
+a considerable inclination, and often deposited in irregular curved
+layers. These interior converging beds, as well as the proper,
+diverging crateriform strata, are represented in figure No. 13, a rude,
+sectional sketch of the headlands, forming this Cove. The internal and
+external strata differ little in composition, and the former have
+evidently resulted from the wear and tear, and redeposition of the
+matter forming the external crateriform strata. From the great
+development of these inner beds, a person walking round the rim of this
+crater might fancy himself on a circular anticlinal ridge of stratified
+sandstone and conglomerate. The sea is wearing away the inner and outer
+strata, and especially the latter; so that the inwardly converging
+strata will, perhaps, in some future age, be left standing alone—a case
+which might at first perplex a geologist.[12]
+
+ [12] I believe that this case actually occurs in the Azores, where Dr.
+ Webster (“Description,” p. 185) has described a basin-formed, little
+ island, composed of _strata of tuff_, dipping inwards and bounded
+ externally by steep sea-worn cliffs. Dr. Daubeny supposes (on
+ Volcanoes, p. 266), that this cavity must have been formed by a
+ circular subsidence. It appears to me far more probable, that we here
+ have strata which were originally deposited within the hollow of a
+ crater, of which the exterior walls have since been removed by the
+ sea.
+
+
+No. 13
+
+
+[Illustration: Sectional sketch of headlands forming Banks’ Cove.]
+
+A sectional sketch of the headlands forming Banks’ Cove, showing the
+diverging craterform strata, and the converging stratified talus. The
+highest point of these hills is 817 feet above the sea.
+
+JAMES ISLAND.—Two craters of tuff on this island are the only remaining
+ones which require any notice. One of them lies a mile and a half
+inland from Puerto Grande: it is circular, about the third of a mile in
+diameter, and 400 feet in depth. It differs from all the other
+tuff-craters which I examined, in having the lower part of its cavity,
+to the height of between one hundred and one hundred and fifty feet,
+formed by a precipitous wall of basalt, giving to the crater the
+appearance of having burst through a solid sheet of rock. The upper
+part of this crater consists of strata of the altered tuff, with a
+semi-resinous fracture. Its bottom is
+occupied by a shallow lake of brine, covering layers of salt, which
+rest on deep black mud. The other crater lies at the distance of a few
+miles, and is only remarkable from its size and perfect condition. Its
+summit is 1,200 feet above the level of the sea, and the interior
+hollow is 600 feet deep. Its external sloping surface presented a
+curious appearance from the smoothness of the wide layers of tuff,
+which resembled a vast plastered floor. Brattle Island is, I believe,
+the largest crater in the Archipelago composed of tuff; its interior
+diameter is nearly a nautical mile. At present it is in a ruined
+condition, consisting of little more than half a circle open to the
+south; its great size is probably due, in part, to internal
+degradation, from the action of the sea.
+
+No. 14
+
+
+[Illustration: Segment of very small orifice of eruption.]
+
+Segment of a very small orifice of eruption, on the beach of
+Fresh-water Bay.
+
+_Segment of a basaltic crater._—One side of Fresh-water Bay, in James
+Island, is bounded by a promontory, which forms the last wreck of a
+great crater. On the beach of this promontory, a quadrant-shaped
+segment of a small subordinate point of eruption stands exposed. It
+consists of nine separate little streams of lava piled upon each other;
+and of an irregular pinnacle, about fifteen feet high, of
+reddish-brown, vesicular basalt, abounding with large crystals of
+glassy albite, and with fused augite. This pinnacle, and some adjoining
+paps of rock on the beach, represent the axis of the crater. The
+streams of lava can be followed up a little ravine, at right angles to
+the coast, for between ten and fifteen yards, where they are hidden by
+detritus: along the beach they are visible for nearly eighty yards, and
+I do not believe that they extend much further. The three lower streams
+are united to the pinnacle; and at the point of junction (as shown in
+figure No. 14, a rude sketch made on the spot), they are slightly
+arched, as if in the act of flowing over the lip of the crater. The six
+upper streams no doubt were originally united to this same column
+before it was worn down by the sea. The lava of these streams is of
+similar composition with that of the pinnacle, excepting that the
+crystals of albite appear to be more comminuted, and the grains of
+fused augite are absent. Each stream is separated from the one above it
+by a few inches, or at most by one or two feet in thickness, of loose
+fragmentary scoriæ,
+apparently derived from the abrasion of the streams in passing over
+each other. All these streams are very remarkable from their thinness.
+I carefully measured several of them; one was eight inches thick, but
+was firmly coated with three inches above, and three inches below, of
+red scoriaceous rock (which is the case with all the streams), making
+altogether a thickness of fourteen inches: this thickness was preserved
+quite uniformly along the entire length of the section. A second stream
+was only eight inches thick, including both the upper and lower
+scoriaceous surfaces. Until examining this section, I had not thought
+it possible that lava could have flowed in such uniformly thin sheets
+over a surface far from smooth. These little streams closely resemble
+in composition that great deluge of lava at Albemarle Island, which
+likewise must have possessed a high degree of fluidity.
+
+_Pseudo-extraneous, ejected fragments._—In the lava and in the scoriæ
+of this little crater, I found several fragments, which, from their
+angular form, their granular structure, their freedom from air-cells,
+their brittle and burnt condition, closely resembled those fragments of
+primary rocks which are occasionally ejected, as at Ascension, from
+volcanoes. These fragments consist of glassy albite, much mackled, and
+with very imperfect cleavages, mingled with semi-rounded grains, having
+tarnished, glossy surfaces, of a steel-blue mineral. The crystals of
+albite are coated by a red oxide of iron, appearing like a residual
+substance; and their cleavage-planes also are sometimes separated by
+excessively fine layers of this oxide, giving to the crystals the
+appearance of being ruled like a glass micrometer. There was no quartz.
+The steel-blue mineral, which is abundant in the pinnacle, but which
+disappears in the streams derived from the pinnacle, has a fused
+appearance, and rarely presents even a trace of cleavage; I obtained,
+however, one measurement, which proved that it was augite; and in one
+other fragment, which differed from the others, in being slightly
+cellular, and in gradually blending into the surrounding matrix the
+small grains of this mineral were tolerably well crystallised. Although
+there is so wide a difference in appearance between the lava of the
+little streams, and especially of their red scoriaceous crusts, and one
+of these angular ejected fragments, which at first sight might readily
+be mistaken for syenite, yet I believe that the lava has originated
+from the melting and movement of a mass of rock of absolutely similar
+composition with the fragments. Besides the specimen above alluded to,
+in which we see a fragment becoming slightly cellular, and blending
+into the surrounding matrix, some of the grains of the steel-blue
+augite also have their surfaces becoming very finely vesicular, and
+passing into the nature of the surrounding paste; other grains are
+throughout, in an intermediate condition. The paste seems to consist of
+the augite more perfectly fused, or, more probably, merely disturbed in
+its softened state by the movement of the mass, and mingled with the
+oxide of iron and with finely comminuted, glassy albite. Hence probably
+it is that the fused albite, which is abundant in the pinnacle,
+disappears in the streams. The albite is in exactly the same state,
+with the exception of most of the crystals being smaller in the lava
+and in the embedded fragments; but
+in the fragments they appear to be less abundant: this, however, would
+naturally happen from the intumescence of the augitic base, and its
+consequent apparent increase in bulk. It is interesting thus to trace
+the steps by which a compact granular rock becomes converted into a
+vesicular, pseudo-porphyritic lava, and finally into red scoriæ. The
+structure and composition of the embedded fragments show that they are
+parts either of a mass of primary rock which has undergone considerable
+change from volcanic action, or more probably of the crust of a body of
+cooled and crystallised lava, which has afterwards been broken up and
+re-liquified; the crust being less acted on by the renewed heat and
+movement.
+
+_Concluding remarks on the tuff-craters._—These craters, from the
+peculiarity of the resin-like substance which enters largely into their
+composition, from their structure, their size and number, present the
+most striking feature in the geology of this Archipelago. The majority
+of them form either separate islets, or promontories attached to the
+larger islands; and those which now stand at some little distance from
+the coast are worn and breached, as if by the action of the sea. From
+this general circumstance of their position, and from the small
+quantity of ejected ashes in any part of the Archipelago, I am led to
+conclude, that the tuff has been chiefly produced, by the grinding
+together of fragments of lava within active craters, communicating with
+the sea. In the origin and composition of the tuff, and in the frequent
+presence of a central lake of brine and of layers of salt, these
+craters resemble, though on a gigantic scale, the “salses,” or hillocks
+of mud, which are common in some parts of Italy and in other
+countries.[13] Their closer connection, however, in this Archipelago,
+with ordinary volcanic action, is shown by the pools of solidified
+basalt, with which they are sometimes filled up.
+
+ [13] D’Aubuisson’s “Traité de Géognosie,” tome i, p. 189. I may
+ remark, that I saw at Terceira, in the Azores, a crater of tuff or
+ peperino, very similar to these of the Galapagos Archipelago. From the
+ description given in Freycinet “Voyage,” similar ones occur at the
+ Sandwich Islands; and probably they are present in many other places.
+
+
+It at first appears very singular, that all the craters formed of tuff
+have their southern sides, either quite broken down and wholly removed,
+or much lower than the other sides. I saw and received accounts of
+twenty-eight of these craters; of these, twelve form separate
+islets,[14] and now exist as mere crescents quite open to the south,
+with occasionally a few points of rock marking their former
+circumference: of the remaining sixteen, some form promontories, and
+others stand at a little distance inland from the shore; but all have
+their southern sides either the lowest, or quite broken down. Two,
+however, of the sixteen had
+their northern sides also low, whilst their eastern and western sides
+were perfect. I did not see, or hear of, a single exception to the
+rule, of these craters being broken down or low on the side, which
+faces a point of the horizon between S.E. and S.W. This rule does not
+apply to craters composed of lava and scoriæ. The explanation is
+simple: at this Archipelago, the waves from the trade-wind, and the
+swell propagated from the distant parts of the open ocean, coincide in
+direction (which is not the case in many parts of the Pacific), and
+with their united forces attack the southern sides of all the islands;
+and consequently the southern slope, even when entirely formed of hard
+basaltic rock, is invariably steeper than the northern slope. As the
+tuff-craters are composed of a soft material, and as probably all, or
+nearly all, have at some period stood immersed in the sea, we need not
+wonder that they should invariably exhibit on their exposed sides the
+effects of this great denuding power. Judging from the worn condition
+of many of these craters, it is probable that some have been entirely
+washed away. As there is no reason to suppose, that the craters formed
+of scoriæ and lava were erupted whilst standing in the sea, we can see
+why the rule does not apply to them. At Ascension, it was shown that
+the mouths of the craters, which are there all of terrestrial origin,
+have been affected by the trade-wind; and this same power might here,
+also, aid in making the windward and exposed sides of some of the
+craters originally the lowest.
+
+ [14] These consist of the three Crossman Islets, the largest of which
+ is 600 feet in height; Enchanted Island; Gardner Island (760 feet
+ high); Champion Island (331 feet high); Enderby Island; Brattle
+ Island; two islets near Indefatigable Island; and one near James
+ Island. A second crater near James Island (with a salt lake in its
+ centre) has its southern side only about twenty feet high, whilst the
+ other parts of the circumference are about three hundred feet in
+ height.
+
+
+_Mineralogical composition of the rocks._—In the northern islands, the
+basaltic lavas seem generally to contain more albite than they do in
+the southern half of the Archipelago; but almost all the streams
+contain some. The albite is not unfrequently associated with olivine. I
+did not observe in any specimen distinguishable crystals of hornblende
+or augite; I except the fused grains in the ejected fragments, and in
+the pinnacle of the little crater, above described. I did not meet with
+a single specimen of true trachyte; though some of the paler lavas,
+when abounding with large crystals of the harsh and glassy albite,
+resemble in some degree this rock; but in every case the basis fuses
+into a black enamel. Beds of ashes and far-ejected scoriæ, as
+previously stated, are almost absent; nor did I see a fragment of
+obsidian or of pumice. Von Buch[15] believes that the absence of pumice
+on Mount Etna is consequent on the feldspar being of the Labrador
+variety; if the presence of pumice depends on the constitution of the
+feldspar, it is remarkable, that it should be absent in this
+archipelago, and abundant in the Cordillera of South America, in both
+of which regions the feldspar is of the albitic variety. Owing to the
+absence of ashes, and the general indecomposable character of the lava
+in this Archipelago, the islands are slowly clothed with a poor
+vegetation, and the scenery has a desolate and frightful aspect.
+
+ [15] “Description des Isles Canaries,” p. 328.
+
+_Elevation of the land._—Proofs of the rising of the land are scanty
+and imperfect. At Chatham Island, I noticed some great blocks of lava,
+cemented by calcareous matter, containing recent shells; but they
+occurred at the height of only a few feet above high-water mark. One
+of the officers gave me some fragments of shells, which he found
+embedded several hundred feet above the sea, in the tuff of two
+craters, distant from each other. It is possible, that these fragments
+may have been carried up to their present height in an eruption of mud;
+but as, in one instance, they were associated with broken
+oyster-shells, almost forming a layer, it is more probable that the
+tuff was uplifted with the shells in mass. The specimens are so
+imperfect that they can be recognised only as belonging to recent
+marine genera. On Charles Island, I observed a line of great rounded
+blocks, piled on the summit of a vertical cliff, at the height of
+fifteen feet above the line, where the sea now acts during the heaviest
+gales. This appeared, at first, good evidence in favour of the
+elevation of the land; but it was quite deceptive, for I afterwards saw
+on an adjoining part of this same coast, and heard from eye-witnesses,
+that wherever a recent stream of lava forms a smooth inclined plane,
+entering the sea, the waves during gales have the power of _rolling up
+rounded_ blocks to a great height, above the line of their ordinary
+action. As the little cliff in the foregoing case is formed by a stream
+of lava, which, before being worn back, must have entered the sea with
+a gently sloping surface, it is possible or rather it is probable, that
+the rounded boulders, now lying on its summit, are merely the remnants
+of those which had been _rolled up_ during storms to their present
+height.
+
+_Direction of the fissures of eruption._—The volcanic orifices in this
+group cannot be considered as indiscriminately scattered. Three great
+craters on Albermarle Island form a well-marked line, extending N.W. by
+N. and S.E. by S. Narborough Island, and the great crater on the
+rectangular projection of Albemarle Island, form a second parallel
+line. To the east, Hood’s Island, and the islands and rocks between it
+and James Island, form another nearly parallel line, which, when
+prolonged, includes Culpepper and Wenman Islands, lying seventy miles
+to the north. The other islands lying further eastward, form a less
+regular fourth line. Several of these islands, and the vents on
+Albemarle Island, are so placed, that they likewise fall on a set of
+rudely parallel lines, intersecting the former lines at right angles;
+so that the principal craters appear to lie on the points where two
+sets of fissures cross each other. The islands themselves, with the
+exception of Albemarle Island, are not elongated in the same direction
+with the lines on which they stand. The direction of these islands is
+nearly the same with that which prevails in so remarkable a manner in
+the numerous archipelagoes of the great Pacific Ocean. Finally, I may
+remark, that amongst the Galapagos Islands there is no one dominant
+vent much higher than all the others, as may be observed in many
+volcanic archipelagoes: the highest is the great mound on the
+south-western extremity of Albemarle Island, which exceeds by barely a
+thousand feet several other neighbouring craters.
+
+
+
+
+Chapter VI TRACHYTE AND BASALT.—DISTRIBUTION OF VOLCANIC ISLES.
+
+
+The sinking of crystals in fluid lava.—Specific gravity of the
+constituent parts of trachyte and of basalt, and their consequent
+separation.—Obsidian.—Apparent non-separation of the elements of
+plutonic rocks.—Origin of trap-dikes in the plutonic
+series.—Distribution of volcanic islands; their prevalence in the great
+oceans.—They are generally arranged in lines.—The central volcanoes of
+Von Buch doubtful.—Volcanic islands bordering continents.—Antiquity of
+volcanic islands, and their elevation in mass.—Eruptions on parallel
+lines of fissure within the same geological period.
+
+
+_On the separation of the constituent minerals of lava, according to
+their specific gravities._—One side of Fresh-water Bay, in James
+Island, is formed by the wreck of a large crater, mentioned in the last
+chapter, of which the interior has been filled up by a pool of basalt,
+about two hundred feet in thickness. This basalt is of a grey colour,
+and contains many crystals of glassy albite, which become much more
+numerous in the lower, scoriaceous part. This is contrary to what might
+have been expected, for if the crystals had been originally
+disseminated in equal numbers, the greater intumescence of this lower
+scoriaceous part would have made them appear fewer in number. Von
+Buch[1] has described a stream of obsidian on the Peak of Teneriffe, in
+which the crystals of feldspar become more and more numerous, as the
+depth or thickness increases, so that near the lower surface of the
+stream the lava even resembles a primary rock. Von Buch further states,
+that M. Dree, in his experiments in melting lava, found that the
+crystals of feldspar always tended to precipitate themselves to the
+bottom of the crucible. In these cases, I presume there can be no
+doubt[2] that the crystals sink from their weight. The specific gravity
+of feldspar varies[3] from 2·4 to 2·58, whilst obsidian seems commonly
+to be from 2·3 to 2·4; and in a fluidified state its specific gravity
+would probably be less, which would
+facilitate the sinking of the crystals of feldspar. At James Island,
+the crystals of albite, though no doubt of less weight than the grey
+basalt, in the parts where compact, might easily be of greater specific
+gravity than the scoriaceous mass, formed of melted lava and bubbles of
+heated gas.
+
+ [1] “Description des Isles Canaries,” pp. 190 and 191.
+
+
+ [2] In a mass of molten iron, it is found (_Edinburgh New
+ Philosophical Journal_, vol. xxiv, p. 66) that the substances, which
+ have a closer affinity for oxygen than iron has, rise from the
+ interior of the mass to the surface. But a similar cause can hardly
+ apply to the separation of the crystals of these lava-streams. The
+ cooling of the surface of lava seems, in some cases, to have affected
+ its composition; for Dufrenoy (“Mém. pour servir,” tome iv, p. 271)
+ found that the interior parts of a stream near Naples contained
+ two-thirds of a mineral which was acted on by acids, whilst the
+ surface consisted chiefly of a mineral unattackable by acids.
+
+
+ [3] I have taken the specific gravities of the simple minerals from
+ Von Kobell, one of the latest and best authorities, and of the rocks
+ from various authorities. Obsidian, according to Phillips, is 2·35;
+ and Jameson says it never exceeds 2·4; but a specimen from Ascension,
+ weighed by myself, was 2·42.
+
+
+The sinking of crystals through a viscid substance like molten rock, as
+is unequivocally shown to have been the case in the experiments of M.
+Drée, is worthy of further consideration, as throwing light on the
+separation of the trachytic and basaltic series of lavas. Mr. P. Scrope
+has speculated on this subject; but he does not seem to have been aware
+of any positive facts, such as those above given; and he has overlooked
+one very necessary element, as it appears to me, in the
+phenomenon—namely, the existence of either the lighter or heavier
+mineral in globules or in crystals. In a substance of imperfect
+fluidity, like molten rock, it is hardly credible, that the separate,
+infinitely small atoms, whether of feldspar, augite, or of any other
+mineral, would have power from their slightly different gravities to
+overcome the friction caused by their movement; but if the atoms of any
+one of these minerals became, whilst the others remained fluid, united
+into crystals or granules, it is easy to perceive that from the
+lessened friction, their sinking or floating power would be greatly
+increased. On the other hand, if all the minerals became granulated at
+the same time, it is scarcely possible, from their mutual resistance,
+that any separation could take place. A valuable, practical discovery,
+illustrating the effect of the granulation of one element in a fluid
+mass, in aiding its separation, has lately been made: when lead
+containing a small proportion of silver, is constantly stirred whilst
+cooling, it becomes granulated, and the grains of imperfect crystals of
+nearly pure lead sink to the bottom, leaving a residue of melted metal
+much richer in silver; whereas if the mixture be left undisturbed,
+although kept fluid for a length of time, the two metals show no signs
+of separating.[4] The sole use of the stirring seems to be, the
+formation of detached granules. The specific gravity of silver is 10·4,
+and of lead 11·35: the granulated lead, which sinks, is never
+absolutely pure, and the residual fluid metal contains, when richest,
+only 1/119 part of silver. As the difference in specific gravity,
+caused by the different proportions of the two metals, is so
+exceedingly small, the separation is probably aided in a great degree
+by the difference in gravity between the lead, when granular though
+still hot, and when fluid.
+
+ [4] A full and interesting account of this discovery, by Mr.
+ Pattinson, was read before the British Association in September 1838.
+ In some alloys, according to Turner (“Chemistry,” p. 210), the
+ heaviest metal sinks, and it appears that this takes place whilst both
+ metals are fluid. Where there is a considerable difference in gravity,
+ as between iron and the slag formed during the fusion of the ore, we
+ need not be surprised at the atoms separating, without either
+ substance being granulated.
+
+
+In a body of liquified volcanic rock, left for some time without any
+violent disturbance, we might expect, in accordance with the above
+facts, that if one of the constituent minerals became aggregated into
+crystals or granules, or had been enveloped in this state from some
+previously existing mass, such crystals or granules would rise or sink,
+according to their specific gravity. Now we have plain evidence of
+crystals being embedded in many lavas, whilst the paste or basis has
+continued fluid. I need only refer, as instances, to the several,
+great, pseudo-porphyritic streams at the Galapagos Islands, and to the
+trachytic streams in many parts of the world, in which we find crystals
+of feldspar bent and broken by the movement of the surrounding,
+semi-fluid matter. Lavas are chiefly composed of three varieties of
+feldspar, varying in specific gravity from 2·4 to 2·74; of hornblende
+and augite, varying from 3·0 to 3·4; of olivine, varying from 3·3 to
+3·4; and lastly, of oxides of iron, with specific gravities from 4·8 to
+5·2. Hence crystals of feldspar, enveloped in a mass of liquified, but
+not highly vesicular lava, would tend to rise to the upper parts; and
+crystals or granules of the other minerals, thus enveloped, would tend
+to sink. We ought not, however, to expect any perfect degree of
+separation in such viscid materials. Trachyte, which consists chiefly
+of feldspar, with some hornblende and oxide of iron, has a specific
+gravity of about 2·45;[5] whilst basalt, composed chiefly of augite and
+feldspar, often with much iron and olivine, has a gravity of about 3·0.
+Accordingly we find, that where both trachytic and basaltic streams
+have proceeded from the same orifice, the trachytic streams have
+generally been first erupted owing, as we must suppose, to the molten
+lava of this series having accumulated in the upper parts of the
+volcanic focus. This order of eruption has been observed by Beudant,
+Scrope, and by other authors; three instances, also, have been given in
+this volume. As the later eruptions, however, from most volcanic
+mountains, burst through their basal parts, owing to the increased
+height and weight of the internal column of molten rock, we see why, in
+most cases, only the lower flanks of the central, trachytic masses, are
+enveloped by basaltic streams. The separation of the ingredients of a
+mass of lava, would, perhaps, sometimes take place within the body of a
+volcanic mountain, if lofty and of great dimensions, instead of within
+the underground focus; in which case, trachytic streams might be poured
+forth, almost contemporaneously, or at short recurrent intervals, from
+its summit, and basaltic streams from its base: this seems to have
+taken place at Teneriffe.[6] I need only further remark, that from
+violent disturbances the separation of the two series, even under
+otherwise favourable conditions, would naturally often be prevented,
+and likewise their usual order of eruption be inverted. From the high
+degree of fluidity of most basaltic lavas, these perhaps, alone, would
+in many cases reach the surface.
+
+ [5] Trachyte from Java was found by Von Buch to be 2·47; from
+ Auvergne, by De la Beche, it was 2·42; from Ascension, by myself, it
+ was 2·42. Jameson and other authors give to basalt a specific gravity
+ of 3·0; but specimens from Auvergne were found, by De la Beche, to be
+ only 2·78; and from the Giant’s Causeway, to be 2·91.
+
+
+ [6] Consult Von Buch’s well-known and admirable “Description Physique”
+ of this island, which might serve as a model of descriptive geology.
+
+
+As we have seen that crystals of feldspar, in the instance described by
+Von Buch, sink in obsidian, in accordance with their known greater
+specific gravity, we might expect to find in every trachytic district,
+where obsidian has flowed as lava, that it had proceeded from the upper
+or highest orifices. This, according to Von Buch, holds good in a
+remarkable manner both at the Lipari Islands and on the Peak of
+Teneriffe; at this latter place obsidian has never flowed from a less
+height than 9,200 feet. Obsidian, also, appears to have been erupted
+from the loftiest peaks of the Peruvian Cordillera. I will only further
+observe, that the specific gravity of quartz varies from 2·6 to 2·8;
+and therefore, that when present in a volcanic focus, it would not tend
+to sink with the basaltic bases; and this, perhaps, explains the
+frequent presence, and the abundance of this mineral, in the lavas of
+the trachytic series, as observed in previous parts of this volume.
+
+An objection to the foregoing theory will, perhaps, be drawn from the
+plutonic rocks not being separated into two evidently distinct series,
+of different specific gravities; although, like the volcanic, they have
+been liquified. In answer, it may first be remarked, that we have no
+evidence of the atoms of any one of the constituent minerals in the
+plutonic series having been aggregated, whilst the others remained
+fluid, which we have endeavoured to show is an almost necessary
+condition of their separation; on the contrary, the crystals have
+generally impressed each other with their forms.[7]
+
+ [7] The crystalline paste of phonolite is frequently penetrated by
+ long needles of hornblende; from which it appears that the hornblende,
+ though the more fusible mineral, has crystallised before, or at the
+ same time with a more refractory substance. Phonolite, as far as my
+ observations serve, in every instance appears to be an injected rock,
+ like those of the plutonic series; hence probably, like these latter,
+ it has generally been cooled without repeated and violent
+ disturbances. Those geologists who have doubted whether granite could
+ have been formed by igneous liquefaction, because minerals of
+ different degrees of fusibility impress each other with their forms,
+ could not have been aware of the fact of crystallised hornblende
+ penetrating phonolite, a rock undoubtedly of igneous origin. The
+ viscidity, which it is now known, that both feldspar and quartz retain
+ at a temperature much below their points of fusion, easily explains
+ their mutual impressment. Consult on this subject Mr. Horner’s paper
+ on Bonn, “Geolog. Transact.,” vol. iv, p. 439; and “L’Institut,” with
+ respect to quartz, 1839, p. 161.
+
+In the second place, the perfect tranquillity, under which it is
+probable that the plutonic masses, buried at profound depths, have
+cooled, would, most likely, be highly unfavourable to the separation of
+their constituent minerals; for, if the attractive force, which during
+the progressive cooling draws together the molecules of the different
+minerals, has power sufficient to keep them together, the friction
+between such half-formed crystals or pasty globules would effectually
+prevent the heavier ones from sinking, or the lighter ones from rising.
+On the other hand, a small amount of disturbance, which would probably
+occur in most volcanic foci, and which we have seen does not prevent
+the separation of granules of lead from a mixture of molten lead and
+silver, or crystals of feldspar from streams of lava, by breaking
+and dissolving the less perfectly formed globules, would permit the
+more perfect and therefore unbroken crystals, to sink or rise,
+according to their specific gravity.
+
+Although in plutonic rocks two distinct species, corresponding to the
+trachytic and basaltic series, do not exist, I much suspect that a
+certain amount of separation of their constituent parts has often taken
+place. I suspect this from having observed how frequently dikes of
+greenstone and basalt intersect widely extended formations of granite
+and the allied metamorphic rocks. I have never examined a district in
+an extensive granitic region without discovering dikes; I may instance
+the numerous trap-dikes, in several districts of Brazil, Chile, and
+Australia, and at the Cape of Good Hope: many dikes likewise occur in
+the great granitic tracts of India, in the north of Europe, and in
+other countries. Whence, then, has the greenstone and basalt, forming
+these dikes, come? Are we to suppose, like some of the elder
+geologists, that a zone of trap is uniformly spread out beneath the
+granitic series, which composes, as far as we know, the foundations of
+the earth’s crust? Is it not more probable, that these dikes have been
+formed by fissures penetrating into partially cooled rocks of the
+granitic and metamorphic series, and by their more fluid parts,
+consisting chiefly of hornblende, oozing out, and being sucked into
+such fissures? At Bahia, in Brazil, in a district composed of gneiss
+and primitive greenstone, I saw many dikes, of a dark augitic (for one
+crystal certainly was of this mineral) or hornblendic rock, which, as
+several appearances clearly proved, either had been formed before the
+surrounding mass had become solid, or had together with it been
+afterwards thoroughly softened.[8] On both sides of one of these dikes,
+the gneiss was penetrated, to the distance of several yards, by
+numerous, curvilinear threads or streaks of dark matter, which
+resembled in form clouds of the class called cirrhi-comæ; some few of
+these threads could be traced to their junction with the dike. When
+examining them, I doubted whether such hair-like and curvilinear veins
+could have been injected, and I now suspect, that instead of having
+been injected from the dike, they were its feeders. If the foregoing
+views of the origin of trap-dikes in widely extended granitic regions
+far from rocks of any other formation, be admitted as probable, we may
+further admit, in the case of a great body of plutonic rock, being
+impelled by repeated movements into the axis of a mountain-chain, that
+its more liquid constituent parts might drain into deep and unseen
+abysses; afterwards, perhaps, to be brought to the surface under the
+form, either of injected masses of greenstone and augitic porphyry,[9]
+or of basaltic eruptions. Much of
+the difficulty which geologists have experienced when they have
+compared the composition of volcanic with plutonic formations, will, I
+think, be removed, if we may believe that most plutonic masses have
+been, to a certain extent, drained of those comparatively weighty and
+easily liquified elements, which compose the trappean and basaltic
+series of rocks.
+
+ [8] Portions of these dikes have been broken off, and are now
+ surrounded by the primary rocks, with their laminæ conformably winding
+ round them. Dr. Hubbard also (_Silliman’s Journal,_ vol. xxxiv, p.
+ 119), has described an interlacement of trap-veins in the granite of
+ the White Mountains, which he thinks must have been formed when both
+ rocks were soft.
+
+
+ [9] Mr. Phillips (“Lardner’s Encyclop.,” vol. ii, p. 115) quotes Von
+ Buch’s statement, that augitic porphyry ranges parallel to, and is
+ found constantly at the base of, great chains of mountains. Humboldt,
+ also, has remarked the frequent occurrence of trap-rock, in a similar
+ position; of which fact I have observed many examples at the foot of
+ the Chilian Cordillera. The existence of granite in the axes of great
+ mountain chains is always probable, and I am tempted to suppose, that
+ the laterally injected masses of augitic porphyry and of trap, bear
+ nearly the same relation to the granitic axes which basaltic lavas
+ bear to the central trachytic masses, round the flanks of which they
+ have so frequently been erupted.
+
+
+_On the distribution of volcanic islands._—During my investigations on
+coral-reefs, I had occasion to consult the works of many voyagers, and
+I was invariably struck with the fact, that with rare exceptions, the
+innumerable islands scattered throughout the Pacific, Indian, and
+Atlantic Oceans, were composed either of volcanic, or of modern
+coral-rocks. It would be tedious to give a long catalogue of all the
+volcanic islands; but the exceptions which I have found are easily
+enumerated: in the Atlantic, we have St. Paul’s Rock, described in this
+volume, and the Falkland Islands, composed of quartz and clay-slate;
+but these latter islands are of considerable size, and lie not very far
+from the South American coast:[10] in the Indian Ocean, the Seychelles
+(situated in a line prolonged from Madagascar) consist of granite and
+quartz: in the Pacific Ocean, New Caledonia, an island of large size,
+belongs (as far as is known) to the primary class. New Zealand, which
+contains much volcanic rock and some active volcanoes, from its size
+cannot be classed with the small islands, which we are now considering.
+The presence of a small quantity of non-volcanic rock, as of clay-slate
+on three of the Azores,[11] or of tertiary limestone at Madeira, or of
+clay-slate at Chatham Island in the Pacific, or of lignite at Kerguelen
+Land, ought not to exclude such islands or archipelagoes, if formed
+chiefly of erupted matter, from the volcanic class.
+
+ [10] Judging from Forster’s imperfect observation, perhaps Georgia is
+ not volcanic. Dr. Allan is my informant with regard to the Seychelles.
+ I do not know of what formation Rodriguez, in the Indian Ocean, is
+ composed.
+
+
+ [11] This is stated on the authority of Count V. de Bedemar, with
+ respect to Flores and Graciosa (Charlsworth, “Magazine of Nat. Hist.,”
+ vol. i, p. 557). St. Maria has no volcanic rock, according to Captain
+ Boyd (Von Buch “Descript.,” p. 365). Chatham Island has been described
+ by Dr. Dieffenbach in the “Geographical Journal,” 1841, p. 201. As yet
+ we have received only imperfect notices on Kerguelen Land, from the
+ Antarctic Expedition.
+
+The composition of the numerous islands scattered through the great
+oceans being with such rare exceptions volcanic, is evidently an
+extension of that law, and the effect of those same causes, whether
+chemical or mechanical, from which it results, that a vast majority of
+the volcanoes now in action stand either as islands in the sea, or near
+its shores. This fact of the ocean-islands being so generally volcanic
+is also interesting in relation to the nature of the mountain-chains on
+our
+continents, which are comparatively seldom volcanic; and yet we are led
+to suppose that where our continents now stand an ocean once extended.
+Do volcanic eruptions, we may ask, reach the surface more readily
+through fissures formed during the first stages of the conversion of
+the bed of the ocean into a tract of land?
+
+Looking at the charts of the numerous volcanic archipelagoes, we see
+that the islands are generally arranged either in single, double, or
+triple rows, in lines which are frequently curved in a slight
+degree.[12] Each separate island is either rounded, or more generally
+elongated in the same direction with the group in which it stands, but
+sometimes transversely to it. Some of the groups which are not much
+elongated present little symmetry in their forms; M. Virlet[13] states
+that this is the case with the Grecian Archipelago: in such groups I
+suspect (for I am aware how easy it is to deceive oneself on these
+points), that the vents are generally arranged on one line, or on a set
+of short parallel lines, intersecting at nearly right angles another
+line, or set of lines. The Galapagos Archipelago offers an example of
+this structure, for most of the islands and the chief orifices on the
+largest island are so grouped as to fall on a set of lines ranging
+about N.W. by N., and on another set ranging about W.S.W.: in the
+Canary Archipelago we have a simpler structure of the same kind: in the
+Cape de Verde group, which appears to be the least symmetrical of any
+oceanic volcanic archipelago, a N.W. and S.E. line formed by several
+islands, if prolonged, would intersect at right angles a curved line,
+on which the remaining islands are placed. Von Buch[14] has classed all
+volcanoes under two heads, namely, _central volcanoes_, round which
+numerous eruptions have taken place on all sides, in a manner almost
+regular, and _volcanic chains._ In the examples given of the first
+class, as far as position is concerned, I can see no grounds for their
+being called “central;” and the evidence of any difference in
+mineralogical nature between _central volcanoes_ and _volcanic chains_
+appears slight. No doubt some one island in most small volcanic
+archipelagoes is apt to be considerably higher than the others; and in
+a similar manner, whatever the cause may be, that on the same island
+one vent is generally higher than all the others. Von Buch does not
+include in his class of volcanic chains small archipelagoes, in which
+the islands are admitted by him, as at the Azores, to be arranged in
+lines; but when viewing on a map of the world how perfect a series
+exists from a few volcanic islands placed in a row to a train of linear
+archipelagoes following each other in a straight line, and so on to a
+great wall like the Cordillera of America, it is difficult to believe
+that there exists any essential difference between short and long
+volcanic chains. Von Buch[15] states that his volcanic chains surmount,
+or are closely connected with, mountain-ranges of primary formation:
+but if trains of linear archipelagoes are, in the course of time, by
+the long-continued action of the elevatory and volcanic forces,
+converted into mountain-ranges, it would naturally result that the
+inferior primary rocks would often be uplifted and brought into view.
+
+ [12] Professors William and Henry Darwin Rogers have lately insisted
+ much, in a memoir read before the American Association, on the
+ regularly curved lines of elevation in parts of the Appalachian range.
+
+
+ [13] “Bulletin de la Soc. Géolog.,” tome iii, p. 110.
+
+
+ [14] “Description des Isles Canaries,” p. 324.
+
+
+ [15] _Idem,_ p. 393.
+
+Some authors have remarked that volcanic islands occur scattered,
+though at very unequal distances, along the shores of the great
+continents, as if in some measure connected with them. In the case of
+Juan Fernandez, situated 330 miles from the coast of Chile, there was
+undoubtedly a connection between the volcanic forces acting under this
+island and under the continent, as was shown during the earthquake of
+1835. The islands, moreover, of some of the small volcanic groups which
+thus border continents, are placed in lines, related to those along
+which the adjoining shores of the continents trend; I may instance the
+lines of intersection at the Galapagos, and at the Cape de Verde
+Archipelagoes, and the best marked line of the Canary Islands. If these
+facts be not merely accidental, we see that many scattered volcanic
+islands and small groups are related not only by proximity, but in the
+direction of the fissures of eruption to the neighbouring continents—a
+relation, which Von Buch considers, characteristic of his great
+volcanic chains.
+
+In volcanic archipelagoes, the orifices are seldom in activity on more
+than one island at a time; and the greater eruptions usually recur only
+after long intervals. Observing the number of craters, that are usually
+found on each island of a group, and the vast amount of matter which
+has been erupted from them, one is led to attribute a high antiquity
+even to those groups, which appear, like the Galapagos, to be of
+comparatively recent origin. This conclusion accords with the
+prodigious amount of degradation, by the slow action of the sea, which
+their originally sloping coasts must have suffered, when they are worn
+back, as is so often the case, into grand precipices. We ought not,
+however, to suppose, in hardly any instance, that the whole body of
+matter, forming a volcanic island, has been erupted at the level, on
+which it now stands: the number of dikes, which seem invariably to
+intersect the interior parts of every volcano, show, on the principles
+explained by M. Elie de Beaumont, that the whole mass has been uplifted
+and fissured. A connection, moreover, between volcanic eruptions and
+contemporaneous elevations in mass[16] has, I think, been shown to
+exist in my work on Coral-Reefs, both from the frequent presence of
+upraised organic remains, and from the structure of the accompanying
+coral-reefs. Finally, I may remark, that in the same Archipelago,
+eruptions have taken place within the historical period on more than
+one of the parallel lines of fissure: thus, at the Galapagos
+Archipelago, eruptions have taken place from a vent on Narborough
+Island, and from one on Albemarle Island, which vents do not fall on
+the same line; at the Canary Islands, eruptions have taken place in
+Teneriffe and Lanzarote; and at
+the Azores, on the three parallel lines of Pico, St. Jorge, and
+Terceira. Believing that a mountain-axis differs essentially from a
+volcano, only in plutonic rocks having been injected, instead of
+volcanic matter having been ejected, this appears to me an interesting
+circumstance; for we may infer from it as probable, that in the
+elevation of a mountain-chain, two or more of the parallel lines
+forming it may be upraised and injected within the same geological
+period.
+
+ [16] A similar conclusion is forced on us, by the phenomena, which
+ accompanied the earthquake of 1835, at Concepcion, and which are
+ detailed in my paper (vol. v, p. 601) in the “Geological
+ Transactions.”
+
+
+
+
+Chapter VII AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.
+
+
+New South Wales.—Sandstone formation.—Embedded pseudo-fragments of
+shale.—Stratification.—Current-cleavage.—Great valleys.—Van Diemen’s
+Land.—Palæozoic formation.—Newer formation with volcanic
+rocks.—Travertin with leaves of extinct plants.—Elevation of the
+land.—New Zealand.—King George’s Sound.—Superficial ferruginous
+beds.—Superficial calcareous deposits, with casts of branches.—Their
+origin from drifted particles of shells and corals.—Their extent.—Cape
+of Good Hope.—Junction of the granite and clay-slate.—Sandstone
+formation.
+
+The _Beagle_, in her homeward voyage, touched at New Zealand,
+Australia, Van Diemen’s Land, and the Cape of Good Hope. In order to
+confine the Third Part of these Geological Observations to South
+America, I will here briefly describe all that I observed at these
+places worthy of the attention of geologists.
+
+_New South Wales._—My opportunities of observation consisted of a ride
+of ninety geographical miles to Bathurst, in a W.N.W. direction from
+Sydney. The first thirty miles from the coast passes over a sandstone
+country, broken up in many places by trap-rocks, and separated by a
+bold escarpment overhanging the river Nepean, from the great sandstone
+platform of the Blue Mountains. This upper platform is 1,000 feet high
+at the edge of the escarpment, and rises in a distance of twenty-five
+miles to between three and four thousand feet above the level of the
+sea. At this distance the road descends to a country rather less
+elevated, and composed in chief part of primary rocks. There is much
+granite, in one part passing into a red porphyry with octagonal
+crystals of quartz, and intersected in some places by trap-dikes. Near
+the Downs of Bathurst I passed over much pale-brown, glossy clay-slate,
+with the shattered laminæ running north and south; I mention this fact,
+because Captain King informs me that, in the country a hundred miles
+southward, near Lake George, the mica-slate ranges so invariably north
+and south that the inhabitants take advantage of it in finding their
+way through the forests.
+
+The sandstone of the Blue Mountains is at least 1,200 feet thick, and
+in some parts is apparently of greater thickness; it consists of
+small grains of quartz, cemented by white earthy matter, and it abounds
+with ferruginous veins. The lower beds sometimes alternate with shales
+and coal: at Wolgan I found in carbonaceous shale leaves of the
+_Glossopteris Brownii_, a fern which so frequently accompanies the coal
+of Australia. The sandstone contains pebbles of quartz; and these
+generally increase in number and size (seldom, however, exceeding an
+inch or two in diameter) in the upper beds: I observed a similar
+circumstance in the grand sandstone formation at the Cape of Good Hope.
+On the South American coast, where tertiary and supra-tertiary beds
+have been extensively elevated, I repeatedly noticed that the uppermost
+beds were formed of coarser materials than the lower: this appears to
+indicate that, as the sea became shallower, the force of the waves or
+currents increased. On the lower platform, however, between the Blue
+Mountains and the coast, I observed that the upper beds of the
+sandstone frequently passed into argillaceous shale,—the effect,
+probably, of this lower space having been protected from strong
+currents during its elevation. The sandstone of the Blue Mountains
+evidently having been of mechanical origin, and not having suffered any
+metamorphic action, I was surprised at observing that, in some
+specimens, nearly all the grains of quartz were so perfectly
+crystallised with brilliant facets that they evidently had not in their
+_present_ form been aggregated in any previously existing rock.[1] It
+is difficult to imagine how these crystals could have been formed; one
+can hardly believe that they were separately precipitated in their
+present crystallised state. Is it possible that rounded grains of
+quartz may have been acted on by a fluid corroding their surfaces, and
+depositing on them fresh silica? I may remark that, in the sandstone
+formation of the Cape of Good Hope, it is evident that silica has been
+profusely deposited from aqueous solution.
+
+ [1] I have lately seen, in a paper by Smith (the father of English
+ geologists), in the _Magazine of Natural History_, that the grains of
+ quartz in the millstone grit of England are often crystallised. Sir
+ David Brewster, in a paper read before the British Association, 1840,
+ states, that in old decomposed glass, the silex and metals separate
+ into concentric rings, and that the silex regains its crystalline
+ structure, as is shown by its action on light.
+
+In several parts of the sandstone I noticed patches of shale which
+might at the first glance have been mistaken for extraneous fragments;
+their horizontal laminæ, however, being parallel with those of the
+sandstone, showed that they were the remnants of thin, continuous beds.
+One such fragment (probably the section of a long narrow strip) seen in
+the face of a cliff, was of greater vertical thickness than breadth,
+which proves that this bed of shale must have been in some slight
+degree consolidated, after having been deposited, and before being worn
+away by the currents. Each patch of the shale shows, also, how slowly
+many of the successive layers of sandstone were deposited. These
+pseudo-fragments of shale will perhaps explain, in some cases, the
+origin of apparently extraneous fragments in crystalline metamorphic
+rocks. I mention this, because I found near Rio de Janeiro a
+well-defined angular fragment, seven yards long by two yards in
+breadth, of gneiss
+containing garnets and mica in layers, enclosed in the ordinary,
+stratified, porphyritic gneiss of the country. The laminæ of the
+fragment and of the surrounding matrix ran in exactly the same
+direction, but they dipped at different angles. I do not wish to affirm
+that this singular fragment (a solitary case, as far as I know) was
+originally deposited in a layer, like the shale in the Blue Mountains,
+between the strata of the porphyritic gneiss, before they were
+metamorphosed; but there is sufficient analogy between the two cases to
+render such an explanation possible.
+
+_Stratification of the escarpment._—The strata of the Blue Mountains
+appear to the eye horizontal; but they probably have a similar
+inclination with the surface of the platform, which slopes from the
+west towards the escarpment over the Nepean, at an angle of one degree,
+or of one hundred feet in a mile.[2] The strata of the escarpment dip
+almost conformably with its steeply inclined face, and with so much
+regularity, that they appear as if thrown into their present position;
+but on a more careful examination, they are seen to thicken and to thin
+out, and in the upper part to be succeeded and almost capped by
+horizontal beds. These appearances render it probable, that we here see
+an original escarpment, not formed by the sea having eaten back into
+the strata, but by the strata having originally extended only thus far.
+Those who have been in the habit of examining accurate charts of
+sea-coasts, where sediment is accumulating, will be aware, that the
+surfaces of the banks thus formed, generally slope from the coast very
+gently towards a certain line in the offing, beyond which the depth in
+most cases suddenly becomes great. I may instance the great banks of
+sediment within the West Indian Archipelago,[3] which terminate in
+submarine slopes, inclined at angles of between thirty and forty
+degrees, and sometimes even at more than forty degrees: every one knows
+how steep such a slope would appear on the land. Banks of this nature,
+if uplifted, would probably have nearly the same external form as the
+platform of the Blue Mountains, where it abruptly terminates over the
+Nepean.
+
+ [2] This is stated on the authority of Sir T. Mitchell, in his
+ “Travels,” vol. ii, p. 357.
+
+
+ [3] I have described these very curious banks in the Appendix to my
+ volume on the structure of Coral-Reefs. I have ascertained the
+ inclination of the edges of the banks, from information given me by
+ Captain B. Allen, one of the surveyors, and by carefully measuring the
+ horizontal distances between the last sounding on the bank and the
+ first in the deep water. Widely extended banks in all parts of the
+ West Indies have the same general form of surface.
+
+
+_Current-cleavage._—The strata of sandstone in the low coast country,
+and likewise on the Blue Mountains, are often divided by cross or
+current laminæ, which dip in different directions, and frequently at an
+angle of forty-five degrees. Most authors have attributed these cross
+layers to successive small accumulations on an inclined surface; but
+from a careful examination in some parts of the New Red Sandstone of
+England, I believe that such layers generally form parts of a series of
+curves, like gigantic tidal ripples, the tops of which have since been
+cut off, either by nearly horizontal layers, or by another set of great
+ripples, the folds of which do not exactly coincide with those below
+them. It is well-known to surveyors that mud and sand are disturbed
+during storms at considerable depths, at least from three hundred to
+four hundred and fifty feet,[4] so that the nature of the bottom even
+becomes temporarily changed; the bottom, also, at a depth between sixty
+and seventy feet, has been observed[5] to be broadly rippled. One may,
+therefore, be allowed to suspect, from the appearance just mentioned in
+the New Red Sandstone, that at greater depths, the bed of the ocean is
+heaped up during gales into great ripple-like furrows and depressions,
+which are afterwards cut off by the currents during more tranquil
+weather, and again furrowed during gales.
+
+ [4] See Martin White, on “Soundings in the British Channel,” pp. 4 and
+ 166.
+
+
+ [5] M. Siau on the “Action of Waves,” _Edin. New Phil. Journ.,_ vol.
+ xxxi, p. 245.
+
+
+_Valleys in the sandstone platforms._—The grand valleys, by which the
+Blue Mountains and the other sandstone platforms of this part of
+Australia are penetrated, and which long offered an insuperable
+obstacle to the attempts of the most enterprising colonist to reach the
+interior country, form the most striking feature in the geology of New
+South Wales. They are of grand dimensions, and are bordered by
+continuous links of lofty cliffs. It is not easy to conceive a more
+magnificent spectacle, than is presented to a person walking on the
+summit-plains, when without any notice he arrives at the brink of one
+of these cliffs, which are so perpendicular, that he can strike with a
+stone (as I have tried) the trees growing, at the depth of between one
+thousand and one thousand five hundred feet below him; on both hands he
+sees headland beyond headland of the receding line of cliff, and on the
+opposite side of the valley, often at the distance of several miles, he
+beholds another line rising up to the same height with that on which he
+stands, and formed of the same horizontal strata of pale sandstone. The
+bottoms of these valleys are moderately level, and the fall of the
+rivers flowing in them, according to Sir T. Mitchell, is gentle. The
+main valleys often send into the platform great baylike arms, which
+expand at their upper ends; and on the other hand, the platform often
+sends promontories into the valley, and even leaves in them great,
+almost insulated, masses. So continuous are the bounding lines of
+cliff, that to descend into some of these valleys, it is necessary to
+go round twenty miles; and into others, the surveyors have only lately
+penetrated, and the colonists have not yet been able to drive in their
+cattle. But the most remarkable point of structure in these valleys,
+is, that although several miles wide in their upper parts, they
+generally contract towards their mouths to such a degree as to become
+impassable. The Surveyor-General, Sir T. Mitchell,[6] in vain
+endeavoured, first on foot and then by crawling between the great
+fallen
+fragments of sandstone, to ascend through the gorge by which the river
+Grose joins the Nepean; yet the valley of the Grose in its upper part,
+as I saw, forms a magnificent basin some miles in width, and is on all
+sides surrounded by cliffs, the summits of which are believed to be
+nowhere less than 3,000 feet above the level of the sea. When cattle
+are driven into the valley of the Wolgan by a path (which I descended)
+partly cut by the colonists, they cannot escape; for this valley is in
+every other part surrounded by perpendicular cliffs, and eight miles
+lower down, it contracts, from an average width of half a mile, to a
+mere chasm impassable to man or beast. Sir T. Mitchell[7] states, that
+the great valley of the Cox river with all its branches contracts,
+where it unites with the Nepean, into a gorge 2,200 yards wide, and
+about one thousand feet in depth. Other similar cases might have been
+added.
+
+ [6] “Travels in Australia,” vol. i, p. 154.—I must express my
+ obligation to Sir T. Mitchell for several interesting personal
+ communications on the subject of these great valleys of New South
+ Wales.
+
+
+ [7] _Idem_, vol. ii, p. 358.
+
+
+The first impression, from seeing the correspondence of the horizontal
+strata, on each side of these valleys and great amphitheatre-like
+depressions, is that they have been in chief part hollowed out, like
+other valleys, by aqueous erosion; but when one reflects on the
+enormous amount of stone, which on this view must have been removed, in
+most of the above cases through mere gorges or chasms, one is led to
+ask whether these spaces may not have subsided. But considering the
+form of the irregularly branching valleys, and of the narrow
+promontories, projecting into them from the platforms, we are compelled
+to abandon this notion. To attribute these hollows to alluvial action,
+would be preposterous; nor does the drainage from the summit-level
+always fall, as I remarked near the Weatherboard, into the head of
+these valleys, but into one side of their bay-like recesses. Some of
+the inhabitants remarked to me, that they never viewed one of these
+baylike recesses, with the headlands receding on both hands, without
+being struck with their resemblance to a bold sea-coast. This is
+certainly the case; moreover, the numerous fine harbours, with their
+widely branching arms, on the present coast of New South Wales, which
+are generally connected with the sea by a narrow mouth, from one mile
+to a quarter of a mile in width, passing through the sandstone
+coast-cliffs, present a likeness, though on a miniature scale, to the
+great valleys of the interior. But then immediately occurs the
+startling difficulty, why has the sea worn out these great, though
+circumscribed, depressions on a wide platform, and left mere gorges,
+through which the whole vast amount of triturated matter must have been
+carried away? The only light I can throw on this enigma, is by showing
+that banks appear to be forming in some seas of the most irregular
+forms, and that the sides of such banks are so steep (as before stated)
+that a comparatively small amount of subsequent erosion would form them
+into cliffs: that the waves have power to form high and precipitous
+cliffs, even in landlocked harbours, I have observed in many parts of
+South America. In the Red Sea, banks with an extremely irregular
+outline and composed of sediment, are penetrated by the most singularly
+shaped creeks with narrow mouths: this is likewise the case, though on
+a larger scale,
+with the Bahama Banks. Such banks, I have been led to suppose,[8] have
+been formed by currents heaping sediment on an irregular bottom. That
+in some cases, the sea, instead of spreading out sediment in a uniform
+sheet, heaps it round submarine rocks and islands, it is hardly
+possible to doubt, after having examined the charts of the West Indies.
+To apply these ideas to the sandstone platforms of New South Wales, I
+imagine that the strata might have been heaped on an irregular bottom
+by the action of strong currents, and of the undulations of an open
+sea; and that the valley-like spaces thus left unfilled might, during a
+slow elevation of the land, have had their steeply sloping flanks worn
+into cliffs; the worn-down sandstone being removed, either at the time
+when the narrow gorges were cut by the retreating sea, or subsequently
+by alluvial action.
+
+ [8] See the “Appendix” to the Part on Coral-Reefs. The fact of the sea
+ heaping up mud round a submarine nucleus, is worthy of the notice of
+ geologists: for outlyers of the same composition with the coast banks
+ are thus formed; and these, if upheaved and worn into cliffs, would
+ naturally be thought to have been once connected together.
+
+_Van Diemen’s Land._
+
+The southern part of this island is mainly formed of mountains of
+greenstone, which often assumes a syenitic character, and contains much
+hypersthene. These mountains, in their lower half, are generally
+encased by strata containing numerous small corals and some shells.
+These shells have been examined by Mr. G. B. Sowerby, and have been
+described by him: they consist of two species of Producta, and of six
+of Spirifera; two of these, namely, _P. rugata_ and _S. rotundata_,
+resemble, as far as their imperfect condition allows of comparison,
+British mountain-limestone shells. Mr. Lonsdale has had the kindness to
+examine the corals; they consist of six undescribed species, belonging
+to three genera. Species of these genera occur in the Silurian,
+Devonian, and Carboniferous strata of Europe. Mr. Lonsdale remarks,
+that all these fossils have undoubtedly a Palæozoic character, and that
+probably they correspond in age to a division of the system above the
+Silurian formations.
+
+The strata containing these remains are singular from the extreme
+variability of their mineralogical composition. Every intermediate form
+is present, between flinty-slate, clay-slate passing into grey wacke,
+pure limestone, sandstone, and porcellanic rock; and some of the beds
+can only be described as composed of a siliceo-calcareo-clay-slate. The
+formation, as far as I could judge, is at least a thousand feet in
+thickness: the upper few hundred feet usually consist of a siliceous
+sandstone, containing pebbles and no organic remains; the inferior
+strata, of which a pale flinty slate is perhaps the most abundant, are
+the most variable; and these chiefly abound with the remains. Between
+two beds of hard crystalline limestone, near Newtown, a layer of white
+soft calcareous matter is quarried, and is used for whitewashing
+houses. From information given to me by Mr. Frankland, the
+Surveyor-General,
+it appears that this Palæozoic formation is found in different parts of
+the whole island; from the same authority, I may add, that on the
+north-eastern coast and in Bass’ Straits primary rocks extensively
+occur.
+
+The shores of Storm Bay are skirted, to the height of a few hundred
+feet, by strata of sandstone, containing pebbles of the formation just
+described, with its characteristic fossils, and therefore belonging to
+a subsequent age. These strata of sandstone often pass into shale, and
+alternate with layers of impure coal; they have in many places been
+violently disturbed. Near Hobart Town, I observed one dike, nearly a
+hundred yards in width, on one side of which the strata were tilted at
+an angle of 60 degrees, and on the other they were in some parts
+vertical, and had been altered by the effects of the heat. On the west
+side of Storm Bay, I found these strata capped by streams of basaltic
+lava with olivine; and close by there was a mass of brecciated scoriæ,
+containing pebbles of lava, which probably marks the place of an
+ancient submarine crater. Two of these streams of basalt were separated
+from each other by a layer of argillaceous wacke, which could be traced
+passing into partially altered scoriæ. The wacke contained numerous
+rounded grains of a soft, grass-green mineral, with a waxy lustre, and
+translucent on its edges: under the blowpipe it instantly blackened,
+and the points fused into a strongly magnetic, black enamel. In these
+characters, it resembles those masses of decomposed olivine, described
+at St. Jago in the Cape de Verde group; and I should have thought that
+it had thus originated, had I not found a similar substance, in
+cylindrical threads, within the cells of the vesicular basalt,—a state
+under which olivine never appears; this substance,[9] I believe, would
+be classed as bole by mineralogists.
+
+ [9] Chlorophæite, described by Dr. MacCulloch (“Western Islands,” vol.
+ i, p. 504) as occurring in a basaltic amygdaloid, differs from this
+ substance, in remaining unchanged before the blowpipe, and in
+ blackening from exposure to the air. May we suppose that olivine, in
+ undergoing the remarkable change described at St. Jago, passes through
+ several states?
+
+_Travertin with extinct plants._—Behind Hobart Town there is a small
+quarry of a hard travertin, the lower strata of which abound with
+distinct impressions of leaves. Mr. Robert Brown has had the kindness
+to look at my specimens, and he informed me that there are four or five
+kinds, none of which he recognises as belonging to existing species.
+The most remarkable leaf is palmate, like that of a fan-palm, and no
+plant having leaves of this structure has hitherto been discovered in
+Van Diemen’s Land. The other leaves do not resemble the most usual form
+of the Eucalyptus (of which tribe the existing forests are chiefly
+composed), nor do they resemble that class of exceptions to the common
+form of the leaves of the Eucalyptus, which occur in this island. The
+travertin containing this remnant of a lost vegetation, is of a pale
+yellow colour, hard, and in parts even crystalline; but not compact,
+and is everywhere penetrated by minute, tortuous, cylindrical pores. It
+contains a very few pebbles of quartz, and occasionally layers of
+chalcedonic nodules, like those of chert in our Greensand. From
+the pureness of this calcareous rock, it has been searched for in other
+places, but has never been found. From this circumstance, and from the
+character of the deposit, it was probably formed by a calcareous spring
+entering a small pool or narrow creek. The strata have subsequently
+been tilted and fissured; and the surface has been covered by a
+singular mass, with which, also, a large fissure has been filled up,
+formed of balls of trap embedded in a mixture of wacke and a white,
+earthy, alumino-calcareous substance. Hence it would appear, as if a
+volcanic eruption had taken place on the borders of the pool, in which
+the calcareous matter was depositing, and had broken it up and drained
+it.
+
+_Elevation of the land._—Both the eastern and western shores of the
+bay, in the neighbourhood of Hobart Town, are in most parts covered to
+the height of thirty feet above the level of high-water mark, with
+broken shells, mingled with pebbles. The colonists attribute these
+shells to the aborigines having carried them up for food: undoubtedly,
+there are many large mounds, as was pointed out to me by Mr. Frankland,
+which have been thus formed; but I think from the numbers of the
+shells, from their frequent small size, from the manner in which they
+are thinly scattered, and from some appearances in the form of the
+land, that we must attribute the presence of the greater number to a
+small elevation of the land. On the shore of Ralph Bay (opening into
+Storm Bay) I observed a continuous beach about fifteen feet above
+high-water mark, clothed with vegetation, and by digging into it,
+pebbles encrusted with Serpulæ were found: along the banks, also, of
+the river Derwent, I found a bed of broken sea-shells above the surface
+of the river, and at a point where the water is now much too fresh for
+sea-shells to live; but in both these cases, it is just possible, that
+before certain spits of sand and banks of mud in Storm Bay were
+accumulated, the tides might have risen to the height where we now find
+the shells.[10]
+
+ [10] It would appear that some changes are now in progress in Ralph
+ Bay, for I was assured by an intelligent farmer, that oysters were
+ formerly abundant in it, but that about the year 1834 they had,
+ without any apparent cause, disappeared. In the “Transactions of the
+ Maryland Academy” (vol. i, part i, p. 28) there is an account by Mr.
+ Ducatel of vast beds of oysters and clams having been destroyed by the
+ gradual filling up of the shallow lagoons and channels, on the shores
+ of the southern United States. At Chiloe, in South America, I heard of
+ a similar loss, sustained by the inhabitants, in the disappearance
+ from one part of the coast of an edible species of Ascidia.
+
+
+Evidence more or less distinct of a change of level between the land
+and water, has been detected on almost all the land on this side of the
+globe. Captain Grey, and other travellers, have found in Southern
+Australia upraised shells, belonging either to the recent, or to a late
+tertiary period. The French naturalists in Baudin’s expedition, found
+shells similarly circumstanced on the S.W. coast of Australia. The Rev.
+W. B. Clarke[11] finds proofs of the elevation of the land, to the
+amount of 400 feet, at the Cape of Good Hope. In the neighbourhood
+of the Bay of Islands in New Zealand,[12] I observed that the shores
+were scattered to some height, as at Van Diemen’s Land, with
+sea-shells, which the colonists attribute to the natives. Whatever may
+have been the origin of these shells, I cannot doubt, after having seen
+a section of the valley of the Thames River (37 degrees S.), drawn by
+the Rev. W. Williams, that the land has been there elevated: on the
+opposite sides of this great valley, three step-like terraces, composed
+of an enormous accumulation of rounded pebbles, exactly correspond with
+each other: the escarpment of each terrace is about fifty feet in
+height. No one after having examined the terraces in the valleys on the
+western shores of South America, which are strewed with sea-shells, and
+have been formed during intervals of rest in the slow elevation of the
+land, could doubt that the New Zealand terraces have been similarly
+formed. I may add, that Dr. Dieffenbach, in his description of the
+Chatham Islands[13] (S.W. of New Zealand), states that it is manifest
+“that the sea has left many places bare which were once covered by its
+waters.”
+
+ [11] “Proceedings of the Geological Society,” vol. iii, p. 420.
+
+
+ [12] I will here give a catalogue of the rocks which I met with near
+ the Bay of Islands, in New Zealand:—1st, Much basaltic lava, and
+ scoriform rocks, forming distinct craters;—2nd, A castellated hill of
+ horizontal strata of flesh-coloured limestone, showing when fractured
+ distinct crystalline facets: the rain has acted on this rock in a
+ remarkable manner, corroding its surface into a miniature model of an
+ Alpine country: I observed here layers of chert and clay ironstone;
+ and in the bed of a stream, pebbles of clay-slate;—3rd, The shores of
+ the Bay of Islands are formed of a feldspathic rock, of a bluish-grey
+ colour, often much decomposed, with an angular fracture, and crossed
+ by numerous ferruginous seams, but without any distinct stratification
+ or cleavage. Some varieties are highly crystalline, and would at once
+ be pronounced to be trap; others strikingly resembled clay-slate,
+ slightly altered by heat: I was unable to form any decided opinion on
+ this formation.
+
+
+ [13] _Geographical Journal_, vol. xi, pp. 202, 205.
+
+_King George’s Sound._
+
+This settlement is situated at the south-western angle of the
+Australian continent: the whole country is granitic, with the
+constituent minerals sometimes obscurely arranged in straight or curved
+laminæ. In these cases, the rock would be called by Humboldt,
+gneiss-granite, and it is remarkable that the form of the bare conical
+hills, appearing to be composed of great folding layers, strikingly
+resembles, on a small scale, those composed of gneiss-granite at Rio de
+Janeiro, and those described by Humboldt at Venezuela. These plutonic
+rocks are, in many places, intersected by trappean-dikes; in one place,
+I found ten parallel dikes ranging in an E. and W. line; and not far
+off another set of eight dikes, composed of a different variety of
+trap, ranging at right angles to the former ones. I have observed in
+several primary districts, the occurrence of systems of dikes parallel
+and close to each other.
+
+_Superficial ferruginous beds._—The lower parts of the country are
+everywhere covered by a bed, following the inequalities of the surface,
+of a honeycombed sandstone, abounding with oxides of iron. Beds of
+nearly similar composition are common, I believe, along the whole
+western coast of Australia, and on many of the East Indian islands. At
+the Cape of Good Hope, at the base of the mountains formed of granite
+and capped with sandstone, the ground is everywhere coated either by a
+fine-grained, rubbly, ochraceous mass, like that at King George’s
+Sound, or by a coarser sandstone with fragments of quartz, and rendered
+hard and heavy by an abundance of the hydrate of iron, which presents,
+when freshly broken, a metallic lustre. Both these varieties have a
+very irregular texture, including spaces either rounded or angular,
+full of loose sand: from this cause the surface is always honeycombed.
+The oxide of iron is most abundant on the edges of the cavities, where
+alone it affords a metallic fracture. In these formations, as well as
+in many true sedimentary deposits, it is evident that iron tends to
+become aggregated, either in the form of a shell, or of a network. The
+origin of these superficial beds, though sufficiently obscure, seems to
+be due to alluvial action on detritus abounding with iron.
+
+_ Superficial calcareous deposit._—A calcareous deposit on the summit
+of Bald Head, containing branched bodies, supposed by some authors to
+have been corals, has been celebrated by the descriptions of many
+distinguished voyagers.[14] It folds round and conceals irregular
+hummocks of granite, at the height of 600 feet above the level of the
+sea. It varies much in thickness; where stratified, the beds are often
+inclined at high angles, even as much as at thirty degrees, and they
+dip in all directions. These beds are sometimes crossed by oblique and
+even-sided laminæ. The deposit consists either of a fine, white
+calcareous powder, in which not a trace of structure can be discovered,
+or of exceedingly minute, rounded grains, of brown, yellowish, and
+purplish colours; both varieties being generally, but not always, mixed
+with small particles of quartz, and being cemented into a more or less
+perfect stone. The rounded calcareous grains, when heated in a slight
+degree, instantly lose their colours; in this and in every other
+respect, closely resembling those minute, equal-sized particles of
+shells and corals, which at St. Helena have been drifted up the side of
+the mountains, and have thus been winnowed of all coarser fragments. I
+cannot doubt that the coloured calcareous particles here have had a
+similar origin. The impalpable powder has probably been derived from
+the decay of the rounded particles; this certainly is possible, for on
+the coast of Peru, I have traced _large unbroken_ shells gradually
+falling into a substance as fine as powdered chalk. Both of the
+above-mentioned varieties of calcareous sandstone frequently alternate
+with, and blend into, thin layers of a hard substalagmitic[15] rock,
+which, even
+when the stone on each side contains particles of quartz, is entirely
+free from them: hence we must suppose that these layers, as well as
+certain vein-like masses, have been formed by rain dissolving the
+calcareous matter and re-precipitating it, as has happened at St.
+Helena. Each layer probably marks a fresh surface, when the, now firmly
+cemented, particles existed as loose sand. These layers are sometimes
+brecciated and re-cemented, as if they had been broken by the slipping
+of the sand when soft. I did not find a single fragment of a sea-shell;
+but bleached shells of the _Helix melo_, an existing land species,
+abound in all the strata; and I likewise found another Helix, and the
+case of an Oniscus.
+
+ [14] I visited this hill, in company with Captain Fitzroy, and we came
+ to a similar conclusion regarding these branching bodies.
+
+
+ [15] I adopt this term from Lieutenant Nelson’s excellent paper on the
+ Bermuda Islands (“Geolog. Trans.,” vol. v, p. 106), for the hard,
+ compact, cream- or brown-coloured stone, without any crystalline
+ structure, which so often accompanies superficial calcareous
+ accumulations. I have observed such superficial beds, coated with
+ substalagmitic rock, at the Cape of Good Hope, in several parts of
+ Chile, and over wide spaces in La Plata and Patagonia. Some of these
+ beds have been formed from decayed shells, but the origin of the
+ greater number is sufficiently obscure. The causes which determine
+ water to dissolve lime, and then soon to redeposit it, are not, I
+ think, known. The surface of the substalagmitic layers appears always
+ to be corroded by the rain-water. As all the above-mentioned countries
+ have a long dry season, compared with the rainy one, I should have
+ thought that the presence of the substalagmitic was connected with the
+ climate, had not Lieutenant Nelson found this substance forming under
+ sea-water. Disintegrated shell seems to be extremely soluble; of which
+ I found good evidence, in a curious rock at Coquimbo in Chile, which
+ consisted of small, pellucid, empty husks, cemented together. A series
+ of specimens clearly showed that these husks had originally contained
+ small rounded particles of shells, which had been enveloped and
+ cemented together by calcareous matter (as often happens on
+ sea-beaches), and which subsequently had decayed, and been dissolved
+ by water, that must have penetrated through the calcareous husks,
+ without corroding them,—of which processes every stage could be seen.
+
+
+The branches are absolutely undistinguishable in shape from the broken
+and upright stumps of a thicket; their roots are often uncovered, and
+are seen to diverge on all sides; here and there a branch lies
+prostrate. The branches generally consist of the sandstone, rather
+firmer than the surrounding matter, with the central parts filled,
+either with friable, calcareous matter, or with a substalagmitic
+variety; this central part is also frequently penetrated by linear
+crevices, sometimes, though rarely, containing a trace of woody matter.
+These calcareous, branching bodies, appear to have been formed by fine
+calcareous matter being washed into the casts or cavities, left by the
+decay of branches and roots of thickets, buried under drifted sand. The
+whole surface of the hill is now undergoing disintegration, and hence
+the casts, which are compact and hard, are left projecting. In
+calcareous sand at the Cape of Good Hope, I found the casts, described
+by Abel, quite similar to these at Bald Head; but their centres are
+often filled with black carbonaceous matter not yet removed. It is not
+surprising, that the woody matter should have been almost entirely
+removed from the casts on Bald Head; for it is certain, that many
+centuries must have elapsed since the thickets were buried; at present,
+owing to the form and height of the narrow promontory, no sand is
+drifted up, and the whole surface, as I have remarked, is wearing away.
+We must, therefore,
+look back to a period when the land stood lower, of which the French
+naturalists[16] found evidence in upraised shells of recent species,
+for the drifting on Bald Head of the calcareous and quartzose sand, and
+the consequent embedment of the vegetable remains. There was only one
+appearance which at first made me doubt concerning the origin of the
+cast,—namely, that the finer roots from different stems sometimes
+became united together into upright plates or veins; but when the
+manner is borne in mind in which fine roots often fill up cracks in
+hard earth, and that these roots would decay and leave hollows, as well
+as the stems, there is no real difficulty in this case. Besides the
+calcareous branches from the Cape of Good Hope, I have seen casts, of
+exactly the same forms, from Madeira[17] and from Bermuda; at this
+latter place, the surrounding calcareous rocks, judging from the
+specimens collected by Lieutenant Nelson, are likewise similar, as is
+their subaerial formation. Reflecting on the stratification of the
+deposit on Bald Head,—on the irregularly alternating layers of
+substalagmitic rock,—on the uniformly sized, and rounded particles,
+apparently of sea-shells and corals,—on the abundance of land-shells
+throughout the mass,—and finally, on the absolute resemblance of the
+calcareous casts, to the stumps, roots, and branches of that kind of
+vegetation, which would grow on sand-hillocks, I think there can be no
+reasonable doubt, notwithstanding the different opinion of some
+authors, that a true view of their origin has been here given.
+
+ [16] See M. Péron’s “Voyage,” tome i, p. 204.
+
+
+ [17] Dr. J. Macaulay has fully described (_Edinb. New Phil. Journ.,_
+ vol. xxix, p. 350) the casts from Madeira. He considers (differently
+ from Mr. Smith of Jordan Hill) these bodies to be corals, and the
+ calcareous deposit to be of subaqueous origin. His arguments chiefly
+ rest (for his remarks on their structure are vague) on the great
+ quantity of the calcareous matter, and on the casts containing animal
+ matter, as shown by their evolving ammonia. Had Dr. Macaulay seen the
+ enormous masses of rolled particles of shells and corals on the beach
+ of Ascension, and especially on coral-reefs; and had he reflected on
+ the effects of long-continued, gentle winds, in drifting up the finer
+ particles, he would hardly have advanced the argument of quantity,
+ which is seldom trustworthy in geology. If the calcareous matter has
+ originated from disintegrated shells and corals, the presence of
+ animal matter is what might have been expected. Mr. Anderson analysed
+ for Dr. Macaulay part of a cast, and he found it composed of—
+
+Carbonate of lime	73·15 Silica	11·90 Phosphate of lime	8·81
+Animal matter	4·25 Sulphate of lime	a trace ——— 98·11
+
+Calcareous deposits, like these of King George’s Sound, are of vast
+extent on the Australian shores. Dr. Fitton remarks, that “recent
+calcareous breccia (by which term all these deposits are included) was
+found during Baudin’s voyage, over a space of no less than twenty-five
+degrees
+of latitude and an equal extent of longitude, on the southern, western,
+and north-western coasts.”[18] It appears also from M. Peron, with
+whose observations and opinions on the origin of the calcareous matter
+and branching casts mine entirely accord, that the deposit is generally
+much more continuous than near King George’s Sound. At Swan River,
+Archdeacon Scott[19] states that in one part it extends ten miles
+inland. Captain Wickham, moreover, informs me that during his late
+survey of the western coast, the bottom of the sea, wherever the vessel
+anchored, was ascertained, by crowbars being let down, to consist of
+white calcareous matter. Hence it seems that along this coast, as at
+Bermuda and at Keeling Atoll, submarine and subaerial deposits are
+contemporaneously in process of formation, from the disintegration of
+marine organic bodies. The extent of these deposits, considering their
+origin, is very striking; and they can be compared in this respect only
+with the great coral-reefs of the Indian and Pacific Oceans. In other
+parts of the world, for instance in South America, there are
+_superficial_ calcareous deposits of great extent, in which not a trace
+of organic structure is discoverable; these observations would lead to
+the inquiry, whether such deposits may not, also, have been formed from
+disintegrated shells and corals.
+
+ [18] For ample details on this formation consult Dr. Fitton’s
+ “Appendix to Captain King’s Voyage.” Dr. Fitton is inclined to
+ attribute a concretionary origin to the branching bodies: I may
+ remark, that I have seen in beds of sand in La Plata cylindrical stems
+ which no doubt thus originated; but they differed much in appearance
+ from these at Bald Head, and the other places above specified.
+
+
+ [19] “Proceedings of the Geolog. Soc.,” vol. i, p. 320.
+
+_Cape of Good Hope._
+
+After the accounts given by Barrow, Carmichael, Basil Hall, and W. B.
+Clarke of the geology of this district, I shall confine myself to a few
+observations on the junction of the three principal formations. The
+fundamental rock is granite,[20] overlaid by clay-slate: the latter is
+generally hard, and glossy from containing minute scales of mica; it
+alternates with, and passes into, beds of slightly crystalline,
+feldspathic, slaty rock. This clay-slate is remarkable from being in
+some places (as on the Lion’s Rump) decomposed, even to the depth of
+twenty feet, into a pale-coloured, sandstone-like rock, which has been
+mistaken, I believe, by some observers, for a separate formation. I was
+guided by Dr. Andrew Smith to a fine junction at Green Point between
+the granite and clay-slate: the latter at the distance of a quarter of
+a mile
+from the spot, where the granite appears on the beach (though,
+probably, the granite is much nearer underground), becomes slightly
+more compact and crystalline. At a less distance, some of the beds of
+clay-slate are of a homogeneous texture, and obscurely striped with
+different zones of colour, whilst others are obscurely spotted. Within
+a hundred yards of the first vein of granite, the clay-slate consists
+of several varieties; some compact with a tinge of purple, others
+glistening with numerous minute scales of mica and imperfectly
+crystallised feldspar; some obscurely granular, others porphyritic with
+small, elongated spots of a soft white mineral, which being easily
+corroded, gives to this variety a vesicular appearance. Close to the
+granite, the clay-slate is changed into a dark-coloured, laminated
+rock, having a granular fracture, which is due to imperfect crystals of
+feldspar, coated by minute, brilliant scales of mica.
+
+ [20] In several places I observed in the granite, small dark-coloured
+ balls, composed of minute scales of black mica in a tough basis. In
+ another place, I found crystals of black schorl radiating from a
+ common centre. Dr. Andrew Smith found, in the interior parts of the
+ country, some beautiful specimens of granite, with silvery mica
+ radiating or rather branching, like moss, from central points. At the
+ Geological Society, there are specimens of granite with crystallised
+ feldspar branching and radiating in like manner.
+
+
+The actual junction between the granitic and clay-slate districts
+extends over a width of about two hundred yards, and consists of
+irregular masses and of numerous dikes of granite, entangled and
+surrounded by the clay-slate: most of the dikes range in a N.W. and
+S.E. line, parallel to the cleavage of the slate. As we leave the
+junction, thin beds, and lastly, mere films of the altered clay-slate
+are seen, quite isolated, as if floating, in the coarsely crystallised
+granite; but although completely detached, they all retain traces of
+the uniform N.W. and S.E. cleavage. This fact has been observed in
+other similar cases, and has been advanced by some eminent
+geologists,[21] as a great difficulty on the ordinary theory, of
+granite having been injected whilst liquified; but if we reflect on the
+probable state of the lower surface of a laminated mass, like
+clay-slate, after having been violently arched by a body of molten
+granite, we may conclude that it would be full of fissures parallel to
+the planes of cleavage; and that these would be filled with granite, so
+that wherever the fissures were close to each other, mere parting
+layers or wedges of the slate would depend into the granite. Should,
+therefore, the whole body of rock afterwards become worn down and
+denuded, the lower ends of these dependent masses or wedges of slate
+would be left quite isolated in the granite; yet they would retain
+their proper lines of cleavage, from having been united, whilst the
+granite was fluid, with a continuous covering of clay-slate.
+
+ [21] See M. Keilhau’s “Theory on Granite” translated in the _Edinburgh
+ New Philosophical Journal,_ vol.xxiv, p. 402.
+
+Following, in company with Dr. A. Smith, the line of junction between
+the granite and the slate, as it stretched inland, in a S.E. direction,
+we came to a place, where the slate was converted into a fine-grained,
+perfectly characterised gneiss, composed of yellow-brown granular
+feldspar, of abundant black brilliant mica, and of few and thin laminæ
+of quartz. From the abundance of the mica in this gneiss, compared with
+the small quantity and excessively minute scales, in which it exists in
+the glossy clay-slate, we must conclude, that it has been here formed
+by the metamorphic action—a circumstance doubted, under nearly similar
+circumstances, by some authors. The laminæ of the clay-slate are
+straight; and it was interesting to observe, that as they assumed
+the character of gneiss, they became undulatory with some of the
+smaller flexures angular, like the laminæ of many true metamorphic
+schists.
+
+_Sandstone formation._—This formation makes the most imposing feature
+in the geology of Southern Africa. The strata are in many parts
+horizontal, and attain a thickness of about two thousand feet. The
+sandstone varies in character; it contains little earthy matter, but is
+often stained with iron; some of the beds are very fine-grained and
+quite white; others are as compact and homogeneous as quartz rock. In
+some places I observed a breccia of quartz, with the fragments almost
+dissolved in a siliceous paste. Broad veins of quartz, often including
+large and perfect crystals, are very numerous; and it is evident in
+nearly all the strata, that silica has been deposited from solution in
+remarkable quantity. Many of the varieties of quartzite appeared quite
+like metamorphic rocks; but from the upper strata being as siliceous as
+the lower, and from the undisturbed junctions with the granite, which
+in many places can be examined, I can hardly believe that these
+sandstone-strata have been exposed to heat.[22] On the lines of
+junction between these two great formations, I found in several places
+the granite decayed to the depth of a few inches, and succeeded, either
+by a thin layer of ferruginous shale, or by four or five inches in
+thickness of the re-cemented crystals of the granite, on which the
+great pile of sandstone immediately rested.
+
+ [22] The Rev. W. B. Clarke, however, states, to my surprise (“Geolog.
+ Proceedings,” vol. iii, p. 422), that the sandstone in some parts is
+ penetrated by granitic dikes: such dikes must belong to an epoch
+ altogether subsequent to that when the molten granite acted on the
+ clay-slate.
+
+
+Mr. Schomburgk has described[23] a great sandstone formation in
+Northern Brazil, resting on granite, and resembling to a remarkable
+degree, in composition and in the external form of the land, this
+formation of the Cape of Good Hope. The sandstones of the great
+platforms of Eastern Australia, which also rest on granite, differ in
+containing more earthy and less siliceous matter. No fossil remains
+have been discovered in these three vast deposits. Finally, I may add
+that I did not see any boulders of far-transported rocks at the Cape of
+Good Hope, or on the eastern and western shores of Australia, or at Van
+Diemen’s Land. In the northern island of New Zealand, I noticed some
+large blocks of greenstone, but whether their parent rock was far
+distant, I had no opportunity of determining.
+
+ [23] _Geographical Journal,_ vol. x, p. 246.
+
+
+
+
+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 palæontology 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 palæontological 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
+palæontological 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 Caribæus_, 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,[1] according to M.
+d’Orbigny, now live at the mouth of the estuary. 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[2] 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.
+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,[3] of the _Azara labiata_, lying at about forty feet above the
+level of the river, and distant between two and three miles from it.
+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:—
+
+Buccinanops globulosum, d’Orbigny.
+
+Olivancillaria auricularia, d’Orbigny.
+
+Venus flexuosa, Lam.
+
+Cytheræa (imperfect).
+
+Mactra Isabellei, d’Orbigny.
+
+Ostrea pulchella, d’Orbigny.
+
+
+Besides these, Sir W. Parish procured[4] (as named by Mr. G. B.
+Sowerby) the following shells:—
+
+Voluta colocynthis.
+
+Voluta angulata.
+
+Buccinum (not spec.?).
+
+
+ [1] “Voyage dans l’Amérique Mérid.: Part. Géolog.,” p. 21.
+
+
+ [2] _Ibid._, p. 43.
+
+
+ [3] “Buenos Ayres,” etc., by Sir Woodbine Parish, p. 168.
+
+
+ [4] “Buenos Ayres,” etc., by Sir W. Parish, p. 168.
+
+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,[5] however, has described groups of sand dunes
+scattered over the wide extent of the Pampas southward of Buenos Ayres,
+which M. d’Orbigny attributes with much probability to the action of
+the sea, before the plains were raised above its level.[6]
+
+ [5] D’Orbigny’s “Voyage Géolog.,” p. 44.
+
+
+ [6] 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 (lat. 24° 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.,” p. 35, and Dr. Meigs, in
+ “Trans. Amer. Phil. Soc.”), 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
+ (lat. 13° 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 (lat. 8° 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 lat. 39°
+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 scoriæ: 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° 40′ S.) a little south of the mouth of the Colorado,
+M. d’Orbigny[7] found fourteen species of existing shells (six of them
+identical with those from Bahia Blanca), embedded in their natural
+positions. 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.
+
+ [7] “Voyage,” etc., p. 54.
+
+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.
+
+In the following diagrams:
+Baseline is Level of sea.
+Scale is 1/20 of inch to 100 feet vertical.
+
+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.
+
+No. 1
+Section of step-formed plains south of Nuevo Gulf.
+
+
+[Illustration: Section of step-formed plains south of Nuevo Gulf.]
+
+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.
+
+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 No. 1.
+
+No. 2
+Section of plains in the Bay of St. George.
+
+
+[Illustration: Section of plains in the Bay of St. George.]
+
+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 No. 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. 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
+following section:—
+
+No. 3
+Section of plains at Port Desire.
+
+
+[Illustration: Section of plains at Port Desire.]
+
+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.
+
+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); where we have the following section:—
+
+No. 4
+Section of plains at Port St. Julian.
+
+
+[Illustration: Section of plains at Port St. Julian.]
+
+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.
+
+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:—
+
+No. 5
+Section of plains at the mouth of the Rio Santa Cruz.
+
+
+[Illustration: Section of plains at the mouth of the Rio Santa Cruz.]
+
+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.[8] 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 (long.
+71° 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.
+
+ [8] I have described this formation in a paper in the “Geological
+ Transactions,” vol. vi, p. 415.
+
+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.
+
+I have said that the valley in its whole course is bordered by
+gravel-capped plains. The section (diagram No. 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 (_n_) was 869
+above the river: very near to where A (_n_) was measured, C (_n_) was
+639 above the same level: the terrace D (_n_) was nowhere measured: the
+lowest E (_n_) 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 (_s_), however was traced continuously for a great distance.
+The terrace B (_n_), 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 (_n_) to A (_s_) being generally nearly double the width of from
+B (_n_) to B (_s_).
+
+No. 6
+North and South Section across the terraces bounding the valley of the
+River Santa Cruz, high up its course.
+
+
+[Illustration: North and South Section across the terraces bounding the
+River Santa Cruz.]
+
+Low down the valley, the summit-plain A (_s_) is continuous with the
+840 feet plain on the coast, but it is soon lost or unites with the
+escarpment of B (_s_). The corresponding plain A (_n_), 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 (_n_), 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 Patellæ and Mytili,
+scattered on the surface and partially embedded in earth. On the
+eastern coast, also, of Tierra del Fuego, in latitude 53° 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[9] on the surface, that two sea-channels connected the
+Strait of Magellan both with Sebastian Bay and with Otway Water.
+
+ [9] “Geolog. Transactions,” vol. vi, p. 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° 40′ to 53° 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° 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.[10]
+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.[11] 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.
+
+ [10] Captain King, “Voyages of _Adventure_ and _Beagle_,” vol. i, 1
+ pp. 6 and 133.
+
+
+ [11] See Mr. Lyell “Proofs of a Gradual Rising in Sweden,” in the
+ “Philosoph. Transact.,” 1835, p. 1. See also Mr. Smith of Jordan Hill
+ in the _Edin. New Phil. Journal_, vol. xxv, p. 393.
+
+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.[12] Moreover, we know
+that in Tierra del Fuego the boulder formation has been uplifted within
+the recent period, and a similar formation occurs[13] on the
+north-western shores (Byron Sound) of these islands. 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,[14] 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;[15] 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.
+
+ [12] “Voyages of the _Adventure_ and _ Beagle_,” vol. ii, p. 227. And
+ Bougainville’s “Voyage,” tome i, p. 112.
+
+
+ [13] 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 (p. 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.
+
+
+ [14] 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.
+
+
+ [15] See Sir W. Parish’s work on “La Plata,” p. 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.
+
+
+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:—
+
+ 	Feet 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 the table below, indicate the much greater extension of
+780 miles:—
+
+ 	Feet 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 lat. 54° 30′ to the mouth of the
+Rio Negro, in lat. 31° (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.
+The following table gives the average result of many soundings off the
+Santa Cruz:—
+
+Under two miles from the shore, many of the pebbles were of large size,
+mingled with some small ones.
+
+
+Distance in miles from shore	Depth in fathoms	Size of Pebbles 3
+to 4	11 to 12	As large as walnuts; mingled in every case with
+some smaller ones. 6 to 7	17 to 19	As large as hazel-nuts. 10 to
+11	23 to 25	From three- to four-tenths of an inch in diameter.
+12	30 to 40	Two-tenths of an inch. 22 to 150	45 to
+65	One-tenth of an inch, to the finest sand.
+
+I particularly attended to the size of the pebbles on the 355 feet
+Santa Cruz plain, and I noticed that on the summit-edge of the present
+sea cliffs many were as large as half a man’s head; and in crossing
+from these cliffs to the foot of the next highest escarpment, a
+distance of six miles, I could not observe any increase in their size.
+We shall presently see that the theory of a slow and almost insensible
+rise of the land, will explain all the facts connected with the
+gravel-capped terraces, better than the theory of sudden elevations of
+from one to two hundred feet.
+
+M. d’Orbigny has argued, from the upraised shells at San Blas being
+embedded in the positions in which they lived, and from the valves of
+the _Azara labiata_ high on the banks of the Parana being united and
+unrolled, that the elevation of Northern Patagonia and of La Plata must
+have been sudden; for he thinks, if it had been gradual, these shells
+would all have been rolled on successive beach-lines. But in
+_protected_ bays, such as in that of Bahia Blanca, wherever the sea is
+accumulating extensive mud-banks, or where the winds quietly heap up
+sand-dunes, beds of shells might assuredly be preserved buried in the
+positions in which they had lived, even whilst the land retained the
+same level; any, the smallest, amount of elevation would directly aid
+in their preservation. I saw a multitude of spots in Bahia Blanca where
+this might have been effected; and at Maldonado it almost certainly has
+been effected. In speaking of the elevation of the land having been
+slow, I do not wish to exclude the small starts which accompany
+earthquakes, as on the coast of Chile; and by such movements beds of
+shells might easily be uplifted, even in positions exposed to a heavy
+surf, without undergoing any attrition: for instance, in 1835, a rocky
+flat off the island of Santa Maria was at one blow upheaved above
+high-water mark, and was left covered with gaping and putrefying
+mussel-shells, still attached to the bed on which they had lived. If M.
+d’Orbigny had been aware of the many long parallel lines of
+sand-hillocks, with infinitely numerous shells of the Mactra and Venus,
+at
+a low level near the Uruguay; if he had seen at Bahia Blanca the
+immense sand-dunes, with water-worn pebbles of pumice, ranging in
+parallel lines, one behind the other, up a height of at least 120 feet;
+if he had seen the sand-dunes, with the countless Paludestrinas, on the
+low plain near the Fort at this place, and that long line on the edge
+of the cliff, sixty feet higher up; if he had crossed that long and
+great belt of parallel sand-dunes, eight miles in width, standing at
+the height of from forty to fifty feet above the Colorado, where sand
+could not now collect,—I cannot believe he would have thought that the
+elevation of this great district had been sudden. Certainly the
+sand-dunes (especially when abounding with shells), which stand in
+ranges at so many different levels, must all have required long time
+for their accumulation; and hence I do not doubt that the last 100 feet
+of elevation of La Plata and Northern Patagonia has been exceedingly
+slow.
+
+If we extend this conclusion to Central and Southern Patagonia, the
+inclination of the successively rising gravel-capped plains can be
+explained quite as well, as by the more obvious view already given of a
+few comparatively great and sudden elevations; in either case we must
+admit long periods of rest, during which the sea ate deeply into the
+land. Let us suppose the present coast to rise at a nearly equable,
+slow rate, yet sufficiently quick to prevent the waves quite removing
+each part as soon as brought up; in this case every portion of the
+present bed of the sea will successively form a beach-line, and from
+being exposed to a like action will be similarly affected. It cannot
+matter to what height the tides rise, even if to forty feet as at Santa
+Cruz, for they will act with equal force and in like manner on each
+successive line. Hence there is no difficulty in the fact of the 355
+feet plain at Santa Cruz sloping up 108 feet to the foot of the next
+highest escarpment, and yet having no marks of any one particular
+beach-line on it; for the whole surface on this view has been a beach.
+I cannot pretend to follow out the precise action of the tidal-waves
+during a rise of the land, slow, yet sufficiently quick to prevent or
+check denudation: but if it be analogous to what takes place on
+protected parts of the present coast, where gravel is now accumulating
+in large quantities,[16] an inclined surface, thickly capped by
+well-rounded pebbles of about the same size, would be ultimately left.
+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.
+
+ [16] 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.
+
+
+_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,[17] during
+which the sea corroded deeply, as it is still corroding, into the land.
+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 lat. 50° 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.
+
+ [17] 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.)
+
+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
+lat. 54° 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.[18] 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.
+
+ [18] 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.
+
+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. A table has already been given, showing how very uniformly and
+gradually[19] the pebbles decrease in size with the gradually seaward
+increasing depth and distance. 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,[20] 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. Groundswells, which are believed to be caused by
+distant gales, seem especially to affect the bottom: at such times,
+according to Sir R. Schomburgk,[21] 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.
+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.[22] 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.[23]
+
+ [19] 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,” pp. 4, 6, 175; also Captain Beechey’s “Voyage to the
+ Pacific,” chap. xviii.
+
+
+ [20] “Soundings in the Channel,” pp. 4, 166. M. Siau states (_Edin.
+ New Phil. Jour._, vol. xxxi, p. 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.”
+
+
+ [21] _Journal of Royal Geograph. Soc.,_ vol. v, p. 25. It appears from
+ Mr. Scott Russell’s investigations (see Mr. Murchison’s “Anniver.
+ Address Geolog. Soc.,” 1843, p. 40), that in waves of translation the
+ motion of the particles of water is nearly as great at the bottom as
+ at the top.
+
+
+ [22] (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.
+
+
+ [23] 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_,” vol. i, p. 375 and “Appendix,” p.
+ 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,[24] 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. 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.
+
+ [24] “Philosophical Transactions,” 1834, p. 576.
+
+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.
+
+No. 7
+Section of coast-cliffs and bottom of sea, off the island of St.
+Helena.
+
+
+[Illustration: Section of clast-cliffs and bottom of sea, off the
+island of St. Helena.]
+
+_Formation of Cliffs._—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 No. 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 lat. 46° 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 lat. 47° and 48°), Byron[1] remarks,
+“We thought it very strange, that upon the summits of the highest hills
+were found beds of shells, a foot or two thick.” In the Chonos
+Archipelago, the island of Lemus (lat. 44° 30′) was, according to M.
+Coste,2 suddenly elevated eight feet, during the earthquake of 1829: he
+adds, “Des roches jadis toujours couvertes par la mer, restant
+aujourd’hui constamment decouvertes.” In other parts of this
+archipelago, I observed two terraces of gravel, abutting to the foot of
+each other: at Lowe’s Harbour (43° 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.
+
+ [1] “Narrative of the Loss of the _Wager_.”
+
+
+ [2] “Comptes Rendus,” October 1838, p. 706.
+
+_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 Serpulæ
+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[3] to
+the names of promontories, traditionally said to have been islands; in
+Sumatra, as in Chiloe, there are upraised recent shells. The Bay of
+Carelmapu, on the mainland north of Chiloe, according to Agüerros,[4]
+was in 1643 a good harbour; it is now quite useless, except for boats.
+
+ [3] Marsden’s “Sumatra,” p. 31.
+
+
+ [4] “Descripcion Hist. de la Provincia de Chiloé,” p. 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.
+
+
+_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 Cytheræa, too
+imperfect to be specifically recognised. At Imperial, seventy miles
+north of Valdivia, Agüerros[5] states that there are large beds of
+shells, at a considerable distance from the coast, which are burnt for
+lime. The island of Mocha, lying a little north of Imperial, was
+uplifted two feet,[6] during the earthquake of 1835.
+
+ [5] _Ibid.,_ p. 25.
+
+
+ [6] “Voyages of _Adventure_ and _Beagle_,” vol. ii, p. 415.
+
+
+_Concepcion._—I cannot add anything to the excellent account by Captain
+Fitzroy[7] of the elevation of the land at this place, which
+accompanied the earthquake of 1835. 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, patellæ, 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.
+
+ [7] _Ibid.,_ vol. ii, p. 412, _et seq._ In vol. v, p. 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.
+
+
+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 Flustræ; 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.[8] 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.
+
+ [8] “Zoology of Captain Beechey’s Voyage,” p. 162.
+
+
+_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[9] found here recent shells at the distance
+of two leagues from the shore. 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.
+
+ [9] “Annales des Scienc. Nat.,” Avril 1833.
+
+_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, Patellæ, Crepidulæ,
+and of _Mytilus Magellanicus_ (?):[10] 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,[11] 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.
+
+ [10] 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.
+
+
+ [11] 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.
+
+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 Patellæ, Mytili, and other species. I
+found similar microscopical fragments in earth filling up the central
+orifices of some large Fissurellæ. 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.[12]
+
+ [12] In the “Proceedings of the Geolog. Soc.,” vol. ii, p. 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,[13] which accompanied the earthquake of 1822:
+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.
+
+ [13] Dr. Meyen (“Reise um Erde,” Th. I, 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.
+
+
+_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,[14] to a height of 500 feet, and at a distance of three leagues
+from the coast: 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.
+
+ [14] “Travels in Chile,” vol. i, pp. 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.
+
+_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,[15] 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. 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.[16]
+
+ [15] 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.
+
+
+ [16] 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:—
+
+Venus opaca.
+
+Mulinia Byronensis.
+
+Pecten purpuratus.
+
+Mesodesma donaciforme.
+
+Turritella cingulata.
+
+Monoceros costatum.
+
+Concholepas Peruviana.
+
+Trochus (common Valparaiso species).
+
+Calyptræa 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 _Calyptræa
+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.
+
+No. 8
+Section of plain of Coquimbo.
+
+
+[Illustration: Section of plain of Coquimbo.]
+
+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:—
+
+Calyptræa radians.
+
+Turritella cingulata.
+
+Oliva Peruviana.
+
+Murex labiosus, var.
+
+Nassa (identical with a living species).
+
+Solen Dombeiana.
+
+Pecten purpuratus.
+
+Venus Chilensis.
+
+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.)
+
+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,[17] that a rise has
+taken place here within the last century and a half; 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 Feuillèe, were _à 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 Feuillèe
+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.
+
+ [17] “Proceedings of the Geological Society,” vol. ii, p. 446.
+
+
+_Valley 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.[18] 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.
+
+ [18] “Principles of Geology” (1st edit.), vol. iii, p. 131.
+
+
+No. 9
+East and west section through the terraces at Coquimbo, where they
+debouch from the valley, and front the sea.
+
+
+[Illustration: East and west section through terraces at Coquimbo.]
+
+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.
+
+We come now to the terraces on the opposite sides of the east and west
+valley of Coquimbo: the section in figure No. 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.
+
+No. 10
+North and south section across the valley of Coquimbo.
+
+
+[Illustration: North and south section across the valley of Coquimbo.]
+
+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 No. 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 No. 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 No. 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.
+
+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 two last
+sections 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.
+
+[Illustration: A bay in the 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,[19] 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.
+
+ [19] “Philosophical Transactions,” 1839, p. 39.
+
+
+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,[20] 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,[21] 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 showing that the
+proportional difference in the amount of elevation on the coast and in
+the interior, varied at different periods.
+
+ [20] Mr. Place in the _Quarterly Journal of Science,_ 1824, vol. xvii,
+ p. 42.
+
+
+ [21] “Voyages de la Comm. du Nord,” etc., also “Comptes Rendus,” Oct.
+ 1842.
+
+_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.
+
+No. 12
+North and south section across the valley of Guasco, and of a plain
+north of it.
+
+
+[Illustration: North and south section across the valley of Guasco.]
+
+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[22] 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 _Calyptræa 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.
+
+ [22] “Reise um die Erde,” Th. I, s. 372, _et seq._
+
+
+Northward of Copiapo, in lat. 26° S., the old voyager Wafer[23] found
+immense numbers of sea-shells some miles from the coast. At Cobija
+(lat. 22° 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, Calyptræa, 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:[24] 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 (lat. 20° 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[25] 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.” 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 (lat. 18° 28′), M. d’Orbigny[26]
+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. Lieutenant
+Freyer has given some more precise facts: he states[27] 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 Milleporæ
+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[28] found numerous recent sea shells in sand,
+at a considerable distance from the sea.
+
+ [23] Burnett’s “Collection of Voyages,” vol. iv, p. 193.
+
+
+ [24] “Voyage, Part Géolog.,” p. 94. M. d’Orbigny (p. 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 mètres; il faudrait supposer encore
+ que ce soulèvement n’a point été graduel; . . . mais qu’il résulterait
+ d’une seule et même 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, Calyptræa, 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.
+
+
+ [25] _Silliman’s Amer. Journ. of Science,_ vol. xliv, p. 2.
+
+
+ [26] “Voyage,” etc., p. 101.
+
+
+ [27] In a letter to Mr. Lyell, “Geolog. Proc.,” vol. ii, p. 179.
+
+
+ [28] _Edin. New Phil. Journ.,_ vol. xxx, p. 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.[29] 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 Serpulæ.
+All, according to Mr. G.B. Sowerby, are recent species: they consist
+of:—
+
+Mytilus Magellanicus: same as that found at Valparaiso, and there
+stated to be probably distinct from the true _M. Magellanicus_ of the
+east coast.
+
+Venus costellata, Sowerby “Zoological Proceedings.”
+
+Pecten purpuratus, Lam.
+
+Chama, probably echinulata, Brod.
+
+Calyptræa Byronensis, Gray.
+
+Calyptræa radians (Trochus, Lam.)
+
+Fissurella affinis, Gray.
+
+Fissurella biradiata, Trembly.
+
+Purpura chocolatta, Duclos.
+
+Purpura Peruviana, Gray.
+
+Purpura labiata, Gray.
+
+Purpura buxea (Murex, Brod.).
+
+Concholepas Peruviana.
+
+Nassa, related to reticulata.
+
+Triton rudis, Brod.
+
+Trochus, not yet described, but well-known and very common.
+
+and 18. Balanus, two species, both common on the coast.
+
+
+ [29] M. Chevalier, in the “Voyage of the _Bonite_,” observed these
+ shells; but his specimens were lost.—“L’Institut,” 1838, p. 151.
+
+
+These upraised shells appear to be nearly in the same proportional
+numbers—with the exception of the Crepidulæ 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 Fissurellæ 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.[30] 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.
+
+ [30] 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” (vol.
+ ii, Eng. Trans., p. 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° 46′); this rain
+ entirely ruined (“Ulloa,” etc., p. 18) the mud houses of the
+ inhabitants.
+
+
+_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,[31] 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[32] of the earthquake of 1835, 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.
+
+ [31] Mr. Smith of Jordan Hill found pieces of seaweed in an upraised
+ pleistocene deposit in Scotland. See his admirable Paper in the _Edin.
+ New Phil. Journal,_ vol. xxv, p. 384.
+
+
+ [32] I have described this in my “Journal of Researches,” p. 303, 2nd
+ edit.
+
+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[33] 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
+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[34] 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.
+
+ [33] 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.
+
+
+ [34] “Observaciones sobre el Clima del Lima” par Dr. H. Unanùe, p.
+ 4.—Ulloa’s “Voyage,” vol. ii, Eng. Trans., p. 97.—For Mr. Cruikshank’s
+ observations, see Mr. Lyell’s “Principles of Geology” (1st edition)
+ vol. iii, p. 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.[35] The other saline bodies
+may perhaps have been partially thus derived, but chiefly, as I
+conclude from the following facts, through a different means.
+
+ [35] See “Volcanic Islands,” etc., by the Author.
+
+
+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,[36] and slightly moistened, partially decompose each other:
+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.
+
+ [36] 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’ Chron.,_
+ 1845, p. 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’
+ Chron.,_ p. 675, 1844.
+
+
+_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 lat. 45° 35′ to 12° 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.[37] 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.
+
+ [37] 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.
+
+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[1] since the
+time of Molina. 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[2] above the surface of the
+surrounding plain.
+
+ [1] This plain is partially separated into two basins by a range of
+ hills; the southern half, according to Meyen (“Reise um Erde,” Th. i,
+ s. 274), falls in height, by an abrupt step, of between fifteen and
+ twenty feet.
+
+
+ [2] 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. i, 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;[3] 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.[4] 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.
+
+ [3] The plain of San Fernando has, according to MM. Meyen and Gay
+ “Reise,” etc., Th. i, 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.
+
+
+ [4] It appears from Captain Herbert’s account of the Diluvium of the
+ Himalaya, (“Gleanings of Science,” Calcutta, vol. ii, p. 164), that
+ precisely similar remarks apply to the drainage of the plains or
+ valleys between those great mountains.
+
+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.
+
+No. 13
+Section of the plain at the eastern foot of the Chilian Cordillera.
+
+
+[Illustration: Section of the plain at the eastern foot of the Chilian
+Cordillera.]
+
+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._—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.
+
+[Illustration: Ground plan of a bifurcating valley in the Cordillera.]
+
+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[5] states that the granite does not occur in situ within
+a distance of twenty leagues; 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.
+
+ [5] “Annales des Science Nat.” (I. séries, 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.
+
+
+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,[6] in the Tinguirica. XIn 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.
+
+ [6] “Reise,” etc., Th. I, s. 302.
+
+
+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.[7] 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.[8] 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.
+
+ [7] Sloping terraces of precisely similar structure have been
+ described by me (“Philosoph. Transactions,” 1839, p. 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 Edin. Phil. Journal,_ vol. xxi,
+ p. 187) of North Wales.
+
+
+ [8] I have attempted to explain this process in a more detailed
+ manner, in a letter to Mr. Maclaren, published in the _Edinburgh New
+ Phil. Journal,_ vol. xxxv, p. 288.
+
+
+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,[9] 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. 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,[10] 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.
+
+ [9] “Compendio de la Hist.,” etc., tome i, p. 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 (“Geolog. Transac.,” vol. vi, p. 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.
+
+
+ [10] 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.[11] 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[12] substance had probably been derived
+through common alluvial action from the layers of salt which occur
+interstratified in the surrounding mountains: 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[13] observed analogous
+saline beds extending from Cobija for five degrees of latitude
+northward, and at heights varying from six hundred to nine hundred
+feet: 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.
+
+ [11] It is singular how slowly, according to the observations of M.
+ Cordier on the salt-mountain of Cardona in Spain (“Ann. des Mines,
+ Transl. of Geolog. Mem.” by De la Beche, p. 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.
+
+
+ [12] “Journal of Researches,” p. 444, first edit.
+
+
+ [13] “Voyage,” etc., p. 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 edit., p. 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.
+
+
+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.[14] 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[15] 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. 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.
+
+ [14] “Volcanic Islands,” etc., p. 87.
+
+
+ [15] “Voyage Géolog.,” etc., p. 95.
+
+
+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,[16] 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. 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.
+
+ [16] See an admirable paper “Geolog. and Miscell. Notices of
+ Tarapaca,” in _Silliman’s American Journal_, vol. xliv, p. 1.
+
+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[17] to contain much muriate of lime. 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.[18]
+
+ [17] _Literary Gazette_, 1841, p. 475.
+
+
+ [18] (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:—
+
+Entire export 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[19] state
+that they occur at intervals over the vast plains extending from the
+mouth of the Plata to Rioja and Catamarca. 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[20] has analysed it;
+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.
+
+ [19] Azara (“Travels,” vol. i, p. 55) considers that the Parana is the
+ eastern boundary of the saliferous region; but I heard of “salitrales”
+ in the Province of Entre Rios.
+
+
+ [20] M. d’Orbigny’s “Voyage,” etc., Part. Hist., tome i, p. 664.
+
+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 (lat. 49° to 50°) at which salinas are known to
+occur.[21] The depressions, in which these salt-lakes lie, are from a
+few feet to sixty metres, as asserted by M. d’Orbigny,[22] below the
+surface of the surrounding plains; 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.
+
+ [21] According to Azara (“Travels,” vol. i, p. 56) there are
+ salt-lakes as far north as Chaco (lat. 25°), on the banks of the
+ Vermejo. The salt-lakes of Siberia appear (Pallas’s “Travels,” English
+ Trans., vol. i, p. 284) to occur in very similar depressions to those
+ of Patagonia.
+
+
+ [22] “Voyage Géolog.,” p. 63.
+
+
+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.[23] 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[24] states that some of these crystals are
+acicular and more than even nine inches in length; 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.[25] 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.
+
+ [23] Professor Ehrenberg examined some of this muddy sand, but was
+ unable to find in it any infusoria.
+
+
+ [24] “Voyage Géolog.,” p. 64.
+
+
+ [25] This is what might have been expected; for M. Ballard asserts
+ (_Acad. des Sciences_, Oct. 7, 1844, that sulphate of soda is
+ precipitated from solution more readily from water containing muriate
+ of soda in excess, than from pure water.
+
+
+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,[26] that those salts answer best for
+preserving cheese which contain most of the deliquescent chlorides.[27]
+
+ [26] _Hort. and Agricult. Gazette_, 1845, p. 93.
+
+
+ [27] 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,[28] and is sometimes found in small lakes, quite close to these
+salinas. 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.
+
+ [28] Sir W. Parish states (“Buenos Ayres,” etc., pp. 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.
+
+
+
+
+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.[1]
+
+ [1] Schmidtmeyer (“Travels in Chile,” p. 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[2]) 840 feet above the
+sea-level. 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.
+
+ [2] “La Plata,” etc., by Sir W. Parish, p. 146.
+
+
+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,[3] beds of
+clay two yards thick. 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[4] as composed
+of solid tosca-rock; 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.
+
+ [3] M. d’Orbigny, “Voyage,” Part. Géolog., pp. 47, 48.
+
+
+ [4] _Ibid._
+
+
+_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.[5] 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 vertebræ, limbs, ribs, and other bones of two rodents;
+sixthly, bones of the extremities of some great megatheroid
+quadruped.[6] 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.
+
+ [5] The following list is given in the “Monatsberichten der könig.
+ Akad. zu Berlin,” April 1845:—
+
+POLYGASTRICA.
+    Fragilaria rhabdosoma.
+    Gallionella distans.
+    Pinnularia?
+PHYTOLITHARIA.
+    Lithodontium Bursa.
+    Lithodontium furcatum.
+    Lithostylidium exesum.
+    Lithostylidium rude.
+    Lithostylidium Serra.
+    Spongolithis Fustis?
+
+
+ [6] See “Fossil Mammalia” (p. 109) by Professor Owen, in the “Zoology
+ of the Voyage of the _Beagle_;” and Catalogue (p. 36) of Fossil
+ Remains in Museum of Royal College of Surgeons.
+
+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.
+
+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 No. 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.
+
+No. 15
+Section of beds with recent shells and extinct mammifers, at Punta Alta
+in Bahia Blanca.
+
+
+[Illustration: Section of beds at Punta Alta in Bahia Blanca.]
+
+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:—
+
+
+Volutella angulata, d’Orbigny, “Voyage” Mollusq. and Pal.
+
+Voluta Braziliana, Sol.
+
+Olicancilleria Braziliensis, d’Orbigny.
+
+Olicancilleria auricularia, d’Orbigny.
+
+Olivina puelchana, d’Orbigny.
+
+Buccinanops cochlidium, d’Orbigny.
+
+Buccinanops globulosum, d’Orbigny.
+
+Colombella sertulariarum, d’Orbigny.
+
+Trochus Patagonicus, and var. of ditto, d’Orbigny.
+
+Paludestrina Australis, d’Orbigny.
+
+Fissurella Patagonica, d’Orbigny.
+
+Crepidula muricata, Lam.
+
+Venus purpurata, Lam.
+
+Venus rostrata, Phillippi.
+
+Mytilus Darwinianus, d’Orbigny.
+
+Nucula semiornata, d’Orbigny.
+
+Cardita Patagonica, d’Orbigny.
+
+Corbula Patagonica (?), d’Orbigny.
+
+Pecten tethuelchus, d’Orbigny.
+
+Ostrea puelchana, d’Orbigny.
+
+A living species of Balanus.
+
+and 23. An Astræ 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,[7] 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.
+
+ [7] 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, vertebræ,
+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 Palæotherium, 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, Serpulæ, and corallines are attached to many of
+the bones, but I neglected to observe[8] whether these might not have
+grown on them since being exposed to the present tidal action; 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.
+
+ [8] 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.
+
+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 vertebræ 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[9] 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. 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.
+
+ [9] “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.
+
+
+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[10] that all these mammiferous remains may have
+been washed out of the Pampean formation, and afterwards redeposited
+together with the recent shells. 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.
+
+ [10] “Voyage,” Part. Géolog., p. 49.
+
+
+_Buenos Ayres to St. Fé 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.”[11] 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.
+
+ [11] 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.
+
+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,[12] all of them, I believe, with two exceptions, already
+known species. 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.
+
+ [12] “Monatsberichten der könig. Akad. zu Berlin,” April 1845. The
+ list consists of:—
+
+ POLYGASTRICA.
+    Campylodiscus clypeus.
+    Coscinodiscus subtilis.
+    Coscinodiscus al. sp.
+    Eunotia.
+    Gallionella granulata.
+    Himantidium gracile.
+    Pinnularia borealis.
+
+
+At _St. Fé 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.[13] Above this there is a thick bed of yellowish sandy clay,
+with much crystallised gypsum and many shells of Ostreæ, Pectens, and
+Arcæ: 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_[14] chiefly abounds. 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:—
+
+Ostrea Patagonica, d’Orbigny, “Voyage,” Part. Pal.
+
+Ostrea Alvarezii, d’Orbigny, “Voyage,” Part. Pal.
+
+Pecten Paranensis, d’Orbigny, “Voyage,” Part. Pal.
+
+Pecten Darwinianus, d’Orbigny, “Voyage,” Part. Pal.
+
+Venus Munsterii, d’Orbigny, “Voyage,” Pal.
+
+Arca Bonplandiana, d’Orbigny, “Voyage,” Pal.
+
+Cardium Platense, d’Orbigny, “Voyage,” Pal.
+
+Tellina, probably nov. spec., 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.
+
+ [13] M. d’Orbigny has given (“Voyage,” Part. Géolog., p. 37) 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.
+
+
+ [14] 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.
+
+
+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[15] from North America, than to
+those of any other species of Equus.
+
+ [15] Lyell’s “Travels in North America,” vol. i, p. 164 and “Proc. of
+ Geolog. Soc.,” vol. iv, p. 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. Fé 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,[16] 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.[17]
+
+ [16] 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.
+
+
+ [17] Liebig (“Chemistry of Agriculture,” p. 194) states that fresh dry
+ bones contain from 32 to 33 per cent of dry gelatine. See also Dr.
+ Daubeny, in _Edin. New Phil. Journ._, vol. xxxvii, p. 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. Fé Bajada; at P.
+Gorda,[18] 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_: 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. Fé, 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. Fé Bajada, I could not be deceived in
+the mineralogical character of the Pampean deposit.
+
+ [18] In my “Journal” (p. 171, 1st edit.), 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.
+
+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 scoriæ; 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 scoriæ 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,[19] where the Parana makes a remarkable bend, M. Bonpland found
+some singular amygdaloidal rocks, which perhaps may belong to this same
+epoch. 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.
+
+ [19] M. d’Orbigny, “Voyage,” Part. Géolog., p. 29.
+
+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 Fé 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.
+
+No. 16
+Section of the lowest plain at Port S. Julian.
+
+
+[Illustration: Section of the lowest plain at Port S. Julian.]
+
+_Earthy mass, with extinct mammiferous remains, over the porphyritic
+gravel at S. Julian, lat. 49° 14′ S., in Patagonia._—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
+above.
+
+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 No. 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 Palæotherium, but with
+affinities to the Ruminants, especially to the American division of the
+Camelidæ. Several of the vertebræ 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.[20]
+
+ [20] 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[21] 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. 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.[22] 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.
+
+ [21] “Geological Proceedings,” vol. iv, p. 36.
+
+
+ [22] 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.
+
+_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.[23]
+
+ [23] It is almost superfluous to give the numerous cases (for
+ instance, in Sumatra; Lyell’s “Principles,” vol. iii, p. 325, sixth
+ edit.), 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 Soc.,” 1845, June 21st), of oxen, deer, and bears being
+ carried into the Gulf of Cambray; see also the account in my “Journal”
+ (2nd edit., p. 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:[24] 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.
+
+ [24] 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., p. 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
+ 13th, 1840, and abstracted in the _ Athenæum_, p. 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.[25]
+
+ [25] M. d’Orbigny believes (“Voyage,” Part. Géolog., p. 81) that this
+ formation, though “très voisine de la nôtre, est néanmoins de beaucoup
+ antérieure à notre création.”
+
+At St. Fé 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. Fé 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. Fé. 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[26] to at least as far south as
+Tarija; 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
+edit., p. 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 edit., p. 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.
+
+ [26] 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.
+
+
+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.[27] 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 lat. 39° 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 lat. 35° south. The case of
+the Mastodon Andium is one of more difficulty, for it is found from
+lat. 36° 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.
+
+ [27] Many original observations, and a summary on this subject, are
+ given in Mr. Lyell’s paper in the “Geolog. Proc.,” vol. iv, p. 3 and
+ in his “Travels in North America,” vol. i, p. 164 and vol. ii, p. 60.
+ For the European analogous cases see Mr. Lyell’s “Principles of
+ Geology” (6th edit.), vol. i, p. 37.
+
+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, Hydrochærus,
+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., p. 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_ p. 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., p. 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,” p. 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” (p. 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 (vol. i, p. 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,” p. 322) says, many bones have been found near the R. Negro,
+and on the R. Arapey, an affluent of the Paraguay, in lat. 30° 40′
+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,” Géolog., p. 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[1]
+on the sandstone formation of this district. 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.
+
+ [1] “Voyage,” Part. Géolog., pp. 57-65.
+
+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:—
+
+Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé, and whole
+coast of Patagonia).
+
+Ostrea Ferrarisi, d’Orbigny, “Voyage, Pal.”
+
+Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. Fé, and S.
+Josef).
+
+Pecten Patagoniensis, d’Orbigny, “Voyage, Pal.”
+
+Venus Munsterii, d’Orbigny, “Voyage, Pal.” (also at St. Fé).
+
+Arca Bonplandiana, d’Orbigny, “Voyage, Pal.” (also at St. Fé).
+
+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 Ostræa Patagonica, often marked with
+dendritic manganese and small coral-lines, were extraordinarily
+numerous. I found here the following shells:—
+
+Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé and whole
+coast of Patagonia).
+
+Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. Fé and R.
+Negro).
+
+Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at St. Fé, S.
+Julian, and Port Desire).
+
+Pecten Darwinianus, d’Orbigny, “Voyage, Pal.” (also at St. Fé).
+
+Pecten actinodes, G. B. Sowerby.
+
+Terebratula Patagonica, G. B. Sowerby (also S. Julian).
+
+Casts of a Turritella.
+
+The four first of these species occur at St. Fé 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 laminæ 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 _Ostrea 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:—
+
+
+Ostrea Patagonica, d’Orbigny, (also at St. Fé, and whole coast of
+Patagonia).
+
+Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at St. Fé, S. Josef,
+S. Julian).
+
+Pecten centralis, G. B. Sowerby (also at S. Julian and S. Cruz).
+
+Cucullæa alta, G. B. Sowerby (also at S. Cruz).
+
+Nucula ornata, G. B. Sowerby.
+
+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 laminæ 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 laminæ) of gypsum. Under the microscope, according to
+Professor Ehrenberg,[2] it consists of minute, triturated, cellular,
+glassy fragments of pumice, with some broken crystals. 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.
+
+ [2] “Monatsberichten de könig. Akad. zu Berlin,” vom April 1845.
+
+_Port S. Julian._—On the south side of the harbour, the following
+section (figure No. 17) gives the nature of the beds seen in the cliffs
+of the ninety feet plain. Beginning at the top:—first, 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 Scutellæ;
+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 Balanidæ. Out of
+these three beds, I procured the following twelve species, of which the
+two first were exceedingly numerous in individuals, as were the
+Terebratulæ and Turritellæ in certain layers:—
+
+Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé, and whole
+coast of Patagonia).
+
+Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (St. Fé, S. Josef, Port
+Desire).
+
+Pecten centralis, G. B. Sowerby (also at Port Desire and S. Cruz).
+
+Pecten geminatus, G. B. Sowerby.
+
+Terebratula Patagonica, G. B. Sowerby (also S. Josef).
+
+Struthiolaria ornata, G. B. Sowerby (also S. Cruz).
+
+Fusus Patagonicus, G. B. Sowerby.
+
+Fusus Noachinus, G. B. Sowerby.
+
+Scalaria rugulosa, G. B. Sowerby.
+
+Turritella ambulacrum, G. B. Sowerby (also S. Cruz).
+
+Pyrula, cast of, like P. ventricosa of Sowerby, Tank Cat.
+
+Balanus varians, G. B. Sowerby.
+
+Scutella, differing from the species from Nuevo Gulf.
+
+
+No. 17
+Section of the strata exhibited in the cliffs of the ninety feet plain
+at Port S. Julian.
+
+
+[Illustration: Section of the strata exhibited in the cliffs of the
+ninety feet plain at Port S. Julian.]
+
+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 laminæ 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:—
+
+Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. Fé and whole
+coast of Patagonia).
+
+Pecten centralis, G. B. Sowerby (also P. Desire and S. Julian).
+
+Venus meridionalis of G. B. Sowerby.
+
+Crassatella Lyellii, G. B. Sowerby.
+
+Cardium puelchum, G. B. Sowerby.
+
+Cardita Patagonica, G. B. Sowerby.
+
+Mactra rugata, G. B. Sowerby.
+
+Mactra Darwinii, G. B. Sowerby.
+
+Cucullæa alta, G. B. Sowerby (also P. Desire).
+
+Trigonocelia insolita, G. B. Sowerby.
+
+Nucula (?) glabra, G. B. Sowerby.
+
+Crepidula gregaria, G. B. Sowerby.
+
+Voluta alta, G. B. Sowerby.
+
+Trochus collaris, G. B. Sowerby.
+
+Natica solida (?), G. B. Sowerby.
+
+Struthiolaria ornata, G. B. Sowerby (also P. Desire).
+
+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 _
+Cucullæa alta_ (found at Port Desire in the lower fossiliferous mass)
+and of the _Mactra rugata_, which latter shell has been partially
+converted into gypsum.
+
+No. 18
+Section of the plain at Patagonia, on the banks of the S. Cruz.
+
+
+[Illustration: Section of the plain at Patagonia, on the banks of the
+S. Cruz.]
+
+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,[3] 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 one hundred miles from the coast, that is
+at a central point between the Atlantic and the Cordillera, we have the
+section in figure No. 18.
+
+ [3] 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° 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?
+
+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[4] (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._ 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.
+
+ [4] 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.
+
+_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° 7′ 20″: 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° 10′ 53″; 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° 7′ 52″: 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° 1′ 22″; 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° 30′, is that of the great subaerial eruption in
+1783 from Skaptar Jukul in Iceland; and M. E. de Beaumont shows[5] that
+it must have flowed down a mean inclination of less than 0° 20′. 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°
+7′ 52″; 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° 7′ 20″. 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.
+
+ [5] “Mémoires pour servir,” etc., pp. 178 and 217.
+
+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 (lat. 51° 35′, 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,[6] 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 laminæ of the muddy sandstone;
+they belong, as I am informed by Dr. J. D. Hooker,[7] to three species
+of deciduous beech, different from the two species which compose the
+great proportion of trees in this forest-clad land. From these facts it
+is difficult to conjecture, whether we here see the basal part of the
+great Patagonian formation, or some later deposit.
+
+ [6] Described in the “Geological Transactions,” vol. vi, p. 415.
+
+
+ [7] “Botany of the Antarctic Voyage,” p. 212.
+
+_Summary on the Patagonian tertiary formation._—Four out of the seven
+fossil shells, from St. Fé 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, Cucullæa, 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. Fé. 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 (lat. 40° 30′ S.), I extracted the four
+following extinct species of shells:—
+
+Turritella suturalis, G. B. Sowerby (also Navidad).
+
+Sigaretus subglobosus, G. B. Sowerby (also Navidad).
+
+Cytheræa (?) sulculosa (?), G. B. Sowerby (also Chiloe and Huafo?).
+
+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°, 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:—
+
+Bulla cosmophila, G. B. Sowerby.
+
+Pleurotoma subæqualis, G. B. Sowerby.
+
+Fusus cleryanus, d’Orbigny, “Voyage Pal.” (also at Coquimbo).
+
+Triton leucostomoides, G. B. Sowerby.
+
+Turritella Chilensis, G. B. Sowerby (also Mocha).
+
+Venus, probably a distinct species, but very imperfect.
+
+Cytheræa (?) sulculosa (?), probably a distinct species, but very
+imperfect.
+
+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:—
+
+Tellinides (?) oblonga, G. B. Sowerby (a solenella in M. d’Orbigny’s
+opinion).
+
+Natica striolata, G. B. Sowerby.
+
+Natica (?) pumila, G. B. Sowerby.
+
+Cytheræa (?) 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.[8] 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.
+
+ [8] 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 laminæ 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 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 No. 19):—
+
+No. 19
+
+
+[Illustration: Section at Point Tenuy]
+
+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.
+
+No. 20
+Ground plan showing the relation between veins and concretionary zones
+in a mass of tuff.
+
+
+[Illustration: 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,[9] 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.
+
+ [9] The frequent tendency in iron to form hollow concretions or shell
+ containing incoherent matter is singular; D’Aubuisson (“Traité de
+ Géogn.” tome i, p. 318) remarks on this circumstance.
+
+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°, 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° 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
+(lat. 38° 20′), but Mr. Stokes brought me specimens of the grey,
+fine-grained, slightly calcareous sandstone, precisely like that of
+Huafo, containing lignite and numerous Turritellæ. 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:—
+
+Turritella Chilensis, G. B. Sowerby (also at Huafo).
+
+Fusus, very imperfect, somewhat resembling F. subreflexus of Navidad,
+but probably different.
+
+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°; near Concepcion they are also inclined: at the
+northern end of Quiriquina they have been tilted at an angle of 30°,
+and at the southern end at angles varying from 15° to 40°: 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:—
+
+Scalaria Chilensis, d’Orbigny, “Voyage, Part Pal.”
+
+Natica Araucana, d’Orbigny, “Voyage, Part Pal.”
+
+Natica australis, d’Orbigny, “Voyage, Part Pal.”
+
+Fusus difficilis, d’Orbigny, “Voyage, Part Pal.”
+
+Pyrula longirostra, d’Orbigny, “Voyage, Part Pal.”
+
+Pleurotoma Araucana, d’Orbigny, “Voyage, Part Pal.”
+
+Cardium auca, d’Orbigny, “Voyage, Part Pal.”
+
+Cardium acuticostatum, d’Orbigny, “Voyage, Part Pal.”
+
+Venus auca, d’Orbigny, “Voyage, Part Pal.”
+
+Mactra cecileana, d’Orbigny, “Voyage, Part Pal.”
+
+Mactra Araucana, d’Orbigny, “Voyage, Part Pal.”
+
+Arca Araucana, d’Orbigny, “Voyage, Part Pal.”
+
+Nucula Largillierti, d’Orbigny, “Voyage, Part Pal.”
+
+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:—
+
+
+Natica australis, d’Orbigny, “Voyage, Part Pal.”
+
+Mactra Araucana, d’Orbigny, “Voyage, Part Pal.”
+
+Trigonia Hanetiana, d’Orbigny, “Voyage, Part Pal.”
+
+Pecten, fragments of, probably two species, but too imperfect for
+description.
+
+Baculites vagina, E. Forbes.
+
+Nautilus d’Orbignyanus, E. Forbes.
+
+
+Besides these shells, Captain Belcher[10] 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 vertebræ of some very large animal. 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.
+
+ [10] “Zoology of Captain Beechey’s Voyage,” p. 163.
+
+
+_Navidad._[11]—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:—
+
+Gastridium cepa, G. B. Sowerby.
+
+Monoceros, fragments of, considered by M. d’Orbigny as a new species.
+
+Voluta alta, G. B. Sowerby (considered by M. d’Orbigny as distinct from
+the V. alta of Santa Cruz).
+
+Voluta triplicata, G. B. Sowerby.
+
+Oliva dimidiata, G. B. Sowerby.
+
+Pleurotoma discors, G. B. Sowerby.
+
+Pleurotoma turbinelloides, G. B. Sowerby.
+
+Fusus subreflexus, G. B. Sowerby.
+
+Fusus pyruliformis, G. B. Sowerby.
+
+Fusus, allied to F. regularis (considered by M. d’Orbigny as a distinct
+species).
+
+Turritella suturalis, G. B. Sowerby.
+
+Turritella Patagonica, G. B. Sowerby (fragments of).
+
+Trochus lævis, G. B. Sowerby.
+
+Trochus collaris, G. B. Sowerby (considered by M. d’Orbigny as the
+young of the T. lævis).
+
+Cassis monilifer, G. B. Sowerby.
+
+Pyrula distans, G. B. Sowerby.
+
+Triton verruculosus, G. B. Sowerby.
+
+Sigaretus subglobosus, G. B. Sowerby.
+
+Natica solida, G. B. Sowerby. (It is doubtful whether the Natica solida
+of S. Cruz is the same species with this.)
+
+Terebra undulifera, G. B. Sowerby.
+
+Terebra costellata, G. B. Sowerby.
+
+Bulla (fragments of).
+
+Dentalium giganteum, do.
+
+Dentalium sulcosum, do.
+
+Corbis (?) lævigata, do.
+
+Cardium multiradiatum, do.
+
+Venus meridionalis, do.
+
+Pectunculus dispar, (?) Desh. (considered by M. d’Orbigny as a distinct
+species).
+
+and 30. Cytheræa and Mactra, fragments of (considered by M. d’Orbigny
+as new species).
+
+Pecten, fragments of.
+
+ [11] 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.
+
+_Coquimbo._—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:—
+
+No. 21
+Section of the tertiary formation at Coquimbo.
+
+
+[Illustration: Section of the tertiary formation at Coquimbo.]
+
+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:—
+
+Bulla ambigua, d’Orbigny, “Voyage,” Pal.
+
+Monoceros Blainvillii, d’Orbigny, “Voyage,” Pal.
+
+Cardium auca, d’Orbigny, “Voyage,” Pal.
+
+Panopæa Coquimbensis, d’Orbigny, “Voyage,” Pal.
+
+Perna Gaudichaudi, d’Orbigny, “Voyage,” Pal.
+
+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.
+
+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.
+
+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 Balanidæ; it includes some pebbles, and layers of
+yellowish-brown mudstone. The embedded shells consist of:—
+
+Monoceros Blainvillii, d’Orbigny, “Voyage” Pal.
+
+Monoceros ambiguus, G. B. Sowerby.
+
+Anomia alternans, G. B. Sowerby.
+
+Pecten rudis, G. B. Sowerby.
+
+Perna Gaudichaudi, d’Orbigny, “Voyage” Pal.
+
+Ostrea Patagonica (?), d’Orbigny, “Voyage” Pal.
+
+Ostrea, small species, in imperfect state; it appeared to me like a
+small kind now living in, but very rare in the bay.
+
+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.
+
+Balanus Coquimbensis, G. B. Sowerby.
+
+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:—
+
+Fusus Cleryanus, d’Orbigny, “Voyage” Pal.
+
+Fusus petitianus, d’Orbigny, “Voyage” Pal.
+
+Venus hanetiana, d’Orbigny, “Voyage” Pal.
+
+Venus incerta (?) d’Orbigny, “Voyage” Pal.
+
+Venus Cleryana, d’Orbigny, “Voyage” Pal.
+
+Venus petitiana, d’Orbigny, “Voyage” Pal.
+
+Venus Chilensis, d’Orbigny, “Voyage” Pal.
+
+Solecurtus hanetianus, d’Orbigny, “Voyage” Pal.
+
+Mactra auca, d’Orbigny, “Voyage” Pal.
+
+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 Chañeral, 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:—
+
+Rostellaria Gaudichaudi, d’Orbigny, “Voyage” Pal.
+
+Pectunculus Paytensis, d’Orbigny, “Voyage” Pal.
+
+Venus petitiana, d’Orbigny, “Voyage” Pal.
+
+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 Cytheræa 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 lat. 27°,
+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.
+
+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 Palæontological 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. Panopæa,
+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 the table on the following page, 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?
+
+Genera, with living and tertiary species on the west coast of S.
+America.[12]	Latitudes, in which found fossil on the coasts of Chile
+and Peru.	Southernmost latitude, in which found living on the west
+coast of
+S. America. Bulla	30° to 43° 30′	12° near Lima.
+Cassis	34°	1° 37′ Pyrula	34° (and 36° 30′ at Concepcion)	5°
+Payta Fusus	30° to 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 lat. 43°
+south; from this latitude a species is found as far south as Tierra del
+Fuego.
+
+ [12] 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 lat. 20°, by Mr. Cuming;
+ Arca, also, was found by Captain King, at Juan Fernandez, in lat. 33°
+ 30′.
+
+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 lat. 34°, 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 the table given in the previous page, 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.[13]
+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.
+
+ [13] It may be worth while to mention that the shells living at the
+ present day on this eastern side of South America, in lat. 40°, 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.
+
+_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 S. 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 S. 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 S. 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.[14] 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.
+
+ [14] Professor H. D. Rogers, in his excellent address to the
+ Association of American Geologists (_Silliman’s Journal,_ vol. xlvii,
+ p. 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.)
+
+
+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 S. 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.[15]
+
+ [15] Professor Forbes has some admirable remarks on this subject, in
+ his “Report on the Shells of the Ægean Sea.” In a letter to Mr.
+ Maclaren (_Edinburgh New Phil. 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, Palæozoic 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: lat. 13° 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 laminæ
+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,[1] these latter also
+having been at the same time bent and softened.
+
+ [1] Professor Hitchcock (“Geology of Massachusetts,” vol. ii, p. 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° N.) and S.W. by S. line,
+corresponding nearly with the coast-line northwards of the bay. I may
+add that Mr. Gardner[2] 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° N.; and in Guyana according to
+Sir R. Schomburgk, the same rock strikes E. 57° N. Again, Humboldt
+describes the gneiss-granite over an immense area in Venezuela and even
+in Colombia, as striking E. 50° N., and dipping to the N.W. at an angle
+of fifty degrees. 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.[3]
+
+ [2] “Geological Section of the Brit. Assoc.,” 1840. For Sir R.
+ Schomburgk’s observations see _Geograph. Journal,_ 1842, p. 190. See
+ also Humboldt’s discussion on Loxodrism in the “Personal Narrative.”
+
+
+ [3] 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 vol.
+ (1841) p. 258 of the _ London and Edin. Phil. Magazine._
+    ABROLHOS ISLETS, _lat. 18° 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° 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 laminæ.
+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°)
+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.
+
+No. 22
+Fragment of gneiss embedded in another variety of the same rock.
+
+
+[Illustration: 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 here represented.
+
+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[4] 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. Near
+Rio, every mineral except the quartz has been completely softened, in
+some places to a depth little less than one hundred feet.[5] 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.
+
+ [4] 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’s “Voyage.”
+
+
+ [5] Dr. Benza describes granitic rock (_Madras Journal of Lit.,_ etc.,
+ Oct. 183? p. 246), in the Neelgherries, decomposed to a depth of forty
+ feet.
+
+
+_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.[6] 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
+laminæ 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.
+
+ [6] M. Isabelle, “Voyage à Buenos Ayres,” p. 479.
+
+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 laminæ 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[7]—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° 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.
+
+ [7] Near the Pan de Azucar there is some greenish porphyry, in one
+ place amygdaloidal with agate.
+
+
+_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 laminæ with laminæ 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 laminæ 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.[8]
+
+ [8] Mr. Greenough (p. 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” (p. 236), remarks on
+ alternating laminæ 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 laminæ 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°: 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[9] 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.
+
+ [9] M. d’Orbigny’s “Voyage,” Part. Géolog., p. 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 edit.), p. 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
+laminæ 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° N.
+line, dipping southerly at an angle of 45° and upwards. The principal
+line of mountains, with some quite subordinate parallel ridges, range
+about W. 45° N.: but at their S.E. termination, only W. 25° 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 (lat.
+43° 50′), 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° 56′. 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;[10]
+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.
+
+ [10] 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.
+
+_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 Palæozoic 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°, 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,[11] by the parallelism in the clay-slate districts of
+the shores and channels, ranging in a line between [W. 20° to 40° N.]
+and [E. 20° to 40° S.].
+
+ [11] _Geographical Journal,_ vol. i, p. 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:—
+
+Ancyloceras simplex, d’Orbigny, “Pal Franc,” Mount Tarn.
+
+Fusus (in imperfect state), d’Orbigny, “Pal Franc,” Mount Tarn.
+
+Natica, d’Orbigny, “Pal Franc,” Mount Tarn.
+
+Pentacrimus, d’Orbigny, “Pal Franc,” Mount Tarn.
+
+Lucina excentrica, G. B. Sowerby, Port Famine.
+
+Venus (in imperfect state), G. B. Sowerby, Port Famine.
+
+Turbinolia (?), G. B. Sowerby, Port Famine.
+
+Hamites elatior, G. B. Sowerby, Port Famine.
+
+
+M. d’Orbigny states[12] that MM. Hombron and Grange found in this
+neighbourhood an Ancyloceras, perhaps _A. simplex_, an Ammonite, a
+Plicatula and Modiola. 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 Palæozoic
+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.
+
+ [12] “Voyage,” Part Géolog., p. 242.
+
+
+The clay-slate in many parts of Tierra del Fuego, is broken by
+dikes[13] and by great masses of greenstone, often highly hornblendic:
+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 laminæ 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.
+
+ [13] 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.
+
+
+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 scoriæ, 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[14] to consist of
+porphyritic lava. 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.[15] 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.
+
+ [14] Determined by Professor Jameson. Weddell’s “Voyage,” p. 169.
+
+
+ [15] See Mr. Brooke’s Paper in the _London Phil. Mag.,_ vol. x. This
+ mineral occurs in an ancient volcanic rock near Rome.
+
+
+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-laminæ 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,_[16] seem, together with granite, chiefly to prevail along the
+western coast as far north as the entrance of the St. of Magellan: 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.
+
+ [16] See the Paper by Captain King in the _ Geograph. Journal_; also a
+ Letter to Dr. Fitton in “Geolog. Proc.,” vol. i, p. 29; also some
+ observations by Captain Fitzroy, “Voyages,” vol. i, p. 375. I am
+ indebted also to Mr. Lyell for a series of specimens collected by
+ Lieutenant Graves.
+
+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 laminæ ranging with extreme regularity, W.N.W.
+and E.S.E., seldom varying more than one point of the compass from this
+direction.[17] 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° and 90°, 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 laminæ 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-laminæ, though to the eye appearing straight, being
+parts of large abrupt curves, with their summits cut off and worn down.
+
+ [17] 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-laminæ was
+ in two cases 45° and in two others 79°.
+
+In several places I was particularly struck with the fact, that the
+fine laminæ 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 laminæ 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[18] 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.” 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.
+
+ [18] _Geograph. Journal,_ vol. i, p. 170.
+
+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 lat. 47° 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[19] 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. 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.
+
+ [19] 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.
+
+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°
+W. and S. 19° 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° W. and
+the dip, generally at an angle of about 40° to W. 25° 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° to 22° W.] and [S. 11° to
+22° 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° W. to even as
+much as N. 45° W.: in an analogous manner at Vallenar Bay, where the
+dip was westerly (viz. on an average directed to W. 25° S.), as soon as
+the angle became very high, the planes struck in a line more than 25°
+west of north. The average result from all the observations on this 200
+miles of coast, is a strike of N. 19° W. and S. 19° 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° E., whereas, as we have just seen, the
+average strike of the foliation is N. 19° W.
+
+There is one interesting exception to the uniformity in the strike of
+the foliation. At the northern point of Tres Montes (lat. 45° 52′) a
+bold chain of granite, between two and three thousand feet in height,
+runs from the coast far into the interior,[20] in an E.S.E. line, or
+more strictly E. 28° S. and W. 28° N. 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°
+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° to 20°, but sometimes mounting up to 30°. The southern flank
+consists of bare granite. The mica-slate is penetrated by small
+veins[21] of granite, branching from the main body. 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° 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 laminæ 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 laminæ, dipping at an angle of 40
+degrees to W. 19° S., and let an axis of elevation traverse it in an E.
+28° S. line, what will the position of the laminæ be on the northern
+flank after a tilt, we will first suppose, of 45°? Mr. Hopkins informs
+me, that the angle of the dip will be 28° 31′, and its direction to
+north 30° 33′ west.[22] By varying the supposed angle of the tilt, our
+previously inclined folia can be thrown into any angle between 26°,
+which is the least possible angle, and 90°; 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.
+
+ [20] 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.
+
+
+ [21] 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.
+
+
+ [22] 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° 55′,
+ directed to west 35° 33′ 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°, would here be only
+ 86° 50′ apart.
+
+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°: 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° (in one spot, however, at 60°), 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° and 22° 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:[23] but here, as at Valdivia, a N.W. and S.E. strike seemed to
+be the most frequent one. 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.
+
+ [23] I observed in some parts that the tops of the laminæ of the
+ clay-slate (_b_ of the diagram) under the superficial detritus and
+ soil (_a_) were bent, sometimes without being broken, as represented
+ in the accompanying diagram, which is copied from one given by Sir H.
+ De la Beche (p. 42 “Geological Manual”) of an exactly similar
+ phenomenon in Devonshire. Mr. R. A. C. Austen, also, in his excellent
+ paper on S.E. Devon (“Geolog. Transact.,” vol. vi, p. 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 Phil. Mag.,_ vol. xxi, p. 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.
+
+
+[Illustration: Diagram described in note 23.]
+
+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.[24]
+
+ [24] “Geolog. Trans.,” vol. vi, pp. 602 and 617. “Journal of
+ Researches” (2nd edit.), p. 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.[25]
+
+ [25] Humboldt (“Personal Narrative,” vol. iv, p. 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°: 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-laminæ range over wide areas with remarkable uniformity,
+cutting straight through the planes of stratification,[26] but yet
+being parallel in strike to the main axes of elevation, and generally
+to the outlines of the coast. 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.
+
+ [26] In my paper on the Falkland Islands (_Geological Journal,_ vol.
+ iii, p. 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 fact of the cleavage-laminæ 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,[27] where glaciers had passed over
+the truncated edges of the highly inclined laminæ of clay-slate, that
+the surface, though smooth, was worn into small parallel undulations,
+caused by the competent laminæ being of slightly different degrees of
+hardness. 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.”[28] 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.
+
+ [27] _London Phil. Mag._, vol. xxi, p. 182.
+
+
+ [28] “Geological Trans.,” vol. iii, p. 471.
+
+
+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-laminæ, the folia preserve over very large areas a uniform
+strike: thus Humboldt[29] 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° to N.W.; 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° W. and S. 19° 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-laminæ 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,[30] the
+coincidence between its strike and that of the main stratification
+seems sometimes to fail. Foliation and cleavage resemble each other in
+the planes winding round concretions, and in becoming tortuous where
+veins of quartz abound.[31] 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,[32] in
+which the folia in the central crests are vertical and on the two
+flanks inclined inwards. 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.[33]
+
+ [29] “Personal Narrative,” vol. vi, p. 59 _et seq._
+
+
+ [30] Cases are given by Mr. Jukes in his “Geology of Newfoundland,” p.
+ 130.
+
+
+ [31] I have seen in Brazil and Chile concretions thus enfolded by
+ foliated gneiss; and Macculloch (“Highlands,” vol. i, p. 64) has
+ described a similar case. For analogous cases in clay-slate, see
+ Professor Henslow’s Memoir in “Cambridge Phil. Trans.,” vol. i, p.
+ 379, and Macculloch’s “Class. of Rocks,” p. 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,” p. 78.
+
+
+ [32] Studer in _Edin. New Phil. Journal,_ vol. xxiii, p. 144.
+
+
+ [33] 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,[34] we clearly see
+that folia of distinct minerals can originate through the metamorphosis
+of a homogeneous fissile rock. 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.[35] 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-laminæ,
+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-laminæ of closely allied rocks, which none would imagine had
+ever been horizontal, dip at nearly the same angle, to nearly the same
+point?
+
+ [34] I have described in “Volcanic Islands” a good instance of such a
+ passage at the Cape of Good Hope.
+
+
+ [35] See some excellent remarks on this subject, in D’Aubuisson’s
+ “Traité de Géog.,” tome i, p. 297. Also some remarks by Mr. Dana in
+ _Silliman’s American Journ.,_ vol. xlv, p. 108.
+
+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.[36] 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.
+
+ [36] Macculloch states (“Classification of Rocks,” p. 364) states that
+ primary limestones are often found in irregular masses or great
+ nodules, “which can scarcely be said to possess a stratified shape!”
+
+
+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-laminæ: 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 laminæ 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.[37]
+
+ [37] 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,[38] 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. 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-laminæ 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.
+
+ [38] In “Volcanic Islands.”
+
+
+[Illustration: Geological sections through the Cordilleras.]
+
+For enlargements of the above plate use the following links:
+left section
+center section
+right section
+
+
+
+
+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,[1] 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.
+
+ [1] Meyen “Reise um Erde” Th. I, s. 235.
+
+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-laminæ 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;[2] 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.
+
+ [2] 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.)
+
+
+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.[3] 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.
+
+ [3] 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 (_Edin. New Phil. Journ.,_ vol. xli, p.
+ 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. I, s. 316) from Chile, as consisting of crystallised
+    quartz, with crystals of stilbite within, and lined externally by
+    green earth.
+
+
+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;[4] 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.
+
+ [4] 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 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[5] 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. 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 scoriæ 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.
+
+ [5] 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.
+
+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[6] (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. 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[7] states that
+it occurs in Eastern Kamtschatka. 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.
+
+ [6] I here, and elsewhere, call by this name, those feldspathic
+ minerals which cleave like albite: but it now appears (_Edin. New
+ Phil. Journal.,_ vol. xxiv, p. 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.
+
+
+ [7] _Geograph. Journal,_ vol. ix, p. 510.
+
+
+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,[8] 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. 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.
+
+ [8] “Reise um Erde,” Th. I, ss. 338 and 362.
+
+_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.
+Plate V sections (between  and 441) 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,[9] is 13,210
+feet; 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.
+
+ [9] _Journal of Nat. and Geograph. Science,_ August 1830.
+
+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°, and less than a mile south of it at 67°:
+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 our section [Plate V] 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°. 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[10] met with
+a hill
+of pumice containing mica. 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.
+
+ [10] “Reise um Erde,” Th. I, ss. 338, 341.)
+
+Beyond the junction of the Yeso and Volcan, the porphyritic strata
+appear to dip towards the hillocks of andesite at an angle of 40°; 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° 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° 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°, 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°,
+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° 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°, but at the eastern or farther end of the series it increases to
+60°. 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:—
+
+Ammonite, indeterminable, near to _A. recticostatus,_ d’Orbigny, “Pal.
+Franc.” (Neocomian formation).
+
+Gryphæa, near to _G. Couloni_ (Neocomian formations of France and
+Neufchâtel).
+
+Natica, indeterminable.
+
+Cyprina rostrata, d’Orbigny, “Pal. Franc.” (Neocomian formation).
+
+Rostellaria angulosa (?), d’Orbigny, “Pal. de l’Amér. Mer.”
+
+Terebratula (?).
+
+
+Some of the fragments of Ammonites were as thick as a man’s arm: the
+Gryphæa is much the most abundant shell. These fossils M. d’Orbigny
+considers as belonging to the Neocomian stage of the Cretaceous system.
+Dr. Meyen,[11] 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: 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,[12] of:—
+
+Exogyra (Gryphæa) Couloni, absolutely identical with specimens from the
+Jura and South of France.
+
+Trigonia costata, identical with those found in the upper Jurassic beds
+at Hildesheim.
+
+Pecten striatus, identical with those found in the upper Jurassic beds
+at Hildesheim.
+
+Cucullæa, corresponding in form to _C. longirostris,_ so frequent in
+the upper Jurassic beds of Westphalia.
+
+Ammonites resembling _A. biplex._
+
+
+ [11] “Reise um Erde,” etc., Th. I, s. 355.
+
+
+ [12] “Descript. Phys. des Iles Canaries,” p. 471.
+
+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
+hich 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°, which further on increases
+to about 45°. 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,[13] however, collected
+here specimens of ribboned jasper, magnesian limestone, and other
+minerals. 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.
+
+ [13] “Travels,” etc., vol. i, p. 308.
+
+
+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 laminæ. The streams are separated from each other by beds
+of fragmentary brown scoriæ, 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.[14] 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.
+
+ [14] 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.
+
+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° 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 V, Section 1/2, [see map page
+440] 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°. 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° 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°, and its eastern side by a
+mountain of similar strata [G] inclined westward at 70°, and
+superimposed by an oblique fault on another mass of the same strata
+[H], also inclined westward, but at an angle of about 30°: 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°,
+occasionally mounting up to 45°, 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[15] (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° to the
+west. 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.
+
+ [15] At this place, there are some hot and cold springs, the warmest
+ having a temperature, according to Lieutenant Brand (“Travels,” p.
+ 240), of 91°; 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.
+
+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 recemented. 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:—
+    Gryphæa, 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[16] 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.
+
+ [16] “Description Phys. des Iles Can.,” p. 472.
+
+Two of these shells, namely, the Gryphæa 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 palæontologists 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[17] 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°, and in
+some spots even at a higher angle. 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.
+
+ [17] I find that Mr. Smith of Jordan Hill has described (_Edinburgh
+ New Phil. Journ.,_ vol. xxv, p. 392) beds of sand and gravel, near
+ Edinburgh, tilted at an angle of 60°, and dislocated by miniature
+ faults.
+
+
+_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° to the
+N.E.,[18] 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.
+
+ [18] 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.
+
+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° to the west, fronted
+by strata [O] inclined at 45° 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° to the west; the inclination in some places
+being only 19°, in some few others as much as 45°. 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° 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°, 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 laminæ of this latter
+formation, on the extreme eastern flank, are generally nearly vertical;
+further inwards they become inclined from 45° to 80° 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 laminæ 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.[19]
+
+ [19] 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° 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°:
+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,[20] 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 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.
+
+ [20] 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.
+
+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.[21] 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.
+
+ [21] Very easily fusible into a jet-black bead, attracted by the
+ magnet: the crystals are too much tarnished to be measured by the
+ goniometer.
+
+
+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,[22]
+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. 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° 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.
+
+ [22] 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.
+
+
+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,[23] which I
+estimated at 1,000 feet in thickness, and which probably has been
+formed by more than one stream. 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.[24]
+
+ [23] 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.
+
+
+ [24] 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[25] when cooling, and by its having been thus more easily
+sucked into fissures than the other constituent minerals of granite?
+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.
+
+ [25] See a paper by M. Elie de Beaumont, “Soc. Philomath.,” May 1839
+ (“L’Institut.,” 1839, p. 161.)
+
+
+_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.[26] 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.
+
+ [26] See on this subject, “Volcanic Islands,” etc., by the Author.
+
+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° 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,[1] generally
+embedded in sulphate of barytes. 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.
+
+ [1] See the Report on M. Domeyko’s account of those mines, in the
+ “Comptes Rendus,” tome xiv, p. 560.
+
+_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 Gryphæa, 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° 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°, 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 ænigma, d’Orbigny, “Voyage” etc. (Pl. XXII, Figs. 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. tetrædra, and that the varieties amongst them are
+such as are found in that variable species. Generally speaking, the
+American specimens of T. ænigma may be distinguished from the British
+T. tetrædra, 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 ænigma, 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. ænigma. 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 Terebratulæ. 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 Gryphæa appears to Professor
+Forbes to be identical with a species, associated in Southern India
+with unquestionably cretaceous forms. On the other hand, the two
+Terebratulæ 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. ænigma 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 Terebratulæ 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, Gryphæa, Ostrea, Pecten, Turritella,
+Nautilus, two Terebratulæ, 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;[2]—both of which genera occur at the Puente del Inca.
+
+ [2] D’Orbigny, “Voyage,” Part Géolog., p. 242.
+
+_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 ænigma, d’Orbigny, “Voyage,” Part Pal.
+This shell M. d’Orbigny does not consider identical with his T. ænigma,
+but near to T. obsoleta. Professor Forbes thinks that it is certainly a
+variety of T. ænigma: 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 Terebratulæ, 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,
+Fig. 3 in Plate V, 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,[3] of sulphate of iron under two forms, and of the sulphates of
+copper and alumina: 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.
+
+ [3] Meyen’s “Reise,” etc., Th. I, s. 394.
+
+_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°. 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 Las 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°. 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.
+Gryphæa Darwinii, E. Forbes.
+An intermediate form between G. gigantea and G. incurva.
+
+Gryphæa 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 ænigma, 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. ænigma, and related to
+T. obsoleta.
+
+Plagiostoma and Ammonites, fragments of.
+
+The lower layers of the limestone contained thousands of the Gryphæa;
+and the upper ones as many of the Turritella, with the Gryphæa (nov.
+species) and Serpulæ 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 ænigma 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. ænigma, 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 ænigma.
+    Spirifer linguiferoides.
+
+Same bed, near Arqueros:
+    Hippurites Chilensis.
+    Gryphæa orientalis.
+
+Collected by M. Domeyko from the same locality, apparently near
+Arqueros:
+    Terebratula ænigma 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 ænigma, _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 ænigma, _var._, as at Guasco.
+    Astarte Darwinii.
+    Gryphæa Darwinii.
+    Gryphæa nov. species?
+    Perna Americana.
+    Avicula, nov. species.
+
+Main valley of Copiapo, apparently same formation with that of
+Amolanas:
+    Terebratula ænigma (true).
+
+In the same bed, high up the great lateral valley of the Despoblado, in
+the ravine of Maricongo:
+    Terebratula ænigma (true).
+    Pecten Dufreynoyi.
+    Gryphæa 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, Gryphæa
+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 Gryphæa 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 ænigma, 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 ænigma, 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;[4] 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.
+
+ [4] I have endeavoured to show in my “Journal,” etc. (2nd edit.), p.
+ 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.
+
+
+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;[5] 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.
+
+ [5] 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.
+
+
+No. 40
+
+
+[Illustration]
+
+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 no. 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° 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°: so that at this point the beds
+have been turned through an angle of 135°; and here there is a kind of
+anticlinal axis, with the strata on both sides dipping to opposite
+points at an angle of 45°, 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.[6] 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,[7]consists of laminated, impure, argillaceous,
+purplish-grey limestone, associated, I believe, with some purple
+sandstone. In the limestone shells are found: the three following
+species were given me:—
+
+    Lucina Americana, E. Forbes.
+    Terebratula inca, E. Forbes.
+    Terebratula ænigma, D’Orbigny.
+
+ [6] The lowest point, where the road crosses the coast-escarpment, is
+ 1,900 feet by the barometer above the level of the sea.
+
+
+ [7] Mr. Bollaert has described (“Geolog. Proceedings,” vol. ii, p.
+ 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.
+
+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.[8] 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.
+
+ [8] 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’s
+    “Anniversary Address to Geolog. Soc., 1843,” p. 65.)
+
+_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.];[9] 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 laminæ 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.[10] 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[11] 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. 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.
+
+ [9] 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.].
+
+
+ [10] The same fact has been observed by Mr. Taylor in Cuba: _London
+ Phil. Journ.,_ vol. xi, p. 21.
+
+
+ [11] “Travels in Chile,” p. 29.
+
+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,[12] 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.
+
+ [12] 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.
+
+
+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.[13] 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.
+
+ [13] Sir R. Murchison and his fellow travellers have given some
+ striking facts on this subject in their account of the Ural Mountains
+ (“Geolog. Proc.,” vol. iii, p. 748.
+
+_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
+palæontologists 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,[14] 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 ænigma,_
+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.
+
+ [14] Von Buch’s “Descript. Physique des Iles Canaries,” p. 428.
+
+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.[15] 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.
+
+ [15] Sir R. Murchison and his companions state (“Geolog. Proc.,” vol.
+ iii, p. 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.
+
+
+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[16] as in volcanic archipelagoes lavas are contemporaneously
+ejected on the parallel lines of fissure. 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.[17] 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[18] 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.
+
+ [16] “Volcanic Islands,” etc.)
+
+
+ [17] I have endeavoured to show in my “Journal” (2nd edit., p. 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 Phil.
+ Journal,_ vol. xxviii, pp. 33 and 44.
+
+
+ [18] “Reise in Peru,” Band 2, s. 8: Author’s “Journal,” 2nd edit., p.
+ 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,[19] 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.
+
+ [19] “The Structure, etc., 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,[20] appears to me highly
+important, for no explanation of the one phenomenon can be considered
+as satisfactory which is not applicable to the other. 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.
+
+ [20] On the Connection of certain Volcanic Phenomena in South America:
+ “Geolog. Transact.,” vol. v, p. 609.
+
+
+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,[21] 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. 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.
+
+ [21] “Geolog. Transact.,” vol. v, p. 626.
+
+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-Gürtelthiere in den Dünenhügeln bei Bahia
+Blanca, beides in Patagonien, unveränderte brakische
+Süsswasserbildungen sind, die einst wohl sämmtlich zum obersten
+Fluthgebiethe des Meeres im tieferen Festlande
+gehörten.”—_Monatsberichten der königl. Akad., etc.,_ zu Berlin vom
+April 1845.
+
+
+
+
+INDEX TO CORAL-REEFS.
+
+
+The names in italics are all names of places, and refer exclusively to
+the Appendix: in well-defined archipelagoes, or groups of islands, the
+name of each separate island is not given.
+
+Abrolhos, Brazil, coated by corals 50
+_Abrolhos (Australia)_ 130
+Absence of coral-reefs from certain coasts 51
+_Acaba, gulf of_ 147
+_Admiralty group_ 124
+Africa, east coast, fringing-reef of 48
+—— Madreporitic rock of 101
+_Africa, east coast_ 141
+Age of individual corals 57, 64
+_Aiou_ 128
+_Aitutaki_ 114
+_Aldabra_ 139
+_Alert reef_ 123
+_Alexander, Grand Duke, island_ 115
+Allan, Dr., on Holuthuriæ feeding on corals 21
+—— on quick growth of corals at Madagascar 62
+—— on reefs affected by currents 9
+_Alloufatou_ 119
+_Alphonse_ 139
+_Amargoura (Amargura)_ 119
+_Amboina_ 128
+_America, west coast_ 111
+_Amirantes_ 138
+_Anachorites_ 125
+_Anambas_ 133
+Anamouka, description of 99
+_Anamouka_ 119
+_Anadaman islands_ 132
+_Antilles_ 153
+_Appoo reef_ 134
+_Arabia Felix_ 143
+Areas, great extent of, interspersed with low islands
+—— of subsidence and of elevation 106
+—— of subsidence appear to be elongated 106
+—— of subsidence alternating with areas of elevation 108
+_Arru group_ 128
+_Arzobispo_ 127
+Ascidia, depth at which found 67
+_Assomption_ 139
+_Astova_ 139
+_Atlantic islands_ 121
+Atolls, breaches in their reefs 31, 81
+—— dimensions of 25
+—— dimensions of groups of 71
+—— not based on craters or on banks of sediment, or of ck 69, 71, 72,
+73, 108
+—— of irregular forms 25, 84
+—— steepness of their flanks 26
+—— width of their reef and islets 25
+—— their lowness 70
+—— lagoons 29
+—— general range 94
+—— with part of their reef submerged, and theory of 29, 81
+_Augustine, St._ 120
+Aurora island, an upraised atoll 64, 71, 104
+_Aurora_ 112
+Austral islands, recently elevated 99
+_Austral islands_ 114
+_Australia, N.W. coast_ 130
+Australian barrier-reef 42, 93
+_Australian barrier_ 123
+
+_Babuyan group_ 134
+_Bahama banks_ 149, 150
+_Balahac_ 133
+_Bally_ 131
+_Baring_ 121
+Barrier-reef of Australia 42, 93
+—— of New Caledonia 44
+Barrier-reefs, breaches through 77
+—— not based on worn down margin of rock 43
+—— on banks of sediment 43
+—— on submarine craters 44
+—— steepness of their flanks 39
+—— their probable vertical thickness 43, 76
+—— theory of their formation 76, 78
+_Bampton shoal_ 123
+_Banks islands_ 122
+_Banks in the West Indies_ 147
+_Bashee islands_ 134
+_Bass island_ 115
+_Batoa_ 119
+_Beaupré reef_ 123
+Beechey, Captain, obligations of the author to 26
+—— on submerged reefs 27
+—— account of Matilda island 60
+Belcher, Captain, on boring through coral-reef 59
+_Belize reef, off_ 151
+_Bellinghausen_ 113
+_Bermuda islands_ 153
+_Beveridge reef_ 118
+_Bligh_ 122
+Bolabola, view of 12
+_Bombay shoal_ 136
+_Bonin Bay_ 131
+_Bonin group_ 127
+Borings through coral-reefs 59
+Borneo, W. coast, recently elevated 101
+_Borneo, E. coast_ 131
+—— _S.W. and W. coast_ 133
+—— _N. coast_ 133
+—— _western bank_ 136
+_Boscawen_ 119
+_Boston_ 121
+_Bouka_ 124
+_Bourbon_ 138
+_Bourou_ 128
+_Bouton_ 132
+Brazil, fringing-reefs on coast of 48
+Breaches through barrier-reefs 71
+_Brook_ 115
+_Bunker_ 115
+_Bunoa_ 133
+Byron 121
+
+_Cagayanes_ 133
+_Candelaria_ 124
+_Cargados Carajos_ 138
+_Caroline archipelago_ 125
+_Caroline island_ 115
+_Carteret shoal_ 128
+Caryophyllia, depth at which it lives 66
+_Cavilli_ 133
+_Cayman island_ 152
+_Celebes_ 129
+_Ceram_ 128
+Ceylon, recently elevated 101
+_Ceylon_ 137
+Chagos Great Bank, description and theory of 37, 85
+Chagos group 86
+_Chagos group_ 137
+Chama-shells embedded in coral-rock 68
+Chamisso, on corals preferring the surf 52
+Changes in the state of Keeling atoll 21
+—— of atolls 74
+Channels leading into the lagoons of atolls 30, 82
+—— —— into the Maldiva atolls 33, 35
+—— through barrier-reefs 77
+_Chase_ 120
+_China sea_ 135
+Christmas atoll 60, 97
+_Christmas atoll_ 116
+_Christmas island_ (Indian Ocean) 137
+_Clarence_ 116
+_Clipperton rock_ 111
+Cocos, or Keeling atoll 15
+_Cocos (or Keeling)_ 137
+_Cocos island_ (Pacific) 111
+Cochin China, encroachments of the sea on the coast 95
+_Cochin China_ 183
+_Coetivi_ 139
+_Comoro group_ 139
+Composition of coral-formations 88
+Conglomerate coral-rock on Keeling atoll 20
+—— on other atolls 28
+—— coral-rock 88
+Cook islands, recently elevated 98, 103
+_Cook islands_ 114
+Coral-blocks bored by vermiform animals 21, 88
+Coral-reefs, their distribution and absence from certain areas 50
+—— destroyed by loose sediment 53
+Coral-rock at Keeling atoll 20
+—— Mauritius 47
+—— organic remains of 88
+Corals dead but upright in Keeling lagoon 22
+—— depths at which they live 64
+—— off Keeling atoll 17
+—— killed by a short exposure 16
+—— living in the lagoon of Keeling atoll 20
+—— quick growth of, in Keeling lagoon 21
+—— merely coating the bottom of the sea 50
+—— standing exposed in the Low archipelago 96
+Corallian sea 94
+_Corallian sea_ 123
+_Cornwallis_ 121
+_Cosmoledo_ 139
+Couthouy, Mr., alleged proofs of recent elevation of the Low
+archipelago 96
+—— on coral-rock at Mangaia and Aurora islands 64
+—— on external ledges round coral-islands 80
+—— remarks confirmatory of the author’s theory 96
+Crescent-formed reefs 84
+_Cuba_ 150
+Cuming, Mr., on the recent elevation of the Philippines 101
+
+_Dangerous, or Low archipelago_ 111
+_Danger islands_ 116
+Depths at which reef-building corals live 63
+—— at Mauritius, the Red Sea, and in the Maldiva archipelago 66
+—— at which other corals and corallines can live 67
+_Dhalac group_ 144
+Diego Garcia, slow growth of reef 56
+Dimensions of the larger groups of atolls 71
+Disseverment of the Maldiva atolls, and theory of 37, 82
+Distribution of coral-reefs 50
+_Domingo, St._ 152
+Dory, Port, recently elevated 100
+_Dory, Port_ 127
+_Duff islands_ 122
+_Durour_ 125
+
+_Eap_ 126
+arthquakes at Keeling atoll 23
+—— in groups of atolls 75
+—— in Navigator archipelago 100
+ast Indian archipelago, recently elevated 100
+_Easter_ 111
+_Echequier_ 125
+hrenberg, on the banks of the Red Sea 49, 143
+—— on depths at which corals live in the Red Sea 66
+—— on corals preferring the surf 53
+—— on the antiquity of certain corals 57
+_Eimeo_ 112
+levated reef of Mauritius 47
+levations, recent proofs of 98
+—— immense areas of 106
+_Elivi_ 126
+lizabeth island 59
+—— recently elevated 98, 104
+_Elizabeth island_ 112
+_Ellice group_ 120
+ncircled islands, their height 41
+—— geological composition 42, 44
+ua, description of 99
+_Eoua_ 118
+upted matter probably not associated with thick masses of coral-rock 89
+
+Fais, recently elevated 100, 104
+_Fais_ 126
+_Fanning_ 116
+_Farallon de Medinilla_ 127
+_Farson group_ 144
+_Fataka_ 122
+Fiji archipelago 119
+Fish, feeding on corals 21
+—— killed in Keeling lagoon by heavy rain 24
+Fissures across coral-islands 75
+Fitzroy, Captain, on a submerged shed at Keeling atoll 23
+—— on an inundation in the Low archipelago 74
+_Flint_ 115
+_Flores_ 130
+_Florida_ 149
+_Folger_ 127
+_Formosa_ 135
+Forster, theory of coral-formations 73
+_Frederick reef_ 123
+_Freewill_ 128
+Friendly group recently elevated 99, 105
+_Friendly archipelago_ 118
+Fringing-reefs, absent where coast precipitous 5
+—— breached in front of streams 54
+—— described by MM. Quoy and Gaimard 98
+—— not closely attached to shelving coasts 46
+—— of east coast of Africa
+—— of Cuba 48
+—— of Mauritius 45
+—— on worn down banks of rock 9
+—— on banks of sediment 49
+—— their appearance when elevated 7
+—— their growth influenced by currents 49
+—— by shallowness of sea 49
+
+_Galapagos archipelago_ 111
+_Galega_ 139
+Gambier islands, section of 43
+_Gambier islands_ 112
+_Gardner_ 116
+_Gaspar rico_ 121
+Geological composition of coral-formations
+
+_Gilbert archipelago_ 120
+_Gilolo_ 129
+_Glorioso_ 139
+Gloucester island 74
+_Glover reef_ 152
+_Gomez_ 111
+_Gouap_ 126
+_Goulou_ 126
+_Grampus_ 127
+_Gran Cocal_ 120
+Great Chagos Bank, description and theory of 37, 85
+Grey, Captain, on sandbars 46
+Grouping of the different classes of reefs 93
+_Guedes_ 128
+
+Hall, Captain B., on Loo Choo 101
+Harvey islands, recently elevated 104
+Height of encircled islands 41
+_Hermites_ 125
+_Hervey or Cook islands_ 114
+_Hogoleu_ 125
+Holothuriæ (Holuthuriæ) feeding on coral 21
+Houden island, height of 71
+_Honduras, reef off_ 151
+_Horn_ 119
+_Houtman Abrolhos_ 130
+Huaheine; alleged proofs of its recent elevation 103
+_Huaheine_ 113
+_Humphrey_ 115
+_Hunter_ 119
+Hurricanes, effects of, on coral-islands 74
+
+_Immaum_ 143
+_Independence_ 120
+India, west coast, recently elevated 101
+_India_ 143
+Irregular reefs in shallow seas 49
+Islets of coral-rock, their formation 19
+—— their destruction in the Maldiva atolls 36
+
+_Jamaica_ 152
+_Jarvis_ 115
+Java, recently elevated 100
+_Java_ 131
+_Johnston island_ 116
+_Juan de Nova_ 139
+_Juan de Nova (Madagascar)_ 140
+
+_Kalatoa_ 131
+Kamtschatka, proofs of its recent elevation 105
+_Karkalang_ 129
+Keeling atoll, section of reef 15
+_Keeling, south atoll_ 137
+—— _north atoll_ 137
+_Keffing_ 128
+_Kemin_ 115, 116
+_Kennedy_ 123
+_Keppel_ 119
+_Kumi_ 135
+
+_Laccadive group_ 137
+Ladrones, or Marianas, recently elevated 100
+_Ladrones archipelago_ 127
+Lagoon of Keeling atoll 20
+Lagoons bordered by inclined ledges and walls, and theory of their
+formation 32, 79
+—— of small atolls filled up with sediment 32
+Lagoon-channels within barrier-reefs 40
+Lagoon-reefs, all submerged in some atolls, and rising to the surface
+in others 55
+_Lancaster reef_ 115
+_Latte_ 119
+_Lauglan islands_ 123
+Ledges round certain lagoons 32, 79
+_Lette_ 129
+_Lighthouse reef_ 152
+Lloyd, Mr., on corals refixing themselves 62
+Loo Choo, recently elevated 101
+_Loo Choo_ 135
+_Louisiade_ 123
+Low archipelago, alleged proofs of its recent elevation 96
+_Low archipelago_ 111
+Lowness of coral-islands 70
+_Loyalty group_ 123
+_Lucepara_ 133
+Lutké, Admiral, on fissures across coral-islands 75
+Luzon, recently elevated 101
+_Luzon_ 134
+Lyell, Mr., on channels into the lagoons of atolls 31
+—— on the lowness of their leeward sides 82
+—— on the antiquity of certain corals 58
+—— on the apparent continuity of distinct coral-islands 89
+—— on the recently elevated beds of the Red Sea 102
+—— on the outline of the areas of subsidence 106
+
+_Macassar strait_ 131
+_Macclesfield bank_ 136
+Madagascar, quick growth of corals at 62
+—— madreporitic rock of 101
+_Madagascar_ 140
+_Madjiko-sima_ 135
+_Madura (Java)_ 131
+_Madura (India)_ 137
+Mahlos Mahdoo, theory of formation 88
+Malacca, recently elevated 100
+_Malacca_ 133
+Malcolmson, Dr., on recent elevation of W. coast of India 100
+—— on recent elevation of Camaran island 102
+_Malden_ 115
+Maldiva atolls, and theory of their formation 33, 80, 82
+—— steepness of their flanks 26
+—— growth of coral at 62
+_Maldiva archipelago_ 137
+Mangaia island 64
+—— recently elevated 99, 104
+_Mangaia_ 114
+_Mangs_ 127
+Marianas, recently elevated 100
+_Mariana archipelago_ 127
+_Mariere_ 126
+_Marquesas archipelago_ 113
+_Marshall archipelago_ 121
+_Marshall island_ 127
+_Martinique_ 153
+_Martires_ 126
+Mary’s St. in Madagascar, harbour made in reefs 54
+_Mary island_ 116
+_Matia, or Aurora_ 112
+Matilda atoll 60
+Mauritius, fringing-reefs of 45
+—— depths at which corals live there 64
+—— recently elevated 101
+_Mauritius_ 138
+Maurua, section of 43
+_Maurua_ 113
+Menchikoff atoll 25,
+
+_Mendana archipelago_ 113
+_Mendana isles_ 122
+_Mexico, gulf of_ 149
+Millepora complanata at Keeling atoll 16
+_Mindoro_ 134
+_Mohilla (Mohila)_ 139
+Molucca islands, recently elevated 100
+_Mopeha_ 113
+Moresby, Captain, on boring through coral-reefs 59
+_Morty_ 129
+_Mosquito coast_ 152
+Musquillo atoll 84
+_Mysol_ 129
+
+Namourrek group 84
+_Natunas_ 133
+Navigator archipelago, elevation of 99
+_Navigator archipelago_ 117
+_Nederlandisch_ 120
+Nelson, Lieutenant, on the consolidation of coral-rocks under water 59
+—— theory of coral-formations 73
+—— on the Bermuda islands 154
+_New Britain_ 124
+New Caledonia, steepness of its reefs 39
+—— —— barrier-reef of , 79, 83, 93
+_New Caledonia_ 123
+_New Guinea (E. end)_ 124
+_New Guinea (W. end)_ 127
+_New Hanover_ 124
+New Hebrides, recently elevated 100
+_New Hebrides_ 121
+New Ireland, recently elevated 100
+_New Ireland_ 124
+_New Nantucket_ 116
+_Nicobar islands_ 132
+_Niouha_ 119
+Nulliporæ at Keeling atoll 18
+—— on the reefs of atolls 28
+—— on barrier-reefs 39
+—— their wide distribution and abundance 68
+
+Objections to the theory of subsidence 7
+_Ocean islands_ 117, 121
+_Ono_ 120
+_Onouafu (Onouafou)_ 119
+_Ormuz_ 143
+_Oscar group_ 120
+Oscillations of level 103, 108
+_Ouallan, or Ualan (Oualan)_ 125
+Ouluthy atoll 60
+_Outong Java_ 124
+
+_Palawan, S.W. coast 133
+—— N.W. coast 134
+—— western bank 136
+_ Palmerston 114
+_Palmyra_ 116
+_Paracells_ 136
+_Paraquas_ 136
+_Patchow_ 135
+_Pelew islands_ 126
+Pemba island, singular form of 102
+_Pemba_ 142
+_Penrhyn_ 115
+_Peregrino_ 115
+Pernambuco, bar of sandstone at 47
+Persian gulf, recently elevated 102
+_Persian gulf_ 143
+Pescado 115
+_Pescadores_ 135
+_Peyster group_ 120
+_Philip_ 126
+Philippine archipelago, recently elevated 101
+_Philippine archipelago_ 134
+_Phœnix_ 116
+_Piguiram_ 126
+_Pitcairn_ 112
+Pitt’s bank 86
+_Pitt island_ 120
+_Platte_ 139
+_Pleasant_ 121
+Porites, chief coral on margin of Keeling atoll 16
+_Postillions_ 131
+Pouynipète 95
+—— its probable subsidence 95
+_Pouynipète_ 125
+_Pratas shoal_ 135
+_Proby_ 119
+_Providence_ 139
+_Puerto Rico_ 152
+_Pulo Anna_ 126
+Pumice floated to coral-islands 88
+_Pylstaart_ 118
+Pyrard de Laval, astonishment at the atolls in the Indian Ocean 11
+
+Quoy and Gaimard, depths at which corals live 66
+—— description of reefs applicable only to fringing-reefs 98
+
+Range of atolls 94
+_Rapa_ 115
+_Rearson_ 115
+Red Sea, banks of rock coated by reefs 49
+—— proofs of its recent elevation 102
+—— supposed subsidence of 103
+_Red Sea_ 143
+Reefs, irregular in shallow seas 49
+—— rising to the surface in some lagoons and all submerged in others 55
+—— their distribution 50
+—— their absence from some coasts 51
+_Revilla-gigedo_ 111
+Ring-formed reefs of the Maldiva atolls, and theory of , 80
+_Rodriguez_ 138
+_Rosario_ 127
+_Rose island_ 118
+_Rotches_ 120
+_Rotoumah_ 120
+_Roug_ 125
+_Rowley shoals_ 130
+Rüppell, Dr., on the recent deposits of Red Sea 102
+
+_Sable, ile de_ 138
+_Sahia de Malha_ 137
+_St. Pierre_ 139
+_Sala_ 111
+_Salomon (Solomon) archipelago_ 123
+Samoa, or Navigator archipelago, elevation of 99
+_Samoa archipelago_ 117
+Sand-bars parallel to coasts 46
+_Sandal-wood_ 129
+Sandwich archipelago, recently elevated 98
+_Sandwich archipelago_ 117
+_Sanserot_ 126
+_Santa-Cruz group_ 122
+Savage island, recently elevated 59, 99, 104
+_Savage_ 118
+_Savu_ 129
+_Saya, or Sahia de Malha_ 137
+_Scarborough shoal_ 136
+Scarus feeding on corals 21
+_Schouten_ 124
+_Scilly_ 113
+Scoriæ floated to coral-islands 89
+_Scott’s reef_ 130
+Sections of islands encircled by barrier-reefs 43, 176
+—— of Bolabola 76
+Sediment in Keeling lagoon 21
+—— in other atolls 29, 35
+—— injurious to corals 53
+—— transported from coral-islands far seaward 89
+ _Seniavine_ 125
+_Serangani_ 129
+_Seychelles_ 138
+Ship-bottom quickly coated with coral 62
+_Smyth island_ 116
+Society archipelago, stationary condition of 96
+—— alleged proofs of recent elevation 103
+_Society archipelago_ 112
+_Socotra_ 143
+_Solor_ 130
+Sooloo islands, recently elevated 101
+_Sooloo islands_ 133
+_Souvaroff_ 115
+_Spanish_ 126
+Sponge, depths at which found 67
+_Starbuck (Slarbuck)_ 115
+Stones transported in roots of trees 89
+Storms, effects of, on coral-islands 74
+Stutchbury, Mr., on the growth of an Agaricia 63
+—— on upraised corals in Society archipelago 103
+Subsidence of Keeling atoll 28
+—— extreme slowness of 87, 108
+—— areas of, apparently elongated 106
+—— areas of immense 106
+—— great amount of 108
+_Suez, gulf of_ 147
+_Sulphur islands_ 127
+Sumatra, recently elevated 100
+_Sumatra_ 132
+_Sumbawa_ 130
+Surf favourable to the growth of massive corals 52
+_Swallow shoal_ 136
+_Sydney island_ 116
+
+Tahiti, alleged proofs of its recent elevation 103
+_Tahiti_ 112
+Temperature of the sea at the Galapagos archipelago 51
+_Tenasserim_ 133
+_Tenimber island_ 128
+_Teturoa_ 113
+Theories on coral-formations 69, 73
+Theory of subsidence, and objections to 72, 86
+Thickness, vertical, of barrier-reefs 43, 76
+_Thomas, St._ 153
+_Tikopia_ 122
+Timor, recently elevated 100
+_Timor_ 129
+_Timor-laut_ 128
+_Tokan-Bessees_ 131
+_Tongatabou_ 118
+_Tonquin_ 137
+_Toubai_ 113
+_Toufoa (Toofoa)_ 119
+_Toupoua_ 122
+Traditions of change in coral-islands 73
+Tridacnæ embedded in coral-rock 63
+—— left exposed in the Low archipelago 96
+Tubularia, quick growth of 63
+_Tumbelan_ 133
+_Turneffe reef_ 152
+_Turtle_ 119
+
+_Ualan_ 125
+
+Vanikoro, section of 43
+—— its state and changes in its reefs 95
+_Vanikoro_ 122
+_Vine reef_ 125
+_Virgin Gorda_ 153
+_Viti archipelago_ 119
+Volcanic islands, with living corals on their shores 51
+—— matter, probably not associated with thick masses of coral-rock 88
+Volcanoes, authorities for their position on the map 90
+—— their presence determined by the movements in progress 104
+—— absent or extinct in the areas of subsidence 105
+
+_Waigiou_ 128
+_Wallis island_ 119
+_Washington_ 116
+_Wells’ reef_ 123
+Wellstead, Lieutenant, account of a ship coated with corals 62
+West Indies, banks of sediment fringed by reefs 49
+—— recently elevated 102
+_West Indies_ 147
+Whitsunday island, view of 12
+—— changes in its state 74
+Williams, Rev. J., on traditions of the natives regarding coral-islands
+74
+—— on antiquity of certain corals 64
+_Wolchonsky_ 111
+_Wostock_ 115
+
+_Xulla islands_ 128
+
+_York island_ 116
+_Yucutan, coast of_ 151
+
+Zones of different kinds of corals outside the same reefs 55, 60
+
+
+
+
+INDEX TO VOLCANIC ISLANDS.
+
+
+Abel, M., on calcareous casts at the Cape of Good Hope 261
+ Abingdon island 234
+ Abrolhos islands, incrustation on 188
+ Aeriform explosions at Ascension 191
+ _Albatross_, driven from St. Helena 225
+ Albemarle island 234
+ Albite, at the Galapagos archipelago 234
+ Amygdaloidal cells, half filled 184
+ Amygdaloids, calcareous origin of 176
+ Ascension, arborescent incrustation on rocks of 188
+—— absence of dikes, freedom from volcanic action, and state of
+lava-streams 226
+ Ascidia, extinction of 258
+ Atlantic Ocean, new volcanic focus in 226
+ Augite, fused 239
+ Australia 251
+ Azores 182, 248
+
+ Bahia in Brazil, dikes at 247
+ Bailly, M., on the mountains of Mauritius 185
+ Bald Head 260
+ Banks’ Cove 234, 236
+ Barn, The, St. Helena 216
+ Basalt, specific gravity of 245
+ Basaltic coast-mountains at Mauritiu 185
+—— at St. Helena 218
+—— at St. Jago 178
+ Beaumont, M. Elie de, on circular subsidences in lava 233
+—— on dikes indicating elevation 228
+—— on inclination of lava-streams 227
+—— on laminated dikes 212
+ Bermuda, calcareous rocks of 260, 262
+ Beudant, M., on bombs 191
+—— on jasper 197
+—— on laminated trachyte 211
+—— on obsidian of Hungary 207
+—— on silex in trachyte 270, 197
+ Bole 257
+ Bombs, volcanic 189
+ Bory St. Vincent, on bombs 190
+ Boulders, absence in Australia and Cape of Good Hope 265
+ Brattle island 238
+ Brewster, Sir D., on a calcareo-animal substance 201
+—— on decomposed glass 252
+ Brown, Mr. R., on extinct plants from Van Diemen’s land 257
+—— on sphærulitic bodies in silicified wood 207
+ Buch, Von, on cavernous lava 233
+—— on central volcanoes 249
+—— on crystals sinking in obsidian 243
+—— on laminated lava 209
+—— on obsidian streams 208
+—— on olivine in basalt 234
+—— on superficial calcareous beds in the Canary islands 224
+
+ Calcareous deposit at St. Jago affected by heat 169, 171
+—— fibrous matter, entangled in streaks in scoriæ 174
+—— freestone at Ascension 198
+—— incrustations at Ascension 199
+—— sandstone at St. Helena 222
+—— superficial beds at King George’s sound 260
+ Cape of Good Hope 263
+ Carbonic acid, expulsion of, by heat 171, 176
+ Carmichael, Capt., on glassy coatings to dikes 216
+ Casts, calcareous, of branches 261
+ Chalcedonic nodules 257
+ Chalcedony in basalt and in silicified wood 196
+ Chatham island 231, 235, 241, 248, 259
+ Chlorophæite 257
+ Clarke, Rev. W., on the Cape of Good Hope 258, 263
+ Clay-slate, its decomposition and junction with granite at the Cape of
+ Good Hope 264
+ Cleavage of clay-slate in Australia 252
+ Cleavage, cross, in sandstone 253
+ Coast denudation at St. Helena 226
+ Columnar basalt 173
+ “Comptes Rendus,” account of volcanic phenomena in the Atlantic 226
+ Concepcion, earthquake of 228, 249
+ Concretions in aqueous and igneous rocks compared 206
+—— in tuff 197
+—— of obsidian 206, 208
+ Conglomerate, recent, at St. Jago 181
+ Coquimbo, curious rock of 261
+ Corals, fossil, from Van Diemen’s Land 256
+ Crater, segment of, at the Galapagos 238
+—— great central one at St. Helena 219
+—— internal ledges round, and parapet on 220
+ Craters, basaltic, at Ascension 189
+—— form of, affected by the trade wind 189
+—— of elevation 227
+—— of tuff at Terceira 182
+—— of tuff at the Galapagos archipelago 230, 231, 235, 237
+—— their breached state 240
+—— small basaltic at St. Jago 177
+—— —— at the Galapagos archipelago 232
+ Crystallisation favoured by space 211
+
+ Dartigues, M., on sphærulites 207
+ Daubeny, Dr., on a basin-formed island 237
+—— on fragments in trachyte 193
+ D’Aubuisson on hills of phonolite 222
+—— on the composition of obsidian 206
+—— on the lamination of clay-slate 210
+ De la Beche, Sir H., on magnesia in erupted lime 174
+—— on specific gravity of limestones 198
+ Denudation of coast at St. Helena 226
+ Diana’s Peak, St. Helena 220
+ Dieffenbach, Dr., on the Chatham Islands 259
+ Dikes, truncated, on central crateriform ridge of St. Helena 219
+—— at St. Helena; number of; coated by a glossy layer; uniform
+thickness of 216
+—— great parallel ones at St. Helena 222
+—— not observed at Ascension 226
+—— of tuff 231
+—— of trap in the plutonic series 247
+—— remnants of, extending far into the sea round St. Helena 226
+ Dislocations at Ascension 192
+—— at St. Helena 217, 221
+ Distribution of volcanic islands 248
+ Dolomieu, on decomposed trachyte 182
+—— on laminated lava 210, 211
+—— on obsidian 208
+ Drée, M., on crystals sinking in lava 243
+ Dufrenoy, M., on the composition of the surface of certain
+ lava-streams 209, 243
+—— on the inclination of tuff-strata 236
+
+ Eggs of birds embedded at St. Helena 224
+—— of turtle at Ascension 198
+ Ejected fragments at Ascension 192
+—— at the Galapagos archipelago 239
+ Elevation of St. Helena 225
+—— the Galapagos archipelago 241
+—— Van Diemen’s Land, Cape of Good Hope, New Zealand, Australia, and
+Chatham island 258
+—— of volcanic islands 250
+ Ellis, Rev. W., on ledges within the great crater at Hawaii 220
+—— on marine remains at Otaheite 184
+ Eruption, fissures of 224, 249, 250
+ Extinction of land-shells at St. Helena 224
+
+ Faraday, Mr., on the expulsion of carbonic acid gas 171
+ Feldspar, fusibility of 246
+—— in radiating crystals 263
+—— Labrador, ejected 193
+ Feldspathic lavas 179
+—— at St. Helena 219
+—— rock, alternating with obsidian 202
+—— lamination, and origin of 209
+ Fernando Noronha 181, 210
+ Ferruginous superficial beds 259
+ Fibrous calcareous matter at St. Jago 174
+ Fissures of eruption 242, 249, 250
+ Fitton, Dr., on calcareous breccia 262
+ Flagstaff Hill, St. Helena 216
+ Fleurian de Bellevue on sphærulites 207
+ Fluidity of lavas 234, 235
+ Forbes, Professor, on the structure of glaciers 212
+ Fragments ejected at Ascension 192
+—— at the Galapagos archipelago 239
+ Freshwater Bay 238, 243
+ Fuerteventura (Feurteventura), calcareous beds of 224
+
+ Galapagos archipelago 229
+—— parapets round craters 220
+ Gay Lussac, on the expulsion of carbonic acid gas 171
+ Glaciers, their structure 212
+ Glossiness of texture, origin of 206
+ Gneiss, derived from clay-slate 264
+—— with a great embedded fragment 252
+ Gneiss-granite, form of hills of 259
+ Good Hope, Cape of 263
+ Gorges, narrow, at St. Helena 225
+ Granite, junction with clay-slate, at the Cape of Good Hope 263
+ Granitic ejected fragments 192, 239
+ Gravity, specific, of lavas 243-8
+ Gypsum, at Ascension 201
+—— in volcanic strata at St. Helena 215
+—— on surface of the ground at ditto 223
+
+ Hall, Sir J., on the expulsion of carbonic acid gas 171
+ Heat, action of, on calcareous matter 170
+ Hennah, Mr., on ashes at Ascension 189
+ Henslow, Prof., on chalcedony 197
+ Hoffmann, on decomposed trachyte 182
+ Holland, Dr., on Iceland 228
+ Horner, Mr., on a calcareo-animal substance 201
+—— on fusibility of feldspar 246
+ Hubbard, Dr., on dikes 247
+ Humboldt on ejected fragments 193
+—— on obsidian formations 207, 209
+—— on parapets round craters 220
+—— on sphærulites 210
+ Hutton on amygdaloids 176
+ Hyalite in decomposed trachyte 182
+
+ Iceland, stratification of the circumferential hills 228
+ Islands, volcanic, distribution of 248
+—— their elevation 250
+ Incrustation, on St. Paul’s rocks 187
+ Incrustations, calcareous, at Ascension 199
+
+ Jago, St. 167
+ James island 234, 237, 242
+ Jasper, origin of 196
+ Jonnès, M. Moreau de, on craters affected by wind 189
+ Juan Fernandez 250
+
+ Keilhau, M., on granite 264
+ Kicker Rock 232
+ King George’s sound 259
+
+ Labrador feldspar, ejected 193
+ Lakes at bases of volcanoes 229
+ Lamination of volcanic rocks 209
+ Land-shells, extinct, at St. Helena 224
+ Lanzarote, calcareous beds of 223
+ Lava, adhesion to sides of a gorge 177
+—— feldspathic 179
+—— with cells semi-amygdaloidal 184
+ Lavas, specific gravity of 243, 247
+ Lava-streams blending together at St. Jago 177
+—— composition of surface of 208
+—— differences in the state of their surfaces 244
+—— extreme thinness of 238
+—— heaved up into hillocks at the Galapagos archipelago 233
+—— their fluidity 234, 235
+—— with irregular hummocks at Ascension 189
+ Lead, separation from silver 244
+ Lesson, M., on craters at Ascension 189
+ Leucite 234
+ Lime, sulphate of, at Ascension 200
+ Lonsdale, Mr., on fossil-corals from Van Diemen’s land 256
+ Lot, St. Helena 221
+ Lyell, Mr., on craters of elevation 227
+—— on embedded turtles’ eggs 198
+—— on glossy coating to dikes 216
+
+ Macaulay, Dr., on calcareous casts at Madeira 262
+ MacCulloch, Dr., on an amygdaloid 184
+—— on chlorophæite 287
+—— on laminated pitchstone 209
+ Mackenzie, Sir G., on cavernous lava-streams 233
+—— on glossy coatings to dikes 216
+—— on obsidian streams 208
+—— on stratification in Iceland 228
+ Madeira, calcareous casts at 262
+ _Magazine, Nautical,_—account of volcanic phenomena in the Atlantic
+ 226
+ Marekanite 206
+ Mauritius, crater of elevation of 184, 227
+ Mica, in rounded nodules 168
+—— origin in metamorphic slate 264
+—— radiating form of 263
+ Miller, Prof., on ejected Labrador feldspar 193
+—— on quartz crystals in obsidian beds 202
+ Mitchell, Sir T., on bombs 191
+—— on the Australian valleys 254
+ Mud streams at the Galapagos archipelago 236
+
+ Narborough island 234
+ Nelson, Lieut., on the Bermuda islands 260, 262
+ New Caledonia 248
+ New Red sandstone, cross cleavage of 253
+ New South Wales 251
+ New Zealand 259
+ Nulliporæ (fossil), resembling concretions 169
+
+ Obsidian, absent at the Galapagos archipelago 241
+—— bombs of 191
+—— composition and origin of 207, 208
+—— crystals of feldspar sink in 243
+—— its irruption from lofty craters 246
+—— passage of beds into 202
+—— specific gravity of 243, 246
+—— streams of 208
+ Olivine decomposed at St. Jago 178
+—— at Van Diemen’s land 257
+—— in the lavas at the Galapagos archipelago 234
+ Oolitic structure of recent calcareous beds at St. Helena 223
+ Otaheite 183
+ Oysters, extinction of 258
+
+ Panza islands, laminated trachyte of 209
+ Pattinson, Mr., on the separation of lead and silver 244
+ Paul’s, St., rocks of 187
+ Pearlstone 206
+ Peperino 232
+ Péron, M., on calcareous rocks of Australia 262, 263
+ Phonolite, hills of 179, 181, 221
+—— laminated 210
+—— with more fusible hornblende 246
+ Pitchstone 204
+—— dikes of 209
+ Plants, extinct 257
+ Plutonic rocks, separation of constituent parts of, by gravity 246
+ Porto Praya 167
+ Prevost, M. C., on rarity of great dislocations in volcanic islands
+ 217
+ Prosperous hill, St. Helena 218
+ Pumice, absent at the Galapagos archipelago 241
+—— laminated 209, 210, 211
+ Puy de Dome, trachyte of 193
+
+ Quail island, St. Jago 168, 170, 173
+ Quartz, crystals of, in beds alternating with obsidian 202
+—— crystallised in sandstone 252
+—— fusibility of 246
+—— rock, mottled from metamorphic action with earthy matter 170
+
+ Red hill 173
+ Resin-like altered scoriæ 171
+ Rio de Janeiro, gneiss of 252
+ Robert, M., on strata of Iceland 228
+ Rogers, Professor, on curved lines of elevation 249
+
+ Salses, compared with tuff craters 240
+ Salt deposited by the sea 200
+—— in volcanic strata 201, 215
+—— lakes of, in craters 240
+ Sandstone of Brazil 265
+—— of the Cape of Good Hope 265
+—— platforms of, in New South Wales 252, 265
+ Schorl, radiating 263
+ Scrope, Mr. P., on laminated trachyte 209, 210, 212
+—— on obsidian 208
+—— on separation of trachyte and basalt 244
+—— on silex in trachyte 176
+—— on sphærulites 210
+ Seale, Mr., geognosy of St. Helena 215
+—— on dikes 226
+—— on embedded birds’ bones 225
+ Seale, on extinct shells of St. Helena 224
+ Sedgwick, Professor, on concretions 206
+ Septaria, in concretions in tuff 198
+ Serpulæ on upraised rocks 185
+ Seychelles 248
+ Shells, colour of, affected by light 201
+—— from Van Diemen’s land 256
+—— land, extinct, at St. Helena 224
+—— particles of, drifted by the wind at St. Helena 223
+ Shelly matter deposited by the waves 200
+ Siau, M., on ripples 254
+ Signal Post Hill 168, 175, 176
+ Silica, deposited by steam 182
+—— large proportion of, in obsidian 206, 208
+—— specific gravity of 246
+ Siliceous sinter 196
+ Smith, Dr. A., on junction of granite and clay-slate 264
+ Spallanzani on decomposed trachyte 182
+ Specific gravity of recent calcareous rocks and of limestone 198
+—— of lavas 245
+ Sphærulites in glass and in silicified wood 207
+—— in obsidian 204, 210
+ Sowerby, Mr. G. B., on fossil-shells from Van Diemen’s land 256
+—— from St. Jago 169
+—— land-shells from St. Helena 224
+ St. Helena 214
+—— crater of elevation of 227
+ St. Jago, crater of elevation of 227
+—— effects of calcareous matter on lava 231
+ St. Paul’s rocks 187, 248
+ Stokes, Mr., collections of sphærulites and of obsidians 207, 212
+ Stony-top, Little 218, 222
+—— Great 218
+ Stratification of sandstone in New South Wales 253, 255
+ Streams of obsidian 208
+ Stutchbury, Mr., on marine remains at Otaheite 184
+ Subsided space at Ascension 192
+
+ Tahiti 183
+ Talus, stratified, within tuff craters 236
+ Terceira 182
+ Tertiary deposit of St. Jago 169
+ Trachyte, absent at the Galapagos archipelago 241
+—— at Ascension 193
+—— at Terceira 182
+—— decomposition of, by steam 182
+—— its lamination 200, 210
+—— its separation from basalt 244
+—— softened at Ascension 194
+—— specific gravity of 245
+—— with singular veins 195
+ Trap-dikes in the plutonic series 247
+—— at King George’s sound 259
+ Travertin at Van Diemen’s land 257
+ Tropic-bird, now rare, at St. Helena 225
+ Tuff, craters of 231, 235, 236
+—— their breached state 240
+—— peculiar kind of 231
+ Turner, Mr., on the separation of molten metals 244
+ Tyerman and Bennett on marine remains at Huaheine 184
+
+ Valleys, gorge-like, at St. Helena 225
+—— in New South Wales 254
+—— in St. Jago 180
+ Van Diemen’s land 256
+ Veins in trachyte 195
+—— of jasper 195
+ Vincent, Bory St., on bombs 190
+ Volcanic bombs 189
+—— island in process of formation in the Atlantic 226
+—— islands, their distribution 248
+
+ Wacke, its passage into lava 183, 257
+ Wackes, argillaceous 168, 178
+ Webster, Dr., on a basin-formed island 237
+—— on gypsum at Ascension 201
+ White, Martin, on soundings 254
+ Wind, effects of, on the form of craters
+
+
+
+
+INDEX TO SOUTH AMERICAN GEOLOGY.
+
+
+Abich, on a new variety of feldspar 446
+ Abrolhos islands 415
+ Absence of recent formations on the S. American coasts 409
+ Aguerros on elevation of Imperial 305
+ Albite, constituent mineral in andesite 446
+—— in rocks of Tierra del Fuego 427
+—— in porphyries 444
+—— crystals of, with orthite 447
+ Alison, Mr., on elevation of Valparaiso 307, 310
+ Alumina, sulphate of 439
+Ammonites from Concepcion 400, 405
+ Amolanas, Las 493
+ Amygdaloid, curious varieties of 444
+Amygdaloids of the Uspallata range 471
+—— of Copiapo 498
+ Andesite of Chile 446
+—— in the valley of Maypu 449, 450
+—— of the Cumbre pass 460, 466
+—— of the Uspallata range 475
+—— of Los Hornos 480
+—— of Copiapo 488, 491
+ Anhydrite, concretions of 450, 463
+ Araucaria, silicified wood of 394, 474
+ Arica, elevation of 323
+ Arqueros, mines of 481
+ Ascension, gypsum deposited on 328
+—— laminated volcanic rocks of 439, 440
+ Augite in fragments, in gneiss 414
+—— with albite, in lava 347
+ Austin, Mr. R. A. C., on bent cleavage lamina 434
+ Austin, Captain, on sea-bottom 302
+ Australia, foliated rocks of 438
+ _Azara labiata_, beds of, at San Pedro 277, 352
+
+ _Baculites vagina_ 400
+ Bahia Blanca, elevation of 280
+—— formations near 355
+—— character of living shells of 408
+ Bahia (Brazil), elevation near 280
+—— crystalline rocks of 414
+ Ballard, M., on the precipitation of sulphate of soda 349
+ Banda Oriental, tertiary formations of 365
+—— crystalline rocks of 418
+ Barnacles above sea-level 311
+—— adhering to upraised shells 306
+ Basalt of S. Cruz 389
+—— streams of, in the Portillo range 456
+—— in the Uspallata range 472
+ Basin chains of Chile 333
+ Beagle Channel 427, 430
+ Beaumont, Elie de, on inclination of lava-streams 390, 457
+—— on viscid quartz-rocks 475
+ Beech-tree, leaves of fossil 391
+ Beechey, Captain, on sea-bottom 299
+ Belcher, Lieutenant, on elevated shells from Concepcion 306
+ Bella Vista, plain of 325
+ Benza, Dr., on decomposed granite 417
+ Bettington, Mr., on quadrupeds transported by rivers 374
+ Blake, Mr., on the decay of elevated shells near Iquique 322
+—— on nitrate of soda 346
+ Bole 444
+ Bollaert, Mr., on mines of Iquique 503
+ Bones, silicified 402
+—— fossil, fresh condition of 366
+ Bottom of sea off Patagonia 292, 298
+ Bougainville, on elevation of the Falkland islands 290
+ Boulder formation of S. Cruz 285, 295
+—— of Falkland islands 290
+—— anterior to certain extinct quadrupeds 371
+—— of Tierra del Fuego 391
+ Boulders in the Cordillera 339, 341
+—— transported by earthquake-waves 344
+—— in fine-grained tertiary deposits 401
+ Brande, Mr., on a mineral spring 461
+ Bravais, M., on elevation of Scandinavia 320
+ Brazil, elevation of 279
+—— crystalline rocks of 414, 418
+ Broderip, Mr., on elevated shells from Concepcion 306
+ Brown, Mr. R., on silicified wood of Uspallata range 474
+ Brown, on silicified wood 495
+ Bucalema, elevated shells near 307
+ Buch, Von, on cleavage 438
+—— on cretaceous fossils of the Cordillera 453, 465
+—— on the sulphureous volcanoes of Java 509
+ Buenos Ayres 352
+ Burchell, Mr., on elevated shells of Brazil 279
+ Byron, on elevated shells 303
+
+ Cachapual, boulders in valley of 339, 341
+ Caldcleugh, Mr., on elevation of Coquimbo 314
+—— on rocks of the Portillo range 456
+ Callao, elevation near 323
+—— old town of 327
+ Cape of Good Hope, metamorphic rocks of 439
+ _Carcharias megalodon_ 402
+ Carpenter, Dr., on microscopic organisms 352
+ Castro (Chiloe), beds near 394
+ Cauquenes Baths, boulders near 339, 341
+—— pebbles in porphyry near 443
+—— volcanic formation near 447
+—— stratification near 449
+ Caves above sea-level 303, 307, 322
+ _Cervus pumilus,_ fossil-horns of 304
+ Chevalier, M., on elevation near Lima 323
+ Chile, structure of country between the Cordillera and the Pacific 333
+—— tertiary formations of 337
+—— crystalline rocks in 435
+—— central, geology of 441
+—— northern, geology of 479
+ Chiloe, gravel on coast 294
+—— elevation of 303
+—— tertiary formation of 337, 405
+—— crystalline rocks of 433
+ Chlorite-schist, near M. Video 419
+ Chonos archipelago, tertiary formations of 393
+—— crystalline rocks of 430
+ Chupat, Rio, scoriæ transported by 280
+ Claro, Rio, fossiliferous beds of 485
+ Clay-shale of Los Hornos 480
+ Clay-slate, formation of, Tierra del Fuego 424
+—— of Concepcion 433
+—— feldspathic, of Chile 442, 444, 448
+—— —— of the Uspallata range 468, 470
+—— black siliceous, band of, in porphyritic formations of Chile 445
+ Claystone porphyry, formation of, in Chile 442
+—— origin of 445
+—— eruptive sources of 444
+ Cleavage, definition of 414
+—— at Bahia 415
+—— Rio de Janeiro 415
+—— Maldonado 418
+—— Monte Video 420
+—— S. Guitru-gueyu 421
+—— Falkland I. 424
+—— Tierra del Fuego 428
+—— Chonos I. 434
+—— Chiloe 435
+—— Concepcion 434
+—— Chile 435
+—— discussion on 436
+ Cleavage-laminæ superficially bent 434
+ Cliffs, formation of 301
+ Climate, late changes in 345
+—— of Chile during tertiary period 408
+ Coal of Concepcion 399
+—— S. Lorenzo 504
+ Coast-denudation of St. Helena 301
+ Cobija, elevation of 322
+ Colombia, cretaceous formation of 504
+ Colonia del Sacramiento, elevation of 278
+—— Pampean formation near 355
+ Colorado, Rio, gravel of 295
+—— sand-dunes of 281, 294
+—— Pampean formation near 355
+ Combarbala 479, 481
+ Concepcion, elevation of 305
+—— deposits of 399, 405
+—— crystalline rocks of 433
+ Conchalee, gravel-terraces of 311
+ Concretions of gypsum, at Iquique 345
+—— in sandstone at S. Cruz 387
+—— in tufaceous tuff of Chiloe 387
+—— in gneiss 414
+—— in claystone-porphyry at Port Desire 421
+—— in gneiss at Valparaiso 435
+—— in metamorphic rocks 436
+—— of anhydrite 450
+—— relations of, to veins 473
+ Conglomerate claystone of Chile 443, 445
+—— of Tenuyan 454, 458, 478
+—— of the Cumbre Pass 462, 466
+—— of Rio Claro 485
+—— of Copiapo 496, 499
+ Cook, Captain, on form of sea-bottom 300
+ Copiapo, elevation of 321
+—— tertiary formations of 403
+—— secondary formations of 489
+ Copper, sulphate of 489
+—— native, at Arqueros 482
+—— mines of, at Panuncillo 481
+—— veins, distribution of 505
+ Coquimbo, elevation and terraces of 312
+—— tertiary formations of 404
+—— secondary formations of 482
+ Corallines living on pebbles 299
+ Cordillera, valleys bordered by gravel fringes 337
+—— basal strata of 442
+—— fossils of 453, 465, 486, 487, 493, 503
+—— elevation of 442, 459, 474, 476, 500, 502, 510, 512, 517
+—— gypseous formations of 450, 452, 461, 463, 479, 483, 489, 491, 503
+—— claystone-porphyries of 442
+—— andesitic rocks of 446
+—— volcanoes of 447, 511, 517
+ Coste, M., on elevation of Lemus 303
+ Coy inlet, tertiary formation of 390
+ _Crassatella Lyellii_ 392
+ Cruickshanks, Mr., on elevation near Lima 327
+ Crystals of feldspar, gradual formation of, at Port Desire 422
+ Cumbre, Pass of, in Cordillera 502
+ Cuming, Mr., on habits of the Mesodesma 310
+—— on range of living shells on west coast 407
+
+ Dana, Mr., on foliated rocks 438
+—— on amygdaloids 444
+ Darwin, Mount 427
+ D’Aubuisson, on concretions 397
+—— on foliated rocks 438
+ Decay, gradual, of upraised shells 323, 327
+ Decomposition of granite rocks 417
+ De la Beche, Sir H., his theoretical researches in geology 299
+—— on the action of salt on calcareous rocks 327
+—— on bent cleavage-laminæ 434
+ Denudation on coast of Patagonia 292, 300, 409
+—— great powers of 410
+—— of the Portillo range 456, 458
+ Deposits, saline 344
+ Despoblado, valley of 496, 497, 499
+ Detritus, nature of, in Cordillera 338
+ Devonshire, bent cleavage in 434
+ Dikes, in gneiss of Brazil 414, 418
+—— near Rio de Janeiro 417
+—— pseudo, at Port Desire 423
+—— in Tierra del Fuego 426
+—— in Chonos archipelago, containing quartz 432
+—— near Concepcion, with quartz 434
+—— granitic-porphyritic, at Valparaiso 435
+—— rarely vesicular in Cordillera 347
+—— absent in the central ridges of the Portillo pass 452
+—— of the Portillo range, with grains of quartz 456
+—— intersecting each other often 466
+—— numerous at Copiapo 498
+ Domeyko, M., on the silver mines of Coquimbo 482
+—— on the fossils of Coquimbo 486
+ D’Orbigny, M. A., on upraised shells of Monte Video 278
+—— on elevated shells at St. Pedro 278
+—— on elevated shells near B. Ayres 279
+—— on elevation of S. Blas 281
+—— on the sudden elevation of La Plata 293
+—— on elevated shells near Cobija 322
+—— on elevated shells near Arica 322
+—— on the climate of Peru 324
+—— on salt deposits of Cobija 345
+—— on crystals of gypsum in salt-lakes 349
+—— on absence of gypsum in the Pampean formation 353
+—— on fossil remains from Bahia Blanca 359, 360
+—— on fossil remains from the banks of the Parana 362
+—— on the geology of St. Fé 363
+—— on the age of Pampean formation 367, 376
+—— on the _Mastodon Andium_ 379
+—— on the geology of the Rio Negro 381
+—— on the character of the Patagonian fossils 391
+—— on fossils from Concepcion 399
+—— —— from Coquimbo 404
+—— —— from Payta 405
+—— on fossil tertiary shells of Chile 406
+—— on cretaceous fossils of Tierra del Fuego 426
+—— —— from the Cordillera of Chile 453, 465, 486, 488, 493, 504
+
+ Earth, marine origin of 304, 308
+ Earthenware, fossil 326
+ Earthquake, effect of, at S. Maria 293
+—— elevation during, at Lemus 303
+—— of 1822, at Valparaiso 310
+—— effects of, in shattering surface 325
+—— fissures made by 325
+—— probable effects on cleavage 325
+ Earthquakes in Pampas 290
+ Earthquake-waves, power of, in throwing up shells 310
+—— effects of, near Lima 327
+—— power of, in transporting boulders 344
+ Edmonston, Mr., on depths at which shells live at Valparaiso 309
+ Ehrenberg, Professor, on infusoria in the Pampean formation 355, 359,
+ 362
+—— on infusoria in the Patagonian formation 383, 384, 386, 391, 392
+ Elevation of La Plata 278
+—— Brazil 279
+—— Bahia Blanca 280, 357
+—— San Blas 281
+—— Patagonia 281, 291, 293
+—— Tierra del Fuego 288
+—— Falkland islands 290
+—— Pampas 289, 377
+—— Chonos archipelago 303
+—— Chiloe 304
+—— Chile 304
+—— Valparaiso 307, 310
+—— Coquimbo 312, 320
+—— Guasco 320
+—— Iquique 322
+—— Cobija 322
+—— Lima 323
+—— sudden, at S. Maria 293
+—— —— at Lemus 303
+—— insensible, at Chiloe 304
+—— —— at Valparaiso 311
+—— —— at Coquimbo 314
+—— axes of, at Chiloe 398, 405
+—— —— at P. Rumena 398, 405
+—— —— at Concepcion 398, 405
+—— unfavourable for the accumulation of permanent deposits 410
+—— lines of, parallel to cleavage and foliation 416, 417, 424, 428,
+432, 434, 438
+—— lines of, oblique to foliation 431
+—— areas of, causing lines of elevation and cleavage 441
+—— lines of, in the Cordillera 442
+—— slow, in the Portillo range 475
+—— two periods of, in Cordillera of Central Chile 476
+—— of the Uspallata range 474
+—— two periods of, in Cumbre Pass 476
+—— horizontal, in the Cordillera of Copiapo 500
+—— axes of, coincident with volcanic orifices 503
+—— of the Cordillera, summary on 510, 513, 517
+ Elliott, Captain, on human remains 279
+ Ensenada, elevated shells of 278
+ Entre Rios, geology of 363
+ _Equus curvidens_ 364, 379
+ Epidote in Tierra del Fuego 426
+—— in gneiss 435
+—— frequent in Chile 445
+—— in the Uspallata range 475
+—— in porphyry of Coquimbo 482
+ Erman, M., on andesite 347
+ Escarpments, recent, of Patagonia 301
+ Extinction of fossil mammifers 370
+
+ Falkland islands, elevation of 290
+—— pebbles on coast 297, 299
+—— geology of 424
+ Falkner, on saline incrustations 347
+ Faults, great, in Cordillera 461, 469
+ Feldspar, earthy, metamorphosis of, at Port Desire 422
+—— albitic 347
+—— crystals of, with albite 347
+—— orthitic, in conglomerate of Tenuyan 454
+—— in granite of Portillo range 455
+—— in porphyries in the Cumbre Pass 466
+ Feuillée on sea-level at Coquimbo 314
+ Fissures, relations of, to concretions 397
+—— upfilled, at Port Desire 424
+—— in clay-slate 470
+ Fitton, Dr., on the geology of Tierra del Fuego 427
+ Fitzroy, Captain, on the elevation of the Falkland islands 427
+—— on the elevation of Concepcion 305
+ Foliation, definition of 414
+—— of rocks at Bahia 414
+—— Rio de Janeiro 415
+—— Maldonado 418
+—— Monte Video 420
+—— S. Guitru-gueyu 421
+—— Falkland I. 424
+—— Tierra del Fuego 427
+—— Chonos archipelago 430
+—— Chiloe 433
+—— Concepcion 434
+—— Chile 435
+—— discussion on 435
+ Forbes, Professor E., on cretaceous fossils of Concepcion 400
+—— on cretaceous fossils and subsidence in Cumbre Pass 465
+—— on fossils from Guasco 488
+—— —— from Coquimbo 483, 487
+—— —— from Copiapo 493
+—— on depths at which shells live 409, 496
+ Formation, Pampean 352
+—— —— area of 371
+—— —— estuary origin 373
+—— tertiary of Entre Rios 363
+—— of Banda Oriental 365
+—— volcanic, in Banda Oriental 367
+—— of Patagonia 381
+—— summary on 391
+—— tertiary of Tierra del Fuego 391
+—— —— of the Chonos archipelago 393
+—— —— of Chiloe 394
+—— —— of Chile 394
+—— —— of Concepcion 398, 404
+—— —— of Navidad 400
+—— —— of Coquimbo 402
+—— —— of Peru 404
+—— —— subsidence during 402
+—— volcanic, of Tres Montes 393
+—— —— of Chiloe 394
+—— —— old, near Maldonado 418
+—— —— with laminar structure 440
+—— —— ancient, in Tierra del Fuego 426
+—— recent, absent on S. American coast 409
+—— metamorphic, of claystone-porphyry of Patagonia 421, 440
+—— foliation of 436
+—— plutonic, with laminar structure 440
+—— palaeozoic, of the Falkland I. 424
+—— claystone, at Concepcion 433
+—— Jurassic, of Cordillera 512
+—— Neocomian, of the Portillo Pass 453
+—— volcanic, of Cumbre Pass 465
+—— gypseous, of Los Hornos 479, 487
+—— —— of Coquimbo 482
+—— —— of Guasco 487
+—— —— of Copiapo 488
+—— —— of Iquique 503
+—— cretaceo-oolitic, of Coquimbo 486, 495
+—— —— of Guasco 487, 494
+—— —— of Copiapo 495
+—— —— of Iquique 504
+ Fossils, Neocomian, of Portillo Pass 453
+—— —— of Cumbre Pass 465
+—— secondary, of Coquimbo 485
+—— —— of Guasco 487
+—— —— of Copiapo 494
+—— —— of Iquique 503
+—— palæozoic, from the Falklands 424
+ Fragments of hornblende-rock in gneiss 414
+—— of gneiss in gneiss 416
+ Freyer, Lieutenant, on elevated shells of Arica 323
+ Frezier on sea-level at Coquimbo 314
+
+ Galapagos archipelago, pseudo-dikes of 424
+ Gallegos, Port, tertiary formation of 390
+ Garnets in gneiss 415
+—— in mica-slate 427
+—— at Panuncillo 481
+ Gardichaud, M., on granites of Brazil 417
+ Gay, M., on elevated shells 306
+—— on boulders in the Cordillera 339, 341
+—— on fossils from Cordillera of Coquimbo 487
+ Gill, Mr., on brickwork transported by an earthquake-wave 327
+ Gillies, Dr., on heights in the Cordillera 448
+—— on extension of the Portillo range 458
+ Glen Roy, parallel roads of 319
+—— sloping terraces of 340
+ Gneiss, near Bahia 414
+—— of Rio de Janeiro 415
+—— decomposition of 417
+ Gold, distribution of 506
+ Gorodona, formations near 362
+ Granite, axis of oblique, to foliation 431
+—— andesitic 446
+—— of Portillo range 455
+—— veins of, quartzose 432, 475
+—— pebble of, in porphyritic conglomerate 493
+—— conglomerate 497
+ Grauwacke of Uspallata range 468
+ Gravel at bottom of sea 293, 298
+—— formation of, in Patagonia 295
+—— means of transportation of 298
+—— strata of, inclined 467
+ Gravel-terraces in Cordillera 337
+ Greenough, Mr., on quartz veins 437
+ Greenstone, resulting from metamorphose hornblende-rock 419
+—— of Tierra del Fuego 426
+—— on the summit of the Campana of Quillota 442
+—— porphyry 443
+—— relation of, to clay-slate 443
+ _Gryphæa orientalis_ 483
+ Guasco, elevation of 321
+—— secondary formation of 487
+ Guitru-gueyu, Sierra 421
+ Guyana, gneissic rocks of 415
+ Gypsum, nodules of, in gravel at Rio Negro 296
+—— deposited from sea-water 327
+—— deposits of, at Iquique 345
+—— crystals of, in salt lakes 346
+—— in Pampean formation 353
+—— in tertiary formation of Patagonia 382, , ,
+—— great formation of, in the Portillo Pass 461, 463
+—— —— in the Cumbre Pass 461, 463
+—— —— near Los Hornos 479
+—— —— at Coquimbo 482
+—— —— at Copiapo 490, 492
+—— —— near Iquique 504
+—— of San Lorenzo 504
+
+ Hall, Captain, on terraces at Coquimbo 316
+ Hamilton, Mr., on elevation near Tacna 323
+ Harlan, Dr., on human remains 279
+ Hayes, Mr. A., on nitrate of soda 346
+ Henslow, Professor, on concretions 437
+ Herbert, Captain, on valleys in the Himalaya 335
+ Herradura Bay, elevated shells of 315
+—— tertiary formations of 402
+ Himalaya, valleys in 335
+ _Hippurites Chilensis_ 483, 486
+ Hitchcock, Professor, on dikes 414
+ Honestones, pseudo, of Coquimbo 483
+—— of Copiapo 489
+ Hooker, Dr. J. D., on fossil beech-leaves 391
+ Hopkins, Mr., on axes of elevation oblique to foliation 432
+—— on origin of lines of elevation 440, 512
+ Hornblende-rock, fragments of, in gneiss 414
+ Hornblende-schist, near M. Video 420
+ Hornos, Los, section near 479
+ Hornstone, dike of 433, 434
+ Horse, fossil tooth of 358, 364
+ Huafo island 393, 404
+—— subsidence at 411
+ Huantajaya, mines of 503
+ Humboldt, on saline incrustations 347
+—— on foliations of gneiss 415
+—— on concretions in gneiss 435
+
+ Icebergs, action on cleavage 434, 436
+ Illapele, section near 479
+ Imperial, beds of shells near 305
+ Incrustations, saline 347
+ Infusoria in Pampean formation 352, 355, 360, 363
+—— in Patagonian formation 382, 383, 384, 391
+ Iodine, salts of 347, 348
+ Iquique, elevation of 322
+—— saliferous deposits of 344
+—— cretaceo-oolitic formation of 503
+ Iron, oxide of, in lavas 463, 499
+—— in sedimentary beds 480, 482
+—— tendency in, to produce hollow concretions 398
+—— sulphate of 489
+ Isabelle, M., on volcanic rocks of Banda Oriental 368
+
+ Joints in clay-slate 428
+ Jukes, Mr., on cleavage in Newfoundland 437
+
+ Kamtschatka, andesite of 347
+ Kane, Dr., on the production of carbonate of soda 328
+ King George’s sound, calcareous beds of 312
+
+ Lakes, origin of 300
+—— fresh-water, near salt lakes 350
+ Lava, basaltic, of S. Cruz 389
+—— claystone-porphyry, at Chiloe 395
+—— —— ancient submarine 446
+—— basaltic, of the Portillo range 457
+—— feldspathic, of the Cumbre Pass 463
+—— submarine, of the Uspallata range 471, 473, 476
+—— basaltic, of the Uspallata range 475
+—— submarine, of Coquimbo 484, 486
+—— of Copiapo 490, 496, 499
+ Lemus island 393, 404
+ Lemuy islet 394
+ Lignite of Chiloe 395
+—— of Concepcion 398
+ Lima, elevation of 323
+ Lime, muriate of 328, 344, 347
+ Limestone of Cumbre Pass 462
+—— of Coquimbo 483, 485
+—— of Copiapo 493
+ Lund and Clausen on remains of caves in Brazil 378, 380
+ Lund, M., on granites of Brazil 417
+ Lyell, M., on upraised shells retaining their colours 289
+—— on terraces at Coquimbo 315
+—— on elevation near Lima 327
+—— on fossil horse’s tooth 364
+—— on the boulder-formation being anterior to the extinction of North
+American mammifers 371
+—— on quadrupeds washed down by floods 374
+—— on age of American fossil mammifers 379
+—— on changes of climate 409
+—— on denudation 410
+—— on foliation 438
+
+ MacCulloch, Dr., on concretions 437
+—— on beds of marble 440
+ Maclaren, Mr., letter to, on coral-formations 413
+ _Macrauchenia Patachonica_ 358, 370
+ Madeira, subsidence of 302
+ Magellan, Strait, elevation near, of 288
+ Magnesia, sulphate of, in veins 387
+ Malcolmson, Dr., on trees carried out to sea 475
+ Maldonado, elevation of 277
+—— Pampean formation of 365
+—— crystalline rocks of 418
+ Mammalia, fossil, of Bahia Blanca 356, 364
+—— —— near St. Fé 363
+—— —— of Banda Oriental 366
+—— —— of St. Julian 369
+—— —— at Port Gallegos 391
+—— washed down by floods 373
+—— number of remains of, and range of, in Pampas 376
+ Man, skeletons of (Brazil) 279
+—— remains of, near Lima 325
+—— Indian, antiquity of 325
+ Marble, beds of 418
+ Maricongo, ravine of 500
+ Marsden, on elevation of Sumatra 305
+ _Mastodon Andium_, remains of 362
+—— range of 378
+ Maypu, Rio, mouth of, with upraised shells 307
+—— gravel fringes of 339
+—— debouchement from the Cordillera 449
+ Megalonyx, range of 379
+ Megatherium, range of 379
+ Miers, Mr., on elevated shells 311
+—— on the height of the Uspallata plain 335
+ Minas, Las 418
+ Mocha Island, elevation of 305
+—— tertiary form of 398
+—— subsidence at 411
+ Molina, on a great flood 341
+ Monte Hermoso, elevation of 280
+—— fossils of 355
+ Monte Video, elevation of 278
+—— Pampean formation of 365
+—— crystalline rocks of 419
+ Morris and Sharpe, Messrs., on the palæozoic fossils of the Falklands
+ 424
+ Mud, Pampean 352
+—— long deposited on the same area 376
+ Murchison, Sir R., on cleavage 436
+—— on waves transporting gravel 299
+—— on origin of salt formations 505
+—— on the relations of metalliferous veins and intrusive rocks 507
+—— on the absence of granite in the Ural 512
+
+ _Nautilus d’Orbignyanus_ 400, 405
+ Navidad, tertiary formations of, subsidence of 400, 411
+ Negro, Rio, pumice of pebbles of 281
+—— gravel of 295
+—— salt lakes of 295
+—— tertiary strata of 384
+ North America, fossil remains of 379
+ North Wales, sloping terraces absent in 340
+—— bent cleavage of 434
+ Neuvo Gulf, plains of 282
+—— tertiary formation of 384
+
+ Owen, Professor, on fossil mammiferous remains 356, 358, 364, 366, 370
+
+ Palmer, Mr., on transportation of gravel 300
+ Pampas, elevation of 290
+—— earthquakes of 290
+—— formation of 295, 350
+—— localities in which fossil mammifers have been found 380
+ Panuncillo, mines of 481
+ Parana, Rio, on saline incrustations 347
+—— Pampean formations near 361
+—— on the S. Tandil 420
+ Parish, Sir W., on elevated shells near Buenos Ayres 278, 279
+—— on earthquakes in the Pampas 290
+—— on fresh-water near salt lakes 350
+—— on origin of Pampean formation 373
+ Patagonia, elevation and plains of 281
+—— denudation of 291
+—— gravel-formation of 295
+—— sea-cliffs of 301
+—— subsidence during tertiary period 411
+—— crystalline rocks of 421
+ Payta, tertiary formations of 404
+ Pebbles of pumice 280
+—— decrease in size on the coast of Patagonia 293
+—— means of transportation 298
+—— encrusted with living corallines 299
+—— distribution of, at the eastern foot of Cordillera 337
+—— dispersal of, in the Pampas 354
+—— zoned with colour 443
+ Pentland, Mr., on heights in the Cordillera 460
+—— on fossils of the Cordillera 465
+ Pernambuco 279
+ Peru, tertiary formations of 403
+ Peuquenes, Pass of, in the Cordillera 448
+—— ridge of 452
+ Pholas, elevated shells of 303
+ Pitchstone of Chiloe 395
+—— of Port Desire 421
+—— near Cauquenes 448
+—— layers of, in the Uspallata range 472
+—— of Los Hornos 480
+—— of Coquimbo 483
+ Plains of Patagonia 282, 291
+—— of Chiloe 304
+—— of Chile 333
+—— of Uspallata 335
+—— on eastern foot of Cordillera 336
+—— of Iquique 346
+ Plata, La, elevation of 277
+—— tertiary formation of 295, 353
+—— crystalline rocks of 418
+ Playfair, Professor, on the transportation of gravel 300
+ Pluclaro, axis of 483
+ Pondicherry, fossils of 400
+ Porcelain rocks of Port Desire 422
+—— of the Uspallata range 471, 473, 476
+ Porphyry, pebbles of, strewed over Patagonia 296
+ Porphyry, claystone, of Chiloe 395
+—— —— of Patagonia 421
+—— —— of Chile 442, 445
+—— greenstone, of Chile 444
+—— doubly columnar 448
+—— claystone, rare, on the eastern side of the Portillo Pass 454
+—— brick-red and orthitic, of Cumbre Pass 458, 467
+—— intrusive, repeatedly injected 467
+—— claystone of the Uspallata range 468
+—— —— of Copiapo 489, 499
+—— —— eruptive sources of 502
+ Port Desire, elevation and plains of 283
+—— tertiary formation of 383
+—— porphyries of 421
+ Portillo Pass in the Cordillera 448
+ Portillo chain 454, 458
+—— compared with that of the Uspallata 478
+ Prefil or sea-wall of Valparaiso 310
+ Puente del Inca, section of 461
+ Pumice, pebbles of 230
+—— conglomerate of R. Negro 382
+—— hills of, in the Cordillera 347
+ Punta Alta, elevation of 280
+—— beds of 356
+
+ Quartz-rock of the S. Ventana 421
+—— C. Blanco 421
+—— Falkland islands 424
+—— Portillo range 455
+—— viscidity of 475
+—— veins of, near Monte Video 420
+—— —— in dike of greenstone 426
+—— grains of, in mica slate 430
+—— —— in dikes 432, 434
+—— veins of, relations to cleavage 437
+ Quillota, Campana of 442
+ Quintero, elevation of 311
+ Quiriquina, elevation of 306
+—— deposits of 399
+
+ Rancagua, plain of 334
+ Rapel, R., elevation near 307
+ Reeks, Mr. T., his analysis of decomposed shells 328
+—— his analysis of salts 344
+ Remains, human 324
+ Rio de Janeiro, elevation near 279
+—— crystalline rocks of 415
+ Rivers, small power of transporting pebbles 298
+—— small power of, in forming valleys 343
+—— drainage of, in the Cordillera 449, 513
+ Roads, parallel, of Glen Roy 319
+ Rocks, volcanic, of Banda Oriental 367
+—— Tres Montes 393
+—— Chiloe 394
+—— Tierra del Fuego 426
+—— with laminar structure 440
+ Rodents, fossil, remains of 356
+ Rogers, Professor, address to Association of American Geologists 412
+ Rose, Professor G., on sulphate of iron at Copiapo 489
+
+ S. Blas, elevation of 281
+ S. Cruz, elevation and plains of 284
+—— valley of 285
+—— nature of gravel in valley of 296
+—— boulder formation of 371
+—— tertiary formation of 386
+—— subsidence at 412
+ S. Fé Bajada, formations of 363
+ S. George’s bay, plains of 282
+ S. Helena island, sea-cliffs, and subsidence of 301
+ S. Josef, elevation of 281
+—— tertiary formation of 383
+ S. Juan, elevation near 278
+ S. Julian, elevation and plains of 284
+—— salt lake of 348
+—— earthy deposit with mammiferous remains 369
+—— tertiary formations of 384
+—— subsidence at 411
+ S. Lorenzo, elevation of 323
+—— old salt formation of 504
+ S. Mary, island of, elevation of 305
+ S. Pedro, elevation of 278
+ Salado, R., elevated shells of 279
+—— Pampean formation of 353
+ Salines 348
+ Salt, with upraised shell 324, 327
+—— lakes of 348
+—— purity of, in salt lakes 349
+—— deliquescent, necessary for the preservation of meat 349
+—— ancient formation of, at Iquique 504
+—— —— at S. Lorenzo 504
+—— strata of, origin of 505
+ Salts, superficial deposits of 344
+ Sand-dunes of the Uruguay 279
+—— of the Pampas 281
+—— near Bahia Blanca 281, 293
+—— of the Colorado 281, 294
+—— of S. Cruz 286
+—— of Arica 323
+ Sarmiento, Mount 427
+ Schmidtmeyer on auriferous detritus 506
+ Schomburghk, Sir R., on sea-bottom 299
+—— on the rocks of Guyana 415
+ Scotland, sloping terraces of 340
+ Sea, nature of bottom of, off Patagonia 292
+—— power of, in forming valleys 343
+ Sea cliffs, formation of 301
+ Seale, Mr., model of St. Helena 301
+ Sebastian Bay, tertiary formation of 391
+ Sedgwick, Professor, on cleavage 336
+ Serpentine of Copiapo 489
+ Serpulæ, on upraised rocks 325
+ Shale-rock, of the Portillo Pass 452
+—— of Copiapo 493
+ Shells, upraised state of, in Patagonia 288
+—— elevated, too small for human food 308
+—— transported far inland, for food 309
+—— upraised, proportional numbers varying 312, 324
+—— —— gradual decay of 323, 324, 327
+—— —— absent on high plains of Chile 335
+—— —— near Bahia Blanca 358
+—— preserved in concretions 394, 397
+—— living and fossil range of, on west coast 406, 408
+—— living, different on the east and west coast 411
+ Shingle of Patagonia 295
+ Siau, M., on sea-bottom 299
+ Silver mines of Arqueros 431
+—— of Chanuncillo 494
+—— of Iquique 503
+—— distribution of 506
+ Slip, great, at S. Cruz 387
+ Smith, Mr., of Jordan Hill, on upraised shells retaining their colours
+ 289
+—— on Madeira 302
+—— on elevated seaweed 325
+—— on inclined gravel beds 467
+ Soda, nitrate of 346
+—— sulphate of, near Bahia Blanca 348, 349
+—— carbonate of 347
+ Soundings off Patagonia 293, 299
+—— in Tierra del Fuego 300
+ Spirifers 486, 488
+ Spix and Martius on Brazil 417
+ Sprengel on the production of carbonate of soda 328
+ Springs, mineral, in the Cumbre Pass 461
+ Stratification of sandstone in metamorphic rocks 414
+—— of clay-slate in Tierra del Fuego 428
+—— of the Cordillera of Central Chile 442, 448, 461
+—— little disturbed in Cumbre Pass 460, 466
+—— disturbance of, near Copiapo 501
+ Streams of lava at S. Cruz, inclination of 390
+—— in the Portillo range 457
+ String of cotton with fossil-shells 325
+ _Struthiolaria ornata_ 392
+ Studer, M., on metamorphic rocks 438
+ Subsidence during formation of sea-cliffs 301
+—— near Lima 327
+—— probable, during Pampean formation 376
+—— necessary for the accumulation of permanent deposits 411
+—— during the tertiary formations of Chile and Patagonia 413
+—— probable during the Neocomian formation of the Portillo Pass 453
+—— probable during the formation of conglomerate of Tenuyan 459
+—— during the Neocomian formation of the Cumbre Pass 465
+—— of the Uspallata range 474, 477
+—— great, at Copiapo 496
+—— —— during the formation of the Cordillera 510
+ Sulphur, volcanic exhalations of 509
+ Sumatra, promontories of 305
+ Summary on the recent elevatory movements 259, 329, 514
+—— on the Pampean formation 371, 515
+—— on the tertiary formations of Patagonia and Chile 391, 404, 513
+—— on the Chilean Cordillera 508
+—— on the cretaceo-oolitic formation 508
+—— on the subsidences of the Cordillera 509
+—— on the elevation of the Cordillera 511, 517
+
+ Tacna, elevation of 323
+ Tampico, elevated shells near 329
+ Tandil, crystalline rocks of 420
+ Tapalguen, Pampean formation of 353
+—— crystalline rocks of 420
+ Taylor, Mr., on copper veins of Cuba 506
+ Temperature of Chile during the tertiary period 408
+ Tension, lines of, origin of, axes of elevation and of cleavage 440
+ Tenuy Point, singular section of 395
+ Tenuyan, valley of 454, 478
+ Terraces of the valley of S. Cruz 286
+—— of equable heights throughout Patagonia 290
+—— of Patagonia, formation of 294
+—— of Chiloe 304
+—— at Conchalee 311
+—— of Coquimbo 316
+—— not horizontal at Coquimbo 317
+—— of Guasco 320
+—— of S. Lorenzo 323
+—— of gravel within the Cordillera 337
+ Theories on the origin of the Pampean formation 372
+ Tierra Amarilla 489
+ Tierra del Fuego, form of sea-bottom 300
+—— tertiary formations of 391
+—— clay-slate formation of 424
+—— cretaceous formation of 426
+—— crystalline rocks of 426
+—— cleavage of clay-slate 427, 436
+ Tosca rock 352
+ Trachyte of Chiloe 394
+—— of Port Desire 421
+—— in the Cordillera 347
+ Traditions of promontories having been islands 305
+—— on changes of level near Lima 327
+ Trees buried in plain of Iquique 346
+—— silicified, vertical, of the Uspallata range 473
+ Tres Montes, elevation of 303
+—— volcanic rocks of 393
+ _Trigonocelia insolita_ 392
+ Tristan Arroyo, elevated shells of 278
+ Tschudi, Mr., on subsidence near Lima 327
+ Tuff, calcareous, at Coquimbo 313
+—— on basin-plain near St. Jago 334
+—— structure of, in Pampas 352
+—— origin of, in Pampas 374
+—— pumiceous, of R. Negro 382
+—— Nuevo Gulf 383
+—— Port Desire 383
+—— S. Cruz 386
+—— Patagonia, summary on Chiloe 391
+—— formation of, in Portillo chain 395
+—— great deposit of, at Copiapo 457
+ Tuffs, volcanic, metamorphic, of Uspallata 471
+—— of Coquimbo 484
+
+ Ulloa, on rain in Peru 324
+—— on elevation near Lima 327
+ Uruguay, Rio, elevation of country near 278
+ Uspallata, plain of 335, 515
+—— pass of 459
+—— range of 368
+—— concluding remarks on 476
+
+ Valdivia, tertiary beds of 398
+—— mica-slate of 433
+ Valley of S. Cruz, structure of 285
+—— Coquimbo 314
+—— Guasco, structure of 320
+—— Copiapo, structure of 321
+—— S. Cruz, tertiary formations of 386
+—— Coquimbo, geology of 482
+—— Guasco, secondary formations of 487
+—— Copiapo, secondary formations of 488
+—— Despoblado 496, 497, 499
+ Valleys in the Cordillera bordered by gravel fringes 337
+—— formation of 338
+—— in the Cordillera 449
+ Valparaiso, elevation of 307
+—— gneiss of 435
+ Vein of quartz near Monte Video 419
+—— in mica-slate 430
+—— relations of, to cleavage 437
+—— in a trap dike 426
+—— of granite, quartzose 432, 475
+—— remarkable, in gneiss, near Valparaiso 435
+ Veins, relations of, to concretions 396
+—— metalliferous, of the Uspallata range 475
+—— metalliferous, discussion on 505
+ Venezuela, gneissic rocks of 415
+ Ventana, Sierra, Pampean formation near 353
+—— quartz-rock of 421
+ Villa Vincencio Pass 468
+ Volcan, Rio, mouth of 449
+—— fossils of 453
+ Volcanoes of the Cordillera 392, 447, 511
+—— absent, except near bodies of water 457
+—— ancient submarine, in Cordillera 502
+—— action of, in relation to changes of level 514
+—— long action of, in the Cordillera 517
+
+ Wafer on elevated shells 322
+ Waves caused by earthquakes, power of, in transporting boulders 326,
+ 344
+—— power of, in throwing up shells 309
+ Weaver, Mr., on elevated shells 329
+ White, Martin, on sea-bottom 299
+ Wood, silicified, of Entre Rios 364
+—— S. Cruz 388
+—— Chiloe 394, 396
+—— Uspallata range 473
+—— Los Hornos 479
+—— Copiapo 495, 497
+
+ Yeso, Rio, and plain of 450
+ Ypun Island, tertiary formation of 393
+
+ Zeagonite 426
+
+
+
+
+End of the Project Gutenberg EBook of Coral Reefs, Volcanic Islands, South American Geology, by Charles Darwin
+
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