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| author | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:22:45 -0700 |
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| committer | Roger Frank <rfrank@pglaf.org> | 2025-10-15 05:22:45 -0700 |
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diff --git a/4022-h/4022-h.htm b/4022-h/4022-h.htm new file mode 100644 index 0000000..33956ff --- /dev/null +++ b/4022-h/4022-h.htm @@ -0,0 +1,34666 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" +"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> +<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> +<head> +<meta http-equiv="Content-Type" content="text/html;charset=utf-8" /> +<meta http-equiv="Content-Style-Type" content="text/css" /> +<title>Coral Reefs, Volcanic Islands, South American Geology, by Charles Darwin</title> + +<style type="text/css"> + +body { margin-left: 20%; + margin-right: 20%; + text-align: justify; } + +h1, h2, h3, h4, h5 {text-align: center; font-style: normal; font-weight: +normal; line-height: 1.5; margin-top: .5em; margin-bottom: .5em;} + +h1 {font-size: 300%; + margin-top: 0.6em; + margin-bottom: 0.6em; + letter-spacing: 0.12em; + word-spacing: 0.2em; + text-indent: 0em;} +h2 {font-size: 150%; margin-top: 2em; margin-bottom: 1em;} +h3 {font-size: 150%; margin-top: 2em;} +h4 {font-size: 120%;} +h5 {font-size: 110%;} + +hr {width: 80%; margin-top: 2em; margin-bottom: 2em;} + +div.chapter {page-break-before: always; margin-top: 4em;} + +p {text-indent: 1em; + margin-top: 0.25em; + margin-bottom: 0.25em; } + +.p2 {margin-top: 2em;} + +p.poem {text-indent: 0%; + margin-left: 10%; + font-size: 90%; + margin-top: 1em; + margin-bottom: 1em; } + +p.letter {text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +p.noindent {text-indent: 0% } + +p.center {text-align: center; + text-indent: 0em; + margin-top: 1em; + margin-bottom: 1em; } + +p.right {text-align: right; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +p.footnote {font-size: 90%; + text-indent: 0%; + margin-left: 10%; + margin-right: 10%; + margin-top: 1em; + margin-bottom: 1em; } + +sup { vertical-align: top; font-size: 0.6em; } + +div.fig { display:block; + margin:0 auto; + text-align:center; + margin-top: 1em; + margin-bottom: 1em;} + +a:link {color:blue; text-decoration:none} +a:visited {color:blue; text-decoration:none} +a:hover {color:red} + +</style> + +</head> + +<body> + +<pre> +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 + + + +</pre> + +<h1>Coral Reefs, Volcanic Islands, South American Geology</h1> + +<h2>by Charles Darwin</h2> + +<hr /> + +<p class="center"> +EDITORIAL NOTE +</p> + +<p> +Although in some respects more technical in their subjects and style than +Darwin’s “Journal,” the books here reprinted will never lose their value and +interest for the originality of the observations they contain. Many parts of +them are admirably adapted for giving an insight into problems regarding the +structure and changes of the earth’s surface, and in fact they form a charming +introduction to physical geology and physiography in their application to +special domains. The books themselves cannot be obtained for many times the +price of the present volume, and both the general reader, who desires to know +more of Darwin’s work, and the student of geology, who naturally wishes to know +how a master mind reasoned on most important geological subjects, will be glad +of the opportunity of possessing them in a convenient and cheap form. +</p> + +<p> +The three introductions, which my friend Professor Judd has kindly furnished, +give critical and historical information which makes this edition of special +value. +</p> + +<p class="right"> +G.T.B. +</p> + +<h2>CONTENTS</h2> + +<p class="center"> +<a href="#vol01"><b>THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.</b></a> +</p> + +<p> +<a href="#page3">C<small>RITICAL</small> I<small>NTRODUCTION</small></a> +</p> + +<p> +<a href="#page11">I<small>NTRODUCTION</small></a> +</p> + +<p> +<a href="#chap01">Chapter I—ATOLLS OR LAGOON-ISLANDS.</a> +</p> + +<p> +<a href="#sec01"><i>Section I</i>—DESCRIPTION OF KEELING ATOLL.</a><br/> +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. +</p> + +<p> +<a href="#sec02"><i>Section II</i>—GENERAL DESCRIPTION OF +ATOLLS.</a><br/> 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. +</p> + +<p> +<a href="#sec03"><i>Section III</i>—ATOLLS OF THE MALDIVA +ARCHIPELAGO—GREAT CHAGOS BANK.</a><br/> 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. +</p> + +<p> +<a href="#chap02">Chapter II—BARRIER REEFS.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#chap03">Chapter III—FRINGING OR SHORE-REEFS.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#chap04">Chapter IV—ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS.</a> +</p> + +<p> +<a href="#sec04"><i>Section I</i>—ON THE DISTRIBUTION OF CORAL-REEFS, +AND ON THE CONDITIONS FAVOURABLE TO THEIR INCREASE.</a> +</p> + +<p> +<a href="#sec05"><i>Section II</i>—ON THE RATE OF GROWTH OF CORAL-REEFS.</a> +</p> + +<p> +<a href="#sec06"><i>Section III</i>—ON THE DEPTHS AT WHICH REEF-BUILDING +POLYPIFERS CAN LIVE.</a> +</p> + +<p> +<a href="#chap05">Chapter V—THEORY OF THE FORMATION OF THE DIFFERENT +CLASSES OF CORAL-REEFS.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#chap06">Chapter VI—ON THE DISTRIBUTION OF CORAL-REEFS WITH +REFERENCE TO THE THEORY OF THEIR FORMATION.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#appendix">Appendix</a> +</p> + +<p> +Containing a detailed description of the reefs and islands in Plate III. +</p> + +<p class="center"> +<a href="#vol02"><b>GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.</b></a> +</p> + +<p> +<a href="#page157">C<small>RITICAL</small> I<small>NTRODUCTION</small></a> +</p> + +<p> +<a href="#chap2.01">Chapter I—ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#chap2.02">Chapter II—FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.</a> +</p> + +<p> +F<small>ERNANDO</small> N<small>ORONHA</small>.—Precipitous hill of +phonolite. T<small>ERCEIRA</small>.—Trachytic rocks: their singular +decomposition by steam of high temperature. +T<small>AHITI</small>.—Passage from wacke into trap; singular volcanic +rock with the vesicles half-filled with mesotype. +M<small>AURITIUS</small>.—Proofs of its recent elevation.—Structure +of its more ancient mountains; similarity with St. Jago. S<small>T.</small> +P<small>AUL’S</small> R<small>OCKS</small>.—Not of volcanic +origin.—Their singular mineralogical composition. +</p> + +<p> +<a href="#chap2.03">Chapter III—ASCENSION.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#chap2.04">Chapter IV—ST. HELENA.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#chap2.05">Chapter V—GALAPAGOS ARCHIPELAGO.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#chap2.06">Chapter VI—TRACHYTE AND BASALT.—DISTRIBUTION OF +VOLCANIC ISLES.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#chap2.07">Chapter VII—AUSTRALIA; NEW ZEALAND; CAPE OF GOOD +HOPE.</a> +</p> + +<p> +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. +</p> + +<p class="center"> +<a href="#vol03"><b>GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA.</b></a> +</p> + +<p> +<a href="#page269">C<small>RITICAL</small> I<small>NTRODUCTION</small></a> +</p> + +<p> +<a href="#chap3.01"> +Chapter I—ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.</a> +</p> + +<p> +Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and +Pumice-pebbles.—Step-formed plains of Patagonia, with upraised +shells.—Terrace-bounded valley of Santa Cruz, formerly a +sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth +of the elevated area.—Equability of the movements, as shown by the +similar heights of the plains.—Slowness of the elevatory +process.—Mode of formation of the step-formed +plains.—Summary.—Great shingle formation of Patagonia; its extent, +origin, and distribution.—Formation of sea-cliffs. +</p> + +<p> +<a href="#chap3.02"> +Chapter II—ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.</a> +</p> + +<p> +Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions +of the inhabitants on this subject.—Concepcion, earthquake and elevation +of.—V<small>ALPARAISO</small>, great elevation of, upraised shells, earth +or marine origin, gradual rise of the land within the historical +period.—C<small>OQUIMBO</small>, elevation of, in recent times; terraces +of marine origin, their inclination, their escarpments not +horizontal.—Guasco, gravel terraces +of.—Copiapo.—P<small>ERU</small>.—Upraised shells of Cobija, +Iquique, and Arica.—Lima, shell-beds and sea-beach on San +Lorenzo.—Human remains, fossil earthenware, earthquake debacle, recent +subsidence.—On the decay of upraised shells.—General summary. +</p> + +<p> +<a href="#chap3.03"> +Chapter III—ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS +SUPERFICIAL DEPOSITS.</a> +</p> + +<p> +Basin-like plains of Chile; their drainage, their marine origin.—Marks of +sea-action on the eastern flanks of the Cordillera.—Sloping terrace-like +fringes of stratified shingle within the valleys of the Cordillera; their +marine origin.—Boulders in the valley of Cachapual.—Horizontal +elevation of the Cordillera.—Formation of valleys.—Boulders moved +by earthquake-waves.—Saline superficial deposits.—Bed of nitrate of +soda at Iquique.—Saline incrustations.—Salt-lakes of La Plata and +Patagonia; purity of the salt; its origin. +</p> + +<p> +<a href="#chap3.04"> +Chapter IV—ON THE FORMATIONS OF THE PAMPAS.</a> +</p> + +<p> +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. B<small>ANDA</small> +O<small>RIENTAL</small>.—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. S<small>UMMARY</small>.—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. +</p> + +<p> +<a href="#chap3.05"> +Chapter V—ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#chap3.06"> +Chapter VI—PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#chap3.07"> +Chapter VII—CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.</a> +</p> + +<p> +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. +</p> + +<p> +<a href="#chap3.08"> +Chapter VIII—NORTHERN CHILE.—CONCLUSION.</a> +</p> + +<p> +Section from Illapel to Combarbala; gypseous formation with silicified +wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley; +fossils.—Guasco, fossils of.—Copiapo, section up valley; Las +Amolanas, silicified wood.—Conglomerates, nature of former land, fossils, +thickness of strata, great subsidence.—Valley of Despoblado, fossils, +tufaceous deposit, complicated dislocations of.—Relations between ancient +orifices of eruption and subsequent axes of injection.—Iquique, Peru, +fossils of, salt-deposits.—Metalliferous veins.—Summary on the +porphyritic conglomerate and gypseous formations.—Great subsidence with +partial elevations during the cretaceo-oolitic period.—On the elevation +and structure of the Cordillera.—Recapitulation on the tertiary +series.—Relation between movements of subsidence and volcanic +action.—Pampean formation.—Recent elevatory +movements.—Long-continued volcanic action in the +Cordillera.—Conclusion. +</p> + +<p> +<a href="#index01"><b>Index to “Coral-Reefs”</b></a> +</p> + +<p> +<a href="#index02"><b>Index to “Volcanic Islands”</b></a> +</p> + +<p> +<a href="#index03"><b>Index to “South American Observations”</b></a> +</p> + +<hr /> + +<div class="chapter"> + +<h2><a name="vol01"></a>THE STRUCTURE AND DISTRIBUTION OF CORAL REEFS.</h2> + +</div><!--end chapter--> + +<div class="chapter"> + +<p class="center"> +<a name="page3"></a><b>CRITICAL INTRODUCTION</b> +</p> + +<p>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.</p> + +<p>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.</p> + +<p> +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 +<a name="page4"></a> +words of Flinders, they constitute breastworks, defending the +workshops from whence “infant colonies might be safely sent +forth.” +</p> + +<p>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.</p> + +<p>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.’”</p> + +<p>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.</p> + +<p>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 +<a name="page5"></a> +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.”</p> + +<p>On her homeward voyage, the <i>Beagle</i> 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.</p> + +<p>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 <i>Beagle</i>. 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.</p> + +<p>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 +<a name="page6"></a> +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.</p> + +<p>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.</p> + +<p>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 <i>Beagle</i> 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 +<a name="page7"></a> +elevation and subsidence—topics which a wide range of +travel over the Pacific brought directly and constantly to my +attention.”</p> + +<p>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.</p> + +<p>Another able exponent and illustrator of the theory of +coral-reefs was found in Professor J. B. Jukes, who accompanied +H.M.S. <i>Fly</i>, 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.”</p> + +<p>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.</p> + +<p>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 +<a name="page8"></a> +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.</p> + +<p>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.</p> + +<p>The most serious opposition to Darwin’s coral-reef theory, +however, was that which developed itself after the return of H.M.S. +<i>Challenger</i> 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.</p> + +<p>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 +<a name="page9"></a> +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.</p> + +<p>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 <i>Beagle</i>, 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.”</p> + +<p>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.</p> + +<p>“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.”</p> + +<p>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 +<a name="page10"></a> +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.”</p> + +<p>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.</p> + +<p>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.</p> + +<p>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.</p> + +<p class="right"> +J<small>OHN</small> W. J<small>UDD</small>. +</p> + +<hr /> + +<h2><a name="page11"></a>CORAL-REEFS</h2> + +<hr /> + +<h3>INTRODUCTION</h3> + +<p>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.</p> + +<p>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 +<a name="page12"></a> +inhabitants in the Indian Ocean, and is synonymous with “lagoon- +island.”</p> + +<div class="fig" style="width:100%;"> +<img src="images/p12a.jpg" width="539" height="221" alt="[Illustration: +Whitsunday Island]" /> +</div> + +<p> +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 <i>Coquille</i>,” the reef +is seen from within, from one of the high peaks of the island of Bolabola.<a +href="#fn-1" name="fnref-1" id="fnref-1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-1" id="fn-1"></a> <a href="#fnref-1">[1]</a> +I have taken the liberty of simplifying the foreground, and leaving out a +mountainous island in the far distance. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/p12b.jpg" width="531" height="239" alt="[Illustration: +Island of Bolabola]" /> +</div> + +<p>With respect to fringing, or shore-reefs, there is little in +their structure which needs explanation; and their name expresses +their comparatively +<a name="page13"></a> +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.</p> + +<p> +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<a href="#fn-2" name="fnref-2" +id="fnref-2"><sup>[2]</sup></a> 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. +</p> + +<p> +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.<a +href="#fn-3" name="fnref-3" id="fnref-3"><sup>[3]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-2" id="fn-2"></a> <a href="#fnref-2">[2]</a> +Replaced by numbers in this edition. +</p> + +<p class="footnote"> +<a name="fn-3" id="fn-3"></a> <a href="#fnref-3">[3]</a> +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. +</p> + +<p> +<a name="page14"></a> +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 <i>annular +reef</i>, 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.</p> + +<p class="center"><a name="PlateI"><i>Plate I</i></a>—Map +showing the resemblance in form between barrier coral-reefs +surrounding mountainous islands, and atolls or lagoon islands.</p> + +<div class="fig" style="width:100%;"> +<img src="images/plate1a.jpg" width="361" height="291" alt="[Illustration: +Map showing the resemblance in form.]" /> +</div> + +<p class="capt">Fig. 1—V<small>ANIKORO</small>, situated in +the western part of the South Pacific; taken from the survey by +Captain D’Urville in the <i>Astrolabe</i>; 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.</p> + +<p class="capt">Fig. 2—H<small>OGOLEU</small>, or +R<small>OUG</small>, in the Caroline Archipelago; taken from the +“Atlas of the Voyage of the <i>Astrolabe,</i>” compiled from the +surveys of Captains Duperrey and D’Urville; the depth of the +immense lagoon-like space within the reef is not known.</p> + +<p class="capt">Fig. 3—R<small>AIATEA</small>, in the Society +Archipelago; from the map given in the quarto edition of “Cook’s +First Voyage;” it is probably not accurate.</p> + +<p class="capt">Fig. 4—B<small>OW</small>, or H<small>EYOU +ATOLL</small> (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.</p> + +<p class="capt">Fig. 5—B<small>OLABOLA</small>, in the +Society Archipelago, from the survey of Captain Duperrey in the <i> +Coquille</i>: 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.</p> + +<div class="fig" style="width:100%;"> +<img src="images/plate1b.jpg" width="395" height="396" alt="[Illustration: +Map showing the resemblance in form.]" /> +</div> + +<p class="capt">Fig. 6.—M<small>AURUA</small>, in the Society +Archipelago; from the survey by Captain Duperrey in the <i> +Coquille</i>: height of land about eight hundred feet.</p> + +<p class="capt">Fig. 7—P<small>OUYNIPÈTE</small>, or +S<small>ENIAVINE</small>, in the Caroline Archipelago; from the +survey by Admiral Lutké.</p> + +<p class="capt">Fig. 8—G<small>AMBIER</small> +I<small>SLANDS</small>, 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.</p> + +<p class="capt">Fig. 9—P<small>EROS</small> B<small>ANHOS +ATOLL</small> (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.</p> + +<p class="capt">Fig. 10—K<small>EELING</small>, or +C<small>OCOS ATOLL</small> (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.</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="page15"></a><a name="chap01"></a>Chapter I<br/>ATOLLS OR LAGOON-ISLANDS</h2> + +<h3><a name="sec01"></a><i>Section I</i>—KEELING ATOLL</h3> + +<p class="letter"> +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. +</p> + +<p>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. <i>Beagle</i>, is given in <a href="#PlateI">Plate I</a>, +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.</p> + +<div class="fig" style="width:100%;"> +<img src="images/keeling.jpg" width="443" height="93" alt="[Illustration: +Vertical section of one of the low islets.]" /> +</div> + +<p class="letter"> +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.<br/> +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.<br/> + C.—A flat of coral-rock, covered at high water.<br/> + D.—A low projecting ledge of brecciated coral-rock washed by +the waves at high water.<br/> + 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.<br/> + F.—Level of the lagoon at low water. + </p> + +<p> +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. +<a name="page16"></a> +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.</p> + +<p> +Next, but much inferior in importance to the Porites, is the <i> Millepora +complanata.</i><a href="#fn-1.1" name="fnref-1.1" +id="fnref-1.1"><sup>[1]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-1.1" id="fn-1.1"></a> <a href="#fnref-1.1">[1]</a> +This Millepora (Palmipora of Blainville), as well as the <i>M. alcicornis</i>, +possesses the singular property of stinging the skin where it is delicate, as +on the face and arm. +</p> + +<p>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.</p> + +<p> +<a name="page17"></a> +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°.<a href="#fn-1.2" name="fnref-1.2" +id="fnref-1.2"><sup>[2]</sup></a> 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 <i> Millepora alcicornis</i> 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 <i>M. corymbosa</i>, 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 <i>Millepora +alcicornis</i> 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. +</p> + +<p class="footnote"> +<a name="fn-1.2" id="fn-1.2"></a> <a href="#fnref-1.2">[2]</a> +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. +</p> + +<p>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 +<a name="page18"></a> +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.</p> + +<p> +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.<a href="#fn-1.3" name="fnref-1.3" id="fnref-1.3"><sup>[3]</sup></a> +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. +</p> + +<p class="footnote"> +<a name="fn-1.3" id="fn-1.3"></a> <a href="#fnref-1.3">[3]</a> +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. +</p> + +<p>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 +<a name="page19"></a> +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 <i>Pocillopora verrucosa</i>, 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 <i>M. +pocillifera</i>, 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.</p> + +<p>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: +<a name="page20"></a> +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.</p> + +<p> +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 <i>Seriatapora subulata</i>; two species of +Porites<a href="#fn-1.4" name="fnref-1.4" id="fnref-1.4"><sup>[4]</sup></a> +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 +<a name="page21"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-1.4" id="fn-1.4"></a> <a href="#fnref-1.4">[4]</a> +This Porites has somewhat the habit of <i>P. clavaria</i>, 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. +</p> + +<p>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.</p> + +<p>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 +<a name="page22"></a> +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.</p> + +<p>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 +<a name="page23"></a> +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.</p> + +<p>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 +<a name="page24"></a> +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.</p> + +<p>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.</p> + +<h3><a name="sec02"></a><i>Section II</i>—GENERAL DESCRIPTION OF ATOLLS.</h3> + +<p class="letter"> +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. +</p> + +<p>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 +<a name="page25"></a> +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, <a href= +"#PlateI">Plate I</a>), 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, <a href= +"#PlateII">Plate II</a>), 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.</p> + +<p> +The average width of the annular wreath may be taken as about a quarter of a +mile. Captain Beechey<a href="#fn-1.5" name="fnref-1.5" +id="fnref-1.5"><sup>[5]</sup></a> 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, +<a name="page26"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-1.5" id="fn-1.5"></a> <a href="#fnref-1.5">[5]</a> +Beechey’s “Voyage to the Pacific and Beering’s +Straits,” chapter viii. +</p> + +<p> +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<a href="#fn-1.6" name="fnref-1.6" +id="fnref-1.6"><sup>[6]</sup></a> 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<a href="#fn-1.7" name="fnref-1.7" +id="fnref-1.7"><sup>[7]</sup></a> 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,<a href="#fn-1.8" name="fnref-1.8" id="fnref-1.8"><sup>[8]</sup></a> 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<a +href="#fn-1.9" name="fnref-1.9" id="fnref-1.9"><sup>[9]</sup></a> 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 +<a name="page27"></a> +sandy: nor was there any reason to suspect the existence of submarine cliffs, +as there was at Keeling Island.<a href="#fn-1.10" name="fnref-1.10" +id="fnref-1.10"><sup>[10]</sup></a> 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<a href="#fn-1.11" name="fnref-1.11" +id="fnref-1.11"><sup>[11]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-1.6" id="fn-1.6"></a> <a href="#fnref-1.6">[6]</a> +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.” +</p> + +<p class="footnote"> +<a name="fn-1.7" id="fn-1.7"></a> <a href="#fnref-1.7">[7]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-1.8" id="fn-1.8"></a> <a href="#fnref-1.8">[8]</a> +Cook’s “Third Voyage,” vol. ii, chap. 10. +</p> + +<p class="footnote"> +<a name="fn-1.9" id="fn-1.9"></a> <a href="#fnref-1.9">[9]</a> +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 +<i>Geographical Journal</i>, vol. v, p. 401. +</p> + +<p class="footnote"> +<a name="fn-1.10" id="fn-1.10"></a> <a href="#fnref-1.10">[10]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-1.11" id="fn-1.11"></a> <a href="#fnref-1.11">[11]</a> +“Memoires pour servir à une description Geolog. de France,” tome +iv, p. 216. +</p> + +<p> +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<a href="#fn-1.12" +name="fnref-1.12" id="fnref-1.12"><sup>[12]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-1.12" id="fn-1.12"></a> <a href="#fnref-1.12">[12]</a> +Beechey’s “Voyage,” 4to ed., p. 44. +</p> + +<p> +<a name="page28"></a> +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 <i>wherever the waves beat</i>; +and, under favourable circumstances, assumes a stalactical form,”—a +description perfectly applicable to the margin of Keeling atoll.<a +href="#fn-1.13" name="fnref-1.13" id="fnref-1.13"><sup>[13]</sup></a> 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,<a +href="#fn-1.14" name="fnref-1.14" id="fnref-1.14"><sup>[14]</sup></a> 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,<a href="#fn-1.15" name="fnref-1.15" +id="fnref-1.15"><sup>[15]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-1.13" id="fn-1.13"></a> <a href="#fnref-1.13">[13]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-1.14" id="fn-1.14"></a> <a href="#fnref-1.14">[14]</a> +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 <i>London Phil. Journal,</i> 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. +</p> + +<p class="footnote"> +<a name="fn-1.15" id="fn-1.15"></a> <a href="#fnref-1.15">[15]</a> +Captain Moresby, in his valuable paper “on the Northern atolls of +Maldivas” (<i>Geographical Journal</i>, vol. v), says that the edges of +the reefs there stand above water at low spring-tides. +</p> + +<p> +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<a href="#fn-1.16" name="fnref-1.16" +id="fnref-1.16"><sup>[16]</sup></a> 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 +<a name="page29"></a> +Maldiva atolls, shown me by Captain Moresby,<a href="#fn-1.17" +name="fnref-1.17" id="fnref-1.17"><sup>[17]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-1.16" id="fn-1.16"></a> <a href="#fnref-1.16">[16]</a> +Kotzebue’s “First Voyage,” vol. iii, p. 144. +</p> + +<p class="footnote"> +<a name="fn-1.17" id="fn-1.17"></a> <a href="#fnref-1.17">[17]</a> +See also Moresby on the Northern atolls of the Maldivas, <i>Geographical +Journal</i>, vol v, p. 400. +</p> + +<p> +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 <i>soft +clay</i>; 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.<a href="#fn-1.18" name="fnref-1.18" +id="fnref-1.18"><sup>[18]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-1.18" id="fn-1.18"></a> <a href="#fnref-1.18">[18]</a> +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 <i>Pilote du +Brésil</i>, 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. +</p> + +<p> +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 (<a +href="#PlateI">Plate 1,</a> Fig. 9) in the Chagos group, with Mourileu atoll,<a +href="#fn-1.19" name="fnref-1.19" id="fnref-1.19"><sup>[19]</sup></a> in the +Caroline Archipelago, and with the barrier-reef (<a href="#PlateI">Plate I,</a> +Fig. 8) of the Gambier Islands. I allude to the latter reef, although belonging +to +<a name="page30"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-1.19" id="fn-1.19"></a> <a href="#fnref-1.19">[19]</a> +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. +</p> + +<p> +There is seldom, with the exception of the Maldiva atolls, more than two or +three channels, and generally only one leading into the lagoon, +<a name="page31"></a> +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<a href="#fn-1.20" name="fnref-1.20" +id="fnref-1.20"><sup>[20]</sup></a> 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,<a href="#fn-1.21" +name="fnref-1.21" id="fnref-1.21"><sup>[21]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-1.20" id="fn-1.20"></a> <a href="#fnref-1.20">[20]</a> +“Principles of Geology,” vol. iii, p. 289. +</p> + +<p class="footnote"> +<a name="fn-1.21" id="fn-1.21"></a> <a href="#fnref-1.21">[21]</a> +Beechey’s “Voyage,” 4to ed., vol. i, p. 189. +</p> + +<p> +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,<a +href="#fn-1.22" name="fnref-1.22" id="fnref-1.22"><sup>[22]</sup></a> 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,<a href="#fn-1.23" name="fnref-1.23" +id="fnref-1.23"><sup>[23]</sup></a> 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 +<a name="page32"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-1.22" id="fn-1.22"></a> <a href="#fnref-1.22">[22]</a> +Kotzebue’s “First Voyage,” vol. iii, p. 142. +</p> + +<p class="footnote"> +<a name="fn-1.23" id="fn-1.23"></a> <a href="#fnref-1.23">[23]</a> +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. +</p> + +<p> +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<a href="#fn-1.24" +name="fnref-1.24" id="fnref-1.24"><sup>[24]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-1.24" id="fn-1.24"></a> <a href="#fnref-1.24">[24]</a> +Kotzebue’s “First Voyage,” vol. iii, p. 221. +</p> + +<h3><a name="page33"></a><a name="sec03"></a><i>Section III</i>—ATOLLS OF +THE MALDIVA ARCHIPELAGO—GREAT CHAGOS BANK</h3> + +<p class="letter">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.</p> + +<p>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.</p> + +<p> +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 <a href="#PlateII"> Plate II.</a> 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 (<a href="#PlateII">Plate II,</a> 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 +<a name="page34"></a> +invariably bordered by living coral<a href="#fn-1.25" name="fnref-1.25" +id="fnref-1.25"><sup>[25]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-1.25" id="fn-1.25"></a> <a href="#fnref-1.25">[25]</a> +Captain Moresby informs me that <i>Millepora complanata</i> is one of the +commonest kinds on the outer margin, as it is at Keeling atoll. +</p> + +<p>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.</p> + +<p>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. +<a name="page35"></a> +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!</p> + +<p>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.</p> + +<p>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.</p> + +<p>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 +<a name="page36"></a> +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.</p> + +<p>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.</p> + +<p>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, <a href="#PlateII">Plate II</a>), 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 +<a name="page37"></a> +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, <a href="#PlateII">Plate +II</a>) 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.</p> + +<p class="center"> +<a name="PlateII"><i>Plate II</i></a>—Great Chagos Bank, New Caledonia, +Menchikoff Atoll, etc. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/plate2a.jpg" width="434" height="468" alt="[Illustration: +Great Chagos Bank]" /> +</div> + +<p class="letter"> +Fig. 1.—G<small>REAT</small> C<small>HAGOS</small> B<small>ANK</small>, +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. +</p> + +<p class="letter"> +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. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/plate2b.jpg" width="414" height="467" alt="[Illustration: +New Caledonia, Menchikoff Atoll, etc.]" /> +</div> + +<p class="letter"> +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. +</p> + +<p class="letter"> +Fig. 4.—M<small>AHLOS</small> M<small>AHDOO</small> A<small>TOLL</small>, +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. +</p> + +<p class="letter"> +Fig. 5.—N<small>EW</small> C<small>ALEDONIA</small>, 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 <i>Astrolabe</i>.” +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. +</p> + +<p class="letter"> +Fig. 6.—M<small>ALDIVA</small> A<small>RCHIPELAGO</small>, in the Indian +Ocean; from the survey by Captain Moresby and Lieutenant Powell. +</p> + +<p> +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 <a href= +"#PlateII">Plate II,</a> 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; +<a name="page38"></a> +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, <a href="#PlateII">Plate II</a>), although reduced to so +small a scale, without at once perceiving that the Great Chagos Bank is, in the +words of Captain Moresby,<a href="#fn-1.26" name="fnref-1.26" +id="fnref-1.26"><sup>[26]</sup></a> “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. +</p> + +<p class="footnote"> +<a name="fn-1.26" id="fn-1.26"></a> <a href="#fnref-1.26">[26]</a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap02"></a>Chapter II<br/>BARRIER REEFS</h2> + +<p class="letter"> +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. +</p> + +<p>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 +<a name="page39"></a> +intermediate form between a small encircling reef and the +Australian barrier, which stretches for a thousand miles in nearly +a straight line.</p> + +<p> +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 <a href="#PlateI">Plate I,</a><a href="#fn-2.1" name="fnref-2.1" +id="fnref-2.1"><sup>[1]</sup></a> 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<a href="#fn-2.2" name="fnref-2.2" +id="fnref-2.2"><sup>[2]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-2.1" id="fn-2.1"></a> <a href="#fnref-2.1">[1]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-2.2" id="fn-2.2"></a> <a href="#fnref-2.2">[2]</a> +Dalrymple, “Hydrog. Mem.” vol. iii. +</p> + +<p>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, <a href= +"#PlateI">Plate I</a>), 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.</p> + +<p> +In the Society Archipelago, Ellis<a href="#fn-2.3" name="fnref-2.3" +id="fnref-2.3"><sup>[3]</sup></a> 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 +<a name="page40"></a> +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, <a href="#PlateI">Plate I</a>), in the +Caroline Archipelago,<a href="#fn-2.4" name="fnref-2.4" +id="fnref-2.4"><sup>[4]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-2.3" id="fn-2.3"></a> <a href="#fnref-2.3">[3]</a> +Consult, on this and other points, the “Polynesian Researches,” by +the Rev. W. Ellis, an admirable work, full of curious information. +</p> + +<p class="footnote"> +<a name="fn-2.4" id="fn-2.4"></a> <a href="#fnref-2.4">[4]</a> +See “Hydrographical Mem.” and the “Atlas of the Voyage of the +<i>Astrolabe</i>,” by Captain Dumont D’Urville, p. 428. +</p> + +<p> +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<a href="#fn-2.5" name="fnref-2.5" +id="fnref-2.5"><sup>[5]</sup></a> 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 (<a href="#PlateI">Plate I,</a> +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 <a href="#appendix">Appendix</a>) 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. +</p> + +<p class="footnote"> +<a name="fn-2.5" id="fn-2.5"></a> <a href="#fnref-2.5">[5]</a> +See the chart in vol. i of Hawkesworth’s 4to ed. of “Cook’s +First Voyage.” +</p> + +<p>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 +<a name="page41"></a> +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 (<a href="#PlateI">Plate I,</a> Fig. 6), in the Society +group, it has been almost effected, so that there remains only one +harbour for small craft.</p> + +<p>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.</p> + +<p> +The islands lying within reefs of this class, are of very various heights. +Tahiti<a href="#fn-2.6" name="fnref-2.6" id="fnref-2.6"><sup>[6]</sup></a> 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, <a href="#PlateI">Plate I</a>), 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, <a +href="#PlateI">Plate I</a>); within that of +<a name="page42"></a> +Hogoleu (Fig. 2, <a href="#PlateI">Plate I</a>) nearly a dozen small islands +are scattered over the expanse of one vast lagoon. +</p> + +<p class="footnote"> +<a name="fn-2.6" id="fn-2.6"></a> <a href="#fnref-2.6">[6]</a> +The height of Tahiti is given from Captain Beechey; Maurua from Mr. F. D. +Bennett (<i>Geograph. Journ.</i> vol. viii, p. 220); Aitutaki from measurements +made on board the <i>Beagle</i>; 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. +</p> + +<p>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.</p> + +<p> +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, <a href="#PlateII">Plate II</a>) 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<a href="#fn-2.7" name="fnref-2.7" +id="fnref-2.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-2.7" id="fn-2.7"></a> <a href="#fnref-2.7">[7]</a> +Flinders’ “Voyage to Terra Australis,” vol. ii, p. 88. +</p> + +<p> +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. +<a name="page43"></a> +</p> + +<div class="fig" style="width:100%;"> +<img src="images/vanik.jpg" width="429" height="222" alt="[Illustration: +Vertical thickness of Vanikoro, Gambier and Maurua.]" /> +</div> + +<p class="letter"> +1. V<small>ANIKORO</small>, from the “Atlas of the Voyage of the +<i>Astrolabe</i>,” by D. D’Urville.<br/> +2. G<small>AMBIER</small> I<small>SLAND</small>, from Beechey.<br/> +3. M<small>AURUA</small>, from the “Atlas of the Voyage of the <i> +Coquille</i>,” by Duperrey.<br/> +<br/> +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.<br/> +AA.—Outer edge of the coral-reefs, where the sea breaks.<br/> +BB.—The shore of the encircled islands. +</p> + +<p> +I must observe that if the sections had been taken in any other direction +across these islands, or across other encircled islands,<a href="#fn-2.8" +name="fnref-2.8" id="fnref-2.8"><sup>[8]</sup></a> 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 <i>apparent</i> +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,<a href="#fn-2.9" name="fnref-2.9" id="fnref-2.9"><sup>[9]</sup></a> +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, +<a name="page44"></a> +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 <a +href="#PlateII">Plate II</a>, 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 <a href="#PlateII">Plate II</a>, 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. +</p> + +<p class="footnote"> +<a name="fn-2.8" id="fn-2.8"></a> <a href="#fnref-2.8">[8]</a> +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 <i>Coquille</i>,” by Duperrey. The depth of the lagoon-channel is +exaggerated. +</p> + +<p class="footnote"> +<a name="fn-2.9" id="fn-2.9"></a> <a href="#fnref-2.9">[9]</a> +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. +</p> + +<p> +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;<a +href="#fn-2.10" name="fnref-2.10" id="fnref-2.10"><sup>[10]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-2.10" id="fn-2.10"></a> <a href="#fnref-2.10">[10]</a> +I have been informed that this is the case by Dr. Allan of Forres, who has +visited this group. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="page45"></a><a name="chap03"></a>Chapter III<br/>FRINGING OR SHORE-REEFS</h2> + +<p class="letter"> +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. +</p> + +<p>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.</p> + +<p> +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,<a href="#fn-3.1" name="fnref-3.1" +id="fnref-3.1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-3.1" id="fn-3.1"></a> <a href="#fnref-3.1">[1]</a> +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 <i>Bot. Misc.</i> vol. ii, p. +316) on one part of the coast there is a space for sixteen miles without a +reef. +</p> + +<p>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 +<a name="page46"></a> +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.</p> + +<p>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.</p> + +<p> +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<a href="#fn-3.2" +name="fnref-3.2" id="fnref-3.2"><sup>[2]</sup></a> 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 +<a name="page47"></a> +water thrown beyond the line, on which the waves break with the greatest force. +At Pernambuco a bar of hard sandstone,<a href="#fn-3.3" name="fnref-3.3" +id="fnref-3.3"><sup>[3]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-3.2" id="fn-3.2"></a> <a href="#fnref-3.2">[2]</a> +Captain Grey’s “Journal of Two Expeditions,” vol. i, p. 369. +</p> + +<p class="footnote"> +<a name="fn-3.3" id="fn-3.3"></a> <a href="#fnref-3.3">[3]</a> +I have described this singular structure in the <i>Lond. and Edin. Phil. +Mag.,</i> October 1841. +</p> + +<p> +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<a href="#fn-3.4" name="fnref-3.4" +id="fnref-3.4"><sup>[4]</sup></a> 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 +<i>une espèce d’effoncement</i> 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. +</p> + +<p class="footnote"> +<a name="fn-3.4" id="fn-3.4"></a> <a href="#fnref-3.4">[4]</a> +“Voyage à l’Isle de France, par un Officier du Roi,” part i, +pp. 192, 200. +</p> + +<p> +<a name="page48"></a> +Many islands<a href="#fn-3.5" name="fnref-3.5" +id="fnref-3.5"><sup>[5]</sup></a> 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<a href="#fn-3.6" +name="fnref-3.6" id="fnref-3.6"><sup>[6]</sup></a> Islands, where it is only +fifty yards in width: the sea round these islands is very deep. +</p> + +<p class="footnote"> +<a name="fn-3.5" id="fn-3.5"></a> <a href="#fnref-3.5">[5]</a> +I may give Cuba, as another instance; Mr. Taylor (<i>Loudon’s Mag. of +Nat. Hist.,</i> 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. +</p> + +<p class="footnote"> +<a name="fn-3.6" id="fn-3.6"></a> <a href="#fnref-3.6">[6]</a> +Mauti is described by Lord Byron in the voyage of H.M.S. <i> Blonde</i>, and +Elizabeth Island by Captain Beechey. +</p> + +<p> +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,<a href="#fn-3.7" +name="fnref-3.7" id="fnref-3.7"><sup>[7]</sup></a> “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. +</p> + +<p class="footnote"> +<a name="fn-3.7" id="fn-3.7"></a> <a href="#fnref-3.7">[7]</a> +Owen’s “Africa,” vol. i, p. 357, from which work the +foregoing facts are likewise taken. +</p> + +<p> +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 +<a name="page49"></a> +of Pernambuco; but a few miles south of this latter city, the reef follows<a +href="#fn-3.8" name="fnref-3.8" id="fnref-3.8"><sup>[8]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-3.8" id="fn-3.8"></a> <a href="#fnref-3.8">[8]</a> +See Baron Roussin’s “Pilote du Brésil,” and accompanying +hydrographical memoir. +</p> + +<p>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 +<a name="page50"></a> +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.</p> + +<p>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 (<a href="#PlateIII">Plate +III</a>) is coloured, the exceptional cases will be enumerated.</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap04"></a>Chapter IV<br/>ON THE DISTRIBUTION AND GROWTH OF CORAL-REEFS</h2> + +<p>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.</p> + +<p>These subjects have an important bearing on the theory of the +origin of the different classes of coral-reefs.</p> + +<h3><a name="sec04"></a><i>Section I</i>—ON THE DISTRIBUTION OF +CORAL-REEFS, AND ON THE CONDITIONS FAVOURABLE TO THEIR INCREASE</h3> + +<p>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.</p> + +<p>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. +<a name="page51"></a> +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.</p> + +<p> +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 <i>Beagle</i> 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<a href="#fn-4.1" name="fnref-4.1" +id="fnref-4.1"><sup>[1]</sup></a> 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,<a href="#fn-4.2" name="fnref-4.2" +id="fnref-4.2"><sup>[2]</sup></a> to the coldness of the currents from the +south, but the Gulf of Panama is one of the hottest pelagic districts in the +world.<a href="#fn-4.3" name="fnref-4.3" id="fnref-4.3"><sup>[3]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-4.1" id="fn-4.1"></a> <a href="#fnref-4.1">[1]</a> +I have been informed that this is the case, by Lieutenant Ryder, R.N., and +others who have had ample opportunities for observation. +</p> + +<p class="footnote"> +<a name="fn-4.2" id="fn-4.2"></a> <a href="#fnref-4.2">[2]</a> +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 <i>Beagle</i>, 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. +</p> + +<p class="footnote"> +<a name="fn-4.3" id="fn-4.3"></a> <a href="#fnref-4.3">[3]</a> +Humboldt’s “Personal Narrative,” vol. vii, p. 434. +</p> + +<p>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.</p> + +<p> +I have been assured by several people, that there are no coral-reefs on the +west coast of Africa,<a href="#fn-4.4" name="fnref-4.4" +id="fnref-4.4"><sup>[4]</sup></a> 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, +<a name="page52"></a> +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.</p> + +<p class="footnote"> +<a name="fn-4.4" id="fn-4.4"></a> <a href="#fnref-4.4">[4]</a> +It might be concluded, from a paper by Captain Owen (<i>Geograph. Journ.</i>, +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. +</p> + +<p> +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,<a href="#fn-4.5" name="fnref-4.5" id="fnref-4.5"><sup>[5]</sup></a> +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. +</p> + +<p> +<a name="page53"></a> +Ehrenberg<a href="#fn-4.6" name="fnref-4.6" id="fnref-4.6"><sup>[6]</sup></a> +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.<a href="#fn-4.7" +name="fnref-4.7" id="fnref-4.7"><sup>[7]</sup></a> 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:<a href="#fn-4.8" +name="fnref-4.8" id="fnref-4.8"><sup>[8]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-4.5" id="fn-4.5"></a> <a href="#fnref-4.5">[5]</a> +Kotzebue’s “First Voyage” (Eng. Trans.), vol. iii, pp. 142, +143, 331. +</p> + +<p class="footnote"> +<a name="fn-4.6" id="fn-4.6"></a> <a href="#fnref-4.6">[6]</a> +Ehrenberg, “Über die Natur und Bildung der Corallen Bänke im rothen +Meere,” p. 49. +</p> + +<p class="footnote"> +<a name="fn-4.7" id="fn-4.7"></a> <a href="#fnref-4.7">[7]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-4.8" id="fn-4.8"></a> <a href="#fnref-4.8">[8]</a> +“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. +</p> + +<p> +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,<a href="#fn-4.9" name="fnref-4.9" id="fnref-4.9"><sup>[9]</sup></a> +have been observed. +</p> + +<p class="footnote"> +<a name="fn-4.9" id="fn-4.9"></a> <a href="#fnref-4.9">[9]</a> +Ehrenberg, “Über die Natur,” etc., p. 46. +</p> + +<p> +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.<a href="#fn-4.10" name="fnref-4.10" +id="fnref-4.10"><sup>[10]</sup></a> An experiment, +<a name="page54"></a> +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,<a +href="#fn-4.11" name="fnref-4.11" id="fnref-4.11"><sup>[11]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-4.10" id="fn-4.10"></a> <a href="#fnref-4.10">[10]</a> +<i>Ibid</i>., p. 49. +</p> + +<p class="footnote"> +<a name="fn-4.11" id="fn-4.11"></a> <a href="#fnref-4.11">[11]</a> +Captain Owen on the Geography of the Maldiva Islands, <i>Geograph. Journal</i>, +vol. ii, p. 88. +</p> + +<p> +In the last chapter I remarked that fringing-reefs are almost universally +breached, where streams enter the sea.<a href="#fn-4.12" name="fnref-4.12" +id="fnref-4.12"><sup>[12]</sup></a> 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,<a href="#fn-4.13" name="fnref-4.13" +id="fnref-4.13"><sup>[13]</sup></a> 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 +<a name="page55"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-4.12" id="fn-4.12"></a> <a href="#fnref-4.12">[12]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-4.13" id="fn-4.13"></a> <a href="#fnref-4.13">[13]</a> +Cook’s “First Voyage,” vol. ii, p. 271 (Hawkesworth’s +edit.) +</p> + +<p> +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 <i>Porites</i> and <i>Millepora +complanata</i> 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 <i>Millepora +alcicornis</i> 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,<a href="#fn-4.14" name="fnref-4.14" +id="fnref-4.14"><sup>[14]</sup></a>between the great branching corals of the +Red Sea, and those on the reefs of the Maldiva atolls. +</p> + +<p class="footnote"> +<a name="fn-4.14" id="fn-4.14"></a> <a href="#fnref-4.14">[14]</a> +Captain Moresby on the Northern Maldiva atolls, <i>Geograph. Journal</i>, vol. +v, p. 401. +</p> + +<p> +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 +<a name="page56"></a> +very steep sides;<a href="#fn-4.15" name="fnref-4.15" +id="fnref-4.15"><sup>[15]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-4.15" id="fn-4.15"></a> <a href="#fnref-4.15">[15]</a> +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. +</p> + +<p>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.</p> + +<p>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 +<a name="page57"></a> +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.</p> + +<h3><a name="sec05"></a><i>Section II</i>—ON THE RATE OF GROWTH OF +CORAL-REEFS</h3> + +<p> +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<a href="#fn-4.16" name="fnref-4.16" +id="fnref-4.16"><sup>[16]</sup></a> has stated, that in +<a name="page58"></a> +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,<a +href="#fn-4.17" name="fnref-4.17" id="fnref-4.17"><sup>[17]</sup></a> with +respect to the thickness of some upraised beds of coral, which they examined at +Timor and some other places. Ehrenberg<a href="#fn-4.18" name="fnref-4.18" +id="fnref-4.18"><sup>[18]</sup></a> 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<a href="#fn-4.19" +name="fnref-4.19" id="fnref-4.19"><sup>[19]</sup></a> 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<a href="#fn-4.20" +name="fnref-4.20" id="fnref-4.20"><sup>[20]</sup></a> 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<a +href="#fn-4.21" name="fnref-4.21" id="fnref-4.21"><sup>[21]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-4.16" id="fn-4.16"></a> <a href="#fnref-4.16">[16]</a> +Ehrenberg, as before cited, pp. 39, 46, and 50. +</p> + +<p class="footnote"> +<a name="fn-4.17" id="fn-4.17"></a> <a href="#fnref-4.17">[17]</a> +“Annales des Sciences Nat.” tom. vi, p. 28. +</p> + +<p class="footnote"> +<a name="fn-4.18" id="fn-4.18"></a> <a href="#fnref-4.18">[18]</a> +Ehrenberg, <i>ut sup.</i>, p. 42. +</p> + +<p class="footnote"> +<a name="fn-4.19" id="fn-4.19"></a> <a href="#fnref-4.19">[19]</a> +Lyell’s “Principles of Geology,” book iii, ch. xviii. +</p> + +<p class="footnote"> +<a name="fn-4.20" id="fn-4.20"></a> <a href="#fnref-4.20">[20]</a> +Beechey’s “Voyage to the Pacific,” ch. viii. +</p> + +<p class="footnote"> +<a name="fn-4.21" id="fn-4.21"></a> <a href="#fnref-4.21">[21]</a> +Ehrenberg, <i>ut sup.</i>, p. 43. +</p> + +<p>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 +<a name="page59"></a> +Lieutenant Nelson,<a href="#fn-4.22" name="fnref-4.22" +id="fnref-4.22"><sup>[22]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-4.22" id="fn-4.22"></a> <a href="#fnref-4.22">[22]</a> +“Geological Transactions,” vol. v, p. 113. +</p> + +<p> +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,<a href="#fn-4.23" +name="fnref-4.23" id="fnref-4.23"><sup>[23]</sup></a> to have a similar +structure, and its shores are about forty feet high: some of the Cook Islands +also appear<a href="#fn-4.24" name="fnref-4.24" +id="fnref-4.24"><sup>[24]</sup></a> 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 +<i> creamy</i> 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. +</p> + +<p class="footnote"> +<a name="fn-4.23" id="fn-4.23"></a> <a href="#fnref-4.23">[23]</a> +Forster’s “Voyage round the World with Cook,” vol. ii, pp. +163, 167. +</p> + +<p class="footnote"> +<a name="fn-4.24" id="fn-4.24"></a> <a href="#fnref-4.24">[24]</a> +Williams’s “Narrative of Missionary Enterprise,” p. 30. +</p> + +<p> +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 <i> +Millepora complanata,</i> as is now taking place; nor these latter without +encroaching on the strongly branched Madreporet, the <i> Millepora +alcicornis,</i> 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 +<a name="page60"></a> +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,<a href="#fn-4.25" name="fnref-4.25" +id="fnref-4.25"><sup>[25]</sup></a> “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.”<a href="#fn-4.26" name="fnref-4.26" +id="fnref-4.26"><sup>[26]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-4.25" id="fn-4.25"></a> <a href="#fnref-4.25">[25]</a> +Cook’s “Third Voyage,” book III, ch. x. +</p> + +<p class="footnote"> +<a name="fn-4.26" id="fn-4.26"></a> <a href="#fnref-4.26">[26]</a> +Beechey’s “Voyage to the Pacific,” ch. vii and viii. +</p> + +<p> +After these considerations, I attach little importance, as indicating the +ordinary and still less the possible rate of <i> outward</i> 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,<a href="#fn-4.27" +name="fnref-4.27" id="fnref-4.27"><sup>[27]</sup></a>—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. +</p> + +<p class="footnote"> +<a name="fn-4.27" id="fn-4.27"></a> <a href="#fnref-4.27">[27]</a> +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. +</p> + +<p>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 +<a name="page61"></a> +under favourable circumstances of the <i>upward</i> 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.</p> + +<p>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 +“<i>entirely covered with live coral and Madrepore.</i>” 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.</p> + +<p>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.</p> + +<p> +It may, however, be inferred from the following facts, that the rate +<a name="page62"></a> +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 <i>three feet deep at low water.</i> 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<a +href="#fn-4.28" name="fnref-4.28" id="fnref-4.28"><sup>[28]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-4.28" id="fn-4.28"></a> <a href="#fnref-4.28">[28]</a> +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. +</p> + +<p> +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, <i>two feet</i> 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 +<a name="page63"></a> +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.<a href="#fn-4.29" +name="fnref-4.29" id="fnref-4.29"><sup>[29]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-4.29" id="fn-4.29"></a> <a href="#fnref-4.29">[29]</a> +Mr. Stutchbury (<i>West of England Journal</i>, 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 +<i>Beagle</i>, 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 <i> Chama gigas</i> 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 <i>Coquille</i>”), has remarked, “que constamment ses valves +étaient engagés complétement dans la masse des Madrepores.” +</p> + +<p>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.</p> + +<h3><a name="sec06"></a><i>Section III</i>—ON THE DEPTHS AT WHICH +REEF-BUILDING POLYPIFERS CAN LIVE</h3> + +<p> +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 +<a name="page64"></a> +of branching Madrepores, which chiefly consist of two species,—apparently +<i>M. corymbosa</i> and <i> pocillifera</i>,—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<a href="#fn-4.30" +name="fnref-4.30" id="fnref-4.30"><sup>[30]</sup></a> 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 <i>Millepora alcicornis,</i> 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 <i>S. +subulata</i>, 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, <i> Pocillopora +verrucosa</i>, 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 <i>M. pocillifera</i>, 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 +<a name="page65"></a> +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 +<i>gradual</i> 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. +</p> + +<p class="footnote"> +<a name="fn-4.30" id="fn-4.30"></a> <a href="#fnref-4.30">[30]</a> +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. +</p> + +<p> +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. +<a name="page66"></a> +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.”<a href="#fn-4.31" name="fnref-4.31" +id="fnref-4.31"><sup>[31]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-4.31" id="fn-4.31"></a> <a href="#fnref-4.31">[31]</a> +Ehrenberg, “Über die Natur,” etc., p. 50. +</p> + +<p> +MM. Quoy and Gaimard<a href="#fn-4.32" name="fnref-4.32" +id="fnref-4.32"><sup>[32]</sup></a> 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. <i>Millepora alcicornis</i> 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 <i>Sideropora scabra</i> (Porites of Lamarck) brought up +alive from 17 fathoms. Mr. Couthouy<a href="#fn-4.33" name="fnref-4.33" +id="fnref-4.33"><sup>[33]</sup></a> 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:<a href="#fn-4.34" name="fnref-4.34" +id="fnref-4.34"><sup>[34]</sup></a> this is the most remarkable fact with which +I am acquainted, showing the depth at which a genus of +<a name="page67"></a> +corals often found on reefs, can exist.<a href="#fn-4.35" name="fnref-4.35" +id="fnref-4.35"><sup>[35]</sup></a> We ought, however, to feel less +<a name="page68"></a> +surprise at this fact, as Caryophyllia alone of the lamelliform genera, ranges +far beyond the tropics; it is found in Zetland<a href="#fn-4.36" +name="fnref-4.36" id="fnref-4.36"><sup>[36]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-4.32" id="fn-4.32"></a> <a href="#fnref-4.32">[32]</a> +“Annales des Sci. Nat.” tom. vi. +</p> + +<p class="footnote"> +<a name="fn-4.33" id="fn-4.33"></a> <a href="#fnref-4.33">[33]</a> +“Remarks on Coral Formations,” p. 12. +</p> + +<p class="footnote"> +<a name="fn-4.34" id="fn-4.34"></a> <a href="#fnref-4.34">[34]</a> +I am indebted to Mr. Stokes for having kindly communicated this fact to me, +together with much other valuable information. +</p> + +<p class="footnote"> +<a name="fn-4.35" id="fn-4.35"></a> <a href="#fnref-4.35">[35]</a> +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.<br/> + Ellis (“Nat. Hist. of Coralline,” p. 96) states that +Ombellularia was procured in latitude 79° N. <i> sticking</i> to a +<i>line</i> 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. +</p> + +<p class="footnote"> +<a name="fn-4.36" id="fn-4.36"></a> <a href="#fnref-4.36">[36]</a> +Fleming’s “British Animals,” genus Caryophyllia. +</p> + +<table width="100%" border="1" cellspacing="0" +cellpadding="4" summary= +"Name of Zoophyte, Depth in fathoms, Country and S. Latitute, Authority."> +<tr valign="bottom"> +<td align="center">Name of Zoophyte</td> +<td align="center">Depth in<br/> +Fathoms</td> +<td align="center">Country and<br/> +S. Latitude</td> +<td align="center">Authority</td> +</tr> + +<tr valign="top"> +<td>Sertularia</td> +<td align="center">40</td> +<td align="center">Cape Horn 66°</td> +<td>(Where none is given, the observation is my own.)</td> +</tr> + +<tr> +<td>Cellaria</td> +<td align="center">Ditto</td> +<td align="center">Ditto</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>Cellaria. A minute scarlet encrusted species, found living</td> +<td align="center">190</td> +<td align="center">Keeling Atoll 12°</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>Cellaria. An allied, small stony sub-generic form</td> +<td align="center">48</td> +<td align="center">S. Cruz River 50°</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>A coral allied to Vincularia, with eight rows of cells</td> +<td align="center">40</td> +<td align="center">Cape Horn</td> +<td> </td> +</tr> + +<tr> +<td>Tubulipora, near to T. patima</td> +<td align="center">Ditto</td> +<td align="center">Ditto</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>Ditto</td> +<td align="center">94</td> +<td align="center">East Chiloe 43°</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>Cellepora, several species, and allied sub-generic forms</td> +<td align="center">40</td> +<td align="center">Cape Horn</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>Ditto</td> +<td align="center">40 and 57</td> +<td align="center">Chonos Arch. 45°</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>Ditto</td> +<td align="center">48</td> +<td align="center">S. Cruz 50°</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>Eschara</td> +<td align="center">30</td> +<td align="center">Tierra del Fuego 53°</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>Ditto</td> +<td align="center">48</td> +<td align="center">S. Cruz R. 50°</td> +<td> </td> +</tr> + +<tr> +<td>Retepora</td> +<td align="center">40</td> +<td align="center">Cape Horn</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>Ditto</td> +<td align="center">100</td> +<td align="center">C. Good Hope 34°</td> +<td>Quoy and Gaimard, <i>Ann. Scien. Nat.,</i> t. vi, p. 284.</td> +</tr> + +<tr valign="top"> +<td>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</td> +<td align="center">94 and 30</td> +<td align="center">E. Chiloe 43°<br/> +Tierra del Fuego 53°</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>Coralium</td> +<td align="center">120</td> +<td align="center">Barbary 33° N.</td> +<td>Peyssonel in paper read to Royal society May 1752.</td> +</tr> + +<tr> +<td>Antipathes</td> +<td align="center">16</td> +<td align="center">Chonos 45°</td> +<td> </td> +</tr> + +<tr valign="top"> +<td>Gorgonia (or an allied form)</td> +<td align="center">160</td> +<td align="center">Abrolhos on the coast of Brazil 18°</td> +<td>Capt. Beechey informed me of this fact in a letter.</td> +</tr> +</table> + +<p>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.</p> + +<p> +It has been argued<a href="#fn-4.37" name="fnref-4.37" +id="fnref-4.37"><sup>[37]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-4.37" id="fn-4.37"></a> <a href="#fnref-4.37">[37]</a> +<i>Journal of the Royal Geographical Society,</i> 1831, p. 218. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="page69"></a><a name="chap05"></a>Chapter V<br/>THEORY OF THE +FORMATION OF THE DIFFERENT CLASSES OF CORAL-REEFS</h2> + +<p class="letter"> +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. +</p> + +<p>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 (<a href="#PlateI">Plate I</a>, Fig. 4); or +like that of Menchikoff Island (<a href="#PlateII">Plate II</a>, +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.</p> + +<p> +An earlier and better theory was proposed by Chamisso;<a href="#fn-5.1" +name="fnref-5.1" id="fnref-5.1"><sup>[1]</sup></a> 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 +<a name="page70"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-5.1" id="fn-5.1"></a> <a href="#fnref-5.1">[1]</a> +Kotzebue’s “First Voyage,” vol. iii, p. 331. +</p> + +<p>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.</p> + +<p> +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<a href="#fn-5.2" +name="fnref-5.2" id="fnref-5.2"><sup>[2]</sup></a> 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 +<a name="page71"></a> +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<a href="#fn-5.3" name="fnref-5.3" +id="fnref-5.3"><sup>[3]</sup></a> 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 +<a name="page72"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-5.2" id="fn-5.2"></a> <a href="#fnref-5.2">[2]</a> +See also Captain Owen’s and Lieutenant Wood’s papers in the +<i>Geographical Journal</i>, 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. +</p> + +<p class="footnote"> +<a name="fn-5.3" id="fn-5.3"></a> <a href="#fnref-5.3">[3]</a> +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 <i>Beagle</i>, +whilst sailing by, is 114 feet from the <i>summit of the trees</i> to the +water’s edge. This island appeared to resemble the other atolls of the +group. +</p> + +<p> +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,<a +href="#fn-5.4" name="fnref-5.4" id="fnref-5.4"><sup>[4]</sup></a> +<a name="page73"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-5.4" id="fn-5.4"></a> <a href="#fnref-5.4">[4]</a> +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! +</p> + +<p> +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,<a href="#fn-5.5" name="fnref-5.5" +id="fnref-5.5"><sup>[5]</sup></a> 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 +<a name="page74"></a> +nations how long might not a slow movement, even of elevation such as that now +affecting Scandinavia, have escaped attention! +</p> + +<p class="footnote"> +<a name="fn-5.5" id="fn-5.5"></a> <a href="#fnref-5.5">[5]</a> +“Principles of Geology,” sixth edition, vol. iii, p. 386. +</p> + +<p> +Mr. Williams<a href="#fn-5.6" name="fnref-5.6" +id="fnref-5.6"><sup>[6]</sup></a> 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.”<a +href="#fn-5.7" name="fnref-5.7" id="fnref-5.7"><sup>[7]</sup></a> A storm +lately entirely swept away two of the Caroline islands, and converted them into +shoals; it partly, also, destroyed two other islands.<a href="#fn-5.8" +name="fnref-5.8" id="fnref-5.8"><sup>[8]</sup></a> 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.”<a href="#fn-5.9" +name="fnref-5.9" id="fnref-5.9"><sup>[9]</sup></a> 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,<a href="#fn-5.10" +name="fnref-5.10" id="fnref-5.10"><sup>[10]</sup></a> “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 +<a name="page75"></a> +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:”<a href="#fn-5.11" +name="fnref-5.11" id="fnref-5.11"><sup>[11]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-5.6" id="fn-5.6"></a> <a href="#fnref-5.6">[6]</a> +Williams’s “Narrative of Missionary Enterprise,” p. 31. +</p> + +<p class="footnote"> +<a name="fn-5.7" id="fn-5.7"></a> <a href="#fnref-5.7">[7]</a> +Kotzebue’s “First Voyage,” vol. iii, p. 168. +</p> + +<p class="footnote"> +<a name="fn-5.8" id="fn-5.8"></a> <a href="#fnref-5.8">[8]</a> +M. Desmoulins in “Comptes Rendus,” 1840, p. 837. +</p> + +<p class="footnote"> +<a name="fn-5.9" id="fn-5.9"></a> <a href="#fnref-5.9">[9]</a> +<i>West of England Journal</i>, No. I, p. 35. +</p> + +<p class="footnote"> +<a name="fn-5.10" id="fn-5.10"></a> <a href="#fnref-5.10">[10]</a> +Beechey’s “Voyage to the Pacific,” chap. vii, and +Wallis’s “Voyage in the <i>Dolphin</i>,” chap. iv. +</p> + +<p class="footnote"> +<a name="fn-5.11" id="fn-5.11"></a> <a href="#fnref-5.11">[11]</a> +See an extract from Pyrard’s Voyage in Captain Owen’s paper on the +Maldiva Archipelago, in the <i>Geographical Journal</i>, vol. ii, p. 84. +</p> + +<p> +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<a href="#fn-5.12" name="fnref-5.12" +id="fnref-5.12"><sup>[12]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-5.12" id="fn-5.12"></a> <a href="#fnref-5.12">[12]</a> +See Chamisso, in Kotzebue’s “First Voyage,” vol. iii, p. 182 +and 136. +</p> + +<p>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?</p> + +<p> +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, +<a name="page76"></a> +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<a href="#fn-5.13" +name="fnref-5.13" id="fnref-5.13"><sup>[13]</sup></a> east and west through the +highest point of the encircled island of Bolabola; of which a plan is given in +<a href="#PlateI">Plate I</a>, 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. +</p> + +<p class="footnote"> +<a name="fn-5.13" id="fn-5.13"></a> <a href="#fnref-5.13">[13]</a> +The section has been made from the chart given in the “Atlas of the +Voyage of the <i>Coquille</i>.” 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. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no4.jpg" width="472" height="141" alt="[Illustration: +Vertical section of an island of Bolabola.]" /> +</div> + +<p class="letter"> +AA—Outer edge of the reef at the level of the sea.<br/> +BB—Shores of the island.<br/> +A′A′—Outer edge of the reef, after its upward growth during +a period of subsidence.<br/> +CC—The lagoon-channel between the reef and the shores of the now +encircled land.<br/> +B′B′—The shores of the encircled island.<br/> +<br/> +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. +</p> + +<p>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, +<a name="page77"></a> +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.</p> + +<p>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.</p> + +<div class="fig" style="width:100%;"> +<img src="images/no5.jpg" width="446" height="179" alt="[Illustration: +Vertical section of an island of Bolabola.]" /> +</div> + +<p class="letter"> +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.<br/> +CC—The lagoon-channel.<br/> +B′B′—The shores of the island, generally formed of low +alluvial land and of coral detritus from the lagoon-channel.<br/> +A″A″—The outer edges of the reef now forming an atoll.<br/> +C′—The lagoon of the newly formed atoll. According to the scale, +the depth of the lagoon and of the lagoon-channel is exaggerated. +</p> + +<p> +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 +<a name="page78"></a> +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.</p> + +<p>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.</p> + +<p>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 +<a name="page79"></a> +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 (<a +href="#PlateII">Plate II</a>, 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.</p> + +<p>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.</p> + +<p> +<i>Step-formed ledges round certain lagoons.</i>—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 +<a name="page80"></a> +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.<a href="#fn-5.14" name="fnref-5.14" +id="fnref-5.14"><sup>[14]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-5.14" id="fn-5.14"></a> <a href="#fnref-5.14">[14]</a> +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.? +</p> + +<p><i>The ring or basin-formed reefs of the northern Maldiva +atolls.</i>—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 +<a name="page81"></a> +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.</p> + +<p> +<i>Submerged and dead reefs.</i>—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 +<a name="page82"></a> +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,<a href="#fn-5.15" +name="fnref-5.15" id="fnref-5.15"><sup>[15]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-5.15" id="fn-5.15"></a> <a href="#fnref-5.15">[15]</a> +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. +</p> + +<p>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 (<a href="#PlateI">Plate I</a>, +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.</p> + +<p><i>The disseverment of the larger Maldiva atolls.</i>—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 +<a name="page83"></a> +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 (<a href="#PlateII">Plate II</a>, 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 (<a href= +"#PlateII">Plate II</a>, 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 (<a href="#PlateII">Plate II</a>, 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.</p> + +<p> +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 (<a href="#PlateII">Plate II</a>, +Fig. 5), for if, in imagination, we complete the subsidence of that great +island, we might anticipate from the present broken condition of the +<a name="page84"></a> +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.<a href="#fn-5.16" +name="fnref-5.16" id="fnref-5.16"><sup>[16]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-5.16" id="fn-5.16"></a> <a href="#fnref-5.16">[16]</a> +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. +</p> + +<p><i>Irregularly formed atolls.</i>—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, <a href="#PlateII">Plate II</a>) 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 +<a name="page85"></a> +subsidence on part of the circumference of an atoll, whilst on +the other parts it continued to grow up till it reached the +surface.</p> + +<p><i>The Great Chagos Bank.</i>—I have already shown that +the submerged condition of the Great Chagos Bank (<a href= +"#PlateII">Plate II</a>, 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.</p> + +<p> +<a name="page86"></a> +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.</p> + +<p>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.</p> + +<p>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.</p> + +<p><i>Objections to the theory of the formation of atolls and +barrier-reefs.</i>—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 +<a name="page87"></a> +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.</p> + +<p> +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 +<a name="page88"></a> +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.<a href="#fn-5.17" name="fnref-5.17" +id="fnref-5.17"><sup>[17]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-5.17" id="fn-5.17"></a> <a href="#fnref-5.17">[17]</a> +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. +</p> + +<p class="center"> +<a name="PlateIII"><i> Plate III</i></a>—Map showing the distribution of +coral-reefs and active volcanoes. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/plate3a.jpg" width="457" height="473" alt="[Illustration: +Map showing distribution of coral-reefs and active volcanoes.]" /> +</div> + +<div class="fig" style="width:100%;"> +<img src="images/plate3b.jpg" width="464" height="476" alt="[Illustration: +Map showing distribution of coral-reefs and active volcanoes.]" /> +</div> + +<div class="fig" style="width:100%;"> +<img src="images/plate3c.jpg" width="490" height="478" alt="[Illustration: +Map showing distribution of coral-reefs and active volcanoes.]" /> +</div> + +<p class="letter"> +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 <i>Coral Reefs.</i> The names not printed in upper case in the +Index refer to the Appendix. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="page89"></a><a name="chap06"></a>Chapter VI<br/>ON THE DISTRIBUTION +OF CORAL-REEFS WITH REFERENCE TO THE THEORY OF THEIR FORMATION</h2> + +<p class="letter">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.</p> + +<p> +It will be convenient to give here a short account of the appended map (Plate +III):<a href="#fn-6.1" name="fnref-6.1" id="fnref-6.1"><sup>[1]</sup></a> a +fuller one, with the data for colouring each spot, is reserved +<a name="page90"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-6.1" id="fn-6.1"></a> <a href="#fnref-6.1">[1]</a> +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. +</p> + +<p>The <i>dark blue colour</i> [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.</p> + +<p>The <i>pale blue colour</i> [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.</p> + +<p>The <i>red colour</i> (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 <i>breached</i> +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.</p> + +<p> +The <i>vermillion spots</i>, 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.<a href="#fn-6.2" name="fnref-6.2" +id="fnref-6.2"><sup>[2]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-6.2" id="fn-6.2"></a> <a href="#fnref-6.2">[2]</a> +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 <i>Beagle</i> (“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 <i>Pandora</i>” (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 <i>Astrolabe</i>.” 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 <i>opposite</i> 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 <i> Silliman’s North American Journal</i>, 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 (<i>Naut. Mag.</i> 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. +</p> + +<p> +<a name="page91"></a> +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 +<a name="page92"></a> +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. +</p> + +<p>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 +<a name="page93"></a> +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.</p> + +<p> +<i>On the grouping of the different classes of reefs.</i>—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<a href="#fn-6.3" +name="fnref-6.3" id="fnref-6.3"><sup>[3]</sup></a> 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<a href="#fn-6.4" name="fnref-6.4" id="fnref-6.4"><sup>[4]</sup></a> +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 +<a name="page94"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-6.3" id="fn-6.3"></a> <a href="#fnref-6.3">[3]</a> +Sailing directions, appended to vol. ii of his “Surveying Voyage to +Australia.” +</p> + +<p class="footnote"> +<a name="fn-6.4" id="fn-6.4"></a> <a href="#fnref-6.4">[4]</a> +“Voyage to Terra Australis,” vol. ii, p. 336. +</p> + +<p>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 +<a name="page95"></a> +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.</p> + +<p> +<i>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.</i>—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,<a href="#fn-6.5" name="fnref-6.5" +id="fnref-6.5"><sup>[5]</sup></a> 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 +<a name="page96"></a> +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.<a href="#fn-6.6" name="fnref-6.6" id="fnref-6.6"><sup>[6]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-6.5" id="fn-6.5"></a> <a href="#fnref-6.5">[5]</a> +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 (<a href="#PlateI">Plate +I</a>, Fig. 7), in the Caroline Archipelago, being encircled by a barrier-reef, +must have subsided. In the <i> New S. Wales Lit. Advert.</i> 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, +<i>now only</i> accessible by boats, the waves <i>reaching to the steps of the +houses.</i>” 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. +</p> + +<p class="footnote"> +<a name="fn-6.6" id="fn-6.6"></a> <a href="#fnref-6.6">[6]</a> +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.”<br/> + 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. +</p> + +<p> +<a name="page97"></a> +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 +<a name="page98"></a> +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<a href="#fn-6.7" name="fnref-6.7" +id="fnref-6.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-6.7" id="fn-6.7"></a> <a href="#fnref-6.7">[7]</a> +“Annales des Sciences Nat.” tom. vi, p. 279, etc. +</p> + +<p> +In the eastern half of the Pacific, the <i> Sandwich Islands</i> 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<a +href="#fn-6.8" name="fnref-6.8" id="fnref-6.8"><sup>[8]</sup></a> 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<a href="#fn-6.9" +name="fnref-6.9" id="fnref-6.9"><sup>[9]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-6.8" id="fn-6.8"></a> <a href="#fnref-6.8">[8]</a> +“Zoology of Captain Beechey’s Voyage,” p. 176. See also MM. +Quoy and Gaimard in “Annales de Scien. Nat.” tom. vi. +</p> + +<p class="footnote"> +<a name="fn-6.9" id="fn-6.9"></a> <a href="#fnref-6.9">[9]</a> +“Remarks on Coral Formations,” p. 51. +</p> + +<p> +In the southern part of the Low Archipelago, Elizabeth Island is described by +Captain Beechey,<a href="#fn-6.10" name="fnref-6.10" +id="fnref-6.10"><sup>[10]</sup></a> 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 +<a name="page99"></a> +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 <i>Cook and Austral Islands</i> +(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<a +href="#fn-6.11" name="fnref-6.11" id="fnref-6.11"><sup>[11]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-6.10" id="fn-6.10"></a> <a href="#fnref-6.10">[10]</a> +Beechey’s “Voyage in the Pacific,” p. 46, 4to ed. +</p> + +<p class="footnote"> +<a name="fn-6.11" id="fn-6.11"></a> <a href="#fnref-6.11">[11]</a> +Couthouy’s “Remarks,” p. 34. +</p> + +<p> +<i>Savage Island</i> (S.E. of the Friendly group), is about forty feet in +height. Forster<a href="#fn-6.12" name="fnref-6.12" +id="fnref-6.12"><sup>[12]</sup></a> describes the plants as already growing out +of the dead, but still upright and spreading trees of coral; and the younger +Forster<a href="#fn-6.13" name="fnref-6.13" id="fnref-6.13"><sup>[13]</sup></a> +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 +<i>friendly group</i>, 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.”<a +href="#fn-6.14" name="fnref-6.14" id="fnref-6.14"><sup>[14]</sup></a> 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<a href="#fn-6.15" name="fnref-6.15" +id="fnref-6.15"><sup>[15]</sup></a> 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 <i>Navigator +Archipelago</i>, Mr. Couthouy<a href="#fn-6.16" name="fnref-6.16" +id="fnref-6.16"><sup>[16]</sup></a> 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 +<a name="page100"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-6.12" id="fn-6.12"></a> <a href="#fnref-6.12">[12]</a> +“Observations made during Voyage round the World,” p. 147. +</p> + +<p class="footnote"> +<a name="fn-6.13" id="fn-6.13"></a> <a href="#fnref-6.13">[13]</a> +“Voyage,” vol. ii, p. 163. +</p> + +<p class="footnote"> +<a name="fn-6.14" id="fn-6.14"></a> <a href="#fnref-6.14">[14]</a> +Cook’s “Third Voyage” (4to ed.), vol. i, p. 314. +</p> + +<p class="footnote"> +<a name="fn-6.15" id="fn-6.15"></a> <a href="#fnref-6.15">[15]</a> +<i>Ibid</i>., vol. i, p. 235. +</p> + +<p class="footnote"> +<a name="fn-6.16" id="fn-6.16"></a> <a href="#fnref-6.16">[16]</a> +“Remarks on Coral-Formations,” p. 50. +</p> + +<p> +Still proceeding westward we come to the <i>New Hebrides</i>; 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 <i>Santa Cruz</i>, and the <i>Solomon Archipelago</i>, 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<a href="#fn-6.17" name="fnref-6.17" +id="fnref-6.17"><sup>[17]</sup></a> 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 <i> +Marianas</i> form the main part. Of these Guam, Rota, Tiniam, Saypan, and some +islets farther north, are described by Quoy and Gaimard,<a href="#fn-6.18" +name="fnref-6.18" id="fnref-6.18"><sup>[18]</sup></a> and Chamisso,<a +href="#fn-6.19" name="fnref-6.19" id="fnref-6.19"><sup>[19]</sup></a> 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. <i>Fais</i>, 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.<a href="#fn-6.20" name="fnref-6.20" +id="fnref-6.20"><sup>[20]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-6.17" id="fn-6.17"></a> <a href="#fnref-6.17">[17]</a> +“Voyage de la <i>Coquille</i>,” Part. Zoolog. +</p> + +<p class="footnote"> +<a name="fn-6.18" id="fn-6.18"></a> <a href="#fnref-6.18">[18]</a> +Freycinet’s “Voyage autour du Monde.” See also the +“Hydrographical Memoir,” p. 215. +</p> + +<p class="footnote"> +<a name="fn-6.19" id="fn-6.19"></a> <a href="#fnref-6.19">[19]</a> +Kotzebue’s “First Voyage.” +</p> + +<p class="footnote"> +<a name="fn-6.20" id="fn-6.20"></a> <a href="#fnref-6.20">[20]</a> +Lutké’s “Voyage,” vol. ii, p. 304. +</p> + +<p> +In the <i>East Indian Archipelago</i>, many authors have recorded proofs of +recent elevation. M. Lesson<a href="#fn-6.21" name="fnref-6.21" +id="fnref-6.21"><sup>[21]</sup></a> 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<a href="#fn-6.22" name="fnref-6.22" id="fnref-6.22"><sup>[22]</sup></a> +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,”<a +href="#fn-6.23" name="fnref-6.23" id="fnref-6.23"><sup>[23]</sup></a> 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<a href="#fn-6.24" name="fnref-6.24" +id="fnref-6.24"><sup>[24]</sup></a> has described some upraised shells and +corals, apparently recent, on Pulo Nias off <i>Sumatra</i>; 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 +<a name="page101"></a> +coast of <i>Borneo</i> and at the <i>Sooloo Islands</i>, the form of the land, +the nature of the soil, and the water-washed rocks, present appearances<a +href="#fn-6.25" name="fnref-6.25" id="fnref-6.25"><sup>[25]</sup></a> (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 <i>Philippines</i>, 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<a href="#fn-6.26" name="fnref-6.26" +id="fnref-6.26"><sup>[26]</sup></a> of the lines of inland reefs, and walls of +coral-rock worn into caves, above the present reach of the waves, at the <i>Loo +Choo</i> Islands, there can be little doubt that they have been upraised at no +very remote period. +</p> + +<p class="footnote"> +<a name="fn-6.21" id="fn-6.21"></a> <a href="#fnref-6.21">[21]</a> +Partie Zoolog., “Voyage de la <i>Coquille</i>.” +</p> + +<p class="footnote"> +<a name="fn-6.22" id="fn-6.22"></a> <a href="#fnref-6.22">[22]</a> +“Ann. des Scien. Nat.” tom. vi, p. 281. +</p> + +<p class="footnote"> +<a name="fn-6.23" id="fn-6.23"></a> <a href="#fnref-6.23">[23]</a> +Translated by Windsor Earl, chapters vi, vii. +</p> + +<p class="footnote"> +<a name="fn-6.24" id="fn-6.24"></a> <a href="#fnref-6.24">[24]</a> +“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 (<i>Ibid</i>., part i, p. +131) mounds of shells lying two miles inland on this line of coast. +</p> + +<p class="footnote"> +<a name="fn-6.25" id="fn-6.25"></a> <a href="#fnref-6.25">[25]</a> +“Notices of the East Indian Arch.” Singapore, 1828, p. 6, and +Append., p. 43. +</p> + +<p class="footnote"> +<a name="fn-6.26" id="fn-6.26"></a> <a href="#fnref-6.26">[26]</a> +Captain B. Hall, “Voyage to Loo Choo,” Append., pp. xxi and xxv. +Captain Beechey’s “Voyage,” p. 496. +</p> + +<p> +Dr. Davy<a href="#fn-6.27" name="fnref-6.27" +id="fnref-6.27"><sup>[27]</sup></a> describes the northern province of +<i>Ceylon</i> 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.<a href="#fn-6.28" +name="fnref-6.28" id="fnref-6.28"><sup>[28]</sup></a> <i>Mauritius</i> has +certainly been upraised within the recent period, as I have stated in the +chapter on fringing-reefs. The northern extremity of <i>Madagascar</i> is +described by Captain Owen<a href="#fn-6.29" name="fnref-6.29" +id="fnref-6.29"><sup>[29]</sup></a> as formed of madreporitic rock, as likewise +are the shores and outlying islands along an immense space of <i> Eastern +Africa</i>, 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 +<a name="page102"></a> +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 +<i>Seychelles</i>, which come under our fringed class. +</p> + +<p class="footnote"> +<a name="fn-6.27" id="fn-6.27"></a> <a href="#fnref-6.27">[27]</a> + “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 <i>Chevrette</i>, as one of immense extent, and belonging to the + latest tertiary period. +</p> + +<p class="footnote"> +<a name="fn-6.28" id="fn-6.28"></a> <a href="#fnref-6.28">[28]</a> +Dr. Benza, in his “Journey through the N. Circars” (the <i> Madras +Lit. and Scient. Journ.</i> 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. +</p> + +<p class="footnote"> +<a name="fn-6.29" id="fn-6.29"></a> <a href="#fnref-6.29">[29]</a> +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. +</p> + +<p> +The nature of the formations round the shores of the <i>Red Sea</i>, 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<a +href="#fn-6.30" name="fnref-6.30" id="fnref-6.30"><sup>[30]</sup></a> 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<a href="#fn-6.31" name="fnref-6.31" +id="fnref-6.31"><sup>[31]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-6.30" id="fn-6.30"></a> <a href="#fnref-6.30">[30]</a> +Rüppell, “Reise in Abyssinien,” Band i., s. 141. +</p> + +<p class="footnote"> +<a name="fn-6.31" id="fn-6.31"></a> <a href="#fnref-6.31">[31]</a> +Lyell’s “Principles of Geology,” 5th ed., vol. iv, p. 25. +</p> + +<p> +The <i>Persian Gulf</i> 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<a href="#fn-6.32" +name="fnref-6.32" id="fnref-6.32"><sup>[32]</sup></a> says that the land is +worn into terraces, and that the beds contain organic remains of existing +forms. The <i>West Indian Archipelago</i> 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.<a href="#fn-6.33" name="fnref-6.33" +id="fnref-6.33"><sup>[33]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-6.32" id="fn-6.32"></a> <a href="#fnref-6.32">[32]</a> +Ainsworth’s “Assyria and Babylon,” p. 217. +</p> + +<p class="footnote"> +<a name="fn-6.33" id="fn-6.33"></a> <a href="#fnref-6.33">[33]</a> +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. +</p> + +<p> +<a name="page103"></a> +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.</p> + +<p> +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<a href="#fn-6.34" name="fnref-6.34" id="fnref-6.34"><sup>[34]</sup></a> +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, +<a name="page104"></a> +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,<a href="#fn-6.35" +name="fnref-6.35" id="fnref-6.35"><sup>[35]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-6.34" id="fn-6.34"></a> <a href="#fnref-6.34">[34]</a> +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, <i>West of England Journal</i>, 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. +</p> + +<p class="footnote"> +<a name="fn-6.35" id="fn-6.35"></a> <a href="#fnref-6.35">[35]</a> +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. +</p> + +<p>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.</p> + +<p><i>On the absence of active volcanoes in the areas of +subsidence, and on their frequent presence in the areas of +elevation.</i>—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 +<a name="page105"></a> +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.</p> + +<p>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.</p> + +<p> +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<a href="#fn-6.36" name="fnref-6.36" +id="fnref-6.36"><sup>[36]</sup></a> 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 +<a name="page106"></a> +every geologist.<a href="#fn-6.37" name="fnref-6.37" +id="fnref-6.37"><sup>[37]</sup></a> 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.<a href="#fn-6.38" name="fnref-6.38" +id="fnref-6.38"><sup>[38]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-6.36" id="fn-6.36"></a> <a href="#fnref-6.36">[36]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-6.37" id="fn-6.37"></a> <a href="#fnref-6.37">[37]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-6.38" id="fn-6.38"></a> <a href="#fnref-6.38">[38]</a> +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 <i>tendency</i> to an upward movement in the +earth’s surface, and certainly no movement of subsidence. +</p> + +<p> +<i>On the relations of the areas of subsidence and elevation.</i>—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,<a href="#fn-6.39" name="fnref-6.39" +id="fnref-6.39"><sup>[39]</sup></a> so that the prevailing movements now in +progress, seem to accord with the actual states of surface of the great +divisions of the world. +</p> + +<p class="footnote"> +<a name="fn-6.39" id="fn-6.39"></a> <a href="#fnref-6.39">[39]</a> +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. +</p> + +<p>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 +<a name="page107"></a> +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.</p> + +<p> +I infer from Mr. Hopkins’ researches,<a href="#fn-6.40" name="fnref-6.40" +id="fnref-6.40"><sup>[40]</sup></a> 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<a href="#fn-6.41" name="fnref-6.41" id="fnref-6.41"><sup>[41]</sup></a> +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. +</p> + +<p class="footnote"> +<a name="fn-6.40" id="fn-6.40"></a> <a href="#fnref-6.40">[40]</a> +“Researches in Physical Geology,” Transact. Cambridge Phil. Soc., +vol. vi, part i. +</p> + +<p class="footnote"> +<a name="fn-6.41" id="fn-6.41"></a> <a href="#fnref-6.41">[41]</a> +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. +</p> + +<p>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 +<a name="page108"></a> +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.</p> + +<p><i>Recapitulation.</i>—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 +<a name="page109"></a> +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.</p> + +<p> +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. +</p> + +<p>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 +<a name="page110"></a> +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.</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="page111"></a><a name="appendix"></a>APPENDIX.<br/> +<small>CONTAINING A DETAILED DESCRIPTION OF THE REEFS AND ISLANDS IN <a +href="#PlateIII">PLATE III.</a></small></h2> + +<p>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.</p> + +<p>The W<small>ESTERN</small> S<small>HORES</small> <small> +OF</small> A<small>MERICA</small> appear to be entirely without +coral-reefs; south of the equator the survey of the <i>Beagle</i>, +and north of it, the published charts show that this is the case. +Even in the Bay of <i>Panama</i>, where corals flourish, there are +no true coral-reefs, as I have been informed by Mr. Lloyd. There +are no coral-reefs in the <i>Galapagos</i> Archipelago, as I know +from personal inspection; and I believe there are none on the <i> +Cocos, Revilla-gigedo</i>, and other neighbouring islands. <i> +Clipperton</i> 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 <i>E</i>, and +<i>Sala</i>, and <i>Gomez</i> Islands, which do not appear to be +surrounded by reefs.</p> + +<p>The L<small>OW</small> A<small>RCHIPELAGO</small>.—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 (<i>Wolchonsky</i>) 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 +<a name="page112"></a> +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 <i>Beagle</i>; and notices +of other atolls are scattered through several publications. The <i> +Actæon</i> group in this archipelago has lately been +discovered (<i>Geograph. Journ.</i>, vol. vii, p. 454); it consists +of three small and low islets, one of which has a lagoon. Another +lagoon-island has been discovered (<i>Naut. Mag.</i>, 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: <i> +Elizabeth</i> 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. <i>Pitcairn</i> 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. <i>Gambier</i> Islands +(see <a href="#PlateI">Plate I</a> Fig. 8), are encircled by a +barrier-reef; the greatest depth within is thirty-eight fathoms; +coloured pale blue. <i>Aurora</i> 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 (<a href="#page71"></a>), 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.</p> + +<p>The S<small>OCIETY</small> 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 <i>Coquille’s</i> Voyage” there is a good general +chart of the group, and separate plans of some of the islands. <i> +Tahiti</i>, 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 +(<i>Naut. Mag.</i>, 1836, p. 264) on the N.E. coast of the island, +where none are laid down by Cook. At <i>Eimeo</i> 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. +<i>Tapamanoa</i> 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. +<a name="page113"></a> +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 <i> +Teturoa</i> 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 (<i>West of England Journal</i>, vol. i, p. 54) +describes it as consisting of a mere narrow ridge, I have left it +uncoloured. <i>Maitea</i>, 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 <i>Coquille’s</i> Voyage.” <i>Maurua</i> 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 <i> +Coquille’s</i> 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 <i>Huaheine</i> 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 <i>Toubai</i> (Motou-iti +of the “<i>Coquille’s</i> 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 <a href="#PlateI"> +Plate I.</a> There are three low coral-groups lying a little E. of +the Society Archipelago, and almost forming part of it, namely <i> +Bellinghausen</i>, which is said by Kotzebue (“Second Voyage,” vol. +ii, p. 255), to be a lagoon-island; <i>Mopeha</i>, which, from +Cook’s description (“Second Voyage,” book iii, chap. i), no doubt +is an atoll; and the <i>Scilly</i> Islands, which are said by +Wallis (“Voyage,” chap. ix) to form a <i>group</i> of <i>low</i> +islets and shoals, and, therefore, probably, they compose an atoll: +the two former have been coloured blue, but not the latter.</p> + +<p>M<small>ENDANA</small> <small>OR</small> +M<small>ARQUESAS</small> 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 <i>Geograph. Journ.</i> 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, <i>Dominicana, Christiana,</i> and <i>Roapoa</i>; their +beaches are strewed with rounded masses of coral, and +<a name="page114"></a> +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.</p> + +<p>C<small>OOK</small> or H<small>ARVEY</small> and +A<small>USTRAL</small> +I<small>SLAND</small>.—<i>Palmerston</i> Island is minutely +described as an atoll by Captain Cook during his voyage in 1774; +coloured blue. <i>Aitutaki</i> was partially surveyed by the <i> +Beagle</i> (see map accompanying “Voyages of <i>Adventure</i> and +<i>Beagle</i>”); 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 <i>Beagle</i> +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.—<i>Manouai</i> or <i>Harvey</i> 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 <i>Mitiaro</i> Island, as I am informed by Mr. +Williams, the reef is attached to the shore; coloured +red.—<i>Mauki</i> or Maouti; the reef round this island +(under the name of Parry Island, in the “Voyage of H.M.S. <i> +Blonde</i>,” 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.—<i>Aitu</i>, 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.—<i>Fenoua-iti</i>; 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 +“<i>Coquille’s</i> 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.—<i>Rarotonga</i>; Mr. Williams informs me that it is a +lofty basaltic island with an attached reef; coloured +red.—There are three islands, <i>Rourouti, Roxburgh</i>, and +<i>Hull</i>, 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.—<i>Mangaia</i>; +height about three hundred feet; “the surrounding reef joins the +shore” (Williams, “Narrative,” p. 18); coloured +red.—<i>Rimetara</i>; 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 (<i>Naut. Mag.</i>, 1839, p. 738); coloured +red.—<i>Rurutu</i>; 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 +<a name="page115"></a> +surrounded, like the neighbouring islands by a reef; he must +have meant a distant reef.—<i>Toubouai</i>; 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.—<i>Raivaivai</i>, 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.—<i>Lancaster</i> Reef, described in <i> +Naut. Mag.</i>, 1833 (p. 693), as an extensive crescent-formed +coral-reef. I have not coloured it.—<i>Rapa</i>, or Oparree; +from the accounts given of it by Ellis and Vancouver, there does +not appear to be any reef.—<i>I. de Bass</i> is an adjoining +island, of which I cannot find any account.—<i>Kemin</i> +Island; Krusenstern seems hardly to know its position, and gives no +further particulars.</p> + +<h4>I<small>SLANDS</small> <small>BETWEEN</small> <i>the Low +and Gilbert Archipelagoes.</i></h4> + +<p><i>Caroline</i> Island (10° S., 150 deg W.) is described by +Mr. F. D. Bennett (<i>Geograph. Journ.</i>, vol. vii, p. 225) as +containing a fine lagoon; coloured blue.—<i>Flint</i> 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.—<i>Wostock</i> 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.—<i>Penrhyn</i> 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.—<i>Slarbuck</i> Island (5° S., +156° W.) is described in Byron’s “Voyage in the <i>Blonde</i>” +(p. 206) as formed of a flat coral-rock, with no trees; the height +not given; not coloured.—<i>Malden</i> 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.—<i>Jarvis</i>, or <i>Bunker</i> Island (0° 20′ S., +160° W.) is described by Mr. F. D. Bennett (<i>Geograph. +Journ.</i>, vol. vii, p. 227) as a narrow, low strip of +coral-formation; not coloured.—<i>Brook</i>, is a small low +island between the two latter; the position, and perhaps even the +existence of it is doubtful; not coloured.—<i>Pescado</i> and +<i>Humphrey</i> Islands; I can find out nothing about these +islands, except that the latter appears to be small and low; not +coloured.—<i>Rearson</i>, or Grand Duke Alexander’s (10° +S., 161° W.); an atoll, of which a plan is given by +Bellinghausen; blue.—<i>Souvoroff</i> 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 +<a name="page116"></a> +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.—<i>Danger</i> 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.—<i>Clarence</i> Island (9° S., 172° W.); +discovered in the <i>Pandora</i> (G. Hamilton’s “Voyage,” p. 75): +it is said, “in running along the land, we saw several canoes +crossing the <i>lagoons</i>;” 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. <i>York</i> Island (8° S., 172° W.) is +described by Commodore Byron (chap. x of his “Voyage”) as an atoll; +blue.—<i>Sydney</i> 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.—<i>Phoenix</i> 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.—<i>New Nantucket</i> (0° 15′ N., +174° W.). From the French chart it must be a low island; I can +find nothing more about it or about <i>Mary</i> Island; both +uncoloured.—<i>Gardner</i> Island (5° S., 174° W.) +from its position is certainly the same as <i>Kemin</i> Island +described (Krusenstern, p. 435, Appen. to Mem., published 1827) as +having a lagoon in its centre; blue.</p> + +<h4>I<small>SLANDS</small> <small>SOUTH</small> <i>of the +Sandwich Archipelago.</i></h4> + +<p><i>Christmas</i> 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 (<i>Geograph. Journ.</i>, 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.—<i>Fanning</i> 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.—<i>Washington</i> +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.—<i>Palmyra</i> 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.—<i>Smyth’s</i> or Johnston’s +<a name="page117"></a> +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.</p> + +<p> +S<small>ANDWICH</small> +A<small>RCHIPELAGO</small>.—<i>Hawaii</i>; 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.—<i>Maui</i>; 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.—<i>Morotoi</i>, 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.—<i>Oahu</i>; 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.—<i>Atooi</i>, 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.—<i>Oneehow</i>; 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 <i> +Gledstanes</i> 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 <i> +Ocean</i> 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.</p> + +<p>S<small>AMOA</small> or N<small>AVIGATOR</small> +G<small>ROUP</small>.—Kotzebue, in his “Second Voyage,” +<a name="page118"></a> +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 <i>Savaii, Maouna, Orosenga</i>, and <i> +Manua</i>, 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 <i>Oyolava</i>, +and returning again to it: all these islands have been coloured +red.—A chart of <i>Rose</i> 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.</p> + +<p><i>Beveridge</i> Reef, 20° S., 167° W., is described in +the <i>Naut. Mag.</i> (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.</p> + +<p><i>Savage</i> 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.</p> + +<p>F<small>RIENDLY</small> +A<small>RCHIPELAGO</small>.—<i>Pylstaart</i> 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.—<i>Tongatabou</i>: In the “Atlas of the Voyage of +the <i>Astrolabe</i>,” 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 <i>Eoaigee</i>, is represented as +fringed; coloured red.—<i>Eoua.</i> 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.</p> + +<p> +<a name="page119"></a> +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 <i>Anamouka, Komango, Kotou, Lefouga, Foa</i>, +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.—<i>Toufoa and Kao</i>, forming the western part +of the group, according to Forster have no reefs; the former is an +active volcano.—<i>Vavao.</i> 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 <i>Latte</i> and +<i>Amargura</i>, for I have not seen plans on a large scale of +them, and do not know whether they are fringed.</p> + +<p><i>Niouha</i>, 16° S., 174° W., or <i>Keppel</i> Island +of Wallis, or <i>Cocos</i> 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 +<i>Boscawen</i> Island, immediately adjoining, has no reef of any +kind; uncoloured.</p> + +<p><i>Wallis</i> 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 <i>Naut. Mag.</i>, July 1833, p. +376, shows the same fact; blue.</p> + +<p><i>Alloufatou</i>, or <i>Horn</i> Island, <i>Onouafu</i>, or <i> +Proby</i> Island, and <i>Hunter</i> Islands, lie between the +Navigator and Fidji groups. I can find no distinct accounts of +them.</p> + +<p> +F<small>IDJI</small> or V<small>ITI</small> G<small>ROUP</small>.—The +best chart of the numerous islands of this group, will be found in the +“Atlas of the <i> Astrolabe’s</i> 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 <i>Astrolabe</i> 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 <i> +Batoa</i>, or <i>Turtle</i> 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 (<i>Ibid</i>., 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 +<a name="page120"></a> +often mentioned,”—namely, atolls. South of Batoa, lies the +high island of <i>Ono</i>, 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.</p> + +<p><i>Rotoumah</i>, 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.</p> + +<p><i>Independence</i> Island, 10° S., 179° E., is +described by Mr. G. Bennett, (<i>United Service Journ.</i>, 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.</p> + +<p>E<small>LLICE</small> G<small>ROUP</small>.—<i>Oscar, +Peyster</i>, and <i>Ellice</i> Islands are figured in Arrowsmith’s +“Chart of the Pacific” (corrected to 1832) as atolls, and are said +to be very low; blue.—<i>Nederlandisch</i> 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.—<i>Gran +Cocal</i> 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.—<i>St. Augustin.</i> From a +chart and view of it, given in the “Atlas of the <i>Coquille’s</i> +Voyage,” it appears to be a small atoll, with its lagoon partly +filled up; coloured blue.</p> + +<p>G<small>ILBERT</small> G<small>ROUP</small>.—The chart of +this group, given in the “Atlas of the <i>Coquille’s</i> Voyage,” +at once shows that it is composed of ten well characterised atolls. +In D’Urville and Lottin’s chart, <i>Sydenham</i> 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 “<i>Coquille’s</i> +Atlas.” Some narrow strip-like reefs project from the southern side +of <i>Drummond</i> atoll, and render it irregular. The southern +island of the group is called <i>Chase</i> (in some charts, <i> +Rotches</i>); of this I can find no account, but Mr. F. D. Bennett +discovered (<i>Geograph. Journ.</i>, 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.—<i>Pitt</i> Island, at the extreme northern point of +the group, is left uncoloured, as its exact position and nature +<a name="page121"></a> +is not known.—<i>Byron</i> 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.</p> + +<p><i>Ocean, Pleasant</i>, and <i>Atlantic</i> 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.</p> + +<p>M<small>ARSHALL</small> G<small>ROUP</small>.—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 <i>little</i> 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 <i>Gaspar Rico</i>, or <i>Cornwallis</i> 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 <i>Baring</i> Island, of which little +is known (see Krusenstern’s “Appendix,” 1835, p. 149). I have left +it uncoloured; but <i>Boston</i> Island I have coloured blue, as it +is described (<i>Ibid</i>.) as consisting of fourteen small +islands, which, no doubt, enclose a lagoon, as represented in a +chart in the “‘Coquille’s’ Atlas.”—Two islands, <i>Aur +Kawen</i> and <i>Gaspar Rico</i>, 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.</p> + +<p>N<small>EW</small> H<small>EBRIDES</small>.—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.—<i>Matthew’s +Rock</i>, an active volcano, some way south of the group (of which +a plan is given in the “Atlas of the <i>Astrolabe’s</i> Voyage”) +does not appear to have reefs of any kind about +it.—<i>Annatom</i>, the southernmost of the Hebrides; from a +rough woodcut given in the <i>United Service Journal</i> (1831, +part iii, p. 190), accompanying a paper by Mr. Bennett, it appears +that the shore is fringed; coloured red.—<i>Tanna.</i> +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 +<a name="page122"></a> +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.—<i>Immer</i> +is described (<i>United Service Journal,</i> 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.—<i>Erromango</i> Island; Cook (“Second Voyage,” vol. +ii, p. 45, 4to ed.) speaks of rocks everywhere <i>lining</i> 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 +<i>Singapore Chron.</i>, alludes to the <i>reefs</i> 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.—<i>Sandwich</i> 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.—<i>Mallicollo.</i> 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.—<i>Aurora</i> and <i>Pentecost</i> Islands, according to +Bougainville, apparently have no reefs; nor has the large island of +<i>S. Espiritu</i>, nor <i>Bligh</i> Island or <i>Banks’</i> +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.</p> + +<p>S<small>ANTA</small> C<small>RUZ</small> +G<small>ROUP</small>.—<i>Vanikoro</i> (Fig. 1, <a href= +"#PlateI">Plate I</a>) offers a striking example of a barrier- +reef: it was first described by the Chevalier Dillon, in his +voyage, and was surveyed in the <i>Astrolabe</i>; coloured pale +blue.—<i>Tikopia</i> and <i>Fataka</i> Islands appear, from +the descriptions of Dillon and D’Urville, to have no reefs; <i> +Anouda</i> is a low, flat island, surrounded by cliffs +(“<i>Astrolabe</i> Hydrog.” and Krusenstern, “Mem.” vol. ii, p. +432); these are uncoloured. <i>Toupoua</i> (<i>Otooboa</i> 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.—<i>Santa-Cruz.</i> 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.—<i>Tinakoro</i> is a constantly active +volcano without reefs.—<i>Mendana Isles</i> (mentioned by +Dillon under the name of <i>Mammee</i>, etc.); said by Krusenstern +to be low, and intertwined with reefs. I do not believe they +include a lagoon; I have left them uncoloured.—<i>Duff’s</i> +Islands compose a small group +<a name="page123"></a> +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. <i>Kennedy</i> +Island, N.E. of Duff’s. I have been unable to find any account of +it.</p> + +<p>N<small>EW</small> C<small>ALEDONIA</small>.—The great +barrier-reefs on the shores of this island have already been +described (Fig. 5, <a href="#PlateII">Plate II</a>). 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 <i>Loyalty</i> group is situated +eastward of this island; from the chart and description given in +the “Voyage of the <i>Astrolabe</i>,” they do not appear to have +any reefs; north of this group, there are some extensive low reefs +(called <i>Astrolabe</i> and <i>Beaupré</i>,) which do not +seem to be atoll-formed; these are left uncoloured.</p> + +<p>A<small>USTRALIAN</small> +B<small>ARRIER</small>-R<small>EEF</small>.—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, +<i>Bampton</i> shoal, <i>Frederic, Vine</i> or Horse-shoe, and <i> +Alert</i> reefs; these have been coloured dark blue.</p> + +<p>L<small>OUISIADE</small>.—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 <i>Laughlan</i> +Islands, the reefs round which are engraved in the “Atlas of the +Voyage of the <i>Astrolabe</i>,” 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 <i>Wells</i> reef, +described in G. Hamilton’s “Voyage in H.M.S. <i>Pandora</i>” (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.</p> + +<p>S<small>OLOMON</small> A<small>RCHIPELAGO</small>.—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 +<a name="page124"></a> +“Voyage,” regarding the southern islands of the group, so have +left them uncoloured.—<i>Malayta</i> Island in a rough MS. +chart in the Admiralty has its northern shore +fringed.—<i>Ysabel</i> 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.—<i>Choiseul</i> Island. In Bougainville’s +“Chart of Choiseul Bay,” parts of the shores are fringed by +coral-reefs.—<i>Bougainville</i> 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.—<i>Bouka</i> 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 +<i>Coquille’s</i> 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.</p> + +<p>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 <i>Candelaria.—Outong Java</i>, +according to the Spanish navigator, Maurelle, is thus +characterised; but this is the only one which I have ventured to +colour blue.</p> + +<p>N<small>EW</small> I<small>RELAND</small>.—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 <i>Coquille</i> and <i>Astrolabe</i>.” M. Lesson observes that +the reefs are open in front of each streamlet. The <i>Duke of +York’s</i> Island is also fringed; but with regard to the other +parts of <i>New Ireland, New Hanover</i>, 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.</p> + +<p>N<small>EW</small> B<small>RITAIN AND THE</small> +N<small>ORTHERN</small> S<small>HORE OF</small> N<small>EW</small> +G<small>UINEA</small>.—From the charts in the “Voyage of the +<i>Astrolabe</i>,” and from the “Hydrog. Memoir,” it appears that +these coasts are entirely without reefs, as are the <i>Schouten</i> +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.</p> + +<p>A<small>DMIRALTY</small> G<small>ROUP</small>.—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 <i> +Elizabeth</i> 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 +<a name="page125"></a> +containing a lagoon.—Near here, also lies <i> +Circular-reef</i> (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 <i> +Circular-reef</i> dark blue.—The <i>Anachorites, +Echequier</i>, and <i>Hermites</i>, 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 +<i>Durour</i> Island, which is described by Carteret as low.</p> + +<p> +The C<small>AROLINE</small> A<small>RCHIPELAGO</small> 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, <a href="#PlateI">Plate I.</a> Commencing with the eastern part; +the encircling reef round <i>Ualen</i> 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 <i>Coquille’s</i> +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.—<i>Pouynipète</i>, 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.—<i>Hogoleu</i> or <i>Roug</i>. 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 <i>Astrolabe</i>,” 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<a href="#fn-7.1" +name="fnref-7.1" id="fnref-7.1"><sup>[1]</sup></a> 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 <i> Macaskill</i> and <i>Duperrey</i>, of which +plans are given in the +<a name="page126"></a> +“Atlas of the <i>Coquille’s</i> Voyage.” There are, however, some low small +islands of coral-formation, namely <i>Ollap, Tamatam, Bigali, Satahoual</i>, +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 <i>Coquille</i>,” 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 <i> +L’Uranie</i> 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 <i> Piguiram</i>, is not coloured, because I have +found no account of it. <i>Nougouor</i>, or <i>Monte Verdison</i>, which was +not visited by Lutké, is described and figured by Mr. Bennett (<i>United +Service Journal</i>, January 1832) as an atoll. All the above-mentioned islands +have been coloured blue. +</p> + +<p class="footnote"> +<a name="fn-7.1" id="fn-7.1"></a> <a href="#fnref-7.1">[1]</a> +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. +</p> + +<p>W<small>ESTERN</small> P<small>ART OF THE</small> +C<small>AROLINE</small> +A<small>RCHIPELAGO</small>.—<i>Fais</i> 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.—<i>Philip</i> Island., I believe, is low; but +Hunter, in his “Historical Journal,” gives no clear account of it; +uncoloured.—<i>Elivi</i>; from the manner in which the islets +on the reefs are engraved, in the “Atlas of the <i>Astrolabe’s</i> +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.—<i>Gouap</i> (<i>Eap</i> of +Chamisso), is a high island with a reef (see chart in “Voyage of +the <i>Astrolabe</i>”), 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. <i>Astrolabe</i> +Voyage,” p. 436) for ships within the reef, if a passage could be +found; coloured pale blue.—<i>Goulou</i>, from the chart in +the “<i>Astrolabe’s</i> Atlas,” appears to be an atoll. D’Urville +(“Hydrog. Descript.” p. 437) speaks of the low islets on the reef; +coloured dark blue.</p> + +<p>P<small>ELEW</small> I<small>SLANDS</small>.—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.—<i>Spanish, Martires, Sanserot, Pulo +Anna</i> and <i>Mariere</i> Islands are not coloured, because I +know nothing about them, excepting that according to Krusenstern, +the second, third, and fourth mentioned, are +<a name="page127"></a> +low, placed on coral-reefs, and therefore, perhaps, contain +lagoons; but Pulo Mariere is a little higher.</p> + +<p>M<small>ARIANA</small> A<small>RCHIPELAGO</small>, or +L<small>ADRONES</small>.—<i>Guahan.</i> 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.—<i>Rota</i>. “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.—<i>Tinian. The eastern</i> coast is +precipitous, and is without reefs; but the western side is fringed +like the last island; coloured red.—<i>Saypan</i>. 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.—<i>Farallon de Medinilla</i>, +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.—<i>Mangs</i>, 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 <i>Folger</i> and <i> +Marshall</i> 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.</p> + +<p>B<small>ONIN OR</small> A<small>RZOBISPO</small> +G<small>ROUP</small>.—<i>Peel</i> 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.—<i>Grampus</i> Island to the eastward, does not appear +(Meare’s “Voyage,” p. 95) to have any reefs, nor does <i> +Rosario</i> Island (from Lutké’s chart), which lies to the +westward. Respecting the few other islands in this part of the sea, +namely the <i>Sulphur</i> 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.</p> + +<p>W<small>EST</small> E<small>ND OF</small> N<small>EW</small> +G<small>UINEA</small>.—<i>Port Dory.</i> From the charts in +the “Voyage of the <i>Coquille</i>,” it would appear that the coast +in this part +<a name="page128"></a> +is fringed by coral-reefs; M. Lesson, however, remarks that the +coral is sickly; coloured red.—<i>Waigiou.</i> 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 <i>Guedes</i> (or <i> +Freewill</i>, or <i>St. David’s</i>), 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.—<i>Carteret’s +Shoals</i>, in 2° 53′ N., are described as circular, with stony +points showing all round, with deeper water in the middle; coloured +blue.—<i>Aiou</i>; the plan of this group, given in the +“Atlas of the Voyage of the <i>Astrolabe</i>,” 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 <i>Arru, Timor-laut</i>, and <i> +Tenimber</i> 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 <i>Ceram-laut, +Goram</i> and <i>Keffing</i> 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).</p> + +<p>C<small>ERAM</small>.—In various charts which I have +examined, several parts of the coast are represented as fringed by +reefs.—<i>Manipa</i> 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.—<i>Bourou</i>; +parts of this island appear fringed by coral-reefs, namely, the +eastern coast, as seen in Freycinet’s chart; and <i>Cajeli Bay</i>, +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 A<small>MBOINA</small> G<small>ROUP</small> are fringed +by reefs; for instance, <i>Noessa, Harenca</i>, and <i>Ucaster</i>, +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 <i> +Xulla</i> Isles extends: I have not been able to find out anything +about them, excepting +<a name="page129"></a> +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.—<i>Mysol Group</i>; 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.—<i>Guebe</i>, 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.—<i>Gilolo</i>. In a chart published by Dalrymple, the +numerous islands on the western, southern (<i>Batchian</i> and the +<i>Strait of Patientia</i>), 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 <i>dans les plupart</i> 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.—<i>Morty</i> 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.—<i>Celebes.</i> 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 <i>Straits of Limbe</i>, and parts of the adjoining shore, +appear to be fringed: the east side of the bay of <i>Manado</i>, +has deep water, and is fringed by sand and coral (“<i>Astrol.</i> +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 <i> +Serangani</i>, 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 +<i>Karkalang</i>, 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.</p> + +<p>I<small>SLANDS NEAR</small> T<small>IMOR</small>.—The +account of the following islands is taken from Captain D. Kolff’s +“Voyage,” in 1825, translated by Mr. W. Earl, from the +Dutch.—<i>Lette</i> has “reefs extending along shore at the +distance of half a mile from the land.”—<i>Moa</i> has reefs +on the S.W. part.—<i>Lakor</i> has a reef lining its shore; +these islands are coloured red.—Still more eastward, <i> +Luan</i> 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.—<i>Kissa</i>, 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.—<i>Timor</i>; 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.—<i>Savu</i>, +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.—<i>Sandalwood</i> 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, +<a name="page130"></a> +and generally bold, this probably is a barrier-reef, but I have +not ventured to colour it.</p> + +<p>N.W. C<small>OAST OF</small> A<small>USTRALIA</small>.—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 <i>Naut. Mag.</i> 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.—<i>Rowley Shoals.</i> 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.—<i>Scott’s Reefs</i>, +lying north of Rowley Shoals, are briefly described by Captain +Wickham (<i>Naut. Mag.</i> 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.</p> + +<p>Proceeding westward along the great volcanic chain of the East +Indian Archipelago, <i>Solor Strait</i> is represented in a chart +published by Dalrymple from a Dutch MS., as fringed; as are parts +of <i>Flores</i>, of <i>Adenara</i>, and of <i>Solor.</i> 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 <i>Sapy</i> Bay. From the same +authority it appears (p. 610) that reefs fringe the island of <i> +Timor-Young</i>, on the N. shore of Sumbawa; and, likewise (p. +600), +<a name="page131"></a> +that <i>Bally</i> town in <i>Lombock</i>, 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.—<i>Bally</i> 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.</p> + +<p>J<small>AVA</small>.—My information regarding the reefs of +this great island is derived from the chart just mentioned. The +greater part of <i>Maduara</i> 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 <i>Sourabaya.</i> The islets and parts of the N. coast of +Java, west of <i>Point Buang</i>, or <i>Japara</i>, are fringed by +reefs, said to be of coral. <i>Lubeck</i>, or <i>Bavian</i> +Islands, lying at some distance from the shore of Java, are +regularly fringed by coral-reefs. <i>Carimon Java</i> 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 <i>Sunda Strait</i>, where +the water is from forty to eighty fathoms deep, and the islets near +<i>Batavia</i> 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 <i> +Segorrowodee</i> Bay, and the extreme S.E. and E. portions are +likewise fringed by coral-reefs; all the above-mentioned places +coloured red.</p> + +<p><i>Macassar Strait</i>; the east coast of Borneo appears, in +most parts, free from reefs, and where they occur, as on the east +coast of <i>Pamaroong</i>, the sea is very shallow; hence no part +is coloured. In <i>Macassar</i> 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 <i>Postillions, Kalatoa, Tokan-Bessees</i>, +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 <i>Bonin</i>, between the two +southern arms of Celebes, there are numerous coral-reefs; +<a name="page132"></a> +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 <i>Bouton</i> 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 <i>Coquille</i>,” 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.</p> + +<p>S<small>UMATRA</small>.—Commencing with the west coast and +outlying islands, <i>Engano Island</i> 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.—<i>Rat Island</i> +(3° 51′ S.) is surrounded by reefs of coral, partly dry at low +water, (Horsburgh, vol. ii, p. 96).—<i>Trieste Island</i> +(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.—<i>Pulo Dooa</i> (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.—<i>Pulo +Pisang</i>; 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.—<i>Pulo Mintao</i> is +lined with reefs on its west side (Horsburgh, vol. ii, p. +107).—<i>Pulo Baniak</i>; the same authority (vol. ii, p. +105), speaking of a part, says it is faced with +coral-rocks.—<i>Minguin</i> (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).—<i>Pulo Brassa</i> (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.</p> + +<p>N<small>ICOBAR</small> I<small>SLANDS</small>.—The chart +represents the islands of this group as fringed by reefs. With +regard to <i>Great Nicobar</i>, Captain Moresby informs me, that it +is fringed by reefs of coral, extending between two and three +hundred yards from the shore. The <i>Northern Nicobars</i> appear +so regularly fringed in the published charts, that I have no doubt +the reefs are of coral. This group, therefore, is coloured red.</p> + +<p>A<small>NDAMAN</small> I<small>SLANDS</small>.—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 +<a name="page133"></a> +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.</p> + +<p>The coast of <i>Malacca, Tenasserim</i> and the coasts +northward, appear in the greater part to be low and muddy: where +reefs occur, as in parts of <i>Malacca Straits</i>, and near <i> +Singapore</i>, 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 <i>Natunas</i> +and the <i>Anambas</i> Islands are represented in the charts on a +large scale, published in the “Atlas of the Voyage of the <i> +Favourite</i>,” as fringed by reefs of coral, with very shoal water +within them.—<i>Tumbelan</i> and <i>Bunoa</i> Islands (1° +N.) are represented in the English charts as surrounded by a very +regular fringe.—<i>St. Barbes</i> (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 <i>Borneo</i> +at <i>Tunjong Apee</i> 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 <i>Pulo Leat</i>, in Gaspar +Strait, <i>Lucepara</i>, and <i>Carimata</i>; but as the sea is +confined and shallow, and the reefs not very regular, I have left +them uncoloured.</p> + +<p>The water shoals gradually towards the whole west coast of <i> +Borneo</i>: 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 <i>Palawan</i>, are fringed by very distant coral-reefs; thus +the reefs in the case of <i>Balabac</i> 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 <i>Sooloo Archipelago</i>, about which I have been able to +obtain very little information; <i>Pangootaran</i>, 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.—<i>Tahow Bank</i>, 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 <i> +Basselan</i>, 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, <i> +Cagayanes</i> and <i>Cavilli</i>, 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 +<a name="page134"></a> +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 <i>Palawan</i>, 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.</p> + +<p>P<small>HILIPPINE</small> A<small>RCHIPELAGO</small>.—A +chart on a large scale of <i>Appoo Shoal</i>, 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.—<i>Mindoro</i>: the N.W. coast is represented +in several charts, as fringed by a reef, and <i>Luban Island</i> is +said, by Horsburgh (vol. ii, p. 436), to be “lined by a +reef.”—<i>Luzon</i>: 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 <i> +Masbate</i>, and nearly the whole of <i>Bohol</i>; which are both +coloured red. On the S. shore of <i>Magindanao</i>, 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.</p> + +<p>B<small>ABUYAN</small> I<small>SLANDS</small>.—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.—B<small>ASHEE</small> I<small>SLANDS</small>: 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 +<a name="page135"></a> +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.—F<small>ORMOSA</small>. 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.—P<small>ATCHOW, OR</small> +M<small>ADJIKO</small>-S<small>IMA</small> G<small>ROUPS</small>. +<i>Patchuson</i> 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.—<i>Taypin-san</i>; 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 <i>Kumi</i> (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.—L<small>OO</small> +C<small>HOO</small>. 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.—P<small>ESCADORES</small> (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.</p> + +<p>C<small>HINA</small> S<small>EA</small>.—Proceeding from +north to south, we first meet the <i>Pratas Shoal</i> (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, +<a name="page136"></a> +there appear to be several parts where a ship might find +anchorage outside the breakers;” coloured blue.—The <i> +Paracells</i> 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 +(<i>Prattle, Robert, Drummond</i>, etc.) are so arranged round a +moderately shallow space, as to appear as if they had once formed +one large atoll.—<i>Bombay Shoal</i> (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.—<i>Discovery Shoal</i>, +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.—<i>Macclesfield +Bank</i>: 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.—<i>Scarborough Shoal</i>: 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: <i>Swallow Shoal</i>, according to Horsburgh +(vol. ii, p. 431) “is formed, <i>like most</i> of the shoals +hereabouts, of a belt of coral-rocks, “with a basin of deep water +within.”—<i>Half-Moon Shoal</i> 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.—<i>Bombay Shoal</i> appears (Horsburgh, vol. ii, p. +432) “to be a basin of smooth water surrounded by breakers.” These +three shoals I have coloured blue.—The <i>Paraquas Shoals</i> +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 <i>Borneo</i>, and to thirty miles from the +northern part of <i>Palawan.</i> 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 +<a name="page137"></a> +is not very deep, these reefs may have grown up from +inequalities on the bank: I have not coloured them.—The coast +of <i>China, Tonquin</i>, and <i>Cochin-China</i>, 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 <i> +Favourite</i>.”</p> + +<p>I<small>NDIAN</small> O<small>CEAN</small>.—<i>South +Keeling</i> atoll has been specially described. Nine miles north of +it lies North Keeling, a very small atoll, surveyed by the <i> +Beagle</i>, the lagoon of which is dry at low +water.—<i>Christmas Island</i>, lying to the east, is a high +island, without, as I have been informed by a person who passed it, +any reefs at all.—C<small>EYLON</small>: a space about eighty +miles in length of the south-western and southern shores of these +islands has been described by Mr. Twynam (<i>Naut. Mag.</i> 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 <i>Madura.</i></p> + +<p>C<small>HAGOS</small>, M<small>ALDIVA</small>, <small> +AND</small> L<small>ACCADIVE</small> +A<small>RCHIPELAGOES</small>.—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 <i>Chagos</i> and <i>Maldiva</i> +groups are entirely formed of great atolls, or lagoon-formed reefs, +surmounted by islets. In the <i>Laccadive</i> group, this structure +is less evident; the islets are low, not exceeding the usual height +of coral-formations (see Lieutenant Wood’s account, <i>Geograph. +Journ.</i>, 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.</p> + +<p><i>Sahia de Malha</i> 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 +<a name="page138"></a> +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.—<i>Cargados Carajos Bank.</i> 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.—<i>Ile de Sable</i> is a +little island, lying west of C. Carajos, only some toises in height +(“Voyage of the <i>Favourite</i>,” 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.—<i>Mauritius.</i> The reefs round this island +have been described in the chapter on fringing-reefs; coloured +red.—<i>Rodriguez.</i> 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.—<i>Bourbon.</i> 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.</p> + +<p>S<small>EYCHELLES</small>.—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 <i>Favourite</i>,” it appears that the east side of +<i>Mahe</i> and the adjoining islands of <i>St. Anne</i> and <i> +Cerf</i>, are regularly fringed by coral-reefs. A portion of the +S.E. part of <i>Curieuse Island</i>, the N., and part of the S.W. +shore of <i>Praslin Island</i>, and the whole west side of <i>Digue +Island</i>, appear fringed. From a MS. account of these islands by +Captain F. Moresby, in the Admiralty, it appears that <i> +Silhouette</i> 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. <i> +Amirantes Islands</i>: 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 +<a name="page139"></a> +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—<i>Platte, Alphonse, Coetivi, Galega, +Providence, St. Pierre, Astova, Assomption</i>, and <i> +Glorioso</i>, 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, <i>Juan de +Nova</i>, which appears from some plans and accounts to be an +atoll; but from others does not appear to be so; not coloured. +Secondly <i>Cosmoledo</i>; “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, <i>Aldabra</i>; 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.</p> + +<p>C<small>OMORO</small> +G<small>ROUP</small>.—<i>Mayotta</i>, 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.—<i>Johanna</i>; 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.—<i>Mohilla</i>; on the S. side of this island there +is +<a name="page140"></a> +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.—<i>Great Comoro Island</i> 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 <i>shore</i>, 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.</p> + +<p>M<small>ADAGASCAR</small>.—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 <i>Star +Bank</i> (in lat. 25° S.) the coast for ten miles is fringed by +a reef; coloured red. The shore immediately S. of <i>St. +Augustine’s Bay</i> appears fringed; but <i>Tullear</i> 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 +<i>Juan de Nova</i> (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 <i>Narrenda</i> +and <i>Passandava</i> Bays (14° 40′) and the islands in front +of <i>Radama Harbour</i> are represented in the plans as regularly +fringed, and have accordingly been coloured red. With respect to +the <i>East coast of Madagascar</i>, Dr. Allan informs me in a +letter, that the whole line of coast, from <i>Tamatave</i>, in +18° 12′, to <i>C. Amber</i>, at the extreme northern point of +the island, is bordered by coral-reefs. The land is low, +uneven, +<a name="page141"></a> +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 <i>British Sound</i>, +and near <i>Ngoncy</i>, of the above line of coast might have been +inferred. Lieutenant Boteler (vol. i, p. 155) speaks of “the reef +surrounding the island of <i>St. Mary’s</i> 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.</p> + +<p>E<small>AST</small> C<small>OAST OF</small> +A<small>FRICA</small>.—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 <i>Mukdeesha</i> (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 <i> +Juba</i>, a little S. of the equator, to <i>Lamoo</i> (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 <i> +Dundas Islands</i>), 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 <i>Patta</i>, 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 +<a name="page142"></a> +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.—<i>Maleenda</i> +(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.—<i>Mombas</i> (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.—<i>Pemba</i> (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.—<i>Zanzibar</i> 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.—<i>Latham’s Bank</i> is a small island, fringed by +coral-reefs; but being only ten feet high, it has not been +coloured.—<i>Monfeea</i> 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.—<i>Keelwa.</i> 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 <i>Lindy</i> and <i>Monghow</i> (9° 59′ and +10° 7′ S.) has the same structure; coloured +red.—<i>Querimba Islands</i> (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 +<a name="page143"></a> +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.—<i>Monabila</i> +(13° 32′ S.). In the plan of this harbour, the headlands +outside are fringed by reefs apparently of coral; coloured +red.—<i>Mozambique</i> (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 <i> +Mozambique</i> to <i>Delagoa Bay</i> 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.</p> + +<p>P<small>ERSIAN</small> G<small>ULF</small>.—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 <i>Ormuz</i> and <i>Larrack</i>, +appear so regularly fringed, that I have coloured them red. There +are certainly no atolls in the Persian Gulf. The shores of <i> +Immaum</i>, 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 <i>Arabia Felix</i>, and the +shores of <i>Socotra</i> appear from the charts and memoir of +Captain Haines (<i>Geograph. Journ.</i>, 1839, p. 125) to be +without any reefs. I believe there are no extensive coral-reefs on +any part of the coasts of <i>India</i>, except on the low +promontory of <i>Madura</i> (as already mentioned) in front of +Ceylon.</p> + +<p>R<small>ED</small> S<small>EA</small>.—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 +<a name="page144"></a> +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.</p> + +<p> +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 <i>Dhalac</i> 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 <i> Farsan Island</i>, 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 +<a name="page145"></a> +beds, containing but little broken coral,<a href="#fn-7.2" name="fnref-7.2" +id="fnref-7.2"><sup>[2]</sup></a> their shores are fringed by living +coral-reefs. +</p> + +<p class="footnote"> +<a name="fn-7.2" id="fn-7.2"></a> <a href="#fnref-7.2">[2]</a> +Rüppell, “Reise in Abyssinie,” Band. i, S. 247. +</p> + +<p> +From the account given by Rüppell<a href="#fn-7.3" name="fnref-7.3" +id="fnref-7.3"><sup>[3]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-7.3" id="fn-7.3"></a> <a href="#fnref-7.3">[3]</a> +<i>Ibid</i>., S. 245. +</p> + +<p>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.</p> + +<p><i>The west coast of the Red Sea between lat. 19° and +22°.</i>—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 +<a name="page146"></a> +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 <i> +Macowa</i> (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.</p> + +<p><i>The west coast from lat. 22° to 24°.</i>—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. <i>On the west coast from lat. +19° to 17°</i> (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 +<a name="page147"></a> +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.</p> + +<p><i>Eastern coast.</i>—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.</p> + +<p>In the <i>southern parts</i> 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 +<i>northern parts of the western coast</i> (north of lat. 24° +30′), and likewise the shores of the chief part of the <i>Gulf of +Suez.</i> In the <i>Gulf of Acaba</i>, as I am informed by Captain +Moresby there are no coral-reefs, and the water is profoundly +deep.</p> + +<p>W<small>EST</small> I<small>NDIES</small>.—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.</p> + +<p>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 +<a name="page148"></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.</p> + +<p>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), +<a name="page149"></a> +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.</p> + +<p>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.</p> + +<p> +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,<a href="#fn-7.4" name="fnref-7.4" id="fnref-7.4"><sup>[4]</sup></a> 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 +<a name="page150"></a> +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<a +href="#fn-7.5" name="fnref-7.5" id="fnref-7.5"><sup>[5]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-7.4" id="fn-7.4"></a> <a href="#fnref-7.4">[4]</a> +In the <i>London and Edinburgh Philosophical Journal,</i> 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. +</p> + +<p class="footnote"> +<a name="fn-7.5" id="fn-7.5"></a> <a href="#fnref-7.5">[5]</a> +In the ordinary sea-charts, no lagoons appear on the coast of Florida, north of +26°; but Major Whiting (<i>Silliman’s Journal</i>, 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. +</p> + +<p>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 <i>Bahama Banks</i> 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 <i>Florida</i> are similarly fringed; +coloured red. C<small>UBA</small>: 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 “<i>baxo</i>,” 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 “<i>cays</i>,” 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 +<a name="page151"></a> +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 <i>Colorado Shoals</i> (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 <i>Colorados</i>, 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 <i>Isle of Pines</i> 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.</p> + +<p>Y<small>UCUTAN</small>.—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 <i>Honduras</i>) 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 +<a name="page152"></a> +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, <i>Turneffe, +Lighthouse</i>, and <i>Glover</i> 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. <i>Turneffe Reef</i> 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 <i>Northern Triangles</i>, on which there +is an islet between seventy and eighty feet, and to <i>Cozumel</i> +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.</p> + +<p>In front of the eastern <i>Mosquito</i> 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.</p> + +<p><i>Cayman Island:</i> 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.—<i>Jamaica:</i> 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.—<i>St. Domingo:</i> 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.—<i>Puerto Rico:</i> 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 +<a name="page153"></a> +southern side of the Island of <i>St. Thomas</i> is fringed; +most of the <i>Virgin Gorda</i> Islands, as I am informed by Mr. +Schomburgk, are fringed; the shores of <i>Anegada</i>, 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 +<i>Santa Cruz</i> appears in the Danish survey to be fringed (see +also Prof. Hovey’s account of this island, in <i>Silliman’s +Journal</i>, 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 <i> +Antilles</i>, 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, <i>Barbuda</i> and the western coasts of <i>Antigua</i> +and <i>Mariagalante</i> appear to be fringed: this is also the case +with <i>Barbadoes</i>, 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 <i>Martinique</i>, 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.</p> + +<p>T<small>HE</small> B<small>ERMUDA</small> I<small>SLANDS</small> +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 +<a name="page154"></a> +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, <i> +Ibid</i>., 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.</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="vol02"></a>GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.</h2> + +<h3><a name="page157"></a>CRITICAL INTRODUCTION.</h3> + +<p>The preparation of the series of works published under the +general title “Geology of the Voyage of the <i>Beagle</i>” 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 <i>Beagle</i> 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 <i>Beagle</i> 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.</p> + +<p>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 <i>Beagle.</i> His friend Henslow +too, on parting with him, had given him the advice to procure and +read the recently published first volume of the +<a name="page158"></a> +“Principles of Geology,” though he warned him against accepting +the views advocated by its author. During the time the <i> +Beagle</i> 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!</p> + +<p>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.”</p> + +<p>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 +<a name="page159"></a> +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.”</p> + +<p>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.”</p> + +<p>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 <i>Beagle</i>! 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.</p> + +<p>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 +<a name="page160"></a> +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.”</p> + +<p>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.</p> + +<p>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.</p> + +<p>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.”</p> + +<p>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.”</p> + +<p>Every geologist knows how full of interest and suggestiveness +is +<a name="page161"></a> +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.</p> + +<p>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.”</p> + +<p>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.”</p> + +<p>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.</p> + +<p>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 +<a name="page162"></a> +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.</p> + +<p>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.</p> + +<p>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.</p> + +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.<br/> +<br/> + +<p>One of the most striking parts of the book is that in which he +deals with the effects of denudation in producing “basal +wrecks” +<a name="page163"></a> +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.”</p> + +<p>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.</p> + +<p>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 +<a name="page164"></a> +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.</p> + +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.<br/> +<br/> + +<p>Some years before the voyage of the <i>Beagle,</i> 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.”</p> + +<p>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. <i>Challenger,</i> many of the islands studied +by +<a name="page165"></a> +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 <i>Challenger</i> 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.</p> + +<p class="right"> +J<small>OHN</small> W. J<small>UDD</small>. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="page167"></a><a name="chap2.01"></a>Chapter I<br/>ST. JAGO, IN THE +CAPE DE VERDE ARCHIPELAGO.</h2> + +<p class="letter">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.</p> + +<p> +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<a +href="#fn-8.1" name="fnref-8.1" id="fnref-8.1"><sup>[1]</sup></a>) 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. +</p> + +<p class="footnote"> +<a name="fn-8.1" id="fn-8.1"></a> <a href="#fnref-8.1">[1]</a> +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. +<i>Leven.</i> The square-topped hills (A, B, C, etc.) are put in merely by eye, +to illustrate my description. +</p> + +<p>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 +<a name="page168"></a> +intimately related in mineralogical composition, and in other +respects, with the lowest series of the coast-rocks, with which +they seem to be continuous.</p> + +<div class="fig" style="width:100%;"> +<img src="images/jago.jpg" width="399" height="256" alt="[Illustration: +Part of St. Jago, one of the Cape de Verde islands.]" /> +</div> + +<p class="letter"> +Part of St. Jago, one of the Cape de Verde islands. +</p> + +<p><i>Mineralogical description of the rocks of the lowest +series.</i>—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 +<a name="page169"></a> +surface, also, had previously been denuded to a great extent, as +is shown by many truncated dikes.</p> + +<p><i>Description of the calcareous deposit overlying the foregoing +volcanic rocks.</i>—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 <i>débris</i>, 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.</p> + +<p><i>Effects produced by the flowing of the superincumbent +basaltic lava over the calcareous deposit.</i>—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, +<a name="page170"></a> +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.</p> + +<p>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.</p> + +<p> +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 +<a name="page171"></a> +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,<a href="#fn-8.2" name="fnref-8.2" +id="fnref-8.2"><sup>[2]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-8.2" id="fn-8.2"></a> <a href="#fnref-8.2">[2]</a> +I am much indebted to Mr. E. W. Brayley in having given me the following +references to papers on this subject: Faraday in the <i>Edinburgh New +Philosophical Journal</i>, vol. xv, p. 398; Gay-Lussac in <i>Annales de Chem. +et Phys.,</i> tome lxiii, p. 219, translated in the <i>London and Edinburgh +Philosophical Magazine,</i> vol. x, p. 496. +</p> + +<p>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.</p> + +<p><i>The extent and horizontality of the calcareous +stratum.</i>—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 +<a name="page172"></a> +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.</p> + +<p class="center"> +Fig. 2 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/sighill.jpg" width="403" height="55" alt="[Illustration: +Signal Post Hill]" /> +</div> + +<p class="letter"> +SIGNAL POST HILL<br/> +A—Ancient volcanic rocks. B—Calcareous stratum. + C—Upper balastic lava. +</p> + +<p>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 +<a name="page173"></a> +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.</p> + +<p><i>The basaltic lava, superincumbent on the calcareous +deposit.</i>—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.</p> + +<p><i>Volcanic eruptions subsequent to the elevation of the +coastland; the ejected matter associated with earthy +lime.</i>—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, <i>Red Hill</i>, 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 +<a name="page174"></a> +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.</p> + +<p>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.</p> + +<p>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 +<a name="page175"></a> +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.</p> + +<p>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.</p> + +<p> +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 +<a name="page176"></a> +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.<a href="#fn-8.3" name="fnref-8.3" id="fnref-8.3"><sup>[3]</sup></a> 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 <i>débris</i> of organic remains, at the bottom of a shallow sea. +</p> + +<p class="footnote"> +<a name="fn-8.3" id="fn-8.3"></a> <a href="#fnref-8.3">[3]</a> +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. +</p> + +<p> +<i>Signal Post Hill.</i>—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 (<a href="#page172">Fig. +2</a>), dips under it into the sea. It has a broad summit, with obscure traces +of a crateriform structure, and is composed of basaltic rocks,<a href="#fn-8.4" +name="fnref-8.4" id="fnref-8.4"><sup>[4]</sup></a> 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 +<a name="page177"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-8.4" id="fn-8.4"></a> <a href="#fnref-8.4">[4]</a> +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. +</p> + +<p> +The stream of lava, which fills the narrow gorge<a href="#fn-8.5" +name="fnref-8.5" id="fnref-8.5"><sup>[5]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-8.5" id="fn-8.5"></a> <a href="#fnref-8.5">[5]</a> +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. +</p> + +<p><i>Inland hills of more ancient volcanic rocks.</i>—These +hills are laid down by eye, and marked as A, B, C, etc., in <a +href="#page168">Map 1.</a> They are related in mineralogical +composition, and are probably directly +<a name="page178"></a> +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 <i>Beagle</i>; 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.</p> + +<p>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.</p> + +<p> +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 +<a name="page179"></a> +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.<a href="#fn-8.6" name="fnref-8.6" +id="fnref-8.6"><sup>[6]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-8.6" id="fn-8.6"></a> <a href="#fnref-8.6">[6]</a> +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. +</p> + +<p>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.</p> + +<p> +<a name="page180"></a> +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?</p> + +<p> +In the centre of the island<a href="#fn-8.7" name="fnref-8.7" +id="fnref-8.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-8.7" id="fn-8.7"></a> <a href="#fnref-8.7">[7]</a> +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. +</p> + +<p><i>Valleys near the coast.</i>—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 +<a name="page181"></a> +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.</p> + +<p><i>Recent conglomerate.</i>—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.</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap2.02"></a>Chapter II<br/>FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.</h2> + +<p class="letter"> +F<small>ERNANDO</small> N<small>ORONHA</small>.—Precipitous hill of +phonolite. T<small>ERCEIRA</small>.—Trachytic rocks: their singular +decomposition by steam of high temperature. +T<small>AHITI</small>.—Passage from wacke into trap; singular volcanic +rock with the vesicles half-filled with mesotype. +M<small>AURITIUS</small>.—Proofs of its recent elevation. Structure of +its more ancient mountains; similarity with St. Jago. S<small>T</small>. +P<small>AUL’S</small> R<small>OCKS</small>.—Not of volcanic origin. Their +singular mineralogical composition. +</p> + +<p><i>Fernando Noronha.</i>—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 +<a name="page182"></a> +fragments of compact basalt, of which rock a distant +façade of columns seemed to be formed.</p> + +<p><i>Terceira in the Azores.</i>—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.</p> + +<p> +<i>Effects of steam on the trachytic rocks.</i>—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.<a +href="#fn-9.1" name="fnref-9.1" id="fnref-9.1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-9.1" id="fn-9.1"></a> <a href="#fnref-9.1">[1]</a> +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. +</p> + +<p>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, +<a name="page183"></a> +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.</p> + +<p><i>Tahiti (Otaheite).</i>—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.</p> + +<p> +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 +<a name="page184"></a> +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<a href="#fn-9.2" name="fnref-9.2" id="fnref-9.2"><sup>[2]</sup></a> +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. +</p> + +<p class="footnote"> +<a name="fn-9.2" id="fn-9.2"></a> <a href="#fnref-9.2">[2]</a> +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. +</p> + +<p>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.”</p> + +<p> +<i>Mauritius.</i>—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<a href="#fn-9.3" name="fnref-9.3" +id="fnref-9.3"><sup>[3]</sup></a> have described masses of upraised coral-rock +round the greater part of the circumference of the island. Between Tamarin Bay +<a name="page185"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-9.3" id="fn-9.3"></a> <a href="#fnref-9.3">[3]</a> +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.” +</p> + +<p>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.</p> + +<p> +The mountains composed of the more compact and crystalline basalt, form the +main skeleton of the island. M. Bailly<a href="#fn-9.4" name="fnref-9.4" +id="fnref-9.4"><sup>[4]</sup></a> 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 +<a name="page186"></a> +them perfectly correct.<a href="#fn-9.5" name="fnref-9.5" +id="fnref-9.5"><sup>[5]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-9.4" id="fn-9.4"></a> <a href="#fnref-9.4">[4]</a> +“Voyage aux Terres Australes,” tome i, p. 54. +</p> + +<p class="footnote"> +<a name="fn-9.5" id="fn-9.5"></a> <a href="#fnref-9.5">[5]</a> +M. Lesson, in his account of this island, in the “Voyage of the +<i>Coquille</i>,” seems to follow M. Bailly’s views. +</p> + +<p>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.</p> + +<p> +<a name="page187"></a> +<i>St. Paul’s Rocks.</i>—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.</p> + +<p>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.</p> + +<p> +<i>A glossy incrustation.</i>—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.<a href="#fn-9.6" name="fnref-9.6" +id="fnref-9.6"><sup>[6]</sup></a> 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 +<a name="page188"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-9.6" id="fn-9.6"></a> <a href="#fnref-9.6">[6]</a> +In my “Journal” I have described this substance; I then believed +that it was an impure phosphate of lime. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap2.03"></a>Chapter III<br/>ASCENSION.</h2> + +<p class="letter"> +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. +</p> + +<p> +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<a href="#fn-10.1" +name="fnref-10.1" id="fnref-10.1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.1" id="fn-10.1"></a> <a href="#fnref-10.1">[1]</a> +<i>Geographical Journal</i>, vol. v, p. 243. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/ascen.jpg" width="457" height="363" alt="Illustration: +Island of Ascension" /> +<p class="caption"><b>P<small>LATE</small> IV.</b><br/></p> +</div> + +<p><i>Basaltic rocks.</i>—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, +<a name="page189"></a> +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.</p> + +<p> +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.<a href="#fn-10.2" name="fnref-10.2" +id="fnref-10.2"><sup>[2]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.2" id="fn-10.2"></a> <a href="#fnref-10.2">[2]</a> +M. Lesson in the “Zoology of the Voyage of the <i>Coquille</i>,” 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. +</p> + +<p> +<i>Volcanic bombs.</i>—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 +<a name="page190"></a> +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<a href="#fn-10.3" name="fnref-10.3" +id="fnref-10.3"><sup>[3]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.3" id="fn-10.3"></a> <a href="#fnref-10.3">[3]</a> +Nichol’s “Architecture of the Heavens.” +</p> + +<p class="center"> +No. 3 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no3a.jpg" width="261" height="283" alt="[Illustration: +Fragment of a spherical volcanic bomb.]" /> +</div> + +<p class="letter"> +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. +</p> + +<p class="center"> +No. 4 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no4a.jpg" width="359" height="139" alt="[Illustration: +Volcanic bomb of obsidian from Australia.]" /> +</div> + +<p class="letter"> +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. +</p> + +<p> +M. Bory St. Vincent<a href="#fn-10.4" name="fnref-10.4" +id="fnref-10.4"><sup>[4]</sup></a> 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 +<a name="page191"></a> +I have given; for he supposes that they have rolled, like snowballs, down the +sides of the crater. M. Beudant,<a href="#fn-10.5" name="fnref-10.5" +id="fnref-10.5"><sup>[5]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.4" id="fn-10.4"></a> <a href="#fnref-10.4">[4]</a> +“Voyage aux Quatre Isles d’Afrique” tome i, p. 222. +</p> + +<p class="footnote"> +<a name="fn-10.5" id="fn-10.5"></a> <a href="#fnref-10.5">[5]</a> +“Voyage en Hongrie,” tome ii, p. 214. +</p> + +<p> +<i>Aeriform explosions.</i>—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,<a href="#fn-10.6" name="fnref-10.6" +id="fnref-10.6"><sup>[6]</sup></a> and the upper beds of great +<a name="page192"></a> +loose fragments, with alternating finer beds.<a href="#fn-10.7" +name="fnref-10.7" id="fnref-10.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.6" id="fn-10.6"></a> <a href="#fnref-10.6">[6]</a> +Some of this peperino, or tuff, is sufficiently hard not to be broken by the +greatest force of the fingers. +</p> + +<p class="footnote"> +<a name="fn-10.7" id="fn-10.7"></a> <a href="#fnref-10.7">[7]</a> +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. +</p> + +<p> +<i>Ejected granitic fragments.</i>—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. <i>First</i>, 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. <i>Secondly</i>, 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. +<a name="page193"></a> +<i>Thirdly</i>, 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. <i>Fourthly</i>, a rock, +which at first appears a simple aggregation of distinct and large-sized +crystals of dusty-coloured Labrador feldspar;<a href="#fn-10.8" +name="fnref-10.8" id="fnref-10.8"><sup>[8]</sup></a> 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<a href="#fn-10.9" name="fnref-10.9" +id="fnref-10.9"><sup>[9]</sup></a> to find granitic rocks ejected from +volcanoes with their <i>minerals unchanged</i>, 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. +</p> + +<p class="footnote"> +<a name="fn-10.8" id="fn-10.8"></a> <a href="#fnref-10.8">[8]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.9" id="fn-10.9"></a> <a href="#fnref-10.9">[9]</a> +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.” +</p> + +<p> +<i>Trachytic series of rocks.</i>—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, +<a name="page194"></a> +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,<a +href="#fn-10.10" name="fnref-10.10" id="fnref-10.10"><sup>[10]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.10" id="fn-10.10"></a> <a href="#fnref-10.10">[10]</a> +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.” +</p> + +<p> +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.<a href="#fn-10.11" name="fnref-10.11" +id="fnref-10.11"><sup>[11]</sup></a> 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,<a href="#fn-10.12" +name="fnref-10.12" id="fnref-10.12"><sup>[12]</sup></a> and in other compact +lavas, this circumstance is not any real argument for the sedimentary origin of +the white earthy stone.<a href="#fn-10.13" name="fnref-10.13" +id="fnref-10.13"><sup>[13]</sup></a> The insensible passage of the greenish +variety +<a name="page195"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.11" id="fn-10.11"></a> <a href="#fnref-10.11">[11]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.12" id="fn-10.12"></a> <a href="#fnref-10.12">[12]</a> +D’Aubuisson, “Traité de Géognosie,” tome ii, p. 548. +</p> + +<p class="footnote"> +<a name="fn-10.13" id="fn-10.13"></a> <a href="#fnref-10.13">[13]</a> +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. +</p> + +<p><i>Veins in the earthy trachytic masses.</i>—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 +<a name="page196"></a> +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.</p> + +<p> +<i>Siliceous sinter and jasper.</i>—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 +<a name="page197"></a> +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.<a href="#fn-10.14" +name="fnref-10.14" id="fnref-10.14"><sup>[14]</sup></a> 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;<a href="#fn-10.15" name="fnref-10.15" +id="fnref-10.15"><sup>[15]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.14" id="fn-10.14"></a> <a href="#fnref-10.14">[14]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.15" id="fn-10.15"></a> <a href="#fnref-10.15">[15]</a> +Beudant (“Voyage Min.,” tome iii, p. 507) enumerates cases in +Hungary, Germany, Central France, Italy, Greece, and Mexico. +</p> + +<p><i>Concretions in pumiceous tuff.</i>—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 +<a name="page198"></a> +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.</p> + +<p> +<i>Formation of calcareous rocks on the sea-coast.</i>—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.<a href="#fn-10.16" name="fnref-10.16" +id="fnref-10.16"><sup>[16]</sup></a> 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<a href="#fn-10.17" name="fnref-10.17" +id="fnref-10.17"><sup>[17]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.16" id="fn-10.16"></a> <a href="#fnref-10.16">[16]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.17" id="fn-10.17"></a> <a href="#fnref-10.17">[17]</a> +“Researches in Theoretical Geology,” p. 12. +</p> + +<p>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 +<a name="page199"></a> +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.</p> + +<p><i>A frondescent calcareous incrustation.</i>—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!</p> + +<p class="center"> +No. 5 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no5a.jpg" width="309" height="211" alt="[Illustration: +An incrustration of calcareous and animal matter.]" /> +</div> + +<p class="letter"> +An incrustation of calcareous and animal matter, coating the tidal-rocks at +Ascension. +</p> + +<p> +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, +<a name="page200"></a> +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.<a href="#fn-10.18" name="fnref-10.18" +id="fnref-10.18"><sup>[18]</sup></a> From this source +<a name="page201"></a> +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,<a href="#fn-10.19" +name="fnref-10.19" id="fnref-10.19"><sup>[19]</sup></a> 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.<a +href="#fn-10.20" name="fnref-10.20" id="fnref-10.20"><sup>[20]</sup></a> This +appears to me to be an interesting physiological fact.<a href="#fn-10.21" +name="fnref-10.21" id="fnref-10.21"><sup>[21]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-10.18" id="fn-10.18"></a> <a href="#fnref-10.18">[18]</a> +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 <i>Chanticleer,</i>” 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.) +</p> + +<p class="footnote"> +<a name="fn-10.19" id="fn-10.19"></a> <a href="#fnref-10.19">[19]</a> +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.) +</p> + +<p class="footnote"> +<a name="fn-10.20" id="fn-10.20"></a> <a href="#fnref-10.20">[20]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.21" id="fn-10.21"></a> <a href="#fnref-10.21">[21]</a> +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. +</p> + +<p> +<a name="page202"></a> +<i>Singular laminated beds alternating with and passing into +obsidian.</i>—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.</p> + +<p> +First.—A pale grey, irregularly and coarsely laminated,<a +href="#fn-10.22" name="fnref-10.22" id="fnref-10.22"><sup>[22]</sup></a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.22" id="fn-10.22"></a> <a href="#fnref-10.22">[22]</a> +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.” +</p> + +<p>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.</p> + +<p> +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,<a href="#fn-10.23" name="fnref-10.23" +id="fnref-10.23"><sup>[23]</sup></a> 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 +<a name="page203"></a> +crystalline, and some fine, white, granular, crystalline matter which is +probably feldspar. Minute fragments of this rock are easily fusible. +</p> + +<p class="footnote"> +<a name="fn-10.23" id="fn-10.23"></a> <a href="#fnref-10.23">[23]</a> +Professor Miller informs me that the crystals which he measured had the faces +<i>P, z, m</i> 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 <i>r</i> of the regular six-sided prism. +</p> + +<p>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.</p> + +<p>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.</p> + +<p>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 +<a name="page204"></a> +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.</p> + +<p>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.</p> + +<p class="center"> +No. 6</p> + +<div class="fig" style="width:100%;"> +<img src="images/no6.jpg" width="310" height="169" alt="[Illustration: +Opaque brown sphærulites, drawn on an enlarged scale.]" /> +</div> + +<p class="letter"> +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. +</p> + +<p class="center"> +No. 7 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no7.jpg" width="393" height="155" alt="[Illustration: +A layer, formed by the union of minute brown sphærulites." /> +</div> + +<p class="letter"> +A layer, formed by the union of minute brown sphærulites, intersecting two +other similar layers: the whole represented of nearly the natural size. +</p> + +<p>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 +<a name="page205"></a> +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 +<a name="page206"></a> +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.</p> + +<p> +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.<a href="#fn-10.24" +name="fnref-10.24" id="fnref-10.24"><sup>[24]</sup></a> 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.<a href="#fn-10.25" name="fnref-10.25" +id="fnref-10.25"><sup>[25]</sup></a> 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,<a href="#fn-10.26" name="fnref-10.26" +id="fnref-10.26"><sup>[26]</sup></a> 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,<a href="#fn-10.27" name="fnref-10.27" +id="fnref-10.27"><sup>[27]</sup></a> 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<a href="#fn-10.28" name="fnref-10.28" +id="fnref-10.28"><sup>[28]</sup></a> in giving glassiness of +<a name="page207"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.24" id="fn-10.24"></a> <a href="#fnref-10.24">[24]</a> +“Geological Transactions,” vol. 3, part i, p. 37. +</p> + +<p class="footnote"> +<a name="fn-10.25" id="fn-10.25"></a> <a href="#fnref-10.25">[25]</a> +The foregoing analyses are taken from Beudant “Traité de +Minéralogie,” tome ii, p. 113; and one analysis of obsidian from +Phillips’s “Mineralogy.” +</p> + +<p class="footnote"> +<a name="fn-10.26" id="fn-10.26"></a> <a href="#fnref-10.26">[26]</a> +These analyses are taken from Von Kobell’s “Grundzüge der +Mineralogie,” 1838. +</p> + +<p class="footnote"> +<a name="fn-10.27" id="fn-10.27"></a> <a href="#fnref-10.27">[27]</a> +“Traité de Géogn.,” tome ii, p. 535. +</p> + +<p class="footnote"> +<a name="fn-10.28" id="fn-10.28"></a> <a href="#fnref-10.28">[28]</a> +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. +</p> + +<p> +The natural sphærulites in these rocks<a href="#fn-10.29" name="fnref-10.29" +id="fnref-10.29"><sup>[29]</sup></a> 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,<a href="#fn-10.30" name="fnref-10.30" +id="fnref-10.30"><sup>[30]</sup></a> 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<a href="#fn-10.31" name="fnref-10.31" +id="fnref-10.31"><sup>[31]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.29" id="fn-10.29"></a> <a href="#fnref-10.29">[29]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.30" id="fn-10.30"></a> <a href="#fnref-10.30">[30]</a> +<i>Journal de Physique,</i> tome 59 (1804), pp. 10, 12. +</p> + +<p class="footnote"> +<a name="fn-10.31" id="fn-10.31"></a> <a href="#fnref-10.31">[31]</a> +<i>Idem,</i> tome 60 (1805), p. 418. +</p> + +<p> +<i>Comparison of the obsidian beds and alternating strata of ascension, with +those of other countries.</i>—I have been struck with much surprise, how +closely the excellent description of the obsidian rocks of Hungary, given by +Beudant,<a href="#fn-10.32" name="fnref-10.32" +id="fnref-10.32"><sup>[32]</sup></a> and that by Humboldt, of the same +formation in +<a name="page208"></a> +Mexico and Peru,<a href="#fn-10.33" name="fnref-10.33" +id="fnref-10.33"><sup>[33]</sup></a> and likewise the descriptions given by +several authors<a href="#fn-10.34" name="fnref-10.34" +id="fnref-10.34"><sup>[34]</sup></a> 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.<a href="#fn-10.35" +name="fnref-10.35" id="fnref-10.35"><sup>[35]</sup></a> 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<a href="#fn-10.36" name="fnref-10.36" +id="fnref-10.36"><sup>[36]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.32" id="fn-10.32"></a> <a href="#fnref-10.32">[32]</a> +“Voyage en Hongrie,” tome i, p. 330; tome ii, pp. 221 and 315; tome +iii, pp. 369, 371, 377, 381. +</p> + +<p class="footnote"> +<a name="fn-10.33" id="fn-10.33"></a> <a href="#fnref-10.33">[33]</a> +“Essai Géognostique,” pp. 176, 326, 328. +</p> + +<p class="footnote"> +<a name="fn-10.34" id="fn-10.34"></a> <a href="#fnref-10.34">[34]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.35" id="fn-10.35"></a> <a href="#fnref-10.35">[35]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.36" id="fn-10.36"></a> <a href="#fnref-10.36">[36]</a> +Beudant’s “Voyage,” tome iii, p. 373. +</p> + +<p> +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.<a href="#fn-10.37" +name="fnref-10.37" id="fnref-10.37"><sup>[37]</sup></a> In these cases, the +superficial parts are the most +<a name="page209"></a> +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<a href="#fn-10.38" name="fnref-10.38" +id="fnref-10.38"><sup>[38]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.37" id="fn-10.37"></a> <a href="#fnref-10.37">[37]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-10.38" id="fn-10.38"></a> <a href="#fnref-10.38">[38]</a> +“Mémoires pour servir a une Descript. Géolog. de la France,” tome +iv, p. 371. +</p> + +<h3><i>Lamination of volcanic rocks of the trachytic series.</i></h3> + +<p> +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<a href="#fn-10.39" name="fnref-10.39" +id="fnref-10.39"><sup>[39]</sup></a> into ridges and furrows, corresponding +with the zones of different degrees of glassiness: Humboldt,<a href="#fn-10.40" +name="fnref-10.40" id="fnref-10.40"><sup>[40]</sup></a> 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<a +href="#fn-10.41" name="fnref-10.41" id="fnref-10.41"><sup>[41]</sup></a> 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<a href="#fn-10.42" name="fnref-10.42" +id="fnref-10.42"><sup>[42]</sup></a> has described at Teneriffe, a stream of +lava +<a name="page210"></a> +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<a href="#fn-10.43" name="fnref-10.43" +id="fnref-10.43"><sup>[43]</sup></a> 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.<a href="#fn-10.44" name="fnref-10.44" +id="fnref-10.44"><sup>[44]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.39" id="fn-10.39"></a> <a href="#fnref-10.39">[39]</a> +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.” +</p> + +<p class="footnote"> +<a name="fn-10.40" id="fn-10.40"></a> <a href="#fnref-10.40">[40]</a> +“Personal Narrative,” vol. i, p. 222. +</p> + +<p class="footnote"> +<a name="fn-10.41" id="fn-10.41"></a> <a href="#fnref-10.41">[41]</a> +“Geological Transactions,” vol. ii (second series), p. 195. +</p> + +<p class="footnote"> +<a name="fn-10.42" id="fn-10.42"></a> <a href="#fnref-10.42">[42]</a> +“Description des Iles Canaries,” p. 184. +</p> + +<p class="footnote"> +<a name="fn-10.43" id="fn-10.43"></a> <a href="#fnref-10.43">[43]</a> +“Voyage aux Isles de Lipari,” pp. 35 and 85. +</p> + +<p class="footnote"> +<a name="fn-10.44" id="fn-10.44"></a> <a href="#fnref-10.44">[44]</a> +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. +</p> + +<p> +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:<a +href="#fn-10.45" name="fnref-10.45" id="fnref-10.45"><sup>[45]</sup></a> 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 +<a name="page211"></a> +described other cavities behind fragments embedded in his laminated trachyte, +and which he supposes to have been produced during the movement of the mass.<a +href="#fn-10.46" name="fnref-10.46" id="fnref-10.46"><sup>[46]</sup></a> 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,<a href="#fn-10.47" +name="fnref-10.47" id="fnref-10.47"><sup>[47]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.45" id="fn-10.45"></a> <a href="#fnref-10.45">[45]</a> +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 <i>Proc. of the Geolog. Soc.</i> June 1838. +</p> + +<p class="footnote"> +<a name="fn-10.46" id="fn-10.46"></a> <a href="#fnref-10.46">[46]</a> +“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. +</p> + +<p class="footnote"> +<a name="fn-10.47" id="fn-10.47"></a> <a href="#fnref-10.47">[47]</a> +Dolomieu’s “Voyage,” p. 64. +</p> + +<p> +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.<a href="#fn-10.48" name="fnref-10.48" +id="fnref-10.48"><sup>[48]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.48" id="fn-10.48"></a> <a href="#fnref-10.48">[48]</a> +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. +</p> + +<p>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 +<a name="page212"></a> +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.</p> + +<p>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.</p> + +<p> +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,<a href="#fn-10.49" name="fnref-10.49" +id="fnref-10.49"><sup>[49]</sup></a> 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’<a href="#fn-10.50" name="fnref-10.50" +id="fnref-10.50"><sup>[50]</sup></a> 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,<a href="#fn-10.51" +name="fnref-10.51" id="fnref-10.51"><sup>[51]</sup></a> in precisely the same +manner as Professor Forbes believes, that the ice of moving glaciers is +stretched and fissured. In both cases, +<a name="page213"></a> +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,<a href="#fn-10.52" +name="fnref-10.52" id="fnref-10.52"><sup>[52]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-10.49" id="fn-10.49"></a> <a href="#fnref-10.49">[49]</a> +“Mém. pour servir,” etc., tome iv, p. 131. +</p> + +<p class="footnote"> +<a name="fn-10.50" id="fn-10.50"></a> <a href="#fnref-10.50">[50]</a> +<i>Edinburgh New Phil. Journal,</i> 1842, p. 350. +</p> + +<p class="footnote"> +<a name="fn-10.51" id="fn-10.51"></a> <a href="#fnref-10.51">[51]</a> +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.” +</p> + +<p class="footnote"> +<a name="fn-10.52" id="fn-10.52"></a> <a href="#fnref-10.52">[52]</a> +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. +</p> + +<p>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 +<a name="page214"></a> +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.</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap2.04"></a>Chapter IV<br/>ST. HELENA</h2> + +<p class="letter"> +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. +</p> + +<p> +The whole island is of volcanic origin; its circumference, according to +Beatson,<a href="#fn-11.1" name="fnref-11.1" id="fnref-11.1"><sup>[1]</sup></a> +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. +</p> + +<p class="footnote"> +<a name="fn-11.1" id="fn-11.1"></a> <a href="#fnref-11.1">[1]</a> +Governor Beatson’s “Account of St. Helena.” +</p> + +<p> +<a name="page215"></a> +<i>Basaltic series.</i>—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.</p> + +<p> +<i>Basal submarine lavas.</i>—The lavas of this basal series lie +immediately beneath both the basaltic and feldspathic rocks. According to Mr. +Seale,<a href="#fn-11.2" name="fnref-11.2" id="fnref-11.2"><sup>[2]</sup></a> +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 +<a name="page216"></a> +strata of the basal series, whenever I examined them, were intersected by an +extraordinary number of dikes. +</p> + +<p class="footnote"> +<a name="fn-11.2" id="fn-11.2"></a> <a href="#fnref-11.2">[2]</a> +“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. +</p> + +<p><i>Flagstaff Hill and the Barn.</i>—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.</p> + +<p class="center"> +No. 8 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no8.jpg" width="406" height="107" alt="[Illustration: +Flagstaff Hill and the Barn.]" /> +</div> + +<p class="letter"> +The double lines represent the basaltic strata; the single, the basal submarine +strata; the dotted, the upper feldspathic strata; the dikes are shaded +transversely. +</p> + +<p> +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,<a href="#fn-11.3" name="fnref-11.3" +id="fnref-11.3"><sup>[3]</sup></a> 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 +<a name="page217"></a> +they seem to preserve a nearly uniform thickness:<a href="#fn-11.4" +name="fnref-11.4" id="fnref-11.4"><sup>[4]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-11.3" id="fn-11.3"></a> <a href="#fnref-11.3">[3]</a> +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.” +</p> + +<p class="footnote"> +<a name="fn-11.4" id="fn-11.4"></a> <a href="#fnref-11.4">[4]</a> +“Geognosy of the Island of St. Helena,” plate 5. +</p> + +<p> +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<a +href="#fn-11.5" name="fnref-11.5" id="fnref-11.5"><sup>[5]</sup></a> 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 name="page218"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-11.5" id="fn-11.5"></a> <a href="#fnref-11.5">[5]</a> +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.” +</p> + +<p class="center"> +No. 9 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no9.jpg" width="390" height="79" alt="[Illustration: +Prosperous Hill and The Barn.]" /> +</div> + +<p class="letter"> +The double lines represent the basaltic strata; the single, the basal submarine +strata; the dotted, the upper feldspathic strata. +</p> + +<p><i>Turk’s Cap and Prosperous Bays.</i>—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.</p> + +<p><i>Basaltic ring.</i>—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 +<a name="page219"></a> +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.</p> + +<p> +<i>The central curved ridge.</i>—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 +<a name="page220"></a> +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.<a href="#fn-11.6" +name="fnref-11.6" id="fnref-11.6"><sup>[6]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-11.6" id="fn-11.6"></a> <a href="#fnref-11.6">[6]</a> +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. +</p> + +<p class="center"> +No. 10 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no10.jpg" width="167" height="73" alt="[Illustration: Dike]" /> +</div> + +<p class="letter"> +1—Grey feldspathic lava.<br/> +2—A layer, one inch in thickness, of a reddish earthy matter.<br/> +3—Brecciated, red, argillaceous tuff. +</p> + +<p> +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<a +href="#fn-11.7" name="fnref-11.7" id="fnref-11.7"><sup>[7]</sup></a> 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<a href="#fn-11.8" name="fnref-11.8" id="fnref-11.8"><sup>[8]</sup></a> +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. +</p> + +<p class="footnote"> +<a name="fn-11.7" id="fn-11.7"></a> <a href="#fnref-11.7">[7]</a> +“Personal Narrative,” vol. i, p. 171. +</p> + +<p class="footnote"> +<a name="fn-11.8" id="fn-11.8"></a> <a href="#fnref-11.8">[8]</a> +Humboldt’s “Picturesque Atlas,” folio, pl. 10. +</p> + +<p> +<a name="page221"></a> +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.</p> + +<p> +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;<a +href="#fn-11.9" name="fnref-11.9" id="fnref-11.9"><sup>[9]</sup></a> the +uncovered portion is 197 feet<a href="#fn-11.10" name="fnref-11.10" +id="fnref-11.10"><sup>[10]</sup></a> in height, and its horizontal section +gives an oval figure. The phonolite is of a greenish-grey +<a name="page222"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-11.9" id="fn-11.9"></a> <a href="#fnref-11.9">[9]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-11.10" id="fn-11.10"></a> <a href="#fnref-11.10">[10]</a> +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. +</p> + +<p> +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<a href="#fn-11.11" name="fnref-11.11" id="fnref-11.11"><sup>[11]</sup></a> +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? +</p> + +<p class="footnote"> +<a name="fn-11.11" id="fn-11.11"></a> <a href="#fnref-11.11">[11]</a> +D’Aubuisson, in his “Traité de Géognosie” (tome ii, p. 540) +particularly remarks that this is the case. +</p> + +<p><i>Superficial deposits.</i>—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 +<a name="page223"></a> +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.</p> + +<p> +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<a href="#fn-11.12" +name="fnref-11.12" id="fnref-11.12"><sup>[12]</sup></a> 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,<a +href="#fn-11.13" name="fnref-11.13" id="fnref-11.13"><sup>[13]</sup></a> 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 name="page224"></a> +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,<a href="#fn-11.14" name="fnref-11.14" +id="fnref-11.14"><sup>[14]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-11.12" id="fn-11.12"></a> <a href="#fnref-11.12">[12]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-11.13" id="fn-11.13"></a> <a href="#fnref-11.13">[13]</a> +“Description des Isles Canaries,” p. 293. +</p> + +<p class="footnote"> +<a name="fn-11.14" id="fn-11.14"></a> <a href="#fnref-11.14">[14]</a> +<i>Idem,</i> pp. 314 and 374. +</p> + +<p> +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,<a href="#fn-11.15" +name="fnref-11.15" id="fnref-11.15"><sup>[15]</sup></a> 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, <i> Cochlogena fossilis</i> and <i>Helix biplicata</i>, +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. +</p> + +<p class="footnote"> +<a name="fn-11.15" id="fn-11.15"></a> <a href="#fnref-11.15">[15]</a> +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. +</p> + +<p> +<i>Beds of extinct land-shells.</i>—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 <i>Cochlogena +aurisvulpina.</i> 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 +<a name="page225"></a> +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,”<a href="#fn-11.16" +name="fnref-11.16" id="fnref-11.16"><sup>[16]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-11.16" id="fn-11.16"></a> <a href="#fnref-11.16">[16]</a> +“Journal of Researches,” p. 582. +</p> + +<p> +<i>Elevation of the land.</i>—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<a +href="#fn-11.17" name="fnref-11.17" id="fnref-11.17"><sup>[17]</sup></a> +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 +<a name="page226"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-11.17" id="fn-11.17"></a> <a href="#fnref-11.17">[17]</a> +A fissure-like gorge, near Stony-top, is said by Mr. Seale to be 840 feet deep, +and only 115 feet in width. +</p> + +<p> +<i>Coast denudation.</i>—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.<a href="#fn-11.18" name="fnref-11.18" +id="fnref-11.18"><sup>[18]</sup></a> 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 +<a name="page227"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-11.18" id="fn-11.18"></a> <a href="#fnref-11.18">[18]</a> +In the <i>Nautical Magazine</i> 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. +</p> + +<h3><i>Craters of Elevation.</i></h3> + +<p>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.</p> + +<p> +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,<a href="#fn-11.19" name="fnref-11.19" +id="fnref-11.19"><sup>[19]</sup></a> 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 +<a name="page228"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-11.19" id="fn-11.19"></a> <a href="#fnref-11.19">[19]</a> +“Principles of Geology” (fifth edit.), vol. ii, p. 171. +</p> + +<p> +A conjecture, including the above circumstances, occurred to me, when,— +with my mind fully convinced, from the phenomena of 1835 in South America,<a +href="#fn-11.20" name="fnref-11.20" id="fnref-11.20"><sup>[20]</sup></a> 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,<a href="#fn-11.21" name="fnref-11.21" +id="fnref-11.21"><sup>[21]</sup></a> but that they are separated from it by +curved faults. +<a name="page229"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-11.20" id="fn-11.20"></a> <a href="#fnref-11.20">[20]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-11.21" id="fn-11.21"></a> <a href="#fnref-11.21">[21]</a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap2.05"></a>Chapter V<br/>GALAPAGOS ARCHIPELAGO.</h2> + +<p class="letter"> +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. +</p> + +<p> +This archipelago is situated under the equator, at a distance of between five +and six hundred miles from the west coast of South +<a name="page230"></a> +America. It consists of five principal islands, and of several small ones, +which together are equal in area,<a href="#fn-12.1" name="fnref-12.1" +id="fnref-12.1"><sup>[1]</sup></a> 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 <i>Beagle.</i> 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. +</p> + +<p class="footnote"> +<a name="fn-12.1" id="fn-12.1"></a> <a href="#fnref-12.1">[1]</a> +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. +</p> + +<p class="center"> +No. 11 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no11.jpg" width="403" height="323" alt="[Illustration: +Galapagos Archipelago.]" /> +</div> + +<p class="letter"> +Galapagos Archipelago +</p> + +<p> +<a name="page231"></a> +C<small>HATHAM</small> I<small>SLAND.</small> <i>Craters composed of a singular +kind of tuff.</i>—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.<a +href="#fn-12.2" name="fnref-12.2" id="fnref-12.2"><sup>[2]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-12.2" id="fn-12.2"></a> <a href="#fnref-12.2">[2]</a> +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. +</p> + +<p> +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, +<a name="page232"></a> +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.<a +href="#fn-12.3" name="fnref-12.3" id="fnref-12.3"><sup>[3]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-12.3" id="fn-12.3"></a> <a href="#fnref-12.3">[3]</a> +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.” +</p> + +<p class="center"> +No. 12 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no12.jpg" width="369" height="195" alt="[Illustration: +The Kicker Rock.]" /> +</div> + +<p class="letter"> +The Kicker Rock, 400 feet high. +</p> + +<p>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.</p> + +<p><i>Small basaltic craters.</i>—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 +<a name="page233"></a> +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.</p> + +<p> +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.<a href="#fn-12.4" name="fnref-12.4" id="fnref-12.4"><sup>[4]</sup></a> +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.<a href="#fn-12.5" name="fnref-12.5" +id="fnref-12.5"><sup>[5]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-12.4" id="fn-12.4"></a> <a href="#fnref-12.4">[4]</a> +(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. +</p> + +<p class="footnote"> +<a name="fn-12.5" id="fn-12.5"></a> <a href="#fnref-12.5">[5]</a> +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? +</p> + +<p> +<a name="page234"></a> +A<small>LBEMARLE</small> I<small>SLAND</small>.—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.</p> + +<p> +<i>Fluidity of different lavas.</i>—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,<a href="#fn-12.6" name="fnref-12.6" +id="fnref-12.6"><sup>[6]</sup></a> 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,<a href="#fn-12.7" name="fnref-12.7" id="fnref-12.7"><sup>[7]</sup></a> 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;<a href="#fn-12.8" name="fnref-12.8" +id="fnref-12.8"><sup>[8]</sup></a> 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 +<a name="page235"></a> +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,<a href="#fn-12.9" name="fnref-12.9" +id="fnref-12.9"><sup>[9]</sup></a> 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 +<i>apparent</i> 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.<a href="#fn-12.10" +name="fnref-12.10" id="fnref-12.10"><sup>[10]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-12.6" id="fn-12.6"></a> <a href="#fnref-12.6">[6]</a> +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, +<i>albite</i>, 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. +</p> + +<p class="footnote"> +<a name="fn-12.7" id="fn-12.7"></a> <a href="#fnref-12.7">[7]</a> +“Description des Isles Canaries,” p. 295. +</p> + +<p class="footnote"> +<a name="fn-12.8" id="fn-12.8"></a> <a href="#fnref-12.8">[8]</a> +Humboldt mentions that he mistook a green augitic mineral, occurring in the +volcanic rocks of the Cordillera of Quito, for olivine. +</p> + +<p class="footnote"> +<a name="fn-12.9" id="fn-12.9"></a> <a href="#fnref-12.9">[9]</a> +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, <i>as well as an even-sized grain</i>, the vesicles are internally +smooth and spherical. +</p> + +<p class="footnote"> +<a name="fn-12.10" id="fn-12.10"></a> <a href="#fnref-12.10">[10]</a> +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. +</p> + +<p> +<i>Craters of tuff.</i>—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 +<a name="page236"></a> +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.<a href="#fn-12.11" name="fnref-12.11" +id="fnref-12.11"><sup>[11]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-12.11" id="fn-12.11"></a> <a href="#fnref-12.11">[11]</a> +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°. +</p> + +<p>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.</p> + +<p> +<i>Banks’ Cove.</i>—This harbour occupies part of the interior of a +shattered crater of tuff larger than that last described. All the tuff is +<a name="page237"></a> +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.<a +href="#fn-12.12" name="fnref-12.12" id="fnref-12.12"><sup>[12]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-12.12" id="fn-12.12"></a> <a href="#fnref-12.12">[12]</a> +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 <i>strata of tuff</i>, 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. +</p> + +<p class="center"> +No. 13 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no13.jpg" width="439" height="148" alt="[Illustration: +Sectional sketch of headlands forming Banks’ Cove.]" /> +</div> + +<p class="letter"> +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. +</p> + +<p>J<small>AMES</small> I<small>SLAND</small>.—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 +<a name="page238"></a> +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.</p> + +<p class="center"> +No. 14 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/no14.jpg" width="352" height="171" alt="[Illustration: +Segment of very small orifice of eruption.]" /> +</div> + +<p class="letter"> +Segment of a very small orifice of eruption, on the beach of Fresh-water Bay. +</p> + +<p><i>Segment of a basaltic crater.</i>—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, +<a name="page239"></a> +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.</p> + +<p><i>Pseudo-extraneous, ejected fragments.</i>—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 +<a name="page240"></a> +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.</p> + +<p> +<i>Concluding remarks on the tuff-craters.</i>—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.<a +href="#fn-12.13" name="fnref-12.13" id="fnref-12.13"><sup>[13]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-12.13" id="fn-12.13"></a> <a href="#fnref-12.13">[13]</a> +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. +</p> + +<p> +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,<a href="#fn-12.14" +name="fnref-12.14" id="fnref-12.14"><sup>[14]</sup></a> 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 +<a name="page241"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-12.14" id="fn-12.14"></a> <a href="#fnref-12.14">[14]</a> +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. +</p> + +<p> +<i>Mineralogical composition of the rocks.</i>—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<a href="#fn-12.15" name="fnref-12.15" +id="fnref-12.15"><sup>[15]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-12.15" id="fn-12.15"></a> <a href="#fnref-12.15">[15]</a> +“Description des Isles Canaries,” p. 328. +</p> + +<p><i>Elevation of the land.</i>—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 +<a name="page242"></a> +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 <i>rolling up rounded</i> 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 <i>rolled up</i> during storms to their +present height.</p> + +<p><i>Direction of the fissures of eruption.</i>—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.</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="page243"></a><a name="chap2.06"></a>Chapter VI<br/>TRACHYTE AND +BASALT.—DISTRIBUTION OF VOLCANIC ISLES.</h2> + +<p class="letter"> +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. +</p> + +<p> +<i>On the separation of the constituent minerals of lava, according to their +specific gravities.</i>—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<a +href="#fn-13.1" name="fnref-13.1" id="fnref-13.1"><sup>[1]</sup></a> 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<a href="#fn-13.2" name="fnref-13.2" +id="fnref-13.2"><sup>[2]</sup></a> that the crystals sink from their weight. +The specific gravity of feldspar varies<a href="#fn-13.3" name="fnref-13.3" +id="fnref-13.3"><sup>[3]</sup></a> 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 +<a name="page244"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-13.1" id="fn-13.1"></a> <a href="#fnref-13.1">[1]</a> +“Description des Isles Canaries,” pp. 190 and 191. +</p> + +<p class="footnote"> +<a name="fn-13.2" id="fn-13.2"></a> <a href="#fnref-13.2">[2]</a> +In a mass of molten iron, it is found (<i>Edinburgh New Philosophical +Journal</i>, 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. +</p> + +<p class="footnote"> +<a name="fn-13.3" id="fn-13.3"></a> <a href="#fnref-13.3">[3]</a> +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. +</p> + +<p> +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.<a href="#fn-13.4" +name="fnref-13.4" id="fnref-13.4"><sup>[4]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-13.4" id="fn-13.4"></a> <a href="#fnref-13.4">[4]</a> +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. +</p> + +<p> +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 +<a name="page245"></a> +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;<a href="#fn-13.5" name="fnref-13.5" +id="fnref-13.5"><sup>[5]</sup></a> 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.<a +href="#fn-13.6" name="fnref-13.6" id="fnref-13.6"><sup>[6]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-13.5" id="fn-13.5"></a> <a href="#fnref-13.5">[5]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-13.6" id="fn-13.6"></a> <a href="#fnref-13.6">[6]</a> +Consult Von Buch’s well-known and admirable “Description +Physique” of this island, which might serve as a model of descriptive +geology. +</p> + +<p> +<a name="page246"></a> +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.</p> + +<p> +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.<a href="#fn-13.7" +name="fnref-13.7" id="fnref-13.7"><sup>[7]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-13.7" id="fn-13.7"></a> <a href="#fnref-13.7">[7]</a> +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. +</p> + +<p>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 +<a name="page247"></a> +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.</p> + +<p> +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.<a href="#fn-13.8" +name="fnref-13.8" id="fnref-13.8"><sup>[8]</sup></a> 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,<a href="#fn-13.9" +name="fnref-13.9" id="fnref-13.9"><sup>[9]</sup></a> or of basaltic eruptions. +Much of +<a name="page248"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-13.8" id="fn-13.8"></a> <a href="#fnref-13.8">[8]</a> +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 (<i>Silliman’s Journal,</i> 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. +</p> + +<p class="footnote"> +<a name="fn-13.9" id="fn-13.9"></a> <a href="#fnref-13.9">[9]</a> +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. +</p> + +<p> +<i>On the distribution of volcanic islands.</i>—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:<a href="#fn-13.10" name="fnref-13.10" +id="fnref-13.10"><sup>[10]</sup></a> 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,<a href="#fn-13.11" +name="fnref-13.11" id="fnref-13.11"><sup>[11]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-13.10" id="fn-13.10"></a> <a href="#fnref-13.10">[10]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-13.11" id="fn-13.11"></a> <a href="#fnref-13.11">[11]</a> +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. +</p> + +<p>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 +<a name="page249"></a> +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?</p> + +<p> +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.<a href="#fn-13.12" +name="fnref-13.12" id="fnref-13.12"><sup>[12]</sup></a> 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<a href="#fn-13.13" name="fnref-13.13" +id="fnref-13.13"><sup>[13]</sup></a> 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<a +href="#fn-13.14" name="fnref-13.14" id="fnref-13.14"><sup>[14]</sup></a> has +classed all volcanoes under two heads, namely, <i>central volcanoes</i>, round +which numerous eruptions have taken place on all sides, in a manner almost +regular, and <i>volcanic chains.</i> 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 <i>central volcanoes</i> and <i>volcanic chains</i> 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<a href="#fn-13.15" name="fnref-13.15" id="fnref-13.15"><sup>[15]</sup></a> +states that his volcanic chains surmount, +<a name="page250"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-13.12" id="fn-13.12"></a> <a href="#fnref-13.12">[12]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-13.13" id="fn-13.13"></a> <a href="#fnref-13.13">[13]</a> +“Bulletin de la Soc. Géolog.,” tome iii, p. 110. +</p> + +<p class="footnote"> +<a name="fn-13.14" id="fn-13.14"></a> <a href="#fnref-13.14">[14]</a> +“Description des Isles Canaries,” p. 324. +</p> + +<p class="footnote"> +<a name="fn-13.15" id="fn-13.15"></a> <a href="#fnref-13.15">[15]</a> +<i>Idem,</i> p. 393. +</p> + +<p>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.</p> + +<p> +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<a +href="#fn-13.16" name="fnref-13.16" id="fnref-13.16"><sup>[16]</sup></a> 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 +<a name="page251"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-13.16" id="fn-13.16"></a> <a href="#fnref-13.16">[16]</a> +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.” +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap2.07"></a>Chapter VII<br/>AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.</h2> + +<p class="letter"> +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. +</p> + +<p>The <i>Beagle</i>, 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.</p> + +<p><i>New South Wales.</i>—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.</p> + +<p> +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 +<a name="page252"></a> +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 <i>Glossopteris Brownii</i>, +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 <i>present</i> form been aggregated +in any previously existing rock.<a href="#fn-14.1" name="fnref-14.1" +id="fnref-14.1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-14.1" id="fn-14.1"></a> <a href="#fnref-14.1">[1]</a> +I have lately seen, in a paper by Smith (the father of English geologists), in +the <i>Magazine of Natural History</i>, 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. +</p> + +<p>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 +<a name="page253"></a> +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.</p> + +<p> +<i>Stratification of the escarpment.</i>—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.<a +href="#fn-14.2" name="fnref-14.2" id="fnref-14.2"><sup>[2]</sup></a> 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,<a +href="#fn-14.3" name="fnref-14.3" id="fnref-14.3"><sup>[3]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-14.2" id="fn-14.2"></a> <a href="#fnref-14.2">[2]</a> +This is stated on the authority of Sir T. Mitchell, in his +“Travels,” vol. ii, p. 357. +</p> + +<p class="footnote"> +<a name="fn-14.3" id="fn-14.3"></a> <a href="#fnref-14.3">[3]</a> +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. +</p> + +<p> +<i>Current-cleavage.</i>—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 +<a name="page254"></a> +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,<a +href="#fn-14.4" name="fnref-14.4" id="fnref-14.4"><sup>[4]</sup></a> 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<a href="#fn-14.5" +name="fnref-14.5" id="fnref-14.5"><sup>[5]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-14.4" id="fn-14.4"></a> <a href="#fnref-14.4">[4]</a> +See Martin White, on “Soundings in the British Channel,” pp. 4 and +166. +</p> + +<p class="footnote"> +<a name="fn-14.5" id="fn-14.5"></a> <a href="#fnref-14.5">[5]</a> +M. Siau on the “Action of Waves,” <i>Edin. New Phil. Journ.,</i> +vol. xxxi, p. 245. +</p> + +<p> +<i>Valleys in the sandstone platforms.</i>—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,<a +href="#fn-14.6" name="fnref-14.6" id="fnref-14.6"><sup>[6]</sup></a> in vain +endeavoured, first on foot and then by crawling between the great fallen +<a name="page255"></a> +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<a +href="#fn-14.7" name="fnref-14.7" id="fnref-14.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-14.6" id="fn-14.6"></a> <a href="#fnref-14.6">[6]</a> +“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. +</p> + +<p class="footnote"> +<a name="fn-14.7" id="fn-14.7"></a> <a href="#fnref-14.7">[7]</a> +<i>Idem</i>, vol. ii, p. 358. +</p> + +<p> +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, +<a name="page256"></a> +with the Bahama Banks. Such banks, I have been led to suppose,<a +href="#fn-14.8" name="fnref-14.8" id="fnref-14.8"><sup>[8]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-14.8" id="fn-14.8"></a> <a href="#fnref-14.8">[8]</a> +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. +</p> + +<h4><i>Van Diemen’s Land.</i></h4> + +<p>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, +<i>P. rugata</i> and <i>S. rotundata</i>, 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.</p> + +<p>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, +<a name="page257"></a> +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.</p> + +<p> +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,<a href="#fn-14.9" +name="fnref-14.9" id="fnref-14.9"><sup>[9]</sup></a> I believe, would be +classed as bole by mineralogists. +</p> + +<p class="footnote"> +<a name="fn-14.9" id="fn-14.9"></a> <a href="#fnref-14.9">[9]</a> +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? +</p> + +<p><i>Travertin with extinct plants.</i>—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 +<a name="page258"></a> +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.</p> + +<p> +<i>Elevation of the land.</i>—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.<a href="#fn-14.10" name="fnref-14.10" +id="fnref-14.10"><sup>[10]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-14.10" id="fn-14.10"></a> <a href="#fnref-14.10">[10]</a> +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. +</p> + +<p> +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<a href="#fn-14.11" +name="fnref-14.11" id="fnref-14.11"><sup>[11]</sup></a> finds proofs of the +elevation of the land, to the amount of 400 feet, at the Cape of Good Hope. In +the neighbourhood +<a name="page259"></a> +of the Bay of Islands in New Zealand,<a href="#fn-14.12" name="fnref-14.12" +id="fnref-14.12"><sup>[12]</sup></a> 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<a href="#fn-14.13" +name="fnref-14.13" id="fnref-14.13"><sup>[13]</sup></a> (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.” +</p> + +<p class="footnote"> +<a name="fn-14.11" id="fn-14.11"></a> <a href="#fnref-14.11">[11]</a> +“Proceedings of the Geological Society,” vol. iii, p. 420. +</p> + +<p class="footnote"> +<a name="fn-14.12" id="fn-14.12"></a> <a href="#fnref-14.12">[12]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-14.13" id="fn-14.13"></a> <a href="#fnref-14.13">[13]</a> +<i>Geographical Journal</i>, vol. xi, pp. 202, 205. +</p> + +<h4><i>King George’s Sound.</i></h4> + +<p>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.</p> + +<p><i>Superficial ferruginous beds.</i>—The lower parts of +the country are everywhere covered by a bed, following the +inequalities of the surface, +<a name="page260"></a> +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.</p> + +<p> +<i> +Superficial calcareous deposit.</i>—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.<a href="#fn-14.14" name="fnref-14.14" +id="fnref-14.14"><sup>[14]</sup></a> 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 <i>large unbroken</i> +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<a href="#fn-14.15" +name="fnref-14.15" id="fnref-14.15"><sup>[15]</sup></a> rock, which, even +<a name="page261"></a> +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 <i>Helix melo</i>, an existing land +species, abound in all the strata; and I likewise found another Helix, and the +case of an Oniscus. +</p> + +<p class="footnote"> +<a name="fn-14.14" id="fn-14.14"></a> <a href="#fnref-14.14">[14]</a> +I visited this hill, in company with Captain Fitzroy, and we came to a similar +conclusion regarding these branching bodies. +</p> + +<p class="footnote"> +<a name="fn-14.15" id="fn-14.15"></a> <a href="#fnref-14.15">[15]</a> +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. +</p> + +<p> +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, +<a name="page262"></a> +look back to a period when the land stood lower, of which the French +naturalists<a href="#fn-14.16" name="fnref-14.16" +id="fnref-14.16"><sup>[16]</sup></a> 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<a +href="#fn-14.17" name="fnref-14.17" id="fnref-14.17"><sup>[17]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-14.16" id="fn-14.16"></a> <a href="#fnref-14.16">[16]</a> +See M. Péron’s “Voyage,” tome i, p. 204. +</p> + +<p class="footnote"> +<a name="fn-14.17" id="fn-14.17"></a> <a href="#fnref-14.17">[17]</a> +Dr. J. Macaulay has fully described (<i>Edinb. New Phil. Journ.,</i> 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— +</p> + +<table style="margin-left: 10%" border="0" summary="Analysis of cast"> +<tr> +<td align="left"><small>Carbonate of lime</small></td> +<td align="right"><small>73·15</small></td> +</tr> + +<tr> +<td><small>Silica</small></td> +<td align="right"><small>11·90</small></td> +</tr> + +<tr> +<td><small>Phosphate of lime</small></td> +<td align="right"><small>8·81</small></td> +</tr> + +<tr> +<td><small>Animal matter</small></td> +<td align="right"><small>4·25</small></td> +</tr> + +<tr> +<td><small>Sulphate of lime</small></td> +<td align="right"><small>a trace</small></td> +</tr> + +<tr> +<td> </td> +<td align="right">———</td> +</tr> + +<tr> +<td> </td> +<td align="right"><small>98·11</small></td> +</tr> +</table> + +<p> +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 +<a name="page263"></a> +of latitude and an equal extent of longitude, on the southern, western, and +north-western coasts.”<a href="#fn-14.18" name="fnref-14.18" +id="fnref-14.18"><sup>[18]</sup></a> 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<a +href="#fn-14.19" name="fnref-14.19" id="fnref-14.19"><sup>[19]</sup></a> 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 <i>superficial</i> 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. +</p> + +<p class="footnote"> +<a name="fn-14.18" id="fn-14.18"></a> <a href="#fnref-14.18">[18]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-14.19" id="fn-14.19"></a> <a href="#fnref-14.19">[19]</a> +“Proceedings of the Geolog. Soc.,” vol. i, p. 320. +</p> + +<h4><i>Cape of Good Hope.</i></h4> + +<p> +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,<a href="#fn-14.20" name="fnref-14.20" id="fnref-14.20"><sup>[20]</sup></a> +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 +<a name="page264"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-14.20" id="fn-14.20"></a> <a href="#fnref-14.20">[20]</a> +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. +</p> + +<p> +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,<a +href="#fn-14.21" name="fnref-14.21" id="fnref-14.21"><sup>[21]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-14.21" id="fn-14.21"></a> <a href="#fnref-14.21">[21]</a> +See M. Keilhau’s “Theory on Granite” translated in the +<i>Edinburgh New Philosophical Journal,</i> vol.xxiv, p. 402. +</p> + +<p>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 +<a name="page265"></a> +the character of gneiss, they became undulatory with some of the +smaller flexures angular, like the laminæ of many true +metamorphic schists.</p> + +<p> +<i>Sandstone formation.</i>—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.<a href="#fn-14.22" name="fnref-14.22" +id="fnref-14.22"><sup>[22]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-14.22" id="fn-14.22"></a> <a href="#fnref-14.22">[22]</a> +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. +</p> + +<p> +Mr. Schomburgk has described<a href="#fn-14.23" name="fnref-14.23" +id="fnref-14.23"><sup>[23]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-14.23" id="fn-14.23"></a> <a href="#fnref-14.23">[23]</a> +<i>Geographical Journal,</i> vol. x, p. 246. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="vol03"></a>GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA.</h2> + +<h3><a name="page269"></a>CRITICAL INTRODUCTION.</h3> + +<p>Of the remarkable “trilogy” constituted by Darwin’s writings +which deal with the geology of the <i>Beagle</i>, 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 <i>Beagle</i>, and the completion +of his geological books being, it is sad to relate, “lost through +illness!”</p> + +<p>Concerning the “Geological Observations on South America,” +Darwin wrote to his friend Lyell, as follows:—“My volume will +be about 240 pages, dreadfully dull, yet much condensed. I think +whenever you have time to look through it, you will think the +collection of facts on the elevation of the land and on the +formation of terraces pretty good.”</p> + +<p>“Much condensed” is the verdict that everyone must endorse, on +rising from the perusal of this remarkable book; but by no means +“dull.” The three and a half years from April 1832 to September +1835, were spent by Darwin in South America, and were devoted to +continuous scientific work; the problems he dealt with were either +purely geological or those which constitute the borderland between +the geological and biological sciences. It is impossible to read +the journal which he kept during this time without being impressed +by the conviction +<a name="page270"></a> +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 +<i>hurrah for my species work!</i>”</p> + +<p>It would seem that the feeling of disappointment, which Darwin +so often experienced in comparing a book when completed, with the +observations and speculations which had inspired it, was more +keenly felt in the case of his volume on South America than any +other. To one friend he writes, “I have of late been slaving extra +hard, to the great discomfiture of wretched digestive organs, at +South America, and thank all the fates, I have done three-fourths +of it. Writing plain English grows with me more and more difficult, +and never attainable. As for your pretending that you will read +anything so dull as my pure geological descriptions, lay not such a +flattering unction on my soul, for it is incredible.” To another +friend he writes, “You do not know what you threaten when you +propose to read it—it is purely geological. I said to my +brother, ‘You will of course read it,’ and his answer was, ‘Upon my +life, I would sooner even buy it.’”</p> + +<p>In spite of these disparaging remarks, however, we are strongly +inclined to believe that this book, despised by its author, and +neglected by his contemporaries, will in the end be admitted to be +one of Darwin’s chief titles to fame. It is, perhaps, an +unfortunate circumstance that the great success which he attained +in biology by the publication of the “Origin of Species” has, to +some extent, overshadowed the fact that Darwin’s claims as a +geologist, are of the very highest order. It is not too much to say +that, had Darwin not been a geologist, the “Origin of Species” +could never have been written by him. But apart from those +geological questions, which have an important bearing on biological +thought and speculation, such as the proofs of +<a name="page271"></a> +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.</p> + +<p>But in the case of Darwin’s principal geological work—that +relating to the origin of the crystalline schists,—geologists +were not at the time prepared to receive his revolutionary +teachings. The influence of powerful authority was long exercised, +indeed, to stifle his teaching, and only now, when this unfortunate +opposition has disappeared, is the true nature and importance of +Darwin’s purely geological work beginning to be recognised.</p> + +<p>The two first chapters of the “Geological Observations on South +America,” deal with the proofs which exist of great, but frequently +interrupted, movements of elevation during very recent geological +times. In connection with this subject, Darwin’s particular +attention was directed to the relations between the great +earthquakes of South America—of some of which he had +impressive experience—and the permanent changes of elevation +which were taking place. He was much struck by the rapidity with +which the evidence of such great earth movements is frequently +obliterated; and especially with the remarkable way in which the +action of rain-water, percolating through deposits on the earth’s +surface, removes all traces of shells and other calcareous +organisms. It was these considerations which were the parents of +the generalisation that a 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.</p> + +<p>In his third chapter, Darwin gives a number of interesting +facts, collected during his visits to the plains and valleys of +Chili, which bear on the question of the origin of saliferous +deposits—the accumulation of salt, gypsum, and nitrate of +soda. This is a +<a name="page272"></a> +problem that has excited much discussion among geologists, and +which, in spite of many valuable observations, still remains to a +great extent very obscure. Among the important considerations +insisted upon by Darwin is that relating to the absence of marine +shells in beds associated with such deposits. He justly argues that +if the strata were formed in shallow waters, and then exposed by +upheaval to subaerial action, all shells and other calcareous +organisms would be removed by solution.</p> + +<p>Following Lyell’s method, Darwin proceeds from the study of +deposits now being accumulated on the earth’s surface, to those +which have been formed during the more recent periods of the +geological history.</p> + +<p>His account of the great Pampean formation, with its wonderful +mammalian remains—<i>Mastodon, Toxodon, Scelidotherium, +Macrauchenia, Megatherium, Megalonyx, Mylodon,</i> and <i> +Glyptodon</i>—this full of interest. His discovery of the +remains of a true <i>Equus</i> afforded a remarkable confirmation +of the fact—already made out in North America—that +species of horse had existed and become extinct in the New World, +before their introduction by the Spaniards in the sixteenth +century. Fully perceiving the importance of the microscope in +studying the nature and origin of such deposits as those of the +Pampas, Darwin submitted many of his specimens both to Dr. +Carpenter in this country, and to Professor Ehrenberg in Berlin. +Many very important notes on the microscopic organisms contained in +the formation will be found scattered through the chapter.</p> + +<p>Darwin’s study of the older tertiary formations, with their +abundant shells, and their relics of vegetable life buried under +great sheets of basalt, led him to consider carefully the question +of climate during these earlier periods. In opposition to prevalent +views on this subject, Darwin points out that his observations are +opposed to the conclusion that a higher temperature prevailed +universally over the globe during early geological periods. He +argues that “the causes which gave to the older tertiary +productions of the quite temperate zones of Europe a tropical +character, <i>were of a local character and did not affect the +whole globe.</i>” 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 +<a name="page273"></a> +geologists whose studies had been circumscribed within the +limits of Western Europe.</p> + +<p>In the Cordilleras of Northern and Central Chili, Darwin, when +studying still older formations, clearly recognised that they +contain an admixture of the forms of life, which in Europe are +distinctive of the Cretaceous and Jurassic periods respectively. He +was thus led to conclude that the classification of geological +periods, which fairly well expresses the facts that had been +discovered in the areas where the science was first studied, is no +longer capable of being applied when we come to the study of widely +distant regions. This important conclusion led up to the further +generalisation that each great geological period has exhibited a +geographical distribution of the forms of animal and vegetable +life, comparable to that which prevails in the existing fauna and +flora. To those who are familiar with the extent to which the +doctrine of universal formations has affected geological thought +and speculation, both long before and since the time that Darwin +wrote, the importance of this new standpoint to which he was able +to attain will be sufficiently apparent. Like the idea of the +extreme imperfection of the Geological Record, the doctrine of <i> +local</i> geological formations is found permeating and moulding +all the palæontological reasonings of his great work.</p> + +<p>In one of Darwin’s letters, written while he was in South +America, there is a passage we have already quoted, in which he +expresses his inability to decide between the rival claims upon his +attention of “the old crystalline group of rocks,” and “the softer +fossiliferous beds” respectively. The sixth chapter of the work +before us, entitled “Plutonic and Metamorphic Rocks—Cleavage +and Foliation,” contains a brief summary of a series of +observations and reasonings upon these crystalline rocks, which +are, we believe, calculated to effect a revolution in geological +science, and—though their value and importance have long been +overlooked—are likely to entitle Darwin in the future to a +position among geologists, scarcely, if at all, inferior to that +which he already occupies among biologists.</p> + +<p>Darwin’s studies of the great rock-masses of the Andes convinced +him of the close relations between the granitic or Plutonic rocks, +and those which were undoubtedly poured forth as lavas. Upon his +return, he set to work, with the aid of Professor Miller, to make a +careful study of the minerals composing the granites and those +which occur in the lavas, and he was able to show that in all +essential respects they are identical. He was further able to +<a name="page274"></a> +prove that there is a complete gradation between the highly +crystalline or granitic rock-masses, and those containing more or +less glassy matter between their crystals, which constitute +ordinary lavas. The importance of this conclusion will be realised +when we remember that it was then the common creed of +geologists—and still continues to be so on the +Continent—that all highly crystalline rocks are of great +geological antiquity, and that the igneous ejections which have +taken place since the beginning of the tertiary periods differ +essentially, in their composition, their structure, and their mode +of occurrence, from those which have made their appearance at +earlier periods of the world’s history.</p> + +<p>Very completely have the conclusions of Darwin upon these +subjects been justified by recent researches. In England, the +United States, and Italy, examples of the gradual passage of rocks +of truly granitic structure into ordinary lavas have been +described, and the reality of the transition has been demonstrated +by the most careful studies with the microscope. Recent researches +carried on in South America by Professor Stelzner, have also shown +the existence of a class of highly crystalline rocks—the +“Andengranites”—which combine in themselves many of the +characteristics which were once thought to be distinctive of the +so-called Plutonic and volcanic rocks. No one familiar with recent +geological literature—even in Germany and France, where the +old views concerning the distinction of igneous products of +different ages have been most stoutly maintained—can fail to +recognise the fact that the principles contended for by Darwin bid +fair at no distant period to win universal acceptance among +geologists all over the globe.</p> + +<p>Still more important are the conclusions at which Darwin arrived +with respect to the origin of the schists and gneisses which cover +so large an area in South America.</p> + +<p>Carefully noting, by the aid of his compass and clinometer, at +every point which he visited, the direction and amount of +inclination of the parallel divisions in these rocks, he was led to +a very important generalisation—namely, that over very wide +areas the direction (strike) of the planes of cleavage in slates, +and of foliation in schists and gneisses, remained constant, though +the amount of their inclination (dip) often varied within wide +limits. Further than this it appeared that there was always a close +correspondence between the strike of the cleavage and foliation and +the direction of the great axes along which elevation had taken +place in the district.</p> + +<p> +<a name="page275"></a> +In Tierra del Fuego, Darwin found striking evidence that the +cleavage intersecting great masses of slate-rocks was quite +independent of their original stratification, and could often, +indeed, be seen cutting across it at right angles. He was also able +to verify Sedgwick’s observation that, in some slates, glossy +surfaces on the planes of cleavage arise from the development of +new minerals, chlorite, epidote or mica, and that in this way a +complete graduation from slates to true schists may be traced.</p> + +<p>Darwin further showed that in highly schistose rocks, the folia +bend around and encircle any foreign bodies in the mass, and that +in some cases they exhibit the most tortuous forms and complicated +puckerings. He clearly saw that in all cases the forces by which +these striking phenomena must have been produced were persistent +over wide areas, and were connected with the great movements by +which the rocks had been upheaved and folded.</p> + +<p>That the distinct folia of quartz, feldspar, mica, and other +minerals composing the metamorphic schists could not have been +separately deposited as sediment was strongly insisted upon by +Darwin; and in doing so he opposed the view generally prevalent +among geologists at that time. He was thus driven to the conclusion +that foliation, like cleavage, is not an original, but a +superinduced structure in rock-masses, and that it is the result of +re-crystallisation, under the controlling influence of great +pressure, of the materials of which the rock was composed.</p> + +<p>In studying the lavas of Ascension, as we have already seen, +Darwin was led to recognise the circumstance that, when igneous +rocks are subjected to great differential movements during the +period of their consolidation, they acquire a foliated structure, +closely analogous to that of the crystalline schists. Like his +predecessor in this field of inquiry, Mr. Poulett Scrope, Charles +Darwin seems to have been greatly impressed by these facts, and he +argued from them that the rocks exhibiting the foliated structure +must have been in a state of plasticity, like that of a cooling +mass of lava. At that time the suggestive experiments of Tresca, +Daubree, and others, showing that solid masses under the influence +of enormous pressure become actually plastic, had not been +published. Had Darwin been aware of these facts he would have seen +that it was not necessary to assume a state of imperfect solidity +in rock-masses in order to account for their having yielded to +pressure and tension, and, in doing so, acquiring the new +characters which distinguish the crystalline schists.</p> + +<p> +<a name="page276"></a> +The views put forward by Darwin on the origin of the crystalline +schists found an able advocate in Mr. Daniel Sharpe, who in 1852 +and 1854 published two papers, dealing with the geology of the +Scottish Highlands and of the Alps respectively, in which he showed +that the principles arrived at by Darwin when studying the South +American rocks afford a complete explanation of the structure of +the two districts in question.</p> + +<p>But, on the other hand, the conclusions of Darwin and Sharpe +were met with the strongest opposition by Sir Roderick Murchison +and Dr. A. Geikie, who in 1861 read a paper before the Geological +Society “On the Coincidence between Stratification and Foliation in +the Crystalline Rocks of the Scottish Highlands,” in which they +insisted that their observations in Scotland tended to entirely +disprove the conclusions of Darwin that foliation in rocks is a +secondary structure, and entirely independent of the original +stratification of the rock-masses.</p> + +<p>Now it is a most significant circumstance that, no sooner did +the officers of the Geological Survey commence the careful and +detailed study of the Scottish Highlands than they found themselves +compelled to make a formal retraction of the views which had been +put forward by Murchison and Geikie in opposition to the +conclusions of Darwin. The officers of the Geological Survey have +completely abandoned the view that the foliation of the Highland +rocks has been determined by their original stratification, and +admit that the structure is the result of the profound movements to +which the rocks have been subjected. The same conclusions have +recently been supported by observations made in many different +districts—among which we may especially refer to those of Dr. +H. Reusch in Norway, and those of Dr. J. Lehmann in Saxony. At the +present time the arguments so clearly stated by Darwin in the work +before us, have, after enduring opposition or neglect for a whole +generation, begun to “triumph all along the line,” and we may look +forward confidently to the near future, when his claim to be +regarded as one of the greatest of geological discoverers shall be +fully vindicated.</p> + +<p class="right"> +J<small>OHN</small> W. J<small>UDD</small>. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="page277"></a><a name="chap3.01"></a>Chapter I<br/>ON THE ELEVATION +OF THE EASTERN COAST OF SOUTH AMERICA.</h2> + +<p class="letter"> +Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and +Pumice-pebbles.—Step-formed plains of Patagonia, with upraised +Shells.—Terrace-bounded Valley of Santa Cruz, formerly a +Sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth +of the elevated area.—Equability of the movements, as shown by the +similar heights of the plains.—Slowness of the elevatory +process.—Mode of formation of the step-formed +plains.—Summary.—Great Shingle Formation of Patagonia; its extent, +origin, and distribution.—Formation of sea-cliffs. +</p> + +<p>In the following Volume, which treats of the geology of South +America, and almost exclusively of the parts southward of the +Tropic of Capricorn, I have arranged the chapters according to the +age of the deposits, occasionally departing from this order, for +the sake of geographical simplicity.</p> + +<p>The elevation of the land within the recent period, and the +modifications of its surface through the action of the sea (to +which subjects I paid particular attention) will be first +discussed; I will then pass on to the tertiary deposits, and +afterwards to the older rocks. Only those districts and sections +will be described in detail which appear to me to deserve some +particular attention; and I will, at the end of each chapter, give +a summary of the results. We will commence with the proofs of the +upheaval of the eastern coast of the continent, from the Rio Plata +southward; and, in the Second Chapter, follow up the same subject +along the shores of Chile and Peru.</p> + +<p class="p2"> +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 <i>Azara labiata</i>, d’Orbigny, fragments of +<i>Mytilus eduliformis</i>, d’Orbigny, <i>Paludestrina +Isabellei</i>, d’Orbigny, and the <i>Solen Caribæus</i>, +Lam., which last was embedded vertically in the position in which +it had lived. These shells lie at the height of only two feet above +the lake, nor would they have been worth mentioning, except in +connection with analogous facts.</p> + +<p> +<a name="page278"></a> +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,<a href="#fn-15.1" name="fnref-15.1" +id="fnref-15.1"><sup>[1]</sup></a> 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 <i>Mactra Isabellei</i>, d’Orbigny, mingled with +few of <i>Venus sinuosa</i>, 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<a href="#fn-15.2" +name="fnref-15.2" id="fnref-15.2"><sup>[2]</sup></a> 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 <i>Azara +labiata</i>, 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,<a +href="#fn-15.3" name="fnref-15.3" id="fnref-15.3"><sup>[3]</sup></a> of the +<i>Azara labiata</i>, 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 +<i>Venus sinuosa</i>, 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 <i>A. +labiata.</i> 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 +<a name="page279"></a> +appear to have lain on the surface. Sir W. Parish has given me some of these +shells, and M. d’Orbigny pronounces them to be:— +</p> + +<ol> +<li>Buccinanops globulosum, d’Orbigny.</li> + +<li>Olivancillaria auricularia, d’Orbigny.</li> + +<li>Venus flexuosa, Lam.</li> + +<li>Cytheræa (imperfect).</li> + +<li>Mactra Isabellei, d’Orbigny.</li> + +<li>Ostrea pulchella, d’Orbigny.</li> +</ol> + +<p> +Besides these, Sir W. Parish procured<a href="#fn-15.4" name="fnref-15.4" +id="fnref-15.4"><sup>[4]</sup></a> (as named by Mr. G. B. Sowerby) the +following shells:— +</p> + +<ol> +<li>Voluta colocynthis.</li> + +<li>Voluta angulata.</li> + +<li>Buccinum (not spec.?).</li> +</ol> + +<p class="footnote"> +<a name="fn-15.1" id="fn-15.1"></a> <a href="#fnref-15.1">[1]</a> +“Voyage dans l’Amérique Mérid.: Part. Géolog.,” p. 21. +</p> + +<p class="footnote"> +<a name="fn-15.2" id="fn-15.2"></a> <a href="#fnref-15.2">[2]</a> +<i>Ibid.</i>, p. 43. +</p> + +<p class="footnote"> +<a name="fn-15.3" id="fn-15.3"></a> <a href="#fnref-15.3">[3]</a> +“Buenos Ayres,” etc., by Sir Woodbine Parish, p. 168. +</p> + +<p class="footnote"> +<a name="fn-15.4" id="fn-15.4"></a> <a href="#fnref-15.4">[4]</a> +“Buenos Ayres,” etc., by Sir W. Parish, p. 168. +</p> + +<p>All these species (with, perhaps, the exception of the last) are +recent, and live on the South American coast. These shell-beds +extend from one league to six leagues from the Plata, and must lie +many feet above its level. I heard, also, of beds of shells on the +Somborombon, and on the Rio Salado, at which latter place, as M. +d’Orbigny informs me, the <i>Mactra Isabellei</i> and <i>Venus +sinuosa</i> are found.</p> + +<p> +During the elevation of the Provinces of La Plata, the waters of the ancient +estuary have but little affected (with the exception of the sand-hills on the +banks of the Parana and Uruguay) the outline of the land. M. Parchappe,<a +href="#fn-15.5" name="fnref-15.5" id="fnref-15.5"><sup>[5]</sup></a> 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.<a href="#fn-15.6" name="fnref-15.6" id="fnref-15.6"><sup>[6]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-15.5" id="fn-15.5"></a> <a href="#fnref-15.5">[5]</a> +D’Orbigny’s “Voyage Géolog.,” p. 44. +</p> + +<p class="footnote"> +<a name="fn-15.6" id="fn-15.6"></a> <a href="#fnref-15.6">[6]</a> +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. +</p> + +<p> +<a name="page280"></a> +<i>Southward of the Plata.</i>—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.</p> + +<p>The main plain surrounding Bahia Blanca I estimated at from two +hundred to three hundred feet; it insensibly rises towards the +distant Sierra Ventana. There are in this neighbourhood some other +and lower plains, but they do not abut one at the foot of the +other, in the manner hereafter to be described, so characteristic +of Patagonia. The plain on which the settlement stands is crossed +by many low sand-dunes, abounding with the minute shells of the <i> +Paludestrina australis</i>, d’Orbigny, which now lives in the bay. +This low plain is bounded to the south, at +<a name="page281"></a> +the Cabeza del Buey, by the cliff-formed margin of a wide plain +of the Pampean formation, which I estimated at sixty feet in +height. On the summit of this cliff there is a range of high +sand-dunes extending several miles in an east and west line.</p> + +<p>Southward of Bahia Blanca, the river Colorado flows between two +plains, apparently from thirty to forty feet in height. Of these +plains, the southern one slopes up to the foot of the great +sandstone plateau of the Rio Negro; and the northern one against an +escarpment of the Pampean deposit; so that the Colorado flows in a +valley fifty miles in width, between the upper escarpments. I state +this, because on the low plain at the foot of the northern +escarpment, I crossed an immense accumulation of high sand-dunes, +estimated by the Gauchos at no less than eight miles in breadth. +These dunes range westward from the coast, which is twenty miles +distant, to far inland, in lines parallel to the valley; they are +separated from each other by argillaceous flats, precisely like +those on the northern shore of Bahia Blanca. At present there is no +source whence this immense accumulation of sand could proceed; but +if, as I believe, the upper escarpments once formed the shores of +an estuary, in that case the sandstone formation of the river Negro +would have afforded an inexhaustible supply of sand, which would +naturally have accumulated on the northern shore, as on every part +of the coast open to the south winds between Bahia Blanca and +Buenos Ayres.</p> + +<p> +At San Blas (40° 40′ S.) a little south of the mouth of the +Colorado, M. d’Orbigny<a href="#fn-15.7" name="fnref-15.7" +id="fnref-15.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-15.7" id="fn-15.7"></a> <a href="#fnref-15.7">[7]</a> +“Voyage,” etc., p. 54. +</p> + +<p>Ten miles southward, but 120 miles to the west, at Port S. +Antonio, the Officers employed on the Survey assured me that they +saw many old sea-shells strewed on the surface of the ground, +similar to those found on other parts of the coast of Patagonia. At +San Josef, ninety miles south in nearly the same longitude, I +found, above the gravel, which caps an old tertiary formation, an +irregular bed and hillock of sand, several feet in thickness, +abounding with shells of <i>Patella deaurita, Mytilus +Magellanicus,</i> the latter retaining much of its colour; <i>Fusus +Magellanicus</i> (and a variety of the same), and a large Balanus +(probably <i>B. Tulipa</i>), 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 +<a name="page282"></a> +sufficient to show the general uniformity of the whole line of +coast. I may here state, that the whole of Patagonia consists of a +tertiary formation, resting on and sometimes surrounding hills of +porphyry and quartz: the surface is worn into many wide valleys and +into level step-formed plains, rising one above another, all capped +by irregular beds of gravel, chiefly composed of porphyritic rocks. +This gravel formation will be separately described at the end of +the chapter.</p> + +<p>In the following diagrams:<br/> +<small>Baseline is Level of sea.<br/> +Scale is 1/20 of inch to 100 feet vertical.</small></p> + +<p>Height is shown in feet thus:<br/> +<small>An. M. always stands for angular or trigonometrical +measurement.<br/> +Ba. M. always stands for barometrical measurement.<br/> +Est. always stands for estimation by the Officers of the +Survey.</small></p> + +<p class="center"> +No. 1<br/> +Section of step-formed plains south of Nuevo Gulf. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono1.jpg" width="396" height="66" alt="[Illustration: +Section of step-formed plains south of Nuevo Gulf.]" /> +</div> + +<p>My object in giving the following measurements of the plains, as +taken by the Officers of the Survey, is, as will hereafter be seen, +to show the remarkable equability of the recent elevatory +movements. Round the southern parts of Nuevo Gulf, as far as the +River Chupat (seventy miles southward of San Josef), there appear +to be several plains, of which the best defined are here +represented.</p> + +<p>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.</p> + +<p class="center"> +No. 2<br/> +Section of plains in the Bay of St. George. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono2.jpg" width="452" height="108" alt="[Illustration: +Section of plains in the Bay of St. George.]" /> +</div> + +<p>Near the north headland of the great Bay of St. George (100 +miles south of the Chupat), two well-marked plains of 250 and 330 +feet were measured: these are said to sweep round a great part of +the Bay. At its south headland, 120 miles distant from the north +headland, the 250 feet plain was again measured. In the middle of +the bay, a higher plain was +<a name="page283"></a> +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:—</p> + +<p class="center"> +No. 3<br/> +Section of plains at Port Desire. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono3.jpg" width="425" height="100" alt="[Illustration: +Section of plains at Port Desire.]" /> +</div> + +<p>In many places, even at the distance of three and four miles +from the coast, I found on the gravel-capped surface of the 245-255 +feet, and of the 330 feet plain, shells of <i>Mytilus Magellanicus, +M. edulis, Patella deaurita</i>, 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 +<a name="page284"></a> +nature of the plains, apply to the following cases, so need not +be repeated.</p> + +<p>Southward of Port Desire, the plains have been greatly denuded, +with only small pieces of tableland marking their former extension. +But opposite Bird Island, two considerable step-formed plains were +measured, and found respectively to be 350 and 590 feet in height. +This latter plain extends along the coast close to Port St. Julian +(110 miles south of Port Desire); where we have the following +section:—</p> + +<p class="center"> +No. 4<br/> +Section of plains at Port St. Julian. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono4.jpg" width="457" height="90" alt="[Illustration: +Section of plains at Port St. Julian.]" /> +</div> + +<p>The lowest plain was estimated at ninety feet: it is remarkable +from the usual gravel-bed being deeply worn into hollows, which are +filled up with, as well as the general surface covered by, sandy +and reddish earthy matter: in one of the hollows thus filled up, +the skeleton of the Macrauchenia Patachonica, as will hereafter be +described, was embedded. On the surface and in the upper parts of +this earthy mass, there were numerous shells of Mytilus +Magellanicus and M. edulis, Patella deaurita, and fragments of +other species. This plain is tolerably level, but not extensive; it +forms a promontory seven or eight miles long, and three or four +wide. The upper plains in Diagram 4 were measured by the Officers +of the Survey; they were all capped by thick beds of gravel, and +were all more or less denuded; the 950 plain consists merely of +separate, truncated, gravel-capped hills, two of which, by +measurement, were found to differ only three feet. The 430 feet +plain extends, apparently with hardly a break, to near the northern +entrance of the Rio Santa Cruz (fifty miles to the south); but it +was there found to be only 330 feet in height.</p> + +<p>On the southern side of the mouth of the Santa Cruz we have +Diagram 5, which I am able to give with more detail than in the +foregoing cases:—</p> + +<p class="center"> +No. 5<br/> +Section of plains at the mouth of the Rio Santa Cruz. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono5.jpg" width="460" height="85" alt="[Illustration: +Section of plains at the mouth of the Rio Santa Cruz.]" /> +</div> + +<p>The plain marked 355 feet (as ascertained by the barometer and +by angular measurement) is a continuation of the above-mentioned +330 +<a name="page285"></a> +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 <i>Mytilus Magellanicus</i> and of a Mytilus, which Mr. Sowerby +informs me is yet unnamed, though well-known as recent on this +coast; <i>Patella deaurita</i>; <i>Fusus</i>, I believe, <i> +Magellanicus</i>, 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 <i>V. ancilla</i>) partially embedded in the +superficial sandy earth. All these shells had the same ancient +appearance with those from the foregoing localities. As the tides +along this part of the coast rise at the Syzygal period forty feet, +and therefore form a well-marked beach-line, I particularly looked +out for ridges in crossing this plain, which, as we have seen, +rises 108 feet in about six miles, but I could not see any traces +of such. The next highest plain is 710 feet above the sea; it is +very narrow, but level, and is capped with gravel; it abuts to the +foot of the 840 feet plain. This summit-plain extends as far as the +eye can range, both inland along the southern side of the valley of +the Santa Cruz, and southward along the Atlantic.</p> + +<p> +<i>The Valley of the R. Santa Cruz.</i>—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.<a href="#fn-15.8" +name="fnref-15.8" id="fnref-15.8"><sup>[8]</sup></a> 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 +<a name="page286"></a> +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 <i>Patella deaurita.</i> 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 +<i>Voluta ancilla</i>, still retaining traces of their colours; and one of the +<i>Patella deaurita.</i> It appeared that these shells had been washed from the +banks into the river; considering the distance from the sea, the desert and +absolutely unfrequented character of the country, and the very ancient +appearance of the shells (exactly like those found on the plains nearer the +coast), there is, I think, no cause to suspect that they could have been +brought here by Indians. +</p> + +<p class="footnote"> +<a name="fn-15.8" id="fn-15.8"></a> <a href="#fnref-15.8">[8]</a> +I have described this formation in a paper in the “Geological +Transactions,” vol. vi, p. 415. +</p> + +<p>The plain at the head of the valley is tolerably level, but +water-worn, and with many sand-dunes on it like those on a +sea-coast. At the highest point to which we ascended, it was +sixteen miles wide in a north and south line; and forty-five miles +in length in an east and west line. It is bordered by the +escarpments, one above the other, of two plains, which diverge as +they approach the Cordillera, and consequently resemble, at two +levels, the shores of great bays facing the mountains; and these +mountains are breached in front of the lower plain by a remarkable +gap. The valley, therefore, of the Santa Cruz consists of a +straight broad cut, about ninety miles in length, bordered by +gravel-capped terraces and plains, the escarpments of which at both +ends diverge or expand, one over the other, after the manner of the +shores of great bays. Bearing in mind this peculiar form of the +land—the sand-dunes on the plain at the head of the +valley—the gap in the Cordillera, in front of it—the +presence in two places of very ancient shells of existing +species—and lastly, the circumstance of the 355-453 feet +plain, with the numerous marine remains on its surface, sweeping +from the Atlantic coast, far up the valley, I think we must admit, +that within the recent period, the course of the Santa Cruz formed +a sea-strait intersecting the continent. At this period, the +southern part of South America consisted of an archipelago of +islands 360 miles in a north and south line. We shall presently +see, that two other straits also, since closed, then cut through +Tierra del Fuego; I may add, that one of them must at that time +have expanded at the foot of the Cordillera into a great bay (now +Otway Water) like that which formerly covered the 440 feet plain at +the head of the Santa Cruz.</p> + +<p>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 +<a name="page287"></a> +the stream, I found by measurement and estimation that B +(<i>n</i>) was 869 above the river: very near to where A (<i>n</i>) +was measured, C (<i>n</i>) was 639 above the same level: the +terrace D (<i>n</i>) was nowhere measured: the lowest E (<i>n</i>) +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 (<i>s</i>), however was traced continuously for a +great distance. The terrace B (<i>n</i>), 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 (<i>n</i>) to A +(<i>s</i>) being generally nearly double the width of from B +(<i>n</i>) to B (<i>s</i>).</p> + +<p class="center"> +No. 6<br/> +North and South Section across the terraces bounding the valley of the River +Santa Cruz, high up its course. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono6.jpg" width="455" height="168" alt="[Illustration: +North and South Section across the terraces bounding the River Santa Cruz.]" /> +</div> + +<p>Low down the valley, the summit-plain A (<i>s</i>) is continuous +with the 840 feet plain on the coast, but it is soon lost or unites +with the escarpment of B (<i>s</i>). The corresponding plain A +(<i>n</i>), 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 (<i>n</i>), measured at two points twenty-four miles +apart, was found to have risen 185 feet. From several reasons I +suspect, that this gradual inclination of the plains up the valley, +has been chiefly caused by the elevation of the continent in mass, +having been the greater the nearer to the Cordillera.</p> + +<p>All the terraces are capped with well-rounded gravel, which +rests either on the denuded and sometimes furrowed surface of the +soft +<a name="page288"></a> +tertiary deposits, or on the basaltic lava. The difference in +height between some of the lower steps or terraces seems to be +entirely owing to a difference in the thickness of the capping +gravel. Furrows and inequalities in the gravel, where such occur, +are filled up and smoothed over with sandy earth. The pebbles, +especially on the higher plains, are often whitewashed, and even +cemented together by a white aluminous substance, and I +occasionally found this to be the case with the gravel on the +terrace D. I could not perceive any trace of a similar deposition +on the pebbles now thrown up by the river, and therefore I do not +think that terrace D was river-formed. As the terrace E generally +stands about twenty feet above the bed of the river, my first +impression was to doubt whether even this lowest one could have +been so formed; but it should always be borne in mind, that the +horizontal upheaval of a district, by increasing the total descent +of the streams, will always tend to increase, first near the +sea-coast and then further and further up the valley, their +corroding and deepening powers: so that an alluvial plain, formed +almost on a level with a stream, will, after an elevation of this +kind, in time be cut through, and left standing at a height never +again to be reached by the water. With respect to the three upper +terraces of the Santa Cruz, I think there can be no doubt, that +they were modelled by the sea, when the valley was occupied by a +strait, in the same manner (hereafter to be discussed) as the +greater step-formed, shell-strewed plains along the coast of +Patagonia.</p> + +<p class="p2"> +To return to the shores of the Atlantic: the 840 feet plain, at +the mouth of the Santa Cruz, is seen extending horizontally far to +the south; and I am informed by the Officers of the Survey, that +bending round the head of Coy Inlet (sixty-five miles southward), +it trends inland. Outliers of apparently the same height are seen +forty miles farther south, inland of the river Gallegos; and a +plain comes down to Cape Gregory (thirty-five miles southward), in +the Strait of Magellan, which was estimated at between eight +hundred and one thousand feet in height, and which, rising towards +the interior, is capped by the boulder formation. South of the +Strait of Magellan, there are large outlying masses of apparently +the same great tableland, extending at intervals along the eastern +coast of Tierra del Fuego: at two places here, 110 miles a part, +this plain was found to be 950 and 970 feet in height.</p> + +<p> +From Coy Inlet, where the high summit-plain trends inland, a plain estimated at +350 feet in height, extends for forty miles to the river Gallegos. From this +point to the Strait of Magellan, and on each side of that Strait, the country +has been much denuded and is less level. It consists chiefly of the boulder +formation, which rises to a height of between one hundred and fifty and two +hundred and fifty feet, and is often capped by beds of gravel. At N.S. Gracia, +on the north side of the Inner Narrows of the Strait of Magellan, I found on +the summit of a cliff, 160 feet in height, shells of existing 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 +<a name="page289"></a> +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<a +href="#fn-15.9" name="fnref-15.9" id="fnref-15.9"><sup>[9]</sup></a> on the +surface, that two sea-channels connected the Strait of Magellan both with +Sebastian Bay and with Otway Water. +</p> + +<p class="footnote"> +<a name="fn-15.9" id="fn-15.9"></a> <a href="#fnref-15.9">[9]</a> +“Geolog. Transactions,” vol. vi, p. 419. +</p> + +<p> +<i>Concluding remarks on the recent elevation of the south-eastern coasts of +America, and on the action of the sea on the land.</i>—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.<a +href="#fn-15.10" name="fnref-15.10" id="fnref-15.10"><sup>[10]</sup></a> 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.<a href="#fn-15.11" +name="fnref-15.11" id="fnref-15.11"><sup>[11]</sup></a> Most of the shells are +broken; I nowhere found two valves united; the fragments are not rounded, at +least in none of the specimens which I brought home. +</p> + +<p class="footnote"> +<a name="fn-15.10" id="fn-15.10"></a> <a href="#fnref-15.10">[10]</a> +Captain King, “Voyages of <i>Adventure</i> and <i>Beagle</i>,” vol. +i, 1 pp. 6 and 133. +</p> + +<p class="footnote"> +<a name="fn-15.11" id="fn-15.11"></a> <a href="#fnref-15.11">[11]</a> +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 <i>Edin. New Phil. Journal</i>, vol. xxv, p. 393. +</p> + +<p>With respect to the breadth of the upraised area in an east and +west line, we know from the shells found at the Inner Narrows of +the +<a name="page290"></a> +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.<a href="#fn-15.12" name="fnref-15.12" +id="fnref-15.12"><sup>[12]</sup></a> Moreover, we know that in Tierra del Fuego +the boulder formation has been uplifted within the recent period, and a similar +formation occurs<a href="#fn-15.13" name="fnref-15.13" +id="fnref-15.13"><sup>[13]</sup></a> 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,<a href="#fn-15.14" +name="fnref-15.14" id="fnref-15.14"><sup>[14]</sup></a> 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;<a href="#fn-15.15" name="fnref-15.15" +id="fnref-15.15"><sup>[15]</sup></a> whereas at Mendoza, at the eastern foot of +the Cordillera, only gentle oscillations, transmitted from the shores of the +Pacific, have ever been experienced. Hence the elevation of the Pampas may be +due to several distinct axes of movement; and we cannot judge, from the +upraised shells round the estuary of the Plata, of the breadth of the area +uplifted within the recent period. +</p> + +<p class="footnote"> +<a name="fn-15.12" id="fn-15.12"></a> <a href="#fnref-15.12">[12]</a> +“Voyages of the <i>Adventure</i> and <i> Beagle</i>,” vol. ii, p. +227. And Bougainville’s “Voyage,” tome i, p. 112. +</p> + +<p class="footnote"> +<a name="fn-15.13" id="fn-15.13"></a> <a href="#fnref-15.13">[13]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-15.14" id="fn-15.14"></a> <a href="#fnref-15.14">[14]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-15.15" id="fn-15.15"></a> <a href="#fnref-15.15">[15]</a> +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. +</p> + +<p> +<a name="page291"></a> +Not only has the above specified long range of coast been +elevated within the recent period, but I think it may be safely +inferred from the similarity in height of the gravel-capped plains +at distant points, that there has been a remarkable degree of +equability in the elevatory process. I may premise, that when I +measured the plains, it was simply to ascertain the heights at +which shells occurred; afterwards, comparing these measurements +with some of those made during the Survey, I was struck with their +uniformity, and accordingly tabulated all those which represented +the summit-edges of plains. The extension of the 330 to 355 feet +plain is very striking, being found over a space of 500 +geographical miles in a north and south line. A table (Table 1) of +the measurements is given below. The angular measurements and all +the estimations (in feet) are by the Officers of the Survey; the +barometrical ones by myself:—</p> + +<table border="0" width="90%" summary= +"Table of measurements in feet of the plains"> +<tr> +<td> </td> +<td>Feet</td> +</tr> + +<tr> +<td>Gallegos River to Coy Inlet (partly angular partly estimation)</td> +<td>350</td> +</tr> + +<tr> +<td>South Side of Santa Cruz (angular and barometric)</td> +<td>355</td> +</tr> + +<tr> +<td>North Side of Santa Cruz (angular and barometric)</td> +<td>330</td> +</tr> + +<tr> +<td>Bird Island, plain opposite to (angular)</td> +<td>350</td> +</tr> + +<tr> +<td>Port Desire, plain extending far along coast (barometric)</td> +<td>330</td> +</tr> + +<tr> +<td>St. George’s Bay, north promontory (angular)</td> +<td>330</td> +</tr> + +<tr> +<td>Table Land, south of New Bay (angular)</td> +<td>350</td> +</tr> +</table> + +<p>A plain, varying from 245 to 255 feet, seems to extend with much +uniformity from Port Desire to the north of St. George’s Bay, a +distance of 170 miles; and some approximate measurements (in feet), +also given in the table below, indicate the much greater extension +of 780 miles:—</p> + +<table border="0" width="90%" summary="Further measurements"> +<tr> +<td> </td> +<td>Feet</td> +</tr> + +<tr> +<td>Coy Inlet, south of (partly angular and partly estimation)</td> +<td>200 to 300</td> +</tr> + +<tr> +<td>Port Desire (barometric)</td> +<td>245 to 255</td> +</tr> + +<tr> +<td>C. Blanco (angular)</td> +<td>250</td> +</tr> + +<tr> +<td>North Promontory of St. George’s Bay (angular)</td> +<td>250</td> +</tr> + +<tr> +<td>South of New Bay (angular)</td> +<td>200 to 220</td> +</tr> + +<tr> +<td>North of S. Josef (estimation)</td> +<td>200 to 300</td> +</tr> + +<tr> +<td>Plain of Rio Negro (angular)</td> +<td>200 to 220</td> +</tr> + +<tr> +<td>Bahia Blanca (estimation)</td> +<td>200 to 300</td> +</tr> +</table> + +<p>The extension, moreover, of the 560 to 580, and of the 80 to 100 +feet, plains is remarkable, though somewhat less obvious than in +the former cases. Bearing in mind that I have not picked these +measurements out of a series, but have used all those which +represented the edges of plains, I think it scarcely possible that +these coincidences in height should be accidental. We must +therefore conclude that the action, whatever it may have been, by +which these plains have been modelled into their present forms, has +been singularly uniform.</p> + +<p>These plains or great terraces, of which three and four often +rise like steps one behind the other, are formed by the denudation +of the old Patagonian tertiary beds, and by the deposition on their +surfaces of a mass of well-rounded gravel, varying, near the coast, +from ten to thirty-five +<a name="page292"></a> +feet in thickness, but increasing in thickness towards the +interior. The gravel is often capped by a thin irregular bed of +sandy earth. The plains slope up, though seldom sensibly to the +eye, from the summit edge of one escarpment to the foot of the next +highest one. Within a distance of 150 miles, between Santa Cruz to +Port Desire, where the plains are particularly well developed, +there are at least seven stages or steps, one above the other. On +the three lower ones, namely, those of 100 feet, 250 feet, and 350 +feet in height, existing littoral shells are abundantly strewed, +either on the surface, or partially embedded in the superficial +sandy earth. By whatever action these three lower plains have been +modelled, so undoubtedly have all the higher ones, up to a height +of 950 feet at S. Julian, and of 1,200 feet (by estimation) along +St. George’s Bay. I think it will not be disputed, considering the +presence of the upraised marine shells, that the sea has been the +active power during stages of some kind in the elevatory +process.</p> + +<p>We will now briefly consider this subject: if we look at the +existing coast-line, the evidence of the great denuding power of +the sea is very distinct; for, from Cape St. Diego, in 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.</p> + +<p>The chief difficulty (for there are other inconsiderable ones) +on this view, is the fact,—as far as I can trust two +continuous lines of soundings carefully taken between Santa Cruz +and the Falkland Islands, and several scattered observations on +this and other coasts,—that the pebbles at the bottom of the +sea <i>quickly</i> and <i>regularly</i> 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. +<a name="page293"></a> +The following table gives the average result of many soundings +off the Santa Cruz:—</p> + +<p>Under two miles from the shore, +many of the pebbles were of large size,<br/> +mingled with some small ones.</p> + +<br/> + +<table border="1" width="90%" cellpadding="6" summary= +"Distance, Depth, and Size of Pebbles"> +<tr> +<td align="center">Distance in miles from shore</td> +<td align="center">Depth in fathoms</td> +<td align="center">Size of Pebbles</td> +</tr> + +<tr valign="top"> +<td>3 to 4</td> +<td>11 to 12</td> +<td>As large as walnuts; mingled in every case with some smaller +ones.</td> +</tr> + +<tr valign="top"> +<td>6 to 7</td> +<td>17 to 19</td> +<td>As large as hazel-nuts.</td> +</tr> + +<tr valign="top"> +<td>10 to 11</td> +<td>23 to 25</td> +<td>From three- to four-tenths of an inch in diameter.</td> +</tr> + +<tr valign="top"> +<td>12</td> +<td>30 to 40</td> +<td>Two-tenths of an inch.</td> +</tr> + +<tr valign="top"> +<td>22 to 150</td> +<td>45 to 65</td> +<td>One-tenth of an inch, to the finest sand.</td> +</tr> +</table> + +<p>I particularly attended to the size of the pebbles on the 355 +feet Santa Cruz plain, and I noticed that on the summit-edge of the +present sea cliffs many were as large as half a man’s head; and in +crossing from these cliffs to the foot of the next highest +escarpment, a distance of six miles, I could not observe any +increase in their size. We shall presently see that the theory of a +slow and almost insensible rise of the land, will explain all the +facts connected with the gravel-capped terraces, better than the +theory of sudden elevations of from one to two hundred feet.</p> + +<p>M. d’Orbigny has argued, from the upraised shells at San Blas +being embedded in the positions in which they lived, and from the +valves of the <i>Azara labiata</i> 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 <i>protected</i> 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 name="page294"></a> +a low level near the Uruguay; if he had seen at Bahia Blanca the +immense sand-dunes, with water-worn pebbles of pumice, ranging in +parallel lines, one behind the other, up a height of at least 120 +feet; if he had seen the sand-dunes, with the countless +Paludestrinas, on the low plain near the Fort at this place, and +that long line on the edge of the cliff, sixty feet higher up; if +he had crossed that long and great belt of parallel sand-dunes, +eight miles in width, standing at the height of from forty to fifty +feet above the Colorado, where sand could not now collect,—I +cannot believe he would have thought that the elevation of this +great district had been sudden. Certainly the sand-dunes +(especially when abounding with shells), which stand in ranges at +so many different levels, must all have required long time for +their accumulation; and hence I do not doubt that the last 100 feet +of elevation of La Plata and Northern Patagonia has been +exceedingly slow.</p> + +<p> +If we extend this conclusion to Central and Southern Patagonia, the inclination +of the successively rising gravel-capped plains can be explained quite as well, +as by the more obvious view already given of a few comparatively great and +sudden elevations; in either case we must admit long periods of rest, during +which the sea ate deeply into the land. Let us suppose the present coast to +rise at a nearly equable, slow rate, yet sufficiently quick to prevent the +waves quite removing each part as soon as brought up; in this case every +portion of the present bed of the sea will successively form a beach-line, and +from being exposed to a like action will be similarly affected. It cannot +matter to what height the tides rise, even if to forty feet as at Santa Cruz, +for they will act with equal force and in like manner on each successive line. +Hence there is no difficulty in the fact of the 355 feet plain at Santa Cruz +sloping up 108 feet to the foot of the next highest escarpment, and yet having +no marks of any one particular beach-line on it; for the whole surface on this +view has been a beach. I cannot pretend to follow out the precise action of the +tidal-waves during a rise of the land, slow, yet sufficiently quick to prevent +or check denudation: but if it be analogous to what takes place on protected +parts of the present coast, where gravel is now accumulating in large +quantities,<a href="#fn-15.16" name="fnref-15.16" +id="fnref-15.16"><sup>[16]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-15.16" id="fn-15.16"></a> <a href="#fnref-15.16">[16]</a> +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. +</p> + +<p> +<a name="page295"></a> +<i>Summary of results.</i>—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,<a href="#fn-15.17" name="fnref-15.17" +id="fnref-15.17"><sup>[17]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-15.17" id="fn-15.17"></a> <a href="#fnref-15.17">[17]</a> +I say <i>comparative</i> and not <i> absolute</i> 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.) +</p> + +<p>Lastly, considering the great upward movements which this long +line of coast has undergone, and the proximity of its southern half +to the volcanic axis of the Cordillera, it is highly remarkable +that in the many fine sections exposed in the Pampean, Patagonian +tertiary, and Boulder formations, I nowhere observed the smallest +fault or abrupt curvature in the strata.</p> + +<h4><i>Gravel Formation of Patagonia.</i></h4> + +<p>I will here describe in more detail than has been as yet +incidentally done, the nature, origin, and extent of the great +shingle covering of Patagonia: but I do not mean to affirm that all +of this shingle, especially that on the higher plains, belongs to +the recent period. A thin bed of sandy earth, with small pebbles of +various porphyries and of quartz, covering a low plain on the north +side of the Rio Colorado, is the extreme northern limit of this +formation. These little pebbles have probably been derived from the +denudation of a more regular bed of gravel, capping the old +tertiary sandstone plateau of the Rio Negro. The gravel-bed near +the Rio Negro is, on an average, about ten or +<a name="page296"></a> +twelve feet in thickness; and the pebbles are larger than on the +northern side of the Colorado, being from one or two inches in +diameter, and composed chiefly of rather dark-tinted porphyries. +Amongst them I here first noticed a variety often to be referred +to, namely, a peculiar gallstone-yellow siliceous porphyry, +frequently, but not invariably, containing grains of quartz. The +pebbles are embedded in a white, gritty, calcareous matrix, very +like mortar, sometimes merely coating with a whitewash the separate +stones, and sometimes forming the greater part of the mass. In one +place I saw in the gravel concretionary nodules (not rounded) of +crystallised gypsum, some as large as a man’s head. I traced this +bed for forty-five miles inland, and was assured that it extended +far into the interior. As the surface of the calcareo-argillaceous +plain of Pampean formation, on the northern side of the wide valley +of the Colorado, stands at about the same height with the +mortar-like cemented gravel capping the sandstone on the southern +side, it is probable, considering the apparent equability of the +subterranean movements along this side of America, that this gravel +of the Rio Negro and the upper beds of the Pampean formation +northward of the Colorado, are of nearly contemporaneous origin, +and that the calcareous matter has been derived from the same +source.</p> + +<p>Southward of the Rio Negro, the cliffs along the great bay of S. +Antonio are capped with gravel: at San Josef, I found that the +pebbles closely resembled those on the plain of the Rio Negro, but +that they were not cemented by calcareous matter. Between San Josef +and Port Desire, I was assured by the Officers of the Survey that +the whole face of the country is coated with gravel. At Port Desire +and over a space of twenty-five miles inland, on the three +step-formed plains and in the valleys, I everywhere passed over +gravel which, where thickest, was between thirty and forty feet. +Here, as in other parts of Patagonia, the gravel, or its sandy +covering, was, as we have seen, often strewed with recent marine +shells. The sandy covering sometimes fills up furrows in the +gravel, as does the gravel in the underlying tertiary formations. +The pebbles are frequently whitewashed and even cemented together +by a peculiar, white, friable, aluminous, fusible substance, which +I believe is decomposed feldspar. At Port Desire, the gravel rested +sometimes on the basal formation of porphyry, and sometimes on the +upper or the lower denuded tertiary strata. It is remarkable that +most of the porphyritic pebbles differ from those varieties of +porphyry which occur here abundantly <i>in situ.</i> The peculiar +gallstone-yellow variety was common, but less numerous than at Port +S. Julian, where it formed nearly one-third of the mass of the +gravel; the remaining part there consisting of pale grey and +greenish porphyries with many crystals of feldspar. At Port S. +Julian, I ascended one of the flat-topped hills, the denuded +remnant of the highest plain, and found it, at the height of 950 +feet, capped with the usual bed of gravel.</p> + +<p>Near the mouth of the Santa Cruz, the bed of gravel on the 355 +feet plain is from twenty to about thirty-five feet in thickness. +The pebbles vary from minute ones to the size of a hen’s egg, and +even to that of half a man’s head; they consist of paler varieties +of porphyry +<a name="page297"></a> +than those found further northward, and there are fewer of the +gallstone-yellow kind; pebbles of compact black clay-slate were +here first observed. The gravel, as we have seen, covers the +step-formed plains at the mouth, head, and on the sides of the +great valley of the Santa Cruz. At a distance of 110 miles from the +coast, the plain has risen to the height of 1,416 feet above the +sea; and the gravel, with the associated great boulder formation, +has attained a thickness of 212 feet. The plain, apparently with +its usual gravel covering, slopes up to the foot of the Cordillera +to the height of between 3,200 and 3,300 feet. In ascending the +valley, the gravel gradually becomes entirely altered in character: +high up, we have pebbles of crystalline feldspathic rocks, compact +clay-slate, quartzose schists, and pale-coloured porphyries; these +rocks, judging both from the gigantic boulders in the surface and +from some small pebbles embedded beneath 700 feet in thickness of +the old tertiary strata, are the prevailing kinds in this part of +the Cordillera; pebbles of basalt from the neighbouring streams of +basaltic lava are also numerous; there are few or none of the +reddish or of the gallstone-yellow porphyries so common near the +coast. Hence the pebbles on the 350 feet plain at the mouth of the +Santa Cruz cannot have been derived (with the exception of those of +compact clay-slate, which, however, may equally well have come from +the south) from the Cordillera in this latitude; but probably, in +chief part, from farther north.</p> + +<p> +Southward of the Santa Cruz, the gravel may be seen continuously capping the +great 840 feet plain: at the Rio Gallegos, where this plain is succeeded by a +lower one, there is, as I am informed by Captain Sulivan, an irregular covering +of gravel from ten to twelve feet in thickness over the whole country. The +district on each side of the Strait of Magellan is covered up either with +gravel or the boulder formation: it was interesting to observe the marked +difference between the perfectly rounded state of the pebbles in the great +shingle formation of Patagonia, and the more or less angular fragments in the +boulder formation. The pebbles and fragments near the Strait of Magellan nearly +all belong to rocks known to occur in Fuegia. I was therefore much surprised in +dredging south of the Strait to find, in 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.<a href="#fn-15.18" name="fnref-15.18" +id="fnref-15.18"><sup>[18]</sup></a> The distribution of the pebbles of this +peculiar porphyry, which I venture to affirm is not found <i>in situ</i> either +in Fuegia, the Falkland Islands, or on the coast of Patagonia, is very +remarkable, for they are +<a name="page298"></a> +found over a space of 840 miles in a north and south line, and at the +Falklands, 300 miles eastward of the coast of Patagonia. Their occurrence in +Fuegia and the Falklands may, however, perhaps be due to the same ice-agency by +which the boulders have been there transported. +</p> + +<p class="footnote"> +<a name="fn-15.18" id="fn-15.18"></a> <a href="#fnref-15.18">[18]</a> +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 <i>in situ</i> in the +Falklands; one pebble belonged to the peculiar yellow siliceous porphyry; +thirty were of doubtful origin. +</p> + +<p>We have seen that porphyritic pebbles of a small size are first +met with on the northern side of the Rio Colorado, the bed becoming +well developed near the Rio Negro: from this latter point I have +every reason to believe that the gravel extends uninterruptedly +over the plains and valleys of Patagonia for at least 630 nautical +miles southward to the Rio Gallegos. From the slope of the plains, +from the nature of the pebbles, from their extension at the Rio +Negro far into the interior, and at the Santa Cruz close up to the +Cordillera, I think it highly probable that the whole breadth of +Patagonia is thus covered. If so, the average width of the bed must +be about two hundred miles. Near the coast the gravel is generally +from ten to thirty feet in thickness; and as in the valley of Santa +Cruz it attains, at some distance from the Cordillera, a thickness +of 214 feet, we may, I think, safely assume its average thickness +over the whole area of 630 by 200 miles, at fifty feet!</p> + +<p>The transportal and origin of this vast bed of pebbles is an +interesting problem. From the manner in which they cap the +step-formed plains, worn by the sea within the period of existing +shells, their deposition, at least on the plains up to a height of +400 feet, must have been a recent geological event. From the form +of the continent, we may feel sure that they have come from the +westward, probably, in chief part from the Cordillera, but, +perhaps, partly from unknown rocky ridges in the central districts +of Patagonia. That the pebbles have not been transported by rivers, +from the interior towards the coast, we may conclude from the +fewness and smallness of the streams of Patagonia: moreover, in the +case of the one great and rapid river of Santa Cruz, we have good +evidence that its transporting power is very trifling. This river +is from two to three hundred yards in width, about seventeen feet +deep in its middle, and runs with a singular degree of uniformity +five knots an hour, with no lakes and scarcely any still reaches: +nevertheless, to give one instance of its small transporting power, +upon careful examination, pebbles of compact basalt could not be +found in the bed of the river at a greater distance than ten miles +below the point where the stream rushes over the debris of the +great basaltic cliffs forming its shore: fragments of the <i> +cellular</i> varieties have been washed down twice or thrice as +far. That the pebbles in Central and Northern Patagonia have not +been transported by ice-agency, as seems to have been the case to a +considerable extent farther south, and likewise in the northern +hemisphere, we may conclude, from the absence of all angular +fragments in the gravel, and from the complete contrast in many +other respects between the shingle and neighbouring boulder +formation.</p> + +<p> +Looking to the gravel on any one of the step-formed plains, I cannot doubt, +from the several reasons assigned in this chapter, that it has been spread out +and leveled by the long-continued action of the sea, probably during the slow +rise of the land. The smooth and perfectly rounded +<a name="page299"></a> +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<a href="#fn-15.19" name="fnref-15.19" +id="fnref-15.19"><sup>[19]</sup></a> 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,<a +href="#fn-15.20" name="fnref-15.20" id="fnref-15.20"><sup>[20]</sup></a> 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,<a href="#fn-15.21" +name="fnref-15.21" id="fnref-15.21"><sup>[21]</sup></a> 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.<a href="#fn-15.22" name="fnref-15.22" +id="fnref-15.22"><sup>[22]</sup></a> Hence we must conclude +<a name="page300"></a> +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 <i>tending</i> +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 <i>breaking</i> 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.<a href="#fn-15.23" name="fnref-15.23" +id="fnref-15.23"><sup>[23]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-15.19" id="fn-15.19"></a> <a href="#fnref-15.19">[19]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-15.20" id="fn-15.20"></a> <a href="#fnref-15.20">[20]</a> +“Soundings in the Channel,” pp. 4, 166. M. Siau states (<i>Edin. +New Phil. Jour.</i>, 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.” +</p> + +<p class="footnote"> +<a name="fn-15.21" id="fn-15.21"></a> <a href="#fnref-15.21">[21]</a> +<i>Journal of Royal Geograph. Soc.,</i> 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. +</p> + +<p class="footnote"> +<a name="fn-15.22" id="fn-15.22"></a> <a href="#fnref-15.22">[22]</a> +(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. +</p> + +<p class="footnote"> +<a name="fn-15.23" id="fn-15.23"></a> <a href="#fnref-15.23">[23]</a> +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 <i> Adventure</i> and <i>Beagle</i>,” 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. +</p> + +<p> +The sea acts in another and distinct manner in the distribution of pebbles, +namely by the waves on the beach. Mr. Palmer,<a href="#fn-15.24" +name="fnref-15.24" id="fnref-15.24"><sup>[24]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-15.24" id="fn-15.24"></a> <a href="#fnref-15.24">[24]</a> +“Philosophical Transactions,” 1834, p. 576. +</p> + +<p>I will not pretend to assign to these several and complicated +agencies their shares in the distribution of the Patagonian +shingle: but from the several considerations given in this chapter, +and I may add, from the frequency of a capping of gravel on +tertiary deposits in all parts of the world, as I have myself +observed and seen stated in the works of +<a name="page301"></a> +various authors, I cannot doubt that the power of widely +dispersing gravel is an ordinary contingent on the action of the +sea; and that even in the case of the great Patagonian shingle-bed +we have no occasion to call in the aid of debacles. I at one time +imagined that perhaps an immense accumulation of shingle had +originally been collected at the foot of the Cordillera; and that +this accumulation, when upraised above the level of the sea, had +been eaten into and partially spread out (as off the present line +of coast); and that the newly-spread out bed had in its turn been +upraised, eaten into, and re-spread out; and so onwards, until the +shingle, which was first accumulated in great thickness at the foot +of the Cordillera, had reached in thinner beds its present +extension. By whatever means the gravel formation of Patagonia may +have been distributed, the vastness of its area, its thickness, its +superficial position, its recent origin, and the great degree of +similarity in the nature of its pebbles, all appear to me well +deserving the attention of geologists, in relation to the origin of +the widely-spread beds of conglomerate belonging to past +epochs.</p> + +<p class="center"> +No. 7<br/> +Section of coast-cliffs and bottom of sea, off the island of St. Helena. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono7.jpg" width="454" height="163" alt="[Illustration: +Section of clast-cliffs and bottom of sea, off the island of St. Helena.]" /> +</div> + +<p><i>Formation of Cliffs.</i>—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.</p> + +<p>If, as seems probable, the basaltic streams were originally +prolonged with nearly their present inclination, they must, as +shown by the dotted line in the section, once have extended at +least to a point, now covered +<a name="page302"></a> +by the sea to a depth of nearly thirty fathoms: but I have every +reason to believe they extended considerably further, for the +inclination of the streams is less near the coast than further +inland. It should also be observed, that other sections on the +coast of this island would have given far more striking results, +but I had not the exact measurements; thus, on the windward side, +the cliffs are about two thousand feet in height and the cut-off +lava streams very gently inclined, and the bottom of the sea has +nearly a similar slope all round the island. How, then, has all the +hard basaltic rock, which once extended beneath the surface of the +sea, been worn away? According to Captain Austin, the bottom is +uneven and rocky only to that very small distance from the beach +within which the depth is from five to six fathoms; outside this +line, to a depth of about one hundred fathoms, the bottom is +smooth, gently inclined, and formed of mud and sand; outside the +one hundred fathoms, it plunges suddenly into unfathomable depths, +as is so very commonly the case on all coasts where sediment is +accumulating. At greater depths than the five or six fathoms, it +seems impossible, under existing circumstances, that the sea can +both have worn away hard rock, in parts to a thickness of at least +150 feet, and have deposited a smooth bed of fine sediment. Now, if +we had any reason to suppose that St. Helena had, during a long +period, gone on slowly subsiding, every difficulty would be +removed: for looking at the diagram, and imagining a fresh amount +of subsidence, we can see that the waves would then act on the +coast-cliffs with fresh and unimpaired vigour, whilst the rocky +ledge near the beach would be carried down to that depth, at which +sand and mud would be deposited on its bare and uneven surface: +after the formation near the shore of a new rocky shoal, fresh +subsidence would carry it down and allow it to be smoothly covered +up. But in the case of the many cliff-bounded islands, for instance +in some of the Canary Islands and of Madeira, round which the +inclination of the strata shows that the land once extended far +into the depths of the sea, where there is no apparent means of +hard rock being worn away—are we to suppose that all these +islands have slowly subsided? Madeira, I may remark, has, according +to Mr. Smith of Jordan Hill, subsided. Are we to extend this +conclusion to the high, cliff-bound, horizontally stratified shores +of Patagonia, off which, though the water is not deep even at the +distance of several miles, yet the smooth bottom of pebbles +gradually decreasing in size with the increasing depth, and derived +from a foreign source, seem to declare that the sea is now a +depositing and not a corroding agent? I am much inclined to +suspect, that we shall hereafter find in all such cases, that the +land with the adjoining bed of the sea has in truth subsided: the +time will, I believe, come, when geologists will consider it as +improbable, that the land should have retained the same level +during a whole geological period, as that the atmosphere should +have remained absolutely calm during an entire season.</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="page303"></a><a name="chap3.02"></a>Chapter II<br/>ON THE +ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.</h2> + +<p class="letter"> +Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions +of the inhabitants on this subject.—Concepcion, earthquake and elevation +of.—V<small>ALPARAISO</small>, great elevation of, upraised shells, earth +of marine origin, gradual rise of the land within the historical +period.—C<small>OQUIMBO</small>, elevation of, in recent times; terraces +of marine origin, their inclination, their escarpments not +horizontal.—Guasco, gravel terraces +of.—Copiapo.—P<small>ERU</small>.—Upraised shells of Cobija, +Iquique, and Arica.—Lima, shell-beds and sea-beach on San Lorenzo, human +remains, fossil earthenware, earthquake debacle, recent subsidence. On the +decay of upraised shells.—General summary. +</p> + +<p> +Commencing at the south and proceeding northward, the first place at which I +landed, was at Cape Tres Montes, in 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<a href="#fn-16.1" +name="fnref-16.1" id="fnref-16.1"><sup>[1]</sup></a> 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,<a href="#fn-16.2" +name="fnref-16.2" id="fnref-16.2"><sup>[2]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-16.1" id="fn-16.1"></a> <a href="#fnref-16.1">[1]</a> +“Narrative of the Loss of the <i>Wager</i>.” +</p> + +<p class="footnote"> +<a name="fn-16.2" id="fn-16.2"></a> <a href="#fnref-16.2">[2]</a> +“Comptes Rendus,” October 1838, p. 706. +</p> + +<p><i>The Island of Chiloe.</i>—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 +<a name="page304"></a> +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 <i>Venus costellata</i> 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 <i>Venus costellata</i>, and the +Ostrea (<i>O. edulis</i>, 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 <i>Cervus +humilis</i>, which now inhabits Chiloe.</p> + +<p> +The eastern or inland side of Chiloe, with its many adjacent islets, consists +of tertiary and boulder deposits, worn into irregular plains capped by gravel. +Near Castro, and for ten miles southward, and on the islet of Lemuy, I found +the surface of the ground to a height of between twenty and thirty feet above +high-water mark, and in several places apparently up to fifty feet, thickly +coated by much comminuted shells, chiefly of the <i>Venus costellata</i> and +<i>Mytilus Chiloensis</i>; 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 +<a name="page305"></a> +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<a href="#fn-16.3" name="fnref-16.3" +id="fnref-16.3"><sup>[3]</sup></a> 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,<a href="#fn-16.4" name="fnref-16.4" +id="fnref-16.4"><sup>[4]</sup></a> was in 1643 a good harbour; it is now quite +useless, except for boats. +</p> + +<p class="footnote"> +<a name="fn-16.3" id="fn-16.3"></a> <a href="#fnref-16.3">[3]</a> +Marsden’s “Sumatra,” p. 31. +</p> + +<p class="footnote"> +<a name="fn-16.4" id="fn-16.4"></a> <a href="#fnref-16.4">[4]</a> +“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. +</p> + +<p> +<i>Valdivia.</i>—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<a href="#fn-16.5" name="fnref-16.5" id="fnref-16.5"><sup>[5]</sup></a> +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,<a href="#fn-16.6" name="fnref-16.6" +id="fnref-16.6"><sup>[6]</sup></a> during the earthquake of 1835. +</p> + +<p class="footnote"> +<a name="fn-16.5" id="fn-16.5"></a> <a href="#fnref-16.5">[5]</a> +<i>Ibid.,</i> p. 25. +</p> + +<p class="footnote"> +<a name="fn-16.6" id="fn-16.6"></a> <a href="#fnref-16.6">[6]</a> +“Voyages of <i>Adventure</i> and <i>Beagle</i>,” vol. ii, p. 415. +</p> + +<p> +<i>Concepcion.</i>—I cannot add anything to the excellent account by +Captain Fitzroy<a href="#fn-16.7" name="fnref-16.7" +id="fnref-16.7"><sup>[7]</sup></a> 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 +<a name="page306"></a> +evidence of this considerable upheaval; one spot must be excepted, where there +was a detached rock which before the earthquake had always been covered by the +sea, but afterwards was left uncovered. +</p> + +<p class="footnote"> +<a name="fn-16.7" id="fn-16.7"></a> <a href="#fnref-16.7">[7]</a> +<i>Ibid.,</i> vol. ii, p. 412, <i>et seq.</i> 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. +</p> + +<p> +On the island of Quiriquina (in the Bay of Concepcion), I found, at an +estimated height of four hundred feet, extensive layers of shells, mostly +comminuted, but some perfectly preserved and closely packed in black vegetable +mould; they consisted of Concholepas, Fissurella, Mytilus, Trochus, and +Balanus. Some of these layers of shells rested on a thick bed of bright-red, +dry, friable earth, capping the surface of the tertiary sandstone, and +extending, as I observed whilst sailing along the coast, for 150 miles +southward: at Valparaiso, we shall presently see that a similar red earthy +mass, though quite like terrestrial mould, is really in chief part of recent +marine origin. On the flanks of this island of Quiriquina, at a less height +than the 400 feet, there were spaces several feet square, thickly strewed with +fragments of similar shells. During a subsequent visit of the <i>Beagle</i> 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 <i>Balanus psittacus.</i> 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.<a href="#fn-16.8" name="fnref-16.8" +id="fnref-16.8"><sup>[8]</sup></a> In some places round the bay, Mr. Kent +observed that there were beds formed exclusively of the <i>Mytilus +Chiloensis</i>: 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, +<i>Mytilus Chiloensis</i>, and a Turbo. These shells were softer and more +brittle than those from the height of 164 feet; and these latter had obviously +a much more ancient appearance than the same species from the height of only +twenty feet. +</p> + +<p class="footnote"> +<a name="fn-16.8" id="fn-16.8"></a> <a href="#fnref-16.8">[8]</a> +“Zoology of Captain Beechey’s Voyage,” p. 162. +</p> + +<p> +<i>Coast north of Concepcion.</i>—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<a href="#fn-16.9" name="fnref-16.9" id="fnref-16.9"><sup>[9]</sup></a> +found here recent shells at the distance +<a name="page307"></a> +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 <i>Mesodesma donaciforme</i>, Desh., lying on a bed of +muddy sand: this shell now lives associated together in great numbers, on +tidal-flats on the coast of Chile. +</p> + +<p class="footnote"> +<a name="fn-16.9" id="fn-16.9"></a> <a href="#fnref-16.9">[9]</a> +“Annales des Scienc. Nat.,” Avril 1833. +</p> + +<h4><i>Valparaiso.</i></h4> + +<p> +During two successive years I carefully examined, part of the time in company +with Mr. Alison, into all the facts connected with the recent elevation of this +neighbourhood. In very many parts a beach of broken shells, about fourteen or +fifteen feet above high-water mark, may be observed; and at this level the +coast-rocks, where precipitous, are corroded in a band. At one spot, Mr. +Alison, by removing some birds’ dung, found at this same level barnacles +adhering to the rocks. For several miles southward of the bay, almost every +flat little headland, between the heights of 60 and 230 feet (measured by the +barometer), is smoothly coated by a thick mass of comminuted shells, of the +same species, and apparently in the same proportional numbers with those +existing in the adjoining sea. The Concholepas is much the most abundant, and +the best preserved shell; but I extracted perfectly preserved specimens of the +<i>Fissurella biradiata</i>, a Trochus and Balanus (both well-known, but +according to Mr. Sowerby yet unnamed) and parts of the <i>Mytilus +Chiloensis.</i> 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 <i>many</i> consisted (and likewise from lesser heights) of +very young and small +<a name="page308"></a> +specimens of the still living Concholepas, Trochus, Patellæ, Crepidulæ, and of +<i>Mytilus Magellanicus</i> (?):<a href="#fn-16.10" name="fnref-16.10" +id="fnref-16.10"><sup>[10]</sup></a> 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 +<i>Mytilus Chiloensis</i>, all embedded in reddish-brown mould: I also found +these same species, with fragments of an Echinus and of <i>Balanus +psittacus</i>, on a hill 1,000 feet high. Above this height, shells became very +rare, though on a hill 1,300 feet high,<a href="#fn-16.11" name="fnref-16.11" +id="fnref-16.11"><sup>[11]</sup></a> I collected the Concholepas, Trochus, +Fissurella, and a Patella. At these greater heights the shells are almost +invariably embedded in mould, and sometimes are exposed only by tearing up +bushes. These shells obviously had a very much more ancient appearance than +those from the lesser heights; the apices of the Trochi were often worn down; +the little holes made by burrowing animals were greatly enlarged; and the +Concholepas was often perforated quite through, owing to the inner plates of +shell having scaled off. +</p> + +<p class="footnote"> +<a name="fn-16.10" id="fn-16.10"></a> <a href="#fnref-16.10">[10]</a> +Mr. Cuming informs me that he does not think this species identical with, +though closely resembling, the true <i>M. Magellanicus</i> of the southern and +eastern coast of South America; it lives abundantly on the coast of Chile. +</p> + +<p class="footnote"> +<a name="fn-16.11" id="fn-16.11"></a> <a href="#fnref-16.11">[11]</a> +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. +</p> + +<p>Many of these shells, as I have said, were packed in, and were +quite filled with, blackish or reddish-brown earth, resting on the +granitic detritus. I did not doubt until lately that this mould was +of purely terrestrial origin, when with a microscope examining some +of it from the inside of a Concholepas from the height of about one +hundred feet, I found that it was in considerable part composed of +minute fragments of the spines, mouth-bones, and shells of Echini, +and of minute fragments, of chiefly very young 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.</p> + +<p> +Being well aware from what I have seen at Chiloe and in Tierra del +<a name="page309"></a> +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 <i>not lying in heaps</i>,—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.<a href="#fn-16.12" name="fnref-16.12" +id="fnref-16.12"><sup>[12]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-16.12" id="fn-16.12"></a> <a href="#fnref-16.12">[12]</a> +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. +</p> + +<p>With respect to the position in which the shells lie, I was +repeatedly struck here, at Concepcion, and at other places, with +the frequency of their occurrence on the summits and edges either +of separate hills, or of little flat headlands often terminating +precipitously over the sea. The several above-enumerated species of +mollusca, which are found strewed on the surface of the land from a +few feet above the level of the sea up to the height of 1,300 feet, +all now live either on the beach, or at only a few fathoms’ depth: +Mr. Edmondston, in a letter to Professor E. Forbes, states that in +dredging in the Bay of Valparaiso, he found the common species of +Concholepas, Fissurella, Trochus, Monoceros, Chitons, etc., living +in abundance from the beach to a depth of seven fathoms; and dead +shells occurred only a few fathoms deeper. The common <i>Turritella +cingulata</i> 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 +<a name="page310"></a> +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 <i>Mytilus +Chiloensis</i>, 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 <i>Mesodesma donaciforme</i>,—a +shell which, as I am informed by Mr. Cuming, inhabits sandbanks at +the level of the lowest tides. But even in the case of shells +having the habits of this Mytilus and Mesodesma, beds of them, +wherever the sea gently throws up sand or mud, and thus protects +its own accumulations, might be upraised by the slowest movement, +and yet remain undisturbed by the waves of each new beach-line.</p> + +<p>It is worthy of remark, that nowhere near Valparaiso above the +height of twenty feet, or rarely of fifty feet, I saw any lines of +erosion on the solid rocks, or any beds of pebbles; this, I +believe, may be accounted for by the disintegrating tendency of +most of the rocks in this neighbourhood. Nor is the land here +modelled into terraces: Mr. Alison, however, informs me, that on +both sides of one narrow ravine, at the height of 300 feet above +the sea, he found a succession of rather indistinct step-formed +beaches, composed of broken shells, which together covered a space +of about eighty feet vertical.</p> + +<p> +I can add nothing to the accounts already published of the elevation of the +land at Valparaiso,<a href="#fn-16.13" name="fnref-16.13" +id="fnref-16.13"><sup>[13]</sup></a> 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 +<a name="page311"></a> +to stand nineteen feet six inches above the highest beach-line; so that we see +in a period of 220 years, the elevation cannot have been as much as nineteen +feet six inches. From the facts given with respect to the sea-wall, and from +the testimony of the elder inhabitants, it appears certain that the change in +level began to be manifest about the year 1817. The only sudden elevation of +which there is any record occurred in 1822, and this seems to have been less +than three feet. Since that year, I was assured by several competent observers, +that part of an old wreck, which is firmly embedded near the beach, has +sensibly emerged; hence here, as at Chiloe, a slow rise of the land appears to +be now in progress. It seems highly probable that the rocks which are corroded +in a band at the height of fourteen feet above the sea were acted on during the +period, when by tradition the base of S. Augustin church, now nineteen feet six +inches above the highest water-mark, was occasionally washed by the waves. +</p> + +<p class="footnote"> +<a name="fn-16.13" id="fn-16.13"></a> <a href="#fnref-16.13">[13]</a> +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. +</p> + +<p> +<i>Valparaiso to Coquimbo.</i>—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 <i>Mesodesma donaciforme</i>, packed in +sandy earth; they abound chiefly about fifteen feet above high-water, but +shells are here found, according to Mr. Miers,<a href="#fn-16.14" +name="fnref-16.14" id="fnref-16.14"><sup>[14]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-16.14" id="fn-16.14"></a> <a href="#fnref-16.14">[14]</a> +“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. +</p> + +<h4><a name="page312"></a><i>Coquimbo.</i></h4> + +<p>A narrow fringe-like plain, gently inclined towards the sea, +here extends for eleven miles along the coast, with arms stretching +up between the coast-mountains, and likewise up the valley of +Coquimbo: at its southern extremity it is directly connected with +the plain of Limari, out of which hills abruptly rise like islets, +and other hills project like headlands on a coast. The surface of +the fringe-like plain appears level, but differs insensibly in +height, and greatly in composition, in different parts.</p> + +<p> +At the mouth of the valley of Coquimbo, the surface consists wholly of gravel, +and stands from 300 to 350 feet above the level of the sea, being about one +hundred feet higher than in other parts. In these other and lower parts the +superficial beds consist of calcareous matter, and rest on ancient tertiary +deposits hereafter to be described. The uppermost calcareous layer is +cream-coloured, compact, smooth-fractured, sub-stalactiform, and contains some +sand, earthy matter, and recent shells. It lies on, and sends wedge-like veins +into,<a href="#fn-16.15" name="fnref-16.15" +id="fnref-16.15"><sup>[15]</sup></a> 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 +<i>losa</i>, 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.<a href="#fn-16.16" +name="fnref-16.16" id="fnref-16.16"><sup>[16]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-16.15" id="fn-16.15"></a> <a href="#fnref-16.15">[15]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-16.16" id="fn-16.16"></a> <a href="#fnref-16.16">[16]</a> +I have incidentally described this rock in the above work on Volcanic Islands. +</p> + +<p>The shells embedded in the calcareous beds forming the surface +of this fringe-like plain, at the height of from 200 to 250 feet +above the sea, consist of:—</p> + +<ol> +<li>Venus opaca.</li> + +<li>Mulinia Byronensis.</li> + +<li>Pecten purpuratus.</li> + +<li>Mesodesma donaciforme.</li> + +<li>Turritella cingulata.</li> + +<li>Monoceros costatum.</li> + +<li>Concholepas Peruviana.</li> + +<li>Trochus (common Valparaiso species).</li> + +<li>Calyptræa Byronensis.</li> +</ol> + +<p>Although these species are all recent, and are all found in the +neighbouring sea, yet I was particularly struck with the difference +in the +<a name="page313"></a> +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 +<i>Calyptræa radians</i>, of which I did not find one in the +calcareous beds. I will not pretend to determine how far this +difference in the proportional numbers depends on the age of the +deposit, and how far on the difference in nature between the +present sandy beaches and the calcareous bottom, on which the +embedded shells must have lived.</p> + +<p class="center"> +No. 8<br/> +Section of plain of Coquimbo. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono8.jpg" width="450" height="178" alt="[Illustration: +Section of plain of Coquimbo.]" /> +</div> + +<p>On the bare surface of the calcareous plain, or in a thin +covering of sand, there were lying, at a height from 200 to 252 +feet, many recent shells, which had a much fresher appearance than +the embedded ones: fragments of the Concholepas, and of the common +Mytilus, still retaining a tinge of its colour, were numerous, and +altogether there was manifestly a closer approach in proportional +numbers to those now lying on the beach. In a mass of stratified, +slightly agglutinated sand, which in some places covers up the +lower half of the seaward escarpment of the plain, the included +shells appeared to be in exactly the same proportional numbers with +those on the beach. On one side of a steep-sided ravine, cutting +through the plain behind Herradura Bay, I observed a narrow strip +of stratified sand, containing similar shells in similar +proportional numbers; a section of the ravine is represented in +Diagram 8, which serves also to show the general composition of the +plain. I mention this case of the ravine chiefly because without +the evidence of the marine shells in the sand, any one would have +supposed that it had been hollowed out by simple alluvial +action.</p> + +<p>The escarpment of the fringe-like plain, which stretches for +eleven miles along the coast, is in some parts fronted by two or +three narrow, step-formed terraces, one of which at Herradura Bay +expands into a small plain. Its surface was there formed of gravel, +cemented together by calcareous matter; and out of it I extracted +the following recent +<a name="page314"></a> +shells, which are in a more perfect condition than those from +the upper plain:—</p> + +<ol> +<li>Calyptræa radians.</li> + +<li>Turritella cingulata.</li> + +<li>Oliva Peruviana.</li> + +<li>Murex labiosus, var.</li> + +<li>Nassa (identical with a living species).</li> + +<li>Solen Dombeiana.</li> + +<li>Pecten purpuratus.</li> + +<li>Venus Chilensis.</li> + +<li>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.)</li> + +<li>Balanus (identical with living species).</li> +</ol> + +<p>On the syenitic ridge, which forms the southern boundary of +Herradura Bay and Plain, I found the Concholepas and <i>Turritella +cingulata</i> (mostly in fragments), at the height of 242 feet +above the sea. I could not have told that these shells had not +formerly been brought up by man, if I had not found one very small +mass of them cemented together in a friable calcareous tuff. I +mention this fact more particularly, because I carefully looked, in +many apparently favourable spots, at lesser heights on the side of +this ridge, and could not find even the smallest fragment of a +shell. This is only one instance out of many, proving that the +absence of sea-shells on the surface, though in many respects +inexplicable, is an argument of very little weight in opposition to +other evidence on the recent elevation of the land. The highest +point in this neighbourhood at which I found upraised shells of +existing species was on an inland calcareous plain, at the height +of 252 feet above the sea.</p> + +<p> +It would appear from Mr. Caldcleugh’s researches,<a href="#fn-16.17" +name="fnref-16.17" id="fnref-16.17"><sup>[17]</sup></a> 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 <i>à fleur d’eau</i>, 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. +</p> + +<p class="footnote"> +<a name="fn-16.17" id="fn-16.17"></a> <a href="#fnref-16.17">[17]</a> +“Proceedings of the Geological Society,” vol. ii, p. 446. +</p> + +<p> +<i>Valley of Coquimbo.</i>—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 +<a name="page315"></a> +Hall, to the parallel roads of Glen Roy in Scotland: their origin has been ably +discussed by Mr. Lyell.<a href="#fn-16.18" name="fnref-16.18" +id="fnref-16.18"><sup>[18]</sup></a> The first section which I will give +(Figure 9), is not drawn across the valley, but in an east and west line at its +mouth, where the step-formed terraces debouch and present their very gently +inclined surfaces towards the Pacific. +</p> + +<p class="footnote"> +<a name="fn-16.18" id="fn-16.18"></a> <a href="#fnref-16.18">[18]</a> + “Principles of Geology” (1st edit.), vol. iii, p. 131. +</p> + +<p class="center"> +No. 9<br/> +East and west section through the terraces at Coquimbo, where they debouch from +the valley, and front the sea. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono9.jpg" width="342" height="92" alt="[Illustration: +East and west section through terraces at Coquimbo.]" /> +</div> + +<p>The bottom plain (A) is about a mile in width, and rises quite +insensibly from the beach to a height of twenty-five feet at the +foot of the next plain; it is sandy, and abundantly strewed with +shells.</p> + +<p>Plain or terrace B is of small extent, and is almost concealed +by the houses of the town, as is likewise the escarpment of terrace +C. On both sides of a ravine, two miles south of the town, there +are two little terraces, one above the other, evidently +corresponding with B and C; and on them marine remains of the +species already enumerated were plentiful. Terrace E is very +narrow, but quite distinct and level; a little southward of the +town there were traces of a terrace D intermediate between E and C. +Terrace F is part of the fringe-like plain, which stretches for the +eleven miles along the coast; it is here composed of shingle, and +is 100 feet higher than where composed of calcareous matter. This +greater height is obviously due to the quantity of shingle, which +at some former period has been brought down the great valley of +Coquimbo.</p> + +<p>Considering the many shells strewed over the terraces A, B, and +C, and a few miles southward on the calcareous plain, which is +continuously united with the upper step-like plain F, there cannot, +I apprehend, be any doubt, that these six terraces have been formed +by the action of the sea; and that their five escarpments mark so +many periods of comparative rest in the elevatory movement, during +which the sea wore into the land. The elevation between these +periods may have been sudden and on <i>an average</i> 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 +<a name="page316"></a> +movement has been slow. The existence of these successive +escarpments, or old cliff-lines, is in another respect highly +instructive, for they show periods of comparative rest in the +elevatory movement, and of denudation, which would never even have +been suspected from a close examination of many miles of coast +southward of Coquimbo.</p> + +<p>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.</p> + +<p class="center"> +No. 10<br/> +North and south section across the valley of Coquimbo. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono10.jpg" width="456" height="110" alt="[Illustration: +North and south section across the valley of Coquimbo.]" /> +</div> + +<p class="letter"> +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.<br/> +<br/> + <b>AA.</b> 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.<br/> + <b>B.</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.<br/> + <b>C.</b> 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.<br/> + <b>D</b> 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.<br/> + <b>E</b> 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.<br/> + <b>F.</b> This is the surface-plain, and is continuously united +with that which stretches like a fringe along the coast. In +ascending the valley it gradually becomes narrower, and is at last, +at the distance of about ten miles from the sea, reduced to a row +of flat-topped patches on the sides of the mountains. None of the +lower terraces extend so far up the valley.</p> + +<p>These five terraces are formed of shingle and sand; three of +them, as marked by Captain B. Hall (namely, B, C, and F), are much +more conspicuous than the others. From the marine remains copiously +strewed at the mouth of the valley on the lower terraces, and +southward of the town on the upper one, they are, as before +remarked, undoubtedly of marine origin; but within the valley, and +this fact well deserves notice, at a distance of from only a mile +and a half to three or four miles from the sea, I could not find +even a fragment of a shell.</p> + +<p> +<a name="page317"></a> +<i>On the inclination of the terraces of Coquimbo, and on the +upper and basal edges of their escarpments not being +horizontal.</i>—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.</p> + +<p>The direction of the diagonal inclination in the different +terraces being different,—in some being directed more towards +the middle of the valley, in others more towards its +mouth,—naturally follows on the view of each terrace, being +an accumulation of successive beach-lines round bays, which must +have been of different forms and sizes when the land stood at +different levels: for if we look to the actual beach of a narrow +creek, its slope is directed towards the middle; whereas, in an +open bay, or slight concavity on a coast, the slope is towards the +mouth, that is, almost directly seaward; hence as a bay alters in +form and size, so will the direction of the inclination of its +successive beaches become changed. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono11.jpg" width="257" height="262" alt="[Illustration: +A bay in the district which has begun slowly rising.]" /> +</div> + +<p>If it were possible to trace any one of the many beach-lines, +composing each sloping terrace, it would of course be horizontal; +but the only lines of demarcation are the summit and basal edges of +the escarpments. Now the summit-edge of one of these escarpments +marks the furthest line or point to which the sea has cut into a +mass of gravel sloping seaward; and as the sea will generally have +greater power at the mouth than at the protected head of the bay, +so will the escarpment at the mouth be cut deeper into the land, +and its summit-edge be higher; consequently it will not be +horizontal. With respect to the basal or lower edges of the +escarpments, from picturing in one’s mind ancient bays <i> +entirely</i> 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 +<a name="page318"></a> +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 +<a name="page319"></a> +continue very gradually to fail to reach the cliffs, which are +now in the diagram represented as washed by the sea, and which, +consequently, will be left standing higher and higher above its +level; whilst at the still exposed mouth, it might well happen that +the waves might be enabled to cut deeper and deeper, both down and +into the cliffs, as the land slowly rose.</p> + +<p>The greater or lesser destroying power of the waves at the +mouths of successive bays, comparatively with this same power in +their upper and protected parts, will vary as the bays become +changed in form and size, and therefore at different levels, at +their mouths and heads, more or less of the surfaces between the +escarpments (that is, the accumulated beach-lines or terraces) will +be left undestroyed: from what has gone before we can see that, +according as the elevatory movements after each cessation +recommence more or less slowly, according to the amount of detritus +delivered by the river at the heads of the successive bays, and +according to the degree of protection afforded by their altered +forms, so will a greater or less extent of terrace be accumulated +in the upper part, to which there will be no surface at a +corresponding level at the mouth: hence we can perceive why no one +terrace, taken in its whole breadth and followed up the valley, is +horizontal, though each separate beach-line must have been so; and +why the inclination of the several terraces, both transversely, and +longitudinally up the valley, is not alike.</p> + +<p> +I have entered into this case in some detail, for I was long perplexed (and +others have felt the same difficulty) in understanding how, on the idea of an +equable elevation with the sea at intervals eating into the land, it came that +neither the terraces nor the upper nor lower edges of the escarpments were +horizontal. Along lines of coast, even of great lengths, such as that of +Patagonia, if they are nearly uniformly exposed, the corroding power of the +waves will be checked and conquered by the elevatory movement, as often as it +recommences, at about the same period; and hence the terraces, or accumulated +beach-lines, will commence being formed at nearly the same levels: at each +succeeding period of rest, they will, also, be eaten into at nearly the same +rate, and consequently there will be a much closer coincidence in their levels +and inclinations, than in the terraces and escarpments formed round bays with +their different parts very differently exposed to the action of the sea. It is +only where the waves are enabled, after a long lapse of time, slowly to corrode +hard rocks, or to throw up, owing to the supply of sediment being small and to +the surface being steeply inclined, a narrow beach or mound, that we can +expect, as at Glen Roy in Scotland,<a href="#fn-16.19" name="fnref-16.19" +id="fnref-16.19"><sup>[19]</sup></a> 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 +<a name="page320"></a> +than in exposed situations, and sooner where there is copious supply of +detritus than where there is little; on the other hand, the steps or +escarpments are formed during the stationary periods, and are more deeply cut +down and into the coast-land in exposed than in protected situations;—the +cutting action, moreover, being prolonged in the most exposed parts, both +during the beginning and ending, if slow, of the upward movement. +</p> + +<p class="footnote"> +<a name="fn-16.19" id="fn-16.19"></a> <a href="#fnref-16.19">[19]</a> +“Philosophical Transactions,” 1839, p. 39. +</p> + +<p> +Although in the foregoing discussion I have assumed the elevation to have been +horizontal, it may be suspected, from the considerable seaward slope of the +terraces, both up the valley of S. Cruz and up that of Coquimbo, that the +rising has been greater inland than nearer the coast. There is reason to +believe,<a href="#fn-16.20" name="fnref-16.20" +id="fnref-16.20"><sup>[20]</sup></a> 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,<a href="#fn-16.21" name="fnref-16.21" +id="fnref-16.21"><sup>[21]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-16.20" id="fn-16.20"></a> <a href="#fnref-16.20">[20]</a> +Mr. Place in the <i>Quarterly Journal of Science,</i> 1824, vol. xvii, p. 42. +</p> + +<p class="footnote"> +<a name="fn-16.21" id="fn-16.21"></a> <a href="#fnref-16.21">[21]</a> +“Voyages de la Comm. du Nord,” etc., also “Comptes +Rendus,” Oct. 1842. +</p> + +<p><i>Coquimbo to Guasco.</i>—In this distance of ninety +miles, I found in almost every part marine shells up to a height of +apparently from two hundred to three hundred feet. The desert plain +near Choros is thus covered; it is bounded by the escarpment of a +higher plain, consisting of pale-coloured, earthy, calcareous +stone, like that of Coquimbo, with the same recent shells embedded +in it. In the valley of Chaneral, a similar bed occurs in which, +differently from that of Coquimbo, I observed many shells of the +Concholepas: near Guasco the same calcareous bed is likewise met +with.</p> + +<p>In the valley of Guasco, the step-formed terraces of gravel are +displaced in a more striking manner than at any other point. I +followed the valley for thirty-seven miles (as reckoned by the +inhabitants) from the coast to Ballenar; in nearly the whole of +this distance, five grand terraces, running at corresponding +heights on both sides of the broad valley, are more conspicuous +than the three best-developed ones at Coquimbo. They give to the +landscape the most singular and formal aspect; and when the clouds +hung low, hiding the neighbouring mountains, the valley resembled +in the most striking manner that of Santa Cruz. The whole thickness +of these terraces or plains seems composed of gravel, rather firmly +aggregated together, with occasional parting seams of clay: the +pebbles on the upper plain are often whitewashed with an aluminous +substance, as in Patagonia. Near the coast I observed many +sea-shells on the lower plains. At Freyrina (twelve miles up the +valley), there are six terraces beside the bottom-surface of the +valley: the two lower ones are here only from two +<a name="page321"></a> +hundred to three hundred yards in width, but higher up the +valley they expand into plains; the third terrace is generally +narrow; the fourth I saw only in one place, but there it was +distinct for the length of a mile; the fifth is very broad; the +sixth is the summit-plain, which expands inland into a great basin. +Not having a barometer with me, I did not ascertain the height of +these plains, but they appeared considerably higher than those at +Coquimbo. Their width varies much, sometimes being very broad, and +sometimes contracting into mere fringes of separate flat-topped +projections, and then quite disappearing: at the one spot, where +the fourth terrace was visible, the whole six terraces were cut off +for a short space by one single bold escarpment. Near Ballenar +(thirty-seven miles from the mouth of the river), the valley +between the summit-edges of the highest escarpments is several +miles in width, and the five terraces on both sides are broadly +developed: the highest cannot be less than six hundred feet above +the bed of the river, which itself must, I conceive, be some +hundred feet above the sea.</p> + +<p class="center"> +No. 12<br/> +North and south section across the valley of Guasco, and of a plain north of +it. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono12.jpg" width="421" height="83" alt="[Illustration: +North and south section across the valley of Guasco.]" /> +</div> + +<p>On the northern side of the valley the summit-plain of gravel +(A) has two escarpments, one facing the valley, and the other a +great basin-like plain (B), which stretches for several leagues +northward. This narrow plain (A) with the double escarpment, +evidently once formed a spit or promontory of gravel, projecting +into and dividing two great bays, and subsequently was worn on both +sides into steep cliffs. Whether the several escarpments in this +valley were formed during the same stationary periods with those of +Coquimbo, I will not pretend to conjecture; but if so the +intervening and subsequent elevatory movements must have been here +much more energetic, for these plains certainly stand at a much +higher level than do those of Coquimbo.</p> + +<p> +<i>Copiapo.</i>—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<a +href="#fn-16.22" name="fnref-16.22" id="fnref-16.22"><sup>[22]</sup></a> +between fifty and seventy feet in height, of which the upper part consists +chiefly of gravel, abounding with recent shells, chiefly of the Concholepas, +<i>Venus Dombeyi</i>, and <i>Calyptræa trochiformis.</i> A little +<a name="page322"></a> +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 <i>Mulinia +Byronensis</i> and <i>Pecten purpuratus</i> were the most numerous. The lower +plain stretches for some miles southward, and for an unknown distance +northward, but not far up the valley; its seaward face, according to Meyen, is +worn into caves above the level of the present beach. The valley of Copiapo is +much less steeply inclined and less direct in its course than any other valley +which I saw in Chile; and its bottom does not generally consist of gravel: +there are no step-formed terraces in it, except at one spot near the mouth of +the great lateral valley of the Despoblado where there are only two, one above +the other: lower down the valley, in one place I observed that the solid rock +had been cut into the shape of a beach, and was smoothed over with shingle. +</p> + +<p class="footnote"> +<a name="fn-16.22" id="fn-16.22"></a> <a href="#fnref-16.22">[22]</a> +“Reise um die Erde,” Th. I, s. 372, <i>et seq.</i> +</p> + +<p> +Northward of Copiapo, in lat. 26° S., the old voyager Wafer<a +href="#fn-16.23" name="fnref-16.23" id="fnref-16.23"><sup>[23]</sup></a> 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:<a href="#fn-16.24" name="fnref-16.24" +id="fnref-16.24"><sup>[24]</sup></a> 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<a href="#fn-16.25" name="fnref-16.25" +id="fnref-16.25"><sup>[25]</sup></a> described these +<a name="page323"></a> +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<a href="#fn-16.26" +name="fnref-16.26" id="fnref-16.26"><sup>[26]</sup></a> 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<a +href="#fn-16.27" name="fnref-16.27" id="fnref-16.27"><sup>[27]</sup></a> 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<a href="#fn-16.28" +name="fnref-16.28" id="fnref-16.28"><sup>[28]</sup></a> found numerous recent +sea shells in sand, at a considerable distance from the sea. +</p> + +<p class="footnote"> +<a name="fn-16.23" id="fn-16.23"></a> <a href="#fnref-16.23">[23]</a> +Burnett’s “Collection of Voyages,” vol. iv, p. 193. +</p> + +<p class="footnote"> +<a name="fn-16.24" id="fn-16.24"></a> <a href="#fnref-16.24">[24]</a> +“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. +</p> + +<p class="footnote"> +<a name="fn-16.25" id="fn-16.25"></a> <a href="#fnref-16.25">[25]</a> +<i>Silliman’s Amer. Journ. of Science,</i> vol. xliv, p. 2. +</p> + +<p class="footnote"> +<a name="fn-16.26" id="fn-16.26"></a> <a href="#fnref-16.26">[26]</a> +“Voyage,” etc., p. 101. +</p> + +<p class="footnote"> +<a name="fn-16.27" id="fn-16.27"></a> <a href="#fnref-16.27">[27]</a> +In a letter to Mr. Lyell, “Geolog. Proc.,” vol. ii, p. 179. +</p> + +<p class="footnote"> +<a name="fn-16.28" id="fn-16.28"></a> <a href="#fnref-16.28">[28]</a> +<i>Edin. New Phil. Journ.,</i> vol. xxx, p. 155. +</p> + +<h4><i>Lima.</i></h4> + +<p> +Northward of Arica, I know nothing of the coast for about a space of five +degrees of latitude; but near Callao, the port of Lima, there is abundant and +very curious evidence of the elevation of the land. The island of San Lorenzo +is upwards of one thousand feet high; the basset edges of the strata composing +the lower part are worn into three obscure, narrow, sloping steps or ledges, +which can be seen only when standing on them: they probably resemble those +described by Lieutenant Freyer at Arica. The surface of the lower ledge, which +extends from a low cliff overhanging the sea to the foot of the next upper +escarpment, is covered by an enormous accumulation of recent shells.<a +href="#fn-16.29" name="fnref-16.29" id="fnref-16.29"><sup>[29]</sup></a> 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 <i> insides</i> were incrusted with Balani and +Serpulæ. +<a name="page324"></a> +All, according to Mr. G.B. Sowerby, are recent species: they consist of:— +</p> + +<ol> +<li>Mytilus Magellanicus: same as that found at Valparaiso, and +there stated to be probably distinct from the true <i>M. +Magellanicus</i> of the east coast.</li> + +<li>Venus costellata, Sowerby “Zoological Proceedings.”</li> + +<li>Pecten purpuratus, Lam.</li> + +<li>Chama, probably echinulata, Brod.</li> + +<li>Calyptræa Byronensis, Gray.</li> + +<li>Calyptræa radians (Trochus, Lam.)</li> + +<li>Fissurella affinis, Gray.</li> + +<li>Fissurella biradiata, Trembly.</li> + +<li>Purpura chocolatta, Duclos.</li> + +<li>Purpura Peruviana, Gray.</li> + +<li>Purpura labiata, Gray.</li> + +<li>Purpura buxea (Murex, Brod.).</li> + +<li>Concholepas Peruviana.</li> + +<li>Nassa, related to reticulata.</li> + +<li>Triton rudis, Brod.</li> + +<li>Trochus, not yet described, but well-known and very +common.</li> + +<li>and 18. Balanus, two species, both common on the coast.</li> +</ol> + +<p class="footnote"> +<a name="fn-16.29" id="fn-16.29"></a> <a href="#fnref-16.29">[29]</a> +M. Chevalier, in the “Voyage of the <i>Bonite</i>,” observed these +shells; but his specimens were lost.—“L’Institut,” +1838, p. 151. +</p> + +<p> +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.<a href="#fn-16.30" name="fnref-16.30" +id="fnref-16.30"><sup>[30]</sup></a> On the highest parts of the ledge, small +fragments of the shells were mingled with, and evidently in process of +reduction into, a yellowish-white, soft, calcareous powder, tasting strongly of +salt, and in some places as fine as prepared medicinal chalk. +</p> + +<p class="footnote"> +<a name="fn-16.30" id="fn-16.30"></a> <a href="#fnref-16.30">[30]</a> +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. +</p> + +<p> +<i>Fossil-remains of human art.</i>—In the midst of these shells on San +<a name="page325"></a> +Lorenzo, I found light corallines, the horny ovule-cases of Mollusca, roots of +seaweed,<a href="#fn-16.31" name="fnref-16.31" +id="fnref-16.31"><sup>[31]</sup></a> bones of birds, the heads of Indian corn +and other vegetable matter, a piece of woven rushes, and another of nearly +decayed <i>cotton</i> 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 <i>cotton</i> 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 <i>Indian man +inhabited Peru.</i> 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<a href="#fn-16.32" name="fnref-16.32" +id="fnref-16.32"><sup>[32]</sup></a> of the earthquake of 1835, in absolutely +shattering as if by gunpowder the <i>surface</i> of the primary rocks near +Concepcion, that a smooth bare surface of stone was left by the sea covered by +the shelly mass, and that afterwards when upraised, it was superficially +shattered by the severe shocks so often experienced here. +</p> + +<p class="footnote"> +<a name="fn-16.31" id="fn-16.31"></a> <a href="#fnref-16.31">[31]</a> +Mr. Smith of Jordan Hill found pieces of seaweed in an upraised pleistocene +deposit in Scotland. See his admirable Paper in the <i>Edin. New Phil. +Journal,</i> vol. xxv, p. 384. +</p> + +<p class="footnote"> +<a name="fn-16.32" id="fn-16.32"></a> <a href="#fnref-16.32">[32]</a> +I have described this in my “Journal of Researches,” p. 303, 2nd +edit. +</p> + +<p>The very low land surrounding the town of Callao, is to the +south joined by an obscure escarpment to a higher plain (south of +Bella Vista), which stretches along the coast for a length of about +eight miles. This plain appears to the eye quite level; but the +sea-cliffs show that its height varies (as far as I could estimate) +from seventy to one hundred and twenty feet. It is composed of +thin, sometimes waving, beds of clay, often of bright red and +yellow colours, of layers of impure sand, and in one part with a +great stratified mass of granitic pebbles. These +<a name="page326"></a> +beds are capped by a remarkable mass, varying from two to six +feet in thickness, of reddish loam or mud, containing many +scattered and broken fragments of recent marine shells, sometimes +though rarely single large round pebble, more frequently short +irregular layers of fine gravel, and very many pieces of red coarse +earthenware, which from their curvatures must once have formed +parts of large vessels. The earthenware is of Indian manufacture; +and I found exactly similar pieces accidentally included within the +bricks, of which the neighbouring ancient Peruvian burial-mounds +are built. These fragments abounded in such numbers in certain +spots, that it appeared as if waggon-loads of earthenware had been +smashed to pieces. The broken sea-shells and pottery are strewed +both on the surface, and throughout the whole thickness of this +upper loamy mass. I found them wherever I examined the cliffs, for +a space of between two and three miles, and for half a mile inland; +and there can be little doubt that this same bed extends with a +smooth surface several miles further over the entire plain. Besides +the little included irregular layers of small pebbles, there are +occasionally very obscure traces of stratification.</p> + +<p>At one of the highest parts of the cliff, estimated 120 feet +above the sea, where a little ravine came down, there were two +sections, at right angles to each other, of the floor of a shed or +building. In both sections or faces, two rows, one over the other, +of large round stones could be distinctly seen; they were packed +close together on an artificial layer of sand two inches thick, +which had been placed on the natural clay-beds; the round stones +were covered by three feet in thickness of the loam with broken +sea-shells and pottery. Hence, before this widely spread-out bed of +loam was deposited, it is certain that the plain was inhabited; and +it is probable, from the broken vessels being so much more abundant +in certain spots than in others, and from the underlying clay being +fitted for their manufacture, that the kilns stood here.</p> + +<p>The smoothness and wide extent of the plain, the bulk of matter +deposited, and the obscure traces of stratification seem to +indicate that the loam was deposited under water; on the other +hand, the presence of sea-shells, their broken state, the pebbles +of various sizes, and the artificial floor of round stones, almost +prove that it must have originated in a rush of water from the sea +over the land. The height of the plain, namely, 120 feet, renders +it improbable that an earthquake-wave, vast as some have here been, +could have broken over the surface at its present level; but when +the land stood eighty-five feet lower, at the period when the +shells were thrown up on the ledge at S. Lorenzo, and when as we +know man inhabited this district, such an event might well have +occurred; and if we may further suppose, that the plain was at that +time converted into a temporary lake, as actually occurred, during +the earthquakes of 1713 and 1746, in the case of the low land round +Callao owing to its being encircled by a high shingle-beach, all +the appearances above described will be perfectly explained. I must +add, that at a lower level near the point where the present low +land round Callao joins the higher plain, there are appearances of +two +<a name="page327"></a> +distinct deposits both apparently formed by debacles: in the +upper one, a horse’s tooth and a dog’s jaw were embedded; so that +both must have been formed after the settlement of the Spaniards: +according to Acosta, the earthquake-wave of 1586 rose eighty-four +feet.</p> + +<p> +The inhabitants of Callao do not believe, as far as I could ascertain, that any +change in level is now in progress. The great fragments of brickwork, which it +is asserted can be seen at the bottom of the sea, and which have been adduced +as a proof of a late subsidence, are, as I am informed by Mr. Gill, a resident +engineer, loose fragments; this is probable, for I found on the beach, and not +near the remains of any building, masses of brickwork, three and four feet +square, which had been washed into their present places, and smoothed over with +shingle during the earthquake of 1746. The spit of land, on which the ruins of +<i>Old</i> 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<a href="#fn-16.33" name="fnref-16.33" +id="fnref-16.33"><sup>[33]</sup></a> 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<a +href="#fn-16.34" name="fnref-16.34" id="fnref-16.34"><sup>[34]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-16.33" id="fn-16.33"></a> <a href="#fnref-16.33">[33]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-16.34" id="fn-16.34"></a> <a href="#fnref-16.34">[34]</a> +“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. +</p> + +<p> +<i>On the decay of upraised sea-shells.</i>—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 +<a name="page328"></a> +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.<a href="#fn-16.35" name="fnref-16.35" +id="fnref-16.35"><sup>[35]</sup></a> The other saline bodies may perhaps have +been partially thus derived, but chiefly, as I conclude from the following +facts, through a different means. +</p> + +<p class="footnote"> +<a name="fn-16.35" id="fn-16.35"></a> <a href="#fnref-16.35">[35]</a> +See “Volcanic Islands,” etc., by the Author. +</p> + +<p> +On most parts of the second ledge or old sea-beach, at a height of 170 feet, +there is a layer of white powder of variable thickness, as much in some parts +as two inches, lying on the angular, salt-cemented fragments of sandstone and +under about four inches of earth, which powder, from its close resemblance in +nature to the upper and most decayed parts of the shelly mass, I can hardly +doubt originally existed as a bed of shells, now much collapsed and quite +disintegrated. I could not discover with the microscope a trace of organic +structure in it; but its chemical constituents, according to Mr. Reeks, are the +same as in the powder extracted from amongst the decaying shells on the lower +ledge, with the marked exception that the carbonate of lime is present in only +very small quantity. On the third and highest ledge, I observed some of this +powder in a similar position, and likewise occasionally in small patches at +considerably greater heights near the summit of the island. At Iquique, where +the whole face of the country is covered by a highly saliferous alluvium, and +where the climate is extremely dry, we have seen that, according to Mr. Blake, +the shells which are perfect near the beach become, in ascending, gradually +less and less perfect, until scarcely a trace of their original structure can +be discovered. It is known that carbonate of lime and common salt left in a +mass together,<a href="#fn-16.36" name="fnref-16.36" +id="fnref-16.36"><sup>[36]</sup></a> 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 +<a name="page329"></a> +undissolved, is much less favourable to the preservation of upraised shells +than a humid climate. However this may be, it is interesting to know the manner +in which masses of shells, gradually upraised above the sea-level, decay and +finally disappear. +</p> + +<p class="footnote"> +<a name="fn-16.36" id="fn-16.36"></a> <a href="#fnref-16.36">[36]</a> +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 (<i>Gardeners’ Chron.,</i> 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 <i>Gardeners’ Chron.,</i> p. 675, 1844. +</p> + +<p> +<i>Summary on the recent elevation of the west coast of South +America.</i>—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.<a href="#fn-16.37" name="fnref-16.37" +id="fnref-16.37"><sup>[37]</sup></a> 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 +<a name="page330"></a> +of elevation, from shells being found on the surface or in superficial beds at +different heights; for we do not know on what their rate of decay depends; and +at Coquimbo one instance out of many has been given, of a promontory, which, +from the occurrence of one very small collection of lime-cemented shells, has +indisputably been elevated 242 feet, and yet on which, not even a fragment of +shell could be found on careful examination between this height and the beach, +although many sites appeared very favourable for the preservation of organic +remains: the absence, also, of shells on the gravel-terraces a short distance +up the valley of Coquimbo, though abundant on the corresponding terraces at its +mouth, should be borne in mind. +</p> + +<p class="footnote"> +<a name="fn-16.37" id="fn-16.37"></a> <a href="#fnref-16.37">[37]</a> +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. +</p> + +<p>There are other epochs, besides that of the existence of recent +Mollusca, by which to judge of the changes of level on this coast. +At Lima, as we have just seen, the elevation has been at least +eighty-five feet, within the Indo-human period; and since the +arrival of the Spaniards in 1530, there has apparently been a +sinking of the surface. At Valparaiso, in the course of 220 years, +the rise must have been less than nineteen feet; but it has been as +much as from ten to eleven feet in the seventeen years subsequently +to 1817, and of this rise only a part can be attributed to the +earthquake of 1822, the remainder having been insensible and +apparently still, in 1834, in progress. At Chiloe the elevation has +been gradual, and about four feet during four years. At Coquimbo, +also, it has been gradual, and in the course of 150 years has +amounted to several feet. The sudden small upheavals, accompanied +by earthquakes, as in 1822 at Valparaiso, in 1835 at Concepcion, +and in 1837 in the Chonos Archipelago, are familiar to most +geologists, but the gradual rising of the coast of Chile has been +hardly noticed; it is, however, very important, as connecting +together these two orders of events.</p> + +<p>The rise of Lima, having been eighty-five feet within the period +of man, is the more surprising if we refer to the eastern coast of +the continent, for at Port S. Julian, in Patagonia, there is good +evidence (as we shall hereafter see) that when the land stood +ninety feet lower, the Macrauchenia, a mammiferous beast, was +alive; and at Bahia Blanca, when it stood only a few feet lower +than it now does, many gigantic quadrupeds ranged over the +adjoining country. But the coast of Patagonia is some way distant +from the Cordillera, and the movement at Bahia Blanca is perhaps +noways connected with this great range, but rather with the +tertiary volcanic rocks of Banda Oriental, and therefore the +elevation at these places may have been infinitely slower than on +the coast of Peru. All such speculations, however, must be vague, +for as we know with certainty that the elevation of the whole coast +of Patagonia has been interrupted by many and long pauses, who will +pretend to say that, in such cases, many and long periods of +subsidence may not also have been intercalated?</p> + +<p>In many parts of the coast of Chile and Peru there are marks of +the action of the sea at successive heights on the land, showing +that the elevation has been interrupted by periods of comparative +rest in the upward movement, and of denudation in the action of the +sea. These +<a name="page331"></a> +are plainest at Chiloe, where, in a height of about five hundred +feet, there are three escarpments,—at Coquimbo, where in a +height of 364 feet, there are five,—at Guasco, where there +are six, of which five may perhaps correspond with those at +Coquimbo, but if so, the subsequent and intervening elevatory +movements have been here much more energetic,—at Lima, where, +in a height of about 250 feet there are three terraces, and others, +as it is asserted, at considerably greater heights. The almost +entire absence of ancient marks of sea-action at defined levels +along considerable spaces of coast, as near Valparaiso and +Concepcion, is highly instructive, for as it is improbable that the +elevation at these places alone should have been continuous, we +must attribute the absence of such marks to the nature and form of +the coast-rocks. Seeing over how many hundred miles of the coast of +Patagonia, and on how many places on the shores of the Pacific, the +elevatory process has been interrupted by periods of comparative +rest, we may conclude, conjointly with the evidence drawn from +other quarters of the world, that the elevation of the land is +generally an intermittent action. From the quantity of matter +removed in the formation of the escarpments, especially of those of +Patagonia, it appears that the periods of rest in the movement, and +of denudation of the land, have generally been very long. In +Patagonia, we have seen that the elevation has been equable, and +the periods of denudation synchronous over very wide spaces of +coast; on the shores of the Pacific, owing to the terraces chiefly +occurring in the valleys, we have not equal means of judging on +this point; and the very different heights of the upraised shells +at Coquimbo, Valparaiso, and Concepcion seem directly opposed to +such a conclusion.</p> + +<p>Whether on this side of the continent the elevation, between the +periods of comparative rest when the escarpments were formed, has +been by small sudden starts, such as those accompanying recent +earthquakes, or, as is most probable, by such starts conjointly +with a gradual upward movement, or by great and sudden upheavals, I +have no direct evidence. But as on the eastern coast, I was led to +think, from the analogy of the last hundred feet of elevation in La +Plata, and from the nearly equal size of the pebbles over the +entire width of the terraces, and from the upraised shells being +all littoral species, that the elevation had been gradual; so do I +on this western coast, from the analogy of the movements now in +progress, and from the vast numbers of shells now living +exclusively on or close to the beach, which are strewed over the +whole surface of the land up to very considerable heights, +conclude, that the movement here also has been slow and gradual, +aided probably by small occasional starts. We know at least that at +Coquimbo, where five escarpments occur in a height of 364 feet, the +successive elevations, if they have been sudden, cannot have been +very great. It has, I think, been shown that the occasional +preservation of shells, unrolled and unbroken, is not improbable +even during a quite gradual rising of the land; and their +preservation, if the movement has been aided by small starts, is +quite conformable with what actually takes place during recent +earthquakes.</p> + +<p> +<a name="page332"></a> +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 +<i>comparative rest</i>, 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 <i>narrow</i> mound of +detritus, that we can confidently assert that the land at that +level and at that period long remained absolutely stationary. In +the case of terraces formed of gravel or sand, although the +elevation may have been strictly horizontal, it may well happen +that no one level beach-line may be traceable, and that neither the +terraces themselves nor the summit nor basal edges of their +escarpments may be horizontal.</p> + +<p>Finally, comparing the extent of the elevated area, as deduced +from the upraised recent organic remains, on the two sides of the +continent, we have seen that on the Atlantic, shells have been +found at intervals from Eastern Tierra del Fuego for 1,180 miles +northward, and on the Pacific for a space of 2,075 miles. For a +length of 775 miles, they occur in the same latitudes on both sides +of the continent. Without taking this circumstance into +consideration, it is probable from the reasons assigned in the last +chapter, that the entire breadth of the continent in Central +Patagonia has been uplifted in mass; but from other reasons there +given, it would be hazardous to extend this conclusion to La Plata. +From the continent being narrow in the southern-most parts of +Patagonia, and from the shells found at the Inner Narrows of the +Strait of Magellan, and likewise far up the valley of the Santa +Cruz, +<a name="page333"></a> +it is probable that the southern part of the western coast, +which was not visited by me, has been elevated within the period of +recent Mollusca: if so, the shores of the Pacific have been +continuously, recently, and in a geological sense synchronously +upraised, from Lima for a length of 2,480 nautical miles +southward,—a distance equal to that from the Red Sea to the +North Cape of Scandinavia!</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap3.03"></a>Chapter III<br/>ON THE PLAINS AND VALLEYS OF +CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS.</h2> + +<p class="letter"> +Basin-like plains of Chile; their drainage, their marine origin.—Marks of +sea-action on the eastern flanks of the Cordillera.—Sloping terrace-like +fringes of stratified shingle within the valleys of the Cordillera; their +marine origin.—Boulders in the valley of Cachapual.—Horizontal +elevation of the Cordillera.—Formation of valleys.—Boulders moved +by earthquake-waves.—Saline superficial deposits.—Bed of nitrate of +soda at Iquique.—Saline incrustations.—Salt-lakes of La Plata and +Patagonia; purity of the salt; its origin. +</p> + +<p>The space between the Cordillera and the coast of Chile is on a +rude average from eighty to above one hundred miles in width; it is +formed, either of an almost continuous mass of mountains, or more +commonly of several nearly parallel ranges, separated by plains; in +the more southern parts of this province the mountains are quite +subordinate to the plains; in the northern part the mountains +predominate.</p> + +<p> +The basin-like plains at the foot of the Cordillera are in several respects +remarkable; that on which the capital of Chile stands is fifteen miles in +width, in an east and west line, and of much greater length in a north and +south line; it stands 1,750 feet above the sea; its surface appears smooth, but +really falls and rises in wide gentle undulations, the hollows corresponding +with the main valleys of the Cordillera: the striking manner in which it +abruptly comes up to the foot of this great range has been remarked by every +author<a href="#fn-17.1" name="fnref-17.1" id="fnref-17.1"><sup>[1]</sup></a> +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 +<a name="page334"></a> +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<a +href="#fn-17.2" name="fnref-17.2" id="fnref-17.2"><sup>[2]</sup></a> above the +surface of the surrounding plain. +</p> + +<p class="footnote"> +<a name="fn-17.1" id="fn-17.1"></a> <a href="#fnref-17.1">[1]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-17.2" id="fn-17.2"></a> <a href="#fnref-17.2">[2]</a> +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. +</p> + +<p> +To the south this basin-like plain contracts, and rising scarcely perceptibly +with a smooth surface, passes through a remarkable level gap in the mountains, +forming a true land-strait, and called the Angostura. It then immediately +expands into a second basin-formed plain: this again to the south contracts +into another land-strait, and expands into a third basin, which, however, falls +suddenly in level about forty feet. This third basin, to the south, likewise +contracts into a strait, and then again opens into the great plain of San +Fernando, stretching so far south that the snowy peaks of the distant +Cordillera are seen rising above its horizon as above the sea. These plains, +near the Cordillera, are generally formed of a thick stratified mass of +shingle;<a href="#fn-17.3" name="fnref-17.3" id="fnref-17.3"><sup>[3]</sup></a> +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 +<a name="page335"></a> +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.<a href="#fn-17.4" name="fnref-17.4" +id="fnref-17.4"><sup>[4]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-17.3" id="fn-17.3"></a> <a href="#fnref-17.3">[3]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-17.4" id="fn-17.4"></a> <a href="#fnref-17.4">[4]</a> +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. +</p> + +<p>The origin and manner of formation of the thick beds of gravel, +sandy clay, volcanic detritus, and calcareous tuff, composing these +basin-like plains, is very important; because, as we shall +presently show, they send arms or fringes far up the main valleys +of the Cordillera. Many of the inhabitants believe that these +plains were once occupied by lakes, suddenly drained; but I +conceive that the number of the separate breaches at nearly the +same level in the mountains surrounding them quite precludes this +idea. Had not such distinguished naturalists as MM. Meyen and Gay +stated their belief that these deposits were left by great debacles +rushing down from the Cordillera, I should not have noticed a view, +which appears to me from many reasons improbable in the highest +degree—namely, from the vast accumulation of <i>well-rounded +pebbles</i>—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 +<a name="page336"></a> +at various levels, and their general similarity in form +and composition with the many plains near the coast, which are +either similarly marked or are strewed with upraised marine +remains, fully convince me that the mountains bounding these +basin-plains were breached, their islet-like projecting rocks worn, +and the loose stratified detritus forming their now level surfaces +deposited, by the sea, as the land slowly emerged. It is hardly +possible to state too strongly the perfect resemblance in outline +between these basin-like, long, and narrow plains of Chile +(especially when in the early morning the mists hanging low +represented water), and the creeks and fiords now intersecting the +southern and western shores of the continent. We can on this view +of the sea, when the land stood lower, having long and tranquilly +occupied the spaces between the mountain-ranges, understand how the +boundaries of the separate basins were breached in more than one +place; for we see that this is the general character of the inland +bays and channels of Tierra del Fuego; we there, also, see in the +sawing action of the tides, which flow with great force in the +cross channels, a power sufficient to keep the breaches open as the +land emerged. We can further see that the waves would naturally +leave the smooth bottom of each great bay or channel, as it became +slowly converted into land, gently inclined to as many points as +there were mouths, through which the sea finally retreated, thus +forming so many watersheds, without any marked ridges, on a nearly +level surface. The absence of marine remains in these high inland +plains cannot be properly adduced as an objection to their marine +origin: for we may conclude, from shells not being found in the +great shingle beds of Patagonia, though copiously strewed on their +surfaces, and from many other analogous facts, that such deposits +are eminently unfavourable for the embedment of such remains; and +with respect to shells not being found strewed on the surface of +these basin-like plains, it was shown in the last chapter that +remains thus exposed in time decay and disappear.</p> + +<p class="center"> +No. 13<br/> +Section of the plain at the eastern foot of the Chilian Cordillera. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono13.jpg" width="417" height="70" alt="[Illustration: +Section of the plain at the eastern foot of the Chilian Cordillera.]" /> +</div> + +<p>I observed some appearances on the plains at the eastern and +opposite foot of the Cordillera which are worth notice, as showing +that the sea there long acted at nearly the same level as on the +basin-plains of Chile. The mountains on this eastern side are +exceedingly abrupt; they rise out of a smooth, talus-like, very +gentle, slope, from five to ten miles in width (as represented in +Figure 13), entirely composed of perfectly rounded pebbles, often +white-washed with an aluminous substance like decomposed feldspar. +This sloping plain or talus blends into a perfectly flat space a +few miles in width, composed +<a name="page337"></a> +of reddish impure clay, with small calcareous concretions as in +the Pampean deposit,—of fine white sand with small pebbles in +layers,—and of the above-mentioned white aluminous earth, all +interstratified together. This flat space runs as far as Mendoza, +thirty miles northward, and stands probably at about the same +height, namely, 2,700 feet (Pentland and Miers) above the sea. To +the east it is bounded by an escarpment, eighty feet in height, +running for many miles north and south, and composed of perfectly +round pebbles, and loose, white-washed, or embedded in the +aluminous earth: behind this escarpment there is a second and +similar one of gravel. Northward of Mendoza, these escarpments +become broken and quite obliterated; and it does not appear that +they ever enclosed a lake-like area: I conclude, therefore, that +they were formed by the sea, when it reached the foot of the +Cordillera, like the similar escarpments occurring at so many +points on the coasts of Chile and Patagonia.</p> + +<p>The talus-like plain slopes up with a smooth surface into the +great dry valleys of the Cordillera. On each hand of the Portillo +valley, the mountains are formed of red granite, mica-slate, and +basalt, which all have suffered a truly astonishing amount of +denudation; the gravel in the valley, as well as on the talus-like +plain in front of it, is composed of these rocks; but at the mouth +of the valley, in the middle (height probably about three thousand +five hundred feet above the sea), a few small isolated hillocks of +several varieties of porphyry project, round which, on all sides, +smooth and often white-washed pebbles of these same porphyries, to +the exclusion of all others, extend to a circumscribed distance. +Now, it is difficult to conceive any other agency, except the quiet +and long-continued action of the sea on these hillocks, which could +have rounded and whitewashed the fragments of porphyry, and caused +them to radiate from such small and quite insignificant centres, in +the midst of that vast stream of stones which has descended from +the main Cordillera.</p> + +<p><i>Sloping terraces of gravel in the valleys of the +Cordillera.</i>—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 +<a name="page338"></a> +forming these fringes, on a rude average, may be said to vary +from thirty to sixty or eighty feet; but near the mouths of the +valleys it was in several places from two to three hundred feet. +The amount of matter removed by the torrents has been immense; yet +in the lower parts of the valleys the terraces have seldom been +entirely worn away on either side, nor has the solid underlying +rock been reached: higher up the valleys, the terraces have +frequently been removed on one or the other side, and sometimes on +both sides; but in this latter case they reappear after a short +interval on the line, which they would have held had they been +unbroken. Where the solid rock has been reached, it has been cut +into deep and narrow gorges. Still higher up the valleys, the +terraces gradually become more and more broken, narrower, and less +thick, until, at a height of from seven to nine thousand feet, they +become lost, and blended with the piles of fallen detritus.</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono14.jpg" width="213" height="303" alt="[Illustration: +Ground plan of a bifurcating valley in the Cordillera.]" /> +</div> + +<p>I carefully examined in many places the state of the gravel, and +almost everywhere found the pebbles equally and perfectly rounded, +occasionally with great blocks of rock, and generally distinctly +stratified, often with parting seams of sand. The pebbles were +sometimes coated with a white aluminous, and less frequently with a +calcareous, crust. At great heights up the valleys the pebbles +become less rounded; and as the terraces become obliterated, the +whole mass passes into the nature of ordinary detritus. I was +repeatedly struck with the great difference between this detritus +high up the valleys, and the gravel of the terraces low down, +namely, in the greater number of the quite angular fragments in the +detritus,—in the unequal degree to which the other fragments +have been rounded,—in the quantity of associated +earth,—in the absence of stratification,—and in the +irregularity of the upper surfaces. This difference was likewise +well shown at points low down the valleys, where precipitous +ravines, cutting through mountains of highly coloured rock, have +thrown down wide, fan-shaped accumulations of detritus on the +terraces: in such cases, the line of separation between the +detritus and the terrace could be pointed out to within an inch or +two; the detritus consisting entirely of angular and only partially +rounded fragments of the adjoining coloured rocks; the stratified +shingle (as I ascertained by close inspection, especially in one +case, in the valley of the River Mendoza) containing only a small +proportion of these fragments, and those few well rounded.</p> + +<p> +<a name="page339"></a> +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 <i>in situ</i>: 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 <i>in situ</i>), 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<a +href="#fn-17.5" name="fnref-17.5" id="fnref-17.5"><sup>[5]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-17.5" id="fn-17.5"></a> <a href="#fnref-17.5">[5]</a> +“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. +</p> + +<p> +These fringes of stratified gravel occur along all the great valleys of the +Cordillera, as well as along their main branches; they are strikingly developed +in the valleys of the Maypu, Mendoza, Aconcagua, Cachapual, and according to +Meyen,<a href="#fn-17.6" name="fnref-17.6" id="fnref-17.6"><sup>[6]</sup></a> +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 <i>every</i> valley, and all their branches, +eastward and westward from the central pinnacles of the Cordillera, we must +admit from the following +<a name="page340"></a> +reasons,—from the distinct stratification of the mass,—its smooth +upper surface,—the well-rounded and sometimes encrusted state of the +pebbles, so different from the loose debris on the mountains,—and +especially from the terraces preserving their uniform inclination round the +most abrupt bends. To suppose that as the land now stands, the rivers deposited +the shingle along the course of every valley, and all their main branches, +appears to me preposterous, seeing that these same rivers not only are now +removing and have removed much of this deposit, but are everywhere tending to +cut deep and narrow gorges in the hard underlying rocks. +</p> + +<p class="footnote"> +<a name="fn-17.6" id="fn-17.6"></a> <a href="#fnref-17.6">[6]</a> +“Reise,” etc., Th. I, s. 302. +</p> + +<p> +I have stated that these fringes of gravel, the origin of which are +inexplicable on the notion of debacles or of ordinary alluvial action, are +directly continuous with the similarly-composed basin-like plains at the foot +of the Cordillera, which, from the several reasons before assigned, I cannot +doubt were modelled by the agency of the sea. Now if we suppose that the sea +formerly occupied the valleys of the Chilean Cordillera, in precisely the same +manner as it now does in the more southern parts of the continent, where deep +winding creeks penetrate into the very heart of, and in the case of Obstruction +Sound quite through, this great range; and if we suppose that the mountains +were upraised in the same slow manner as the eastern and western coasts have +been upraised within the recent period, then the origin and formation of these +sloping, terrace-like fringes of gravel can be simply explained. For every part +of the bottom of each valley will, on this view, have long stood at the head of +a sea creek, into which the then existing torrents will have delivered +fragments of rocks, where, by the action of the tides, they will have been +rolled, sometimes encrusted, rudely stratified, and the whole surface levelled +by the blending together of the successive beach lines.<a href="#fn-17.7" +name="fnref-17.7" id="fnref-17.7"><sup>[7]</sup></a> 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 +<a name="page341"></a> +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.<a href="#fn-17.8" name="fnref-17.8" +id="fnref-17.8"><sup>[8]</sup></a> Hence, after the final conversion of the +creeks into valleys, almost the only difference in the terraces at those points +at which the sea stood long, will be a somewhat more gentle inclination, with +river-worn instead of sea-worn detritus on the surface. +</p> + +<p class="footnote"> +<a name="fn-17.7" id="fn-17.7"></a> <a href="#fnref-17.7">[7]</a> +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 (<i>London and Edin. Phil. Journal,</i> vol. xxi, +p. 187) of North Wales. +</p> + +<p class="footnote"> +<a name="fn-17.8" id="fn-17.8"></a> <a href="#fnref-17.8">[8]</a> +I have attempted to explain this process in a more detailed manner, in a letter +to Mr. Maclaren, published in the <i>Edinburgh New Phil. Journal,</i> vol. +xxxv, p. 288. +</p> + +<p> +I know of only one difficulty on the foregoing view, namely, the +far-transported blocks of rock high on the sides of the valley of the +Cachapual: I will not attempt any explanation of this phenomenon, but I may +state my belief that a mountain-ridge near the Baths of Cauquenes has been +upraised long subsequently to all the other ranges in the neighbourhood, and +that when this was effected the whole face of the country must have been +greatly altered. In the course of ages, moreover, in this and other valleys, +events may have occurred like, but even on a grander scale than, that described +by Molina,<a href="#fn-17.9" name="fnref-17.9" +id="fnref-17.9"><sup>[9]</sup></a> 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 +<a name="page342"></a> +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,<a +href="#fn-17.10" name="fnref-17.10" id="fnref-17.10"><sup>[10]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-17.9" id="fn-17.9"></a> <a href="#fnref-17.9">[9]</a> +“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. +</p> + +<p class="footnote"> +<a name="fn-17.10" id="fn-17.10"></a> <a href="#fnref-17.10">[10]</a> +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 <i>great</i> and <i>abrupt</i> 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 +<i>after the elevation</i> of this part of the valley above the level of the +sea. The valley here is almost blocked up by a pile about one thousand feet in +thickness, formed, as far as I could judge, from three sides, entirely, or at +least in chief part, of gravel and detritus. On the south side, the river has +cut quite through this mass; on the northern side, and on the very summit, deep +ravines, parallel to the line of the valley, are worn, as if the drainage from +the valley above had passed by these two lines before following its present +course. +</p> + +<h4><i>Formation of Valleys.</i></h4> + +<p>The bulk of solid rock which has been removed in the lower parts +of the valleys of the Cordillera has been enormous. It is only by +reflecting on such cases as that of the gravel beds of Patagonia, +covering so many thousand square leagues of surface, and which, if +heaped into a ridge, would form a mountain-range almost equal to +the Cordillera, that the amount of denudation becomes credible. The +valleys within this range often follow anticlinal but rarely +synclinal lines; that is, the strata on the two sides more often +dip from the line of valley than towards it. On the flanks of the +range, the valleys most frequently run neither along anticlinal nor +synclinal axes, but along lines of flexure or faults: that is, the +strata on both sides dip in the same direction, but with different, +though often only slightly different, inclinations. As most of the +nearly parallel ridges which together form the Cordillera run +approximately north and south, the east and west valleys cross them +in zig-zag lines, bursting through the points where the strata have +been +<a name="page343"></a> +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 <i>angular</i> upheaval to a period +subsequent to the elevation of the western ridge; and it is +likewise probable that many of the other parallel ridges have been +angularly upheaved at different periods; consequently many parts of +the surfaces of these mountains must formerly have been exposed to +the full force of the waves, which, if the Cordillera were now sunk +into the sea, would be protected by parallel chains of islands. The +torrents in the valleys certainly have great power in wearing the +rocks; as could be told by the dull rattling sound of the many +fragments night and day hurrying downwards; and as was attested by +the vast size of certain fragments, which I was assured had been +carried onwards during floods; yet we have seen in the lower parts +of the valleys, that the torrents have seldom removed all the +sea-checked shingle forming the terraces, and have had time since +the last elevation in mass only to cut in the underlying rocks, +gorges, deep and narrow, but quite insignificant in dimensions +compared with the entire width and depth of the valleys.</p> + +<p>Along the shores of the Pacific, I never ceased during my many +and long excursions to feel astonished at seeing every valley, +ravine, and even little inequality of surface, both in the hard +granitic and soft tertiary districts, retaining the exact outline, +which they had when the sea left their surfaces coated with organic +remains. When these remains shall have decayed, there will be +scarcely any difference in appearance between this line of +coast-land and most other countries, which we are accustomed to +believe have assumed their present features chiefly through the +agency of the weather and fresh-water streams. In the old granitic +districts, no doubt it would be rash to attribute all the +modifications of outline exclusively to the sea-action; for who can +say how often this lately submerged coast may not previously have +existed as land, worn by running streams and washed by rain? This +source of doubt, however, does not apply to the districts +superficially formed of the modern tertiary deposits. The valleys +worn by the sea, through the softer formations, both on the +Atlantic and Pacific sides of the continent, are generally broad, +winding, and flat-bottomed: the only district of this nature now +penetrated by arms of the sea, is the island of Chiloe.</p> + +<p>Finally, the conclusion at which I have arrived, with respect to +the relative powers of rain and sea water on the land, is, that the +latter is far the most efficient agent, and that its chief tendency +is to widen the valleys; whilst torrents and rivers tend to deepen +them, and to remove the wreck of the sea’s destroying action. As +the waves have more +<a name="page344"></a> +power, the more open and exposed the space may be, so will they +always tend to widen more and more the mouths of valleys compared +with their upper parts: hence, doubtless, it is, that most valleys +expand at their mouths,—that part, at which the rivers +flowing in them, generally have the least wearing power.</p> + +<p>When reflecting on the action of the sea on the land at former +levels, the effect of the great waves, which generally accompany +earthquakes, must not be overlooked: few years pass without a +severe earthquake occurring on some part of the west coast of South +America; and the waves thus caused have great power. At Concepcion, +after the shock of 1835, I saw large slabs of sandstone, one of +which was six feet long, three in breadth, and two in thickness, +thrown high up on the beach; and from the nature of the marine +animals still adhering to it, it must have been torn up from a +considerable depth. On the other hand, at Callao, the recoil-wave +of the earthquake of 1746 carried great masses of brickwork, +between three and four feet square, some way out seaward. During +the course of ages, the effect thus produced at each successive +level, cannot have been small; and in some of the tertiary deposits +on this line of coast, I observed great boulders of granite and +other neighbouring rocks, embedded in fine sedimentary layers, the +transportal of which, except by the means of earthquake-waves, +always appeared to me inexplicable.</p> + +<h4><i>Superficial Saline Deposits.</i></h4> + +<p> +This subject may be here conveniently treated of: I will begin with the most +interesting case, namely, the superficial saline beds near Iquique in Peru. The +porphyritic mountains on the coast rise abruptly to a height of between one +thousand nine hundred and three thousand feet: between their summits and an +inland plain, on which the celebrated deposit of nitrate of soda lies, there is +a high undulatory district, covered by a remarkable superficial saliferous +crust, chiefly composed of common salt, either in white, hard, opaque nodules, +or mingled with sand, in this latter case forming a compact sandstone. This +saliferous superficial crust extends from the edge of the coast-escarpment, +over the whole face of the country; but never attains, as I am assured by Mr. +Bollaert (long resident here) any great thickness. Although a very slight +shower falls only at intervals of many years, yet small funnel-shaped cavities +show that the salt has been in some parts dissolved.<a href="#fn-17.11" +name="fnref-17.11" id="fnref-17.11"><sup>[11]</sup></a> 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 +<a name="page345"></a> +it rested, I imagined that this<a href="#fn-17.12" name="fnref-17.12" +id="fnref-17.12"><sup>[12]</sup></a> 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<a href="#fn-17.13" name="fnref-17.13" +id="fnref-17.13"><sup>[13]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-17.11" id="fn-17.11"></a> <a href="#fnref-17.11">[11]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-17.12" id="fn-17.12"></a> <a href="#fnref-17.12">[12]</a> +“Journal of Researches,” p. 444, first edit. +</p> + +<p class="footnote"> +<a name="fn-17.13" id="fn-17.13"></a> <a href="#fnref-17.13">[13]</a> +“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. +</p> + +<p> +Associated with the salt in the superficial beds, there are numerous, thin, +horizontal layers of impure, dirty-white, friable, gypseous and calcareous +tuffs. The gypseous beds are very remarkable, from abounding with, so as +sometimes to be almost composed of, irregular concretions, from the size of an +egg to that of a man’s head, of very hard, compact, heavy gypsum, in the +form of anhydrite. This gypsum contains some foreign particles of stone; it is +stained, judging from its action with borax, with iron, and it exhales a strong +aluminous odour. The surfaces of the concretions are marked by sharp, +radiating, or bifurcating ridges, as if they had been (but not really) +corroded: internally they are penetrated by branching veins (like those of +calcareous spar in the septaria of the London clay) of pure white anhydrite. +These veins might naturally have been thought to have been formed by subsequent +infiltration, had not each little embedded fragment of rock been likewise edged +in a very remarkable manner by a narrow border of the same white anhydrite: +this shows that the veins must have been formed by a process of segregation, +and not of infiltration. Some of the little included and <i>cracked</i> +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 +<a name="page346"></a> +separate small fragments of rock from the larger masses.<a href="#fn-17.14" +name="fnref-17.14" id="fnref-17.14"><sup>[14]</sup></a> 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<a href="#fn-17.15" +name="fnref-17.15" id="fnref-17.15"><sup>[15]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-17.14" id="fn-17.14"></a> <a href="#fnref-17.14">[14]</a> +“Volcanic Islands,” etc., p. 87. +</p> + +<p class="footnote"> +<a name="fn-17.15" id="fn-17.15"></a> <a href="#fnref-17.15">[15]</a> +“Voyage Géolog.,” etc., p. 95. +</p> + +<p> +The plain or basin, on the borders of which the famous bed of nitrate of soda +lies, is situated at the distance of about thirty miles from the sea, being +separated from it by the saliferous district just described. It stands at a +height of 3,300 feet; its surface is level, and some leagues in width; it +extends forty miles northward, and has a total length (as I was informed by Mr. +Belford Wilson, the Consul-General at Lima) of 420 miles. In a well near the +works, thirty-six yards in depth, sand, earth, and a little gravel were found: +in another well, near Almonte, fifty yards deep, the whole consisted, according +to Mr. Blake,<a href="#fn-17.16" name="fnref-17.16" +id="fnref-17.16"><sup>[16]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-17.16" id="fn-17.16"></a> <a href="#fnref-17.16">[16]</a> +See an admirable paper “Geolog. and Miscell. Notices of Tarapaca,” +in <i>Silliman’s American Journal</i>, vol. xliv, p. 1. +</p> + +<p>The nitrate of soda varies in purity in different parts, and +often contains nodules of common salt. According to Mr. Blake, the +proportion of nitrate of soda varies from 20 to 75 per cent. An +analysis by Mr. A. Hayes, of an average specimen, gave:—</p> + +<table border="0" width="40%" summary="Analysis of specimen"> +<tr> +<td>Nitrate of Soda</td> +<td></td> +<td align="right">64·98</td> +</tr> + +<tr> +<td>Sulphate of Soda</td> +<td></td> +<td align="right">3·00</td> +</tr> + +<tr> +<td>Chloride of Soda</td> +<td></td> +<td align="right">28·69</td> +</tr> + +<tr> +<td>Iodic Salts</td> +<td></td> +<td align="right">0·63</td> +</tr> + +<tr valign="top"> +<td>Shells and Marl</td> +<td></td> +<td align="right">2·60<br/> +———<br/> +99.90</td> +</tr> +</table> + +<p class="noindent"> +<a name="page347"></a> +The “mother-water” at some of the refineries is very rich in iodic +salts, and is supposed<a href="#fn-17.17" name="fnref-17.17" +id="fnref-17.17"><sup>[17]</sup></a> 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.<a +href="#fn-17.18" name="fnref-17.18" id="fnref-17.18"><sup>[18]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-17.17" id="fn-17.17"></a> <a href="#fnref-17.17">[17]</a> +<i>Literary Gazette</i>, 1841, p. 475. +</p> + +<p class="footnote"> +<a name="fn-17.18" id="fn-17.18"></a> <a href="#fnref-17.18">[18]</a> +(From an official document, shown me by Mr. Belford Wilson, it +appears that the first export of nitrate of soda to Europe was in +July 1830, on French account, in a British ship:— +</p> + +<table style="margin-left: 10%" border="0" width="40%" summary= +"Export quantities of nitrate of soda"> +<tr> +<td><small>Entire export in</small></td> +<td></td> +<td align="right"><small>Quintals</small></td> +</tr> + +<tr> +<td><small>1830</small></td> +<td></td> +<td align="right"><small>17,300</small></td> +</tr> + +<tr> +<td><small>1831</small></td> +<td></td> +<td align="right"><small>40,885</small></td> +</tr> + +<tr> +<td><small>1832</small></td> +<td></td> +<td align="right"><small>51,400</small></td> +</tr> + +<tr> +<td><small>1833</small></td> +<td></td> +<td align="right"><small>91,335</small></td> +</tr> + +<tr> +<td><small>1834</small></td> +<td></td> +<td align="right"><small>149,538</small></td> +</tr> +</table> + +<p class="footnote"> +The Spanish quintal nearly equals 100 English pounds. +</p> + +<p> +<i>Thin, superficial, saline incrustations.</i>—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<a +href="#fn-17.19" name="fnref-17.19" id="fnref-17.19"><sup>[19]</sup></a> 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<a href="#fn-17.20" name="fnref-17.20" +id="fnref-17.20"><sup>[20]</sup></a> 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 +<a name="page348"></a> +the common salt, by some unknown process, becomes in time changed into the +sulphate. Friable, calcareous matter is here abundant, and the case of the +apparent double decomposition of the shells and salt on San Lorenzo, should not +be forgotten. +</p> + +<p class="footnote"> +<a name="fn-17.19" id="fn-17.19"></a> <a href="#fnref-17.19">[19]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-17.20" id="fn-17.20"></a> <a href="#fnref-17.20">[20]</a> +M. d’Orbigny’s “Voyage,” etc., Part. Hist., tome i, p. +664. +</p> + +<p>The saline incrustations, near Bahia Blanca, are not confined +to, though most abundant on, the low muddy flats; for I noticed +some on a calcareous plain between thirty and forty feet above the +sea, and even a little occurs in still higher valleys. Low alluvial +tracts in the valleys of the Rivers Negro and Colorado are also +encrusted, and in the latter valley such spaces appeared to be +occasionally overflowed by the river. I observed saline +incrustations in some of the valleys of Southern Patagonia. At Port +Desire a low, flat, muddy valley was thickly incrusted by salts, +which on analysis by Mr. T. Reeks, are found to consist of a +mixture of sulphate and muriate of soda, with carbonate of lime and +earthy matter. On the western side of the continent, the southern +coasts are much too humid for this phenomenon; but in Northern +Chile I again met with similar incrustations. On the hardened mud, +in parts of the broad, flat-bottomed valley of Copiapo, the saline +matter encrusts the ground to the thickness of some inches: +specimens, sent by Mr. Bingley to Apothecaries’ Hall for analysis, +were said to consist of carbonate and sulphate of soda. Much +sulphate of soda is found in the desert of Atacama. In all parts of +South America, the saline incrustations occur most frequently on +low damp surfaces of mud, where the climate is rather dry; and +these low surfaces have, in almost every case, been upraised above +the level of the sea, within the recent period.</p> + +<p> +<i>Salt-lakes of Patagonia and La Plata.</i>—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.<a +href="#fn-17.21" name="fnref-17.21" id="fnref-17.21"><sup>[21]</sup></a> The +depressions, in which these salt-lakes lie, are from a few feet to sixty +metres, as asserted by M. d’Orbigny,<a href="#fn-17.22" +name="fnref-17.22" id="fnref-17.22"><sup>[22]</sup></a> 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 name="page349"></a> +a part, of the salt is dissolved, being redeposited during the succeeding dry +season. At this period the appearance of the snow-white expanse of salt +crystallised in great cubes, is very striking. In a large salina, northward of +the Rio Negro, the salt at the bottom, during the whole year, is between two +and three feet in thickness. +</p> + +<p class="footnote"> +<a name="fn-17.21" id="fn-17.21"></a> <a href="#fnref-17.21">[21]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-17.22" id="fn-17.22"></a> <a href="#fnref-17.22">[22]</a> +“Voyage Géolog.,” p. 63. +</p> + +<p> +The salt rests almost always on a thick bed of black muddy sand, which is +fetid, probably from the decay of the burrowing worms inhabiting it.<a +href="#fn-17.23" name="fnref-17.23" id="fnref-17.23"><sup>[23]</sup></a> 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<a href="#fn-17.24" +name="fnref-17.24" id="fnref-17.24"><sup>[24]</sup></a> 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 <i>padre del sal</i>, and +those of the sulphate of soda, the <i>madre del sal</i>; 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.<a href="#fn-17.25" +name="fnref-17.25" id="fnref-17.25"><sup>[25]</sup></a> The association of +gypsum and salt in this case, as well as in the superficial deposits of +Iquique, appears to me interesting, considering how generally these substances +are associated in the older stratified formations. +</p> + +<p class="footnote"> +<a name="fn-17.23" id="fn-17.23"></a> <a href="#fnref-17.23">[23]</a> +Professor Ehrenberg examined some of this muddy sand, but was unable to find in +it any infusoria. +</p> + +<p class="footnote"> +<a name="fn-17.24" id="fn-17.24"></a> <a href="#fnref-17.24">[24]</a> +“Voyage Géolog.,” p. 64. +</p> + +<p class="footnote"> +<a name="fn-17.25" id="fn-17.25"></a> <a href="#fnref-17.25">[25]</a> +This is what might have been expected; for M. Ballard asserts (<i>Acad. des +Sciences</i>, 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. +</p> + +<p> +Mr. Reeks has analysed for me some of the salt from the salina near the Rio +Negro; he finds it composed entirely of chloride of sodium, with the exception +of 0·26 of sulphate of lime and of 0·22 of earthy matter: there +are no traces of iodic salts. Some salt from the salina Chiquitos, in the +Pampean formation, is equally pure. It is a singular fact, that the salt from +these salinas does not serve so well for preserving meat, <a +name="page350"></a> 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,<a +href="#fn-17.26" name="fnref-17.26" id="fnref-17.26"><sup>[26]</sup></a> that +those salts answer best for preserving cheese which contain most of the +deliquescent chlorides.<a href="#fn-17.27" name="fnref-17.27" +id="fnref-17.27"><sup>[27]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-17.26" id="fn-17.26"></a> <a href="#fnref-17.26">[26]</a> +<i>Hort. and Agricult. Gazette</i>, 1845, p. 93. +</p> + +<p class="footnote"> +<a name="fn-17.27" id="fn-17.27"></a> <a href="#fnref-17.27">[27]</a> +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 <i> mother-water</i> during the refinement of the nitrate of soda, +is annually thrown away. +</p> + +<p> +With respect to the origin of the salt in the salinas, the foregoing analysis +seems opposed to the view entertained by M. d’Orbigny and others, and +which seems so probable considering the recent elevation of this line of coast, +namely, that it is due to the evaporation of sea-water and to the drainage from +the surrounding strata impregnated with sea-salt. I was informed (I know not +whether accurately) that on the northern side of the salina on the Rio Negro, +there is a small brine spring which flows at all times of the year: if this be +so, the salt in this case at least, probably is of subterranean origin. It at +first appears very singular that fresh water can often be procured in wells,<a +href="#fn-17.28" name="fnref-17.28" id="fnref-17.28"><sup>[28]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-17.28" id="fn-17.28"></a> <a href="#fnref-17.28">[28]</a> +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. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap3.04"></a>Chapter IV<br/>ON THE FORMATIONS OF THE PAMPAS.</h2> + +<p class="letter"> +Mineralogical constitution.—Microscopical structure.—Buenos Ayres, +shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—San +Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta, +shells, bones, and infusoria of; co-existence of the recent shells and extinct +mammifers.—Buenos Ayres to Santa Fé.—Skeletons of +Mastodon.—Infusoria.—Inferior marine tertiary strata, their +age.—Horse’s tooth. B<small>ANDA</small> +O<small>RIENTAL</small>.—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. S<small>UMMARY</small>.—Area of Pampean +formation.—Theories of origin.—Source of sediment.—Estuary +origin.— +<a name="page351"></a> +Contemporaneous with existing +mollusca.—Relations to underlying tertiary strata.—Ancient deposit +of estuary origin.—Elevation and successive deposition of the Pampean +formation.—Number and state of the remains of mammifers; their +habitation, food, extinction, and range.—Conclusion.—Localities in +Pampas at which mammiferous remains have been found. +</p> + +<p>The Pampean formation is highly interesting from its vast +extent, its disputed origin, and from the number of extinct +gigantic mammifers embedded in it. It has upon the whole a very +uniform character: consisting of a more or less dull reddish, +slightly indurated, argillaceous earth or mud, often, but not +always, including in horizontal lines concretions of marl, and +frequently passing into a compact marly rock. The mud, wherever I +examined it, even close to the concretions, did not contain any +carbonate of lime. The concretions are generally nodular, sometimes +rough externally, sometimes stalactiformed; they are of a compact +structure, but often penetrated (as well as the mud) by hair-like +serpentine cavities, and occasionally with irregular fissures in +their centres, lined with minute crystals of carbonate of lime; +they are of white, brown, or pale pinkish tints, often marked by +black dendritic manganese or iron; they are either darker or +lighter tinted than the surrounding mass; they contain much +carbonate of lime, but exhale a strong aluminous odour, and leave, +when dissolved in acids, a large but varying residue, of which the +greater part consists of sand. These concretions often unite into +irregular strata; and over very large tracts of country, the entire +mass consists of a hard, but generally cavernous marly rock: some +of the varieties might be called calcareous tuffs.</p> + +<p>Dr. Carpenter has kindly examined under the microscope, sliced +and polished specimens of these concretions, and of the solid +marl-rock, collected in various places between the Colorado and +Santa Fe Bajada. In the greater number, Dr. Carpenter finds that +the whole substance presents a tolerably uniform amorphous +character, but with traces of incipient crystalline metamorphosis; +in other specimens he finds microscopically minute rounded +concretions of an amorphous substance (resembling in size those in +oolitic rocks, but not having a concentric structure), united by a +cement which is often crystalline. In some, Dr. Carpenter can +perceive distinct traces of shells, corals, Polythalamia, and +rarely of spongoid bodies. For the sake of comparison, I sent Dr. +Carpenter specimens of the calcareous rock, formed chiefly of +fragments of recent shells, from Coquimbo in Chile: in one of these +specimens, Dr. Carpenter finds, besides the larger fragments, +microscopical particles of shells, and a varying quantity of opaque +amorphous matter; in another specimen from the same bed, he finds +the whole composed of the amorphous matter, with layers showing +indications of +<a name="page352"></a> +an incipient crystalline metamorphosis: hence these latter +specimens, both in external appearance and in microscopical +structure, closely resemble those of the Pampas. Dr. Carpenter +informs me that it is well known that chemical precipitation throws +down carbonate of lime in the opaque amorphous state; and he is +inclined to believe that the long-continued attrition of a +calcareous body in a state of crystalline or semi-crystalline +aggregation (as, for instance, in the ordinary shells of Mollusca, +which, when sliced, are transparent) may yield the same result. +From the intimate relations between all the Coquimbo specimens, I +can hardly doubt that the amorphous carbonate of lime in them has +resulted from the attrition and decay of the larger fragments of +shell: whether the amorphous matter in the marly rocks of the +Pampas has likewise thus originated, it would be hazardous to +conjecture.</p> + +<p>For convenience’ sake, I will call the marly rock by the name +given to it by the inhabitants, namely, Tosca-rock; and the reddish +argillaceous earth, Pampean mud. This latter substance, I may +mention, has been examined for me by Professor Ehrenberg, and the +result of his examination will be given under the proper +localities.</p> + +<p>I will commence my descriptions at a central spot, namely, at +Buenos Ayres, and thence proceed first southward to the extreme +limit of the deposit, and afterwards northward. The plain on which +Buenos Ayres stands is from thirty to forty feet in height. The +Pampean mud is here of a rather pale colour, and includes small +nearly white nodules, and other irregular strata of an unusually +arenaceous variety of tosca-rock. In a well at the depth of seventy +feet, according to Ignatio Nunez, much tosca-rock was met with, and +at several points, at one hundred feet deep, beds of sand have been +found. I have already given a list of the recent marine and estuary +shells found in many parts on the surface near Buenos Ayres, as far +as three or four leagues from the Plata. Specimens from near +Ensenada, given me by Sir W. Parish, where the rock is quarried +just beneath the surface of the plain, consist of broken bivalves, +cemented by and converted into white crystalline carbonate of lime. +I have already alluded, in the first chapter, to a specimen (also +given me by Sir W. Parish) from the A. del Tristan, in which +shells, resembling in every respect the <i>Azara labiata</i>, +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 +<i>Azara labiata</i> (and I believe of <i>Venus sinuosa</i>) occur +embedded in a stratified earthy mass, including small marly +concretions, and said to be precisely like the great Pampean +deposit. Hence we may conclude that the mud of the Pampas continued +to be deposited to within the period of this existing estuary +shell. Although this formation is of such immense extent, I know of +no other instance of the presence of shells in it.</p> + +<p> +<a name="page353"></a> +<i>Buenos Ayres to the Rio Colorado.</i>—With the +exception of a few metamorphic ridges, the country between these +two points, a distance of 400 geographical miles, belongs to the +Pampean formation, and in the southern part is generally formed of +the harder and more calcareous varieties. I will briefly describe +my route: about twenty-five miles S.S.W. of the capital, in a well +forty yards in depth, the upper part, and, as I was assured, the +entire thickness, was formed of dark red Pampean mud without +concretions. North of the River Salado, there are many lakes; and +on the banks of one (near the Guardia) there was a little cliff +similarly composed, but including many nodular and stalactiform +concretions: I found here a large piece of tessellated armour, like +that of the Glyptodon, and many fragments of bones. The cliffs on +the Salado consist of pale-coloured Pampean mud, including and +passing into great masses of tosca-rock: here a skeleton of the +Megatherium and the bones of other extinct quadrupeds (see the list +at the end of this chapter) were found. Large quantities of +crystallised gypsum (of which specimens were given me) occur in the +cliffs of this river; and likewise (as I was assured by Mr. Lumb) +in the Pampean mud on the River Chuelo, seven leagues from Buenos +Ayres: I mention this because M. d’Orbigny lays some stress on the +supposed absence of this mineral in the Pampean formation.</p> + +<p>Southward of the Salado the country is low and swampy, with +tosca-rock appearing at long intervals at the surface. On the +banks, however, of the Tapalguen (sixty miles south of the Salado) +there is a large extent of tosca-rock, some highly compact and even +semi-crystalline, overlying pale Pampean mud with the usual +concretions. Thirty miles further south, the small quartz-ridge of +Tapalguen is fringed on its northern and southern flank, by little, +narrow, flat-topped hills of tosca-rock, which stand higher than +the surrounding plain. Between this ridge and the Sierra of +Guitru-gueyu, a distance of sixty miles, the country is swampy, +with the tosca-rock appearing only in four or five spots: this +sierra, precisely like that of Tapalguen, is bordered by +horizontal, often cliff-bounded, little hills of tosca-rock, higher +than the surrounding plain. Here, also, a new appearance was +presented in some extensive and level banks of alluvium or detritus +of the neighbouring metamorphic rocks; but I neglected to observe +whether it was stratified or not. Between Guitru-gueyu and the +Sierra Ventana, I crossed a dry plain of tosca-rock higher than the +country hitherto passed over, and with small pieces of denuded +tableland of the same formation, standing still higher.</p> + +<p> +The marly or calcareous beds not only come up nearly horizontally to the +northern and southern foot of the great quartzose mountains of the Sierra +Ventana, but interfold between the parallel ranges. The superficial beds (for I +nowhere obtained sections more than twenty feet deep) retain, even close to the +mountains, their usual character: the uppermost layer, however, in one place +included pebbles of quartz, and rested on a mass of detritus of the same rock. +At the very foot of the mountains, there were some few piles of quartz and +tosca-rock detritus, including land-shells; but at the distance of only half a +mile +<a name="page354"></a> +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.<a href="#fn-18.1" name="fnref-18.1" +id="fnref-18.1"><sup>[1]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-18.1" id="fn-18.1"></a> <a href="#fnref-18.1">[1]</a> +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. +</p> + +<p> +The highest peak of the St. Ventana is, by Captain Fitzroy’s measurement, +3,340 feet, and the calcareous plain at its foot (from observations taken by +some Spanish officers<a href="#fn-18.2" name="fnref-18.2" +id="fnref-18.2"><sup>[2]</sup></a>) 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. +</p> + +<p class="footnote"> +<a name="fn-18.2" id="fn-18.2"></a> <a href="#fnref-18.2">[2]</a> +“La Plata,” etc., by Sir W. Parish, p. 146. +</p> + +<p> +Southward of the Ventana, the plain extends farther than the eye can range; its +surface is not very level, having slight depressions with no drainage exits; it +is generally covered by a few feet in thickness of sandy earth; and in some +places, according to M. Parchappe,<a href="#fn-18.3" name="fnref-18.3" +id="fnref-18.3"><sup>[3]</sup></a> 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<a href="#fn-18.4" +name="fnref-18.4" id="fnref-18.4"><sup>[4]</sup></a> 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 +<a name="page355"></a> +this place, there is a remarkable ridge extending W. by N. and E. by S., formed +of small, separate, flat-topped, steep-sided hills, rising between one hundred +and two hundred feet above the Pampean plain at its southern base, which plain +is a little lower than that to the north. The uppermost stratum in this ridge +consists of pale, highly calcareous, compact tosca-rock, resting (as seen in +one place) on reddish Pampean mud, and this again on a paler kind: at the foot +of the ridge, there is a well in reddish clay or mud. I have seen no other +instance of a chain of hills belonging to the Pampean formation; and as the +strata show no signs of disturbance, and as the direction of the ridge is the +same with that common to all the metamorphic lines in this whole area, I +suspect that the Pampean sediment has in this instance been accumulated on and +over a ridge of hard rocks, instead of, as in the case of the above-mentioned +Sierras, round their submarine flanks. South of this little chain of +tosca-rock, a plain of Pampean mud declines towards the banks of the Colorado: +in the middle a well has been dug in red Pampean mud, covered by two feet of +white, softish, highly calcareous tosca-rock, over which lies sand with small +pebbles three feet in thickness—the first appearance of that vast shingle +formation described in the First Chapter. In the first section after crossing +the Colorado, an old tertiary formation, namely, the Rio Negro sandstone (to be +described in the next chapter), is met with: but from the accounts given me by +the Gauchos, I believe that at the mouth of the Colorado the Pampean formation +extends a little further southwards. +</p> + +<p class="footnote"> +<a name="fn-18.3" id="fn-18.3"></a> <a href="#fnref-18.3">[3]</a> +M. d’Orbigny, “Voyage,” Part. Géolog., pp. 47, 48. +</p> + +<p class="footnote"> +<a name="fn-18.4" id="fn-18.4"></a> <a href="#fnref-18.4">[4]</a> +<i>Ibid.</i> +</p> + +<p> +<i>Bahia Blanca.</i>—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.<a href="#fn-18.5" +name="fnref-18.5" id="fnref-18.5"><sup>[5]</sup></a> Of these, only one +(<i>Spongolithis +<a name="page356"></a> +Fustis?</i>) 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.<a href="#fn-18.6" name="fnref-18.6" +id="fnref-18.6"><sup>[6]</sup></a> The number of the remains of rodents gives +to this collection a peculiar character, compared with those found in any other +locality. All these bones are compact and heavy; many of them are stained red, +with their surfaces polished; some of the smaller ones are as black as jet. +</p> + +<p class="footnote"> +<a name="fn-18.5" id="fn-18.5"></a> <a href="#fnref-18.5">[5]</a> +The following list is given in the “Monatsberichten der könig. Akad. zu +Berlin,” April 1845:—<br/> +<br/> +P<small>OLYGASTRICA.</small><br/> + Fragilaria rhabdosoma.<br/> + Gallionella distans.<br/> + Pinnularia?<br/> +P<small>HYTOLITHARIA.</small><br/> + Lithodontium Bursa.<br/> + Lithodontium furcatum.<br/> + Lithostylidium exesum.<br/> + Lithostylidium rude.<br/> + Lithostylidium Serra.<br/> + Spongolithis Fustis?</p> + +<p class="footnote"> +<a name="fn-18.6" id="fn-18.6"></a> <a href="#fnref-18.6">[6]</a> +See “Fossil Mammalia” (p. 109) by Professor Owen, in the +“Zoology of the Voyage of the <i>Beagle</i>;” and Catalogue (p. 36) +of Fossil Remains in Museum of Royal College of Surgeons. +</p> + +<p>Monte Hermoso is between fifty and sixty miles distant in a S.E. +line from the Ventana, with the intermediate country gently rising +towards it, and all consisting of the Pampean formation. What +relation, then, do these beds, at the level of the sea and under +it, bear to those on the flanks of the Ventana, at the height of +840 feet, and on the flanks of the other neighbouring sierras, +which, from the reasons already assigned, do not appear to owe +their greater height to unequal elevation? When the tosca-rock was +accumulating round the Ventana, and when, with the exception of a +few small rugged primary islands, the whole wide surrounding plains +must have been under water, were the strata at Monte Hermoso +depositing at the bottom of a great open sea, between eight hundred +and one thousand feet in depth? I much doubt this; for if so, the +almost perfect carcasses of the several small rodents, the remains +of which are so very numerous in so limited a space, must have been +drifted to this spot from the distance of many hundred miles. It +appears to me far more probable, that during the Pampean period +this whole area had commenced slowly rising (and in the cliffs, at +several different heights we have proofs of the land having been +exposed to sea-action at several levels), and that tracts of land +had thus been formed of Pampean sediment round the Ventana and the +other primary ranges, on which the several rodents and other +quadrupeds lived, and that a stream (in which perhaps the extinct +aquatic Hydrochoerus lived) drifted their bodies into the adjoining +sea, into which the Pampean mud continued to be poured from the +north. As the land continued to rise, it appears that this source +of sediment was cut off; and in its place sand and pebbles were +borne down by stronger currents, and conformably deposited over the +Pampean strata.</p> + +<p>Punta Alta is situated about thirty miles higher up on the +northern +<a name="page357"></a> +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.</p> + +<p class="center"> +No. 15<br/> +Section of beds with recent shells and extinct mammifers, at Punta Alta in +Bahia Blanca. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono15.jpg" width="357" height="66" alt="[Illustration: +Section of beds at Punta Alta in Bahia Blanca.]" /> +</div> + +<p>The second bed (B) is about fifteen feet in thickness, but +towards both extremities of the cliff (not included in the diagram) +it either thins out and dies away, or passes insensibly into an +overlying bed of gravel. It consists of red, tough clayey mud, with +minute linear cavities; it is marked with faint horizontal shades +of colour; it includes a few pebbles, and rarely a minute particle +of shell: in one spot, the dermal armour and a few bones of a +Dasypoid quadruped were embedded in it: it fills up furrows in the +underlying gravel. With the exception of the few pebbles and +particles of shells, this bed resembles the true Pampean mud; but +it still more closely resembles the clayey flats (mentioned in the +First Chapter) separating the successively rising parallel ranges +of sand-dunes.</p> + +<p>The bed (C) is of stratified gravel, like the lowest one; it +fills up furrows in the underlying red mud, and is sometimes +interstratified with it, and sometimes insensibly passes into it; +as the red mud thins out, this upper gravel thickens. Shells are +more numerous in it than in the lower gravel; but the bones, though +some are still present, are less numerous. In one part, however, +where this gravel and the red mud passed into each other, I found +several bones and a tolerably perfect head of the Megatherium. Some +of the large Volutas, though embedded in the gravel-bed (C), were +filled with the red mud, including great numbers of the little +recent <i>Paludestrina australis.</i> These three lower beds are +covered by an unconformable mantle (D) of stratified sandy earth, +including many pebbles of quartz, pumice and phonolite, land and +sea-shells.</p> + +<p>M. d’Orbigny has been so obliging as to name for me the twenty +species of Mollusca embedded in the two gravel beds: they consist +of:— +<a name="page358"></a> +</p> + +<ol> +<li>Volutella angulata, d’Orbigny, “Voyage” Mollusq. and Pal.</li> + +<li>Voluta Braziliana, Sol.</li> + +<li>Olicancilleria Braziliensis, d’Orbigny.</li> + +<li>Olicancilleria auricularia, d’Orbigny.</li> + +<li>Olivina puelchana, d’Orbigny.</li> + +<li>Buccinanops cochlidium, d’Orbigny.</li> + +<li>Buccinanops globulosum, d’Orbigny.</li> + +<li>Colombella sertulariarum, d’Orbigny.</li> + +<li>Trochus Patagonicus, and var. of ditto, d’Orbigny.</li> + +<li>Paludestrina Australis, d’Orbigny.</li> + +<li>Fissurella Patagonica, d’Orbigny.</li> + +<li>Crepidula muricata, Lam.</li> + +<li>Venus purpurata, Lam.</li> + +<li>Venus rostrata, Phillippi.</li> + +<li>Mytilus Darwinianus, d’Orbigny.</li> + +<li>Nucula semiornata, d’Orbigny.</li> + +<li>Cardita Patagonica, d’Orbigny.</li> + +<li>Corbula Patagonica (?), d’Orbigny.</li> + +<li>Pecten tethuelchus, d’Orbigny.</li> + +<li>Ostrea puelchana, d’Orbigny.</li> + +<li>A living species of Balanus.</li> + +<li>and 23. An Astræ and encrusting Flustra, apparently +identical with species now living in the bay.</li> +</ol> + +<p> +All these shells now live on this coast, and most of them in this same bay. I +was also struck with the fact, that the proportional numbers of the different +kinds appeared to be the same with those now cast up on the beach: in both +cases specimens of Voluta, Crepidula, Venus, and Trochus are the most abundant. +Four or five of the species are the same with the upraised shells on the Pampas +near Buenos Ayres. All the specimens have a very ancient and bleached +appearance,<a href="#fn-18.7" name="fnref-18.7" +id="fnref-18.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-18.7" id="fn-18.7"></a> <a href="#fnref-18.7">[7]</a> +A Bulinus, mentioned in the Introduction to the “Fossil Mammalia” +in the “Zoology of the Voyage of the <i> Beagle</i>” has so much +fresher an appearance, than the marine species, that I suspect it must have +fallen amongst the others, and been collected by mistake. +</p> + +<p> +The remains of the extinct mammiferous animals, from the two gravel beds have +been described by Professor Owen in the “Zoology of the Voyage of the +<i>Beagle</i>:” they consist of, 1st, one nearly perfect head and three +fragments of heads of the <i> Megatherium Cuvierii</i>; 2nd, a lower jaw of +<i>Megalonyx Jeffersonii</i>; 3rd, lower jaw of <i>Mylodon Darwinii</i>; 4th, +fragments of a head of some gigantic Edental quadruped; 5th, an almost entire +skeleton of the great <i>Scelidotherium leptocephalum</i>, 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 <i>T. Platensis</i>, or to a second species +lately discovered near Buenos Ayres; 7th, a tooth of <i>Equus curvidens</i>; +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 <i> Macrauchenia +Patagonica</i> 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 +<a name="page359"></a> +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<a href="#fn-18.8" name="fnref-18.8" +id="fnref-18.8"><sup>[8]</sup></a> 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 <i>wholly</i> covered up by the gravel. Besides the remains in +the condition here described, I found one single fragment of bone very much +rolled, and as black as jet, so as perfectly to resemble some of the remains +from Monte Hermoso. +</p> + +<p class="footnote"> +<a name="fn-18.8" id="fn-18.8"></a> <a href="#fnref-18.8">[8]</a> +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. +</p> + +<p>Very many of the bones had been broken, abraded, and rolled, +before being embedded. Others, even some of those included in the +coarsest parts of the the now hard conglomerate, still retain all +their minutest prominences perfectly preserved; so that I conclude +that they probably were protected by skin, flesh, or ligaments, +whilst being covered up. In the case of the Scelidotherium, it is +quite certain that the whole skeleton was held together by its +ligaments, when deposited in the gravel in which I found it. Some +cervical 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.</p> + +<p> +Professor Ehrenberg<a href="#fn-18.9" name="fnref-18.9" +id="fnref-18.9"><sup>[9]</sup></a> has examined for me specimens of the finer +<a name="page360"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-18.9" id="fn-18.9"></a> <a href="#fnref-18.9">[9]</a> +“Monatsberichten der Akad. zu Berlin,” April 1845. The list +consists of:—<br/> +<br/> +P<small>OLYGASTRICA</small>.<br/> + Gallionella sulcata.<br/> + Stauroptera aspera? fragm.<br/> + P<small>HYTOLITHARIA</small>.<br/> + Lithasteriscus tuberculatus.<br/> + Lithostylidium Clepsammidium.<br/> + Lithostylidium quadratum.<br/> + Lithostylidium rude.<br/> + Lithostylidium unidentatum.<br/> + Spongolithis acicularis.</p> + +<p> +We will now see what conclusions may be drawn from the facts above detailed. It +is certain that the gravel-beds and intermediate red mud were deposited within +the period, when existing species of Mollusca held to each other nearly the +same relative proportions as they do on the present coast. These beds, from the +number of littoral species, must have been accumulated in shallow water; but +not, judging from the stratification of the gravel and the layers of marl, on a +beach. From the manner in which the red clay fills up furrows in the underlying +gravel, and is in some parts itself furrowed by the overlying gravel, whilst in +other parts it either insensibly passes into, or alternates with, this upper +gravel, we may infer several local changes in the currents, perhaps caused by +slight changes, up or down, in the level of the land. By the elevation of these +beds, to which period the alluvial mantle with pumice-pebbles, land and +sea-shells belongs, the plain of Punta Alta, from twenty to thirty feet in +height, was formed. In this neighbourhood there are other and higher sea-formed +plains and lines of cliffs in the Pampean formation worn by the denuding action +of the waves at different levels. Hence we can easily understand the presence +of rounded masses of tosca-rock in this lowest plain; and likewise, as the +cliffs at Monte Hermoso with their mammiferous remains stand at a higher level, +the presence of the one much-rolled fragment of bone which was as black as jet: +possibly some few of the other much-rolled bones may have been similarly +derived, though I saw only the one fragment, in the same condition with those +from Monte Hermoso. M. d’Orbigny has suggested<a href="#fn-18.10" +name="fnref-18.10" id="fnref-18.10"><sup>[10]</sup></a> 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 <i>Equus curvidens</i>, 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 +<a name="page361"></a> +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 <i>apparently</i> been embedded in their proper relative +position: but will any one be so bold as to affirm that it is possible, that a +piece of the thin tessellated armour of a Dasypoid quadruped, at least three +feet long and two in width, and now so tender that I was unable with the utmost +care to extract a fragment more than two or three inches square, could have +been washed out of one bed, and re-embedded in another, together with some of +the small bones of the feet, without having been dashed into atoms? We must +then wholly reject M. d’Orbigny’s supposition, and admit as +certain, that the Scelidotherium and the large Dasypoid quadruped, and as +highly probable, that the Toxodon, Megatherium, etc., some of the bones of +which are perfectly preserved, were embedded for the first time, and in a fresh +condition, in the strata in which they were found entombed. These gigantic +quadrupeds, therefore, though belonging to extinct genera and families, +coexisted with the twenty above-enumerated Mollusca, the barnacle and two +corals, still living on this coast. From the rolled fragment of black bone, and +from the plain of Punta Alta being lower than that of Monte Hermoso, I conclude +that the coarse sub-littoral deposits of Punta Alta, are of subsequent origin +to the Pampean mud of Monte Hermoso; and the beds at this latter place, as we +have seen, are probably of subsequent origin to the high tosca-plain round the +Sierra Ventana: we shall, however, return, at the end of this chapter, to the +consideration of these several stages in the great Pampean formation. +</p> + +<p class="footnote"> +<a name="fn-18.10" id="fn-18.10"></a> <a href="#fnref-18.10">[10]</a> +“Voyage,” Part. Géolog., p. 49. +</p> + +<p> +<i>Buenos Ayres to St. Fé Bajada, in Entre Rios.</i>—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 (<i>Ctenomys Bonariensis</i> +and <i>Kerodon antiquus</i>) 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.”<a href="#fn-18.11" +name="fnref-18.11" id="fnref-18.11"><sup>[11]</sup></a> The plain northward of +this point is very +<a name="page362"></a> +level, but with some depressions and lakes; I estimated its height at from +forty to sixty feet above the Parana. At the A. Medio the bright red Pampean +mud contains scarcely any tosca-rock; whilst at a short distance the stream of +the Pabon, forms a cascade, about twenty feet in height, over a cavernous mass +of two varieties of tosca-rock; of which one is very compact and +semi-crystalline, with seams of crystallised carbonate of lime: similar compact +varieties are met with on the Salidillo and Seco. The absolute identity (I +speak after a comparison of my specimens) between some of these varieties, and +those from Tapalguen, and from the ridge south of Bahia Blanca, a distance of +400 miles of latitude, is very striking. +</p> + +<p class="footnote"> +<a name="fn-18.11" id="fn-18.11"></a> <a href="#fnref-18.11">[11]</a> +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. +</p> + +<p>At Rosario there is but little tosca-rock: near this place I +first noticed at the edge of the river traces of an underlying +formation, which, twenty-five miles higher up in the estancia of +Gorodona, consists of a pale yellowish clay, abounding with +concretionary cylinders of a ferruginous sandstone. This bed, which +is probably the equivalent of the older tertiary marine strata, +immediately to be described in Entre Rios, only just rises above +the level of the Parana when low. The rest of the cliff at +Gorodona, is formed of red Pampean mud, with, in the lower part, +many concretions of tosca, some stalacti-formed, and with only a +few in the upper part: at the height of six feet above the river, +two gigantic skeletons of the <i>Mastodon Andium</i> 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 <i>Toxodon Platensis</i>: above this there was an +unusual bed of very soft impure sandstone. In this neighbourhood I +noticed many single embedded bones, and I heard of others having +been found in so perfect a state that they were long used as +gate-posts: the Jesuit Falkner found here the dermal armour of some +gigantic Edental quadruped.</p> + +<p> +In some of the red mud scraped from a tooth of one of the Mastodons at +Gorodona, Professor Ehrenberg finds seven Polygastrica and thirteen +Phytolitharia,<a href="#fn-18.12" name="fnref-18.12" +id="fnref-18.12"><sup>[12]</sup></a> all of them, I believe, with two +exceptions, already +<a name="page363"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-18.12" id="fn-18.12"></a> <a href="#fnref-18.12">[12]</a> +“Monatsberichten der könig. Akad. zu Berlin,” April 1845. The list +consists of:—<br/> +<br/> + P<small>OLYGASTRICA</small>.<br/> + Campylodiscus clypeus.<br/> + Coscinodiscus subtilis.<br/> + Coscinodiscus al. sp.<br/> + Eunotia.<br/> + Gallionella granulata.<br/> + Himantidium gracile.<br/> + Pinnularia borealis. +</p> + +<p> +At <i>St. Fé Bajada</i>, 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.<a href="#fn-18.13" +name="fnref-18.13" id="fnref-18.13"><sup>[13]</sup></a> 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 +<i>Ostrea Patagonica</i><a href="#fn-18.14" name="fnref-18.14" +id="fnref-18.14"><sup>[14]</sup></a> 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:— +</p> + +<ol> +<li>Ostrea Patagonica, d’Orbigny, “Voyage,” Part. Pal.</li> + +<li>Ostrea Alvarezii, d’Orbigny, “Voyage,” Part. Pal.</li> + +<li>Pecten Paranensis, d’Orbigny, “Voyage,” Part. Pal.</li> + +<li>Pecten Darwinianus, d’Orbigny, “Voyage,” Part. Pal.</li> + +<li>Venus Munsterii, d’Orbigny, “Voyage,” Pal.</li> + +<li>Arca Bonplandiana, d’Orbigny, “Voyage,” Pal.</li> + +<li>Cardium Platense, d’Orbigny, “Voyage,” Pal.</li> + +<li>Tellina, probably nov. spec., but too imperfect for +description.</li> +</ol> + +<p>P<small>HYTOLITHARIA</small>.<br/> + Lithasteriscus tuberculatus.<br/> + Lithodontium bursa.<br/> + Lithodontium furcatum.<br/> + Lithodontium rostratum.<br/> + Lithostylidium Amphiodon.<br/> + Lithostylidium Clepsammidium.<br/> + Lithostylidium Hamus.<br/> + Lithostylidium polyedrum.<br/> + Lithostylidium quadratum.<br/> + Lithostylidium rude.<br/> + Lithostylidium Serra.<br/> + Lithostylidium unidentatum.<br/> + Spongolithis Fustis.</p> + +<p class="footnote"> +<a name="fn-18.13" id="fn-18.13"></a> <a href="#fnref-18.13">[13]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-18.14" id="fn-18.14"></a> <a href="#fnref-18.14">[14]</a> +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. +</p> + +<p> +<a name="page364"></a> +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 <i>T. +Platensis</i> of the Pampean formation. Much silicified wood is +found on the banks of the Parana (and likewise on the Uruguay), and +I was informed that they come out of these lower beds; four +specimens collected by myself are dicotyledonous.</p> + +<p> +The upper half of the cliff, to a thickness of about thirty feet, consists of +Pampean mud, of which the lower part is pale-coloured, and the upper part of a +brighter red, with some irregular layers of an arenaceous variety of tosca, and +a few small concretions of the ordinary kind. Close above the marine limestone, +there is a thin stratum with a concretionary outline of white hard tosca-rock +or marl, which may be considered either as the uppermost bed of the inferior +deposits, or the lowest of the Pampean formation; at one time I considered this +bed as marking a passage between the two formations: but I have since become +convinced that I was deceived on this point. In the section on the Parana, I +did not find any mammiferous remains; but at two miles distance on the A. Tapas +(a tributary of the Conchitas), they were extremely numerous in a low cliff of +red Pampean mud with small concretions, precisely like the upper bed on the +Parana. Most of the bones were solitary and much decayed; but I saw the dermal +armour of a gigantic Edental quadruped, forming a caldron-like hollow, four or +five feet in diameter, out of which, as I was informed, the almost entire +skeleton had been lately removed. I found single teeth of the <i>Mastodon +Andium, Toxodon Platensis</i>, and <i>Equus curvidens</i>, 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<a href="#fn-18.15" +name="fnref-18.15" id="fnref-18.15"><sup>[15]</sup></a> from North America, +than to those of any other species of Equus. +</p> + +<p class="footnote"> +<a name="fn-18.15" id="fn-18.15"></a> <a href="#fnref-18.15">[15]</a> +Lyell’s “Travels in North America,” vol. i, p. 164 and +“Proc. of Geolog. Soc.,” vol. iv, p. 39. +</p> + +<p>The underlying marine tertiary strata extend over a wide area: I +was assured that they can be traced in ravines in an east and west +line across Entre Rios to the Uruguay, a distance of about 135 +miles. In a S.E. direction I heard of their existence at the head +of the R. Nankay; and at P. Gorda in Banda Oriental, a distance +of +<a name="page365"></a> +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 <i>Toxodon Paranensis</i>, found (according to M. +d’Orbigny) in still lower strata.</p> + +<p><i>Banda Oriental.</i>—This province lies on the northern +side of the Plata, and eastward of the Uruguay: it has a gentle +undulatory surface, with a basis of primary rocks; and is in most +parts covered up with an unstratified mass, of no great thickness, +of reddish Pampean mud. In the eastern half, near Maldonado, this +deposit is more arenaceous than in the Pampas, it contains many +though small concretions of marl or tosca-rock, and others of +highly ferruginous sandstone; in one section, only a few yards in +depth, it rested on stratified sand. Near Monte Video this deposit +in some spots appears to be of greater thickness; and the remains +of the Glyptodon and other extinct mammifers have been found in it. +In the long line of cliffs, between fifty and sixty feet in height, +called the Barrancas de S. Gregorio, which extend westward of the +Rio S. Lucia, the lower half is formed of coarse sand of quartz and +feldspar without mica, like that now cast up on the beach near +Maldonado; and the upper half of Pampean mud, varying in colour and +containing honeycombed veins of soft calcareous matter and small +concretions of tosca-rock arranged in lines, and likewise a few +pebbles of quartz. This deposit fills up hollows and furrows in the +underlying sand; appearing as if water charged with mud had invaded +a sandy beach. These cliffs extend far westward, and at a distance +of sixty miles, near Colonia del Sacramiento, I found the Pampean +deposit resting in some places on this sand, and in others on the +primary rocks: between the sand and the reddish mud, there appeared +to be interposed, but the section was not a very good one, a thin +bed of shells of an existing Mytilus, still partially retaining +their colour. The Pampean formation in Banda Oriental might readily +be mistaken for an alluvial deposit: compared with that of the +Pampas, it is often more sandy, and contains small fragments of +quartz; the concretions are much smaller, and there are no +extensive masses of tosca-rock.</p> + +<p> +In the extreme western parts of this province, between the Uruguay and a line +drawn from Colonia to the R. Perdido (a tributary of the R. Negro), the +formations are far more complicated. Besides primary rocks, we meet with +extensive tracts and many flat-topped, horizontally +<a name="page366"></a> +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 <i>low</i> 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 <i>Toxodon Platensis</i>, another of a +Mylodon,<a href="#fn-18.16" name="fnref-18.16" +id="fnref-18.16"><sup>[16]</sup></a> perhaps <i>M. Darwinii</i>, and a large +piece of dermal armour, differing from that of the <i>Glyptodon clavipes.</i> +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.<a href="#fn-18.17" name="fnref-18.17" +id="fnref-18.17"><sup>[17]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-18.16" id="fn-18.16"></a> <a href="#fnref-18.16">[16]</a> +This head was at first considered by Professor Owen (in the “Zoology of +the <i>Beagle</i>’s Voyage”) as belonging to a distinct genus, +namely, Glossotherium. +</p> + +<p class="footnote"> +<a name="fn-18.17" id="fn-18.17"></a> <a href="#fnref-18.17">[17]</a> +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 +<i>Edin. New Phil. Journ.</i>, vol. xxxvii, p. 293. +</p> + +<p> +The older tertiary strata, forming the higher isolated hills and extensive +tracts of country, vary, as I have said, extraordinarily in composition: within +the distance of a few miles, I sometimes passed over crystalline limestone with +agate, calcareous tuffs, and marly rocks, all passing into each +other,—red and pale mud with concretions of tosca-rock, quite like the +Pampean formation,—calcareous conglomerates and sandstones,—bright +red sandstones passing either into red conglomerate, or into white +sandstone,—hard siliceous sandstones, jaspery and chalcedonic rocks, and +numerous other subordinate varieties. I was unable to mark out the relations of +all these strata, and will describe only a few distinct sections:—in the +cliffs between P. Gorda on the Uruguay and the A. de Vivoras, the upper bed is +crystalline +<a name="page367"></a> +cellular limestone often passing into calcareous sandstone, with impressions of +some of the same shells as at St. Fé Bajada; at P. Gorda,<a href="#fn-18.18" +name="fnref-18.18" id="fnref-18.18"><sup>[18]</sup></a> 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 <i>Ostrea +Patagonica</i>: 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 <i>appeared</i> 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 <i>Venus Munsterii</i>, +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 +<i>Ostrea Patagonica</i> (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. +</p> + +<p class="footnote"> +<a name="fn-18.18" id="fn-18.18"></a> <a href="#fnref-18.18">[18]</a> +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 <i>over</i> the limestone at P. Gorda; I should have said +that there was reason to infer that it was a subsequent or superior deposit. +</p> + +<p>Moreover, in a precipitous part of the ravine of Las Bocas, a +red sandstone is distinctly seen to overlie a thick bed of pale +mud, also quite like the Pampean formation, abounding with +concretions of true tosca-rock. This sandstone extends over many +miles of country: it is as red as the brightest volcanic +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, +<a name="page368"></a> +very similar to those fringing the primary ranges in the great +plain south of Buenos Ayres: they are composed—1st, of +calcareous tuff with many particles of quartz, sometimes passing +into a coarse conglomerate; 2nd, of a stone undistinguishable on +the closest inspection from the compacter varieties of tosca-rock; +and 3rd, of semi-crystalline limestone, including nodules of agate: +these three varieties pass insensibly into each other, and as they +form the uppermost stratum in this district, I believe that they, +also, are the equivalents of the pure crystalline limestone, and of +the red and white sandstones and conglomerates.</p> + +<p>Between these points and Mercedes on the Rio Negro, there are +scarcely any good sections, the road passing over limestone, +tosca-rock, calcareous and bright red sandstones, and near the +source of the San Salvador over a wide extent of jaspery rocks, +with much milky agate, like that in the limestone near San Juan. In +the estancia of Berquelo, the separate, flat-topped, cliff-bounded +hills are rather higher than in the other parts of the country; +they range in a N.E. and S.W. direction; their uppermost beds +consist of the same bright red sandstone, passing sometimes into a +conglomerate, and in the lower part into soft white sandstone, and +even into loose sand: beneath this sandstone, I saw in two places +layers of calcareous and marly rocks, and in one place red +Pampean-like earth; at the base of these sections, there was a +hard, stratified, white sandstone, with chalcedonic layers. Near +Mercedes, beds of the same nature and apparently of the same age, +are associated with compact, white, crystalline limestone, +including much botryoidal agate, and singular masses, like +porcelain, but really composed of a calcareo-siliceous paste. In +sinking wells in this district the chalcedonic strata seem to be +the lowest. Beds, such as there described, occur over the whole of +this neighbourhood; but twenty miles further up the R. Negro, in +the cliffs of Perika, which are about fifty feet in height, the +upper bed is a prettily variegated chalcedony, mingled with a pure +white tallowy limestone; beneath this there is a conglomerate of +quartz and granite; beneath this many sandstones, some highly +calcareous; and the whole lower two-thirds of the cliff consists of +earthy calcareous beds of various degrees of purity, with one layer +of reddish Pampean-like mud.</p> + +<p> +When examining the agates, the chalcedonic and jaspery rocks, some of the +limestones, and even the bright red sandstones, I was forcibly struck with +their resemblance to deposits formed in the neighbourhood of volcanic action. I +now find that M. Isabelle, in his “Voyage a Buenos Ayres,” has +described closely similar beds on Itaquy and Ibicuy (which enter the Uruguay +some way north of the R. Negro) and these beds include fragments of red +decomposed true 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,<a href="#fn-18.19" name="fnref-18.19" +id="fnref-18.19"><sup>[19]</sup></a> 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 +<a name="page369"></a> +condition of the blocks of rock in the above-described conglomerates, masses of +primary formation probably existed at this tertiary period above water: there +is, also, according to M. Isabelle, much conglomerate further north, at Salto. +</p> + +<p class="footnote"> +<a name="fn-18.19" id="fn-18.19"></a> <a href="#fnref-18.19">[19]</a> +M. d’Orbigny, “Voyage,” Part. Géolog., p. 29. +</p> + +<p>From whatever source and through whatever means the great +Pampean formation originated, we here have, I must repeat, +unequivocal evidence of a similar action at a period before that of +the deposition of the marine tertiary strata with extinct shells, +at Santa 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.</p> + +<p class="center"> +No. 16<br/> +Section of the lowest plain at Port S. Julian. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono16.jpg" width="449" height="146" alt="[Illustration: +Section of the lowest plain at Port S. Julian.]" /> +</div> + +<p><i>Earthy mass, with extinct mammiferous remains, over the +porphyritic gravel at S. Julian, lat. 49° 14′ S., in +Patagonia.</i>—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.</p> + +<p> +The lower old tertiary strata (to be described in the next chapter) are covered +by the usual gravel bed; and this by an irregular earthy, sometimes sandy mass, +seldom more than two or three feet in thickness, except where it fills up +furrows or gullies worn not only through the underlying gravel, but even +through the upper tertiary beds. This earthy mass is of a pale reddish colour, +like the less pure varieties of Pampean mud in Banda Oriental; it includes +small calcareous concretions, like those of tosca-rock but more arenaceous, and +other +<a name="page370"></a> +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 <i>Macrauchenia Patachonica</i>—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.<a href="#fn-18.20" name="fnref-18.20" +id="fnref-18.20"><sup>[20]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-18.20" id="fn-18.20"></a> <a href="#fnref-18.20">[20]</a> +In the succeeding chapter I shall have to refer to a great deposit of extinct +mammiferous remains, lately discovered by Captain Sulivan, R.N., at a point +still further south, namely, at the R. Gallegos; their age must at present +remain doubtful. +</p> + +<p>With respect to the age of the Macrauchenia, the shells on the +surface prove that the mass in which the skeleton was enveloped has +been elevated above the sea within the recent period: I did not see +any of the shells embedded at a sufficient depth to assure me +(though it be highly probable) that the whole thickness of the mass +was contemporaneous with these <i>individual specimens.</i> 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 +<a name="page371"></a> +place subsequently to the elevation of the 250 feet plain, on +which recent shells are also strewed. Hence there cannot be any +doubt that the Macrauchenia, which certainly was entombed in a +fresh state, and which must have been alive after the spreading out +of the gravel on the 90 feet plain, existed, not only subsequently +to the upraised shells on the surface of the 250 feet plain, but +also to those on the 350 to 400 feet plain: these shells, eight in +number (namely, three species of Mytilus, two of Patella, one +Fusus, Voluta, and Balanus), are undoubtedly recent species, and +are the commonest kinds now living on this coast. At Punta Alta in +B. Blanca, I remarked how marvellous it was, that the Toxodon, a +mammifer so unlike to all known genera, should have co-existed with +twenty-three still living marine animals; and now we find that the +Macrauchenia, a quadruped only a little less anomalous than the +Toxodon, also co-existed with eight other still existing Mollusca: +it should, moreover, be borne in mind, that a tooth of a +pachydermatous animal was found with the other remains at Punta +Alta, which Professor Owen thinks almost certainly belonged to the +Macrauchenia.</p> + +<p> +Mr. Lyell<a href="#fn-18.21" name="fnref-18.21" +id="fnref-18.21"><sup>[21]</sup></a> 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.<a href="#fn-18.22" +name="fnref-18.22" id="fnref-18.22"><sup>[22]</sup></a> Mr. Lyell’s +conclusion, therefore, is thus far confirmed in the southern hemisphere; and it +is the more important, as one is naturally tempted to admit so simple an +explanation, that it was the ice-period that caused the extinction of the +numerous great mammifers which so lately swarmed over the two Americas. +</p> + +<p class="footnote"> +<a name="fn-18.21" id="fn-18.21"></a> <a href="#fnref-18.21">[21]</a> +“Geological Proceedings,” vol. iv, p. 36. +</p> + +<p class="footnote"> +<a name="fn-18.22" id="fn-18.22"></a> <a href="#fnref-18.22">[22]</a> +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. +</p> + +<p><i>Summary and concluding remarks on the Pampean +formation.</i>—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 +<a name="page372"></a> +the latitude of the Plata, I examined this formation at +intervals over an east and west line of 300 miles from Maldonado to +the R. Carcarana; and M. d’Orbigny believes it extends 100 miles +further inland: from Mr. Caldcleugh’s travels, however, I should +have thought that it had extended, south of the Cordovese range, to +near Mendoza, and I may add that I heard of great bones having been +found high up the R. Quinto. Hence the area of the Pampean +formation, as remarked by M. d’Orbigny, is probably at least equal +to that of France, and perhaps twice or thrice as great. In a +basin, surrounded by gravel-cliff (at a height of nearly three +thousand feet), south of Mendoza, there is, as described in the +Third Chapter, a deposit very like the Pampean, interstratified +with other matter; and again at S. Julian’s, in Patagonia, 560 +miles south of the Colorado, a small irregular bed of a nearly +similar nature contains, as we have just seen, mammiferous remains. +In the provinces of Moxos and Chiquitos (1,000 miles northward of +the Pampas), and in Bolivia, at a height of 4,000 metres, M. +d’Orbigny has described similar deposits, which he believes to have +been formed by the same agency contemporaneously with the Pampean +formation. Considering the immense distances between these several +points, and their different heights, it appears to me infinitely +more probable, that this similarity has resulted not from +contemporaneousness of origin, but from the similarity of the rocky +framework of the continent: it is known that in Brazil an immense +area consists of gneissic rocks, and we shall hereafter see, over +how great a length the plutonic rocks of the Cordillera, the +overlying purple porphyries, and the trachytic ejections, are +almost identical in nature.</p> + +<p>Three theories on the origin of the Pampean formation have been +propounded:—First, that of a great debacle by M. d’Orbigny; +this seems founded chiefly on the absence of stratification, and on +the number of embedded remains of terrestrial quadrupeds. Although +the Pampean formation (like so many argillaceous deposits) is not +divided into distinct and separate strata, yet we have seen that in +one good section it was striped with horizontal zones of colour, +and that in several specified places the upper and lower parts +differed, not only considerably in colour, but greatly in +constitution. In the southern part of the Pampas the upper mass (to +a certain extent stratified) generally consists of hard tosca-rock, +and the lower part of red Pampean mud, often itself divided into +two or more masses, varying in colour and in the quantity of +included calcareous matter. In Western Banda Oriental, beds of a +similar nature, but of a greater age, conformably underlie and are +intercalated with the regularly stratified tertiary formation. As a +general rule, the marly concretions are arranged in horizontal +lines, sometimes united into irregular strata: surely, if the mud +had been tumultuously deposited in mass, the included calcareous +matter would have segregated itself irregularly, and not into +nodules arranged in horizontal lines, one above the other and often +far apart: this arrangement appears to me to prove that mud, +differing slightly in composition, was successively and quietly +deposited. On the theory of a debacle, a prodigious amount of mud, +without a single pebble, is supposed to have been borne over the +wide surface of the Pampas, when under water: on the other hand, +over the +<a name="page373"></a> +whole of Patagonia, the same or another debacle is supposed to +have borne nothing but gravel,—the gravel and the fine mud in +the neighbourhood of the Rios Negro and Colorado having been borne +to an equal distance from the Cordillera, or imagined line of +disturbance: assuredly directly opposite effects ought not to be +attributed to the same agency. Where, again, could a mass of fine +sediment, charged with calcareous matter in a fit state for +chemical segregation, and in quantity sufficient to cover an area +at least 750 miles long, and 400 miles broad, to a depth of from +twenty to thirty feet to a hundred feet, have been accumulated, +ready to be transported by the supposed debacle? To my mind it is +little short of demonstration, that a great lapse of time was +necessary for the production and deposition of the enormous amount +of mudlike matter forming the Pampas; nor should I have noticed the +theory of a debacle, had it not been adduced by a naturalist so +eminent as M. d’Orbigny.</p> + +<p>A second theory, first suggested, I believe, by Sir W. Parish, +is that the Pampean formation was thrown down on low and marshy +plains by the rivers of this country before they assumed their +present courses. The appearance and composition of the deposit, the +manner in which it slopes up and round the primary ranges, the +nature of the underlying marine beds, the estuary and sea-shells on +the surface, the overlying sandstone beds at M. Hermoso, are all +quite opposed to this view. Nor do I believe that there is a single +instance of a skeleton of one of the extinct mammifers having been +found in an upright position, as if it had been mired.</p> + +<p> +The third theory, of the truth of which I cannot entertain the smallest doubt, +is that the Pampean formation was slowly accumulated at the mouth of the former +estuary of the Plata and in the sea adjoining it. I have come to this +conclusion from the reasons assigned against the two foregoing theories, and +from simple geographical considerations. From the numerous shells of the +<i>Azara labiata</i> 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 +<a name="page374"></a> +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.<a href="#fn-18.23" +name="fnref-18.23" id="fnref-18.23"><sup>[23]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-18.23" id="fn-18.23"></a> <a href="#fnref-18.23">[23]</a> +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. +</p> + +<p> +With respect to the first origin of the reddish mud, I will only remark, that +the enormous area of Brazil consists in chief part of gneissic and other +granitic rocks, which have suffered decomposition, and been converted into a +red, gritty, argillaceous mass, to a greater depth than in any other country +which I have seen. The mixture of rounded grains, and even of small fragments +and pebbles of quartz, in the Pampean mud of Banda Oriental, is evidently due +to the neighbouring and underlying primary rocks. The estuary mud was drifted +during the Pampean period in a much more southerly course, owing probably to +the east and west primary ridges south of the Plata not having been then +elevated, than the mud of the Plata at present is; for it was formerly +deposited as far south as the Colorado. The quantity of calcareous matter in +this formation, especially in those large districts where the whole mass passes +into tosca-rock, is very great: I have already remarked on the close +resemblance in external and microscopical appearance, between this tosca-rock +and the strata at Coquimbo, which have certainly resulted from the decay and +attrition of recent shells:<a href="#fn-18.24" name="fnref-18.24" +id="fnref-18.24"><sup>[24]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-18.24" id="fn-18.24"></a> <a href="#fnref-18.24">[24]</a> +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 <i> +Athenæum</i>, 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. +</p> + +<p> +The Pampean formation, judging from its similar composition, and from the +apparent absolute specific identity of some of its mammiferous +<a name="page375"></a> +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 <i>Azara labiata</i> 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 <i>Macrauchenia +Patachonica</i> (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.<a +href="#fn-18.25" name="fnref-18.25" id="fnref-18.25"><sup>[25]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-18.25" id="fn-18.25"></a> <a href="#fnref-18.25">[25]</a> +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.” +</p> + +<p>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 +<a name="page376"></a> +disputes this view (as given in my “Journal”), and I admit that +it is erroneous, though in some degree excusable, from their +conformability and from both abounding with calcareous matter. It +would, indeed, have been a great anomaly if there had been a true +passage between a deposit contemporaneous with existing species of +mollusca, and one in which all the mollusca appear to be extinct. +Northward of Santa Fe, M. d’Orbigny met with ferruginous +sandstones, marly rocks, and other beds, which he considers as a +distinct and lower formation; but the evidence that they are not +parts of the same with an altered mineralogical character, does not +appear to me quite satisfactory.</p> + +<p>In Western Banda Oriental, while the marine tertiary strata were +accumulating, there were volcanic eruptions, much silex and lime +were precipitated from solution, coarse conglomerates were formed, +being derived probably from adjoining land, and layers of red mud +and marly rocks, like those of the Pampean formation, were +occasionally deposited. The true Pampean deposit, with mammiferous +remains, instead of as at Santa Fe overlying conformably the +tertiary strata, is here seen at a lower level folding round and +between the flat-topped, cliff-bounded hills, formed by a upheaval +and denudation of these same tertiary strata. The upheaval, having +occurred here earlier than at Santa Fe, may be naturally accounted +for by the contemporaneous volcanic action. At the Barrancas de S. +Gregorio, the Pampean deposit, as we have seen, overlies and fills +up furrows in coarse sand, precisely like that now accumulating on +the shores near the mouth of the Plata. I can hardly believe that +this loose and coarse sand is contemporaneous with the old tertiary +and often crystalline strata of the more western parts of the +province; and am induced to suspect that it is of subsequent +origin. If that section near Colonia could be implicitly trusted, +in which, at a height of only fifteen feet above the Plata, a bed +of fresh-looking mussels, of an existing <i>littoral</i> species, +appeared to lie between the sand and the Pampean mud, I should +conclude that Banda Oriental must have stood, when the coarse sand +was accumulating, at only a little below its present level, and had +then subsided, allowing the estuary Pampean mud to cover far and +wide its surface up to a height of some hundred feet; and that +after this subsidence the province had been uplifted to its present +level.</p> + +<p>In Western Banda Oriental, we know, from two unequivocal +sections that there is a mass, absolutely undistinguishable from +the true Pampean deposit, beneath the old tertiary strata. This +inferior mass must be very much more ancient than the upper deposit +with its mammiferous remains, for it lies beneath the tertiary +strata in which all the shells are extinct. Nevertheless, the lower +and upper masses, as well as some intermediate layers, are so +similar in mineralogical character, that I cannot doubt that they +are all of estuary origin, and have been derived from the same +great source. At first it appeared to me extremely improbable, that +mud of the same nature should have been deposited on nearly the +same spot, during an immense lapse of time, namely, from a period +equivalent perhaps to the Eocene of Europe to that of the Pampean +formation. But as, at the very commencement of the Pampean +<a name="page377"></a> +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.</p> + +<p>The elevation of the Pampas, at least of the southern parts, has +been slow and interrupted by several periods of rest, as may be +inferred from the plains, cliffs, and lines of sand-dunes (with +shells and pumice-pebbles) standing at different heights. I +believe, also, that the Pampean mud continued to be deposited, +after parts of this formation had already been elevated, in the +same manner as mud would continue to be deposited in the estuary of +the Plata, if the mud-banks on its shores were now uplifted and +changed into plains: I believe in this from the improbability of so +many skeletons and bones having been accumulated at one spot, where +M. Hermoso now stands, at a depth of between eight hundred and one +thousand feet, and at a vast distance from any land except small +rocky islets,—as must have been the case, if the high +tosca-plain round the Ventana and adjoining Sierras, had not been +already uplifted and converted into land, supporting mammiferous +animals. At Punta Alta we have good evidence that the +gravel-strata, which certainly belong to the true Pampean period, +were accumulated after the elevation in that neighbourhood of the +main part of the Pampean deposit, whence the rounded masses of +tosca-rock were derived, and that rolled fragment of black bone in +the same peculiar condition with the remains at Monte Hermoso.</p> + +<p class="p2"> +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 +<a name="page378"></a> +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?</p> + +<p> +It may be naturally asked, where did these numerous animals live? From the +remarkable discoveries of MM. Lund and Clausen, it appears that some of the +species found in the Pampas inhabited the highlands of Brazil: the <i>Mastodon +Andium</i> is embedded at great heights in the Cordillera from north of the +equator<a href="#fn-18.26" name="fnref-18.26" +id="fnref-18.26"><sup>[26]</sup></a> 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 +<a name="page379"></a> +view the rarity of any particular species as an ordinary event, not requiring +any extraordinary agency. +</p> + +<p class="footnote"> +<a name="fn-18.26" id="fn-18.26"></a> <a href="#fnref-18.26">[26]</a> +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. +</p> + +<p> +The several mammifers embedded in the Pampean formation, which mostly belong to +extinct genera, and some even to extinct families or orders, and which differ +nearly, if not quite, as much as do the Eocene mammifers of Europe from living +quadrupeds having existed contemporaneously with mollusca, all still inhabiting +the adjoining sea, is certainly a most striking fact. It is, however, far from +being an isolated one; for, during the late tertiary deposits of Britain, an +elephant, rhinoceros, and hippopotamus co-existed with many recent land and +fresh-water shells; and in North America, we have the best evidence that a +mastodon, elephant, megatherium, megalonyx, mylodon, an extinct horse and ox, +likewise co-existed with numerous land, fresh-water, and marine recent +shells.<a href="#fn-18.27" name="fnref-18.27" +id="fnref-18.27"><sup>[27]</sup></a> 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 <i>Megatherium, Megalonyx, +Equus</i> (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. +</p> + +<p class="footnote"> +<a name="fn-18.27" id="fn-18.27"></a> <a href="#fnref-18.27">[27]</a> +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. +</p> + +<p>At the period, so recent in a geological sense, when these +extinct mammifers existed, the two Americas must have swarmed with +quadrupeds, many of them of gigantic size; for, besides those more +particularly referred to in this chapter, we must include in this +same period those wonderfully numerous remains, some few of them +specifically, and others generically related to those of the +Pampas, discovered by +<a name="page380"></a> +MM. Lund and Clausen in the caves of Brazil. Finally, the facts +here given show how cautious we ought to be in judging of the +antiquity of a formation from even a great amount of difference +between the extinct and living species in any one class of +animals;—we ought even to be cautious in accepting the +general proposition, that change in organic forms and lapse of time +are at all, necessarily, correlatives.</p> + +<hr /> + +<p class="center"> +<i>Localities within the region of the Pampas where great bones have been +found.</i> +</p> + +<p><small>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. <i>bones</i>), 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 <i>Glyptodon clavipes</i>, 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 (<i>vide</i> +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 <i>Glyptodon +clavipes, G. ornatus</i>, and <i>G. reticulatus</i> 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 <i>Mylodon robustus</i> and <i>Glyptodon ornatus.</i> +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 <i>Mastodon Andium</i>; of Macrauchenia; of a second species +of Toxodon, different from <i>T. Platensis</i>; 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 <i>Toxodon Platensis.</i></small></p> + +<p><small>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 (<i>British Packet</i>, 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 +<a name="page381"></a> +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 <i>Mastodon +Andium</i>, 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 <i>heard</i> of fossil bones, was high up +on the banks of R. Quinto.</small></p> + +<p><small>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.</small></p> + +<p><small>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.</small></p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap3.05"></a>Chapter V<br/>ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.</h2> + +<p class="letter"> +Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with +infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P. +Gallegos.—Eastern Tierra del Fuego; leaves of extinct +beech-trees.—Summary on the Patagonian tertiary +formations.—Tertiary formations of the Western Coast.—Chonos and +Chiloe groups, volcanic rocks +of.—Concepcion.—Navidad.—Coquimbo.—Summary.—Age +of the tertiary formations.—Lines of elevation.—Silicified +wood.—Comparative ranges of the extinct and living mollusca on the West +Coast of S. America.—Climate of the tertiary period.—On the causes +of the absence of recent conchiferous deposits on the coast of S. +America.—On the contemporaneous deposition and preservation of +sedimentary formations. +</p> + +<p> +<i>Rio Negro.</i>—I can add little to the details given by M. +d’Orbigny<a href="#fn-19.1" name="fnref-19.1" +id="fnref-19.1"><sup>[1]</sup></a> on the sandstone formation of this district. +The cliffs to the south of the +<a name="page382"></a> +river are about two hundred feet in height, and are composed of sandstone of +various tints and degrees of hardness. One layer, which thinned out at both +ends, consisted of earthy matter, of a pale reddish colour, with some gypsum, +and very like (I speak after comparison of the specimens brought home) Pampean +mud: above this was a layer of compact marly rock with dendritic manganese. +Many blocks of a conglomerate of pumice-pebbles embedded in hard sandstone were +strewed at the foot of the cliff, and had evidently fallen from above. A few +miles N.E. of the town, I found, low down in the sandstone, a bed, a few inches +in thickness, of a white, friable, harsh-feeling sediment, which adheres to the +tongue, is of easy fusibility, and of little specific gravity; examined under +the microscope, it is seen to be pumiceous tuff, formed of broken transparent +crystals. In the cliffs south of the river, there is, also, a thin layer of +nearly similar nature, but finer grained, and not so white; it might easily +have been mistaken for a calcareous tuff, but it contains no lime: this +substance precisely resembles a most widely extended and thick formation in +Southern Patagonia, hereafter to be described, and which is remarkable for +being partially formed of infusoria. These beds, conjointly with the +conglomerate of pumice, are interesting, as showing the nature of the volcanic +action in the Cordillera during this old tertiary period. +</p> + +<p class="footnote"> +<a name="fn-19.1" id="fn-19.1"></a> <a href="#fnref-19.1">[1]</a> +“Voyage,” Part. Géolog., pp. 57-65. +</p> + +<p>In a bed at the base of the southern cliffs, M. d’Orbigny found +two extinct fresh-water shells, namely, a Unio and Chilina. This +bed rested on one with bones of an extinct rodent, namely, the <i> +Megamys Patagoniensis</i>; and this again on another with extinct +marine shells. The species found by M. d’Orbigny in different parts +of this formation consist of:—</p> + +<ol> +<li>Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. +Fé, and whole coast of Patagonia).</li> + +<li>Ostrea Ferrarisi, d’Orbigny, “Voyage, Pal.”</li> + +<li>Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. +Fé, and S. Josef).</li> + +<li>Pecten Patagoniensis, d’Orbigny, “Voyage, Pal.”</li> + +<li>Venus Munsterii, d’Orbigny, “Voyage, Pal.” (also at St. +Fé).</li> + +<li>Arca Bonplandiana, d’Orbigny, “Voyage, Pal.” (also at St. +Fé).</li> +</ol> + +<p>According to M. d’Orbigny, the sandstone extends westward along +the coast as far as Port S. Antonio, and up the R. Negro far into +the interior: northward I traced it to the southern side of the Rio +Colorado, where it forms a low denuded plain. This formation, +though contemporaneous with that of the rest of Patagonia, is quite +different in mineralogical composition, being connected with it +only by the one thin white layer: this difference may be reasonably +attributed to the sediment brought down in ancient times by the Rio +Negro; by which agency, also, we can understand the presence of the +fresh-water shells, and of the bones of land animals. Judging from +the identity of four of the above shells, this formation is +contemporaneous (as remarked by M. d’Orbigny) with that under the +Pampean deposit in Entre Rios and in Banda Oriental. The gravel +capping the sandstone plain, with its calcareous cement and nodules +of gypsum, is probably, from the reasons +<a name="page383"></a> +given in the First Chapter, contemporaneous with the uppermost +beds of the Pampean formation on the upper plain north of the +Colorado.</p> + +<p><i>San Josef.</i>—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:—</p> + +<ol> +<li>Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. +Fé and whole coast of Patagonia).</li> + +<li>Ostrea Alvarezii, d’Orbigny, “Voyage, Pal.” (also at St. +Fé and R. Negro).</li> + +<li>Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at St. +Fé, S. Julian, and Port Desire).</li> + +<li>Pecten Darwinianus, d’Orbigny, “Voyage, Pal.” (also at St. +Fé).</li> + +<li>Pecten actinodes, G. B. Sowerby.</li> + +<li>Terebratula Patagonica, G. B. Sowerby (also S. Julian).</li> + +<li>Casts of a Turritella.</li> +</ol> + +<p>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.</p> + +<p><i>Nuevo Gulf.</i>—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 <i>Pecten Paranensis</i>, d’Orbigny, casts of a +Turritella and Scutella. On this there are two strata of the pale +brown mudstone, also like that of <i>S. Josef</i>, separated by a +darker-coloured, more argillaceous variety, including the <i>Ostrea +Patagonica.</i> Professor Ehrenberg has examined this mudstone for +me: he finds in it three already known microscopic organisms, +enveloped in a fine-grained pumiceous tuff, which I shall have +immediately to describe in detail. Specimens brought to me from the +uppermost bed, north of the Rio Chupat, consist of this same +substance, but of a whiter colour.</p> + +<p>Tertiary strata, such as here described, appear to extend along +the whole coast between Rio Chupat and Port Desire, except where +interrupted by the underlying claystone porphyry, and by some +metamorphic rocks; these hard rocks, I may add, are found at +intervals over a space of about five degrees of latitude, from +Point Union to a point between Port S. Julian and S. Cruz, and will +be described in the ensuing chapter. Many gigantic specimens of the +<i>Ostrea Patagonica</i> were collected in the Gulf of St. +George.</p> + +<p><i>Port Desire.</i>—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:— +<a name="page384"></a> +</p> + +<ol> +<li>Ostrea Patagonica, d’Orbigny, (also at St. Fé, and whole +coast of Patagonia).</li> + +<li>Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (also at St. +Fé, S. Josef, S. Julian).</li> + +<li>Pecten centralis, G. B. Sowerby (also at S. Julian and S. +Cruz).</li> + +<li>Cucullæa alta, G. B. Sowerby (also at S. Cruz).</li> + +<li>Nucula ornata, G. B. Sowerby.</li> + +<li>Turritella Patagonica, G. B. Sowerby.</li> +</ol> + +<p> +The fossiliferous strata, when not denuded, are conformably covered by a +considerable thickness of the fine-grained pumiceous mudstone, divided into two +masses: the lower half is very fine-grained, slightly unctuous, and so compact +as to break with a semi-conchoidal fracture, though yielding to the nail; it +includes 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,<a href="#fn-19.2" +name="fnref-19.2" id="fnref-19.2"><sup>[2]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-19.2" id="fn-19.2"></a> <a href="#fnref-19.2">[2]</a> +“Monatsberichten de könig. Akad. zu Berlin,” vom April 1845. +</p> + +<p><i>Port S. Julian.</i>—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 +<a name="page385"></a> +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 <i> +Pecten Paranensis</i>; 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:—</p> + +<ol> +<li>Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. +Fé, and whole coast of Patagonia).</li> + +<li>Pecten Paranensis, d’Orbigny, “Voyage, Pal.” (St. Fé, S. +Josef, Port Desire).</li> + +<li>Pecten centralis, G. B. Sowerby (also at Port Desire and S. +Cruz).</li> + +<li>Pecten geminatus, G. B. Sowerby.</li> + +<li>Terebratula Patagonica, G. B. Sowerby (also S. Josef).</li> + +<li>Struthiolaria ornata, G. B. Sowerby (also S. Cruz).</li> + +<li>Fusus Patagonicus, G. B. Sowerby.</li> + +<li>Fusus Noachinus, G. B. Sowerby.</li> + +<li>Scalaria rugulosa, G. B. Sowerby.</li> + +<li>Turritella ambulacrum, G. B. Sowerby (also S. Cruz).</li> + +<li>Pyrula, cast of, like P. ventricosa of Sowerby, Tank Cat.</li> + +<li>Balanus varians, G. B. Sowerby.</li> + +<li>Scutella, differing from the species from Nuevo Gulf.</li> +</ol> + +<p class="center"> +No. 17<br/> +Section of the strata exhibited in the cliffs of the ninety feet plain at Port +S. Julian. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono17.jpg" width="356" height="80" alt="[Illustration: +Section of the strata exhibited in the cliffs of the ninety feet plain at Port +S. Julian.]" /> +</div> + +<p>At the head of the inner harbour of Port S. Julian, the +fossiliferous mass is not displayed, and the sea-cliffs from the +water’s edge to a height of between one and two hundred feet are +formed of the white pumiceous mudstone, which here includes +innumerable, far-extended, sometimes horizontal, sometimes inclined +or vertical 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, <i>Pecten +Paranensis</i> and <i>Turritella ambulacrum</i>, are +interstratified; +<a name="page386"></a> +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 “<i>Chalk Hills.</i>”</p> + +<p><i>Santa Cruz.</i>—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 <i>Pecten Paranensis</i>, so +common in all other parts of the coast, is here absent: the shells +consist of:—</p> + +<ol> +<li>Ostrea Patagonica, d’Orbigny, “Voyage, Pal.” (also at St. +Fé and whole coast of Patagonia).</li> + +<li>Pecten centralis, G. B. Sowerby (also P. Desire and S. +Julian).</li> + +<li>Venus meridionalis of G. B. Sowerby.</li> + +<li>Crassatella Lyellii, G. B. Sowerby.</li> + +<li>Cardium puelchum, G. B. Sowerby.</li> + +<li>Cardita Patagonica, G. B. Sowerby.</li> + +<li>Mactra rugata, G. B. Sowerby.</li> + +<li>Mactra Darwinii, G. B. Sowerby.</li> + +<li>Cucullæa alta, G. B. Sowerby (also P. Desire).</li> + +<li>Trigonocelia insolita, G. B. Sowerby.</li> + +<li>Nucula (?) glabra, G. B. Sowerby.</li> + +<li>Crepidula gregaria, G. B. Sowerby.</li> + +<li>Voluta alta, G. B. Sowerby.</li> + +<li>Trochus collaris, G. B. Sowerby.</li> + +<li>Natica solida (?), G. B. Sowerby.</li> + +<li>Struthiolaria ornata, G. B. Sowerby (also P. Desire).</li> + +<li>Turritella ambulacrum, G. B. Sowerby (also P. S. Julian).<br/> +Imperfect fragments of the genera Byssoarca, Artemis, and +Fusus.</li> +</ol> + +<p>The upper part of the cliff is generally divided into three +great strata, differing slightly in composition, but essentially +resembling the pumiceous mudstone of the places farther north; the +deposit, however, here is more arenaceous, of greater specific +gravity, and not so white: it is interlaced with numerous thin +veins, partially or quite filled with transverse fibres of gypsum; +these fibres were too short to reach across the vein, have their +extremities curved or bent: in the same veins with the gypsum, and +likewise in separate veins as well as in little nests, there is +much powdery sulphate of magnesia (as ascertained by Mr. Reeks) in +an uncompressed form: I believe that this salt has not +heretofore +<a name="page387"></a> +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 <i> +Cucullæa alta</i> (found at Port Desire in the lower +fossiliferous mass) and of the <i>Mactra rugata</i>, which latter +shell has been partially converted into gypsum.</p> + +<p class="center"> +No. 18<br/> +Section of the plain at Patagonia, on the banks of the S. Cruz. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono18.jpg" width="474" height="287" alt="[Illustration: +Section of the plain at Patagonia, on the banks of the S. Cruz.]" /> +</div> + +<p> +In ascending the valley of the S. Cruz, the upper strata of the coast-cliffs +are prolonged, with nearly the same characters, for fifty miles: at about this +point, they begin in the most gradual and scarcely perceptible manner, to be +banded with white lines; and after ascending ten miles farther, we meet with +distinct thin layers of whitish, greenish, and yellowish fine-grained, fusible +sediments. At eighty miles from the coast,<a href="#fn-19.3" name="fnref-19.3" +id="fnref-19.3"><sup>[3]</sup></a> in a cliff thus composed, there were a few +layers of ferruginous +<a name="page388"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-19.3" id="fn-19.3"></a> <a href="#fnref-19.3">[3]</a> +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? +</p> + +<p>The upper half of the sedimentary mass, under the basaltic lava, +consists of innumerable zones of perfectly white bright green, +yellowish and brownish, fine-grained, sometimes incoherent, +sedimentary matter. The white, pumiceous, trachytic tuff-like +varieties are of rather greater specific gravity than the pumiceous +mudstone on the coast to the north; some of the layers, especially +the browner ones, are coarser, so that the broken crystals are +distinguishable with a weak lens. The layers vary in character in +short distances. With the exception of a few of the <i>Ostrea +Patagonica</i>, which appeared to have rolled down from the cliff +above, no organic remains were found. The chief difference between +these layers taken as a whole, and the upper beds both at the mouth +of the river and on the coast northward, seems to lie in the +occasional presence of more colouring matter, and in the supply +having been intermittent; these characters, as we have seen, very +gradually disappear in descending the valley, and this fact may +perhaps be accounted for by the currents of a more open sea having +blended together the sediment from a distant and intermittent +source.</p> + +<p> +The coloured layers in the foregoing section rest on a mass, apparently of +great thickness (but much hidden by the talus), of soft sandstone, almost +composed of minute pebbles, from one-tenth to two-tenths of an inch in +diameter, of the rocks (with the entire exception of the basaltic lava) +composing the great boulders on the surface of the plain, and probably +composing the neighbouring Cordillera. Five miles higher up the valley, and +again thirty miles higher up<a href="#fn-19.4" name="fnref-19.4" +id="fnref-19.4"><sup>[4]</sup></a> (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 <i>Ostrea Patagonica.</i> 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; +<a name="page389"></a> +but as amongst the pebbles there were none of basalt, although the cliffs on +both sides of the valley are composed of this rock, I believe that the +sandstone does belong to this formation. At the highest point to which we +ascended, twenty miles distant from the nearest slope of the Cordillera, I +could see the horizontally zoned white beds, stretching under the black +basaltic lava, close up to the mountains; so that the valley of the S. Cruz +gives a fair idea of the constitution of the whole width of Patagonia. +</p> + +<p class="footnote"> +<a name="fn-19.4" id="fn-19.4"></a> <a href="#fnref-19.4">[4]</a> +I found at both places, but not <i>in situ</i>, quantities of coniferous and +ordinary dicotyledonous silicified wood, which was examined for me by Mr. R. +Brown. +</p> + +<p><i>Basaltic lava of the S. Cruz.</i>—This formation is +first met with sixty-seven miles from the mouth of the river; +thence it extends uninterruptedly, generally but not exclusively on +the northern side of the valley, close up to the Cordillera. The +basalt is generally black and fine-grained, but sometimes grey and +laminated; it contains some olivine, and high up the valley much +glassy feldspar, where, also, it is often amygdaloidal; it is never +highly vesicular, except on the sides of rents and on the upper and +lower, spherically laminated surfaces. It is often columnar; and in +one place I saw magnificent columns, each face twelve feet in +width, with their interstices filled up with calcareous tuff. The +streams rest conformably on the white sedimentary beds, but I +nowhere saw the actual junction; nor did I anywhere see the white +beds actually superimposed on the lava; but some way up the valley +at the foot of the uppermost escarpments, they must be thus +superimposed. Moreover, at the lowest point down the valley, where +the streams thin out and terminate in irregular projections, the +spaces or intervals between these projections are filled up to the +level of the now denuded and gravel-capped surfaces of the plains, +with the white-zoned sedimentary beds; proving that this matter +continued to be deposited after the streams had flowed. Hence we +may conclude that the basalt is contemporaneous with the upper +parts of the great tertiary formation.</p> + +<p>The lava where first met with is 130 feet in thickness: it there +consists of two, three, or perhaps more streams, divided from each +other by vesicular spheroids like those on the surface. From the +streams having, as it appears, extended to different distances, the +terminal points are of unequal heights. Generally the surface of +the basalt is smooth them in one part high up the valley, it was so +uneven and hummocky, that until I afterwards saw the streams +extending continuously on both sides of the valley up to a height +of about three thousand feet close to the Cordillera, I thought +that the craters of eruption were probably close at hand. This +hummocky surface I believe to have been caused by the crossing and +heaping up of different streams. In one place, there were several +rounded ridges about twenty feet in height, some of them as broad +as high, and some broader, which certainly had been formed whilst +the lava was fluid, for in transverse sections each ridge was seen +to be concentrically laminated, and to be composed of imperfect +columns radiating from common centres, like the spokes of +wheels.</p> + +<p>The basaltic mass where first met with is, as I have said, 130 +feet in thickness, and, thirty-five miles higher up the valley, it +increases to +<a name="page390"></a> +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 <i>now</i> (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.</p> + +<p> +This great deluge of lava is worthy, in its dimensions, of the great continent +to which it belongs. The aggregate streams have flowed from the Cordillera to a +distance (unparalleled, I believe, in any case yet known) of about one hundred +geographical miles. Near their furthest extremity their total thickness is 130 +feet, which increase thirty-five miles farther inland, as we have just seen, to +322 feet. The least inclination given by M. E. de Beaumont of the upper surface +of a lava-stream, namely 0° 30′, is that of the great subaerial +eruption in 1783 from Skaptar Jukul in Iceland; and M. E. de Beaumont shows<a +href="#fn-19.5" name="fnref-19.5" id="fnref-19.5"><sup>[5]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-19.5" id="fn-19.5"></a> <a href="#fnref-19.5">[5]</a> +“Mémoires pour servir,” etc., pp. 178 and 217. +</p> + +<p>Southward of the S. Cruz, the cliffs of the 840 feet plain +extend to Coy Inlet, and owing to the naked patches of the white +sediment, they are said on the charts to be “like the coast of +Kent.” At Coy Inlet the high plain trends inland, leaving +flat-topped outliers. At Port Gallegos (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 +<a name="page391"></a> +three hundred feet in height, formed of numerous strata, some +fine-grained and pale-coloured, like the upper beds at the mouth of +the S. Cruz, others rather dark and coarser, so as to resemble +gritstones or tuffs; these latter include rather large fragments of +apparently decomposed volcanic rocks; there are, also, included +layers of gravel. This formation is highly remarkable, from +abounding with mammiferous remains, which have not as yet been +examined by Professor Owen, but which include some large, but +mostly small, species of Pachydermata, Edentata, and Rodentia. From +the appearance of the pale-coloured, fine-grained beds, I was +inclined to believe that they corresponded with the upper beds of +the S. Cruz; but Professor Ehrenberg, who has examined some of the +specimens, informs me that the included microscopical organisms are +wholly different, being fresh and brackish-water forms. Hence the +two to three hundred feet plain at Port Gallegos is of unknown age, +but probably of subsequent origin to the great Patagonian tertiary +formation.</p> + +<p> +<i>Eastern Tierra del Fuego.</i>—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,<a href="#fn-19.6" name="fnref-19.6" +id="fnref-19.6"><sup>[6]</sup></a> 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 <i>V. lenticularis</i>, 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,<a href="#fn-19.7" +name="fnref-19.7" id="fnref-19.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-19.6" id="fn-19.6"></a> <a href="#fnref-19.6">[6]</a> +Described in the “Geological Transactions,” vol. vi, p. 415. +</p> + +<p class="footnote"> +<a name="fn-19.7" id="fn-19.7"></a> <a href="#fnref-19.7">[7]</a> +“Botany of the Antarctic Voyage,” p. 212. +</p> + +<p><i>Summary on the Patagonian tertiary formation.</i>—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 +<a name="page392"></a> +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 <i> +Fusus Patagonicus</i>; and, according to Mr. Sowerby, with the <i> +Pyrula</i>, the <i>Venus meridionalis</i>, the <i>Crepidula +gregaria</i>, and the <i>Turritella ambulacrum</i>, and <i>T. +Patagonica.</i> 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.</p> + +<p>The white pumiceous mudstone, with its abundant gypsum, belongs +to the same general epoch with the underlying fossiliferous mass, +as may be inferred from the shells included in the intercalated +layers at Nuevo Gulf, S. Julian, and S. Cruz. Out of the +twenty-seven marine microscopic structures found by Professor +Ehrenberg in the specimens from S. Julian and Port Desire, ten are +common to these two places: the three found at Nuevo Gulf are +distinct. I have minutely described this deposit, from its +remarkable characters and its wide extension. From Coy Inlet to +Port Desire, a distance of 230 miles, it is certainly continuous; +and I have reason to believe that it likewise extends to the Rio +Chupat, Nuevo Gulf, and San Josef, a distance of 570 miles: we +have, also, seen that a single layer occurs at the Rio Negro. At +Port S. Julian it is from eight to nine hundred feet in thickness; +and at S. Cruz it extends, with a slightly altered character, up to +the Cordillera. From its microscopic structure, and from its +analogy with other formations in volcanic districts, it must be +considered as originally of volcanic origin: it may have been +formed by the long-continued attrition of vast quantities of +pumice, or judging from the manner in which the mass becomes, in +ascending the valley of S. Cruz, divided into variously coloured +layers, from the long-continued eruption of clouds of fine ashes. +In either case, we must conclude, that the southern volcanic +orifices of the Cordillera, now in a dormant state, were at about +this period over a wide space, and for a great length of time, in +action. We have evidence of this fact, in the latitude of the Rio +Negro, in the sandstone-conglomerate with pumice, and demonstrative +proof of it, at S. Cruz, in the vast deluges of basaltic lava: at +this same tertiary period, also, there is distinct evidence of +volcanic action in Western Banda Oriental.</p> + +<p>The Patagonian tertiary formation extends continuously, +judging +<a name="page393"></a> +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.</p> + +<h4><i>Tertiary Formations on the West Coast.</i></h4> + +<p><i>Chonos Archipelago.</i>—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:—</p> + +<ol> +<li>Turritella suturalis, G. B. Sowerby (also Navidad).</li> + +<li>Sigaretus subglobosus, G. B. Sowerby (also Navidad).</li> + +<li>Cytheræa (?) sulculosa (?), G. B. Sowerby (also Chiloe +and Huafo?).</li> + +<li>Voluta, fragments of.</li> +</ol> + +<p>In the northern parts of this group there are some cliffs of +gravel and of the boulder formation. In the southern part (at P. +Andres in Tres Montes), there is a volcanic formation, probably of +tertiary origin. The lavas attain a thickness of from two to three +hundred feet; they are extremely variable in colour and nature, +being compact, or brecciated, or cellular, or amygdaloidal with +zeolite, agate and bole, or porphyritic with glassy albitic +feldspar. There is also much imperfect rubbly pitchstone, with the +interstices charged with powdery carbonate of lime apparently of +contemporaneous origin. These lavas are conformably associated with +strata of breccia and of brown tuff containing lignite. The whole +mass has been broken up and tilted at an angle of 45°, by a +series of great volcanic dikes, one of which was thirty yards in +breadth. This volcanic formation resembles one, presently to be +described, in Chiloe.</p> + +<p><i>Huafo.</i>—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 +<a name="page394"></a> +muddy sandstones, with fragments of lignite and +concretions of calcareous sandstone. I collected the following +extinct shells, of which the Turritella was in great +numbers:—</p> + +<ol> +<li>Bulla cosmophila, G. B. Sowerby.</li> + +<li>Pleurotoma subæqualis, G. B. Sowerby.</li> + +<li>Fusus cleryanus, d’Orbigny, “Voyage Pal.” (also at +Coquimbo).</li> + +<li>Triton leucostomoides, G. B. Sowerby.</li> + +<li>Turritella Chilensis, G. B. Sowerby (also Mocha).</li> + +<li>Venus, probably a distinct species, but very imperfect.</li> + +<li>Cytheræa (?) sulculosa (?), probably a distinct species, +but very imperfect.</li> + +<li>Dentalium majus, G. B. Sowerby.</li> +</ol> + +<p><i>Chiloe.</i>—This fine island is about one hundred miles +in length. The entire southern part, and the whole western coast, +consists of mica-schist, which likewise is seen in the ravines of +the interior. The central mountains rise to a height of 3,000 feet, +and are said to be partly formed of granite and greenstone: there +are two small volcanic districts. The eastern coast, and large +parts of the northern extremity of the island are composed of +gravel, the boulder formation, and underlying horizontal strata. +The latter are well displayed for twenty miles north and south of +Castro; they vary in character from common sandstone to +fine-grained, laminated mudstones: all the specimens which I +examined are easily fusible, and some of the beds might be called +volcanic grit-stones. These latter strata are perhaps related to a +mass of columnar trachyte which occurs behind Castro. The sandstone +occasionally includes pebbles, and many fragments and layers of +lignite; of the latter, some are apparently formed of wood and +others of leaves: one layer on the N.W. side of Lemuy is nearly two +feet in thickness. There is also much silicified wood, both common +dicotyledonous and coniferous: a section of one specimen in the +direction of the medullary rays has, as I am informed by Mr. R. +Brown, the discs in a double row placed alternately, and not +opposite as in the true Araucaria. I found marine remains only in +one spot, in some concretions of hard calcareous sandstone: in +several other districts I have observed that organic remains were +exclusively confined to such concretions; are we to account for +this fact, by the supposition that the shells lived only at these +points, or is it not more probable that their remains were +preserved only where concretions were formed? The shells here are +in a bad state, they consist of:—</p> + +<ol> +<li>Tellinides (?) oblonga, G. B. Sowerby (a solenella in M. +d’Orbigny’s opinion).</li> + +<li>Natica striolata, G. B. Sowerby.</li> + +<li>Natica (?) pumila, G. B. Sowerby.</li> + +<li>Cytheræa (?) sulculosa, G. B. Sowerby (also Ypun and +Huafo?).</li> +</ol> + +<p> +At the northern extremity of the island, near S. Carlos, there is a large +volcanic formation, between five and seven hundred feet in thickness. The +commonest lava is blackish-grey or brown, either vesicular, or amygdaloidal +with calcareous spar and bole: most even of +<a name="page395"></a> +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.<a +href="#fn-19.8" name="fnref-19.8" id="fnref-19.8"><sup>[8]</sup></a> The beds +are obscurely separated from each other; they are sometimes parted by seams of +tuff and layers of pebbles. In one place they rested on, and in another place +were capped by, tuffs and girt-stones, apparently of submarine origin. +</p> + +<p class="footnote"> +<a name="fn-19.8" id="fn-19.8"></a> <a href="#fnref-19.8">[8]</a> +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! +</p> + +<p>The neighbouring peninsula of Lacuy is almost wholly formed of +tufaceous deposits, connected probably in their origin with the +volcanic hills just described. The tuffs are pale-coloured, +alternating with laminated mudstones and sandstones (all easily +fusible), and passing sometimes into fine-grained white beds +strikingly resembling the great upper infusorial deposit of +Patagonia, and sometimes into brecciolas with pieces of pumice in +the last stage of decay; these again pass into ordinary coarse +breccias and conglomerates of hard rocks. Within very short +distances, some of the finer tuffs often passed into each other in +a peculiar manner, namely, by irregular polygonal concretions of +one variety increasing so much and so suddenly in size, that the +second variety, instead of any longer forming the entire mass, was +left merely in thin veins between the concretions. In a straight +line of cliffs, at Point Tenuy, I examined the following remarkable +section (figure No. 19):—</p> + +<p class="center"> +No. 19 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono19.jpg" width="282" height="269" alt="[Illustration: +Section at Point Tenuy]" /> +</div> + +<p>On the left hand, the lower part (AA) consists of regular, +alternating strata of brown tuffs and greenish laminated mudstone, +gently inclined to the right, and conformably covered by a mass (B +<i>left</i>) of a white, tufaceous and brecciolated deposit. On the +right hand, the whole cliff (BB <i>right</i>) 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 <i>left</i>) 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 +<a name="page396"></a> +been accidentally not represented steep enough, for I +particularly noticed that before the beds had been tilted to the +right, this line must have been nearly vertical. It appears that a +current of water cut for itself a deep and steep submarine channel, +and at the same time or afterwards filled it up with the tufaceous +and brecciolated matter, and spread the same over the surrounding +submarine beds; the matter becoming stratified in these more +distant and less troubled parts, and being moreover subsequently +covered up by other strata (like AA) not shown in the diagram. It +is singular that three of the beds (of AA) are prolonged in their +proper direction, as represented, beyond the line of junction into +the white tufaceous matter: the prolonged portions of two of the +beds are rounded; in the third, the terminal fragment has been +pushed upwards: how these beds could have been left thus prolonged, +I will not pretend to explain. In another section on the opposite +side of a promontory, there was at the foot of this same line of +junction, that is at the bottom of the old submarine channel, a +pile of fragments of the strata (AA), with their interstices filled +up with white tufaceous matter: this is exactly what might have +been anticipated under such circumstances.</p> + +<p class="center"> +No. 20<br/> +Ground plan showing the relation between veins and concretionary zones in a +mass of tuff. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono20.jpg" width="341" height="199" alt="[Illustration: +Ground plan showing the relation between veins and concretionary zones in a +mass of tuff.]" /> +</div> + +<p> +The various tufaceous and other beds at this northern end of Chiloe probably +belong to about the same age with those near Castro, and they contain, as +there, many fragments of black lignite and of silicified and pyritous wood, +often embedded close together. They also contain many and singular concretions: +some are of hard calcareous sandstone, in which it would appear that broken +volcanic crystals and scales of mica have been better preserved (as in the case +of the +<a name="page397"></a> +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,<a href="#fn-19.9" name="fnref-19.9" +id="fnref-19.9"><sup>[9]</sup></a> 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. +<a name="page398"></a> +It may be seen how obviously the fissures determine (or have been determined +by) the shape, sometimes of the whole concretion, and sometimes only of its +central parts. The fissures also determine the curvatures of the long +undulating zones of concretionary action. From the varying composition of the +veins and concretions, the amount of chemical action which the mass has +undergone is surprisingly great; and it would likewise appear from the +difference in size in the particles of the concretionary zones, that the mass, +also, has been subjected to internal mechanical movements. +</p> + +<p class="footnote"> +<a name="fn-19.9" id="fn-19.9"></a> <a href="#fnref-19.9">[9]</a> +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. +</p> + +<p>In the peninsula of Lacuy, the strata over a width of four miles +have been upheaved by three distinct, and some other indistinct, +lines of elevation, ranging within a point of north and south. One +line, about two hundred feet in height, is regularly anticlinal, +with the strata dipping away on both sides, at an angle of 15°, +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.</p> + +<p><i>Valdivia: Island of Mocha.</i>—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:—</p> + +<ol> +<li>Turritella Chilensis, G. B. Sowerby (also at Huafo).</li> + +<li>Fusus, very imperfect, somewhat resembling F. subreflexus of +Navidad, but probably different.</li> + +<li>Venus, fragments of.</li> +</ol> + +<p><i>Concepcion.</i>—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 +<a name="page399"></a> +are all uniclinal, the strata in each dipping to a point between +S. and S.S.E. with an inclination in the central lines of about +forty degrees, and in the outer ones of under twenty degrees. This +band of symmetrically troubled country is about eight miles in +width.</p> + +<p>The island of Quiriquina, in the Bay of Concepcion, is formed of +various soft and often ferruginous sandstones, with bands of +pebbles, and with the lower strata sometimes passing into a +conglomerate resting on the underlying metamorphic schists. These +beds include subordinate layers of greenish impure clay, soft +micaceous and calcareous sandstones, and reddish friable earthy +matter with white specks like decomposed crystals of feldspar; they +include, also, hard concretions, fragments of shells, lignite, and +silicified wood. In the upper part they pass into white, soft +sediments and brecciolas, very like those described at Chiloe; as +indeed is the whole formation. At Lirguen and other places on the +eastern side of the bay, there are good sections of the lower +sandstones, which are generally ferruginous, but which vary in +character, and even pass into an argillaceous nature; they contain +hard concretions, fragments of lignite, silicified wood, and +pebbles (of the same rocks with the pebbles in the sandstones of +Quiriquina), and they alternate with numerous, often very thin +layers of imperfect coal, generally of little specific gravity. The +main bed here is three feet thick; and only the coal of this one +bed has a glossy fracture. Another irregular, curvilinear bed of +brown, compact lignite, is remarkable for being included in a mass +of coarse gravel. These imperfect coals, when placed in a heap, +ignite spontaneously. The cliffs on this side of the bay, as well +as on the island of Quiriquina, are capped with red friable earth, +which, as stated in the Second Chapter, is of recent formation. The +stratification in this neighbourhood is generally horizontal; but +near Lirguen the beds dip N.W. at an angle of 23°; 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.</p> + +<p>A collection of shells, from the island of Quiriquina, has been +described by M. d’Orbigny: they are all extinct, and from their +generic character, M. d’Orbigny inferred that they were of tertiary +origin: they consist of:—</p> + +<ol> +<li>Scalaria Chilensis, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Natica Araucana, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Natica australis, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Fusus difficilis, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Pyrula longirostra, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Pleurotoma Araucana, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Cardium auca, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Cardium acuticostatum, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Venus auca, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Mactra cecileana, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Mactra Araucana, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Arca Araucana, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Nucula Largillierti, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Trigonia Hanetiana, d’Orbigny, “Voyage, Part Pal.”</li> +</ol> + +<p>During a second visit of the <i>Beagle</i> 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:— +<a name="page400"></a> +</p> + +<ol> +<li>Natica australis, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Mactra Araucana, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Trigonia Hanetiana, d’Orbigny, “Voyage, Part Pal.”</li> + +<li>Pecten, fragments of, probably two species, but too imperfect +for description.</li> + +<li>Baculites vagina, E. Forbes.</li> + +<li>Nautilus d’Orbignyanus, E. Forbes.</li> +</ol> + +<p> +Besides these shells, Captain Belcher<a href="#fn-19.10" name="fnref-19.10" +id="fnref-19.10"><sup>[10]</sup></a> found here an <i>Ammonite</i>, 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 <i>Baculites vagina</i> 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 <i>Baculites vagina</i>, 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 <i>Nautilus +d’Orbignyanus</i>, 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 <i>Cardium auca</i> is found also at Coquimbo, the beds at which place, +there can be no doubt, are tertiary. +</p> + +<p class="footnote"> +<a name="fn-19.10" id="fn-19.10"></a> <a href="#fnref-19.10">[10]</a> +“Zoology of Captain Beechey’s Voyage,” p. 163. +</p> + +<p> +<i>Navidad.</i><a href="#fn-19.11" name="fnref-19.11" +id="fnref-19.11"><sup>[11]</sup></a>—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 +<a name="page401"></a> +hard calcareous sandstone. In one part, there were many pebbles of the common +metamorphic porphyries of the Cordillera: and near the base of the cliff, I +observed a single rounded boulder of greenstone, nearly a yard in diameter. I +traced this sandstone formation beneath the superficial covering of gravel, for +some distance inland: the strata are slightly inclined from the sea towards the +Cordillera, which apparently has been caused by their having been accumulated +against or round outlying masses of granite, of which some points project near +the coast. The sandstone contains fragments of wood, either in the state of +lignite or partially silicified, sharks’ teeth, and shells in great +abundance, both high up and low down the sea-cliffs. Pectunculus and Oliva were +most numerous in individuals, and next to them Turritella and Fusus. I +collected in a short time, though suffering from illness, the following +thirty-one species, all of which are extinct, and several of the genera do not +now range (as we shall hereafter show) nearly so far south:— +</p> + +<ol> +<li>Gastridium cepa, G. B. Sowerby.</li> + +<li>Monoceros, fragments of, considered by M. d’Orbigny as a new +species.</li> + +<li>Voluta alta, G. B. Sowerby (considered by M. d’Orbigny as +distinct from the V. alta of Santa Cruz).</li> + +<li>Voluta triplicata, G. B. Sowerby.</li> + +<li>Oliva dimidiata, G. B. Sowerby.</li> + +<li>Pleurotoma discors, G. B. Sowerby.</li> + +<li>Pleurotoma turbinelloides, G. B. Sowerby.</li> + +<li>Fusus subreflexus, G. B. Sowerby.</li> + +<li>Fusus pyruliformis, G. B. Sowerby.</li> + +<li>Fusus, allied to F. regularis (considered by M. d’Orbigny as a +distinct species).</li> + +<li>Turritella suturalis, G. B. Sowerby.</li> + +<li>Turritella Patagonica, G. B. Sowerby (fragments of).</li> + +<li>Trochus lævis, G. B. Sowerby.</li> + +<li>Trochus collaris, G. B. Sowerby (considered by M. d’Orbigny as +the young of the T. lævis).</li> + +<li>Cassis monilifer, G. B. Sowerby.</li> + +<li>Pyrula distans, G. B. Sowerby.</li> + +<li>Triton verruculosus, G. B. Sowerby.</li> + +<li>Sigaretus subglobosus, G. B. Sowerby.</li> + +<li>Natica solida, G. B. Sowerby. (It is doubtful whether the +Natica solida of S. Cruz is the same species with this.)</li> + +<li>Terebra undulifera, G. B. Sowerby.</li> + +<li>Terebra costellata, G. B. Sowerby.</li> + +<li>Bulla (fragments of).</li> + +<li>Dentalium giganteum, do.</li> + +<li>Dentalium sulcosum, do.</li> + +<li>Corbis (?) lævigata, do.</li> + +<li>Cardium multiradiatum, do.</li> + +<li>Venus meridionalis, do.</li> + +<li>Pectunculus dispar, (?) Desh. (considered by M. d’Orbigny as a +distinct species).</li> + +<li><a name="page402"></a>and 30. Cytheræa and Mactra, fragments of (considered by +M. d’Orbigny as new species).</li> + +<li>Pecten, fragments of.</li> +</ol> +<p class="footnote"> +<a name="fn-19.11" id="fn-19.11"></a> <a href="#fnref-19.11">[11]</a> +I was guided to this locality by the Report on M. Gay’s “Geological +Researches,” in the “Annales des Scienc. Nat.” (1st series, +tome 28. +</p> + +<p><i>Coquimbo.</i>—For more than two hundred miles northward +of Navidad, the coast consists of plutonic and metamorphic rocks, +with the exception of some quite insignificant superficial beds of +recent origin. At Tonguay, twenty-five miles south of Coquimbo, +tertiary beds recommence. I have already minutely described in the +Second Chapter, the step-formed plains of Coquimbo, and the upper +calcareous beds (from twenty to thirty feet in thickness) +containing shells of recent species, but in different proportions +from those on the beach. There remains to be described only the +underlying ancient tertiary beds, represented in Figure 21 by the +letters F and E:—</p> + +<p class="center"> +No. 21<br/> +Section of the tertiary formation at Coquimbo. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono21.jpg" width="400" height="147" alt="[Illustration: +Section of the tertiary formation at Coquimbo.]" /> +</div> + +<p>I obtained good sections of bed (F) only in Herradura Bay: it +consists of soft whitish sandstone, with ferruginous veins, some +pebbles of granite, and concretionary layers of hard calcareous +sandstone. These concretions are remarkable from the great number +of large silicified bones, apparently of cetaceous animals, which +they contain; and likewise of a shark’s teeth, closely resembling +those of the <i>Carcharias megalodon.</i> Shells of the following +species, of which the gigantic Oyster and Perna are the most +conspicuous, are numerously embedded in the concretions:—</p> + +<ol> +<li>Bulla ambigua, d’Orbigny, “Voyage,” Pal.</li> + +<li>Monoceros Blainvillii, d’Orbigny, “Voyage,” Pal.</li> + +<li>Cardium auca, d’Orbigny, “Voyage,” Pal.</li> + +<li>Panopæa Coquimbensis, d’Orbigny, “Voyage,” Pal.</li> + +<li>Perna Gaudichaudi, d’Orbigny, “Voyage,” Pal.</li> + +<li>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.</li> + +<li>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.</li> + +<li>Fragments of a Venus and Natica.</li> +</ol> + +<p> +<a name="page403"></a> +The cliffs on one side of Herradura Bay are capped by a mass of +stratified shingle, containing a little calcareous matter, and I +did not doubt that it belonged to the same recent formation with +the gravel on the surrounding plains, also cemented by calcareous +matter, until to my surprise, I found in the midst of it, a single +thin layer almost entirely composed of the above gigantic +oyster.</p> + +<p>At a little distance inland, I obtained several sections of the +bed (E), which, though different in appearance from the lower bed +(F), belongs to the same formation: it consists of a highly +ferruginous sandy mass, almost composed, like the lowest bed at +Port S. Julian, of fragments of Balanidæ; it includes some +pebbles, and layers of yellowish-brown mudstone. The embedded +shells consist of:—</p> + +<ol> +<li>Monoceros Blainvillii, d’Orbigny, “Voyage” Pal.</li> + +<li>Monoceros ambiguus, G. B. Sowerby.</li> + +<li>Anomia alternans, G. B. Sowerby.</li> + +<li>Pecten rudis, G. B. Sowerby.</li> + +<li>Perna Gaudichaudi, d’Orbigny, “Voyage” Pal.</li> + +<li>Ostrea Patagonica (?), d’Orbigny, “Voyage” Pal.</li> + +<li>Ostrea, small species, in imperfect state; it appeared to me +like a small kind now living in, but very rare in the bay.</li> + +<li>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.</li> + +<li>Balanus Coquimbensis, G. B. Sowerby.</li> + +<li>Balanus psittacus? King. This appears to Mr. Sowerby and myself +identical with a very large and common species now living on the +coast.</li> +</ol> + +<p>The uppermost layers of this ferrugino-sandy mass are +conformably covered by, and impregnated to the depth of several +inches with, the calcareous matter of the bed (D) called <i> +losa</i>: hence I at one time imagined that there was a gradual +passage between them; but as all the species are recent in the bed +(D), whilst the most characteristic shells of the uppermost layers +of (E) are the extinct Perna, Pecten, and Monoceros, I agree with +M. d’Orbigny, that this view is erroneous, and that there is only a +mineralogical passage between them, and no gradual transition in +the nature of their organic remains. Besides the fourteen species +enumerated from these two lower beds, M. d’Orbigny has described +ten other species given to him from this locality; +namely:—</p> + +<ol> +<li>Fusus Cleryanus, d’Orbigny, “Voyage” Pal.</li> + +<li>Fusus petitianus, d’Orbigny, “Voyage” Pal.</li> + +<li>Venus hanetiana, d’Orbigny, “Voyage” Pal.</li> + +<li>Venus incerta (?) d’Orbigny, “Voyage” Pal.</li> + +<li>Venus Cleryana, d’Orbigny, “Voyage” Pal.</li> + +<li>Venus petitiana, d’Orbigny, “Voyage” Pal.</li> + +<li>Venus Chilensis, d’Orbigny, “Voyage” Pal.</li> + +<li>Solecurtus hanetianus, d’Orbigny, “Voyage” Pal.</li> + +<li>Mactra auca, d’Orbigny, “Voyage” Pal.</li> + +<li>Oliva serena, d’Orbigny, “Voyage” Pal.</li> +</ol> + +<p>Of these twenty-four shells, all are extinct, except, according +to Mr. Sowerby, the <i>Artemis ponderosa, Mytilus Chiloensis,</i> +and probably the great Balanus.</p> + +<p><i>Coquimbo to Copiapo.</i>—A few miles north of Coquimbo, +I met with +<a name="page404"></a> +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.</p> + +<p><i>Coast of Peru.</i>—With the exception of deposits +containing recent shells and of quite insignificant dimensions, no +tertiary formations have been observed on this coast, for a space +of twenty-two degrees of latitude north of Copiapo, until coming to +Payta, where there is said to be a considerable calcareous deposit: +a few fossils have been described by M. d’Orbigny from this place, +namely:—</p> + +<ol> +<li>Rostellaria Gaudichaudi, d’Orbigny, “Voyage” Pal.</li> + +<li>Pectunculus Paytensis, d’Orbigny, “Voyage” Pal.</li> + +<li>Venus petitiana, d’Orbigny, “Voyage” Pal.</li> + +<li>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.</li> +</ol> + +<p><i>Concluding Remarks.</i>—The formations described in +this chapter, have, in the case of Chiloe and probably in that of +Concepcion and Navidad, apparently been accumulated in troughs +formed by submarine ridges extending parallel to the ancient shores +of the continent; in the case of the islands of Mocha and Huafo it +is highly probable, and in that of Ypun and Lemus almost certain, +that they were accumulated round isolated rocky centres or nuclei, +in the same manner as mud and sand are now collecting round the +outlying islets and reefs in the West Indian Archipelago. Hence, I +may remark, it does not follow that the outlying tertiary masses of +Mocha and Huafo were ever continuously united at the same level +with the formations on the mainland, though they may have been of +contemporaneous origin, and been subsequently upraised to the same +height. In the more northern parts of Chile, the tertiary strata +seem to have been separately accumulated in bays, now forming the +mouths of valleys.</p> + +<p>The relation between these several deposits on the shores of the +Pacific, is not nearly so clear as in the case of the tertiary +formations on the Atlantic. Judging from the form and height of the +land (evidence which I feel sure is here much more trustworthy than +it can ever be in such broken continents as that of Europe), from +the identity of mineralogical composition, from the presence of +fragments of lignite and of silicified wood, and from the +intercalated layers of imperfect coal, I must believe +<a name="page405"></a> +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 <i>Cardium auca</i> in common with +Concepcion, and <i>Fusus Cleryanus</i> with Huafo; I may add, that +Coquimbo has <i>Venus petitiana</i>, 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.</p> + +<p>From these facts, and from the generic resemblance of the +fossils from the different localities, I cannot avoid the suspicion +that they all belong to nearly the same epoch, which epoch, as we +shall immediately see, must be a very ancient tertiary one. But as +the Baculite, especially considering its apparent identity with the +Cretaceous Pondicherry species, and the presence of an Ammonite, +and the resemblance of the Nautilus to two upper greensand species, +together afford very strong evidence that the formation of +Concepcion is a Secondary one; I will, in my remarks on the fossils +from the other localities, put on one side those from Concepcion +and from Eastern Chiloe, which, whatever their age may be, appear +to me to belong to one group. I must, however, again call attention +to the fact that the <i>Cardium auca</i> is found both at +Concepcion and in the undoubtedly tertiary strata of Coquimbo: nor +should the possibility be overlooked, that as Trigonia, though +known in the northern hemisphere only as a Secondary genus, has +living representatives in the Australian seas, so a Baculite, +Ammonite, and Trigonia may have survived in this remote part of the +southern ocean to a somewhat later period than to the north of the +equator.</p> + +<p>Before passing in review the fossils from the other localities, +there are two points, with respect to the formations between +Concepcion and Chiloe, which deserve some notice. First, that +though the strata are generally horizontal, they have been upheaved +in Chiloe in a set of parallel anticlinal and uniclinal lines +ranging north and south,—in the district near P. Rumena by +eight or nine far-extended, most symmetrical, uniclinal lines +ranging nearly east and west,—and in the neighbourhood of +Concepcion by less regular single lines, directed both N.E. and +S.W., and N.W. and S.E. This fact is of some interest, as showing +that within a period which cannot be considered as very ancient in +relation to the history of the continent, the strata between the +Cordillera and the Pacific have been broken up in the same +variously directed manner as have the old plutonic and metamorphic +rocks in this same district. The second point is, that the +sandstone between Concepcion and Southern Chiloe is everywhere +lignitiferous, and includes much silicified wood; whereas the +formations in Northern Chile do not +<a name="page406"></a> +include beds of lignite or coal, and in place of the fragments +of silicified wood there are silicified bones. Now, at the present +day, from Cape Horn to near Concepcion, the land is entirely +concealed by forests, which thin out at Concepcion, and in Central +and Northern Chile entirely disappear. This coincidence in the +distribution of the fossil wood and the living forests may be quite +accidental; but I incline to take a different view of it; for, as +the difference in climate, on which the presence of forests +depends, is here obviously in chief part due to the form of the +land, and as the Cordillera undoubtedly existed when the +lignitiferous beds were accumulating, I conceive it is not +improbable that the climate, during the lignitiferous period, +varied on different parts of the coast in a somewhat similar manner +as it now does. Looking to an earlier epoch, when the strata of the +Cordillera were depositing, there were islands which even in the +latitude of Northern Chile, where now all is irreclaimably desert, +supported large coniferous forests.</p> + +<p>Seventy-nine species of fossil shells, in a tolerably +recognisable condition, from the coast of Chile and Peru, are +described in this volume, and in the 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 <i>Turritella Chilensis</i> from Huafo and Mocha, +the <i>T. Patagonica</i> and <i>Venus meridionalis</i> from +Navidad, come very near to recent South American shells, namely, +the two Turritellas to <i>T. cingulata</i>, and the Venus to <i>V. +exalbida</i>: 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.</p> + +<p>When we consider that very few, if any, of the fifty-nine fossil +shells are identical with, or make any close approach to, living +species; when we consider that some of the genera do not now exist +on the west coast of South America, and that no less than twelve +genera out of the thirty-two formerly ranged very differently from +the existing species of the +<a name="page407"></a> +same genera, we must admit that these deposits are of +considerable antiquity, and that they probably verge on the +commencement of the tertiary era. May we not venture to believe, +that they are of nearly contemporaneous origin with the Eocene +formations of the northern hemisphere?</p> + +<table border="1" width="100%" cellpadding="4" summary= +"Genera of fossils, Latitudes in which found, Southernmost latitude in which found"> +<tr valign="bottom"> +<td>Genera, with living and tertiary +species on the west coast of +S. America.<a href="#fn-19.12" name="fnref-19.12" id="fnref-19.12"><sup>[12]</sup></a></td> +<td>Latitudes, in which found fossil on +the coasts of Chile and Peru.</td> +<td>Southernmost latitude, in which +found living on the west coast of<br/> +S. America.</td> +</tr> + +<tr> +<td>Bulla</td> +<td>30° to 43° 30′</td> +<td>12° near Lima.</td> +</tr> + +<tr> +<td>Cassis</td> +<td>34°</td> +<td>1° 37′</td> +</tr> + +<tr valign="top"> +<td>Pyrula</td> +<td>34° (and 36° 30′ at Concepcion)</td> +<td>5° Payta</td> +</tr> + +<tr valign="top"> +<td>Fusus</td> +<td>30° to 43° 30′</td> +<td>23° Mexillones; reappears at the St. of Magellan</td> +</tr> + +<tr valign="top"> +<td>Pleurotoma</td> +<td>34° to 43° 30′</td> +<td>2° 18′ St. Elena</td> +</tr> + +<tr valign="top"> +<td>Terebra</td> +<td>34°</td> +<td>5° Payta</td> +</tr> + +<tr valign="top"> +<td>Sigaretus</td> +<td>34° to 44° 30′</td> +<td>12° Lima</td> +</tr> + +<tr valign="top"> +<td>Anomia</td> +<td>30°</td> +<td>7° 48′</td> +</tr> + +<tr valign="top"> +<td>Perna</td> +<td>30°</td> +<td>1° 23′ Xixappa</td> +</tr> + +<tr valign="top"> +<td>Cardium</td> +<td>30° to 34° (and 36° 30′ at Concepcion)</td> +<td>5° Payta</td> +</tr> + +<tr valign="top"> +<td>Artemis</td> +<td>30°</td> +<td>5° Payta</td> +</tr> + +<tr valign="top"> +<td>Voluta</td> +<td>34° to 44° 30′</td> +<td>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.</td> +</tr> +</table> + +<p class="footnote"> +<a name="fn-19.12" id="fn-19.12"></a> <a href="#fnref-19.12">[12]</a> +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′. +</p> + +<p>Comparing the fossil remains from the coast of Chile (leaving +out, as before, Concepcion and Chiloe) with those from Patagonia, +we may conclude, from their generic resemblance, and from the small +number of the species which from either coast approach closely to +living forms, that the formations of both belong to nearly the same +epoch; and this is the opinion of M. D’Orbigny. Had not a single +fossil shell been common to the two coasts, it could not have been +argued that the formations belonged to different ages; for Messrs. +Cuming and Hinds have found, on the comparison of nearly two +thousand living species from the opposite sides of South America, +only one in common, namely, the <i>Purpura lapillus</i> 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 +<a name="page408"></a> +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, <i> +Voluta alta, Turritella Patagonica, Trochus collaris, Venus +meridionalis,</i> perhaps (Natica solida), and perhaps the large +oyster from Coquimbo, are considered by Mr. Sowerby as identical +with species from Santa Cruz and P. Desire. M. d’Orbigny, however, +admits the perfect identity only of the Trochus.</p> + +<p> +<i>On the temperature of the Tertiary period.</i>—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 <i>Trochus collaris</i>, +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.<a +href="#fn-19.13" name="fnref-19.13" id="fnref-19.13"><sup>[13]</sup></a> +<a name="page409"></a> +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, +<i>were of a local character and did not affect the entire globe.</i> On the +other hand, I have endeavoured to show, in the “Geological +Transactions,” that, at a much later period, Europe and North and South +America were nearly contemporaneously subjected to ice-action, and consequently +to a colder, or at least more equable, climate than that now characteristic of +the same latitudes. +</p> + +<p class="footnote"> +<a name="fn-19.13" id="fn-19.13"></a> <a href="#fnref-19.13">[13]</a> +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. +</p> + +<p><i>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.</i>—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 <i>possibly</i> 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 +<a name="page410"></a> +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.</p> + +<p>Can any light be thrown on this remarkable absence of recent +conchiferous deposits on these coasts, on which, at an ancient +tertiary epoch, strata abounding with organic remains were +extensively accumulated? I think there can, namely, by considering +the conditions necessary for the preservation of a formation to a +distant age. Looking to the enormous amount of denudation which on +all sides of us has been effected,—as evidenced by the lofty +cliffs cutting off on so many coasts horizontal and once +far-extended strata of no great antiquity (as in the case of +Patagonia),—as evidenced by the level surface of the ground +on both sides of great faults and dislocations,—by inland +lines of escarpments, by outliers, and numberless other facts, and +by that argument of high generality advanced by Mr. Lyell, namely, +that every <i>sedimentary</i> 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 <i>conchiferous</i> 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.</p> + +<p> +<a name="page411"></a> +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 <i>conchiferous</i> deposits, of sufficient +thickness, extension, and hardness, to resist the average powers of +denudation.</p> + +<p>We have seen that at an ancient tertiary epoch, fossiliferous +deposits were extensively deposited on the coasts of 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 +<a name="page412"></a> +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.</p> + +<p> +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 +<i>conchiferous</i> 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.<a href="#fn-19.14" +name="fnref-19.14" id="fnref-19.14"><sup>[14]</sup></a> 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 <i>durable</i> +fossiliferous deposits accumulating. +</p> + +<p class="footnote"> +<a name="fn-19.14" id="fn-19.14"></a> <a href="#fnref-19.14">[14]</a> +Professor H. D. Rogers, in his excellent address to the Association of American +Geologists (<i>Silliman’s Journal,</i> vol. xlvii, p. 277) makes the +following remark: “I question if we are at all aware how +<i>completely</i> 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 <i>has written +its own history</i>, 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.) +</p> + +<p> +<a name="page413"></a> +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.<a href="#fn-19.15" name="fnref-19.15" +id="fnref-19.15"><sup>[15]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-19.15" id="fn-19.15"></a> <a href="#fnref-19.15">[15]</a> +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 (<i>Edinburgh New Phil. Journal,</i> January 1843), I partially +entered into this discussion, and endeavoured to show that it was highly +improbable, that upraised atolls or barrier-reefs, though of great thickness, +should, owing to their small extension or breadth, be preserved to a distant +future period. +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap3.06"></a>Chapter VI<br/>PLUTONIC AND METAMORPHIC +ROCKS:—CLEAVAGE AND FOLIATION.</h2> + +<p class="letter"> +Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of +foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in, +decomposition of.—La Plata, metamorphic and old volcanic rocks +of.—S. Ventana.—Claystone porphyry formation of Patagonia; singular +metamorphic rocks; pseudo-dikes.—Falkland Islands, 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. +</p> + +<p>The metamorphic and plutonic formations of the several districts +visited by the <i>Beagle</i> will be here chiefly treated of, but +only such cases as appear to me new, or of some special interest, +will be +<a name="page414"></a> +described in detail; at the end of the chapter I will sum up all +the facts on cleavage and foliation,—to which I particularly +attended.</p> + +<p> +<i>Bahia, Brazil: lat. 13° south.</i>—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,<a href="#fn-20.1" name="fnref-20.1" id="fnref-20.1"><sup>[1]</sup></a> +these latter also having been at the same time bent and softened. +</p> + +<p class="footnote"> +<a name="fn-20.1" id="fn-20.1"></a> <a href="#fnref-20.1">[1]</a> +Professor Hitchcock (“Geology of Massachusetts,” vol. ii, p. 673, +gives a closely similar case of a greenstone dike in syenite. +</p> + +<p>I must here take the opportunity of premising, that by the term +<i>cleavage</i> I imply those planes of division which render a +rock, appearing to the eye quite or nearly homogeneous, fissile. By +the term <i>foliation</i>, 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 <i> +stratification</i>, as applied to these formations, I mean those +alternate, parallel, large masses of different composition, which +are themselves frequently either foliated or fissile,—such as +the alternating so-called strata of mica-slate, gneiss, glossy +clay-slate, and marble.</p> + +<p> +The folia of the gneiss within a few miles round Bahia generally +<a name="page415"></a> +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<a href="#fn-20.2" +name="fnref-20.2" id="fnref-20.2"><sup>[2]</sup></a> 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.<a href="#fn-20.3" name="fnref-20.3" +id="fnref-20.3"><sup>[3]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-20.2" id="fn-20.2"></a> <a href="#fnref-20.2">[2]</a> +“Geological Section of the Brit. Assoc.,” 1840. For Sir R. +Schomburgk’s observations see <i>Geograph. Journal,</i> 1842, p. 190. See +also Humboldt’s discussion on Loxodrism in the “Personal +Narrative.” +</p> + +<p class="footnote"> +<a name="fn-20.3" id="fn-20.3"></a> <a href="#fnref-20.3">[3]</a> +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 <i> London and Edin. Phil. +Magazine.</i><br/> + A<small>BROLHOS</small> I<small>SLETS</small>, <i>lat. 18° S. off the +coast of Brazil.</i>—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. +</p> + +<p><i>Rio de Janeiro.</i>—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 “<i>grain</i>,” 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 name="page416"></a> +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.</p> + +<p class="center"> +No. 22<br/> +Fragment of gneiss embedded in another variety of the same rock. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono22.jpg" width="334" height="238" alt="[Illustration: +Fragment of gneiss embedded in another variety of the same rock.]" /> +</div> + +<p>On a bare gently inclined surface of the porphyritic gneiss in +Botofogo Bay, I observed the appearance here represented.</p> + +<p>A fragment seven yards long and two in width, with angular and +distinctly defined edges, composed of a peculiar variety of gneiss +with dark layers of mica and garnets, is surrounded on all sides by +the ordinary gneiss-granite; both having been dislocated by a +granitic vein. The folia in the fragment and in the surrounding +rock strike in the same N.N.E. and S.S.W. line; but in the fragment +they are vertical, whereas in the gneiss-granite they dip at a +small angle, as shown by the arrows, to S.S.E. This fragment, +considering its great size, its solitary position, and its foliated +structure parallel to that of the surrounding rock, is, as far as I +know, a unique case: and I will not attempt any explanation of its +origin.</p> + +<p> +<a name="page417"></a> +The numerous travellers<a href="#fn-20.4" name="fnref-20.4" +id="fnref-20.4"><sup>[4]</sup></a> 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.<a href="#fn-20.5" name="fnref-20.5" +id="fnref-20.5"><sup>[5]</sup></a> The minerals retain their positions in folia +ranging in the usual direction; and fractured quartz veins may be traced from +the solid rock, running for some distance into the softened, mottled, highly +coloured, argillaceous mass. It is said that these decomposed rocks abound with +gems of various kinds, often in a fractured state, owing, as some have +supposed, to the collapse of geodes, and that they contain gold and diamonds. +At Rio, it appeared to me that the gneiss had been softened before the +excavation (no doubt by the sea) of the existing, broad, flat-bottomed valleys; +for the depth of decomposition did not appear at all conformable with the +present undulations of the surface. The porphyritic gneiss, where now exposed +to the air, seems to withstand decomposition remarkably well; and I could see +no signs of any tendency to the production of argillaceous masses like those +here described. I was also struck with the fact, that where a bare surface of +this rock sloped into one of the quiet bays, there were no marks of erosion at +the level of the water, and the parts both beneath and above it preserved a +uniform curve. At Bahia, the gneiss rocks are similarly decomposed, with the +upper parts insensibly losing their foliation, and passing, without any +distinct line of separation, into a bright red argillaceous earth, including +partially rounded fragments of quartz and granite. From this circumstance, and +from the rocks appearing to have suffered decomposition before the excavation +of the valleys, I suspect that here, as at Rio, the decomposition took place +under the sea. The subject appeared to me a curious one, and would probably +well repay careful examination by an able mineralogist. +</p> + +<p class="footnote"> +<a name="fn-20.4" id="fn-20.4"></a> <a href="#fnref-20.4">[4]</a> +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.” +</p> + +<p class="footnote"> +<a name="fn-20.5" id="fn-20.5"></a> <a href="#fnref-20.5">[5]</a> + Dr. Benza describes granitic rock (<i>Madras Journal of Lit.,</i> etc., Oct. + 183? p. 246), in the Neelgherries, decomposed to a depth of forty feet. +</p> + +<p> +<i>The Northern Provinces of La Plata.</i>—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.<a +href="#fn-20.6" name="fnref-20.6" id="fnref-20.6"><sup>[6]</sup></a> 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 +<a name="page418"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-20.6" id="fn-20.6"></a> <a href="#fnref-20.6">[6]</a> +M. Isabelle, “Voyage à Buenos Ayres,” p. 479. +</p> + +<p>Northward of Maldonado, and south of Las Minas, there is an E. +and W. hilly band of country, some miles in width, formed of +siliceous clay-slate, with some quartz, rock, and limestone, having +a tortuous irregular cleavage, generally ranging east and west. E. +and S.E. of Las Minas there is a confused district of imperfect +gneiss and laminated quartz, with the hills ranging in various +directions, but with each separate hill generally running in the +same line with the folia of the rocks of which it is composed: this +confusion appears to have been caused by the intersection of the +[E. and W.] and [N.N.E. and S.S.W.] strikes. Northward of Las +Minas, the more regular northerly ranges predominate: from this +place to near Polanco, we meet with the coarse-grained mixture of +quartz and feldspar, often with the imperfect hornblende, and then +becoming foliated in a N. and S. line—with imperfect +clay-slate, including 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.</p> + +<p> +Some ancient submarine volcanic rocks are worth mentioning, from their rarity +on this eastern side of the continent. In the valley of the Tapas (fifty or +sixty miles N. of Maldonado) there is a tract three or four miles in length, +composed of various trappean rocks with glassy feldspar—of apparently +metamorphosed grit-stones—of purplish +<a name="page419"></a> +amygdaloids with large kernels of carbonate of lime<a href="#fn-20.7" +name="fnref-20.7" id="fnref-20.7"><sup>[7]</sup></a>—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. +</p> + +<p class="footnote"> +<a name="fn-20.7" id="fn-20.7"></a> <a href="#fnref-20.7">[7]</a> +Near the Pan de Azucar there is some greenish porphyry, in one place +amygdaloidal with agate. +</p> + +<p> +<i>Monte Video.</i>—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 <i>blend</i> +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.<a href="#fn-20.8" name="fnref-20.8" +id="fnref-20.8"><sup>[8]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-20.8" id="fn-20.8"></a> <a href="#fnref-20.8">[8]</a> +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. +</p> + +<p>The Mount, a hill believed to be 450 feet in height, from which +the place takes its name, is much the highest land in this +neighbourhood: it consists of hornblendic slate, which (except on +the eastern and disturbed base) has an east and west nearly +vertical cleavage; the longer axis of the hill also ranges in this +same line. Near the summit the hornblende-slate gradually becomes +more and more coarsely crystallised, and less plainly laminated, +until it passes into a heavy, sonorous greenstone, with a slaty +conchoidal fracture; the laminæ on the north +<a name="page420"></a> +and south sides near the summit dip inwards, as if this upper +part had expanded or bulged outwards. This greenstone must, I +conceive, be considered as metamorphosed hornblende-slate. The +Cerrito, the next highest, but much less elevated point, is almost +similarly composed. In the more western parts of the province, +besides gneiss, there is quartz-rock, syenite, and granite; and at +Colla, I heard of marble.</p> + +<p>Near M. Video, the space which I more accurately examined was +about fifteen miles in an east and west line, and here I found the +foliation of the gneiss and the cleavage of the slates generally +well developed, and extending parallel to the alternating strata +composed of the gneiss, hornblendic and chloritic schists. These +planes of division all range within one point of east and west, +frequently east by south and west by north; their dip is generally +almost vertical, and scarcely anywhere under 45°: this fact, +considering how slightly undulatory the surface of the country is, +deserves attention. Westward of M. Video, towards the Uruguay, +wherever the gneiss is exposed, the highly inclined folia are seen +striking in the same direction; I must except one spot where the +strike was N.W. by W. The little Sierra de S. Juan, formed of +gneiss and laminated quartz, must also be excepted, for it ranges +between [N. to N.E.] and [S. to S.W.] and seems to belong to the +same system with the hills in the Maldonado district. Finally, we +have seen that, for many miles northward of Maldonado and for +twenty-five miles westward of it, as far as the S. de las Animas, +the foliation, cleavage, so-called stratification and lines of +hills, all range N.N.E. and S.S.W., which is nearly coincident with +the adjoining coast of the Atlantic. Westward of the S. de las +Animas, as far as even the Uruguay, the foliation, cleavage, and +stratification (but not lines of hills, for there are no defined +ones) all range about E. by S. and W. by N., which is nearly +coincident with the direction of the northern shore of the Plata; +in the confused country near Las Minas, where these two great +systems appear to intersect each other, the cleavage, foliation, +and stratification run in various directions, but generally +coincide with the line of each separate hill.</p> + +<p> +<i>Southern La Plata.</i>—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<a href="#fn-20.9" name="fnref-20.9" +id="fnref-20.9"><sup>[9]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-20.9" id="fn-20.9"></a> <a href="#fnref-20.9">[9]</a> +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. +</p> + +<p> +<a name="page421"></a> +The Sierra Guitru-gueyu is situated sixty miles south of the S. +Tapalguen: it consists of numerous parallel, sometimes blended +together ridges, about twenty-three miles in width, and five +hundred feet in height above the plain, and extending in a N.W. and +S.E. direction. Skirting round the extreme S.E. termination, I +ascended only a few points, which were composed of a fine-grained +gneiss, almost composed of feldspar with a little mica, and passing +in the upper parts of the hills into a rather compact purplish +clay-slate. The cleavage was nearly vertical, striking in a N.W. by +W. and S.E. by E. line, nearly, though not quite, coincident with +the direction of the parallel ridges.</p> + +<p>The Sierra Ventana lies close south of that of Guitru-gueyu; it +is remarkable from attaining a height, very unusual on this side of +the continent, of 3,340 feet. It consists up to its summit, of +quartz, generally pure and white, but sometimes reddish, and +divided into thick 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.</p> + +<p><i>Patagonia.</i>—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.</p> + +<p>At Port Desire there is an extensive formation of the claystone +porphyry, stretching at least twenty-five miles into the interior: +it has been denuded and deeply worn into gullies before being +covered up by the tertiary deposits, through which it here and +there projects in hills; those north of the bay being 440 feet in +height. The strata have in several places been tilted at small +angles, generally either to N.N.W. or S.S.E. By gradual passages +and alternations, the porphyries change incessantly in nature. I +will describe only some of the principal +<a name="page422"></a> +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 <i>fibrous structure</i>, then +passing into opaque imperfectly shaped crystals, and lastly, into +perfect glassy crystals. When these crystals have appeared, and +when the basis has become compact, the rock in many places could +not be distinguished from a true claystone porphyry without a trace +of mechanical structure.</p> + +<p>In some parts, these earthy or tufaceous beds pass into jaspery +and into beautifully mottled and banded porcelain rocks, which +break into splinters, translucent at their edges, hard enough to +scratch glass, and fusible into white transparent beads: grains of +quartz included in the porcelainous varieties can be seen melting +into the surrounding paste. In other parts, the earthy or tufaceous +beds either insensibly pass into, or alternate with, breccias +composed of large and small fragments of various purplish +porphyries, with the matrix generally porphyritic: these breccias, +though their subaqueous origin is in many places shown both by the +arrangement of their smaller particles and by an oblique or current +lamination, also pass into porphyries, in which every trace of +mechanical origin and stratification has been obliterated.</p> + +<p>Some highly porphyritic though coarse-grained masses, +evidently +<a name="page423"></a> +of sedimentary origin, and divided into thin layers, differing +from each other chiefly in the number of embedded grains of quartz, +interested me much from the peculiar manner in which here and there +some of the layers terminated in abrupt points, quite unlike those +produced by a layer of sediment naturally thinning out, and +apparently the result of a subsequent process of metamorphic +aggregation. In another common variety of a finer texture, the +aggregating process had gone further, for the whole mass consisted +of quite short, parallel, often slightly curved layers or patches, +of whitish or reddish finely granulo-crystalline feldspathic +matter, generally terminating at both ends in blunt points; these +layers or patches further tended to pass into wedge or +almond-shaped little masses, and these finally into true crystals +of feldspar, with their centres often slightly drusy. The series +was so perfect that I could not doubt that these large crystals, +which had their longer axes placed parallel to each other, had +primarily originated in the metamorphosis and aggregation of +alternating layers of tuff; and hence their parallel position must +be attributed (unexpected though the conclusion may be), not to +laws of chemical action, but to the original planes of deposition. +I am tempted briefly to describe three other singular allied +varieties of rock; the first without examination would have passed +for a stratified porphyritic breccia, but all the included angular +fragments consisted of a border of pinkish crystalline feldspathic +matter, surrounding a dark translucent siliceous centre, in which +grains of quartz not quite blended into the paste could be +distinguished: this uniformity in the nature of the fragments shows +that they are not of mechanical, but of concretionary origin, +having resulted perhaps from the self-breaking up and aggregation +of layers of indurated tuff containing numerous grains of +quartz,—into which, indeed, the whole mass in one part +passed. The second variety is a reddish non-porphyritic claystone, +quite full of spherical cavities, about half an inch in diameter, +each lined with a collapsed crust formed of crystals of quartz. The +third variety also consists of a pale purple non-porphyritic +claystone, almost wholly formed of concretionary balls, obscurely +arranged in layers, of a less compact and paler coloured claystone; +each ball being on one side partly hollow and lined with crystals +of quartz.</p> + +<p> +<i>Pseudo-dikes.</i>—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 <i> rounded</i> +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 +<a name="page424"></a> +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;<a href="#fn-20.10" +name="fnref-20.10" id="fnref-20.10"><sup>[10]</sup></a> but if we reflect on +the suction which would result from a deep-seated fissure being formed, we may +admit that if the fissure were in any part open to the surface, mud and water +might well be drawn into it along its whole course. The third dike consisted of +a hard, rough, white rock, almost composed of broken crystals of glassy +feldspar, with numerous scales of black mica, cemented in a scanty base; there +was little in the appearance of this rock, to preclude the idea of its having +been a true injected feldspathic dike. The matter composing these three +pseudo-dikes, especially the second one, appears to have suffered, like the +surrounding strata, a certain degree of metamorphic action; and this has much +aided the deceptive appearance. At Bahia, in Brazil, we have seen that a true +injected hornblendic dike, not only has suffered metamorphosis, but has been +dislocated and even diffused in the surrounding gneiss, under the form of +separate crystals and of fragments. +</p> + +<p class="footnote"> +<a name="fn-20.10" id="fn-20.10"></a> <a href="#fnref-20.10">[10]</a> +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. +</p> + +<p><i>Falkland Islands.</i>—I have described these islands in +a paper published in the third volume of the <i>Geological +Journal.</i> 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.</p> + +<p> +<i>Tierra del Fuego.</i>—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 +<a name="page425"></a> +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,<a href="#fn-20.11" +name="fnref-20.11" id="fnref-20.11"><sup>[11]</sup></a> 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.]. +</p> + +<p class="footnote"> +<a name="fn-20.11" id="fn-20.11"></a> <a href="#fnref-20.11">[11]</a> +<i>Geographical Journal,</i> vol. i, p. 155. +</p> + +<p>The clay-slate is generally fissile, sometimes siliceous or +ferruginous, with veins of quartz and calcareous spar; it often +assumes, especially on the loftier mountains, an altered +feldspathic character, passing into feldspathic porphyry: +occasionally it is associated with breccia and grauwacke. At Good +Success Bay, there is a little intercalated black crystalline +limestone. At Port Famine much of the clay-slate is calcareous, and +passes either into a mudstone or into grauwacke, including +odd-shaped concretions of dark argillaceous limestone. Here alone, +on the shore a few miles north of Port Famine, and on the summit of +Mount Tarn (2,600 feet high), I found organic remains; they consist +of:—</p> + +<ol> +<li>Ancyloceras simplex, d’Orbigny, “Pal Franc,” Mount Tarn.</li> + +<li>Fusus (in imperfect state), d’Orbigny, “Pal Franc,” Mount +Tarn.</li> + +<li>Natica, d’Orbigny, “Pal Franc,” Mount Tarn.</li> + +<li>Pentacrimus, d’Orbigny, “Pal Franc,” Mount Tarn.</li> + +<li>Lucina excentrica, G. B. Sowerby, Port Famine.</li> + +<li>Venus (in imperfect state), G. B. Sowerby, Port Famine.</li> + +<li>Turbinolia (?), G. B. Sowerby, Port Famine.</li> + +<li>Hamites elatior, G. B. Sowerby, Port Famine.</li> +</ol> + +<p> +M. d’Orbigny states<a href="#fn-20.12" name="fnref-20.12" +id="fnref-20.12"><sup>[12]</sup></a> that MM. Hombron and Grange found in this +neighbourhood an Ancyloceras, perhaps <i>A. simplex</i>, 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 <i>A. simplex</i> 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 <i>Hamites elatior</i> 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. +</p> + +<p class="footnote"> +<a name="fn-20.12" id="fn-20.12"></a> <a href="#fnref-20.12">[12]</a> +“Voyage,” Part Géolog., p. 242. +</p> + +<p> +<a name="page426"></a> +The clay-slate in many parts of Tierra del Fuego, is broken by dikes<a +href="#fn-20.13" name="fnref-20.13" id="fnref-20.13"><sup>[13]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-20.13" id="fn-20.13"></a> <a href="#fnref-20.13">[13]</a> +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. +</p> + +<p> +In Southern Tierra del Fuego, the clay-slate towards its S.W. boundary, becomes +much altered and feldspathic. Thus on Wollaston Island slate and grauwacke can +be distinctly traced passing into feldspathic rocks and greenstones, including +iron pyrites and epidote, but still retaining traces of cleavage with the usual +strike and dip. One such metamorphosed mass was traversed by large vein-like +masses of a beautiful mixture (as ascertained by Professor Miller) of green +epidote, garnets, and white calcareous spar. On the northern point of this same +island, there were various ancient submarine volcanic rocks, consisting of +amygdaloids with dark bole and agate,—of basalt with decomposed +olivine—of compact lava with glassy feldspar,—and of a coarse +conglomerate of red 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<a href="#fn-20.14" name="fnref-20.14" +id="fnref-20.14"><sup>[14]</sup></a> 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.<a href="#fn-20.15" +name="fnref-20.15" id="fnref-20.15"><sup>[15]</sup></a> In the midst of these +singular rocks, no doubt of ancient submarine volcanic origin, a high hill of +feldspathic clay-slate projected, retaining +<a name="page427"></a> +its usual cleavage. Near this point, there was a small hillock, having the +aspect of granite, but formed of white albite, brilliant crystals of hornblende +(both ascertained by the reflecting goniometer) and mica; but with no quartz. +No recent volcanic district has been observed in any part of Tierra del Fuego. +</p> + +<p class="footnote"> +<a name="fn-20.14" id="fn-20.14"></a> <a href="#fnref-20.14">[14]</a> +Determined by Professor Jameson. Weddell’s “Voyage,” p. 169. +</p> + +<p class="footnote"> +<a name="fn-20.15" id="fn-20.15"></a> <a href="#fnref-20.15">[15]</a> +See Mr. Brooke’s Paper in the <i>London Phil. Mag.,</i> vol. x. This +mineral occurs in an ancient volcanic rock near Rome. +</p> + +<p> +Five miles west of the bifurcation of the Beagle Channel, the slate-formation, +instead of becoming, as in the more southern parts of Tierra del Fuego, +feldspathic, and associated with trappean or old volcanic rocks, passes by +alternations into a great underlying mass of fine gneiss and glossy clay-slate, +which at no great distance is succeeded by a grand formation of mica-slate +containing garnets. The folia of these metamorphic schists strike parallel to +the cleavage-planes of the clay-slate, which have a very uniform direction over +the whole of this part of the country: the folia, however, are undulatory and +tortuous, whilst the cleavage-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 <i>strata</i> 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 +<i>Adventure</i> and <i>Beagle,</i><a href="#fn-20.16" name="fnref-20.16" +id="fnref-20.16"><sup>[16]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-20.16" id="fn-20.16"></a> <a href="#fnref-20.16">[16]</a> +See the Paper by Captain King in the <i> Geograph. Journal</i>; 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. +</p> + +<p>Only one other rock, met with in both arms of the Beagle +Channel, deserves any notice, namely a granulo-crystalline mixture +of white albite, black hornblende (ascertained by measurement of +the crystals, and confirmed by Professor Miller), and more or less +of brown mica, but without any quartz. This rock occurs in large +masses, closely resembling in external form granite or syenite: in +the southern arm of the Channel, one such mass underlies the +mica-slate, on which clay-slate was superimposed: this peculiar +plutonic rock which, as we have +<a name="page428"></a> +seen, occurs also in Hardy Peninsula, is interesting, from its +perfect similarity with that (hereafter often to be referred to +under the name of andesite) forming the great injected axes of the +Cordillera of Chile.</p> + +<p> +The stratification of the clay-slate is generally very obscure, whereas the +cleavage is remarkably well defined: to begin with the extreme eastern parts of +Tierra del Fuego; the cleavage-planes near the St. of Le Maire strike either W. +and E. or W.S.W. and E.N.E., and are highly inclined; the form of the land, +including Staten Island, indicates that the axes of elevation have run in this +same line, though I was unable to distinguish the planes of stratification. +Proceeding westward, I accurately examined the cleavage of the clay-slate on +the northern, eastern, and western sides (thirty-five miles apart) of Navarin +Island, and everywhere found the laminæ ranging with extreme regularity, W.N.W. +and E.S.E., seldom varying more than one point of the compass from this +direction.<a href="#fn-20.17" name="fnref-20.17" +id="fnref-20.17"><sup>[17]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-20.17" id="fn-20.17"></a> <a href="#fnref-20.17">[17]</a> +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°. +</p> + +<p>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, +<a name="page429"></a> +which appear only on the weathered surfaces, are the last +vestiges of the original planes of stratification, now almost +obliterated by the highly fissile and altered structure which the +mass has assumed.</p> + +<p>The clay-slate cleaves in the same W.N.W. and E.S.E. direction, +as on Navarin Island, on both sides of the Beagle Channel, on the +eastern side of Hoste Island, on the N.E. side of Hardy Peninsula, +and on the northern point of Wollaston Island; although in these +two latter localities the cleavage has been much obscured by the +metamorphosed and feldspathic condition of the slate. Within the +area of these several islands, including Navarin Island, the +direction of the stratification and of the mountain-chains is very +obscure; though the mountains in several places appeared to range +in the same W.N.W. line with the cleavage: the outline of the +coast, however, does not correspond with this line. Near the +bifurcation of the Beagle Channel, where the underlying metamorphic +schists are first seen, they are foliated (with some +irregularities), in this same W.N.W. line, and parallel, as before +stated, to the main mountain-axis of this part of the country. +Westward of this main range, the metamorphic schists are foliated, +though less plainly, in the same direction, which is likewise +common to the zone of old erupted trappean rocks, forming the +outermost islets. Hence the area, over which the cleavage of the +slate and the foliation of the metamorphic schists extends with an +average W.N.W. and E.S.E. strike, is about forty miles in a north +and south line, and ninety miles in an east and west line.</p> + +<p> +Further northward, near Port Famine, the stratification of the clay-slate and +of the associated rocks, is well defined, and there alone the cleavage and +strata-planes are parallel. A little north of this port there is an anticlinal +axis ranging N.W. (or a little more westerly) and S.E.: south of the port, as +far as Admiralty Sound and Gabriel Channel, the outline of the land clearly +indicates the existence of several lines of elevation in this same N.W. +direction, which, I may add, is so uniform in the western half of the St. of +Magellan, that, as Captain King<a href="#fn-20.18" name="fnref-20.18" +id="fnref-20.18"><sup>[18]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-20.18" id="fn-20.18"></a> <a href="#fnref-20.18">[18]</a> +<i>Geograph. Journal,</i> vol. i, p. 170. +</p> + +<p>Finally, we have seen that in the extreme eastern point of +Tierra del Fuego, the cleavage and coast-lines extend W. and E. and +even W.S.W. and E.N.E.: over a large area westward, the cleavage, +the main range of mountains, and some subordinate ranges, but not +the outlines of the coast, strike W.N.W., and E.S.E.: in the +central and western parts of the St. of Magellan, the +stratification, the mountain-ranges, the +<a name="page430"></a> +outlines of the coast, and the cleavage all strike nearly N.W. +and S.E. North of the strait, the outline of the coast, and the +mountains on the mainland, run nearly north and south. Hence we +see, at this southern point of the continent, how gradually the +Cordillera bend, from their north and south course of so many +thousand miles in length, into an E. and even E.N.E. direction.</p> + +<p> +<i>West coast, from the Southern Chonos Islands to Northern +Chile.</i>—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 <i>Beagle</i> 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<a href="#fn-20.19" name="fnref-20.19" +id="fnref-20.19"><sup>[19]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-20.19" id="fn-20.19"></a> <a href="#fnref-20.19">[19]</a> +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. +</p> + +<p>The cleavage of the homogeneous schists, the foliation of those +composed of more or less distinct minerals in layers, and the +planes of alternation of the different varieties or so-called +stratification, are all parallel, and preserve over this 200 miles +of coast a remarkable degree of uniformity in direction. At the +northern end of the group, at Low’s Harbour, the well-defined folia +of mica-schist everywhere ranged within eight degrees (or less than +one point of the compass) of N. 19° 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 name="page431"></a> +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.</p> + +<p> +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,<a href="#fn-20.20" name="fnref-20.20" +id="fnref-20.20"><sup>[20]</sup></a> 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 +<a name="page432"></a> +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<a href="#fn-20.21" +name="fnref-20.21" id="fnref-20.21"><sup>[21]</sup></a> 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.<a +href="#fn-20.22" name="fnref-20.22" id="fnref-20.22"><sup>[22]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-20.20" id="fn-20.20"></a> <a href="#fnref-20.20">[20]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-20.21" id="fn-20.21"></a> <a href="#fnref-20.21">[21]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-20.22" id="fn-20.22"></a> <a href="#fnref-20.22">[22]</a> +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. +</p> + +<p>Dikes are frequent in the metamorphic schists of the Chonos +Islands, +<a name="page433"></a> +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.</p> + +<p> +<i>Chiloe, Valdivia, Concepcion.</i>—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 +<a name="page434"></a> +glossy clay-slate had its cleavage often slightly curvilinear, and inclined, +seldom at a high angle, towards various points of the compass:<a +href="#fn-20.23" name="fnref-20.23" id="fnref-20.23"><sup>[23]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-20.23" id="fn-20.23"></a> <a href="#fnref-20.23">[23]</a> +I observed in some parts that the tops of the laminæ of the clay-slate +(<i>b</i> of the diagram) under the superficial detritus and soil (<i>a</i>) +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 (<i>London Phil. Mag.,</i> 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. +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono23.jpg" width="177" height="87" alt="[Illustration: +Diagram described in note 23.]" /> +</div> + +<p> +At the northern end of Quiriquina Island, in the Bay of Concepcion, at least +eight rudely parallel dikes, which have been guided to a certain extent by the +cleavage of the slate, occur within the space of a quarter of a mile. They vary +much in composition, resembling in many respects the dikes at Low’s +Harbour: the greater number consist of feldspathic porphyries, sometimes +containing grains of quartz: one, however, was black and brilliant, like an +augitic rock, but really formed of feldspar; others of a feldspathic nature +were perfectly white, with either an earthy or crystalline fracture, and +including grains and regular octagons of quartz; these white varieties passed +into ordinary greenstones. Although, both here and at Low’s Harbour, the +nature of the rock varied considerably in the same dike, yet I cannot but think +that at these two places and in other parts of the Chonos group, where the +dikes, though close to each other and running parallel, are of different +composition, that they must have been formed at different periods. In the case +of Quiriquina this is a rather interesting conclusion, for these eight parallel +dikes cut through the metamorphic schists in a N.W. and S.E. line, and since +their injection the overlying cretaceous or tertiary strata have been tilted +(whilst still under the sea) from a N.W. by N. and S.E. by S. line; and again, +during the great earthquake of February 1835, the ground in this neighbourhood +was fissured in N.W. and S.E. lines; and from the manner in which buildings +were +<a name="page435"></a> +thrown down, it was evident that the surface undulated in this same +direction.<a href="#fn-20.24" name="fnref-20.24" +id="fnref-20.24"><sup>[24]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-20.24" id="fn-20.24"></a> <a href="#fnref-20.24">[24]</a> +“Geolog. Trans.,” vol. vi, pp. 602 and 617. “Journal of +Researches” (2nd edit.), p. 307. +</p> + +<p><i>Central and Northern Chile.</i>—Northward of +Concepcion, as far as Copiapo, the shores of the Pacific consist, +with the exception of some small tertiary basins, of gneiss, +mica-schist, altered clay-slate, granite, greenstone and syenite: +hence the coast from Tres Montes to Copiapo, a distance of 1,200 +miles, and I have reason to believe for a much greater space, is +almost similarly constituted.</p> + +<p> +Near Valparaiso the prevailing rock is gneiss, generally including much +hornblende: concretionary balls formed of feldspar, hornblende and mica, from +two or three feet in diameter, are in very many places conformably enfolded by +the foliated gneiss: veins of quartz and feldspar, including black schorl and +well-crystallised epidote, are numerous. Epidote likewise occurs in the gneiss +in thin layers, parallel to the foliation of the mass. One large vein of a +coarse granitic character was remarkable from in one part quite changing its +character, and insensibly passing into a blackish porphyry, including acicular +crystals of glassy feldspar and of hornblende: I have never seen any other such +case.<a href="#fn-20.25" name="fnref-20.25" +id="fnref-20.25"><sup>[25]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-20.25" id="fn-20.25"></a> <a href="#fnref-20.25">[25]</a> +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. +</p> + +<p>I shall in the few following remarks on the rocks of Chile +allude exclusively to their foliation and cleavage. In the gneiss +round Valparaiso the strike of the foliation is very variable, but +I think about N. by W. and S. by E. is the commonest direction; +this likewise holds good with the cleavage of the altered +feldspathic clay-slates, occasionally met with on the coast for +ninety miles north of Valparaiso. Some feldspathic slate, +alternating with strata of claystone porphyry in the Bell of +Quillota and at Jajuel, and therefore, perhaps, belonging to a +later period than the metamorphic schists on the coast, cleaved in +this same direction. In the Eastern Cordillera, in the Portillo +Pass, there is a grand mass of mica-slate, foliated in a north and +south line, and with a high westerly dip: in the Uspallata range, +clay-slate and grauwacke have a highly inclined, nearly north and +south cleavage, though in some parts the strike is irregular: in +the main or Cumbre range, the direction of the cleavage in the +feldspathic clay-slate is N.W. and S.E.</p> + +<p>Between Coquimbo and Guasco there are two considerable +formations of mica-slate, in one of which the rock passed sometimes +into common clay-slate and sometimes into a glossy black variety, +very like that in the Chonos Archipelago. The folia and cleavage of +these rocks ranged between [N. and N.W. by N.] and [S. and S.W. by +S.]. Near the Port of Guasco several varieties of altered +clay-slate have a quite irregular cleavage. Between Guasco and +Copiapo, there are some siliceous and talcaceous slates cleaving in +a north and south line, with an easterly dip of between 60° and +70°: high up, also, the main valley of Copiapo, there is +mica-slate with a high easterly dip. In the whole space +<a name="page436"></a> +between Valparaiso and Copiapo an easterly dip is much more +common than an opposite or westerly one.</p> + +<h4><i>Concluding Remarks on Cleavage and Foliation.</i></h4> + +<p> +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,<a href="#fn-20.26" +name="fnref-20.26" id="fnref-20.26"><sup>[26]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-20.26" id="fn-20.26"></a> <a href="#fnref-20.26">[26]</a> +In my paper on the Falkland Islands (<i>Geological Journal,</i> 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. +</p> + +<p> +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,<a href="#fn-20.27" name="fnref-20.27" +id="fnref-20.27"><sup>[27]</sup></a> 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.”<a href="#fn-20.28" +name="fnref-20.28" id="fnref-20.28"><sup>[28]</sup></a> In some of those +<a name="page437"></a> +glossy and hard varieties of clay-slate, which may often be seen passing into +mica-schist, it has appeared to me that the cleavage-planes were formed of +excessively thin, generally slighted convoluted, folia, composed of +microscopically minute scales of mica. From these several facts, and more +especially from the case of the clay-slate in Tierra del Fuego, it must, I +think, be concluded, that the same power which has impressed on the slate its +fissile structure or cleavage has tended to modify its mineralogical character +in parallel planes. +</p> + +<p class="footnote"> +<a name="fn-20.27" id="fn-20.27"></a> <a href="#fnref-20.27">[27]</a> +<i>London Phil. Mag.</i>, vol. xxi, p. 182. +</p> + +<p class="footnote"> +<a name="fn-20.28" id="fn-20.28"></a> <a href="#fnref-20.28">[28]</a> +“Geological Trans.,” vol. iii, p. 471. +</p> + +<p> +Let us now turn to the foliation of the metamorphic schists, a subject which +has been much less attended to. As in the case of cleavage-laminæ, the folia +preserve over very large areas a uniform strike: thus Humboldt<a +href="#fn-20.29" name="fnref-20.29" id="fnref-20.29"><sup>[29]</sup></a> 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,<a href="#fn-20.30" name="fnref-20.30" +id="fnref-20.30"><sup>[30]</sup></a> 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.<a href="#fn-20.31" name="fnref-20.31" +id="fnref-20.31"><sup>[31]</sup></a> On the flanks of +<a name="page438"></a> +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,<a href="#fn-20.32" name="fnref-20.32" +id="fnref-20.32"><sup>[32]</sup></a> 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.<a href="#fn-20.33" +name="fnref-20.33" id="fnref-20.33"><sup>[33]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-20.29" id="fn-20.29"></a> <a href="#fnref-20.29">[29]</a> +“Personal Narrative,” vol. vi, p. 59 <i>et seq.</i> +</p> + +<p class="footnote"> +<a name="fn-20.30" id="fn-20.30"></a> <a href="#fnref-20.30">[30]</a> +Cases are given by Mr. Jukes in his “Geology of Newfoundland,” p. +130. +</p> + +<p class="footnote"> +<a name="fn-20.31" id="fn-20.31"></a> <a href="#fnref-20.31">[31]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-20.32" id="fn-20.32"></a> <a href="#fnref-20.32">[32]</a> +Studer in <i>Edin. New Phil. Journal,</i> vol. xxiii, p. 144. +</p> + +<p class="footnote"> +<a name="fn-20.33" id="fn-20.33"></a> <a href="#fnref-20.33">[33]</a> +I have given a case in Australia. See my “Volcanic Islands.” +</p> + +<p> +With respect to the origin of the folia of quartz, mica, feldspar, and other +minerals composing the metamorphic schists, Professor Sedgwick, Mr. Lyell, and +most authors believe, that the constituent parts of each layer were separately +deposited as sediment, and then metamorphosed. This view, in the majority of +cases, I believe to be quite untenable. In those not uncommon instances, where +a mass of clay-slate, in approaching granite, gradually passes into gneiss,<a +href="#fn-20.34" name="fnref-20.34" id="fnref-20.34"><sup>[34]</sup></a> 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.<a href="#fn-20.35" +name="fnref-20.35" id="fnref-20.35"><sup>[35]</sup></a> 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; +<a name="page439"></a> +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? +</p> + +<p class="footnote"> +<a name="fn-20.34" id="fn-20.34"></a> <a href="#fnref-20.34">[34]</a> +I have described in “Volcanic Islands” a good instance of such a +passage at the Cape of Good Hope. +</p> + +<p class="footnote"> +<a name="fn-20.35" id="fn-20.35"></a> <a href="#fnref-20.35">[35]</a> +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 +<i>Silliman’s American Journ.,</i> vol. xlv, p. 108. +</p> + +<p>Seeing, then, that foliated schists indisputably are sometimes +produced by the metamorphosis of homogeneous fissile rocks; seeing +that foliation and cleavage are so closely analogous in the several +above-enumerated respects; seeing that some fissile and almost +homogeneous rocks show incipient mineralogical changes along the +planes of their cleavage, and that other rocks with a fissile +structure alternate with, and pass into varieties with a foliated +structure, I cannot doubt that in most cases foliation and cleavage +are parts of the same process: in cleavage there being only an +incipient separation of the constituent minerals; in foliation a +much more complete separation and crystallisation.</p> + +<p> +The fact often referred to in this chapter, of the foliation and the so-called +strata in the metamorphic series,—that is, the alternating masses of +different varieties of gneiss, mica-schist, and hornblende-slate, +etc.,—being parallel to each other, at first appears quite opposed to the +view, that the folia have no relation to the planes of original deposition. +Where the so-called beds are not very thick and of widely different +mineralogical composition from each other, I do not think that there is any +difficulty in supposing that they have originated in an analogous manner with +the separate folia. We should bear in mind what thick strata, in ordinary +sedimentary masses, have obviously been formed by a concretionary process. In a +pile of volcanic rocks on the Island of Ascension, there are strata, differing +quite as much in appearance as the ordinary varieties of the metamorphic +schists, which undoubtedly have been produced, not by successive flowings of +lava, but by internal molecular changes. Near Monte Video, where the +stratification, as it would be called, of the metamorphic series is, in most +parts, particularly well developed, being as usual, parallel to the foliation, +we have seen that a mass of chloritic schist, netted with quartz-veins, is +entangled in gneiss, in such a manner as to show that it had certainly +originated in some process of segregation: again, in another spot, the gneiss +tended to pass into hornblendic schist by alternating with layers of quartz; +but these layers of quartz almost certainly had never been separately +deposited, for they were absolutely continuous with the numerous intersecting +veins of quartz. I have never had an opportunity of tracing for any distance, +along the line both of strike and of dip, the so-called beds in the metamorphic +schists, but I strongly suspect that they would not be found to extend with the +same character, very far in the line either of their dip or strike. Hence I am +led to believe, that most of the so-called beds are of the nature of complex +folia, and have not been separately deposited. Of course, this view cannot be +extended to <i>thick</i> 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 +<a name="page440"></a> +strata.<a href="#fn-20.36" name="fnref-20.36" +id="fnref-20.36"><sup>[36]</sup></a> Such strata, however, will almost always +strike in the same direction with the folia, owing to the axes of elevation +being in most countries parallel to the strike of the foliation; but they will +generally dip at a different angle from that of the foliation; and the angle of +the foliation in itself almost always varies much: hence, in crossing a +metamorphosed schistose district, it would require especial attention to +discriminate between true strata of deposition and complex foliated masses. The +mere presence of true strata in the midst of a set of metamorphic schists, is +no argument that the foliation is of sedimentary origin, without it be further +shown in each case, that the folia not only strike, but dip throughout in +parallel planes with those of the true stratification. +</p> + +<p class="footnote"> +<a name="fn-20.36" id="fn-20.36"></a> <a href="#fnref-20.36">[36]</a> +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!” +</p> + +<p> +As in some cases it appears that where a fissile rock has been exposed to +partial metamorphic action, for instance from the irruption of granite, the +foliation has supervened on the already existing cleavage-planes; so perhaps in +some instances, the foliation of a rock may have been determined by the +original planes of deposition or of oblique current-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.<a href="#fn-20.37" name="fnref-20.37" +id="fnref-20.37"><sup>[37]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-20.37" id="fn-20.37"></a> <a href="#fnref-20.37">[37]</a> +As remarked in a former part of this chapter, I suspect that the boldly conical +mountains of gneiss-granite, near Rio de Janeiro, in which the constituent +minerals are arranged in parallel planes, are of intrusive origin. We must not, +however, forget the lesson of caution taught by the curious claystone +porphyries of Port Desire, in which we have seen that the breaking up and +aggregation of a thinly stratified tufaceous mass, has yielded a rock +semi-porphyritic with crystals of feldspar, arranged in the planes of original +deposition. +</p> + +<p> +Finally, we have seen that the planes of cleavage and of foliation, that is, of +the incipient process and of the final result, generally strike parallel to the +principal axes of elevation, and to the outline of the land: the strike of the +axes of elevation (that is, of the lines of fissures with the strata on their +edges upturned), according to the reasoning of Mr. Hopkins, is determined by +the form of the area undergoing changes of level, and the consequent direction +of the lines of tension and fissure. Now, in that remarkable pile of volcanic +rocks at Ascension, which has +<a name="page441"></a> +several times been alluded to (and in some other cases), I have endeavoured to +show,<a href="#fn-20.38" name="fnref-20.38" +id="fnref-20.38"><sup>[38]</sup></a> 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, <i>after</i> the main fissures or axes of upheavement had +been formed, but <i>before</i> the final consolidation of the mass and the +total cessation of all molecular movement. +</p> + +<p class="footnote"> +<a name="fn-20.38" id="fn-20.38"></a> <a href="#fnref-20.38">[38]</a> +In “Volcanic Islands.” +</p> + +<div class="fig" style="width:100%;"> +<img src="images/plate5.jpg" width="682" height="240" alt="[Illustration: +Geological sections through the Cordilleras.]" /> +</div> + +<p class="letter"> +For enlargements of the above plate use the following links:<br/> +<a href="images/plate5a.jpg">left section</a><br/> +<a href="images/plate5b.jpg">center section</a><br/> +<a href="images/plate5c.jpg">right section</a><br/> +</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="chap3.07"></a>Chapter VII<br/>CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.</h2> + +<p class="letter"> +Central Chile.—Basal formations of the Cordillera.—Origin of the +porphyritic clay-stone conglomerate.—Andesite.—Volcanic +rocks.—Section of the Cordillera by the Peuquenes are Portillo +Pass.—Great gypseous formation.—Peuquenes line; thickness of +strata, fossils of.—Portillo line.—Conglomerate, orthitic granite, +mica-schist, volcanic rocks of.—Concluding remarks on the denudation and +elevation of the Portillo line.—Section by the Cumbre, or Uspallata +Pass.—Porphyries.—Gypseous strata.—Section near the Puente +del Inca; fossils of.—Great subsidence.—Intrusive +porphyries.—Plain of Uspallata.—Section of the Uspallata +chain.—Structure and nature of the strata.—Silicified vertical +trees.—Great subsidence.—Granitic rocks of axis.—Concluding +remarks on the Uspallata range; origin subsequent to that of the main +Cordillera; two periods of subsidence; comparison with the Portillo chain. +</p> + +<p> +The district between the Cordillera and the Pacific, on a rude average, is from +about eighty to one hundred miles in width. It is crossed by many chains of +mountains, of which the principal ones, in the latitude of Valparaiso and +southward of it, range nearly north and south; but in the more northern parts +of the province, they run in almost every possible direction. Near the Pacific, +the mountain-ranges are generally +<a name="page442"></a> +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,<a href="#fn-21.1" name="fnref-21.1" +id="fnref-21.1"><sup>[1]</sup></a> and Aconcagua) are formed partly of granitic +rocks, and partly of purple porphyritic conglomerates, claystone porphyry, +greenstone porphyry, and other rocks, such as we shall immediately see, form +the basal strata of the main Cordillera. In the more northern parts of Chile, +this porphyritic series extends over large tracts of country far from the +Cordillera; and even in Central Chile such occasionally occur in outlying +positions. +</p> + +<p class="footnote"> +<a name="fn-21.1" id="fn-21.1"></a> <a href="#fnref-21.1">[1]</a> +Meyen “Reise um Erde” Th. I, s. 235. +</p> + +<p>I will describe the Campana of Quillota, which stands only +fifteen miles from the Pacific, as an instance of one of these +outlying masses. This hill is conspicuous from rising to the height +of 6,400 feet: its summit shows a nucleus, uncovered for a height +of 800 feet, of fine greenstone, including epidote and octahedral +magnetic iron ore; its flanks are formed of great strata of +porphyritic claystone conglomerate associated with various true +porphyries and amygdaloids, alternating with thick masses of a +highly feldspathic, sometimes porphyritic, pale-coloured slaty +rock, with its cleavage-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.</p> + +<p>In Central Chile, the flanks of the main Cordillera, into which +I penetrated by four different valleys, generally consist of +distinctly stratified rocks. The strata are inclined at angles +varying from sometimes even under ten, to twenty degrees, very +rarely exceeding forty degrees: in some, however, of the quite +small, exterior, spur-like ridges, the inclination was not +unfrequently greater. The dip of the strata in the main outer lines +was usually outwards or from the Cordillera, but in Northern Chile +frequently inwards,—that is, their basset-edges fronted the +Pacific. Dikes occur in extraordinary numbers. In the great, +central, loftiest ridges, the strata, as we shall presently see, +are almost always highly inclined and often vertical. Before giving +a detailed account of my two sections across the Cordillera, it +will, I think, be convenient to describe the basal strata as seen, +often to a thickness of four or five thousand feet, on the flanks +of the outer lines.</p> + +<p> +<i>Basal strata of the Cordillera.</i>—The prevailing rock is a purplish +or greenish, porphyritic claystone conglomerate. The embedded fragments +<a name="page443"></a> +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;<a href="#fn-21.2" name="fnref-21.2" +id="fnref-21.2"><sup>[2]</sup></a> in some parts the rounded, in others the +angular fragments prevail, and usually both kinds are mixed together: hence the +word <i>breccia</i> ought strictly to be appended to the term <i>porphyritic +conglomerate.</i> The fragments consist of many varieties of claystone +porphyry, usually of nearly the same colour with the surrounding basis, namely, +purplish-reddish, brownish, mottled or bright green; occasionally fragments of +a laminated, pale-coloured, feldspathic rock, like altered clay-slate are +included; as are sometimes grains of quartz, but only in one instance in +Central Chile (namely, at the mines of Jajuel) a few pebbles of quartz. I +nowhere observed mica in this formation, and rarely hornblende; where the +latter mineral did occur, I was generally in doubt whether the mass really +belonged to this formation, or was of intrusive origin. Calcareous spar +occasionally occurs in small cavities; and nests and layers of epidote are +common. In some few places in the finer-grained varieties (for instance, at +Quillota), there were short, interrupted layers of earthy feldspar, which could +be traced, exactly as at Port Desire, passing into large crystals of feldspar: +I doubt, however, whether in this instance the layers had ever been separately +deposited as tufaceous sediment. +</p> + +<p class="footnote"> +<a name="fn-21.2" id="fn-21.2"></a> <a href="#fnref-21.2">[2]</a> +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.) +</p> + +<p> +All the varieties of porphyritic conglomerates and breccias pass into each +other, and by innumerable gradations into porphyries no longer retaining the +least trace of mechanical origin: the transition appears to have been effected +much more easily in the finer-grained, than in the coarser-grained varieties. +In one instance, near Cauquenes, I noticed that a porphyritic conglomerate +assumed a spheroidal structure, and tended to become columnar. Besides the +porphyritic conglomerates and the perfectly characterised porphyries, of +metamorphic origin, there are other porphyries, which, though differing not at +all or only slightly in composition, certainly have had a different origin: +these consist of pink or purple claystone porphyries, sometimes including +grains of quartz,—of greenstone porphyry, and of other dusky rocks, all +generally porphyritic with fine, large, tabular, opaque crystals, often placed +crosswise, of feldspar cleaving like albite (judging from several +measurements), and often amygdaloidal with silex, agate, carbonate of +<a name="page444"></a> +lime, green and brown bole.<a href="#fn-21.3" name="fnref-21.3" +id="fnref-21.3"><sup>[3]</sup></a> These several porphyritic and amygdaloidal +varieties never show any signs of passing into masses of sedimentary origin: +they occur both in great and small intrusive masses, and likewise in strata +alternating with those of the porphyritic conglomerate, and with the planes of +junction often quite distinct, yet not seldom blended together. In some of +these intrusive masses, the porphyries exhibit, more or less plainly, a +brecciated structure, like that often seen in volcanic masses. These brecciated +porphyries could generally be distinguished at once from the metamorphosed, +porphyritic breccia-conglomerates, by all the fragments being angular and being +formed of the same variety, and by the absence of every trace of aqueous +deposition. One of the porphyries above specified, namely, the greenstone +porphyry with large tabular crystals of albite, is particularly abundant, and +in some parts of the Cordillera (as near St. Jago) seemed more common even than +the purplish porphyritic conglomerate. Numerous dikes likewise consist of this +greenstone porphyry; others are formed of various fine-grained trappean rocks; +but very few of claystone porphyry: I saw no true basaltic dikes. +</p> + +<p class="footnote"> +<a name="fn-21.3" id="fn-21.3"></a> <a href="#fnref-21.3">[3]</a> +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 +(<i>Edin. New Phil. Journ.,</i> 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.<br/> + 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. +</p> + +<p> +In several places in the lower part of the series, but not everywhere, thick +masses of a highly feldspathic, often porphyritic, slaty rock occur +interstratified with the porphyritic conglomerate; I believe in one or two +cases blackish limestone has been found in a similar position. The feldspathic +rock is of a pale grey or greenish colour; it is easily fusible; +<a name="page445"></a> +where porphyritic, the crystals of feldspar are generally small and vitreous: +it is distinctly laminated, and sometimes includes parallel layers of +epidote;<a href="#fn-21.4" name="fnref-21.4" id="fnref-21.4"><sup>[4]</sup></a> +the lamination appears to be distinct from stratification. Occasionally this +rock is somewhat curious; and at one spot, namely, at the C. of Quillota, it +had a brecciated structure. Near the mines of Jajuel, in a thick stratum of +this feldspathic, porphyritic slate, there was a layer of hard, blackish, +siliceous, infusible, compact clay-slate, such as I saw nowhere else; at the +same place I was able to follow for a considerable distance the junction +between the slate and the conformably underlying porphyritic conglomerate, and +they certainly passed gradually into each other. Wherever these slaty +feldspathic rocks abound, greenstone seems common; at the C. of Quillota a bed +of well-crystallised greenstone lay conformably in the midst of the feldspathic +slate, with the upper and lower junctions passing insensibly into it. From this +point, and from the frequently porphyritic condition of the slate, I should +perhaps have considered this rock as an erupted one (like certain laminated +feldspathic lavas in the trachytic series), had I not seen in Tierra del Fuego +how readily true clay-slate becomes feldspathic and porphyritic, and had I not +seen at Jajuel the included layer of black, siliceous clay-slate, which no one +could have thought of igneous origin. The gentle passage of the feldspathic +slate, at Jajuel, into the porphyritic conglomerate, which is certainly of +aqueous origin, should also be taken in account. +</p> + +<p class="footnote"> +<a name="fn-21.4" id="fn-21.4"></a> <a href="#fnref-21.4">[4]</a> +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. +</p> + +<p> +The alternating strata of porphyries and porphyritic conglomerate, and with the +occasionally included beds of feldspathic slate, together make a grand +formation; in several places within the Cordillera, I estimated its thickness +at from six to seven thousand feet. It extends for many hundred miles, forming +the western flank of the Chilean Cordillera; and even at Iquique in Peru, 850 +miles north of the southernmost point examined by me in Chile, the +coast-escarpment which rises to a height of between two and three thousand feet +is thus composed. In several parts of Northern Chile this formation extends +much further towards the Pacific, over the granitic and metamorphic lower +rocks, than it does in Central Chile; but the main Cordillera may be considered +as its central line, and its breadth in an east and west direction is never +great. At first the origin of this thick, massive, long but narrow formation, +appeared to me very anomalous: whence were derived, and how were dispersed the +innumerable fragments, often of large size, sometimes angular and sometimes +rounded, and almost invariably composed of porphyritic rocks? Seeing that the +interstratified porphyries are never vesicular and often not even amygdaloidal, +we must conclude that the pile was formed in deep water; how then came so many +fragments to be well rounded and so many to remain angular, sometimes the two +kinds being equally mingled, sometimes one and sometimes the other +preponderating? That the claystone, +<a name="page446"></a> +greenstone, and other porphyries and amygdaloids, which lie <i>conformably</i> +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<a href="#fn-21.5" +name="fnref-21.5" id="fnref-21.5"><sup>[5]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-21.5" id="fn-21.5"></a> <a href="#fnref-21.5">[5]</a> +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. +</p> + +<p>This whole great pile of rock has suffered much metamorphic +action, as is very obvious in the gradual formation and appearance +of the crystals of albitic feldspar and of epidote—in the +bending together of the fragments—in the appearance of a +laminated structure in the feldspathic slate—and, lastly, in +the disappearance of the planes of stratification, which could +sometimes be seen on the same mountain quite distinct in the upper +part, less and less plain on the flanks, and quite obliterated at +the base. Partly owing to this metamorphic action, and partly to +the close relationship in origin, I have seen fragments of +porphyries—taken from a metamorphosed conglomerate—from +a neighbouring stream of lava—from the nucleus or centre (as +it appeared to me) of the whole submarine volcano—and lastly +from an intrusive mass of quite subsequent origin, all of which +were absolutely undistinguishable in external characters.</p> + +<p> +One other rock, of plutonic origin, and highly important in the history of the +Cordillera, from having been injected in most of the great axes of elevation, +and from having apparently been instrumental in metamorphosing the +superincumbent strata, may be conveniently described in this preliminary +discussion. It has been called by some authors <i>Andesite</i>: it mainly +consists of well-crystallised white albite<a href="#fn-21.6" name="fnref-21.6" +id="fnref-21.6"><sup>[6]</sup></a> (as determined with the goniometer in +numerous specimens both by +<a name="page447"></a> +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<a +href="#fn-21.7" name="fnref-21.7" id="fnref-21.7"><sup>[7]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-21.6" id="fn-21.6"></a> <a href="#fnref-21.6">[6]</a> +I here, and elsewhere, call by this name, those feldspathic minerals which +cleave like albite: but it now appears (<i>Edin. New Phil. Journal.,</i> 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 <i>Andesine.</i> 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. +</p> + +<p class="footnote"> +<a name="fn-21.7" id="fn-21.7"></a> <a href="#fnref-21.7">[7]</a> +<i>Geograph. Journal,</i> vol. ix, p. 510. +</p> + +<p> +The few still active volcanoes in Chile are confined to the central and +loftiest ranges of the Cordillera; and volcanic matter, such as appears to have +been of subaerial eruption, is everywhere rare. According to Meyen,<a +href="#fn-21.8" name="fnref-21.8" id="fnref-21.8"><sup>[8]</sup></a> 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 +<a name="page448"></a> +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 <i>vesicular</i> rock, approaching in character to +trachyte; in no other part of Chile did I observe vesicular or amygdaloidal +dikes, though these are so common in ordinary volcanic districts. +</p> + +<p class="footnote"> +<a name="fn-21.8" id="fn-21.8"></a> <a href="#fnref-21.8">[8]</a> +“Reise um Erde,” Th. I, ss. 338 and 362. +</p> + +<h4><i>Passage of the Andes by the Portillo or Pequenes Pass.</i></h4> + +<p> +Although I crossed the Cordillera only once by this pass, and only once by that +of the Cumbre or Uspallata (presently to be described), riding slowly and +halting occasionally to ascend the mountains, there are many circumstances +favourable to obtaining a more faithful sketch of their structure than would at +first be thought possible from so short an examination. The mountains are steep +and absolutely bare of vegetation; the atmosphere is resplendently clear; the +stratification distinct; and the rocks brightly and variously coloured: some of +the natural sections might be truly compared for distinctness to those coloured +ones in geological works. Considering how little is known of the structure of +this gigantic range, to which I particularly attended, most travellers having +collected only specimens of the rocks, I think my sketch-sections, though +necessarily imperfect, possess some interest. <a href="images/plate5.jpg">Plate +V</a> sections (between <a href="#page440"></a> 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 <i> +exceedingly</i> 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,<a href="#fn-21.9" name="fnref-21.9" +id="fnref-21.9"><sup>[9]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-21.9" id="fn-21.9"></a> <a href="#fnref-21.9">[9]</a> +<i>Journal of Nat. and Geograph. Science,</i> August 1830. +</p> + +<p>The Cordillera here, and indeed I believe throughout Chile, +consist of several parallel, anticlinal and uniclinal +mountain-lines, ranging north, or north with a little westing, and +south. Some exterior and much lower ridges often vary considerably +from this course, projecting like oblique spurs from the main +ranges: in the district towards the Pacific, the mountains, as +before remarked, extend in various directions, even east and west. +In the main exterior lines, the strata, as also before remarked, +are seldom inclined at a high angle; but in the central lofty +ridges they are almost always highly inclined, broken by many +<a name="page449"></a> +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.</p> + +<p>The valleys, by which the Cordillera are drained, follow the +anticlinal or rarely synclinal troughs, which deviate most from the +usual north and south course; or still more commonly those lines of +faults or of unequal curvature (that is, lines with the strata on +both hands dipping in the same direction, but at a somewhat +different angle) which deviate most from a northerly course. +Occasionally the torrents run for some distance in the north and +south valleys, and then recover their eastern or western course by +bursting through the ranges at those points where the strata have +been least inclined and the height consequently is less. Hence the +valleys, along which the roads run, are generally zigzag; and, in +drawing an east and west section, it is necessary to contract +greatly that which is actually seen on the road.</p> + +<p> +Commencing at the western end of 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<a +href="#fn-21.10" name="fnref-21.10" id="fnref-21.10"><sup>[10]</sup></a> met +with a hill +<a name="page450"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-21.10" id="fn-21.10"></a> <a href="#fnref-21.10">[10]</a> +“Reise um Erde,” Th. I, ss. 338, 341.) +</p> + +<p>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.</p> + +<p>In the valley basin of the Yeso, which I estimated at 7,000 feet +above the level of the sea, we first reach at [F] the gypseous +formation. Its thickness is very great. It consists in most parts +of snow-white, hard, compact gypsum, which breaks with a saccharine +fracture, having translucent edges; under the blowpipe gives out +much vapour; it frequently includes nests and exceedingly thin +layers of crystallised, blackish carbonate of lime. Large, +irregularly shaped concretions (externally still exhibiting lines +of aqueous deposition) of blackish-grey, but sometimes white, +coarsely and brilliantly crystallised, hard anhydrite, abound +within the common gypsum. Hillocks, formed of the hardest and +purest varieties of the white gypsum, stand up above the +surrounding parts, and have their surfaces cracked and marked, just +like newly baked bread. There is much pale brown, soft +argillaceous +<a name="page451"></a> +gypsum; and there were some intercalated green beds which I had +not time to reach. I saw only one fragment of selenite or +transparent gypsum, and that perhaps may have come from some +subsequently formed vein. From the mineralogical characters here +given, it is probable that these gypseous beds have undergone some +metamorphic action. The strata are much hidden by detritus, but +they appeared in most parts to be highly inclined; and in an +adjoining lofty pinnacle they could be distinctly seen bending up, +and becoming vertical, conformably with the underlying porphyritic +conglomerate. In very many parts of the great mountain-face [F], +composed of thin gypseous beds, there were innumerable masses, +irregularly shaped and not like dikes, yet with well-defined edges, +of an imperfectly granular, pale greenish, or yellowish-white rock, +essentially composed of feldspar, with a little chlorite or +hornblende, epidote, iron-pyrites, and ferruginous powder: I +believe that these curious trappean masses have been injected from +the not far distant mountain-mass [E] of andesite whilst still +fluid, and that owing to the softness of the gypseous strata they +have not acquired the ordinary forms of dikes. Subsequently to the +injection of these feldspathic rocks, a great dislocation has taken +place; and the much shattered gypseous strata here overlie a +hillock [G], composed of vertical strata of impure limestone and of +black highly calcareous shale including threads of gypsum: these +rocks, as we shall presently see, belong to the upper parts of the +gypseous series, and hence must here have been thrown down by a +vast fault.</p> + +<p>Proceeding up the valley-basin of the Yeso, and taking our +section sometimes on one hand and sometimes on the other, we come +to a great hill of stratified porphyritic conglomerate [H] dipping +at 45° to the west; and a few hundred yards farther on, we have +a bed between three or four hundred feet thick of gypsum [I] +dipping eastward at a very high angle: here then we have a fault +and anticlinal axis. On the opposite side of the valley, a vertical +mass of red conglomerate, conformably underlying the gypsum, +appears gradually to lose its stratification and passes into a +mountain of porphyry. The gypsum [I] is covered by a bed [K], at +least 1,000 feet in thickness, of a purplish-red, compact, heavy, +fine-grained sandstone or mudstone, which fuses easily into a white +enamel, and is seen under a lens to contain triturated crystals. +This is succeeded by a bed [L], 1,000 feet thick (I believe I +understate the thickness) of gypsum, exactly like the beds before +described; and this again is capped by another great bed [M] of +purplish-red sandstone. All these strata dip eastward; but the +inclination becomes less and less, as we leave the first and almost +vertical bed [I] of gypsum.</p> + +<p>Leaving the basin-plain of Yeso, the road rapidly ascends, +passing by mountains composed of the gypseous and associated beds, +with their stratification greatly disturbed and therefore not +easily intelligible: hence this part of the section has been left +uncoloured. Shortly before reaching the great Pequenes ridge, the +lowest stratum visible [N] is a red sandstone or mudstone, capped +by a vast thickness of black, compact, calcareous, shaly rock [O], +which has been thrown into four lofty, +<a name="page452"></a> +though small ridges: looking northward, the strata in these +ridges are seen gradually to rise in inclination, becoming in some +distant pinnacles absolutely vertical.</p> + +<p>The ridge of Pequenes, which divides the waters flowing into the +Pacific and Atlantic Oceans, extends in a nearly N.N.W. and S.S.E. +line; its strata dip eastward at an angle of between 30° and +45°, but in the higher peaks bending up and becoming almost +vertical. Where the road crosses this range, the height is 13,210 +feet above the sea-level, and I estimated the neighbouring +pinnacles at from fourteen to fifteen thousand feet. The lowest +stratum visible in this ridge is a red stratified sandstone [P]; on +it are superimposed two great masses [Q and S] of black, hard, +compact, even having a conchoidal fracture, calcareous, more or +less laminated shale, passing into limestone: this rock contains +organic remains, presently to be enumerated. The compacter +varieties fuse easily in a white glass; and this I may add is a +very general character with all the sedimentary beds in the +Cordillera: although this rock when broken is generally quite +black, it everywhere weathers into an ash-grey tint. Between these +two great masses [Q and S], a bed [R] of gypsum is interposed, +about three hundred feet in thickness, and having the same +characters as heretofore described. I estimated the total thickness +of these three beds [Q, R, S] at nearly three thousand feet; and to +this must be added, as will be immediately seen, a great overlying +mass of red sandstone.</p> + +<p>In descending the eastern slope of this great central range, the +strata, which in the upper part dip eastward at about an angle of +40°, 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 +<a name="page453"></a> +the gypseous and intercalated beds are lens-shaped, and that +they thin out, even where very thick, and disappear in short +horizontal distances: it is quite possible that the black shales +and red sandstones may be contemporaneous, but it is more probable +that the former compose the uppermost parts of the series.</p> + +<p>The fossils above alluded to in the black calcareous shales are +few in number, and are in an imperfect condition; they consist, as +named for me by M. d’Orbigny, of:—</p> + +<ol> +<li>Ammonite, indeterminable, near to <i>A. recticostatus,</i> +d’Orbigny, “Pal. Franc.” (Neocomian formation).</li> + +<li>Gryphæa, near to <i>G. Couloni</i> (Neocomian formations +of France and Neufchâtel).</li> + +<li>Natica, indeterminable.</li> + +<li>Cyprina rostrata, d’Orbigny, “Pal. Franc.” (Neocomian +formation).</li> + +<li>Rostellaria angulosa (?), d’Orbigny, “Pal. de l’Amér. +Mer.”</li> + +<li>Terebratula (?).</li> +</ol> + +<p> +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,<a href="#fn-21.11" name="fnref-21.11" +id="fnref-21.11"><sup>[11]</sup></a> 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,<a href="#fn-21.12" +name="fnref-21.12" id="fnref-21.12"><sup>[12]</sup></a> of:— +</p> + +<ol> +<li>Exogyra (Gryphæa) Couloni, absolutely identical with +specimens from the Jura and South of France.</li> + +<li>Trigonia costata, identical with those found in the upper +Jurassic beds at Hildesheim.</li> + +<li>Pecten striatus, identical with those found in the upper +Jurassic beds at Hildesheim.</li> + +<li>Cucullæa, corresponding in form to <i>C. +longirostris,</i> so frequent in the upper Jurassic beds of +Westphalia.</li> + +<li>Ammonites resembling <i>A. biplex.</i></li> +</ol> + +<p class="footnote"> +<a name="fn-21.11" id="fn-21.11"></a> <a href="#fnref-21.11">[11]</a> +“Reise um Erde,” etc., Th. I, s. 355. +</p> + +<p class="footnote"> +<a name="fn-21.12" id="fn-21.12"></a> <a href="#fnref-21.12">[12]</a> +“Descript. Phys. des Iles Canaries,” p. 471. +</p> + +<p>Von Buch concludes that this formation is intermediate between +the limestone of the Jura and the chalk, and that it is analogous +with the uppermost Jurassic beds forming the plains of Switzerland. +Hence M. D’Orbigny and Von Buch, under different terms, compare +these fossils to those from the same late stage in the secondary +formations of Europe.</p> + +<p>Some of the fossils which I collected were found a good way down +the western slope of the main ridge, and hence must originally have +been covered up by a great thickness of the black shaly rock, +independently of the now denuded, thick, overlying masses of red +sandstone. I neglected at the time to estimate how many hundred or +rather thousand feet thick the superincumbent strata must have +been: and I will not now attempt to do so. This, however, would +have been a highly +<a name="page454"></a> +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.</p> + +<p><i>The Portillo or Eastern Chain.</i>—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 [P<sup>2</sup>] 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 <i>containing some of the same fossil +shells</i>; also very many pebbles of quartz, some of micaceous +schist, and numerous, broken, rounded crystals of a reddish +orthitic or potash feldspar (as determined by Professor Miller), +and these from their size must have been derived from a +coarse-grained rock, probably granite. From this feldspar being +orthitic, and even from its external appearance, I venture +positively to affirm that it has not been derived from the rocks of +the western ranges; but, on the other hand, it may well have come, +together with the quartz and metamorphic schists, from the eastern +or Portillo line, for this line mainly consists of coarse orthitic +granite. The pebbles of the fossiliferous slate and of the purple +sandstone, certainly have been derived from the Peuquenes or +western ranges.</p> + +<p>The road crosses the valley of Tenuyan in a nearly east and west +line, and for several miles we have on both hands the conglomerate, +everywhere dipping west and forming separate great mountains. The +strata, where first met with, after leaving the gypseous formation, +are inclined westward at an angle of only 20°, 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, +<a name="page455"></a> +which in its general aspect and colour, resembles the +porphyritic conglomerate formation, so rare on this side of the +Peuquenes line and so grandly developed throughout the western +ranges.</p> + +<p>The conglomerate is of great thickness: I do not suppose that +the strata forming the separate mountain-masses [V, V, V] have ever +been prolonged over each other, but that one mass has been broken +up by several, distinct, parallel, uniclinal lines of elevation. +Judging therefore of the thickness of the conglomerate, as seen in +the separate mountain-masses, I estimated it at least from one +thousand five hundred to two thousand feet. The lower beds rest +conformably on some singularly coloured, soft strata [W], which I +could not reach to examine; and these again rest conformably on a +thick mass of micaceous, thinly laminated, siliceous sandstone [X], +associated with a little black clay-slate. These lower beds are +traversed by several dikes of decomposing porphyry. The laminated +sandstone is directly superimposed on the vast masses of granite +[Y, Y] which mainly compose the Portillo range. The line of junction +between this latter rock, which is of a bright red colour, and the +whitish sandstone was beautifully distinct; the sandstone being +penetrated by numerous, great, tortuous dikes branching from the +granite, and having been converted into a granular quartz rock +(singularly like that of the Falkland Islands), containing specks +of an ochrey powder, and black crystalline atoms, apparently of +imperfect mica. The quartzose strata in one spot were folded into a +regular dome.</p> + +<p>The granite which composes the magnificent bare pinnacles and +the steep western flank of the Portillo chain, is of a brick-red +colour, coarsely crystallised, and composed of orthitic or potash +feldspar, quartz, and imperfect mica in small quantity, sometimes +passing into chlorite. These minerals occasionally assume a laminar +or foliated arrangement. The fact of the feldspar being orthitic in +this range, is very remarkable, considering how rare, or rather, as +I believe, entirely absent, this mineral is throughout the western +ranges, in which soda-feldspar, or at least a variety cleaving like +albite, is so extremely abundant. In one spot on the western flank, +and on the eastern flank near Los Manantiales and near the crest, I +noticed some great masses of a whitish granite, parts of it fine- +grained, and parts containing large crystals of feldspar; I +neglected to collect specimens, so I do not know whether this +feldspar is also orthitic, though I am inclined to think so from +its general appearance. I saw also some syenite and one mass which +resembled andesite, but of which I likewise neglected to collect +specimens. From the manner in which the whitish granites formed +separate mountain-masses in the midst of the brick-red variety, and +from one such mass near the crest being traversed by numerous veins +of flesh-coloured and greenish eurite (into which I occasionally +observed the brick-red granite insensibly passing), I conclude that +the white granites probably belong to an older formation, almost +overwhelmed and penetrated by the red granite.</p> + +<p> +On the crest I saw also, at a short distance, some coloured stratified beds, +apparently like those [W] at the western base, but was prevented +<a name="page456"></a> +examining them by a snowstorm: Mr. Caldcleugh,<a href="#fn-21.13" +name="fnref-21.13" id="fnref-21.13"><sup>[13]</sup></a> 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 [Y<sup>2</sup>], exclusively +composed of white granite. Near Los Arenales, the mountains on all sides are +composed of the mica-slate; and looking backwards from this point up to the +bare gigantic peaks above, the view was eminently interesting. The colours of +the red granite and the black mica-slate are so distinct, that with a bright +light these rocks could be readily distinguished even from the Pampas, at a +level of at least 9,000 feet below. The red granite, from being divided by +parallel joints, has weathered into sharp pinnacles, on some of which, even on +some of the loftiest, little caps of mica-schist could be clearly seen: here +and there isolated patches of this rock adhered to the mountain-flanks, and +these often corresponded in height and position on the opposite sides of the +immense valleys. Lower down the schist prevailed more and more, with only a few +quite small points of granite projecting through. Looking at the entire eastern +face of the Portillo range, the red colour far exceeds in area the black; yet +it was scarcely possible to doubt that the granite had once been almost wholly +encased by the mica-schist. +</p> + +<p class="footnote"> +<a name="fn-21.13" id="fn-21.13"></a> <a href="#fnref-21.13">[13]</a> +“Travels,” etc., vol. i, p. 308. +</p> + +<p> +At Los Arenales, low down on the eastern flank, the mica-slate is traversed by +several closely adjoining, broad dikes, parallel to each other and to the +foliation of the schist. The dikes are formed of three different varieties of +rock, of which a pale brown feldspathic porphyry with grains of quartz was much +the most abundant. These dikes with their granules of quartz, as well as the +mica-schist itself, strikingly resemble the rocks of the Chonos Archipelago. At +a height of about twelve hundred feet above the dikes, and perhaps connected +with them, +<a name="page457"></a> +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.<a href="#fn-21.14" name="fnref-21.14" +id="fnref-21.14"><sup>[14]</sup></a> On the opposite and northern side of the +valley there is another line of lava-cliffs at a corresponding height; the +valley between being of considerable breadth, and as nearly as I could estimate +1,500 feet in depth. This field of lava is confined on both sides by the +mountains of mica-schist, and slopes down rapidly but irregularly to the edge +of the Pampas, where, having a thickness of about two hundred feet, it +terminates against a little range of claystone porphyry. The valley in this +lower part expands into a bay-like, gentle slope, bordered by the cliffs of +lava, which must certainly once have extended across this wide expanse. The +inclination of the streams from Los Arenales to the mouth of the valley is so +great, that at the time (though ignorant of M. Elie de Beaumont’s +researches on the extremely small slope over which lava can flow, and yet +retain a compact structure and considerable thickness) I concluded that they +must subsequently to their flowing have been upheaved and tilted from the +mountains; of this conclusion I can now entertain not the smallest doubt. +</p> + +<p class="footnote"> +<a name="fn-21.14" id="fn-21.14"></a> <a href="#fnref-21.14">[14]</a> +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. +</p> + +<p>At the mouth of the valley, within the cliffs of the above +lava-field, there are remnants, in the form of separate small +hillocks and of lines of low cliffs, of a considerable deposit of +compact white tuff (quarried for filtering-stones), composed of +broken pumice, volcanic crystals, scales of mica, and fragments of +lava. This mass has suffered much denudation; and the hard +mica-schist has been deeply worn, since the period of its +deposition; and this period must have been subsequent to the +denudation of the basaltic lava-streams, as attested by their +encircling cliffs standing at a higher level. At the present day, +under the existing arid climate, ages might roll past without a +square yard of rock of any kind being denuded, except perhaps in +the rarely moistened drainage-channel of the valley. Must we then +look back to that ancient period, when the waves of the sea beat +against the eastern foot of the Cordillera, for a power sufficient +to denude extensively, though superficially, this tufaceous +deposit, soft although it be?</p> + +<p>There remains only to mention some little water-worn hillocks +[BB], +<a name="page458"></a> +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.</p> + +<p><i>Concluding remarks on the Portillo range.</i>—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. +<a name="page459"></a> +Late, therefore, in a geological sense, as must be the age of +the main part of the red granite, I can conceive nothing more +impressive than the eastern view of this great range, as forcing +the mind to grapple with the idea of the thousands of thousands of +years requisite for the denudation of the strata which originally +encased it,—for that the fluidified granite was once encased, +its mineralogical composition and structure, and the bold conical +shape of the mountain-masses, yield sufficient evidence. Of the +encasing strata we see the last vestiges in the coloured beds on +the crest, in the little caps of mica-schist on some of the +loftiest pinnacles, and in the isolated patches of this same rock +at corresponding heights on the now bare and steep flanks.</p> + +<p>The lava-streams at the eastern foot of the Portillo are +interesting, not so much from the great denudation which they have +suffered at a comparatively late period as from the evidence they +afford by their inclination taken conjointly with their thickness +and compactness, that after the great range had assumed its present +general outline, it continued to rise as an axis of elevation. The +plains extending from the base of the Cordillera to the Atlantic +show that the continent has been upraised in mass to a height of +3,500 feet, and probably to a much greater height, for the smooth +shingle-covered margin of the Pampas is prolonged in a gentle +unbroken slope far up many of the great valleys. Nor let it be +assumed that the Peuquenes and Portillo ranges have undergone only +movements of elevation; for we shall hereafter see, that the bottom +of the sea subsided several thousand feet during the deposition of +strata, occupying the same relative place in the Cordillera, with +those of the Peuquenes ridge; moreover, we shall see from the +unequivocal evidence of buried upright trees, that at a somewhat +later period, during the formation of the Uspallata chain, which +corresponds geographically with that of the Portillo, there was +another subsidence of many thousand feet: here, indeed, in the +valley of Tenuyan, the accumulation of the coarse stratified +conglomerate to a thickness of fifteen hundred or two thousand +feet, offers strong presumptive evidence of subsidence; for all +existing analogies lead to the belief that large pebbles can be +transported only in shallow water, liable to be affected by +currents and movements of undulation—and if so, the shallow +bed of the sea on which the pebbles were first deposited must +necessarily have sunk to allow of the accumulation of the +superincumbent strata. What a history of changes of level, and of +wear and tear, all since the age of the latter secondary formations +of Europe, does the structure of this one great mountain-chain +reveal!</p> + +<h4><i>Passage of the Andes by the Cumbre or Uspallata Pass.</i></h4> + +<p>This Pass crosses the Andes about sixty miles north of that just +described: the section given in <a href="images/plate5.jpg">Plate +V,</a> Section 1/2, [<a href="#page440">see map page 440</a>] 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 +<a name="page460"></a> +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.</p> + +<p> +The Cumbre, where the road crosses it, is, according to Mr. +Pentland, 12,454 feet above the sea; and the neighbouring peaks, +composed of dark purple and whitish porphyries, some obscurely +stratified with a westerly dip, and others without a trace of +stratification, must exceed 13,000 feet in height. Descending the +eastern slope of the Cumbre, the structure becomes very +complicated, and generally differs on the two sides of the east and +west line of road and section. First we come to a great mass [A] of +nearly vertical, singularly contorted strata, composed of highly +compact red sandstones, and of often calcareous conglomerates, and +penetrated by green, yellow, and reddish dikes; but I shall +presently have an opportunity of describing in some detail an +analogous pile of strata. These vertical beds are abruptly +succeeded by others [B], of apparently nearly the same nature but +more metamorphosed, +<a name="page461"></a> +alternating with porphyries and limestones; these dip +for a short space westward, but there has been here an +extraordinary dislocation, which, on the north side of the road, +appears to have determined the excavation of the north and south +valley of the R. de las Cuevas. On this northern side of the road, +the strata [B] are prolonged till they come in close contact with a +jagged lofty mountain [D] of dark-coloured, unstratified, intrusive +porphyry, where the beds have been more highly inclined and still +more metamorphosed. This mountain of porphyry seems to form a short +axis of elevation, for south of the road in its line there is a +hill [C] of porphyritic conglomerate with absolutely vertical +strata.</p> + +<p>We now come to the gypseous formation: I will first describe the +structure of the several mountains, and then give in one section a +detailed account of the nature of the rocks. On the north side of +the road, which here runs in an east and west valley, the mountain +of porphyry [D] is succeeded by a hill [E] formed of the upper +gypseous strata tilted, at an angle of between 70° and 80° +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.</p> + +<p> +The uniclinal axis [I] intersected near the Puente del Inca<a href="#fn-21.15" +name="fnref-21.15" id="fnref-21.15"><sup>[15]</sup></a> (of which +<a name="page462"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-21.15" id="fn-21.15"></a> <a href="#fnref-21.15">[15]</a> +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. +</p> + +<p>1st. The lowest mass is the altered clay-slate described in the +preliminary discussion, and which in this line of section was here +first met with. Lower down the valley, at the R. de las Vacas, I +had a better opportunity of examining it; it is there in some parts +well characterised, having a distinct, nearly vertical, tortuous +cleavage, ranging N.W. and S.E., and intersected by quartz veins: +in most parts, however, it is crystalline and feldspathic, and +passes into a true greenstone often including grains of quartz. The +clay-slate, in its upper half, is frequently brecciated, the +embedded angular fragments being of nearly the same nature with the +paste.</p> + +<p>2nd. Several strata of purplish porphyritic conglomerate, of no +very great thickness, rest conformably upon the feldspathic slate. +A thick bed of fine, purple, claystone porphyry, obscurely +brecciated (but not of metamorphosed sedimentary origin), and +capped by porphyritic conglomerate, was the lowest bed actually +examined in this section at the Puente del Inca.</p> + +<p>3rd. A stratum, eighty feet thick, of hard and very compact +impure whitish limestone, weathering bright red, with included +layers brecciated and recemented. Obscure marks of shell are +distinguishable in it.</p> + +<p>4th. A red, quartzose, fine-grained conglomerate, with grains of +quartz, and with patches of white earthy feldspar, apparently due +to some process of concretionary crystalline action; this bed is +more compact and metamorphosed than any of the overlying +conglomerates.</p> + +<p>5th. A whitish cherty limestone, with nodules of bluish +argillaceous limestone.</p> + +<p>6th. A white conglomerate, with many particles of quartz, almost +blending into the paste.</p> + +<p>7th. Highly siliceous, fine-grained white sandstone.</p> + +<p>8th and 9th. Red and white beds not examined.</p> + +<p>10th. Yellow, fine-grained, thinly stratified, magnesian +(judging from its slow dissolution in acids) limestone: it includes +some white quartz pebbles, and little cavities, lined with +calcareous spar, some retaining the form of shells.</p> + +<p> +<a name="page463"></a> +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 <i>numerous</i> crystals of +whitish feldspar, with black mica and little spots of soft +ferruginous matter: evidently a submarine lava.</p> + +<p>12th. Yellow magnesian limestone, as before, part-stained +purple.</p> + +<p>13th. A most singular rock; basis purplish grey, obscurely +crystalline, easily fusible into a dark green glass, not hard, +thickly speckled with crystals more or less perfect of white +carbonate of lime, of red hydrous oxide of iron, of a white and +transparent mineral like analcime, and of a green opaque mineral +like soap-stone; the basis is moreover amygdaloidal with many +spherical balls of white crystallised carbonate of lime, of which +some are coated with the red oxide of iron. I have no doubt, from +the examination of a superincumbent stratum (19), that this is a +submarine lava; though in Northern Chile, some of the metamorphosed +sedimentary beds are almost as crystalline, and of as varied +composition.</p> + +<p>14th. Red sandstone, passing in the upper part into a coarse, +hard, red conglomerate, 300 feet thick, having a calcareous cement, +and including grains of quartz and broken crystals of feldspar; +basis infusible; the pebbles consist of dull purplish porphyries, +with some of quartz, from the size of a nut to a man’s head. This +is the coarsest conglomerate in this part of the Cordillera: in the +middle there was a white layer not examined.</p> + +<p>15th. Grand thick bed, of a very hard, yellowish-white rock, +with a crystalline feldspathic base, including large crystals of +white feldspar, many little cavities mostly full of soft +ferruginous matter, and numerous hexagonal plates of black mica. +The upper part of this great bed is slightly cellular; the lower +part compact: the thickness varied a little in different parts. +Manifestly a submarine lava; and is allied to bed 11.</p> + +<p>16th and 17th. Dull purplish, calcareous, fine-grained, compact +sandstones, which pass into coarse white conglomerates with +numerous particles of quartz.</p> + +<p>18th. Several alternations of red conglomerate, purplish +sandstone, and submarine lava, like that singular rock forming bed +13.</p> + +<p>19th. A very heavy, compact, greenish-black stone, with a +fine-grained obviously crystalline basis, containing a few specks +of white calcareous spar, many specks of the crystallised hydrous +red oxide of iron, and some specks of a green mineral; there are +veins and nests filled with epidote: certainly a submarine +lava.</p> + +<p>20th. Many thin strata of compact, fine-grained, pale purple +sandstone.</p> + +<p>21st. Gypsum in a nearly pure state, about three hundred feet in +thickness: this bed, in its concretions of anhydrite and layers of +small blackish crystals of carbonate of lime, exactly resembles the +great gypseous beds in the Peuquenes range.</p> + +<p>22nd. Pale purple and reddish sandstone, as in bed 20: about +three hundred feet in thickness.</p> + +<p>23rd. A thick mass composed of layers, often as thin as paper +and +<a name="page464"></a> +convoluted, of pure gypsum with others very impure, of a +purplish colour.</p> + +<p>24th. Pure gypsum, thick mass.</p> + +<p>25th. Red sandstones, of great thickness.</p> + +<p>26th. Pure gypsum, of great thickness.</p> + +<p>27th. Alternating layers of pure and impure gypsum, of great +thickness.</p> + +<p>I was not able to ascend to these few last great strata, which +compose the neighbouring loftiest pinnacles. The thickness, from +the lowest to the uppermost bed of gypsum, cannot be less than +2,000 feet: the beds beneath I estimated at 3,000 feet, and this +does not include either the lower parts of the porphyritic +conglomerate, or the altered clay-slate; I conceive the total +thickness must be about six thousand feet. I distinctly observed +that not only the gypsum, but the alternating sandstones and +conglomerates were lens-shaped, and repeatedly thinned out and +replaced each other: thus in the distance of about a mile, a bed +300 feet thick of sandstone between two beds of gypsum, thinned out +to nothing and disappeared. The lower part of this section differs +remarkably,—in the much greater diversity of its +mineralogical composition,—in the abundance of calcareous +matter,—in the greater coarseness of some of the +conglomerates,—and in the numerous particles and well-rounded +pebbles, sometimes of large size, of quartz,— from any other +section hitherto described in Chile. From these peculiarities and +from the lens-form of the strata, it is probable that this great +pile of strata was accumulated on a shallow and very uneven bottom, +near some pre-existing land formed of various porphyries and +quartz-rock. The formation of porphyritic claystone conglomerate +does not in this section attain nearly its ordinary thickness; this +may be PARTLY attributed to the metamorphic action having been here +much less energetic than usual, though the lower beds have been +affected to a certain degree. If it had been as energetic as in +most other parts of Chile, many of the beds of sandstone and +conglomerate, containing rounded masses of porphyry, would +doubtless have been converted into porphyritic conglomerate; and +these would have alternated with, and even blended into, +crystalline and porphyritic strata without a trace of mechanical +structure,—namely, into those which, in the present state of +the section, we see are unquestionably submarine lavas.</p> + +<p>The beds of gypsum, together with the red alternating sandstones +and conglomerates, present so perfect and curious a resemblance +with those seen in our former section in the basin-valley of Yeso, +that I cannot doubt the identity of the two formations: I may add, +that a little westward of the P. del Inca, a mass of gypsum passed +into a fine-grained, hard, brown sandstone, which contained some +layers of black, calcareous, compact, shaly rock, precisely like +that seen in such vast masses on the Peuquenes range.</p> + +<p>Near the Puente del Inca, numerous fragments of limestone, +containing some fossil remains, were scattered on the ground: these +fragments so perfectly resemble the limestone of bed No. 3, in +which I saw impressions of shells, that I have no doubt they have +fallen from it.</p> + +<p> +<a name="page465"></a> +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:—<br/> +<small> Gryphæa, near to <i>G. +Couloni</i> (Neocomian formation).<br/> + Arca, perhaps <i>A. Gabrielis,</i> +d’Orbigny, “Pal. Franc.” (Neocomian formation).</small></p> + +<p> +Mr. Pentland made a collection of shells from this same spot, and Von Buch<a +href="#fn-21.16" name="fnref-21.16" id="fnref-21.16"><sup>[16]</sup></a> +considers them as consisting of:—<br/> +<small> Trigonia, resembling in form <i>T. +costata.</i><br/> + Pholadomya, like one found by M. Dufresnoy +near Alencon.<br/> + Isocardi excentrica, Voltz., identical with +that from the Jura.</small></p> + +<p class="footnote"> +<a name="fn-21.16" id="fn-21.16"></a> <a href="#fnref-21.16">[16]</a> +“Description Phys. des Iles Can.,” p. 472. +</p> + +<p>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.</p> + +<p>It is well worthy of remark that these shells, or at least those +of which I saw impressions in the limestone (bed No. 3), must have +been covered up, on the <i>least</i> 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 <i>coarse,</i> well-rounded conglomerates, which it is +scarcely possible to believe were spread out in profoundly deep +water, and which, especially those containing pebbles of quartz, +could hardly have been rounded in submarine craters and afterwards +ejected from them, as I believe to have been the case with much of +the porphyritic conglomerate formation. I may add that, in +Professor Forbes’s opinion, the above-enumerated species of +mollusca probably did not live at a much greater depth than twenty +fathoms, that is only 120 feet.</p> + +<p>To return to our section down the valley; standing on the great +N. by W. and S. by E. uniclinal axis of the Puente del Inca, of +which a section has just been given, and looking north-east, +greater tabular +<a name="page466"></a> +masses of gypseous formation (KK) could be seen in the distance, +very slightly inclined towards the east. Lower down the valley, the +mountains are almost exclusively composed of porphyries, many of +them of intrusive origin and non-stratified, others stratified, but +with the stratification seldom distinguishable except in the upper +parts. Disregarding local disturbances, the beds are either +horizontal or inclined at a small angle eastwards: hence, when +standing on the plain of Uspallata and looking to the west or +backwards, the Cordillera appear composed of huge, square, nearly +horizontal, tabular masses: so wide a space, with such lofty +mountains so equably elevated, is rarely met with within the +Cordillera. In this line of section, the interval between the +Puente del Inca and the neighbourhood of the Cumbre, includes all +the chief axes of dislocation.</p> + +<p> +The altered clay-slate formation, already described, is seen in several parts +of the valley as far down as Las Vacas, underlying the porphyritic +conglomerate. At the Casa de Pujios [L], there is a hummock of (andesitic?) +granite; and the stratification of the surrounding mountains here changes from +W. by S. to S.W. Again, near the R. Vacas there is a larger formation of +(andesitic?) granite [M], which sends a meshwork of veins into the +superincumbent clay-slate, and which locally throws off the strata, on one side +to N.W. and on the other to S.E. but not at a high angle: at the junction, the +clay-slate is altered into fine-grained greenstone. This granitic axis is +intersected by a green dike, which I mention, because I do not remember having +elsewhere seen dikes in this lowest and latest intrusive rock. From the R. +Vacas to the plain of Uspallata, the valley runs N.E., so that I have had to +contract my section; it runs exclusively through porphyritic rocks. As far as +the Pass of Jaula, the claystone conglomerate formation, in most parts highly +porphyritic, and crossed by numerous dikes of greenstone porphyry, attains a +great thickness: there is also much intrusive porphyry. From the Jaula to the +plain, the stratification has been in most places obliterated, except near the +tops of some of the mountains; and the metamorphic action has been extremely +great. In this space, the number and bulk of the intrusive masses of +differently coloured porphyries, injected one into another and intersected by +dikes, is truly extraordinary. I saw one mountain of whitish porphyry, from +which two huge dikes, thinning out, branched <i>downwards</i> 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 +<a name="page467"></a> +partially decomposed, I was unable to measure them; and they certainly are +quite unlike the variety, so abundantly met with in almost all the other rocks +of this line of section, and which, wherever I tried it, cleaved like albite. +This brick-red porphyry appears to have burst through all the other porphyries, +and numerous red dikes traversing the neighbouring mountains have proceeded +from it: in some few places, however, it was intersected by white dikes. From +this posteriority of intrusive origin,—from the close general resemblance +between this red porphyry and the red granite of the Portillo line, the only +difference being that the feldspar here is less perfectly granular, and that +soapstone replaces the mica, which is there imperfect and passes into +chlorite,—and from the Portillo line a little southward of this point +appearing to blend (according to Dr. Gillies) into the western ranges,—I +am strongly urged to believe (as formerly remarked) that the grand +mountain-masses composed of this brick-red porphyry belong to the same axis of +injection with the granite of the Portillo line; if so, the injection of this +porphyry probably took place, as long subsequently to the several axes of +elevation in the gypseous formation near the Cumbre, as the injection of the +Portillo granite has been shown to have been subsequent to the elevation of the +gypseous strata composing the Peuquenes range; and this interval, we have seen, +must have been a very long one. +</p> + +<p> +The Plain of Uspallata has been briefly described in Chapter 3; it resembles +the basin-plains of Chile; it is ten or fifteen miles wide, and is said to +extend for 180 miles northward; its surface is nearly six thousand feet above +the sea; it is composed, to a thickness of some hundred feet of loosely +aggregated, stratified shingle, which is prolonged with a gently sloping +surface up the valleys in the mountains on both sides. One section in this +plain [Z] is interesting, from the unusual<a href="#fn-21.17" +name="fnref-21.17" id="fnref-21.17"><sup>[17]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-21.17" id="fn-21.17"></a> <a href="#fnref-21.17">[17]</a> +I find that Mr. Smith of Jordan Hill has described (<i>Edinburgh New Phil. +Journ.,</i> vol. xxv, p. 392) beds of sand and gravel, near Edinburgh, tilted +at an angle of 60°, and dislocated by miniature faults. +</p> + +<p> +<a name="page468"></a> +<i>The Uspallata range.</i>—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.,<a href="#fn-21.18" +name="fnref-21.18" id="fnref-21.18"><sup>[18]</sup></a> 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 +<a name="page469"></a> +and rests conformably on it. These porphyries, moreover, with the exception of +the one blackish stratum, and of the one indurated, white tufaceous bed, differ +from the beds composing the Uspallata range in the line of the Villa Vicencio +Pass. +</p> + +<p class="footnote"> +<a name="fn-21.18" id="fn-21.18"></a> <a href="#fnref-21.18">[18]</a> +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. +</p> + +<p>I will now give, first, a sketch of the structure of the range, +as represented in the section, and will then describe its +composition and interesting history. At its western foot, a hillock +[N] is seen to rise out of the plain, with its strata dipping at +70° 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, +<a name="page470"></a> +project in a north-easterly line, in the same oblique manner as +do the ridges on the western foot, and as is so frequently the case +with those at the base of the main Cordillera.</p> + +<p> +I will now describe the nature of the beds, beginning at the base on the +eastern side. First, for the clay-slate formation: the slate is generally hard +and bluish, with the 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.<a +href="#fn-21.19" name="fnref-21.19" id="fnref-21.19"><sup>[19]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-21.19" id="fn-21.19"></a> <a href="#fnref-21.19">[19]</a> +I infer this from the height of V. Vicencio, which was ascertained by Mr. Miers +to be 5,328 feet above the sea. +</p> + +<p> +Secondly: the most usual bed on the clay-slate is a coarse, white, slightly +calcareous conglomerate, of no great thickness, including broken crystals of +feldspar, grains of quartz, and numerous pebbles of brecciated claystone +porphyry, but without any pebbles of the underlying clay-slate. I nowhere saw +the actual junction between this bed and the clay-slate, though I spent a whole +day in endeavouring to discover their relations. In some places I distinctly +saw the white conglomerate and overlying beds inclined at from 25° to +30° 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,<a href="#fn-21.20" name="fnref-21.20" +id="fnref-21.20"><sup>[20]</sup></a> in some cases gradually and entirely +disappeared in approaching the overlying white conglomerate; +<a name="page471"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-21.20" id="fn-21.20"></a> <a href="#fnref-21.20">[20]</a> +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. +</p> + +<p>Thirdly: on the white conglomerate, strata several hundred feet +in thickness are superimposed, varying much in nature in short +distances: the commonest variety is a white, much indurated tuff, +sometimes slightly calcareous, with ferruginous spots and +water-lines, often passing into whitish or purplish compact, +fine-grained grit or sandstones; other varieties become +semi-porcellanic, and tinted faint green or blue; others pass into +an indurated shale: most of these varieties are easily fusible.</p> + +<p>Fourthly: a bed, about one hundred feet thick of a compact, +partially columnar, pale-grey, feldspathic lava, stained with iron, +including very numerous crystals of opaque feldspar, and with some +crystallised and disseminated calcareous matter. The tufaceous +stratum on which this feldspathic lava rests is much hardened, +stained purple, and has a spherico-concretionary structure; it here +contains a good many pebbles of claystone porphyry.</p> + +<p>Fifthly: thin beds, 400 feet in thickness, varying much in +nature, consisting of white and ferruginous tuffs, in some parts +having a concretionary structure, in others containing rounded +grains and a few pebbles of quartz; also passing into hard +gritstones and into greenish mudstones: there is, also, much of a +bluish-grey and green semi-porcellanic stone.</p> + +<p>Sixthly: a volcanic stratum, 250 feet in thickness, of so +varying a nature that I do not believe a score of specimens would +show all the varieties; much is highly amygdaloidal, much compact; +there are greenish, blackish, purplish, and grey varieties, rarely +including crystals of green augite and minute acicular ones of +feldspar, but often crystals and amygdaloidal masses of white, red, +and black carbonate of lime. Some of the blackish varieties of this +rock have a conchoidal fracture and resemble basalt; others have an +irregular fracture. Some of the grey and purplish varieties are +thickly speckled with green earth and with white crystalline +carbonate of lime; others are largely amygdaloidal with green earth +and calcareous spar. Again, other earthy varieties, of greenish, +purplish and grey tints, contain much iron, and are almost half +composed of amygdaloidal balls of dark brown bole, of a whitish +indurated feldspathic matter, of bright green earth, of agate, and +of black and white crystallised carbonate of lime. All these +varieties are easily fusible. Viewed from a distance, the line of +junction with the underlying semi-porcellanic strata was distinct; +but when examined +<a name="page472"></a> +closely, it was impossible to point out within a foot where the +lava ended and where the sedimentary mass began: the rock at the +time of junction was in most places hard, of a bright green colour, +and abounded with irregular amygdaloidal masses of ferruginous and +pure calcareous spar, and of agate.</p> + +<p>Seventhly: strata, eighty feet in thickness, of various +indurated tuffs, as before; many of the varieties have a fine basis +including rather coarse extraneous particles; some of them are +compact and semi-porcellanic, and include vegetable +impressions.</p> + +<p>Eighthly: a bed, about fifty feet thick, of greenish-grey, +compact, feldspathic lava, with numerous small crystals of opaque +feldspar, black augite, and oxide of iron. The junction with the +bed on which it rested, was ill defined; balls and masses of the +feldspathic rock being enclosed in much altered tuff.</p> + +<p>Ninthly: indurated tuffs, as before.</p> + +<p>Tenthly: a conformable layer, less than two feet in thickness, +of pitchstone, generally brecciated, and traversed by veins of +agate and of carbonate of lime: parts are composed of apparently +concretionary fragments of a more perfect variety, arranged in +horizontal lines in a less perfectly characterised variety. I have +much difficulty in believing that this thin layer of pitchstone +flowed as lava.</p> + +<p>Eleventhly: sedimentary and tufaceous beds as before, passing +into sandstone, including some conglomerate: the pebbles in the +latter are of claystone porphyry, well rounded, and some as large +as cricket-balls.</p> + +<p>Twelfthly: a bed of compact, sonorous, feldspathic lava, like +that of bed No. 8, divided by numerous joints into large angular +blocks.</p> + +<p>Thirteenthly: sedimentary beds as before.</p> + +<p>Fourteenthly: a thick bed of greenish or greyish black, compact +basalt (fusing into a black enamel), with small crystals, +occasionally distinguishable, of feldspar and augite: the junction +with the underlying sedimentary bed, differently from that in most +of the foregoing streams, here was quite distinct:—the lava +and tufaceous matter preserving their perfect characters within two +inches of each other. This rock closely resembles certain parts of +that varied and singular lava-stream No. 6; it likewise resembles, +as we shall immediately see, many of the great upper beds on the +western flank and on the summit of this range.</p> + +<p> +The pile of strata here described attains a great thickness; and above the +last-mentioned volcanic stratum, there were several other great tufaceous beds +alternating with submarine lavas, which I had not time to examine; but a +corresponding series, several thousand feet in thickness, is well exhibited on +the crest and western flank of the range. Most of the lava-streams on the +western side are of a jet-black colour and basaltic nature; they are either +compact and fine-grained, including minute crystals of augite and feldspar, or +they are coarse-grained and abound with rather large coppery-brown crystals of +an augitic mineral.<a href="#fn-21.21" name="fnref-21.21" +id="fnref-21.21"><sup>[21]</sup></a> Another variety was of a dull-red colour, +having a claystone brecciated basis, including specks of oxide of iron and of +calcareous spar, and +<a name="page473"></a> +amygdaloidal with green earth: there were apparently several other varieties. +These submarine lavas often exhibit a spheroidal, and sometimes an imperfect +columnar structure: their upper junctions are much more clearly defined than +their lower junctions; but the latter are not so much blended into the +underlying sedimentary beds as is the case in the eastern flank. On the crest +and western flank of the range, the streams, viewed as a whole, are mostly +basaltic; whilst those on the eastern side, which stand lower in the series, +are, as we have seen, mostly feldspathic. +</p> + +<p class="footnote"> +<a name="fn-21.21" id="fn-21.21"></a> <a href="#fnref-21.21">[21]</a> +Very easily fusible into a jet-black bead, attracted by the magnet: the +crystals are too much tarnished to be measured by the goniometer. +</p> + +<p> +The sedimentary strata alternating with the lavas on the crest and western +side, are of an almost infinitely varying nature; but a large proportion of +them closely resemble those already described on the eastern flank: there are +white and brown, indurated, easily fusible tuffs,—some passing into pale +blue and green semi-porcellanic rocks,—others into brownish and purplish +sandstones and gritstones, often including grains of quartz,—others into +mudstone containing broken crystals and particles of rock, and occasionally +single large pebbles. There was one stratum of a bright red, coarse, volcanic +gritstone; another of conglomerate; another of a black, indurated, carbonaceous +shale marked with imperfect vegetable impressions; this latter bed, which was +thin, rested on a submarine lava, and followed all the considerable +inequalities of its upper surface. Mr. Miers states that coal has been found in +this range. Lastly, there was a bed (like No. 10 on the eastern flank) +evidently of sedimentary origin, and remarkable from closely approaching in +character to an imperfect pitchstone, and from including extremely thin layers +of perfect pitchstone, as well as nodules and irregular fragments (but not +resembling extraneous fragments) of this same rock arranged in horizontal +lines: I conceive that this bed, which is only a few feet in thickness, must +have assumed its present state through metamorphic and concretionary action. +Most of these sedimentary strata are much indurated, and no doubt have been +partially metamorphosed: many of them are extraordinarily heavy and compact; +others have agate and crystalline carbonate of lime disseminated throughout +them. Some of the beds exhibit a singular concretionary arrangement, with the +curves determined by the lines of fissure. There are many veins of agate and +calcareous spar, and innumerable ones of iron and other metals, which have +blackened and curiously affected the strata to considerable distances on both +sides. +</p> + +<p> +Many of these tufaceous beds resemble, with the exception of being more +indurated, the upper beds of the Great Patagonian tertiary formation, +especially those variously coloured layers high up the River Santa Cruz, and in +a remarkable degree the tufaceous formation at the northern end of Chiloe. I +was so much struck with this resemblance, that I particularly looked out for +silicified wood, and found it under the following extraordinary circumstances. +High up on this western flank,<a href="#fn-21.22" name="fnref-21.22" +id="fnref-21.22"><sup>[22]</sup></a> +<a name="page474"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-21.22" id="fn-21.22"></a> <a href="#fnref-21.22">[22]</a> +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. +</p> + +<p> +The green and brown mudstone beds including the trees, are conformably covered +by much indurated, compact, white or ferruginous tuffs, which pass upwards into +a fine-grained, purplish sedimentary rock: these strata, which, together, are +from four to five hundred feet in thickness, rest on a thick bed of submarine +lava, and are conformably covered by another great mass of fine-grained +basalt,<a href="#fn-21.23" name="fnref-21.23" +id="fnref-21.23"><sup>[23]</sup></a> 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, +<a name="page475"></a> +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.<a href="#fn-21.24" name="fnref-21.24" +id="fnref-21.24"><sup>[24]</sup></a> +</p> + +<p class="footnote"> +<a name="fn-21.23" id="fn-21.23"></a> <a href="#fnref-21.23">[23]</a> +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. +</p> + +<p class="footnote"> +<a name="fn-21.24" id="fn-21.24"></a> <a href="#fnref-21.24">[24]</a> +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 <i>several thousand</i> 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 <i>grouped in a +clump</i>, and from their all standing vertically to the strata, it is +superfluous to speculate on the chance of the trees having been drifted from +adjoining land, and deposited upright: I may, however, mention that the late +Dr. Malcolmson assured me, that he once met in the Indian Ocean, fifty miles +from land, several cocoa-nut trees floating upright, owing to their roots being +loaded with earth. +</p> + +<p> +In nearly the middle of the range, there are some hills [Q], before alluded to, +formed of a kind of granite externally resembling andesite, and consisting of a +white, imperfectly granular, feldspathic basis, including some perfect crystals +apparently of albite (but I was unable to measure them), much black mica, +epidote in veins, and very little or no quartz. Numerous small veins branch +from this rock into the surrounding strata; and it is a singular fact that +these veins, though composed of the same kind of feldspar and small scales of +mica as in the solid rock, abound with innumerable minute <i>rounded</i> 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<a +href="#fn-21.25" name="fnref-21.25" id="fnref-21.25"><sup>[25]</sup></a> when +cooling, and by its having been thus more easily +<a name="page476"></a> +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. +</p> + +<p class="footnote"> +<a name="fn-21.25" id="fn-21.25"></a> <a href="#fnref-21.25">[25]</a> +See a paper by M. Elie de Beaumont, “Soc. Philomath.,” May 1839 +(“L’Institut.,” 1839, p. 161.) +</p> + +<p> +<i>Concluding remarks on the Uspallata range.</i>—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.<a href="#fn-21.26" name="fnref-21.26" +id="fnref-21.26"><sup>[26]</sup></a> Both the basaltic and feldspathic +submarine streams are very compact; none being vesicular, and only a few +amygdaloidal: the effects which some of them, especially those low in the +series, have produced on the tufaceous beds over which they have flowed is +highly curious. Independently of this local metamorphic action, all the strata +undoubtedly display an indurated and altered character; and all the rocks of +this range—the lavas, the alternating sediments, the intrusive granite +and porphyries, and the underlying clay-slate—are intersected by +metalliferous veins. The lava-strata can often be seen extending for great +distances, conformably with the under and overlying beds; and it was obvious +that they thickened towards the west. Hence the points of eruption must have +been situated westward of the present range, in the direction of the main +Cordillera: as, however, the flanks of the Cordillera are entirely composed of +various porphyries, chiefly claystone and greenstone, some intrusive, and +others belonging to the porphyritic conglomerate formation, but all quite +unlike these submarine lava-streams, we must in all probability look to the +plain of Uspallata for the now deeply buried points of eruption. +</p> + +<p class="footnote"> +<a name="fn-21.26" id="fn-21.26"></a> <a href="#fnref-21.26">[26]</a> +See on this subject, “Volcanic Islands,” etc., by the Author. +</p> + +<p>Comparing our section of the Uspallata range with that of the +Cumbre, we see, with the exception of the underlying clay-slate, +and perhaps of the intrusive rocks of the axes, a striking +dissimilarity in the strata composing them. The great porphyritic +conglomerate formation +<a name="page477"></a> +has not extended as far as this range; nor have we here any of +the gypseous strata, the magnesian and other limestones, the red +sandstones, the siliceous beds with pebbles of quartz, and +comparatively little of the conglomerates, all of which form such +vast masses over the basal series in the main Cordillera. On the +other hand, in the Cordillera, we do not find those endless +varieties of indurated tuffs, with their numerous veins and +concretionary arrangement, and those grit and mud stones, and +singular semi-porcellanic rocks, so abundant in the Uspallata +range. The submarine lavas, also, differ considerably; the +feldspathic streams of the Cordillera contain much mica, which is +absent in those of the Uspallata range: in this latter range we +have seen on how grand a scale, basaltic lava has been poured +forth, of which there is not a trace in the Cordillera. This +dissimilarity is the more striking, considering that these two +parallel chains are separated by a plain only between ten and +fifteen miles in width; and that the Uspallata lavas, as well as no +doubt the alternating tufaceous beds, have proceeded from the west, +from points apparently between the two ranges. To imagine that +these two piles of strata were contemporaneously deposited in two +closely adjoining, very deep, submarine areas, separated from each +other by a lofty ridge, where a plain now extends, would be a +gratuitous hypothesis. And had they been contemporaneously +deposited, without any such dividing ridge, surely some of the +gypseous and other sedimentary matter forming such immensely thick +masses in the Cordillera, would have extended this short distance +eastwards; and surely some of the Uspallata tuffs and basalts also +accumulated to so great a thickness, would have extended a little +westward. Hence I conclude, that it is far from probable that these +two series are not contemporaneous; but that the strata of one of +the chains were deposited, and even the chain itself uplifted, +before the formation of the other:—which chain, then, is the +oldest? Considering that in the Uspallata range the lowest strata +on the western flank lie unconformably on the clay-slate, as +probably is the case with those on the eastern flank, whereas in +the Cordillera all the overlying strata lie conformably on this +formation:—considering that in the Uspallata range some of +the beds, both low down and high up in the series, are marked with +vegetable impressions, showing the continued existence of +neighbouring land;—considering the close general resemblance +between the deposits of this range and those of tertiary origin in +several parts of the continent;—and lastly, even considering +the lesser height and outlying position of the Uspallata +range,—I conclude that the strata composing it are in all +probability of subsequent origin, and that they were accumulated at +a period when a deep sea studded with submarine volcanoes washed +the eastern base of the already partially elevated Cordillera.</p> + +<p>This conclusion is of much importance, for we have seen that in +the Cordillera, during the deposition of the Neocomian strata, the +bed of the sea must have subsided many thousand feet: we now learn +that at a later period an adjoining area first received a great +accumulation of strata, and was upheaved into land on which +coniferous trees grew, and that this area then subsided several +thousand feet to receive the superincumbent +<a name="page478"></a> +submarine strata, afterwards being broken up, denuded, +and elevated in mass to its present height. I am strengthened in +this conclusion of there having been two distinct, great periods of +subsidence, by reflecting on the thick mass of coarse stratified +conglomerate in the valley of Tenuyan, between the Peuquenes and +Portillo lines; for the accumulation of this mass seems to me, as +previously remarked, almost necessarily to have required a +prolonged subsidence; and this subsidence, from the pebbles in the +conglomerate having been to a great extent derived from the +gypseous or Neocomian strata of the Peuquenes line, we know must +have been quite distinct from, and subsequent to, that sinking +movement which probably accompanied the deposition of the Peuquenes +strata, and which certainly accompanied the deposition of the +equivalent beds near the Puente del Inca, in this line of +section.</p> + +<p>The Uspallata chain corresponds in geographical position, though +on a small scale, with the Portillo line; and its clay-slate +formation is probably the equivalent of the mica-schist of the +Portillo, there metamorphosed by the old white granites and +syenites. The coloured beds under the conglomerate in the valley of +Tenuyan, of which traces are seen on the crest of the Portillo, and +even the conglomerate itself, may perhaps be synchronous with the +tufaceous beds and submarine lavas of the Uspallata range; an open +sea and volcanic action in the latter case, and a confined channel +between two bordering chains of islets in the former case, having +been sufficient to account for the mineralogical dissimilarity of +the two series. From this correspondence between the Uspallata and +Portillo ranges, perhaps in age and certainly in geographical +position, one is tempted to consider the one range as the +prolongation of the other; but their axes are formed of totally +different intrusive rocks; and we have traced the apparent +continuation of the red granite of the Portillo in the red +porphyries diverging into the main Cordillera. Whether the axis of +the Uspallata range was injected before, or as perhaps is more +probable, after that of the Portillo line, I will not pretend to +decide; but it is well to remember that the highly inclined +lava-streams on the eastern flank of the Portillo line, prove that +its angular upheavement was not a single and sudden event; and +therefore that the anticlinal elevation of the Uspallata range may +have been contemporaneous with some of the later angular movements +by which the gigantic Portillo range gained its present height +above the adjoining plain.</p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="page479"></a><a name="chap3.08"></a>Chapter VIII<br/>NORTHERN +CHILE.—CONCLUSION.</h2> + +<p class="letter"> +Section from Illapel to Combarbala; gypseous formation with silicified +wood.—Panuncillo.—Coquimbo; mines of Arqueros; section up valley; +fossils.—Guasco, fossils of.—Copiapo, section up valley; Las +Amolanas, silicified wood.—Conglomerates, nature of former land, fossils, +thickness of strata, great subsidence.—Valley of Despoblado, fossils, +tufaceous deposit, complicated dislocations of.—Relations between ancient +orifices of eruption and subsequent axes of injection.—Iquique, Peru, +fossils of, salt-deposits.—Metalliferous veins.—Summary on the +porphyritic conglomerate and gypseous formations.—Great subsidence with +partial elevations during the cretaceo-oolitic period.—On the elevation +and structure of the Cordillera.—Recapitulation on the tertiary +series.—Relation between movements of subsidence and volcanic +action.—Pampean formation.—Recent elevatory movements. +Long-continued volcanic action in the Cordillera.—Conclusion. +</p> + +<p><i>Valparaiso to Coquimbo.</i> I have already described the +general nature of the rocks in the low country north of Valparaiso, +consisting of granites, syenites, greenstones, and altered +feldspathic clay-slate. Near Coquimbo there is much hornblendic +rock and various dusky-coloured porphyries. I will describe only +one section in this district, namely, from near Illapel in a N.E. +line to the mines of Los Hornos, and thence in a north by east +direction to Combarbala, at the foot of the main Cordillera.</p> + +<p>Near Illapel, after passing for some distance over granite, +andesite, and andesitic porphyry, we come to a greenish stratified +feldspathic rock, which I believe is altered clay-slate, +conformably capped by porphyries and porphyritic conglomerate of +great thickness, dipping at an average angle of 20° to N.E. by +N. The uppermost beds consist of conglomerates and sandstone only a +little metamorphosed, and conformably covered by a gypseous +formation of very great thickness, but much denuded. This gypseous +formation, where first met with, lies in a broad valley or basin, a +little southward of the mines of Los Hornos: the lower half alone +contains gypsum, not in great masses as in the Cordillera, but in +innumerable thin layers, seldom more than an inch or two in +thickness. The gypsum is either opaque or transparent, and is +associated with carbonate of lime. The layers alternate with +numerous varying ones of a calcareous clay-shale (with strong +aluminous odour, adhering to the tongue, easily fusible into a pale +green glass), more or less indurated, either earthy and +cream-coloured, or greenish and hard. The more indurated varieties +have a compact, homogeneous, almost crystalline fracture, and +contain granules of crystallised oxide of iron. Some of the +varieties almost resemble honestones. There is also a little black, +hardly fusible, siliceo-calcareous clay-slate, like some of the +varieties alternating with gypsum on the Peuquenes range.</p> + +<p>The upper half of this gypseous formation is mainly formed of +the +<a name="page480"></a> +same calcareous clay-shale rock, but without any gypsum, and +varying extremely in nature: it passes from a soft, coarse, earthy, +ferruginous state, including particles of quartz, into compact +claystones with crystallised oxide of iron,—into porcellanic +layers, alternating with seams of calcareous matter,—and into +green porcelain-jasper, excessively hard, but easily fusible. +Strata of this nature alternate with much black and brown +siliceo-calcareous slate, remarkable from the wonderful number of +huge embedded logs of silicified wood. This wood, according to Mr. +R. Brown, is (judging from several specimens) all coniferous. Some +of the layers of the black siliceous slate contained irregular +angular fragments of imperfect pitchstone, which I believe, as in +the Uspallata range, has originated in a metamorphic process. There +was one bed of a marly tufaceous nature, and of little specific +gravity. Veins of agate and calcareous spar are numerous. The whole +of this gypseous formation, especially the upper half, has been +injected, metamorphosed, and locally contorted by numerous hillocks +of intrusive porphyries crowded together in an extraordinary +manner. These hillocks consist of purple claystone and of various +other porphyries, and of much white feldspathic greenstone passing +into andesite; this latter variety included in one case crystals of +orthitic and albitic feldspar touching each other, and others of +hornblende, chlorite, and epidote. The strata surrounding these +intrusive hillocks at the mines of Los Hornos, are intersected by +many veins of copper-pyrites, associated with much micaceous +iron-ore, and by some of gold: in the neighbourhood of these veins +the rocks are blackened and much altered. The gypsum near the +intrusive masses is always opaque. One of these hillocks of +porphyry was capped by some stratified porphyritic conglomerate, +which must have been brought up from below, through the whole +immense thickness of the overlying gypseous formation. The lower +beds of the gypseous formation resemble the corresponding and +probably contemporaneous strata of the main Cordillera; whilst the +upper beds in several respects resemble those of the Uspallata +chain, and possibly may be contemporaneous with them; for I have +endeavoured to show that the Uspallata beds were accumulated +subsequently to the gypseous or Neocomian formations of the +Cordillera.</p> + +<p>This pile of strata dips at an angle of about 20 degrees to N.E. +by N., close up to the foot of the Cuesta de Los Hornos, a crooked +range of mountains formed of intrusive rocks of the same nature +with the above described hillocks. Only in one or two places, on +this south-eastern side of the range, I noticed a narrow fringe of +the upper gypseous strata brushed up and inclined south-eastward +from it. On its north-eastern flank, and likewise on a few of the +summits, the stratified porphyritic conglomerate is inclined N.E.: +so that, if we disregard the very narrow anticlinal fringe of +gypseous strata at its S.E. foot, this range forms a second +uniclinal axis of elevation. Proceeding in a north-by-east +direction to the village of Combarbala, we come to a third +escarpment of the porphyritic conglomerate, dipping eastwards, and +forming the outer range of the main Cordillera. The lower beds were +here more jaspery than usual, and they included some white cherty +strata and red +<a name="page481"></a> +sandstones, alternating with purple claystone porphyry. Higher +up in the Cordillera there appeared to be a line of andesitic +rocks; and beyond them, a fourth escarpment of the porphyritic +conglomerate, again dipping eastwards or inwards. The overlying +gypseous strata, if they ever existed here, have been entirely +removed.</p> + +<p><i>Copper mines of Panuncillo.</i>—From Combarbala to +Coquimbo, I traversed the country in a zigzag direction, crossing +and recrossing the porphyritic conglomerate and finding in the +granitic districts an unusual number of mountain-masses composed of +various intrusive, porphyritic rocks, many of them andesitic. One +common variety was greenish-black, with large crystals of blackish +albite. At Panuncillo a short N.N.W. and S.S.E. ridge, with a +nucleus formed of greenstone and of a slate-coloured porphyry +including crystals of glassy feldspar, deserves notice, from the +very singular nature of the almost vertical strata composing it. +These consist chiefly of a finer and coarser granular mixture, not +very compact, of white carbonate of lime, of protoxide of iron and +of yellowish garnets (ascertained by Professor Miller), each grain +being an almost perfect crystal. Some of the varieties consist +exclusively of granules of the calcareous spar; and some contain +grains of copper ore, and, I believe, of quartz. These strata +alternate with a bluish, compact, fusible, feldspathic rock. Much +of the above granular mixture has, also, a pseudo-brecciated +structure, in which fragments are obscurely arranged in planes +parallel to those of the stratification, and are conspicuous on the +weathered surfaces. The fragments are angular or rounded, small or +large, and consist of bluish or reddish compact feldspathic matter, +in which a few acicular crystals of feldspar can sometimes be seen. +The fragments often blend at their edges into the surrounding +granular mass, and seem due to a kind of concretionary action.</p> + +<p>These singular rocks are traversed by many copper veins, and +appear to rest conformably on the granular mixture (in parts as +fine-grained as a sandstone) of quartz, mica, hornblende, and +feldspar; and this on fine-grained, common gneiss; and this on a +laminated mass, composed of pinkish <i>orthitic</i> feldspar, +including a few specks of hornblende; and lastly, this on granite, +which together with andesitic rocks, form the surrounding +district.</p> + +<p><i>Coquimbo: Mining district of Arqueros.</i>—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 +<a name="page482"></a> +variety is a dull-red, compact, finely brecciated stone, +containing much iron and innumerable white crystallised particles +of carbonate of lime, and minute extraneous fragments. Another +variety is almost equally common near S. Rosa; it has a bright +green, scanty basis, including distinct crystals and patches of +white carbonate of lime, and grains of red, semi-micaceous oxide of +iron; in parts the basis becomes dark green, and assumes an obscure +crystalline arrangement, and occasionally in parts it becomes soft +and slightly translucent like soapstone. These red and green rocks +are often quite distinct, and often pass into each other; the +passage being sometimes affected by a fine brecciated structure, +particles of the red and green matter being mingled together. Some +of the varieties appear gradually to become porphyritic with +feldspar; and all of them are easily fusible into pale or +dark-coloured beads, strongly attracted by the magnet. I should +perhaps have mistaken several of these stratified rocks for +submarine lavas, like some of those described at the Puente del +Inca, had I not examined, a few leagues eastward of this point, a +fine series of analogous but less metamorphosed, sedimentary beds +belonging to the gypseous formation, and probably derived from a +volcanic source.</p> + +<p> +This formation is intersected by numerous metalliferous veins, running, though +irregularly, N.W. and S.E., and generally at right angles to the many dikes. +The veins consist of native silver, of muriate of silver, an amalgam of silver, +cobalt, antimony, and arsenic,<a href="#fn-22.1" name="fnref-22.1" +id="fnref-22.1"><sup>[1]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-22.1" id="fn-22.1"></a> <a href="#fnref-22.1">[1]</a> +See the Report on M. Domeyko’s account of those mines, in the +“Comptes Rendus,” tome xiv, p. 560. +</p> + +<p><i>Section eastward, up the Valley of Coquimbo.</i>—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 +<a name="page483"></a> +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 <i>Hippurites +Chilensis,</i> 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 <i>G. +Orientalis,</i> Forbes MS.,—a cretaceous species (probably +upper greensand) from Verdachellum, in Southern India. These +fossils seem to occupy nearly the same position with those at the +Puente del Inca,—namely, at the top of the porphyritic +conglomerate, and at the base of the gypseous formation.</p> + +<p>A little above the Hacienda of Pluclaro, I made a detour on the +northern side of the valley, to examine the superincumbent gypseous +strata, which I estimated at 6,000 feet in thickness. The uppermost +beds of the porphyritic conglomerate, on which the gypseous strata +conformably rest, are variously coloured, with one very singular +and beautiful stratum composed of purple pebbles of various kinds +of porphyry, embedded in white calcareous spar, including cavities +lined with bright-green crystallised epidote. The whole pile of +strata belonging to both formations is inclined, apparently from +the above-mentioned axis of Pluclaro, at an angle of between 20 and +30 degrees to the east. I will here give a section of the principal +beds met with in crossing the entire thickness of the gypseous +strata.</p> + +<p>Firstly: above the porphyritic conglomerate formation, there is +a fine-grained, red, crystalline sandstone.</p> + +<p>Secondly: a thick mass of smooth-grained, calcareo-aluminous, +shaly rock, often marked with dendritic manganese, and having, +where most compact, the external appearance of honestone. It is +easily fusible. I shall for the future, for convenience’ sake, call +this variety pseudo-honestone. Some of the varieties are quite +black when freshly broken, but all weather into a yellowish-ash +coloured, soft, earthy substance, precisely as is the case with the +compact shaly rocks of the Peuquenes range. This stratum is of the +same general nature with many of the beds near Los Hornos in the +Illapel section. In this second bed, or in the underlying red +sandstone (for the surface was partially concealed by detritus), +there was a thick mass of gypsum, having the same mineralogical +characters with the great beds described in our sections across the +Cordillera.</p> + +<p>Thirdly: a thick stratum of fine-grained, red, sedimentary +matter, easily fusible into a white glass, like the basis of +claystone porphyry; but in parts jaspery, in parts brecciated, and +including crystalline specks of carbonate of lime. In some of the +jaspery layers, and in some of the black siliceous slaty bands, +there were irregular seams of imperfect pitchstone, undoubtedly of +metamorphic origin, and other seams of brown, crystalline +limestone. Here, also, were masses, externally resembling +ill-preserved silicified wood.</p> + +<p> +<a name="page484"></a> +Fourthly and fifthly: calcareous pseudo-honestone; and a thick +stratum concealed by detritus.</p> + +<p>Sixthly: a thinly stratified mass of bright green, compact, +smooth-grained, calcareo-argillaceous stone, easily fusible, and +emitting a strong aluminous odour: the whole has a highly +angulo-concretionary structure; and it resembles, to a certain +extent, some of the upper tufaceo-infusorial deposits of the +Patagonian tertiary formation. It is in its nature allied to our +pseudo-honestone, and it includes well characterised layers of that +variety; and other layers of a pale green, harder, and brecciated +variety; and others of red sedimentary matter, like that of bed +Three. Some pebbles of porphyries are embedded in the upper +part.</p> + +<p>Seventhly: red sedimentary matter or sandstone like that of bed +One, several hundred feet in thickness, and including jaspery +layers, often having a finely brecciated structure.</p> + +<p>Eighthly: white, much indurated, almost crystalline tuff, +several hundred feet in thickness, including rounded grains of +quartz and particles of green matter like that of bed Six. Parts +pass into a very pale green, semi-porcellanic stone.</p> + +<p>Ninthly: red or brown coarse conglomerate, three or four hundred +feet thick, formed chiefly of pebbles of porphyries, with volcanic +particles, in an arenaceous, non-calcareous, fusible basis: the +upper two feet are arenaceous without any pebbles.</p> + +<p>Tenthly: the last and uppermost stratum here exhibited, is a +compact, slate-coloured porphyry, with numerous elongated crystals +of glassy feldspar, from one hundred and fifty to two hundred feet +in thickness; it lies strictly conformably on the underlying +conglomerate, and is undoubtedly a submarine lava.</p> + +<p>This great pile of strata has been broken up in several places +by intrusive hillocks of purple claystone porphyry, and by dikes of +porphyritic greenstone: it is said that a few poor metalliferous +veins have been discovered here. From the fusible nature and +general appearance of the finer-grained strata, they probably owe +their origin (like the allied beds of the Uspallata range, and of +the Upper Patagonian tertiary formations), to gentle volcanic +eruptions, and to the abrasion of volcanic rocks. Comparing these +beds with those in the mining district of Arqueros, we see at both +places rocks easily fusible, of the same peculiar bright green and +red colours, containing calcareous matter, often having a finely +brecciated structure, often passing into each other, and often +alternating together: hence I cannot doubt that the only difference +between them, lies in the Arqueros beds having been more +metamorphosed (in conformity with their more dislocated and +injected condition), and consequently in the calcareous matter, +oxide of iron and green colouring matter, having been segregated +under a more crystalline form.</p> + +<p>The strata are inclined, as before stated, from 20° to +30° 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 +<a name="page485"></a> +met with, apparently a southern prolongation of the above chain; +and eastward of it we have an escarpment of the porphyritic +conglomerate, with the strata inclined at a small angle eastward, +which makes the third escarpment, including that nearest the coast. +Proceeding up the valley we come to another north and south line of +granite, andesite, and blackish porphyry, which seem to lie in an +irregular trough of the porphyritic conglomerate. Again, on the +south side of the R. Claro, there are some irregular granitic +hills, which have thrown off the strata of porphyritic conglomerate +to the N.W. by W.; but the stratification here has been much +disturbed. I did not proceed any farther up the valley, and this +point is about two-thirds of the distance between the Pacific and +the main Cordillera.</p> + +<p>I will describe only one other section, namely, on the north +side of the R. Claro, which is interesting from containing fossils: +the strata are much dislocated by faults and dikes, and are +inclined to the north, towards a mountain of andesite and porphyry, +into which they appear to become almost blended. As the beds +approach this mountain, their inclination increases up to an angle +of 70°, and in the upper part, the rocks become highly +metamorphosed. The lowest bed visible in this section, is a +purplish hard sandstone. Secondly, a bed two or three hundred feet +thick, of a white siliceous sandstone, with a calcareous cement, +containing seams of slaty sandstone, and of hard yellowish-brown +(dolomitic?) limestone; numerous, well-rounded, little pebbles of +quartz are included in the sandstone. Thirdly, a dark coloured +limestone with some quartz pebbles, from fifty to sixty feet in +thickness, containing numerous silicified shells, presently to be +enumerated. Fourthly, very compact, calcareous, jaspery sandstone, +passing into (fifthly) a great bed, several hundred feet thick, of +conglomerate, composed of pebbles of white, red, and purple +porphyries, of sandstone and quartz, cemented by calcareous matter. +I observed that some of the finer parts of this conglomerate were +much indurated within a foot of a dike eight feet in width, and +were rendered of a paler colour with the calcareous matter +segregated into white crystallised particles; some parts were +stained green from the colouring matter of the dike. Sixthly, a +thick mass, obscurely stratified, of a red sedimentary stone or +sandstone, full of crystalline calcareous matter, imperfect +crystals of oxide of iron, and I believe of feldspar, and therefore +closely resembling some of the highly metamorphosed beds at +Arqueros: this bed was capped by, and appeared to pass in its upper +part into, rocks similarly coloured, containing calcareous matter, +and abounding with minute crystals, mostly elongated and glassy, of +reddish albite. Seventhly, a conformable stratum of fine reddish +porphyry with large crystals of (albitic?) feldspar; probably a +submarine lava. Eighthly, another conformable bed of green +porphyry, with specks of green earth and cream-coloured crystals of +feldspar. I believe that there are other superincumbent crystalline +strata and submarine lavas, but I had not time to examine them.</p> + +<p>The upper beds in this section probably correspond with parts of +the great gypseous formation; and the lower beds of red sandstone +conglomerate and fossiliferous limestone no doubt are the +equivalents of +<a name="page486"></a> +the Hippurite stratum, seen in descending from Arqueros to +Pluclaro, which there lies conformably upon the porphyritic +conglomerate formation. The fossils found in the third bed, consist +of:—</p> + +<p>Pecten Dufreynoyi, d’Orbigny, “Voyage, Part Pal.”<br/> +<small>This species, which occurs here in vast numbers, according +to M. D’Orbigny, resembles certain cretaceous forms.</small></p> + +<p>Ostrea hemispherica, d’Orbigny, “Voyage” etc.<br/> +<small>Also resembles, according to the same author, cretaceous +forms.</small></p> + +<p>Terebratula ænigma, d’Orbigny, “Voyage” etc. (Pl. XXII, +Figs. 10-12.)<br/> +<small>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.</small></p> + +<p>Spirifer linguiferoides, E. Forbes.<br/> +<small>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.</small></p> + +<p>Ammonites, imperfect impression of.</p> + +<p>M. Domeyko has sent to France a collection of fossils, which, I +presume, from the description given, must have come from the +neighbourhood of Arqueros; they consist of:—</p> + +<p>Pecten Dufreynoyi, d’Orbigny, “Voyage” Part Pal.<br/> +Ostrea hemispherica, d’Orbigny, “Voyage” Part Pal.<br/> +Turritella Andii, d’Orbigny, “Voyage” Part Pal. (Pleurotomaria +Humboldtii of Von Buch).<br/> +Hippurites Chilensis, d’Orbigny, “Voyage” Part Pal.<br/> +<small>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.</small></p> + +<p>Nautilus Domeykus, d’Orbigny, “Voyage” Part Pal.<br/> +Terebratula ænigma, d’Orbigny, “Voyage” Part Pal.<br/> +Terebratula ignaciana, d’Orbigny, “Voyage” Part Pal.<br/> +<small>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</small> +<a name="page487"></a> +<small>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.</small></p> + +<p> +The remarks given on the several foregoing shells, show that, in M. +d’Orbigny’s opinion, the Pecten, Ostrea, Turritella, and Hippurite +indicate the cretaceous period; and the 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;<a +href="#fn-22.2" name="fnref-22.2" id="fnref-22.2"><sup>[2]</sup></a>—both +of which genera occur at the Puente del Inca. +</p> + +<p class="footnote"> +<a name="fn-22.2" id="fn-22.2"></a> <a href="#fnref-22.2">[2]</a> +D’Orbigny, “Voyage,” Part Géolog., p. 242. +</p> + +<p><i>Coquimbo to Guasco.</i>—The rocks near the coast, and +some way inland, do not differ from those described northwards of +Valparaiso: we have +<a name="page488"></a> +much greenstone, syenite, feldspathic and jaspery slate, and +grauwackes having a basis like that of claystone; there are some +large tracts of granite, in which the constituent minerals are +sometimes arranged in folia, thus composing an imperfect gneiss. +There are two large districts of mica-schists, passing into glossy +clay-slate, and resembling the great formation in the Chonos +Archipelago. In the valley of Guasco, an escarpment of porphyritic +conglomerate is first seen high up the valley, about two leagues +eastward of the town of Ballenar. I heard of a great gypseous +formation in the Cordillera; and a collection of shells made there +was given me. These shells are all in the same condition, and +appear to have come from the same bed: they consist of:—</p> + +<p>Turritella Andii, d’Orbigny, “Voyage,” Part Pal.<br/> +Pecten Dufreynoyi, d’Orbigny, “Voyage,” Part Pal.<br/> +Terebatula ignaciana, d’Orbigny, “Voyage,” Part Pal.<br/> +<small>The relations of these species have been given under the +head of Coquimbo.</small></p> + +<p>Terebratula ænigma, d’Orbigny, “Voyage,” Part Pal.<br/> +<small>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.</small></p> + +<p>Spirifer Chilensis, E. Forbes.<br/> +<small>Professor Forbes remarks that this fossil resembles several +carboniferous limestone Spirifers; and that it is also related to +some liassic species, as S. Wolcotii.</small></p> + +<p><br/> +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.</p> + +<p> +<i>Valley of Copiapo.</i>—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 +<a name="page489"></a> +not more than thirty miles, but along the valley the distance is much greater. +Wherever the valley trended very southerly, I have endeavoured to contract the +section into its true proportion. This valley, I may add, rises much more +gently than any other valley which I saw in Chile. +</p> + +<p> +To commence with our section, for a short distance above the town we have hills +of the granitic series, together with some of that rock [A], which I suspect to +be altered clay-slate, but which Professor G. Rose, judging from specimens +collected by Meyen at P. Negro, states is serpentine passing into greenstone. +We then come suddenly to the great gypseous formation [B], without having +passed over, differently from, in all the sections hitherto described, any of +the porphyritic conglomerate. The strata are at first either horizontal or +gently inclined westward; then highly inclined in various directions, and +contorted by underlying masses of intrusive rocks; and lastly, they have a +regular eastward dip, and form a tolerably well pronounced north and south line +of hills. This formation consists of thin strata, with innumerable +alternations, of black, calcareous slate-rock, of calcareo-aluminous stones +like those at Coquimbo, which I have called pseudo-honestones of green jaspery +layers, and of pale-purplish, calcareous, soft rotten-stone, including seams +and veins of gypsum. These strata are conformably overlaid by a great thickness +of thinly stratified, compact limestone with included crystals of carbonate of +lime. At a place called Tierra Amarilla, at the foot of a mountain thus +composed there is a broad vein, or perhaps stratum, of a beautiful and curious +crystallised mixture, composed, according to Professor G. Rose,<a +href="#fn-22.3" name="fnref-22.3" id="fnref-22.3"><sup>[3]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-22.3" id="fn-22.3"></a> <a href="#fnref-22.3">[3]</a> +Meyen’s “Reise,” etc., Th. I, s. 394. +</p> + +<p><i>Second axis of elevation.</i>—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. <i>Third axis of +elevation.</i>—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 +<a name="page490"></a> +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, +<i>and i believe that they are subaqueous volcanic bombs</i>; they +differ from <i>subaerial</i> bombs only in not being vesicular. Bed +4; a dull purplish-red, hard conglomerate, with crystallised +particles and veins of carbonate of lime, from three hundred to +four hundred feet in thickness. The pebbles are of claystone +porphyries of many varieties; they are tolerably well rounded, and +vary in size from a large apple to a man’s head. This bed includes +three layers of coarse, black, calcareous, somewhat slaty rock: the +upper part passes into a compact red sandstone.</p> + +<p>In a formation so highly variable in mineralogical nature, any +division not founded on fossil remains, must be extremely +arbitrary: nevertheless, the beds below the last conglomerate may, +in accordance with all the sections hitherto described, be +considered as belonging to the porphyritic conglomerate, and those +above it to the gypseous formation, marked [E] in the section. The +part of the valley in which the following beds are seen is near +Potrero Seco. Bed 5, compact, fine-grained, pale greenish-grey, +non-calcareous, indurated mudstone, easily fusible into a pale +green and white glass. Bed 6, purplish, coarse-grained, hard +sandstone, with broken crystals of feldspar and crystallised +particles of carbonate of lime; it possesses a slightly nodular +structure. Bed 7, blackish-grey, much indurated, calcareous +mudstone, with extraneous particles of unequal size; the whole +being in parts finely brecciated. In this mass there is a stratum, +twenty feet in thickness, of impure gypsum. Bed 8, a greenish +mudstone, with several layers of gypsum. Bed 9, +<a name="page491"></a> +a highly indurated, easily fusible, white tuff, thickly mottled +with ferruginous matter, and including some white semi-porcellanic +layers, which are interlaced with ferruginous veins. This stone +closely resembles some of the commonest varieties in the Uspallata +chain. Bed 10, a thick bed of rather bright green, indurated +mudstone or tuff, with a concretionary nodular structure so +strongly developed that the whole mass consists of balls. I will +not attempt to estimate the thickness of the strata in the gypseous +formation hitherto described, but it must certainly be very many +hundred feet. Bed 11 is at least 800 feet in thickness: it consists +of thin layers of whitish, greenish, or more commonly brown, +fine-grained, indurated tuffs, which crumble into angular +fragments: some of the layers are semi-porcellanic, many of them +highly ferruginous, and some are almost composed of carbonate of +lime and iron with drusy cavities lined with quartzf-crystals. Bed +12, dull purplish or greenish or dark-grey, very compact and much +indurated mudstone: estimated at 1,500 feet in thickness: in some +parts this rock assumes the character of an imperfect coarse +clay-slate; but viewed under a lens, the basis always has a mottled +appearance, with the edges of the minute component particles +blending together. Parts are calcareous, and there are numerous +veins of highly crystalline carbonate of lime charged with iron. +The mass has a nodular structure, and is divided by only a few +planes of stratification: there are, however, two layers, each +about eighteen inches thick, of a dark brown, finer-grained stone, +having a conchoidal, semi-porcellanic fracture, which can be +followed with the eye for some miles across the country.</p> + +<p>I believe this last great bed is covered by other nearly similar +alternations; but the section is here obscured by a tilt from the +next porphyritic chain, presently to be described. I have given +this section in detail, as being illustrative of the general +character of the mountains in this neighbourhood; but it must not +be supposed that any one stratum long preserves the same character. +At a distance of between only two and three miles the green +mudstones and white indurated tuffs are to a great extent replaced +by red sandstone and black calcareous shaly rocks, alternating +together. The white indurated tuff, bed 11, here contains little or +no gypsum, whereas on the northern and opposite side of the valley, +it is of much greater thickness and abounds with layers of gypsum, +some of them alternating with thin seams of crystalline carbonate +of lime. The uppermost, dark-coloured, hard mudstone, bed 12, is in +this neighbourhood the most constant stratum. The whole series +differs to a considerable extent, especially in its upper part, +from that met with at [BB], in the lower part of the valley; +nevertheless, I do not doubt that they are equivalents. <i>Fourth +axis of elevation (Valley of Copiapo).</i>—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 +<a name="page492"></a> +central great mass of this latter rock, is covered on the +eastern side by a black, fine-grained, highly micaceous slate, +which, together with the succeeding mountains of porphyry, are +traversed by numerous white dikes, branching from the andesite, and +some of them extending in straight lines, to a distance of at least +two miles. The mountains of porphyry eastward of the micaceous +schist soon, but gradually, assume (as observed in so many other +cases) a stratified structure, and can then be recognised as a part +of the porphyritic conglomerate formation. These strata [G] are +inclined at a high angle to the S.E., and form a mass from fifteen +hundred to two thousand feet in thickness. The gypseous masses to +the west already described, dip directly towards this axis, with +the strata only in a few places (one of which is represented in the +section) thrown from it: hence this fourth axis is mainly uniclinal +towards the S.E., and just like our third axis, only locally +anticlinal.</p> + +<p>The above strata of porphyritic conglomerate [G] with their +south-eastward dip, come abruptly up against beds of the gypseous +formation [H], which are gently, but irregularly, inclined +westward: so that there is here a synclinal axis and great fault. +Further up the valley, here running nearly north and south, the +gypseous formation is prolonged for some distance; but the +stratification is unintelligible, the whole being broken up by +faults, dikes, and metalliferous veins. The strata consist chiefly +of red calcareous sandstones, with numerous veins in the place of +layers, of gypsum; the sandstone is associated with some black +calcareous slate-rock, and with green pseudo-honestones, passing +into porcelain-jasper. Still further up the valley, near Las +Amolanas [I], the gypseous strata become more regular, dipping at +an angle of between 30 and 40 degrees to W.S.W., and conformably +overlying, near the mouth of the ravine of Jolquera, strata [K] of +porphyritic conglomerate. The whole series has been tilted by a +partially concealed axis [L], of granite, andesite, and a granitic +mixture of white feldspar, quartz, and oxide of iron.</p> + +<p><i>Fifth axis of elevation (Valley of Copiapo, near Las +Amolanas).</i>—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.</p> + +<p>Above these porphyries comes a coarse, arenaceous conglomerate, +the lower half white and the upper half of a pink colour, composed +chiefly of pebbles of various porphyries, but with some of red +sandstone +<a name="page493"></a> +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 <i> +Las Amolanas</i>), and certainly belonging to the gypseous +formation, as does probably the immediately underlying +conglomerate. In this yellowish sandstone there are layers of white +and pale-red siliceous conglomerate; other layers with small, +well-rounded pebbles of white quartz, like the bed at the R. Claro +at Coquimbo; others of a greenish, fine-grained, less siliceous +stone, somewhat resembling the pseudo-honestones lower down the +valley; and lastly, others of a black calcareous shale-rock. In one +of the layers of conglomerate, there was embedded a fragment of +mica-slate, of which this is the first instance; hence perhaps, it +is from a formation of mica-slate, that the numerous small pebbles +of quartz, both here and at Coquimbo, have been derived. Not only +does the siliceous sandstone include layers of the black, thinly +stratified, not fissile, calcareous shale-rock, but in one place +the whole mass, especially the upper part, was, in a marvellously +short horizontal distance, after frequent alternations, replaced by +it. When this occurred, a mountain-mass, several thousand feet in +thickness was thus composed; the black calcareous shale-rock, +however, always included some layers of the pale-yellowish +siliceous sandstone, of the red conglomerate, and of the greenish +jaspery and pseudo-honestone varieties. It likewise included three +or four widely separated layers of a brown limestone, abounding +with shells immediately to be described. This pile of strata was in +parts traversed by many veins of gypsum. The calcareous shale-rock, +though when freshly broken quite black, weathers into an ash- +colour: in which respect and in general appearance, it perfectly +resembles those great fossiliferous beds of the Peuquenes range, +alternating with gypsum and red sandstone, described in the last +chapter.</p> + +<p>The shells out of the layers of brown limestone, included in the +black calcareous shale-rock, which latter, as just stated, replaces +the white siliceous sandstone, consist of:—</p> + +<p>Pecten Dufreynoyi, d’Orbigny, “Voyage,” Part Pal.<br/> +Turritella Andii, d’Orbigny, “Voyage,” Part Pal.<br/> +<br/> + Astarte Darwinii, E. Forbes.<br/> +Gryphæa Darwinii, E. Forbes.<br/> +<small>An intermediate form between G. gigantea and G. +incurva.</small></p> + +<p>Gryphæa nov. spec.?, E. Forbes.<br/> +Perna Americana, E. Forbes.<br/> +Avicula, nov. spec.<br/> +<small>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.</small></p> + +<p> +<a name="page494"></a> +Terebratula ænigma, d’Orbigny, (var. of do. E. +Forbes.)<br/> +<small>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.</small></p> + +<p>Plagiostoma and Ammonites, fragments of.</p> + +<p>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 <i> +true</i> 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.</p> + +<p>I will reiterate the evidence on the association of these +several shells in the several localities.</p> + +<p class="center"> +<i>Coquimbo.</i> +</p> + +<p><small>In the same bed, Rio Claro:<br/> + Pecten Dufreynoyi.<br/> + Ostrea hemispherica.<br/> + Terebratula ænigma.<br/> + Spirifer linguiferoides.</small></p> + +<p><small>Same bed, near Arqueros:<br/> + Hippurites Chilensis.<br/> + Gryphæa orientalis.</small></p> + +<p><small>Collected by M. Domeyko from the same locality, +apparently near Arqueros:<br/> + Terebratula ænigma and Terebratula +ignaciana, in same block of limestone.<br/> + Pecten Dufreynoyi.<br/> + Ostrea hemispherica.<br/> + Hippurites Chilensis.<br/> + Turritella Andii.<br/> + Nautilus Domeykus.</small></p> + +<p class="center"> +<i>Guasco.</i> +</p> + +<p><small>In a collection from the Cordillera, given me: the +specimens all in the same condition:<br/> + Pecten Dufreynoyi.<br/> + Turritella Andii.<br/> + Terebratula ignaciana.<br/> + Terebratula ænigma, <i>var.</i><br/> + Spirifer Chilensis.</small></p> + +<p class="center"> +<a name="page495"></a><i>Copiapo.</i> +</p> + +<p><small>Mingled together in alternating beds in the main valley +of Copiapo near Las Amolanas, and likewise higher up the +valley:<br/> + Pecten Dufreynoyi.<br/> + Turritella Andii.<br/> + Terebratula ænigma, <i>var.</i>, as +at Guasco.<br/> + Astarte Darwinii.<br/> + Gryphæa Darwinii.<br/> + Gryphæa nov. species?<br/> + Perna Americana.<br/> + Avicula, nov. species.</small></p> + +<p><small>Main valley of Copiapo, apparently same formation with +that of Amolanas:<br/> + Terebratula ænigma +(true).</small></p> + +<p><small>In the same bed, high up the great lateral valley of the +Despoblado, in the ravine of Maricongo:<br/> + Terebratula ænigma (true).<br/> + Pecten Dufreynoyi.<br/> + Gryphæa Darwinii?</small></p> + +<p>Considering this table, I think it is impossible to doubt that +all these fossils belong to the same formation. If, however, the +species from Las Amolanas, in the Valley of Copiapo, had, as in the +case of those from Guasco, been separately examined, they would +probably have been ranked as oolitic; for, although no Spirifers +were found here, all the other species, with the exception of the +Pecten, Turritella, and Astarte, have a more ancient aspect than +cretaceous forms. On the other hand, taking into account the +evidence derived from the cretaceous character of these three +shells, and of the Hippurites, 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.</p> + +<p>To return to our section near Las Amolanas:—Above the +yellow siliceous sandstone, or the equivalent calcareous +slate-rock, with its bands of fossil-shells, according as the one +or other prevails, there is a pile of strata, which cannot be less +than from two to three thousand feet in thickness, in main part +composed of a coarse, bright red conglomerate, with many +intercalated beds of red sandstone, and some of green and other +coloured porcelain-jaspery layers. The included pebbles are +well-rounded, varying from the size of an egg to that of a +cricket-ball, with a few larger; and they consist chiefly of +porphyries. The basis of the conglomerate, as well as some of the +alternating thin beds, are formed of a red, rather harsh, easily +fusible sandstone, with crystalline calcareous particles. This +whole great pile is remarkable from the thousands of huge, +embedded, silicified trunks of trees, one of which was eight feet +long, and another eighteen feet in circumference: how marvellous it +is, that every vessel in so thick a mass of wood should have been +converted into silex! I brought home many specimens, and all of +them, according to Mr. R. Brown, present a coniferous +structure.</p> + +<p>Above this great conglomerate, we have from two to three hundred +feet in thickness of red sandstone; and above this, a stratum of +black calcareous slate-rock, like that which alternates with +and +<a name="page496"></a> +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 <i>above</i> the porphyritic +conglomerate at 7,000 feet.</p> + +<p>The fossils before enumerated, from the limestone-layers in the +whitish siliceous sandstone, are now covered, on the least +computation, by strata from 5,000 to 6,000 feet in thickness. +Professor E. Forbes thinks that these shells probably lived at a +depth of from about 30 to 40 fathoms, that is from 180 to 240 feet; +anyhow, it is impossible that they could have lived at the depth of +from 5,000 to 6,000 feet. Hence in this case, as in that of the +Puente del Inca, we may safely conclude that the bottom of the sea +on which the shells lived, subsided, so as to receive the +superincumbent submarine strata: and this subsidence must have +taken place during the existence of these shells; for, as I have +shown, some of them occur high up as well as low down in the +series. That the bottom of the sea subsided, is in harmony with the +presence of the layers of coarse, well-rounded pebbles included +throughout this whole pile of strata, as well as of the great upper +mass of conglomerate from 2,000 to 3,000 feet thick; for coarse +gravel could hardly have been formed or spread out at the profound +depths indicated by the thickness of the strata. The subsidence, +also, must have been slow to have allowed of this often-recurrent +spreading out of the pebbles. Moreover, we shall presently see that +the surfaces of some of the streams of porphyritic lava beneath the +gypseous formation, are so highly amygdaloidal that it is scarcely +possible to believe that they flowed under the vast pressure of a +deep ocean. The conclusion of a +<a name="page497"></a> +great subsidence during the existence of these cretaceo-oolitic +fossils, may, I believe, be extended to the district of Coquimbo, +although owing to the fossiliferous beds there not being directly +covered by the upper gypseous strata, which in the section north of +the valley are about 6,000 feet in thickness, I did not there +insist on this conclusion.</p> + +<p>The pebbles in the above conglomerates, both in the upper and +lower beds, are all well rounded, and, though chiefly composed of +various porphyries, there are some of red sandstone and of a +jaspery stone, both like the rocks intercalated in layers in this +same gypseous formation; there was one pebble of mica-slate and +some of quartz, together with many particles of quartz. In these +respects there is a wide difference between the gypseous +conglomerates and those of the porphyritic-conglomerate formation, +in which latter, angular and rounded fragments, almost exclusively +composed of porphyries, are mingled together, and which, as already +often remarked, probably were ejected from craters deep under the +sea. From these facts I conclude, that during the formation of the +conglomerates, land existed in the neighbourhood, on the shores of +which the innumerable pebbles were rounded and thence dispersed, +and on which the coniferous forests flourished—for it is +improbable that so many thousand logs of wood should have drifted +from any great distance. This land, probably islands, must have +been mainly formed of porphyries, with some mica-slate, whence the +quartz was derived, and with some red sandstone and jaspery rocks. +This latter fact is important, as it shows that in this district, +even previously to the deposition of the lower gypseous or +cretaceo-oolitic beds, strata of an analogous nature had elsewhere, +no doubt in the more central ranges of the Cordillera, been +elevated; thus recalling to our minds the relations of the Cumbre +and Uspallata chains. Having already referred to the great lateral +valley of the Despoblado, I may mention that above the 2,700 feet +of red and white sandstone and dark mudstone, there is a vast mass +of coarse, hard, red conglomerate, some thousand feet in thickness, +which contains much silicified wood, and evidently corresponds with +the great upper conglomerate at Las Amolanas: here, however, the +conglomerate consists almost exclusively of pebbles of granite, and +of disintegrated crystals of reddish feldspar and quartz firmly +recemented together. In this case, we may conclude that the land +whence the pebbles were derived, and on which the now silicified +trees once flourished, was formed of granite.</p> + +<p>The mountains near Las Amolanas, composed of the +cretaceo-oolitic strata, are interlaced with dikes like a spider’s +web, to an extent which I have never seen equalled, except in the +denuded interior of a volcanic crater: north and south lines, +however, predominate. These dikes are composed of green, white, and +blackish rocks, all porphyritic with feldspar, and often with large +crystals of hornblende. The white varieties approach closely in +character to andesite, which composes as we have seen, the injected +axes of so many of the lines of elevation. Some of the green +varieties are finely laminated, parallel to the walls of the +dikes.</p> + +<p><i>Sixth axis of elevation (Valley of Copiapo).</i>—This +axis consists of +<a name="page498"></a> +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 <i> +Turritella Andii, Pecten Dufreynoyi, Terebratula ænigma, +var.,</i> 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.</p> + +<p>Eastward of the broad andesitic axis of this sixth line, and +penetrated by many dikes from it, there is a great formation [P] of +mica-schist, with its usual variations, and passing in one part +into a ferruginous quartz-rock. The folia are curved and highly +inclined, generally dipping eastward. It is probable that this +mica-schist is an old formation, connected with the granitic rocks +and metamorphic schists near the coast; and that the one fragment +of mica-slate, and the pebbles of quartz low down in the gypseous +formation at Las Amolanas, have been derived from it. The +mica-schist is succeeded by stratified porphyritic conglomerate [Q] +of great thickness, dipping eastward with a high inclination: I +have included this latter mountain-mass in the same anticlinal axis +with the porphyritic streams [NN]; but I am far from sure that the +two masses may not have been independently upheaved.</p> + +<p><i>Seventh axis of elevation.</i>—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, +<a name="page499"></a> +that the porphyritic beds, or submarine lavas, preponderate +greatly in bulk over the alternating sedimentary layers, which have +been but little metamorphosed: these latter consist of fine-grained +red tuffs and of whitish volcanic grit-stones, together with much +of a singular, compact rock, having an almost crystalline basis, +finely brecciated with red and green fragments, and occasionally +including a few large pebbles. The porphyritic lavas are highly +amygdaloidal, both on their upper and lower surfaces; they consist +chiefly of claystone porphyry, but with one common variety, like +some of the streams at the Puente del Inca, having a grey mottled +basis, abounding with crystals of red hydrous oxide of iron, green +ones apparently of epidote, and a few glassy ones of feldspar. This +pile of strata differs considerably from the basal strata of the +Cordillera in Central Chile, and may possibly belong to the upper +and gypseous series: I saw, however, in the bed of the valley, one +fragment of porphyritic breccia-conglomerate, exactly like those +great masses met with in the more southern parts of Chile.</p> + +<p>Finally, I must observe, that though I have described between +the town of Copiapo and the western flank of the main Cordillera +seven or eight axes of elevation, extending nearly north and south, +it must not be supposed that they all run continuously for great +distances. As was stated to be the case in our sections across the +Cordillera of Central Chile, so here most of the lines of +elevation, with the exception of the first, third, and fifth, are +very short. The stratification is everywhere disturbed and +intricate; nowhere have I seen more numerous faults and dikes. The +whole district, from the sea to the Cordillera, is more or less +metalliferous; and I heard of gold, silver, copper, lead, mercury, +and iron veins. The metamorphic action, even in the lower strata, +has certainly been far less here than in Central Chile.</p> + +<p><i>Valley of the Despoblado.</i>—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 +<a name="page500"></a> +mountains composed of a new kind, having a compact, smooth, +cream-coloured basis, including only a few crystals of feldspar, +and mottled with dendritic spots of oxide of iron. There were also +some mountains of a porphyry with a brick-red basis, containing +irregular, often lens-shaped, patches of compact feldspar, and +crystals of feldspar, which latter to my surprise I find to be +orthite.</p> + +<p>At the foot of the first ridge of the main Cordillera, in the +ravine of Maricongo, and at an elevation which, from the extreme +coldness and appearance of the vegetation, I estimated at about ten +thousand feet, I found beds of white sandstone and of limestone +including the Pecten Dufreynoyi, Terebratula 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.</p> + +<p> +The crest of this first main ridge and the surrounding mountains, with the +exception of a few lofty pinnacles, are capped by a great thickness of a +horizontally stratified, tufaceous deposit. The lowest bed is of a pale purple +colour, hard, fine-grained, and full of broken crystals of feldspar and scales +of mica. The middle bed is coarser, and less hard, and hence weathers into very +sharp pinnacles; it includes very small fragments of granite, and innumerable +ones of all sizes of grey vesicular trachyte, some of which were distinctly +rounded. The uppermost bed is about two hundred feet in thickness, of a darker +colour and apparently hard: but I had not time to ascend to it. These three +horizontal beds may be seen for the distance of many leagues, especially +westward or in the direction of the Pacific, capping the summits of the +mountains, and standing on the opposite sides of the immense valleys at exactly +corresponding heights. If united they would form a plain, inclined very +slightly towards the Pacific; the beds become thinner in this direction, and +the tuff (judging from one point to which I ascended, some way down the valley) +finer-grained and of less specific gravity, though still compact and sonorous +under the hammer. The gently inclined, almost horizontal stratification, the +presence of some rounded pebbles, and the compactness of the lowest bed, though +rendering it probable, would not have convinced me that this mass had been of +subaqueous origin, for it is known that volcanic ashes falling on land and +moistened by rain often become hard and stratified; but beds thus originating, +and owing their consolidation to atmospheric moisture, would have covered +almost equally every neighbouring summit, high and low, and would not have left +those above a certain exact level absolutely bare; this circumstance seems to +me to prove that the volcanic ejections were arrested at their present, widely +extended, equable level, and there consolidated by some other means than simple +atmospheric moisture; and this no doubt must have been a sheet of water. A lake +at this great height, and without a barrier on any one side, is out of the +question; consequently we must conclude that the tufaceous matter was anciently +deposited beneath the sea. It was certainly +<a name="page501"></a> +deposited before the excavation of the valleys, or at least before their final +enlargement;<a href="#fn-22.4" name="fnref-22.4" +id="fnref-22.4"><sup>[4]</sup></a> and I may add, that Mr. Lambert, a gentleman +well acquainted with this country, informs me, that in ascending the ravine of +Santandres (which branches off from the Despoblado) he met with streams of lava +and much erupted matter capping all the hills of granite and porphyry, with the +exception of some projecting points; he also remarked that the valleys had been +excavated subsequently to these eruptions. +</p> + +<p class="footnote"> +<a name="fn-22.4" id="fn-22.4"></a> <a href="#fnref-22.4">[4]</a> +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. +</p> + +<p> +This volcanic formation, which I am informed by Mr. Lambert extends far +northward, is of interest, as typifying what has taken place on a grander scale +on the corresponding western side of the Cordillera of Peru. Under another +point of view, however, it possesses a far higher interest, as confirming that +conclusion drawn from the structure of the fringes of stratified shingle which +are prolonged from the plains at the foot of the Cordillera far up the +valleys,—namely, that this great range has been elevated in mass to a +height of between eight and nine thousand feet;<a href="#fn-22.5" +name="fnref-22.5" id="fnref-22.5"><sup>[5]</sup></a> and now, judging from this +tufaceous deposit, we may conclude that the horizontal elevation has been in +the district of Copiapo about ten thousand feet. +</p> + +<p class="footnote"> +<a name="fn-22.5" id="fn-22.5"></a> <a href="#fnref-22.5">[5]</a> +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. +</p> + +<p class="center"> +No. 40 +</p> + +<div class="fig" style="width:100%;"> +<img src="images/geono40.jpg" width="371" height="121" alt="[Illustration]" /> +</div> + +<p>In the valley of the Despoblado, the stratification, as before +remarked has been much disturbed, and in some points to a greater +degree than I have anywhere else seen. I will give two cases: a +very thick mass of thinly stratified red sandstone, including beds +of conglomerate, has been crushed together (as represented in +figure 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 +<a name="page502"></a> +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.</p> + +<p><i>On the eruptive sources of the porphyritic claystone and +greenstone lavas.</i>—In Central Chile, from the extreme +metamorphic action, it is in most parts difficult to distinguish +between the streams of porphyritic lava and the porphyritic +breccia-conglomerate, but here, at Copiapo, they are generally +perfectly distinct, and in the Despoblado, I saw for the first +time, two great strata of purple claystone porphyry, after having +been for a considerable space closely united together, one above +the other, become separated by a mass of fragmentary matter, and +then both thin out;—the lower one more rapidly than the upper +and greater stream. Considering the number and thickness of the +streams of porphyritic lava, and the great thickness of the beds of +breccia-conglomerate, there can be little doubt that the sources of +eruption must originally have been numerous: nevertheless, it is +now most difficult even to conjecture the precise point of any one +of the ancient submarine craters. I have repeatedly observed +mountains of porphyries, more or less distinctly stratified towards +their summits or on their flanks, without a trace of stratification +in their central and basal parts: in most cases, I believe this is +simply due either to the obliterating effects of metamorphic +action, or to such parts having been mainly formed of intrusive +porphyries, or to both causes conjoined; in some instances, +however, it appeared to me very probable that the great central +unstratified masses of porphyry were the now partially denuded +nuclei of the old submarine volcanoes, and that the stratified +parts marked the points whence the streams flowed. In one case +alone, and it was in this Valley of the Despoblado, I was able +actually to trace a thick stratum of purplish porphyry, which for a +space of some miles conformably overlay the usual alternating beds +of breccia-conglomerates and claystone lavas, until it became +united with, and blended into, a mountainous mass of various +unstratified porphyries.</p> + +<p> +<a name="page503"></a> +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, <i>that the original points of +eruption tend to become the points of injection.</i> This in itself +does not seem improbable; for where the earth’s crust has once +yielded, it would be liable to yield again, though the liquified +intrusive matter might not be any longer enabled to reach the +submarine surface and flow as lava. I have been led to this +conclusion, from having so frequently observed that, where part of +an unstratified mountain-mass resembled in mineralogical character +the adjoining streams or strata, there were several other kinds of +intrusive porphyries and andesitic rocks injected into the same +point. As these intrusive mountain-masses form most of the +axes-lines in the Cordillera, whether anticlinal, uniclinal, or +synclinal, and as the main valleys have generally been hollowed out +along these lines, the intrusive masses have generally suffered +much denudation. Hence they are apt to stand in some degree +isolated, and to be situated at the points where the valleys +abruptly bend, or where the main tributaries enter. On this view of +there being a tendency in the old points of eruption to become the +points of subsequent injection and disturbance, and consequently of +denudation, it ceases to be surprising that the streams of lava in +the porphyritic claystone conglomerate formation, and in other +analogous cases, should most rarely be traceable to their actual +sources.</p> + +<p> +<i>Iquique, Southern Peru.</i>—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.<a href="#fn-22.6" name="fnref-22.6" +id="fnref-22.6"><sup>[6]</sup></a> 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,<a href="#fn-22.7" name="fnref-22.7" +id="fnref-22.7"><sup>[7]</sup></a>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:— +</p> + +<p><small> Lucina Americana, E. Forbes.<br/> + Terebratula inca, E. Forbes.<br/> + Terebratula ænigma, +D’Orbigny.</small></p> + +<p class="footnote"> +<a name="fn-22.6" id="fn-22.6"></a> <a href="#fnref-22.6">[6]</a> +The lowest point, where the road crosses the coast-escarpment, is 1,900 feet by +the barometer above the level of the sea. +</p> + +<p class="footnote"> +<a name="fn-22.7" id="fn-22.7"></a> <a href="#fnref-22.7">[7]</a> +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. +</p> + +<p>This latter species we have seen associated with the fossils of +which +<a name="page504"></a> +lists have been given in this chapter, in two places in the +valley of Coquimbo, and in the ravine of Maricongo at Copiapo. +Considering this fact, and the superposition of these beds on the +porphyritic conglomerate formation; and, as we shall immediately +see, from their containing much gypsum, and from their otherwise +close general resemblance in mineralogical nature with the strata +described in the valley of Copiapo, I have little doubt that these +fossiliferous beds of Iquique belong to the great cretaceo-oolitic +formation of Northern Chile. Iquique is situated seven degrees +latitude north of Copiapo; and I may here mention, that an +Ammonites, nov. species, and an Astarte, nov. species, were given +me from the Cerro Pasco, about ten degrees of latitude north of +Iquique, and M. D’Orbigny thinks that they probably indicate a +Neocomian formation. Again, fifteen degrees of latitude northward, +in Colombia, there is a grand fossiliferous deposit, now well known +from the labours of Von Buch, Lea, d’Orbigny, and Forbes, which +belongs to the earlier stages of the cretaceous system. Hence, +bearing in mind the character of the few fossils from Tierra del +Fuego, there is some evidence that a great portion of the +stratified deposits of the whole vast range of the South American +Cordillera belongs to about the same geological epoch.</p> + +<p> +Proceeding from the coast escarpment inwards, I crossed, in a space of about +thirty miles, an elevated undulatory district, with the beds dipping in various +directions. The rocks are of many kinds,—white laminated, sometimes +siliceous sandstone,—purple and red sandstone, sometimes so highly +calcareous as to have a crystalline fracture,—argillaceous +limestone,—black calcareous slate-rock, like that so often described at +Copiapo and other places,—thinly laminated, fine-grained, greenish, +indurated, sedimentary, fusible rocks, approaching in character to the +so-called pseudo-honestone of Chile, including thin contemporaneous veins of +gypsum,—and lastly, much calcareous, laminated porcelain jasper, of a +green colour, with red spots, and of extremely easy fusibility: I noticed one +conformable stratum of a freckled-brown, feldspathic lava. I may here mention +that I heard of great beds of gypsum in the Cordillera. The only novel point in +this formation, is the presence of innumerable thin layers of rock-salt, +alternating with the laminated and hard, but sometimes earthy, yellowish, or +bright red and ferruginous sandstones. The thickest layer of salt was only two +inches, and it thinned out at both ends. On one of these saliferous masses I +noticed a stratum about twelve feet thick, of dark-brown, hard brecciated, +easily fusible rock, containing grains of quartz and of black oxide of iron, +together with numerous imperfect fragments of shells. The problem of the origin +of salt is so obscure, that every fact, even geographical position, is worth +recording.<a href="#fn-22.8" name="fnref-22.8" +id="fnref-22.8"><sup>[8]</sup></a> With the exception of +<a name="page505"></a> +these saliferous beds, most of the rocks as already remarked, present a +striking general resemblance with the upper parts of the gypseous or +cretaceo-oolitic formation of Chile. +</p> + +<p class="footnote"> +<a name="fn-22.8" id="fn-22.8"></a> <a href="#fnref-22.8">[8]</a> +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.<br/> + 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.) +</p> + +<h4><i>Metalliferous Veins.</i></h4> + +<p> +I have only a few remarks to make on this subject: in nine mining districts, +some of them of considerable extent, which I visited in <i>Central</i> Chile, I +found the <i>principal</i> veins running from between [N. and N.W.] to [S. and +S.E.];<a href="#fn-22.9" name="fnref-22.9" id="fnref-22.9"><sup>[9]</sup></a> +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 <i>granitic</i> 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. +<a name="page506"></a> +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.<a href="#fn-22.10" name="fnref-22.10" +id="fnref-22.10"><sup>[10]</sup></a> 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<a href="#fn-22.11" name="fnref-22.11" +id="fnref-22.11"><sup>[11]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-22.9" id="fn-22.9"></a> <a href="#fnref-22.9">[9]</a> +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.]. +</p> + +<p class="footnote"> +<a name="fn-22.10" id="fn-22.10"></a> <a href="#fnref-22.10">[10]</a> +The same fact has been observed by Mr. Taylor in Cuba: <i>London Phil. +Journ.,</i> vol. xi, p. 21. +</p> + +<p class="footnote"> +<a name="fn-22.11" id="fn-22.11"></a> <a href="#fnref-22.11">[11]</a> +“Travels in Chile,” p. 29. +</p> + +<p>Silver, in the form of a chloride, sulphuret, or an amalgam, or +in its native state, and associated with lead and other metals, and +at Arqueros with pure native copper, occurs chiefly in the upper +great gypseous or cretaceo-oolitic formation which forms probably +the richest mass in Chile. We may instance the mining districts of +Arqueros near Coquimbo, and of nearly the whole valley of Copiapo, +and of Iquique (where the principal veins run N.E. by E. and S.W. +by W.), in Peru. Hence comes Molina’s remark, that silver is born +in the cold and solitary deserts of the Upper Cordillera. There +are, however, exceptions to +<a name="page507"></a> +this rule: at Paral (S.E. of Coquimbo) silver is found in the +porphyritic conglomerate formation; as I suspect is likewise the +case at S. Pedro de Nolasko in the Peuquenes Pass. Rich +argentiferous lead is found in the clay-slate of the Uspallata +range; and I saw an old silver-mine in a hill of syenite at the +foot of the Bell of Quillota: I was also assured that silver has +been found in the andesitic and porphyritic region between the town +of Copiapo and the Pacific. I have stated in a previous part of +this chapter, that in two neighbouring mines at Arqueros the veins +in one were productive when they traversed the singular green +sedimentary beds, and unproductive when crossing the reddish beds; +whereas at the other mine exactly the reverse takes place; I have +also described the singular and rare case of numerous particles of +native silver and of the chloride being disseminated in the green +rock at the distance of a yard from the vein. Mercury occurs with +silver both at Arqueros and at Copiapo: at the base of C. de los +Hornos (S.E. of Coquimbo, a different place from Los Hornos, before +mentioned) I saw in a syenitic rock numerous quartzose veins, +containing a little cinnabar in nests: there were here other +parallel veins of copper and of a ferrugino-auriferous ore. I +believe tin has never been found in Chile.</p> + +<p> +From information given me by Mr. Nixon of Yaquil,<a href="#fn-22.12" +name="fnref-22.12" id="fnref-22.12"><sup>[12]</sup></a> 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 <i>rich</i> in metals; such a +vein is called a <i>veta real.</i> More commonly the mines are worked only +where one, two, or more thin veins or strings running in a different direction, +intersect a <i>poor</i> “veta real:” it is unanimously believed +that at such points of intersection (<i>cruceros</i>), the quantity of metal is +much greater than that contained in other parts of the intersecting veins. In +some <i> cruceros</i> or points of intersection, the metals extend even beyond +the walls of the main, broad, stony vein. It is said that the greater the angle +of intersection, the greater the produce; and that nearly parallel strings +attract each other; in the Uspallata range, I observed that numerous thin +auri-ferruginous veins repeatedly ran into knots, and then branched out again. +I have already described the remarkable manner in which rocks of the Uspallata +range are indurated and blackened (as if by a blast of gunpowder) to a +considerable distance from the metallic veins. +</p> + +<p class="footnote"> +<a name="fn-22.12" id="fn-22.12"></a> <a href="#fnref-22.12">[12]</a> +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. +</p> + +<p> +Finally, I may observe, that the presence of metallic veins seems obviously +connected with the presence of intrusive rocks, and with the degree of +metamorphic action which the different districts of Chile have undergone.<a +href="#fn-22.13" name="fnref-22.13" id="fnref-22.13"><sup>[13]</sup></a> 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 +<a name="page508"></a> +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 <i>not +metamorphosed,</i> which compose the numerous islands in the Pacific, Indian, +and Atlantic Oceans; for in these islands metals are entirely absent, and their +nature even unknown to the aborigines. +</p> + +<p class="footnote"> +<a name="fn-22.13" id="fn-22.13"></a> <a href="#fnref-22.13">[13]</a> +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. +</p> + +<h4><i>Summary of the Geological History of the Chilean Cordillera, and of the +Southern Parts of South America.</i></h4> + +<p>We have seen that the shores of the Pacific, for a space of +1,200 miles from Tres Montes to Copiapo, and I believe for a very +much greater distance, are composed, with the exception of the +tertiary basins, of metamorphic schists, plutonic rocks, and more +or less altered clay-slate. On the floor of the ocean thus +constituted, vast streams of various purplish claystone and +greenstone porphyries were poured forth, together with great +alternating piles of angular and rounded fragments of similar rocks +ejected from the submarine craters. From the compactness of the +streams and fragments, it is probable that, with the exception of +some districts in Northern Chile, the eruptions took place in +profoundly deep water. The orifices of eruption appear to have been +studded over a breadth, with some outliers, of from fifty to one +hundred miles: and closely enough together, both north and south, +and east and west, for the ejected matter to form a continuous +mass, which in Central Chile is more than a mile in thickness. I +traced this mould-like mass, for only 450 miles; but judging from +what I saw at Iquique, from specimens, and from published accounts, +it appears to have a manifold greater length. In the basal parts of +the series, and especially towards the flanks of the range, mud, +since converted into a feldspathic slaty rock, and sometimes into +greenstone, was occasionally deposited between the beds of erupted +matter: with this exception the uniformity of the porphyritic rocks +is very remarkable. 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, +<a name="page509"></a> +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.</p> + +<p> +The strata in this formation, composed of black calcareous shaly-rocks of red +and white, and sometimes siliceous sandstone, of coarse conglomerates, +limestones, tuffs, dark mudstones, and those singular fine-grained rocks which +I have called pseudo-honestones, vast beds of gypsum, and many other jaspery +and scarcely describable varieties, vary and replace each other in short +horizontal distances, to an extent, I believe, unequalled even in any tertiary +basin. Most of these substances are easily fusible, and have apparently been +derived either from volcanoes still in quiet action, or from the attrition of +volcanic products. If we picture to ourselves the bottom of the sea, rendered +uneven in an extreme degree, with numerous craters, some few occasionally in +eruption, but the greater number in the state of solfataras, discharging +calcareous, siliceous, ferruginous matters, and gypsum or sulphuric acid to an +amount surpassing, perhaps, even the existing sulphureous volcanoes of Java,<a +href="#fn-22.14" name="fnref-22.14" id="fnref-22.14"><sup>[14]</sup></a> 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 <i>Terebratula ænigma,</i> at Iquique, it extends from four to five +hundred miles further: and perhaps even for ten degrees of latitude north of +Iquique to the Cerro Pasco, not far from Lima: again, we know that a cretaceous +formation, abounding with fossils, is largely developed north of the equator, +in Colombia: in Tierra del Fuego, at about this same period, a wide district of +clay-slate was deposited, which in its mineralogical characters and external +features, might be compared to the Silurian regions of North Wales. The +gypseous formation, like that of the porphyritic breccia-conglomerate on which +it rests, is of inconsiderable breadth; though of greater breadth in Northern +than in Central Chile. +</p> + +<p class="footnote"> +<a name="fn-22.14" id="fn-22.14"></a> <a href="#fnref-22.14">[14]</a> +Von Buch’s “Descript. Physique des Iles Canaries,” p. 428. +</p> + +<p>As the fossil shells in this formation are covered, in the +Peuquenes ridge, by a great thickness of strata; at the Puente del +Inca, by at least five thousand feet; at Coquimbo, though the +superposition there is less plainly seen, by about six thousand +feet; and at Copiapo, certainly by five or +<a name="page510"></a> +six thousand, and probably by seven thousand feet (the same +species there recurring in the upper and lower parts of the +series), we may feel confident that the bottom of the sea subsided +during this cretaceo-oolitic period, so as to allow of the +accumulation of the superincumbent submarine strata. This +conclusion is confirmed by, or perhaps rather explains, the +presence of the many beds at many levels of coarse conglomerate, +the well-rounded pebbles in which we cannot believe were +transported in very deep water. Even the underlying porphyries at +Copiapo. with their highly amygdaloidal surfaces, do not appear to +have flowed under great pressure. The great sinking movement thus +plainly indicated, must have extended in a north and south line for +at least four hundred miles, and probably was co-extensive with +the gypseous formation.</p> + +<p>The beds of conglomerate just referred to, and the +extraordinarily numerous silicified trunks of fir-trees at Los +Hornos, perhaps at Coquimbo and at two distant points in the valley +of Copiapo, indicate that land existed at this period in the +neighbourhood. This land, or islands, in the northern part of the +district of Copiapo, must have been almost exclusively composed, +judging from the nature of the pebbles of granite: in the southern +parts of Copiapo, it must have been mainly formed of claystone +porphyries, with some mica-schist, and with much sandstone and +jaspery rocks exactly like the rocks in the gypseous formation, and +no doubt belonging to its basal series. In several other places +also, during the accumulation of the gypseous formation, its basal +parts and the underlying porphyritic conglomerate must likewise +have been already partially upheaved and exposed to wear and tear; +near the Puente del Inca and at Coquimbo, there must have existed +masses of mica-schist or some such rock, whence were derived the +many small pebbles of opaque quartz. It follows from these facts, +that in some parts of the Cordillera the upper beds of the gypseous +formation must lie unconformably on the lower beds; and the whole +gypseous formation, in parts, unconformably on the porphyritic +conglomerate; although I saw no such cases, yet in many places the +gypseous formation is entirely absent; and this, although no doubt +generally caused by quite subsequent denudation, may in others be +due to the underlying porphyritic conglomerate having been locally +upheaved before the deposition of the gypseous strata, and thus +having become the source of the pebbles of porphyry embedded in +them. In the porphyritic conglomerate formation, in its lower and +middle parts, there is very rarely any evidence, with the exception +of the small quartz pebbles at Jajuel near Aconcagua, and of the +single pebble of granite at Copiapo, of the existence of +neighbouring land: in the upper parts, however, and especially in +the district of Copiapo, the number of thoroughly well-rounded +pebbles of compact porphyries make me believe, that, as during the +prolonged accumulation of the gypseous formation the lower beds had +already been locally upheaved and exposed to wear and tear, so it +was with the porphyritic conglomerate. Hence in following thus far +the geological history of the Cordillera, it may be inferred that +the bed of a deep and open, or nearly open, ocean was filled up by +porphyritic +<a name="page511"></a> +eruptions, aided probably by some general and some local +elevations, to that comparatively shallow level at which the +cretaceo-oolitic shells first lived. At this period, the submarine +craters yielded at intervals a prodigious supply of gypsum and +other mineral exhalations, and occasionally, in certain places +poured forth lavas, chiefly of a feldspathic nature: at this +period, islands clothed with fir-trees and composed of porphyries, +primary rocks, and the lower gypseous strata had already been +locally upheaved, and exposed to the action of the waves;—the +general movement, however, at this time having been over a very +wide area, one of slow subsidence, prolonged till the bed of the +sea sank several thousand feet.</p> + +<p>In Central Chile, after the deposition of a great thickness of +the gypseous strata, and after their upheaval, by which the Cumbre +and adjoining ranges were formed, a vast pile of tufaceous matter +and submarine lava was accumulated, where the Uspallata chain now +stands; also after the deposition and upheaval of the equivalent +gypseous strata of the Peuquenes range, the great thick mass of +conglomerate in the valley of Tenuyan was accumulated: during the +deposition of the Uspallata strata, we know absolutely, from the +buried vertical trees, that there was a subsidence of some thousand +feet; and we may infer from the nature of the conglomerate in the +valley of Tenuyan, that a similar and perhaps contemporaneous +movement there took place. We have, then, evidence of a second +great period of subsidence; and, as in the case of the subsidence +which accompanied the accumulation of the cretaceo-oolitic strata, +so this latter subsidence appears to have been complicated by +alternate or local elevatory movement— for the vertical +trees, buried in the midst of the Uspallata strata, must have grown +on dry land, formed by the upheaval of the lower submarine beds. +Presently I shall have to recapitulate the facts, showing that at a +still later period, namely, at nearly the commencement of the old +tertiary deposits of Patagonia and of Chile, the continent stood at +nearly its present level, and then, for the third time, slowly +subsided to the amount of several hundred feet, and was afterwards +slowly re-uplifted to its present level.</p> + +<p>The highest peaks of the Cordillera appear to consist of active +or more commonly dormant volcanoes,—such as Tupungato, Maypu, +and Aconcagua, which latter stands 23,000 feet above the level of +the sea, and many others. The next highest peaks are formed of the +gypseous and porphyritic strata, thrown into vertical or highly +inclined positions. Besides the elevation thus gained by angular +displacements, I infer, without any hesitation—from the +stratified gravel-fringes which gently slope up the valleys of the +Cordillera from the gravel-capped plains at their base, which +latter are connected with the plains, still covered with recent +shells on the coast—that this great range has been upheaved +in mass by a slow movement, to an amount of at least 8,000 feet. In +the Despoblado Valley, north of Copiapo, the horizontal elevation, +judging from the compact, stratified tufaceous deposit, capping the +distant mountains at corresponding heights, was about ten thousand +feet. It is very possible, or rather probable, that this elevation +in mass may not have +<a name="page512"></a> +been strictly horizontal, but more energetic under the +Cordillera, than towards the coast on either side; nevertheless, +movements of this kind may be conveniently distinguished from those +by which strata have been abruptly broken and upturned. When +viewing the Cordillera, before having read Mr. Hopkins’s profound +“Researches on Physical Geology,” the conviction was impressed on +me, that the angular dislocations, however violent, were quite +subordinate in importance to the great upward movement in mass, and +that they had been caused by the edges of the wide fissures, which +necessarily resulted from the tension of the elevated area, having +yielded to the inward rush of fluidified rock, and having thus been +upturned.</p> + +<p> +The ridges formed by the angularly upheaved strata are seldom of great length: +in the central parts of the Cordillera they are generally parallel to each +other, and run in north and south lines; but towards the flanks they often +extend more or less obliquely. The angular displacement has been much more +violent in the central than in the exterior <i>main</i> lines; but it has +likewise been violent in some of the <i>minor</i> 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.<a href="#fn-22.15" +name="fnref-22.15" id="fnref-22.15"><sup>[15]</sup></a> The close similarity of +the andesitic granites and porphyries, throughout Chile, Tierra del Fuego, and +even in Peru, is very remarkable. The prevalence of feldspar cleaving like +albite, is common not only to the andesites, but (as I infer from the high +authority of Professor G. Rose, as well as from my own measurements) to the +various claystone and greenstone porphyries, and to the trachytic lavas of the +Cordillera. The andesitic rocks have in most cases been the last injected ones, +and they probably form a continuous dome under this great range: they stand in +intimate relationship with the modern lavas; and they seem to have been the +immediate agent in metamorphosing the porphyritic conglomerate formation, and +often likewise the gypseous strata, to the extraordinary extent to which they +have suffered. +</p> + +<p class="footnote"> +<a name="fn-22.15" id="fn-22.15"></a> <a href="#fnref-22.15">[15]</a> +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. +</p> + +<p> +With respect to the age at which the several parallel ridges composing the +Cordillera were upthrown, I have little evidence. Many of them may have been +contemporaneously elevated and injected in the same manner<a href="#fn-22.16" +name="fnref-22.16" id="fnref-22.16"><sup>[16]</sup></a> 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, +<a name="page513"></a> +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.<a href="#fn-22.17" +name="fnref-22.17" id="fnref-22.17"><sup>[17]</sup></a> 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<a href="#fn-22.18" name="fnref-22.18" +id="fnref-22.18"><sup>[18]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-22.16" id="fn-22.16"></a> <a href="#fnref-22.16">[16]</a> +“Volcanic Islands,” etc.) +</p> + +<p class="footnote"> +<a name="fn-22.17" id="fn-22.17"></a> <a href="#fnref-22.17">[17]</a> +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 <i> Edinburgh New Phil. Journal,</i> vol. xxviii, pp. 33 and 44. +</p> + +<p class="footnote"> +<a name="fn-22.18" id="fn-22.18"></a> <a href="#fnref-22.18">[18]</a> +“Reise in Peru,” Band 2, s. 8: Author’s +“Journal,” 2nd edit., p. 359. +</p> + +<p>Ascending to the older tertiary formations, I will not again +recapitulate the remarks already given at the end of the Fifth +Chapter,—on their great extent, especially along the shores +of the Atlantic—on their antiquity, perhaps corresponding +with that of the eocene deposits of Europe,—on the almost +entire dissimilarity, though the formations are apparently +contemporaneous, of the fossils from the eastern and western +coasts, as is likewise the case, even in a still more marked +degree, with the shells now living in these opposite though +approximate +<a name="page514"></a> +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 <i> +extensive</i> formations containing recent shells, either on the +eastern or western coasts of the continent,—though these +coasts now abound with living mollusca,—though they are, and +apparently have always been, as favourable for the deposition of +sediment as they were when the tertiary formations were copiously +deposited,—and though they have been upheaved to an amount +quite sufficient to bring up strata from the depths the most +fertile for animal life—can be explained in accordance with +the above proposition. As a deduction, it was also attempted to be +shown, first, that the want of close sequence in the fossils of +successive formations, and of successive stages in the same +formation, would follow from the improbability of the same area +continuing slowly to subside from one whole period to another, or +even during a single entire period; and secondly, that certain +epochs having been favourable at distant points, in the same +quarter of the world for the synchronous accumulation of +fossiliferous strata, would follow from movements of subsidence +having apparently, like those of elevation, contemporaneously +affected very large areas.</p> + +<p> +There is another point which deserves some notice, namely, the analogy between +the upper parts of the Patagonian tertiary formation, as well as of the upper +possibly contemporaneous beds at Chiloe and Concepcion, with the great gypseous +formation of Cordillera; for in both formations, the rocks, in their fusible +nature, in their containing gypsum, and in many other characters, show a +connection, either intimate or remote, with volcanic action; and as the strata +in both were accumulated during subsidence, it appears at first natural to +connect this sinking movement with a state of high activity in the neighbouring +volcanoes. During the cretaceo-oolitic period this certainly appears to have +been the case at the Puente del Inca, judging from the number of intercalated +lava-streams in the lower 3,000 feet of strata; but generally, the volcanic +orifices seem at this time to have existed as submarine solfataras, and were +certainly quiescent compared with their state +<a name="page515"></a> +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,<a href="#fn-22.19" name="fnref-22.19" +id="fnref-22.19"><sup>[19]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-22.19" id="fn-22.19"></a> <a href="#fnref-22.19">[19]</a> +“The Structure, etc., of Coral Reefs.” +</p> + +<p>At a later period, the Pampean mud, of estuary origin, was +deposited over a wide area,—in one district conformably on +the underlying old tertiary strata, and in another district +unconformably on them, after their upheaval and denudation. During +and before the accumulation, however, of these old tertiary strata, +and, therefore, at a very remote period, sediment, strikingly +resembling that of the Pampas, was deposited; showing during how +long a time in this case the same agencies were at work in the same +area. The deposition of the Pampean estuary mud was accompanied, at +least in the southern parts of the Pampas, by an elevatory +movement, so that the M. Hermoso beds probably were accumulated +after the upheaval of those round the S. Ventana; and those at P. +Alta after the upheaval of the M. Hermoso strata; but there is some +reason to suspect that one period of subsidence intervened, during +which mud was deposited over the coarse sand of the Barrancas de S. +Gregorio, and on the higher parts of Banda Oriental. The +mammiferous animals characteristic of this formation, many of which +differ as much from the present inhabitants of South America, as do +the eocene mammals of Europe from the present ones of that quarter +of the globe, certainly co-existed at B. Blanca with twenty species +of mollusca, one balanus, and two corals, all now living in the +adjoining sea: this is likewise the case in Patagonia with the +Macrauchenia, which co-existed with eight shells, still the +commonest kinds on that coast. I will not repeat what I have +elsewhere said, on the place of habitation, food, wide range, and +extinction of the numerous gigantic mammifers, which at this late +period inhabited the two Americas.</p> + +<p>The nature and grouping of the shells embedded in the old +tertiary formations of Patagonia and Chile show us, that the +continent at that period must have stood only a few fathoms below +its present level, and +<a name="page516"></a> +that afterwards it subsided over a wide area, seven or eight +hundred feet. The manner in which it has since been rebrought up to +its actual level, was described in detail in the First and Second +Chapters. It was there shown that recent shells are found on the +shores of the Atlantic, from Tierra del Fuego northward for a space +of at least 1,180 nautical miles, and at the height of about 100 +feet in La Plata, and of 400 feet in Patagonia. The elevatory +movements on this side of the continent have been slow; and the +coast of Patagonia, up to the height in one part of 950 feet and in +another of 1,200 feet, is modelled into eight great, step-like, +gravel-capped plains, extending for hundreds of miles with the same +heights; this fact shows that the periods of denudation (which, +judging from the amount of matter removed, must have been long +continued) and of elevation were synchronous over surprisingly +great lengths of coasts. On the shores of the Pacific, upraised +shells of recent species, generally, though not always, in the same +proportional numbers as in the adjoining sea, have actually been +found over a north and south space of 2,075 miles, and there is +reason to believe that they occur over a space of 2,480 miles. The +elevation on this western side of the continent has not been +equable; at Valparaiso, within the period during which upraised +shells have remained undecayed on the surface, it has been 1,300 +feet, whilst at Coquimbo, 200 miles northward, it has been within +this same period only 252 feet. At Lima, the land has been uplifted +at least 80 feet since Indian man inhabited that district; but the +level within historical times apparently has subsided. At Coquimbo, +in a height of 364 feet, the elevation has been interrupted by five +periods of comparative rest. At several places the land has been +lately, or still is, rising both insensibly and by sudden starts of +a few feet during earthquake-shocks; this shows that these two +kinds of upward movement are intimately connected together. For a +space of 775 miles, upraised recent shells are found on the two +opposite sides of the continent; and in the southern half of this +space, it may be safely inferred from the slope of the land up to +the Cordillera, and from the shells found in the central part of +Tierra del Fuego, and high up the River Santa Cruz, that the entire +breadth of the continent has been uplifted. From the general +occurrence on both coasts of successive lines of escarpments, of +sand-dunes and marks of erosion, we must conclude that the +elevatory movement has been normally interrupted by periods, when +the land either was stationary, or when it rose at so slow a rate +as not to resist the average denuding power of the waves, or when +it subsided. In the case of the present high sea-cliffs of +Patagonia and in other analogous instances, we have seen that the +difficulty in understanding how strata can be removed at those +depths under the sea, at which the currents and oscillations of the +water are depositing a smooth surface of mud, sand, and sifted +pebbles, leads to the suspicion that the formation or denudation of +such cliffs has been accompanied by a sinking movement.</p> + +<p>In South America, everything has taken place on a grand scale, +and all geological phenomena are still in active operation. We know +how violent at the present day the earthquakes are, we have seen +how great +<a name="page517"></a> +an area is now rising, and the plains of tertiary origin are of +vast dimensions; an almost straight line can be drawn from Tierra +del Fuego for 1,600 miles northward, and probably for a much +greater distance, which shall intersect no formation older than the +Patagonian deposits; so equable has been the upheaval of the beds, +that throughout this long line, not a fault in the stratification +or abrupt dislocation was anywhere observable. Looking to the +basal, metamorphic, and plutonic rocks of the continent, the areas +formed of them are likewise vast; and their planes of cleavage and +foliation strike over surprisingly great spaces in uniform +directions. The Cordillera, with its pinnacles here and there +rising upwards of twenty thousand feet above the level of the sea, +ranges in an unbroken line from Tierra del Fuego, apparently to the +Arctic circle. This grand range has suffered both the most violent +dislocations, and slow, though grand, upward and downward movements +in mass; I know not whether the spectacle of its immense valleys, +with mountain-masses of once liquified and intrusive rocks now +bared and intersected, or whether the view of those plains, +composed of shingle and sediment hence derived, which stretch to +the borders of the Atlantic Ocean, is best adapted to excite our +astonishment at the amount of wear and tear which these mountains +have undergone.</p> + +<p> +The Cordillera from Tierra del Fuego to Mexico, is penetrated by volcanic +orifices, and those now in action are connected in great trains. The intimate +relation between their recent eruptions and the slow elevation of the continent +in mass,<a href="#fn-22.20" name="fnref-22.20" +id="fnref-22.20"><sup>[20]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-22.20" id="fn-22.20"></a> <a href="#fnref-22.20">[20]</a> +On the Connection of certain Volcanic Phenomena in South America: +“Geolog. Transact.,” vol. v, p. 609. +</p> + +<p> +Recurring to the evidence by which it was shown that some at least of the +parallel ridges, which together compose the Cordillera, were successively and +slowly upthrown at widely different periods; and that the whole range certainly +once, and almost certainly twice, subsided some thousand feet, and being then +brought up by a slow movement +<a name="page518"></a> +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,<a href="#fn-22.21" name="fnref-22.21" +id="fnref-22.21"><sup>[21]</sup></a> 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. +</p> + +<p class="footnote"> +<a name="fn-22.21" id="fn-22.21"></a> <a href="#fnref-22.21">[21]</a> +“Geolog. Transact.,” vol. v, p. 626. +</p> + +<p><small>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.”—<i>Monatsberichten der königl. Akad., +etc.,</i> zu Berlin vom April 1845.</small></p> + +<hr /> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="index01">INDEX<br/>TO CORAL-REEFS.</a></h2> + +<p> +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. +</p> + +<p class="noindent"> +<small>Abrolhos, Brazil, coated by corals <a href="#page50"> +50</a><br/> +<i>Abrolhos (Australia)</i> <a href="#page130">130</a><br/> +Absence of coral-reefs from certain coasts <a href="#page51"> +51</a><br/> +<i>Acaba, gulf of</i> <a href="#page147">147</a><br/> +<i>Admiralty group</i> <a href="#page124">124</a><br/> +Africa, east coast, fringing-reef of <a href="#page48">48</a><br/> +—— Madreporitic rock of <a href="#page101">101</a><br/> +<i>Africa, east coast</i> <a href="#page141">141</a><br/> +Age of individual corals <a href="#page57">57</a>, <a href= +"#page64">64</a><br/> +<i>Aiou</i> <a href="#page128">128</a><br/> +<i>Aitutaki</i> <a href="#page114">114</a><br/> +<i>Aldabra</i> <a href="#page139">139</a><br/> +<i>Alert reef</i> <a href="#page123">123</a><br/> +<i>Alexander, Grand Duke, island</i> <a href="#page115"> +115</a><br/> +Allan, Dr., on Holuthuriæ feeding on corals <a href= +"#page21">21</a><br/> +—— on quick growth of corals at Madagascar <a href= +"#page62">62</a><br/> +—— on reefs affected by currents <a href="#page49"> +9</a><br/> +<i>Alloufatou</i> <a href="#page119">119</a><br/> +<i>Alphonse</i> <a href="#page139">139</a><br/> +<i>Amargoura (Amargura)</i> <a href="#page119">119</a><br/> +<i>Amboina</i> <a href="#page128">128</a><br/> +<i>America, west coast</i> <a href="#page111">111</a><br/> +<i>Amirantes</i> <a href="#page138">138</a><br/> +<i>Anachorites</i> <a href="#page125">125</a><br/> +<i>Anambas</i> <a href="#page133">133</a><br/> +Anamouka, description of <a href="#page99">99</a><br/> +<i>Anamouka</i> <a href="#page119">119</a><br/> +<i>Anadaman islands</i> <a href="#page132">132</a><br/> +<i>Antilles</i> <a href="#page153">153</a><br/> +<i>Appoo reef</i> <a href="#page134">134</a><br/> +<i>Arabia Felix</i> <a href="#page143">143</a><br/> +Areas, great extent of, interspersed with low islands<br/> +—— of subsidence and of elevation <a href="#page106"> +106</a><br/> +—— of subsidence appear to be elongated <a href= +"#page106">106</a><br/> +—— of subsidence alternating with areas of elevation <a +href="#page108">108</a><br/> +<i>Arru group</i> <a href="#page128">128</a><br/> +<i>Arzobispo</i> <a href="#page127">127</a><br/> +Ascidia, depth at which found <a href="#page67">67</a><br/> +<i>Assomption</i> <a href="#page139">139</a><br/> +<i>Astova</i> <a href="#page139">139</a><br/> +<i>Atlantic islands</i> <a href="#page121">121</a><br/> +Atolls, breaches in their reefs <a href="#page31">31</a>, <a href= +"#page81">81</a><br/> +—— dimensions of <a href="#page25">25</a><br/> +—— dimensions of groups of <a href="#page71"> +71</a><br/> +—— not based on craters or on banks of sediment, or of +ck <a href="#page69">69</a>, <a href="#page71">71</a>, <a href= +"#page72">72</a>, <a href="#page73">73</a>, <a href="#page108"> +108</a><br/> +—— of irregular forms <a href="#page25">25</a>, <a +href="#page84">84</a><br/> +—— steepness of their flanks <a href="#page26"> +26</a><br/> +—— width of their reef and islets <a href="#page25"> +25</a><br/> +—— their lowness <a href="#page70">70</a><br/> +—— lagoons <a href="#page29">29</a><br/> +—— general range <a href="#page94">94</a><br/> +—— with part of their reef submerged, and theory of <a +href="#page29">29</a>, <a href="#page81">81</a><br/> +<i>Augustine, St.</i> <a href="#page120">120</a><br/> +Aurora island, an upraised atoll <a href="#page64">64</a>, <a +href="#page71">71</a>, <a href="#page104">104</a><br/> +<i>Aurora</i> <a href="#page112">112</a><br/> +Austral islands, recently elevated <a href="#page99">99</a><br/> +<i>Austral islands</i> <a href="#page114">114</a><br/> +<i>Australia, N.W. coast</i> <a href="#page130">130</a><br/> +Australian barrier-reef <a href="#page42">42</a>, <a href= +"#page93">93</a><br/> +<i>Australian barrier</i> <a href="#page123">123</a><br/> +<br/> +<i>Babuyan group</i> <a href="#page134">134</a><br/> +<i>Bahama banks</i> <a href="#page149">149</a>, <a href= +"#page150">150</a><br/> +<i>Balahac</i> <a href="#page133">133</a><br/> +<i>Bally</i> <a href="#page131">131</a><br/> +<i>Baring</i> <a href="#page121">121</a><br/> +Barrier-reef of Australia <a href="#page42">42</a>, <a href= +"#page93">93</a><br/> +—— of New Caledonia <a href="#page44">44</a><br/> +Barrier-reefs, breaches through <a href="#page77">77</a><br/> +—— not based on worn down margin of rock <a href= +"#page43">43</a><br/> +—— on banks of sediment <a href="#page43">43</a><br/> +—— on submarine craters <a href="#page44">44</a><br/> +—— steepness of their flanks <a href="#page39"> +39</a><br/> +—— their probable vertical thickness <a href= +"#page43">43</a>, <a href="#page76">76</a><br/> +—— theory of their formation <a href="#page76">76</a>, +<a href="#page78">78</a><br/> +<i>Bampton shoal</i> <a href="#page123">123</a><br/> +<i>Banks islands</i> <a href="#page122">122</a><br/> +<i>Banks in the West Indies</i> <a href="#page147">147</a><br/> +<i>Bashee islands</i> <a href="#page134">134</a><br/> +<i>Bass island</i> <a href="#page115">115</a><br/> +<i>Batoa</i> <a href="#page119">119</a><br/> +<i>Beaupré reef</i> <a href="#page123">123</a><br/> +Beechey, Captain, obligations of the author to <a href="#page26"> +26</a><br/> +—— on submerged reefs <a href="#page27">27</a><br/> +—— account of Matilda island <a href="#page60"> +60</a><br/> +Belcher, Captain, on boring through coral-reef <a href="#page59"> +59</a><br/> +<i>Belize reef, off</i> <a href="#page151">151</a><br/> +<i>Bellinghausen</i> <a href="#page113">113</a><br/> +<i>Bermuda islands</i> <a href="#page153">153</a><br/> +<i>Beveridge reef</i> <a href="#page118">118</a><br/> +<i>Bligh</i> <a href="#page122">122</a><br/> +Bolabola, view of <a href="#page12">12</a><br/> +<i>Bombay shoal</i> <a href="#page136">136</a><br/> +<i>Bonin Bay</i> <a href="#page131">131</a><br/> +<i>Bonin group</i> <a href="#page127">127</a><br/> +Borings through coral-reefs <a href="#page59">59</a><br/> +Borneo, W. coast, recently elevated <a href="#page101">101</a><br/> +<i>Borneo, E. coast</i> <a href="#page131">131</a><br/> +—— <i>S.W. and W. coast</i> <a href="#page133"> +133</a><br/> +—— <i>N. coast</i> <a href="#page133">133</a><br/> +—— <i>western bank</i> <a href="#page136">136</a><br/> +<i>Boscawen</i> <a href="#page119">119</a><br/> +<i>Boston</i> <a href="#page121">121</a><br/> +<i>Bouka</i> <a href="#page124">124</a><br/> +<i>Bourbon</i> <a href="#page138">138</a><br/> +<i>Bourou</i> <a href="#page128">128</a><br/> +<i>Bouton</i> <a href="#page132">132</a><br/> +Brazil, fringing-reefs on coast of <a href="#page48">48</a><br/> +Breaches through barrier-reefs <a href="#page71">71</a><br/> +<i>Brook</i> <a href="#page115">115</a><br/> +<i>Bunker</i> <a href="#page115">115</a><br/> +<i>Bunoa</i> <a href="#page133">133</a><br/> +Byron <a href="#page121">121</a><br/> +<br/> +<i>Cagayanes</i> <a href="#page133">133</a><br/> +<i>Candelaria</i> <a href="#page124">124</a><br/> +<i>Cargados Carajos</i> <a href="#page138">138</a><br/> +<i>Caroline archipelago</i> <a href="#page125">125</a><br/> +<i>Caroline island</i> <a href="#page115">115</a><br/> +<i>Carteret shoal</i> <a href="#page128">128</a><br/> +Caryophyllia, depth at which it lives <a href="#page66">66</a><br/> +<i>Cavilli</i> <a href="#page133">133</a><br/> +<i>Cayman island</i> <a href="#page152">152</a><br/> +<i>Celebes</i> <a href="#page129">129</a><br/> +<i>Ceram</i> <a href="#page128">128</a><br/> +Ceylon, recently elevated <a href="#page101">101</a><br/> +<i>Ceylon</i> <a href="#page137">137</a><br/> +Chagos Great Bank, description and theory of <a href="#page37"> +37</a>, <a href="#page85">85</a><br/> +Chagos group <a href="#page86">86</a><br/> +<i>Chagos group</i> <a href="#page137">137</a><br/> +Chama-shells embedded in coral-rock <a href="#page68">68</a><br/> +Chamisso, on corals preferring the surf <a href="#page52"> +52</a><br/> +Changes in the state of Keeling atoll <a href="#page21">21</a><br/> +—— of atolls <a href="#page74">74</a><br/> +Channels leading into the lagoons of atolls <a href="#page30"> +30</a>, <a href="#page82">82</a><br/> +—— —— into the Maldiva atolls <a href= +"#page33">33</a>, <a href="#page35">35</a><br/> +—— through barrier-reefs <a href="#page77">77</a><br/> +<i>Chase</i> <a href="#page120">120</a><br/> +<i>China sea</i> <a href="#page135">135</a><br/> +Christmas atoll <a href="#page60">60</a>, <a href="#page97"> +97</a><br/> +<i>Christmas atoll</i> <a href="#page116">116</a><br/> +<i>Christmas island</i> (Indian Ocean) <a href="#page137"> +137</a><br/> +<i>Clarence</i> <a href="#page116">116</a><br/> +<i>Clipperton rock</i> <a href="#page111">111</a><br/> +Cocos, or Keeling atoll <a href="#page15">15</a><br/> +<i>Cocos (or Keeling)</i> <a href="#page137">137</a><br/> +<i>Cocos island</i> (Pacific) <a href="#page111">111</a><br/> +Cochin China, encroachments of the sea on the coast <a href= +"#page95">95</a><br/> +<i>Cochin China</i> <a href="#page183">183</a><br/> +<i>Coetivi</i> <a href="#page139">139</a><br/> +<i>Comoro group</i> <a href="#page139">139</a><br/> +Composition of coral-formations <a href="#page88">88</a><br/> +Conglomerate coral-rock on Keeling atoll <a href="#page20"> +20</a><br/> +—— on other atolls <a href="#page28">28</a><br/> +—— coral-rock <a href="#page88">88</a><br/> +Cook islands, recently elevated <a href="#page98">98</a>, <a href= +"#page103">103</a><br/> +<i>Cook islands</i> <a href="#page114">114</a><br/> +Coral-blocks bored by vermiform animals <a href="#page21">21</a>, +<a href="#page88">88</a><br/> +Coral-reefs, their distribution and absence from certain areas <a +href="#page50">50</a><br/> +—— destroyed by loose sediment <a href="#page53"> +53</a><br/> +Coral-rock at Keeling atoll <a href="#page20">20</a><br/> +—— Mauritius <a href="#page47">47</a><br/> +—— organic remains of <a href="#page88">88</a><br/> +Corals dead but upright in Keeling lagoon <a href="#page22"> +22</a><br/> +—— depths at which they live <a href="#page64"> +64</a><br/> +—— off Keeling atoll <a href="#page17">17</a><br/> +—— killed by a short exposure <a href="#page16"> +16</a><br/> +—— living in the lagoon of Keeling atoll <a href= +"#page20">20</a><br/> +—— quick growth of, in Keeling lagoon <a href= +"#page21">21</a><br/> +—— merely coating the bottom of the sea <a href= +"#page50">50</a><br/> +—— standing exposed in the Low archipelago <a href= +"#page96">96</a><br/> +Corallian sea <a href="#page94">94</a><br/> +<i>Corallian sea</i> <a href="#page123">123</a><br/> +<i>Cornwallis</i> <a href="#page121">121</a><br/> +<i>Cosmoledo</i> <a href="#page139">139</a><br/> +Couthouy, Mr., alleged proofs of recent elevation of the Low +archipelago <a href="#page96">96</a><br/> +—— on coral-rock at Mangaia and Aurora islands <a href= +"#page64">64</a><br/> +—— on external ledges round coral-islands <a href= +"#page80">80</a><br/> +—— remarks confirmatory of the author’s theory <a href= +"#page96">96</a><br/> +Crescent-formed reefs <a href="#page84">84</a><br/> +<i>Cuba</i> <a href="#page150">150</a><br/> +Cuming, Mr., on the recent elevation of the Philippines <a href= +"#page101">101</a><br/> +<br/> +<i>Dangerous, or Low archipelago</i> <a href="#page111"> +111</a><br/> +<i>Danger islands</i> <a href="#page116">116</a><br/> +Depths at which reef-building corals live <a href="#page63"> +63</a><br/> +—— at Mauritius, the Red Sea, and in the Maldiva +archipelago <a href="#page66">66</a><br/> +—— at which other corals and corallines can live <a +href="#page67">67</a><br/> +<i>Dhalac group</i> <a href="#page144">144</a><br/> +Diego Garcia, slow growth of reef <a href="#page56">56</a><br/> +Dimensions of the larger groups of atolls <a href="#page71"> +71</a><br/> +Disseverment of the Maldiva atolls, and theory of <a href= +"#page37">37</a>, <a href="#page82">82</a><br/> +Distribution of coral-reefs <a href="#page50">50</a><br/> +<i>Domingo, St.</i> <a href="#page152">152</a><br/> +Dory, Port, recently elevated <a href="#page100">100</a><br/> +<i>Dory, Port</i> <a href="#page127">127</a><br/> +<i>Duff islands</i> <a href="#page122">122</a><br/> +<i>Durour</i> <a href="#page125">125</a><br/> +<br/> +<i>Eap</i> <a href="#page126">126</a><br/> +arthquakes at Keeling atoll <a href="#page23">23</a><br/> +—— in groups of atolls <a href="#page75">75</a><br/> +—— in Navigator archipelago <a href="#page100"> +100</a><br/> +ast Indian archipelago, recently elevated <a href="#page100"> +100</a><br/> +<i>Easter</i> <a href="#page111">111</a><br/> +<i>Echequier</i> <a href="#page125">125</a><br/> +hrenberg, on the banks of the Red Sea <a href="#page49">49</a>, <a +href="#page143">143</a><br/> +—— on depths at which corals live in the Red Sea <a +href="#page66">66</a><br/> +—— on corals preferring the surf <a href="#page53"> +53</a><br/> +—— on the antiquity of certain corals <a href= +"#page57">57</a><br/> +<i>Eimeo</i> <a href="#page112">112</a><br/> +levated reef of Mauritius <a href="#page47">47</a><br/> +levations, recent proofs of <a href="#page98">98</a><br/> +—— immense areas of <a href="#page106">106</a><br/> +<i>Elivi</i> <a href="#page126">126</a><br/> +lizabeth island <a href="#page59">59</a><br/> +—— recently elevated <a href="#page98">98</a>, <a +href="#page104">104</a><br/> +<i>Elizabeth island</i> <a href="#page112">112</a><br/> +<i>Ellice group</i> <a href="#page120">120</a><br/> +ncircled islands, their height <a href="#page41">41</a><br/> +—— geological composition <a href="#page42">42</a>, <a +href="#page44">44</a><br/> +ua, description of <a href="#page99">99</a><br/> +<i>Eoua</i> <a href="#page118">118</a><br/> +upted matter probably not associated with thick masses of +coral-rock <a href="#page89">89</a><br/> +<br/> +Fais, recently elevated <a href="#page100">100</a>, <a href= +"#page104">104</a><br/> +<i>Fais</i> <a href="#page126">126</a><br/> +<i>Fanning</i> <a href="#page116">116</a><br/> +<i>Farallon de Medinilla</i> <a href="#page127">127</a><br/> +<i>Farson group</i> <a href="#page144">144</a><br/> +<i>Fataka</i> <a href="#page122">122</a><br/> +Fiji archipelago <a href="#page119">119</a><br/> +Fish, feeding on corals <a href="#page21">21</a><br/> +—— killed in Keeling lagoon by heavy rain <a href= +"#page24">24</a><br/> +Fissures across coral-islands <a href="#page75">75</a><br/> +Fitzroy, Captain, on a submerged shed at Keeling atoll <a href= +"#page23">23</a><br/> +—— on an inundation in the Low archipelago <a href= +"#page74">74</a><br/> +<i>Flint</i> <a href="#page115">115</a><br/> +<i>Flores</i> <a href="#page130">130</a><br/> +<i>Florida</i> <a href="#page149">149</a><br/> +<i>Folger</i> <a href="#page127">127</a><br/> +<i>Formosa</i> <a href="#page135">135</a><br/> +Forster, theory of coral-formations <a href="#page73">73</a><br/> +<i>Frederick reef</i> <a href="#page123">123</a><br/> +<i>Freewill</i> <a href="#page128">128</a><br/> +Friendly group recently elevated <a href="#page99">99</a>, <a +href="#page105">105</a><br/> +<i>Friendly archipelago</i> <a href="#page118">118</a><br/> +Fringing-reefs, absent where coast precipitous <a href="#page45"> +5</a><br/> +—— breached in front of streams <a href="#page54"> +54</a><br/> +—— described by MM. Quoy and Gaimard <a href= +"#page98">98</a><br/> +—— not closely attached to shelving coasts <a href= +"#page46">46</a><br/> +—— of east coast of Africa <a href="#page48"></a><br/> +—— of Cuba <a href="#page48">48</a><br/> +—— of Mauritius <a href="#page45">45</a><br/> +—— on worn down banks of rock <a href="#page49"> +9</a><br/> +—— on banks of sediment <a href="#page49">49</a><br/> +—— their appearance when elevated <a href="#page47"> +7</a><br/> +—— their growth influenced by currents <a href= +"#page49">49</a><br/> +—— by shallowness of sea <a href="#page49">49</a><br/> +<br/> +<i>Galapagos archipelago</i> <a href="#page111">111</a><br/> +<i>Galega</i> <a href="#page139">139</a><br/> +Gambier islands, section of <a href="#page43">43</a><br/> +<i>Gambier islands</i> <a href="#page112">112</a><br/> +<i>Gardner</i> <a href="#page116">116</a><br/> +<i>Gaspar rico</i> <a href="#page121">121</a><br/> +Geological composition of coral-formations <a href="#page88"> +</a><br/> +<i>Gilbert archipelago</i> <a href="#page120">120</a><br/> +<i>Gilolo</i> <a href="#page129">129</a><br/> +<i>Glorioso</i> <a href="#page139">139</a><br/> +Gloucester island <a href="#page74">74</a><br/> +<i>Glover reef</i> <a href="#page152">152</a><br/> +<i>Gomez</i> <a href="#page111">111</a><br/> +<i>Gouap</i> <a href="#page126">126</a><br/> +<i>Goulou</i> <a href="#page126">126</a><br/> +<i>Grampus</i> <a href="#page127">127</a><br/> +<i>Gran Cocal</i> <a href="#page120">120</a><br/> +Great Chagos Bank, description and theory of <a href="#page37"> +37</a>, <a href="#page85">85</a><br/> +Grey, Captain, on sandbars <a href="#page46">46</a><br/> +Grouping of the different classes of reefs <a href="#page93"> +93</a><br/> +<i>Guedes</i> <a href="#page128">128</a><br/> +<br/> +Hall, Captain B., on Loo Choo <a href="#page101">101</a><br/> +Harvey islands, recently elevated <a href="#page104">104</a><br/> +Height of encircled islands <a href="#page41">41</a><br/> +<i>Hermites</i> <a href="#page125">125</a><br/> +<i>Hervey or Cook islands</i> <a href="#page114">114</a><br/> +<i>Hogoleu</i> <a href="#page125">125</a><br/> +Holothuriæ (Holuthuriæ) feeding on coral <a href= +"#page21">21</a><br/> +Houden island, height of <a href="#page71">71</a><br/> +<i>Honduras, reef off</i> <a href="#page151">151</a><br/> +<i>Horn</i> <a href="#page119">119</a><br/> +<i>Houtman Abrolhos</i> <a href="#page130">130</a><br/> +Huaheine; alleged proofs of its recent elevation <a href= +"#page103">103</a><br/> +<i>Huaheine</i> <a href="#page113">113</a><br/> +<i>Humphrey</i> <a href="#page115">115</a><br/> +<i>Hunter</i> <a href="#page119">119</a><br/> +Hurricanes, effects of, on coral-islands <a href="#page74"> +74</a><br/> +<br/> +<i>Immaum</i> <a href="#page143">143</a><br/> +<i>Independence</i> <a href="#page120">120</a><br/> +India, west coast, recently elevated <a href="#page101"> +101</a><br/> +<i>India</i> <a href="#page143">143</a><br/> +Irregular reefs in shallow seas <a href="#page49">49</a><br/> +Islets of coral-rock, their formation <a href="#page19">19</a><br/> +—— their destruction in the Maldiva atolls <a href= +"#page36">36</a><br/> +<br/> +<i>Jamaica</i> <a href="#page152">152</a><br/> +<i>Jarvis</i> <a href="#page115">115</a><br/> +Java, recently elevated <a href="#page100">100</a><br/> +<i>Java</i> <a href="#page131">131</a><br/> +<i>Johnston island</i> <a href="#page116">116</a><br/> +<i>Juan de Nova</i> <a href="#page139">139</a><br/> +<i>Juan de Nova (Madagascar)</i> <a href="#page140">140</a><br/> +<br/> +<i>Kalatoa</i> <a href="#page131">131</a><br/> +Kamtschatka, proofs of its recent elevation <a href="#page105"> +105</a><br/> +<i>Karkalang</i> <a href="#page129">129</a><br/> +Keeling atoll, section of reef <a href="#page15">15</a><br/> +<i>Keeling, south atoll</i> <a href="#page137">137</a><br/> +—— <i>north atoll</i> <a href="#page137">137</a><br/> +<i>Keffing</i> <a href="#page128">128</a><br/> +<i>Kemin</i> <a href="#page115">115</a>, <a href="#page116"> +116</a><br/> +<i>Kennedy</i> <a href="#page123">123</a><br/> +<i>Keppel</i> <a href="#page119">119</a><br/> +<i>Kumi</i> <a href="#page135">135</a><br/> +<br/> +<i>Laccadive group</i> <a href="#page137">137</a><br/> +Ladrones, or Marianas, recently elevated <a href="#page100"> +100</a><br/> +<i>Ladrones archipelago</i> <a href="#page127">127</a><br/> +Lagoon of Keeling atoll <a href="#page20">20</a><br/> +Lagoons bordered by inclined ledges and walls, and theory of their +formation <a href="#page32">32</a>, <a href="#page79">79</a><br/> +—— of small atolls filled up with sediment <a href= +"#page32">32</a><br/> +Lagoon-channels within barrier-reefs <a href="#page40">40</a><br/> +Lagoon-reefs, all submerged in some atolls, and rising to the +surface in others <a href="#page55">55</a><br/> +<i>Lancaster reef</i> <a href="#page115">115</a><br/> +<i>Latte</i> <a href="#page119">119</a><br/> +<i>Lauglan islands</i> <a href="#page123">123</a><br/> +Ledges round certain lagoons <a href="#page32">32</a>, <a href= +"#page79">79</a><br/> +<i>Lette</i> <a href="#page129">129</a><br/> +<i>Lighthouse reef</i> <a href="#page152">152</a><br/> +Lloyd, Mr., on corals refixing themselves <a href="#page62"> +62</a><br/> +Loo Choo, recently elevated <a href="#page101">101</a><br/> +<i>Loo Choo</i> <a href="#page135">135</a><br/> +<i>Louisiade</i> <a href="#page123">123</a><br/> +Low archipelago, alleged proofs of its recent elevation <a href= +"#page96">96</a><br/> +<i>Low archipelago</i> <a href="#page111">111</a><br/> +Lowness of coral-islands <a href="#page70">70</a><br/> +<i>Loyalty group</i> <a href="#page123">123</a><br/> +<i>Lucepara</i> <a href="#page133">133</a><br/> +Lutké, Admiral, on fissures across coral-islands <a href= +"#page75">75</a><br/> +Luzon, recently elevated <a href="#page101">101</a><br/> +<i>Luzon</i> <a href="#page134">134</a><br/> +Lyell, Mr., on channels into the lagoons of atolls <a href= +"#page31">31</a><br/> +—— on the lowness of their leeward sides <a href= +"#page82">82</a><br/> +—— on the antiquity of certain corals <a href= +"#page58">58</a><br/> +—— on the apparent continuity of distinct coral-islands +<a href="#page89">89</a><br/> +—— on the recently elevated beds of the Red Sea <a +href="#page102">102</a><br/> +—— on the outline of the areas of subsidence <a href= +"#page106">106</a><br/> +<br/> +<i>Macassar strait</i> <a href="#page131">131</a><br/> +<i>Macclesfield bank</i> <a href="#page136">136</a><br/> +Madagascar, quick growth of corals at <a href="#page62">62</a><br/> +—— madreporitic rock of <a href="#page101">101</a><br/> +<i>Madagascar</i> <a href="#page140">140</a><br/> +<i>Madjiko-sima</i> <a href="#page135">135</a><br/> +<i>Madura (Java)</i> <a href="#page131">131</a><br/> +<i>Madura (India)</i> <a href="#page137">137</a><br/> +Mahlos Mahdoo, theory of formation <a href="#page88">88</a><br/> +Malacca, recently elevated <a href="#page100">100</a><br/> +<i>Malacca</i> <a href="#page133">133</a><br/> +Malcolmson, Dr., on recent elevation of W. coast of India <a href= +"#page100">100</a><br/> +—— on recent elevation of Camaran island <a href= +"#page102">102</a><br/> +<i>Malden</i> <a href="#page115">115</a><br/> +Maldiva atolls, and theory of their formation <a href="#page33"> +33</a>, <a href="#page80">80</a>, <a href="#page82">82</a><br/> +—— steepness of their flanks <a href="#page26"> +26</a><br/> +—— growth of coral at <a href="#page62">62</a><br/> +<i>Maldiva archipelago</i> <a href="#page137">137</a><br/> +Mangaia island <a href="#page64">64</a><br/> +—— recently elevated <a href="#page99">99</a>, <a +href="#page104">104</a><br/> +<i>Mangaia</i> <a href="#page114">114</a><br/> +<i>Mangs</i> <a href="#page127">127</a><br/> +Marianas, recently elevated <a href="#page100">100</a><br/> +<i>Mariana archipelago</i> <a href="#page127">127</a><br/> +<i>Mariere</i> <a href="#page126">126</a><br/> +<i>Marquesas archipelago</i> <a href="#page113">113</a><br/> +<i>Marshall archipelago</i> <a href="#page121">121</a><br/> +<i>Marshall island</i> <a href="#page127">127</a><br/> +<i>Martinique</i> <a href="#page153">153</a><br/> +<i>Martires</i> <a href="#page126">126</a><br/> +Mary’s St. in Madagascar, harbour made in reefs <a href="#page54"> +54</a><br/> +<i>Mary island</i> <a href="#page116">116</a><br/> +<i>Matia, or Aurora</i> <a href="#page112">112</a><br/> +Matilda atoll <a href="#page60">60</a><br/> +Mauritius, fringing-reefs of <a href="#page45">45</a><br/> +—— depths at which corals live there <a href= +"#page64">64</a><br/> +—— recently elevated <a href="#page101">101</a><br/> +<i>Mauritius</i> <a href="#page138">138</a><br/> +Maurua, section of <a href="#page43">43</a><br/> +<i>Maurua</i> <a href="#page113">113</a><br/> +Menchikoff atoll <a href="#page25">25</a>, <a href="#page84"> +</a><br/> +<i>Mendana archipelago</i> <a href="#page113">113</a><br/> +<i>Mendana isles</i> <a href="#page122">122</a><br/> +<i>Mexico, gulf of</i> <a href="#page149">149</a><br/> +Millepora complanata at Keeling atoll <a href="#page16">16</a><br/> +<i>Mindoro</i> <a href="#page134">134</a><br/> +<i>Mohilla (Mohila)</i> <a href="#page139">139</a><br/> +Molucca islands, recently elevated <a href="#page100">100</a><br/> +<i>Mopeha</i> <a href="#page113">113</a><br/> +Moresby, Captain, on boring through coral-reefs <a href="#page59"> +59</a><br/> +<i>Morty</i> <a href="#page129">129</a><br/> +<i>Mosquito coast</i> <a href="#page152">152</a><br/> +Musquillo atoll <a href="#page84">84</a><br/> +<i>Mysol</i> <a href="#page129">129</a><br/> +<br/> +Namourrek group <a href="#page84">84</a><br/> +<i>Natunas</i> <a href="#page133">133</a><br/> +Navigator archipelago, elevation of <a href="#page99">99</a><br/> +<i>Navigator archipelago</i> <a href="#page117">117</a><br/> +<i>Nederlandisch</i> <a href="#page120">120</a><br/> +Nelson, Lieutenant, on the consolidation of coral-rocks under water +<a href="#page59">59</a><br/> +—— theory of coral-formations <a href="#page73"> +73</a><br/> +—— on the Bermuda islands <a href="#page154"> +154</a><br/> +<i>New Britain</i> <a href="#page124">124</a><br/> +New Caledonia, steepness of its reefs <a href="#page39">39</a><br/> +—— —— barrier-reef of <a href="#page44"> +</a>, <a href="#page79">79</a>, <a href="#page83">83</a>, <a +href="#page93">93</a><br/> +<i>New Caledonia</i> <a href="#page123">123</a><br/> +<i>New Guinea (E. end)</i> <a href="#page124">124</a><br/> +<i>New Guinea (W. end)</i> <a href="#page127">127</a><br/> +<i>New Hanover</i> <a href="#page124">124</a><br/> +New Hebrides, recently elevated <a href="#page100">100</a><br/> +<i>New Hebrides</i> <a href="#page121">121</a><br/> +New Ireland, recently elevated <a href="#page100">100</a><br/> +<i>New Ireland</i> <a href="#page124">124</a><br/> +<i>New Nantucket</i> <a href="#page116">116</a><br/> +<i>Nicobar islands</i> <a href="#page132">132</a><br/> +<i>Niouha</i> <a href="#page119">119</a><br/> +Nulliporæ at Keeling atoll <a href="#page18">18</a><br/> +—— on the reefs of atolls <a href="#page28">28</a><br/> +—— on barrier-reefs <a href="#page39">39</a><br/> +—— their wide distribution and abundance <a href= +"#page68">68</a><br/> +<br/> +Objections to the theory of subsidence <a href="#page87">7</a><br/> +<i>Ocean islands</i> <a href="#page117">117</a>, <a href= +"#page121">121</a><br/> +<i>Ono</i> <a href="#page120">120</a><br/> +<i>Onouafu (Onouafou)</i> <a href="#page119">119</a><br/> +<i>Ormuz</i> <a href="#page143">143</a><br/> +<i>Oscar group</i> <a href="#page120">120</a><br/> +Oscillations of level <a href="#page103">103</a>, <a href= +"#page108">108</a><br/> +<i>Ouallan, or Ualan (Oualan)</i> <a href="#page125">125</a><br/> +Ouluthy atoll <a href="#page60">60</a><br/> +<i>Outong Java</i> <a href="#page124">124</a><br/> +<br/> +<i>Palawan, S.W. coast <a href="#page133">133</a><br/> +—— N.W. coast <a href="#page134">134</a><br/> +—— western bank <a href="#page136">136</a><br/> +</i> Palmerston <a href="#page114">114</a><br/> +<i>Palmyra</i> <a href="#page116">116</a><br/> +<i>Paracells</i> <a href="#page136">136</a><br/> +<i>Paraquas</i> <a href="#page136">136</a><br/> +<i>Patchow</i> <a href="#page135">135</a><br/> +<i>Pelew islands</i> <a href="#page126">126</a><br/> +Pemba island, singular form of <a href="#page102">102</a><br/> +<i>Pemba</i> <a href="#page142">142</a><br/> +<i>Penrhyn</i> <a href="#page115">115</a><br/> +<i>Peregrino</i> <a href="#page115">115</a><br/> +Pernambuco, bar of sandstone at <a href="#page47">47</a><br/> +Persian gulf, recently elevated <a href="#page102">102</a><br/> +<i>Persian gulf</i> <a href="#page143">143</a><br/> +Pescado <a href="#page115">115</a><br/> +<i>Pescadores</i> <a href="#page135">135</a><br/> +<i>Peyster group</i> <a href="#page120">120</a><br/> +<i>Philip</i> <a href="#page126">126</a><br/> +Philippine archipelago, recently elevated <a href="#page101"> +101</a><br/> +<i>Philippine archipelago</i> <a href="#page134">134</a><br/> +<i>Phœnix</i> <a href="#page116">116</a><br/> +<i>Piguiram</i> <a href="#page126">126</a><br/> +<i>Pitcairn</i> <a href="#page112">112</a><br/> +Pitt’s bank <a href="#page86">86</a><br/> +<i>Pitt island</i> <a href="#page120">120</a><br/> +<i>Platte</i> <a href="#page139">139</a><br/> +<i>Pleasant</i> <a href="#page121">121</a><br/> +Porites, chief coral on margin of Keeling atoll <a href="#page16"> +16</a><br/> +<i>Postillions</i> <a href="#page131">131</a><br/> +Pouynipète <a href="#page95">95</a><br/> +—— its probable subsidence <a href="#page95"> +95</a><br/> +<i>Pouynipète</i> <a href="#page125">125</a><br/> +<i>Pratas shoal</i> <a href="#page135">135</a><br/> +<i>Proby</i> <a href="#page119">119</a><br/> +<i>Providence</i> <a href="#page139">139</a><br/> +<i>Puerto Rico</i> <a href="#page152">152</a><br/> +<i>Pulo Anna</i> <a href="#page126">126</a><br/> +Pumice floated to coral-islands <a href="#page88">88</a><br/> +<i>Pylstaart</i> <a href="#page118">118</a><br/> +Pyrard de Laval, astonishment at the atolls in the Indian Ocean <a +href="#page11">11</a><br/> +<br/> +Quoy and Gaimard, depths at which corals live <a href="#page66"> +66</a><br/> +—— description of reefs applicable only to +fringing-reefs <a href="#page98">98</a><br/> +<br/> +Range of atolls <a href="#page94">94</a><br/> +<i>Rapa</i> <a href="#page115">115</a><br/> +<i>Rearson</i> <a href="#page115">115</a><br/> +Red Sea, banks of rock coated by reefs <a href="#page49"> +49</a><br/> +—— proofs of its recent elevation <a href="#page102"> +102</a><br/> +—— supposed subsidence of <a href="#page103"> +103</a><br/> +<i>Red Sea</i> <a href="#page143">143</a><br/> +Reefs, irregular in shallow seas <a href="#page49">49</a><br/> +—— rising to the surface in some lagoons and all +submerged in others <a href="#page55">55</a><br/> +—— their distribution <a href="#page50">50</a><br/> +—— their absence from some coasts <a href="#page51"> +51</a><br/> +<i>Revilla-gigedo</i> <a href="#page111">111</a><br/> +Ring-formed reefs of the Maldiva atolls, and theory of <a href= +"#page34"></a>, <a href="#page80">80</a><br/> +<i>Rodriguez</i> <a href="#page138">138</a><br/> +<i>Rosario</i> <a href="#page127">127</a><br/> +<i>Rose island</i> <a href="#page118">118</a><br/> +<i>Rotches</i> <a href="#page120">120</a><br/> +<i>Rotoumah</i> <a href="#page120">120</a><br/> +<i>Roug</i> <a href="#page125">125</a><br/> +<i>Rowley shoals</i> <a href="#page130">130</a><br/> +Rüppell, Dr., on the recent deposits of Red Sea <a href= +"#page102">102</a><br/> +<br/> +<i>Sable, ile de</i> <a href="#page138">138</a><br/> +<i>Sahia de Malha</i> <a href="#page137">137</a><br/> +<i>St. Pierre</i> <a href="#page139">139</a><br/> +<i>Sala</i> <a href="#page111">111</a><br/> +<i>Salomon (Solomon) archipelago</i> <a href="#page123"> +123</a><br/> +Samoa, or Navigator archipelago, elevation of <a href="#page99"> +99</a><br/> +<i>Samoa archipelago</i> <a href="#page117">117</a><br/> +Sand-bars parallel to coasts <a href="#page46">46</a><br/> +<i>Sandal-wood</i> <a href="#page129">129</a><br/> +Sandwich archipelago, recently elevated <a href="#page98"> +98</a><br/> +<i>Sandwich archipelago</i> <a href="#page117">117</a><br/> +<i>Sanserot</i> <a href="#page126">126</a><br/> +<i>Santa-Cruz group</i> <a href="#page122">122</a><br/> +Savage island, recently elevated <a href="#page59">59</a>, <a +href="#page99">99</a>, <a href="#page104">104</a><br/> +<i>Savage</i> <a href="#page118">118</a><br/> +<i>Savu</i> <a href="#page129">129</a><br/> +<i>Saya, or Sahia de Malha</i> <a href="#page137">137</a><br/> +<i>Scarborough shoal</i> <a href="#page136">136</a><br/> +Scarus feeding on corals <a href="#page21">21</a><br/> +<i>Schouten</i> <a href="#page124">124</a><br/> +<i>Scilly</i> <a href="#page113">113</a><br/> +Scoriæ floated to coral-islands <a href="#page89">89</a><br/> +<i>Scott’s reef</i> <a href="#page130">130</a><br/> +Sections of islands encircled by barrier-reefs <a href="#page43"> +43</a>, <a href="#page176">176</a><br/> +—— of Bolabola <a href="#page76">76</a><br/> +Sediment in Keeling lagoon <a href="#page21">21</a><br/> +—— in other atolls <a href="#page29">29</a>, <a href= +"#page35">35</a><br/> +—— injurious to corals <a href="#page53">53</a><br/> +—— transported from coral-islands far seaward <a href= +"#page89">89</a><br/> + <i>Seniavine</i> <a href="#page125">125</a><br/> +<i>Serangani</i> <a href="#page129">129</a><br/> +<i>Seychelles</i> <a href="#page138">138</a><br/> +Ship-bottom quickly coated with coral <a href="#page62">62</a><br/> +<i>Smyth island</i> <a href="#page116">116</a><br/> +Society archipelago, stationary condition of <a href="#page96"> +96</a><br/> +—— alleged proofs of recent elevation <a href= +"#page103">103</a><br/> +<i>Society archipelago</i> <a href="#page112">112</a><br/> +<i>Socotra</i> <a href="#page143">143</a><br/> +<i>Solor</i> <a href="#page130">130</a><br/> +Sooloo islands, recently elevated <a href="#page101">101</a><br/> +<i>Sooloo islands</i> <a href="#page133">133</a><br/> +<i>Souvaroff</i> <a href="#page115">115</a><br/> +<i>Spanish</i> <a href="#page126">126</a><br/> +Sponge, depths at which found <a href="#page67">67</a><br/> +<i>Starbuck (Slarbuck)</i> <a href="#page115">115</a><br/> +Stones transported in roots of trees <a href="#page89">89</a><br/> +Storms, effects of, on coral-islands <a href="#page74">74</a><br/> +Stutchbury, Mr., on the growth of an Agaricia <a href="#page63"> +63</a><br/> +—— on upraised corals in Society archipelago <a href= +"#page103">103</a><br/> +Subsidence of Keeling atoll <a href="#page28">28</a><br/> +—— extreme slowness of <a href="#page87">87</a>, <a +href="#page108">108</a><br/> +—— areas of, apparently elongated <a href="#page106"> +106</a><br/> +—— areas of immense <a href="#page106">106</a><br/> +—— great amount of <a href="#page108">108</a><br/> +<i>Suez, gulf of</i> <a href="#page147">147</a><br/> +<i>Sulphur islands</i> <a href="#page127">127</a><br/> +Sumatra, recently elevated <a href="#page100">100</a><br/> +<i>Sumatra</i> <a href="#page132">132</a><br/> +<i>Sumbawa</i> <a href="#page130">130</a><br/> +Surf favourable to the growth of massive corals <a href="#page52"> +52</a><br/> +<i>Swallow shoal</i> <a href="#page136">136</a><br/> +<i>Sydney island</i> <a href="#page116">116</a><br/> +<br/> +Tahiti, alleged proofs of its recent elevation <a href="#page103"> +103</a><br/> +<i>Tahiti</i> <a href="#page112">112</a><br/> +Temperature of the sea at the Galapagos archipelago <a href= +"#page51">51</a><br/> +<i>Tenasserim</i> <a href="#page133">133</a><br/> +<i>Tenimber island</i> <a href="#page128">128</a><br/> +<i>Teturoa</i> <a href="#page113">113</a><br/> +Theories on coral-formations <a href="#page69">69</a>, <a href= +"#page73">73</a><br/> +Theory of subsidence, and objections to <a href="#page72">72</a>, +<a href="#page86">86</a><br/> +Thickness, vertical, of barrier-reefs <a href="#page43">43</a>, <a +href="#page76">76</a><br/> +<i>Thomas, St.</i> <a href="#page153">153</a><br/> +<i>Tikopia</i> <a href="#page122">122</a><br/> +Timor, recently elevated <a href="#page100">100</a><br/> +<i>Timor</i> <a href="#page129">129</a><br/> +<i>Timor-laut</i> <a href="#page128">128</a><br/> +<i>Tokan-Bessees</i> <a href="#page131">131</a><br/> +<i>Tongatabou</i> <a href="#page118">118</a><br/> +<i>Tonquin</i> <a href="#page137">137</a><br/> +<i>Toubai</i> <a href="#page113">113</a><br/> +<i>Toufoa (Toofoa)</i> <a href="#page119">119</a><br/> +<i>Toupoua</i> <a href="#page122">122</a><br/> +Traditions of change in coral-islands <a href="#page73">73</a><br/> +Tridacnæ embedded in coral-rock <a href="#page63">63</a><br/> +—— left exposed in the Low archipelago <a href= +"#page96">96</a><br/> +Tubularia, quick growth of <a href="#page63">63</a><br/> +<i>Tumbelan</i> <a href="#page133">133</a><br/> +<i>Turneffe reef</i> <a href="#page152">152</a><br/> +<i>Turtle</i> <a href="#page119">119</a><br/> +<br/> +<i>Ualan</i> <a href="#page125">125</a><br/> +<br/> +Vanikoro, section of <a href="#page43">43</a><br/> +—— its state and changes in its reefs <a href= +"#page95">95</a><br/> +<i>Vanikoro</i> <a href="#page122">122</a><br/> +<i>Vine reef</i> <a href="#page125">125</a><br/> +<i>Virgin Gorda</i> <a href="#page153">153</a><br/> +<i>Viti archipelago</i> <a href="#page119">119</a><br/> +Volcanic islands, with living corals on their shores <a href= +"#page51">51</a><br/> +—— matter, probably not associated with thick masses of +coral-rock <a href="#page88">88</a><br/> +Volcanoes, authorities for their position on the map <a href= +"#page90">90</a><br/> +—— their presence determined by the movements in +progress <a href="#page104">104</a><br/> +—— absent or extinct in the areas of subsidence <a +href="#page105">105</a><br/> +<br/> +<i>Waigiou</i> <a href="#page128">128</a><br/> +<i>Wallis island</i> <a href="#page119">119</a><br/> +<i>Washington</i> <a href="#page116">116</a><br/> +<i>Wells’ reef</i> <a href="#page123">123</a><br/> +Wellstead, Lieutenant, account of a ship coated with corals <a +href="#page62">62</a><br/> +West Indies, banks of sediment fringed by reefs <a href="#page49"> +49</a><br/> +—— recently elevated <a href="#page102">102</a><br/> +<i>West Indies</i> <a href="#page147">147</a><br/> +Whitsunday island, view of <a href="#page12">12</a><br/> +—— changes in its state <a href="#page74">74</a><br/> +Williams, Rev. J., on traditions of the natives regarding +coral-islands <a href="#page74">74</a><br/> +—— on antiquity of certain corals <a href="#page64"> +64</a><br/> +<i>Wolchonsky</i> <a href="#page111">111</a><br/> +<i>Wostock</i> <a href="#page115">115</a><br/> +<br/> +<i>Xulla islands</i> <a href="#page128">128</a><br/> +<br/> +<i>York island</i> <a href="#page116">116</a><br/> +<i>Yucutan, coast of</i> <a href="#page151">151</a><br/> +<br/> +Zones of different kinds of corals outside the same reefs <a href= +"#page55">55</a>, <a href="#page60">60</a><br/> +</small></p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="index02">INDEX TO VOLCANIC ISLANDS.</a></h2> + +<p class="noindent"> +<small>Abel, M., on calcareous casts at the Cape of Good Hope <a +href="#page261">261</a><br/> + Abingdon island <a href="#page234">234</a><br/> + Abrolhos islands, incrustation on <a href="#page188">188</a><br/> + Aeriform explosions at Ascension <a href="#page191">191</a><br/> + <i>Albatross</i>, driven from St. Helena <a href="#page225"> +225</a><br/> + Albemarle island <a href="#page234">234</a><br/> + Albite, at the Galapagos archipelago <a href="#page234"> +234</a><br/> + Amygdaloidal cells, half filled <a href="#page184">184</a><br/> + Amygdaloids, calcareous origin of <a href="#page176">176</a><br/> + Ascension, arborescent incrustation on rocks of <a href= +"#page188">188</a><br/> +—— absence of dikes, freedom from volcanic action, and +state of lava-streams <a href="#page226">226</a><br/> + Ascidia, extinction of <a href="#page258">258</a><br/> + Atlantic Ocean, new volcanic focus in <a href="#page226"> +226</a><br/> + Augite, fused <a href="#page239">239</a><br/> + Australia <a href="#page251">251</a><br/> + Azores <a href="#page182">182</a>, <a href="#page248"> +248</a><br/> +<br/> + Bahia in Brazil, dikes at <a href="#page247">247</a><br/> + Bailly, M., on the mountains of Mauritius <a href="#page185"> +185</a><br/> + Bald Head <a href="#page260">260</a><br/> + Banks’ Cove <a href="#page234">234</a>, <a href="#page236"> +236</a><br/> + Barn, The, St. Helena <a href="#page216">216</a><br/> + Basalt, specific gravity of <a href="#page245">245</a><br/> + Basaltic coast-mountains at Mauritiu <a href="#page185"> +185</a><br/> +—— at St. Helena <a href="#page218">218</a><br/> +—— at St. Jago <a href="#page178">178</a><br/> + Beaumont, M. Elie de, on circular subsidences in lava <a href= +"#page233">233</a><br/> +—— on dikes indicating elevation <a href="#page228"> +228</a><br/> +—— on inclination of lava-streams <a href="#page227"> +227</a><br/> +—— on laminated dikes <a href="#page212">212</a><br/> + Bermuda, calcareous rocks of <a href="#page260">260</a>, <a href= +"#page262">262</a><br/> + Beudant, M., on bombs <a href="#page191">191</a><br/> +—— on jasper <a href="#page197">197</a><br/> +—— on laminated trachyte <a href="#page211"> +211</a><br/> +—— on obsidian of Hungary <a href="#page202"> +207</a><br/> +—— on silex in trachyte <a href="#page170">270</a>, <a +href="#page197">197</a><br/> + Bole <a href="#page257">257</a><br/> + Bombs, volcanic <a href="#page189">189</a><br/> + Bory St. Vincent, on bombs <a href="#page190">190</a><br/> + Boulders, absence in Australia and Cape of Good Hope <a href= +"#page265">265</a><br/> + Brattle island <a href="#page238">238</a><br/> + Brewster, Sir D., on a calcareo-animal substance <a href= +"#page201">201</a><br/> +—— on decomposed glass <a href="#page252">252</a><br/> + Brown, Mr. R., on extinct plants from Van Diemen’s land <a href= +"#page257">257</a><br/> +—— on sphærulitic bodies in silicified wood <a +href="#page207">207</a><br/> + Buch, Von, on cavernous lava <a href="#page233">233</a><br/> +—— on central volcanoes <a href="#page249">249</a><br/> +—— on crystals sinking in obsidian <a href="#page243"> +243</a><br/> +—— on laminated lava <a href="#page209">209</a><br/> +—— on obsidian streams <a href="#page208">208</a><br/> +—— on olivine in basalt <a href="#page234">234</a><br/> +—— on superficial calcareous beds in the Canary islands +<a href="#page224">224</a><br/> +<br/> + Calcareous deposit at St. Jago affected by heat <a href= +"#page169">169</a>, <a href="#page171">171</a><br/> +—— fibrous matter, entangled in streaks in scoriæ +<a href="#page174">174</a><br/> +—— freestone at Ascension <a href="#page198"> +198</a><br/> +—— incrustations at Ascension <a href="#page199"> +199</a><br/> +—— sandstone at St. Helena <a href="#page222"> +222</a><br/> +—— superficial beds at King George’s sound <a href= +"#page260">260</a><br/> + Cape of Good Hope <a href="#page263">263</a><br/> + Carbonic acid, expulsion of, by heat <a href="#page171">171</a>, +<a href="#page176">176</a><br/> + Carmichael, Capt., on glassy coatings to dikes <a href= +"#page216">216</a><br/> + Casts, calcareous, of branches <a href="#page261">261</a><br/> + Chalcedonic nodules <a href="#page257">257</a><br/> + Chalcedony in basalt and in silicified wood <a href="#page196"> +196</a><br/> + Chatham island <a href="#page231">231</a>, <a href="#page235"> +235</a>, <a href="#page241">241</a>, <a href="#page248">248</a>, +<a href="#page259">259</a><br/> + Chlorophæite <a href="#page257">257</a><br/> + Clarke, Rev. W., on the Cape of Good Hope <a href="#page258"> +258</a>, <a href="#page263">263</a><br/> + Clay-slate, its decomposition and junction with granite at the +Cape of Good Hope <a href="#page264">264</a><br/> + Cleavage of clay-slate in Australia <a href="#page252"> +252</a><br/> + Cleavage, cross, in sandstone <a href="#page253">253</a><br/> + Coast denudation at St. Helena <a href="#page226">226</a><br/> + Columnar basalt <a href="#page173">173</a><br/> + “Comptes Rendus,” account of volcanic phenomena in the Atlantic <a +href="#page226">226</a><br/> + Concepcion, earthquake of <a href="#page228">228</a>, <a href= +"#page249">249</a><br/> + Concretions in aqueous and igneous rocks compared <a href= +"#page206">206</a><br/> +—— in tuff <a href="#page197">197</a><br/> +—— of obsidian <a href="#page206">206</a>, <a href= +"#page208">208</a><br/> + Conglomerate, recent, at St. Jago <a href="#page181">181</a><br/> + Coquimbo, curious rock of <a href="#page261">261</a><br/> + Corals, fossil, from Van Diemen’s Land <a href="#page256"> +256</a><br/> + Crater, segment of, at the Galapagos <a href="#page238"> +238</a><br/> +—— great central one at St. Helena <a href="#page219"> +219</a><br/> +—— internal ledges round, and parapet on <a href= +"#page220">220</a><br/> + Craters, basaltic, at Ascension <a href="#page189">189</a><br/> +—— form of, affected by the trade wind <a href= +"#page189">189</a><br/> +—— of elevation <a href="#page227">227</a><br/> +—— of tuff at Terceira <a href="#page182">182</a><br/> +—— of tuff at the Galapagos archipelago <a href= +"#page230">230</a>, <a href="#page231">231</a>, <a href= +"#page235">235</a>, <a href="#page237">237</a><br/> +—— their breached state <a href="#page240">240</a><br/> +—— small basaltic at St. Jago <a href="#page177"> +177</a><br/> +—— —— at the Galapagos archipelago <a href= +"#page232">232</a><br/> + Crystallisation favoured by space <a href="#page211">211</a><br/> +<br/> + Dartigues, M., on sphærulites <a href="#page207"> +207</a><br/> + Daubeny, Dr., on a basin-formed island <a href="#page237"> +237</a><br/> +—— on fragments in trachyte <a href="#page193"> +193</a><br/> + D’Aubuisson on hills of phonolite <a href="#page222">222</a><br/> +—— on the composition of obsidian <a href="#page206"> +206</a><br/> +—— on the lamination of clay-slate <a href="#page210"> +210</a><br/> + De la Beche, Sir H., on magnesia in erupted lime <a href= +"#page174">174</a><br/> +—— on specific gravity of limestones <a href= +"#page198">198</a><br/> + Denudation of coast at St. Helena <a href="#page226">226</a><br/> + Diana’s Peak, St. Helena <a href="#page220">220</a><br/> + Dieffenbach, Dr., on the Chatham Islands <a href="#page259"> +259</a><br/> + Dikes, truncated, on central crateriform ridge of St. Helena <a +href="#page219">219</a><br/> +—— at St. Helena; number of; coated by a glossy layer; +uniform thickness of <a href="#page216">216</a><br/> +—— great parallel ones at St. Helena <a href= +"#page222">222</a><br/> +—— not observed at Ascension <a href="#page226"> +226</a><br/> +—— of tuff <a href="#page231">231</a><br/> +—— of trap in the plutonic series <a href="#page247"> +247</a><br/> +—— remnants of, extending far into the sea round St. +Helena <a href="#page226">226</a><br/> + Dislocations at Ascension <a href="#page192">192</a><br/> +—— at St. Helena <a href="#page217">217</a>, <a href= +"#page221">221</a><br/> + Distribution of volcanic islands <a href="#page248">248</a><br/> + Dolomieu, on decomposed trachyte <a href="#page182">182</a><br/> +—— on laminated lava <a href="#page210">210</a>, <a +href="#page211">211</a><br/> +—— on obsidian <a href="#page208">208</a><br/> + Drée, M., on crystals sinking in lava <a href="#page243"> +243</a><br/> + Dufrenoy, M., on the composition of the surface of certain +lava-streams <a href="#page209">209</a>, <a href="#page243"> +243</a><br/> +—— on the inclination of tuff-strata <a href= +"#page236">236</a><br/> +<br/> + Eggs of birds embedded at St. Helena <a href="#page224"> +224</a><br/> +—— of turtle at Ascension <a href="#page198"> +198</a><br/> + Ejected fragments at Ascension <a href="#page192">192</a><br/> +—— at the Galapagos archipelago <a href="#page239"> +239</a><br/> + Elevation of St. Helena <a href="#page225">225</a><br/> +—— the Galapagos archipelago <a href="#page241"> +241</a><br/> +—— Van Diemen’s Land, Cape of Good Hope, New Zealand, +Australia, and Chatham island <a href="#page258">258</a><br/> +—— of volcanic islands <a href="#page250">250</a><br/> + Ellis, Rev. W., on ledges within the great crater at Hawaii <a +href="#page220">220</a><br/> +—— on marine remains at Otaheite <a href="#page184"> +184</a><br/> + Eruption, fissures of <a href="#page224">224</a>, <a href= +"#page249">249</a>, <a href="#page250">250</a><br/> + Extinction of land-shells at St. Helena <a href="#page224"> +224</a><br/> +<br/> + Faraday, Mr., on the expulsion of carbonic acid gas <a href= +"#page171">171</a><br/> + Feldspar, fusibility of <a href="#page246">246</a><br/> +—— in radiating crystals <a href="#page263"> +263</a><br/> +—— Labrador, ejected <a href="#page193">193</a><br/> + Feldspathic lavas <a href="#page179">179</a><br/> +—— at St. Helena <a href="#page219">219</a><br/> +—— rock, alternating with obsidian <a href="#page202"> +202</a><br/> +—— lamination, and origin of <a href="#page209"> +209</a><br/> + Fernando Noronha <a href="#page181">181</a>, <a href="#page210"> +210</a><br/> + Ferruginous superficial beds <a href="#page259">259</a><br/> + Fibrous calcareous matter at St. Jago <a href="#page174"> +174</a><br/> + Fissures of eruption <a href="#page242">242</a>, <a href= +"#page249">249</a>, <a href="#page250">250</a><br/> + Fitton, Dr., on calcareous breccia <a href="#page262">262</a><br/> + Flagstaff Hill, St. Helena <a href="#page216">216</a><br/> + Fleurian de Bellevue on sphærulites <a href="#page207"> +207</a><br/> + Fluidity of lavas <a href="#page234">234</a>, <a href= +"#page235">235</a><br/> + Forbes, Professor, on the structure of glaciers <a href= +"#page212">212</a><br/> + Fragments ejected at Ascension <a href="#page192">192</a><br/> +—— at the Galapagos archipelago <a href="#page239"> +239</a><br/> + Freshwater Bay <a href="#page238">238</a>, <a href="#page243"> +243</a><br/> + Fuerteventura (Feurteventura), calcareous beds of <a href= +"#page224">224</a><br/> +<br/> + Galapagos archipelago <a href="#page229">229</a><br/> +—— parapets round craters <a href="#page220"> +220</a><br/> + Gay Lussac, on the expulsion of carbonic acid gas <a href= +"#page171">171</a><br/> + Glaciers, their structure <a href="#page212">212</a><br/> + Glossiness of texture, origin of <a href="#page206">206</a><br/> + Gneiss, derived from clay-slate <a href="#page264">264</a><br/> +—— with a great embedded fragment <a href="#page252"> +252</a><br/> + Gneiss-granite, form of hills of <a href="#page259">259</a><br/> + Good Hope, Cape of <a href="#page263">263</a><br/> + Gorges, narrow, at St. Helena <a href="#page225">225</a><br/> + Granite, junction with clay-slate, at the Cape of Good Hope <a +href="#page263">263</a><br/> + Granitic ejected fragments <a href="#page192">192</a>, <a href= +"#page239">239</a><br/> + Gravity, specific, of lavas <a href="#page243">243-8</a><br/> + Gypsum, at Ascension <a href="#page201">201</a><br/> +—— in volcanic strata at St. Helena <a href= +"#page215">215</a><br/> +—— on surface of the ground at ditto <a href= +"#page223">223</a><br/> +<br/> + Hall, Sir J., on the expulsion of carbonic acid gas <a href= +"#page171">171</a><br/> + Heat, action of, on calcareous matter <a href="#page170"> +170</a><br/> + Hennah, Mr., on ashes at Ascension <a href="#page189">189</a><br/> + Henslow, Prof., on chalcedony <a href="#page197">197</a><br/> + Hoffmann, on decomposed trachyte <a href="#page182">182</a><br/> + Holland, Dr., on Iceland <a href="#page228">228</a><br/> + Horner, Mr., on a calcareo-animal substance <a href="#page201"> +201</a><br/> +—— on fusibility of feldspar <a href="#page246"> +246</a><br/> + Hubbard, Dr., on dikes <a href="#page247">247</a><br/> + Humboldt on ejected fragments <a href="#page193">193</a><br/> +—— on obsidian formations <a href="#page207">207</a>, +<a href="#page209">209</a><br/> +—— on parapets round craters <a href="#page220"> +220</a><br/> +—— on sphærulites <a href="#page210">210</a><br/> + Hutton on amygdaloids <a href="#page176">176</a><br/> + Hyalite in decomposed trachyte <a href="#page182">182</a><br/> +<br/> + Iceland, stratification of the circumferential hills <a href= +"#page228">228</a><br/> + Islands, volcanic, distribution of <a href="#page248">248</a><br/> +—— their elevation <a href="#page250">250</a><br/> + Incrustation, on St. Paul’s rocks <a href="#page187">187</a><br/> + Incrustations, calcareous, at Ascension <a href="#page199"> +199</a><br/> +<br/> + Jago, St. <a href="#page167">167</a><br/> + James island <a href="#page234">234</a>, <a href="#page237"> +237</a>, <a href="#page242">242</a><br/> + Jasper, origin of <a href="#page196">196</a><br/> + Jonnès, M. Moreau de, on craters affected by wind <a href= +"#page189">189</a><br/> + Juan Fernandez <a href="#page250">250</a><br/> +<br/> + Keilhau, M., on granite <a href="#page264">264</a><br/> + Kicker Rock <a href="#page232">232</a><br/> + King George’s sound <a href="#page259">259</a><br/> +<br/> + Labrador feldspar, ejected <a href="#page193">193</a><br/> + Lakes at bases of volcanoes <a href="#page229">229</a><br/> + Lamination of volcanic rocks <a href="#page209">209</a><br/> + Land-shells, extinct, at St. Helena <a href="#page224"> +224</a><br/> + Lanzarote, calcareous beds of <a href="#page223">223</a><br/> + Lava, adhesion to sides of a gorge <a href="#page177">177</a><br/> +—— feldspathic <a href="#page179">179</a><br/> +—— with cells semi-amygdaloidal <a href="#page184"> +184</a><br/> + Lavas, specific gravity of <a href="#page243">243</a>, <a href= +"#page247">247</a><br/> + Lava-streams blending together at St. Jago <a href="#page177"> +177</a><br/> +—— composition of surface of <a href="#page208"> +208</a><br/> +—— differences in the state of their surfaces <a href= +"#page244">244</a><br/> +—— extreme thinness of <a href="#page238">238</a><br/> +—— heaved up into hillocks at the Galapagos archipelago +<a href="#page233">233</a><br/> +—— their fluidity <a href="#page234">234</a>, <a href= +"#page235">235</a><br/> +—— with irregular hummocks at Ascension <a href= +"#page189">189</a><br/> + Lead, separation from silver <a href="#page244">244</a><br/> + Lesson, M., on craters at Ascension <a href="#page189"> +189</a><br/> + Leucite <a href="#page234">234</a><br/> + Lime, sulphate of, at Ascension <a href="#page200">200</a><br/> + Lonsdale, Mr., on fossil-corals from Van Diemen’s land <a href= +"#page256">256</a><br/> + Lot, St. Helena <a href="#page221">221</a><br/> + Lyell, Mr., on craters of elevation <a href="#page227"> +227</a><br/> +—— on embedded turtles’ eggs <a href="#page198"> +198</a><br/> +—— on glossy coating to dikes <a href="#page216"> +216</a><br/> +<br/> + Macaulay, Dr., on calcareous casts at Madeira <a href="#page262"> +262</a><br/> + MacCulloch, Dr., on an amygdaloid <a href="#page184">184</a><br/> +—— on chlorophæite <a href="#page287"> +287</a><br/> +—— on laminated pitchstone <a href="#page209"> +209</a><br/> + Mackenzie, Sir G., on cavernous lava-streams <a href="#page233"> +233</a><br/> +—— on glossy coatings to dikes <a href="#page216"> +216</a><br/> +—— on obsidian streams <a href="#page208">208</a><br/> +—— on stratification in Iceland <a href="#page228"> +228</a><br/> + Madeira, calcareous casts at <a href="#page262">262</a><br/> + <i>Magazine, Nautical,</i>—account of volcanic phenomena in +the Atlantic <a href="#page226">226</a><br/> + Marekanite <a href="#page206">206</a><br/> + Mauritius, crater of elevation of <a href="#page184">184</a>, <a +href="#page227">227</a><br/> + Mica, in rounded nodules <a href="#page168">168</a><br/> +—— origin in metamorphic slate <a href="#page264"> +264</a><br/> +—— radiating form of <a href="#page263">263</a><br/> + Miller, Prof., on ejected Labrador feldspar <a href="#page193"> +193</a><br/> +—— on quartz crystals in obsidian beds <a href= +"#page202">202</a><br/> + Mitchell, Sir T., on bombs <a href="#page191">191</a><br/> +—— on the Australian valleys <a href="#page254"> +254</a><br/> + Mud streams at the Galapagos archipelago <a href="#page236"> +236</a><br/> +<br/> + Narborough island <a href="#page234">234</a><br/> + Nelson, Lieut., on the Bermuda islands <a href="#page260"> +260</a>, <a href="#page262">262</a><br/> + New Caledonia <a href="#page248">248</a><br/> + New Red sandstone, cross cleavage of <a href="#page253"> +253</a><br/> + New South Wales <a href="#page251">251</a><br/> + New Zealand <a href="#page259">259</a><br/> + Nulliporæ (fossil), resembling concretions <a href= +"#page169">169</a><br/> +<br/> + Obsidian, absent at the Galapagos archipelago <a href="#page241"> +241</a><br/> +—— bombs of <a href="#page191">191</a><br/> +—— composition and origin of <a href="#page207"> +207</a>, <a href="#page208">208</a><br/> +—— crystals of feldspar sink in <a href="#page243"> +243</a><br/> +—— its irruption from lofty craters <a href= +"#page246">246</a><br/> +—— passage of beds into <a href="#page202">202</a><br/> +—— specific gravity of <a href="#page243">243</a>, <a +href="#page246">246</a><br/> +—— streams of <a href="#page208">208</a><br/> + Olivine decomposed at St. Jago <a href="#page178">178</a><br/> +—— at Van Diemen’s land <a href="#page257">257</a><br/> +—— in the lavas at the Galapagos archipelago <a href= +"#page234">234</a><br/> + Oolitic structure of recent calcareous beds at St. Helena <a href= +"#page223">223</a><br/> + Otaheite <a href="#page183">183</a><br/> + Oysters, extinction of <a href="#page258">258</a><br/> +<br/> + Panza islands, laminated trachyte of <a href="#page209"> +209</a><br/> + Pattinson, Mr., on the separation of lead and silver <a href= +"#page244">244</a><br/> + Paul’s, St., rocks of <a href="#page187">187</a><br/> + Pearlstone <a href="#page206">206</a><br/> + Peperino <a href="#page232">232</a><br/> + Péron, M., on calcareous rocks of Australia <a href= +"#page262">262</a>, <a href="#page263">263</a><br/> + Phonolite, hills of <a href="#page179">179</a>, <a href= +"#page181">181</a>, <a href="#page221">221</a><br/> +—— laminated <a href="#page210">210</a><br/> +—— with more fusible hornblende <a href="#page246"> +246</a><br/> + Pitchstone <a href="#page204">204</a><br/> +—— dikes of <a href="#page209">209</a><br/> + Plants, extinct <a href="#page257">257</a><br/> + Plutonic rocks, separation of constituent parts of, by gravity <a +href="#page246">246</a><br/> + Porto Praya <a href="#page167">167</a><br/> + Prevost, M. C., on rarity of great dislocations in volcanic +islands <a href="#page217">217</a><br/> + Prosperous hill, St. Helena <a href="#page218">218</a><br/> + Pumice, absent at the Galapagos archipelago <a href="#page241"> +241</a><br/> +—— laminated <a href="#page209">209</a>, <a href= +"#page210">210</a>, <a href="#page211">211</a><br/> + Puy de Dome, trachyte of <a href="#page193">193</a><br/> +<br/> + Quail island, St. Jago <a href="#page168">168</a>, <a href= +"#page170">170</a>, <a href="#page173">173</a><br/> + Quartz, crystals of, in beds alternating with obsidian <a href= +"#page202">202</a><br/> +—— crystallised in sandstone <a href="#page252"> +252</a><br/> +—— fusibility of <a href="#page246">246</a><br/> +—— rock, mottled from metamorphic action with earthy +matter <a href="#page170">170</a><br/> +<br/> + Red hill <a href="#page173">173</a><br/> + Resin-like altered scoriæ <a href="#page171">171</a><br/> + Rio de Janeiro, gneiss of <a href="#page252">252</a><br/> + Robert, M., on strata of Iceland <a href="#page228">228</a><br/> + Rogers, Professor, on curved lines of elevation <a href= +"#page249">249</a><br/> +<br/> + Salses, compared with tuff craters <a href="#page240">240</a><br/> + Salt deposited by the sea <a href="#page200">200</a><br/> +—— in volcanic strata <a href="#page201">201</a>, <a +href="#page215">215</a><br/> +—— lakes of, in craters <a href="#page240">240</a><br/> + Sandstone of Brazil <a href="#page265">265</a><br/> +—— of the Cape of Good Hope <a href="#page265"> +265</a><br/> +—— platforms of, in New South Wales <a href= +"#page252">252</a>, <a href="#page265">265</a><br/> + Schorl, radiating <a href="#page263">263</a><br/> + Scrope, Mr. P., on laminated trachyte <a href="#page209">209</a>, +<a href="#page210">210</a>, <a href="#page212">212</a><br/> +—— on obsidian <a href="#page208">208</a><br/> +—— on separation of trachyte and basalt <a href= +"#page244">244</a><br/> +—— on silex in trachyte <a href="#page176">176</a><br/> +—— on sphærulites <a href="#page210">210</a><br/> + Seale, Mr., geognosy of St. Helena <a href="#page215">215</a><br/> +—— on dikes <a href="#page226">226</a><br/> +—— on embedded birds’ bones <a href="#page225"> +225</a><br/> + Seale, on extinct shells of St. Helena <a href="#page24"> +224</a><br/> + Sedgwick, Professor, on concretions <a href="#page206"> +206</a><br/> + Septaria, in concretions in tuff <a href="#page198">198</a><br/> + Serpulæ on upraised rocks <a href="#page185">185</a><br/> + Seychelles <a href="#page248">248</a><br/> + Shells, colour of, affected by light <a href="#page201"> +201</a><br/> +—— from Van Diemen’s land <a href="#page256"> +256</a><br/> +—— land, extinct, at St. Helena <a href="#page224"> +224</a><br/> +—— particles of, drifted by the wind at St. Helena <a +href="#page223">223</a><br/> + Shelly matter deposited by the waves <a href="#page200"> +200</a><br/> + Siau, M., on ripples <a href="#page254">254</a><br/> + Signal Post Hill <a href="#page168">168</a>, <a href="#page175"> +175</a>, <a href="#page176">176</a><br/> + Silica, deposited by steam <a href="#page182">182</a><br/> +—— large proportion of, in obsidian <a href= +"#page206">206</a>, <a href="#page208">208</a><br/> +—— specific gravity of <a href="#page246">246</a><br/> + Siliceous sinter <a href="#page196">196</a><br/> + Smith, Dr. A., on junction of granite and clay-slate <a href= +"#page264">264</a><br/> + Spallanzani on decomposed trachyte <a href="#page182">182</a><br/> + Specific gravity of recent calcareous rocks and of limestone <a +href="#page198">198</a><br/> +—— of lavas <a href="#page245">245</a><br/> + Sphærulites in glass and in silicified wood <a href= +"#page207">207</a><br/> +—— in obsidian <a href="#page204">204</a>, <a href= +"#page210">210</a><br/> + Sowerby, Mr. G. B., on fossil-shells from Van Diemen’s land <a +href="#page256">256</a><br/> +—— from St. Jago <a href="#page169">169</a><br/> +—— land-shells from St. Helena <a href="#page224"> +224</a><br/> + St. Helena <a href="#page214">214</a><br/> +—— crater of elevation of <a href="#page227"> +227</a><br/> + St. Jago, crater of elevation of <a href="#page227">227</a><br/> +—— effects of calcareous matter on lava <a href= +"#page231">231</a><br/> + St. Paul’s rocks <a href="#page187">187</a>, <a href="#page248"> +248</a><br/> + Stokes, Mr., collections of sphærulites and of obsidians <a +href="#page207">207</a>, <a href="#page212">212</a><br/> + Stony-top, Little <a href="#page218">218</a>, <a href= +"#page222">222</a><br/> +—— Great <a href="#page218">218</a><br/> + Stratification of sandstone in New South Wales <a href= +"#page253">253</a>, <a href="#page255">255</a><br/> + Streams of obsidian <a href="#page208">208</a><br/> + Stutchbury, Mr., on marine remains at Otaheite <a href= +"#page184">184</a><br/> + Subsided space at Ascension <a href="#page192">192</a><br/> +<br/> + Tahiti <a href="#page183">183</a><br/> + Talus, stratified, within tuff craters <a href="#page236"> +236</a><br/> + Terceira <a href="#page182">182</a><br/> + Tertiary deposit of St. Jago <a href="#page169">169</a><br/> + Trachyte, absent at the Galapagos archipelago <a href="#page241"> +241</a><br/> +—— at Ascension <a href="#page193">193</a><br/> +—— at Terceira <a href="#page182">182</a><br/> +—— decomposition of, by steam <a href="#page182"> +182</a><br/> +—— its lamination <a href="#page200">200</a>, <a href= +"#page210">210</a><br/> +—— its separation from basalt <a href="#page244"> +244</a><br/> +—— softened at Ascension <a href="#page194"> +194</a><br/> +—— specific gravity of <a href="#page245">245</a><br/> +—— with singular veins <a href="#page195">195</a><br/> + Trap-dikes in the plutonic series <a href="#page247">247</a><br/> +—— at King George’s sound <a href="#page259"> +259</a><br/> + Travertin at Van Diemen’s land <a href="#page257">257</a><br/> + Tropic-bird, now rare, at St. Helena <a href="#page225"> +225</a><br/> + Tuff, craters of <a href="#page231">231</a>, <a href="#page235"> +235</a>, <a href="#page236">236</a><br/> +—— their breached state <a href="#page240">240</a><br/> +—— peculiar kind of <a href="#page231">231</a><br/> + Turner, Mr., on the separation of molten metals <a href= +"#page244">244</a><br/> + Tyerman and Bennett on marine remains at Huaheine <a href= +"#page184">184</a><br/> +<br/> + Valleys, gorge-like, at St. Helena <a href="#page225">225</a><br/> +—— in New South Wales <a href="#page254">254</a><br/> +—— in St. Jago <a href="#page180">180</a><br/> + Van Diemen’s land <a href="#page256">256</a><br/> + Veins in trachyte <a href="#page195">195</a><br/> +—— of jasper <a href="#page195">195</a><br/> + Vincent, Bory St., on bombs <a href="#page190">190</a><br/> + Volcanic bombs <a href="#page189">189</a><br/> +—— island in process of formation in the Atlantic <a +href="#page226">226</a><br/> +—— islands, their distribution <a href="#page248"> +248</a><br/> +<br/> + Wacke, its passage into lava <a href="#page183">183</a>, <a href= +"#page257">257</a><br/> + Wackes, argillaceous <a href="#page168">168</a>, <a href= +"#page178">178</a><br/> + Webster, Dr., on a basin-formed island <a href="#page237"> +237</a><br/> +—— on gypsum at Ascension <a href="#page201"> +201</a><br/> + White, Martin, on soundings <a href="#page254">254</a><br/> + Wind, effects of, on the form of craters <a href="#page189"> +</a><br/> +</small></p> + +</div><!--end chapter--> + +<div class="chapter"> + +<h2><a name="index03">INDEX TO SOUTH AMERICAN GEOLOGY.</a></h2> + +<p class="noindent"> +<small>Abich, on a new variety of feldspar <a href="#page446"> +446</a><br/> + Abrolhos islands <a href="#page415">415</a><br/> + Absence of recent formations on the S. American coasts <a href= +"#page409">409</a><br/> + Aguerros on elevation of Imperial <a href="#page305">305</a><br/> + Albite, constituent mineral in andesite <a href="#page446"> +446</a><br/> +—— in rocks of Tierra del Fuego <a href="#page427"> +427</a><br/> +—— in porphyries <a href="#page444">444</a><br/> +—— crystals of, with orthite <a href="#page447"> +447</a><br/> + Alison, Mr., on elevation of Valparaiso <a href="#page307"> +307</a>, <a href="#page310">310</a><br/> + Alumina, sulphate of <a href="#page439">439</a><br/> +Ammonites from Concepcion <a href="#page400">400</a>, <a href= +"#page405">405</a><br/> + Amolanas, Las <a href="#page493">493</a><br/> + Amygdaloid, curious varieties of <a href="#page444">444</a><br/> +Amygdaloids of the Uspallata range <a href="#page471">471</a><br/> +—— of Copiapo <a href="#page498">498</a><br/> + Andesite of Chile <a href="#page446">446</a><br/> +—— in the valley of Maypu <a href="#page449">449</a>, +<a href="#page450">450</a><br/> +—— of the Cumbre pass <a href="#page460">460</a>, <a +href="#page466">466</a><br/> +—— of the Uspallata range <a href="#page475"> +475</a><br/> +—— of Los Hornos <a href="#page480">480</a><br/> +—— of Copiapo <a href="#page488">488</a>, <a href= +"#page491">491</a><br/> + Anhydrite, concretions of <a href="#page450">450</a>, <a href= +"#page463">463</a><br/> + Araucaria, silicified wood of <a href="#page394">394</a>, <a +href="#page474">474</a><br/> + Arica, elevation of <a href="#page323">323</a><br/> + Arqueros, mines of <a href="#page481">481</a><br/> + Ascension, gypsum deposited on <a href="#page328">328</a><br/> +—— laminated volcanic rocks of <a href="#page439"> +439</a>, <a href="#page440">440</a><br/> + Augite in fragments, in gneiss <a href="#page414">414</a><br/> +—— with albite, in lava <a href="#page347">347</a><br/> + Austin, Mr. R. A. C., on bent cleavage lamina <a href="#page434"> +434</a><br/> + Austin, Captain, on sea-bottom <a href="#page302">302</a><br/> + Australia, foliated rocks of <a href="#page438">438</a><br/> + <i>Azara labiata</i>, beds of, at San Pedro <a href="#page277"> +277</a>, <a href="#page352">352</a><br/> +<br/> + <i>Baculites vagina</i> <a href="#page400">400</a><br/> + Bahia Blanca, elevation of <a href="#page280">280</a><br/> +—— formations near <a href="#page355">355</a><br/> +—— character of living shells of <a href="#page408"> +408</a><br/> + Bahia (Brazil), elevation near <a href="#page280">280</a><br/> +—— crystalline rocks of <a href="#page414">414</a><br/> + Ballard, M., on the precipitation of sulphate of soda <a href= +"#page349">349</a><br/> + Banda Oriental, tertiary formations of <a href="#page365"> +365</a><br/> +—— crystalline rocks of <a href="#page418">418</a><br/> + Barnacles above sea-level <a href="#page311">311</a><br/> +—— adhering to upraised shells <a href="#page306"> +306</a><br/> + Basalt of S. Cruz <a href="#page389">389</a><br/> +—— streams of, in the Portillo range <a href= +"#page456">456</a><br/> +—— in the Uspallata range <a href="#page472"> +472</a><br/> + Basin chains of Chile <a href="#page333">333</a><br/> + Beagle Channel <a href="#page427">427</a>, <a href="#page430"> +430</a><br/> + Beaumont, Elie de, on inclination of lava-streams <a href= +"#page390">390</a>, <a href="#page457">457</a><br/> +—— on viscid quartz-rocks <a href="#page475"> +475</a><br/> + Beech-tree, leaves of fossil <a href="#page391">391</a><br/> + Beechey, Captain, on sea-bottom <a href="#page299">299</a><br/> + Belcher, Lieutenant, on elevated shells from Concepcion <a href= +"#page306">306</a><br/> + Bella Vista, plain of <a href="#page325">325</a><br/> + Benza, Dr., on decomposed granite <a href="#page417">417</a><br/> + Bettington, Mr., on quadrupeds transported by rivers <a href= +"#page374">374</a><br/> + Blake, Mr., on the decay of elevated shells near Iquique <a href= +"#page322">322</a><br/> +—— on nitrate of soda <a href="#page346">346</a><br/> + Bole <a href="#page444">444</a><br/> + Bollaert, Mr., on mines of Iquique <a href="#page503">503</a><br/> + Bones, silicified <a href="#page402">402</a><br/> +—— fossil, fresh condition of <a href="#page366"> +366</a><br/> + Bottom of sea off Patagonia <a href="#page292">292</a>, <a href= +"#page298">298</a><br/> + Bougainville, on elevation of the Falkland islands <a href= +"#page290">290</a><br/> + Boulder formation of S. Cruz <a href="#page285">285</a>, <a href= +"#page295">295</a><br/> +—— of Falkland islands <a href="#page290">290</a><br/> +—— anterior to certain extinct quadrupeds <a href= +"#page371">371</a><br/> +—— of Tierra del Fuego <a href="#page391">391</a><br/> + Boulders in the Cordillera <a href="#page339">339</a>, <a href= +"#page341">341</a><br/> +—— transported by earthquake-waves <a href="#page344"> +344</a><br/> +—— in fine-grained tertiary deposits <a href= +"#page401">401</a><br/> + Brande, Mr., on a mineral spring <a href="#page461">461</a><br/> + Bravais, M., on elevation of Scandinavia <a href="#page320"> +320</a><br/> + Brazil, elevation of <a href="#page279">279</a><br/> +—— crystalline rocks of <a href="#page414">414</a>, <a +href="#page418">418</a><br/> + Broderip, Mr., on elevated shells from Concepcion <a href= +"#page306">306</a><br/> + Brown, Mr. R., on silicified wood of Uspallata range <a href= +"#page474">474</a><br/> + Brown, on silicified wood <a href="#page495">495</a><br/> + Bucalema, elevated shells near <a href="#page307">307</a><br/> + Buch, Von, on cleavage <a href="#page438">438</a><br/> +—— on cretaceous fossils of the Cordillera <a href= +"#page453">453</a>, <a href="#page465">465</a><br/> +—— on the sulphureous volcanoes of Java <a href= +"#page509">509</a><br/> + Buenos Ayres <a href="#page352">352</a><br/> + Burchell, Mr., on elevated shells of Brazil <a href="#page279"> +279</a><br/> + Byron, on elevated shells <a href="#page303">303</a><br/> +<br/> + Cachapual, boulders in valley of <a href="#page339">339</a>, <a +href="#page341">341</a><br/> + Caldcleugh, Mr., on elevation of Coquimbo <a href="#page314"> +314</a><br/> +—— on rocks of the Portillo range <a href="#page456"> +456</a><br/> + Callao, elevation near <a href="#page323">323</a><br/> +—— old town of <a href="#page327">327</a><br/> + Cape of Good Hope, metamorphic rocks of <a href="#page439"> +439</a><br/> + <i>Carcharias megalodon</i> <a href="#page402">402</a><br/> + Carpenter, Dr., on microscopic organisms <a href="#page352"> +352</a><br/> + Castro (Chiloe), beds near <a href="#page394">394</a><br/> + Cauquenes Baths, boulders near <a href="#page339">339</a>, <a +href="#page341">341</a><br/> +—— pebbles in porphyry near <a href="#page443"> +443</a><br/> +—— volcanic formation near <a href="#page447"> +447</a><br/> +—— stratification near <a href="#page449">449</a><br/> + Caves above sea-level <a href="#page303">303</a>, <a href= +"#page307">307</a>, <a href="#page322">322</a><br/> + <i>Cervus pumilus,</i> fossil-horns of <a href="#page304"> +304</a><br/> + Chevalier, M., on elevation near Lima <a href="#page323"> +323</a><br/> + Chile, structure of country between the Cordillera and the Pacific +<a href="#page333">333</a><br/> +—— tertiary formations of <a href="#page337"> +337</a><br/> +—— crystalline rocks in <a href="#page435">435</a><br/> +—— central, geology of <a href="#page441">441</a><br/> +—— northern, geology of <a href="#page479">479</a><br/> + Chiloe, gravel on coast <a href="#page294">294</a><br/> +—— elevation of <a href="#page303">303</a><br/> +—— tertiary formation of <a href="#page337">337</a>, +<a href="#page405">405</a><br/> +—— crystalline rocks of <a href="#page433">433</a><br/> + Chlorite-schist, near M. Video <a href="#page419">419</a><br/> + Chonos archipelago, tertiary formations of <a href="#page393"> +393</a><br/> +—— crystalline rocks of <a href="#page430">430</a><br/> + Chupat, Rio, scoriæ transported by <a href="#page280"> +280</a><br/> + Claro, Rio, fossiliferous beds of <a href="#page485">485</a><br/> + Clay-shale of Los Hornos <a href="#page480">480</a><br/> + Clay-slate, formation of, Tierra del Fuego <a href="#page424"> +424</a><br/> +—— of Concepcion <a href="#page433">433</a><br/> +—— feldspathic, of Chile <a href="#page442">442</a>, +<a href="#page444">444</a>, <a href="#page448">448</a><br/> +—— —— of the Uspallata range <a href= +"#page468">468</a>, <a href="#page470">470</a><br/> +—— black siliceous, band of, in porphyritic formations +of Chile <a href="#page445">445</a><br/> + Claystone porphyry, formation of, in Chile <a href="#page442"> +442</a><br/> +—— origin of <a href="#page445">445</a><br/> +—— eruptive sources of <a href="#page444">444</a><br/> + Cleavage, definition of <a href="#page414">414</a><br/> +—— at Bahia <a href="#page415">415</a><br/> +—— Rio de Janeiro <a href="#page415">415</a><br/> +—— Maldonado <a href="#page418">418</a><br/> +—— Monte Video <a href="#page420">420</a><br/> +—— S. Guitru-gueyu <a href="#page421">421</a><br/> +—— Falkland I. <a href="#page424">424</a><br/> +—— Tierra del Fuego <a href="#page428">428</a><br/> +—— Chonos I. <a href="#page434">434</a><br/> +—— Chiloe <a href="#page435">435</a><br/> +—— Concepcion <a href="#page434">434</a><br/> +—— Chile <a href="#page435">435</a><br/> +—— discussion on <a href="#page436">436</a><br/> + Cleavage-laminæ superficially bent <a href="#page434"> +434</a><br/> + Cliffs, formation of <a href="#page301">301</a><br/> + Climate, late changes in <a href="#page345">345</a><br/> +—— of Chile during tertiary period <a href="#page408"> +408</a><br/> + Coal of Concepcion <a href="#page399">399</a><br/> +—— S. Lorenzo <a href="#page504">504</a><br/> + Coast-denudation of St. Helena <a href="#page301">301</a><br/> + Cobija, elevation of <a href="#page322">322</a><br/> + Colombia, cretaceous formation of <a href="#page504">504</a><br/> + Colonia del Sacramiento, elevation of <a href="#page278"> +278</a><br/> +—— Pampean formation near <a href="#page355"> +355</a><br/> + Colorado, Rio, gravel of <a href="#page295">295</a><br/> +—— sand-dunes of <a href="#page281">281</a>, <a href= +"#page294">294</a><br/> +—— Pampean formation near <a href="#page355"> +355</a><br/> + Combarbala <a href="#page479">479</a>, <a href="#page481"> +481</a><br/> + Concepcion, elevation of <a href="#page305">305</a><br/> +—— deposits of <a href="#page399">399</a>, <a href= +"#page405">405</a><br/> +—— crystalline rocks of <a href="#page433">433</a><br/> + Conchalee, gravel-terraces of <a href="#page311">311</a><br/> + Concretions of gypsum, at Iquique <a href="#page345">345</a><br/> +—— in sandstone at S. Cruz <a href="#page387"> +387</a><br/> +—— in tufaceous tuff of Chiloe <a href="#page387"> +387</a><br/> +—— in gneiss <a href="#page414">414</a><br/> +—— in claystone-porphyry at Port Desire <a href= +"#page421">421</a><br/> +—— in gneiss at Valparaiso <a href="#page435"> +435</a><br/> +—— in metamorphic rocks <a href="#page436">436</a><br/> +—— of anhydrite <a href="#page450">450</a><br/> +—— relations of, to veins <a href="#page473"> +473</a><br/> + Conglomerate claystone of Chile <a href="#page443">443</a>, <a +href="#page445">445</a><br/> +—— of Tenuyan <a href="#page454">454</a>, <a href= +"#page458">458</a>, <a href="#page478">478</a><br/> +—— of the Cumbre Pass <a href="#page462">462</a>, <a +href="#page466">466</a><br/> +—— of Rio Claro <a href="#page485">485</a><br/> +—— of Copiapo <a href="#page496">496</a>, <a href= +"#page499">499</a><br/> + Cook, Captain, on form of sea-bottom <a href="#page300"> +300</a><br/> + Copiapo, elevation of <a href="#page321">321</a><br/> +—— tertiary formations of <a href="#page403"> +403</a><br/> +—— secondary formations of <a href="#page489"> +489</a><br/> + Copper, sulphate of <a href="#page489">489</a><br/> +—— native, at Arqueros <a href="#page482">482</a><br/> +—— mines of, at Panuncillo <a href="#page481"> +481</a><br/> +—— veins, distribution of <a href="#page505"> +505</a><br/> + Coquimbo, elevation and terraces of <a href="#page312"> +312</a><br/> +—— tertiary formations of <a href="#page404"> +404</a><br/> +—— secondary formations of <a href="#page482"> +482</a><br/> + Corallines living on pebbles <a href="#page299">299</a><br/> + Cordillera, valleys bordered by gravel fringes <a href= +"#page337">337</a><br/> +—— basal strata of <a href="#page442">442</a><br/> +—— fossils of <a href="#page453">453</a>, <a href= +"#page465">465</a>, <a href="#page486">486</a>, <a href= +"#page487">487</a>, <a href="#page493">493</a>, <a href= +"#page503">503</a><br/> +—— elevation of <a href="#page442">442</a>, <a href= +"#page459">459</a>, <a href="#page474">474</a>, <a href= +"#page476">476</a>, <a href="#page500">500</a>, <a href= +"#page502">502</a>, <a href="#page510">510</a>, <a href= +"#page512">512</a>, <a href="#page517">517</a><br/> +—— gypseous formations of <a href="#page450">450</a>, +<a href="#page452">452</a>, <a href="#page461">461</a>, <a href= +"#page463">463</a>, <a href="#page479">479</a>, <a href= +"#page483">483</a>, <a href="#page489">489</a>, <a href= +"#page491">491</a>, <a href="#page503">503</a><br/> +—— claystone-porphyries of <a href="#page442"> +442</a><br/> +—— andesitic rocks of <a href="#page446">446</a><br/> +—— volcanoes of <a href="#page447">447</a>, <a href= +"#page511">511</a>, <a href="#page517">517</a><br/> + Coste, M., on elevation of Lemus <a href="#page303">303</a><br/> + Coy inlet, tertiary formation of <a href="#page390">390</a><br/> + <i>Crassatella Lyellii</i> <a href="#page392">392</a><br/> + Cruickshanks, Mr., on elevation near Lima <a href="#page327"> +327</a><br/> + Crystals of feldspar, gradual formation of, at Port Desire <a +href="#page422">422</a><br/> + Cumbre, Pass of, in Cordillera <a href="#page502">502</a><br/> + Cuming, Mr., on habits of the Mesodesma <a href="#page310"> +310</a><br/> +—— on range of living shells on west coast <a href= +"#page407">407</a><br/> +<br/> + Dana, Mr., on foliated rocks <a href="#page438">438</a><br/> +—— on amygdaloids <a href="#page444">444</a><br/> + Darwin, Mount <a href="#page427">427</a><br/> + D’Aubuisson, on concretions <a href="#page397">397</a><br/> +—— on foliated rocks <a href="#page438">438</a><br/> + Decay, gradual, of upraised shells <a href="#page323">323</a>, <a +href="#page327">327</a><br/> + Decomposition of granite rocks <a href="#page417">417</a><br/> + De la Beche, Sir H., his theoretical researches in geology <a +href="#page299">299</a><br/> +—— on the action of salt on calcareous rocks <a href= +"#page327">327</a><br/> +—— on bent cleavage-laminæ <a href="#page434"> +434</a><br/> + Denudation on coast of Patagonia <a href="#page292">292</a>, <a +href="#page300">300</a>, <a href="#page409">409</a><br/> +—— great powers of <a href="#page410">410</a><br/> +—— of the Portillo range <a href="#page456">456</a>, +<a href="#page458">458</a><br/> + Deposits, saline <a href="#page344">344</a><br/> + Despoblado, valley of <a href="#page496">496</a>, <a href= +"#page497">497</a>, <a href="#page499">499</a><br/> + Detritus, nature of, in Cordillera <a href="#page338">338</a><br/> + Devonshire, bent cleavage in <a href="#page434">434</a><br/> + Dikes, in gneiss of Brazil <a href="#page414">414</a>, <a href= +"#page418">418</a><br/> +—— near Rio de Janeiro <a href="#page417">417</a><br/> +—— pseudo, at Port Desire <a href="#page423"> +423</a><br/> +—— in Tierra del Fuego <a href="#page426">426</a><br/> +—— in Chonos archipelago, containing quartz <a href= +"#page432">432</a><br/> +—— near Concepcion, with quartz <a href="#page434"> +434</a><br/> +—— granitic-porphyritic, at Valparaiso <a href= +"#page435">435</a><br/> +—— rarely vesicular in Cordillera <a href="#page347"> +347</a><br/> +—— absent in the central ridges of the Portillo pass <a +href="#page452">452</a><br/> +—— of the Portillo range, with grains of quartz <a +href="#page456">456</a><br/> +—— intersecting each other often <a href="#page466"> +466</a><br/> +—— numerous at Copiapo <a href="#page498">498</a><br/> + Domeyko, M., on the silver mines of Coquimbo <a href="#page482"> +482</a><br/> +—— on the fossils of Coquimbo <a href="#page486"> +486</a><br/> + D’Orbigny, M. A., on upraised shells of Monte Video <a href= +"#page278">278</a><br/> +—— on elevated shells at St. Pedro <a href="#page278"> +278</a><br/> +—— on elevated shells near B. Ayres <a href= +"#page279">279</a><br/> +—— on elevation of S. Blas <a href="#page281"> +281</a><br/> +—— on the sudden elevation of La Plata <a href= +"#page293">293</a><br/> +—— on elevated shells near Cobija <a href="#page322"> +322</a><br/> +—— on elevated shells near Arica <a href="#page322"> +322</a><br/> +—— on the climate of Peru <a href="#page324"> +324</a><br/> +—— on salt deposits of Cobija <a href="#page345"> +345</a><br/> +—— on crystals of gypsum in salt-lakes <a href= +"#page349">349</a><br/> +—— on absence of gypsum in the Pampean formation <a +href="#page353">353</a><br/> +—— on fossil remains from Bahia Blanca <a href= +"#page359">359</a>, <a href="#page360">360</a><br/> +—— on fossil remains from the banks of the Parana <a +href="#page362">362</a><br/> +—— on the geology of St. Fé <a href="#page363"> +363</a><br/> +—— on the age of Pampean formation <a href="#page367"> +367</a>, <a href="#page376">376</a><br/> +—— on the <i>Mastodon Andium</i> <a href="#page379"> +379</a><br/> +—— on the geology of the Rio Negro <a href="#page381"> +381</a><br/> +—— on the character of the Patagonian fossils <a href= +"#page391">391</a><br/> +—— on fossils from Concepcion <a href="#page399"> +399</a><br/> +—— —— from Coquimbo <a href="#page404"> +404</a><br/> +—— —— from Payta <a href="#page405"> +405</a><br/> +—— on fossil tertiary shells of Chile <a href= +"#page406">406</a><br/> +—— on cretaceous fossils of Tierra del Fuego <a href= +"#page426">426</a><br/> +—— —— from the Cordillera of Chile <a href= +"#page453">453</a>, <a href="#page465">465</a>, <a href= +"#page486">486</a>, <a href="#page488">488</a>, <a href= +"#page493">493</a>, <a href="#page504">504</a><br/> +<br/> + Earth, marine origin of <a href="#page304">304</a>, <a href= +"#page308">308</a><br/> + Earthenware, fossil <a href="#page326">326</a><br/> + Earthquake, effect of, at S. Maria <a href="#page293">293</a><br/> +—— elevation during, at Lemus <a href="#page303"> +303</a><br/> +—— of 1822, at Valparaiso <a href="#page310"> +310</a><br/> +—— effects of, in shattering surface <a href= +"#page325">325</a><br/> +—— fissures made by <a href="#page325">325</a><br/> +—— probable effects on cleavage <a href="#page325"> +325</a><br/> + Earthquakes in Pampas <a href="#page290">290</a><br/> + Earthquake-waves, power of, in throwing up shells <a href= +"#page310">310</a><br/> +—— effects of, near Lima <a href="#page327"> +327</a><br/> +—— power of, in transporting boulders <a href= +"#page344">344</a><br/> + Edmonston, Mr., on depths at which shells live at Valparaiso <a +href="#page309">309</a><br/> + Ehrenberg, Professor, on infusoria in the Pampean formation <a +href="#page355">355</a>, <a href="#page359">359</a>, <a href= +"#page362">362</a><br/> +—— on infusoria in the Patagonian formation <a href= +"#page383">383</a>, <a href="#page384">384</a>, <a href= +"#page386">386</a>, <a href="#page391">391</a>, <a href= +"#page392">392</a><br/> + Elevation of La Plata <a href="#page278">278</a><br/> +—— Brazil <a href="#page279">279</a><br/> +—— Bahia Blanca <a href="#page280">280</a>, <a href= +"#page357">357</a><br/> +—— San Blas <a href="#page281">281</a><br/> +—— Patagonia <a href="#page281">281</a>, <a href= +"#page291">291</a>, <a href="#page293">293</a><br/> +—— Tierra del Fuego <a href="#page288">288</a><br/> +—— Falkland islands <a href="#page290">290</a><br/> +—— Pampas <a href="#page289">289</a>, <a href= +"#page377">377</a><br/> +—— Chonos archipelago <a href="#page303">303</a><br/> +—— Chiloe <a href="#page304">304</a><br/> +—— Chile <a href="#page304">304</a><br/> +—— Valparaiso <a href="#page307">307</a>, <a href= +"#page310">310</a><br/> +—— Coquimbo <a href="#page312">312</a>, <a href= +"#page320">320</a><br/> +—— Guasco <a href="#page320">320</a><br/> +—— Iquique <a href="#page322">322</a><br/> +—— Cobija <a href="#page322">322</a><br/> +—— Lima <a href="#page323">323</a><br/> +—— sudden, at S. Maria <a href="#page293">293</a><br/> +—— —— at Lemus <a href="#page303"> +303</a><br/> +—— insensible, at Chiloe <a href="#page304"> +304</a><br/> +—— —— at Valparaiso <a href="#page311"> +311</a><br/> +—— —— at Coquimbo <a href="#page314"> +314</a><br/> +—— axes of, at Chiloe <a href="#page398">398</a>, <a +href="#page405">405</a><br/> +—— —— at P. Rumena <a href="#page398"> +398</a>, <a href="#page405">405</a><br/> +—— —— at Concepcion <a href="#page398"> +398</a>, <a href="#page405">405</a><br/> +—— unfavourable for the accumulation of permanent +deposits <a href="#page410">410</a><br/> +—— lines of, parallel to cleavage and foliation <a +href="#page416">416</a>, <a href="#page417">417</a>, <a href= +"#page424">424</a>, <a href="#page428">428</a>, <a href= +"#page432">432</a>, <a href="#page434">434</a>, <a href= +"#page438">438</a><br/> +—— lines of, oblique to foliation <a href="#page431"> +431</a><br/> +—— areas of, causing lines of elevation and cleavage <a +href="#page441">441</a><br/> +—— lines of, in the Cordillera <a href="#page442"> +442</a><br/> +—— slow, in the Portillo range <a href="#page475"> +475</a><br/> +—— two periods of, in Cordillera of Central Chile <a +href="#page476">476</a><br/> +—— of the Uspallata range <a href="#page474"> +474</a><br/> +—— two periods of, in Cumbre Pass <a href="#page476"> +476</a><br/> +—— horizontal, in the Cordillera of Copiapo <a href= +"#page500">500</a><br/> +—— axes of, coincident with volcanic orifices <a href= +"#page503">503</a><br/> +—— of the Cordillera, summary on <a href="#page510"> +510</a>, <a href="#page513">513</a>, <a href="#page517"> +517</a><br/> + Elliott, Captain, on human remains <a href="#page279">279</a><br/> + Ensenada, elevated shells of <a href="#page278">278</a><br/> + Entre Rios, geology of <a href="#page363">363</a><br/> + <i>Equus curvidens</i> <a href="#page364">364</a>, <a href= +"#page379">379</a><br/> + Epidote in Tierra del Fuego <a href="#page426">426</a><br/> +—— in gneiss <a href="#page435">435</a><br/> +—— frequent in Chile <a href="#page445">445</a><br/> +—— in the Uspallata range <a href="#page475"> +475</a><br/> +—— in porphyry of Coquimbo <a href="#page482"> +482</a><br/> + Erman, M., on andesite <a href="#page347">347</a><br/> + Escarpments, recent, of Patagonia <a href="#page301">301</a><br/> + Extinction of fossil mammifers <a href="#page370">370</a><br/> +<br/> + Falkland islands, elevation of <a href="#page290">290</a><br/> +—— pebbles on coast <a href="#page297">297</a>, <a +href="#page299">299</a><br/> +—— geology of <a href="#page424">424</a><br/> + Falkner, on saline incrustations <a href="#page347">347</a><br/> + Faults, great, in Cordillera <a href="#page461">461</a>, <a href= +"#page469">469</a><br/> + Feldspar, earthy, metamorphosis of, at Port Desire <a href= +"#page422">422</a><br/> +—— albitic <a href="#page347">347</a><br/> +—— crystals of, with albite <a href="#page347"> +347</a><br/> +—— orthitic, in conglomerate of Tenuyan <a href= +"#page454">454</a><br/> +—— in granite of Portillo range <a href="#page455"> +455</a><br/> +—— in porphyries in the Cumbre Pass <a href= +"#page466">466</a><br/> + Feuillée on sea-level at Coquimbo <a href="#page314"> +314</a><br/> + Fissures, relations of, to concretions <a href="#page397"> +397</a><br/> +—— upfilled, at Port Desire <a href="#page424"> +424</a><br/> +—— in clay-slate <a href="#page470">470</a><br/> + Fitton, Dr., on the geology of Tierra del Fuego <a href= +"#page427">427</a><br/> + Fitzroy, Captain, on the elevation of the Falkland islands <a +href="#page427">427</a><br/> +—— on the elevation of Concepcion <a href="#page305"> +305</a><br/> + Foliation, definition of <a href="#page414">414</a><br/> +—— of rocks at Bahia <a href="#page414">414</a><br/> +—— Rio de Janeiro <a href="#page415">415</a><br/> +—— Maldonado <a href="#page418">418</a><br/> +—— Monte Video <a href="#page420">420</a><br/> +—— S. Guitru-gueyu <a href="#page421">421</a><br/> +—— Falkland I. <a href="#page424">424</a><br/> +—— Tierra del Fuego <a href="#page427">427</a><br/> +—— Chonos archipelago <a href="#page430">430</a><br/> +—— Chiloe <a href="#page433">433</a><br/> +—— Concepcion <a href="#page434">434</a><br/> +—— Chile <a href="#page435">435</a><br/> +—— discussion on <a href="#page435">435</a><br/> + Forbes, Professor E., on cretaceous fossils of Concepcion <a href= +"#page400">400</a><br/> +—— on cretaceous fossils and subsidence in Cumbre Pass +<a href="#page465">465</a><br/> +—— on fossils from Guasco <a href="#page488"> +488</a><br/> +—— —— from Coquimbo <a href="#page483"> +483</a>, <a href="#page487">487</a><br/> +—— —— from Copiapo <a href="#page493"> +493</a><br/> +—— on depths at which shells live <a href="#page409"> +409</a>, <a href="#page496">496</a><br/> + Formation, Pampean <a href="#page352">352</a><br/> +—— —— area of <a href="#page371"> +371</a><br/> +—— —— estuary origin <a href="#page373"> +373</a><br/> +—— tertiary of Entre Rios <a href="#page363"> +363</a><br/> +—— of Banda Oriental <a href="#page365">365</a><br/> +—— volcanic, in Banda Oriental <a href="#page367"> +367</a><br/> +—— of Patagonia <a href="#page381">381</a><br/> +—— summary on <a href="#page391">391</a><br/> +—— tertiary of Tierra del Fuego <a href="#page391"> +391</a><br/> +—— —— of the Chonos archipelago <a href= +"#page393">393</a><br/> +—— —— of Chiloe <a href="#page394"> +394</a><br/> +—— —— of Chile <a href="#page394"> +394</a><br/> +—— —— of Concepcion <a href="#page398"> +398</a>, <a href="#page404">404</a><br/> +—— —— of Navidad <a href="#page400"> +400</a><br/> +—— —— of Coquimbo <a href="#page402"> +402</a><br/> +—— —— of Peru <a href="#page404"> +404</a><br/> +—— —— subsidence during <a href= +"#page402">402</a><br/> +—— volcanic, of Tres Montes <a href="#page393"> +393</a><br/> +—— —— of Chiloe <a href="#page394"> +394</a><br/> +—— —— old, near Maldonado <a href= +"#page418">418</a><br/> +—— —— with laminar structure <a href= +"#page440">440</a><br/> +—— —— ancient, in Tierra del Fuego <a href= +"#page426">426</a><br/> +—— recent, absent on S. American coast <a href= +"#page409">409</a><br/> +—— metamorphic, of claystone-porphyry of Patagonia <a +href="#page421">421</a>, <a href="#page440">440</a><br/> +—— foliation of <a href="#page436">436</a><br/> +—— plutonic, with laminar structure <a href= +"#page440">440</a><br/> +—— palaeozoic, of the Falkland I. <a href="#page424"> +424</a><br/> +—— claystone, at Concepcion <a href="#page433"> +433</a><br/> +—— Jurassic, of Cordillera <a href="#page512"> +512</a><br/> +—— Neocomian, of the Portillo Pass <a href="#page453"> +453</a><br/> +—— volcanic, of Cumbre Pass <a href="#page465"> +465</a><br/> +—— gypseous, of Los Hornos <a href="#page479">479</a>, +<a href="#page487">487</a><br/> +—— —— of Coquimbo <a href="#page482"> +482</a><br/> +—— —— of Guasco <a href="#page487"> +487</a><br/> +—— —— of Copiapo <a href="#page488"> +488</a><br/> +—— —— of Iquique <a href="#page503"> +503</a><br/> +—— cretaceo-oolitic, of Coquimbo <a href="#page486"> +486</a>, <a href="#page495">495</a><br/> +—— —— of Guasco <a href="#page487"> +487</a>, <a href="#page494">494</a><br/> +—— —— of Copiapo <a href="#page495"> +495</a><br/> +—— —— of Iquique <a href="#page504"> +504</a><br/> + Fossils, Neocomian, of Portillo Pass <a href="#page453"> +453</a><br/> +—— —— of Cumbre Pass <a href="#page465"> +465</a><br/> +—— secondary, of Coquimbo <a href="#page485"> +485</a><br/> +—— —— of Guasco <a href="#page487"> +487</a><br/> +—— —— of Copiapo <a href="#page494"> +494</a><br/> +—— —— of Iquique <a href="#page503"> +503</a><br/> +—— palæozoic, from the Falklands <a href= +"#page424">424</a><br/> + Fragments of hornblende-rock in gneiss <a href="#page414"> +414</a><br/> +—— of gneiss in gneiss <a href="#page416">416</a><br/> + Freyer, Lieutenant, on elevated shells of Arica <a href= +"#page323">323</a><br/> + Frezier on sea-level at Coquimbo <a href="#page314">314</a><br/> +<br/> + Galapagos archipelago, pseudo-dikes of <a href="#page424"> +424</a><br/> + Gallegos, Port, tertiary formation of <a href="#page390"> +390</a><br/> + Garnets in gneiss <a href="#page415">415</a><br/> +—— in mica-slate <a href="#page427">427</a><br/> +—— at Panuncillo <a href="#page481">481</a><br/> + Gardichaud, M., on granites of Brazil <a href="#page417"> +417</a><br/> + Gay, M., on elevated shells <a href="#page306">306</a><br/> +—— on boulders in the Cordillera <a href="#page339"> +339</a>, <a href="#page341">341</a><br/> +—— on fossils from Cordillera of Coquimbo <a href= +"#page487">487</a><br/> + Gill, Mr., on brickwork transported by an earthquake-wave <a href= +"#page327">327</a><br/> + Gillies, Dr., on heights in the Cordillera <a href="#page448"> +448</a><br/> +—— on extension of the Portillo range <a href= +"#page458">458</a><br/> + Glen Roy, parallel roads of <a href="#page319">319</a><br/> +—— sloping terraces of <a href="#page340">340</a><br/> + Gneiss, near Bahia <a href="#page414">414</a><br/> +—— of Rio de Janeiro <a href="#page415">415</a><br/> +—— decomposition of <a href="#page417">417</a><br/> + Gold, distribution of <a href="#page506">506</a><br/> + Gorodona, formations near <a href="#page362">362</a><br/> + Granite, axis of oblique, to foliation <a href="#page431"> +431</a><br/> +—— andesitic <a href="#page446">446</a><br/> +—— of Portillo range <a href="#page455">455</a><br/> +—— veins of, quartzose <a href="#page432">432</a>, <a +href="#page475">475</a><br/> +—— pebble of, in porphyritic conglomerate <a href= +"#page493">493</a><br/> +—— conglomerate <a href="#page497">497</a><br/> + Grauwacke of Uspallata range <a href="#page468">468</a><br/> + Gravel at bottom of sea <a href="#page293">293</a>, <a href= +"#page298">298</a><br/> +—— formation of, in Patagonia <a href="#page295"> +295</a><br/> +—— means of transportation of <a href="#page298"> +298</a><br/> +—— strata of, inclined <a href="#page467">467</a><br/> + Gravel-terraces in Cordillera <a href="#page337">337</a><br/> + Greenough, Mr., on quartz veins <a href="#page437">437</a><br/> + Greenstone, resulting from metamorphose hornblende-rock <a href= +"#page419">419</a><br/> +—— of Tierra del Fuego <a href="#page426">426</a><br/> +—— on the summit of the Campana of Quillota <a href= +"#page442">442</a><br/> +—— porphyry <a href="#page443">443</a><br/> +—— relation of, to clay-slate <a href="#page443"> +443</a><br/> + <i>Gryphæa orientalis</i> <a href="#page483">483</a><br/> + Guasco, elevation of <a href="#page321">321</a><br/> +—— secondary formation of <a href="#page487"> +487</a><br/> + Guitru-gueyu, Sierra <a href="#page421">421</a><br/> + Guyana, gneissic rocks of <a href="#page415">415</a><br/> + Gypsum, nodules of, in gravel at Rio Negro <a href="#page296"> +296</a><br/> +—— deposited from sea-water <a href="#page327"> +327</a><br/> +—— deposits of, at Iquique <a href="#page345"> +345</a><br/> +—— crystals of, in salt lakes <a href="#page346"> +346</a><br/> +—— in Pampean formation <a href="#page353">353</a><br/> +—— in tertiary formation of Patagonia <a href= +"#page382">382</a>, <a href="#page383"></a>, <a href="#page385"> +</a>, <a href="#page386"></a><br/> +—— great formation of, in the Portillo Pass <a href= +"#page461">461</a>, <a href="#page463">463</a><br/> +—— —— in the Cumbre Pass <a href= +"#page461">461</a>, <a href="#page463">463</a><br/> +—— —— near Los Hornos <a href="#page479"> +479</a><br/> +—— —— at Coquimbo <a href="#page482"> +482</a><br/> +—— —— at Copiapo <a href="#page490"> +490</a>, <a href="#page492">492</a><br/> +—— —— near Iquique <a href="#page504"> +504</a><br/> +—— of San Lorenzo <a href="#page504">504</a><br/> +<br/> + Hall, Captain, on terraces at Coquimbo <a href="#page316"> +316</a><br/> + Hamilton, Mr., on elevation near Tacna <a href="#page323"> +323</a><br/> + Harlan, Dr., on human remains <a href="#page279">279</a><br/> + Hayes, Mr. A., on nitrate of soda <a href="#page346">346</a><br/> + Henslow, Professor, on concretions <a href="#page437">437</a><br/> + Herbert, Captain, on valleys in the Himalaya <a href="#page335"> +335</a><br/> + Herradura Bay, elevated shells of <a href="#page315">315</a><br/> +—— tertiary formations of <a href="#page402"> +402</a><br/> + Himalaya, valleys in <a href="#page335">335</a><br/> + <i>Hippurites Chilensis</i> <a href="#page483">483</a>, <a href= +"#page486">486</a><br/> + Hitchcock, Professor, on dikes <a href="#page414">414</a><br/> + Honestones, pseudo, of Coquimbo <a href="#page483">483</a><br/> +—— of Copiapo <a href="#page489">489</a><br/> + Hooker, Dr. J. D., on fossil beech-leaves <a href="#page391"> +391</a><br/> + Hopkins, Mr., on axes of elevation oblique to foliation <a href= +"#page432">432</a><br/> +—— on origin of lines of elevation <a href="#page440"> +440</a>, <a href="#page512">512</a><br/> + Hornblende-rock, fragments of, in gneiss <a href="#page414"> +414</a><br/> + Hornblende-schist, near M. Video <a href="#page420">420</a><br/> + Hornos, Los, section near <a href="#page479">479</a><br/> + Hornstone, dike of <a href="#page43">433</a>, <a href= +"#page434">434</a><br/> + Horse, fossil tooth of <a href="#page358">358</a>, <a href= +"#page364">364</a><br/> + Huafo island <a href="#page393">393</a>, <a href="#page404"> +404</a><br/> +—— subsidence at <a href="#page411">411</a><br/> + Huantajaya, mines of <a href="#page503">503</a><br/> + Humboldt, on saline incrustations <a href="#page347">347</a><br/> +—— on foliations of gneiss <a href="#page415"> +415</a><br/> +—— on concretions in gneiss <a href="#page435"> +435</a><br/> +<br/> + Icebergs, action on cleavage <a href="#page434">434</a>, <a href= +"#page436">436</a><br/> + Illapele, section near <a href="#page479">479</a><br/> + Imperial, beds of shells near <a href="#page305">305</a><br/> + Incrustations, saline <a href="#page347">347</a><br/> + Infusoria in Pampean formation <a href="#page352">352</a>, <a +href="#page355">355</a>, <a href="#page360">360</a>, <a href= +"#page363">363</a><br/> +—— in Patagonian formation <a href="#page382">382</a>, +<a href="#page383">383</a>, <a href="#page384">384</a>, <a href= +"#page391">391</a><br/> + Iodine, salts of <a href="#page347">347</a>, <a href="#page348"> +348</a><br/> + Iquique, elevation of <a href="#page322">322</a><br/> +—— saliferous deposits of <a href="#page344"> +344</a><br/> +—— cretaceo-oolitic formation of <a href="#page503"> +503</a><br/> + Iron, oxide of, in lavas <a href="#page463">463</a>, <a href= +"#page499">499</a><br/> +—— in sedimentary beds <a href="#page480">480</a>, <a +href="#page482">482</a><br/> +—— tendency in, to produce hollow concretions <a href= +"#page398">398</a><br/> +—— sulphate of <a href="#page489">489</a><br/> + Isabelle, M., on volcanic rocks of Banda Oriental <a href= +"#page368">368</a><br/> +<br/> + Joints in clay-slate <a href="#page428">428</a><br/> + Jukes, Mr., on cleavage in Newfoundland <a href="#page437"> +437</a><br/> +<br/> + Kamtschatka, andesite of <a href="#page347">347</a><br/> + Kane, Dr., on the production of carbonate of soda <a href= +"#page328">328</a><br/> + King George’s sound, calcareous beds of <a href="#page312"> +312</a><br/> +<br/> + Lakes, origin of <a href="#page300">300</a><br/> +—— fresh-water, near salt lakes <a href="#page350"> +350</a><br/> + Lava, basaltic, of S. Cruz <a href="#page389">389</a><br/> +—— claystone-porphyry, at Chiloe <a href="#page395"> +395</a><br/> +—— —— ancient submarine <a href= +"#page446">446</a><br/> +—— basaltic, of the Portillo range <a href="#page457"> +457</a><br/> +—— feldspathic, of the Cumbre Pass <a href="#page463"> +463</a><br/> +—— submarine, of the Uspallata range <a href= +"#page471">471</a>, <a href="#page473">473</a>, <a href= +"#page476">476</a><br/> +—— basaltic, of the Uspallata range <a href= +"#page475">475</a><br/> +—— submarine, of Coquimbo <a href="#page484">484</a>, +<a href="#page486">486</a><br/> +—— of Copiapo <a href="#page490">490</a>, <a href= +"#page496">496</a>, <a href="#page499">499</a><br/> + Lemus island <a href="#page393">393</a>, <a href="#page404"> +404</a><br/> + Lemuy islet <a href="#page394">394</a><br/> + Lignite of Chiloe <a href="#page395">395</a><br/> +—— of Concepcion <a href="#page398">398</a><br/> + Lima, elevation of <a href="#page323">323</a><br/> + Lime, muriate of <a href="#page328">328</a>, <a href="#page344"> +344</a>, <a href="#page347">347</a><br/> + Limestone of Cumbre Pass <a href="#page462">462</a><br/> +—— of Coquimbo <a href="#page483">483</a>, <a href= +"#page485">485</a><br/> +—— of Copiapo <a href="#page493">493</a><br/> + Lund and Clausen on remains of caves in Brazil <a href= +"#page378">378</a>, <a href="#page380">380</a><br/> + Lund, M., on granites of Brazil <a href="#page417">417</a><br/> + Lyell, M., on upraised shells retaining their colours <a href= +"#page289">289</a><br/> +—— on terraces at Coquimbo <a href="#page315"> +315</a><br/> +—— on elevation near Lima <a href="#page327"> +327</a><br/> +—— on fossil horse’s tooth <a href="#page364"> +364</a><br/> +—— on the boulder-formation being anterior to the +extinction of North American mammifers <a href="#page371"> +371</a><br/> +—— on quadrupeds washed down by floods <a href= +"#page374">374</a><br/> +—— on age of American fossil mammifers <a href= +"#page379">379</a><br/> +—— on changes of climate <a href="#page409"> +409</a><br/> +—— on denudation <a href="#page410">410</a><br/> +—— on foliation <a href="#page438">438</a><br/> +<br/> + MacCulloch, Dr., on concretions <a href="#page437">437</a><br/> +—— on beds of marble <a href="#page440">440</a><br/> + Maclaren, Mr., letter to, on coral-formations <a href="#page413"> +413</a><br/> + <i>Macrauchenia Patachonica</i> <a href="#page358">358</a>, <a +href="#page370">370</a><br/> + Madeira, subsidence of <a href="#page302">302</a><br/> + Magellan, Strait, elevation near, of <a href="#page288"> +288</a><br/> + Magnesia, sulphate of, in veins <a href="#page387">387</a><br/> + Malcolmson, Dr., on trees carried out to sea <a href="#page475"> +475</a><br/> + Maldonado, elevation of <a href="#page277">277</a><br/> +—— Pampean formation of <a href="#page365">365</a><br/> +—— crystalline rocks of <a href="#page418">418</a><br/> + Mammalia, fossil, of Bahia Blanca <a href="#page356">356</a>, <a +href="#page364">364</a><br/> +—— —— near St. Fé <a href= +"#page363">363</a><br/> +—— —— of Banda Oriental <a href= +"#page366">366</a><br/> +—— —— of St. Julian <a href="#page369"> +369</a><br/> +—— —— at Port Gallegos <a href="#page391"> +391</a><br/> +—— washed down by floods <a href="#page373"> +373</a><br/> +—— number of remains of, and range of, in Pampas <a +href="#page376">376</a><br/> + Man, skeletons of (Brazil) <a href="#page279">279</a><br/> +—— remains of, near Lima <a href="#page325"> +325</a><br/> +—— Indian, antiquity of <a href="#page325">325</a><br/> + Marble, beds of <a href="#page418">418</a><br/> + Maricongo, ravine of <a href="#page500">500</a><br/> + Marsden, on elevation of Sumatra <a href="#page305">305</a><br/> + <i>Mastodon Andium</i>, remains of <a href="#page362">362</a><br/> +—— range of <a href="#page378">378</a><br/> + Maypu, Rio, mouth of, with upraised shells <a href="#page307"> +307</a><br/> +—— gravel fringes of <a href="#page339">339</a><br/> +—— debouchement from the Cordillera <a href= +"#page449">449</a><br/> + Megalonyx, range of <a href="#page379">379</a><br/> + Megatherium, range of <a href="#page379">379</a><br/> + Miers, Mr., on elevated shells <a href="#page311">311</a><br/> +—— on the height of the Uspallata plain <a href= +"#page335">335</a><br/> + Minas, Las <a href="#page418">418</a><br/> + Mocha Island, elevation of <a href="#page305">305</a><br/> +—— tertiary form of <a href="#page398">398</a><br/> +—— subsidence at <a href="#page411">411</a><br/> + Molina, on a great flood <a href="#page341">341</a><br/> + Monte Hermoso, elevation of <a href="#page280">280</a><br/> +—— fossils of <a href="#page355">355</a><br/> + Monte Video, elevation of <a href="#page278">278</a><br/> +—— Pampean formation of <a href="#page365">365</a><br/> +—— crystalline rocks of <a href="#page419">419</a><br/> + Morris and Sharpe, Messrs., on the palæozoic fossils of the +Falklands <a href="#page424">424</a><br/> + Mud, Pampean <a href="#page352">352</a><br/> +—— long deposited on the same area <a href="#page376"> +376</a><br/> + Murchison, Sir R., on cleavage <a href="#page436">436</a><br/> +—— on waves transporting gravel <a href="#page299"> +299</a><br/> +—— on origin of salt formations <a href="#page505"> +505</a><br/> +—— on the relations of metalliferous veins and +intrusive rocks <a href="#page507">507</a><br/> +—— on the absence of granite in the Ural <a href= +"#page512">512</a><br/> +<br/> + <i>Nautilus d’Orbignyanus</i> <a href="#page400">400</a>, <a +href="#page405">405</a><br/> + Navidad, tertiary formations of, subsidence of <a href= +"#page400">400</a>, <a href="#page411">411</a><br/> + Negro, Rio, pumice of pebbles of <a href="#page281">281</a><br/> +—— gravel of <a href="#page295">295</a><br/> +—— salt lakes of <a href="#page295">295</a><br/> +—— tertiary strata of <a href="#page384">384</a><br/> + North America, fossil remains of <a href="#page379">379</a><br/> + North Wales, sloping terraces absent in <a href="#page340"> +340</a><br/> +—— bent cleavage of <a href="#page434">434</a><br/> + Neuvo Gulf, plains of <a href="#page282">282</a><br/> +—— tertiary formation of <a href="#page384"> +384</a><br/> +<br/> + Owen, Professor, on fossil mammiferous remains <a href= +"#page356">356</a>, <a href="#page358">358</a>, <a href= +"#page364">364</a>, <a href="#page366">366</a>, <a href= +"#page370">370</a><br/> +<br/> + Palmer, Mr., on transportation of gravel <a href="#page300"> +300</a><br/> + Pampas, elevation of <a href="#page290">290</a><br/> +—— earthquakes of <a href="#page290">290</a><br/> +—— formation of <a href="#page295">295</a>, <a href= +"#page350">350</a><br/> +—— localities in which fossil mammifers have been found +<a href="#page380">380</a><br/> + Panuncillo, mines of <a href="#page481">481</a><br/> + Parana, Rio, on saline incrustations <a href="#page347"> +347</a><br/> +—— Pampean formations near <a href="#page361"> +361</a><br/> +—— on the S. Tandil <a href="#page420">420</a><br/> + Parish, Sir W., on elevated shells near Buenos Ayres <a href= +"#page278">278</a>, <a href="#page279">279</a><br/> +—— on earthquakes in the Pampas <a href="#page290"> +290</a><br/> +—— on fresh-water near salt lakes <a href="#page350"> +350</a><br/> +—— on origin of Pampean formation <a href="#page373"> +373</a><br/> + Patagonia, elevation and plains of <a href="#page281">281</a><br/> +—— denudation of <a href="#page291">291</a><br/> +—— gravel-formation of <a href="#page295">295</a><br/> +—— sea-cliffs of <a href="#page301">301</a><br/> +—— subsidence during tertiary period <a href= +"#page411">411</a><br/> +—— crystalline rocks of <a href="#page421">421</a><br/> + Payta, tertiary formations of <a href="#page404">404</a><br/> + Pebbles of pumice <a href="#page280">280</a><br/> +—— decrease in size on the coast of Patagonia <a href= +"#page293">293</a><br/> +—— means of transportation <a href="#page298"> +298</a><br/> +—— encrusted with living corallines <a href= +"#page299">299</a><br/> +—— distribution of, at the eastern foot of Cordillera +<a href="#page337">337</a><br/> +—— dispersal of, in the Pampas <a href="#page354"> +354</a><br/> +—— zoned with colour <a href="#page443">443</a><br/> + Pentland, Mr., on heights in the Cordillera <a href="#page460"> +460</a><br/> +—— on fossils of the Cordillera <a href="#page465"> +465</a><br/> + Pernambuco <a href="#page279">279</a><br/> + Peru, tertiary formations of <a href="#page403">403</a><br/> + Peuquenes, Pass of, in the Cordillera <a href="#page448"> +448</a><br/> +—— ridge of <a href="#page452">452</a><br/> + Pholas, elevated shells of <a href="#page303">303</a><br/> + Pitchstone of Chiloe <a href="#page395">395</a><br/> +—— of Port Desire <a href="#page421">421</a><br/> +—— near Cauquenes <a href="#page448">448</a><br/> +—— layers of, in the Uspallata range <a href= +"#page472">472</a><br/> +—— of Los Hornos <a href="#page480">480</a><br/> +—— of Coquimbo <a href="#page483">483</a><br/> + Plains of Patagonia <a href="#page282">282</a>, <a href= +"#page291">291</a><br/> +—— of Chiloe <a href="#page304">304</a><br/> +—— of Chile <a href="#page333">333</a><br/> +—— of Uspallata <a href="#page335">335</a><br/> +—— on eastern foot of Cordillera <a href="#page336"> +336</a><br/> +—— of Iquique <a href="#page346">346</a><br/> + Plata, La, elevation of <a href="#page277">277</a><br/> +—— tertiary formation of <a href="#page295">295</a>, +<a href="#page353">353</a><br/> +—— crystalline rocks of <a href="#page418">418</a><br/> + Playfair, Professor, on the transportation of gravel <a href= +"#page300">300</a><br/> + Pluclaro, axis of <a href="#page483">483</a><br/> + Pondicherry, fossils of <a href="#page400">400</a><br/> + Porcelain rocks of Port Desire <a href="#page422">422</a><br/> +—— of the Uspallata range <a href="#page471">471</a>, +<a href="#page473">473</a>, <a href="#page476">476</a><br/> + Porphyry, pebbles of, strewed over Patagonia <a href="#page296"> +296</a><br/> + Porphyry, claystone, of Chiloe <a href="#page395">395</a><br/> +—— —— of Patagonia <a href="#page421"> +421</a><br/> +—— —— of Chile <a href="#page442">442</a>, +<a href="#page445">445</a><br/> +—— greenstone, of Chile <a href="#page444">444</a><br/> +—— doubly columnar <a href="#page448">448</a><br/> +—— claystone, rare, on the eastern side of the Portillo +Pass <a href="#page454">454</a><br/> +—— brick-red and orthitic, of Cumbre Pass <a href= +"#page458">458</a>, <a href="#page467">467</a><br/> +—— intrusive, repeatedly injected <a href="#page467"> +467</a><br/> +—— claystone of the Uspallata range <a href= +"#page468">468</a><br/> +—— —— of Copiapo <a href="#page489"> +489</a>, <a href="#page499">499</a><br/> +—— —— eruptive sources of <a href= +"#page502">502</a><br/> + Port Desire, elevation and plains of <a href="#page283"> +283</a><br/> +—— tertiary formation of <a href="#page383"> +383</a><br/> +—— porphyries of <a href="#page421">421</a><br/> + Portillo Pass in the Cordillera <a href="#page448">448</a><br/> + Portillo chain <a href="#page454">454</a>, <a href="#page458"> +458</a><br/> +—— compared with that of the Uspallata <a href= +"#page478">478</a><br/> + Prefil or sea-wall of Valparaiso <a href="#page310">310</a><br/> + Puente del Inca, section of <a href="#page461">461</a><br/> + Pumice, pebbles of <a href="#page230">230</a><br/> +—— conglomerate of R. Negro <a href="#page382"> +382</a><br/> +—— hills of, in the Cordillera <a href="#page347"> +347</a><br/> + Punta Alta, elevation of <a href="#page280">280</a><br/> +—— beds of <a href="#page356">356</a><br/> +<br/> + Quartz-rock of the S. Ventana <a href="#page421">421</a><br/> +—— C. Blanco <a href="#page421">421</a><br/> +—— Falkland islands <a href="#page424">424</a><br/> +—— Portillo range <a href="#page455">455</a><br/> +—— viscidity of <a href="#page475">475</a><br/> +—— veins of, near Monte Video <a href="#page420"> +420</a><br/> +—— —— in dike of greenstone <a href= +"#page426">426</a><br/> +—— grains of, in mica slate <a href="#page430"> +430</a><br/> +—— —— in dikes <a href="#page432">432</a>, +<a href="#page434">434</a><br/> +—— veins of, relations to cleavage <a href="#page437"> +437</a><br/> + Quillota, Campana of <a href="#page442">442</a><br/> + Quintero, elevation of <a href="#page311">311</a><br/> + Quiriquina, elevation of <a href="#page306">306</a><br/> +—— deposits of <a href="#page399">399</a><br/> +<br/> + Rancagua, plain of <a href="#page334">334</a><br/> + Rapel, R., elevation near <a href="#page307">307</a><br/> + Reeks, Mr. T., his analysis of decomposed shells <a href= +"#page328">328</a><br/> +—— his analysis of salts <a href="#page344"> +344</a><br/> + Remains, human <a href="#page324">324</a><br/> + Rio de Janeiro, elevation near <a href="#page279">279</a><br/> +—— crystalline rocks of <a href="#page415">415</a><br/> + Rivers, small power of transporting pebbles <a href="#page298"> +298</a><br/> +—— small power of, in forming valleys <a href= +"#page343">343</a><br/> +—— drainage of, in the Cordillera <a href="#page449"> +449</a>, <a href="#page513">513</a><br/> + Roads, parallel, of Glen Roy <a href="#page319">319</a><br/> + Rocks, volcanic, of Banda Oriental <a href="#page367">367</a><br/> +—— Tres Montes <a href="#page393">393</a><br/> +—— Chiloe <a href="#page394">394</a><br/> +—— Tierra del Fuego <a href="#page426">426</a><br/> +—— with laminar structure <a href="#page440"> +440</a><br/> + Rodents, fossil, remains of <a href="#page356">356</a><br/> + Rogers, Professor, address to Association of American Geologists +<a href="#page412">412</a><br/> + Rose, Professor G., on sulphate of iron at Copiapo <a href= +"#page489">489</a><br/> +<br/> + S. Blas, elevation of <a href="#page281">281</a><br/> + S. Cruz, elevation and plains of <a href="#page284">284</a><br/> +—— valley of <a href="#page285">285</a><br/> +—— nature of gravel in valley of <a href="#page296"> +296</a><br/> +—— boulder formation of <a href="#page371">371</a><br/> +—— tertiary formation of <a href="#page386"> +386</a><br/> +—— subsidence at <a href="#page412">412</a><br/> + S. Fé Bajada, formations of <a href="#page363">363</a><br/> + S. George’s bay, plains of <a href="#page282">282</a><br/> + S. Helena island, sea-cliffs, and subsidence of <a href= +"#page301">301</a><br/> + S. Josef, elevation of <a href="#page281">281</a><br/> +—— tertiary formation of <a href="#page383"> +383</a><br/> + S. Juan, elevation near <a href="#page278">278</a><br/> + S. Julian, elevation and plains of <a href="#page284">284</a><br/> +—— salt lake of <a href="#page348">348</a><br/> +—— earthy deposit with mammiferous remains <a href= +"#page369">369</a><br/> +—— tertiary formations of <a href="#page384"> +384</a><br/> +—— subsidence at <a href="#page411">411</a><br/> + S. Lorenzo, elevation of <a href="#page323">323</a><br/> +—— old salt formation of <a href="#page504"> +504</a><br/> + S. Mary, island of, elevation of <a href="#page305">305</a><br/> + S. Pedro, elevation of <a href="#page278">278</a><br/> + Salado, R., elevated shells of <a href="#page279">279</a><br/> +—— Pampean formation of <a href="#page353">353</a><br/> + Salines <a href="#page348">348</a><br/> + Salt, with upraised shell <a href="#page324">324</a>, <a href= +"#page327">327</a><br/> +—— lakes of <a href="#page348">348</a><br/> +—— purity of, in salt lakes <a href="#page349"> +349</a><br/> +—— deliquescent, necessary for the preservation of meat +<a href="#page349">349</a><br/> +—— ancient formation of, at Iquique <a href= +"#page504">504</a><br/> +—— —— at S. Lorenzo <a href="#page504"> +504</a><br/> +—— strata of, origin of <a href="#page505">505</a><br/> + Salts, superficial deposits of <a href="#page344">344</a><br/> + Sand-dunes of the Uruguay <a href="#page279">279</a><br/> +—— of the Pampas <a href="#page281">281</a><br/> +—— near Bahia Blanca <a href="#page281">281</a>, <a +href="#page293">293</a><br/> +—— of the Colorado <a href="#page281">281</a>, <a +href="#page294">294</a><br/> +—— of S. Cruz <a href="#page286">286</a><br/> +—— of Arica <a href="#page323">323</a><br/> + Sarmiento, Mount <a href="#page427">427</a><br/> + Schmidtmeyer on auriferous detritus <a href="#page506"> +506</a><br/> + Schomburghk, Sir R., on sea-bottom <a href="#page299">299</a><br/> +—— on the rocks of Guyana <a href="#page415"> +415</a><br/> + Scotland, sloping terraces of <a href="#page340">340</a><br/> + Sea, nature of bottom of, off Patagonia <a href="#page292"> +292</a><br/> +—— power of, in forming valleys <a href="#page343"> +343</a><br/> + Sea cliffs, formation of <a href="#page301">301</a><br/> + Seale, Mr., model of St. Helena <a href="#page301">301</a><br/> + Sebastian Bay, tertiary formation of <a href="#page391"> +391</a><br/> + Sedgwick, Professor, on cleavage <a href="#page336">336</a><br/> + Serpentine of Copiapo <a href="#page489">489</a><br/> + Serpulæ, on upraised rocks <a href="#page325">325</a><br/> + Shale-rock, of the Portillo Pass <a href="#page452">452</a><br/> +—— of Copiapo <a href="#page493">493</a><br/> + Shells, upraised state of, in Patagonia <a href="#page288"> +288</a><br/> +—— elevated, too small for human food <a href= +"#page308">308</a><br/> +—— transported far inland, for food <a href= +"#page309">309</a><br/> +—— upraised, proportional numbers varying <a href= +"#page312">312</a>, <a href="#page324">324</a><br/> +—— —— gradual decay of <a href="#page323"> +323</a>, <a href="#page324">324</a>, <a href="#page327"> +327</a><br/> +—— —— absent on high plains of Chile <a +href="#page335">335</a><br/> +—— —— near Bahia Blanca <a href= +"#page358">358</a><br/> +—— preserved in concretions <a href="#page394"> +394</a>, <a href="#page397">397</a><br/> +—— living and fossil range of, on west coast <a href= +"#page406">406</a>, <a href="#page408">408</a><br/> +—— living, different on the east and west coast <a +href="#page411">411</a><br/> + Shingle of Patagonia <a href="#page295">295</a><br/> + Siau, M., on sea-bottom <a href="#page299">299</a><br/> + Silver mines of Arqueros <a href="#page431">431</a><br/> +—— of Chanuncillo <a href="#page494">494</a><br/> +—— of Iquique <a href="#page503">503</a><br/> +—— distribution of <a href="#page506">506</a><br/> + Slip, great, at S. Cruz <a href="#page387">387</a><br/> + Smith, Mr., of Jordan Hill, on upraised shells retaining their +colours <a href="#page289">289</a><br/> +—— on Madeira <a href="#page302">302</a><br/> +—— on elevated seaweed <a href="#page325">325</a><br/> +—— on inclined gravel beds <a href="#page467"> +467</a><br/> + Soda, nitrate of <a href="#page346">346</a><br/> +—— sulphate of, near Bahia Blanca <a href="#page348"> +348</a>, <a href="#page349">349</a><br/> +—— carbonate of <a href="#page347">347</a><br/> + Soundings off Patagonia <a href="#page293">293</a>, <a href= +"#page299">299</a><br/> +—— in Tierra del Fuego <a href="#page300">300</a><br/> + Spirifers <a href="#page486">486</a>, <a href="#page488"> +488</a><br/> + Spix and Martius on Brazil <a href="#page417">417</a><br/> + Sprengel on the production of carbonate of soda <a href= +"#page328">328</a><br/> + Springs, mineral, in the Cumbre Pass <a href="#page461"> +461</a><br/> + Stratification of sandstone in metamorphic rocks <a href= +"#page414">414</a><br/> +—— of clay-slate in Tierra del Fuego <a href= +"#page428">428</a><br/> +—— of the Cordillera of Central Chile <a href= +"#page442">442</a>, <a href="#page448">448</a>, <a href= +"#page461">461</a><br/> +—— little disturbed in Cumbre Pass <a href="#page460"> +460</a>, <a href="#page466">466</a><br/> +—— disturbance of, near Copiapo <a href="#page501"> +501</a><br/> + Streams of lava at S. Cruz, inclination of <a href="#page390"> +390</a><br/> +—— in the Portillo range <a href="#page457"> +457</a><br/> + String of cotton with fossil-shells <a href="#page325"> +325</a><br/> + <i>Struthiolaria ornata</i> <a href="#page392">392</a><br/> + Studer, M., on metamorphic rocks <a href="#page438">438</a><br/> + Subsidence during formation of sea-cliffs <a href="#page301"> +301</a><br/> +—— near Lima <a href="#page327">327</a><br/> +—— probable, during Pampean formation <a href= +"#page376">376</a><br/> +—— necessary for the accumulation of permanent deposits +<a href="#page411">411</a><br/> +—— during the tertiary formations of Chile and +Patagonia <a href="#page413">413</a><br/> +—— probable during the Neocomian formation of the +Portillo Pass <a href="#page453">453</a><br/> +—— probable during the formation of conglomerate of +Tenuyan <a href="#page459">459</a><br/> +—— during the Neocomian formation of the Cumbre Pass <a +href="#page465">465</a><br/> +—— of the Uspallata range <a href="#page474">474</a>, +<a href="#page477">477</a><br/> +—— great, at Copiapo <a href="#page496">496</a><br/> +—— —— during the formation of the +Cordillera <a href="#page510">510</a><br/> + Sulphur, volcanic exhalations of <a href="#page509">509</a><br/> + Sumatra, promontories of <a href="#page305">305</a><br/> + Summary on the recent elevatory movements <a href="#page259"> +259</a>, <a href="#page329">329</a>, <a href="#page514"> +514</a><br/> +—— on the Pampean formation <a href="#page371"> +371</a>, <a href="#page515">515</a><br/> +—— on the tertiary formations of Patagonia and Chile <a +href="#page391">391</a>, <a href="#page404">404</a>, <a href= +"#page513">513</a><br/> +—— on the Chilean Cordillera <a href="#page508"> +508</a><br/> +—— on the cretaceo-oolitic formation <a href= +"#page508">508</a><br/> +—— on the subsidences of the Cordillera <a href= +"#page509">509</a><br/> +—— on the elevation of the Cordillera <a href= +"#page511">511</a>, <a href="#page517">517</a><br/> +<br/> + Tacna, elevation of <a href="#page323">323</a><br/> + Tampico, elevated shells near <a href="#page329">329</a><br/> + Tandil, crystalline rocks of <a href="#page420">420</a><br/> + Tapalguen, Pampean formation of <a href="#page353">353</a><br/> +—— crystalline rocks of <a href="#page420">420</a><br/> + Taylor, Mr., on copper veins of Cuba <a href="#page506"> +506</a><br/> + Temperature of Chile during the tertiary period <a href= +"#page408">408</a><br/> + Tension, lines of, origin of, axes of elevation and of cleavage <a +href="#page440">440</a><br/> + Tenuy Point, singular section of <a href="#page395">395</a><br/> + Tenuyan, valley of <a href="#page454">454</a>, <a href= +"#page478">478</a><br/> + Terraces of the valley of S. Cruz <a href="#page286">286</a><br/> +—— of equable heights throughout Patagonia <a href= +"#page290">290</a><br/> +—— of Patagonia, formation of <a href="#page294"> +294</a><br/> +—— of Chiloe <a href="#page304">304</a><br/> +—— at Conchalee <a href="#page311">311</a><br/> +—— of Coquimbo <a href="#page316">316</a><br/> +—— not horizontal at Coquimbo <a href="#page317"> +317</a><br/> +—— of Guasco <a href="#page320">320</a><br/> +—— of S. Lorenzo <a href="#page323">323</a><br/> +—— of gravel within the Cordillera <a href="#page337"> +337</a><br/> + Theories on the origin of the Pampean formation <a href= +"#page372">372</a><br/> + Tierra Amarilla <a href="#page489">489</a><br/> + Tierra del Fuego, form of sea-bottom <a href="#page300"> +300</a><br/> +—— tertiary formations of <a href="#page391"> +391</a><br/> +—— clay-slate formation of <a href="#page424"> +424</a><br/> +—— cretaceous formation of <a href="#page426"> +426</a><br/> +—— crystalline rocks of <a href="#page426">426</a><br/> +—— cleavage of clay-slate <a href="#page427">427</a>, +<a href="#page436">436</a><br/> + Tosca rock <a href="#page352">352</a><br/> + Trachyte of Chiloe <a href="#page394">394</a><br/> +—— of Port Desire <a href="#page421">421</a><br/> +—— in the Cordillera <a href="#page347">347</a><br/> + Traditions of promontories having been islands <a href= +"#page305">305</a><br/> +—— on changes of level near Lima <a href="#page327"> +327</a><br/> + Trees buried in plain of Iquique <a href="#page346">346</a><br/> +—— silicified, vertical, of the Uspallata range <a +href="#page473">473</a><br/> + Tres Montes, elevation of <a href="#page303">303</a><br/> +—— volcanic rocks of <a href="#page393">393</a><br/> + <i>Trigonocelia insolita</i> <a href="#page392">392</a><br/> + Tristan Arroyo, elevated shells of <a href="#page278">278</a><br/> + Tschudi, Mr., on subsidence near Lima <a href="#page327"> +327</a><br/> + Tuff, calcareous, at Coquimbo <a href="#page313">313</a><br/> +—— on basin-plain near St. Jago <a href="#page334"> +334</a><br/> +—— structure of, in Pampas <a href="#page352"> +352</a><br/> +—— origin of, in Pampas <a href="#page374">374</a><br/> +—— pumiceous, of R. Negro <a href="#page382"> +382</a><br/> +—— Nuevo Gulf <a href="#page383">383</a><br/> +—— Port Desire <a href="#page383">383</a><br/> +—— S. Cruz <a href="#page386">386</a><br/> +—— Patagonia, summary on Chiloe <a href="#page391"> +391</a><br/> +—— formation of, in Portillo chain <a href="#page395"> +395</a><br/> +—— great deposit of, at Copiapo <a href="#page457"> +457</a><br/> + Tuffs, volcanic, metamorphic, of Uspallata <a href="#page471"> +471</a><br/> +—— of Coquimbo <a href="#page484">484</a><br/> +<br/> + Ulloa, on rain in Peru <a href="#page324">324</a><br/> +—— on elevation near Lima <a href="#page327"> +327</a><br/> + Uruguay, Rio, elevation of country near <a href="#page278"> +278</a><br/> + Uspallata, plain of <a href="#page335">335</a>, <a href= +"#page515">515</a><br/> +—— pass of <a href="#page459">459</a><br/> +—— range of <a href="#page368">368</a><br/> +—— concluding remarks on <a href="#page476"> +476</a><br/> +<br/> + Valdivia, tertiary beds of <a href="#page398">398</a><br/> +—— mica-slate of <a href="#page433">433</a><br/> + Valley of S. Cruz, structure of <a href="#page285">285</a><br/> +—— Coquimbo <a href="#page314">314</a><br/> +—— Guasco, structure of <a href="#page320">320</a><br/> +—— Copiapo, structure of <a href="#page321"> +321</a><br/> +—— S. Cruz, tertiary formations of <a href="#page386"> +386</a><br/> +—— Coquimbo, geology of <a href="#page482">482</a><br/> +—— Guasco, secondary formations of <a href="#page487"> +487</a><br/> +—— Copiapo, secondary formations of <a href= +"#page488">488</a><br/> +—— Despoblado <a href="#page496">496</a>, <a href= +"#page497">497</a>, <a href="#page499">499</a><br/> + Valleys in the Cordillera bordered by gravel fringes <a href= +"#page337">337</a><br/> +—— formation of <a href="#page338">338</a><br/> +—— in the Cordillera <a href="#page449">449</a><br/> + Valparaiso, elevation of <a href="#page307">307</a><br/> +—— gneiss of <a href="#page435">435</a><br/> + Vein of quartz near Monte Video <a href="#page419">419</a><br/> +—— in mica-slate <a href="#page430">430</a><br/> +—— relations of, to cleavage <a href="#page437"> +437</a><br/> +—— in a trap dike <a href="#page426">426</a><br/> +—— of granite, quartzose <a href="#page432">432</a>, +<a href="#page475">475</a><br/> +—— remarkable, in gneiss, near Valparaiso <a href= +"#page435">435</a><br/> + Veins, relations of, to concretions <a href="#page396"> +396</a><br/> +—— metalliferous, of the Uspallata range <a href= +"#page475">475</a><br/> +—— metalliferous, discussion on <a href="#page505"> +505</a><br/> + Venezuela, gneissic rocks of <a href="#page415">415</a><br/> + Ventana, Sierra, Pampean formation near <a href="#page353"> +353</a><br/> +—— quartz-rock of <a href="#page421">421</a><br/> + Villa Vincencio Pass <a href="#page468">468</a><br/> + Volcan, Rio, mouth of <a href="#page449">449</a><br/> +—— fossils of <a href="#page453">453</a><br/> + Volcanoes of the Cordillera <a href="#page392">392</a>, <a href= +"#page447">447</a>, <a href="#page511">511</a><br/> +—— absent, except near bodies of water <a href= +"#page457">457</a><br/> +—— ancient submarine, in Cordillera <a href= +"#page502">502</a><br/> +—— action of, in relation to changes of level <a href= +"#page514">514</a><br/> +—— long action of, in the Cordillera <a href= +"#page517">517</a><br/> +<br/> + Wafer on elevated shells <a href="#page322">322</a><br/> + Waves caused by earthquakes, power of, in transporting boulders <a +href="#page326">326</a>, <a href="#page344">344</a><br/> +—— power of, in throwing up shells <a href="#page309"> +309</a><br/> + Weaver, Mr., on elevated shells <a href="#page329">329</a><br/> + White, Martin, on sea-bottom <a href="#page299">299</a><br/> + Wood, silicified, of Entre Rios <a href="#page364">364</a><br/> +—— S. Cruz <a href="#page388">388</a><br/> +—— Chiloe <a href="#page394">394</a>, <a href= +"#page396">396</a><br/> +—— Uspallata range <a href="#page473">473</a><br/> +—— Los Hornos <a href="#page479">479</a><br/> +—— Copiapo <a href="#page495">495</a>, <a href= +"#page497">497</a><br/> +<br/> + Yeso, Rio, and plain of <a href="#page450">450</a><br/> + Ypun Island, tertiary formation of <a href="#page393">393</a><br/> +<br/> + Zeagonite <a href="#page426">426</a></small></p> + +</div><!--end chapter--> + +<pre> + + + + + +End of the Project Gutenberg EBook of Coral Reefs, Volcanic Islands, South American Geology, by Charles Darwin + +*** END OF THIS PROJECT GUTENBERG EBOOK CORAL REEFS, VOLCANIC ISLANDS, SOUTH AMERICAN GEOLOGY *** + +***** This file should be named 4022-h.htm or 4022-h.zip ***** +This and all associated files of various formats will be found in: + http://www.gutenberg.org/4/0/2/4022/ + +Produced by Sue Asscher + +Updated editions will replace the previous one--the old editions will +be renamed. + +Creating the works from print editions not protected by U.S. copyright +law means that no one owns a United States copyright in these works, +so the Foundation (and you!) can copy and distribute it in the United +States without permission and without paying copyright +royalties. 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