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Geological Observations on South America

Geological Observations on South America

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Etext Prepared by Sue Asscher asschers@dingoblue.net.au Urangan, 17 June, 2001

GEOLOGICAL OBSERVATIONS ON SOUTH AMERICA

by CHARLES DARWIN

EDITORIAL NOTE.

Although in some respects more technical in their subjects and style than Darwin's "Journal," the books here reprinted will never lose their value and interest for the originality of the observations they contain. Many parts of them are admirably adapted for giving an insight into problems regarding the structure and changes of the earth's surface, and in fact they form a charming introduction to physical geology and physiography in their application to special domains. The books themselves cannot be obtained for many times the price of the present volume, and both the general reader, who desires to know more of Darwin's work, and the student of geology, who naturally wishes to know how a master mind reasoned on most important geological subjects, will be glad of the opportunity of possessing them in a convenient and cheap form.

The three introductions, which my friend Professor Judd has kindly furnished, give critical and historical information which makes this edition of special value.

G.T.B.

PLATE I. GEOLOGICAL SECTIONS THROUGH THE CORDILLERAS.

SECTION 1/1. SECTION OF THE PEUQUENES OR PORTILLO PASS OF THE CORDILLERA.
SECTION 1/2. SECTION OF THE CUMBRE OR USPALLATA PASS.
SECTION 1/3. SECTION OF THE VALLEY OF COPIAPO TO THE BASE OF THE MAIN CORDILLERA.

PLATE II. MAP OF SOUTHERN PORTION OF SOUTH AMERICA.

TABLE OF CONTENTS.

CRITICAL INTRODUCTION.

CHAPTER I.—ON THE ELEVATION OF THE EASTERN COAST OF SOUTH AMERICA.

Upraised shells of La Plata.—Bahia Blanca, Sand-dunes and Pumice-pebbles.- -Step-formed plains of Patagonia, with upraised shells.—Terrace-bounded valley of Santa Cruz, formerly a sea-strait.—Upraised shells of Tierra del Fuego.—Length and breadth of the elevated area.—Equability of the movements, as shown by the similar heights of the plains.—Slowness of the elevatory process.—Mode of formation of the step-formed plains.—Summary.- -Great shingle formation of Patagonia; its extent, origin, and distribution.—Formation of sea-cliffs.

CHAPTER II.—ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AMERICA.

Chonos Archipelago.—Chiloe, recent and gradual elevation of, traditions of the inhabitants on this subject.—Concepcion, earthquake and elevation of.- -VALPARAISO, great elevation of, upraised shells, earth or marine origin, gradual rise of the land within the historical period.—COQUIMBO, elevation of, in recent times; terraces of marine origin, their inclination, their escarpments not horizontal.—Guasco, gravel terraces of.—Copiapo.—PERU.— Upraised shells of Cobija, Iquique, and Arica.—Lima, shell-beds and sea- beach on San Lorenzo.—Human remains, fossil earthenware, earthquake debacle, recent subsidence.—On the decay of upraised shells.—General summary.

CHAPTER III.—ON THE PLAINS AND VALLEYS OF CHILE:—SALIFEROUS SUPERFICIAL DEPOSITS.

Basin-like plains of Chile; their drainage, their marine origin.—Marks of sea-action on the eastern flanks of the Cordillera.—Sloping terrace-like fringes of stratified shingle within the valleys of the Cordillera; their marine origin.—Boulders in the valley of Cachapual.—Horizontal elevation of the Cordillera.—Formation of valleys.—Boulders moved by earthquake- waves.—Saline superficial deposits.—Bed of nitrate of soda at Iquique.— Saline incrustations.—Salt-lakes of La Plata and Patagonia; purity of the salt; its origin.

CHAPTER IV.—ON THE FORMATIONS OF THE PAMPAS.

Mineralogical constitution.—Microscopical structure.—Buenos Ayres, shells embedded in tosca-rock.—Buenos Ayres to the Colorado.—S. Ventana.—Bahia Blanca; M. Hermoso, bones and infusoria of; P. Alta, shells, bones, and infusoria of; co-existence of the recent shells and extinct mammifers.— Buenos Ayres to St. Fe.—Skeletons of Mastodon.—Infusoria.—Inferior marine tertiary strata, their age.—Horse's tooth. BANDA ORIENTAL.— Superficial Pampean formation.—Inferior tertiary strata, variation of, connected with volcanic action; Macrauchenia Patachonica at S. Julian in Patagonia, age of, subsequent to living mollusca and to the erratic block period. SUMMARY.—Area of Pampean formation.—Theories of origin.—Source of sediment.—Estuary origin.—Contemporaneous with existing mollusca.— Relations to underlying tertiary strata. Ancient deposit of estuary origin.—Elevation and successive deposition of the Pampean formation.— Number and state of the remains of mammifers; their habitation, food, extinction, and range.—Conclusion.—Supplement on the thickness of the Pampean formation.—Localities in Pampas at which mammiferous remains have been found.

CHAPTER V.—ON THE OLDER TERTIARY FORMATIONS OF PATAGONIA AND CHILE.

Rio Negro.—S. Josef.—Port Desire, white pumiceous mudstone with infusoria.—Port S. Julian.—Santa Cruz, basaltic lava of.—P. Gallegos.— Eastern Tierra del Fuego; leaves of extinct beech-trees.—Summary on the Patagonian tertiary formations.—Tertiary formations of the Western Coast.- -Chonos and Chiloe groups, volcanic rocks of.—Concepcion.—Navidad.— Coquimbo.—Summary.—Age of the tertiary formations.—Lines of elevation.— Silicified wood.—Comparative ranges of the extinct and living mollusca on the West Coast of S. America.—Climate of the tertiary period.—On the causes of the absence of recent conchiferous deposits on the coasts of South America.—On the contemporaneous deposition and preservation of sedimentary formations.

CHAPTER VI.—PLUTONIC AND METAMORPHIC ROCKS:—CLEAVAGE AND FOLIATION. Brazil, Bahia, gneiss with disjointed metamorphosed dikes.—Strike of foliation.—Rio de Janeiro, gneiss-granite, embedded fragment in, decomposition of.—La Plata, metamorphic and old volcanic rocks of.—S. Ventana.—Claystone porphyry formation of Patagonia; singular metamorphic rocks; pseudo-dikes.—Falkland Islands, palaeozoic fossils of.—Tierra del Fuego, clay-slate formation, cretaceous fossils of; cleavage and foliation; form of land.—Chonos Archipelago, mica-schists, foliation disturbed by granitic axis; dikes.—Chiloe.—Concepcion, dikes, successive formation of.—Central and Northern Chile.—Concluding remarks on cleavage and foliation.—Their close analogy and similar origin.—Stratification of metamorphic schists.—Foliation of intrusive rocks.—Relation of cleavage and foliation to the lines of tension during metamorphosis.

CHAPTER VII.—CENTRAL CHILE:—STRUCTURE OF THE CORDILLERA.

Central Chile.—Basal formations of the Cordillera.—Origin of the porphyritic clay-stone conglomerate.—Andesite.—Volcanic rocks.—Section of the Cordillera by the Peuquenes or Portillo Pass.—Great gypseous formation.—Peuquenes line;

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