Volcanic Islands

by the Classic British Author

Charles Darwin

Free Public Domain Book from the
Classic Literature Library

Volcanic Islands Page 01

VOLCANIC ISLANDS

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.

VOLCANIC ISLANDS.

TABLE OF CONTENTS.

CRITICAL INTRODUCTION.

CHAPTER I.--ST. JAGO, IN THE CAPE DE VERDE ARCHIPELAGO.

Rocks of the lowest series.--A calcareous sedimentary deposit, with recent shells, altered by the contact of superincumbent lava, its horizontality and extent.--Subsequent volcanic eruptions, associated with calcareous matter in an earthy and fibrous form, and often enclosed within the separate cells of the scoriae.--Ancient and obliterated orifices of eruption of small size.--Difficulty of tracing over a bare plain recent streams of lava.--Inland hills of more ancient volcanic rock.--Decomposed olivine in large masses.--Feldspathic rocks beneath the upper crystalline basaltic strata.--Uniform structure and form of the more ancient volcanic hills.--Form of the valleys near the coast.--Conglomerate now forming on the sea beach.

CHAPTER II.--FERNANDO NORONHA; TERCEIRA; TAHITI, ETC.

FERNANDO NORONHA.--Precipitous hill of phonolite.

TERCEIRA.--Trachytic rocks: their singular decomposition by steam of high temperature.

TAHITI.--Passage from wacke into trap; singular volcanic rock with the vesicles half-filled with mesotype.

MAURITIUS.--Proofs of its recent elevation.--Structure of its more ancient mountains; similarity with St. Jago.

ST. PAUL'S ROCKS.--Not of volcanic origin.--Their singular mineralogical composition.

CHAPTER III.--ASCENSION.

Basaltic lavas.--Numerous craters truncated on the same side.--Singular structure of volcanic bombs.--Aeriform explosions.--Ejected granite fragments.--Trachytic rocks.--Singular veins.--Jasper, its manner of formation.--Concretions in pumiceous tuff.--Calcareous deposits and frondescent incrustations on the coast.--Remarkable laminated beds, alternating with, and passing into obsidian.--Origin of obsidian.-- Lamination of volcanic rocks.

CHAPTER IV.--ST. HELENA.

Lavas of the feldspathic, basaltic, and submarine series.--Section of Flagstaff Hill and of the Barn.--Dikes.--Turk's Cap and Prosperous Bays.-- Basaltic ring.--Central crateriform ridge, with an internal ledge and a parapet.--Cones of phonolite.--Superficial beds of calcareous sandstone.-- Extinct land-shells.--Beds of detritus.--Elevation of the land.-- Denudation.--Craters of elevation.

CHAPTER V.--GALAPAGOS ARCHIPELAGO.

Chatham Island.--Craters composed of a peculiar kind of tuff.--Small basaltic craters, with hollows at their bases.--Albemarle Island; fluid lavas, their composition.--Craters of tuff; inclination of their exterior diverging strata, and structure of their interior converging strata.--James Island, segment of a small basaltic crater; fluidity and composition of its lava-streams, and of its ejected fragments.--Concluding remarks on the craters of tuff, and on the breached condition of their southern sides.-- Mineralogical composition of the rocks of the archipelago.--Elevation of the land.--Direction of the fissures of eruption.

CHAPTER VI.--TRACHYTE AND BASALT.--DISTRIBUTION OF VOLCANIC ISLES.

The sinking of crystals in fluid lava.--Specific gravity of the constituent parts of trachyte and of basalt, and their consequent separation.-- Obsidian.--Apparent non-separation of the elements of plutonic rocks.-- Origin of trap-dikes in the plutonic series.--Distribution of volcanic islands; their prevalence in the great oceans.--They are generally arranged in lines.--The central volcanoes of Von Buch doubtful.--Volcanic islands bordering continents.--Antiquity of volcanic islands, and their elevation in mass.--Eruptions on parallel lines of fissure within the same geological period.

CHAPTER VII.--AUSTRALIA; NEW ZEALAND; CAPE OF GOOD HOPE.

New South Wales.--Sandstone formation.--Embedded pseudo-fragments of shale.--Stratification.--Current-cleavage.--Great valleys.--Van Diemen's Land.--Palaeozoic 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.

INDEX.

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