Geological Observations of Volcanic Islands | Page 3

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This etext was prepared by Sue Asscher

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