Geological Observations of Volcanic Islands | Page 4

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.

GEOLOGICAL OBSERVATIONS ON VOLCANIC ISLANDS.
CRITICAL INTRODUCTION.
The preparation of the series of works published under the general title

"Geology of the Voyage of the 'Beagle'" occupied a great part of
Darwin's time during the ten years that followed his return to England.
The second volume of the series, entitled "Geological Observations on
Volcanic Islands, with Brief Notices on the Geology of Australia and
the Cape of Good Hope," made its appearance in 1844. The materials
for this volume were collected in part during the outward voyage, when
the "Beagle" called at St. Jago in the Cape de Verde Islands, and St.
Paul's Rocks, and at Fernando Noronha, but mainly during the
homeward cruise; then it was that the Galapagos Islands were surveyed,
the Low Archipelago passed through, and Tahiti visited; after making
calls at the Bay of Islands, in New Zealand, and also at Sydney, Hobart
Town and King George's Sound in Australia, the "Beagle" sailed across
the Indian Ocean to the little group of the Keeling or Cocos Islands,
which Darwin has rendered famous by his observations, and thence to
Mauritius; calling at the Cape of Good Hope on her way, the ship then
proceeded successively to St. Helena and Ascension, and revisited the
Cape de Verde Islands before finally reaching England.
Although Darwin was thus able to gratify his curiosity by visits to a
great number of very interesting volcanic districts, the voyage opened
for him with a bitter disappointment. He had been reading Humboldt's
"Personal Narrative" during his last year's residence in Cambridge, and
had copied out from it long passages about Teneriffe. He was actually
making inquiries as to the best means of visiting that island, when the
offer was made to him to accompany Captain Fitzroy in the "Beagle. "
His friend Henslow too, on parting with him, had given him the advice
to procure and read the recently published first volume of the
"Principles of Geology," though he warned him against accepting the
views advocated by its author. During the time the "Beagle" was
beating backwards and forwards when the voyage commenced, Darwin,
although hardly ever able to leave his berth, was employing all the
opportunities which the terrible sea-sickness left him, in studying
Humboldt and Lyell. We may therefore form an idea of his feelings
when, on the ship reaching Santa Cruz, and the Peak of Teneriffe
making its appearance among the clouds, they were suddenly informed
that an outbreak of cholera would prevent any landing!

Ample compensation for this disappointment was found, however,
when the ship reached Porta Praya in St. Jago, the largest of the Cape
de Verde Islands. Here he spent three most delightful weeks, and really
commenced his work as a geologist and naturalist. Writing to his father
he says, "Geologising in