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Overview
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.
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