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.