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.