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.