Garnet, 16 Geological causes, how far uniform, 94 Geological causes, slow operation of, 95 Geological investigations aided by displacement of strata, 92 Geological periods, prolonged, 63 shown by amount of strata, 63 shown by duration of species, 64 shown by amount of organic matter, 64 shown by microscopic accumulations, 65 Geology and Revelation, 65 Giant's Causeway, 30 Glacial period, 90 Glacial theory, 124 Glaciers-- how formed, 120 cause of motion, 121 when they decrease, 122 earthy matter on them, 122 lateral moraines, 123 surfaces grooved by, 123 terminal moraines, 123 Gneiss, 18, 30 Gorge, 69 Graham Island, 131 Granite, 16 varieties of, 16 thickness of, 23 structure of, 23 formation of, 67 igneous origin of, 138 Granite veins, 24 in granite, 24 Granite of different ages, 25 Granitic axes of mountains, 24, 140 Greensand, 19, 52 Greenstone, 18, 30 Grooved surfaces of rock, 54, 87, 126 Gypsum, 15 in new red sandstone, 47 beds, how produced, 99 H.