Oceanic mountains, 75 Ocean level, nearly permanent, 74 Old red sandstone, 38 fossils of, 39 extent of, 39 Oölite, 19 Oölitic structure, 49 Oölite system, 49 calcareous, 49 fossils of, 50 localities of, 51 Opal, 14 Organic causes, 101 Orthoceras, 36, 40 Outcrop, 71 Oxide of iron, 16 Oxygen, 11 P. Paleotherium, 55 Papandayang, 95, 142 Permian system, 47 Plesiosaurus, 51 Porphyritic structure, 17 Potassium, 12 Primary limestone, 19 Pterodactyle, 51 Pumice-stone, 17 Pyroxene, 15 Q. Quartz, 14 rock, 32 R. Raindrops, impressions of, 48 Raised beaches, 76 Ravine, 69 Recent elevation in Europe, 79 Recent formation, 57 Ripple marks, 48 Rivers-- beds of, raised, 113 continued into the sea, 112 abrading action of, 105 abrading action of promoted by foreign substances, 106 Rock crystal, 14 Rock salt, 16, 118 Rocks, denned, 16 Rose quartz, 14 S.