496 
O 
Obelisk, weathering of, 109. 
Obsidian, 60, 61, 68. 
Ocean, 15; agents at work in, 220, 239; 
burial of fossils in, 389; deposition in, 
16, 243; erosion in, 220, 239, 241. 
Ocean basins, 18; permanence of, 318. 
Ocean currents, 16, 235, 239, 394. 
Oil, rock, 418. 
Old red sandstone, 397. 
Oneida strata, 454. 
Onondaga strata, 402, 456. 
Odlites, 95, 97, 276. 
Oolitic rock, 95. 
Order of superposition, 267, 396. 
Ordovician, 398, 402; geography of, 
451; life of, 408. 
Ore deposits, 378. 
Ores, 382; of iron, 47. 
Organic deposits in the sea, 248, 
Organic rocks, 73, 89, 97. 
Organisms of Cambrian age, 407; Car- 
boniferous, 419; Cenozoic, 433; Devo- 
nian, 414; influence of in ocean 
erosion, 233, 239; Mesozoic, 426; Or- 
dovician, 408; Silurian, 410. 
Oriskany strata, 402, 457. 
Orthoclase, 40, 66. 
Outcrop, 283. 
Overburdened rivers, 160. 
Overthrust faults, 291, 292, 324. 
Overturned folds, 286, 288, 311, 313, 
324. 
Oxidation, 15, 26, 118, 128. 
Oxygen, 14, 24, 26, 28, 30. 
Oysters, Cretaceous, 427. 
= 
Pahoehoe, 334, 335. 
Paleolithic stage, 400. 
Paleozoic, 402; life in, 406 425. 
Palisades of the Hudson, 70, 349, 466; 
joint planes of, 278. 
Peaks, mountain, 305, 306, 314; volcanic, 
329. 
Peat bogs, 93, 190, 194. 
Pebble beach, 77. 
Pebbles carried by rivers, 159, 
ELEMENTARY GEOLOGY 
Pebbly rocks, 75, 97. 
Percolating water, 117, 128. 
Period, 401, 402. 
Permanence of continents and ocean 
basins, 318. 
Permanence of mountains, 317. 
Permian, 397, 402,424; climate of, 463. 
Peru, changes in level of, 295. 
Petrified wood, 382. 
Petroleum, 31, 418. 
Phosphate rocks, 91. 
Phosphates, 32. 
Phosphorus, 24, 32. 
Physical geography of the past, 394. 
Physiographic geology, 8. 
Piedmont glaciers, 216, 218. 
Pike’s Peak, 308. 
Placer gold, 380. 
Plagioclase, 40, 66. 
Planets, 13. 
Plant deposits, 92; in lakes, 190, 194; in 
the sea, 248. 
Plant fossils, 94. 
Plants, aid of, in weathering, 114, 119, 
128; Cambrian, 407; Carboniferous, 
420, 422, 423, 424, 460; destruction by 
volcanic eruption, 345; Devonian, 415, 
417; influence on ocean erosion, 233, 
239; Mesozoic, 4382; Ordovician, 410; 
Silurian, 412. 
Plateaus, 317. 
Pleistocene, 398, 402. 
Plugs, voleanic, 346. 
Plutonic rocks, 55, 62, 66, 404; meta- 
morphism by, 372. 
Polar currents, 236. 
Polyps, 252. 
Pompeii, 345. 
Popocatepetl, 331. 
Porous lava, 56, 69. 
Porphyritic rock, 65, 67. 
Potassium, 30. 
Pot-holes in river beds, 163. 
Potsdam, 402. 
Primary strata, 397. 
Primitive strata, 397. 
Pterodactylus, 430. 
Pteropod ooze, 258. 
Pumice, 57. 
Pumiceous lava, 68. 
