444 J. BAKRELL REC0G>'1T10X OF A^•C1E:^'T DELTA DEPOSITS 



"\Yliite Eiver formations of Oligocene age which skirt the eastern base of 

 the Eockj Mountains and extend outward for distances of from 200 to 

 300 miles are subdivided into, first, the Titanotherium sandstones and 

 clays, overlaid by, second, the Oreodon clays, including the Metamynodon 

 sandstones, and these in turn by, third, the Leptauehenia clays, includ- 

 ing the Protoceras sandstones. Hatcher gives reasons on structural 

 grounds and apart from the fossils for regarding the Oreodon and Lep- 

 tauehenia clays as of floodplain origin. The clays hold, furthermore, 

 thin layers of limestone which mark the former presence of small ponds 

 and lakes. The included Metamynod©n and Protoceras sandstones are 

 the deposits of the river channels. Each of these subdivisions holds a 

 distinct fauna and flora. The clays contain numerous remains of giant 

 land tortoises and the scattered and fragmental skeletons of such great 

 mammals as the Titanotheres. The limestone lenses within the clays 

 are rich in the remains of fresh-water plants and mollusca, whose habi- 

 tats are swamps and ponds. The sandstones are more sparingly fossil- 

 iferous, but contain the remains of aquatic turtles, fishes, and crocodiles, 

 and in one locality the casts of unios were observed in great numbers. 

 Hatcher comments, further, that the bodies of animals Avill only be swept 

 into lakes or the sea while they are intact and distended by gases and 

 will ordinarily become buried as complete skeletons.^' It would seem 

 possible, however, that even large terrestrial animals might be preserved 

 intact within the zone of terrestrial deposition, seeking shelter in grou])s 

 in the lee of cliffs, or dunes, or mired in water holes. A priori it miglit 

 be expected that but little significance could be attached to the place ut 

 burial of a large mammal; but, considering the truly terrestrial fauna, 

 the observations of Hatcher and general geological experience point to 

 the conclusion that their fossils are but rarely found entombed in ancient 

 lake or sea bottoms. 



Fixed terrestrial fossils, plants and animals. — These are evidences of 

 the highest value as to terrestrial conditions of origin. Progress in 

 paleobotanical studies has shown that the great majority of coal seams 

 consist of the debris of fresh-water swamps in place, though in the case 

 of bogheads and cannels the seam represents metamorphosed sapropelic 

 deposits of lacustrine origin, and some deposits of coal may be due to 

 material drifted into large lake basins by river agency."*^ Xo salt-water 

 adaptations of coal plants have been demonstrated, so tliat these as well 



*^ Origin of tlae Oligocene and Miocene deposits of tlie Great riains. American Phil. 

 See. Proc, vol. xli, 1902, pp. 113-131. 



^ F. E. Weiss : Address to the Botanical Section, Brit. Assoc. Adv. Sci. Science, n. s., 

 vol. xxxiv, 1911, p. 475. 



