88 



THE AGE OF MAMMALS 



pally flood plain depositions, delta, overflow, and swamp deposits with 

 relatively limited areas of shallow lakes. According to this theory the prin- 

 cipal true lakes of Eocene and Miocene times are the Green River and the 

 Florissant. The famous Green River formation consists of impure lime- 

 stones and thin fissile calcareous shales often as finely laminated as paper, 

 between the leaves of which we discover in beautiful preservation the re- 

 mains of plants, insects, and fishes, but none of mammals except in the form 

 of footprints. Occasional layers of gypsum indicate periods of evaporation 

 and salinity. In the Bridger beds the frequent occurrence of large selenite 



General Comparison of some of the Mountain and Plains Formations ' 



MOUNTAIN BASIN DEPOSITS 



Geologic. — Partly of erosion 

 materials; largely of vol- 

 canic materials, partly 

 eolian, partly deposited 

 in water. 



Faunistic. — Extinct mam- 

 mals, chiefly inhabiting a 

 mountainous, hilly, for- 

 ested, lake- and river- 

 border, well-watered coun- 

 try. 



GREAT PLAINS DEPOSITS 



Geologic. — Largely of wa- 

 ter-erosion and wind-ero- 

 sion materials; partly of 

 volcanic materials. 



Faunistic. — E.xtinct mam- 

 mals, chiefly of an open- 

 plains country, traversed 

 by broad, slow-moving 

 rivers, savannah, partly 

 forested, with shallow 

 lakes and decreasing rain 

 supply. 



Middle Pliocene 

 Lower Pliocene . 



Rattlesnake, Greg. 



Upper Miocene . 

 Middle Miocene 



Lower Miocene 

 and Upper Oli- 

 gocene 



Upper Oligocene 



Lower Oligocene 



Upper Eocene 

 Middle Eocene 

 Lower Eocene 

 Basal Eocene 



Deep River, Mont.; Mas- 

 call, Oreg. 



"Fort Logan," Mont. . . 



John Day (upper part), 

 Oreg. 



John Day (middle and 

 lower parts), Oreg. 



Deposits on Pipestone 

 Creek, Mont. 



Uinta, northern Utah 

 f "Washakie," Wyo. . 

 { Bridger, Wyo. . . . 

 l Wind River, Wyo. . 



Wasatch, N. Mex. and Wy 

 f Torrejon, N. Mex. . 

 ■I Puerco, N. Mex. . . 

 I Fort Union, Mont. . 



Blanco, Tex. 



Ogalalla, in part ("Repub- 

 lican River"), Nebr. 



Ogalalla ("Nebraska"), 

 Nebr. 



"Pawnee Creek," Colo. 



Arikaree (Gering, Rose- 

 bud), Nebr., S. Dak. 



White River, S. Dak. (up- 

 per part). 



White River (lower part), 

 of the western plains of 

 South Dakota, Nebraska, 

 etc. 



Second deposition (or 

 Neocene) period of 

 very widespread flu- 

 viatile, flood-plain, 

 and eolian deposits, 

 chiefly erosion and 

 volcanic materials, 

 on the Great Plains of 

 Dakota, Nebraska, 

 Colorado, and west- 

 ern Kansas. Limited 

 and scattered depos- 

 its in the Rocky 

 Mountain region. 



First deposition (or 

 Eocene) period of 

 lacustrine, river, and 

 flood-plain deposits, 

 largely of volcanic 

 materials in the 

 Rocky Moimtain ba- 

 sins, chiefly in the 

 ancient drainage ba- 

 sin of Colorado Riv- 

 er. Plains deposits 

 of this period eroded 

 away, buried, or un- 

 known. 



crystals proves that the water was occasionally uu'luly saline through evapo- 

 ration. The existence of swamps in the Bridger is indicated by stretches of 



' From p. 22 of Osborn, Cenozoic Mammal Horizons of Western North America. U.S. 

 Geol. Sun. Bull., 361. Washington, 1909. 



