206 



THE AGE OF MAMMALS 



plain origin, with periods of backwater, lagoon, and shallow lake conditions, 

 and even of aeolian conditions. Matthew and Hatcher pointed out that 

 the great Badlands are composed partly of coarse sandstones and con- 

 glomerates, indicating river formations, and partly of so-called clays, indi' 

 eating still water or aeolian conditions in which horizontal banded deposits 

 were laid down. Especially interesting is the demonstration by Matthew 

 that the river channel sandstones contain chiefly the remains of forest- and 



Fig. 95. — Lower Oligoceiie overlying Upper Eocene horizons on the Beaver Divide at 

 Wagon-bed Spring, near Hailey, Wyo. Diplacodon Zone (Eocene) below ; Titanotherium 

 Zone (Oligocene) above. Photograph by American Museum of Natural History, expedition 

 of 1909. 



river-living animals, while the fine clays contain the remains of plains- 

 living or cursorial animals. 



The accompanying panorama prepared bj^ Osborn after a personal 

 survey of this wonderful region in 1907 is designed to indicate how the 

 fluviatile 'Titanotherium,' 'Metamynodon,' and 'Protoceras' sandstones 

 traverse the outlying fine deposits or clays and prove the existence of 

 great river channels from seven hundred feet to a mile in width. These 

 rivers flowed eastward, and bore down from the mountains coarse ma- 

 terials; they occasionally overflowed in broad shallow sheets of water, too 

 transitory to support any of the aquatic animals. They caused such dep- 

 ositions as are left by the annual overflows of the Nile. 



A picture of the plains region in Oligocene times is that of broad, gentle 

 eastward slopes from the Rocky Mountains, plane or gently undulating 



