204 



THE AGE OF MAMMALS 



ences between these two proboscideans, and indicate that we may look 

 for other radiations of the proboscidean stock in Africa; possibly the 

 river-living sirenians may prove to be one of these radiations. Certain 

 of the paliEomastodons attained an imposing size, but none of them rivaled 

 the arsinoitheres. 



III. OLIGOCENE LIFE OF AMERICA 



Geologic conditions. — Widely contrasting with the limited and scattered 

 deposits of Europe are the vast Badlands, or Mauvaises Terres, of the 



OLIGOCENE DEPOSITS 



Fig. 94. — Chief Oligocene deposits of fossil mammals in the Mountain Region of North 

 America. 1. John Day, Oreg. 2. White River, S. Dak., Neb., Wyo. 3. Horsetail Creek 

 and Cedar Creek, Col. 4. Pipestone Creek and Threeforks, Mont. 6. White Buttes, 

 N. Dak. 6. Swift Current Creek, Assiniboia. 7. Bate's Hole, Wyo. 



western plains region which, as we now believe, represent the vestiges of 

 extensive flood plains similar to those of many existing rivers in India and 

 South America. Scattered over the surface at different points from British 

 Columbia on the north to the Mexican plateau on the south are areas from 

 two to three hundred miles east of the Rocky Mountains. For the most 

 part they overlie not the Eocene, but the worn upper surfaces of the Cre- 

 taceous (Fort Pierre), proving that while the Rocky Mountain basin de- 

 posits were forming, the region of the Great Plains was an open, slightly 

 undulating country, traversed by rivers and streams. 



