288 



HISTORICAL GEOLOGY 



moderate thickness, occupy extensive areas in Wyoming, western 

 South Dakota, western Nebraska, and eastern Colorado. The 

 formation consists of clays and sandstones, with some limestone and 

 volcanic ash, variously deposited in lakes, by rivers, by wind, etc. 

 The Miocene, far less thick than the Eocene, is represented 

 toward the base by the Arikaree formation of chiefly soft sand- 



Fig. 178 

 Eocene-Oligocene strata as seen in the Wind River 

 Basin of Wyoming. 3, Eocene sandstone; 4, 5, 6, 7, 

 Eocene sandstone and shale; 8, 9, Oligocene volcanic 

 dust and marl. (After Sinclair and Granger, Amer. 

 Mus. Nat. Hist., Bui. 30.) 



stones some hundreds of feet thick in South Dakota, Nebraska, 

 and Wyoming; toward the middle by the Florissant beds, which 

 consist of laminated shales formed by deposition of fine volcanic 

 ash in a small lake in Colorado and remarkable for the great num- 

 ber of insects and plants contained in it; and toward the top by 

 the Loup Fork beds, which form thin deposits of fine sands and 

 marls (both subaerial and lacustrine) over extensive areas from 

 South Dakota to Mexico. 



