CRETACEOUS AND TERTIARY FORMATIONS 547 



discovered on Sentinel Butte by Dr. C. A. White, who visited 

 the locality in 1882 and published an account of the deposit 

 containing them. They were described by E. D. Cope as belong- 

 ing to a new genus and were named by him Plioplarchus Whitei 

 Cope and Plioplarchus sexspinosus Cope. 



Since the fishes were not closely related to any previously 

 described they did not serve to indicate the age of the beds in 

 which they were found, but upon stratigraphic grounds Dr. White 

 referred the strata to the Green River group of the Eocene, though 

 he was by no means confident that this was their true position. 

 In the light of more recent discoveries it seems much more probable 

 that these beds on Sentinel Butte belong to the White River 

 division of the Oligoeene. It is now known that less than forty 

 miles to the southeast are other deposits which rest directly on 

 the upper sandstone of the Fort Union and which are known from 

 their fossils to belong to the White River group. On the other 

 hand, no beds of the Green River group are found any nearer 

 than southwestern Wyoming and it is not at all likely that they 

 ever extended this far north and east, while the White River 

 beds cover considerable areas in South Dakota and Montana. 

 The extensive erosion to which this region has been subjected 

 during many ages, and which is known to have removed at least 

 from 800 to 1,000 feet of strata over a large area, has left only a 

 few remnants of the White River deposits. 



In southeastern Custer County, Montana, in the district 

 known as the Long Pine Hills between the Little Missouri River 

 and Big Box Elder Creek, the White River beds are known to occur. 

 They here have a thickness of at least 150 feet and are composed 

 of fine-grained, greenish gray calcareous clay, soft, compact, 

 white limestone, and calcareous clay. They resemble the White 

 River beds of the Slim Buttes in South Dakota. 



The Oligoeene beds of North Dakota and Montana are believed 

 to be in part lake deposits and in part river deposits. The lack 

 of uniformity, the cross-bedding, and the coarseness of the materials 

 in some portions of the formation are probably the result of deposi- 

 tion through river action. In other areas, as those of Sentinel 

 Butte and Long Pine Hills, the materials were perhaps laid down 

 in the more quiet water of a lake. 



