OLIGOCENE OF EUROPE, NORTH AFRICA, AND NORTH AMERICA 217 



Montana will be more fully described in the Miocene section (p. 279). 

 The Lower Oligocene beds of White River age overlie the Basal Eocene 

 or Fort Union. They were apparently deposited in streams, lakes, and 

 marshes in ancient river valleys, cut into the Fort Union. Douglass deter- 

 mines beds of both Titanotherium and Oreodon Zones. The Pipestone 

 Creek beds, discovered in 1899, belong in the Titanotherium level, and 

 have yielded a rich fauna of small mammals. 



Similarly, in North Dakota there are restricted areas of Lower Oligo- 

 cene overlying the Fort Union, especially at White Butte, throughout a 



Fig. 105. — ' Big Badlands,' head of Corral Draw, South Dakota. Lower Oreodon Zone, 

 and river channel beds of Metamynodon sandstones in the foreground, overlaid by the Upper 

 Oreodon Zone and capped by the Leptauchenia Zone and river channel beds of the Protoceras 

 sandstones. Photograph by American Museum of Natural History, 1906. 



section two hundred and ten feet in thickness, apparently including the 

 Titanotherium Zone below and Oreodon Zone above. Another section 

 affords a thickness of three hundred and twenty feet, which, however, 

 includes the Oreodon and overlying Protoceras and Leptauchenia Zones. 

 These White River formations in North Dakota are l^elieved to represent 

 deposits made in the old river valley traversed by streams originating in 

 the Black Hills. 



The giant pigs, or entelodonts. — The family tree of the giant pigs 

 has recently been studied by Peterson,' who traces these animals from 

 lower Oligocene ancestors {Entelodon in Europe, ArchcEotherium in North 

 America), which may have sprung alike from an unknowni northern or 

 Holarctic form. Related, are the Eocene giant pigs {Achcenodon) of the 

 W^ashakie and Uinta (Upper Eocene of the Rocky Mountains), too special- 



' Peterson, O. A., A Revision of the Entelodontidse. Mem. Carneg. Mus., Vol. IV, 

 no. .3, May, 1909, pp. 41-158, Pis. liv-lxii. 



