South Dakota School of Mines 3* 



in the whole White River (Oligocene) formation. This is a buff- 

 colored clay carrying numerous calcareous nodules in which are 

 imbedded remains of turtles and oreodons. The fossils are 

 almost invariably covered with a scale of ferruginous oxide 

 when first removed from the matrix, and are of decidedly red- 

 dish cast. Upon this account this stratum is known to .the col- 

 lector as the 'red-layer.' It is situated somewhere between 40 

 and 50 feet above the top of the Titanotherium beds and can 

 almost always be easily identified. It varies in thickness from 

 10 to 20 feet, and in some rare instances it is replaced by sand- 

 stone. I have also found it without the nodules in places, but 

 this is also quite a rare occurence."* 



Another tolerably constant fossiliferous nodular layer oc- 

 curs at from 75 to 100 feet above the nodular layer just de- 

 scribed. This higher horizon was provisionally considered 

 as marking the top of the Oreodon beds. The present 

 tendency is to extend the Oreodon Beds upward so as to include 

 the series of non-fossiliferous clays about 100 feet thick, lying 

 just above the upper nodular layer. The total thickness of the 

 beds in the vicinity of Sheep Mountain is from 250 to 300 feet. 

 The stratigraphy in Pine Ridge differs in some important 

 respects lithologically from that of the Big Badlands and the 

 exact equivalent there of the Oreodon beds does not yet seem 

 clear. 



The Protoceras Beds. The Protoceras beds, earlier con- 

 sidered as part of the Oreodon Beds, were first differentiated 

 by J. L. Wortman as a result of field work done during the 

 summer of 1892 for the American Museum of Natural History. 

 The name is derived from the characteristic and highly interest- 

 ing extinct animal, the Protoceras, which occurs in the sand- 

 stones of these beds in considerable abundance. 



Lithologically the beds are made up of isolated patches of 

 coarse, lenticular sandstones, fine-grained clays, and nodular lay- 

 ers. The sandstones occur at different levels and are usually fos- 

 siliferous. They are seldom continuous for any great distance 

 and often change abruptly into fine-grained barren clays. Immed- 

 iately overlying the sandstones there is a pinkish colored nodule- 

 bearing clay, containing abundant remains of Leptauchenia and 

 other forms, hence the name Leptauchenia zone often used in 

 connection with these beds. The Protoceras beds have been 



* Wortman, J. L. On the Divisions of the White River or Lower 

 Miocene of Dakota. Am. Mus. Nat. Hist., Bull. Vol. 5, 1893, pp. 95- 

 105. 



