SOUTH DAKOTA SCHOOL OF MINES 41 



come a rather striking feature. Their thickness varies from 

 an inch or less to occasionally several feet. Sometimes they 

 are repeated in rapid succession without great contrasts in 

 color. More often a few bands stand out with prominence, 

 especially if moistened by recent rains and, seen from some 

 commanding point, may be traced for long distances. 



The sandstones being of a lenticular nature are often 

 absent or of little consequence, but in many localities they 

 reach considerable thicknesses. One series near the middle 

 of the bed is of particular importance. It reaches in the 

 Big Badlands a thickness of twenty feet or more, and ac- 

 cording to Wortman, covers an area approximately twelve 

 miles in length and a mile or a mile and a half in width. 

 It contains fossil remains in abundance of the ancestral 

 rhinoceros, Metamynodon, hence is commonly known as the 

 Metamynodon sandstone. 



Of the nodular layers, one just above the Metamynodon 

 sandstone is of paramount importance. For description of 

 this I quote from Mr. Wortman, 1893 : "There is one layer 

 found in the Oreodon Beds which is highly characteristic and 

 is perhaps more constant and widely distributed than any 

 other single stratum in the whole White Kiver (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 reddish cast. Upon this 

 account this stratum is known to the collector 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 occurrence." 



Another tolerably constant fossiliferous nodular layer 

 occurs at from 75 to 100 feet above the nodular layer just 

 described. This higher horizon was provisionally con- 

 sidered as marking the top of the Oreodon beds. The pres- 

 ent 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 



