U.'^ 



PROCEEDINGS 



OF THE 



AMERICAN PHILOSOPHICAL SOCIETY 



HELD AT PHILADELPHIA FOR PROMOTING USEFUL EKOIVLEDGE. 



Vol. XLI. April, 1902. No. 169. 



ORIGIN OF THE OLIGOCENE AND MIOCENE DEPOSITS 

 OF THE GREAT PLAINS. 



BY J. B. HATCHER. 

 {Read April 3, 1902.) 



Skirting the base of the Rocky Mountains and covering the* sur- 

 face of the plains for some two or three hundred miles to the east- 

 ward is a series of Tertiary clays and sandstones with a combined 

 maximum thickness of over 1700 feet. This extends from the Rio 

 Grande in southern Texas to and beyond the northern limits of 

 the Black Hills in South Dakota, and covers the greater portion of 

 the plains of eastern New Mexico and Colorado, southeastern 

 Wyoming and western Texas, Oklahoma, Kansas, Nebraska and 

 South Dakota. Within this 1700 feet of Tertiary deposits there are 

 a number of different horizons, which are usually quite distinct 

 both faunally and lithologically. The more important of these 

 were long ago differentiated and given appropriate names by Hay- 

 den, Leidy, Cope, and others. If we exclude the Equus beds and 

 certain other deposits at the top, of Pliocene and Pleistocene age, 

 and which do not fall within the limits of this paper, this entire 

 series of rocks has been considered to belong to two formations, 

 the White River, or Oligocene, and the Loup Fork, or Miocene. 

 The White River, so named from a stream in northwestern Nebraska 

 and southwestern South Dakota, where it is particularly well repre- 

 sented, is the lowermost, and therefore the older of these two 

 formations. It has a maximum thickness of about 700 feet and 

 consists for the most part of very fine and usually unlaminated clays, 

 with frequent lenses of sandstones which in places become so coarse 



PROC. AMER. PHILOS. SOC. XLI. 169. H. PRINTED MAY 24, 1902. 



