400 N. H. DARTON — STRATIGRAPHY OF THE BLACK HILLS, ETC. 



formation, or several hundred feet below the Greenhorn limestone hori- 

 zon. This series I shall designate the Mowrie beds in the Bighorn 

 region from Mowrie creek northwest of Buffalo. The top member of 

 the Benton group comprises 200 feet or more of dark shales, with occa- 

 sional fossiliferous layers containing Benton forms, and at their top there 

 is a series from 20 to 30 feet thick, containing lens-shaped concretions, 

 of buff color when weathered, in greater part 2 to 4 feet in diameter. 

 These concretions carry occasional remains of Prionotropis ivoolgari, a 

 species characteristic of the upper part of the Benton (Carlile) beds 

 about the Black hills and elsewhere. 



NIOBRARA FORMATION 



The impure chalks and calcareous shales of this formation, which are 

 so conspicuous in the region east and south, are not clearly distinct in 

 the vicinity of the Bighorn mountains. Overlying the Prionotropis 

 horizon above referred to, there are about 200 feet of light gray shales 

 extending to the lowest dark gray shales containing Pierre fossils, which 

 are believed to represent the Niobrara. Ostrea congesta was found in 

 this series in the southern portion of the range, but was not observed 

 northward. 



PIERRE SHALE 



This shale outcrops all along the Bighorn uplift, presenting its usual 

 features of dark gray shales or clays containing occasional concretions 

 filled with characteristic fossils. A few thin beds of sandstone occur in 

 the northern part of the region, which develop to the southward and 

 become conspicuous in the district south of Powder river, where they 

 appear to constitute the principal oil-bearing horizon. The thickness 

 of the formation varies from 2,000 to 3,000 feet, but it is possibly that 

 this apparent unusual thickness is due partly to the crumpling of the 

 beds in the steeper slopes of the uplift where most of my measurements 

 were made. 



FOX HILLS SANDSTONE 



The Pierre shales give place rapidly to the Fox Hills sandstones, which 

 have a thickness of 200 feet or more and consist mainly of fine grained, 

 light buff rock, with numerous marine fossils. The bedding is massive 

 in greater part, and the lithification of the sandstone is irregular, some 

 portions being hard and the others so soft that it can be dug with a 

 shovel. Hard concretions occur, mainly in elongated rounded forms of 

 dark gray color. These concretions are similar to those which some, 

 times occur in the soft sandstones of the lower part of the Laramie for. 



