446 N. H. DARTON STRATIGRAPHY OF THE BLACK HILLS, ETC. 



fish scales, weathering light gray, appear first in southeastern Wyoming 

 and are a conspicuous feature northward at a horizon a short distance 

 above that of the sandstone in the lower part of the Graneros beds. 

 They attain their greatest prominence along the Bighorns and around 

 the Black hills. 



The Carlile formation does not vary greatly in thickness through Colo- 

 rado and southern Wyoming, but it expands greatly in the Black hills 

 to over 500 feet in most parts of the uplift. It is not, however, charac- 

 terized in the Bighorn uplift, but doubtless is represented in the gray 

 shales lying not far above the Mowry beds. Owing to the absence of the 

 Greenhorn limestone in this uplift, the lower limit of the formation is not 

 indicated, but its upper portion is characterized by a zone of sandy con- 

 cretions containing Prionotropis woolgari, a horizon which is characteristic 

 as marking the upper limit of the Benton group throughout its course. 

 These concretions are especially numerous around the Black hills and 

 in Kansas. The top of the Carlile is also often marked by sandy sedi- 

 ments, and a top sandstone is a prominent feature in eastern South 

 Dakota, through Colorado, and in southeastern Wyoming. In a por- 

 tion of the Arkansas valley the upper sandstone is replaced by a thin 

 bed of purplish limestone carrying Prionocyclas. 



In eastern South Dakota the Benton group comprises a thin mass of 

 Graneros black shales below, the Greenhorn limestone associated with 

 some chalkstone, and a considerable thickness of Carlile shales. At 

 or near the top of the Carlile formation there are concretions and a 

 nearly general bed of sandstone from 15 to 50 feet thick, and in places 

 considerably thicker. 



NIOBRARA FORMATION 



This deposit occupies a wide area in the central Great Plains region, 

 succeeding the Carlile formation without unconformity, and, excepting 

 in the vicinity of the Bighorn mountains, it consists largely of carbonate 

 of lime. Its thickness varies considerably from apparently less than 

 100 feet in some portions of eastern South Dakota to 700 feet in central 

 southeastern Colorado. 



At the type locality on the Missouri river, at the mouth of the Nio- 

 brara, the formation is represented by chalk rock having a thickness of 

 about 200 feet. In southern Nebraska and Kansas, where it appears 

 extensively, the amount is considerably greater, 350 feet being the esti- 

 mate by the Kansas geologists. The formation usually presents purer 

 and harder carbonate of lime deposits near its base, constituting the 

 Fort Hays limestone in Kansas and the Timpas formation in Colorado. 

 The characteristic fossil of this horizon is the Inoceramus deformis, which 



