390 N. H. DAKTON — STRATIGRAPHY OF THE BLACK HILLS, ETC. 



stones and was originally comprised in the " Dakota sandstone " group. 

 It has been found to extend throughout the Black Hills uplift and into 

 other regions. Its thickness averages about 50 feet, but varies consider- 

 ably and is greatest in the northern and southern extremities of the uplift. 

 The material consists mostly of a mixture of clay and fine sand usually 

 massively bedded. Some beds of sandstone are included, and there are 

 extensive deposits of nearly pure shale. The predominant color is 

 white or gray, but buff, purple, and maroon tints are often conspicuous. 

 One of the most characteristic exposures in the southern hills is at the 

 falls of Cheyenne river, where the following beds are shown : 



Feet 

 Dakota sandstone 



Dark sandy shale 4 



Soft gray slabby sandstone, plants 6 



Compact white mndstone 8 



Dark green clay 1 



Dark gray compact mudstone 25 



Very compact white mudstone 2J 



Gray mudstone 6 



Harder white mudstone 9 



Purplish shale 1 



White fine grained sandstone 5-12 



Purple shale 6- 8 



Light buff massive sandstone 25 



Dark buff coarser sandstone, much honeycombed by 



weathering 25 



In Fuson canyon, on the east side of the hills, a typical locality, the 

 uppermost bed is a moderately hard sandstone underlain by 10 feet of 

 purplish gray shale, then 10 feet of massive white shale, and at the base 

 20 feet of bright purple shale. Farther north the formation varies from 

 50 feet of white massive shale, on as Dry creek, to about the same amount 

 of buff and purple shale overlain by several feet of alternations of buff 

 shale and thin sandstone. Near Rapid the thickness is 100 feet, and in 

 the northern hills it varies from 60 to 100 feet. In this region, in the 

 vicinity of Hay creek, the formation has yielded large numbers of fossil 

 plants of Lower Cretaceous age, described by L. F. Ward.* They were 

 collected by W. P. Jenny from u the upper part of division number 2." 



DAKOTA SANDSTONE 



This formation is the uppermost member of the series, formerly des- 

 ignated " Dakota sandstone," in the Black Hills region. Being rarely 

 over 100 feet thick, it constitutes only a small part of the mass of the 



* Loc. cit., p. 946. 



