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



canyon, below the town of Deadwood. It thins out to the southward 

 and disappears in the central Black hills. 



The rock is a massive, tough limestone of buff color, with brownish 

 spots or mottlings. It contains large Endoceras, Maclureas, and corals 

 of Ordovician age lying on the Cambrian shales, to which it conforms 

 in attitude, but from which it is separated by a planation unconformity 

 representing later Cambrian time. In part of the area it is overlain by 

 several feet of greenish shales which may possibly be of Devonian age, 

 but no fossils have as yet been found in them. 



ENGLEWOOD LIMESTONE 



This basal member of the Mississippian division of the Carboniferous 

 extends throughout the Black Hills area. It is a thin bedded, pale 

 pinkish buff limestone, from 25 to 50 feet in thickness. It is abruptly 

 separated from the underlying Deadwood or Whitewood formation, 

 but although there is an intervening unconformity, probably repre- 

 senting all of Silurian and Devonian time, there is no discordance 

 in dip and no evidence of channeling. Above, it merges rapidly into 

 the overlying Pahasapa limestone, in some cases with a few feet of im- 

 pure buff limestone beds of passage. Fossils usually occur throughout 

 down to the Cambrian or Ordovician contact. The following forms 

 have been reported : Fenestella, Orthothetes, Leptsena, Spirifer, Chonetes 

 logani, Reticularia peculiaris, Syringothyris carteri, and crinoids. It is 

 correlated with the Chouteau or Kinderhook of the Mississippi valley. 



The formation was differentiated by T. A. Jaggar, Jr., and named 

 from extensive exposures at Englewood, in the northern Black hills. 



PAHASAPA LIMESTONE 



This is the prominent " gray limestone v of the high plateaus in the 

 Black Hills uplift. The rock is in massive beds, having a total thick- 

 ness of from about 250 feet in the southern hills to 400 feet northward, 

 with a maximum amount of 700 feet reported by Jaggar in the vicinity 

 of Spearfish canyon. The character of the rock is relatively uniform, 

 but the upper portion of the formation is somewhat more silicious and 

 flinty. 



Fossils occur sparingly throughout the formation, including the Spi- 

 rifer rockymontanus, Seminula dawsoni (Athyris subtilita), Productus, and 

 Zaphrentis, a fauna which indicates Mississippian (Lower Carboniferous) 

 age. 



MINNELVSA FORMATION 



The Minnelusa formation consists mainly of thick beds of gray, buff, 

 and reddish sandstones, usually fine grained, most of which, in their 



