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



formation are usually red sandstones, the beds varying in thickness from 

 4 to 13 inches. Gypsum occurs at a few points in beds which are neither 

 thick nor extensive. 



The age of the formation has not been definitely determined, for so 

 far it has yielded no fossils. From the fact that the overlying Minne- 

 kahta limestone is of Permian age, and the deposition of gypsiferous 

 Red beds in other regions began in Permian time, the formation is 

 provisionally assigned to that division. 



MINNEKAHTA LIMESTONE 



Overlying the Opeche Red beds there is a limestone persistent over a 

 wide area in the Northwest, which I have designated the Minnekahta 

 limestone. Though thin, averaging less than 50 feet in thickness, it is 

 hard, flexible, and, by the easy erosion of the Red beds in which it is 

 inclosed, outcrops in long slopes and prominent ridges. 



The rock is uniform in character throughout, being a thin bedded, 

 light colored limestone, containing magnesia and more or less clay. 

 Its thin bedding is a characteristic feature, although the thin layers 

 are so cemented together that the outcropping ledges present a massive 

 appearance, as shown in plate 24. On weathering, it breaks into slabs 

 usually 2 to 3 inches in thickness. On the western side of the hills its 

 coloring is slightly darker, varying from a dove color to lead gray, and 

 some of the beds present a semi-nodular structure. An increased ad- 

 mixture of clay is also observed in some layers. The general appearance 

 of the formation is always slightly pinkish, with a tinge of purple, from 

 which fact the old term " purple limestone " originated. 



The limestone contains fossils at a number of localities, but the forms 

 are not well preserved and not altogether decisive as to the age of the 

 deposits. At a locality 15 miles west-northwest of Hot Springs there 

 were observed a Bakewellia and Edmundia similar to those observed in 

 the Kansas Permian, and from this evidence the limestone is assigned to 

 the Permian. Near Sturgis similar fossils occur. 



SPEARFISH FORMATION 



The designation Spearfish formation has been applied to the main 

 body of gypsiferous Red beds which outcrop in a broad zone encircling 

 the Black Hills uplift. This formation consists of from 350 to 700 feet 

 of red, sandy clays, with intercalated beds of gypsum which sometimes 

 are 30 feet thick. The bright red color of the shales and the snowy white 

 of the gypsum are striking features of the formation. Were it not for 

 the gypsum the formation would present no noticeable features of stratig- 

 raphy, as the sedimentary material is almost entirely a red shale con- 



