MINNELUSA FORMATION 385 



unweathered condition, contain a considerable proportion of carbonate 

 of lime. Thin sheets of limestone occur in some districts, and, less fre- 

 quently, sandy shales of red or gray color. Some layers are cherty. 

 The formation is prominent on the outer slopes of the higher ridges of 

 the encircling rim of the hills. It is thickest on the western side, where 

 it is fully 750 feet, and it thins gradually to the south and east, being 

 about 400 feet thick in the southeastern portion of the uplift. 



The formation presents local variations. There is generally a lower 

 member of buff, slabby sandstones often 100 feet in thickness. Next 

 above there are usually more massive sandstones, in part brecciated and 

 often of red color. In the northeastern portion of the area, the Minne- 

 lusa beds comprise a thick mass of coffee colored sandstone at the top, 

 reddish buff sandstone with some thin, interbedded limestone layers 

 next below, and the basal member of gray sandstones. On the western 

 side of the Black hills the upper sandstones are light colored, often pure 

 white, the medial member consists of red and buff sandstones and the 

 basal member is gray, calcareous sandstone with reddish shale partings 

 and considerable chert. Sandstone predominates everywhere in the 

 outcrops, but in the borings from the deep well at Cambria, on the west 

 side of the hills, it is found that nearly all of the rocks contain consider- 

 able carbonate of lime in their unweathered condition. At the base of 

 the formation there usually occurs a red shaly bed of slight thickness, 

 containing oval, fossiliferous concretions of hard silica, from 6 inches to 

 2 feet in diameter in greater part. 



Very few fossils have been obtained from the Minnelusa formation, 

 and the only determinable ones so far discovered were obtained from 

 upper beds west of Hot Springs, where, among some indistinct casts 

 Productus semireticidatus and Seminula daivsonii (Athyris subtilila) were 

 recognized with a fair degree of certainty. It is on the basis of this not 

 very conclusive evidence that the upper part of the formation, at least, 

 is referred to the Pennsylvanian division of the Carboniferous, a deter- 

 mination which is somewhat strengthened by the similarity of the de- 

 posits to other formations in the Northwest which yield Upper Carbon- 

 iferous fossils. There are some reasons, which will be presented later, 

 for believing that the lower beds may represent the Mississippian. 



OPE CHE FORMATION 



In this formation the first of the Red beds makes its appearance in 

 the Black Hills region. The materials are soft, red sandstones, mainly 

 thin bedded and containing variable amounts of clay admixture, having 

 a thickness varying from 120 feet in the southeastern part of the hills to 

 about half that amount to the northwestward. The basal beds of the 



