496 INDEX TO THE STRATIGKAPHY OF NOETH AMEEICA. 



mations. The Opeche and Minnekahta are now regarded as Permian, but the Spear- 

 fish is beheved to be probably of Triassic age. Below the red sandstones of the 

 Opeche occur other sandstones which have yielded "indistinct casts of Productus 

 semireticulatus and Seminula dawsonii (Athyris subtilita)" and which are provision- 

 ally assigned to the Pennsylvanian. Concerning the formations Barton^"" says: 



OpecJie formation. — In this formation the &st of the red beds makes its appearance in the 

 Black Hills region. The materials are soft red sandstones, mainly thin bedded and containiag 

 variable amounts of clay admixture, having a thickness varying from 120 feet in the south- 

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

 the 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. 



Tlie age of the formation has not been definitely determined, for so far it has yielded no 

 fossUs. From the fact that the overlying Minnekahta 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 mas- 

 sive appearance. * * * Qjj weathering, it breaks into slabs usually 2 to 3 inches in thick- 

 ness. 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 seminodular structure. An increased admixture 

 of clay is also observed in some layers. The general appearance of the formation is always 

 shghtly 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 pre- 

 served and not altogether decisive as to the age of the deposits. At a locality 15 miles west- 

 northwest of Hot Springs there were observed 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. 



SpearfisJi formation. — The designation Spearfish formation has been apphed to the main 

 body of gypsiferous red beds which outcrop in a broad zone encirchng the Black HiUs uplift. 

 This formation consists of from 350 to 700 feet of red sandy clays, with intercalated beds of gyp- 

 sum 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 stratigraphy, as the sedimentary material is almost 

 entirely a red shale containing varying amounts of fine sand admixture. It is generally thin 

 bedded. The gypsum occurs in beds at various horizons, some of the larger beds extending con- 

 tinuously over wide areas. There is also throughout the formation more or less secondary 

 deposition of gypsum in small veins. 



The thickness of the Spearfish formation can seldom be determined with accuracy, owing 

 to the softness of the material and the predominance of low, variable dips which are difficult to 

 measure. Along the east side of the Black Hills the formation appears to have a thickness of 

 from 350 to 400 feet, but the amount increases to the northward to 492 feet in the well at Cam- 

 bria, 695 feet in a well at Sturgis, and at least 650 feet in a deep boring at Aladdin. To the south- 

 eastward the principal bed of gypsum generally varies from 5 to 15 feet, increasing southward 

 in the vicinity of Hot Springs to the maximum development, in which the principal beds have 

 a thickness of 33 i feet, with a 10-foot parting of red shale between. * * * Along the west 



