438 N. H. DARTON STRATIGRAPHY OF THE BLACK HILLS, ETC. 



nate fossils. Next above is the Tensleep sandstone, very like the top 

 sandstone of the Minnelusa in most portions of the Black hills, and 

 similarly overlain by a great body of Red beds. In the Black hills these 

 Red beds have three distinct subdivisions: the Opeche formation at the 

 base, 60 to 120 feet thick ; the Minnekahta or " Purple limestone," as it 

 was termed b} r Newton, about 50 feet thick and of very characteristic 

 aspect, and at the top several hundred feet of red, sandy shales, which 

 have been designated the Spearfish formation. It is succeeded uncon- 

 formably by the marine Jurassic. In the Minnekahta limestone fossils 

 occur at many localities, comprising forms which are regarded as Per- 

 mian in age. In the Bighorn uplift the Red Bed series is slightly thicker, 

 but its character is nearly the same as in the Black hills. It has been 

 designated the Chug water formation. The Opeche series appears to be 

 present at the base, but it is only 20 feet thick. It is succeeded by only 

 a few feet of limestone, a probable representative of the Minnekahta, but 

 unfortunately without fossils, so far as observed, and then a thousand feet 

 or more of Red beds, believed to represent the Spearfish formation, and 

 unconformably overlain by marine Jurassic. These Red beds contain 

 gypsum deposits, as in the Black hills, but they differ in including several 

 thin beds of limestone near their top. Fossils of several species are abun- 

 dant in these limestones, but they do not indicate whether the age is Tri- 

 assic or Permian. Passing south in Wyoming to the Laramie range, the 

 lower members, the Upper Carboniferous series, change considerably, but 

 the Red beds present the features which characterize them in the Black 

 hills. The limestone on the Casper mountain and ranges southwest of 

 Douglas are of Upper Carboniferous age and presumably represent the 

 Amsden formation of the Bighorn uplift. Locally they lie on a sand- 

 stone which probably is Cambrian, but may be younger, and they are 

 separated from the Chugwater Red beds by typical Tensleep sandstone. 

 The tripartite subdivision of the Chugwater Red beds is established by 

 the occurrence of typical Minnekahta limestone, notably in the big bend 

 of North Platte river 6 miles south of Douglas and farther down the river 

 in the basin northwest of Hartville. In the Hartville area the basal Red 

 beds (Opeche) are underlain by the Hartville limestone, which is Penn- 

 sylvanian in its upper part and Mississippian at base. The top beds are 

 sandy, suggesting Tensleep sandstone, and the limestones below, with 

 cherty and sandstone layers, doubtless represent the Amsden formation 

 of the Bighorns and the Minnelusa of the Black hills. No special search 

 has been made yet for evidence of unconformity in the Hartville lime- 

 stone at the top of the Pennsylvanian. 



In the Front range, northwest of Cheyenne, the stratigraphy is some- 

 what variable. The lower limestones lie directly on the granites and 



