152 GEOLOGY OF THE BLACK HILLS. 



its strata from the base upward are well characterized by fossils, and, except- 

 ing on the eastern side, they occur in abundance. 



The Jura of the Northwest has a very uniform character, consisting 

 generally of gray or ash-colored marls, marly or arenaceous limestones, and 

 soft sandstones. The greatest thickness yet found is in the Wind River 

 Mountains, where the formation is fully 1,000 feet thick, but elsewhere it 

 ranges from 50 to 100 or 200 feet. In the southwest, however, in southern 

 Nevada, Utah, Arizona, and New Mexico, though in places absent, it is 

 found to range as high as 2,000 feet or even more. In the localities of its 

 maximum development it is composed of highly colored marls, shales, and 

 soft sandstone, with some limestone and considerable gypsum. 



The Jura of the Black Hills consists primarily of gray or ash-colored 

 clays or marls, with occasional bands of green and red. Interbedded with 

 these are soft sandstones, more or less argillaceous, and a few restricted 

 thin bands of impure limestone. The formation is so soft and weathers so 

 easily that its outcrops are largely covered, by the talus of the overlying 

 sandstone, and the exposures are generally imperfect. Its study was there- 

 fore not so satisfactory as that of the other formations; still, by com- 

 bining the data gathered at many different points the general features 

 were well made out. 



The thickness of the formation in the main region of the Hills is about 

 200 feet, but it shows a remarkable increase toward the north and north- 

 west, attaining in the Belle Fourche Valley a depth of nearly 600 feet. Its 

 strata occupy the greater part of the inner slope of the foothills which 

 form the outer rim of the Red Valley, and are overlaid by the heavy sand- 

 stone of the base of the Cretaceous, which everywhere forms the capping 

 rock of the foothills. In the northwest, where the character and continuity 

 of the foothills are somewhat interrupted by the Bear Lodge range, the 

 Jurassic outcrop leaves the margin of the Red Valley and runs over to the 

 Belle Fourche, where it encircles the outlying outcrop of the Red Beds. 



On the north and west, and to a less degree on the south, the forma- 

 tion is well exposed and characterized by a greater or less abundance of 

 fossils. On the southeast and east it is less plainly seen, being usually 



