132 GEOLOGY OF THE BLACK HILLS 



section], and passing down into a very hard, compact, concretionary sand- 

 stone, 250 to 300 feet" — thus embracing both the red arenaceous clay (1 of 

 the above section) and the colored sandstones and occasional limestones 

 which form the upper member of our section of the Carboniferous. 



On the other hand, Professor Winchell (Black Hills of Dakota, Lud- 

 low, 1874, p. 65), while exaggerating very considerably the thickness of the 

 purple limestone (2 of the above section), has included it, together with the 

 underlying red clay and sandstones, whose thickness he greatly underesti- 

 mates, in the Carboniferous system, indicating, however, their reference to 

 that formation with some doubt. 



The reasons for adopting the division here made between the Car- 

 boniferous and Red Beds have already been full}^ expressed in the section 

 on the Carboniferous. There is one statement by Dr. Hayden which, 

 if substantiated, would completely overthrow his own classification and 

 at the same time establish Professor Winchell's. He speaks of finding in 

 the purple limestone in the Hills Spirifera (like S. lineata), Pleurotomaria, 

 Macroclieilus, and Belleroplion. Unfortunately in our most diligent search, 

 examining and breaking the rock almost by the cart-load and in many 

 localities, not the least trace of organic remains was found, and it is reluc- 

 tantly inferred that there was some error in the reference of the fossils to 

 this limestone. 



The purple limestone is very conspicuous in the geological structure 

 of the Black Hills and an important element in the topography. It 

 forms the outer slope of the main portion of the Hills, and in a sloping 

 belt from half a mile to three or four miles in width, dipping outward 

 from 20° to 30°, completely encircles them. It preserves the underlying 

 red clay and sandstone from denudation and defines the inner margin 

 of the great encircling Red Valley. Its inner edge marks with consider- 

 able accurac}' the border of the timbered region of the Hills, though a 

 few trees are in places scattered over its slope. It is so much harder 

 than the overlying clay and lies at so steep an angle, that it is swept 

 clean not only of the clay, but of all soil, and forms a bare and slippery 

 pavement of rock, which, by its seams and peculiar weathering, has 

 acquired a tesselated appearance. Were it not for the breaks caused by 



