114 GEOLOGY OF THE BLACK HILLS. 



included as the upper member of the Carboniferous, together with the red 

 arenaceous clay underlying the limestone of the Red Beds were placed by 

 him in the Red Bed series, their thickness being given as 250 to 300 feet. 



Professor "Winchell, despite the rapidity of his passage through the 

 Hills (he was in the region less than four weeks), was enabled to make 

 some interesting observations on the geology. The route by which he 

 entered them was not, however, a favorable one for geological purposes, and 

 several of the records and observations made by him are now susceptible of 

 a different interpretation. He includes in the Carboniferous two elements 

 which are here placed in the Red Bed series: first the purple limestone, and 

 second the lower red clay, which is not separated by him from the under- 

 lying variegated sandstone. This sandstone, the upper part of our fourth 

 member of the Carboniferous, is called by him the "Minne Lusa" sand- 

 stone, while the underlying sandstones and limestones, 150TcT200'feet in 

 thickness, do not appear to have been separated by him. Excluding the 

 beds that are here placed in the Red Beds, the Carboniferous, as estimated 

 by Professor Winchell, has a thickness of about 300 feet, while the meas- 

 urements we were enabled to make give the thickness as ranging from 570 

 to over 700 feet* 



The character and development of the Carboniferous have had a con- 

 trolling influence in the production of some of the most prominent topo- 

 graphical features in the Hills Though limestone is soluble to a certain 

 extent by ordinary water, it is still one of the most durable of the sedimentary 

 rocks The Carboniferous limestone is not only durable but thick, and its 

 influence on the topography is heightened by the softness and incoherence 

 of the overlying sandstones and clays. The latter have yielded to the 

 action of the elements and disappeared from all the high places and by so 

 doing have laid bare the limestone. The limestone stubbornly resists, and 

 yields only as it is undermined by the disintegration of the shaly and are- 

 naceous beds at its base. It survives over half of the flat-topped uplift 

 which constitutes the Hills, forming- a great plateau, and surrounds the other 

 half in a continuous belt of cliffs. 



* One of the most serious of Winchell's errors was the announcement of an unconformity between 

 the purple, limestone and the gypsiferous clay. If that unconformity really existed his separation of 

 the Paleozoic and Mcsozoic would be preferable to Newton's, — Ed, 



