CONCENTRIC RIDGES. 175 



Black Hills. — Prominently developed, forming the capping rock to the 

 foothills that surround the Hills on all sides ; appears with its characteristic 

 composition — coarse yellow or red sandstones with discontinuous variegated 

 clays. At places a considerable thickness of very soft and fine white sand- 

 stone appears at the base. Elsewhere considerable portions are of hard, 

 dense quartzite. No animal fossils were found, but many remnants of 

 plants — in no case more than mere coaly fragments. 



Thickness, 250 to 400 feet. 



The rim of Cretaceous strata which encircles the Hills dips outward 

 on all sides or away from the axis of upheaval. The strata begin with the 

 foothills that border the outer edge of the valley. The Dakota sand- 

 stone, resting conformably upon the Jura, forms the capping rock of the 

 foothill ridge, and dips outward at various angles from 10° to 40°. Just 

 as the Cretaceous encircles the outcrop of the underlying Jura and Trias, 

 so the different overlying groups of the Cretaceous — the Fort Benton, the 

 Niobrara, the Fort Pierre, and the Fox Hills — succeed each other in regular 

 order, forming a series of concentric ridges that decrease in altitude as the 

 distance from the Hills increases. 



One who journeys from the Hills without following a drainage line 

 passes over a series of ridges of which each one is lower than the last, but 

 is composed of strata higher in the geological scale. He finds, too, that the 

 dip of each ridge-forming stratum is less than the dip of the last, until 

 finally he reaches one that lies nearly or quite horizontal. 



It has already been remarked that the concentration of attention on 

 the main body of the Hills prevented a thorough study of the Jura. In a 

 still greater degree it reduced our opportunities for the examination of the 

 Cretaceous. Very few of our excursions penetrated more than the basal 

 member, and the only examinations of the entire series were in our rapid 

 approach to the Hills via Beaver Creek and on our return march. So far, 

 however, as our observations extended, it was evident that the upper Missouri 

 section of Meek and Hayden was applicable without essential modification. 



Upon the geological map no attempt has been made to distinguish any 

 individual member of the formation except the Dakota; all the upper 

 divisions, from No. 2 to No. 5, inclusive, are represented by one shade. 



