THE BLACK HILLS AN EPITOME OF THE ROCKY MOUNTAINS. 39 



sedimentary rocks, he is given a piece of mountain geology of great beauty, 

 simplicity, and ease of elucidation. 



Usually in explorations in the West or elsewhere the field of work of 

 the geologist at any one time or season is but a part, and commonly a very 

 small part, of a great system that extends over vast areas of country. 

 Thus, in explorations in the Rocky Mountains, the most assiduous labor of 

 the geologist can cover thoroughly in one season but a small part of the 

 great range, and his discussion of results cannot be complete in itself, 

 but must depend largely upon work in the adjoining regions. Rarely, 

 then, does the geologist find his work so admirably circumscribed by 

 nature as did those to whom the exploration of the Black Hills was com- 

 mitted. 



The area of the Hills, to a discussion of whose geological features the 

 present chapter is devoted, includes only that country between the forks of 

 the Cheyenne, limited on the north and west by the Belle Fourche, on the 

 south and west by Beaver Creek and the South Fork, and on the east by 

 the South Fork. A somewhat detailed account has already been given 

 in Chapter II of the principal geological features observed in going to and 

 returning from the Hills — a narration which could not be introduced in this 

 place without in a measure marring the unity of the plan. 



Generally and simply the geological structure of the Black Hills is 

 as follows: Around a nucleal area of metamorphic slates and schists, con- 

 taining masses of granite, the various members of the sedimentary series 

 of rocks, the Potsdam, Carboniferous, Trias or Red Beds, Jura, Creta- 

 ceous, and Tertiary, lie in rudely concentric belts or zones of varying 

 width, dipping on all sides away from the elevatory axis or region of the 

 Hills. From the Hills outward the inclination of the beds gradually 

 diminishes until all evidence of the elevation is lost in the usual rolling 

 configuration of the Plains. At numerous points, also, within the area of 

 the Hills are centers of volcanic eruption of an age probably coincident 

 with that of the elevation of the mountains themselves. 



The succession of the different formations as found in the Hills, 

 together with their character and approximate thickness, is exhibited in the 

 following synopsis : 



