44 GEOLOGY OF THE BLACK HILLS. 



Tertiary strata of the White River group are abundantly exposed on 

 our routes to and from the Hills, as well as on lower Rapid Creek. Many 

 excellent sections, especially of the lower members, were obtained, though 

 little of importance was learned in addition to the published results of 

 observers who have been able to examine the region more thoroughly. 

 Few fragments of vertebrated fossils, and those of little interest, were 

 obtained. 



The Quaternary deposits have had little interest geologically, as they 

 consist merely of certain local deposits of gravel along a portion of the 

 foothills, and the alluvial deposits of bowlders, gravel, sand, and clay, 

 forming the bottoms or floors of the valleys of the creeks. The latter, how- 

 ever, are of very considerable moment by reason of the contained gold, 

 and in certain regions have yielded large returns to the miners. The 

 deposits all belong to very recent geological time, and are the result of the 

 wearing and abrading action of existing streams. Of the true glacial drift 

 we could find no evidence. 



The peaks and ejections of volcanic rock which are found at numerous 

 points in the northern end of the Hills are probably all of similar age, and 

 consist chiefly of sanidin-trachytes and rhyolites. The effect of the extru- 

 sion of the volcanic masses on the surrounding strata has been exceed- 

 ingly local, and if the elevation of the Hills took place synchronously with 

 their ejection the topography was probably little affected by the accompa- 

 nying seismic throes. It is possible, however, that the elevation of the 

 Hills was entirely subsequent to the volcanic extrusions ; for we have no 

 evidence that will define the time of the elevation closer than that it 

 belongs to a date subsequent to Cretaceous and prior to middle Tertiary 

 time, while of the age of the volcanic outbursts we can only say that they 

 were subsequent to the deposition of the Cretaceous. 



In this brief resume an endeavor has been made to give the most 

 comprehensive view of the formations which constitute the Hills. The 

 remainder of the chapter will be devoted to a more detailed description of 

 the same formations, together with a discussion of all peculiar or interest- 

 ing features and of the general history and origin of the Hills. 



Notwithstanding the smallness of our corps, only two members of which 



