214 GEOLOGY OF THE BLACK HILLS. 



touches the circle of its outcrop about the main body of the Hills, and each 

 is broken for a short space at the contact. 



3. — The age of the hills. 



The age of the Hills is the age of the uplift. There is some reason to 

 suspect, as we have said in another place, that the locality was marked by 

 an elevation or ridge before the Potsdam sandstone was deposited, but the 

 present existence of the Hills as an upland district depends upon and dates 

 from the disturbances which took place after the accumulation of the entire 

 series of conforming strata. From the base of the Potsdam to the top of 

 the Cretaceous all of the strata are parallel and conformable, and above the 

 Potsdam they comprise none of those fragmental deposits which mark the 

 neighborhood of mountains undergoing erosion. All of the strata are 

 upbent together in the flanks of the great arch. If the arch had been 

 formed during the accumulation of the series the beds deposited after the 

 formation would not conform to the dip of the others, but would either 

 abut against them or else overlap their worn edges. We are assured there- 

 fore that the movement did not begin before the end of the period repre- 

 sented by the Cretaceous strata. 



The next formation succeeding the Cretaceous in the region of the 

 Hills is the Fort Union group, but our single observation of the group 

 afforded no means of judging whether it shared the movements of the older 

 beds. Following it in order of time is the White River group, and of this 

 our knowledge is more satisfactoiy. The conglomerate which was found 

 at its base on the east side of the Hills, and only fifteen miles distant from 

 the foothills, proves by the size of its pebbles that a mountain was under- 

 going degradation close at hand during its formation, and leaves no doubt 

 that the Black Hills were then in existence. The discovery by Mr. Jenney 

 of debris from the granite, mingled with the predominant quartz and 

 quartzite of the conglomerate, shows further that the degradation of the 

 uplifted mountain had progressed so far that at some point a water-course 

 had pierced all the sedimentary beds and laid bare one of the granite masses. 

 The time consumed by this erosion could not have been inconsiderable and 

 it preceded the earlier deposits of the White River Miocene lake. The 



