BLACK HILLS GEOLOGY. 241 



D. CONCLUSIONS. 



If we bar out the pustular theory of Newton, which is ren- 

 dered improbable by the complete absence of superficial features 

 in the rocks composing the igneous peaks, we are left with two 

 almost contradictory assumptions — one that the magmas in- 

 truded were of great viscidity, and hence took the form of 

 " plutonic plugs " ; the other that they were extremely fluid, 

 and were thus enabled to spread out into thin sheets. How 

 can we reconcile these opposing hypotheses, and the facts ad- 

 vanced in support of them ? If we are to accept Russel's view 

 that the outlying masses of Bear butte, Mato Tepee, etc., are 

 " plutonic plugs " it is necessary to explain the occurrence of 

 the vast number of intrusive sheets and dikes in the region 

 about Terry peak. 



It might be suggested that the Terry peak region is near the 

 center of the uplift, and that for this reason the pressure of the 

 overlying sediments may have been relieved by erosion, while 

 the sediments remained in their full development on the out- 

 lying portions of the hills. This then might enable us to ex- 

 plain the predominance of fluid magmas in the central region 

 and more viscid upon the borders of the hills, on the ground 

 that the latter were the more deeply buried. The laccolitic 

 peaks are not, however, confined to the borders of the hills. 

 Crow peak, which is one of the most typical and is cited both 

 by Newton and Russell, is situated well up on the border of the 

 eruptive center. The uplifts known as the Needles, Terry peak 

 and Ragged Top (and the latter is more plug-like in its aspect 

 than any of the other masses that the writer has seen) are 

 directly in the center of the region most thickly seamed with 

 dikes and sheets. 



Again, we cannot explain the occurrence of these laccolitic 

 masses by the argument that, they are of a more acidic rock 

 than those which form the sheets and dikes in their immediate 

 vicinity, for these latter types of intrusives frequently range 

 well up over 70 per cent, in silica. In the case of the Ragged 

 Top mass, moreover, widespread, thin and markedly conform- 



