1888.] 499 [Crosby. 



range in thickness from a foot or less, to perhaps five hundred feet. 

 They are often branching and sometimes enclose masses of schist. 

 Being much harder than the schists, the granite masses have been 

 left in high and rugged relief by the agents of erosion. Over an 

 area of many square miles about Harney Peak, the granite rises 

 above the schists from one hundred to five hundred feet in precip- 

 itous and even wall-like, craggy ridges, which are transversely cleft 

 to form gigantic saw-teeth or jutting towers and pinnacles, the sum- 

 mits of which are often quite inaccessible. The effect of this topog- 

 raphy is not only to make the Harney Range exceedingly difficult 

 to traverse ; but also largely to conceal the intervening schists and 

 gneiss beneath the debris of the granite. It may have been this 

 circumstance that led Newton to underestimate the importance of 

 the stratified rocks in this area. 



Newton's discussion of the origin of the granite is directed 

 chiefly to showing that it can not be regarded as metamorphic, 

 as having been derived from the associated schists and gneiss, 

 and that it must, therefore, be eruptive. We may, however, 

 readily grant the first conclusion without accepting the second ; 

 for it seems not to have occurred to Newton that there is a third 

 alternative. But the following description of the granite, con- 

 densed, with only slight verbal changes, from his pages, will, it is 

 believed, suffice to convince every student of the crystalline rocks 

 of New England that the granitic masses of the Black Hills are 

 not essentially unlike the great segregated veins intercalated 

 with our Montalban schists and gneisses, and which are the 

 source in many localities of coarsely crystalline mica, quartz and 

 feldspar for commercial uses. 



" The granite of the Black Hills varies but little in character. 

 Its texture is so extremely coarse that it would scarcely be rec- 

 ognized as granite by one accustomed to the fine-grained varieties 

 of New England or the Rock}' Mountains. It is granite on a 

 large scale, with all the elements of that rock — feldspar, quartz 

 and mica — present, but instead of their being mixed with toler- 

 able uniformity throughout the mass each constituent is very 

 highly crystalline and aggregated by itself. Feldspar forms 70 or 

 75 per cent of the rock, and presents large crystal faces and cleav- 

 age masses. The quartz is commonly glassy and clear, but many 

 large masses are of a delicate pink or rose color. It composes 

 about 20 per cent of the granite, and often penetrates the feld- 



