228 GEOLOGY OF THE BLACK HILLS. 



action on a layer of the schists, but the granite ridges often occur travers- 

 ing the park as a broken range of lenticular peaks on the same line of 

 fracture, resembling, on an enormously-magnified scale, a well-known 

 form of segregated quartz-veins. Narrow dikes of granite tra\'erse the 

 schists, often not more than 2 or 3 feet in width and of regular thick- 

 ness, continuing uniformly for several hundred yards parallel to the stratifi- 

 cation, and being harder than the schists, have been left by erosion stand- 

 ing like walls several feet above the surface of the ground. Where the 

 surface of contact of the granite and the schists is exposed, the sides of the 

 dike are seen to be smooth surfaces, often beautifully marked with slicken- 

 sides exceeding in the polished and striated markings any specimens of the 

 walls of mineral veins which have ever come under my notice. 



The sides of the lenticular masses of granite forming the low ridges 

 show slickensides and vertical striated surfaces, but not nearly in so marked 

 a degree as the narrow dikes. 



Tlie granite is very coarsely crystalline, largely composed of wliite or 

 flesh-colored feldspar, intermixed with quartz and mica (muscovite), with 

 black tourmaline in large hexagonal crystals as an associated mineral. 

 Quartz occurs disseminated through the mass of the feldspar in small grains 

 and in nodules between the crystals. Seorrefjated veins and irrejrular 

 masses of quartz occur in the granite ridges, generally white, vitreous, or 

 translucent, frequently of a beautiful deep rose-color, but everywhere appar- 

 ently free from any traces of valuable minerals or any considerable quan- 

 tity of gold. 



The schistose rocks are in great variety, mica-schist being the prevail- 

 ing form, often garnetiferous, and merging into talcose, chlorite, and quartz 

 schists. 



The minerals found in the schists are few in number. Small garnet- 

 crystals are very abundant; a few specimens of hornblende, epidote, and 

 staurotide were seen, but they occur but rarely in these rocks. Veins and 

 ledges of ferruginous quartz are found traversing the schists between the 

 granite ridges or dikes to which they are parallel. These ledges are not 

 true fissure-veins ; that is, they do not intersect the strata at an angle to the 

 stratification, neither are they segregated veins, but occur filling continuous 



