RELATIONS OF GRANITE ASD SCHISTS. 231 



derived also yield mien abundantly. In the slates and schists of the Black 

 Hills these three minerals occur together, but it has been seen that much, if 

 not all, of the mica is authigenic. The specific gravity of quartz and feld- 

 spar differ but little, and it requires very favorable natural conditions to 

 perfectly separate these minerals. That these peculiar conditions not in- 

 frequently maintain for a time is shown by the interlamiuations of pure 

 quartzose sediments and those composed of quartz and feldspar, which is 

 found in many localities. Mica, while having a higher specific gravity than 

 quartz or feldspar, usually floats longer than particles of the same weight 

 of these minerals, because of its ready separation into thin flakes, and 

 may thus be carried to more quiescent deeper water. The conditions of 

 sedimentation in the Black Hills have ordinarily been such that the mica 

 has been separated from the quartz and feldspar, while in certain layers 

 represented by the quartzites the quartz has been practically freed from 

 other minerals. It often happens in other localities that, mingled with quartz- 

 feldspar detritus, is a good deal of clastic mica. The original character of 

 the ferruginous quartz-rocks will not be discussed. They are regarded as 

 chemical or organic sediments, or both combined. 



Bearing of Microscopical Study upon the Origin of the Granite. 



Recurring to the question of the origin of the granite, it would seem that 

 the foregoing microscopic study of the crystalline schists affords additional 

 indication that it is in the main eruptive. It will be remembered that within 

 the central granite mass are contained areas of the schists which appear as 

 though they had been caught in an eruptive rock ; that the schists form a 

 zone about the grauite, striking parallel with and dipping away from the 

 main mass ; and that radiating from the Harney peak core are numerous 

 dike-like ridges which become less frequent and smaller in size as it is receded 

 from. The lithological study shows that the schists become coarser, more 

 foliated, and much more crystalline adjacent to the granite, and also that 

 here are abundant garnet, staurolite, and tourmaline, minerals which are 

 very often produced by the contact of an eruptive with a sedimentary rock. 

 Upon the hypothesis that the granite is eruptive and the cause of the present 

 structure of the surrounding crystalline schists, not only the distribution of 

 the latter is explained, but the peculiar relations of the slates and schists 

 themselves. The strike of the slates is in general in a north and south di- 

 rection. The schistose structure parallel to and north of the grauite is trans- 

 verse to this ; but it has been seen that the slates grade into the schists. Be- 

 ginning with the slate area and passing toward the grauite area to the south, 

 the slaty cleavage in a north and south direction becomes less and less promi- 

 nent. After a time a schistose structure is found cuttiug across the slaty 



