210 C. R. VAN 1 1 1 s J : — PRE-CAMBRIAN OF THE BLACK BILLS. 



The only mineralogies] difference mentioned between the two series is the 

 ater abundance of garnet and mica in the schists than in the slates, and 

 the rare occurrence of staurolite in the latter.* It so happens that the best 

 occurrence of coarse garnetiferous and staurolitic mica-schists which I know 

 are uorth of the main mass of granite — i. e., in the slate area as mapped by 

 Newton. In .-hurt, do evidence was found that there are two distinct pre 

 ( lambrian sn-ics in the Black Hills. r However, from my brief examination, 

 I would not venture to assert that there are not two or nioie, for 1 realize 

 that the true structure of such ancient crystalline and semi-crystalline rocks 

 can only he certainly determined, if at all, by the most detailed study : hut it 

 appears t" he a safe conclusion that the separation of these rocks into two 

 series upon Newton's basis and with his distribution is not warranted by the 

 tact- now at our disposal. 



Origin of the Granite. 



We now coiiie to a question upon which the various writers on the Black 

 Hills hold different opinions. 



. Newton maps a considerable area as solid granite. South of this are 

 found upon his map other detached areas of the same rock. It does not 

 appear, however, from his descriptions that he considers these areas wholly 

 of granite, hut that it i- predominant. This mapping has been criticised by 

 Crosby and Carpenter upon the ground that within these areas is found a 

 quantity of crystalline schists. This is unquestionably true, hut the fact 

 remains that about Harney peak is a verv considerable area which is practi- 

 cally solid granite, although within two miles from this point are found here 

 ami there patches of schist. The relation- a- I saw them are these: In 

 passing away from the central granitic area the schists appear included by 

 the granite. They become more abundant in receding from the central core, 

 until they are finally predominant The granite is then contained in the 

 schists in a series of veins or dikes, which often run in parallel directions. 

 For instance, near Custer City fifteen parallel ridges of granite were counted 

 from one point within a short distance. As the granite core becomes more 

 distant tin- ridges become less ami less prominent and of smaller size, and 

 finally disappear. While no such ruck is mentioned by Newton as occurring 



in the slate area -th of the granites, ridges of it are found here as elsewh< rt 



about the main area. 



Newton, in discussing the origin of the granite, states thai it often contains 

 irregular fragments of schist -some of small, 3ome of great Bize. He finds 



i I 



• ■- in the pre Cambrian an urn-. I to Pro 



15 him if ii" had evei irdance between 



• had not, and thai - itimes he doubled whethei there 





