212 C. R. VAN FIISE — PRE-CAMBRIAN OF THE BLACK HILLS. 



schists are due to the metamorphosing effects of the modern volcanics them- 

 selves, or to the existence at do greal depth of a mass of granite like that al 

 Harney peak, as possibly indicated by the presence of fragments similar 

 to it iii the newer intrusives. 



A.GE OF THE GR V.NITE. 



The relations of the granite to the schists in the southern hills Buggesl the 



■ 



possibility thai its intrusion attended the present Black Hills uplift. How- 

 ever. Newton, in discussing the age of the granite, showed that this could 

 not be the case. He found at the French creek section (I use bis words 



that— 



•A continuous sheet oi the Potsdam passes from a surface of eroded schists to a 

 surface of granite. There was found no intrusion of the granite along the parting 

 between the Potsdam and the schists, and there was found no metamorphism of the 



Potsdam nt the surfai f contact with the granite. In these particulars the relations 



of the granite are strongly contrasted with those of the trachyte of the Hills. 

 Wherever the trachyte appears beneath the Potsdam the latter is uplifted as though 

 by the insertion of the trachyte between it and the Archean, and its lowest beds arc 

 at tic- Bame time metamorphosed as though by the heat of the molten intrusion. The 

 fact tliat tlio -Tanitc did not at this locality affect the- form and constitution <>t' the 



D *■ 



•dam strata in a manner similar to the trachyti i well accord with the idea 



that it was introduced under similar conditions and during the sane geological 

 period." 



Also, he discovered feldspathic debris, which apparently came from the 

 granite, in the basal conglomerate of the Potsdam sandstone. Prof — r 

 Headden mentioned similar phenomena in the vicinity of Hay ward, on Battle 

 creek. He kindly undertook to re-examine the locality for me, and from 

 his account the following is taken : At the first exposure below Hayward, 

 ( lambrian rocks are found to rest upon schists and " granite lenses or dik< s." 

 Ajb to the next exposure below, he Bays thai there can be no question thai 

 the Potsdam is unconformable to the schist nor that it rests upon the granite, 

 " for here a large mass of granite is covered for perhaps more than a hun- 

 dred feel by the conglomerate, and the same i< to be seen in several pla 

 on a .-mailer scale." Further, Professor Headden finds in the Potsdam con- 

 glomerate above Hayward, besides quartz, mica, and feldspar, rather abun- 

 dant crystals of tourmaline. Since no crystals of this mineral, except of 

 minute size, have been found anywhere but in the granite, this is additional 

 proof thai this rock has furnished detritus for the Cambrian basal 

 conglomerate. 

 The foregoing evidence is conclusive aa to the pre-Cambrian age of the 

 anite. The zone of schists about it was then developed and deeply eroded 

 re the I" ginning of Paleozoic time. 



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