186 Proceedings, 



the stratigraphy and structure of the Archaean rocks of that region, and to 

 get at the genesis of the crystalline schists there exposed.* Only the most 

 salient points on the geology of the Hills were noted. 



1. The physical characters of the region were briefly outlined, and 

 the area examined during the visit was pointed out. Specimens of the rocks 

 constituting the central core of this great uplift were exhibited and their 

 field relations to each other were stated, viz : the rocks beneath the Cam- 

 brian sandstones and conglomerates which apparently entirely surround the 

 central, Pre-Cambrian, core consist of coarse sandstones and ©onglomerates 

 which, toward the north and the south, in the neighborhood of eruptive 

 rocks, become genuine crystalline schists, the northerly ones somewhat 

 sericitic. 



2. Therefore the crystalline schists of the southwestern portion of the 

 Hills are doubtless the same formations which in the northern Hills are 

 conglomeratic and sericitic with some ferruginous bands, the change from 

 conglomerates to crystalline schists being brought about by the heat, pressure, 

 displacement and lesser movements contemporaneous with the outflow of 

 the granites. This view naturally removes the necessity of regarding these 

 rocks as separable " into a western series or group of schists and an eastern 

 series or group of schists."t 



3. So far as the speaker's observations went there occur no true Lau- 

 rentian granites in the Black Hills. The granites occurring in such enor- 

 mous quantities in the Southern Hills are of eruptive origin. The strike 

 and dip phenomena so conspicuous around the borders of the Harney Peak 

 core of granite, that are respectively parallel to and away from the borders 

 of the granitic mass, could not well be explained in any other way than by 

 assuming the eruptive origin of the core. 



4. The tin ore, cassiterite, of the granitic area Avill probably never be 

 found to occur in paying quantities save in occasional placers into which the 

 ore of removed areas of granite has been collected, or in occasional lumps of 

 granite (pegmatites) having a larger quantity of the ore than the average of 

 the granitic mass — tbis fact, of the non-existance of the cassiterite in pay- 

 ing quantities, being due to the igneous origin of these rocks and the con- 

 sequent non-segreijafced condition of the ore in vast masses of the carrying 

 rock.J 



♦Bulletin Geol. Soe. Am., Vol. i, pp. 203-244, contains an admirable paper by Pro- 

 lessor Van Hise, entitled " The Pre-Cambrian Rocks or the Black Hills," read December 

 28, 1889. 



tSee Newton and Jenney,;" Report on the Geology and Resources of the Black Hills of 

 Dakota," p. 50. 



JCompare in this connection a paragraph by Professor Van Hise, on p. 211 , of the 

 paper already cited. 



