BLACK HILLS GEOLOGY. 197 



tail and Nevada gulches. On the narrow divides, between the 

 gulches, are found the basal quartzites of the Cambrian. The 

 slates appear again still farther up Nevada gulch in a large ex- 

 posure and have evidently been raised to a higher elevation by 

 a fault whose down-throw is toward the mouth of the gulch. 

 Thence the line of contact passes around the northeastern slopes 

 of Bald mountain and across the headwaters of Deadwood 

 gulch. It then bends out to the west of a large quartz por- 

 phyry mass into the head of East Squaw creek. Thence it 

 crosses the divide to the west of this stream into the valley of 

 Squaw creek itself. The exposure in the bed of this stream is 

 a dense, fine-grained, greenish amphibolite which extends far to- 

 ward the northwest and eventually disappears beneath the west- 

 wardly dipping Cambrian quartzite. Other exposures of am- 

 phibolite occur in the form of dikes, which are conformable to 

 the slates and are present all along White-tail gulch. The gar- 

 netiferous schists are best exposed below Central City, in Dead- 

 wood gulch. The dip of the schistosity of the Algonkian is 

 nearly vertical throughout, but such inclination as may be de- 

 tected in a large number of observations, seems to be to the 

 east. 



The term Algonkian is substituted for Archean because, with 

 the exception of the amphibolites, the series is of undoubted 

 sedimentary origin. That the statement is true is shown by the 

 development of slaty cleavage at an angle to the original bed- 

 ding. These relations are very marked in many localities. A 

 photograph of slate taken from the De Smet cut will be seen 

 in Plate VII. As further proof Professor Crosby ^ has mentioned 

 metamorphic conglomerates which occur near Galena and Pro- 

 fessor Van Hise has still further mentioned slaty cleavage cut- 

 ting the original sedimentary banding. There is then no question 

 that the slate series originally consisted of mechanical sediments 

 which have attained their present crystalline condition through 

 the agency of metamorphism. Whether the schists will like- 

 wise prove to be referable to the Algonkian is, as yet doubtful. 

 It will depend upon the validity of the two-fold grouping of the 



1 Proceedings of the Boston Society of Natural History, XXIH, 494 1888. 



