90 



GEOLOGY OF THE BLACK HILLS. 



color is grayish or sometimes reddish. It is not a persistent stratum, but is 

 devoloped only in the southern and southeastern parts of the Hills. 



The relation of the quartzite in the canon of French 

 Creek to the other members of the Potsdam and to the 

 underlying rocks is shown in Figure 12, where the fol- 

 lowing strata are exposed. 



Carboniferous. 

 7. White limestone. 

 G. Impure, pink, shaly and arenaceous limestone with crinoidal 



stems. 



Potsdam. 



t>. 



Fig. 12. — Section of Pots- 

 dam sandstone on 

 lower French Creek, 

 with underlying Arch- 

 aean rocks. 



Fine, shaly, calcareous, reddish sandstone, containing green 



glauconitic particles and some mica. 

 Yellowish or grayish quartzite, preserving somewhat the lines 

 of stratification but with cracks or fractures perpendicular 

 thereto. 

 Coarse yellowish and red sandstone with Potsdam fossils ; 

 slicken sided in places. 

 2. Hard yellowish quartzite, with quartz pebbles. Has a vertical cleavage. Dip 3° 

 to 5° east. 



Archaean. 



1. Coarse, red, feldspathic granite, at top very much decomposed and changed into a 

 soft clay, almost like a fluccan ; associated in other places near the creek with 

 schists, hornblende rock, quartzites, etc., dipping to the southeast at a high angle. 



At various places in the Rocky Mountains quartzites are found to oc- 

 cupy persistently a similar relation to the Archaean, and have been referred 

 by various observers to the Potsdam formation. Thus on the mountain 

 ranges of the Gallatin and Madison forks of the Missouri and elsewhere 

 in the region of Yellowstone Lake, the Teton Range, and the Shoshone 

 Mountains, quartzites lie unconformably on the metamorphic rocks, and 

 are referred by Hayden, Peale, and Bradley to the Potsdam.* 



They are found also in the Bighorn and in the Wind River Mountains, 

 where Comstock describes a thick formation of compact red sandstone, 

 and refers it to the Potsdam.f 



The Wasatch Mountains exhibit a great thickness of quartzite- — 1,500 



* Geological Survey of the Territories, 1872. pp. 25, 63, 163, etc. 



t Northwest Wyoming and Yellowstone National Park. Jones, 1873. p. 107. 



