206 



GEOLOGY. 



Archean, so far as surface exposures show, and in the latter but little. In both 

 localities there is igneous rock associated with the sedimentary. Locally the 

 quartzite has been metamorphosed by dynamic action into quartz schist l (Fig. 

 81). In these places, and especially in Wisconsin, the superior hardness of the 

 quartzite, as compared with the surrounding Cambrian sandstone, has led to the 

 development of striking topographic features. Post-Cambrian erosion, by re- 

 moving the later formations, has again exposed the pre-Cambrian monadnocks 

 of Baraboo quartzite, much as they were before being buried beneath the later 

 sediments. 



Fig. 80. 



-The Palisades, Minnehaha County, S. D. Sioux quartzite. 

 la. Geol. Surv.) 



(Photo, by Bain 



Rocks of probable Proterozoic age are also known in the Black Hills of South 

 Dakota, 2 making the core of the mountains, and occupying an area about 60 

 miles long by 20 broad. They consist of closely folded meta-sedimentary rocks, 

 such as mica slates, quartzites, schist-conglomerates, ferruginous chert, mica 

 schists and gneisses, and intrusive masses of granite and basic rocks. From the 

 granite batholiths, the largest of which is eight or ten miles long and nearly as 

 broad, numerous dikes penetrate the clastic beds. The latter afford good illus- 



*Van Hise, Jour, of Geol., Vol. I, pp. 347-355. 



2 In addition to general references, see Jaggar, 21st Ann. Rept., Pt. Ill, U. S. 

 Geol. Surv. 



