326 



closely with those of the Lake Superior region. This is true 

 chiefly because the necessary fossils are lacking. 



In the Grand Canon of Arizona Proterozoic strata are 

 again well exposed, but they are unlike the Lake Superior 

 formations in details. The lower walls of the canon reveal 

 the complex schists of the Archaean. Upon these rests un- 

 conformably a tilted pile of sedimentary strata (Fig. 324). 



FIG. 324. Ideal cross section of the Grand Canon of the Colorado River 



in Arizona. 



In spite of their great age they are neither folded nor notably 

 metamorphosed. These in turn were largely removed during 

 a still later period of erosion, so that the Cambrian sandstone 

 was deposited horizontally, not only upon the beveled edges 

 of the Proterozoic formations, but out over the Archaean also. 

 Proterozoic rocks are well known in Scotland, Sweden, and 

 China (Fig. 325), and have been studied in considerable 



FIG. 325. A section through the ancient rocks at a point in Northern 

 China, showing the Archeozoic rocks (A), overlain by a thick series of 

 folded beds of Proterozoic age (B), and upon both resting Proterozoic lime- 

 stone and shale (C), much less folded. The Cambrian rocks (D) rest un- 

 conformably on the others. 



detail. In each case there appear to be two or more systems 

 between the Archaean and the Cambrian, separated from 

 each by a pronounced unconformity. Where there are two 

 systems the older is usually intensely folded and metamor- 

 phosed, although still plainly made up of sedimentary rocks ; 



