THE PROTEROZOIC ERA. 



173 



may completely cover the Proterozoic beds on one side of an area of 

 exposed Archean, while they fail to do so on the other (Figs. 54 and 55). 



Fig. 54. — Diagram showing how Proterozoic rock {AT) may outcrop on one side of 

 an area of Archean ( M ) and not on the other. 



Belts and patches of Proterozoic rock within areas of Archean (Fig. 56) 

 are often remnants left by erosion, and indicate the former extension 

 of the younger group beyond its present limits. 



The occurrence of Proterozoic formations with little surface associa- 



m-- 



Fig. 55. — Diagram showing in ground plan the relations expressed in section in Fig. 

 54. 2R, Archean; Al, Proterozoic; €*, Cambrian. 



tion with Archean, is illustrated in the southeastern part of South 

 Dakota and the adjacent parts of Minnesota (Sioux quartzite). 

 The Proterozoic rocks in this region may be presumed to lie on an 

 Archean floor, and an adjacent area of Archean, now mostly covered 

 by younger formations, may be supposed to have furnished the sedi- 



