456 



GEOLOGY 



That the Proterozoic formations should be exposed most com- 

 monly about the borders of the Archean is made clear by Fig. 348, 

 which shows, in section, the general relations of the Pro'terozoic 

 systems (Al) to the Archean (^t) below, and to younger formations 

 () above. The same relations are shown in ground-plan in Fig. 

 349. While the relations shown in these diagrams are common, 

 there are areas of Archean not surrounded or bordered by exposed 

 Proterozoic formations, and areas of the latter not associated with 





Fig. 350. Diagram showing how Proterozoic rock (Al) may fail to outcrop 



about Archean 



Fig. 351. Diagram showing how Proterozoic rock (Al) may outcrop on one 

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



exposed Archean. Various relations of the two are illustrated by 

 Figs. 350 and 351. 



It is to be borne in mind that the map (Fig. 341) shows only 

 the exposed areas (so far as known) of the Archean and Proterozoic. 

 The Archean is presumably universal, beneath other formations. 

 The Proterozoic system is not universal, but its real extent is much 

 greater than the area where it appears at the surface. Thus the 

 Proterozoic beds of Wisconsin are probably continuous beneath 

 younger formations with the Proterozoic beds of southwestern 

 Minnesota, and these in turn with those of Missouri and Texas on 

 the south, and with those of the Black Hills and Rocky Mountains 

 on the west. 



THE PROTEROZOIC OP THE LAKE SUPERIOR REGION 



The Proterozoic formations have been most carefully studied 

 and their relations are best understood in the region about Lake 



