33 



PROTEROZOIC ERA 



the ripple- and other shallow-water marks on the surfaces of the 

 layers, and by their lamination and stratification, all of which are 

 similar to those of sediments now being deposited. 



Geographic relations of exposed Proterozoic and Archean. 

 Proterozoic rocks appear at the surface in many parts of North 

 America, but they have been clearly separated from the Archean in 



Fig. 302. Diagram showing a common surface relationship between Archean 

 (JR), Proterozoic (A I), and Cambrian (-). The Proterozoic formations appear at 

 the surface between younger and older formations. 



few regions. Fig. 295 shows the area where rocks of known Proter- 

 ozoic age lie at the surface, together with areas where they have not 

 been differentiated from the Archean. In many places, the Protero- 

 zoic rocks at the surface are near areas of exposed Archean. 



That the Proterozoic formations should be exposed most com- 

 monly about the borders of the Archean is made clear by Fig. 302, 



which shows, in section, the 

 general relations of the Prote- 

 rozoic systems (Al) to the 

 Archean (^R) below, and to 

 younger formations (} above. 

 The same relations are shown 

 in ground-plan in Fig. 303. 

 While the relations shown in 

 these diagrams are common, 

 there are areas of Archean not 

 surrounded or bordered by ex- 

 posed Proterozoic formations, 

 and areas of the latter not as- 

 sociated with exposed Archean. Various relations of the two are 

 illustrated by Figs. 304 and 305. 



Fig. 303. Map of the formations 

 shown in section in Fig. 302. 



It is to be borne in mind that the map (Fig. 295) shows only the exposed areas 

 (as now known) of Archean and Proterozoic. The Archean is presumably uni- 

 versal, beneath other formations. The Proterozoic is not universal, but its extent 

 is much greater than the area where it appears at the surface. Thus the Proterozoic 



