THE PROTEROZOIC ERA 



455 



the waters was extracted and deposited. Figs. 346 and 347 show 

 sections of the Proterozoic rocks in the Lake Superior region. 



The inference that these ancient sediments were deposited in 

 the same manner as the sediments of modern times is supported 

 by the ripple- and other shallow-water marks on the sandstones and 

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

 similar to those of sediments now being deposited. 



Fig. 348. Diagram showing a common surface relationship between Ar- 

 chean (JR), Proterozoic (al), and Cambrian (). The Proterozoic 

 (Algonkian) formations appear at the surface between younger and older 

 formations. 



Fig. 349. Diagrammatic map showing the formations shown in section in 

 Fig. 348. 



Geographic relations of the exposed Proterozoic to the Archean. 



Proterozoic rocks appear at the surface in many parts of North 

 America, but in few regions have they been clearly separated from 

 the Archean, and in very few have their subdivisions been worked 

 out. Fig. 341 shows the area where rocks of know T n Proterozoic 

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

 differentiated from the Archean. In many places, the Proterozoic 

 rocks at the surface are near areas of exposed Archean. 



