326 



PROTEROZOIC ERA 



unconformity between the Archean and the Proterozoic is probably 

 not universal, for there are doubtless places where the surface of the 

 Archean did not suffer notable erosion before the deposition of 

 Proterozoic sediments upon it. 



tSea. Z^eve/ 



fe^x u-xVJ- i-' ' '13 



V' r^^^/^'^^n^T^ 



^' ^ /^-^</^C -y-.-^v . ^//--r i \ ,^"J-^.= 



Fig. 296. Diagram showing Archean land (&) with sedimentation, a, along its 

 borders. (Compare Fig. 297.) 



Fig. 297. Diagram representing the same region as Fig. 296, after subsidence- 

 The a of this figure corresponds to a of Fig. 296. 



Subdivisions. No existing classification of the Proterozoic 

 formations has general application, but in the Lake Superior region, 

 where these rocks are best known, four great unconformable systems 

 are referred to this era. In some other regions the number is three 

 (Fig. 298), in others two, and in still others but one. In most places 

 each system is thousands of feet thick. These thick systems of 



Fig. 298. Diagram showing Proterozoic where it is composed of three systems 

 of rock in the Lake Superior region. H, Huronian; A , Animikean; K, Keweenawan. 

 The diagram also shows the relation of these Proterozoic systems to the Archean 

 (-.32) below and to the Cambrian () above. The cross-pattern represents igneous 

 rock. The lines, dots, etc., above the Archean represent sedimentary beds. 



