THE PROTEROZOIC ERA. 



175 



believed to be very great, much greater than the area where they are 

 exposed. 



The Pkoterozoic of the Lake Superior Region. 



The Proterozoic formations have been most carefully studied and 

 their relations are best understood in the region about Lake Superior, 



Fig. 59. — Diagram showing the general relations of the three Proterozoic 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 

 (At) below and to the Cambrian (■€) above. The cross-pattern represents igneous 

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



and the formations of this region have been made the basis for the 

 classification of the Proterozoic group as a whole. The Proterozoic rocks 

 of this region are of great thickness, and, as already indicated (p. 165), 

 they are divisible into at least three great unconformable systems, the 

 relations of which to one another, to the Archean below, and to the 

 Cambrian above, are shown diagrammatically in Fig. 59. 



During at least some portion of the Proterozoic era there was prob- 

 ably an island of Archean rock in northern Wisconsin. North of Lake 

 Superior there is a much larger area of Archean rock not known to have 

 been submerged during this period. So far as present knowledge goes, 

 these land-areas may have been sometimes united as a result of the ele- 

 vation, relative or absolute, of the intervening areas. These land-areas 

 were the chief sources of the Proterozoic sediments deposited in the 

 Lake Superior region. 



The Huronian system. 

 (Lower and Middle Huronian of page 161.) 



Composition and thickness. — The rocks of this system are chiefly 

 sedimentary. They include all the common sorts of clastic rocks or 



