450 



GEOLOGY 



Though unconformity between the Archean and the Protero- 

 zoic is wide-spread, it is presumably not universal. There were 

 probably places, even on the land-areas, where the surface of the 

 Archean did not suffer notable erosion before the deposition of 

 Proterozoic sediments upon it, and there were quite certainly such 

 places in the areas continuously covered by the sea. 



Subdivisions. No 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. 344. Diagram showing Proterozoic where it is composed of three sys- 

 tems of rock in the Lake Superior region. H, Huronian; A, Animi- 

 kean; K, Keweenawan. The diagram also shows the relation of these 

 Proterozoic systems to the Archean (^) below and to the Cambrian 

 (-) above. The cross-pattern represents igneous rock. The lines, 

 dots, etc., above the Archean represent sedimentary beds. 



(Fig. 344), in others two and in still others but one. In most places 

 each system is thousands of feet in thickness, but, in spite of this, 

 they do not constitute a complete record of the era. The uncon- 

 formities between them show that, after the formation of each, 

 there was a disturbance of relations between the sources of the 

 sediments (the lands) and the sites of their deposition (cliiefly the 

 sea). Each unconformity appears to mark a prolonged period of 

 erosion in the region where the formations are exposed, and depo- 

 sition somewhere else. The only record of these periods is the un- 

 conformities themselves. 



Proterozoic sedimentation. The surface of the Archean on 

 which the Proterozoic sediments were deposited was probably 

 comparable to existing land surfaces of crystalline rock which have 

 been long exposed to weathering and other phases of erosion. The 



