44 



HISTORICAL GEOLOGY 



that though in many regions rocks may be confidently referred 

 to the Archean system, nevertheless, such rocks may really rep- 

 resent vast age differences within that system. 



Distribution of the Archean. — So far as known, Archean 

 rocks appear to be universally present at or below the earth's 



Fig. 22 

 Map showing the surface distribution of pre-Cambrian (Archeozoic and 

 Proterozoic) rocks in North America. Largest area shown by dotted 

 pattern; smaller areas by solid black. (Modified by W. J. M. after 

 Willis, U. S. Geological Survey.) 



