THE ARCHEOZOIC ERA 45 



surface. If this be true, and all evidence strongly favors such a 

 view, it is a most remarkable characteristic of the Archean, since 

 no other rock system has such a distribution. 



A rock formation may be so distributed in the earth's crust as 

 to be present (1) at the surface where mere superficial deposits, 

 such as mantle rock, glacial drift, etc., are disregarded; (2) under 

 cover of later rocks, but where its presence is certainly known from 

 surface observations, well borings, etc., and (3) under cover of later 

 formations, but where its presence cannot be definitely proved. 

 Considering all regions which have been geologically explored, 

 the Archean is estimated to appear at the surface over about one- 

 fifth of the land area of the earth. 



On the accompanying map (Fig. 22) the surface distribution 

 only of the Archean rocks in North America is shown. In the 

 Canadian region especially, some portions mapped as Archean may 

 really belong to the Algonkian or even later systems, because so 

 much of that area has not been mapped in detail. At any rate, the 

 map shows the greatest area of Archean in North America to be 

 around Hudson Bay. This vast area of fully 2,000,000 square 

 miles consists mostly of Archean. Among the principal smaller 

 areas are those of Newfoundland, New England states, Adirondack 

 Mountains, Piedmont Plateau district, Michigan, Wisconsin, 

 Minnesota, and numerous small areas in the Rocky Mountain 

 district (including Alaska) and westward. In drilling deep wells 

 in many places, particularly in the upper Mississippi Valley, rocks 

 of the Archean complex have been encountered, and so we may be 

 confident of the presence of Archean under cover of thousands of 

 square miles of later rocks. These facts of distribution, together 

 with the fact that wherever erosion has gone deep enough the 

 Archean never fails, leave little room for doubt concerning the 

 universal presence of the Archean in North America. 



Foreign Archean. — Judging by exposures along its borders, 

 Greenland appears to be largely occupied by Archean rocks. 



The Highlands of Scotland show one of the most clearly 

 exposed areas of Archean in the world, and detailed studies have 

 shown it to be remarkably like that of the Lake Superior 

 region. 



Scandinavia exhibits the largest area of Archean rocks in 

 Europe, and considerable study has shown the rocks to be very 

 similar to those of North America. 



