148 GEOLOGY. 



and its arms embracing the Hudson Bay basin, but the simile is not 

 very accurate. The distribution may be seen in truer form and in 

 greater detail on the accompanying map (Fig. 38), in which the white 

 areas indicate Archcan rock at the surface, the black areas Proterozoic 

 rocks, and the lined areas either Archean or Proterozoic rocks under- 

 lying later formations. As indicated above, the Archean rocks are 

 believed to underlie all later formations everywhere, but the lined 

 areas of the map are intended to represent only those portions that 

 are believed to have constituted a part of the continental platform, 

 and not improbably were exposed at some time as land. It is to be 

 understood that this map is no more than a rough approximation, 

 representative of the present stage of mapping and interpretation. 

 C-Osely related to this great Canadian area are tracts in Michigan and 

 Wisconsin, in Minnesota, and in the Adirondack region of New York, 

 though at least some of the rocks of this last region are probably 

 younger (Proterozoic). 



Lying rudely parallel to the great Canadian area on the southeast 

 is a broken series of probably Archean tracts extending from New- 

 foundland to Alabama. Similarly, on the southwest side there is 

 a belt of detached areas stretching from New Mexico to Alaska. In 

 few places within these belts have the ancient rocks been studied in 

 great detail. 



Greenland, so far as can be judged from its known borders, is 

 occupied by crystalline rocks of probable Archean age, and the same 

 may be said of large tracts imperfectly known on the west side of 

 Baffin bay. 



Partly because of the inherent difficulties, and partly because 

 of the inaccessibility of a large part of this great Archean tract, careful 

 detailed work has as yet covered but a small part of it. The vicinity 

 of Lake Superior in Canada, Michigan, Wisconsin, and Minnesota 

 (Fig. 39), the original Huronian area north of Lake Huron, and the 

 Ottawa region in Ontario, have been the classic areas of investigation . 



Local development. 1 — A definite conception of the character of the Archean 

 is best obtained by the study of its local development in typical regions that 



1 In addition to the references given in connection with the following paragraphs, 

 the Archean appears on the maps of the following folios of the U. S. Geol. Surv. : 

 Arizona, Globe and Bisbee; Colorado, Ten Mile, Anthracite-Crested Butte, Pueblo, 

 Walsenburg, and Pike's Peak; District of Columbia, Washington; Michigan, Menomi- 



