444 GEOLOGY 



lies in Canada (Fig. 341), but it is to be noted that formations of 

 the Proterozoic and later eras occupy numerous small tracts within 

 the area shown on the map, though the Archean underlies them 

 at no great depth. Lying rudely parallel to the great Canadian 

 area on the southeast is an interrupted series of probable Archean 

 tracts, extending from Newfoundland to Alabama. Similarly, on 

 the southwest, there is a belt of detached areas stretching from 

 Mexico to Alaska. In few places within these belts have the 

 ancient rocks been studied in great detail. Lesser areas of Archean 

 rock appear in Michigan and Wisconsin, in Minnesota, and in the 

 Adirondack region of New York. In some of these regions, Arche- 

 ozoic rocks have not been carefully separated from Proterozoic. 



Detailed work has covered but a small part of this great tract 

 of Archean rock. The vicinity of Lake Superior in Canada, Mich- 

 igan, Wisconsin, and Minnesota, the area north of Lake Huron, 

 and the Ottawa region in Ontario, are the areas where the system 

 is best known. 



Summary. By way of summary it may be said that the Archean 

 system consists in some places of rocks which are mainly massive 

 (igneous intrusions); in other places, of rocks which are mainly 

 gneissic (chiefly meta-igneous) ; and in still others, of rocks (largely 

 meta-igneous and subordinately meta-sedimentary) in which a 

 schistose structure predominates. Furthermore, the rocks of each 

 of these structural types have a wide range in composition, from 

 acid on the one hand to basic on the other. Rocks of all these classes 

 are often intimately associated, and any one may predominate over 

 the others. In some places the rocks of the several structural types 

 graduate into one another so completely as to leave no line of sepa- 

 ration, while in others their definition is sharp. Thus massive 

 rock sometimes appears in distinct dikes in the gneisses and 

 schists, while schists are frequently in dike-like sheets in rocks which 

 are more massive. Furthermore, the relations of these several 

 sorts of rock have been enormously complicated by the distortion 

 to which they have been subject. The structure and relations of 

 the several sorts of rock in the system indicate that it was (1) by 

 successive intrusions, large and small, of rocks of different chemical 

 composition into (2) still older rocks which were originally (a) 



