THE ARCHEAN ROCKS 321 



that area, and that between the joints of exposure it is the upper- 

 inost formation beneath the mantle rock. In this sense, the Archean 

 rorks an* estimated to appear at the surface over about one-fifth of 

 tlu' area of the land; but since great areas in some continents have 

 been reconnoitered only, geologically speaking, this figure is only 

 a rough estimate. 



Concerning the real, as distinct from the surface distribution, the 

 Archean is the one accessible rock system which, theoretically, 

 envelops the globe completely. No later system does this, for 

 wherever the Archean comes to the surface, later formations are 

 necessarily absent. 



In North America, 1 by far the largest area of Archean rock is 

 in Canada (Fig. 295). 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 probably-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 

 detail. Lesser areas of Archean rock appear in northern Michigan, 

 Wisconsin, and Minnesota, and in the Adirondack region, but in 

 some of these places, Archeozoic rocks have not been carefully 

 separated from Proterozoic. The vicinity of Lake Superior in 

 Canada, Michigan, Wisconsin, and Minnesota, the area north of 

 Lake Huron, and the Ottawa region in Ontario, are the areas where 

 the system is best known. 



The Archean system contains some iron ore (p. 332) and some 

 ores of other metals, but not as a rule of great richness. Gold 

 especially is widespread 2 , but in few places is it known to occur in 

 workable quantities. 



/// other countries, the general characters and relations of the 

 Archean of North America seem to be duplicated. A corresponding 

 system of rocks, made up primarily of meta-igneous rocks, but 

 subordinately of meta-sedimentary rocks inextricably involved 

 with them, is known in all continents. The general characteristics 

 and relations of the Archean therefore appear to be world- wide. 



1 Van Hise, Pt. II, i6th Ann. Kept., U. S. Geol. Surv., pp. 744-843, and 

 Van Hise & Leith, Monograph LII. 

 2 Op. cit., p. 295. 



