146 GEOLOGY. 



the whole surface of the globe. No later system does this. Wherever 

 the Archean comes to the surface all later formations are absent, 

 and some of them are absent elsewhere. In speaking of the distribu- 

 tion of a formation, it is generally understood, unless otherwise indi- 

 cated, that its distribution at the surface is meant, and in speaking of 

 its surface distribution, the mantle rock, glacial drift, etc., which 

 overlie and conceal it are usually ignored, unless they are so deep 

 as to make the underlying formation indeterminable. When the 

 surface distribution of the formation is given, therefore, it is not to 

 be understood that the formation is literally at the surface everywhere 

 within the area specified, but that it is actually exposed here and there 

 within that area, and that between the points of exposure it is the 

 uppermost formation beneath the mantle rock. In this sense the 

 Archean rocks are estimated to appear at the surface over about one 

 fifth of the area of the land; but since much of Asia, Africa, and South 

 America, and the less accessible portions of other grand divisions have 

 only been reconnoitered geologically, this figure has only the value of 

 a general estimate. 



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

 in Canada. It occupies nearly the whole of the Labrador peninsula, 

 and stretches thence in a broad tract southwestward to the line of 

 the Great Lakes, and thence northwesterly to the Arctic ocean, east 

 of the Mackenzie valley. This general statement is to be qualified by 

 observing that formations of the Proterozoic era occupy numerous 

 tracts within this area, and that many of them have not been separated 

 from the Archean. A few small areas of still later formations overlie 

 the Archean at certain points within this tract, but as the Archean 

 underlies such formations at no great depth, the general statement 

 made above is not seriously incorrect. This Canadian area is often 

 likened to a great "'V" or "U" with its apex resting on Lake Superior, 



1 The literature on the Archean (as well as Algonkian [Proterozoic]) of North 

 America was summarized by Van Hise in Bull. 86, U. S. Geol. Surv., in 1892. This 

 publication gives full bibliography to its date. A later (1896) and briefer summary 

 of the same subjects by the same author was published, Pt. II, 16th Ann. Rept., U. S. 

 Geol. Surv., pp. 744-843. The pre-Cambrian literature since 1892 has been sum- 

 marized from time to time by the same author, and by Leith, in the Jour, of Geol., 

 as follows: Vol. I, pp. 304 and 532; II, pp. 109 and 444; III, pp. 227 and 709; IV, 

 pp. 362 and 744; VI, pp. 527, 739, and 840; VII, pp. 190, 406, 702, and 790; VIII, p. 512; 

 IX, pp. 79 and 441; and XII, pp. 63 and 161. 



