446 GEOLOGY 



chiefly extrusive-igneous and of varying chemical composition, 

 but (b) subordinately sedimentary; and (3) by successive dynamic 

 movements resulting in various degrees of metamorphism and 

 deformation of the various parts, that the intricate structure and 

 composition of the Archean complex was attained. 



Though the variations in the rocks of the Archean system are 

 great, there is, nevertheless, a certain homogeneity in the hetero- 

 geneity of the whole. No one considerable part of the system is 

 very different from any other considerable part, and no definite 

 and orderly relationship between the different parts has been made 

 out over any considerable area. There appears to be no traceable 

 succession of beds, and no definite stratigraphic sequence, such 

 as can be made out in great series of meta-sedimentary rocks, how- 

 ever much folded and metamorphosed. 



The Archean in other countries. The general characters and 

 relations of the Archean in North America seem to be duplicated 

 in other continents. Corresponding systems of rocks, made up 

 primarily of meta-igneous rocks, but subordinately of meta-sedi- 

 mentary rocks inextricably involved with them, are known in all 

 continents. The general characteristics and relations of the 

 Archean, as developed in North America, therefore appear to be 

 essentially world-wide. 



Bearing of the Archean on the theory of the origin of the earth. 

 With the essential facts concerning the constitution and structure 

 of the system in mind, it is in order to inquire to what hypothesis 

 of the earth's origin they best adjust themselves. The constitu- 

 tion of the system makes it clear that it does not represent the 

 original crust of the earth or its downward extension. It cannot 

 be affirmed, however, that no part of what is now classed as Archean 

 is referable to the original crust; that is, it cannot be affirmed that 

 no part of the igneous or meta-igneous rock of the Archean is 

 referable to an azoic or prezoic period, strong as the evidence 

 against such reference may seem. On the other hand, all the facts 

 now known concerning the Archean adjust themselves to the 

 planetesimal hypothesis, or to the modified form of the gaseo- 

 molten hypothesis. They cannot, however, be said to establish 

 either, or to preclude other hypotheses of the origin of the earth. 



