THE ARCHEOZOIC ERA. 155 



was the more altered. 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) 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. 



While, therefore, the variations in the rock of the Archean complex 

 are great, there is, nevertheless, a certain homogeneity in the heterogene- 

 ity 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, however much folded and metamorphosed. 

 So similar are the rocks of the Archean throughout the various areas 

 where the system occurs, that a suite of unlabeled specimens from one 

 region could hardly be asserted not to have come from any other. 1 

 This striking similarity affects not only the general features of the 

 rocks concerned, but also many of the details of their structure and 

 composition. The minerals of which they are composed everywhere 

 (broadly considered) give evidence of having been subject to great 

 dynamic action. They are often broken and distorted, and microscopic 

 study reveals the effects of dynamic action in many cases where it is 

 not visible to the unaided eye. The mineralogical and chemical con- 

 stitution of the series, taken as a whole, appear to be essentially con- 

 stant, in the sense that the variations in any one area are very much 

 the same as those in any other. 



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

 If any accessible system of rocks can be made to throw light on the 

 origin and early history of the earth, it is the Archean. 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 

 1 Van Hise, Bull. 86, U. S. Geol. Surv., p. 476. 



