320 ARCHEOZOIC ERA 



tural types graduate into one another so completely as to leave no 

 line of separation, while in others their definition is sharp. Thus 

 massive rock appears in distinct dikes in gneisses and schists in 

 some places, while in others schists are in dike-like sheets in rocks 

 which are more massive. Furthermore, the relations of these sev- 

 eral sorts of rock have been complicated greatly 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 (i) 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 

 (6) subordinately sedimentary; and (3) by successive dynamic 

 movements resulting in various degrees of metamorphism and de- 

 formation 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 yet a certain homogeneity in the heterogeneity of the 

 whole. No large part of the system is very different from any other 

 large 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 

 younger meta-sedimentary rocks. 



Earlier views concerning the Archean. In explanation of the Archean 

 system, many hypotheses have been suggested at one time and another, most of 

 them starting with the Laplacian hypothesis as a beginning. One of them is that 

 the Archean rocks are wholly of metamorphosed sediments, a second, that they are 

 igneous rocks produced by the fusion of sediments, and a third, that they are 

 igneous rocks intruded beneath the oldest sedimentary rocks after the deposition 

 of the latter. These hypotheses have historic interest, but are not now generally 

 held by geologists. 1 



DISTRIBUTION 



In speaking of the distribution of a formation, its distribution 

 at the surface generally is meant, and in speaking of its surface 

 distribution, the mantle rock (glacial drift, etc.) which overlies 

 and conceals it is usually ignored unless it is so thick as to make 

 the underlying formation indeterminable. When the surface dis- 

 tribution of, the formation is given, therefore, it is not to be under- 

 stood that the formation is literally at the surface everywhere within 

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



1 See the authors' Earth History, Vol. II. 



