318 



HISTORICAL GEOLOGY 



The downward limit of the Archaean rocks is unknown ; 

 probably it will never be attained. The upper limit is the 

 surface which separates it from the Proterozoic group. 1 



Complexity of the Archaean. The study of the Archaean 

 rocks shows that they have been profoundly disturbed and 

 their original form greatly altered. They have been folded, 

 crumpled, and contorted in the most intricate way. They 

 are broken by faults and interrupted by masses of igneous 

 rocks which have been intruded into them. Great batho- 



liths of granite 

 are so common 

 in them as to be 

 almost character- 

 istic of the Ar- 

 chaean system. 

 Study of the in- 



FIG. 318. Successive intrusions of igneous rock. tril<J : nTlc rAwnls 

 How do the intrusions rank in order of age ? 



the fact that 



some have broken through others, showing that they are of 

 many different ages (Fig. 318). Many of these structures 

 have been folded, and broken, since the volcanic activity 

 ceased. 



The known Archaean rocks have been greatly changed. 

 In an earlier chapter (pp. 67, 77) it has been said that un- 

 der the tremendous weight of miles of overlying rock even the 

 strongest materials may be crushed, squeezed out into thin 

 plates, and crumpled like leaves of paper. At the same time 

 certain portions of the rocks are dissolved bit by bit and 

 rearranged into new minerals which are better suited to the 

 conditions of high pressure and temperature of the depths. 

 Thus, black basalts become glistening green schists, granites 

 may become gneisses, limestone crystallizes in the form of 

 marble, and shales are transformed into schists spangled with 

 flakes of mica. 



1 By some geologists Archaean is made to include all the rocks below the 

 Cambrian, but this usage is not common to-day. 



