144 GEOLOGY. 



formed not only before that series, but before the erosion which affected 

 the Archean prior to the deposition of the base of the Proterozoic. 



The diastrophism of the Archeozoic era. — It is probable that the 

 intrusion of the great granitic masses was in itself a notable factor 

 in the distortion which the schist series suffered, but it can hardly 

 be assumed that the development of the schistose structure in great 

 bodies of massive igneous rock was effected simply by the intrusion 

 of lavas. Much less can the crushing down of extensive bodies of 

 granite into gneiss be assigned to their intrusion in the molten state. 

 The simplest and most satisfactory explanation of the prevalent foliated 

 structure, of the interleaved igneous sheets and of the batholiths seems 

 to be that previously suggested (p. 130), which refers all such structures 

 chiefly to the shearing zone between the outer shell and the massive 

 interior. If this be not entertained, some help toward an explanation 

 is the conception that the extensive introduction of batholithic masses 

 in the earlier series of surface formations, resulting in much disruption 

 and distortion, would leave the whole in a condition well suited to 

 develop extreme irregularity and intricacy of structure, whenever 

 sufficient diastrophism was brought to bear upon the whole. Under 

 these conditions, the included bosses of granitic rock would be peculiarly 

 well situated for the development of the concentric schistose structure 

 which is a common feature of the Archean system. 



In still another way (p. 117) igneous action probably aided in the 

 deformation of the rocks. The transfer of so much material from 

 below, and its introduction into the outer parts of the earth, developed 

 lateral pressure, not only by the intrusion and by the heat accom- 

 panying it, but also by causing the outer parts to settle down to take 

 the place of the material transferred from below. The weight of the 

 lavas spread upon the surface tended to the same end. The natural 

 result would be lateral thrust in the outer parts of the earth, and this 

 thrust would be relieved by the bending and crumpling of the rocks 

 (Fig. 37), and by shear. Under the circumstances, this would be very 

 irregular and intimate. Superposed upon this were the general forces of 

 distortion which were operative with peculiar intensity in the earlier 

 stages of the earth's evolution, as previously indicated (p. 122). 



Metamorphism. — That the rocks should have undergone extraor- 

 dinary metamorphism under the conditions described is most natural. 

 It has been demonstrated that what were once massive igneous rocks 



