REPORT OF THE DIRECTOR 1920-21 /7 



In regard to a possible computation of the crustal shortening 

 expressed in the Precambrian folds, this master of analysis states: 

 " No one, so far as I know, has thus far had the temerity to offer an 

 estimate of the amount of shortening implied by the intricate 

 crumpling of these old formations on any great circle of the earth. 

 That it was large, however, goes without the saying." 



Like Chamberlin, Barrell has also seen in the Precambrian folds 

 " Manifestations of mountain-making forces on a prodigious scale" 

 (1915, p. 19). 



Suess based his " Leitlinien " or directrices of the recent con- 

 tinents in linking mountain chain to mountain chain, mainly on 

 the strikes of the axes of the folds, but also on the strike of schistosity, 

 and on the trend of intrusive rock masses, because he knew that the 

 three are intimately related; are expressions of the same deep-seated 

 forces and therefore bound into the uniform parallelism of direction 

 so generally observed. Willis (1920, p. 289) points out that the 

 foliation of the metamorphic rocks falls into the same class of phe- 

 nomena and exhibits the same parallelism with the directrices. It 

 is, however, to be regarded as the primary response of the rock to 

 deformative movements. While, as Barrell (1915, p. 512), Willis 

 (1920, p. 289) and others show, the normal mode of yielding in the 

 upper zone of fracture of the lithosphere which is thin, brittle and 

 relatively weak, is by jointing and faulting, and also by folding in 

 stratified rocks; the rocks of deeper origin yield by massive flowage 

 as their foliated structures and crystalline textures show when they 

 are exposed by erosion. It seems not essential for our inquiry to 

 decide here whether the foliation observed in metamorphosed 

 plutonic rocks (igneous gneisses, etc.) arises, as Miller believes, 

 during the process of consolidation of the magma, resulting from 

 its efforts to shoulder aside the Grenville gneiss with which it is 

 intrusive, or whether, as Willis (1920, p. 290) states, the batholiths 

 in their linear form and arrangement follow an earlier foliation 

 parallel to their present trend. The important fact for our inquiry 

 is that of the parallelism, in the Archean rocks, between foliation 

 and folding and direction of intrusives. As Chamberlin and Salis- 

 bury, in their textbook of geology, state, " the most satisfactory 

 explanation of the prevalent foliated structure of the Archean 

 seems to be that which refers it to the movements of the outer 

 part of the earth, in Archeozoic and later time." We therefore 

 can properly link the foliation with the folding and the longitudinal 

 extension of the early intrusive masses as various expressions of the 

 same deformative and compressive forces. 



