REPORT OF THE DIRECTOR I92O-2I 131 



taneous intense revolutionary folding such as is usually suggested 

 to observers by the closely folded, crumpled condition of the 

 Precambrian rocks, is found in the observations that laccolithic 

 intrusions presuppose an orographic flexing to support the super- 

 incumbent strata (Keyes, 1918, p. 75) and that the large Precambrian 

 complexes of generally laccolithic form at Duluth, Sudbury etc. 

 resulted from intrusions extending over a large period of time and 

 that they were developed where the lateral pressure was not great 

 (Benson, 1920, p. 144). This, as also Daly's work, suggests that the 

 universal development of batholiths, laccoliths and sheet intrusions 

 marking several of the Precambrian revolutions may, in every case, 

 also have been an extremely slow process that developed pari passu 

 with the gradual folding of the arch-continent. 



It may be mentioned in this connection that the metamorphism 

 and the foliation of the rocks which are generally considered as a 

 direct evidence of the intensity and profoundness of the Precambrian 

 revolutions have lately been shown to be explainable by less intensive 

 action. Thus Adams (19 18, p. 180) has demonstrated by experi- 

 ments that in certain types of rock schistosity or foliation may be 

 produced by pressure alone, without heat and in the absence of 

 moisture, and Barrell (1921, p. 11) has found that " metamorphism 

 is not caused by deformation (folding), but recrystallization is 

 largely and directly related to batholithic heat and emanations, 

 while " the folding and mashing " are regarded " as related but 

 independent processes, due to crustal compression, but going on 

 most readily in the weak and recrystallizing roofs of batholithic 

 chambers." Barrell is further arguing for a shallow depth for the 

 Precambrian metamorphism, basing his argument on the com- 

 pleteness of the Archean metamorphism and the salt of the ocean 

 as a measure of erosion. 



The combined inference from these recent conclusions and obser- 

 vations is that the Precambrian revolutions possibly were, even 

 though involving the whole crust of the earth, in kind and intensity 

 not greatly different from the later orogenic revolutions of the 

 earth. This, however, would again point to a probably gradual 

 extension of the Precambrian orogenic revolutions over the whole 

 earth. 



2 Simultaneous, worldwide Precambrian folding 



A simultaneous, worldwide Precambrian folding by terrestrial 

 forces is assumed by some European writers (see above), who 

 having made a close study of the Precambrian folding in Bohemia 

 5 



