698 \y. J. MILLER PRE-CAMBRIAN FOLDING IN NORTH AMERICA 



show that decidedly more of the pre- Cambrian trend-lines run from east- 

 west to about northeast rather than approximately north-northwest. It 

 is, therefore^, a very significant fact that the prevailing direction of pre- 

 Cambrian folding and foliation is across the Cordilleran region rather 

 than parallel with it. Considering this fact, together with the meager 

 structural evidence from western Canada and the practical absence of 

 definite evidence from Alaska and Mexico, it is clear that there are no 

 sufficient grounds for advocating a general north-south trend of pre- 

 Cambrian folds and foliation through western North America, as has 

 been done by Ruedemann. This matter is all the more sigiiificant when 

 it is realized that the western half of the whole continent is involved ! 



General Conclusions 



Some of the main conclusions in regard to the pre-Cambrian folded 

 and foliated structures of North America which seem to be warranted 

 as a result of the evidence presented in this paper are as follows : 



1. The older known pre-Cambrian rocks of JSTorth America have not 

 been universally intensely folded, as has heretofore been commonly as- 

 sumed. In some districts they have never been more than very moder- 

 ately folded, and in other districts they have been moderately or highly 

 folded only since pre-Cambrian time. In many districts they were, how- 

 ever, highly deformed in pre-Cambrian time. In many cases the proba- 

 bility of the development of steep dips and more or less folding through 

 the agency of magmatic intrusion, under the influence of but little lateral 

 pressure, should be considered. 



2. In any attempt to decipher pre-Cambrian diastrophism in North 

 America, careful discrimination should be made between folded struc- 

 tures of pre-Cambrian rocks developed in pre-Cambrian time and those 

 subsequently produced. In many districts this has not been done and in 

 some cases it is impossible. 



3. Because of the vast length of time involved and the great uncertain- 

 ties of subdivision and correlation of pre-Cambrian rocks in various parts 

 of the continent, we are still in the dark in regard to the number, extent, 

 severity, and correlation of crustal deformations of pre-Cambrian time. 

 More particularly, the facts do not warrant the conclusion that there 

 was a great continent-wide so-called "Laurentian Revolution" marking 

 the close of a rather definite portion of pre-Cambrian time. 



4. The plotting of many definitely recorded strikes of pre-Cambrian 

 folding and foliation (see accompanying map) does not indicate "a grand 

 and simple curvature,'^ changing from a northeast direction in the east- 

 ern part of the continent through east-west in the interior to approxi- 



