GENERAL CONSIDERATIONS 683 



In a very recent paper, *^ dealing with the southernmost Rocky Moun- 

 tains of Canada, MacKenzie says : 



"From the earliest pre-Cambrian known until at least the beginning of the 

 Tertiary, the accumulation of sediments proceeded without interruption by 

 pronounced deformation, varied from time to time by broad and relatively 

 slight oscillations which caused local erosion and resulted in disconformities." 



In the eastern Cordilleran district of the United States earlier pre- 

 Cambrian rocks often show steep dips, but these rocks have usually been 

 more or less intimately intruded by igneous material, which, in part at 

 least, may have been the main cause of the steep dips. 



Throughout the Appalachian and western New England regions the 

 pre-Cambrian rocks are more or less highly folded and associated with 

 intrusives, and it is doubtful, as pointed out beyond, whether these rocks 

 were subjected to more than very moderate compression prior to the 

 severe Paleozoic deformation. 



From the above statements we may therefore conclude that various 

 portions of the earlier pre-Cambrian rocks have been folded little, if any 

 at all ; that various portions have been only moderately folded ; that vari- 

 ous portions have been highly folded; and that still other portions are 

 not proved to have been much folded in pre-Cambrian time. In many 

 cases, even where steep dips are recorded, the writer believes that the 

 deformation may reasonably be explained as a direct result of magmatic 

 intrusion and injection, accompanied by not more than moderate com- 

 pressive forces. 



Barrell,^ who has advocated a similar explanation, says : 



"The gnarled and twisted rocks of the Archean speak of the presence be- 

 neath them of molten magmas rather than of an enormous degree of com- 

 pressive forces upon them." 



For the reasons given, the writer can not subscribe to the statement 

 that the earlier kno"«Ti pre-Cambrian rocks were everywhere highly com- 

 pressed and folded in pre-Cambrian time. 



POST-CAMBRIAN FOLDING OF PRE-CAMBRIAN ROCKS 



In any discussion of pre-Cambrian folding, discrimination must be 

 made between those rocks which were folded in pre-Cambrian time and 

 those which were folded in later time. In many cases this can be done, 

 and often earlier and later pre-Cambrian deformations can be recognized. 

 This is particularly true of many parts of the Canadian Shield. In other 



•5 J. D. MacKenzie : Transactions Royal Soc. Can., vol. 16, 1922, p. 98. 

 " J. Barrel! : Jour. Geol., vol. 23, 1915, p. 511. 



