KELATION OF NORTH AMERICA TO EURASIA 



203 



affected the upper or outer parts of the crustal sheet and produced the 

 Appalachian folds. But these authors make no distinction between the 

 relatively superficial thrust movements and the deeper general creeping 

 movement which involved the whole crustal sheet of the continent, as it 

 was then, and reached downward to or into the zone of rock flowage. The 

 variously directed thrusts which affect the more superficial parts may, 

 according to local conditions, act in any horizontal direction, but usually 

 either in the same direction as the general crustal movement or in the 

 opposite direction. The same idea was later applied to the Eocky Moun- 

 tains and to the Cordilleran ranges bordering the Pacific, and the thrust 

 forces there were derived from the Pacific depression. 



Figure 3. — Alaska 



Showing the raountain knot, part of the Aleutian Island arc, and part of the Cor- 

 dilleran ranges of British Columbia and Alaska. The arrows show the supposed direc- 

 tion of crustal movements. 



It seems certain, however, that the relation of the Cordilleran to the 

 Aleutian range in the mountain knot of Alaska shows that the general 

 crustal movement of North America in the Tertiary age was toward the 

 Pacific, and hence that the great eastward and northeastward overthrusts 

 of the Eocky Mountain region are in reality underthrusts, or reflex over- 

 thrusts, directed backward over the southwestward general crustal move- 

 ment. At these localities the southwestward underthrust of the deeper 



