THE BUILDING OF THE COLORADO ROCKIES 245 



deformation steadily increases, thus necessitating greater and 

 greater stress diiferences in order to cause folding. Unless the 

 stress differences increase as rapidly as does the resistance with 

 increasing depth below the surface, folding, instead of becoming 

 easier in the deeper levels as the time-honored doctrines have 

 taught, becomes more and more difficult and hence less effective."^ 

 Daly has called attention to the fact that in the Selkirk Range of 

 British Columbia the Shuswap terrane of presumable Archean age 

 has escaped much of the folding which so powerfully affected the 

 younger rock systems. The basement rocks show comparatively 

 slight deformation, from which Daly concluded that "the earth 

 shell engaged in the post-Shuswap orogeny was only a few miles, 

 perhaps 6 or 8 miles, in depth. Over the Shuswap terrane this 

 shell was thrust and crumpled. "^ 



Beneath the surface the sharpness of folding probably at first 

 increases down to a level of greatest folding, below which it again 

 diminishes. With increasing depth below the level of sharpest 

 folding, the folds of the wrinkled shell may either die out gradually 

 or the adjustment may be accomphshed by shearing between the 

 upper, more movable portion and the lower, less movable portion. 

 Fig. 13 is based on the assumption of undiminished intensity of 

 folding throughout the full depth of each deformed block, and 

 hence calls for adjustment by shearing below. In the actual case 

 the adjustment was probably accomplished, not by shearing alone, 

 but by some combination of the two processes. But at present 

 the relative importance of the two cannot be decided, except upon 

 arbitrary assumptions. Too much weight must, therefore, not be 

 given to the precise figures for the depth of the various blocks. 



Estimates of the depth of deformed shell by this method are 

 subject to modification for such uplifts as were not occasioned by 

 the horizontal compression. In this section of the Colorado 

 Rockies it is quite possible that upbowing has recurred along the 



^ F. D. Adams and J. A. Bancroft, " On the Amount of Internal Friction Developed 

 in Rocks during Deformation and on the Relative Plasticity of Different Types of 

 Rocks," Jour. Geol., XXV (1917), 597-637. 



^ Reginald A. Daly, "A Geological Reconnaissance between Golden and Kamloops, 

 B.C., along the Canadian Pacific Railway," Geol. Survey of Canada, Mem. 68 (1915), 

 PP- 53-55- 



