128 THE ROYAL SOCIETY OF CANADA 



the beds nearer the old land and to have thrown them into low folds, 

 leaving the eastern zones virtually unaffected except perhaps for a 

 slight uplift. On continued compression, as illustrated in Section 

 C, the western folds were steepened, the lower, more competent strata 

 were raised higher, and a greater stress was transmitted to the eastern 

 zones, the first effect of this being to gently fold the Mesozoic measures. 

 At this stage there may have been more severe folding in the upper 

 strata in the western zones, accompanied by overthrusting, but as 

 these Mesozoic beds have been almost entirely removed their condi- 

 tion at this time is a matter of inference only. At the stage repre- 

 sented by Section D, the western zones were stronger by reason of 

 thickening by folding, and because the lower more competent Palae- 

 ozoic and Precambrian strata have been raised higher, so that a greater 

 stress is transmitted to the eastern zones, in which the less competent 

 Mesozoic strata are exposed. The effect of this stress is supposed to 

 have caused reverse faults to develop, dipping westward at something 

 over 30 degrees, this being the angle between the strata and the fault 

 surfaces as previously described. The westernmost of these breaks 

 is considered to be the largest, as it is nearest to the stress and it is 

 shown at depth ending in an anticline in the lower strata. 



Fewer breaks appear to have taken place in the boundary section 

 than farther north, and it is possible that here the stresses were relieved 

 by a large amount of slipping along one surface rather than lesser 

 slipping along several breaks. Sections Eand F illustrate progressive 

 changes caused by continued compression, one effect of which is to 

 cause steepening of the reverse faults of the eastern structural area. 

 This steepening will be explained in connection with the North 

 Kootenay pass section. In Section F a large displacement along the 

 Lewis thrust is indicated, caused in part by the greater opposition 

 to further deformation by the already deformed Mesozoic strata to 

 the east. At the stage of Section F, representing the end of the com- 

 pression of the Laramide revolution, the western structural area was 

 still high, and not greatly deformed. The strata were disposed in 

 moderate folds, for there is no evidence of generally severe folding in 

 the rocks of the MacDonald and Galton ranges (12,n). There may 

 be an exception to this general moderate folding in the case of the 

 structures near the present Flathead and Kootenay valleys. Daly 

 maps limestone fault blocks, presumably of Palaeozoic age, in these 

 valleys (12, n) and there are downfaulted blocks of Mesozoic rocks in 

 the Flathead valley north of the boundary (26), (32). The structure 

 iS; therefore, more complex in detail than the sections show. 



