122 THE ROYAL SOCIETY OF CANADA 



of that valley. In these Appalachian cases we cannot doubt that the upper facets 

 are of Cretaceous date, the lower peneplains of relatively late Tertiary date; that 

 is, they have a great contrast of age, and one which is significantly like that suggested 

 by the writer for the flat erosion -surface of the Great Plains and the adjacent blocks 

 of the Front ranges. Furthermore, the eastern slope of each Front range is generally 

 a retreating escarpment and, as already noted, the retreat is to be measured by 

 miles, perhaps by many miles in some places. The structure of the region, with 

 soft underlying hard at the Lewis thrust, necessarily involves a steep retreating 

 mountain-front so long as the thrust-plane remains above base-level. The case is 

 again analogous to the Catskill or Niagara escarpment exfcept that in those cases 

 the erosional undermining is controlled by bedding and not by a flat plane of over- 

 thrust. 



Again, the dissection of the Front range blocks is just of the order of magnitude 

 expected from the analogy of lithologically somewhat similar Appalachian terranes, 

 which have been maturely dissected in a well dated erosion cycle occupying the larger 

 part of Tertiary time. 



Since the character of the drainage is apparently that to be expected on the 

 one-cycle hypothesis for the region, it seems that all the essential topographic 

 features are explained by that hypothesis. The writer believes that no proved 

 structural relation in the bed rocks needs the two-cycle hypothesis for its explana- 

 tion. In conclusion, therefore, he would state his belief that the Front ranges, as 

 well as the Galton-MacDonald group, were uplifted in the one episode of the Lara- 

 mide orogenic revolution and have undergone steady erosion ever since, this erosion 

 reaching maturity and no later stage. It is possible that an horizontal thrust has 

 deformed the unconsolidated Miocene clays of the Flathead trough, but there is 

 no clear evidence that this movement aff^ected the great blocks to east and west 

 in any essential way. 



The argument has been dwelt upon not only because the physiographic history 

 is also the geological history of the Rocky Mountains proper, but also because a 

 similar history may be credited to the broad Purcell Mountain system, to the brief 

 discussion of which we may turn. 



Suggested Explanation of the Structure 



The present structural features of the region can be explained by 

 a 1 hypothesis which assumes that they were caused by compressive 

 stresses acting in an easterly direction during a single orogenic episode 

 (35, a). It is, however, not vital to the hypothesis that the pressure 

 from the west was continuous from its inception to its decline ; it may 

 be, and probably was, rhythmic, but there v/as no long period of 

 quiescence between pulsations, and the episode is single in the sense 

 that from the time the compressive forces of the Laramide revolution 

 began to deform the strata until they finally ceased to act, continual 

 deformation characterized this geologic event. 



The mass on which the compressive stresses acted consisted of a 

 virtually parallel accumulation of strata many thousands of feet 

 thick, in general more massive and, therefore, more competent in 

 the Precambrian lower and Palseozoi: middle portions than in the 



