610 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



Triassic sandstone 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 baselevel. The case is again analogous to 

 the Catskill or Niagara escarpment except that in those cases the erosional 

 undermining is controlled by bedding and not by a flat plane of overthrust. • 



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

 magnitude expected from the analogy of lithologically somewhat similar Appa- 

 lachian 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 

 explanation. 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 Laramide orogenic revolution and have undergone steady erosion 

 ever since, this erosion reaching maturity and no later stage. It is possible 

 that a horizontal thrust has deformed the unconsolidated Miocene clays of the 

 Flathead trough, but there is no clear evidence that this movement affected 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 Mmmtains 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. 



Purcell Compound Horst. — The relief of the Purcell system is indicated by 

 the elevations of the local baselevels as compared with the highest sum m its. 

 The Kootenay river at Gateway is about 2,300 feet above sea, and at Porthill, 

 about 1,750 feet above sea. The highest peak in the Boundary belt between the 

 two crossings of the river is mapped as 7,518 feet in height. 



This broad, compound horst is throughout composed of exceedingly strong 

 rocks, chiefly quartzites, though the thick sills of g'abbro are perhaps somewhat 

 stronger than the quartzites, and the Purcell Lava is certainly stronger than 

 the associated metargillites. The lava makes strong scarps on the limbs of the 

 broad syncline of the McGillivray range and forms a strong ridge on the eastern 

 limb of the anticlinal fold just where the stratified series plunges under the 

 surface deposits of the Rocky Mountain Trench. Another hint at differential 

 hardness is found in the development of the steep escarpment facing the Moyie 

 sills, and it is possible that the steepness of the McKim cliff is partly due to the 



