118 THE ROYAL SOCIETY OF CANADA 



essentially with the bedding of the Algonkian series, (b) It coincides 

 essentially with the highest peneplain in the Cretaceous rocks, (c) The 

 antecedent structures of the Lewis thrust were determined by con- 

 ditions of deposition." 



Of these three assumptions, the first may be admitted to be 

 substantially true in the direction of the strike at any rate, though 

 the rise of the fault surface from the boundary northward as described 

 above, is to be noted. This of itself does not invalidate Willis' 

 hypothesis. The third assumption which is a generalized statement 

 of Willis' theory of initial dips, may be admitted to have been a con- 

 trolling factor in the localization of certain structures, though it may 

 not have functioned in the way premised by Willis. The second 

 assumption, that the thrust surface coincides with an early Tertiary 

 peneplain, is the one really essential to Willis' hypothesis. The 

 existence of this peneplain is necessary to his explanation of the 

 Lewis overthrust as an erosion thrust and if the peneplain did not 

 exist his hypothesis is not tenable. An examination of the evidence 

 based on my own field work and that of others leads me to believe 

 that this Tertiary peneplain in the Rocky mountains did not exist, 

 and consequently some other explanation of the structures and 

 present relief of the region is required. The evidence may be sum- 

 marized here. 



The stratigraphie facts given in a preceding part of this paper are 

 indicative of virtually continuous sedimentation from the later 

 Cretaceous up to the Paskapoo beds of Fort Union age. The Fort 

 Union is generally accepted as Eocene (47, a), though Schuchert, as 

 noted above, argues from Stanton's evidence (47) for its Cretaceous 

 age. The Paskapoo beds are the latest ones deformed by the Lara- 

 mide revolution. As no general erosion took place before their de- 

 formation, it is clear that the supposed peneplanation must have 

 taken place after the uplift consequent on their deformation. If 

 Schuchert be correct, this deformation may be of pre-Eocene age; if 

 the generally accepted view be taken, it is of early Eocene date. 



It was demonstrated in a previous section that the Lewis thrust 

 was completed before the end of the Eocene. The time available 

 for the peneplanation that Willis supposes to have preceded the 

 Lewis thrust is thus narrowed to a limited interval during the Eocene. 

 When one reflects that on any hypothesis, all of Quaternary, and at 

 least half of Tertiary time have sufficed only to produce the present 

 extreme relief, the time allowed according to the stratigraphie evi- 

 dence given above, seems insufficient to cause peneplanation. The 

 stratigraphie facts further indicate that the Lewis thrust followed a 

 period of deposition, rather than one of erosion. 



