REPORT OF TEE CHIEF ASTRONOMER 609 



SESSIONAL PAPER No. 25a 



are not now very different, and probably have not been very different, in the 

 two mountain-chains throughout the Tertiary. It seems, therefore, hard to 

 believe that the exceptionally tough rocks of the Front ranges at the Forty- 

 ninth Parallel have been peneplained once and maturely dissected afterwards 

 since the close of the Laramie period. 



Again, the general lack of stream adjustment in the entire section from 

 the Great Plains to the Flathead trough is a valid reason for rejecting the 

 two-cycle hypothesis. Difficult as it is to be sure in the case, it seems that 

 most of the drainage is of consequent origin. Contrast with this condition 

 that of the middle Appalachians, where subsequent drainage is probably domi- 

 nant over all other kinds of drainage! In this region of two cycles there has 

 been time enough for head-waters to lengthen the streams by gnawing back 

 into the soft belts for even scores of miles. Yet the second important cycle is 

 still not past maturity. Well-developed subsequent drainage is the rule in 

 many parts of the Appalachians where the rocks are all hard in an absolute 

 sense, though differing relatively in power to resist erosion. In the Front 

 ranges of the Cordillera the rocks are all strong but he is bold who 

 would deny that some are notably weaker than others and should thus ultimately 

 guide headward growth of streams in a two-cycle period of time. Failing such 

 manifest guidance along the strike of certain beds of the Lewis series, it must be 

 said that this well recognized criterion of multiple cycles (so justly emphasized 

 by Davis and others) does not favour the idea of a mid-Tertiary peneplain in 

 the Front ranges. 



Finally, the one-cycle hypothesis, whereby only one major episode of 

 deformation (the Laramide) and one erosion-cycle (including all of Tertiary 

 time) are postulated, seems competent to explain the present topography. 



The accordance of summit levels is here partly implied in the relatively 

 small degree of deformation other than uplift; for the rest, it is explicable on 

 the composite hypothesis discussed at the close of the chapter. 



The bevelled surface of the Cretaceous may truly mean a widespread pene- 

 plain on the soft rocks of the Great Plains, but it by no means implies a pene- 

 plain on the much harder rocks of the Front ranges. The erosion of both 

 provinces* has been chiefly occasioned by rivers and creeks issuing from the 

 mountains. In the mountains these streams have high gradients but small 

 volume; outside the mountains, tolerably swift currents and much greater 

 volume. It seems necessary to believe that on the plains those streams would, 

 through lateral corrasion, develop a peneplained surface with relative rapidity. 

 In the mountains the threads of water must develop such a surface from rocks 

 -like those of the Lewis series, with immense slowness. Willis's argument that 

 it is unlikely that the peneplain formed on the Cretaceous of the plains should 

 not adjoin a rugged, scarped mountain range of contemporaneous development 

 seems to be a very doubtful one, in view of the fact that the precisely similar 

 relation is seen in the case of the dissected Niagara escarpment overlooking 

 the Tertiary lowland of New York and Ontario. Similarly, the Catskill escarp- 

 ment overlooks the Tertiary lowland of the Hudson valley, and the crystalline 

 terranes on each side of the Connecticut valley dominate the peneplained 



