[MACKENZIE] HISTORICAL AND STRUCTURAL GEOLOGY 121 



a second cycle. All post-Cretaceous time has not been enough to destro3' the 

 large monadnocks on the well-established Cretaceous peneplain of the Appalachians, 

 though their rocks are' not sensibly stronger than those of the Front ranges of the 

 Cordillera. In most of the Appalachian belt a very large percentage of all Tertiary 

 time has sufficed to do no more than form mature or submature topography through 

 the dissection of the generally well elevated Cretaceous peneplain. Yet the climatic 

 and other erosion conditions 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 after- 

 wards 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 Appala- 

 chians, where subsequent drainage is probably dominant 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 sub- 

 sequent drainage is the role in many parts oî 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 defor- 

 mation (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 peneplain 

 on the soft rocks of the Great Plains, but it by no means implies a peneplain on the 

 much harder rocks of the Front ranges. The erosion of both provinces has been 

 chiefly occasioned byriversand 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 these 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 escarpment 

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

 on each side of the Connecticut valley dominate the peneplained Triassic sandstone 



