608 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



seen that a great thickness of the Mississippian limestone persists in the fault- 

 blocks of the MacDonald range just across the Flathead. Nowhere in the 

 eastern part of the Cordillera north of Colorado is there evidence of notable 

 deformation of the Rocky Mountain Geosynclinal between Mississippian and 

 Laramie times. It seems likely, therefore, that a great thickness of the Missis- 

 sippian limestone was present in the MacDonald range area before the Laramide 

 or post-Laramie faulting dropped the large masses of the limestone into lateral 

 contact with the Altyn formation of the MacDonald range. If this be granted, 

 it follows that little erosion had been accomplished by erosion in this latitude 

 during the Mesozoic. The Mesozoic erosion-cycle could not have very great 

 significance in the region. 



Returning to the main theme, we may note that Willis's evidences for the 

 mid-Tertiary peneplanation are: (a) the truncation of the crumpled Cretaceous; 

 (&) the presence of accordant levels among the summits of the Galton-Mac- 

 Donald mountain group. Concerning the first point, it is not made certain 

 that the truncation of the Cretaceous was observed outside the area which may 

 reasonably be supposed to have been overridden by the overthrust block of the 

 Front ranges. This thrust, as shown at Chief mountain very clearly, has not 

 only crumpled the Cretaceous beds but has sheared them off sharply at the 

 plane of the Lewis thrust. In some measure the observed truncation elsewhere 

 may be attributed to this constructional process, for there is clear evidence 

 that the original eastern edge of the overthrust block lay several miles to the 

 eastward of the existing frontal escarpments of the Lewis and Clarke ranges. 

 Of course, erosion has modified the surface of scission thus exposed by the 

 retreat of the escarpments, but its base-levelling effect must here have been 

 vastly inferior to that which was demanded on the hard quartzites and silicious 

 dolomites of the Lewis series. 



The argument from the accordance of summit levels cannot, in the writer's 

 opinion, be safely applied in any one of the four ranges now in discussion. 

 In no one of them is there any notable remnant plateau which can fairly be 

 said to prove general baselevelling in a former erosion cycle. The writer has 

 already published the grounds of his protest against using the accordance of 

 peaks and ridges as an evidence of two erosion cycles; a full abstract of that 

 publication will be given at the close of this chapter, to which the reader may 

 turn. In brief, the point is made that sub-equality of heights is to be expected 

 from the early stage in the history of every alpine mountain range. 



The evidences against the hypothesis of a mid-Tertiary peneplain on the 

 Front ranges seem to be powerful. First, the time allowed is not sufficient for 

 peneplanation or even past-mature development, followed by uplift and mature 

 dissection in a second cycle. All post-Cretaceous time has not been enough 

 to destroy 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 



