REPORT OF THE CHIEF ASTRONOMER 605 



SESSIONAL PAPER No. 25a 



adjusted drainage may be partly explained by the relatively small differences 

 of strength in the bedded rocks, but one must suspect that it is also due to the 

 fact that time enough has not been given for a thorough searching out of soft 

 belts by head-water branches. The Carboniferous limestone seems to be dis- 

 tinctly softer than the neighbouring silicious rocks of the Galton series and 

 depressions are begun in the limestone. 



QUESTION OF A TERTIARY PENEPLAIN IN THE ROCKY MOUNTAIN SYSTEM. 



Willis's many-sided, interesting paper contains a clear statement as to the 

 view that the whole Rocky Mountain system from the Rocky Mountain Trench to 

 the Plains, was peneplained in mid-Tertiary time. If this be true, any scientific 

 description of these ranges should be phrased in terms of that fact, just as 

 description of the Appalachians is made at once more simple and more true by 

 assuming the Cretaceous peneplanation of that eastern mountain chain as an 

 event of primary importance. The virtue of the conception is great, for it 

 clarifies both the geological history and the topographic description of the 

 Appalachians in an unequalled manner. 



Unfortunately, the present writer has been unable to accept the hypothesis 

 because of certain grave difficulties which are not felt in the case of the 

 hypothesis of peneplanation in the Appalachians. A brief digression may fitly 

 be made to traverse the problem before proceeding with the notes on he 

 physiography of the Purcell system. 



To present the case as outlined by Willis, it will be well to quote at some 

 length but the reader should consult the original paper in order to appreciate 

 the whole of the argument. On pages 344-349 we read: — 



' Recognition of the tilted attitude of Cretaceous strata and of the 

 even surface extended across their edges is sufficient to demonstrate the 

 character of the Great Plains, at least in the belt adjacent to the Front 

 ranges. The surface is one of planation, independent of structure, and, 

 marine planation being excluded on strong negative grounds, it may be 

 considered a peneplain. Several stages of erosion may be noted in the 

 relief of the Great plains, but the one here referred to is that which is 

 represented by the highest levels and which is the oldest. In the preceding 

 discussion of antecedents of the Lewis thrust [page 92 of this report] it 

 was named Blackfoot peneplain and assigned to a pre-Miocene cycle of 



erosion 



' The rise of the Lewis range above the Blackfoot plain is more than 

 is reasonably attributed to difference of hardness of rocks. Limestones 

 and quartzites could not have maintained such relative altitude so near a 

 lowland in which shale and sandstone were reduced to a plain. The later 

 forms sculptured in the Blackfoot plain are apparently represented by 

 equivalent features in the Front ranges. When their correlation has been 

 worked out, remnants of a surface may be recognized as belonging to the 

 Blackfoot cycle in old age. They may be traced among high shoulders of 

 the peaks, which must then be considered monadnocks, or they may be 



