BE TOUT OF THE CHIEF ASTRONOMER 611 



SESSIONAL PAPER No. 25a 



thick intrusive sheet of gabbro toward the top of the cliff. As a rule, however, 

 the uniformity of the rocks in strength is almost as striking as it is in great 

 batholiths of granite and that strength is nearly of the same order of magni- 

 tude. Such variations in resistance to the weather as might have become mani- 

 fest in the topography if these rocks had been exposed to erosion under arid 

 conditions, are effectually obscured by the fact that the region has been for a 

 long period covered with a heavy forest-cap, which, as usual, blunts the angles 

 of relief whether in profile or ground-plan. For various reasons, therefore, this 

 mountain system nowhere approaches the scenic quality of the Front ranges. 

 The Purcell system is deeply canyoned but lacks the architectural effect of the 

 more easterly ranges. It will be recalled that the Front ranges are composed 

 of the much more heterogeneous rocks equivalent in age to the Purcell system 

 quartzites. 



The structure is essentially that of a series of fault-blocks, the McGillivray 

 range alone showing true folds which themselves are broken by faults. The 

 different blocks have been diversely moved so that the dips run from 0° to 90°, 

 with an average of perhaps 40°. There is no evidence that the faulting belongs 

 to more than one episode of deformation; this may most simply be referred to 

 the Laramide revolution. It is very unlikely that any original fault-scarps 

 are represented in the topography, which has been chiefly determined 

 in details by profound erosion. The obvious difficulty of discovering the 

 form of the constructional surface of this great compound horst makes it 

 difficult to describe the stage of erosion represented by the usual terms of the 

 erosion-cycle; but the degree of relief is about that found in maturely dissected 

 plateaus or other physiographic units where the original form can be recon- 

 structed. With the qualification just made we may profitably speak of this 

 erosion, like that in the Front ranges, as mature. 



The master streams of the region are located on fault-lines. Besides the 

 two wide trenches at Gateway and Porthill, we have the West Fork of the 

 Tahk river and the Moyie river located on either master faults or on zones of 

 faulting. The subordinate valleys are in some instances placed over similar 

 breaks. Among these may be mentioned the east-west valley mapped west of 

 the McGillivray range summit and south of the Boundary; two meridional 

 valleys occurring west of the West Fork of the Yahk; and the meridional valley 

 immediately west of the Moyie river at the Boundary Line. As in the eastern 

 ranges these valleys seem to be located on the lines of depression instituted at 

 the time of faulting, but it cannot be said that they have not in some instances 

 been developed by slow headward growth of streams which lengthened most 

 readily along the relatively weak zones at the fault-planes. This subsequent 

 origin is, however, not probable for the larger rivers. The zones of possible 

 brecciation along the faults is almost certainly very narrow along most of the 

 faults in the mountain system, generally not more than a few feet or scores of 

 feet in width, if the outcrops are to be trusted. It seems therefore unlikely 

 that wide master valleys would have been developed by stream adjustment to 

 the fault-zones unless other and equally wide valleys had been simultaneously 

 25a — vol. iii — 40 



