612 ' DEPARTMENT OF TEE INTERWR 



2 GEORGE V., A. 1912 



formed through streams adjusted to the softer, metargillitic members of the 

 sedimentary rocks. Subsequent streams of the latter class are strikingly rare 

 throughout the mountain system. 



The drainage on the eastern slope of the McGillivray range has the look 

 of consequent streams such as would be initially developed on the long eastern 

 limb of the broad anticline in that part of the section. Similarly, a consequent 

 origin is most plausibly attributed to the north-flowing creek draining the 

 north-pitching axis of the syncline just west of the McGillivray summit. The 

 main fork of the Yahk river is located in an anticlinal belt and it may repre- 

 sent a subsequent stream in this part of its course. 



In summary, it may be said that the existing drainage of the Purcells has 

 the relations of a set of dominant consequent streams and that there is little 

 evidence of stream adjustment in this mountain system. 



Each of the three constituent ranges shows the accordance of summit levels 

 in a very notable way. In no case, however, is there any known remnant plateau 

 of an old, uplifted peneplain. The problem of explaining the accordance of 

 summit levels is the same as in the Gallon range and, in fact, throughout the 

 majority of the ranges crossed by the Forty-ninth Parallel, we have the same 

 phenomenon. The problem's solution in terms of one erosion-cycle has already 

 been partly indicated and will be discussed more fully on later pages. 



Nelson Range Monocline. — The relief of the Selkirks at the Forty-ninth 

 Parallel is given by the following figures. The local baselevels are the Kootenay 

 river (at Porthill) with an altitude above sea of about 1,750 feet; and the 

 Salmon river, at about 2,000 feet above sea. The individual mountains have 

 elevations generally well under 7,800 feet, with Mt. Ripple (7,681 feet) as the 

 highest % in the Boundary belt. 



Again the quality of the topography is that of ' mature ' dissection in a 

 strongly mountain-built region. The structure is chiefly that of a huge mono- 

 cline of conformable strata, steeply upturned, with the exposure of a large area 

 of its foundation, the Priest River terrane. The local uncovering of large 

 batholiths of granitic rock adds an element new to our physiographic section 

 but henceforth to be considered at intervals all the way to the Pacific. The 

 generally very high dips together with the great thickness of the monocline 

 lead to the anticipation of decided differences of strength in the different 

 members ; all of these may contrast in ' hardness ' with the batholithic rocks 

 and with the Priest River terrane which is itself heterogeneous. Field work 

 justifies this view. All the rocks are strong in absolute measure, but there is 

 clear evidence of important differences of strength among the many rock- 

 formations. Among the more resistant members are the Ripple quartzite, the 

 Wolf grit, and the Bayonne granodiorite. The weaker rocks include the Pend 

 D'Oreille schists, the Irene conglomerate, and the many zones of metargillite 

 in the Summit series. 



The bold and fretted ridges and peaks of the , range afford the finest 

 scenery to be found in the Boundary section between the Clarke range and the 

 Hozomeen range. The explanation of its impressiveness lies partly in the 



