REPORT OF THE CHIEF ASTRONOMER 613 



SESSIONAL PAPER No. 25a 



structure of the rocks and the nature of "the erosion, but one cannot resist 

 the suggestion that it may also be conditioned by the unexampled amount of 

 erogenic uplift in this part of the trans-Cordilleran belt. 



The drainage is chiefly transverse and directed into the Kootenay on the 

 one side and the Salmon-Pend D'Oreille system on the other. Boundary, Corn, 

 Summit, Monk, Lost, and Sheep creeks flow in canyons which may reasonably 

 be attributed to streams which initially drained the great monocline — conse- 

 quent streams. Notwithstanding the great variety of strength in the 

 different rock formations, there is, here too, little evidence of adjusted drainage. 

 Upper Priest river, flowing along the contact of the relatively friable Irene 

 conglomerate which is adjoined by the weak phyllites of the Priest River 

 terrane, may represent a short subsequent valley. The course of the Salmon 

 is not easily explicable but may be tentatively considered as locally determined 

 by the break of slope at the eastern foot of the high volcanic pile of Beaver 

 Mountain. The superposed drainage on the granites, including the extensive 

 Bayonne batholith, is transverse and apparently for the most part on the 

 sedimentary cover. The same relation is largely true of the drainage on the 

 Priest River terrane which has been so largely stripped of the overlying Summit 

 series of rocks. 



In general, therefore, the physiographic development of the Nelson range 

 is, to all appearance, parallel to that in the Purcell and Front range systems. 

 The evidence for more than one important erosion cycle since the post- 

 Laramie upturning is practically nil. Considering the enormous amount of 

 erosive work represented in the actual dissection of the monoclinal mass, it 

 would seem that all Tertiary time has been no more than sufficient for the one 

 erosion-cycle carried to the present stage of ' maturity.' 



Bonnington-Bossland Mountain Group. — This field of relatively old, 

 deformed volcanic rocks and of batholithic intrusives may be conveniently 

 treated as a physiographic unit. Its local baselevel is the Columbia at about 

 1,350 feet above sea; the mountains are generally under 6,000 feet, with one 

 notable peak, Old Glory mountain, reaching the height of 7,800 feet. With few 

 exceptions the whole region is heavily forested. 



This region may be described as somewhat past maturity of dissection. 

 Horns are extremely rare ; graded slopes are the rule, with contours and profiles 

 generally well rounded. Nearly all of the Boundary belt has here been glaciated, 

 with the resulting smoothing of angles under the ice-cap both by erosion and, 

 in places, considerable deposition of a drift veneer. The ice-cap has, however, 

 done little to affect the pre-Glacial, late-mature character of this torso landscape. 

 The summits are relatively low here not only because the rocks have wasted 

 somewhat more rapidly than in the more easterly ranges but more especially 

 because the rocks of the Rossland district were not lifted nearly so high as those 

 of the Nelson range at least. 



The drainage history is largely undecipherable. The general arrange- 

 ment of the streams suggests, however, the hypothesis that the original form 

 of the thick Rossland volcanic pile controlled it in some measure, though con- 



25a — vol. iii — 404 



