24 THE ROYAL SOCIETY OF CANADA 



until it joins the Selkirk trench at the northern end of Upper Arrow 

 lake, a drowned portion of the south-flowing Columbia River valley. 

 Daly^ considers the northern extension of the Purcell trench to be the 

 Duncan and Beaver River valleys, while the writer considers the 

 northern portion to be occupied by Lardo river^ Trout lake and Beaton 

 creek which flows into Upper Arrow lake and thus joins Selkirk 

 valley and Purcell trench. 



The following facts support the change in the delineation of the 

 Purcell trench: — 



1. The Purcell trench is a valley formed by an antecedent river 

 and the northern extension occupied by Lardo river. Trout lake and 

 Beaton creek is of similar origin. 



2. The valley of the Duncan river parallels the strike of the rocks 

 and is probably a subsequent river. 



Kootenay lake occupies the central portion of the trench and is 

 drained from a point near its centre by the west-flowing Kootenay 

 river, which in turn empties into the Columbia river. The southern 

 portion of the trench is occupied by the north-flowing Kootenay river. 



Topography 



In a view from one of the higher peaks of the Selkirk range, the 

 most striking feature is the series of almost unbroken ridges, having 

 an approximate elevation of 7,000 feet. The trend of these ridges is 

 in all directions and bears no relationship to the underlying structure. 

 These ridges evidently represent the remnants of an uplifted and 

 dissected peneplain. Numerous peaks having an elevation from 8,000 

 to 9,000 feet project above this old land surface, while great valleys 

 have been carved 6,000 feet below it. The ridges form convenient 

 highways for exploration once this upland surface is reached. In 

 marked contrast to the ridges of the Selkirk mountains, those of the 

 Rocky Mountains to the east have a remarkably constant northwest 

 trend and have a direct relationship to the structure of that system, 

 in that the ridges represent great fault blocks whose trend is in a north- 

 westerly direction. 



Geology 



The Purcell trench is carved in a series of sedimentary rocks rang- 

 ing from the Beltian to the Carboniferous. This series is intruded by 

 masses of granodiorite, offshoots of the west Kootenay batholith of 

 Jurassic age (Plate II). The following is a condensed geological table: 



1 Daly, R. A., Geol. Surv., Can., Mem. 38, 1912, p. 26. 



