IscHOFiELD] ORIGIN OF THE PURCELL TRENCH 27 



From an examination of the above table it will be noticed that 

 the conglomerates are first found in great amount at the base of the 

 Lower Blairmore formation. The pebbles in these conglomerates 

 consist of quartzites and chert derived from the quartzites of the 

 Beltian rocks which make up the great part of the Selkirk range. 

 Evidently in lower Blairmore times the Selkirk range was approaching 

 the maximum of elevation and was undergoing rapid erosion. The 

 upper Blairmore formation also consists of conglomerates and sand- 

 stones, but in addition to the pebbles of quartzite and chert, pebbles 

 of granite occur for the first time and in great abundance. The pres- 

 ence of the granite pebbles at this horizon is interpreted to mean that 

 the Selkirk range was unroofed during the upper Blairmore times and 

 that the Nelson granite batholith which forms the core of the Selkirk 

 range in southern British Columbia was exposed to rapid erosion 

 and furnished the pebbles for the conglomerate. Hence, it is estab- 

 lished that the first intrusion of granodiorite into the Selkirk range 

 took place before the deposition of the Upper Cretaceous. The super- 

 position of the marine Fernie shales upon the marine Devono-Carbon- 

 iferous limestones suggests that the period of stability which prevailed 

 throughout British Columbia until the Triassic was interrupted during 

 the Upper Jurassic period. The Selkirk mountains received their 

 initial form probably at the close of the Jurassic or in early Kootenay 

 times. If mountain-building and igneous intrusion are contempor- 

 aneous, it may be concluded that the first intrusion of granodiorite in 

 the Selkirk range commenced towards the close of the Jurassic and 

 continued until the mountain-building reached its maximum in 

 Kootenay times. 



