220 



Kilometres generally biotitic orthogneisses character- 

 istically occur as sills following a pre-existent, 

 flat-lying foliation in the older rocks. The 

 abundant masses of the later group were them- 

 selves rendered gneissic by a similar type 

 of metamorphism and then injected by myriads 

 of thinner sills, dykes, and chonoliths of white 

 or pink pegmatite and aplite. These youngest 

 members of the complex are less affected by 

 metamorphism, though a gneissic structure 

 parallel to sill-contacts is often seen also 

 in them. 



The whole assemblage of rocks is in striking 

 contrast to that in a normal batholithic province 

 of a post-Cambrian date of intrusion, and one 

 cannot but suspect that some of the conditions 

 of rock formation in this typical "Archean" 

 field were peculiar to an early epoch in the 

 earth's history. 



The gneissic complex is not well exposed in 



the Illecillewaet valley except in a few places 



where forest fires have bared the ledges. An 



119-6 m. example is seen on the 



192 km. Twin Butte Station left, for several kilo- 



Greely Siding metres between Twin 



Butte station and 

 Greely siding (124-2 mis.; 199-5 km.). 



Near the 128th mile-post the river cascades 

 over schists and gneisses on which it has been 

 locally superimposed through its own alluvium. 

 At this point is the power plant of Revelstoke. 

 As the train turns sharply to the right, one sees 

 the fore-set beds of the delta built by the 

 Illecillewaet into the Columbia valley when 

 it was here laked, with a water level about 

 70 metres above that of the present Columbia. 

 It is probable that this water-body was a great 

 expansion of the existing Arrow lakes. 

 130-3 n- Revelstoke — Alt. 1,492 ft. (455 m.) an, 

 209-7 km. important distributing centre in the interior 

 trade of southern British Columbia. 



The orthogneisses, aplites, and pegmatites 

 can be easily studied on the mountain slope 

 rising directly from the railway yard. 



