137 



East of the divide of the Selkirk range, the Cougar 

 formation is, on the whole, thin-bedded and more argil- 

 laceous (originally) than in the section just detailed. 

 The equivalent strata of the Rocky mountains — the Corral 

 Creek formation and the lower part of the Hector formation 

 — are still more argillaceous, consisting of gray, green, 

 purple, and black metargillites with interbeds of rusty 

 quartzite. (See p. 172). The rocks of this general horizon 

 thus become finer-grained, less purely silicious, and more 

 argillaceous as the section is followed from west to east. 

 A similar variation characterizes the Rocky Mountain 

 Geosynclinal rocks at the 49th Parallel section. 



The Nakimu limestone is specially notable as being the 

 most useful horizon-marker in the Selkirk and Purcell 

 mountains. It is truly protean in lithological features, 

 but one is seldom at fault in identifying it in the field. 

 The Caves of Cheops (Caves of Nakimu) have been formed 

 by solution and by the mechanical erosion of Cougar 

 creek, as it follows for some distance a subterranean 

 course in the formation. At that, mc st westerly, outcrop 

 the formation is a light gray, fine-grained crystalline 

 limestone. The lock is comparatively homogeneous, but 

 carries disseminated sericitic mica in many beds. In 

 the outcrops of the eastern Selkirks and of the Purcell 

 mountains, the same gray type of limestone is interbedded 

 with blackish, very carbonaceous limestone and with rusty- 

 weathering, sandy or pebbly, dolomitic limestone. The 

 thickness is quite variable — from as much as perhaps 600 

 feet (183 m.) at the Caves of Cheops to a few feet near 

 Bea vermouth. These differences are in part original; 

 in part they seem to be due to squeezing-out during the 

 uplift of the mountains. 



The Nakimu limestone is conformably overlain by the 

 thick Ross quartzite named from Ross peak, a mountain 

 opposite Cougar creek at its confluence with the Illecill- 

 ewaet river. The lower part of this formation is of Pre- 

 Cambrian age; the upper part is probably to be assigned 

 to the Lower Cambrian. All these admirably exposed beds 

 are conformable not only with one another but also with 

 the definitely Lower Cambrian Sir Donald quartzite above. 



In the section between the Caves of Cheops and Rogers 

 Pass station near the summit of the Selkirks, the Ross 

 formation is relatively homogeneous, with composition as 

 here indicated : 

 35069— 3^A 



