280 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



It is scarcely necessary to remark that the straightness of the bands of 

 colour corresponding on the map to the Summit series formations, is controlled 

 by a structural necessity, namely, the nearly or quite vertical dip which is 

 general throughout the greater part of the monocline. The thrust-plane just 

 described must similarly be nearly vertical. Deep as the canyons are, the out- 

 crops of the different formations deviate but little from the straight line 

 where the bands cross the canyons. 



West of Beehive mountain the Fend D'Oreille series is so thoroughly 

 disordered that the structure section in this part could be represented only in 

 a schematic way. The same procedure is necessary for the continuation of 

 these rocks across the Salmon river. 



Finally, in the Nelson range section we find the outposts of the army of 

 granitic intrusives which cut the stratified rocks of the Cordillera at intervals 

 all the way from the Furcell Trench to the Pacific ocean. In general the 

 sediments of that greater part of the Boundary belt are much younger than 

 the rocks of the Rocky Mountain Geosynclinal; but, because of the inherent 

 weakness of the younger sediments, because of the intrusion of many 

 batholiths, and probably also because of a greater intensity of the orogenic 

 forces in the western half of the Cordillera, these sediments are generally more 

 metamorphosed than those of the older prism. Just east of the Salmon river 

 the Summit series plunges under the Pend D'Oreille group of schists and 

 marbles and the still younger Rossland volcanics, never to reappear in the 

 sections farther west. 



The Nelson range is the greatly worn product of the mightiest crustal 

 upturning on the Forty-ninth Parallel; beside the range is the Purcell Trench, 

 the eroded representative of one of the deepest structural depressions of the 

 Cordillera. 



