2 GEORGE V. SESSIONAL PAPER No. 25a A. 1912 



CHAPTER X. 



INTRUSIVE SILLS OF THE PURCELL MOUNTAIN SYSTEM, 



INTRODUCTION. 



Within the area of the Boundary belt where it crosses the Yahk and Moyie 

 ranges, no extrusive lava was anywhere observed, but intrusive basic masses 

 were found in large development. On the map they form twenty-four bands, 

 covering in all about one-sixth of the area between tbe Kootenay river at 

 Porthill and the main fork of the Yahk. One of them is a true 

 dike: a second is either a dike or a sill; the other twenty -two 

 bands are all more or less certainly to be classified as sills. All the 

 bodies are intrusive into either the Kitchener or C'reston quartzites, and, as 

 noted below, are referred to the Middle Cambrian period. No one of this group 

 of intrusives, so far as known, cuts the Moyie formation. 



The exposure of the igneous bodies has become possible through extensive 

 block faulting and upturning, followed by erosion. The faulting has repeated 

 the outcrops at several points, so that the number of different bodies is less 

 than the number of igneous bands shown on the map. On account of the 

 unusual continuity of the forest cover, obscuring the field relations, and also 

 because of the general lack of easily recognized horizon-markers in the invaded 

 quartzites, it has been impossible to determine the actual amount of this repeti- 

 tion of outcrops through faulting. Its occurrence in certain localities is 

 hypothetically indicated in the general structure section. The rarity of dikes 

 is probably only apparent. If the overwhelming forest-cap were removed from 

 these ranges, dikes in considerable number might be displayed. The sills are, 

 en the average, so large that they were discovered even under the peculiarly 

 difficult field conditions of these mountains. At the same time, the mapping 

 of several of the igneous bands, especially in the eastern half of the Yahk 

 range, must be regarded as merely approximate. 



The sills vary in thickness from fifty feet to about 1,000 feet. The thickest 

 of these is one of a genetically related group of five adjacent sills which are 

 distinguished by peculiar composition and history and may be conveniently 

 referred to as the Moyie sills. Several of the bodies are from 200 to 500 feet 

 in thickness. The dip varies from about 5° to 90°, averaging perhaps 40°. 

 In general, it is a simple matter to locate both top and bottom of each intrusive 

 sheet. On reference to the maps it will be seen that most of the bands hold 

 their respective widths for several miles. In no case was it possible, owing 

 to the conditions of field work, to follow a sill far beyond the limits of the 

 Boundary belt, but, from the fact that the bodies hold nearly uniform thicknesses 



221 



