REPORT OF TEE CHIEF ASTRONOMER 139 



SESSIONAL PAPER No. 25a 



From that point to the Kootenay river the faults shown on the map and 

 section are not so certainly placed. The master-fault following the base of 

 McKim cliff has not been directly observed but is postulated because of the 

 fact that the great sill of gabbro on the west is underlain by rusty quartzite 

 which is believed to belong to the Kitchener formation. If this be the correct 

 interpretation the Purcell Trench is located along a displacement by which 

 the Kitchener formation has been dropped down into lateral contact with strata 

 hear the base of the Creston quartzite as defined in this report. The total 

 displacement of the fault or faults east of Porthill and west of the summit of 

 McKim cliff would thus approach 10,000 feet. The geology of the Selkirk 

 range shows, however, that the zone in which the trench lies has been the scene 

 of still more profound faulting; the evidence is summarized in the next 

 chapter. 



The rocks of the Purcell mountain system have transmitted thrusts of 

 enormous power and have been vigourously upturned at many points. Yet 

 those rocks bear few traces of shearing, cleaving, or dynamic metamorphism. 

 Only in one narrow zone at the Moyie river is cleavage notably developed and 

 that structure is only conspicuous on the weathered ledges. This general 

 failure of metamorphic structures in rocks which have undergone at least once 

 the severe pressures of extensive mountain-building, is amply accounted for by 

 the exceeding strength of the sediments. That strength is m part explained 

 by the homogeneity of the formations and in part by their thorough welding 

 by deep burial and static metamorphism during the immense interval between 

 their deposition and deformation. To the inherent strength of the sedimentary 

 prism has been added the reinforcement by the thick sills which formed so many 

 new, relatively inflexible ribs in the whole mass. Where massive homogeneous 

 quartzite and gabbro predominated (Yahk and Moyie ranges), folding is 

 almost entirely absent and the orogenic pressures produced monoclinal blocks. 

 Farther east, where relatively thin-bedded argillites entered the formation in 

 greater number and where the gabbro sills were not intruded (McGillivray 

 range), the mountain-building produced broad folds rather than upturned 

 fault-blocks. Nevertheless, the rocks of the Purcell series seem everywhere to 

 have much greater average strength than have geosynclinal sediments generally. 



Note added during reading of proof. — Mr. S. J. Schofield has recently shown that 

 a thick, ferruginous quartzite-metargillite series, named the Aldridge formation, 

 underlies the Creston quartzite. It appears probable that the Aldridge is represented 

 in some of the fault-blocks mapped west of the Yahk river. The writer now (1912) 

 suspects that the succession in the sediments immediately east of the Moyie river is 

 normal and that the reversed fault there mapped does not exist. If so, the "over- 

 thrust"''' block of rusty quartzite really belongs to the Aldridge formation and not to 

 the closely similar Kitchener formation. 



