208 DEPARTMENT OF THE INTERIOR 



2 GEORGE V., A. 1912 



to have extended as much as nine miles to the westward of these outcrops, for 

 at that distance the Moyie formation (equivalent to the Gateway) rests directly 

 upon the Kitchener (equivalent, in its upper part, to the Siyeh). Such rela- 

 tions, coupled with the fact that the lava thickens between the western summits 

 of the Galton range and the. summit of the McGillivray range, seems to indi- 

 cate pretty clearly that the greater flows were supplied from vents located near 

 the present summit of the McGillivray range and not far from the Forty-ninth 

 Parallel. One vent seems to be represented in a long, 50-foot dike which cuts 

 the upper beds of the Kitchener formation in a meridional direction, at a point 

 just north of the Boundary line and about 600 yards west of the most westerly 

 band of the lava. In the Galton, Clarke, and Lewis ranges the Purcell lava 

 seems to have issued, in like manner, from local vents, some of which are dikes 

 cutting the underlying Siyeh formation and can be examined in the mountain- 

 walls of all three ranges. Everywhere the lava was emitted in true fissure- 

 eruptions, which were vigorous and wide-spread while they lasted but were not 

 of long duration. The immediate association of dolomites and metargillites 

 both above and below the lava in the Rocky Mountains and the perfect con- 

 formity of these sediments with the lava flows suggest that the eruptions took 

 place on the sea-floor. 



One of the best sections of the formation as exposed in the McGillivray 

 range, occurs at the summit of the 6,583-foot mountain, situated 2,000 yards 

 north of the Boundary line and five miles east of the main fork of the Yahk 

 river. The total thickness is there 465 feet. At the base forty feet of mottled, 

 brecciated lava (zone a) lies directly on the Kitchener (Siyeh) metargillites. 

 This member has, in field appearance, much resemblance to a true tuff or ash- 

 bed and so it was described in the field notes. The microscope has shown, how- 

 ever, that the apparently fragmental masses of porphyrite are cemented in 

 part by an altered glass, bearing feldspar microlites in rough fluidal arrange- 

 ment. In other parts the cement has the composition of metargillite. The 

 writer has concluded that this lowest member is not a product of volcanic 

 explosion but the thick lower shell of a heavy mass of lava Avhich flowed out 

 over the old muds; as it ran, the mass froze and decrepitated, incorporating 

 some of the mud in its progress. 



The zone of overridden block lava is covered by a ten-foot layer (zone h) 

 of compact, slightly vesicular lava of similar composition and of a texture 

 like that of ordinary basalt. This zone also belongs to the chilled, though here 

 not brecciated, lower part of the main flow and passes gradually upward into a 

 massive, eminently porphyritic, non-vesicular phase, 200 feet thick (zone c). 

 Zone c is similarly transitional into the fourth phase, which consists of 220 

 feet of massive, highly amygdaloidal lava devoid of macroscopic phenocrysts 

 (zone d). 



The lava of zones a and b is a dark gray-green rock, originally a basic 

 glass charged with numerous microphenocrysts of labradorite near Ab 4 An... 

 These are usually between 0-5 mm. and 0-8 mm. in length. Octahedra of 

 magnetite represent, the only other primary constituent, unless some of the 



