REPORT OF THE CHIEF ASTRONOMER 211 



SESSIONAL PAPER No. 25a 



zone a is only about twenty feet thick near the summit monument. Zone a 

 includes blocks of quartzite and metargillite, these rocks being torn and 

 slivered as if the sediments were scarcely consolidated when they were over- 

 ridden by the flood of lava. At this section, with the exception of the twenty 

 feet of brecciated lava, the whole formation, again nearly 500 feet thick, is 

 made up of the deep gray-green amygdaloid. The reason for the non-appear- 

 ance of the porphyritic phase in this well-exposed section is not apparent. 



On the east-west ridge two and a half miles south-southeast of the monu- 

 ment, the porphyritic zone reappears at its proper place in the section though 

 it is not so thick as in the type section. No argillitic beds here break the 

 continuity of the lava. The most easterly exposure of the Purcell Lava in the 

 McGillivray range is that at the Kootenay Eiver flats. There the section 

 showed three members with approximate thickness as follows :— 



Top, erosion surface. 

 300+ feet — blackish-green amygdaloid. 



140 porphyritic lava with large phenocrysts of labradorite. 



15 " brecciated lower-contact zone. 



Total... 455+ feet. 



Base, conformable Siyeh metargillite. 



About 220 feet below the base a second sheet of highly sooriaceous amyg- 

 daloid, fifty feet thick, is conformably intercalated in the Siyeh strata. This 

 lava corresponds in all respects with the uppermost member of the Purcell Lava 

 proper. It occurs nowhere else in the Boundary sections and must have been 

 a quite local flow. 



To north and south of the summit monument a twenty-foot flow of 

 rhyolite lies interbedded with the Gateway metargillites. Its base is at a hori- 

 zon about fifty feet above the top of the Purcell amygdaloid. This occurrence 

 of acid lava is unique in the range and has no known parallel in the Galton 

 or Clarke ranges. It can be easily studied at the 6,400-foot contour on the 

 main Commission trail, a half-mile north of the monument. The rock is a 

 greenish-gray, slightly vesicular lava, bearing abundant phenocrysts of quartz 

 and feldspar, from 1 nam. to 5 mm. in diameter; no dark-coloured mineral is 

 macrosciopically visible. 



The thin section shows that the phenocrystic feldspar includes orthoclase 

 (often microperthitic in look) and acid oligoclase. Like the quartz these are 

 idiomorphic. A few small, deep yellow crystals of allanite are accessory. The 

 feldspars are greatly kaolinized. The ground-mass was probably once mostly 

 glass but is now completely devitrified. It is a very pale greenish mass of 

 secondary material enclosing minute feldspar crystals and rounded quartzes, 

 with apatite and altered ilmenite (leucoxene). Rutile needles have developed 

 in the alteration of the ore. The main part of the ground-mass always polarizes, 

 at least faintly. Most of it consists of quartz and a secondary, micaceous 

 mineral, probably sericite, whose abundance seems to explain the relatively high 

 specific gravity of the rock (2-735). The small steam-vesicles are filled with 

 quartz and calcite. 



25a — vol. ii — 14-J- 



