366 DEPARTMENT OF THE INTERIOR 



2 GEORGE V, A. 1912 



along with smaller ones of quartz, biotite, and acid oligoclase, near Ab„ An t . 

 The ground-mass is a macrocrystalline granophyre of quartz and feldspar, 

 carrying a little accessory apatite and magnetite. 



A dike of about the same size and essentially of the same composition 

 (though with minute biotite in the ground-mass) cuts a thick sill-like dike 

 of syenite porphyry outcropping just north of the Boundary line on the Velvet 

 Mine wagon-road. This dike follows a master-joint plane in the older por- 

 phyry. Two hundred yards farther north on the wagon-road a thicker intrusion 

 of the same granite porphyry follows the bedding of the Sophie mountain con- 

 glomerate. 



Missourite Dike. 



In the col between Record mountain and Granite mountain, west-north- 

 west of Rossland, the Coryell syenite is cut by a five-foot dike of rock, which in 

 composition is unique among all the specimens collected during the Boundary 

 survey. It is a dark brownish-green, fine-grained, somewhat porphyritic trap, 

 apparently corresponding mineralogically and chemically to an olivine-free 

 missourite, bearing phenocrysts of pseudoleucite. In the hand-specimen a few, 

 small, black crystals and innumerable glints of light from minute foils of mica 

 may be discerned. The pseudoleucite phenocrysts are conspicuous but do not 

 constitute more than five per cent of the rock by weight. 



In thin section the pyroxene is seen to occur in highly idiomorphic prisms 

 from 1.5 mm. to 0.1 mm. or less in length. The pale greenisl colour of the 

 mineral, a lack of pleochroism, and a high angle of extinction indicate that it is 

 a common augite. The mica is a strongly pleochroic, brown biotite and is also 

 thoroughly idiomorphic. Abundant cubes of magnetite (probably titaniferous) 

 and many, relatively large prismatic crystals of apatite are accessory. All of 

 these minerals are embedded in a pale greenish to brownish matrix, largely 

 composed of the same material as that forming the phenocryst-like areas 

 referred to pseudoleucite. 



The diagnosis of the phenocrysts and of the related ground-mass of the 

 rock has offered considerable difficulty. The phenocrysts, ranging from. 1 mm. 

 to 3 mm. in diameter, have roundish, polygonal outlines of the order expected 

 from idiomorphic leucite. They are habitually wrapped about with foils of 

 fre?h, primary biotite, arranged tangentially about the round phenocrysts. 

 Notwithstanding the perfect freshness of augite and biotite, none of the 

 original substance of the large phenocrysts seems to remain. Each pheno- 

 crystic mass is chiefly made up of pale greenish-gray, spherulitic aggregates of 

 fibrous material showing aggregate polarization, with the black cross in parallel 

 polarized light. The spherules are not clean-cut but fade into each other 

 most irregularly. The long, hair-like elements of the spherulitic substance 

 are so thin that it is difficult to be sure of their proper colour or of their full 

 reaction to polarized light. The single element is probably quite colourless. 

 It always shows negative optical character with respect to its length, and the 

 extinction angle of the crystallite is never more than about 5°, corresponding 



