Dresser — Continbxition to the Geology of Quehec. 45 



been hitlierto mistaken for sedimentaries, and have been 

 accordingly treated on that assumption in all the stratigraphi- 

 cal discussions regarding them. 



(a) In the Stolie Mountain helt the principal rocks exam- 

 ined were taken from the township of Ascot, on the west 

 side of the St. Francis river, between Sherbrooke and Lennox- 

 ville. Others were taken from various points to the south- 

 eastward on the upper Belvidere road and from the vicinity of 

 the Suffield, Sherbrooke and Clark copper mines. The rocks, 

 which are generally fine in texture, are of various shades of 

 green and gray in color, and many are in advanced stages of 

 metamorphism. In the thin section, however, the raicrostructure 

 remains sufficiently distinct to clearly establish the igneous 

 origin of practically all of these rocks, and even to identify 

 the specific characters of several on the merely preliminary 

 examination of them that has yet been made. 



Qiuirtz-^orphyry forms the hanging wall of the Silver Star 

 mine at Suffield. It is of light gray in color, and from the 

 prominence given the quartz phenocrysts by the bleaching of 

 the base of the rock on weathered surfaces, it has been com- 

 monly regarded as a species of sandstone. 



By the aid of the microscope the structure is found to be 

 that typical of an efiEusive rock. A very finely crystalline 

 base holds phenocrysts of quartz, which show in basal sections 

 an uniaxial cross and positive sign ; and also of feldspar, 

 which extinguished in several cases with its principal axes 

 parallel to the planes of the crossed nicols and hence is ortho- 

 clase. A few feldspars are polysyntheticallv twinned and 

 accordingly are plagioclase. Their extinction angles are 

 small. Small rod-like individuals of colorless mica arranged 

 in lines are presumably of secondary origin. They often 

 occur within, or in association with, irregular areas of a 

 rhombohedral carbonate, apparently dolomite. 



Granite-porphyi'y ocQwv^ near Lennoxville on the line of the 

 Canadian Pacific Railway. It differs from the quartz-porphyry 

 chiefly in the more advanced character of its crystallization, 

 both quartz and feldspar being distinguishable in the ground- 

 mass. A little chlorite, pyrite, brown iron oxide, and color- 

 less mica are also present. An incipient cataclastic structure 

 is beautifully shown in both of these rocks. The quartz-pheno- 

 crysts are sometimes reduced to mosaics of quartz grains, or 

 at others are crossed by lines of crushed material, while the 

 crystals on either side of the lines remain unchanged or show 

 undulatory extinction. Being the largest single masses and 

 of the most brittle material, the quartzes thus appear in 

 every case to be the first constituents to show the effects of 

 purely dynamic metamorphism. A much altered rock form- 



