438 INTRUSIVE ROCKS OF THE DISTRICT OF MONTREAL. 
more abundant and appears in slender crystals, with augite and a 
smaller proportion of olivine than the last. A specimen of this 
variety crushed and washed, gave 3.9 p. c. of magnetic iron, and 
10.0 p. c. of a mixture of ilmenite with olivine. The feldspar was 
obtained nearly pure, in the form of slightly yellowish vitreous 
grains having a density of 2.731-2.743. Its analysis gave the com- 
position of labradorite : 
XVII. 
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99-00 
Rougemont.—The rocks from this mountain offer very great 
varieties in composition and appearance. Some portions are a coarse 
grained dolerite in which augite greatly predominates; grains of 
feldspar are present, and a little disseminated carbonate of lime. In 
some specimens the augite crystals are an inch or more in diameter, 
with brilliant cleavages, and grains of pyrites are abundant, with 
calcite, in the interstices. This rock approaches closely to the highly 
augitic dolerite of Montarville. The olivine which characterises the 
latter mountain is also very abundant in two varieties of dolerite 
from Rougemont. One of these consists of a grayish-white finely 
granular feldspathic base, in which are disseminated well defined 
crystalline grains of black augite and amber coloured olivine, the 
latter sometimes in distinct crystals The proportions of these 
elements vary in the same specimen, the feldspar forming more than 
one-half the mass in one part, while im the other the augite and 
olivine predominate. By the action of the weather the feldspar 
acquires an opaque white surface, upon which the black lustrous 
augite and the rusty-red decomposing olivine appear in strong 
contrast. 
Another variety of dolerite from this mountain may be described 
as a fine grained grayish-black basalt enclosing a great number of 
crystals of dark bottle-green translucent olivine, which appear in 
high relief upon the weathered surfaces, and are often half an inch 
in diameter. 
