MINERALOGY OF BLACK LAKE AREA. 
9 
constituents of the rock, plagioclase and pyroxene, having in very large 
measure been replaced by secondary products, including chlorite, epidote, 
quartz, calcite, leucoxene, and occasionally tufted aggregates of actinolite 
needles. Pyrite and magnetite are common accessory minerals. 
It is believed that there was no essential difference in the original 
chemical composition of the gabbro and diabase, but that the same 
magma has given rise to one or other of these rocks, according to the 
conditions of cooling. 
Diabase is well exposed along the Quebec Central railway between 
Black Lake and Thetford Mines, and also near the Roman Catholic 
church at Black Lake. In places, the rock, through loss of pyroxene, 
becomes more acid in composition and passes into porphyrite near the 
outer edges of the masses. 
PORPHYRITE. 
Marginal phases of the diabase frequently exhibit a porphyritic 
structure, in which phenocrysts of plagioclase are distributed through a 
groundmass which consists chiefly of fine granular chlorite and epidote. 
Although unimportant as regards the area it occupies, the porphyrite 
is noteworthy as marking the acid extreme of differentiation in many 
places. 
GRANITE. 
The granite is light-coloured, usually grey, but occasionally it has a 
pinkish tint. As compared with the basic rocks of the serpentine belt, 
it occurs only in small amount, but it is important owing to the favourable 
influence it seems to have had in the formation of the asbestos. Hills of 
granite are found in the northeastern part of Coleraine, and in other 
places the rock appears, probably as a direct differentiate, forming 
isolated masses which grade into diabase or porphyrite. More frequently, 
however, the granite occurs as dykes intersecting the more basic rocks, 
and such dykes may be seen in most of the asbestos pits. 
The rock is composed usually of quartz, feldspar (both orthoclase 
and plagioclase), and hornblende; in general, biotlte is absent, although 
in certain localities it appears as an important essential constituent, 
more especially in the central parts of the larger bosses of granite, and 
there also the structure is often porphyritic. A little iron oxide is 
usually present. 
APLITE. 
Ferromagnesian minerals are absent from some of the dykes. In 
such cases the dykes are composed of quartz and feldspar only, and 
often they are so fine-grained that the constituent minerals cannot be 
distinguished in hand specimens. Such aplite dykes, while comparatively 
