MINERALOGY OF BLACK LAKE AREA. 
U 
these rocks, and, on crystallization, it formed dykes or veins of such 
minerals as diopside, grossularite, and vesuvianite. Present observations- 
indicate that veins of these minerals, containing high percentages of 
lime, are of more frequent occurrence in the chromite pits than elsewhere; 
that is, they are associated with a rock which was originally near pyroxe- 
nite in composition, and which contained an appreciable amount of lime. 
It is believed that the genesis of the colerainite veins is to be explained 
in a similar manner. In this case, however, a hydrous silicate of alumi- 
num and magnesium has been formed, containing no lime, and it is 
interesting to note that colerainite has been found only at asbestos 
pits, where the original country rock was a peridotite, containing little 
or no lime. 
SERPENTINE. 
Serpentine is the mineral most commonly met with in the various 
quarries and pits of the area. In its fibrous form, chrysotile-asbestos, 
it constitutes a product of great economic value. Several other varieties 
of the mineral occur and are described below, but they are only of miner- 
alogical interest. 
* 
Mode of Occurrence of the Massive Serpentine and Ckrysotile. 
Full descriptions of the mode of occurrence of the massive serpentine 
and chrysotile in the area may be found by referring to the report of 
Cirkel 1 and more especially to that of Dresser. The outstanding features 
may be summarized as follows: 
The massive serpentine occurs within the peridotite masses, in the 
form of zones or bands, whose boundaries against the peridotite are 
fairly sharp and are roughly parallel to one another, i.e., they rather 
resemble dykes in outline, and assume all attitudes from vertical to 
horizontal. 
The more prominent bands fall into roughly rectilinear sets, which 
follow the same directions as the main system of joints in the peridotite. 
Others, which are in general narrower and more irregular in their course, 
intersect these and one another at all angles, so that, in some places, the 
peridotite presents a remarkably intricate network of such serpentine 
bands. Of these minor bands, Dresser believes some follow strain 
fractures due to regional compression, and others, cracks caused by 
exfoliation. 
Although these bands are fairly well defined, the peridotite for some 
distance on each side of them is serpentinized to a fairly considerable, 
and gradually decreasing, degree. 
1 Cirkel, Fritz, "Chrysotile-asbestos, its occurrence, exploitation, milling, and uses;" Mines Branch, 
Can,, report No. 69, 1910. 
