MINERALOGY OF BLACK LAKE AREA. 
7 
PERIDOTITE. 
The peridotite is massive, with a dark green colour, weathering to a 
somewhat rusty brown. Locally it is so rich in olivine as to become a 
dunite ; the average type in this area, however, contains 80 to 90 per cent 
of olivine, together with a little pyroxene (usually orthorhombic) and a 
few feldspar individuals; magnetite and chromite are also commonly 
present as disseminated grains. The relative amount of pyroxene 
present in the rock can usually be roughly estimated by mere inspection 
of hand specimens, owing to the cleavage and somewhat bronzy lustre 
exhibited by this mineral; with increase in the amount of pyroxene the 
rock passes into a pyroxenite. 
In thin sections, it is seen that the iron ore crystallized first, followed 
by the olivine. The crystals of olivine all have about the same size, and 
they are sometimes enclosed within the pyroxene, which forms rather 
large individuals, and was the last mineral to crystallize. The olivine 
is seldom entirely fresh; alteration has commenced as usual around the 
margins of the crystals, and along cracks traversing them, and as the 
metamorphism becomes more and more complete the rock passes over 
into serpentine. 
SERPENTINE. 
The serpentine, like the peridotite, is uniformly dark olive-green 
when fresh, weathering to a creamy colour. Since the alteration of the 
olivine of the peridotite commences on the surface of the crystals, and all 
stages of the process are to be met with, from the fresh original rock to the 
completely serpentinized variety, it is not surprising that it is often a 
matter of difficulty to distinguish between peridotite and serpentine in 
exposures, or even in hand specimens. For this reason, the two, as well 
as all partially altered peridotites, are generally referred to collectively 
at the mines as “serpentine,” and, using the term in this sense, it may be 
said that many of the larger hills in the mining district, such as those 
near Black lake, are composed of serpentine (Plate IV). Serpentine 
proper, however, does not form large bodies; it occupies only small 
areas or zones within the peridotite (or related rock) where the meta- 
morphism has been practically complete, and in this way it forms the 
country rock of all the asbestos and chromite mines. 
The rock has a massive compact texture, but contains occasional 
crystals of unaltered or slightly altered pyroxene; on freshly broken 
surfaces these are easily visible, owing to their cleavage and lustre, 
while on weathered surfaces they stand out in relief, as may be well seen 
near the summit of the hill adjacent to Black Lake village. 
When thin sections of the rock are examined, it is usually found that 
the whole of the olivine has been completely altered to serpentine, although 
