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is obscure. At least 80 per cent of the large stock east of Polaris Creek
is highly serpentinized. The most abundant serpentine, which uniformly
pervades most of the peridotites, is a very dark bluish green to black,
dull to waxy lustred material that weathers a bright orange-brown to a
depth of 4 inch or more. In thin section, this material is seen to consist
mainly of bladed, lamellar antigorite, in an irregular, very fine-grained
aggregate. Many sections contain fine fibrous chrysotile, which forms
networks in massive serpentine (serpophite) or surrounding masses pseudo-
morphous after pyroxene (bastite). In common with most serpentinized
bodies, this bright orange-brown weathering serpentine is exceedingly infer-
tile, owing to traces of chromium and nickel in the derived soils (Robinson,
et al., 1935), so that the main part of the ultramafic stocks are almost
completely devoid of plant life, either in the form of sub-alpine forest and
meadowland at lower elevations or of lichen above timber-line; because of
its barrenness and distinctive weathering colour the outlines of the stocks
are conspicuous from a distance.

The successive steps in the process of progressive alteration of fresh
ultramafic rock into serpentinite can be observed in thin section. As is
usually the case, the olivine crystals are more susceptible to serpentiniza-
tion than the pyroxene, and a thin band of what appears to be lamellar
antigorite appears along crystal and fracture faces of olivine in rocks that
otherwise appear fresh. In some rocks iddingsite (or bowlingite) develops
around the olivine at this stage. When the process is farther advanced,
the bands of antigorite(?) around and within the olivine crystals are in
many cases seen to be flanked by zones of bladed and fibrous serpentine;
and at the same time layers of exsolved magnetite in the cleavage planes
of the pyroxene grains indicate the beginning of a gradual replacement of
that mineral by bastite. The alteration of the pyroxene, although com-
mencing somewhat later than that of the olivine, is apparently a relatively
rapid process. Some sections show that the pyroxene has, apparently,
been completely replaced by bastite, whereas the adjacent olivine crystals
have been merely divided into a number of fragments separated by a
network of serpentine and fine-grained magnetite. The magnetite is
commonly expelled to the centre of each widening vein, marking the posi-
tion of the original fracture or crystal boundary. In several sections it
appears that the olivine alters to chrysotile, which is in turn replaced by
antigorite, and that the zone of fibrous serpentine migrates toward the
centre of the grain or fragment as serpentinization proceeds. The last
remnants of the olivine grains seem to be replaced by a massive, structure-
less, nearly isotropic serpentine (serpophite).

Bastite and chrysotile are not common in the serpentinites. This is no
doubt partly because the rocks originally containing relatively more pyro-
xene and thus capable of developing relatively more bastite are uncommon,
and less susceptible to serpentinization than other peridotites, and it may
also indicate a general tendency for antigorite to replace bastite and
chrysotile. ;

In addition to the pervasive, black, orange weathering serpentine, a
great variety of serpentines and related minerals occur in smaller amounts.
Bands up to 120 feet wide composed almost entirely of dark green to black,
very glossy serpentine are found along some contacts between ultramafic
and sedimentary rocks, and flanking the rare corundum-bearing ‘white

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