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primary mineral in the peridotite. Some fresh-appearing rocks are com-
posed almost entirely of hornblende and olivine; others contain hornblende,
olivine, and pyroxene. In most of these rocks the hornblende occurs in
rounded to irregular, in some instances interstitial, grains, with smooth,
sharp contacts against the adjacent minerals. One distinct rock type has a
pronounced porphyritic texture, with subhedral, black hornblende crystals
up to 15 mm. long evenly distributed through a pale olive-green fine-grained
matrix of fresh pyroxene and minor olivine. Other rocks, best described as
hornblende-bearing pyroxenite, consist of fresh, anhedral pyroxene grains
among large, irregular hornblende grains. The ‘primary’ hornblende is
strongly coloured, pleochroic brown to green in thin section. No unusual
optical properties have been observed. The fresh ‘hornblende peridotites’ in
the eastern part of the stock east of Polaris Creek contain hornblende grains
with a maximum extinction angle of 17 degrees, an optic angle of about —80
degrees, and a mean index of refraction (average of only four grains) of
1-670. These properties suggest a relatively high Mg/Fe ratio and an
average Ca/Na ratio as compared with most of the common hornblendes.

Chromite. Chromite occurs in two distinct forms in the ultramafic
rocks. Most common, but inconspicuous, are irregular, cuspoidal or shard-
shaped masses interstitial to olivine and pyroxene. These masses are
distributed throughout almost all the dunites and peridotites, and show a
tendency to be concentrated in bands less than a foot thick. In the richest
of such bands, chromite comprises about 5 per cent of the rock. Chromite
also occurs in rounded to octahedral grains up to 3 mm. in diameter, either
as individual inclusions within olivine and pyroxene crystals, or grouped
into the layers, lenses, and lacy networks that are conspicuous on outcrops
and in hand specimens (See Figures 6 and 7). The composition or range of
composition of the chromite is not known. Most grains show a brown streak,
and many are coffee-brown in thin section. Those tested with the blow-
pipe showed the presence of chromium. Some of the layers rich in chromite
strongly affect a compass needle, but. whether this is due to the chromite
alone or to magnetite developed in the rock as a result of serpentinization
is difficult to determine; specimens of the chromite lenses do not appear
to be appreciably magnetic. It may be that some of the material here called
chromite has such a low CragOg content that it would be more accurately
termed spinel (picotite) or even magnetite; but in order to avoid confusion
with the magnetite that is formed in the same rocks during serpentinization,
all the primary metallic oxides, many of which contain chromium, of these
ultramafic rocks are here referred to as chromite.

Serpentine. Like most bodies of similar composition, the ultramafic
rocks in the Lay Range and near Wasi Lake have been partly to completely
serpentinized. About one-tenth of the outcrop area of these rocks is
underlain by material so completely converted to serpentine, with accessory
magnetite, iddingsite, picotite, and chromite, that no evidence of the original
rock type remains; fully half is underlain by rocks so highly serpentinized
that almost no original minerals are preserved, though the original rock type
can be inferred from the various forms and structures of serpentine present;
the remainder of the ultramafic rocks show varying degrees of serpentiniza-
tion. Unserpentinized ultramafic rocks, although widely distributed and

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