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mineral in the overlying Ingenika group, has been largely replaced by
biotite. Rocks from these beds show, upon microscopic examination, strings
and patches of biotite developing from the chlorite. Material for the
formation of biotite is probably also drawn from the sericite, magnetite, and
quartz present, but the first biotite formed appears to be pseudomorphous
after chlorite. Along with this change is an increasing development of a
banded, instead of a merely schistose, structure; the biotite shows not only
a more perfect parallel orientation of each grain, but a greater tendency
for the grains to be grouped into strings or layers. These rocks would
correspond to Barrow’s ‘biotite zone’. Below about 3,000 feet strati-
graphically beneath the uppermost beds of the Tenakihi group, chlorite is an
insignificant mineral, and is chiefly confined to the purer quartzites, where
it persists in diminishing amounts to the zone of highest grade meta-
morphism,

The presence of two types of biotite—one commonly occurring as
splintery laths, the other as irregular skeletal or ‘poikilitic’? grains—in
some of the Tenakihi group rocks has been noted. Although plates of
chlorite are intergrown with, and may be found between, fractured frag-
ments of the same grain of splintery biotite, this type of biotite appears
to be older than and distinct from chlorite. It is the poikilitic type of
biotite that can be observed in thin section in various stages of formation
from chlorite. The textural relations, the distribution of minute inclusions
of zircon, rutile, tourmaline, etc., and the uniformity of grain size suggest
that some, at least, of the splintery biotite is original detrital material,
whereas the poikilitic biotite is of metamorphic origin produced from
chlorite during a single period of increasingly intense metamorphic
conditions.

Sericite, which is fairly abundant in the less-metamorphosed overlying
Ingenika group rocks, seems to be lacking or relatively inconspicuous below
the upper beds of the Tenakihi group. As successively higher grades of
metamorphism within the ‘biotite zone’ are reached, however, white mica
is reconstituted as coherent flakes of muscovite, which increase in size and
abundance as lower stratigraphic levels (and, therefore, presumably higher
temperatures and pressures during metamorphism) are reached.

Only the uppermost beds of the Tenakihi group can be considered to
fall strictly within the ‘biotite zone’. Within 500 feet of the contact with
the Ingenika group in the Tenakihi Range and near Cutbank Creek, and
more or less coincident with the contact on Swannell River north of Chase
Mountain, rudimentary garnets can be observed in thin sections of the
schist; these, becoming more coherent and conspicuous at slightly lower
stratigraphic levels, mark the advance of the rock into the ‘garnet zone’.
Under the microscope it can be seen that small irregular grains of garnet
form in patches of chlorite at a metamorphic stage very little higher than
that at which biotite becomes conspicuous. Indeed, in a few places garnets
occur in the overlying Ingenika group rocks. No difference in optical
properties was observed between garnets from the lowest and from the
highest grade metamorphic zones in which they are conspicuous in the
Tenakihi group.

Garnet is a conspicuous mineral over a stratigraphic range of about
6,000 feet in the type section of Tenakihi group rocks in the Tenakihi