49

Range; east of Mount Lay, the stratigraphic range is at least 5,500 feet.
Tt seems to form mainly from the decomposition of the remaining chlorite in
the rock, though it probably also draws material from magnetite, and
possibly from biotite. The garnets from the lowest grade metamorphic
zone are irregular in outline and are pseudomorphous after patches or
groups of grains of chlorite. Most of them enclose grains of quartz,
biotite, and feldspar, commonly to such an extent that as much as 90 per
cent of a large skeletal crystal may consist of inclusions. All stages of
growth toward euhedral, dodecahedral crystals relatively free from inclu-
sions can be observed in specimens of successively higher metamorphic
grade. That this growth of crystals was taking place at a time of crustal
movement is shown by spiral lines of inclusions in garnet porphyroblasts
from about the middle of the Tenakihi group section, indicating a rotation
of the grain during growth, and that at least some deformation had already
occurred at the time of crystal growth is shown by helecitic lines of inclu-
sions that preserve and early, wavy, crumpled ‘schistosity’ in garnets in a
rock now almost eompletely recrystallized to coarse, regular layers.

The clinozoisite-epidote-bearing rocks are apparently restricted to
beds of distinctive chemical composition. It has been established (Turner,
1948) that clinozoisite may occur in rocks with or without plagioclase, if
the amount of available lime exceeds the maximum that can enter into
plagioclase under the existing temperature and shearing stress. In almost
all the other rocks in the Tenakihi group plagioclase appears to be the
only essential mineral containing lime.

Kyanite forms conspicuous porphyroblasts in beds about 2,500 feet
below the top of the Tenakihi group section; it is plentiful over a strati-
graphic range of at least 3,000 feet, and persists as a few sparsely distributed
crystals almost to the zone of highest grade metamorphism. Thus, a
large part of the group falls within the ‘kyanite zone’. Coincident with
the most plentiful kyanite, porphyroblasts of staurolite are abundant;
their statigraphic range appears to be limited to about 1,500 feet, and
within this range staurolite is confined to certain beds, presumably those
rich in ferrous iron. Thus no true ‘staurolite zone’ appears to exist; but
staurolite occurs locally within the kyanite zone.

The main change at successively lower horizons within the kyanite
zone is one of slightly increasing grain size and a tendency toward gneissic
foliation. Within 5,000 feet of the lowest beds of the exposed section, in
the Tenakihi Range, and in almost the stratigraphically lowest beds in
the Chase Mountain and Mount Lay areas, small, needle-like crystals
appear in the grains of biotite, muscovite, and quartz. These crystals have
been tentatively identified as sillimanite; although they constitute less
than 1 per cent of the rock in all thin sections examined, the temperature
and pressure conditions appropriate for their formation appear to have
been reached, and the lowermost exposed horizons in all main areas of the
Tenakihi group rocks may be tentatively considered to have entered the
‘sillimanite zone’ . In the stratigraphically lowest rocks of each of these
areas, the crystalloblastic texture is considerably more regular than in
higher beds; the mutually poikilitic texture, in which large irregular grains
of biotite contain inclusions of quartz and feldspar, and large irregular
grains of quartz contain inclusions of biotite and feldspar, has given way to