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with the even degree of metamorphism to which the rocks have been sub-
jected, has resulted in a simple mineralogical composition of the rocks, and
a limited range of rock textures and micro-structures. Metamorphism has
progressed so far that the original differences between successive beds and
groups of beds are preserved only by the different proportions of the prin-
cipal minerals, namely, quartz, the micas, feldspar, and in places garnet and
kyanite; the few accessory minerals occur in almost all beds of suitable
metamorphic grade.

Quartz is the predominant mineral in at least 80 per cent of the
Tenakihi rocks. Except for a few of the larger grains, which may be
detrital, it is entirely recrystallized, or crystalloblastic, with a certain degree
of shape orientation parallel with the schistosity of the rock. In the lower
beds of the group the quartz is mainly clear and unstrained, whereas in the
higher beds it commonly shows wavy extinction due to strain. Many of
the purer quartzites have a well-developed mortar structure, with large,
strained grains rimmed by a profusion of smaller grains, suggesting that
there has been deformation and slight recrystallization of an already
recrystallized quartzite. The feldspar, which composes up to 35 per cent
of individual specimens, is nearly all authigenic, commonly occurring in
irregular, skeletal grains interstitial to, and including, oriented grains of
quartz and the micas. Most of the feldspar grains whose optical properties
could be accurately determined are oligoclase or andesine, but some albite
and orthoclase have been recognized. Biotite is the predominant mica, and
is the mineral mainly responsible for the fissile character and banded
appearance of the Tenakihi group rocks. Viewed in transmitted light, the
biotite is of both brown and green varieties, with the latter type restricted
to the stratigraphically higher beds of the group. The brown biotite occurs
in two distinct forms, commonly present in the same specimen. One form
consists of thin, lath-like flakes, commonly straight, or, if bent, splintered;
these grains show the most perfect parallel orientation of any mineral in
the rock. The other type of biotite is typically crystalloblastic, poikilitic,
or skeletal, surrounding grains of almost all the other minerals in the rock,
including the lath-like form of biotite. Muscovite, as distinct flakes, is an
important mineral only in the lower half of the Tenakihi group stratigraphic
sequence. In the very uppermost beds it reappears as sericite. Chlorite
occurs only in the highest beds, associated with green biotite and garnet.

Garnets are the most abundant and conspicuous porphyroblasts in the
Tenakihi group. They occur throughout the entire upper 6,000 feet of the
section, and in a few beds as much as 38,000 feet stratigraphically lower.
The garnets range in abundance from a few minute crystals in relatively
pure quartzites to nearly half the rock volume in strongly porphyroblastic
garnet-mica schists. Individual crystals range in size up to 24 centimetres
in diameter, though most are 1 to 3 mm., and in form from attenuated net-
works and irregular patches to symmetrical, dodecahedral crystals. Most
crystals are crowded with inclusions of quartz, chlorite, biotite, muscovite,
tourmaline, and magnetite; in many the inclusions are arranged in a spiral
or S-shaped pattern, probably indicating a rotation of grains during growth.
The composition of the garnets appears relatively independent of the posi-
tion in the stratigraphic section or of the lithology of the bed in which the
grains occur; those grains tested are almandine, with about 85 per cent of
the almandite molecule. Kyanite forms porphyroblasts in a fairly definite