97

Examined microscopically, andesine or andesine-labra-
dorite and augite are seen to be the essential minerals; some-
times phenocrysts of labradorite are found. Apatite occurs in
small amounts, and chlorite, calcite, sericite, kaolin, etc., are
the secondary minerals. The andesine is as calcic as AbssAnus,
approaching labradorite, and some of the labradorite phenocrysts
have the composition Ab,;An;;.. The plagioclase is only slightly
altered, and the augite, which is colourless, is often quite fresh.
Alteration is much less noticeable than in the andesites and
dacites.

Biotite Andesite Porphyrite. Rocks classified under this
type were found only in two localities, on upper Hidden creek,
where it is probably intrusive, and in the vicinity of Parry
passage, where a very large dyke of this rock forms all of Lucy
island, Cape Knox, and probably the reefs far to the west of
Cape Knox.

The rock from Hidden creek is a light greenish-grey porphyry
with about 10 per cent of plagioclase phenocrysts up to 4 mm.
and averaging 2 mm. in diameter, and a few, scattered, dark
brown phenocrysts of biotite.

Studied under the microscope, the phenocrysts prove to be
calcic andesine and biotite in a groundmass made up of finely crys-
talline plagioclase laths, presumably andesine, with a small
amount of chloritic material. The composition of the plagio-
clase phenocrysts is andesine, near labradorite, Abs;An4;, and
they are much replaced by kaolin and calcite. The phenocrysts
of biotite are euhedral and of varying shapes, some rectangular,
others rounded-hexagonal, triangular, etc. They vary in size
up to 2 mm., but are mostly smaller. The fresh crystals are
strongly pleochroic, pale green to deep brown, and have a very
small axial angle, with an almost uniaxial interference figure.
Most of the biotite is completely altered to a very finely granular
greenish substance, probably chloritic, with disseminated mag-
netite dust. Various stages of alteration are present, but the
change is usually abrupt, varying amounts of fresh biotite lying
in abrupt contact with the granular chlorite.

The rock from Parry passage is a light grey porphyry, with