General Geology

A breccia from the south slope of Tweedsmuir Peak is a hard, dense,
greenish grey rock containing fragments of black argillite, reddish feldspar
porphyry, and purple, green, and grey volcanic rocks cemented together by
light, fine-grained material of the same kind.

A red tuff from the west side of Morice Lake is composed largely of
particles of purple, red, and dark grey andesitic volcanic material more than
+ inch across. Under the microscope a few small phenocrysts of plagioclase
appear clear but the remainder is clouded by alteration products. Many of
the tuff beds are banded, a characteristic that is emphasized on a weathered
surface. A typical example of these tuffs is exposed on the east slope of
Sibola Peak where they are composed of light grey and maroon bands,
varying in thickness from + to 4 inch.

Fragmental rocks of the types described above form most of the
Hazelton group. Flows and sedimentary strata follow in order of abundance.
The larger fragments of the breccia are commonly angular but may be
subangular to subrounded. The presence of feldspars that occur as pheno-

crysts, commonly § to 4 inch in length, but in places as much as 2 inch,

is typical of both the fragmental types and the flows. This characteristic
is more common in the lavas and was largely responsible for Dawson (1875-

76, 1876) applying the name ‘Porphyrite’ to the group.

Most lava flows of the Hazelton group are of andesitic and basaltic
composition, rhyolites and dacites being much less common. The more basic
lavas are purple, brownish red, red, brown, grey, greenish grey, and less
commonly dark grey and black. The more acidic varieties are lighter in
colour. The andesites and basalts, as mentioned previously, display white
or light coloured phenocrysts of feldspar in a microcrystalline groundmass.
The groundmass commonly exhibits flow structure around the phenocrysts.
The intermediate and basic lavas are medium-grained, normal rocks in flows
up to 50 feet thick commonly well layered. Amygdaloidal and vesicular types
though not common do occur. Many of the andesites are now typical green-
stones with an almost dioritic texture. These rocks are characterized by the
usual feldspar (andesine) phenocrysts and short blocky crystals of horn-
blende and a pyroxene, probably augite. Rhyolites are subordinate in amount
but are present in both the lower and upper volcanic divisions. They may
be cream, white, light green, or light purple. They are fine grained in texture,
but some show a few quartz grains or feldspar phenocrysts in a microcrystal-
line groundmass of quartz and feldspar. Magnetite and biotite are minor
constituents. Dacite lavas are present in the group but rare.

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