General Geology

area suggest that the group may at one time have been more extensive than it
is now.

This unit is composed of rhyolite, trachyte, dacite, minor andesite and basalt,
and related breccias and tufts. The fragmental rocks appear to be equal to the flows
in volume. Sedimentary rocks are not abundant, but some beds of tuffaceous shales
and sands occur, though they are neither thick nor widespread.

The lavas are creamy white, greyish white, pink, lavender, red, yellow, apple-
green, dark green, black, and grey. Flow-banding is common and characteristic
and in places is so even and regular that the rock resembles laminated shale;
elsewhere banding is contorted. Spherulitic structures are common. Perlitic rhyolite
or perlite occurs infrequently. Rhyolites are most common rock type but even
these vary. A typical and widespread rhyolite has phenocrysts of clear quartz up to
one eighth inch in diameter in a white to creamy matrix of cryptocrystalline quartz
and feldspar. Dykes of this type of rock occur north of Chelaslie Lake and in great
numbers along Tetachuck River, flows are present 8 miles north of Fawnie Dome,
southeast of Mount Greer, and on both sides of Cheslatta Lake. A unique rhyolite
occurs north of Borel Lake, consisting of many phenocrysts of smoky quartz to
one eighth inch in diameter in a black cryptocrystalline groundmass. Commonly
flow-banded, mauve rhyolites with phenocrysts of quartz and plagioclase feld-
spar outcrop frequently, particularly north of Knapp Lake. Grey flow rocks
with salmon-coloured potash feldspar phenocrysts outcrop at the west end of
Tetachuck Lake. Dark green or purple andesites with white lath-shaped feldspar
phenocrysts form a minor part of the unit. Dense black basalt occurs infrequently
and may in many places be younger sills or dykes rather than contemporaneous
flows.

Fragmental rocks, tuffs and breccias, are equally abundant. Chalky white
rhyolitic tufts and breccias outcrop as poorly sorted beds up to 50 feet thick near
Marilla, west of Natalkuz Lake, around Lucas Lake, along Chedakuz Creek, and
north of Uduk Lake. Similar fragmental rocks occur in thinner beds elsewhere. Near
Intata and Natalkuz Lakes a coarse breccia outcrops that consists mainly of rhyo-
lite fragments with fragments of other Tertiary and Mesozoic rocks. It has more the
appearance of eroded detritus than of an accumulation of volcanic ejectamenta.
Many other breccia and tuff beds, equally as varied as the related flows, form
important sections.

Normal sedimentary rocks, composed mainly of stream gravels and sands
derived from Mesozoic and Tertiary strata, occur only along major valleys. The
rocks are poorly consolidated, soft, and friable, and are generally less than 25
feet thick. White tuffaceous silt and fine sand have been noted at a few places
not related to major valleys.

Structural Relations

The internal relations of the Ootsa Lake Group are not clear. Where the
rhyolite and andesite units are exposed together the rhyolite unit is younger but

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