Nechako River Map-Area

of Fawnie Range are lithologically similar but their relation to the rhyolite unit
is in doubt.

Andesite forms more than half of this unit, basalt is less common, and dacitic
to rhyolitic flows are least abundant. Pyroclastic rocks are equally as important as
the flows. The total thickness of the unit varies from place to place and is difficult
to estimate; more than 1,500 feet of flows and fragmental rocks are exposed in
Fawnie Range and south of Francois Lake, but near Marilla the total thickness
is less than 500 feet.

The flow rocks are generally grey, green, reddish brown, and black. The
andesites are fresh and consist mainly of porphyritic rocks with phenocrysts of
andesine in a groundmass of biotite, hornblende, epidote, and feldspar. In some
rocks the groundmass is cryptocrystalline. Flow-banding is not common. Basalts
are commonly dense black rocks that would best be described as trap but these
grade in places to porphyries with labradorite or augite phenocrysts. Some basalts
contain much magnetite and commonly most of the groundmass is a dark glass with
embedded feldspar crystals. The basalts may be vesicular and amygdaloidal, par-
ticularly in the rocks near Frangois Lake. Columnar joints are common. The rocks
of this unit in Fawnie Range are characteristically reddish brown to purple.

A dense massive basalt and basalt breccia outcrops near Marilla and there
conformably underlies the rhyolite unit. This basalt also occurs near the outlet of
Ootsa Lake and near the east end of Cheslatta Lake. It lies near the base of the
group.

The fragmental rocks are coarse, poorly sorted, and only crudely stratified.
South of Anzus Lake, where they are best exposed, they are varicoloured and are
made up of volcanic fragments from rocks derived from the unit itself. These
different coloured rocks occur as distinct alternating layers of green, grey, brown,
white, and black fragments or as a single bed with fragments of several colours.
No evidence of water transportation is apparent and this intermixing of rock types
Suggests contemporaneous eruptions of more than one rock type within a small
area.

Sedimentary rocks are exposed only on an island in Chief Louis Bay. Pebble-
conglomerate and greywacke outcrop in beds 1 foot to 5 feet thick, with a total
exposed thickness of 150 feet. These rocks are well consolidated, and are formed
from detritus apparently derived from Mesozoic rocks. So far as the author knows
none of the fragments present is derived from Tertiary rocks and from this it is
concluded that the sediments occur at the base of the Ootsa Lake Group.

Rhyolite Unit

This unit is widespread in the map-area but is most extensive and best
exposed in the northwest quarter where it is essentially an extension of the part of
the Ootsa Lake Group to the west (Duffell, 1959, p. 67) and of similar rocks to
the north (Armstrong, 1949, pp. 71-74). Scattered outcrops in all parts of the

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