General Geology

several hundred feet of arkosic sandstone are present. This sandstone is fawn to
grey, fine grained, and well stratified in beds 2 inches to 2 feet thick. It com-
monly weathers a rusty yellow and contains a few fragmentary plant remains.

The sedimentary strata on Swing Peak are at least 2,000 feet thick at the
east end. They thicken towards the west end of the ridge where they may reach
a thickness of 3,000 feet.

The volcanic rocks overlying the sedimentary sequence consist of red,
green, and brown breccia and tuff and grey, brown, and black andesitic and
basaltic lava flows. The fragments of the pyroclastic rocks range in size from
1 mm. to as much as 6 or 7 inches. Some of the flows are porphyritic with
phenocrysts of plagioclase and hornblende in a fine-grained, purple ground-
mass. Others are amygdaloidal and some show the characteristics of a flow
breccia. The volcanic part of the group has a thickness of about 2,000 feet
and covers the central part of the ridge of Swing Peak.

Structural Relations

All strata on Laventie Mountain and Swing Peak have a general easterly
strike and southerly dip. The dip ranges from 12 to 15 degrees at the eastern
end of the ridge to as high as 65 degrees farther west, but is commonly under
30 degrees. The strike varies from north 40 degrees west to north 80 degrees
west, except where local structures cause a deviation from the general trend.

Both the sedimentary and volcanic sequences are intruded by the stock of
purple feldspar porphyry underlying a part of the eastern end of the Swing
Peak ridge. The whole sequence is also cut by fine-grained porphyritic and
dioritic dykes, and the granitic rocks of the Mount Bolom stock.

The contact of the sedimentary rocks with the underlying Hazelton group
was not observed, though much time and effort was spent in searching for it.
The eastern contact of the group lies close to a small stream along which there
appears to have been some movement and it may be that this contact is a
faulted one. The Hazelton rocks east of the contact have a decidedly different
trend and composition and if the contact is not faulted it is certainly not con-
formable. The Cretaceous rocks outcrop along the south shore of Tahtsa Lake
but were not observed on the north shore. The contact there may also be a
fault that lies under the lake.

Age and Correlation

The mudstones of the Lower Cretaceous series, which are conspicuous
near the base, contain marine invertebrate fossils that definitely establish the
rocks as belonging to the Albian stage of the Lower Cretaceous epoch.

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