43

Most borders of the porphyries are chilled for a few inches at their
contacts with the enclosing Takla and Sustut strata. Near the contacts,
too, the porphyries and adjacent strata may contain a little disseminated
pyrite. The bordering strata have not been greatly altered, but locally they
have been hardened and weather white,

Age

Although the Kastberg porphyries intrude the Sustut strata they
have not been recognized in the upper, Paleocene division of that eroup.
However, as no significant break in deposition is known between the lower
and upper divisions, the Kastberg rocks are probably of post-Paleocene
age. A feldspar-quartz porphyry body 34 miles west-northwest of Mount
Carruthers, lithologically similar to exposures of known Kastberg porphyry,
is cut by dykes of Tertiary or later basalt, and the Kastberg rocks, therefore,
are not younger than Tertiary. Furthermore, the time probably necessary
for ensuing Tertiary events suggests that the porphyries were most probably
emplaced in early Tertiary time.

BASALT NECKS, DYKES, LAVAS, AND PYROCLASTIC ROCKS
Distribution

The youngest known consolidated rocks comprise a distinctive group
of intrusive and extrusive basalts and associated pyroclastic rocks, and
occur as scattered bodies within the south-central part of the map-area.
They are about co-extensive with the Kastberg intrusions. Volcanic necks
were found about 5 miles east-southeast of the Indian village of Bear
Lake, and thence at intervals about east-northeast for nearly 16 miles.
Basalt dykes are numerous but generally small, and are not shown on the
accompanying map (in pocket). Basalt lavas are well exposed 34 miles
east of Bear Lake village, 34 miles east of Saiya Lake, 5 miles east-northeast
of Saiya Lake, and 54 miles south of Sustut Lake: the largest body is
about 3 miles long. Two cone-like pyroclastic deposits lie 13 miles north-
east and 64 miles southeast of The Thumb.

Lithology

Necks, Dykes, and Lavas. So far as known, all rocks of the group may
be corrrelated with the one, more or less continuous, though somewhat
extended, period of voleanic activity, and all are believed to be lithologically
similar. Necks, dykes, and lavas are the common forms, and are com-
posed of hard, fresh, fine-grained, dark grey to nearly black rocks. Most
of these are massive and are weathered light to dark grey, but in places
are reddish and purplish. Parts of the lavas, however, are vesicular, ropy,
or frothy, and most of these have been altered to various shades of brown,
red, or purple. Blobs of black volcanic glass as much as 2 inches in diameter
were noted in many places. Other prominent grains and phenocrysts,
ranging from 7g inch to about an inch, occur here and there and include?:
honey-coloured, well-cleaved, translucent albite; black, well-cleaved hypers-
thene; olive-green, sugary olivine; and black, lustrous magnetite.

1 Identifications by E. Poitevin, Chief, Mineralogical Section, Geological Survey of Canada.
15410—4