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This close association of the dykes with quartz veins suggests that they
are related in origin. In the larger body of dyke-like rock on Quartz
Creek canyon, quartz occurs as large masses of irregular shape. A width
of 100 feet consists roughly of 75 per cent quartz and 25 per cent dyke
rock. Hydrothermal action, presumably associated with the formation of
the quartz veins, has introduced much carbonate and some sericite and
pyrite into the dykes and adjoining volcanic rocks, rendering all the rocks
near the veins light grey in colour.

Microscopic examination shows that the dykes consist mainly of car-
bonate and sericite. Some dykes contain chlorite, epidote, and carbonate.
The outlines of former feldspar crystals remain as long, narrow laths. The
coarser grained part in the interior of one dyke exhibited a type of arrange-
ment of feldspar laths characteristic in diabase.

Narrow, medium- to coarse-grained, basic dykes cut the volcanic
rocks of the McLeod series and the serpentine. They have been altered
to about the same extent as the volcanic rocks of the McLeod series and
are probably related to the igneous activity that produced this series of
rocks.

Dykes, ranging in composition from granite to diorite, intrude the
serpentine, the McLeod series, and the Dease series. These dyke rocks
have approximately the same composition as the batholithic rocks and are
believed to be genetically related to them.

The youngest intrusive rocks are the basaltic dykes and plugs of the
Tuya formation. These dykes are numerous only in the vicinity of cinder
cones of the Tuya formation. Two large dykes of diabase observed in
the northern part of the area are quite unaltered, and their unusual fresh-
ness suggests that they are Tertiary or younger. They are presumably
related in origin to the Tuya eruptives.

TUYA LAVAS AND CINDER CONES

Black cinder cones form one of the most striking topographic features
of the area. The cones are most abundant in the northwesterly trending
belt of sedimentary rocks south of the Cassiar batholith, but Black moun-
tain, a large volcanic cone on the north side of Eagle river, and several
small lava flows in the vicinity of Eagle lake lie entirely on the Cassiar
batholith.

The cones are generally elliptical in plan and conical in section, and
rise to elevations of 6,500 feet. Although their highest summits are 1,000
feet lower than some of the peaks in Cassiar and Hotailuh mountains, their
symmetrical shape and long, black, talus slopes make them conspicuous
landmarks in the region.

The summit of the cones is generally close to a central conduit or
feeding fissure. Slightly consolidated cinders and intermingled bombs
surround the central conduit and make up the greater part of the cone.
Basaltic dykes occur in the cones. They are dark-coloured, fine-grained,
porphyritic rocks containing phenocrysts of olivine. Most of the flows
are vesicular and some are scoriaceous near the base.

Several lines of evidence would indicate that the volcanic cones are
Pleistocene rather than Recent or pre-Pleistocene in age. The cones are