COPPER SILVER VEINS (OF TELKWADISTRICH. S50

estimated by Dawson to be not less than 1,000 feet. With the
exception of a small thickness of sandstone and shale containing
a minor amount of coal, the entire formation is composed of vol-
canic rocks, and of this, 95 per cent. of the thickness, exposed in
the Telkwa district, consists of tuffs. These tuffs are character-
istically red in color, and vary in texture from coarse breccias
with fragments up to a foot in diameter to an extremely fine
cherty rock whose nature could not be determined without the
aid of strong magnification. The prevailing texture, and the
one which was responsible for the formation being called by
Dawson the “ porphyrite group,” consists of a fine, reddish, hard
groundmass plentifully sprinkled through with conspicuous white
feldspar crystal fragments averaging about 3 millimeters in
length.

The chemical composition of the rock is about that of an an-

desite. It consists mainly of small glass fragments in various’

stages of devitrification and containing many minute particles of
hematite. These are cemented with a siliceous matrix also con-
taining a high proportion of hematite powder which gives the
rocks their characteristic color. No ferromagnesian silicates are
present excepting a considerable amount of secondary chlorite.
At the contacts of the recent intrusions the tuffs are metamor-
phosed for distances from the contact up to a mile. The chief
alteration has been the development of a large amount of very
pale green biotite, a considerable amount of sericite and silica,
and the change of hematite to magnetite thus changing the color
from red to black. These rocks underlie nearly the whole of the
district excepting the small areas occupied by the later intrusions.
The oldest intrusive rocks are those of the Coast Range
batholith thought to be of Jurassic age. They form the main
body of the Coast Range in this district and also the small stock-
like intrusion composing the core of the eastern of the two moun-
tainous areas mentioned above, known as the Telkwa Mountains.
The rocks of these two localities are practically identical in com-
position and appearance. They are quartz diorites and grano-
diorites having a medium coarse texture and a light gray color.