105

Trachyte. On the west side of Harrison island, in Juskatla
inlet, prospecting work has been done on a large mass of light
coloured, variegated, dense rock, occurring intercalated in the
basalt flows and agglomerates, which form the remainder of the
island. The rocks of the vicinity strike north 45 degrees east
and dip 70 degrees southeast, in some places slightly contorted,
and the mass of light coloured rock is clearly interbedded with
them. Where it has been blasted, 30 feet of the bed is visible
with no indication of the total thickness. Mr. Arthur Robertson,
owner of the claim, reports that the rock outcrops over an area
several hundred feet wide, crossing the island in a southerly
direction, so that the thickness and lateral extent may be con-
siderable.

The rock varies in appearance in the field. In general, it
is light, creamy white, chalky white where weathered, and pale
bluish or pale sea green where apparently quite fresh. A marked
characteristic of the coloration is the reddish irregular mottlings
which extend throughout the rock in all directions, and make
up varying proportions of it at different places. This colouring
varies from a chocolate red to a light brick red. On joint
planes are brilliant yellow stains of limonite.

The rock shows various gradations in texture from very
dense to fairly porous; and much of it shows a variable lamina-
tion suggestive of flow-structure. The reddened areas mentioned
above are to some degree controlled by this banding. In various
parts of the rock are small cavities, usually under one-fourth of
an inch across, and of no particular regularity of shape, which
are lined with minute crystals and, in rare instances, spherical
concretionary masses of chalcedony have been noted in them.
Disseminated sulphides, like marcasite, are found sparingly.

In general appearance the rock in the field is like a cherty
sediment or silicified tuff, and its true nature was not suspected
until a microscopic study was made. The silicified or cherty
appearance of the rock is enhanced by a slightly vitreous lustre.

Studied in thin section the rock is seen to be composed almost
wholly of feldspar, which is apparently orthoclase. The texture
of the rock is distinctly trachytic, and very fine-grained, the
laths of feldspar averaging 0-03 by 0-003 mm. in size. They