172

CLAY.

Clay, of glacial origin, and of low grade, is found in abun-
dance in the Honna valley, Yakoun valley, underlying the north-
east lowland, and in other localities. It is usually grey, very
plastic, and is suitable for common brick manufacture and
perhaps the lower grades of pottery. High grade clays have not
been observed on Graham island. Some of the clays from the
Skonun formation on the Yakoun river have been tested in the
kilns of the Clayburn Brick Company, but have been found to
melt at low temperatures.

The shales of the Haida formation are usually too sandy for
use in clay industries; but some shales from the tunnel of the
British Pacific Coal Company near Slatechuck creek were col-
lected by Clapp! and tested by Dr. H. Ries. These shales are
found beneath the ‘“‘A’”’ seam. The upper one is a dark carbon-
aceous dense slaty shale, similar to that quarried by the Indians
on Slatechuck creek for carving, and it is 15 feet thick. The
lower shale, 30 feet thick, is a lighter coloured and much softer
rock, and is locally termed fire-clay. Doctor Ries reports on
these clays as follows:

‘The plasticity of the lower shale is fair, that of the upper
shale very poor. A mixture of the two shales in equal proportions
is fairly plastic. The air shrinkage of the mixture averages 4:5
per cent, and the tensile strength averages 50 pounds. At cone
010 (1742 degrees F. or 950 degrees C.) the burnt mixture has a
little ring, poor colour, an absorption of 14 per cent, and a fire
shrinkage of minus 1 per cent. At cone 05 (1922 degrees F.
or 1050 degrees C.) the mixture has a light grey buff colour and an
absorption of 13 per cent. At cone 1 (2102 degrees F. or 1150
degrees C.) the colour is still the same and the fire shrinkage
1-6 per cent. At cone 7 (2318 degrees F. or 1270 degrees C.)
the mixture shows no sign of failing by fusion. These tests
show that the mixture could be used for common and face brick,
although there was not sufficient material supplied to test it in
a stiff-mud die. The lower shale clay alone behaves similarly,
but is more plastic and burns a little denser.”

1 Clapp, C. H., Geol. Surv., Can., Sum. Rept. 1912, p. 40.