104

grains, filling the last spaces left by the crystallization of the
other minerals. Into these spaces the ends of the other minerals
projected and are now surrounded by quartz. The quartz is
estimated to form nearly 5 per cent of the rock.

Tachylyte. This rare! type, a glassy basalt, was found
forming fragments in an agglomerate on the west side of Ship
Kieta island in Masset inlet, and fragments were found in
streams entering Athlow bay on the west coast. A large mass
of it is reported to occur on the west side of Juskatla inlet.

The tachylyte is a black, glassy rock, with a brilliant lustre,
speckled with white, rectangular phenocrysts of feldspar up to a
millimetre in size.

In thin section the phenocrysts are seen to be labradorite,
about AbsAng, in euhedral, equant, and tabular forms, quite
clear and unaltered. Carlsbad twins are combined with albite
twinning. A few phenocrysts of fresh augite are also present.
The groundmass is a brownish, perfectly isotropic glass, con-
taining disseminated grains of a fine black mineral, probably
magnetite, and extremely minute, hair-like crystals, probably
feldspar. The phenocrysts exhibit a glomeroporphyritic ten-
dency. The alteration of this rock is very slight indeed, which
is remarkable considering its unstable condition as a glass.

Tuffs are abundantly represented on the west coast and are
of several varieties, some of which are described in the part of the
chapter on “Economic Geology” treating of the bituminous
rocks in the Masset formation.

Basalt Tuff. On Baddeck river, the Masset flows contain
a number of bright, brick-red, intercalated beds, most of which
are tuffs. The specimen examined is a mottled, bright and dark
red, very dense highly ferruginous rock, clearly of clastic origin.

Studied in thin section, it is seen to consist of sub-rounded
fragments of dense and vitreous basalts, containing phenocrysts
of labradorite closely crowded together. Fragmentary fresh
andesine occurs. The whole rock is permeated and in part
replaced by hematite, giving the red colour and rendering the
rock almost opaque in thin section.

1 Harker, A., ‘Petrology for Students,’’ Univ. Press, Cambridge, 1908, p. 207.
Iddings, J. P., “Igneous Rocks,’ vol. I, Wiley and Sons, New York, N.Y., 1909, p. 378.