96

sericite, kaolin, and the probably zeolitic mineral noted in the
dacites, occur. The andesine varies from Abg;sAn3; to AbgAng,
is often almost unaltered, and has a lath shaped, divergent habit
as in the dacites. The augite is often considerably altered to
chlorite, but much less so than in the dacites. Completely crys-
talline, finely granular, intersertal textures prevail, the augite
in small equant grains being interstitial to the divergent laths of
andesine or, it may be, in some cases including them poiciliti-
cally. Pyrite and calcite impregnations are common.

Andesite Porphyrite. These rocks are very similar to the
andesites, differing only in texture. They have been observed
as dykes only. Phenocrysts of chalky white, equant, euhedral
plagioclase occur and make up in some cases 10 per cent of the
rock.

Examined under the microscope, the phenocrysts are an-
desine, greatly altered to calcite, kaolin, and sericite embedded
in a groundmass resembling the nonporphyritic andesites in
composition and texture, except that in some cases it is almost a
devitrified glass. Ilmenite and leucoxene have been noted.

The thick sills, some of them possibly laccoliths, that occur
near the uppermost exposed beds of the Skidegate formation
south of Lake Stanley, are mostly andesite porphyrite. They
are light, bluish-grey rocks, with phenocrysts forming a much
higher proportion of the rock than usual. One of them, studied
in thin section, was found to contain euhedral to subhedral
phenocrysts of andesine, AbssAns; (about 20 per cent of the rock)
in a groundmass composed of more acid andesine, AbgoAnuag
(about 60 per cent of the rock) the rest being augite almost wholly
altered to chlorite.

On the whole, the alteration of the andesites and andesite
porphyrites, while considerable, is much less marked than in the
more silicic dacites. Much of the andesine is scarcely altered
at all and many rocks contain fresh colourless augite.

Augite Andesite. The augite andesites are usually porphy-
ritic rocks, with phenocrysts of chalky white plagioclase or
augite in a finely crystalline grey, greenish, or dark grey ground-
mass. Phenocrysts in some cases make up as much as 15 per
cent of the rock.

: