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volume of the total components. The composition of the feldspar grains
could be accurately determined only for the larger phenocrysts, but taking
into consideration that in such rocks the phenocrysts, if they differ at all,
are generally more calcic than the feldspar of the groundmass, the average
composition of the plagioclase is probably in the range albite (Anj)
to oligoclase (Ang). No significant amount of undisputedly primary
quartz or potash feldspar was recognized in any of the thin sections studied.
All the rocks thus appear to be normal andesites, and to represent varia-
tions between pyroxene-bearing hornblende andesite and hornblende-
bearing pyroxene andesite.

Amygdaloidal or vesicular rocks constitute only a small percentage
of the Takla group andesites. They are found mainly in breccias on
Thane Creek, and north of Tutizika River, and in fairly fresh flows west
of Lay Creek. In some of these rocks amygdules up to 4 mm. in diameter,
composed of aggregates of chlorite, clinozoisite, calcite, and strained quartz,
are fairly abundant. In general the amygdaloidal rocks are brown, in
contrast with the normal grey-green colour, owing to finely disseminated
ferric oxides in the groundmass.

Sedimentary Rocks

Most of the sedimentary rocks of the Takla group are composed
predominantly of volcanic material. The most abundant type appears
to be tuff, but there are all gradations to greywacke deposits composed
mainly of fragments resulting from erosion of consolidated rocks. The
typical tuffs are grey-green, very fine to medium grained, and well bedded,
with layers ranging from about #, inch to 5 or more feet thick. Tufts
with fragments more than +, inch in diameter are rare. Most of the
tuffs are altered, and show a smooth green surface due to the development
of chlorite, epidote, and clinozoisite; some have a glossy, serpentine-like
appearance. Others are partly carbonatized, and weather a light rusty
orange, although on fresh surfaces the appearance of the rock is little
changed. A few beds have been highly silicified and have a cherty
appearance. In thin sections, the tuffs are seen to consist of various
proportions of broken crystals of feldspar, usually highly altered to
clinozoisite, sericite, and kaolinitic material; pyroxene, probably both
pigeonite and augite; hornblende, in places altered to epidote; and
angular masses of fine-grained volcanic rock composed of the same
minerals. In addition, masses or individual crystals of chlorite (pennine),
clinozoisite, epidote, biotite, quartz, and calcite are abundant in some
tuffs; these minerals are thought to be mainly or entirely secondary.
Likewise, it is probable that some of the feldspar is an alteration product.
The fragments of volcanic rock are typical of the various types of andesites
observed as flows in the Takla group. No glassy fragments were observed
in these tuffs, but several sections show irregular masses of very fine-
grained or amorphous, partly isotropic, dark brown material that could
represent partly devitrified glass. What might be small spherulitic struc-
tures are abundant in one tuff bed near the Vega mineral claims.

The greywackes consist, like the tuffs, almost entirely of fragments
of volcanic rocks and of individual crystals of plagioclase, hornblende,
and pyroxene, with an average composition close to that of the andesite