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against the older rocks to the east, which have a local easterly strike
on Thane Creek near the contact. This contact may be a fault. The
basal conglomerate exposed on Thane Creek consists of well-rounded
pebbles up to 8 inches in diameter of voleanic, intrusive, and sedimentary
rock types in a grey-green, sandy to gritty, greywacke matrix containing
considerable organic matter. About 35 per cent of the rock, by volume,
is composed of pebbles larger than 3 inches in diameter. Of these, about
half are coarse-grained or coarsely porphyritic igneous rock, consisting
of a feldspar porphyry with white feldspar phenocrysts up to 4 inch
long (seen in thin section to be altered albitized plagioclase) in a grey-
green medium-grained groundmass approaching diorite in composition,
and light brown-grey quartz syenite or quartz diorite. Many of these
pebbles resemble those in a late Palwozoic conglomerate in Lay Range (See
page 115). The remainder of the large pebbles are mainly grey-green,
fine- to medium-grained, porphyritic and non-porphyritic andesite, now
largely altered to an albite-chlorite-epidote-amphibole greenstone. The
smaller pebbles are a mixture of the above-mentioned rock types with
lesser amounts of: purple-brown porphyritic basalt, containing white
feldspar phenocrysts; dark green, serpentinized, coarse-grained rock of
gabbroic to ultrabasic composition, possibly recrystallized greenstone, but
conceivably derived from some of the contaminated or hybrid rocks
associated with the ultramafic intrusions; light to dark grey, uniform
and finely banded, fine-grained quartzite; brilliant red cherty tuff; and
many coaly fragments. The coaly material is hard, has a high lustre
and conchoidal fracture, and is easily ignited. The largest lens of such
material observed in this conglomerate is about 4 inches by 2 feet.
Above the basal conglomerate in the southern part of the map-area,
the Takla group is composed mainly of porphyritic andesites and andesite
breccia, with intercalated tuffs and minor greywacke, limestone, and
slate. Recognizable horizon markers are scarce, and different parts of
the group, separated by wide stratigraphic intervals, show remarkable
lithological similarity; these factors, together with abundant faults of
both large and small displacement, in many places too numerous to be
shown on the present map, make it impossible, on the basis of the work
done to date, to establish a general stratigraphic sequence or to estimate
the over-all thickness of the Takla group in this region. In general it is
estimated that about 60 per cent of the outcrop area in this vicinity
consists of grey-green to dark green andesite and andesite breccia. Megas-
copically, the andesite appears quite uniform in mineral composition and
texture. Almost all is porphyritic, with relatively abundant, somewhat
inconspicuous grey or greenish grey feldspar, and less numerous green-
black hornblende or pyroxene phenocrysts 1 to 2 mm. long in a moderately
fine-grained to aphanitic groundmass. The rock weathers light greenish
grey to reddish grey. The characteristic breccias associated with these
porphyries consist of angular fragments averaging about 4 inches to 1
foot in diameter, with exceptional blocks up to 18 feet long, of andesite
generally similar to that of the non-brecciated flows, in a matrix of the
same material. In many places the fragmental character of the breccias
is visible only on the weathered faces of outcrops, fragments and matrix
being almost indistinguishable on freshly broken surfaces. In a few
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