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that weather various shades of light grey, yellow-green, green, and reddish
brown, with a conspicuous banded appearance on outcrops. These tuffs,
which can be seen under the microscope to be composed principally of
chlorite, epidote-clinozoisite, saussuritized plagioclase, and pyroxene
largely or completely altered to amphibole, were probably originally of
andesitic or perhaps in part of basaltic composition. The flows inter-
bedded with the tuffs are grey-green, massive to porphyritic rocks, and are
composed of about the same minerals altered to about the same degree
as in the tuffs. The porphyritic varieties contain phenocrysts of altered
andesine plagioclase, amphibole, or pyroxene, up to 1 inch long. A
relatively few rocks, found as flows and as fragments of flow rocks in the
tuffs, are amygdaloidal, with quartz, chalcedony, chlorite, and carbonate
amygdules up to 4 inch in diameter. Some layers that appear to be flows,
intercalated with the tuffs, are less than 10 feet thick. One exposure of
andesite north of Vega Creek shows excellent pillow structure.

In about the middle of this assemblage, a series of tuff beds with a
stratigraphic thickness of about 100 feet is sharply cut by bodies of
fossiliferous arkosic grit and conglomerate up to 40 feet wide and 200
feet long. The bodies strike almost at right angles to the bedding of the
tuff, against which they have knife-edged, undulating contacts. The
conglomerate is composed of well-rounded to angular fragments up to
1 inch diameter of red and green tuff and andesite, grey-green porphyritic
andesite, blue-grey limestone, and white quartz in a gritty matrix composed
principally of angular white feldspar grains up to 35 inch long, in a fine,
sandy groundmass of quartzite grains cemented by limonite. The arkosic
grit is similar to the matrix of the conglomerate, and like it contains many
fragments of crinoids and bryozoa. These bodies appear to represent some
sort of marine canyon or crack fillings in the tuffs. The overlying tuffs
are not disturbed. Two such bodies are exposed north of Vega Creek on
the east side of Osilinka River Valley, and a similar, smaller body is
found on the east side of Conglomerate Mountain, west of Wasi Lake.

Uniformly banded tuffs constitute the lowermost 600 feet of the section
exposed north of lower Vega Creek. Overlying them are about 400 feet of
uniformly banded tuffs with minor black slaty argillite; and these in turn
are overlain by at least 6,000 feet of banded tuffs, with minor, intercalated,
grey-green, massive to porphyritic andesitic flows and a few small bodies
of gritty arkose and greywacke. Above the highest recognized flow are
about 3,500 feet of uniform, very fine-grained, banded tuffs. One continuous
exposure shows no bed more than 6 inches thick in a stratigraphic section
of 300 feet. The youngest rocks exposed in this section consist of about
800 feet of argillite, grit, greywacke, and banded tufts.

An isolated bluff of conglomerate on the bank of Vega Creek, about
13 miles below the Vega mineral claims, may belong to this assemblage
and may represent a considerably higher horizon in the section. The
conglomerate consists of rounded pebbles of light and dark grey chert,
quartz, micaceous and feldspathic quartzite, brown, fine-grained sandstone,
grey-green and reddish andesites, banded tuff, red jasper, and black cherty
argillite up to 6 inches diameter, in a strong, sandy to gritty matrix
composed mainly of grains of quartzite cemented by limonite. The con-
glomerate is fairly well sorted, with beds ranging from a pea-conglomerate
to a coarse rock with most pebbles more than 2 inches in diameter.