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Conglomerate

Conglomerate is relatively rare in the Lay Range. The largest body
is exposed on the highest peak between the headwaters of Swannell River
and Lay Creek. Here a body at least 340 feet thick consists entirely of
conglomerate, with rounded pebbles and boulders up to 14 inches in
diameter, in a hematitic, gritty matrix. The conglomerate is well bedded,
with several bands up to 2 feet thick of pea-conglomerate and coarse grit.
The pebbles and boulders are all well rounded; most of the matrix is
relatively angular and poorly sorted. For the whole body the following
types and approximate proportions of pebbles are represented:

25 per cent coarse-grained, massive to slightly gneissic, grey, buff, and weathered
grey-brown granodiorite, with minor granite(?), in well-rounded stones up to
1 foot in diameter, and averaging 2 to 4 inches (a representative specimen of
the granodiorite contains: 25 per cent quartz in crystalloblastic, patchy grains;
35 per cent plagioclase feldspar in twinned, partly poikilitic grains zoned from
about Angs to Aniz, 15 per cent microperthite or orthoclase, in large, very
irregular grains, and about 20 per cent hornblende);

30 per cent white, buff, or pale grey quartzite, partly micaceous and feldspathic,
in boulders up to 14 inches in diameter and averaging 2 to 3 inches;

10 per cent dark grey to nearly black quartzite, quartz-mica schist, and chert(?);
average diameter about 2 inches;

15 per cent green, grey-green, and chocolate-brown andesite, massive and porphyri-
tic; some andesite breccia; average size 4 inch to 2 inches;

10 per cent dark grey to nearly black, deeply weathered, schistose rock; looks
like decomposed gneiss; mostly less than 1 inch in diameter;

5 per cent banded grey-green tuffs; much altered to epidote-rich material;
average 1 inch to 3 inches in diameter; and

Minor, splintery fragments of black slaty argillite.

The matrix of this conglomerate constitutes about 20 per cent of the
rock volume, and is a semiangular grit composed mainly of fragments of
quartzite in a hematitic groundmass.

This conglomerate body is overlain by at least 700 feet of sheared,
shattered rock, intensely epidotized and very heavily coated with reddish
purple hematite. The original composition of this rock is doubtful, but
some parts at least were conglomerate bearing granitic pebbles. The
outcrop of conglomerate is 25 miles long. Exposures at apparently equiva-
lent stratigraphic horizons 1 mile to the northwest and 2 miles to the
southeast show no conglomerate, and what may be the same band of
shattering, rich in hematite, lies in normal andesite breccia and tuff.

Smaller bodies of conglomerate are found at higher horizons in this
section. None is known to contain granitic pebbles. Many of these beds
are similar to the conglomerate exposed on Vega Creek at the west edge
of the southern mountain group of this map-unit.

Chert

A few beds of chert are intercalated with the tuffs and sandstones.
Some are dull grey to nearly white; others are bright red jasper. All
beds observed are less than 10 feet thick. Some of these rocks, which
may be seen in thin section to be composed of very fine-grained siliceous
material and chloritic matter, may be siliceous volcanic rocks.