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conchoidal fracture that crosses the bedding. Many of the carbonaceous
films are a mat of fossil leaves, a few of which are well preserved. Occa-
sional layers intercalated with the tuffs on Connelly Range and Sustut
River contain scattered, rounded forms suggestive of amygdules, and may
be lavas.

Microscopic study showed the tuffs to contain abundant brownish
voleanic glass and microcrystalline material formed by the devitrification
of glass; excellent shards appear to be a characteristic feature. The
sharply angular crystal fragments scattered throughout the rock are mainly
plagioclase, at least some of which is andesine; others include quartz and
sericitized orthoclase.

It seems probable that the tuff bands, wherever they occur, are
confined to a stratigraphic interval of perhaps 500 feet closely overlying
the uppermost heavy conglomerate beds; but they are more numerous in
some sections than in others, and in some places, as near the northwest
end of Connelly Range, it is possible that no tuff was laid down in Sustut
time. Thus, about 2 miles northwest of Peggy Peak numerous tuff bands,
distributed throughout an estimated stratigraphic interval of 500 feet,
overlie a prominent conglomerate member and decrease in number to the
northwest so that on Sustut River and beyond only one or two bands
were located; but again these lie just within and above the same con-
glomerate. Also, on either slope of Thutade Lake Valley, three or four
tuff bands, the lowest lying just above the topmost prominent conglomerate,
are separated by a maximum stratigraphic interval of about 475 feet.

Metamorphism

The Sustut strata have been intruded by large bodies of Kastberg
quartz diorite near Peggy and Nep Peaks, where the shales, for as much
as 10 feet from the borders of the intrusions, have been changed to hard,
black, white weathering, slaty rocks. The sandstones throughout this area
are harder and less weathered than elsewhere, and break through rather
than around individual grains and pebbles. They contain a little calcite
and fine-grained, pale brown mica, and are rust stained in places.

Internal Structural Relations

The Sustut strata northeast of Bear Lake and Bear River Valley form
two great, open, northwesterly trending folds, a synclinal axis lying about
2 miles northeast of Bear Lake village, and an anticlinal axis about 6 miles
farther northeast along the southwest side of Saiya Creek Valley. No
minor folds were noted on these major structures northeast from Bear Lake
nearly to the Saiya fault, nor along Sustut River for about 54 miles
upstream from the mouth of Bear River. However, strata exposed in the
succeeding 34 miles up Sustut River to a point a little upstream from
the mouth of Saiya Creek, although also striking northwesterly, display
several reversals of dip and faults of unknown magnitude. These strata
mark a zone of comparatively complex structure that probably extends
some 10 miles northwesterly to the west boundary of the map-area and
may extend southeasterly along the partly drift-covered southwest side
of Saiya Creek Valley. Fold axes in parts of this zone are less than a
mile apart. The remaining formations of the Sustut River section, as