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Basalt specimens examined under the miscroscope contained about
25 per cent colourless to pale brown pyroxene, 60 per cent labradorite,
and 15 per cent magnetite. All minerals are remarkably fresh. The lath-
shaped crystals of labradorite generally form a random felt, but in a few
places were observed to lie more or less parallel with each other as if
oriented in a flowing magma.

Pyroclastic Rocks. These form cone-like deposits and comprise mainly
pumice, ash, cinders, and other voleanic ejectamenta, but contain a little
frothy and fragmental lava. The largest deposit, 64 miles southeast of
The Thumb, is about a mile long and 4 mile wide. In most places these
deposits have weathered to shades of brown, red, or purple. In part they
are well-stratified, loosely consolidated deposits, containing a few rounded,
striated cobbles and boulders of Sustut sandstone and pebbly sandstone.
Elsewhere they are unconsolidated and apparently unsorted.

Structure

The necks are approximately cylindrical, lava-filled conduits or cores
of old voleanoes, and now project as much as several hundred feet above
the more deeply eroded rocks as dark grey or black domes or flat-topped
plugs. The largest, known as The Thumb, is about 1,000 feet in diameter
and forms a conspicuous black knob about 5 miles east-southeast of Bear
Lake village. It consists mainly of well-jointed, massive, black basalt, but
in places near its border contains numerous vesicles and angular fragments
of bleached Sustut sandstone. The surrounding Sustut strata dip very
gently southwest and do not appear to have been disturbed by the intrusion
of the basalt body. A smaller neck or plug of similar basalt lies in a fault
zone about 3 miles northwest of Mount Carruthers and is thought to post-
date the faulting. Although it lies in a small abandoned cirque it has
clearly been shaped by the ice that cut the cirque and is, therefore, older
than the last phases of alpine glaciation in this vicinity.

Most of the dykes are nearly vertical and less than 10 feet wide:
the largest noted was about 200 feet wide. Many exhibit excellent columnar
jointing, with the joint planes about perpendicular to the dyke walls. The
borders are sharp and distinctly chilled, but the adjacent rock is not
appreciably altered. Some of the dykes contain vesicles and fragments of
wall-rock. They cut Sustut strata, which are inclined at angles of as much
as 45 degrees, and project from deeply eroded valley sides, such as the
northeast face of Connelly Range. Evidently the dykes were injected
after the tilting of the Sustut beds but before the period of erosion that
carved the deep valleys such as that now flanking Connelly Range on the
northeast. As previously mentioned, they cut probable Kastberg porphyry

+ miles west-southwest of Mount Carruthers. On the north side of Asitka
River Valley, about 8 miles from the river mouth, a northerly trending
fresh basalt dyke, about 3 feet wide, passes directly through a rusty
crumpled zone believed to mark the Asitka fault, and is thus younger than
both the faulting and the related alteration.

The lavas are horizontal or gently inclined. A cliff 34 miles east of
Bear Lake village displays about 185 fect of basalt, which, together with an
unknown thickness removed by erosion from the upper surface, seems to
comprise a single flow. Another body, 34 miles east of Saiya Lake, is about