:

ARMSTRONG: FORT FRASER MAP-AREA 25

in area. Bordering the peridotite on the east, north, and west are
areas of pyroxenite totalling approximately 12 square miles. Rimming
the pyroxenite on the east and west are areas of gabbroic rocks of
about two square miles each. The Mount Williams ultrabasic mass
consists of 80 square miles of peridotite bordered by small areas of
pyroxenite, of less than three square miles each, on the south-east
and north-west. A small gabbro body cuts the peridotite near the
summit of Mount Williams. Within these peridotite areas there is a
tendency for the olivine and pyroxene grains to collect and form
irregular masses of dunite and pyroxene-rich peridotite respectively.
These have no regular shape or size, and there is no sharp boundary
between them and normal peridotite. They grade into peridotite
simply by an increase or decrease in the proportion of pyroxene. In
the Mount Williams peridotite the dunite segregations predominate
near the centre of the body and the pyroxene segregations are more
abundant nearer the borders.

The pyroxenite-peridotite contacts are believed to be gradational
although serpentinization is everywhere so far advanced that an
actual gradation from relatively unaltered peridotite to pyroxenite
was not observed in the field. Microscopic studies of slides from
serpentinized rocks outcropping in the belt separating identifiable
peridotite and pyroxenite in the east and north-east parts of the
ultrabasic body between Cunningham and Stuart Lakes shows that
intermediate types containing approximately 50 per cent olivine and
50 per cent pyroxene do occur. From a microscopic examination of
thin sections it is also known that similar types occur near the south-
east corner of the Mount Williams body, and in the area east of
Tsitsutl Mountain.

The contact between the pyroxenite and the border phase gabbro
is gradational. In the eastern part of the area of ultrabasic rocks
south of Stuart Lake, a pyroxenite containing no feldspar was ob-
served to grade into a gabbroic rock containing approximately 25
per cent labradorite across an interval of about 200 feet. A thin
section of a rock outcropping near the centre of this belt contains
about 10 per cent feldspar. A student assistant reported that in the
western extremity of the same ultrabasic mass he observed a similar
gradation. A microscopic examination of specimens from the area
west of Tsitsutl Mountain shows that all types from gabbro to
pyroxenite occur there. The relations in the field were not observed.

These facts regarding the two large ultrabasic masses render
necessary the following conclusions: (1) that the peridotite, pyroxenite,
and border phase gabbro are derivatives of a common magma; (2) that