24 | THE ROYAL SOCIETY OF CANADA

amorphous serpentine, and 10 per cent or less of colourless zoisite,
chromite, and secondary carbonate with a few scattered grains of
unaltered pyroxene. The flakes of antigorite are usually arranged
in a rectangular pattern ‘characteristic of serpentine derived from
pyroxene, but unfortunately this is not conclusive evidence that the
original mineral was pyroxene as serpentine derived from olivine
may show a similar arrangement in advanced stages of alteration.

The reasons for considering the serpentine surrounding areas of
unaltered pyroxenite to be derived from that rock are two: (1) The
contacts of the serpentine rock with the pyroxenite are not sharp
but gradational across an interval of a foot or more. In this distance
the pyroxenite can be seen to become gradually more and more
serpentinized; and the outlines of the larger pyroxene crystals are
preserved within the serpentine for distances of ten feet and more
from the unaltered pyroxenite boundary. The above relationship
was best observed in the eastern portion of the ultrabasic mass
between Cunningham and Stuart Lakes. In the western part of the
same mass a student assistant observed outcrops of pyroxenite in a
matrix of serpentine. (2) Thin sections of the serpentine show grains
of unaltered pyroxene. As pyroxene grains in all the thin sections of
peridotite examined are more altered than those of olivine, unaltered
olivine grains might be expected if these rocks had been peridotites.
None was observed. Hence it is concluded that no olivine was
originally present, and the rock, accordingly, must have been pyroxe-
nite.

In the western and eastern extensions of the ultrabasic mass
between Cunningham and Stuart Lakes, gabbroic rocks occur around
the borders of the pyroxenite areas. They are medium-grained, dark
green, and composed of 25 per cent or less of labradorite and 75 per
cent or more of augite. The large gabbro body underlying Deescius
Mountain and the small body cutting the peridotite near the summit
of Mount Williams are medium- to coarse-grained dark green rocks.
They contain from 20 to 40 per cent labradorite which is turbid as
a rule with grains of clinozoisite, paragonite, and calcite. Diallage,
the principal mafic mineral, is commonly altered to uralite and
chlorite. In two thin sections a few grains of olivine were observed.
In many places the gabbro body underlying Deescius Mountain has
an ophitic texture. These gabbros differ from the gabbroic rocks at
the borders of the large ultrabasic masses in their higher proportion
of feldspar and ophitic texture.

The ultrabasic body between Stuart and Cunningham Lakes
consists of a central core of peridotite approximately 50 square miles