ARMSTRONG: FORT FRASER MAP-AREA 27

is believed to be a zone of major thrust faulting. It may be, therefore,
that the upthrust of this batholithic mass was in part at least a
cause of either the anticlinal structure or the faulting or both. A\l-
though the axis of the large ultrabasic body between Stuart and
Cunningham Lakes is elongated parallel to the strike, the Palaeozoic
strata to the north-east and south-west have not an anticlinal structure
as yet known, but there is a little evidence to indicate that Stuart
Lake valley is a fault zone.

SERPENTINIZATION

The peridotites, dunites, and pyroxenites have all undergone
partial to complete alteration to serpentine. They are at least 60
per cent serpentinized. The serpentine is olive or emerald green and
is predominantly antigorite, although veins and veinlets of the fibrous
variety, chrysotile, and the columnar variety, picrolite, were observed
in some localities.

It is possible to study the progress of serpentinization from fresh
peridotite and dunite to serpentine rocks. Alteration of the olivine
and rhombic pyroxene to serpentine commenced at and proceeded
from crystal, fracture, and cleavage faces. The process apparently
began with partial alteration of the olivine crystals to chrysotile
serpentine, and pyroxene grains to bastite. The chrysotile veinlets
fill cracks in olivine. In the centre of each veinlet is an isotropic

strip flanked on each side by perpendicular fibres of chrysotile. Very.

fine-grained magnetite occurs in the central strip. As the alteration
of the olivine proceeded, the chrysotile appears to have been changed
to antigorite. Chrysotile does not occur in the completely serpentin-
ized rocks. Where serpentinization is first completed, the original
crystal boundaries are still recognizable. The mesh texture of anti-
gorite serpentine derived from olivine, and the rectangular texture of
bastite serpentine derived from pyroxene are best developed at this
stage. Then the texture is gradually altered to an aggregate of
bladelike antigorite and brownish-yellow amorphous serpentine in
which the crystal boundaries of the original constituents are either
barely discernible or completely obliterated. The amorphous serpen-
tine contains a fine dust of iron oxide. The characteristic mesh
texture of antigorite derived from olivine is replaced by a lattice
texture in which the blades commonly cross each other at right
angles.

In the Fort Fraser map-area the serpentinized ultrabasic rocks
are locally schistose, the strike of the schistosity being approximately
north-west. These sheared parts form zones up to several thousands