ARMSTRONG: FORT FRASER MAP-AREA 29

younger acid intrusives. Younger acid intrusives border the serpentin-
ized ultrabasic masses near Tsitsutl Mountain, west of Stuart Lake,
and north of Cunningham Lake, but the ultrabasic rocks are no
more altered near these acid intrusives than farther away. If serpen-
tinizing solutions arose from some underlying body of magma, they
would have to ascend through fractures, and hence serpentinization
should be related to faults, joints, and shear zones. This, however,
is not the case. Shear zones, it is true, are completely altered to
antigorite serpentine as described, but the rock alongside of them is
no more altered than farther away. If serpentinization were caused
by solutions from outside, again one might expect that the borders
of the ultrabasic masses would be more serpentinized than the interior;
but they are not. It seems necessary to conclude, therefore, that
serpentinization was not caused by emanations from later intrusive
rocks and therefore it must have been caused by solutions emanating
in the ultrabasic magma itself. Hess (1933, p. 652) finds crystals of
olivine uniformly distributed and about equally corroded in partially
serpentinized ultrabasics. From this he concludes the attack on the
olivine crystals was brought about by a residual liquid present in the
interstices between the grains rather than by solutions concentrated
in more widely spaced channelways. The same thing was observed
in the partially serpentinized peridotites of the Fort Fraser area.

As to the methods by which serpentinization was brought about,
Hess (1933, p. 653) states that if the serpentinizing solutions emanated
from the cooling magma itself they must have been contained within
the volume of the original ultrabasic and not introduced. Assuming
_ the original constituents to be forsterite, enstatite, and serpentinizing
solutions, the alteration of the peridotites in the Fort Fraser area
may be due to one or more of the following reactions:

(1) MgSi0s+Mg.SiO;+-2H,0 = HiMg;Si.0.
enstatite +forsterite +water =serpentine
volume decrease =approximately 2 per cent

(2) 3Mg;FeSi,.Os -+6H,O +0 =3H.Mg,Si.00+Fe:0,
iron-bearing forsterite +water +oxygen =serpentine-+magnetite
volume decrease =approximately 5 per cent

(8) 3MgeSiO4+H4Si04.2H20 = 2HyMg3Six09(Hess 19338, p. 653)
forsterite +silica +water =serpentine
volume decrease =approximately 2 per cent

Reaction (1) could not account for more than a 25 per cent
serpentinization, as the peridotites of the Fort Fraser map-area
seldom contain more than 10 per cent enstatite, and this reaction
would require a peridotite containing at least 40 per cent pyroxene
to account for all the serpentine. Reaction (2) could not account