30 THE ROYAL SOCIETY OF CANADA

for more than 8 per cent serpentinization, because the altered perido-
tites contain less than 1 per cent magnetite, and if all the serpentine
had been formed by this reaction a completely serpentinized rock
should contain at least 12 per cent magnetite by volume. Reaction (3)
may explain the remainder of the serpentinization in the peridotites
of the Fort Fraser map-area. Cooke (1937, pp. 67-68) objects to
reaction (3) on the basis that if the serpentinizing solutions were of
magmatic origin they could hardly have carried a large percentage
of free silica, because this constituent would surely have combined
with magnesia during crystallization to form enstatite after which
serpentinization would go on according to reaction (1). However if
it is assumed, as Hess (1933, pp. 653-654) does, that some combination
of silica and water may have taken place as suggested in reaction (3)
which reacted with olivine to form serpentine rather than enstatite,
and that serpentine also crystallized directly the above objection is
overcome. The volume decrease as indicated by the three chemical
reactions postulated is in accordance with the field evidence.

STEATITIZATION

Between Trembleur Lake and Mount Williams, about 10 per cent
of the serpentinized peridotite has been altered to carbonate-quartz-
mariposite and carbonate-talc rocks. These altered rocks occur in
zones of fracturing ranging in width from 10 to 500 feet and separated
by zones of unaltered serpentinized peridotite of approximately the
same width. Such bands are most numerous on the ridge running
east from Mount Williams.

The carbonate-quartz-mariposite rocks are medium to coarsely
crystalline and weather buff to yellow brown. Buff-coloured carbonate,
quartz, and bright green mariposite? are distinguishable in hand
specimens. On fresh surfaces the rock commonly is a vivid apple
green, due to the presence of mariposite, although this mineral in no
case makes up more than 5 per cent of the rock. The great bulk of
it, 80 to 90 per cent, is an ankeritic carbonate, a variety with indices
of refraction: w=1.698, e=1.539. Mixed with the carbonate is 5 to
10 per cent of quartz probably formed in situ. This mixture is generally
traversed by a network of veinlets of milky white quartz, which
constitute 5 to 10 per cent of the whole rock. In several thin sections
carbonate was observed replacing serpentine. In these sections
angular fragments of serpentine are embedded in a matrix of carbonate.

?This mica is probably mariposite and belongs to the mariposite-fuchsite-
muscovite group which forms part of a gradational series. (2V=approximately
0 degrees indicating mariposite.)