o4

for the gradation, through about 30,000 feet of strata, from the ‘sillimanite
zone’ in the lowest Tenakihi group beds to the ‘chlorite zone’ in the upper
Ingenika group.

Whether the mechanism involves a vertically rising magmatic body, a
large-scale horizontal invasion of an igneous layer at depth, or the presence
of a region of migmatization and anatexis underlying the area at a fairly
uniform depth, is an open question. Whatever the process, for the Tenakihi
group rocks as a whole it would appear probable that magmatic or grantiz-
ing processes were to some extent effective in controlling the temperature of
metamorphism. The available evidence, and comparison with the inter-
pretations advanced for other areas (Cairnes, 1940; Turner, 1948, p. 295),
would seem to suggest that the Tenakihi group rocks were metamorphosed
under conditions of moderately high hydrostatic pressure due to burial of
the lower beds under probably more than 30,000 feet of sediments, of a
temperature gradient due perhaps primarily to the depth of burial but
also probably in part to underlying igneous or anatectic material, and of
stress produced by relatively gentle orogenic deformation that resulted in
differential rock movements between beds of different competency. The
metamorphic rocks developed under these conditions show a systematic
increase in grade of metamorphism with successively lower stratigraphic
horizons, a uniformity of metamorphic grade at approximately the same
stratigraphic position over large areas, and a schistosity parallel or nearly
parallel with the bedding.

ORIGIN

Evidence has been presented to show that the Tenakihi group was
originally an assemblage of sandstones and shales, with very minor con-
glomerates. The regular alternation of arenaceous and _ argillaceous
material, which is characteristically deposited in shallow water, indicates
a fine balance between subsidence of the sedimentary basin and the rate
of supply of sediments. This balance was maintained over an area of
many thousand square miles in north-central British Columbia, through
the deposition of, in the Aiken Lake map-area, at least 13,400 feet of
strata. No evidence bearing on the direction from which the sediments
were supplied was recognized in the map-area. The foliated granitic
pebbles in the two known conglomerates, the relatively coarse grains of
quartz and perthitic feldspar in some of the quartzites and schists, the
grains of biotite that are interpreted as being part of the original sediments,
and the general high silica, high alumina composition of the rocks, suggest
a source area of granitic or possibly in part metamorphic character. The
relatively fresh, apparently unrecrystallized, grains of perthite, and sound,
rounded pebbles in the conglomerate, may mean that the source rocks
were undergoing fairly rapid mechanical disintegration.

AGE AND CORRELATION

The earliest description of the formations in which the Tenakihi group
is found was made by McConnell (1896, pp. 22-31), who found “medium-
grained muscovite gneisses, micaceous and chloritic schists, and quartz-
ites” in the Black Canyon of Omineca River, about 30 miles due east of
the southeast corner of Aiken Lake map-area, and traced them northwest
in the Butler Range along the west side of Finlay River Valley. McConnell