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