eee 35 with the even degree of metamorphism to which the rocks have been sub- jected, has resulted in a simple mineralogical composition of the rocks, and a limited range of rock textures and micro-structures. Metamorphism has progressed so far that the original differences between successive beds and groups of beds are preserved only by the different proportions of the prin- cipal minerals, namely, quartz, the micas, feldspar, and in places garnet and kyanite; the few accessory minerals occur in almost all beds of suitable metamorphic grade. Quartz is the predominant mineral in at least 80 per cent of the Tenakihi rocks. Except for a few of the larger grains, which may be detrital, it is entirely recrystallized, or crystalloblastic, with a certain degree of shape orientation parallel with the schistosity of the rock. In the lower beds of the group the quartz is mainly clear and unstrained, whereas in the higher beds it commonly shows wavy extinction due to strain. Many of the purer quartzites have a well-developed mortar structure, with large, strained grains rimmed by a profusion of smaller grains, suggesting that there has been deformation and slight recrystallization of an already recrystallized quartzite. The feldspar, which composes up to 35 per cent of individual specimens, is nearly all authigenic, commonly occurring in irregular, skeletal grains interstitial to, and including, oriented grains of quartz and the micas. Most of the feldspar grains whose optical properties could be accurately determined are oligoclase or andesine, but some albite and orthoclase have been recognized. Biotite is the predominant mica, and is the mineral mainly responsible for the fissile character and banded appearance of the Tenakihi group rocks. Viewed in transmitted light, the biotite is of both brown and green varieties, with the latter type restricted to the stratigraphically higher beds of the group. The brown biotite occurs in two distinct forms, commonly present in the same specimen. One form consists of thin, lath-like flakes, commonly straight, or, if bent, splintered; these grains show the most perfect parallel orientation of any mineral in the rock. The other type of biotite is typically crystalloblastic, poikilitic, or skeletal, surrounding grains of almost all the other minerals in the rock, including the lath-like form of biotite. Muscovite, as distinct flakes, is an important mineral only in the lower half of the Tenakihi group stratigraphic sequence. In the very uppermost beds it reappears as sericite. Chlorite occurs only in the highest beds, associated with green biotite and garnet. Garnets are the most abundant and conspicuous porphyroblasts in the Tenakihi group. They occur throughout the entire upper 6,000 feet of the section, and in a few beds as much as 38,000 feet stratigraphically lower. The garnets range in abundance from a few minute crystals in relatively pure quartzites to nearly half the rock volume in strongly porphyroblastic garnet-mica schists. Individual crystals range in size up to 24 centimetres in diameter, though most are 1 to 3 mm., and in form from attenuated net- works and irregular patches to symmetrical, dodecahedral crystals. Most crystals are crowded with inclusions of quartz, chlorite, biotite, muscovite, tourmaline, and magnetite; in many the inclusions are arranged in a spiral or S-shaped pattern, probably indicating a rotation of grains during growth. The composition of the garnets appears relatively independent of the posi- tion in the stratigraphic section or of the lithology of the bed in which the grains occur; those grains tested are almandine, with about 85 per cent of the almandite molecule. Kyanite forms porphyroblasts in a fairly definite