300

rounded elongate grains relatively free from inclusions; and the principal
minerals have segregated into bands with a gneissic, rather than schistose,
pattern. In some beds at this stage, particularly in parts of beds near the
crests of folds, the quartz and feldspar have segregated into. lens-shaped
augen 3 cm. or more in length, and the rock becomes a typical augen-
gneiss. Megascopically, many specimens of these rocks have a granitic
appearance, but the bedding, as outlined by alternate layers of eneissic
quartzite and quartz-mica-feldspar gneiss, is well preserved (See Plate
VI B). The gneissic folation is in most cases parallel or nearly parallel
with the bedding.
QUARTZ VEINS

Coincident with the increasing grade of metamorphism of the Tenakihi
rocks, there must have been an expulsion of excess silica, for large quartz
veins are characteristic of all exposures of the group. In some places, in
an area of as much as half a square mile, it was estimated that at least
1 per cent of the area of rock exposed is vein quartz. Most of the quartz
veins are from 3 to 15 feet thick, and from 100 to 300 feet long, although a
few veins have been traced for more than 1,000 feet. The largest body of
vein quartz found in the map-area is in the low hills of Tenakihi Range
about 4 miles east of mile 55 on the Aiken Lake winter road. Here, over
an area 600 feet by 180 feet, the only material outcropping is white vein
quartz; and the ‘depth’ or thickness of the body, as shown by gullies, is at
least 25 feet. The structural relations of this body of quartz to the
surrounding rocks are not known.

Small stringers and veinlets of quartz are scarce. The quartz of the
large veins is opaque, milky white, and almost completely free of rusty dis-
coloration. The only other minerals observed within the veins—muscovite,
biotite, and tourmaline—occur in minute amounts. In some places small
fragments of the surrounding schist or quartzite lie within the vein a few
inches from its walls, and one vein in Jim May Creek Valley contains a
horse of rock about 100 feet long.

Most of the quartz veins parallel the bedding, although a step-like
pattern, in which the veins alternately follow bedding planes and crosseut
the beds to reach and follow a different bedding plane, is not uncommon.
In some of the schist the veins form lenticular masses around which the
planes of schistosity are deformed. Where the beds are transected by the
veins they are sharply terminated, and there is very little small-scale
penetration of quartz along the foliation planes of the host rock.

The remarkable purity of these veins, with an almost total absence of
hydrothermal minerals other than quartz, their lenticular form, lack of
interconnecting ‘feeders’ or branches, and distribution over the entire
exposed area of Tenakihi group rocks, has led to the conclusion that they
are probably composed in large part of quartz expelled from the surround-
ing rocks during metamorphism, without appreciable addition of material
from sources at greater depth.

The quartz veins are, in any one area, usually found within, or along, a
contact of a series of quartzite beds. This position is favourable probably
because of the greater amount of excess silica in the quartzite, and because
the quartzite is a mechanically stronger rock than the associated schists
and is thus more capable of forming and sustaining fractures into which
the siliceous solutions could migrate.