187 
a Bertrand's quartz plate, but not measurable. Sections 
parallel to (010) are relatively fairly numerous. They show 
absolutely no trace of zoning, and so appear quite like ortho- 
clase. The cleavage parallel to (001) and the rough cracks 
and lines of decomposition products permit the orientation of 
the sections to be determined. The extinction angle is + 18°. 
In convergent light the point of emergence of an acute bisec- 
trix is seen within the field, but near the edge. The optical 
sign is positive. The refractive index is less than that of 
quartz at all contacts. The felspar is thus almost pure albite, 
with no perceptible intergrowth with isomorphous compounds. 
Muscovite, both primary and secondary, is abundant. As 
a primary mineral it forms large flakes, whose optical charac- 
ters are quite normai. It is intergrown in the usual way 
with biotite, the basal planes of the two minerals being 
parallel. The mineral is perfectly destitute of colour, and 
exhibits no characteristic microstructure. It contains, as do 
all the minerals of the rock, a few prisms of rutile and apa- 
tite. Тһе secondary mica, though distributed through the 
section, is particularly abundant in the folia containing 
much primary mica, where it forms felt-like masses, with 
strong double refraction. It is derived in part from the 
primary muscovite, as the latter can be observed fraying out 
and breaking up. Some of it is derived from felspar, as por- 
tions of unaltered felspar can be detected amongst it. 
Biotite is primary in all cases. 16 is of the same order of 
size and abundance as the primary muscovite, and, as above 
noted, is intergrown with it. Both minerals are somewhat 
frayed out at the ends, but are not much bent or broken. 
The biotite is light yellowish-brown in colour, and is very 
strongly pleochroic. When the vibrations are perpendicular 
to the cleavage the colour is yellow, the vibrations parallel 
to the cleavage being almost entirely absorbed. Around the 
few small prismatie inclusions (probably rutile) marked halos 
of dark colour are produced. Grains and imperfect crystals 
of magnetite, all of comparatively small size, are present, but 
not abundant. Small prisms of apatite are even scarcer. 
The most abundant accessory mineral is rutile. This is scat- 
tered through the section, and is included by all the essential 
constituents. In most instances the rutile builds short, 
thick prisms, surmounted by pyramids, but occasionally the 
crystal form is not apparent, and the mineral assumes the 
shape of round grains. No certain trace of twinning was ob- 
served. The colour is bright orange by reflected light, 
reddish - brown by transmitted light, though the small- 
est individuals appear somewhat greenish. Тһе refrac- 
tive index is so high that many of the more ` round- 
