188 
ed pieces appear almost opaque. Between crossed nicols only 
the smallest individuals show bright polarisation colours, on 
account of the exceedingly strong double refraction. 
GNEISSIC APLITE, NEAR BASE OF GRANITE RANGE, WEST OF 
Kıng’s Buurr. 
Macroscopic Characters. —The rock is granitic in texture, 
and rather coarse in grain. The mass colour is yellow, but 
it is speckled with small grains of a black material, the ar- 
rangement of which points to a fairly marked foliation. Un- 
der the lens it is seen to consist almost entirely of felspar, 
quartz, and magnetite. Felspar is the most abundant con- 
stivuent. It occurs in pinkish-yellow (tabular) sections up 
to about 8 mm. in diameter, striking on account of the per- 
fection of the cleavage. On cleavages parallel to the base 
(001) twinning after the albite law can be noticed, combine 
occasionally with that after the Carlsbad law. Quartz is pre- 
sent in translucent grains of small size, but is fairly abun- 
dant. No mica or other ferromagnesian mineral is to be 
detected. Тһе abundance of magnetite in so acid a rock is 
remarkable. The grains are up to about I mm. in dia- 
meter, and, as already noted, are arranged in roughly paral- 
lel lines. The rock is obscurely jointed, and a little second- 
ary muscovite has been developed on the joint surfaces. 
Microscopic Uharacters.-—In thin section the rock possesses 
several features in very marked contrast to those previously 
described. In the first place, the absence of primary mica 
of any kind is very striking. Then the evidences of strain 
in the felspar are much more marked here. The abundance of 
magnetite in so acid a rock is also somewhat remarkable. 
The essential minerals present are quartz, albite, and mag- 
netite, the texture being hypidiomorphic granular. 
Albite occurs in relatively large, perfectly allotriomorphic 
sections. These are, on the whole, perfectly clear and colour- 
less, though they contain a considerable amount of dusty de- 
composition product. Examination of the optical properties 
indicates practically pure albite, the measurements being 
practically identical in all cases with those given for the other 
rocks, described and as thoroughly satisfactory. The differ- 
ence of extraction angle between the two portions of a Carls- 
bad twin rises here to about 3°. Lamelle after the pericline 
law are quite frequent and in all cases are certainly of secon- 
dary origin. They appear in sections which have obviously 
been subjected to crushing, and in many instances there is a 
very obvious faulting along their plane of composition. The 
faulted portions, which have a step-like arrangement, con- 
tain pericline lamelle, but no albite lamellæ, whilst in the 
