261 
there are a few broken crystals of brown tourmaline, and grains of zircon 
and brookite. These minerals are set in a paste similar to that already 
described, but containing more secondary quartz. In places there are 
patches of calcite. 
C. Specimens from near the Grano-Diorite. 
These may be divided into altered slates and altered sandstones. 
Taking, first, some samples that present a general resemblance to the 
unaltered slates, they are found to be so hardened that they cannot be 
scratched with a knife. The cleavage is not so well marked, and the 
silky lustre has disappeared. The whole rock is full of small, newh 
formed, crystals, with brightly reflecting cleavage faces. The general 
colour is still dark-grey; but the whole rock is full of little spots rather 
darker than the other parts. Through the microscope these spots appear 
more transparent than the rest of the rock. They are colourless, and 
have a low refractive index. They are elongated in one direction, have 
quite an irregular outline, and are arranged so that the direction of 
greatest length is parallel in all. The central part is,, as a rule, full of 
small inclusions, apparently carbonaceous, arranged in lines parallel to 
the greatest length of the spot. The outer rim is clearer, though it en¬ 
closes flakes of brown mica, minute pleochroic prisms and carbonaceous 
spots. There are also rarer inclusions of muscovite. Examined with 
doubly polarized light, these spots are seen to be doubly refracting, with 
interference colours not unlike those of quartz. Sometimes they extinguish 
all over at the same instant, but sometimes they are resolved into a col¬ 
lection of granules. In one case a spot appears to consist of four sec¬ 
tors, forming an interpenetration twin. There are no cleavages, so the 
extinction angle was measured from the lines of inclusions. It varied 
from o deg. to 43 deg. The line of inclusions may not indicate the trace 
of one of the axial planes, and in that case the spots agree fairly well 
with those identified by Hutchings as cordierite*. The next striking 
mineral present is a colourless pyroxene. One of the prisms is in part 
coloured light-brown, and this part is strongly pleochroic, brown to almost 
colourless. The boundaries are well developed crystal faces, the best 
being (no). The pinacoids are relatively rare. The pyroxene is mono- 
clinic, with a maximum extinction of 35 deg. The interference colours 
are yellow or grey. It is the largest mineral in the rock. Brown mica 
has also been largely developed. It is intensely pleochroic, and appears 
to be a haughtonite. The crystals are most common near the spots, but 
are scattered all through the rock. As a rule, but by no means always, 
they are parallel to the length of the spots. Muscovite has been formed 
in smaller quantities. These minerals are set in a ground mass, consist¬ 
ing of the mineral forming the spots and quartz. It is full of bands and 
lines of dark inclusions. The quartz appears to be all secondary. 
Another altered slate, considerably lighter in colour, has not been 
hardened in the same way. It is yellowish-grey, and full of greenish 
spots. These spots are arranged in the same way as before, but consist 
of small muscovite flakes and small grains of quartz (?). The mica flakes 
are, as a rule, parallel to one another, so that with a low power objective 
the spot appears to extinguish all over at the same moment. The spots 
contain some chlorite, and some larger flakes of muscovite, which do not 
conform to the general arrangement. These larger flakes occur plentifully 
* Geol. Mag., 1894. 
