Microscopical Society of Victoria. 
85 . 
felspars. I also observed small masses of ordinary pyrites ; some 
of these were surrounded by a “ halo” of oxidation. A variation 
of texture occurs in some parts of the dyke, as patches of fine 
grained rock, which, under the lens, is a network of white felspar 
and greenish hornblende crystals. It then resembles some of the 
diorites. 
Microscopic Examination. — The ground-mass of twenty slices 
which were prepared from these dykes is usually microcrystalline, 
and more rarely cryptocrystalline. It is composed of — 
(a) . Granules or minute prisms of felspar, which seem to be 
all triclinic. 
( b) . Small flakes and prisms of amphibole, which are placed 
without order, and lie, as a rule, without any common direction. 
Their appearance is peculiar. The colour is dull yellowish green* 
In carefully passing under review the whole of the slices prepared 
from these dykes, the following points become worthy of notice: 
The flakes are in places of exceedingly minute size, and have a 
rounded outline. In other places, where they are less numerous, 
they occur as isolated irregular lamella*, or as groups of lamellae 
superposed on each other, or as minute prisms, having a tendency 
to arrange themselves in radiating groups. I have observed the 
minute prismatic forms to be bifurcated, or in groups of three, in 
which the length of the middle one is less than that of the 
two others. 
(c) . Grains or imperfect crystals of magnetite or titanic iron. 
( d ) . Prisms and needles of apatite, which lie in all directions in 
the ground-mass. 
(e) . Chlorite (viridite.) 
In rare instances this ground-mass becomes microporphyritic by 
some one constituent preponderating in dimension, and this is 
usually felspar. In the ground-mass are porphyritically situated— 
(a). Felspars. — These are, without exception, triclinic. They 
are highly compound in structure, and also frequently show zonal 
growth in a beautiful maimer. In an example taken from a large 
dyke in the hill above the Bulgoback Post Office, I found the 
angles formed by the plane of vibration in sections in the zone 
o—ii to be 18°, 21°, 25°, 34*, 30'. The angles, I think, point to 
a felspar of the labradorite class. In addition to these, I observed 
also, in the same example, numerous simple crystals, in none of 
