32 Scientifie Proceedings, Royul Dublin Society, 
Under the microscope the anorthite appears very clear and 
fresh, offering a very beautiful chromatic striation under crossed 
Nicols. The angle between the direction of maximum extinction 
and the line of limit of the twinned lamellae was measured to 
35.38° on both sides. It varies, but is not mucb greater or 
smaller than these numbers. Its optical behaviour accords per- 
fectly with the chemical definition as anorthite. The felspar con- 
tains but two kinds of interspaces—empty cavities and fluid 
cavities, often in long lines one after another. The fluid cavities 
contain bubbles, and the very small bubbles are in a very rapid 
movement ; the larger ones are immovable. By heating the 
preparation to 100°C, the fluid shows no expansion and there- 
fore seems to be but a watery solution. 
The pyroxenic mineral appears under the microscope to 
take all the decisive characters of diallage. Its crystalloids are 
sometimes very numerous, but of a very irregular aspect, as they 
are mostly squeezed between the anorthite prisms. The observed 
optical orientation in the different parts of such a crystalloid proves 
it to be but one individual. The colour is pale grey green, without 
a trace of dichroism. ‘The optical character is that of pyroxene ; 
the angle between the direction of maximum extinction of light, 
and the vertical axis of prisms is 39-40 degrees. That is a very 
decisive character as against hypersthene, that being an ortho- 
rhombic mineral, must offer a parallel orientation of the axes of 
elasticity and the crystallographic axis. 
The interstices are also very characteristic, Just as we know in 
other diallages. Very little black bands and needles, or long 
pipe-like cavities are accumulated following the lamellar cleavage 
of the diallage and form long dull stripes alternating with zones 
free of interstices. The brownish interstices of distinct forms 
which we know of in the diallage of Labrador are not entirely 
absent, but are not frequent. They seem to be negative forms of 
the diallage itself, filled with a secondary reddish matter. Fluid 
cavities, with very moveable bubbles and also glassy interstices 
with fixed cavities are very numerous. In some places the diallage 
alters into a green dichroitic matter, which I am disposed to 
take for smaragdite, that amphibolic mineral being associated 
with the pyroxene in some Gabbros and the Eklogites. 
The oliyine appears clearly only in the thin slices, because it 
