Fr 
A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 59 
This mineral occurs in the form of euhedral or ronnded 
subhedral crystals which are bounded by the prismatic faces 
with subordinate pinacoids. The terminal faces are ill 
developed or quite irregular. The crystals are some two or 
three times as long as they are thick and their length may 
reach as much as three and a half millimeters. Basal 
parting in very well developed. They are megascopically 
of a dark green colour and dull lustre. In thin sections 
the colour is light grass green of various intensity. The 
optical arrangement is £ = b, a:c = 35” to 40”. The axial 
colours are « = bluish green, £ pure grass green y yellowish 
green, and the absorption is «> >y. The pleochroism 
is very faint. The birefringence is somewhat variable, from 
about 0.025 to 0.030. In some specimens there is a remar- 
kable zonal structure in the pyroxene grains, the marginal 
- parts having higher birefringence and darker colours than 
the central parts. This phenomenon indicates an increase 
of the aegirite towards the margins.. The optical character 
is positive and the axial angle about 65”. Dispersion of 
the optical axes is hardly noticeable and its character can 
not be stated with certainty. 
The pyroxene contains numerous inclusions. Most of 
these are what may be termed occasional, such as apatite, 
which in most cases is very abundant, and titanite. 
Both these occur in the form of euhedra of rounded minute 
crystals. Epidote grains which are also frequent have pro- 
bably the nature of alteration-products. Most characteristic 
are, however, the inclusions of green hornblende in regular 
intergrowth with the pyroxene. Crystallographical axes 
b and c and the planes of symmetry are common in both 
minerals and the axes a are inclined to the same direction, 
bisectrix y in both lying within the obtuse angle p. 
The analysis of the pyroxene quoted above indicates 
— it to be chiefly a diopside with a low amount of the sesqui- 
oxides, mixed with the aegirite. The ratio of the molecular 
amounts of aegirite and diopside is nearly 1:10. This ratio, 
however, must be variable. The analysis was carried out 
on a variety of rather pale colour and low birefringence. 
