128 
(p. 121), but it is somewhat coarser and the microscopic ap- 
pearance of the felspar therefore approaches that observed 
in the sodalite-foyaite (see below). While in some speci- 
mens the felspar is fresh and clear, in others it is clouded 
with kaolin-like decomposition products; in the unfresh cry- 
stals the twin-structure of the microcline is indistinct or in- 
visible. 
Nepheline occurs rather abundantly. It is idiomorphic when 
bounded by the dark-coloured minerals, but allotriomorphic 
against the felspar crystals. It is of elæolitic habit. In some 
specimens it is partly converted into radiating or irregular ag- 
gregates of cancrinite, in others it has been changed to spreu- 
stein or analcime. 
Sodalite occurs sparingly in this rock, and in some spe- 
cimens it is entirely absent; these specimens, however, are 
rich in spreustein and it is possible that some of the spreu- 
stein-aggregates may have originated from sodalite. One portion 
of the sodalite is in. tolerably idiomorphie crystals, another por- 
tion is interstitial between the felspar crystals. As most spe- 
cimens also contain interstitial analcime the sodalite was 
identified by treating the slices with nitric acid and nitrate of 
silver. 
Ægirine-augite in allotriomorphic anhedra occurs abun- 
dantly in most specimens of the rock. The absorption-tints are 
the same as in the pulaskite (p. 122). In sections parallel to the 
plane of symmetry (010) the extinction-angle с:а averages 30° 
or 40°. This angle has its maximum value (sometimes ex- 
ceeding 50°) in the central part of each crystal and decreases 
gradually towards the outer boundary where it may be less than 
10°. The central portions with large angle of extinction are 
of a rather pale green colour, the marginal zone is intensely 
green. In those portions of the ægirine-augite which are char- 
acterised by an extinction-angle of about 30° a strong disper- 
sion of the axes of optic elasticity is observed: the angle c:a 
