117 
As already mentioned in the geological section the soda- 
granite of Iviangusat is of a rather varying size of grain. 
Coarse-grained and pegmatitic portions alternate irregularly or 
in stripes with medium-grained or fine-grained portions. The 
colour of the rock as a rule is light gray or yellowish. 
Microscopic characters. — In a thin section the main con- 
stituents are seen to be the following: felspar, quartz, cato- 
phorite-like hornblende, and egirine-augite; a little ægirine is 
usually associated with the latter mineral. Small erystals of 
zircon (sometimes with zonary banding) and others of magnetite 
are always present. Apatite is rare. An intensely yellow- 
coloured mineral occurs rather sparingly in small prisms and 
fibrous aggregates; it is optically biaxial and probably titani- 
ferous; it is accompanied by a leucoxene-like decomposition- 
product. 
The felspar is a perthitic microcline-albite of a rather fine 
lamellar structure. The lamelle are of a tolerably regular shape 
and perpendicular to 6 (010) as in ordinary orthoclase-micro- 
perthite. The microcline is irregularly twinned. In many felspar- 
sections the majority of the microcline-areas belongs to one in- 
dividual. The albite shows a fine twin-striation parallel to 6 
(010); its indices of refraction are in all sections found to be 
lower than those of the adjoining quartz. The felspar is rather 
idiomorphic and of a thick-tabular habit. 
The quartz has fluid-cavities with a mobile bubble. It is 
quite allotriomorphic and often occurs in relatively large an- 
hedra each of which encloses a considerable number of felspar 
crystals. This structural feature may even be observed in the 
vicinity of the contact with the sandstone. But immediately at 
the contact it changes into the reverse: here the felspar be- 
comes allotriomorphic and is seen to enclose isolated quartz- 
grains and send out off-shoots between the quartz-grains of 
the adjoining sandstone. 
A catophorite-like hornblende is the most abundant ferro- 
