’ 215 
15°; they are larger along the margin than in the middle of 
the crystals, and the outer shell seems often to consist of pure 
albite. The mean index of refraction is a little lower than that 
of Canada balsam. 
Pseudomorphs from another kind of phenocrysts occur in 
small number. They are smaller than the felspars and often 
show hexagonal outlines. They consist of a microcrystalline 
aggregate of colourless minerals dotted with iron-ore and mica 
scales. 
The ground-mass is of the trachytoid type in regard to 
structure. It is in the main composed of felspar-laths and a 
felt-like aggregate of green biotite; moreover it contains apatite 
and iron-ore. The apatite is in stout prisms. The iron-ore 
occurs in grains and crystals sometimes reaching a diameter of 
0:3 millimeter, but generally much smaller. Most of the iron- 
ore crystals are surrounded by a reaction rim consisting of an 
extremely fine-grained aggregate of a highly refringent and 
strongly birefringent mineral, probably titanite. Many iron-ore 
crystals are almost entirely altered, only a small nucleus being 
left, and in some cases the alteration is complete. The felspar- 
laths of the ground-mass average about 0°4 millimeter in length 
by 0:1 millimeter in breadth. They are simple or once twin- 
ned; more seldom they are polysynthetic. Their mean refrac- 
tion index is slightly lower than that of Canada balsam. The 
maximum extinction-angle in sections at right} angles to the 
albite-lamellæ is about 10°. The felspar-laths of the ground- 
mass thus consist of an acid oligoclase or nearly pure albite. 
It is possible that also other kinds of felspar or felspathoid 
minerals may be present in small amount. The colourless 
minerals of the ground-mass, with the exception of the augite, 
are filled with exceedingly small scales of brown biotite and of 
colourless or greenish needles giving hexagonal sections. The 
latter as a rule attain only 0:01 or 0:02 millimeter in length, 
they are rather highly refringent, but owing to the minuteness 
