The analysis and the mineralogical characters agree in 
showing that the rock belongs to a ralher common type of 
nepheline-syenites. In most parts of the world, indeed, it is 
possible to find rocks which are closely related to the foyaite 
of Korok. It is also noteworthy that rare elements are almost 
entirely absent. On the other hand, if all foyaites of the com- 
monest type be arranged according to the iron contents, the 
foyaite would be placed at that end which is richest in iron. 
Thus the rock may be regarded as a transitional type between 
the ordinary foyaites and the nepheline-syenites rich in iron 
which are represented in the Ilimausak batholite and in the 
lujavrites of a few other occurrences. To illustrate this relation 
an analysis of a lujavrite from Kola is quoted for comparison. 
Most lujavrites, however, and especially those of Ilimausak con- 
tain a still larger quantity of iron. 
Usuk type. 
A different type of nepheline-syenite occurs in the large 
valley east of Usuk. The same type has been found as erratics 
on Mount Iganek. This rock, when fresh, is of a pure gray 
colour; on weathering it becomes reddish-gray. Structurally it 
differs from the Korok type in the size of grain and in the 
form of the felspar crystals. These are Carlsbad twins, deve- 
loped in very thick plates; their thickness frequently exceeds 
one centimeter, while the length only attains three or four centi- 
meters. The nepheline is in anhedra or imperfect crystals, one 
or two centimeters in diameter. The dark-coloured minerals are 
allotriomorphic. 
The mineral constituents of the Usuk type are: felspar, 
nepheline, olivine, ainigmatite, egirine-augite, hornblende, bio- 
tite, iron ore, and apatite; in one specimen a crystal of eudia- 
lyte, about five millimeters long, has also been observed. The 
felspar is a microperthite of the same kind as that of the Korok- 
