219 
ranging from that of an almost pure potash-felspar to that of 
albite. 
Besides the felspar also apatite and iron-ore occur as 
phenocrysts. The apatite is rather abundant in stout prisms 
of 0'6 to 0:2 milimeter. The iron-ore, probably titano-mag- 
netite, occurs in more isometric crystals partially changed to 
titanite. 
The ground-mass consists mainly of small felspar-laths 
and shows a marked flow-structure. The felspar-laths have a 
length of 0°04 to 0°15 millimeter, while the thickness is one 
seventh or one tenth of this. Their index of refraction is a 
little fainter than that of Canada balsam. They show about 
parallel extinction or they are longitudinally once twinned with 
extinction-angles from 0° to 8°. Consequently they are alkali- 
felspar and probably anorthoclase. Besides felspar the ground- 
mass contains a great quantity of colourless needles, grains of 
titanite and scales of muscovite and the whole is finely dotted 
and splashed with innumerable minute grains of iron-ore. The 
colourless needles have a thickness of 0'001 to 0'002 millimeter 
and the length is about ten times as great; the axis of maxi- 
mum optical elasticity apparently coincides with the long axis 
of the microlites. 
Chemical composition. — A chemical analysis of the Ш- 
mausak porphyry is given in No. 28 of the table below. The 
composition is rather peculiar. It may perhaps have been 
slightly modified by the alterations the rock has suffered, but 
it does not seem likely that it should differ greatly from the 
original composition. It is of a type related to the alkali- 
trachytes; on the other hand it shows analogies pointing to- 
wards phonolite and trachydolerite. There is a rather striking 
analogy with the mugearite of Skye an analysis of which is 
quoted for comparison. The main difference is in the alkalies 
which are higher in the Ilimausak rock. There is also some 
analogy with the rhomb-porphyry of South Norway (se analysis 
