221 
C, p. 220). True rhomb-porphyries which cannot macroscopic- 
ally be distinguished from the Norwegian ones are also found 
in the igneous complex of Ilimausak, but they are only found 
as erratics and have not been subjected to closer inspection. 
A peculiarity in the Ilimausak porphyry, which deserves pointing 
out, is its relatively high content of baryta. It is the only 
baryta-bearing igneous rock found in the Ilimausak region. 
QUARTZ-PORPHYRY (COMENDITE). 
Quartz-porphyries are only found as dykes and intrusive 
sheets. But as they belong to the latest rocks in the volcanic 
series, and as the dykes are very numerous and large, it is not 
improbable that effusive rocks of the same composition may 
have been present in those parts of the volcanic series which 
have been entirely removed by erosion. The quartz-porphyries 
have not been so much altered by contact-metamorphism as the 
igneous rocks mentioned in the preceding chapters, still they 
have not quite escaped this influence. 
The commonest type of quartz-porphyry in the Ilimausak 
region has a dense or extremely fine-grained ground-mass of 
grayish violet, grayish red or entirely red colour. There are 
phenocrysts of quartz as well as of felspar. Usually the felspar- 
phenocrysts are 2 or 3 millimeters, and have about the same 
colour as the ground-mass; they have shapes giving short- 
tabular sections and they sometimes occur in groups of 3 or 4 
intergrown crystals. The quartz-phenocrysts are of somewhat 
smaller dimensions and are quite clear and fresh. They have 
the usual bipyramidal shape. Sometimes they are found in groups 
of 2—4 crystals. 
Microscopic characters. — Under the microscope the fel- 
spar-phenocrysts are seen to be strongly clouded by weathering 
products. They consist of an irregular microperthitic inter- 
growth of potash-felspar and albite. The albite is less weathered 
than the potash-felspar and exhibits the usual twin-lamellation. 
