A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. an 
dominant orthoclase. The microcline-perthites, as already 
; stated, are usually rather poor in plagioclase. 
The perthitic feldspar occurs in the form of large pheno- 
crysts which are often idiomorphic Karlsbad twins, tabular 
parallel to M. This is especially the case in the megascopic 
type 1), in which the perthite-crystals are large and the mafic 
minerals play a dominant part in the mineral composition 
of the ground-mass. The smaller the phenocrysts are, the 
less noticable is their idiomorphism. In all cases, even in 
the best developed crystals, their border-lines are fringed 
as if the grains of the granular ground-mass had grown into 
them. Thus they have forms which would be called sub- 
hedral by Iddings. This phenomenon — which is common 
in many porphyritic granites etc. — is probably due to a 
simultaneous crystallization of the marginal parts of the 
phenocrysts and the ground-mass crystals. 
Such coarse-structured microperthite with abundant : 
albitic plagioclase as has been described above is not un- 
common in igneous rocks of the alkaline branch, e. g. in the 
sodalite-foyaite from Ilimausak in Greenland, as recorded 
by Ussing I). Such a perthite has little resemblance to any 
kind of usual fringed perthite in the whole series from homo- 
geneous soda-orthoclase through cryptoperthites to the coar- 
sest perthites occurring in the pegmatites. The theory of 
separation of the perthitic fringes in the solid state (vEnt- 
mischung»), though certainly true in most cases, can hardly 
be accepted as regards such regular intergrowths as we have 
here. As already stated, this perthite bears resemblance to 
Fk the graphic structures which originate by a eutectic crystalli- 
zation and it seems in fact probable that the potash feldspar 
and the plagioclase in this perthite have been separated 
already at their crystallization. Thus they would have the 
same origin as the so-called cryptoperthite and microperthite 
in the Norwegian alkaline-rocks according to J. H. L. Vogt ?). 
The potash feldspar in the granular 
1) N. V. Ussing, »Geology of the country around Julianehaab>, Med- 
delelser om Gronland Vol. XXXVIII, p. 134 (1911). 
?) T. M. P. M., 24, p. 537 (1905). 
