"i M LJ S 
oo A N:o 1) Igneous Rocks of Sviatoy Noss in Transbaikalia. 21 
are found in the central part of the mass as well as near 
the boundaries and in the apophysal intrusions in neighbour- 
ing migmatitic areas. 
As representative of the mineralogical characters and 
structure of the porphyritic granites may be taken the speci- 
men from point 156 A. The rock consists of phenocrysts of 
micro-perthite, 10—20 mm in diameter, embedded in a 
millimeter-grained mass of plagioclase, microcline, quartz, 
biotite, hornblende, titanite, magnetite and apatite. 
oPlagioclase is found as euhedral crystals enclosed 
in the perthitic PASS and as anhedral grains in the 
ground-mass. A section ! PM showed extinction parallel to 
3 the trace of M, SR to the composition Abgp. 
Towards the microcline the grains have albitic border-zones. 
The potash feldsparin the ground-mass is cross- 
hatched microcline destitute of perthitic albite. The pheno- 
crysts of potash feldspar contain abundant albite in perthitic 
intergrowth showing distinct zonal development, the albite 
laths being most abundant in zones arranged along certain 
crystalline faces. The potash feldspar in this zonal perthite 
seems to be orthoclase; at places very fine microcline twinning 
may be observed, but on the whole the quadrille-structure 
if existing has submicroscopic dimensions. There are, how- 
ever, large amounts of coarsely cross-hatched microcline also, 
—— the microcline structure being apparently of secondary origin. 
Often such microcline traverses the whole individual along 
lines that seem to represent deformation zones. Cracks are 
often seen in these. The microcline portions are, in most 
striking manner, very poor if not quite devoid of the perthi- 
tic albite laths. ; 
The other minerals do not need any special description. 
Many other specimens contain only cross-hatched micro- 
. cline as potash feldspar. Myrmekitic intergrowth of 
-plagioclase and quartz is common in such varieties. 
To these microcline-granites also belong the porphyritic 
granites found as intrusives in the migmatite area, e. g. at 
> points 7, 31 and 33. The specimens collected from these 
Points consist of the following minerals: microcline, plagio- 
