pr RN HSA dag oa 
YT EES EG 
REN 
50 Pentti Eskola. | (LXIII 
usually termed perthitic and is here called so, but it differs 
from the usual perthitic intergrowth in some respects. The 
plagioclase builds up portions of very irregular forms which 
can neither be called laths nor fringes. The structure of this 
kind of intergrowth is not unlike the graphic intergrowth 
between feldspar and quartz. 
The potash feldspar of the perthite is orthoclase, having 
in sections normal to M the extinction parallel to the trace 
of M. Many individuals, however, show at places a confused 
microcline twinning which is likely to begin from cracks and 
from these continues some distance inwards. Such ortho- 
clase showing an incipient alteration into microcline was 
found to be the regular constituent of the perthitic feldspar 
in the sviatonossites, in the great majority of the specimens 
examined microscopically. But in a few cases the potash 
feldspar consists of cross-hatched microcline without any 
traces of orthoclase, and the potash feldspar of the granular 
ground-mass was then also microcline. The microcline- 
perthite usually contains less plagioclase than the orthoclase- 
perthite and the fringes are rather irregularly distributed. The 
structure resembles more a usual perthite with narrow fringes. 
The plagioclase in perthitic intergrowth is not pure albite. 
The ground mass plagioclase in the analyzed specimen had 
the composition Abgs. Its refringence as compared with that 
of the Canada balsam was: y' >n; e' <n. The perthitic 
plagioclase showed quite the same relation towards the 
balsam and therefore could not be much more albitic than 
the ground-mass plagioclase. In a mounted section parallel 
to M from another specimen the extinction angle from the 
trace of P was found to be 12”, which indicates the same 
composition: Abgs. In many other instances I found that 
the perthitic plagioclase has nearly the same composition 
as the most sodic part of the granular plagioclase, w which 
often shows a zonal development. 
An estimation by the Rosiwal method, carried out in the 
perthite of the analyzed specimen, gave 52.3 percent by volume 
of plagioclase and 47.7 percent by volume of orthoclase. 
On the other hand, many specimens contain perthite with 
