FY ESAIAS ES Art NOTE rt SR HELA 
pö VOR, Sp AK ERNES Ad 
52 Pentti Bskolg. 6 
ground-mass is mostly orthoclase like the perthitic 
phenocrysts, but when the latter are microcline, the former 
is also. Moreover, I found specimens having orthoclase as 
phenocrysts, but microcline in the ground-mass. At first = 
sight such microcline grains seem to be orthoclase with an 
undulatory exinction; by closer examination with higher 
powers there may be seen portions with better developed 
quadrille-structure. : 
Plagioclase in perthitic intergrowth with the ground-mass 
potash feldspar was found but exceptionally and only in 
trifling amounts, forming narrow fringes irregularly dis- 
tributed in the inner parts of the host. In most specimens 
the potash feldspar is quite pure. 
This potash feldspar occurs like the ground-mass plagio- 
clase in the form of isometric xenomorphic grains. 
The plagioclase in the ground-mass 
"shows the same xenomorphic habit as the. granular potash 
feldspar, and usually shows no twinning structure. When 
twinned, it follows the albite law. In the analyzed specimen 
the plagioclase is fairly uniform in composition. The extinc- 
tion angle in sections normal to PM, determined in 4 indi-' 
viduals, was found to be —7”, corresponding to the compo- 
sition Abgs. Determinations made in other specimens from 
the shore-drift gave slightly variable results as appears from 
. the following data. — All the determinations were made in 
sections | PM and confirmed by the refringence as compared 
with that of the Canada balsam. 
Kind of the 
Thin Kind of the 
ground-mass 
5 ö NE | 
Se RAR eri Ra 
Spar 
76 85 Or Mr Antiperthite in the = 
ground-mass. 
70 82—90 Mr Mr Perthite poor in pla- 
E gioclase. 
71 83—91 = chiefly Or Mr Plagioclase (Abg;) do- 
minant in the per- 
thite. 
72 85—90 Or Or SS 
73 83—87 Or chiefly Or As 71. 
149 36—91 Or chiefly Or Perthite with 52.3 
(analysis) | vol. ”/, plag. 
(LXIII 
