84 PROF. J. H. L. VOGT ON LABEADOEITE-NOEITE [May 1909, 





Labradorite of 

 labradorite-norite. 



Labradorite of 

 labradorite-roek. 



Pheno- 

 cryst. 



Ground- 

 mass. 







No. 1. No. 2. 



No. 3. 



SiOo 



52-42 55-18 



31-25 1 ! 29-02 2 



11-98 J 1002 



3-44 4-88 



097 1-00 



SiC 53-34 



TiO-2 0-29" 



ALO3 



CaO 



I Na 2 



AI0O3 30-19 2 



MgO 0-44 



CaO 1. 1089 



Na 2 , 3-90 



KoO ! 0-82 



KoO 











Totals ... 100-06 



100-10 



Total... 1 99-87 



1 Including a little ferric oxide and titanium-oxide. 



2 Including a very little ferric oxide. 



(3) Extinction-angle. — In slices cut along the cleavage of 

 the phenocrysts the average of many observations was : — 



Large phenocrysts. 



Phenocrysts. 





Centre. 



Outer parts 

 mostly. 



Outer parts, | 

 certain zones. 



Most 

 places. 



Certain 

 zones. 



9^001 



- 6|° 



- 5|° 



' 



~ 6|° 





:£010 



-m° 



-is*' 



down to 

 -14°. 



-194° 



down to 

 -12° 



The porphyritic plagioclase thus shows in most places extinction- 

 angles of about — 6f° and — 19g°. In sections parallel to 010 of 

 the phenocrysts near the border or ' shell/ a few zones are seen 

 exhibiting a smaller extinction-angle and consequently with some- 

 what more albite. In the slice of the ground-mass a section 

 sensibly parallel to 010 gave an extinction-angle of only about —9°. 

 In sections of the phenocrysts parallel to 010, the angle b (between 

 the twin-striation on the pericline-law and the cleavage 001) was 

 determined as about 3°. 



From these observations of the specific gravity, the chemical 

 composition, and the extinction-angles, which correspond well with 

 each other, it is seen that the ground-mass-plagioclase is somewhat 

 richer in albite than the porphyritic plagioclase. 



The porphyritic labradorite consists (according to Analysis 



