NOTES ON SOME IGNEOUS ROCKS OF JAPAN 



557 



in small amounts. The mean index of refraction of the mineral 

 measured by Wright's is n D = 1.526— 1 .531. The extinction 

 angle on (010) is +5 to +9 , and on (001) o to +i°: The 

 characteristic undulatory extinction is well marked. The plane 

 of the optic axes is approximately perpendicular to (010), and the 

 negative acute bisectrix is nearly normal to (201). The apparent 

 optical angle measured on the section nearly parallel to (201), 

 by Mallard-Becke's method, is 84° 44/ and 2V is 52 20', the mean 

 index of refraction being assumed as n D = 1.528. The dispersion 

 is p>v. 



Fig. 1 



Fig. 



The groundmass feldspars are also alkaline feldspar and occur 

 in elongated prisms, simple Carlsbad twinning being commonly 

 present. They are arranged as in trachytic fabric and the fluxion 

 is especially marked around the phenocrysts. 



Aegirine-augite, as phenocrysts, is almost absent, and the largest 

 crystal, which was observed in 5 thin sections, measures 1.5 mm. 

 in length, but the average length of the prisms is 0.2 mm. The 

 mineral is of bluish-green color, and is somewhat pleochroic from 

 bluish-green to the same color with yellowish tint. The greatest 

 extinction angle measured with respect to the c axis, gave 45 . As 

 inclusions magnetic grains are common ; and brown glass, appatite 

 needles, and feldspar laths can be detatched, the last being very 

 scarce. 



