NOTES ON SOME IGNEOUS ROCKS OF JAPAN 



571 



invasion of colorless, low refractive and isotropic substance as in 

 the above types. The alkali-feldspar occurs as the border of almost 

 all crystals of andesine in the groundmass. The augite is light 

 purple in color, and is xenomorphic toward plagioclase. The mag- 

 netite frequently occurs in crystal form. 



II. Alkali-feldspar-bearing basalt.— 



This group differs from the above in the presence of labradorite 

 in the place of andesine, as the essential component. 



The specimen from Madara-shima is dark reddish gray in color 

 with semiwaxy luster. It is holocrystalline, fine granular, and 









It 



Fig. 7. — Microphotograph of the 

 second type of the first group. X30. 



Fig. 8. — Microphotograph of the 

 third type of the first group. X30. 



inconspicuously porphyritic, with not abundant magniphyric 

 feldspar and less pyroxene. 



Under the microscope the rock consists of labradorite, alkali- 

 feldspar, augite, olivine, titaniferous iron ores, and apatite. The 

 labradorite is subhedral to euhedral, twinned according to the 

 Carlsbad and albite laws, and commonly prismatic in shape. 

 Zonal structure is rarely seen. Each of the feldspar crystals com- 

 posing the groundmass is enveloped by a shell of alkali-feldspar. 

 The augite is light greenish yellow with purple tinge, and is sub- 

 hedral to anhedral,- stout prismatic to equant. The larger ones are 

 indistinct phenocrysts; the minute grains are interstitially dis- 

 tributed in the groundmass with magnetite crystals. The olivine 



