572 



S. KOZU 



as microscopic phenocrysts is subhedral to anhedral, and altera- 

 tion into iddingsite is commonly visible along cracks and in marginal 

 portions. The texture of the groundmass is somewhat inter- 

 sertal, and is characterized by divergent arrangement of prisms 

 of plagioclase enveloped by alkali-feldspar, with interstitial gran- 

 ules of augite, olivine, and magnetite (Fig. 9). 



A more distinctly crystalline and coarser type is a specimen 

 from O-shima, an islet, near the Island of Iki. 



Megascopically the rock, more or less decomposed, is evidently 



holocrystalline, but the indi- 

 vidual crystals are scarcely 

 recognizable, though the di- 

 verse arrangement of prismoid 

 feldspars, 1.5 to 2 mm. long, 

 is well marked in the hand 

 specimen. The color is light 

 gray, on account of the 

 abundant feldspars, and is 

 dotted by dull reddish brown 

 spots produced by decompo- 

 sition of the olivine. Rare, 

 inconspicuous phenocrysts are 

 tabular, white feldspar; irreg- 

 ularly shaped black augite; 

 All of them are less than 3 mm. 



Fig. 9. — Microphotograph of the alkali- 

 feldspar-bearing basalt from Madara-shima. 

 X30. 



and equant, dark reddish olivine 

 in diameter. 



Under the microscope (Fig. 10), the texture is transitional from 

 doleritic to intersertal, as the augite is xenomorphic toward feld- 

 spar in one case and automorphic in the other. The mineralogical 

 constituents are as before, but the presence of broad bands of alkali- 

 feldspar enveloping labradorite is especially noticeable (Fig. 11). 

 In some crystals, the alkali-feldspar has more than three times the 

 volume of the labradorite, that is, o. 75 mm. in length and 0.09 mm. 

 in width (Fig. n). In general, the labradorite is in extremely 

 elongated prisms, twinned according to the Carlsbad and albite 

 laws. The augite is anhedral to subhedral, prismatic to equant. 

 In color it is light purple. The magnetite frequently occurs in 



