■94 Vol. XLIIL, Art. 6. — S. Tsaboi : 



The characteristic feature of the groundmass is high granularity, 

 being decimilHmeter-grained and doloritic in texture. 



9) Olivine-bytownite-basalt (ß). (PI. TV. Fig. 2.) 



Mode of Occurrence — This rock constitutes the igneous body 

 to the west of Okata'^ (2a; pp. 41-42; PI. VI. GA 9). 



Megascopic Characters — Megascopically, the rock is seni- 

 patic with abundant phenocrysts of plagioclase, 1-4 mm. in diameter, 

 scattered among the grayish, aphanitic, more or less porous ground- 

 mass. Olivine crystals are found far smaller in size and less in 

 <][uantity. 



Microscopic Characters.— MicA^oscopicsdW, the single or 

 grouped phenocrysts of calcic hytoicmte, Ab^^An,,, («<1.574<f3 

 <1.578</'), are eqiiant, mostly subhedral, and commonly twinned, 

 often exhibiting a very faintly marked zonal structure. 

 Inclusions, sometimes with zonal arrangement, are as usual. 



OUinne phenocrysts, 0.05-0.7 mm. across in average, with 

 rounded corners and indentations, are comparatively common. 

 The refractive indices are l.r)78</?< 1.691, so the composition of 

 the mineral is inferred to bo (Fe.SiOOn-u^Mg.SiOOsz-si. Secondary 

 iron oxide is seen along the margin of the crystal, and iddingsite 

 along the cracks. 



The groundmass is almost holocrystaUine, centimillimeter-grained, 

 and basaltic, being bailt up of prismoid plagioclase (extinction angle 

 >30°), anhedral light greenish augite, and small isometric magnetite. 



(I>) Mocks occurring as Ufjkes. 



Dykes are seen at several places iu the island. On the cliff 

 opposite to Fudeshiraa"'^ (3d), numerous dykes are exposed, as has 

 1) n\w 2j 0^^ 



