92 Vol. XLIII., Alt. G. — S. Tsuboi : 



mass which is high in crystalHnity (nearly holocrystalline) but low 

 in granularity. 



7) Olivine-bytownite-basalt («)• (PI. HI- 'Fig. tl.) 



Mode of Occurrence, — The rock occurs as lavas constituting 

 the ruined igneous body underlying Kazahaya^^ (la; p. 41; PL 

 VI. GA 13). 



Megascopic Characters. — The rock is dark gray and is 

 strongly porphyritic. The phenocrysts are mainly of plagioclase 

 with diameters from 1 to 3 mm. Minute olivine phenocrysts are 

 seen in smaller quantities. Irregular pores with diameters, mostly 

 from 1 to 2 mm. and in maximum 5 mm., are distributed through 

 the rock mass. 



ßlicrosropic Characters. — Microscopically, the rock is dopatic. 

 Phenocrysts of plagioclase, which is calcic bytownite (Abi^Aug^) 

 with the refractive indices ''^•<1.572</3<l,578<r, are generally 

 subhedral, twinned as usual, and often carrying inclusions of 

 augite, uncrystallized groundmass, and fine dusty substances. 

 Zonal structure due to chemical difference is only very faintly 

 developed or almost lacking. 



The microphenocrysts of slightly greenish olii-ine, {Fe,SiO,)^.-^^g 

 (Mg,Si04).7_ci («<1.678<,.Kl.6l)l<r), averaging about 0.2-0.3 w?7i. 

 in length, are anhedral often with deep indentations, remarkably 

 traversed by cleavage lines and cracks perpendicular to the c-axis, 

 and not infrequently surrounded by the alteration product, brownish 

 yellow iron oxide. 



These phenocrysts are scattered through the nearly holocrys- 

 talline, centimillimeter-grained, granular intersertal (jroundmass. The 

 ininerals that build up the groundmass are prismoid plagioclase 



(1) 



