Volcano Osbima, Idzu. 81 



periods. These lavas did not run o\er the brim of the crater but 

 filled the bottom of iV 



Megascopically recognizable minerals are plagioclase and 

 pyroxene. The former occurs abundantly as megaphenocrysts with 

 a diameter varying mostly from 0.2 to 3 mm., while the latter is 

 only sporadically found and is far inferior in size. The colour of 

 the groandmass varies in different parts. In its slaggy part it is 

 jet-black, while in its more compact part it is dark gray. The 

 surface of each lava is often coated with a thin scoriaceous film, 

 brown in colour. 



Microscopically, the rock contains abundant phenocrysts of 

 plagioclase, a few of hypersthene and clino-hypersthene, and a 

 negligible amount of augite. The texture of the groundmass varies. 

 In the megascopic illy gray, comparatively compact part, it is 

 percrystalline in crystallinity and decimiUimeter-grained in granu- 

 larity, consisting of prismoid plagioclase, augite grains, magnetite, 

 and apatite (extremely rare), with only a negligible amount of 

 interstitial glass, arranged in ophitic texture (PI. III. Fig. 1) ; while 

 in the black slaggy part, it is hyalocrystalline or docrystalline, 

 plagioclase and augite swimming in the brown base, and is almost 

 free from visible magnetite crystals (PI. III. Fig. 2). In the part 

 of the intermediate crystallinity, the groundmass is black and 

 opaque in thin section owing to the dissemination of fine grains of 

 magnetite (PI. III. Figs. 3 & 4). 



Characters of the Component Minerals, — Plagioclase occurs 

 as phenocrysts and as a constituent of the groundmass. The 

 plienocrystic one is euhedral and subhedral, and is developed in 

 prismatic and nearly equant habits. Two or three individuals of 



1) Specimens of the earlier lavas which are now entirely biiried were kindly furnished by 

 Prof. Omotj and Mr. Okamit.a to whom the writer extends his grateful acknowledgements. 



