Volcano Oshima, Idzu. 143 



northern coast, several small demolished igneoas bodies now covered 

 by the volcanic materials of the somma. 



(3) The rocks constituting the volcano are of several types as 

 already tabulated on p. 67, but they are readily grouped under 

 three main headings, — basaltic bandaite, miharaite, and basalt. 



Basaltic bandaite predominates amongst the lavas of the 

 somma. In it, the phenocrysts of calcic bytownite are scattered 

 through the granular intersertal groundmass consisting of labra- 

 dorite, augite, magnetite, and a small quantity of glass. Most of 

 the lavas of this type bear a negligible amount of small olivine 

 phenocrysts, though in some this mineral is absent. Hypersthene 

 and augite phenocrysts are cither lacking or present only in small 

 quantities. 



The type which the writer describes under the new name 

 *' miharaite " constitutes the central cone. It does not differ much 

 from the preceding in its pétrographie characters but is charac- 

 terized by an entire absence of olivine. 



Basalt is a variety comparatively rich in phenocrysts of olivine 

 and pyroxene. This constitutes the northwestern demolished 

 igneous bodies, but is very rarely found as the lavas of the 

 somma. 



AU of these rocks are strongly basic. The chemical analyses 

 made of the first two types of the above three — basaltic bandaitô 

 and miharaite — , which exceed the third in mass, show a remark- 

 able character in that the amount of silica, in spite of its low- 

 percentage, is not only enough to form tlie ' highest silicates but ifc 

 is present m excess. This is one of the common characteristics of 

 the most widely spread recent volcanic rocks of Japan, but the rocks 

 of Oshima are peculiar in their more femic and less quaric natures. 



(4) The physiograpMo history of the volcano since its birth 



