70 ;.Vol. XLIII., Art. G. — s. Tsuboi: 



From these values of the refractive indices the composition of 

 tlie oUvine is inferred to be about (Fe, SiO, ),s_3o(Mg, SiO, \._^,^ ac- 

 cording to H. Backlund. 



Pale light greenish aiKjitc and low double-refracting, weakly 

 pleochroic hypersthene are extremely rare as phenocrysts. The 

 former is in subhedral to anhedral forms, sometimes twinned on 

 (100) ; the latter is always normal in optical orientation so far as 

 observed. Sometimes the two occur in parallel intergrowth. 



The r/roundmass consists of prismoid plagioclase, light greenish 

 anhedral augite, and magnetite, usually with more or less interstitial 

 glass. Plagioclase is here labradorite varying from AbjijAnso to 

 Ab.jjAn,. with the mean refractive index 1.558 — 1.5(35. Augite 

 has the refractive indices «< 1.6830 1.6830<i5< 1.6809 l.G975<r, 

 and is comparable with the mineral described as diopside from 

 Taberg in which the medium refractive index is 1.0836. Th 

 texture of the groundmass \aries in different parts. In its mega- 

 scopically black ] art (^-/-type), it is docrystalline ; and fine-grained 

 irregularly shaped, often dendritic, skeletal crystals of magnetite 

 disseminate among the plagioclase prisms and augite grains. The 

 interstitial glass is colourless but sometimes appears l)rown in the 

 vicinity of the vesicles owing to fine dusty inclusions. In its mega 

 scopically gray, compact part (;9-type), the groundmass is nearly 

 liolocrystalhne, granular intersertal, and the felsic and mafic com- 

 ponents are present in equal amount. Here the magnetite crystals 

 are generally large and show their own forms. 



Chemical Characters,— K^ specimen from the uppermost lava 

 (PI. VI. DE 4) exposed on the south shore at the east end of 

 »Sashikiji'^ (2e) was chemically analysed by Mr. Ohashi of the 

 Geological Survey through tlie kindness of Prof. Koto, to both 



1) ^A<\k 



