Volcano Ôshima, Lizi. 89 



amount of phenocrysts of olivine and other mafic minerals in 

 contrast to the somma lavas of the first type (p. t»?), in which 

 these minerals are either entirely wanting or negligible as pheno- 

 crysts. A moderate quantity of olivine in the rocks of the ruined 

 igneous bodies indicates that these may belong to the perfelic order 

 of the quantitative system. From this character as well as fi'om 

 others described below, the rocks are to be classed as " basalt '* 

 following the definition of this term given by Iddings.^^ 



6 Two-pyroxene-olivine-anorthite-basalt. (PI. HI- Fig. 5.) 



Mode of Occurrence. — The rock occurs in several lava flows 

 constituting the ruined igneous body of Cliigasaki"^ (la ; pp. 38- 

 41 ; PI. VI. GA 14). 



Megascopic Characters. — The rock shows very conspicuous 

 phenocrysts of plagioclase, equant, mostly 2 or 3 mm. but some- 

 times as large as 1 or 2 cm. in diameter. Phenocrysts of olivine, 

 hypersthene, and augite are also seen but are far inferior both in 

 size and quantity, to those of plagioclase, being commonly less 

 than 1 mm. in diameter. 



The groundmass is megascopically aphanitic, gray and rather 

 compact. 



Microscoj)ic Characters, — The phenocrystic minerals seen 

 under the microscope are anorthite, olivine, hypersthene, and augite, 

 in quantitative order. 



The large phenocrystic anorthite has the following optical 

 properties : — 



(1) Extiuctiou iiugle ou M (010): -34°8'±32' 



(2) Eefractive indices : a<1.578<^5<l.o82<;' 



(3) Positions of the optic axes relative to M (010) : 



1) Ljneous Hocks, Vol. II., p. Ill, ldl3. 



2) =f-'rti^ or fi^H^ 



