Volcano üshima, Idzu. 121 



That the early separation of olivine would brii^g about an 

 excess of silica in the residual liquid was already pointed out on 



p. 108. 



(4) Micro-allivalite/^— A small ejected block of micro- alii valite 

 was found imbedded in a volcanic ash bed (PI. VI. G-A 12) inter- 

 calated with layers of the lavas of Kazahaya'^ (la ; p. 92). It is 

 a holocrystalline mass consisting of anorthite Ab^AUps and olivine 

 (Fe,SiO,),,_is(Mg,SiO,),s_8, with a small quantity of interstitial matter 

 (p. 99). Both plagioclase and olivine constituting the micro- 

 allivalite belong to the varieties richer in higher -melting components 

 than those in common lavas. This fact is, according to the 

 writer's belief, directly connected with the genesis of this ejected 

 block. 



There was a period when (Diily anorthite an^l olivine (rich in 

 forsterite) were crystallizing from the magma (the second stage, 

 pp. 116-117), so that these had an opportunity of sinking together 

 in the magma reservoir and accumulating in such a manner 

 tliat no change in their compositions toward enrichment in the 

 lower- melting components could be accomplished owing to the small 

 amount of liquid available for the interchange of material between 

 liquid and crystals. Thus, the mass formed at the lower portion 

 of the magma basin would have just the characters now seen in 

 micro-aUivaUte. The block now under consideration may be a piece 

 broken off and ejected from the mass formed by the above process 

 at the lower portion of the reservoir^ 



1) PhaaerocrystaUine éjecta are very common both in our country as well as abroad, and 

 various views on their genèses have been suggested by different writers. Especially on those 

 from the volcanoes of Japan, we have Hieab.vyashi's and FuKrcHi's studies respectively on those 

 of Fuji ("Report on the Geology of Volcanoes Fuji and Ashitaka," h^port Ea.Uq. Invest. Cm., 

 2so. 24, 1883. in Japanese) and o£ some of the Idzu Islands other than Oshima i" On the Phanero- 

 crystftlliTie Bjmb.s from S^me of the Idzu Islands," Jour. Geol. Soc. Takyr,, Vol. VIIL, No. Od, 

 1901, in Japanese). 



2) E^- 



