Volcano Osliima, Idzu. 109 



movement of the crystals in the magma ; consequently if the 

 plagioclase crystals had differed decisively from the magma in 

 density, the movement of the growing crystals and their local 

 accumulation must have taken place in the magma. The inevitable 

 result would be that the plagioclase crystals in different layers in 

 the magma Ijasin would differ in composition, and accordingly the 

 plagioclase phenocrysts in various lavas would not be uniform in 

 their composition. But when we turn to the plagioclase phenocrysts of 

 the Oshima rocks, we find that they are calcic bytownite, Ab^^Angj 

 or very near to it in chemical composition, throughout all the 

 lavas with the only exception that anortliite (Ab,An,,-,) occurs as 

 phenocrysts in the lavas of Chiiiasaki'^ (la ; pp. 80-00). Putting aside 

 for a whilj this exceptional case,'^ the fact tiiat all the plagioclase 

 phenocrysts are nearly of the same composition could be realized only 

 if no appreciable movement of the plagioclase crystals had taken 

 place in the intratelluric stage during the cr>/staUimtion of plagio- 

 clase. From tlie above fact, we can here reasonably deduce that 

 the magma of Oshima {basaltic magma) closely marches the crystals 

 of calcic plagioclase in deiisity. This conclusion as to the density 

 of the basaltic magma is noteworthy in connection with Bowen's 

 statement that " laboratory determination ol densities of calcic 

 plagioclase crystals and of molten gabbro place them very close 

 together, . . . ."'-' 



The degree of porphyricity with respect to tlie plagioclase 

 phenocrysts is quite variable, from only slightly to strongly por- 

 pliyritic, but there is no regularity in the variation which is great 

 even in one flow, ^''ariation in the quantity of the olivine pheno- 

 crysts in the lavas can be explained as due to a gravitative 



1) =f -^1^ or ^idr^ 



2) With regard to this exceptional ca=e, an explanation is given 'ater (p. 110). 



3) "The Tioblem of the Anorthositos," Jour. Geol, Vol. XXV., p. 211, 1917. 



