108 Toi. XLIir., Art. G. — s. Tsuboi: 



discharged from the lower portion, and tlie olivine-poor ones from 

 the upper portion of the magma basin. 



Moreover, the separation of the ohvine crystals from the magma 

 should bring about an excess of silica in the residual lic^uid. That 

 this inference is justified from observed facts, will he shown else- 

 where (p. 120). 



Plagioclase. — Plagioclase also forms a mix-crystal series of 

 Roozeboom's Type I,'^ consisting of albite (NaAlSi.O, , the lower- 

 melting component with the melting point llOOX'.) and anorthite 

 (CaALSi.^Os , the liigher- melting component with the melting point 

 1550^0.). The positions of both solidas ;md liquidus were accurately 

 determined and the exact composition of soHd and liquid in equi- 

 librium witli each other in the binary system became known."'-^ 



The crystallization of plagioclase takes place in one of the four 

 mamiers given on pp. 105-106. 



The plagioclase phenocrysts in the rocks of Oshima show no 

 noticeable zonal sti'uctui'e due to chemical difference, though very 

 faint examples are fre<]ucntly seen. This fact must be construed 

 as meaning that the cooling of the magma of Oshima in the intratel- 

 hiric stage teas e.itremelij sfoic, appi-oafliing the i-ate required to 

 jyroduce perfect equilibrium. Tlie crystals witli only very faint 

 zoning would be formed also by very rapid cooling, l)ut, if so, the 

 centres of crystallization about which plagioclase is precipitated 

 must have been much more numerous, and the plagioclase crystals 

 accordingly would not be so large as they are. 



Such slow cooling as is considered to have prevailed in the 

 magma of Oshima may have afforded an opportunity for the 



1) Day and Allen', " The Isomorijhism nnd Thermal Properties of the Feldspars," Carnegie 

 Institution of Washington, Publ. 'So. 31, 1915. 



2) BowEX, '-The Meltiug Phenomena of the Plagioclase Feldspars," Am. Jour. Sei. [i], VoL 

 XXXV., p. 583, 1913. 



