116 Vol. XLIir., Art. 6. — S. Tsuboi : 



Summary on the Cooling History of the Magma. 



The original magma of Oshima was basaltic, highly flaidal and 

 nearly equal in density to calcic plagioclase. 



The magma proceeded to cool extremely slowly. Its cooling 

 history discussed in the foregoing pages is summarized as follows : — 



(I) The First Stage. 

 Olivine. — The olivine began to crystallize. The first crystals 

 were rich in forsterite, but they had a tendency to change their 

 composition toward enrichment in fayalite, due to the molecular 

 interchange between liquid and crystals. The high specific gravity 

 of olivine crystals, the high fluidity of the magma', and tlie very 

 slow rate of its cooling collectively afforded ample opportunity for 

 the sinking of the olivine crystals in the magma. 



(II) The Second Stage. 



Olivine — The separation and the subsidence of the olivine 

 crystals continued to take place. The early separation of the 

 forsterite rich crystals resulted in the enrichment in fayalite of the 

 residual liquid, accordingly, the later separated crystals became 

 richer and richer in fayalite. The olivine crystals that subsided 

 accumulated toward tlie lower portion of the magma basin, and 

 the comparatively small amount of liquid there available deterred 

 the crystals from continual indefinite change in composition toward 

 enrichment in fayalite. The crystals therefore remained there rich 

 in forsterite. 



Plnffioclase. — The plagioclase crystals began to separate. The 

 earliest ones were very rich in anorthite. Since they closely match 

 the magma in density, they remained practically suspended in the 

 liquid where their composition became more and more sodic. Some 



