Volcano Ôshima, Idzu. 14a- 



cooled gradually, separating out the crystals of olivine, plagioclase, 

 and pyroxene, in order as shown in Fig. 40 (p. 117), its temper- 

 ature becoming at last decisively lower than 1470X\ and probab- 

 ly as low as 1200^-1300=^0. During the course of the cooling 

 of the magma, the separated crystals were affected by gravitation. 

 Thus, plagioclase, matching the magma closely in density, remained 

 practically suspended in the magma basin; while olivine and 

 pyroxene, being much heavier than the magma, subsided in it 

 toward the bottom as they crystallized out. As a result of the 

 separation and the subsidence of the crystals the originally homo- 

 geneous magma became heterogeneous as time passed, the heavy 

 crystals being relatively concentrated at the bottom. All the rocks 

 of Oshima represent, according to the writer's belief, certain stages 

 after the above-mentioned magmatic temperature had been reached 

 and the subsidence of the heavy crystals had gone on to a fair 

 extent. The rock-type rich in phenocrysts of mafic minerals— basalt 

 —may have been extruded from the lower portion of the magma 

 basin where the mafic minerals were relatively concentrated ; while 

 most of the somma lavas in which olivine crystals are either absent- 

 or only very sparingly found— basaltic bandaite— are believed to 

 have come from the upper part of the magma basin at the stage 

 when the olivine crystals had half sunk ; the central cone lavas 

 which are entirely free from olivine— miharaite— are probably of a 

 still later stage when- the subsidence of olivine crystals had pro- 

 gressed still further so that they had no opportunity of entering 

 the lavas as they were extruded. 



The effect likely to be brought about by the cooling of the 

 magma was considered and it was concluded that the magma would 

 change from basic to acidic if the cooling proceeded. All the lavas 

 of Oshima now seen represent only one stage of the above course. 



