Yolcmo Osliima, Idzu. J^Jl 



plagioclase, the residual li(j[uid happened to cool very rapidly, it 

 would consolidate into a crystalline mass with composition corre- 

 sponding to that of the residual liquid, uninfluenced by that of 

 the crystals present in contact with the liquid, there being no 

 opportunity for the adjustment of the equilibrium. 



In the roclvs of Oshima crystals of sodic bytownite or labra- 

 dorite occur as a constituent of the ground mass, and often in a 

 thin layer coating phenocrysts of calcic bytownite. These less 

 calcic crystals are no doubt products formed by the sudden cooling 

 of the residual liquid at the time of extrusion. This indicates that 

 at the last intratelluric stage the bytownite o'ljstals {Ab^^An^.^ or near 

 to It) were in equ'iUbriam icith V^e liquid in which the less calcic 

 ]}lagioclase was one of the components. 



If the magma had cooled slowly further from this stage in the 

 intratelluric reservoir, the plagioclase crystals would have changed 

 their composition, becoming more sodic than AbisAn.g. On the 

 other hand, if the last intratelluric temperature had been higher, 

 the phenocrystic plagioclase would have remained more calcic. 

 Hence, from the fact that all the plagioclase phenocrysts in the 



A 



rocks of Oshima which are considered to have been suspended m 

 the magma have nearly the same composition (near Ab^aAn^is), it 

 is inferred that throughout all the stages in the history of the volcano, 

 so far as represented by the exposed lavas, the magmatic temperature 

 has not changed so much as to affect appreciably the composition of~ 

 the plagioclase crystals suspended in the magma. In other words, 

 the long volcanic history of the pouring out of the observed lavas is 

 represented by a very limited intercal in the cooling history of the 

 magma. 



As to the temperature of the magma at this stage, wo are 

 able to estimate it roughly from the experimental results with 



