204 ^'^^- XLin., Art. 6. — S. Tsuboi : 



fluidity increases inversely their content of silica, and Doelter 

 instanced the magma of plagioclase- basalt as one of the most fluidal 

 magmas. The strongly basic nature of our rocks (SiO^ 51.13-53.01%) 

 suggests that the magma from which they were derived was highly 

 fluidal. 



The behaviour of the magma in the intratelluric stage cannot 

 be discerned, but its behaviour after being extruded as lavas can 

 be inferred from observed facts. As already stated (p. 13), each 

 of the lava flows is very thin, being mostly from a few to 

 several meters, but the crystallinity of the groundmass is very high, 

 percrystalline, in spite of the possible greater tendency to con- 

 solidate as glassy rocks owing to tlie rapid cooling in such thin 

 lavas. This must be due to the high fluidity of the lavas. 



The presence of volatile fluxes and a rise of temperature would 

 naturally diminish the viscosity of the melts. Moreover, according 

 to Vogt's researches on slags, the rate of diminution of viscosity 

 with rise of temperature is greater for those of basic than for those 

 of acidic composition. Thus the fluidity of the basic magma of 

 Oshima, which is very fluidal even in the efiusive stage, must be 

 still greater in the intratelluric stage when the temperature is far 

 higher and an amount of volatile fluxes is present as is indicated 

 by the vesicular nature of the lavas. 



High fluidity of the magma of Oshima is, th(3n, most probable. 

 In such fluidal migma, movement of the crystals is by no means 

 a mere speculative process, but must be necessarily expected. 



Separation of a Mix-Crystal Series. 



Crystallization of the magma of Oshima involves the sep- 

 aration of mix- crystal series as olivine, plagioclase, and py- 

 roxenes. 



