Volcano Ôsbima, Llzu. 29" 



It seems to the writer, however, that the above is not a 

 positive proof that the gap was formed by an explosion. That 

 the momentum of a lava stream is insufficient to form such a gap 

 as that in question is only suggestive that the gap was formed 

 before the central cone lava flowed down to the east coast. More- 

 over, it is not sure whether the recorded detonation in 1112 

 resulted from the eruption in Oshima or not. 



A 



Ohashi,^^ on the other hand, considered that tliis gap might 

 have been formed by depression, basing his view on the absence 

 of explosion products. But this also is not convincing as the later 

 volcanic materials cover the ground so thickly on the eastern part 

 of the island that the explosion products, if there are any, may 

 well be hidden under theso surficial deposits. 



To consider the problem geologically is impossible at present 

 as no data are available, so that in the absence of convincing 

 proof tliat the gap was formed by an explosion, the writer is 

 inclined to agree with Ohashi hi regarding it as the result of 

 simple depression induced by the formation of the caldera. 



The Central Cone. 



The central cone, Miharayama"-' (2c), stands somewhat to the 

 south of the centre of the caldera. It is a perfectly preserved 

 undissected homate. The highest point lies to the east of the 

 crater-wall and rises 755 m. above sea levc4, or about 200 m. 

 above the surrounding surface of the atrio. The sides of the 

 homate averaging 20° in slopes are bare of vegetation and are 

 covered by loose sands, lapilli, bombs, and lava-blocks. 



1) " On the Geology of Volcano Oshima," Jour. Geol. Soc. Tokyo. Vol. XVI., p. 530, 1909 

 (in Japanese). 



2j Hj!:f;u4 



