Volcano Oshima, Itlzu. J3J 



ejection of the basaltic blocks took place after the extrusion of the 

 particular lavas of pyroxene- and hornblende -rhy elites that constitute 

 the masses underlying the basaltic éjecta, and not necessarily thafc 

 all the rhyolitic rocks are older than the basaltic rocks. On the 

 contrary the ejection of the basaltic blocks seems to have been 

 antecedent to the extrusion of the biotite-rhyolites. For, while this 

 basaltic éjecta bed also contains a few blocks of pyroxene- and 

 hornblende-rhyolites, none of biotite-rhyolites are known to bo 

 imbedded ; and it is also not known that the basaltic éjecta 

 cover a biotite-rhyolite mass. Moreover, the rhyolitic éjecta bed, 

 " Shiromama " bed, bears a few basaltic blocks among others. Tho 

 homatholoid of Mukaiyama'^ of biotite-rhyolite in the south of the 

 island rests on the " Shiromama " bed, and show^s a very young 

 morphographic aspect. Thus, it has become clear that rhyolitic 

 and basaltic vulcanism displayed activity alternately in Niijima, 

 accordingly at least some of the rhyolitic rocks must be as young 

 as the basaltic ones. 



All the rhyolitic bodies in Niijima, even the oldest, show far 

 younger morphographic features than those on the main land, and 

 the rocks constituting the former are very fresh with no trace of 

 alteration in contrast to the more or less altered nature of tho 

 rocks of the latter ; . thus the volcanic activity of rhyolites in 

 Niijima seems to have begun far later than that in the Idzii 

 peninsula. Tradition, indeed, speaks of an eruption of Mukai- 

 yama that occurred about 800 years ago. Oral tradition does 

 not constitute very strong evidence, but here it must be noticed 

 that tales of the eruption are told only of the newest volcano and 

 not of the others, which makes us consider the tradition in this 

 case all the more credible. 



1) fSiUi 



