Volcano Ôshima, Idzn. 135 



the rhyolitic rocks of the Idzu islands must have descended from 

 those of the Tertiary period in the Idzu peninsula. 



As stated above, the insular group of Idzu comprises volcanoes 

 of two different lineages— those of basaltic and rhyolitic rocks, 

 descended respectively from the Tertiary andesitic and rhyoKtic in 

 the Idzu peninsula. The writer hereafter distinguishes these two 

 groups of the volcanic islands as " Ôshima group " and " Niijima 

 groupe In the age of the volcanic activity of these two groups 

 there is no disparity, all the volcanoes being very young. Of these, 

 the basaltic Toshima, Udoncshima, and Miki]rashima^\ and the 

 rhyolitic Shikineshima'^ are not known from historic records to 

 have ever been in a state of eruption ; whik^ the basaltic Oshima, 

 Miyakejima, and Hachijôjima^^\ and the rhyolitic Niijima and 

 Kôzushima'^ have displayed activity in historic times. 



The islands of the Ôshima group and the volcanoes Fuji, 

 Ashitaka, Hakone, Amagi, Atami, etc. lie in the zone that trends in 

 the direction N.N.W.— S.S.E., meeting the festoon islands at large 

 angles, and on its southern prolongation the volcanoes Aogashima, 

 Torijima, and Sulphur Islands^\ as weU as the submarine ones near 

 Bayonnaise Rocks, Smith Eocks, North and South Sulphur Islands, 

 etc. are situated. All of these are young volcanoes with historic re- 

 cords of eruptions, and their lavas are auganitic or basaltic. The writer 

 thinks it appropriate to define the term " Fuji volcanic chain " as 

 meaning this row of the Quaternary auganitic and basaltic volcanoes. 

 The insular volcanoes of the Ôshima group form a sub-group 

 in the Fuji volcanic^ chain in geographic contiguity, pétrographie 

 and morphographic resemblance (all konide or homate), and 

 similarity in mode of eruption. Of the volcanoes in the main 



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