Volcjino OsLima, Llzu. 127 



This term '' Fuji volcanic chain " has, however, been variously 

 defined by different authors. In its widest sense it includes all tho 

 volcanoes along the above-mentioned tectonic line, starting fi-om 

 the Myôkô^' volcanic group, passing through Yatsugatake^\ Fuji'^ 

 Ashitaka^', Hakone'\ and then through the volcanoes of the Idzu 

 peninsula and the insular and submarine volcanoes of the Pacific — 

 Shichitô'''^ Ogasawara^\ Sulphur Islands, etc, — to the Mariana Islands. 

 These volcanoes are arranged linearly in the same way as those be- 

 longing to one volcanic chain, but petrographically the nature of their 

 lavas is indefinite, being variously represented by many kinds of 

 rocks, and the forms of the volcanoes are multifarious'^-' ; according- 

 ly it is doubtful whether all of these volcanoes can be reasonably 

 grou}3ed in one volcanic chain. Putting aside for a while a general 

 discussion on the problem of the so-called " Fuji volcanic chain " as a 

 whole, the volcanic lineage of the Idzu islands will be first considered 

 so far as this directly bears on the geologic position of Oshima. 



The insular volcanoes of Idzu are to be divided into two groups 

 in their arrangement and morphographic features, as well as in the 



A ^ 



nature of the rocks which constitute them. Oshima, Toshima, 

 Udoneshima, Miyakejima, Mikurashiraa, and Hachijôjima arrange 

 in the direction N.N.W. — S.S.E. and are conical (homate or konide), 

 being built up of basaltic rocks ; Niijima, Shikineshima, and 

 Kozushima arrange in N.N.E. — S.S.W., to the west of the former 

 group and are flat with comparatively steep sides in their forms 

 (tholoids), being constituted of rhyolitic rocks. 



That the Idzu islands should be classed in two groups was 

 suggested by YAMASAKf^ as early as in 1896, for he stated that 



1) »M 2) A y it 3) fS± 4) '^K 5) ^fg 6) ±m 7) ,^«[^ 



8) B. Koto, "The Volcanoes of Japan," Jour. Gcol. So*: Tokyo, Vol. XXlir., p. 6, 1916. 



9) " Report on the Volcano Oshima." Jieport Etrthq. Invest. Com., N«». "J, 1H96 ^in Japanese). 



