Yolcano Ôshima, Iclzu. X41 



Tertiary sedimentary rocks, while a shallow floor under 300 

 fathoms (550 m.) connects the island to the Idzu peninsula which 

 owes its existence to volcanic action. The materials which con- 

 stitute the sea bottom near Ôshima are, according to Yai\iasaki^\ 

 fragments of volcanic glass with abundant microhtes, pumices, and 

 fine pieces of pyroxenes, and feldspars, mingled with the remains 

 of Foraminifera, Radiolaria, etc., appearing in the main as green 

 mud. 



VII. General Summary. 



(1) Both the position now occupied by the Idzu peninsula 

 and its environs have Ijeen the scene of volcanic agitation since 

 the Tertiary period. Here the vulcanism of the two different pétro- 

 graphie lineages — andesitic and rhyolitic — have displayed their 

 activity, and built up the Idzu peninsula. 



Lavas of these two lineages continued to be extruded down 

 to the Quaternary period. The andesitic or basaltic lavas built up 

 the volcanoes Fuji, Ashitaka, Hakone, Atami, Amagr'\ etc. on the 

 main land and many insular and submarine ones in the Pacific 

 Ocean, along the line that runs in the direction N.N.W. — S.S.E., 

 conforming to the conspicuous " Fuji volcanic chain." The lavas 

 descending from the Tertiary rhyolitic rocks formed the rhyolitic 

 islands off the Idzu peninsula to the west of this volcanic chain. 

 Contrary to the general opinion the writer agrees with T,srii:MURA. 

 that these rhyolitic islands are as young as the volcanoes of the 

 Fuji volcanic chain. 



What is commonly called " Idzu Shichitô " '' comprises the 



1) " Geology of the Bottom of the Pacific Ocean between the Tokyo Bay and Ogasawara," 

 Jour. Gecl. Soc. Tokyo, Vol. XV., pp. 382-383, 1908 (in Japanese). 



2) ^± ^M IWm ^M %m 3) Ps.±^ 



