J 84 Vol. XLIII, Art. 6. — S. Tsuboi : 



to have changed along the Une from acidic to basic as time passed. 

 Tims, according to IsHnvAEA'\ the main I^ody of Amagi'\ a volcano 

 of the early Diluvium, consists of ratlier acidic " pyroxene-andesite " 

 with Gl% of SiO., while its parasitic cones which were formed in 

 the late Diluvium are constituted of " basalt " with 56% of SiO,.'^ 

 HiEABAYASHi*^ also coucluded that the lavas became more and more 

 basic, not only locally but also regionally : from " pyroxene-andesite " 

 to " olivine-pyroxene-andesite " in the volcanoes Hakone, Atami, and 

 Ashitaka"\ and from "olivine-pyroxene-andesite" to " plagioclaso- 

 basalt" with 00% SiO, in the volcano Fuji'^ 



On examining the rocks of the andesitic lineage in the 

 ixniinsula and its environs, it is seen that their pétrographie 

 characters approach more and more to those of the basaltic rocks 

 of the Idzu islands in the course of time, while the lavas of Fuji, 



A 



Omuroyama'\ etc. bear a close resemblance to those of the basaltic 

 hisular volcanoes. It may be reasonably concluded, therefore, that 

 the basaltic Idzu islands belong to the same volcanic lineage as 

 the above-mentioned volcanoes in the main land, and that their 

 lavas descended from the Tertiary andesitic rocks of the southern 

 part of the Idzu peninsula. 



On the other hand, the rliyolitic rocks are not known to have 

 erupted in the Idzu peninsula in tlie post-Tertiary period. But the 

 rocks of the rhyolitic Idzu islands are of the same pétrographie 

 lineage as those of the rhyolitic bodies in the main land, as already 

 suggested by some previous writers ; and their extrusion took place, 

 in all probability, even in historic times. It appears then that 



1) " Volcanoes in the Idzu Peninsula," Jlfport Earthq. Invest. Com., No. 17, 1808 (in Japanese). 



3) B. Koto, '•' Studies on Some Japanese Eocks," Quarledy Journal of Geological Soclelg, 

 Vol. XL., p. 451, 1884. 



4) "Volcanoes Fuji and Asbitaka," Beporl Earth';. lave J. Com. No. 21, 1899. (-Tapanese). 



5) mm BM S-1 G) ^± 7) p«c^.|Il 



