'QO vol. Xl.irf.. Art. n. _S. Tsiboi: 



IV. Petrography. 



Oshima is built up wholly of volcanic rocks. A brief pétro- 

 graphie description of these was given for the first time by Nau- 

 mann'^ in 1877, then by Yamasaki'^ in 1896, and somewhat more 

 in detail by Ohash^^ in 1910. Specially of the lavas erupted in 

 1912, brief notes were published by Okamuea'^ who visited the 

 scene of the volcanic activity and collected some specimens. 



These descriptions, however, are not sufficiently detailed while 

 some are not quite correct ; moreover, there is still a wide field 

 open for further study. 



Of the previous writers, Nau:\iann spoke of sanidine as one 

 of the essential component minerals, citing in his pétrographie 

 description the result of the chemical analysis made by Korschelt 

 which shews a very high percentage of alkalies (K-0 6.28^, 

 NaaO 2.02%"). This would be quite incomprehensible to any one 

 who examined for himself the rocks of Oshima, which are, in 

 reality, characterized by specially high lime and extremely low 

 -alkalies. Nevertheless, this erroneous result iias been reproduced 

 in many later papers by Milne'*^ Wada''\ Nishiyama'\ BACHER''^ and 

 Stark' \ and has become widely known among the petrologists of 



1) "Die Vnlkaninsel Ooshima und ihre jüngste Eruption," Zeitschriß der Deutschen fjeoloij- 

 Âschen Gesellschaft, Bd. XXIX.. 1877. 



2) "Report oii the Volcano Oshima." liepori Eirthq. Invest., Com., No. 9, 1896. (in Japanese), 



3) " On the Geology of Volcano Oshima," Jo^ir. Geol. Sor. Tukijô, Vol. XVII., 1910 (in Japanese). 



4) "Report on the Eruption of Volcano Mihara," liejort Geol. Surv. Japiv, No. 48, 1914 

 •{in Japanese). 



5) "Volcanoes of Jap:in,"' Transactions of tlie Seismological Socie'y cf J^ijia^a, Vol. IX.. l'art 

 IL, 1886; Geological 3Iagazlne, Decade IL, Vol. L, No. 5, 1887. 



6) " Notes on the Volcano Oshima," Gaku(jei Shirin, Vol. I., No. 1, 1877 (in Jajiauese). 



7) " Explanatory Text to the Geologic Sheet of Idzu," 18S6 (in Japanese). 



8) " (""ber die Laven der kleineren Idzu Inseln," München, 1914. 



9) " Petrographische Provinzen," Fortschritte der Mlneralo'jie, Kristallofjraphie u/id I'etro- 

 ■graphie, Bd. IV., 1914. 



