| Abstract |
The average chemical compositions of the volcanic rocks of Indonesia were calculated (Table 2) and compared with those of Japanese volcanic rocks (Ref. Fig. 1). The alkali-lime index for Indonesian vo...lcanic rocks is 60.0 (CaO=Na_3O+K_2O=6.2), though it is 63. 7 for Japanese ones (Taneda, 1952). There is a tendency that alkali-lime index decreases inwards frorri the outer zone of the Indonesia Islands in arc arrangement. Acccrding to the relation between Al_2O_3， Alkaliesand SiO_2 (Kuno, 1960), all or most of the basaltic rocks of Semeru, Kelud, Krakatau Merapi belong to the "high-alu mina basalt", while those of Q.3ngaran and Merapi (partly) to the "alkali basalt". The difference between the lavas of historic times (Table 3) and those of prehistoric times was also noticed (conf. Tables 2 and 3, Figs. 2, 3, 4, 7, 8, and 9). On some volcanoes was considered the mode of lava sequence along with the history of eruption. In addition chemical analyses were made on two specimens collected by the present author from Merapi and, Kelud (Table 1).show more
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