The basement rocks are of Paleogene as well as Neogene beds which are found in the southern half of Shimabara peninsula. Volcanic activity has begun in Pliocene epoch in the form of mesa or aspite of basalt and pyroxene andesite, both of which are seen only in the southern part of the peninsula. The main activities were played from Pleistocene to recent, having formed many tholoid type volcanoes - Volcano Unzen - in the northern part of the peninsula. Volcano Unzen is entirely made of hornblende andesite, which belongs to the hypersthenic rock series, and often includes many hornblende dolerite xenolith. The chemical analyses show that the lavas are in general rich in MgO, Na₂O and K₂O and poor in CaO and total (FeO + Fe₂O₃). The diagrams MgO-(FeO+Fe₂O₃)-(Na₂O+K₂O) and norm An-Ab-Or are discussed. The hornblende lavas from Volcano Unzen were probably influenced by the assimilation of the acidic hornblende magma and the basic lava.

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Volcanic products deposited during and after the caldera formation (about 400 A.D.) in the Oshima Island are called the younger Oshima group which is divided into twelve unit members, each corresponding to a major cycle of eruption (Fig. 1). Thirty-seven samples of lavas, bombs and scoria were collected (Fig. 2) from nine horizons of the younger Oshima group, and their chemical analyses are given in Table 1～6. Although the whole samples are, in a broad sense, similar in chemical composition and almost are nearly aphyric basalt, but several differences in chemical composition are recognizable. Chemical homogeneity of the lavas in the same horizon has been verified in three horizons (Table 1, 3 and 5). There is fairly regular change in composition of the five groups of volcanic rocks successively extruded during the last seven hundred years (Fig. 10). Linear relations between two of the components, SiO₂, TiO₂, Fe₂O₃, MnO, MgO, CaO and K₂O + Na₂O are shown in variation diagrams Figs. 11～14, in spite of the small range of their variation. Some of the relations are held good not only within the younger Oshima group but, the same trends expand out to the somma and the basement volcanics, the latter probably of Pliocene (Fig. 15). Fe/Mg ratio, which was proposed by KATSURA as a good measure of magmatic differentiation in his previous paper, increases gradually from the base to the top of the younger Oshima group (Table 7).

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The bulk density of pumice was measured on about 150 samples collected from the pumice flow deposit of Towada caldera. It varied between 0.211 and 0.777, the average being 0.51, and the porosity varied between 67 and 90%. In spite of these wide differences in the bulk density, mineralogical as well as chemical compositions of these samples are essentially similar, and the true density of homogeneous glasses obtained by melting these samples is also similar. Therefore, the difference in the bulk density of pumice is probably due to the difference in physical conditions, such as temperature and relief of hydrostatic pressure. It is apparent that peculiar mode of grain size distribution of pumice flow deposit of Towada caldera is not genetically related to the difference in bulk density. The results of thermal experiment carried out on these pumice samples at temperatures between 700℃ and 1370℃, are compared with the natural welded tuffs. The obsidian patches in the latter can be correlated to the result obtained at 1200℃, but this gives only the upper limit of formation of welded tuffs, because the experiments do not take into account the hydrostatic pressure or volatile components, both of which play important roles in the formation of welded tuffs. As the bulk density of pumice flows is generally estimated to be less than 1.0, they are not accumulated firmly on the bottom of the ocean, if they are erupted by submarine volcanic activity. Therefore, the welded tuffs may be taken as the feature of terrestrial formation.

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Volcanic rocks distributed in the western part of the San-in region belong to the so-called “Cenozoic alkali suite of Circum-Japan Sea region (TOMITA)”. Their distributions can be divided into three districts: Otsu-gun, Abu-gun and Mi-shima in Yamaguchi prefecture. The followings have been clarified by chemical analyses of 52 specimens and microscopic observations of many specimens of these volcanic rocks. In the Otsu District, volcanic activities of the tholeiitic rock series took place, followed by that of the alkali rock series. As a part of the latter series, the series of iron rich type has been found. In the Abu District, volcanic activities of the alkali rock series took place, followed by the activities of andesites of the calc-alkali rock series, which are formed by successive assimilation of the acidic rocks of the basement with alkali-olivine basalt magma. Large number of xenolithes of gneissose rocks have been found in a part of these lavas. Quartz xenocrysts are found in both of the rock series. In the Mi-shima District, activities of the tholeiitic rock series took place, followed by that of the alkali rock series. The latter rock series are rich in olivine, and picritic basalt is sometimes found. Most of these Cenozoic volcanic rocks erupted out after the Pleistocene Age, but some (Kasa-yama) in the Recent Age. The tholeiitic rock series in the Otsu District show higher Na + K than that in the Izu and Hakone Districts. Almost all of the alkali rock series analysed in this study are plotted in the field of less Fe than of the Circum-Japan Sea province, on the triangular diagram of Fe-Na+K-Mg. Especially, the alkali rock series in the Mi-shima District show high ratio of Mg:Fe. The calc-alkali rock series in the Abu District have similar chemical compositions as of the Chokai volcanic zone (KATSUI) and the Daisen volcanic zone (KURASAWA and TAKAHASHI). The results on the minor elements distributions in these rocks will be reported elsewhere.

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