In order to study the origin of amphibole in alkali basalts, the writer has statistically treated the mineral compositions of basaltic rccks from northern Kyushu, belonging to the “Cenozoic alkali suite of Circum-Japan Sea Region (TOMITA 1935)”. It is always the case that the amphibole-bearing rock is accompanied by the amphibole-free basalt, and the latter is intruded or overlaid by the former. Hence the basalts are classified into the olivine trachybasalt～trachyandesite without amphibole phenocryst (III_b and IV_b types as defined by H. KUNO), and the amphibole-bearing olivine trachybasalt～trachyandesite (VIII_b and IX_b types). Significant differences are noticed between these two types with respect to modal olivine as modal magnetite. Petrochemistry of these two types shows no significant differences between the two types with respect to their ratios. FeO + Fe₂O₃/MgO + FeO + Fe₂O₃, or Fe₂O₃ + TiO₂/FeO + Fe₂O₃ + TiO₂, which are regarded as the indicator of the content of magnetite (+ilmenite). Within the amphibole-bearing rocks, there is a negative correlation between the contents of amphibole and magnetite + ilmenite. From these considerations, the writer is lead to the conclusion that the amphibole is formed by the crystallization of magma during which the iron and titanium components concentrate into the ampoiboles, and consequently the separation of ore minerals is restricted.

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Rhyolitic pumice-fall deposits of Usu volcano, Hokkaido, erupted as the first ejecta during the period of activity characterized by dacitic lava dome extrusions, including O-Usu and Showa-Shinzan, was studied. Prior to the activity, the volcano was dormant for about 1500 years after the formation of a stratovolcano of tholeiitic basalt (Table 1). The pumice-fall (Ub) is distributed in a narrow area extending eastward more than 600km (Fig. 1). Total volume attains 2.2km³ (6.5 × 10⁸ ton), exceeding the 1707 activity of Fuji volcano and the 1783 activity of Asama volcano in magnitude. Sorting of Ub is very good and constant (So = 1.8±), irrespective of the distance from the crater. Tephrochronologic and archeologic investigation revealed the date of the Ub ejection to be older than the oldest record in 1661 and younger than the pottery remains dated as 15th～16th century. The uplift of Ko-Usu lava-dome took place just after the Ub ejection during the same cycle of activity. The pumice-fall deposits occupies one-fourth of the total erupted material of Usu volcano in volume. The sialic material of the volcano, Ub and dome lavas, attains three-fifth of the total material, the rest being basaltic somma lavas (Table 3). Crystallization differentiation of basaltic magma may interprete the wide change of composition of erupted material from basic to acidic (Table 2), however, it cannot fully explain the volume relationship. It is considered that the concentration of vapor (with silica and alkali) at the upper part of the volcanic vent filled up with dacitic magma had given rise to the intense pumice explosion.

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The nature of the volcanic micro-tremor of the second kind and its source condition were investigated. The characteristics of the volcanic micro-tremors at Volcano Aso are closely related to the activity of the volcano. According to K. Sassa, there are four kinds of volcanic micro-tremors when classified from their periods, and these tremors have different characteristics. The volcanic micro-tremor of the “2nd kind” is a long period of 3.5 to 7.0 sec. The period of this tremor is maintained for the duration of unchanged activity of the volcano. The mode of this tremor corresponds to the compressional one and was observed in the form of several groups of regular wave trains during inactive periods, while during active periods it became a continuous train of somewhat irregular waves. The wave forms of this tremor on the seismograms depend on, in general, the characteristics of the active origin of the volcano, the mode in which waves have propagated and observed instruments. The spectrum of waves having a long period and compressional wave forms such as the 2nd kind may be taken to indicate information concerning the nondistorted source conditions. For this reason, an attempt was made to determine the source conditions of the 2nd kind of volcanic micro-tremor from its frequency spectrum.

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