The history of activity of Myoko, an extinct double volcano in central Honshu, Japan, may be divided into the following three stages: 1a Formation of a stratovolcano. 1b Formation of an explosion caldera accompanied by descent of a voluminous mud flow. - erosion interval, enlargement of the caldera - 2 Eruption of Tsubame andesite lava and pyroclastics, and formation of a flat cone within the caldera. - erosion interval - 3 Eruption of central dome lava and pyroclastic flows. The last stage of activity was caused by a hypersthene-hornblende andesite magma. As in the case of Mt. Pelee, the protrusion of the central dome lava was accompanied by the eruption of pyroclastic flows, and the activity seems to have continued for months or years. All pyroclastic flows, except one, which flowed down southward over the caldera rim, advanced eastward through the canyon of the Otagiri river, that cuts the eastern caldera wall, and spread over the eastern foot. The total volume of the pyroclastic flow deposits, except the southern one, whose distribution is not certain is estimated to be 0.6～0.7km³. The pyroclastic flow deposits consist of ashy matrix and angular compact blocks variable in size. Accidental blocks are rare in the deposits. Most of the blocks, matrix material and the dome lava are petrographically similar to one another. Scattered orientations of magnetic dipoles measured from these blocks suggest temperatures of the blocks at the time of deposition were lower than the Curie points of ferromagnetic minerals in the blocks.

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Volcanic earthquakes, volcanic smokes, state of crater, and other volcanic phenomena have been continuously observed since 1923 at Oiwake Volcano Observatory, located 7.6km SSE of the crater and at Karuizawa Weather Station, located 9.1km SE of the same crater. The volcanic earthquakes treated in this report were mainly recorded by seismograph of low magnification since 1923 at Oiwake Volcano Observatory. The volcanic smokes were observed since 1939 from Karuizawa Weather Station. In recent seismometrical investigation of Volcano Asama, the telerecording seismograph is used. Transducer, connected to the recorder at Oiwake Observatory with wire, is set at Chinotaki, situated at 3.75km SSE of the crater. This seismograph is adjusted to 6000 in magnification. The author treated the variation of probability of eruptions, the monthly numbers of earthquakes and tremors originating from the volcano, the probability of occurrence of enormous quantity of smoke and the explosion energies. It is pointed out that the Γ-distribution of the factors participating in the eruptions are often of normal distribution. The correlation between the eruptions and the monthly number of the earthquakes, and that between the eruptions and the quantity of the smoke are studied. It became clear that in the most case an increase of volcanic micro-earthquakes and a remarkable increase of smoke indicated a high probability of great eruption. However, during the active periods, the relation between the eruptions and the number of micro-earthquakes were not so marked.

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By using the seismometrical method, the volcanic micro-tremors of Sakura-jima were observed to investigate their nature and the positions of their epicentres on August 1960. During the observations the summit crater of the volcano kept the ordinary fumarolic state without any explosions. The results of the observations and their analyses are as follows: 1) The later phases of volcanic earthquakes of shallow foci (B type earthquake, after Minakami et al.) and the volcanic pulsations of continuous trains (or micro-tremors) have the same properties in respect to the propagation direction and the mode of oscillation. 2) The micro-tremors oscillate in the directions of their propagation in the horizontal plane and their wave velocities are in the range of 0.4～0.8km/sec. As their oscillations in the vertical plane are not evident in this study, it is not clear whether they propagate in a mode of the surface waves or propagate like the body waves. 3) By using the tripartite station method, the propagation directions of the micro-tremors are observed at the five stations surrounding the summit crater. From these results an epicentre of the micro-tremors is determined at the region near Kita-dake and three epicentres are also determined on the line of vents, as shown in Fig. 9. But the relation between the epicentres in each position is not evident in this study. 4) The periods corresponding to the peaks of the amplitude-period relations of both the micro-tremors and the later phases of deep focused volcanic earthquakes (A type earthquake, after Minakami et al.) range from 0.25 to 0.38sec. On the other hand, each one of the frequency-period relations of the micro-tremors has two peaks at the respective periods ranging 0.19～0.24sec. and 0.28～0.34sec. and the same relation of the later phases of the A type earthquakes has only a peak, whose period is about 0.3sec. If the later phases of the A type earthquakes arise from an effect of the layer system of the observation stations, the shorter period corresponding to one of the peaks of the frequency-period relation of the micro-tremors (0.19～0.24sec.) will represent the period of oscillation at the origin of the micro-tremors.

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It bears an important meaning in considering the evolution of the earth crust and the development of the ocean to clarify the characteristics of isotopic composition of the hydrogen in the earth crust or in magma, and compare them with that of fresh and oceanic water. The isotopic composition of the hydrogen exhaled onto the surface from the depth through volcanic hot springs is considered to show the original composition in the magma, in more un-contaminated form than the case of fresh or oceanic water. In this paper, the result of analyses of the isotopic composition of the hydrogen exhaled by volcanic hydrothermal activity at Noboribetsu, one of the most typical volcanic hot springs in Japan, is described. It is found that the protium in the hot spring shows higher ratio than in fresh water, and that the isotopic composition of the hydrogen varies with the variation of hydrothermal activity. It is considered that this variation in isotopic composition of the hydrogen is closely related to boiling phenomenon of the hydrothermal water.

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