シラス ノ ドシツ リキガクテキ トクセイ ト ソノ ゲンイン : シラス ボウサイ ノ ケンキュウ ダイイチブ ソノ4

Studies and researches have trod into the first half of the 3rd period, that is "the flowering time." In geological, volcanological, and stratigraphic fields researches and arguments concerning volcanic strata became active. R. Ota, S. Aramaki and other members' reports were brought forth. Their theories and opinions sometimes differ one another in various views. But in short, there existed two gigantic past volcanic calderas of Ata and Aira respectively at the mouth and at the blind head of Kagoshima Bay that were already uttered by T. Matsumoto and much pyroclastics flowed out of. The former is considered to be older than the latter. Besides, Ikeda caldera formed after and many other small past volcanos and pyroclastics were discovered one after another throughout the district. Discoverers called them after their own style. Confusion happend, the pumice flow of a researcher may possibly be that of a different discoverer and circumstances were nearly the same with their geological ages. Optical and chemical analyses of pumice flows gave light to the chaos. The refractive index of hypersthene in Ata flows is bigger while that of volcanic glass is inversive. Quantitative distinctions of chemical substances between the above two flows were also revealed. These methods solved and corrected a part of our mistakes in field observation, yet the sight is somewhat obscure, circumstances remain not simple. S. Aramaki, S. Taneda, and Oba et la were main persons in this branch. In 1966, owing to the heavy rain we suffered much damage of landslides in shirasu area. Nevertheless, papers and reports concerned are found few. A brief report on common slopes offers interesting data to us and, be it natural or artificial, most of them are so sharp as in 70°~85°. We have two types of opinion about slope stability, of physical and chemical cause ; the former explains the stability starts from granular interlocking and dilatancy of the material, the latter relates that it is due to chemical setting. The auther gives a short comment to the former. In engineering field T. Yamanouchi showed that shirasu-like sand easily liquefied under repeated loads in triaxial compression test. Judging from this fact, bearing capacity of alluvial shirasu ground was thought to be less than general sand ground. His indication will be of much actual value. The behavior of rain water and underground water in shirasu zone is not yet made clear. Concerning this subject, two papers are taken up in our original