III.—Geological Age of the Ogasawara Group (Bonin Islands) as indicated by the occurrence of Nummulites

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In the geotectonics of the Japanese Islands, neo-volcanic eruptions in chains parallel to the direction of their elongation play an important part. Dr. Harada has counted three of them on the north, and four on the south of Kwantō, where the mountain-systems of northern and southern Japan are confluent. Besides, there are three chains across Japan. They are (1) the Kirishima chain running from N.W. to S.E., in the southern part of Kyushū; (2) the Chishima chain in Hokkaidō running from N.E. to S.W.; and (3) the Fuji chain forming the boundary of northern and southern Japan (Fig. 1). In a part of the Myokō volcanic group forming the northern extremity of the last important chain, there are the volcanoes of Onigajō, Myokō, Kurohime, and Iijuna, all of which have erupted, according to Mr. Yamazaki, through the Neogene Tertiary (probably Miocene). The southern continuation of this group is that of Yatsugatake, consisting of about nine volcanoes. Further on the south stands Mt. Fuji, formed in the Pliocene or Diluvium, like its southern neighbour Ashitaka-yama, which, according to Mr. Hirabayashi, more early reposed. In the Hakone and Atami volcanoes, also in the Fuji chain, eruption took place, according to him, first in the Pliocene epoch, but chiefly in the Diluvial.