296 8. Yoshiivam — On the Bonin Islands. 



III. — Geological Age of the Ogasawara Group (Bonin Islands) 



AS INDICATED BY THE OCCUERENCB OF NUMMULITES. 

 By S. YosHiwABA, Science College, Imperial University, Tokyo, Japan. 



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 Kwant5, 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 Kyushu ; 

 (2) the Chishima chain in HokkaidS 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 Myoko volcanic group 

 forming the northern extremity of the last important chain, there 

 are the volcanoes of Onigajo, Myok5, Kurohime, and lijuna, 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. The Izu peninsula, a part of this chain, described 

 geologically by Mr. Ishihara, has several volcanoes — Necogoe, 

 Jaishi, Amagi, Daruma, Sukumo (?), and Ajiri (?). On the southern 

 part of the Fuji chain lying in the sea, there are found the small 

 islands collectively called Izu-shichito. Tracing further southwards, 

 we have Aoga-shima,- Bayonnaise Eocks, Smith's Island, Tori-shima 

 (Ponafidin Is.), Lot's Wife Eock, the Ogasawara group (Bonin 

 Islands), the Sulphur Is. group (Volcano Islands), and the islands 

 of the Mariana or Ladrone group, all of which, being of neo-volcanic 

 origin, had been until recently regarded as a continuation of the Fuji 

 chain (Fig. 1).^ However, in order to study their true geological 

 relations, I spent about one month (July to August, 1901) in 

 visiting all the islands of the Ogasawara group, except the Muko- 

 jima sub-group, and I came to quite a difierent conclusion. 



The Ogasawara group is divisible into three sub-groups. The 

 northern part is the sub-group of Muko-jima (Parry gr.), containing 

 Kater Island together with Nakodo-jima, Yome-jima, Kitano- 

 shima, Harino-iwa, etc. The middle one is that of Chichi-jima, 

 comprising Chichi-jima (Peel Is.), Ani-jima (Buckland or Hog 

 Is.), Ot5to-jima (Stapleton or North Is.), Higashi-jima (Sandy 

 Beach Is.), Nishi-jima (Little Goat Is.), Minami-jima (Long Is.), 



^ Harada: " Die Japauischen Inseln," 1890. 



^ Shima (pronounced Jima in combiaation) means island or islands. 



' Harada: ''Die Japanischen Inseln," 1890. Kikuchi, ''On Pyroxenite Com- 

 pounds in certain Volcanic Eocks from Bonin Island " : Journ. CoU. Sci. Imp. Univ., 

 vol. iii (1890). 



