8. Yoshiicara — On tJie Bon in Islands. 297 



Hitomaru-jima (Charles Little Is.), Hyotan-jima (Pihi Is.), Yagi- 

 jima (Goat Is.), and Kitano-sMma (Fig. 2). The southern part is 

 the sub-group of Haha-jima (Barley or Coffin gr.), with Haha-jima 

 (Hillsborough Is.), Ane-jima (Perry Is.), Imoto-jima (Kelly Is.), 

 Mei-jima, Hira-shima, Muko-jima (Plymouth Is.), Maru-shima, 

 Futago-jima, Matsunboshi, and others (Fig. 3). All the islets of 

 the Ogasawara group are arranged in a line running 27° 40' N. to 

 26° 38' N. Far to the south-west of Haha-jima there is another 

 group called Sulphur Is. (Volcano Islands), comprising Kita-iw5 

 (San Alessandro Is.), Iwo (Sulphur Is.), and Minami-iwo (San 

 Augustino Is.), (Fig. 1). All the islets of the Ogasawara group 

 show the same geological formation. The chief rock in the Chichi- 

 jima sub-group is agglomerate tuff alternating with audesite lava. 

 On the island of Chichi-jima, a very fine tuff is exposed on the west 

 of Omura and the north of Mikazuki-yama, where we rarely find 

 thin strata of shale. The volcanic rock is almost all augite-andesite. 

 That on the north of Futami harbour (Port Lloyd) in Chichi-jima 

 and on Ani-jima and OtSto-jima is black-coloured with resinous 

 lustre, sometimes changed to a green weathered mass, and is often 

 broken by fissures into large polyhedral fragments. Beautiful 

 crystals of heulandite, apophyllite, analcime, and chabasite are found 

 as druses only in this black andesite. The same rock is also found 

 on the southern coast of Chichi-jima and north of Hatsune-ura on 

 the east coast of the same islet. The andesite exposed on Haha- 

 jima and Kominato in Chichi-jima is blackish-grey to ligh_t-grey, 

 with gas pores filled with white silica. Specimens from Ogiura, 

 Kitafukurozawa, north-east of Tatsumi-ura, and the neighbourhood 

 of Taka-yama in Chichi-jima, belong entirely to vitro-andesite. 

 Mr. Suzuki^ has described serpentine from Kurose in OtSto-jima. 

 Numerous liparite dykes pass from the south-western corner of Ani- 

 jima to the north-eastern of Chichi-jima. The same rock is not 

 found elsewhere on the Ogasawara and Sulphur Is. groups. In the 

 Haha-jima sub-group we find, besides the predominant agglomerate 

 tuff, several varieties of andesite. Vitro-andesite is frequently met 

 with near Oki-mura and in the neighbourhood of Chibusa-yama in 

 Haha-jima. Andesite having large phenocrysts are principally 

 exposed on the west of a line drawn from Kita-mura to Big Bay in 

 Oki-mura. Andesites are also found in the isolated islets of Imoto- 

 jima and Mei-jima. Generally speaking, the Haha-jima sub-group 

 has the same structure as that of Chichi-jima. These iioo sub-groups 

 are typical submarine volcanoes, which sent out numerous streams of 

 lava by subsequent eruptions after frequent periods of repose. With 

 regard to the sub-group of Muko-jima, which I could not personally 

 visit, I have obtained a series of rock-specimens. The islands in 

 this group, situated in the same direction as the Chichi-jima and 

 Haha-jima sub-groups, judging by the specimens, must show the 

 same geological structure. 



1 Suzuki, "Petrography of the Bonin Islands" (in Japanese): Bull. Geol. Soc. 

 Japan, vol. i, No. 1 (1885). 



