300 S. Yoshmara — On the Bonin Islands. 



A special interest is connected with the Nummulites, which are 

 abundantly found in the tuff and agglomerate tuff of the Ogasawara 

 group. A single specimen of Nummulite was brought in 1899 

 to Mr. Wakimizu by a student of the Agricultural College, but 

 at that time the relation to the volcanic rocks of the group was 

 not yet known, nor the occurrence of Nummulites in Japan discussed 

 by Japanese geologists ; thus the subject was one of special 

 interest in my observations. The Nummulites are found, as far 

 as I know, only on the south-western side of Haha-jima, viz., 

 at Nankinyashiki, Cocoa-nut beach, Eosu-dani, Shizuka-dani, Ichino- 

 hashi, Sannohashi, and Nenbutsu-toge. The Foraminifera-bearing 

 tuff of these localities, except the last, belongs to the same horizon 

 and shows a westerly dip (Fig. 3). In some places the rock is 

 found at a few hundred feet above the sea-level, with a thickness 

 of from 30 to 40 feet. That of Nenbutsu-t5ge, only, is found at 

 a height of more than 600 feet, and its relation to that in other 

 localities has not yet been ascertained. 



Our Nummulites are of two distinctly different sizes. Millions of 

 the smaller form a rock closely cemented together with a tufaceous 

 material. Their diameter and thickness scarcely exceed 5'5 mm. 

 and 2"6 mm. respectively. They are identical with Nummulites 

 baguelensis of Verbeek ^ from the Eocene of Java. The larger 

 specimens are again separable into flatter and thicker forms. The 

 thicker ones are identical with Nummulites javanus, Verbeek,^ from 

 the Eocene of Java, and attain a thickness between those of var. 

 y3 and -y of Verbeek. The flatter ones, which belong to either 

 a variety of the other form or to a quite different species, are very 

 large; the largest collected by me has a diameter of 60 mm., and is 

 probably one of the most gigantic Nummulites in the world. On 

 the same horizon as that of the characteristic Foraminifera I found in 

 the tuff of Nishi-ura, near the above-mentioned localities, Schizaster, 

 Pecten, a lai'ge Nerita (somewhat allied to N. schmedeliana, Chem., 

 abundant in the Eocene Nummulitic formation of India), besides less 

 numerous forms as Voluta, Cardium, Tapes, Natica, Bentalium (?), 

 Vermetus, Trochus (?), another species of Nerita, two forms of 

 Ostrea, and many species of corals. I have collected, besides, 

 complete specimens of three species of Lithothamniiim from the same 

 place. In the tuff of Oki-mura, lying a little below the strata of 

 Nummulite-tuff, there are many forms of sharks' teeth, representing 

 at least three different species of Lamna, together with a few 

 specimens of N. baguelensis and N. javanus. It is very noticeable 

 that the Nummulite-tuff is evidently intercalated in the lava-sheets 

 or agglomerate tuff, and some large andesite blocks are sometimes 

 enclosed in the tuff as at Eosu-dani. In Haha-jima I could not 

 find any sign of difference of age to subdivide the rocks of volcanic 

 origin. Thus, all the andesites as well as tuffs of Haha-jima and 

 the other islands of the Ogasawara group must have been formed 

 in the Eocene epoch, 



^ Verbeek et Fennema : " Description geologique de Java at Madoura," t. ii (1896). 



