GEOLOGIC S'l'RUCTURE OF THE RIUKIU CURVE. 



21 



As shown in the abstract of my field notes, all the rocks dip 

 steeply westward and are never horizontal. The strike is XE to 8W 

 which is nearly parallel to the line connecting the island of Oshima 

 with Tokuno-shima and Yoron-jima. The whole interior of the first 

 island is mountainous ; there is a plateau only on the northern end, 

 which has been denuded and partially covered with raised coral reefs. 

 A long narrow inlet with deep water, lying on the south of Edato- 

 jima, is probably due to the formation of a fissure, perpendicular to 

 the axis of the Oshima group. There are greatly contorted strata 

 as well as porphyrite eruptions on both sides of the inlet. The 

 narrow and deep channel between Oshima and Kakeroma-jima, 

 probably of a simihu* origin, extends in the same direction as this 

 inlet, and there is another porphyrite dyke on the northern coast of 

 the western part, lîesides the above mentioned three parallel dykes, 

 exposed at Lken, Kuji, Ikegachi, Ashiken, lîuren, Xagara, Sanen, 

 Kudadon and Keten, there are three other dykes of the same 

 porphyrite running generally NE — SW. They are found in Kake- 

 roma-iima and on the road from Xaijara to Keten in Oshima. That 

 exposed on the south of Naon is properly hornblende-porphyrite. Xo 

 volcanic rocks are found in the islands ; pre-tertiary eruptives are 

 exposed chiefly near the eastern coast. lîiotite-granite is found to 

 have been erupted through the Palœozoic rocks near Yamma, as at 

 Iclii, Todama and other villages. Diorite with a little granite is found 

 near Tekebu on the northern corner of Oshima. 



Tohuno-shima. This island is elonofated from north to south. 



