RAISED CORAL REEFS OF THE RIÜKIU CURVE. H 



and conglomerate in tlie reef coral. At Osliinia, we already find the 

 greater part of the raised reef consisting of coral fragments, as well 

 as of a brownish sand like some Diluvial deposits common in Japan« 

 The fact seems to indicate an incomplete development of the coral 

 formation on these islands and we may fix the limit of the coral reefs 

 of the western T\acific at about 29° X.L. 



When the recent corals are f(3und close to an older reef, their 

 boundary is not sharply defined; however they may be distinguished 

 from each other by position, structure and colour. The older reefs are 

 generally highly elevated while the recent ones are exposed only at 

 low tide. The corals in the raised reefs have mostly lost their struc- 

 ture througli the dissolving action <3f the water. I could only 

 recognize among them the three species of Favia, Pontes and Fimgia ( ?). 

 The older corals are also weathered and show a yellowish brown 

 colour, and have sand-grains filling up the interstices of the skeleton. 

 Finally the older reefs, on the exposed surface, are altered to a mass 

 resembling terra rosa, frequently witli a thickness of several feet. The 

 recent corals show a well-preserved structure, and generally appear 

 whitish in colour, only those on the surfiice being grayish. 



All the raised coral reefs are homogenous in appearance through- 

 out their whole thickness, there being apparently no mark of varying 

 age in their different parts. The terraces in those reefs, as observed 

 at Yonaguni, Ishigaki, Miyako, Irabu, Okinawa, Sesoko, le, Miyagu- 

 suku, Okinoerabu, Kikai, and other islands, may have been formed in 

 the intervals between successive changes of sea-level, or the develop- 

 ment of the coral formation from its beginning to its end may have 

 been not equally extensive. 



The elevations of the principal raised reefs are as follows: 

 Minna-jima 10 — 20 ft. 



