130 ^ol. XLIIL, Art. G. — S. Tsuboi : 



Thus, the almost settled opinion as to the the age relation 

 l^etween the rhyolitic and the basaltic Idzu islands has becomo 

 doubtful, and an interesting problem, which is of first importance 

 in the geology of the Idzu islands, has presented itself for study. 



It is true that the basaltic islands afford many records of 

 recent eruptions, among which those of Oshima in 187 6-' 7 7 and 

 in 19 12-' 14, and of Miyakejima in 1874 are still fresh in our 

 memory ; while the rhyolitic islands have not erupted, at least in 

 very recent times. But it is problematical whether the rhyolitic 

 islands are really so very old as has been hitherto generally 

 supposed and whether their activity had been brought to an end 

 before the basaltic ones appeared. 



In the summer of 1918, the writer visited Niijima, one of the 

 rhyolitic islands, with the object of solving the above question if 

 possible. The following is only a very brief statement on this 

 particular problem. 



The rhyolitic rocks of Niijima arc readily grouped in three 

 main types — pyroxene-rhyolites'\ hornbleiido-rhyolites, and biotite- 

 rhyolites, in order of extrusion. Besides these there is in the north 

 of the island a basaltic éjecta bed which caps the masses of 

 pyroxene- and hornblende- rhy elites. It is this fact which led 

 FuKUCHi to the conclusion that the rhyolitic islands are older than 

 the basaltic ones. This fact, however, only indicates that the 



government with a map of a newly created island, and so Nakamuka considered that the record 

 of the eruption above cited probably related to this new island. Unfortunately the name of the 

 island is not stated, but from the descriptions " jsipag ^'MXMTf^ I'lOI!^'^-»" Omoki and Nak^vmuea 

 are inclined to consider this as meaning an eruption in a rhyolitic island, probably in Niijim,«, 

 for " ®/g- ", or the "silver mountain", may mean white rhyolitic mountain. 



1) Here the rocks are called under the field names. The rocks oE Niijima were deseribeil 

 by FuKTJCHi ("Geology of Niijima," Report Eirlhq. Lweat. Cjm., N>. 39, 1902) as " rbyjlites " ;, 

 while Bacheb ("Über die Laven der kleineren Idzu Inseln," München, lOlt) pointed out that 

 they are "dacites" anl not " rhyolitei ", as they contain no sanidine. The r.jcks are very low 

 in crystalhnity so that to what rî<;k species they babng caanot bj determined by microscopia 

 investigation only. 



