242 "^'oï- XLII[., Art. G. — P. Tsuboi : 



Qaaternary insular volcanoes of both of the above-mentioned 

 lineages, and is to be subdivided into the Oshiraa group and the 

 Niijima group, consisting respectively of basaltic and rhyolitio 

 islands. 



The members of the former group are Oshima, Toshima, 

 Udoneshima, Miyakejima, Mikurashima, and Hachijojima/^ The 

 rocks of these volcanic islands show a very strong resemblance to 

 one another and appear to be closely comagmatic. By correlating 

 these rocks and those in tlie related region in the main land, 

 where the general magmatic change is from acidic ~ to basic, 



o o o 



it appears that the birth of the basaltic insular volcanoes was at 

 a late date, probably late Diluvium, by which time the magma 

 had become strongly basic. 



(2) The insular volcano Oshima consists mainly of double 

 homates — a somma and a central cone — , and is built up of 

 alternating lava-flows, both superfluent and effluent, and éjecta 

 beds. 



The somma lias eight parasitic knobs and two phreatic ex- 

 plosion-craters on its flanks, and is truncated at the top with a 

 ring- wall that surrounds a huge oval caldera, about 3 km. in 

 diameter. The wall is not completely closed but has two gaps, a 

 great one on the northeastern and a small one on the south- 

 western sides. 



The central homate, Miharayama,"^ standing in the caldera, has 

 mi active summit crater, 700 m. across. Its volcanic products 

 cover the ground not only within the inside of the encircling wall 

 but have also spread down to the sea shores through the gaps in 

 the wall. 



Besides the above, there are along the western half of the 



1) :k% m% fêïsm^ H-^^ wBM A5t^ 2) zijr,m 



