Volcano Ôsliima, Idzu. 125 



necessary also to explain tliat the interstitial matter of the latest 

 crystallization is strongly salic, for otherwise it must be far more 

 femic. 



In concluding the present chapter, the writer will remark on 

 the cliange in the composition of the magma which is expected to 

 be brought about by the cooling. All the rocks of Oshima represent 

 only one stage of the cooling of the magma, so that the diflference- 

 between the various rock -types does not correspond to the difference 

 in the course of cooling. The cooling history can only be inferred 

 from tlie observed features of the component minerals. The past 

 course of the magmatic change was summarized on pp. 116-118. 

 The salic interstitial matter in the micro-diorite suggests the future 

 state of the residual liquid after the cooling of the magma has pro- 

 ceeded sufficiently far. Summarizing, it is seen that the composi- 

 tion change of the local magma due to cooling would be from 

 basic to acidic. 



IV. The Geologic Position. 



Nearly along the meridian of 138^E., an important tectonic 

 line— the so-called " fossa magna " or " Fuji line "—traverses the 

 main island of Japan, or Honshu, dividing it into two tectonically 

 very distinct halves, North and South Japan. The line is prolonged 

 in the direction of S. 10°E., being represented by a submarine 

 ridge with a depth of less than 1,000 ?7Z. in the deep of the Pacific 

 Ocean. To the east of this ridge a trench runs parallel with it» 

 to w^hich it may be genetically related (Fig. 41). 



Oshima rests on the above-mentioned submarine ridge with the 

 other members of the Idzu Islands— Toshima, Udoneshima, Niijima,'^' 

 1) m^ mn^^ ^^ 



