Volcano Ôshima, Idzu. 61 



the world. Above all, the last two of these authors laid emphasis 

 on the assumed alkaline nature of this rock, considering it as 

 one of the examples of the sporadic occurrence of alkaline rocks in 

 the region of calci-alkaline rocks, a fact which has an important 

 bearing on the current discussion of the genetical relationship of 

 tliose two branches of igneous rocks. 



It is to be regretted that this erroneous report has led some 

 petrologists to such an important but incorrect conclusion as stated 

 above. ° The writer hopes that the following pétrographie decrip- 

 tions will serve to eUminate this erroneous conception and to add 

 to the meagre pétrographie knowledge of the Idzu islands. 



In applying names to our rocks, the definitions given by 

 IDDINGS in his -Igneous Rocksr Vol. II. have been followed, partly 

 because of the distinctness of his definitions but mainly because 

 his terms seem to be better fitted to our present case than any 

 others yet proposed. 



The rocks of Ôshima may perhaps be called " basalt " according 

 to the loose prevaihng nomenclature. In the typical basalt, how- 

 ever, the amount of silica is generally insufiicient to form the 

 higher silicates ■:' while in the main types of our rocks, the silica, 

 in spite of its low percentage, is more than enougli to form the 

 highest sihcates, leaving some of it still in free state, as indicated 

 l,y the value of /.• in Osann's formula (A->1) and by the presence 

 of normative quartz. 



What has been just stated is one of the cummon characteris- 

 tics of the recent volcanic rocks which are so widely spread through 

 the Japanese arc hipelago and ar e commonly called " pyro xene-an- 



~"l) The type-basalt, calculated by Eobinson {U.S. Geol Surv., Prof. Paper No. 7<;. p. 101, 

 10:3j yields 9.7% of olivine in the norm. 



