Volcano Oshirua, Idz'i. 119 



The main distinguishing characteristics of these types are : 



(1) they are comparatively rich in phenocrysts of mafic minerals ; 



(2) the olivine crystals, when present, are richer in forsterite 

 (Fe SiOJ,.^_,9 (Mg,SiO,)s;_sM than those in the commonest type of the 

 somma lavas. 



These lavas are considered to have been extruded from the 

 lower portion of the magma hasin where the heavy crystals 

 accumulated, remaining rich in the higher- melting components owing 

 to the small amount of licpid present. In some of them, phe- 

 nocrysts of each of the three minerals, olivine, augite, and hyper- 

 sthene, are present ; while in others t'.ey are found only singly or 

 in i>airs. This may be ascribed partly to there being an oppor- 

 tunity for the separate collection of these minerals, the durations 

 of their crystallization being remote. 



As to the occurrence of anorthite phenocrysts in the lava of 

 Chigasaki'^ (la), an explanation has already been given (p. 110). 



(2) Common Type of the Somma Lavas (basaltic bandaite). 

 ^This type is characterized by scarcity in phenocrysts of olivine 

 and pyroxenes. Sometimes these are even lacking. When the 

 olivine crystals are present, they are less rich in forsterite than those 

 in the rocks of the preceding types, about (Fe,SiO,),5_3o(Mg,Si04)-5-r^ 

 in composition. 



The lavas of this type do not differ from those of the preceding 

 types in the times of their extrusion. They are believed to have 

 come from the upper portion of the magma basin where the later 

 formed crystals comparatively rich in the lower-melting components 

 had sunk halfway. At the time of eruption, these crystals happened 

 to come out by chance with the lava flows. That the distribution 

 of the mafic phenocrysts is veiy irregular in these lavas and that 



1) f-yt^ or HyH^ 



