Yolcano Oshirra, Idzu. 75- 



quantity of phenocrysts of both hypersthene and augite besides, 

 those of plagioclase, and by the entire absence of ohvine. 



3fo(le of Occurrence. — The exposure of the lavas of this type 

 is seen only on tlie cliff at Gyôja'\ on the east coast, in three 

 layers, each about 20 in. in thickness, intercalated by layers of 

 éjecta (PL V. 3c III', PL VI. FG 0). The geologic relation be- 

 tween lavas of this type and those of the first type could not be 

 observed owing to tlie accumulation of sands and lapilli of tho 

 central cone blown down by the wind from the caldera through the 

 northeastern gap of the ring- wall of the somma. 



llegascojnc Characters. — The rock is megascopically fine- 

 grained, non-vesicular and is gray, sometimes with a purplish tone 

 due to the partial hematitization of magnetite grains in its ground- 

 mass. It is megascopically dopatic, the phenocrysts being of pla- 

 gioclase, usually less than 2 mm. in diameter, and of dark greenish 

 pyroxenes, smaller and less abundant than the former but not 

 negligible. 



31icroscopic Cliaracters. — Microscopically, the rock is do- 

 crystalline in crystaUinity and seriate porphyritic in fabric. The 

 most conspicuous phenocrysts are of calcic hytomnite near Ab,.An^sv 

 as identified from its optical properties, commonly exhibiting the 

 Carlsbad and albite twins but rarely the pericline, and with rather 

 scanty inclusions of angite microlites, black glass, and fine dust, 

 arranged irregularly or zonally. Sometimes the mineral is coated 

 by a thin layer of less calcic plagioclase (labradorite) with a 

 distinct boundary between them. The core part has the following 

 optical properties : — 



a< 1.571 1.574<j^< 1.577 1.578<r 



2 V = 84° supposiug ,9 = 1.576 Optical character negative. 



1) îf% 



