Volcano Ôsbima, Idzn. gg 



and often forms a group of two or three individaals. The following 

 optical properties were determined for sodium light : — 



ö<1.571 1.572<,9< 1.575 1.578<r 7'-«>0.007 

 2V= S'2^ supposing ß — 1.574 Optical cluiraetei negative. 



The three principal refractive indices were determined with a 

 number of specimens from various localities. The optic axial angle, 

 2F, was measured with a cleavage piec^e of feldspar parallel to 

 M (010) taken from the lava exposed on the sea clifi'to the south 

 of Chigasaki'^ (la ; PL VI. AB 1). From the above optical prop- 

 -erties, the mineral was identified as calcic bytownite with its 

 chemical composition AbigAusB according to Becke."-* Three typas 

 of twinning of this mineral were observed : those according to 

 the Carlsbad, albite, and pericline laws. The former two are very 

 -common, while the last is only rarely met with. Albite- twinned 

 lamellae are generally thick and not numerous. Zonal structure 

 -due to a difference of chemical compositions is not noticeably 

 exhibited. The mineral is generally poor in inclusions, but it 

 sometimes contains uncrystallized or half-crystallized substance 

 which seems to be rock mass imprisoned before consolidation. 



OUcine, which is found in most lavas as phcnccrysts though 

 these are extremely small in number, is always subhedral to an- 

 hedral, occasionally with a resorption- border. Sometimes olivine 

 <îrystals are reduced to very small grains by magmatic resorption. 

 With the grains of this mineral separated from the chemically 

 analysed specimen, the following optical measurement was made : — 



«<1.6906<,9<1.7146<r j'-«>0-024 



1) ^"rm or ?l^m 



2) " Zur Physiograpliie der Gemengteile der krystalLnen Schiefer," Dtiiikschrlften der 

 kniserlîchen Alaidemie der Wtsscnschifkn, 75 Band, S. 103, 1913 (Wien). 



