YolciUio üshima, Idza. 83 



They are more noticeably developed in more hyaline iacies and 



make gradual transition to the plagioclase of common type. Most 



•of them measure from 0.03-0.12 mm. along the longer diagonals 



P^,^^^ r^ç>^ of tlie rhombs. Well defined rhombs 



To^iÇ^M^ <Ï^M^> have angles measuring about 52^, 116°, 



Fig. 37. and 128^, and thus seem to be bounded 



by the faces 31(010), P(OOl), .« (10Ï), and r(lfO) or I (110), as 



shown in Fig. 37, i. e. the lamellae are very thin tabular parallel 



to M(010):^ 



Hypersthene and cUno-hypersthene. — Hypersthene occurs in 

 subordinate amounts as phenocrysts of prismatic form measuring 

 0.2-1 mm. along the longest direction, being nearly euhedral to 

 anhedral. Prismatic cleavage is sometimes noticeably exhibited 

 and transverse cracks are often seen. Axial colours observed in 

 tliin section are : — 



1 1 c . . . . gi-eenisli _L c . . . . browuish 

 Conoscopic examination shows that the tw^o f(3rms of hypersthene 

 are present — the normal one and that of the clino-form, with the 

 trace of the optical plane parallel and transverse to the c-axis respect- 

 ively. In properties other than the optical orientation the two 

 behave quite alike and^ have no other characteristics to distinguish 



1) A similar occurrence of plagioclase is said to have been observed in the basaltic glass 

 of Ogasawarajima (Benin Islands), on which an account was given by the late Dr. Y. KiKUCHr 

 (" On Pyroxenic Components in Certain Volcanic Rocks from Bonin Islands," Jour. Sei. Coll. Imp.- 

 Univ. Japan, Vol. III., pp. 70-7 .3, 1889). There he observed that the "rhombic lamellae" make 

 gradual transition to the porphyritic crystal of plagioclase, as in the present case, and thus 

 he confirmed that they are plagioclase (probably anorthite), as suggested by Penck (•' Studien 

 über lockere vulkanische Auswürflinge," Zeitschrift d. Deutschen geol. Gesellschaft, Bd XXX., S. 9!), 

 1878), Kkeutz ('* Ueber Ve.suvlaven von 1881 und 1883," Tschermak's Min. u. Peti: Milth., Bd. IV., 

 S. 139, 1835), and Doss (" Die basaltischen Laven und TufiEen der Provinz Hauràn und vom Dîret 

 et-Tulnl in Sirien." Tscherm^Ms mn. u. Fetr. Miith., Bd. VII., S. 527, 18S6). Crystal faces ob- 

 served by him are as follows: — 



p^oal), .1/(010), y(20i), o(iiï), p(iii), .••(loi). 



Twinning according to the albite and Carlsbad laAvs was also observed by him in the rook of 

 Ogasawarajima. 



