74 T. KIKUCHI 



It has been pointed out by Judd and iCole * that the Basalt- 

 glasses are of rather rare occurrence in Europe, and that when they 

 occur, they are always found as selvage (Saalband) of dykes, or as 

 small fragments thrown out of volcanic vents and cooled rapidly while 

 passing through the air, whilst in some Pacific Islands (notably in the 

 Sandwich Islands) Basalt-glasses are found as lava-streams. It 

 has also been observed that the glasses from these islands are more 

 transparent in thin sections than those found in Europe. Although it 

 is not likely that the Basalt-glasses in the Bonin Islands have flowed as 

 lava-streams, yet they seem to be nearer to those of the Sandwich Isl- 

 ands, &c, since they are always lightly coloured, transmitting a 

 greenish grey or light brownish colour. Fragments of similar rocks 

 probably derived from lapilli, have also been dreged from the bottom 

 of the Pacific by the Challenger. f 



Another peculiarity of the glasses of Miyanoura and Tatsumiura. 

 is the almost total absence of Magnetite, which seems not to have 

 separated from the magma. Olivine is also entirelv wanting ; the 

 recognisable mineral constituents being Rhombic Pyroxene and Augite, 

 with some Picotite, which is however more generally found as enclos- 

 ures within the Rhombic Pyroxene. 



We shall now proceed to give the characteristics and the relation 

 of the two Pyroxenes which are developed in these glassy rocks. 



Rhombic Pyroxene. 



Owing to the incoherent character of the glassy rocks, in conse- 

 quence of alteration into a palagonitic substance, the crystals of the 

 Rhombic Pyroxene which are unaffected by alteration, can be rather 



* Ou the Basalt-glass (Tachylyte) of the Western Isles of Scotland — Quart. Jour. Geol. Soc. 

 vol. XXXIX, 1883. p. 457. 



f Report ou the Scientific results of the Exploring- Voyage of H. M. S. Challenger — Narra- 

 ti- e of the Cruise— Vol. I, pt 2., 1885 — p. 813. This occurrence is the more interesting as it 

 seems to contain IJhonibie Pvroxene. 



