OX PYROXENIC COMPONENTS IN CERTAIN VOLCANIC ROCKS. 6^ 



of which was given in the Bulletin* of the Geological Society 

 of Japan. His collection besides contains specimens from the 

 neighbouring island of Ototojima (Stapleton Island). A part of his 

 collection he kindly placed at my disposal to supplement my own. 

 The kinds of rocks from Chichijima and Ototojima, as enumerated by 

 Mr. Suzuki are as follows : — 1) Augite-andesite, 2) Augite-andesite 

 glass, 3) Quartz Augite-andesite, and 4) Basalt. It may be noticed 

 however, that the ' Augite-andesite glass ' represents basic glasses, of 

 the nature of Basalt-glass like Tachylyte. The absence of Olivine 

 in these glasses is quite peculiar and noteworthy, its place being 

 taken by the Rhombic Pyroxene and Augite. 



In Chichijima, where I had the best opportunity for observing 

 general geological features, volcanic rocks occur either in a mas- 

 sive state probably consolidated from lava-flows, or as agglom- 

 erates of volcanic pebbles, associated or almost cemented with 

 tuffs. It is usual to find among the former class, a lighter coloured, 

 porphyritic rock, the ground-mass of which is microcrystalline, usu- 

 ally with small quantities of glass ; the essential mineral constituents 

 being Plagioclase, Augite, Rhombic Pyroxene and Magnetite. In fact, 

 it resembles the Augite-andesite which is found in Honshiu. It seems 

 to be especially well developed at Hahajima. Among the pebbles that 

 compose the agglomerates however, we find various remarkable modific- 

 ations. The peculiarity of these rocks is their basic character, with a 

 dark coloured glass basis, in which the Pyroxenes are usually developed 

 as Rhombic and Monocliuic forms. Their mode of occurrence suo-- 

 gests that they were formed as the accumulations of volcanic ejections, 

 which were hurled away from a fluid lava, rapidly cooling on its way, 

 and thus inducing an incipient state of crystallization. In some varieties 

 however, the structure becomes porphyritic ; the crystals of Pyroxenes, 



* Part. B. vol. I (Japanese). 



