70 



Y. KIKUOHI 



and a comparatively few crystals of Plagioclase, being developed within 

 a partly rnicrocrystalline groundmass. The latter consists of a brown 

 coloured basis, containing microcrystals of Plagioclase, filiform Au- 

 gite, and Magnetite. These darker rocks are probably the rocks des- 

 cribed above as Basalt. But they differ from the usual type of Basalt in 

 the absence of Olivine. h\ the more glassy varieties, Plagioclase is 

 absent except as ' Rhombic lamellae.' These rucks are often dark pitch- 

 coloured glass, with numerous round vesicular cavities filled with 

 Zeolites. Under the microscope, they are always found to be of light 

 greyish or brownish coloured glass, in which a light green coloured 

 Augile is developed in slender filiform crystals in innumerable thick 

 clusters. They are often found alternating with sheets of a peculiar 

 greenish rock, resulting from the infiltration of Chalcedony and a green 

 fibrous chloritic mineral through its mass, which thus has the appear- 

 ance of a Jasper, and exhibiting microscopically a diabasic structure. 

 This was especially well observed near Omura in Chichijima. 



The glassy rock mentioned above often shows a most interest- 

 ing modification, in which the basis becomes more glassy and contains 

 very sharply defined crystals of Rhombic Pyroxene. In these varie- 

 ties, the glass often exhibits pearlitic structure under the microscope, 

 while the specimens from Ototojima are in spheroidal masses which 

 have the tendency to separate in concentric layers like an onion. The 

 microscopic porphyritic components are Rhombic Pyroxene, and 

 sometimes monoclinic Augite of a light green colour, together with a 

 microlithic form of the same mineral, Plagioclase and some Picotite. 

 Olivine has not been hitherto recognised. Very characteristic 

 microscopic objects in these glasses are the 'Rhombic lamella?' 

 already referred to. They have been found in basic glasses of other 

 localities. A short account of these lamella?, may be given here. 

 They are found making gradual transition to the porphyritic crystals 



