DR. BUNDJIRO KOTO ON SOME JAPANESE ROCKS. 451 
abundant as interpositions in felspar and augite. A colourless glass- 
basis occupies the wedge-shaped interstices in the ground-mass. 
The dolerite from Ihama also belongs to the preceding type; and 
that of the Asio Pass is characterized by containing olivine rich in 
glass-enclosures and trichites. 
Basaur LAvAs, 
These are numerous, and are apparently homogeneous, black, 
slag-like masses, showing a striking contrast to the previously 
mentioned, dark-grey, porphyritic varieties, although the essential 
components are similar. . 
The typical one is the Omuroyama lava. It is microporphyritic, 
containing plagioclase, olivine, and a few augite crystals in the 
ground-mass. The olivine is fresh but ill-defined; its periphery 
has a granulated appearance and a deep brown colour, due to the 
corrosive action of the once fluid magma. The crystals are free 
from glass- and liquid-enclosures. Magnetite is rare, being contained 
potentially in the brown glass-basis; the latter is full of globulites, 
and is partly devitrified into radially fibrous concentric bodies 
(felsospheerites), which transmit a faint light, like that in the 
anamesite from Steinheim in Germany~*. The rock contains 
56°31 per cent. of silica. In this lava, augite is the predominant 
constituent, and the interstitial spaces of the augite crystals are 
filled up with the brown globulitically devitrified glass-basis. The 
large augites are entirely composed of augite-microliths, which have 
been mechanically disturbed during the overflow of the lava from 
the voleanic funnel, so that these microlith aggregations assume 
fan-shaped or imperfectly radial forms. The felspar also contains 
large brown glass-enclosures with fixed bubbles. All of these 
conditions recur in every lava of the province of Izu, and serve 
to distinguish these rocks from dolerites. The olivine in the 
lavas is almost colourless, with a faint blue tinge, while in the 
dolerites it is greenish yellow. The specimens from Omuroyama, 
Mimuroyama, and Itaru, are essentially the same as those before 
mentioned. 
The pores in these lavas are round or oval, and walled by a deep 
brown glass-magma, which, in turn, is lined bya faintly coloured 
glass, which is not completely isotropic, owing to the strong tension 
to which it has been subjected at the moment of its solidification. 
By subsequent sudden bursting (‘‘Spratzvorgang” of Reyer), during 
the cooling of lavas, these, peculiar forms are disturbed. A perfect 
one is seldom seen in a microscopic section, being usually broken in 
the process of slicing. That the imprisoned gas-pores must have 
been suddenly burst open, is evident from the fact that the felspar- 
microliths have been broken at one end, where they come in contact 
with the margins of the burst bubbles. 
The last of these rocks to be described is a glassy one from the 
Mount Amagi (Omigutsi). It is an obsidian containing 74°327 per 
* Zirkel, ‘ Basaltgesteine,’ p. 145. 
