46 S. KOZU 
prismatic and tabular habits, scattered in an aphanitic ground- 
mass. There are also well-defined prismatic or irregularly out- 
lined cavities having the crystal form of hornblende, in which the 
mineral material is entirely altered to a dark-brown loose sub- 
stance. Their sizes are usually smaller than those of the feldspar 
phenocrysts. In this rock-type, phenocrysts of quartz are entirely 
wanting. Flow-structure is a constant feature; sometimes red 
flow-lines are distinctly marked in the light-colored groundmass. 
Under the microscope the porphyritic character is pronounced. 
Fairly numerous phenocrysts of sanidine and a very small quantity 
of magnetite are scattered through the hyalocrystalline ground- 
mass. Needles of apatite and minute crystals of zircon occur as 
accessory constituents. 
Sanidine phenocrysts, which are the only important constituent 
mineral, occur both in tabular and in prismatic forms, with more or 
less rounded outline. They are simple or twinned, and in many 
instances, inclose clouded patches of glass and minute crystals of 
apatite and iron ores. The characteristic cracks, sometimes filled 
with reddish-brown iron oxide, are also observed. It has a low 
refraction and low double refraction, and exhibits a very small optic 
angle, which is nearly zero. In a specimen of brecciated lava, the 
sanidine is entirely replaced by a colorless substance which is 
isotropic, and has slightly higher refraction than that of Canada 
balsam. From these characters, it appears to be opal. 
The groundmass consists essentially of potash-feldspar and 
devitrified glass in nearly equal proportion though their relative 
amounts vary somewhat in different places. The feldspar is 
variable both in shape and size. Some of the crystals show a dis- 
tinct prismatic form, commonly twinned, but others are irregularly 
outlined. The glass base is more or less densely clouded with 
reddish-black, opaque spots or rods, their presence affecting the 
color of the rock. In rare instances very minute flakes of deep 
reddish-brown mica can be detected among them. 
Chemical characters.—The analysis of the rock, from the western 
foot of Manzoyama, was made by K. Yokoyama in the laboratory 
of the Survey. The result is given as follows: 
