———— Se = 
434 DR. BUNDJIRO KOTO ON SOME JAPANESE ROCKS. 
in the inner zones than in the periphery, indicating that the central 
portion is more basic than the external. 
This optical peculiarity may be partly due to another cause. 
Usually the zonal layers strictly follow and run parallel to the 
contours of the crystals; but here and there the nuclear crystals are 
bounded by entirely different faces. It may possibly be that in this 
case the outer and the inner portion of a crystal are not parallel. 
If this be the case, then the crystal-nuclei must consequently show 
the extinction in another direction, and these differences ought not 
to be exclusively ascribed to chemical discrepancies. 
Under the microscope these thin felspar-sections are usually 
seen to be stripped away in the centre; this is due partly to the 
decomposed state of the centre, the latter being easily obliterated in 
polishing and slicing, while the surrounding parts remain- imtact. 
These facts tend to show that the two respective zones are either 
different in chemical composition or in the molecular arrangement 
of the substance of the felspar. 
The plagioclase is tolerably rich in microscopic interpositions, 
whose arrangements are diverse, viz. central, peripheric (very 
seldom), or in the intermediate zone between the exterior and the 
interior, or, lastly, without any order. They consist of granules 
of augite, fragments of felspar with twin lamelle,magnetite grains 
and crystals, bluish crystallites (probably augite?), iron-glance, tri- 
chites, hyaline and semihyaline enclosures, and stone-cayities. 
It is worth while here to remark that the more recent eruptive 
rocks ate in general free from liquid-enclosures with spontaneously 
moving bubbles; but in the plagioclase of Japanese augite-andesites 
they are not rare, as for example in the andesites from Tsiogigahara, 
Thama, Yawatano, &c. The same is the case in the augite of our 
andesites. The liquid in the enclosures appears to be water, the 
bubbles not being apparently affected by a slight application of heat. 
The glass in the enclosures differs both in colour and in physical 
properties from that of the ground-mass; the enclosed glassy basis 
is sometimes devitrified and no longer isotropic. The felspars are 
often grouped in a confused manner; they are variously bent and 
cracked, owing to mechanical disturbances while fiowing in a once 
semi-fluid magma. 
Sanidine.—This occurs, in thin slices, mostly in square, rect- 
angular, or broad irregularly shaped forms. The crystals are either 
simple or twinned. Sections of the orthodiagonal zone show the 
extinctions which coincide with the orthodiagonal axis and with the 
plane of symmetry. Sanidine occurs only in subordinate amounts, 
or is even entirely absent in many cases. 
Augite is the second essential mineral component, and it claims 
our special attention on acccount of its many peculiarities. It 
occurs in these andesites as grains, as rounded fragments, or in 
the form of well-developed crystals with the combination of a P o, 
co BR «0, P, P, in which the pinacoids are well developed at the 
expense of the prisms. The prismatic faces are, as a general rule, 
unequally developed, one pair being much broader than the other. 
