DR. BUNDJIRO KOTO ON, SOME JAPANESE ROCKS, 439 
sions are tolerably abundant, and at times exhibit the hexagonal 
form of quartz. These glass-enclosures contain fixed bubbles, and 
also black rods which radiate from the centre to the periphery like 
the spokes of a wheel. The augite-andesite from Hosio contains 
quartz, which looks apparently homogeneous, but is really composed 
of many fragments of quartz crystals intimately united, each 
fragment in its optical behaviour being quite independent of the 
others, so that under polarized light the aggregate appears as a 
mosaic. 
The quartz grains are free from liquid-enclosures, while in the 
felspars they are common. When both quartz and felspar occur 
together (as is usually the case), then the presence of liquid-enlco- 
sures in the one and their absence in the other make a striking 
contrast. 
Quartz of secondary origin is by no means rare, and usually 
fills up the vacant spaces*in the ground-mass. In this quartz 
numerous radially arranged air-bubbles occur. The general appear- 
ance is like that of fibrous chalcedonic concretions with vivid 
chromatic polarization. Secondary quartz arises from the decomposi- 
tion of augite and partly from infiltration of silica, 
Hornblende is very rare in these andesites. When it does 
occur its basal section is six-sided, being surrounded marginally 
by opacite. The latter is so arranged as to preserve the contours 
of the characteristic section of heotelleradle. the trace of prismatic 
faces making an angle of about 124°; and by these contours alone 
the original form is recognizable. 
It is usually supposed that this opacite may have been produced 
by the caustic action of the rock-magma before its solidification, 
and that it is not due to subsequent decomposition. Zirkel* was 
the first who advanced this view, which has been partly confirmed 
by the experiments of A. Becker; but nothing definite has been 
settled from his researches. 
The opacite margins appear under low powers as heaps of grey 
grains mixed with some black granules. Where the accumulations 
are thin enough and magnetite grains are scarce, the microscope 
shows that the grey substance consists really of yellowish-brown 
granules. These yellowish-brown particles seem to me to be an 
augitic mineral, and by their colour and general habit they could 
not be distinguished from the augite-grains found in the ground- 
mass. In favourable cases these augite-granules can even be 
optically investigated, the columnar crystals in sections parallel. 
to co P o extinguishing the light at 36° from the axis C., and they 
certainly belong to the pyroxene group. 
As is well known, augite contains more lime than iron, while in 
amphibole the reverse is the case. During the act of transformation 
(probably induced by the caustic action of the once semi-fluid 
* “ Ueber d. kryst. Gesteine langs d. 40 Breitegrade i in N. A.” Ber. d. k. sachs. 
Ges. d. Wiss. 1877, p. 181. 
Tt Neues Jahrb. f. Mineralogie, &c., 1883, ii. ‘‘ Ueber die dunklen Umran- 
dungen der Hornblenden und Biotite in den massigen Gesteinen.” 
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