DR. BUNDJ1RO KOTO ON SOME JAPANESE ROCKS, 441 
partial reabsorption of original crystals by the solvent action of the 
once semifluid rock-magma. Apatite belongs, no doubt, to the 
early-developed crystallized minerals, for the felspar-fragments 
enclose apatite and in turn the felspar is enclosed in the augite. 
Tridymate, or the triclinic modification of crystallized silica, 
occurs in various hand-specimens, being always of microscopic 
dimensions. It fills up the vacant spaces left in the ground-mass, 
and is consequently of a somewhat later origin, except in the case 
of the andesite from Hosio, in which tridymite is one of the normal 
constituents. It possesses the well-known hexagonal or rounded 
forms which lap one over the other, presenting the appearance of 
roofing-tiles. ‘Tridymite is comparatively indifferent to the action 
of polarized light, and between crossed nicols it only transmits a 
faint blue light. 
Scales of tridymite are always colourless, and the mineral 
possesses a vitreous lustre. Another morphological modification of 
it is found in the Japanese rocks, where the mineral is of a somewhat 
large size (0-4 mm.) and shows a fan-like twinning*. A horizontal 
view of the twinned crystals shows a deltoidal form with a line in 
the middle, bisecting the acute angle. This median line represents 
a trace of the joined faces of the twins upon one of the pyramidal 
faces, and the direction of maximum extinction lies at 17° on either 
side of this median line. The included angle measures 35°. Such 
twins are abundantly present in the rocks from the Izu-San. 
Chalcedony occurs as a filling mass in various decomposed rocks, 
and is colourless or brown. It is made up of concentrically and 
radially arranged spherulites presenting faint cruciform figures 
between crossed nicols. By reciprocal compression, the spheru- 
lites assume polygonal outlines. 
Other products of decomposition which deserve to be mentioned, 
are calespar, epidote, and viridite. The first (calespar) is found 
pseudomorphous after augite, and, partly, as a product of the de- 
composition of felspar. It occurs also in the clefts of fibrous 
enstatite, and in the veins of the rock-mass. Veins of ecalespar are 
easily recognized by the rhombohedral cleavage of the mineral ; also 
by its feeble chromatic polarization, and by the twinning striations 
which are caused by the entering and re-entering angles of —3 R. 
It sometimes contains fiuid-enclosures with movable bubbles. 
Epidote (secondary) is found in elongated grains in decomposed 
augites, forming tufts and irregular aggregates. It is of a deep 
lemon-yellow colour and is pleochroic. Its occurrence is always 
associated with that of viridite. Not only in augites, but also in 
decomposed felspars, yellow grains of epidote are met with, and these 
lead the observer to think that epidote may have been derived from 
felspars. Indeed, such an origin has frequently been assigned to it 
in petrographical literature. I think, however, that these epidote 
erains in the augite-andesite are derived from viridite, which often 
* There is no doubt that these twins are the same which have been described 
and figured by Vom Rath (Poggendorff’s Annalen, clii. tab. 1, fig. 1a). The 
twin-face is } P, the mineral being taken as hexagonal: 
oo 
