The volcano NijÔ-san, distant about 25km south-east from the city of Osaka, is a small dissected cone with two isolated peaks (Fig. 1), from which the name was originated. The volcano is formed of 4 different andesitic rocks, among which the biotite andesite contains especially abundant crystals of garnet, less than 2mm in diameter. The number of these crystals is measured about 5 in 1cc of the rock, while in other andesites this mineral is seen only sparingly. The occurrence of the placer garnet in this district was noticed and recorded at such an early time as A.D. 743. It occurs in the recent depcsits in the eastern foot of the mountain, in the vicinity of a village called Anamushi. From their occurrences, the garnet crystals are conveniently divided into two groups: one kind appears as the porphyritic crystals, markedly corroded in the groundmass of the biotite andesite, and also as idiomorphic enclosures in the phenocrystic plagioclases in the same andesite. The other occurs as one of the mineral constituents of xenoliths included in the andesite. The metamorphic minerals in the xenoliths are garnet, plagioclase, quartz, biotite, sillimanite, andalusite, cordierite, staurolite, spinel, magnetite, glass etc. The modes of assemblage of these minerals are shown in Tabs. 2 and 4.
The occurrence of garnets as the enclosures in phenocrystic plagioclases, as stated above, may be taken as an important factor to indicate that the garnet crystallized primarily in the melts of the pieces of argillaceous sediments caught up into the andesite magma in its reservoir. It is conceivable that the magma in the reservoir was agitated strongly and repeatedly by violent activities of volatile components, by which the capture of the wall rocks into the magma was naturally increased and the garnet crystals first developed abundantly in their melts were distributed evenly throughout the magma. In this later stage, the crystals were markedly corroded by the magma forming the groundmass.
The garnets in the andesite are distinguishable from those in the xenoliths by a₀, the prevailing value of the former being 11.544A and that of the latter 11.504A, while the refractive index varies from 1.805 to 1.809 in the two groups and no distinction can be drawn between them (Tab. 5). From these physical properties, we see that the chemical composition of the garnets in the xenoliths must be slightly different from those of the red wine garnets in the andesite. The chemical composition of the latter, analysed by Dr. Y. Kawano in our Institute, is given as Alm=67.66%, Py=13.20%, Sp=4.13%, Gr=8.58% and And 6.43%. From physical constants of the analysed garnets, namely n=1.809, a₀=11.542 A and G= 4.104, the computed composition is Alm=67, 34%, Py=13.07%, Sp=4.51%, Gr=8.64% and And=6.44%. It is worthy to note from a genetic view that the refractive index (γ=1.673)of the biotite enclosed in garnet is practically equal to that of the biotite in the andesite and is noticeably different from that (γ=1.643-8) of the biotite in the xenolith.
The petrographic characters of the xenoliths are widely different in accordance with the grades of thermal metamorphism applied by the andesite magma, that is, from micaceous slate to highly pyrometamorphosed fell. Some of the latter are characterized by brown glass filling up the interstitial spaces between the small well-outlined transparent crystals of plagioclase (ca 45% An) newly formed, which is easily distinguishable from the larger plagioclase (ca 70% An) formerly crystallized in the xenoliths under the influence of the andesite magma. If such pyrometamorphic progresses in the enclosures of the argillaceous rocks under a higher temperature in the andesite magma, the melt in which garnet and plagioclase primarily crystallized will be mixed up gradually with the andesite magma and the garnet crystals will remain in corroded form in the groundmass of andesite.

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