DR. BUNDJIRO KOLO ON SOME JAPANESE ROCKS, 435 
The augites in all hitherto-examined specimens are decidedly 
pleochroic :-— 
C=deep green; A=light green; B=brown; ¢: C=44°—48°. 
A basal section always presents traces of the characteristic 
cleavages which intersect at about 87°. Simple as well as multiple 
twins are of common occurrence, and the polysynthetic twins display 
brilliantly banded polarization-colours similar to those of the twin 
lamellz of plagioclase. 
It is a very singular fact that the triclinic (?) nature of the so-called 
monoclinic normal augite entirely escaped the notice of observers 
until W. Cross lately called attention to this point. Basal sections 
of twins of the common type exhibit always an oblique extinction, 
in such a way that the deviation of direction in the two halves is 
nearly 30° from the plane of composition. If the normal augite be 
really monoclinic, then this should not take place; but in all ob- 
served cases, it indicates the triclinic (?) character rather than the 
monoclinic. Ought this fact to be simply ascribed to an optical 
anomaly ? | 
Another mode of twinning is also observed in the augites of the 
augite-andesite from Tsiogigahara and in the basalt from Komura, 
both in the Izu province. In the microscopic sections, the twinning- 
plane makes an angle of 18°—138° with the traces of cleavage in the 
augite crystals, and consequently this twinning should not be iden- 
tified with that of the common type. The composition-face of the 
twins may be perhaps 0 P2*. Cohen+ and H. Sommerlad t have 
described similar twins in hornblendes. 
Since the publication of the admirable work of M. Fouqué §, 
lithologists have been led to distinguish the pleochroic and non- 
pleochroic augites in augite-andesites as two distinct species of 
pyroxene, the former hypersthene and the latter the normal augite. 
Rosenbusch ||, in his well-known work, discussed this subject very 
critically, and he was led to the conclusion that these (pleochroic 
and non-pleochroic) augites are one and the same species of mono- 
clinic augite, although he did not absolutely deny the presence of 
hypersthene in augite-andesites, but only considered it “ tiberaus 
wahrscheinlich.” Lately a few lithologists 4] have taken up this 
- * H. Sommerlad observed similar twins in the augite of the hornblende- 
basalt from Liebhard, Germany ; but, following the example of Rosenbusch, 
he supposed that the plane of composition of these twins might be a clinodome. 
N. Jahrb. f. Mineralogie, &c. Beilage Band ii. p. 147. 
+t ‘Geognostische Beschreibung der Umgegend von Heidelberg, Heft i. 
. 69. 
7 t “Ueber Hornblendefihrende Basaltgesteine.” N. Jahrb. Beilage Band ii. 
§ ‘ Santorin et ses Eruptions,’ Paris, 1879. 
|| ‘ Massige Gesteine, p. 411. _ 
4 W. Cross, Bulletin of the United States Geological Survey, no. i. 1883, and 
Hague and J. P. Iddings, American Journal of Science, vol. xxvi. September, 
1883; J. J. Harris Teall, Notes on the Cheviot Andesites and Porphyrites, Geol. 
Mag. Dec. ii. vol. x. no. 225, 1883; No. 226, 1883; no. 228, 1883; and on 
Hypersthene Andesite, abcd. no. 230, 1883. 
4) 
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