NOTES ON SOME IGNEOUS ROCKS OF JAPAN 65 
The ferromagnesian minerals belonging to pyroxene and 
amphibole groups are arfvedsonite, barkevikite, aegirite, and 
aegirite-augite. The amphiboles occur as phenocrysts, and the 
pyroxenes both as phenocrysts and in the groundmass. The 
phenocrysts of these minerals are usually found in association. 
Barkevikite occurs irregularly intergrown with the arfvedsonite 
on the one hand, and with the aegirite and aegirite-augite on the 
other. Their shapes are elongated or short prismoid, but their 
outlines are irregular and ragged. No cross-section was found in 
ten thin slices made for the purpose of determining the cross-section. 
So the determination of the optical orientation of the minerals was 
difficult. The aegirite-augite is distinguishable from the aegirite 
by its higher birefringence and larger extinction angle, reaching 
c\ X=37°. From the aegirite, the arfvedsonite is distinguish- 
able by the extinction angle and its direction, compared with those 
of the barkevikite associated with them. The aegirite extinguishes 
at a small angle or nearly zero in #, in which the barkevikite extin- 
guishes also, while the arfvedsonite extinguishes in —8. Ona 
longitudinal section, the extinction of the barkevikite was measured 
as c\Z=2°, and the arfvedsonite showed c\ X=15° in con- 
current direction. Some crystals are strongly decomposed and 
are completely altered into a yellowish-brown substance. There 
are a few minutes rods to be identified as aenigmatite in the ground- 
mass. They are deep brown or almost opaque. 
Chemical character.—A chemical analysis of the rock, made by 
K. Yokoyama in the chemical laboratory of the Survey, compares 
fairly well with those of the comendites from other localities, 
described by different authors. It shows well its characteristic 
properties: high alkalies and nearly equal amounts of soda and 
potash, low lime and magnesia, fairly high iron oxides, and low 
alumina in proportion to alkalies and silica. 
The analysis of the rock from the Oki Islands is given under 
column A in the following table, and is compared with those of the 
comendites from Comende, Sardinia (B)', and Iskagan Bay, 
East Siberia (C).2_ They resemble one another very closely, though 
1H. Rosenbusch, Elemente der Gesteinslehre, 3. Auflage, p. 332. 
2H. S. Washington, American Journal of Science, XIII (1902), 180. 
