Preliminary Notes on Some Igneous Rocks of Japan. II

Abstract

Locality.-Kasa-yama, near Hagi city, prov. Nagato. Occurrence.-The rock occurs as a lava flow erupted at the volcano Kasa-yama, which consists merely of an isolated cone of small size, 112.5 meters above the sea-level and about 1,300 meters in diameter across its base. In the summit, there is a perfectly preserved crater, 25 meters in diameter and 13 meters in depth. This small and regular cone stands in strong contrast to the topography of the environs, where the geology is mainly composed of granites and mesozoic sedimentaries, and especially to that of table-lands or flat islands formed by basalt flows which poured out here and there through the ground. Age.-The eruption of the rock appears to be Diluvium and the latest of the basalt in this region, which seem to have been erupted at the period from the close of Tertiary to Diluvium. Megascopic characters.-The specimen collected from the lava dam near the Shinto shrine at the eastern foot of Kasa-yama is noteworthy for containing abundant quartz as porphyritic grains in a hypocrystalline groundmass. It is black in color and vesicular with small and irregular cavities, but has a high specific gravity. The quartz, varying in size from i mm. to 5 mm., shows an irregular outline, but sometimes almost hexagonal. Though the percentage of quartz grains varies in different portions of the lava, generally they are distributed uniformly and are clearly distinguishable from the groundmass by their color, as seen in the photograph. Besides these there are only a few crystals of yellow