PRELIMINARY NOTES ON SOME IGNEOUS ROCKS 

 OF JAPAN. II 1 



S. KOZU 

 Imperial Geological Survey of Japan 



II. QUARTZ-BASALT 



Locality. — Kasa-yama, near Hagi city, prov. Nagato. 



Occurrence. — The rock occurs as a lava flow erupted at the vol- 

 cano Kasa-yama, which consists merely of an isolated cone of small 

 size, 1 1 2. 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 topog- 

 raphy 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 1 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 dis- 

 tinguishable from the groundmass by their color, as seen in the 

 photograph. Besides these there are only a few crystals of yellow 



1 Published by permission of the Director of the Imperial Geological Survey of 

 Japan. 



561 



