PRELIMINARY NOTES ON SOME IGNEOUS ROCKS OF 



JAPAN. I 1 



S. KOZU 



Imperial Geological Survey of Japan 



I. SODA-TRACHYTE 



Localities. — Matsu-shima and Kakara-jima, two islets, six and 

 a half kilometers northwest of the port of Yobuko, prov. Hizen, 

 Kyushu. 



Occurrence. — As compact lava, associated with an alkaline feld- 

 spar-bearing basaltic rock. 



Age. — Probably near the close of the Tertiary. 



The following notes on the mineralogical and chemical char- 

 acters were made from the specimen collected from Matsu-shima. 



Megascopic characters. — The rock is blackish gray in color with 

 semiwaxy luster, and by weathering easily changes to light brown- 

 ish-gray with greenish tinge. The phenocrysts are of abundant 

 feldspar, and are not easily distinguishable at a glance on fresh 

 fracture surfaces, as their color is not white, but on weathered 

 surfaces exposed to the washing of sea waves the minerals, being 

 less attacked than the matrix, are as well marked as the coarse 

 grains of quartz in weathered sandstone. The mode of weathering 

 is a characteristic feature which enables us to distinguish the present 

 rock from other rocks of the environs. The feldspar phenocrysts 

 are thick tabular or sometimes stout prismatic, from 2 to 5 mm. 

 in length, in rare instances 10 mm., and very light bluish-gray in 

 color, for they contain abundant inclusions. The luster is not 

 purely vitreous but slightly waxy. The groundmass is aphanitic 

 and deep gray with light greenish tinge when freshly fractured, but 

 the color soon changes from dark brownish-gray to light gray by 

 weathering. 



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

 Japan. 



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