PRELIMINARY NOTES ON SOME IGNEOUS ROCKS OF 
JAPAN. V: 
; S. KOZU 
Imperial Geological Survey of Japan 
V. POTASH-RHYOLITE 
Introduction.—The rock is of limited occurrence, forming a hill 
called Manzdyama, with the height of 200 meters above the 
sea-level and the base-area of about 1.62 square kilometers, 
standing near the bay of Shimoda, which is situated at the southern 
end of the Izu Peninsula, projecting southward from the middle 
of the main island of Japan, and is well known as the port first 
visited by Commodore Perry. 
The rock occurs as a lava-flow, not widely extended, but of 
considerable thickness. On the southwestern foot of the hill, a 
distinct prismatic jointing in the lower part of the lava can be 
observed. Its eruption was preceded by that of the so-called 
plagioliparite, and was followed by an enormous outpouring of 
andesitic rocks after a somewhat long interval. The eruptions 
of these rocks seem to have happened about the middle of Tertiary 
time. Tuffites derived from these rocks contain sharks’ teeth 
(mostly Carcharodon megarodon and Lamina sp.), Lithothamnium, 
and several kinds of foraminifera. The age of the formation is 
considered as Miocene. 
Petrographical characters.—Megascopically, the rock is_ char- 
acterized by its color, which varies from brownish-red to light 
reddish-gray with a violet tinge, which distinguishes it from other 
rocks occurring in the region. Its texture is indistinctly por- 
phyritic, owing to the small size of the phenocrysts, from 1 mm. to 
2 mm. in length. The only phenocrysts are feldspar crystals with 
a Published by permission of the Director of the Imperial Geological Survey of 
Japan. 
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