NOTES ON SOME IGNEOUS ROCKS OF JAPAN 661 
is almost entirely replaced by biotite in the fine-grained part. If 
the Tertiary formation, through which the syenite intrudes, proves 
to be Miocene as considered now, the syenite will be probably the 
youngest rock of this kind which has ever been described. 
Megascopic character.—It is medium-grained, evenly granular, 
and almost compact, though miarolitic cavities are present. Ona 
freshly fractured surface, the color is light gray with somewhat 
waxy luster. The rock consists mainly of feldspar, which is light 
gray, due to clouded, minute inclusions; and, in some crystals, a 
faint blue shiller is recognizable. Close examination shows certain 
feldspars developed with a more or less idiomorphic and thick 
tabular habit. Black hornblende occurs in moderate quantity, 
scattered through the feldspathic mass. Its form is irregular, but 
the general shape is prismoid with an average length of 3 mm. 
-Biotite is quite subordinate in amount, and quartz is scarcely 
detected with a lens. 
Microscopical character.—The constituent minerals present in 
the rock specimen used for a chemical analysis are quartz, alkali 
feldspar, plagioclase, hornblende, biotite, diopside, olivine, apatite, 
zircon, ilmenite, and magnetite; besides zeolites in miarolitic cavi- 
ties. The amounts of diopside and olivine are variable even in 
specimens taken from very near one another. A thin section shows 
the presence of allanite. The rock consists essentially of feldspar 
with hornblende; and the other minerals, with the exception of 
allanite, are also constant ingredients, though their quantities are 
subordinate and variable. The texture, for the most part, is 
granitoid, but there are some peculiarities of micrographic inter- 
growth of feldspar and quartz and of miarolitic cavities. 
The feldspars are mostly alkalic (orthoclase, microperthite, and 
cryptoperthite), but plagioclase is not rare. The latter mineral is 
represented by oligoclase and albite, which, besides perthite, is also 
found as the border of the alkali feldspar. The alkali feldspars are 
tabular, parallel to (o10), and are variable in shape, some of them 
being irregularly contoured and others rather euhedral. They are 
clouded with dusty particles and frequently show the zonal struc- 
ture due to chemical differences. The outer shell is usually thin 
and clear, in strong contrast to the clouded inner part. The former 
