660 S. KOZU 
the geological relations of the igneous rocks’ will be given with 
the petrographical notes of each kind of rock. However, some 
striking geological features may be mentioned. Enormous quanti- 
ties of trachydolerites and trachyandesites were erupted near the 
end of the igneous activity in the region, and D6zen was mainly 
composed of these rocks. Their eruptions seem to have occurred 
at several localities. After some interval of time there was a 
depression of the middle of the land which formed the inland sea 
already described. This movement fractured the ground greatly 
and gave rise to the eruption of trachytes. The rocks occur as 
flows and dikes; the flows form most prominently Takuhi-yama, 
and the dikes occur in very remarkable radial arrangement having 
Takuhi-yama as an approximate center, traversing trachydolerites, 
trachyandesites, and the Tertiary formation; but no dike was seen 
cutting the syenite mass. The occurrence of these dikes is analo- 
gous to that of the basic dikes in Rum, Scotland, described by 
Harker? and to that of the basaltic and trachytic dikes in the 
Highwood mountain, Montana, described by Pirsson;3 though the ~ 
petrographical character of the rocks and the number of the dikes 
are not the same. 
QUARTZ-SYENITE 
Field observation.—The rock is found in a small mass, only, at 
the northeastern foot of Takuhi-yama at Dozen. It occurs intruded 
in the Tertiary formation and is covered by glassy trachytes which 
build up Takuhi-yama. Its contact action on the Tertiary rocks 
is clearly seen, and the metamorphosed shaly sandstone is per- 
meated by a brown mica as a contact mineral. The endomorphic 
effect is not so pronounced as that observed in other countries 
where this rock type has been described by several authors. The 
texture becomes gradually fine-grained and inconspicuously 
porphyritic by the development of feldspar 1 cm. in average length. 
Hornblende, which occurs as an essential mineral in the main mass, 
* The petrographical notes on several kinds of rocks from the islands are not 
described in this paper. They will be published in the report of the Imperial Geologi- 
cal Survey of Japan. 
? Harker, Memoirs of the Geological Survey, Scotland, 1908, p. 143. 
3 Pirsson, Bulletin No. 237, U.S. Geol. Surv., 1905, p. 20. 
