NOTES ON SOME IGNEOUS ROCKS OF JAPAN 657 
Mor phological sketch of the Oki Islands.—The Oki Islands, lying 
within 132° 56’-133° 22’ E. long. and 35° 56’—36° 20’ N. lat., are off 
the coast of Izumo, the northwestern province of Honshti, at a 
distance of about 65 kilometers. There are four main islands, in 
two groups. One large island called Dogo is situated to the north- 
west, and is separated by a strait 12 kilometers wide from the other 
three smaller ones—Chiburi-shima, Nishino-shima, and Nakano- 
shima, together known as Dozen. There are also 180 islets and 
rocks included under the name of the Oki Islands. The total land- 
area is about 351 square kilometers. 
The coast line of Dégo is rather regular, and its shape is roughly . 
circular, indented by the bay of Saigo on the southeast and by the 
bay of Fuku-ura on the northwest. Many other smaller narrow 
inlets occur mostly on the southern coast. The coast line of 
Dozen, however, is irregular and is detached into three islands by 
narrow channels called Oguchi, Akanadaseto, and Nakaiguchi. 
These islands are arranged in a triangular position on the arc of 
a circle. 
Dogo has a length of about 21 kilometers from south to north 
and a little shorter breadth from east to west. Its area is about 
245.6 square kilometers, and the coast, in great part, ends abruptly 
against the sea, with elevations varying from 30 meters to 100 
meters. The island is mountainous and morphologically divisible 
into two parts, the western and eastern districts (Fig. 1), separated 
by a watershed trending nearly north and south through the middle 
of the island. ‘The western region consists of gently sloping ridges 
(Fig. 1), composed of rhyolite flows, averaging 300 metersinaltitude. 
These flows have been eroded to deep and narrow valleys diverging 
toward the west and southwest from two isolated peaks, called 
Omine (666 m.) and Yoko-o-yama (568 m.), which.are situated on 
the watershed. It is evident that the region was formerly covered 
by almost horizontal rhyolite flows. The eastern district is char- 
acterized by ragged peaks and isolated knolls, composed of several 
kinds of rocks. The highest point on the conical mountain of 
trachydolerites, named Daimanji (Fig. 1), is 646 meters in altitude. 
The south and east ridges descend gradually, but the mountains 
to the north and west have precipitous slopes and are very irregular. 
