INTERESTING PHENOMENA ON THE ERUPTION OF USU 259 



severe. The Usu eruption is the latest one among the three, 

 which was quite similar to that on Etna in September, 191 1. 

 This volcano is located between the other two, in longitude 

 E. 140 49' 30" and latitude N. 42 33', and lies between "Volcano 

 Bay" on the south and Lake Toya (80 m. higher than sea-level) 

 on the north. Usu is a low, conical, active volcano, 736 m. above 

 the sea, and has a crater 2 km. in diameter, within which there are 



N lS hi-Maruy«ma LAKE TOYA 



Fig. 1. — Map of Volcano Usu, showing craterlets on the northern slope 



two domes occupying respectively the east and the west end of the 

 crater (Fig. 1). The eastern of these domes, O-usu (736 m. AT), 

 looks new, while the western one, Ko-usu (609 m. AT), appears 

 much older. Ko-usu has a few small, steaming pits on the top of 

 its dome, while O-usu has one only on the west side of its dome. 

 The topography, geology, and history of Usu have been well 

 described by Professors F. Omori 1 and D. Sato. 2 Hence only the 

 especially interesting details of the eruptions will be discussed 

 here. 



1 Bulletin 0} the Imperial Earthquake Investigation Committee, V, No. 1 (1911). 



2 Bulletin of the Imperial Geological Survey of Japan, XXIII, No. 1 (1913). 



