286 Y. OINOUYE 



are the weak lines around the foot of Mount Usu. The gases 

 involved within the magma found an exit through the old fissures, 

 the lines of the least resistance, which existed under the lava-flow, 

 and thus the two fissure zones, parallel to the shore line of the lake, 

 were made. As in the case of a viscous substance which is being 

 boiled and shows the evolution of gases in certain restricted points 

 which migrate around over the surface of the liquid, we may assume 

 that the gases evolving from the magma are generated at different 



Fig. 26. — Topographical comparison of the north side of the Usu. Broken line, 

 the slope before the elevation. Taken July 29, 1910. Full line, the present relief. 

 Taken October 25, 1910. Dotted line, the actual difference of altitude. 



places without reference in time to each other. The irregular 

 eruption of the craterlets may be explained on this hypothesis. 



The independent activity of the new craterlets to the old, small, 

 steaming pits on the O-usu and Ko-usu suggests by their lack of 

 sympathy that they do not rise to the surface through the same 

 lava vents and that the reservoirs are not connected. 



3. Origin of the "New Mountain.''''— Professor F. Omori 1 stated 

 that the "New Mountain" is due to the intrusion of lava in the 

 form of a spine or dome, and Professor D. Sato 2 believes the intru- 



1 Bulletin of the Imperial Earthquake Investigation Committee, V, No. 1, p. 1. 



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



