INTERESTING PHENOMENA ON THE ERUPTION OF USU 273 



tain. At the top of the small parasitic cone, Kompirayama, a 

 "graben" was formed about 30 m. in maximum depth, 100 m. 

 wide, and roughly 500 m. in length (Figs. 12, 13). Westward, 

 across the road, the same faulting was found to be "step faulting," 

 with the northern blocks down thrown (Fig. 14). It seems appar- 

 ent, therefore, that the northern fault of the "graben" is a "scissor 

 fault," reversing its throw on crossing the road. Close to the 

 second group 1 of craterlets, at the same time, were formed many 



Fig. 17. — Mud-flow from crater No. VII. Taken August 2, 1910 



faults, among which were some forming fault scarps of 5-7 m. in 

 height. The trend of both systems of faults is west-northwest by 

 east-southeast. It is noteworthy that such a large number of 

 craterlets formed in the ten weeks of the eruption. 



1. Process of explosion. — The order of formation of these 

 craterlets was as follows: In the beginning a cannon-like sound 

 was heard, the ground cracked open in a straight line in the form 

 of a V-shaped crack (Fig. 9), and white smoke issued from the 

 vent. Then followed black smoke together with sand and ashes. 



1 See Fig. 15. 



