Usu-san, Japan. 251 



phenomenon more precisely a graduated level-gauge was at once 

 erected in the lake near the east end of the elevated stretch of coast, 

 and two others at convenient points on either side of the disturbed 

 tract. The two control gauges gave concurrent results, so that it 

 was a simple matter to interpret very precisely the differential 

 movement indicated by the instrument situated in between. It is 

 extremely intei'esting to find that part of the elevation thus 

 determined proved to be of a transient nature. Erora August 6 

 to August 21 an elevation of 34'3 cm. was recorded (additional to 

 that which had occurred previous to the former date). A slight drop 

 was then experienced, but the same height was attained once more on 

 August 27. Thereafter a gradual uneven downward movement set 

 in, and by November 10 the lake bottom at the point investigated 

 stood merely 16"7 cm. higher than on August 6. 



Meanwhile a large inland tract, about 2,700 m. long and 

 600 m. wide, was upheaved. At two places along its inner margin 

 it was bounded by a definite fault-face (Plate XII), and it was at 

 the more westerly of these two localities that the elevation was most 

 pronounced (Plate XIII). Here Omori found the inside of the 

 New Mountain defined by a steaming dislocation surface about 

 94 m. in height and inclined outwards towards the south at angles 

 varying from 30° to 60°. This face, extensive as it was, is said to 

 have consisted entirelj' of soil with embedded fragments of rock. 



The first stages of the elevation passed unnoticed ; in fact, it was 

 not till August 20 that the appearance of the New Mountain attracted 

 general attention. The elevation continued to increase till November. 

 Prom the 8th to the 10th of that month aneroid readings showed 

 a total elevation of 155 m. (510 feet) above the original height, 

 giving, for the time interval of 100 days from the end of July to the 

 commencement of November, an average rate of upheaval of l'55m. 

 (5"1 feet) per day. 



As in the case of the coastal belt, however, the full elevation of the 

 summit of tlie New Mountain was not long maintained: aneroid 

 determinations in April, 1911, showed a relapse of some 36 m. 

 (120 feet). 



Conclusion. Omori regards the upheaval of the New Mountain as 

 due to the intrusion of magma ; the explosions he ascribes to the 

 gaseous emanations from this magma ; and the earthquakes which 

 preceded the explosions he connects with the formation of cracks and 

 fractures below the surface permitting of the advance of the molten 

 material. The situation of the craterlets outside the region of 

 elevation is exactly what might be anticipated according to this 

 interpretation, since the boundary fault of the new mountain is 

 inclined outwards in 'normal' fashion. Altogether the Usu-san 

 eruption furnishes a most suggestive picture of what one might 

 expect at the surface during the injection of a laccolite accompanied 

 by fracturing of the upheaved cover. At the same time without 

 further knowledge of the underground relations of the various rock- 

 masses, new and old, liquid and solid, it would be rash, however 

 tempting, to dogmatize in this direction — even on the evidence so 

 carefully observed and described by Dr. Omori. 



