250 E. B Bailey— The New 3Iountain 0/ 1910 — 



till the end of the year. By Kovember 12 the forty-five craterlets 

 shown on Plate Xll had been blasted out. The relative order of 

 formation of a non-consecutive fifteen from among these forty-five 

 was observed, and is shown by the use of numbers ou the map. The 

 (lata thus afforded are sufficiently complete to indicate that there was 

 no simple connexion between the sequence of the explosions and the 

 spatial distribution of the resulting eruptive centres. 



All the different craterlets were formed in the thick layer of soft 

 earth to which reference has previously been made. Many of the 

 eruptions proceeded (juite quietly and were unaccompanied either by 

 loud detonations or recognizable earthquakes. Ashes and stones were 

 ejected, but never any lava, and 'volcanic fires' are (mly reported 

 iu the case of the first explosion. The majority of the craters had 

 a very brief activit)', and after having thrown out clouds of ashes and 

 stones for a few hours, or a day, were then reduced to complete rest. 

 Some, however, persevered for several days and gradualh' enlarged 

 their diameters to over 200 metres. The spectacle presented by the 

 eruption, as illustrated in Omori's photographs, was often singularly 

 grand with several craterlets in operation at the same time, tossing 

 up smoke and stones 500 m. high into the air. A craterlet at the 

 base of West Maru-yama was somewhat exceptional in its behaviour, 

 for it poured out hot steaming mud after each of its major outbursts ; 

 and eventually this mud accumulated to such an extent that it covered 

 a triangular area 800 m. long and 450 m. in base. 



Plate XII shows the orderly arrangement of the craterlets and Pig. 1, 

 PL XIII, their general appearance. It seems natural to regard them 

 as falling into two divisions, a main group peripheral to the region of 

 elevation and a satellitic group on Kompira-yama. Omori, however, 

 classifies them in three divisions : an arcuate group, concentric with 

 the great crater of Usu-san, and reaching from the flanks of East 

 Maru-yama westwards to the craterlet numbered 5, and two radial 

 groups, the one extending along Kompira-yama and the other along 

 West Maru-yama. There is no need to dwell upon this difference of 

 classification, for it is a mere matter of detail, since Omoi'i is perfectly 

 definite in connecting the origin of the craterlets with the phenomena 

 of elevation which now fall to be described. 



(3) The elevation of the Xew Mountain is the main feature 

 of interest iu the 1910 eruption of Usu-san. It first manifested 

 itself in the behaviour of Lake Toya. Thus, in spite of a light 

 rainfall, the waters of the lake rose more than a foot above their 

 usual level in the latter part of July, that is about the time of the 

 commencement of the eruption, and maintained themselves in this 

 position throughout the following month of August. The accom- 

 panying rise ill the Sobets River, the only outlet of the lake, 

 meanwhile amounted to 2 feet. This apparent increase of the waters 

 of Lake Toya was of a general character and presumably due to 

 a differential upward displacement of some portion of the lake 

 bottom. That local movement of the type required did occur was 

 in fact demonstrated ou August 6, when Omori found that a reach 

 of the coastline about a mile long at the foot of East Maru-yama had 

 been elevated nearly a metre out of the water. To study this 



