266 S. Powers — Volcanic Domes in the Pacific. 



L910, when it was Found that the lake shore for a length of a 

 mile had risen 3 feet. A maximum elevation at the shore of 

 10 feel al the easl end of the uplifted area was reached August 

 21, followed by a gradual settling back of ;">•.> feet by 

 November 10. 



It was not until four days after the change of level of the 

 shore-line was noticed, that attention was called to the forma- 

 tion of New .Mountain north of a fault-scarp which developed 

 between East and West Maruyama (fig. 2). New Mountain 

 rose gradually, as a more or less rectangular block a mile long 

 and 1,000 feet wide, to a maximum height of 310 feet. The 

 fault-scarp disclosed beds of rock fragments and ash bnt no 

 massive lava, and these beds were still steaming in August, 

 1915. The maximum height of 690 feet above Lake Toya 

 (or 950 feet above sea-level) attained by New Mountain was 

 not reached rintil November 9 and by April, 1911, the height 

 had decreased 120 feet. A continuation of the uplift under 

 Lake Toya was indicated by an increase in the amount of water 

 in the lake. 



As indicated above, the force causing the uplift of New 

 Mountain appears to have been the intrusion of a mass of 

 viscous magma which did not reach the surface, although 

 bombs of the new lava were thrown out. The size of the area 

 uplifted may be taken to indicate a large intrusion such as a 

 laccolith, as Bailey 7 has suggested, but the arching characteristic 

 of the beds above a laccolith was lacking. The partial subsi- 

 dence of New Mountain may be compared to the change of 

 form of the top of the Tarumai spine from a rounded dome 

 to a plateau, and may have been caused by a partial withdrawal 

 of the magma, or in part to contraction on cooling and loss of 

 gas. 



The volcanic domes O-Usu and Ko-TTsu rise to heights of 

 975 and 555 feet, respectively, above the floor of the cratei*. 

 O-Usu is the more perfect dome and its symmetry is broken 

 only by an uplifted portion on the south side of the summit 

 (fig. 3). There is no depression on the summit. 8 Ko-TJsu is 

 an older and less perfect dome, similarly composed of hyper- 

 sthene andesite. The east side has apparently been lowered 

 by an explosion and a young crater with a diameter of 200 



7 E. B. Bailey, 'Geol. Mag., vol, ix, pp. 248-252, 1912. 



8 1. Friedlaender (Petermaim's Mitt., vol. lviii (1), pp. 309-12, 1912) 

 cites certain stream-worn pebbles on the summit as evidence that they 

 were elevated to that position with the formation of the dome. It is 

 apparent, however, that the dome was formed by new viscous magma and 

 that the pebbles were carried up by the Ainus. 



