S, Powers — Volcanic Domes in the Pacific. 263 



destroying the domical form. A final explosion on May 12 

 opened a large fissure in the floor of the crater near the dome 

 and spread a covering of ash over the region on the northeast. 

 From this fissure, a crack about 300 feet long with a maximum 

 width of 15 feet and a depth of over 60 feet, steam was rushing 

 in August, 1915. 



Huge blocks of porous pyroxene-hornblende andesite of a 

 light to reddish grey color compose the outside of the dome. 

 The andesite is filled with anorthite crystals ^ to i/o inch in 

 length and it shows banding due to variations in porosity. The 

 dome may be composed of a uniformly solid mass of andesite 

 as figured by Simotomai 4 or it may be formed around a central 

 core as was Perry Peak, Bogoslof, described below. 



Small domes of glassy andesite are reported on Eniwadake, 

 an extinct volcano a few miles west of Tarumai. 5 The domes 

 are located on the northwest side of the summit crater and on 

 a small peninsula in Lake Shikotsu. 



Usudake. The volcano Usu, in Hokkaido, five miles north 

 of Volcano Bay, is situated on the south side of Lake Toya, 

 one of the peculiar depressions like Lake Shikotsu in Hokkaido 

 and Lake Ikeda in Kyushu whose bottom is below sea level. 

 Usu (see fig. 2) consists of a large "somma" rim about 25/0 

 miles in diameter within which are two rounded domes, O-Hsu 

 (fig. 3) on the east, 975 feet high, and Ko-Usu (fig. 4) on the 

 west, 555 feet high. In 1910 an eruption took place north 

 of the somma rim, opening about 45 small craterlets and uptilt- 

 ina; a rectangular block of the mountain in a manner somewhat 

 similar to uplifts which take place at Kilauea on the edge of 

 the crater Halemaumau. O-TTsu and Ko-TJsu are volcanic 

 domes like those of the Puy de Dome region of Prance. The 

 19] uplift was evidently caused by the intrusion of new magna 

 which did not reach the surface. 



The eruption of 1910 commenced with a seismic prelude 

 characteristic of Japanese volcanoes, the first shocks being felt 

 at the time the barometric pressure was low and the outbreak 

 occurring when it was at a maximum (29.99 in.). The first 

 earthquakes were felt July 21 ; 25 were recorded the following 



"E. Reyer (Theoretische Geologie, Stuttgart, 1888, pp. 98-9, 152) sug- 

 gested this structure for certain rounded masses in Bohemia — notably 

 Schlossberg; von Teplitz — first described as Quellkuppen but later shown 

 to be erosion remnants of Miocene intrusives (v. Wolff, Der Vulkanismus, 

 I, p. 484, 1914). 



5 1. Friedlaender, Ueber den Usu in Hokkaido und iiber einige andere 

 Vulkane mit Quellkuppenbiklung, Petermann's Mitt., vol. lviii ( 1 ) , pp. 

 309-12, 1912. 



