LARGE CRATERS AND CRATER-LAKES. 187 



be a good deal influenced by the wind, which often blows the ash so 

 that it accumulates more on one side of the mountain than the other. 

 The steeper inclination is always towards the summit, where it may 

 amount to 20 or 35, the slope gradually diminishing down the 

 flanks, till the level becomes horizontal. Volcanoes are often rendered 

 irregular by the truncation of the cone, and by the development of 

 parasitic cones upon their flanks. According to Sartorius Yon 

 Waltershausen, there are upwards of 700 minor cones on the flanks 

 of Etna ; and Dana mentions several thousand in the island of 

 Hawaii. These minor cones (according to Mr. Scrope) vary in size 

 from that of a hay-stack to 1000 feet in height, and two or three miles 

 in circumference. 



Cones and Craters. All the time that the eruption is in progress, 

 the volcano undergoes changes of form, partly from the accumulation 

 of ejected materials on its flanks, partly from the building up of new 

 lateral cones upon it. But more important changes are developed at 

 the top of the mountain ; for> as the super-heated water rises towards 

 the surface, and flashes into steam in the throat, its explosive force 

 blows out the loose materials of which the cone was composed ; and 

 thus the mountain becomes truncated, and its conical upward termi- 

 nation is often replaced by a funnel-shaped pit, which does not always 

 become entirely obliterated by subsequent eruption. Thus, Monte 

 Somma, on the flank of Vesuvius, is a remnant of the ancient crater, 

 whose size marks the violence of its earlier eruptions. High up on 

 Etna there is a somewhat flattened platform, which probably marks 

 the limits of an ancient truncation of the mountain by the formation 

 of a crater which has since been filled up ; and upon which the upper 

 cone now rises. A still grander example of truncation is furnished 

 by Mauna Loa, which has a horizontal width of 20 miles at 18 feet 

 below its summit. These great craters sometimes remain permanently 

 as pits, and, according to Professor Milne, there are about 20,000 

 people dwelling in the crater of Asosan, in the island of Kiushiu in 

 Japan. This crater is 15 miles in diameter. Many extinct vol- 

 canoes seem to have exhausted their energy in a final effort of this 

 kind, which has blown much of the volcano into the air. Per- 

 haps the most remarkable examples of large craters are seen in the 

 crater lakes to the north of Rome. The lake Bracciano is nearly 

 circular, 6J miles in diameter, and its surface rises 540 feet above 

 the sea. The crater of Monte Albano is 6 miles in internal diameter, 

 almost entirely composed of volcanic dust, and has a central moun- 

 tain in its midst. The lake of Bolsena is over 10 miles long by 9 

 miles broad, approximating to the oval outline common among 

 craters. In its midst rise two islands, which are composed of vol- 

 canic tuffs, and show the characteristic quaquaversal dips which are 

 seen in cinder cones ; so that, large as these excavations are, and 

 vanished as are the ancient volcanoes, we observe no more than 

 extreme stages of truncation of the mountains by explosive forces, 

 accompanied by faint indications of nature's efforts to restore the 

 structures destroyed. 



