

ORIGIN OF ACTIVE VOLCANOES. 



nounced. If the excess be greater still, a dome-shaped hill or 

 mountain will be produced. A greater excess again will cause the 

 disruption of this dome, and its conversion into a crater of eleva- 

 tion. Finally, if the internal force be sufficient to break a way 

 through the entire mass of solid strata which forms the shell of 

 the earth, the fiery fluid central matter, rising through the 

 opening thus made for it, will issue from the holes, crevices, and 

 fissures in the floor of the crater, and overflowing or breaking a 

 way through the surrounding wall, rush in a torrent of fire down 

 the slopes of the dome-shaped hill thus formed. 



61. The volcano thus formed is never uniformly active. The 

 eruptions are only occasional. "When an internal wave or tide of 

 the fiery central ocean passes the base of the opening, a pressure is 

 produced by which the molten matter is forced up and ejected in 

 the form of lava. In its ordinary state, however, when no erup- 

 tion takes place, volumes of smoke more or less dense usually rise 

 from the fissures, and upon looking down into them, the luminous 

 incandescence of their walls and of the matter they include is 

 visible. The light of this illuminating the smoke and ashes, 

 which rise over the crater, often give to them a lurid light which 

 appears like, and is sometimes mistaken for flame. 



62. The intervals of activity and repose of volcanoes are often 

 of very long duration. Thus in the case of Vesuvius, the erup- 

 tions were renewed with unabated force after an interruption of 

 several centuries. In the time of Nero, Etna was considered as 

 approaching to entire extinction, and according to JElian, the 

 summit of the mountain at a later period was gradually sinking, 

 so that it could no longer be seen as a landmark by vessels at sea 

 from the same distances. 



63. Humboldt affirms that it may be considered as a pretty well 

 established law of volcanoes that those which have least elevation 

 are characterised by the most unceasing activity. He proves this 

 .law by many examples, and explains it by the supposition that a 

 less internal force is sufficient to raise the molten masses to low 

 than to high summits. He gives the following series of elevations 

 of the craters of remarkable volcanoes : 



Feet. 



Stromboli '2318 



Guacamayo . . . . ' i . ' . . . 2500 

 Vesuvius . . .... . . 3876 



Etna . . . 10,870 



Peak of Teneriffe . . . . . .12,175 



Cotopaxi 19,070 



64. Now of these, Stromboli has been in a state of activity from 

 the Homeric age to the present, so unceasing that it has served 



171 



