EARTHQUAKES AND VOLCANOES. 



and still serves all the purposes of a stupendous light-house to 

 ships navigating that part of the Mediterranean. The entire 

 island situate off the north coast of Sicily is of volcanic formation. 



65. The activity of Gruacamayo, situate in the province of 

 Quiros, at about 88 miles from Chillo, near Quito, is so con- 

 tinuous that detonations are heard daily from it, even at the 

 distance of Chillo. On the other hand, the eruptions of Vesuvius 

 are comparatively rare, and those of Etna still more so, while 

 those of the lofty volcanic peaks of the Andes are separated by 

 intervals of little less than a century. 



66. As a further evidence of the prevalence of this volcanic law 

 may be mentioned the cases in which volcanoes do not rise, like 

 Etna and Vesuvius, from the midst of extensive plains, but have 

 their bases established on table-lands of great elevation, in which 

 cases it is observed, that even in the most terrible eruptions no 

 lava properly so called is ejected, the matter thrown up being 

 merely ignited scoriae. Yet the violence of the subterranean com- 

 motion is rendered manifest by the terrible detonations which are 

 heard at distances of four hundred miles. Examples of this are 

 presented in the volcano of Popayan, South America, the base of 

 which is 6000 feet above the level of the sea, that of Pasto upon 

 the table-land of the Andes at an elevation of 8500 feet, and those 

 of the Andes near Quito. 



67. Wherever exceptions to this volcanic law have been pre- 

 sented, Humboldt considers them as only apparent, and that they 

 may be explained by the casual obstruction of the fissures of 

 communication between the seat of the volcanic force and the 

 surface. 



68. The volcanic eruptions do not necessarily proceed from the 

 crater. Indeed, in some volcanoes, eruptions from the crater are 

 much less frequent than those which proceed from other parts of 

 the mountain ; as, for example, from the sides where the solid 

 walls of the internal fissure often present less resistance. Lateral 

 fissures and openings are in such cases produced, over which 

 "cones of eruption," as they are called, are formed. In this 

 case, a series, or row of cones, are formed at certain distances 

 along the line of fissure, from each of which the eruption takes- 

 place, all the intermediate part of the fissure being immediately 

 closed up. 



69. The internal forces also often break out at numerous 

 points, distributed over a large space, forming a number of 

 smaller cones which are described as having the form of bells or 

 beehives. The cones produced by the eruption of Vesuvius in 

 October, 1822, offer an example of these, as do also the Hornitos 

 de Jorullo, observed by Humboldt, and delineated by him in his 



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