VOLCANOES, THEIR ACTION AND DISTRIBUTION. 53 



cones have been formed around the main cone. Another example is 

 furnished on the slopes of Etna, where the scoria-cones raised in 

 1865 along a fissure were so close together as to make a long, irregular 

 ridge (Fig. 10). Seven of these cones were formed along this fissure, 



Fig. 10. Fissure formed on the Flanks of Etna during the Eruption of 1865. a, Monte 

 Frumento, an old parasitic cone ; 6, line of fissure ; c c c, new scoria-cones thrown up on the 

 liDe of fissure ; d, lava from the same. 



thirty-six along another fissure during the eruption of 1874. Similar 

 phenomena were witnessed upon the slopes of Vesuvius in 1760, when 

 a fissure opened on the south side of the mountain, and fifteen scoria- 

 cones, which are still visible, were thrown up along it. 



The center of eruption has sometimes shifted itself along a line of 

 fissure, as has taken place at Etna, where the present center is four 

 miles from the old one, and in the Island of Vulcano with the penin- 

 sula of Vulcanello, which affords the best possible example of such 

 a shifting. Whole systems of volcanoes appear to be built up along 

 the lines of such fissures, as is shown by the linear arrangement of 

 volcanoes in different parts of the earth, and strikingly in the Lipari 

 Islands, where the volcanoes are arranged along a series of lines which 

 doubtless mark rents in the earth's crust, and which radiate from a 

 center at which we have proofs of the former existence of a volcano 

 of enormous dimensions. 



There is also good ground for believing that the great linear bands 

 of volcanoes which, as we shall see, stretch for thousands of miles 

 over the earth, have had their positions determined by great lines of 

 fissure in the earth's crust. While, however, the smaller fissures, upon 

 which rows of scoria-cones are thrown up, seem to have been in many 

 cases opened by a single effort of the volcanic forces, the enormous 

 fissures which traverse so large a portion of the surface of the globe 

 are doubtless the result of numerous manifestations of energy extend- 

 ing over vast periods of time. 



It is very hard accurately to estimate the number of volcanoes in the 

 world. They vary in size from immense mountains like Chimborazo 

 and Cotopaxi to mere holes in the ground, letting escape barely per- 

 ceptible columns of vapor. The history of a large proportion of them 

 has been known only for a brief period, and many which are consid- 

 ered extinct because they have never been seen in action may be 

 merely dormant, ready to burst out at any time, as Vesuvius did in 



