IS CHI A AND ITS EARTHQUAKES. 



25 



of this whole space is now concentrated at Vesuvius, and is manifested 

 at other places in the vicinity only by the emanations and thermal 

 springs of which we have spoken, and from time to time, during pe- 

 riods when the volcano is inactive, by violent shocks, of which the 

 terrible disaster of the 28th of July, at Ischia, has just given an im- 

 pressive example. 



Previous to the Christian era, Vesuvius, covered with a rich vege- 

 tation, was wholly inactive. Nothing except the form of the mountain 

 could give a suspicion of the intensity of the fires that were raging 

 beneath it. Volcanic activity, then localized in the Phlegrean fields, 



rni volcanic tutas 



ionsL^I (scoria) 



a tLLUV 



V C'JATEHKIM 3 PLIOCthE 



^Jlavas E23TReHTie tufas 



2 EOCENE LCRETACE0U3 



Fig. 1. Bat op Naples. Geological Map showing the Relations op Ischia with the 



Phlegrean Fields. 



attained its maximum in Ischia, which was its escape-valve during the 

 entire period of Vesuvian quiet. It produced then, through the action 

 of a large number of eruptions taking place within a period of several 

 thousand years, a considerable island, which now rises more than eight 

 hundred metres, or two thousand six hundred feet, above the level of 

 the sea. It is eighty kilometres, or a little less than fifty miles, in cir- 

 cumference at the level of the sea, eight kilometres, or not quite five 

 miles, long from east to west, and eight kilometres, or about three 

 miles, broad. From its center rises Mount Epomeo, which, crowned 

 by an abrupt, semicircular rampart, which is nothing else than the 

 eastern edge of the grand crater, whence have issued all the trachytic 

 projections that now form the greater part of the island, presents the 

 somber aspect of a fire-vomiting mountain. This crater has never 

 given out lavas. Built on masses of pumiceous tufas of slight con- 



