Page:Popular Science Monthly Volume 24.djvu/35

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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 periods when the volcano is inactive, by violent shocks, of which the terrible disaster of the 28th of July, at Ischia, has just given an impressive example.

Previous to the Christian era, Vesuvius, covered with a rich vegetation, 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,

PSM V24 D035 Geological map of naples bay and the surrounding area.jpg

Fig. 1.—Bay of Naples. Geological Map showing the Relations of 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 circumference 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-