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Page:Popular Science Monthly Volume 24.djvu/39

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all the recorded earthquakes, that of 1881, which is still comparatively-fresh in memory, partly destroyed the city of Casamicciola, which has now been obliterated. It gave a warning by which no one knew how to profit. The constitution of the soil of the island, which is composed

PSM V24 D039 Castle of ischia.jpg
Fig. 3.—Castle of Ischia.

chiefly of trachytic tufas and unconsolidated loose matters, is a considerable element in promoting these disasters.

The Ischian earthquakes are narrowly localized. Their origin is not doubtful, but is readily traceable to the efforts which the lavas and the gases, strongly compressed under the earth, make to escape. Their effects never extend to great distances. The catastrophe which has just consummated the destruction of Casamicciola, already severely shaken in 1881, is a striking example of them. A violent shock, quick as the firing of a cannon, was enough to unsettle and partly destroy the whole northern slope of the island. Procida, which was near it, was shaken, but only a few rumblings in the earth were felt on the neighboring coast. The phenomena are marked by vertical shocks, acting only upon a definite point, and violent in proportion as they are limited in extent. These shocks are propagated irregularly, without continuity, by sudden starts, across the trachytic tufas forming the sub-soil of the island. Slides of the ground are thus produced, which carry off with them cultivated fields and buildings. One is sometimes tempted to compare them, on account of the formidable subterranean sounds that accompany them, and of their suddenness, to mine explosions; but the illustration would be badly chosen, for these movements have never caused a sudden rising of the soil, and there is nothing about them comparable to the disturbances produced by an explosion.

They are rather sinkings down, into a soil already cracked and