IS CHI A AND ITS EARTHQUAKES. 29 



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 com- 



Fig. 3. Castle of Ischia. 



posed 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 neigh- 

 boring coast. The phenomena are marked by vertical shocks, acting 

 only upon a definite point, and violent in proportion as they are lim- 

 ited in extent. These shocks are propagated irregularly, without con- 

 tinuity, 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 explo- 

 sion's ; but the illustration would be badly chosen, for these move- 

 ments 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 



