14 ABOUT VOLCANOS AND EARTHQUAKES. 



know what happens when a crack takes place in a high- 

 pressure steam-boiler, with what violence the contents 

 escape, and what havoc takes place. Now there is no 

 doubt that among the minerals of the subterranean 

 world, there is water in abundance, and sulphur, and 

 many other vaporizable substances, all kept subdued 

 and repressed by the enormous pressure. Let this pres- 

 sure be relieved, and forth they rush, and the nearer 

 they approach the surface the more they expand, and 

 the greater is the explosive force they acquire; till at 

 length, after more or fewer preparatory shocks, each 

 accompanied with progressive weakening of the over- 

 lying strata, the surface finally breaks up, and forth rushes 

 the imprisoned power, with all the awful violence of a 

 volcanic eruption. 



(21.) Certainly a volcano does seem to be a very bad 

 neighbour ; and yet it affords a compensation in the ex- 

 traordinary richness of the volcanic soil, and the fertil- 

 izing quality of the ashes thrown out. The flanks of 

 Somma (the exterior crater of Vesuvius) are covered 

 with vineyards producing wonderful wine, and whoever 

 has visited Naples, will not fail to be astonished at the 

 productiveness of the volcanized territory as contrasted 

 with the barrenness of the limestone rocks bordering on 

 it. There you will see the amazing sight (as an English 

 farmer would call it) of a triple crop growing at once 

 on the same soil ; a vineyard, an orchard, and a corn- 

 field all in one. A magnificent wheat crop, five or six 

 feet high, overhung with clustering grape-vines swinging 

 from one apple or pear tree to another in the most luxn- 



