202 EEACTION OF THE INTERIOR OP THE EARTH 



globe no doubt occasion the slight and perfectly harmless 

 tremblings of the crust of the earth lasting several days, (such 

 as those which were experienced in 1816 at Scaccia in Sicily, 

 before the volcanic elevation of the new island of Julia), as well 

 as those terrible explosions which are accompanied by loud 

 noises. Bui the focus of the action, the seat of the moving 

 force, is placed deep below the crust of the earth, and we 

 can as little judge of the depth as we can of the chemical 

 nature of the fluids so powerfully compressed. At the edge 

 of the crater of Vesuvius, and on the towering cliff which 

 rises above the great abyss of the crater of the Pichincha 

 near Quito, I have felt periodical and very regular shocks, oc- 

 curring from twenty to thirty seconds before the eruption of 

 the incandescent scoriee or gases. The shocks were greatest 

 when the explosions were at long intervals, and when, there- 

 fore, the gases were longer in accumulation. Tin's simple 

 experience, confirmed by many travellers, contains the 

 general solution of the phenomenon. Active volcanoes may 

 be regarded as safety-valves for the country in their im- 

 mediate vicinity. The danger increases when the openings 

 of the volcanoes are stopped, and the free communication 

 with the atmosphere impeded ; but the destruction of Lisbon, 

 of Caraccas, of Lima, of Cashmeer in 1554,( 192 ) and of so 

 many towns of Calabria, Syria, and Asia Minor, shews that 

 on the whole the most violent shocks do not usually take 

 place in the vicinity of still active volcanoes. 



As the impeded activity of volcanoes influences the force of 

 earthquakes, so do the latter react on volcanic phenomena. 

 The opening of fissures favours the elevation of cones or 

 craters of eruption, and the chemical processes which take 

 place in the cones by free contact with the atmosphere* 



