GLOLOGY. 171 



From such a view of the subject we are struck with surprise, that so 

 large a portion of the earth's surface is resting upon subterranean fires. 

 Jt is not a hyperbole to say, that these connected chains constitute but 

 one vast volcano, communicating with the surface and the air, by its 

 more than one hundred vents or craters. 



A second fact, which demands our attention, is tlie intimate con- 

 nection which subsists between volcanic action and earthquakes. The 

 geographical relation of earthquakes immediately raises a presumption 

 of their necessary dependence upon volcanic action. So readily does 

 this conviction arise, that the ancient Greeks and other nations of anti- 

 quity referred them to the same cause. They take place most frequently 

 in volcanic districts. The northern portions of South America and 

 some portions of the districts near the Andes, especially the province 

 of Quito, which has often been represented as resting on one vast vol- 

 cano, are almost constantly more or less shaken with earthquakes. The 

 same is true with reference to the countries in Southern Italy and the 

 adjacent islands. Their intimale connexion is farther shown by the re- 

 markable fact^ that when the volcanic vents are open, and belching forth 

 vapor, fire, and lava, the earthquakes are less violent, than when the vol- 

 canos seem to be inactive. "At Naples and at Messina, at the foot of 

 Cotopaxi and the Tunguragua, earthquakes are dreaded only when va- 

 pors and llames do not issue from the mouth of the volcano. In the 

 kingdom of Quito, the catastrophe of Riobamba led many of tlie well 

 informed inhabitants to believe tliat this unfortunate country would be 

 less often disturbed, if the subterranean fire would succeed in destroying 

 the dome of porphyry of Chimborazo, which prevents eruptions fiom 

 its top, and if this colossal mountain should become an active volcano." 

 The reason of these facts seems to be plain \ for when the volcanic 

 forces are repressed and confined, it is reasonable that they should keep 

 the superincumbent crust in a state of violent agitation, and frequently 

 seek egress for themselves, by uplifting and breaking the rocky strata 

 above, but when there is free egress for them through the volcanic vents, 

 they should less disturb the country around. An other fact worthy of 

 remark, and explicable upon the same principles, is that the earth's sur- 

 face is generally more violently disturbed at some distance from the vol- 

 canic vents than at places which are near. Sometimes immediately before 

 the earthquake the volcano ceases its activity. On the morning of No- 

 vember 1st, 1755, on which day the teriible earthquake which destroyed 

 Lisbon and shook one-fouith of the whole northern hemisphere took 

 place, Vesuvius was in a state of excitement, but it became suddenly quiet 

 at the very hour of the shock ; and A'an Hoil relates that the column 



