26 Prof. Bischof on the Natural History of 



canos sometimes in circular groupsand sometimes in double lines, 

 is the most decided proof that their action is not dependent on 

 any trifling causes, lying near the surface, but that they are vast 

 and deeply-seated phenomena. Thus, for example, the whole 

 of the high country of Quito is one volcanic hearth, of which 

 the mountains of Pichincha, Cotopaxi, and Tunguragua, form 

 the summits. The subterranean fire breaks out sometimes from 

 one sometimes from another of these vents, which are usually 

 considered as distinct volcanos. The earthquakes, with which 

 America is so dreadfully visited, are also remarkable proofs of 

 the existence of subterranean communications, not only between 

 countries free from volcanos, as has been long known, but also 

 between volcanic hearths situated at a great distance from each 

 other. All these circumstances prove that the forces do not act 

 at the surface of the crust of the earth, but that, proceeding 

 from the interior of our planet, they communicate contempo- 

 raneously by fissures with the most distant points on the sur- 

 face.* 



Two hypotheses may be proposed respecting the causes of vol- 

 canic phenomena. The one supposes them to be occasioned by in- 

 tense chemical actions taking place between bodies having a very 

 great affinity to each other, and by which so great a heat is pro- 

 duced, that lavas melt and are forced to the surface of the earth 

 by the pressure of elastic fluids. According to the other, the 

 earth at a certain depth is at a white heat, and this heat is the 

 chief cause of volcanic phenomena. 



1. The hypothesis f which ascribes volcanic phenomena to intense chemi 

 col action^ shewn to he untenable. 



AVe will not detain our readers with an account of the earlier 

 hypotheses, which derive volcanic phenomena from the action of 

 iron upon sulphur, or from the combustion of pyrites or coal, 

 as their insufficiency is self evident. But Davy's discovery of the 

 metallic bases of the alkalies and earths was considered as 

 throwing a great light on this subject. 



This distinguished philosopher, who instituted some very 

 interesting experiments at Vesuvius during its eruptions in 



* Von Humboldt's Reisen in die Equinoctial Gegenden des neuen Conti- 

 nents, t. i. p. 49G, t. iii, \\ 24, 2G, and 40, offer many instances of this kind. 



