CAUSES OF VOLCANIC ACTION. 



187 



plate such as have been produced by the agency of 

 fire. 



Volcanic action has been defined to be the influ- 

 ence exerted by the heated interior of the earth on its 

 external covering. Under this definition, therefore, 

 may be inciuded ail the subterranean phenomena, 

 whether of volcanoes, or earthquakes, or those in- 

 sensible movements of the land by which, as Mr. 

 Lyell very plausibly supposes, large districts may 

 be depressed or elevated without convulsions. 



As we have already described the nature of vol- 

 canic products, it is unnecessary to allude to them 

 at present ; a few remarks, however, may not be 

 out of place in relation to the different theories 

 which have been formed for the explanation of vol- 

 canic phenomena. 



The prevailing hypothesis in relation to volcanic 

 action is that of Leibnitz, which is supported by 

 Humboldt, Cordier, and, with some modifications, by 

 Philips, and the most eminent geologists of the 

 present day. This regards volcanic action as the 

 necessary result of the influence exerted by the 

 heated interior upon the cooled exterior masses of 

 the globe. " If the earth," says Philips, " be now 

 generally hot within, it must formerly have been 

 hotter; in the process of cooling, the exterior so- 

 lidified part and the interior fluid parts contract un- 

 equally, a general pressure and tension result, and 

 the crust breaks locally to restore the equilibrium. 

 Hence earthquakes and fissures, on some of which 

 volcanic vents are established, which serve more 

 or less to relieve the subterranean pressure, as 

 earthquakes also do. If, in addition to this general 

 view, we suppose the admission of water through 

 fissures to particular parts of the ' ocean of melted 

 rock,' it is easy to see that the observed mechani- 

 CEil phenomena: of volcanoes and earthquakes will 

 result as the effeqt of a local excitement superadded 

 io a general operation." From extensive observa 



