SECT, xxvi.] VOLCANIC THEORIES. 285 



depth below our feet has been attributed to different causes. 

 By some it is supposed to originate in an ocean of incandes- 

 cent matter, still existing in the central abyss of the earth. 

 Some conceive it to be superficial, and due to chemical action, 

 in strata at no very great depth when compared with the size 

 'of the globe. The more so, as matter on a most extensive 

 scale is passing from old into new combinations, which, if 

 rapidly effected, are capable of producing the most intense 

 heat. According to others, electricity, which is so universally 

 diffused in all its forms throughout the earth, if not the 

 immediate cause of the volcanic phenomena, at least deter- 

 mines the chemical affinities that produce them. It is clear 

 that a subject so involved in mystery must give rise to much 

 speculation, in which every hypothesis is attended with dif- 

 ficulties that observation alone can remove. 



But the views of Mr. Babbage and Sir John Herschel on 

 the general cause of volcanic action, and the changes in the 

 equilibrium of the internal heat of the globe, accord more 

 with the laws of mechanics and radiant caloric than any that 

 have been proposed. The theory of these distinguished 

 philosophers, formed independently of each other, is equally 

 consistent with observed phenomena, whether the earth be a 

 solid crust encompassing a nucleus of liquid lava, or that 

 there is merely a vast reservoir or stratum of melted matter 

 at a moderate depth below the superficial crust. The author 

 is indebted to the kindness of Mr. Lyell for the perusal of a 

 most interesting letter from Sir John Herschel, in which he 

 states his views on the subject. 



Supposing that the globe is merely a solid crust, resting 

 upon fluid or semi-fluid matter, whether extending to the 

 centre or not, the transfer of pressure from one part of its 

 surface to another by the degradation of existing continents, 

 and the formation of new ones, would be sufficient to subvert 

 the equilibrium of heat in the interior, and occasion volcanic 

 irruptions. For, since the internal heat of the earth is trans- 

 mitted outwards by radiation, an accession of new matter on 



