A NEW THEORY OF VOLCANOES. 203 



A NEW THEORY OF VOLCANOES. 



THERE are few subjects less satisfactorily treated in scientific 

 treatises than that which Humboldt calls the Reaction of the 

 Earth's Interior. We find, not merely in the configuration of the 

 earth's crust, but in actual and very remarkable phenomena, evidence 

 of subterranean forces of great activity, and the problems suggested 

 seem in no sense impracticable, yet no theory of the earth's volcanic 

 energy has yet gained general acceptance. While the astronomer 

 tells us of the constitution of orbs millions of times farther away than 

 our own sun, the geologist has hitherto been unable to g'ive an account 

 of the forces which agitate the crust of the orb on which we live. 



A theory has just been put forward respecting volcanic energy, 

 however, by the eminent seismologist Mallet, which promises not 

 merely to take the place of all others, but to gain a degree of accept- 

 ance which has not been accorded to any theory previously enunciated. 

 It is, in principle, exceedingly simple, though many of the details (into 

 which we do not propose to enter) involve questions of considerable 

 difficulty. 



Let us, in the first place, consider briefly the various explanations 

 which had been already advanced. There was first the chemical 

 theory of volcanic energy, the favorite theory of Sir Humphry Davy. 

 It is possible to produce on a small scale nearly all the phenomena due 

 to subterranean activity, by simply bringing together certain sub- 

 stances, and leaving them to undergo the chemical changes due to 

 their association. As a familiar instance of explosive action thus oc- 

 casioned, we need only mention the results experienced when any one, 

 unfamiliar with the methods of treating lime, endeavors over-hastily 

 to "slake" or "slack" it with water. Indeed, one of the strong 

 points of the chemical theory consisted in the circumstance that vol- 

 canoes only occur where water can reach the subterranean regions 

 or as Mallet expresses it, that " without water there is no volcano." 

 But the theory is disposed of by the fact, now generally admitted, that 

 the chemical energies of ou'r earth's materials were almost wholly ex- 

 hausted before the surface was consolidated. 



Another inviting theory is that according to which the earth is re- 

 garded as a mere shell of solid matter surrounding a molten nucleus. 

 There is every reason to believe that the whole interior of the earth 

 is in a state of intense heat ; and if the increase of heat with depth 

 (as shown in our mines) is supposed to continue uniformly, we find 

 that at very moderate depths a degree of heat must prevail sufficient 

 to liquefy any known solids under ordinary conditions. But the con- 

 ditions under which matter exists a few miles only below the surface 

 of the earth are not ordinary ; the pressure enormously exceeds any 



