MALLETS THEORY OF 

 VOLCANOES. 



THERE are few subjects less satisfactorily treated in scien 

 tific treatises than that which Humboldt calls the Reaction 

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

 figuration 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 further 

 away than our own sun, the geologist has hitherto been 

 unable to give an account of the forces which agitate the 

 crust of the orb on which we live. 



The theory put forward respecting volcanic energy, how- 

 ever, by the eminent seismologist Mallet, promises not 

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

 of acceptance which has not been accorded to any theory 

 previously enunciated. It is, in principle, exceedingly 

 simple, though many of the details (into which I 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 

 favourite 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 



