318 



On Mr. Mallet's Theory of Volcanic Energy. 



have any appreciable effect in producing the observed inequa- 

 lities of subterranean temperature. 



We appear, then, to be thrown back upon the old explanation 

 of volcanic phenomena, which supposes them to be manifesta- 

 tions of the intense heat existing at great depths and conveyed 

 to the surface through channels of communication. And it 

 appears to me that the results obtained upon making a comparison 

 between the actual inequalities of the earth's surface fand those 

 which would have been formed if the earth had cooled as a solid 

 body, strengthen this supposition*. They have led me to the 

 conclusion that the interior of the earth has been in some con- 

 dition which has admitted of its shrinking much more than it 

 would have done if it had cooled as a solid ) nay probably more 

 than mere cooling alone can account for. I surmise, therefore, 

 that the nucleus has contained water, which has escaped through 

 volcanic vents as superheated steam, and that the earth's volume 

 has been diminished in that manner. By the presence of such 

 highly heated water I would account for volcanic action still. 

 It is known that in several vents lava remains permanently 

 fused without permanently overflowing. This shows that heat 

 is brought up from below, not by continual supplies of melted 

 rock, but by intensely heated gases passing through the lava. 



Under this view any disruption in the crust which is suffi- 

 cient to permit the passage of steam at an enormous pressure, 

 would originate a volcano. And much of the lava poured out 

 might consist of the materials of the crust itself, fused by the 

 passage of the gases through it, and so vary in its composition 

 at different vents, and even at the same vent at different times. 

 Various kinds of disruption of the crust maybe conceived which 





* See a paper by the author " On the Inequalities of the Earth's Sur- 

 face, viewed in connexion with the Secular Cooling," Cambridge Philcso 



o 



phical Transactions, vol. xn. part. 2. 



