1872.] 



Mr. R. Mallet on Volcanic Energy. 



439 



The chemical theory, which owed its partial acceptance chiefly to the 

 fame of Davy, may he dismissed, as all known facts tend to show that 

 the chemical energies of the materials of our globe were almost wholly 

 exhausted prior to the consolidation of its surface. 



The mechanical theory, which finds in a nucleus still in a state of liquid 

 fusion a store of heat and of lava &c, is only tenable on the admission 

 of a very thin solid crust ; and even through a crust of but 30 miles thick 

 it is difficult to see how surface-water is to gain access to the fused nucleus, 

 yet without water there can be no volcano. More recent investigation on 

 the part of mathematicians has been supposed to prove that the earth's 

 crust is not thin. Attaching little value to the calculations as to this 

 based on precession, the author yet concludes, on other grounds, that the 

 solid crust is probably of great thickness, and that, although there is 

 evidence of a nucleus much hotter than the crust, there is no certainty 

 that any part of it remains liquid ; but if so, it is in any case too deep to 

 render it conceivable that surface-water should make its way down to it. 

 The results of geological speculation and of physico-mathematical reason- 

 ing thus oppose each other, so that some source of volcanic heat closer to 

 the surface remains to be sought. The hypothesis to supply this, pro- 

 posed by Hopkins and adopted by some, viz. of isolated subterranean 

 lakes of liquid matter in fusion at no great depth from the surface re- 

 maining fused for ages, surrounded by colder and solid rock, and with 

 (by hypothesis) access of surface-water, the author views as feeble and un- 

 sustainable. 



A source, then, for volcanic heat remains still to be found ; and if found 

 under conditions admitting to it water, especially of the sea, all known 

 phenomena of volcanic action on our earth's surface are explicable. 



The author points out various relations and points of connexion between 

 volcanic phenomena, seismic phenomena, and the lines of mountain ele- 

 vation, which sufficiently indicate that they are all due to the play of one 

 set of cosmical forces, though different in degree of energy, which has 

 been constantly decaying with time. 



He traces the ways in which the contraction of our globe has been met, 

 from the period of its original fluidity to the present state : first, by de- 

 formation of the spheroid, forming generally the ocean-basins and the 

 land ; afterwards by the foldings over and elevations of the thickened crust 

 into mountain-ranges &c. ; and, lastly, by the mechanism, which he points 

 out as giving rise to volcanic action. The theory of mountain-elevation 

 proposed by C. Prevost was the only true one — that which ascribes this to 

 tangential pressures propagated through a solid crust of sufficient thick- 

 ness to transmit them, those pressures being produced by the relative rate 

 of contraction of the nucleus and of the crust; the former being at the 

 higher temperature, and having a higher coefficient of contraction for equal 

 loss of heat, tends to shrink away from beneath the crust, leaving the latter 

 partially unsupported. This, which during a much more rapid rate of 



