344 G. Poulett-Scrope — Source of Volcanic Heat. 



body can raise the temperature of another by conductive transfer- 

 ence of part of its heat to a temperature as high as its own, and 

 therefore, if the subcortical matter of our globe" (as I have supposed) 

 " passes locally under varying conditions of heat and pressure .... 

 to a liquid or even vaporous state, there must be matter more 

 deeply situated at a still higher temperature, i.e. there must he a 

 liquid nucleus.'" Was there ever such a non sequitur as is contained 

 in this last phrase ? It lays down as a law in physics that " no 

 body " can by conduction reduce another body to fusion, unless it 

 be itself fused — a proposition which every plumber's apprentice 

 and every boy who melts ice with a hot poker practically refute. 

 But if we charitably give Mr. Mallet credit for not intending to 

 broach this palpable absurdity (although his words imply it), but 

 suppose him to have in view not all bodies, but only subterranean 

 rocky matter, can he deny that some rocky strata may, owing to 

 their peculiar composition or structure, or permeation with water, 

 or other conceivable conditions, enter into fusion or igneo-aqueous 

 plasticity at a somewhat lower temperature than that at which some 

 other subjacent or adjacent rock retains its solidity ? Or again, will 

 Mr. Mallet deny that pressure may raise the fusing point of rocky 

 matter composing a deeply buried stratum or zone, so as to maintain 

 it in a solid state at a higher temperature than that at which a body 

 of the same material above it, but exposed to a less amount of 

 pressure, will enter into fusion ? The whole passage I believe to 

 be as false in physics as it is dogmatic in tone. And yet Mr. Mallet 

 looks down with supreme contempt, from what he calls "the lofty 

 point of Thermo-dynamics " (p. 147, op. cit.), upon "the erroneous 

 notions on these subjects commonly current among geologists" ! 



Mr. Mallet dismisses me with a challenge to reduce my views as 

 to the origin of volcanic heat to " a definition," such as that which 

 figures conspicuously in p. 167 of his paper. It is of course easy to 

 give an imposing air to an opinion or conjecture by printing it in 

 italics, with the title of " Definition " above it. And it is perhaps 

 natural, after doing so, to fancy that you have thereby demonstrated 

 its truth — as Mr. Mallet repeatedly affirms that he has done. But it 

 I'emains after all a mere guess — more or less plausible according to 

 the arguments, observations, or experiments by which it can be 

 supported. I have already said that I am not prepared to dogmatize 

 on this obscure and difficult question ; but in my recent letter in 

 the Geol. Mag. for May, 1874, p. 237, I briefly stated what I will 

 here repeat, my preference over Mr. Mallet's theory as to the ex- 

 clusive origin of volcanic heat in the crushing of superficial rocks as 

 they follow downwards a shrinking nucleus, of the opinion that it 

 (volcanic heat) is derived chiefly and directly by conduction or 

 convection, or both, from that intensely heated interior mass of the 

 globe upon which Mr. Mallet's own theory rests as a postulate. 

 Let me briefly dissect Mr. Mallet's " Definition." 

 That the globe is a condensed nebula still gradually cooling from a 

 state effusion, is an hypothesis only; though, I admit, a plausible one. 

 That its nucleus is consequently shrinking, is another hyj)othesis, 



