478 Correspondence — Mr. Robert Mallet. 



C O S-IR-IE S IP O n^ZDE IsT O IE . 



MR. SCROPE'S VIEWS OF VOLCANIC HEAT.i 

 SiK, — My objection to Mr. Scrope's notion of the source of volcanic 

 heat, as enunciated by him in the Geological Magazine for August, 

 page 344, viz. that " volcanic heat is derived chiefly and directly by 

 conduction or convection, or both, from that intensely heated interior 

 mass of the globe," which he says, but erroneously, is necessary to 

 my views, is not so easily se,t aside as Mr. Scrope would have to be 

 believed by the above article. My objection is that this vague 

 notion involves in reality a thin crust and liquid nucleus which Mr. 

 Scrope professes to repudiate, and he rejoins that, with unpardonable 

 ignorance, I have assumed that the liquefied matter filling his reser- 

 voirs and the material of the nucleus have the same melting point. 



My objection does not involve any such assumption, and is equally 

 valid, though the melting point of the nucleus be assumed much 

 above that of the matter filling these reservoirs. What does Mr. 

 Scrope Imoio of the material constituting the deeper portions of our 

 globe or of their fusibility. The only ground for conjecture even as 

 to the latter is derivable from protruded granites, porphyries, elvans, 

 traps, or other ancient fissure-extruded matter, none of which differ 

 greatly in fusibility from modern lavas. But if we assume argumenti 

 gratia that the material of the nticleus at a still greater depth than is 

 indicated b}^ these is fusible only at a temperature twice or thrice 

 that of fused lava, the physical conditions under which alone heat 

 could be conducted from the nucleus to one of these reservoirs 

 through some hundreds of miles of intervening rocky matter, are 

 sufficient to prove that if the lava in the reservoir be thus brought 

 into fusion, the temperature of the nucleus must be so vastly higher 

 that its material, if like anything we are acquainted with, must be in 

 fusion likewise. Is Mr. Scrope aware that, apart from all question 

 of imperfect conductivity, matter heated by conduction from the 

 central parts of a globe decreases in temperature faster than the in- 

 verse square of the radial distance from the centre ? If the heat be 

 transmitted by convection or in vapour, there must be liquid or 

 o-aseous connexion between the nucleus and the i-eservoir, for with- 

 out such, convection is impossible. Thus two of Mr. Scrope's alter- 

 natives directly involve fluidity in the nucleus ; solids do not pass 

 into the state of vapour except through the intermediate stage of 

 liquidity. Thus I reiterate that this notion of reservoirs of matter 

 melted by heat transferred from more highly heated matter, situated at 

 a much greater depth, by conduction or by convection through gaseous 

 or liquid matter, or by both, is only the old notion of a thin crust 

 and liquid nucleus in disguise, and if that be, so cadit questio as to 

 my having misrepresented Mr. Scrope's views. His views are by 

 bis own statement (Geol. Mag. May, page 237-8) entirely different 

 from mine, and it is wholly unimportant to my views what his may 

 be. I have already declined, for want of definition, further dis- 



1 This letter is inserted at Mr. R. Mallet's earnest request. But it is hoped that 

 the discussion will now be allowed to terminate. — Edit. Geol. Mag. 



