294 G. Poulett Scrope—On " Blochf Roch- Surfaces. 



That heat is, and has always been, passmg continually from the 

 interior of the globe into space through the superficial rocks, by 

 conducdon, as well as by the agency of volcanic -eruptions and ex- 

 halations, the upthrust of heated rocky matter, hot springs, etc., is 

 an unquestionable fact. That this escape of heat occasions secular 

 cooling and contraction in the entire globe, is but an hypothesis. It 

 is quite conceivable that the heat so parted with may be restored in 

 some manner, either from within or from without, as, for instance, 

 is the case with the heat of a living animal.^ All the phenomena of 

 Volcanic and Plutonic action are explicable, as I have repeatedly 

 shown, by the local changes of temperature and lateral shifting of 

 heat that must be occasioned in the matter underlying the outer 

 crust by local variations in the thickness and conducting power of 

 the superficial deposits, — producing corresponding variations in the 

 subterranean isothermal planes — which we know to be continually 

 taking place. This, indeed, is admitted by Mr. Fisher, who quotes 

 Mr. Babbage in support of the view that " the deposition of a 

 stratum at the bottom of the ocean would raise the temperature of 

 the rocks underlying the original surface by 2° Fahr. for every 200 

 feet laid down upon it." 



I may mention here that so early as 1825, and consequently some 

 years before the publication of Mr. Babbage's paper in 1834, I had 

 suggested that the very different conductibility of the rocks forming • 

 the crust of the globe — and especially the resistance offered to the 

 escajDc of subterranean heat by the deposition of " sedimentary lime- 

 stones, clays, and sandstones" — must occasion corresponding changes 

 in the temperature of the heated matter beneath, " the internal heat 

 continually endeavouring to put itself in equilibrio." ^ It will be noted 

 that this view is distinct from that of Sir John Herschel (pub- 

 lished in the same year as that of Babbage), which attributed 

 Plutonic and Volcanic movements to variations in the thickness 

 and consequent pressure of the superficial deposits upon a liquid 

 matter beneath, rather than to changes in the amount of internal 

 heat due to variations in the conductibility of the outer covering. 

 Perhaps, indeed, both causes may have co-operated. The increase of 

 local pressure from above may, as already observed, have a mechanical 

 effect in producing lateral movement in the more or less elastic 

 matter beneath. But if the increase of heat, as may well be sup- 

 posed, took place there faster than it could pass off laterally, either 

 by conduction or actual change of place in the highly viscid, if not 

 solid, matter, the results we should anticipate would be local ex- 

 pansion, metamorphism, disturbance, and upthrust, causing super- 

 ficial elevation ; in short, the exact phenomena of which such patent 

 evidences are seen in mountain ranges. 



Without, therefore, denying the possibility of the secular cooling 

 and contraction of the nucleus of the globe, I maintain that the 

 phenomena which it is brought forward to account for can be ex- 

 plained by deductions from ascertained facts without resorting to 

 that hypothesis. 



1 See Lyell's rrinciplos, ed. 1872, p. 233. ^ Volcanos, ed. 1825, p. 30 



