306 The Rev. 0. Fisher on Mr. Mallet's 



plane, if the column were originally weakest at the point of 

 contact and the parts nearer that always weaker than those 

 more remote, it being also provided that it did not bend or 

 "buckle" anywhere. But the column must in any case be 

 isolated; else the supposed "pushing aside" and surrounding of 

 the end of the column by the " hotter portions " could not take 

 place. It seems to me that we here get very far away from the 

 condition of the "rock-masses of nature." If there be any 

 pushing aside of material, it seems that it must happen to the 

 whole vertical thickness affected, and must manifest itself by- 

 extrusion at the surface of matter not melted ; for the tempera- 

 ture is admittedly not high enough for that in the portions first 

 crushed. 



Looking at the matter thus, it seems that, if fusion of the 

 matter of the earth's crust can be induced by the mechanical 

 consequences of gravitation, it must be, as Professor Hilgard 

 suggests, by the friction of rock-masses amongst themselves, 

 but that surface-movements commensurable with the internal 

 friction must in that case manifest themselves. With this 

 aspect of the question Mr. Mallet has not dealt ; and, as Prof. 

 Hilgard observes, " of the complex thermal effects of the move- 

 ments of detrital masses under great pressure, his figures offer 

 no measure whatsoever." 



Nevertheless on the main question Prof. Hilgard thinks that 

 " Mallet's experiments on the contraction of fused rock in cool- 

 ing, and his estimates of the amount of volcanic energy mani- 

 fested on the globe, coupled with that of the earth's annual loss 

 of heat, completed the proof of the quantitative adequacy of the 

 cause invoked by him." And he adds, "the burden of proof of 

 the qualitative inefficiency of the several modes of action that 

 may come into play, would seem to be effectually thrown upon 

 the opponents of the theory." 



In connexion with this branch of the subject it is important 

 to refer to a paper by Mr. Mallet in the Philosophical Trans- 

 actions, entitled an " Addition to the Paper on Volcanic 

 Energy""*. In this he has made a comparison between his 

 estimate of volcanic energy and the particular store of energy 

 from which he supposes it derived, namely the crushing of the 

 rocky matter of the crust. Upon the basis of Sir W. Thomson's 

 estimate, that the heat annually lost by the earth would melt 

 777 cubic miles of ice at 32° P., he estimates the annual descent 

 of the shell on suppositions of its being 100, 200, 400, 800 

 miles thick respectively. The estimates are founded upon the 

 supposition that the amount of heat so lost is derived from the 



* Phil.Trans.vol.clxv.pt. 1. 



