512 E. MALLET ON HIS THEOKY OF VOLCANIC ENEKGY. 



mere "opinion," namely, that the great oceanic and continental 

 areas are on the whole very much unchanged since the period when 

 our earth's crust was extremely thin, he correct or incorrect. The 

 author believes that there is much to support this view, and that 

 the geological circumstances adducible against it present no certain 

 disproof of it ; but he is not called upon here further to refer to it. 

 'Not is it necessary that the author should dwell at any length upon 

 the Eev. 0. Fisher's assertion that in the estimate of the annual 

 supply of heat consumed in supporting existing vulcanicity, Mr. 

 Mallet has not allowed sufficiently for projection of lapilli and dust, 

 nor for corrugation of the earth's crust at present supposed to be 

 taking place, these objections, even if valid, having no direct 

 relation to the Eev. 0. Fisher's objection as to local temperature 

 being insufficient for fusion. The author has doubled the units of 

 heat to allow for waste; and in this, ample allowance is made for 

 dust &c. projected above volcanic summits. In his view there is 

 little or no corrugation now going on, the existing thick crust only 

 admitting of crushing; and even if there were, as the annual 

 demand to supply existing vulcanicity is less than j-^q of the total 

 heat annually dissipated from our globe, we have an ample maga- 

 zine to draw upon for this supposed corrugation, if any such existed 

 (Phil. Trans. 1873, §§ 179-197). The pith of the Eev. 0. Fisher's 

 objections is comprised in the two following extracts in his own 

 words : — 



" Indeed the form in which the objection to Mr. Mallet's reasoning 

 suggested itself to my mind on first reading his paper was simply 

 this. If crushing the rocks can induce fusion, then the cubes experi- 

 mented upon ought to have been fused in the crushing. And I still 

 adhere to this simple mode of expressing my objection." Again, " he 

 considers that the heat so developed may be localized, and that the 

 heat developed by crushing, say ten cubic miles of rock, may fuse 

 one cubic mile; but, I ask, why so? the work is equally distributed 

 throughout. Why should not the heat be so also ? or if not, what 

 determines the localization? For example, suppose a horizontal 

 column ten miles in length and one in sectional area to be 

 crushed by pressure applied at its ends, which of the ten cubic miles 

 is to be the one fused ? But if no cause can assign one rather than 

 another, it is clear that they will all be heated by 170° F.*, and none 

 of them fused." 



If a cube of rock which in free air is found to crush under a 

 certain pressure, be imagined situated deep within a mass of similar 

 rock, and there crushed, it does not admit of dispute that the work 

 necessary to effect crushing must be largely increased ; the particles 

 of the cube and of the entire mass of surrounding rock are under 

 the insistent pressure of the superincumbent rock in a state of 

 elastic equilibrium. It follows therefore that the pressures of the 

 surrounding rock produce the same effect upon the cube, as regards 

 resistance to crushing, as if they were cohesive forces acting within 



( * The mean deduced, apparently, by the Rev. O. Fisher from the lastirve or 

 six experiments in the author's Table I. 



