as applied to the Origin of Mountains. 313 



passed from the molten to the solid state ; for this would involve 

 the liberation of that amount of heat which would be necessary to 

 convert the solid rock into a molten state at the same temperature. 

 If applied to any period during which such changes of phj^sical 

 state took place our equations of conduction woixld simply be 

 talking nonsense ; so that the first important point assumed by 

 Lord Kelvin is that, during the whole period to which he applies 

 his reasoning, the mass under consideration is supposed to be in the 

 same physical state, and therefore, of course, at all times solid. 



Another point to be especialljr noticed, is that there is supposed 

 to be nothing in the nature of discontinuity throughout the mass, 

 or, in other words, that heat is freely conducted from layer to layer. 

 In consequence, any portion of the mass exhibiting such pheuomena 

 would not satisfy the only condition on which our equations can be 

 used ; so that when applied to such portions our equations cannot 

 be regarded as telling us the absolute truth. Now, as far as the 

 interior of the earth is concerned, we can no doubt assume perfect 

 continuity with sufficient exactness ; but when we approach the 

 surface we come upon very heterogeneous rocks, separated by 

 bedding and other separation planes, so that we introduce an 

 element of discontinuity. 



This new element will not, to any appreciable extent, affect the 

 general considerations put forward by Lord Kelvin, but will, on 

 the other hand, render results deduced from our equations in the 

 neighbourhood of the surface necessarily fallacious. 



I attach some considerable importance to this condition as tending 

 to vitiate the practical value of results concerning a level of no- 

 strain to which I shall refer later, and also as exerting a conservative 

 influence in the matter of the outflow of heat, and, in consequence, 

 as tending to lengthen the time from first consolidation. 



Lastly, for purposes of calculation we require to know a certain 

 average value of the coefficient of thermometric conductivity, and 

 this involves the two considerations of conductivity and specific 

 heat. Of the difficulties attending such a determination, it is only 

 necessary to point out that the variations of these two physical 

 quantities with pressure and temperature are as yet very imperfectly 

 known, and that, consequently, the assumption of any such co- 

 efficient for the unknown rocks of the interior must necessarily be 

 an extremely tentative one. But, supposing such an average co- 

 efficient to have been found, it obviously would not agree at all 

 closely with that for rocks near the surface ; for, in its very 

 character as an average coefficient, it must essentially refer to the 

 vast bulk of the interior rocks, and not to the comparatively insig- 

 nificant crust. 



Thus we see another reason why, though our equations may give 

 us exact results with regard to the interior, they cannot, for that 

 very reason, apply rigidly to rocks near the surface. In fine we 

 are not justified in applying Fourier's equations of conduction to 

 rocks near the surface, or, at all events, not equations involving the 

 same constant of conductivity as we have used for the interior. 



