

Major-Gen. McMahon—The " Culm " at Bude. 225 



no doubt, is also needed to aid these changes. Compression is a 

 mechanical source of heat ; but is the supply from this source, I 

 would ask, as important as that due to plutonic causes ? Mr. Harker 

 admits that " under a comparatively small cover of rocks much of 

 the energy " set free by compression " must be lost by conduction." 

 But would not much of it be also lost in the case of deep-seated 

 rocks ? Eocks are bad conductors of heat, but the compression 

 of deep-seated rocks must take place slowly ; and as the amount 

 of heat generated by compression would only be proportional 

 to the reduction of volume, would not much of the heat be lost 

 by conduction ? Mr. Harker makes his voluminal compression 

 depend upon two factors, namely, lateral and vertical pressure, 

 the latter being dependent on the thickness of the cover. When 

 the cover is thin, the energy set free by the lateral pressure may 

 be lost, he explains, by conduction. But a thick cover is not put 

 on suddenly. The deposition of sediment is a slow operation ; 

 and even when lateral pressure is applied after a thick cover has 

 been formed, it would be applied gradually. In the case of rock- 

 mass'es under the conditions supposed by Mr. Harker, namely, those 

 so deeply buried that the lateral pressure would be balanced by 

 the weight of the cover, I cannot conceive of a sudden generation 

 of heat arising from a sudden arrest of motion of the nature of 

 percussion. The heat due to a mechanical cause must be generated 

 by gradual compi*ession ; and where the generation of heat is gradual, 

 the loss of energy by conduction becomes an important element that 

 must not be left out of our calculations. 



As a source of energy it seems to me that plutonic heat supplies 

 all we require. The Rev. 0. Fisher, in his work already referred to, 

 writes : " We have pointed out that, having regard to such depths 

 as artificial excavations reach, the law of increase is on the whole 

 an equable one, amounting on an average to about 1° Fahr. for 

 every 51 feet of descent, if it be not even slightly more rapid." 1 

 The temperature that this rate of increase would give us, at the 

 depths to which we may suppose that the rocks with which we are 

 concerned have been buried, seems sufficient for the work of meta- 

 morphism without our indenting on dynamic agency for our supply 

 of heat. How, moreover, are we to discriminate between plutonic 

 heat and dynamic heat, and say that this, or that, bit of meta- 

 morphic work was done by the energy supplied from a mechanical 

 source alone ? 



Mr. Hutchings, in his reference to M. Spring's experiments, alludes 

 to the "intermixture of minute particles of minerals in the fine silt 

 of which these Bude rocks and similar strata are largely composed," 

 and adds, " all the crushing and grinding has been done in the 

 gentlest and quietest way, and the resulting material has but to lie 

 and await the pressure." But the point 1 desired to make in my 

 paper was that though the material has been finely triturated by 

 Nature's pestle and mortar and judiciously mixed together in Nature's 

 laboratory, and although this mixture has sustained enormous 

 1 I.e. p. 343. 



DECADE III. VOL. VII. — NO. V. 15 



