Corresjjondence — Mr. T. Meliard Reade. 381 



argillaceous than it, the grey sliale having a considerable admixture 

 of arenaceous matter and mica. The difference, however, between 

 these two was so slight that, in the present preliminary researches, 

 when much must be allowed to error, it may be left out of con- 

 sideration altogether. It would appear, then, from these facts, that 

 a certain compactness, accompanied by cleavage, is favourable to the 

 passage of heat through rocks ; and if it be admitted that what is 

 true for small thicknesses is also true for great ones, we may be 

 justified in supposing that the vast masses of clay-slate, and perhaps 

 to a still greater extent their more metamorphosed and crystallized 

 schists (which we know to extend to great depths), are so many 

 points of weakness which must have their influence in the secular 

 cooling of the earth. On the other hand, points of resistance may 

 be assumed to exist, and to be formed by the great sedimentary 

 accumulations of shale, and probably also of clay and other 

 argillaceous unaltered rocks. In a column, therefore, composed in 

 part of cleaved clay-slate and in part of shale, the easy passage of 

 the internal heat outward through the first would be checked 

 through the other in the ratio, roughly speaking, of five to eight. 

 This "becomes a stupendous difference when we apply it to the 

 thicknesses we are acquainted with. If we imagine a thick covering 

 of shale or clay or some other rock with a very loio conductivity 

 which has arrested in its course the heat passing up to it through 

 underlying rocks with a high degree of conductivity — if we imagine 

 such a surface-covering removed (as we know that they frequently 

 have been) by denudation, it is evident that the equilibrium of the 

 heat-resisting covering of the earth will be altered, not only at this 

 particular spot but also wherever the material removed is being 

 redeposited." ^ 



It is obvious that the drying and consolidation of sediment goes 

 on concurrently with its increasing depth, and it is probable that 

 the piles of horizontal sediment miles thick which are laid down as 

 the materials out of which future mountain-ranges are built possess 

 a lower conductivity than the substratum of the crust, largely 

 crystalline, upon which they rest. 



But whether these sediments are better or worse conductors of 

 heat than the average crust does not in the least affect the principle 

 of their action as intercepters and accumulators of the heat out- 

 flowing from the nucleus ; and if Mr. Davison had correctly 

 apprehended this fact, instead of confusing the issue by the 

 repetition of a fundamental misstatement, we might not have 

 been favoured with No. 2 note on the "Expansion Theory of 

 Mountain Evolution." 



Pabk Corner, T. Mellard Eeade, C.E., F.G.S. 



Blundellsands, Liverpool. 



^ British Assoc. Report, 1873, Appendix p. 226 : " Notes on the Conducting-power 

 of certain Rocks," by Herschel and Lebour. 



