326 



REPORT—- 1873. 



kinds of wood, of which the conducting-power is between 1 and 2 heat-imits 

 per hour. 



The following is a list of the rocks of which circular sections of the above uniform 

 size have been provided for this examination : — 



Kilkenny fossil marble. 

 Frosterly fossil marble. 

 Cumberland (Dent) marble. 

 Congleton second gritstone. 

 Red Galashiels sandstone. 

 Kenton sandstone. 

 Ileworth sandstone. 

 Prndham sandstone. 

 Fossiliferous black ) from near 



shale. > Xew- 



Common grey shale. ) castle. 



A. S. IIerschel. 



The foregoing observations are not only of very great interest from a purely 

 physical point of view, but I venture to think have a certain geological importance, 

 especially as regards underground temperature and all the numerous geological 

 problems depending on it. Even with the meagre array of actual readings which 

 it has been possible to arrive at in time for this Meeting, certain results have been 

 obtained which give, I think, great promise of the value of these investigations when 

 we carry them on with the modiKed apparatus already described. It will scarcely 

 be necessary at this early stage of the work to do more than call the attention of 

 the Section to its theoretical bearings as regards geology. 



In the first place, it seems to be proved by our experiments that the conducting- 

 power of different rocks varies strictly according to their lithological character, 

 very crj'stalline rocks, such as granite and sei-joentine and statuary marble, allowed 

 heat to pass rapidly through them ; slate plates, with their uucrystalline compact 

 structure, had a still higher degree of conductivity. The crystalline nature of a 

 rock alone is not, therefore, the lithological test of its conductiNity. The lowest 

 powers of conductivity were found to belong, among 1he specimens experimented 

 on, to shale ; the black shale, which was lower than the grey, is softer and more 

 argillaceous than it, the grey shale 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 consideration 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 supposhig that the vast masses of 

 clay-slate, and perhaps to a still greater extent their more metamorphosed and 

 crystalline 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 Ijy 

 the gi-eat 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 tlirough 

 the first would be checked through the other in the ratio, roughlj- speaking, of 5 to 8. 

 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 low conductivity, which has arrested in its course the heat passing 

 up to it through underlying rocks with a high degree of conductivitj- — if we imagine 

 such a surface-covering removed (as we know that they fi-equently 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. V\'e may say, in other words, that we stand nearer 

 the great central source of heat when we stand on slate than we do when we stand 



A 



