4.28 On tht ROCKS in the 



It has already been obferved, that there are fpots on Salif- 

 bury Craig, where this is not fo apparent as in others, and it 

 very often happens, that fmall feams of clay occur, in a per- 

 fectly foft ftate. In Ireland, at Scrabo, in the county of Down, 

 and at Fairhead in that of Antrim, I found fandftone in the 

 former, cut by veins, and in the latter, overlayed by a bed 

 300 feet thick, where no induration was to be feen. Now, it 

 appears conclutive, that there could not have been a deficiency 

 of induration in any fpeck of Salifbury Craig, far lefs a total 

 abfence, as in the cafes quoted in Ireland, had it in any inftance 

 been effected either by percolation, or by the intermixture of 

 the matter of the vein. The fuperincumbent or included mat- 

 ter, if in a fluid ftate, whatever its chemical powers were, 

 would, to a certain extent, act mechanically, and be, in all cir- 

 cumstances, poflefled of the fame power of communicating its 

 moifture to the unrounding maffes. It is therefore impoffible 

 to conceive, how it fhould have withheld it in one inflance, and 

 parted with it fo amply in another, how it fhould have indu- 

 rated the fandftone, and left the thin feams of clay in a foft 

 and friable ftate. It is quite unimportant, of what bafe the 

 fandftone may be formed ; it is a fubftance, allowed as above 

 to be generally porous, (and, in the cafes alluded to, it certain- 

 ly was fo) ; into that porofity, therefore, the fluid muft have 

 percolated, whatever the bafe may have been. 



On the contrary, according to the Huttonian hypothecs, in- 

 duration diftinctly depends, on the compofkion of the ftrata ex- 

 pofed to the influence of heat. Some ftrata may either whol- 

 ly, or in part, be capable of refifting much higher temperatures 

 than others. It is confequently to the ingredients of which 

 they are formed, that we muft look either for the caufe of in- 

 duration, or the abfence of it. This remark originated in ob- 



ferving, 



