428 On the ROCKS in the 
Ir has already been obferved, that there are {pots 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- 
featly foft ftate. In Ineland, 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 {peck 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- 
cumftances, poflefled of the fame power of communicating its 
moifture to the furrounding mafles. It is therefore impoflible 
to conceive, how it fhould have withheld it in one inftance, and 
parted with it fo amply in another, how it fhould have indu- 
rated the fandftone, and left the thin feams of clay ina 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 hypothefis, it in- 
duration diftin@ly depends, on the compofition of the ftrata ex- 
pofed to the influence of heat. Some {trata 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, 
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