Of Stony Subßances. 417 fiuor fpars, with their tranfparency unimpaired which could not happen if they had been expofed even to the lovveft red heat; we muft fuppoie them. therefore, to have been expelled by the mere force of the exploding elaftic fluids, whofe power is fully equal to fuch an effedt, fince when confined they fhake, agitate, and convulfe whole mountains. ... Earthsofthe argillaceous and muriatic genera expofed to the flighter degrees of heat, as clays and fteatites, are barely hardened ; and ftones of thefe genera lofe their luftre and tranfparency. In a greater they become porous. Stones of the filiceous genus alfo lofe their luftre and! tranfpa rency, and become brittle. Granites have been found in beds of lava in different ftates •, thofe on the furface of the bed have had their felfpar fomewhat altered, bring there expofed to adtual flaine. Bnt thofe in the centcr, having never been in fufion, are only rendered more brittle -f-. In the fame manner porpbyries have been m- volved in beds of lava, even porpbyries that ^ have hornblende for their bafis, without having thereby fuffered any alteration an evident proof öf the low degree of heat which generally prevails in volcanos. In vain have fome philofnphcrs of late at- tempted to account for thefe appearances, by aferibing them to the fufion of granites and por phyries under an immenfe prelfure. Before an effetf can with any degree of probn- bility be aferibed to a given caufe, the exiltence * Dolom. Notes, 227* •j- Dolom. Notes, 76.