Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/279

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269
Dr. Mac Culloch on the Vitrified Forts of Scotland.

adduced are sufficient to prove that some of the fused substances must have been brought to that condition in a heat not less than 60 degrees or upwards of Wedgwood's scale. Such then, at least, is one temperature at which the walls of this fort have been fused. It may have been much greater. It is perfectly evident that if a temperature of 60° existed in one part of the wall, pyrites lying near it must have been decomposed. There could be no such discordance of temperature existing simultaneously, and so near, in a mass of this construction. Hence then it follows, that the wall could not have put on its present appearance by one heating, if it were all actually built previously to the application of the heat. This precludes the possibility of the supposition contained in Mr. Tytler's hypothesis. Had the fire, which he supposes the cause of vitrification, been produced by the burning down of the wooden part of the compound wall which he has imagined, it could not have happened that a vitrification requiring a temperature of 60° should have taken place in one part, while in another such a substance as pyrites remained unchanged. The great heat requisite to effect the vitrification of the pudding stone, is an additional argument against this hypothesis, as it could not have been produced by any quantity of wood capable of entering into such a wall, unless the wood had predominated to an extent incompatible with any idea we can form of its architecture. It is not indeed easy to conceive a plan capable of producing these effects, and certainly none more feasible than the suggestion of Mr. Williams. With him I should rather be inclined to suppose that a sort of furnace was constructed of a double earthen wall, in which the materials were placed, with such a quantity of wood as was sufficient to excite a strong heat, and that this operation was repeated till the wall had gained its wished for elevation. The earthen furnace in which the Africans fuse their ores, seems to countenance this supposition. The imperfect fusion of the upper parts may be