Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/282

272 a design to vitrify can be deduced from this specimen, except that of the great heat required to fuse it, which applies as well to this case as to that of Dun Mac Sniochain.

Having in these two instances detected the existence of a vitrifiable substance in the rocks from which the walls were constructed. I was in hopes that all the other vitrified forts would be found to occur in the vicinity of vitrifiable rocks. No mineralogical notice has accompanied the accounts of those which have been observed in Aberdeenshire, in Ross-shire, and other situations, nor had I an opportunity of inspecting them. But I have since learnt that three or four exist in Arisaik, a country consisting of gneiss and granite rocks only. The refractory nature of these substances would almost lead us to doubt that the buildings are actually vitrified, unless hornblende or other unnoticed vitrifiable matters abound in them.

It is but of late that similar structures have been detected in the southern parts of Scotland. Three of them are found in Galloway; but I had an opportunity of examining only that which lies in the parish of Amworth. It is a rectangular and simple wall, occupying the summit of a steep and strong but low hill, and exhibiting the usual general appearances.

As the whole of this part of the country consists of common grauwacke and grauwacke-slate, I was I confess incredulous about the reported vitrification, on account of the refractory nature of those substances.

On examining the wall, it appeared that although it bore very generally the marks of fire, the vitrification had occurred in very few places and in distant patches. I was at a loss to account for this circumstance, till on accurate examination of the surrounding rocks, I found some places where the grauwacke assumed a peculiar character, exhibiting distinct grains of imbedded carbonate of lime,