Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/277

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Dr. Mac Culloch on the Vitrified Forts of Scotland.

the application of heat as to have bent and conformed itself to the neighbouring protuberances, undergoing at the same time no great change of texture, unless when much impregnated with iron; an appearance perhaps assisting to confirm that explanation of the contortions of the gneiss beds which attributes this effect to the action of heat.

Very little change appears in the specimens of common slate which I have taken from it. If any limestone has found its way into the wall, it has probably been calcined, and subsequently assisted to bring into fusion the refractory earths. It is to the pudding-stone however, that the main part of the vitrification is to be attributed. Without this it would have been only a mass of burnt rocks, and specimens of it may be taken from the wall in every state, from that of a black glass, to a spongy scoria capable of floating in water. There are also many pieces, which having been exposed to a lower heat, exhibit a gradual succession of changes, from incipient calcination to complete fusion. This therefore is the cement of the building; and it has been so mixed through the whole, that there is scarcely a part (I speak of the foundation) which has not been united into a continuous mass by the fusion of this substance. The last stone of which the changes are worth noticing, is the pyritical slate. In general it has become disintegrated in consequence of the sublimation of the sulphur contained in the pyrites. But many specimens may be taken from the wall, where the pyrites has felt no change, proving evidently that it has scarcely undergone the action of heat. In the vitrification therefore of the pudding-stone, and the integrity of the pyrites, we are furnished with the two extreme points of temperature under which this work has been raised. How these are to be reconciled is a new difficulty. It is unnecessary to examine the highest temperature at which pyrites can maintain its

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