Dr. Mac Culloch on the Vitrifed Forts of Scotland. 267 



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 con- 

 tortions 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 spe- 

 cimens 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 con- 

 tinuous mass by the fusion of this substance. The last stone of 

 which the changes are worth noticing, is the pyrltical slate. In 

 general it has become disintegrated in consequence of the sublima- 

 tion 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 diflSculty. It is unnecessary to 

 examine the highest temperature at which pyrites can maintain its 



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