266 Dr. M A c C u L L O c H o« /^<f Vltrifed Forts of Scotland. 



vitrified wall, they rejected that which was unfit for their purposes, 

 however conveniently placed. Had the object been to erect a dry 

 wall of stone and wood the limestone would have equally answered 

 their intentions. This notion of a design to vitrify seems to receive 

 additional strength from the apparent solicitude and labour employed 

 in introducing so much pudding-stone into the work. It is very 

 likely that accident had taught them the vitrifiable nature of this 

 Ingredient ; a piece of knowledge the more probable, if, as there 

 seems little reason to doubt, they were acquainted with the art of 

 making iron, an art which we need not deny to them when it is 

 known to many of the inhabitants of Africa who are in a very low 

 state of civilization. 



Such are the consequences I would try to deduce from the mine- 

 ralogical considerations belonging to this question. 



It is now proper to examine the changes which the several sub- 

 stances have undergone, that we may, if possible, form some rational 

 conjecture on the degree of heat to which they have been subjected, 

 and on the probable means by which that heat was produced. 



Where the quartz has been most exposed to the fire it has become 

 brittle and easily pulverizable. The granite too is brittle and crumbles 

 to pieces. The gneiss and mica-slate are also rotten where they have 

 contained much iron, in consequence of the oxidation of that metal. 

 Where purer they have remained unchanged, as we might expect 

 from the well known properties of some of the varieties of mica-slate. 

 Often where their flat surfaces have been in contact they are agglu- 

 tinated from the superficial vitrification of the quartz which they 

 contain when united to the potash produced by the fuel. This is 

 also the cause of the glazed surface which covers the clay-slate and 

 which has frequently occasioned numerous small pieces to adhere in 

 one lump. In many places the mica-slate has been so softened by 



