292 Dr Hibbert's Observations on Vitrified Forts. 



with the greater ease, and to suffer the fire to act more forci- 

 bly and equally upon the different parts of the wall as it gra- 

 dually consumed, that they were induced to incline the walls 

 so far from a perpendicular position. In an after period, when 

 the woods had gradually been destroyed, and before it was 

 well known how to manufacture peat for fuel, it would be such 

 a difficult matter to procure fuel in abundance, that build- 

 ings of this kind would come to be disused, and the art in a 

 short period, among a people ignorant of letters, be entirely 

 forgotten.'" 



Such is the explanation of the process of vitrifying, as given 

 by Dr Anderson, with the discovery of which he is so delighted 

 that he patriotically adds, " I am disposed to believe that this 

 has been entirely a British invention, and think it probable that 

 the art was never carried out of this country." He then laboured 

 to prove that so admirable an art was unknown to the Danes, 

 who endeavoured to vitrify some walls round the peninsulated 

 rock of Broughhead ; but having blundered by getting hold 

 of some wrong vitrescible ore, the experiment failed. 



On this view of Dr Anderson nothing more may be remarked, 

 than that it is in contradiction with the real facts concerned in 

 this vitrification. All later observers have agreed, that the 

 fusible matter does not consist of any peculiar vitrescible iron 

 ore found in Scotland, but that it is derived for the most part 

 from the alcaline ingredients of well-known rocks of granite, 

 gneiss, mica-slate, clay-slate, hornblende schist, sandstone, &c. 

 &c. ; these rocks being more or less fusible in proportion to 

 the felspar which they contain. 



4thly, The theory of Lord Woodhouselee, that fire has not 

 been employed in the construction, but towards the demolition of' 

 such forts as display the marks of vitrification. 



In a paper published in the year 1787, in the Transactions 

 of the Royal Society of Edinburgh, Lord Woodhouselee suc- 

 cessfully exposed the weakness of the theory first proposed by 

 Williams, and supported to a certain extent by Anderson and 

 other writers. 



His Lordship proceeded upon the correct ground, that the 



vitrification of forts was a very partial occurrence ; that three 



fortified summits which he examined were crowned with stone 



3 



