306 Dr Hibberfs Observations on Vitrified Forts. 



sion to which their burning could be applied. Until the thir- 

 teenth or fourteenth centuries, Scotland was in the exact state 

 of Wales, which called for the particular law of Howel Dha. 

 " There are," says this legislator, " three causes of progres- 

 sion for salutation ; land brought into cultivation, festal games, 

 and the burning of forests ; and no person must obstruct such 

 salutation." 



In fact, the effects indicative of immense piles of blazing 

 forest trees, the vitrifying action of which would be heightened 

 by favouring currents of wind, as by a blast furnace of sur- 

 passing intensity, are most truly marvellous, oft-times appear- 

 ing to vie with the result of volcanic incandescence. But the 

 phenemenon will admit of explanation. The stones which are 

 vitrified generally consist of such materials as granite, gneiss, 

 mica slate, clay slate, or the older sandstones and porphyries ; 

 and these are rendered more or less fusible, in proportion to 

 the felspar which they contain in their composition ; — potash 

 being an ingredient of felspar, occasionally to the amount of 

 14 per cent. The stony fragments which have been subjected 

 to this vitrifying process often exhibit effects which are of par- 

 ticular value to the geologist, as they illustrate various circum- 

 stances connected with the agency of heat, in reference to rocks 

 of much theoretical interest, which all ordinary experiments of 

 the laboratory would fail in achieving. Thus, I might men- 

 tion among the most interesting specimens which I collected 

 from vitrified sites, those which show that heat has the power 

 of causing strata of sandstone to assume the precise character 

 of the more crystalline laminae of gneiss, or of rendering gneiss 

 itself prismatic. * 



* The most beautiful and perfect specimens of prismatic gneiss I found 

 some years ago among the vitrified ramparts of an islet (named the Burnt 

 Island) in the Kyles of Bute. They were small, being about 4 inches in 

 length, and about i an inch in diameter, being formed at nearly right angles 

 to the laminar planes of the rock. 



Since reading this paper, I had the pleasure of observing, in the cabinet 

 of M. Von Leonhard, the celebrated Professor of Mineralogy in the Uni- 

 versity of Heidelberg, several specimens from the vitrified sites of Scotland, 

 which he had collected for the geological information that they conveyed 

 to him of the effects of heat upon certain rocks. I also received a few 

 weeks ago the Heidelberg new journal entitled the " Jahrbuch fur Mi- 

 neralogie, Geognosie," &c. conducted by Messrs Von Leonhard and Bronn, 



