Road Metal. 613 



rocks, such sandstones as the Millstone Grit and Gannister 

 beds of the Coal-measures, are ill adapted for macada- 

 mising roads, for traffic rapidly grinds it into its 

 original state of loose sand. Nevertheless, in some 

 regions they have nothing else to use, and to obviate 

 its defects the following process is used near Barnsley 

 and in other parts of Yorkshire. The rocks in question 

 were made from the debris of granites and gneiss, simi- 

 lar to those of the Scotch Highlands. The stone being 

 quarried in small slabs and fragments, is built in a pile 

 about 30 feet square, and 12 or 14 feet high, somewhat 

 loosely ; and while the building is in progress, brush- 

 wood is mingled with the stones, but not in any great 

 quantity. Two thin layers of coal, about 3 inches thick, 

 at equal distances, are, so to speak, interstratified with 

 the sandstones, and a third layer is strewn over the top. 

 At the bottom facing the prevalent wind, an opening 

 about 2 feet high is left, something like the mouth of 

 an oven. Into this brushwood and a little coal is put 

 and lighted. The tire slowly spreads through the whole 

 pile, and continues burning for about six weeks. After 

 cooling the stack is pulled down, and the stones are 

 found to be vitrified. Slabs originally flat have become 

 bent and contorted like gneiss, and stones originally 

 separate, get, so to speak, glued together in the process 

 of vitrification, aided by the soda, potash, and iron, which 

 form part of the constituents of felspar and mica and 

 act as a flux. 



In the year 1859 I visited a vitrified fort called 

 Knockfarril, near Strathpeifer in Ross-shire, ( and came 

 to the conclusion that the vitrification had been done 

 of set purpose, and that the effect had been pro- 

 duced by burning wood.' In the first volume of Dr. 

 John Hill Burton's 'History of Scotland,' 1866, he ex- 



