OF A GEOLOGIST. 369 



fore, than that the forts of eminences such as Craig Phadrig 

 and Knock Farril, originally mere inclosures of loose, unce- 

 mented stones, may belong to a period not less ancient than 

 that of the first barbarous wars of Scotland, when, though 

 tribe battled with tribe in fierce warfare, like the red men 

 of the West with their brethren ere the European had land- 

 ed on their shores, navigation was yet in so immature a state 

 in Northern Europe as to secure to them an exemption from 

 foreign invasion. In an after age, however, when the roving 

 Vikingr had become formidable, many of the eminences ori- 

 ginally selected, from their inaccessibility, as sites for hill- 

 forts, would come to be chosen, from tlieir prominence in tlie 

 landscape, as stations for beacon-fires. And of course the 

 previously erected ramparts, higher always than the inclosed 

 areas, would furnish on such hills the conspicuous points from 

 which the fires could be best seen. Let us suppose, then, 

 that the rampart-crested eminence of Knock Farril, seen on 

 every side for many miles, has become in the age of northern 

 invasion one of the beacon-posts of the district, and that large 

 fires, abundantly supplied with fuel by the woods of a forest- 

 covered country, and blown at times into intense heat by the 

 strong winds so frequent in that upper stratum of air into 

 which the summit penetrates, have been kindled some six or 

 eight times on some prominent point of the rampart, raised, 

 mayhap, many centuries before. At first the heat has failed 

 to tell on the stubborn quartz and feldspar which forms the 

 preponderating material of the gneisses, granites, quartz 

 rocks, and coarse conglomerate sandstones on which it has 

 been brought to operate ; but each fire throws down into the 

 interstices a considerable amount of the fixed salt of the wood, 

 till at length the heap has become charged with a strong flux ; 

 and then one powerful fire more, fanned to a white heat by 

 a keen, dry breeze, reduces the whole into a semi-fluid mass, 

 The same effects have been produced on the materials of the 



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