_Proc. Soc. Antiq. Scot._, vol. ix. p. 396, vol. x. p. 70, vol. xi. p. 298, and vol. xii. p. 13.
The latest examination of Vitrified Forts is contained in a paper by Dr. Edward Hamilton,[103] in which he gives detailed descriptions of two such structures in Arisaig, one of which is situated on a promontory in Loch na Nuagh. It is an irregular oval occupying the whole summit of the promontory, and measuring about 100 feet in length by about 50 feet in breadth. The enclosing wall varies from 6 feet in thickness and 7 feet in height to about 5 feet in thickness and 3 feet in height. In this case also the wall was not vitrified down to the foundation. Underneath the vitrified portion there was a depth of 3 feet of walling formed of water-worn boulders quite unvitrified. The internal part of the upper or vitrified portion of the wall was also unvitrified. From these appearances Dr. Hamilton concludes that the vitrifaction was the result of fire applied to the upper part of the wall externally.
_Archœological Journal_, vol. xxxvii. p. 227.
From a consideration of these examples it is evident that the Vitrified Forts do not differ in any essential point of their character from the forts that are not vitrified—if vitrifaction be not a feature in the method of their construction. The results of former investigations have not produced evidence sufficient to carry the conclusion that the vitrifaction was accomplished at the time of their construction, or that it was a method of construction. The determination of this question lies at the end of an exhaustive investigation, and can only be obtained from evidence furnished by the phenomena of the structures themselves.
There is one fort in Scotland, at Burghead, in Morayshire, which presents the peculiar feature of being partially constructed of logs of oak alternating with layers of stones. The peculiarity of its dry-built stone rampart is thus described by Dr. Macdonald:—“To strengthen it, beams of solid oak (still measuring from 6 to 12 feet in length) take here and there the place of stones, and similar beams inserted end-ways pass into the mass behind.”[104] We only know the Vitrified Forts from their greatly dilapidated ruins, and it is a legitimate object of investigation whether any of them may yet present evidence of having been constructed with logs and stones in the manner exemplified at Burghead. This method of construction is characteristic of the Celtic or Gaulish forts of France. The rampart of Murcens, on the river Lot is constructed like that of Burghead, of unhewn and uncemented stones. In its mass, at regular intervals, there have been laid courses of oak logs disposed longitudinally and transversely as “binders” and “headers.” The spaces between the logs are filled with stones, and where they cross each other the transverse logs are fastened to the longitudinal rows by massive iron nails. There are two rows of logs laid parallel to the face of the wall and a little apart within its thickness, and these are crossed at every 3 or 4 feet by logs lying transversely and extending the whole thickness of the wall, so that their opposite ends appear in its exterior and interior faces. This is repeated at every 3 or 4 feet of the height of the wall. The same method of construction, with a greater proportion of timber to the mass of the wall, appears in the fort of Impernal, also on the river Lot.[105] It is obvious that by the application of fire to ramparts constructed on this principle, a partially scorified and partially vitrified appearance would be given to their ruins.[106] In the early annals the burning of fortified places appears as the common method of reducing them, and the legendary prophecy of the coming of Birnam Wood to Dunsinnane possessed a peculiarly fateful meaning if its walls were built not of stones alone but of stones and logs.