Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/271

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Dr. Mac Culloch on the Vitrified Forts of Scotland.

was done with a view to strengthen the work, or else that it was an effect of the rude manner in which they were supposed to be erected. It would have been an extraordinary system of defence which should have heaped up a pile of loose stones on the outside of a wall. Modern warfare is satisfied when its ordnance has produced such an addition to the face of an enemy's bastion. A little attention also to the angle which loose stones assume when they are at liberty, might have shown that such a system would not only have prevented the defenders from approaching their own walls, but would in fact in small works, such as those of this fort, have occupied a very considerable portion of the included area.

It is the dilapidation of the unconsolidated parts of the building which has produced this appearance. The thickness of the walls of this fort, as nearly as it can be appreciated, is, as I have already stated, twelve feet. They bear the marks of vitrification throughout their whole extent, but in some places it is more complete than in others. In no case does it seem to have extended more than a foot or two from the foundation, and the most perfect slags are found at the bottom of the wall. As we proceed upwards, we find a mixture of porous slag with stones which having been but partially fused have adhered together in a mass. Higher still we meet with stones which though unvitrified are roasted by the action of the heat, and at length the marks of fire diminish until they almost entirely disappear, leaving only a heap of loose and unconnected stones. The loose part of the wall having fallen through time, has caused that accumulation of rubbish which we find about the vitrified parts. On account of this mixed construction, we have no means of ascertaining the original height of these works; but if a judgment may be formed from the quantity of loose stones which are found at the base of the walls, it was