hat they are _not_
volcanic, broached the theory, still prevalent, as their name testifies,
that they are artificial structures, in which vitrescency was designedly
induced, in order to cement into solid masses accumulations of loose
materials. Lord Woodhouselee advocated an opposite view. Resting on the
fact that the vitrification is but of partial occurrence, be held that
it had been produced, not of design by the builders of the forts, but in
the process of their demolition by a besieging enemy, who, finding, as
he premised, a large portion of the ramparts composed of wood, had
succeeded in setting them on fire. This hypothesis, however, seems quite
as untenable as that of Pennant. Fires not unfrequently occur in cities,
among crowded groups of houses, where walls of stone are surrounded by a
much greater profusion of dry woodwork than could possibly have entered
into the composition of the ramparts of a hill-fort; but who ever saw,
after a city fire, masses of wall from eight to ten feet in thickness
fused throughout? The sandstone columns of the aisles of the Old
Greyfriars in Edinburgh, surrounded by the woodwork of the galleries,
the flooring, the seating, and the roof, were wasted, during the fire
which destroyed the pile, into mere skeletons of their former selves;
but though originally not more than three feet in diameter, they
exhibited no marks of vitrescency. And it does not seem in the least
probable that the stonework of the Knock Farril rampart could, if
surrounded by wood at all, have been surrounded by an amount equally
great, in proportion to its mass, as that which enveloped the
aisle-columns of the Old Greyfriars.

The late Sir George Mackenzie of Coul adopted yet a fourth view. He
held that the vitrification is simply an effect of the ancient
beacon-fires kindled to warn the country of an invading enemy. But how
account, on this hypothesis, for ramparts continuous, as in the case of
Knock Farril, all round the hill? A powerful fire long kept up might
well fuse a heap of loose stones into a solid mass; the bonfire lighted
on the summit of Arthur Seat in 1842, to welcome the Queen on her first
visit to Scotland, particularly fused numerous detached fragments of
basalt, and imparted, in some spots to the depth of about half an inch,
a vesicular structure to the solid rock beneath. But no fire, however
powerful, could have constructed a rampart running without break for
several hundred feet round an in