The vitrification of the rampart which on every side incloses the grassy area has been more variously, but less satisfactorily, accounted for than the green luxuriance within. It was held by Pennant to be an effect of volcanic fire, and that the walls of this and all our other vitrified strongholds are simply the crater-rims of extinct volcanoes,--a hypothesis wholly as untenable in reference to the hill-forts as to the lime-kilns of the country: the vitrified forts are as little volcanic as the vitrified kilns. Williams, the author of the "Mineral Kingdom," and one of our earlier British geologists, after deciding, on data which his peculiar pursuits enabled him to collect and weigh, that 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.