December 9, 1884.-Sheriff DOVE WILSON, President, in the Chair.

Professor GEORGE PIRIE read a paper entitled, "The Vitrified Fort upon the Tap o' Noth."

The Vitrified Fort upon the Tup o' Noth.

By Professor GEORGE PIRIE.

A VITRIFIED Fort is a space generally in a commanding position surrounded by a peculiar wall. This wall, which is in most cases now in a ruinous condition, is made up of materials which bear the marks of fusion. Sometimes the whole looks like lava; sometimes it consists of common stones cemented by molten matter. These forts are peculiar to Scotland. There are about fifty of them, mostly north of the Forth and Clyde. Attention was first directed to them, by a mining engineer called Williams, in 1777. He pronounced them to be artificial structures raised for the purpose of defence, and this is the theory usually accepted now. Others have attributed them to volcanic agency, and others have supposed that they might have been raised to keep in the materials for a beacon, and then fused by the heat of the fire. Neither of these latter suppositions is tenable. The forts, though resembling volcanic craters, are not such, as is proved by the geology of the hills on which they occur. If, again, they had been the walls of a beacon they would have consisted of stones found on the hill.

The Author mentioned that he had accidently noticed that a compass on being brought to the top of the hill called the Tap o' Noth was deflected from the magnetic north. Having on a later occasion ascended the hill with a prismatic compass and taken the bearings of a very distant point from two points on opposite sides of a vitrified mass, he found that the walls of the fort are strongly magnetic, and that, therefore, the molten material must contain iron. A specimen of the material, consisting of both kinds of rock, was submitted to chemical analysis by Professor Brazier.

The materials had therefore been selected on account of their being fusible, and brought from the Cabrach or the upper valley of the Don, where there is plenty of iron stone. They must have been mixed with the stones found on the hill, and built up in the shape of a thick wall with wood below, between, and around, and the whole set on fire, when the ashes of the wood might act as a flux, and assist in the fusion of the mixture. It is confirmatory of this that the chemical analysis shows the presence of potash. The whole would then form one cemented block of enormous strength, with tolerably perpendicular sides. Some light is thrown by these facts on the date of the fort. It must have been in the iron age, for the material used is worthless for the production of iron, while the treatment of it shows a knowledge of the fusibility of ironstone. must also have been before the use of mortar.

It

The fort on the Tap o' Noth is, roughly, a sloping rectangle. The slope of the floor is about 1 in 20, but this is nearly level compared with the sides of the mountain just outside the walls. Towards the lower end there is a hollow, about 3 feet deep, in which a little water generally lies. The old vitrified walls are in most places nearly covered by masses of stones found on the hill. These form, both outside and inside, a steep slope from six to nine feet in height, and are apparently a more recent fortification. Another circumvallation of the same kind-loose stones-runs round the foot of the cone, and a well-marked road winds up the least steep approach, and makes a break in each wall.

NOTE. I.

The accompanying sketch may be taken as an accurate representation of the central line along the top of the wall,

NOTE II.

A paper has since been read before the Huntly Field Club by Mr. Charles Proctor, of the Inland Revenue Laboratory, on the same subject. His conclusions are important. They are (1) that the fused stone is the same as the stone found on the hill; (2) that the stone on the hill is not magnetic, but that it becomes magnetic on being heated, and again loses its magnetism on being fused.