We must cursorily refer to some other fortifications which really scarcely belong to our subject, though certain archæologists claim for them a prehistoric origin. We refer to the vitrified forts, which are strange structures in which stones, such as granite and gneiss, quartzite and basalt, have been subjected to a heat so intense as to produce vitrification. These vitrified forts are _enceintes_, generally of round or elliptical form, carefully erected where they were most needed for defence, and protected by one or more ramparts.28 The ramparts all bear traces of vitrification, more or less complete, which has, so to speak, cemented them together. The vitrification is very unequal, being complete in some parts and scarcely noticeable in others. It is evident that the builders did not know how to direct their fire uniformly. Ever since 1777 vitrified forts have been known in Scotland, and until 1837 they were supposed to exist nowhere else. About that time, however, Professor Zippe called attention to similar ruins in Bohemia, and later it was announced that discoveries of the same kind had been made in various parts of France, Denmark, and Norway. Virchow speaks of the _Schlaken Wälle_, or ramparts of vitrified scoria, near Kern29 and Schaafhausen, and gave an account of them at a meeting of German naturalists at Ratisbon. It would be easy to multiply instances. Vitrified walls are known in the Puy-de-Dôme, in which the facing is of clay, and draught flues, for regulating and fanning the flames, have been made out. At Castel-Sarrazin is a camp refuge with similar dispositions,30 and recently Daubree presented to the Académie des Sciences a piece of porphyry artificially vitrified from the prehistoric _enceinte_ of Hartmannswiller Kopf in Upper Alsace.31 It is in Scotland, however, that are situated the most remarkable vitrified forts. A few years ago no less than forty-four were counted. The most celebrated are those of Barry Hill and Castle Spynie in Invernesshire, Top-O-Noth in Aberdeen, and a small fort which rises from a lofty rock in the midst of the Strait of Bute. Vitrified cairns also occur in the Orkney Islands, notably on the little isle of Sanday, but the most interesting structures of the kind are Craig Phœdrick and Ord Hill of Kissock, which rise up like huge pillars on the hills at the entrance of Moray Firth, at a distance of three miles from each other.32