eminences that had been occupied by hill-forts which, from their want of prominence in the general landscape, had not been selected as beacon-stations. And in the intermediate class of cases we have probably ramparts that were only partially vitrified, because some want of fuel in the neighborhood had starved the customary fires, or because fires had to be less frequently kindled upon them than on the more important stations; or, finally, because these hill-forts, from some disadvantage of situation, were no longer used as places of strength, and so the beacon-keepers had no motive to attempt consolidating them throughout by the piecemeal application of the vitrifying agent. But the old Highland mode of accounting for the present appearance of Knock Farril and its vitrified remains is perhaps, after all, quite as good in its way as any of the modes suggested by the philosophers.[20] I spent some time, agreeably enough, beside the rude rampart of Knock Farril, in marking the various appearances exhibited by the fused and semi-fused materials of which it is composed,--the granites, gneisses, mica-schists, hornblendes, clay-slates, and red sandstones of the locality. One piece of rock, containing much lime, I found resolved into a yellow opaque substance, not unlike the coarse earthenware used in the making of ginger-beer bottles; but though it had been so completely molten that it had dropped into a hollow beneath in long viscid trails, it did not contain a single air-vesicle; while another specimen, apparently a piece of fused mica-schist, was so filled with air-cells, that the dividing partitions were scarcely the tenth of a line in thickness. I found bits of schistose gneiss resolved into green glass; the Old Red Sandstone basis of the Conglomerate, which forms the hill, into a semi-metallic scoria, like that of an iron-smelter's furnace; mica into a gray, waxy-looking stone, that scratched glass; and pure white quartz into porcellanic trails of white, that ran in one instance along the face of a darker-colored rock below, like streaks of cream along the sides of a burnt china jug. In one mass of pale large-grained granite I found that the feldspar, though it had acquired a vitreous gloss on the surface, still retained its peculiar rhomboidal cleavage; while the less stubborn quartz around it had become scarce less vesicular and light than a piece of pumice. On some of the other masses there was impressed, as if by a seal, the stamp of pieces of charcoal; and so sharply was the impression retained, that I could detect on the vitreous surface the mark of the yearly growths, and even of the medullary rays, of the wood. In breaking open some of the others, I detected fragments of the charcoal itself, which, hermetically locked up in the rock, had retained all its original carbon. These last reminded me of specimens not unfrequent among the trap-rocks of the Carboniferous and Oolitic systems. From an intrusive overlying wacke in the