pertain to the same system of rocks. Yet I have reason to suppose that the beds of it which exist in the district of Aberfoyle, alternate with a sort of intermediate rock, which I am inclined to consider as the latest of the primitive, or the oldest of the transition series, a rock approaching at least as near to the character of graywacke, as to that of micaceous schist, if it be possible, as I suspect it is not, to draw any line between these two classes of rocks. In this case, the same limestone will, like clay slate, bear a share in both these artificial divisions, for artificial I must needs consider divisions which nature has separated by a boundary so feeble and so undefinable.
The craig of Ailsa, from which the specimens now exhibited are taken, resembles the summit of a huge mountain rising abruptly out of the deep. Such in fact it is, and not a mere rock, as its name might induce one to suppose. It shelves rapidly into the sea, and is surrounded by deep water on all sides except the south eastern, where a small beach has been formed by the accumulation of its ruins. I regret that the derangement of my barometer, a derangement unfortunately too frequent, prevented me from ascertaining its altitude; but by comparing its appearance with that of Arthur's seat, and computing from the time it required to ascend it, it cannot be much less than 1000 feet in height. It is called 940 in some of the sea charts, but on what authority I do not know.
Its circumference cannot be less than two miles, and it therefore forms a large island, which is covered with verdure, and is the habitation of gulls, awks, gannets, goats and rabbits. Its shape is round and cumbrous when viewed from the north-west, but when seen from the north it assumes an elegant conical figure.
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