The last two portions arc those already alluded to as found in the south beneath the red marl, and as we need take no further notice of these, so we shall say no more of this formation than that some very trifling portions of it are found in different places, as in Fife, in Forfar, in Clydesdale, and very unexpectedly on the north-western shore of the Highlands. In one or two places also, it is accompanied by equally minute por tions of the magnesia]] limestone.
Thus we arrive at the last and uppermost of the Scottish strata, though we ought here to remark what we before omitted, namely, that in some places that limestone which in England immediately follows the red sandstone beneath the coal, also occurs in Scot land, though apparently less commonly. Thus, how ever, it may be seen to the south of Edinburgh, in various parts of Fife, and in the west.
It remains to notice the trap and porphyry of Scot land.
The greatest mass of porphyry is that which lies in the neighbourhood of Inverary, occupying a scattered but considerable space; and next to that is the mass which occurs in Glenco, well known, a dependency of which may be conceived to form the summit of Ben Nevis. We know of no other extensive masses which appear of equal antiquity, and the veins are far too numerous and scattered to admit of description. All the other prophvries we must rank with the later trap rocks.
Of these, Shetland possesses one portion in the island of Papa Stour; but the Western islands display a great extent of the rocks of this class. St. Kilda, is thus formed of augitic grcenstone and syenite. Sky is entirely formed of the same rocks, with the excep tion of the south-eastern quarter, and of another small portion at Sf:rathaird. Here there are all the varieties of this family, and among the rest, that rare substance hypersthenc rock, already mentioned. As dependent on Sky, we may name Rasay, partly formed of por phyry, the Shiant isles, Canna, Egg, Rum, and Muck. Egg also contains pitchstone; and its celebrated Scuir was formerly noticed.
Mull is the next great trap district in the islands, being almost entirely formed of it, and including the adjoining islands, excepting Inch Kenneth, of which Staffa is especially noted. To the south, Kerrera is
also chiefly formed of trap, with part of Seil; but af ter this we meet with nothing.further than veins, till we arrive at Arran, about one half of which consists of the rocks of this class. In the Clyde, Ailsa, the smaller Cumbray, and the southern point of Bute, are of the same materials.
In the Highlands, the chief trap district is Airdna murchan towards the west, and Morven both of them in this sense dependencies on Mull. The minutes portions occurring on the borders of the High land mountains, can only be mentioned thus slightly. But to the south of these, in the middle district, the great range of the Sidlaw displays many insulated por tions of it, as does this country generally; the mass becoming more continuous near Perth, to be renewed in the Ochil, and again in the Campsic Hills, till it meets the highland mountains. The northern shores of Fife display a smaller range; and through this whole county even to Stirling, a crowd of summits and fragments impossible to specify, bespeak the once greater continuity of an extensive central tract of this rock. Thus also, North Berwick Law, Inchkeith, Arthur's Seat, the Pentland, and many more summits which need not be named, indicate the former wider connexions and extent of a mass once lying over the coal field here, and reaching to the westward till it is renewed in a more continuous manner to the west of Glasgow and the confines of Ayr.
Such is a sketch of the positions of a rock, which nothing but a detailed and coloured map could ren der intelligible; while, for want of this, we must also omit all notice of the innumerable veins which main tain this general connexion, and indicate the former greater extent of trap in Scotland. In the south the rocks of this class become rare; and we need only here notice the Eildon Hills, and the Cheviot scarcely appertaining to Scotland.
The following list of some of the rarer Scottish mi nerals was drawn up by the writer of this article, and was first published in Dr. Brewster's Journal of Sci ence, vol. i. p. 225..