In examining the limestone of the mainland, we must commence with Sutherland, as it contains the most extensive tracts of this rock. The westernmost of these is found at I)iurness, occupying the bay to a considerable extent inland, and forming a small island off its entrance. Though we have here enumerated this with the primary limestones, it is rather our opinion that it belongs to the secondary, and is ana logous to the lowest of these, or to the mountain limestone of English geologists. It contains a sub terranean cavern of some interest, called Smow, giv ing passage to a river which plunges into it about a mile inland, to re-appear at the sea shore.
In Loch Eribol there is another tract of the same rock, but of less extent, most conspicuous in a small island at the upper part of this bay. Another of still greater extent. and among the largest in Scotland, is found in the parish of Assynt; and we have already noticed it as formerly wrought for ornamental mar bles. This portion occupies the whole of the valley in which it lies, connected with quartz rock and gneiss like the former; and the ornamental varieties are a black bituminous limestone of a small crystal line grain, varied with red or white calcareous spar. Not very far from this, at Lead Beg, there is a much smaller tract, which produces some pure white mar ble. An attempt was also made to work this; but no large or useful blocks were ever raised.
We must now pass over a great number of small strata scattered here and there in various places south of this, because we find it utterly impossible to indi cate their situations: but we must here remark, part ly as an excuse for this omission, and partly as a geological fact, that such strata are very often not a great many yards wide, and most frequently can scarcely be pursued for half a mile; while having no geographical or political marks near them, there is no mode of indicating their places. For this reason wte can only say generally, that such strata occur in various parts of the mountainous districts, generally associated with gneiss, as that is the prevailing rock, and sometimes with mica slate or quartz rock, very rarely with granite.
The most conspicuous among the few which we can point out, lie above Glen Kindic, near Portsoy ; or Coreen, near Loch Laggan, near Balahulish; or Ran noch, near Aviemore; and so on. In our account of
PERTHSHIRE, we have already detailed at some length the most extensive of those which occur in the mid dle and south Highlands, commencing to the north of Braemar in Aberdeenshire, stretching through Glen Tilt and Blair, aiid crossing by various branches over to Loch Earn. Other parallel branches are found to the south of Killicrankie, in Strath Airdle, and various other parts; the whole presenting a singular prolonged range, combined with an equally singular interruption and ramification.
On the western shore, some slender portions arc found in various places; and a tolerably conspicuous one occurs in Knapdale, occupying among other places the small island of Dana. Of other strata scattered in this neighbourhood through Cantyre, we shall no tice that only near Campbelltown, conspicuous for its extent, and also for the singularity of its crystalline texture. Yet we shall not neglect that of Appin, which may he considered as connected with the great tract of this rock found in Lismore, and which, with little effort of geological continuity, may be consi dered as prolonged to Isla.
The indications of primary limestone in the south of Scotland are so very rare and slender, that they arc undeserving of notice.
'We now arrive at the secondary rocks, and, as first in order, at the lowest or old red sandstone. This occupies a very conspicuous, and often a very continu ous extent in Scotland; hut we shall commence from the north, and with the Shetland islands.
Here, the whole island of Fouls, with the excep tions of granite and gneiss lately mentioned, is an en tire mass of this rock, rising to a mountainous height. On the eastern coast of the Mainland of Shetland it also forms a continuous tract, and occurs in a scat tered manner in one or two other places on the west ern shore. The Orkney isles must be considered as formed fundamentally of this same rock. Among these, Fair isle is conspicuous from the cliffs by which its stratification is displayed, and from• the compara tive purity in which it exists. In all the rest it is intermixed with shales, and often to such an extent, that this far predominates, while many of the islands possess no sandstone whatever. here also its cha racter is considerably different from what it is in Shetland, being more tender, and commonly of a dark grey or red.