The next rock in order is mica slate; and the boun daries of this are somewhat more simple, at least as to the larger mass, though there are many inferior portions of which it would be impossible to convey an accurate idea.
This greater tract may be considered as commenc ing at the Mull of Cantyre, extending northwards, with exceptions from other intervening rocks, as far as Cruachan, and then stretching across the island so as to he hounded southwards by the declivities of the mountains; while at the northward line, its intermix ture and alternations with other rocks arc so numer ous and variable, that it does not admit of definition. We can only remark, that it is gradually extenuated as it proceeds towards the east, and that it at length disap pears.
Independently of this great mass, mica slate occurs dispersedly in various other places. It is interstrati fied with the gneiss on various occasions, and very particularly when they meet in Perthshire. On the west coast, it occurs similarly among the beds of the same rock; as it does in Arran near the granite, and in Banff and Aberdeenshires, both with the granite and the gneiss.
In the islands, it exists, but is little conspicuous. In Shetland, it is found with the gneiss in various places, and it occurs partially in Sutherland, Caith ness, and Ross, both to the eastward and westward; these latter connexions being similar to what it pos sesses in some of the northern islands. In the south-. ern islands, it is most remarkable in Jura and Scarba; though scanty in both, from its mode of interstratifi cation with quartz rock and clay slate. In Bute, it is conspicuous; but this island is so much a part of the adjoining land, that we scarcely think it worth while to consider it separately.
In the south of Scotland this rock is rare; but it occurs occasionally on the confines of the granite, though never forming a separate and noticeable tract, as far as this country has yet been investigated. Mica slate is of no use; but a soft variety, which is proper ly the talc slate of geologists, has been used in build ing the two magnificent houses of Inverary and Tay mouth.
Quartz rock is the next in general order, and it forms a somewhat conspicuous substance in Scotland, though far inferior to mica slate in the extent which it covers. It is at the same time so scattered that we
must pass suddenly from one part of the country to another totally unconnected, in attempting to indicate its places.
In Shetland it is found on the western shore, occu pying a space which, there, must be esteemed conside rable. In the Western islands, there is a considerable tract of it in Sky; but the chief mass lies in Jura and the remainder of that chain, of which it forms a con spicuous portion. The far larger part of Lunga, Scarba, and Jura, consist of quartz rock; and the Paps arc entirely formed of it. In Isla also it consti tutes the much larger portion of the mountainous dis trict.
On the mainland, it is very conspicuous and abun dant in Sutherland to the westward, and is found dis persedly along the coast of Ross-shire; while in the interior country it also forms many mountain summits, occurring without names and incapable of reference. We already noticed in the geography, the conspicuous ridge of Balloch-nan-fey, and may add that it appears in Canasp and many other hills, as also at the eastern extremities of Loch Torridon, Loch Broom, Loch ?three, and other places in this quarter. On the east of Sutherland it is also found in the mountains of Mohr-ben and Scuir-ben. in Banff, it is equally re markable in the Knock-hill and in many other sum mits; as, further south, it forms the summits of Ben y-gloe and other mountains in this neighbourhood, ex tending in a line westward, far into Breadalbane. We formerly remarked that it had been applied to no uses, and we cannot discover that it is known to the south of the Tay and Clyde.
The next rock, clay slate, is more important from its commercial value, on which we already made some remarks in treating of the commerce and manufac tures of the western districts. It is wrought, besides, in many places in the centre of Scotland; as at Callan der and near it, near Loch Lomond, Comrie, Dunkeld, Blairgo•rie, and elsewhere, as well as in some parts of the south of Scotland.