The geography of this rock is very extensive and very scattered, so as to render it impossible to follow the whole of its localities without a very lengthened detail, and the aid of a coloured map. And we must also here premise tat the geological term, clay slate, does not always imply roofing slate, though this ma terial would unquestionably be found in many places where it has never yet been sought.
It abounds in the southern promontory of Shetland, and is also found scattered in many places to the northward, and in many of the smaller islands. The greatest tract here is that which reaches from Sum burgh-head northwards to Scalloway and beyond it. For the others we shall refer to the accurate documents already pointed out. In the Western islands there is a slender tract of this rock on the eastern shore of North Uist, and the adjoining spots; but the only con spicuous tract is that which accompanies the quartz rock already described, of Jura, and the remainder of that chain. This was formerly noticed in a commer cial view. The principal mass occupies the small islands there described, with all those smaller ones lie in the strait that separates these from the chain of Jura. On this side it skirts the eastern shore, even to the extremity of Isla, where it might also be wrought; and it moreover appears on the west ern side of the latter island, in various and extensive strata.
In Arran it accompanies the granite; and in Bute and Inch Marnoch, where it is also wrought, it must be conceived to belong to that extensive range which traverses Scotland; and which we may now describe, as it is impossible to pursue any useful order in this description. This tract is of various breadth, but seldom exceeds a mile or two; and it extends in a tole rably straight line to the east shore, passing through the points formerly indicated as quarries. This is not, however, an entire mass of clay slate, but consists of various schistose rocks, among which that substance seldom occupies more than a third or fourth part. The whole belt is defined with tolerable accuracy on both sides; though, without a coloured map, we could not lay down, nor even approximate to its limits. It remains for those to whom its course and extent are thus pointed out, to render it of far more value than it has yet been, by opening quarries in many other places, where a populous and adjoining country would ensure a regular and sufficient demand.
On the west shore there is a small portion of clay slate near the Crinan canal, which, geologically pos sesses a certain connexion with that of the slate islands of this coast, and tvhich might be wrought were there any demand; but we need not point out any more of these trifling strata till we reach that at Balahulish. The extent of this rock is, in this vici nity, more considerable than is apprehended, but as the convenience of the Glenco quarries supersedes any other, it is not likely to be ever more widely wrought.
The very small quantity of slate which occurs occa sionally on the western shore, as near Loch Carron and elsewhere, commonly interstratificd with gneiss, renders it of no value; and, as a piece of geological topography, it is impossible to define them : but among these we must notice one small portion in Loch Eribol, which might probably be turned to the advantage at least of the surrounding country, where the poor cottagers arc often much troubled to find the means of covering their houses. On the eastern side of Sutherland there is also some slate, near the quartz rock; but no attempts have been made to work it.
Passing the Moray Firth, on this side, we find vari ous beds of slate traversing the country to the south of Cullen and Banff; and some well-known portions of this appear at Portsoy, though no attempts have been made to work it. The topography of these scattered portions is liere inexplicable, but we soon arrive at a much more extensive mass, which stretches eastward from Banff, and penetrates a long way south into the interior country. This is a tract already worked, and which might be wrought much more extensively in this rich and populous county, were its existence through so large a space suspected. It is now quar ried in Fouldan Hill, possibly elsewhere, and its pro duce is of a very good quality. To geological science abstractedly, some of its connexions are interesting; but the details would take us out of the rigid path of the present topographical sketch of Scottish geo logy.