Thus there are in this country, as well as every where else, three things to be distinguished; first, the alpine or elevated country; secondly, the flat or low country; and, thirdly, that which has been of posterior formation to the strata which it traverses, in whatever shape or quality; whether as a mountain, or only as a vein; whether as a basaltes, a porphyry, or a granite, or only as a metal, a siliceous substance, or a spar.
Those three things which are here distinguished do not differ with regard to the chemical character of their substances; for, in each of these, every different substance is to be found, more or less; and it is not in being composed of materials peculiar to itself, that makes an alpine country be distinguished from a flat country; it is chiefly in the changes which the strata of the alpine country have been made to undergo, posterior to their original collection, that the rocks of the alpine country differ from those of the flat country.
But the observation that is most to the purpose of the present subject of bituminous strata, is this; it is chiefly in the strata of the flat country that fossil coal are found; there are none that I know of in all the alpine countries of Scotland; and it is always among the strata peculiar to the flat country that fossil coal is found. Now, this appearance cannot be explained by saying that the materials of mineral coal had not existed in the world while those primary strata were formed in the sea. I have already shown, (chap. 4.) that there had been the same system of a world, producing plants, and thus maintaining animals, while the primary strata were formed in the sea; I have even adduced an example of coal strata among those primary schisti, although this be an extremely rare occurrence: Consequently, we are under the necessity of looking out for some other cause.
If the changes which have been evidently superinduced in the strata of alpine countries arise from the repeated operations of subterranean fire, or to the extreme degree in which those strata have been affected by this consolidating and elevating cause, it will be natural to suppose that the bituminous or combustible part among those stratifications, may have been mostly consumed upon some occasion during those various and long continued operations; whereas, in the flat beds of the low country, although there is the most perfect evidence for the exertion of heat in the consolidation of those strata, the general quantity of this has been a little thing, compared with the universal manifestation of this cause in the operations of the alpine countries, the strata of which have been so much displaced in their situations and positions.
To illustrate this, strata of sand-stone are found in both the alpine and flat countries of Scotland. About Leadhills, for example, there are abundance of those strata; but, in the flat country, the generality of the sand-stone is so little changed as to appear to every enlightened naturalist aquiform strata; whereas the most enlightened of those philosophers will not perhaps attribute the same original to a similar composition in the alpine country, which is so much changed from its original state. It is not because there had been wanting a sufficient degree of heat to consolidate the sand-stone in the coal country; for I can show specimens of sand-stone almost contiguous with coal, that have been extremely much consolidated in this manner. But this is only a particular stratum; and the general appearance of the sand-stone, as well as other strata in the coal countries, is that of having been little affected by those subterranean operations of heat by which those bodies in the alpine country have been changed in their structure, shape, and position.
If we shall thus allow the principle of consolidation, consequently also of induration, to have been much exerted upon the strata of the alpine country, and but moderately or little upon those of the low country of Scotland, we shall evidently see one reason, perhaps the only one, for the lesser elevation of the one country above the level of the sea, than the other. This is because the one resists the powers which have been employed in leveling what has been raised from the bottom of the sea, more than the other; consequently, we find more of the one remaining above the level of the sea than of the other.
Let us now take the map of Scotland, in order to observe the mixture of those two different species of countries, whereof the one is generally low and flat, the other high and mountainous; the one more or less provided with fossil coal, the other not.
From St Abb's Head, on the east of Scotland, to the Mull of Galloway, on the west, there runs a ridge of mountains of granite, quartz, and schistus strata, which contain not coal. On each side of this ridge we find coal countries; Northumberland, on the one side, and, on the other, the shires of Ayr, Lanark, and the Lothians; the one is a mountainous country, the others are comparatively low or flat countries. Let us now draw another alpine line from Buchan and Caithness, upon the east, to the island of Jura, on the west; this traverses a mountainous country destitute of coal, and, so far as I know, of any marks of marine bodies. But, on each side of this great alpine ridge, we find the hard country skirted with one which is lower, flatter, or of a softer nature, in which coal is found, upon the one side, in the shires of Fife, Clackmannan, and Stirling; and, on the other, in that hollow which runs from the Murray Frith south-west, in a straight line, directed upon the end of Mull, and composed, for the most part, of water very little above the level of the sea. Here, to be sure, the coal is scarce, or not so evident; but there is coal upon the sea coast in several places of this great Bay betwixt Buchan and Caithness; and the lowness of the country, across this part of the island, is almost sufficient testimony that it had been composed of softer materials.
Thus the coal country of Scotland may be considered as in one band across the island, and included in the counties of Ayr, Lanark, and all those which border upon the Frith of Forth. Now, in all this tract of coal and tender strata, we do not find ridges of alpine stone or primary mountains, but we find many hills of solid rock, little mountains, from 500 to 1000 feet high; such as that beautiful conical hill North Berwick Law, Torpender Law, Arthur's Seat, the Lowmands, and others of inferior note. That is to say, the whole of this included space, both sea and land, has been invaded from below with melted masses of whin-stone, breaking up through the natural strata of the country, and variously embossing the surface of the earth at present, when all the softer materials, with which those subterranean lavas had been covered, are washed away or removed from those summits of the country. Hence there is scarcely a considerable tubercle, with which this country also abounds, that may not be found containing a mass of whin-stone as a nucleus.
But besides those insulated masses of whinstone that form a gradation from a mountain to a single rock, such, for example, as that on which the Castle of Edinburgh is built, we find immense quantities of the same basaltic rock interjected among the natural strata, always breaking and disordering them, but often apparently following their directions for a considerable space with some regularity. We also find dykes of the same substance bisecting the strata like perpendicular veins of rock; and, in some places, we see the connection of these rocks of the same substance, which thus appear to be placed in such a different form in relation to the strata.