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rocks. When that plain is uplifted en masse to form a plateau it is obvious that epigene action must tend to evolve out of the plateau mountains and ridges which, in their form and alignment, will closely resemble those that existed over the same area before the old plain of erosion had come into existence. The rocks and rock-arrangements, being the same in both cases, must under denudation tend to produce a similar configuration. No doubt there might be certain contrasts, but these would not be due so much to geological structure as to changes in the character of the rocks. The planing away of great mountainmasses might well expose quite a different series of rocks, and these, when the region was again uplifted and carved into hill and valley, would doubtless weather differently from the rock-masses under which they formerly lay buried. But the general geological structure remaining the same, mountains and ridges would necessarily be developed along the old lines.
We may now consider the structure of certain plateaux of erosion which there is every reason for believing existed at one time as plains—plains which had previously replaced mountain-systems. A good example is ready to our hand in the Southern Uplands of Scotland—that belt of high ground which is drained by the Clyde, the Doon, and other streams flowing north-west, and by the Cree, the Dee, the Nith, and the Annan flowing south-east. The north-east section of the region is traversed by the Tweed, with an easterly to north-easterly course; while the extreme south-west portion is watered by the Stinchar, flowing in a south-west direction. The whole area drained by those rivers and streams might be described as a broad undulating plateau, furrowed and trenched by narrower and wider valleys. The mountains are somewhat tame and monotonous—flat-topped elevations with broad, rounded shoulders and smooth grassy slopes. The rocks composing the region consist for the most part of greywackes and shales, the former being usually hard greyish-blue rocks arranged in beds of variable thickness. They are much more abundantly developed than the shales which are associated with them, although now and again the latter attain considerable importance. The strata usually dip at high angles, often approaching the vertical, and, the same beds coming again and again to the surface, it is obvious that we are dealing here with a vast succession of steeply inclined and closely pressed anticlinal and synclinal folds. In many natural exposures, as on the coast and in the valleys, the intensely folded character of the strata is clearly revealed. Obviously the strata have been squeezed together, and affected in precisely the same way as the rocks of the Alps. Frequently, indeed, we find that overthrusting has taken place, the rocks having yielded to tangential pressure by shearing. The general trend or " strike" of the strata is from south-west to north-east, while the dip is sometimes north-west, sometimes south-east, changing now and again very rapidly, at other times remaining constant for long distances. In the former case the folds are not infrequently approximately symmetrical; in the latter they are necessarily unsymmetrical. In a word, the geological structure is that which characterises all mountains of elevation like the Alps. Nor can we reasonably doubt that when the folding and fracturing took place the crust bulged up and a series of superficial ridges and hollows—a true mountain-chain—came into existence. That was a very long time ago, however, for the uplift dates back towards the close of Silurian times. Then followed a protracted period of denudation, during which our mountains of folded rocks must have passed through the various stages of adolescence, maturity, and old age. Much of the region was reduced to the condition of a low plain, diversified in part by swelling hills of less and greater height. All this work had been accomplished, and the degraded hills were continuing to crumble away, when the whole region was once more uplifted, and so converted into a table-land or plateau with an undulating surface. This movement of elevation had been completed, and renewed erosion had furrowed and trenched the plateau to some extent, before the beginning of Old Red Sandstone times, for the lowest or bottom beds of the Old Red Sandstone series here and there occupy valleys carved out of the underlying Silurian greywacke and shale. To what extent the plateau was submerged during the Old Red Sandstone period we cannot tell. Probably the submergence was greatest over the north-east portion of the region, for it is
in that quarter that we meet with the most extensive and continuous accumulations of Old Red Sandstone rocks. Be that as it may, we know that some time before the succeeding Carboniferous period re-elevation ensued and a new cycle of erosion was inaugurated, during which the Old Red Sandstone rocks and the underlying Silurian strata were more or less profoundly denuded. Thereafter followed an epoch of renewed subsidence on a more extensive scale, when much of the plateau was drowned in the Carboniferous sea, and marine sediments of that age were distributed over areas which had probably never been overflowed by the waters of Old Red Sandstone times. Judging from the present distribution of the Carboniferous strata, it seems likely that the plateau was, as before, more deeply submerged towards northeast and south-east than in other directions. So far as we can tell, the region has never since been depressed below the sea, but in succeeding Permian and Triassic times long stretches of inland lakes or seas penetrated into the heart of the plateau, occupying hollows which were certainly in existence during the preceding Carboniferous period.
Such, without going into details, is a general outline of the chief changes which have taken place in the Southern Uplands of Scotland. A plateau which came into existence towards the end of the Silurian period might well be expected to show a highly denuded aspect. It is true that during Old Red Sandstone and Carboniferous times it was considerably depressed, and so escaped much erosion, but in the intervals separating those stages denudation must have been in active progress, as it has continued to be since the final disappearance of marine conditions. No doubt much rock has been removed from the whole surface of the region in question. Not only have wide and deep valleys been excavated, but the broad-backed hills and mountains can hardly fail to have been greatly reduced in height. It is still possible, however, to trace the general configuration of the original surface. The average slope of the plateau appears to have been towards the south-east. This is indicated by the direction of the principal rivers—the Annan, the Nith, the Ken, and the Cree. It is further shown by the distribution of the Old Red Sandstone and later geological formations. Thus strata of Old Red Standstone and Carboniferous age occupy the Merse and the lower reaches of Teviotdale,