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line drawn from Stonehaven in a south-west direction to the Clyde near Helensburgh is at once the geological and geographical boundary of Highlands and Lowlands, while a similar line extending from Dunbar to the coast of Ayrshire near Girvan forms the corresponding boundary of the Lowlands and the
*W Gos/orrt. S.Z.
Owl ft Burn.
Fio. 68. Section On A True Scale Across " Tynedale Fault," Newcastle Coal-field.
Southern Uplands. The lines in question are great dislocations, having in places downthrows of 5000 to 6000 feet. But there can be no doubt that the inequalities at the surface are due not so much to the amount of vertical displacement as to the different character of the rocks on opposite sides of the faults. This is well shown by the fact that the disparity of level along a line of dislocation varies with the character of the rocks which are brought into juxtaposition. Thus, when soft sandstone, as in Strathmore, abut against hard crystalline rocks, the latter■ rise more or less abruptly above the former—the line of demarcation between Highlands and Lowlands is of the fault. However prolonged and intense epigene action may have been, such a fault will nevertheless cause a marked feature at the surface, so long as the general surface of the land remains considerably above the base-level. But when the latter is approached denudation will eventually cease on the low side of the fault, while material will continue to be removed from the high side, and the disparity between the two will thus tend gradually to disappear. In short, the irregularities of surface determined by the presence of faults pass through the same cycle of changes as all other kinds of geological structure. Should the base-level remain undisturbed epigene action must eventually reduce every inequality, no matter what its origin may have been. Again, were such a reduced land-surface to be re-elevated and converted into a plateau, the lines of dislocation that happened to separate areas of hard rock from regions of soft rock would once more determine the boundaries between high and low ground. The surface of the soft rocks would be lowered most readily, while the more durable hard rocks would come to form elevations. In a word, the features that obtained before the land was reduced to base-level would, under the influence of denudation, tend to re-appear.
Fig. 69. Section Across Great Fault Bounding The Highlands
A, "hard " grits and shales; x, relatively " soft " sandstones, etc. Demarcation between
strongly pronounced. But when, as between the val-
Normal faults, as we have seen, have often determined the boundaries between lowlands and highlands. Not infrequently, indeed, it can be shown
Fig. 70. Section Across Great Fault Bounding The
A, " hard " greywackes, etc.; /, "hard" igneous rocks and overlying conglomerate c.
that the dominance of certain mountains is due rather to the sinking down of adjacent low-lying tracts than to bulging up of the crust within the mountain-areas
Fig. 71. Diagram Shction Across Horstgebirge.
a, granite, gneiss, etc., forming the " Horst" ; £, stratified rocks of relatively late age, resting
upon a, dropped down along lines of dislocation //; 0, outlier of b, showing that
the strata b were formerly continuous between A and B.
themselves. Such mountains are, of course, bounded by faults, and are known to German geologists as Horste or Rumpfgcbirgc, the Harz being a good example. The Horste of Middle Europe are composed for the most part of crystalline schists and Palaeozoic rocks, more or less highly flexed and disturbed. The
mountains usually rise somewhat suddenly above the surface of the relatively undisturbed and approximately horizontal Mesozoic strata of the adjacent low grounds, and for a long time it was supposed that these strata in the immediate vicinity of the Horste were littoral deposits. Such, however, is not the case. They are of relatively deep-water origin, and, before faulting supervened, may have covered much of the high lands which now overlook them. It is obvious, in short, that the Horste represent portions of the crust which have maintained their position; they are mountains which testify to a former higher crustal level; the surrounding tracts have broken away from them, and dropped to a lower position.
Probably enough has now been advanced to show that normal faults have had no inconsiderable share in determining surface-features. This, as might have been expected, is most conspicuous in regions of recent crustal deformation and fracture, where epigene action has not had time to effect much modification. In cas.es of very ancient fracture and displacement, however, the surf ace-features, as we have seen, are very greatly modified, and if well-marked disparity of level is still often met with along lines of dislocation, this is mainly due to the fact that rocks of unequal endurance have been brought into juxtaposition. In a case of very considerable displacement it will usually happen, indeed, that crystalline schists, plutonic rocks, or hard Palaeozoic strata will occur upon the high side and relatively softer strata on the low side