acter 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 SE. FIG. 69. SECTION ACROSS GREAT FAULT BOUNDING THE HIGHLANDS h, "hard" grits and shales; s, relatively "soft" sandstones, etc. Demarcation between Highlands and Lowlands well marked. strongly pronounced. But when, as between the valleys of the Earn and the Teith, the hard igneous rocks of the Lowlands are brought against the crystalline schists of the Highlands, the geographical boundary of the two regions is not nearly so well marked the Highland mountains seem to merge gradually into the Lowland hills. And the same phenomena are conspicuously displayed along the margin of the Lowlands and the Southern Uplands. In a word, it is obvious that while the position of the boundaries that separate the Lowlands from the mountain-areas to north and south has been determined by normal faults, the existing configuration is the result of long-continued and profound denudation. The accompanying sketch sections (Figs. 69, 70) will serve to illustrate the foregoing remarks. Normal faults, as we have seen, have often determined the boundaries between lowlands and highlands. Not infrequently, indeed, it can be shown NW. Black Mount Broughton Heights SE FIG. 70. SECTION across GREAT FAULT Bounding tHE h, "hard" greywackés, etc.; ¿, "hard" igneous rocks and overlying conglomerate c. Demarcation between Uplands and Lowlands not well marked. 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 SECTION ACROSS HORSTGEBIRGE. a, granite, gneiss, etc., forming the "Horst"; b, stratified rocks of relatively late age, resting upon a, dropped down along lines of dislocation ƒƒ; o, outlier of b, showing that the strata & 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 Rumpfgebirge, the Harz being a good example. The Horste of Middle Europe are composed for the most part of crystalline schists and Palæozoic 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 cases of very ancient fracture and displacement, however, the surface-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 Palæozoic strata will occur upon the high side and relatively softer strata on the low side. 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. CHAPTER VIII LAND-FORMS DUE DIRECTLY OR INDIRECTLY TO IGNEOUS ACTION OF SURFACE PLUTONIC AND VOLCANIC ROCKS-DEFORMATION -VOLCANOES, STRUCTURE AND FORM OF-MUD-CONES-GEY- IN preceding pages we have had frequent occasion to refer to igneous rocks. These, as we have seen, may be broadly grouped under two heads-Plutonic rocks and Volcanic rocks. The former have cooled and solidified at a less or greater depth below the surface; the latter, on the other hand, have been extruded at or near the surface. No hard and fast line, however, can be drawn between these two groups. All plutonic rocks are indeed intrusive-they have solidified below ground; but the same is true of the sheets and dikes which traverse a volcano, and which, along with the bedded lavas and tuffs they traverse, are properly described as of volcanic origin. It will be understood, then, that the term plutonic is restricted to intrusive rocks which have consolidated at relatively great depths, while the term volcanic includes |