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schistose, they nevertheless sometimes tend to be carved into ridges and ranges, marking the outcrops of the less readily reduced masses. More frequently, however, owing to the direction given to the drainage by the original slopes of the surface, or to the uniform character of the rocks, or, it may be, to complex geological structure, all trace of any definite linear arrangement disappears-parallel ranges and intervening hollows are replaced by amorphous groups of heights and irregularly diverging or radiating valleys. This is frequently due to the presence of great masses of plutonic rocks, such as granite. Igneous intrusions of one kind or another, indeed, often play a not unimportant role in giving variety to the surface of such regions. Lastly, we may note that when the flexured rocks of a plateau are arranged in symmetrical folds, the synclines, by offering greater resistance to denudation than the adjacent anticlines, tend to be developed into synclinal mountains.

As examples of tabular and pyramidal relict mountains we may cite the Red Sandstone Hills of Sutherland, Ingleborough in Yorkshire, the picturesque and often fantastic hills of Saxon Switzerland, the basaltheights of the Faröe Islands and Iceland, and the buttes and mesas of the Colorado Plateau. Throughout the Lowlands of Scotland we meet with diversified features, all the elevations being of subsequent formation, or the result of denudation. The Lowlands are, in short, a plain of erosion, the surface of which has been greatly modified by epigene action. The

more prominent knolls, hills, heights, and ranges of all kinds mark the outcrops of the relatively hard rocks, which in most cases are of igneous origin. Many of the isolated knolls and abrupt eminences are the necks of ancient volcanoes, and these are usually scattered irregularly without reference to the dip of the surrounding strata. Most of the bolder crags and escarpments, however, are formed by the outcrops of sheets and beds of basalt, etc. As the dip is continually changing, such escarpments face almost every point of the compass. When the strike

is more persistent the outcrops of volcanic and intrusive rocks often form considerable ranges, such, for example, as the Ochils, the Sidlaws, the Pentlands, the Bathgate Hills, the Campsie Hills, and others. All these heights might be termed escarpment-hills. So again the outcrops of the calcareous Mesozoic strata of England form still more persistent ranges of escarpment-hills, traversing the country from N.N.E. to S.S.W. The Moors and Wolds of Yorkshire, the Cotswolds, the Chiltern Hills, and the Downs are examples. In all these cases the dip of the strata is moderate. In highly eroded regions of steeply inclined strata the surface-features are sometimes regular, showing a succession of parallel mountain-ranges with intervening hollows. Sometimes, however, they are more or less irregular, the hills and mountains being grouped together without any trace of linear arrangement. The Highlands of

Scotland to some extent illustrate the former class

of relict mountains, the general trend of the ranges and intervening depressions of certain areas being S.W. and N.E. In the Southern Uplands the same linear arrangement is occasionally apparent, but hardly so marked as in some parts of the Highlands. The difference is probably in chief measure due to the fact that throughout the Southern Uplands the rocks show little variety, while in the Highlands the reverse is the case, zones and belts of very different kinds of rock alternating.

The forms assumed by the relict mountains of a highly denuded plateau of erosion do not necessarily differ from those of similarly constructed tectonic mountains. The folded mountains of a region of uplift, after long-continued denudation, eventually become greatly modified, the dominant elevations no longer coinciding with anticlinal axes, but with the outcrops of the more resisting rock-masses, and now and again with synclinal axes. Such highly modified

tectonic mountains, from a certain point of view, might be described as mountains of circumdenudation, but it is better to distinguish them. They should be recognised as tectonic mountains through all the various stages of erosion, until they are reduced to their base-level. Should such a plain of erosion become a plateau, the mountains eventually carved out of it might well repeat the forms and the arrangements of the antecedent tectonic mountains, but they would be true relict mountains-the dominant portions of a highly degraded plateau.

4. Valleys. The term valley has various significations. Usually we mean by it the hollow through which a stream or river flows. But some valleys contain no streams; they are mere elongated depressions—either narrow or broad, shallow or deep. Naturally, however, all depressions in the surface of a land which is not rainless tend to be filled or traversed by running water. By far the great majority of valleys-using the word in its widest meaning-are either the direct result of erosion, or have been greatly modified by it. Nevertheless, not a few valleys owe their origin to other causes. In short, we can recognise at least two kinds of valleys, viz., (a) valleys which have been formed either by hypogene action or by epigene action other than that of running water; and (b) valleys which are true hollows of erosion. These we shall briefly describe as original or tectonic valleys, and subsequent or erosion valleys.

(a) Original or Tectonic Valleys. Of these we distinguish two kinds-valleys which owe their origin to the irregular accumulation or heaping up of materials at the surface, and valleys which are the result of crustal deformation. The former class, or constructional valleys as they may be termed, are of comparatively little importance. They occur sometimes in volcanic regions as depressions in the surface of the various volcanic accumulations, or as hollows separating adjacent cones, sheets of lava, or heaps of ejecta. Similarly the depression lying between lines and ranges of dunes and moraines may be termed

constructional valleys. Sometimes such valleys trend for miles in one and the same direction; more usually, perhaps, they are winding, short, and interrupted. In a word, any hollows at the surface produced by the irregular distribution of materials, whether by volcanic action or by epigene action of any kind, we should class as constructional valleys.

Of much more importance are deformation-valleys. Theoretically we may group these as (1) dislocationvalleys and (2) synclinal valleys. But not infrequently a deformation-valley has been determined partly by fracture and partly by flexure, such as the valley of the Jordan. Dislocation-valleys may extend for long distances between parallel faults, or they may follow the line of one great dislocation alone. Valleys of this kind are approximately straight or gently curved, and are of not infrequent occurrence. The valley of Glen App in Ayrshire and the great hollow traversed by the Caledonian Canal are good examples. The valley of the Rhine between the Vosges and the Black Forest is another. Synclinal valleys, as might have been expected, are best developed in mountains of recent uplift, where the surface-features not infrequently coincide more or less closely with the underground rock-structure. Such valleys naturally trend in the same general direction as the mountains amongst which they occur.

Original or tectonic valleys of all kinds are, of course, liable to modification by erosion. Many constructional valleys, it is true, are dry, and in the

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