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clinal troughs forming conspicuous mountains, while the intermediate anticlines correspond for the most part with valleys and depressions.
If it be true, therefore, that the denudax tion of young mountains, such as the Alps and the Carpathians, has been guided and determined to a large extent by geological structure, we ought to meet with still > stronger evidence of a like kind in mountg , ain-ranges of greater antiquity. The g g mountain-systems we have been considero ^ ing are of Csnozoic age; they are among o | the latest great upheavals of the world. We see in the Appalachian Chain of North < » America a very much older system, for it came into existence about the close of Palaeozoic times. Being of such enormous antiquity, the Appalachians ought to give H « evidence of correspondingly great denudation. All the weak geological structures should have collapsed and disappeared 2 ages ago; the heights ought not to coincide with anticlines. The accompanying section across a portion of the chain in Pennsylvania shows that this has actually £ happened, symmetrical synclines having as usual developed into hills, while anticlines have been degraded.
Similar evidence might be adduced from
many other regions, but enough has been advanced to show that in the process of erosion and denudation of mountains of uplift, anticlines, as compared with synclines, are essentially weak structures. When the flexures are symmetrical the synclines tend to be carved into hills, but when the axes are inclined the strata often give rise to a series of prominent escarpments or to a succession of ridges with intervening hollows, the escarpments and ridges corresponding to the outcrops of the more resistant rocks. (Fig. 55.) Comparing mountain-chain with mountain-chain, we find, as might have been expected, that the oldest mountains, if they are the least prominent, are at the same time the most stable. They have endured so long that much of their primeval elevation has been lost; the weakly built structures have been demolished, and only the stronger now remain. Great rock-falls and landslips are therefore seldom heard of among such mountains. It is quite otherwise with the younger uplifts of the globe. The valleys of the Alps, the Caucasus, the Himalayas, the Cordilleras, and other chains of relatively recent age are cumbered with chaotic heaps of fallen rock-masses. From time to time peaks and whole mountain-sides collapse and slide into the valleys; and this rapid degradation will continue until every weak structure has been removed. The hills and mountains of our own country have long since passed through this phase of unstable equilibrium. In the younger mountain-chains of the globe underground structure and superficial configuration still to a certain extent coincide, but in the more ancient and therefore more highly denuded mountainsystems such coincidence is of very rare occurrence. Anticlinal mountains built up of porous and relatively impermeable strata are restricted to regions of recent uplift, and have no long life before them.
We have seen that in the case of plains and plateaux of accumulation the original surface of the ground is an expression of the geological structure, the general direction of their drainage-systems being determined
Fig. 55. Unsymmetrical Folds, Giving Rise To Escarpments And Ridges.
h A, hard beds ; s s, soft beds.
by the average inclination of the strata. The same is no doubt to a large extent true of regions of mountainous uplift; the shape of the surface and the direction of the streams and rivers must at first have been determined by the arrangement or architecture of the rocks. But while it is comparatively easy to realise the conditions that obtained in a plateau-country during the early stages of its existence, it is very much harder to picture to ourselves the general aspect which a mountain-chain must have presented at the time of its upheaval. We are justified by the evidence in believing that the larger inequalities of the surface must often have coincided with corresponding flexures and other deformations of the strata. But we need not suppose that all the convolutions, fractures, and displacements now laid bare in precipice and gorge actually appeared as such at the surface. Laboratory experiments have shown that a great deal of flexing, folding, contortion, and displacement may take place underground, while the surface simply swells up or bulges. And that may quite well have been the case with many mountain-chains. Yet we cannot ignore the possibility or probability that folding and displacement of strata may sometimes have resulted in wholesale rupture and confusion at the surface. We need not wonder, therefore, if we sometimes find it hard to account for certain vagaries in the drainage-systems of mountain-chains. Even the youngest of these chains has experienced so much denudation, that it is often impossible to realise the surface-conditions which may have determined the initial directions of the rivers. The longitudinal watercourses doubtless follow the axial arrangement of the strata, some of them occupying structural hollows (synclines), while others run along the backs of anticlines, or follow the outcrops of relatively softer rocks. The origin of certain transverse river-courses is harder to understand. Some of these may cut across a succession of great ridges; they break through the mountains in such a way as to suggest that they are perhaps following a line of fracture. Most commonly, however, this is certainly not the case. Sometimes it can be shown, as already indicated, that a transverse stream has simply eaten its way back into the heart of the mountain-ridge, which it has eventually breached or "gapped," and so worn down as to encroach upon the drainage-area of some adjacent longitudinal valley. Transverse streams working back in this way have not infrequently captured longitudinal rivers, which thus appear to mysteriously forsake their own valley in order to breakthrough a mountain-ridge. Perhaps most of the sudden changes in direction of Alpine rivers are illustrations of this system of capture. It is possible, however, as some geologists have supposed, that certain transverse river-courses may have been determined by the presence of a series of minor crustal folds, arranged at right angles to the main or longitudinal flexures of a mountain-chain. But we know so little of the actual conditions of surface that obtained when such a chain was being upheaved, that we must often be content to remain in ignorance of the causes that may have led to the sudden deflection of a river across a mountain-ridge. When we bear in mind, however, that the present lines of drainage can agree only in a general way with those that came into existence at the birth of a chain—that many anticlinal arches, now laid bare and deeply eroded, may never have shown at the original surface—it is not hard to understand how certain transverse river-courses may have come to intersect a succession of ridges. In many cases such courses may really indicate the