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primeval inclination of the ground, the rivers having cut their way at first without any reference to deeply bviried structures, which were only to be exposed later on during the general process of denudation.
Although we may vainly endeavour to trace the history of all the river-courses of a mountain-chain, we need be in no doubt as to the ultimate fate of the mountains themselves. It is more difficult certainly to discover the various stages in the erosion of a mountain-system than in that of a plateau of accumulation; but we are assured that all elevated lands, whatsoever their origin, tend to be lowered to their base-level. Should that base-level be steadfastly maintained, mountains and plateaux alike must eventually be reduced to the condition of plains of erosion. But the modifications of the surface of a mountainregion developed during the process of erosion are infinitely complex. This is due partly to the very varied composition of the rocks, and partly to the complicated geological structure.
The surface-features of a denuded plateau of accumulation have a general sameness; there is little variety in the form of the hills and mountains—all are more or less pyramidal. In regions of gently inclined and undulating strata the features due to erosion are more diversified, and this diversity becomes greater as the dips of the strata increase and change rapidly in direction. The foothills that flank the base of so many mountains of uplift are composed very often of symmetrically folded strata, but as we pass inwards to the main chain the folds become steeper and unsymmetrical, and the structure is rendered still more complex by vast overthrusts and shearing-planes. As the structural complexity increases, and the rocks are thrown and twisted into every possible position, the surface-features are constantly changing, so as to show, often within narrow limits, every variety of cliff and ridge and peak. We see then that it is geological structure chiefly that determines the form of the ground; and since the inclination, the folding, and the shearing of rocks must be attributed to crustal movement, it is clear that hypogene action has played a most important part in the formation of mountains. We may say with truth that all true mountain-ranges owe their origin to deformation of the crust. But the shape which they ultimately assume is solely the result of erosion. It is hypogene action which provides the rough blocks; it is by epigene action that these are subsequently carved and chiselled, the forms of the sculptured masses being determined by the nature and structure of their materials. In regions of recent uplift, the process of sculpturing, although considerably advanced, has not yet sufficed to obliterate the original or primeval shape of all the masses. But in elevated tracts of great antiquity the land-blocks have been entirely remodelled. In the general lowering of the surface by denudation, mountain-masses have been removed, and what were formerly depressed areas now often appear as dominant elevations. Mountains of recent uplift are characterised by steep profiles, by peaks and knife-edged aretes; the structures are often unstable, and yield readily to the agents of erosion, so that rock-falls and landslips are constantly taking place. In regions of ancient uplift, on the other hand, the profiles are generally softer; peaks and sharp-crested ridges are of less frequent occurrence, weak structures have disappeared, and the degradation of the mountains does not advance so rapidly. The levelling process, however, though slower, is quite apparent. The valleys are widened and deepened, the mountains crumble down, and, should the base-level of erosion be retained, the whole area will eventually be flattened out and resolved into a plain of erosion.
Such then are the several stages through which a region of mountain-uplift must pass. First comes the stage of youth, when the surface configuration corresponds more or less closely with the underground structure. Next succeeds the stage of middle-life, when such coincidence is all but obliterated, when the valleys of youth have been exalted and its mountains have been laid low. Last comes old age and final dissolution, when the whole region has been reduced to its base-level. But the decay of a mountain-chain does not always proceed without interruption. Not infrequently the base-level is disturbed; new horizontal movements of the crust take place, and bulgingup of the region is accompanied by further folding and fracturing of the strata. The mountain-system renews its youth. On the other hand, the old baselevel may be destroyed by subsidence of the crust, and the mountains, partially or wholly drowned, may in time become largely buried under new accumulations of sediment. Re-elevation taking place, erosion recommences, and the degradation of the region is resumed. In the structure of not a few mountain
F1g. 56. Structure Of The Ardennls (after Comet and Briart).
MM, the existing surface; the light-shaded area above this level represents the rock-masses removed by denudation. The Silurian rocks at the base of the section are indicated by thin white lines. Above these, on the left-hand side of the section, between C and jW, come Devonian conglomerate, sandstone, shale, and limestone; next in succession follow the Carboniferous strata at and above M; A A, BB, C C, are dislocations.
chains we may read the history of many such vicissitudes.
So completely have some mountains been removed -by denudation, that without some knowledge of geological structure we should never have divined their former existence. An instructive example is furnished by the Carboniferous tracts of Belgium and Northern France. The structure of these regions shows that formerly a considerable range of mountains extended between Boulogne and Aix-la-Chapelle. At or towards the close of Carboniferous times a great earth-movement, acting in a direction from south to north, buckled up the strata, and these, yielding to the pressure, snapped across, and extensive overthrustine followed alone the line referred to, the Carboniferous beds being inverted and overlaid by Devonian strata. The mountains of upheaval which thus came into existence attained a great elevation, the higher parts of the range reaching probably not less than 16,000 or iS.ooo feet. The section (Fig. 56) will show how completely the surface has been remodelled, how mountains of elevation have been replaced by a plain of erosion.