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tudinal river, may now and again succeed in cutting its way back across an anticline so as to tap some adjacent synclinal trough. If the bottom of this trough should chance to be at a higher level than that of the hollow into which the transverse stream makes its way, the river of the invaded syncline may be captured by the stream. Thus we should have the phenomenon of a longitudinal river changing its course and becoming transverse.
The chief point, however, which we have at present to bear in mind is simply this: that anticlinal structures are weak and tend to be reduced ; while synclinal arrangements are relatively strong, and consequently more persistent. We should expect to find, therefore, in all mountains of upheaval, exposed for any time to denudation, that synclinally arranged strata will not infrequently appear in a tolerable state of preservation ; while anticlinal beds will often be deeply eroded. Let us, then, turn our attention to the structures met with in such a region as the Alps, and see how far they bear out these elementary conclusions.
That great chain is a typical example of what are known as mountains of elevation. It consists essentially of a succession of anticlines and synclines, chiefly unsymmetrical. The strata are not only folded and often exceedingly contorted, but the structure is still further complicated by vast thrust-planes and normal faults. Moreover, the chain is the result, not of one, but of many successive earth-movements. But the chief movement—that, namely, to which the mountains owe most of their present elevation—took place at a relatively late geological period. Many of the folded and fractured rocks, indeed, are of no greater antiquity than the soft clays and sands over which London is built. And yet, although the chain belongs to so late a date, its rocks everywhere bear witness to great erosion. Enormous masses of material have been gradually removed, and the original surface, due to folding and displacement, has been more or less profoundly modified.
The sketch-section across the Swiss Alps (Fig. 40, p. 110) gives the general arrangement of the strata, and enables us in some faint measure to appreciate the degree of denudation which has already been experienced by these relatively young mountains. Grant, if you will, that the folding of the strata may have resulted in a kind of chaos at the surface—that the ground along the axes of anticlinal arches may have been ruptured, and the rocks everywhere tumbled in confusion—yet we have still to account for the wholesale removal of the abundant ddbris—the shattered reefs and dislodged mountain-masses. We cannot, in short, escape from the conclusion that an enormous amount of denudation has taken place. So profoundly has the original configuration been modified, that it is only when the mountains are viewed in the broadest way that any coincidence between underground structure and surface-features can be observed. Even where anticlines still form hills and mountains it is obvious that they have yet
Fig. 41. Summit Of Santis, East Side (A. Heim).
trenched—valleys running along their axes, an appearance well shown in Fig. 42. Synclinal hollows