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rivers, the courses of which are determined by the average slope of the land. As valleys are deepened and widened, and the whole surface comes under the influence of the epigene agents, new tributary streams continue from time to time to make their appearance, and eventually a perfect network of drainage-lines is established. Wherever the rocks yield most readily to erosion hollows are formed, and many of these will necessarily coincide with the outcrop or strike of the strata. Longitudinal valleys thus tend to be developed. As denudation proceeds, the capture of streams by rivers and of rivers by streams often takes place, and the hydrographic system becomes more or less modified, but the general direction of the chief lines of drainage remains unchanged. Eventually transverse rivers are found cutting across mountain-ridge after mountain-ridge, the latter having only been developed after the rivers had come into existence. With the deepening and widening of the main valleys, and the continual multiplication of subsidiary hollows by springs, torrents, and streams, the whole plateau eventually becomes cut up into irregular segments of every shape, form, and size—a rolling mountain-land. Waterfalls, rapids, and other irregularities have now disappeared from the courses of the older rivers and streams, except, it may be, towards their heads, where more or less numerous feeders are busy cutting their way back into the mountains. Should the base-level be maintained, the process of denudation must continue until
the rolling mountain-land is finally reduced and resolved once more into a plain of erosion.
It is seldom, however, that a cycle of erosion is allowed to pass through all its stages. The study of many ancient plateaux has shown that the base-level is not infrequently disturbed—sometimes by elevation, at other times by depression. Long before the eroded plateau has been completely reduced, subsidence may ensue, and the drowned land may then become buried under vast accumulations of marine sediments. Should the region be once more upheaved and converted into dry land, streams and rivers will again come into existence, and flow in directions determined by the slopes of the surface. Thus ere long another hydrographic system will be developed which may differ entirely from its predecessor, both as regards direction and arrangement. As the rivers cut their way down through the superimposed marine strata they will eventually reach the buried land-surface, across which they will run without any reference to the former configuration. Should the base-level remain unchanged, a time will come when the overlying marine strata will be entirely removed, but the direction and general arrangement of the river-system acquired when the land was newborn will be maintained. Thus the direction of many transverse rivers, which in ancient plateau-lands are found cutting across mountains of every shape and disposition, have not infrequently been determined by the surface-slope of overlying masses, almost every vestige of which has since disappeared.
LAND-FORMS IN REGIONS AFFECTED BY NORMAL FAULTS OR VERTICAL DISPLACEMENTS
NORMAL FAULTS, GENERAL FEATURES OF THEIR CONNECTION
WITH FOLDS THEIR ORIGIN HOW THEY AFFECT THE SURFACE FAULTS OF THE COLORADO REGION, AND OF THE GREAT
BASIN DEPRESSION OF THE DEAD SEA AND THE JORDAN
LAKE-DEPRESSIONS OF EAST AFRICA FAULTS OF BRITISH
COAL-FIELDS—BOUNDING FAULTS OF SCOTTISH HIGHLANDS
AND LOWLANDS FAULT-BOUNDED MOUNTAINS GENERAL
IN Chapter III. a short account was given of the dislocations or fractures by which rocks are frequently traversed. These, as we saw, are of two kinds—normal faidts and reversed faults or overthrusts. The latter have been sufficiently referred to in connection with the appearances presented by highly flexured strata, amongst which, indeed, they are most usually encountered. Normal faults of various importance may likewise often be seen traversing areas of disturbed and contorted rocks. When such is the case, however, the larger of these faults not infrequently prove to be of later date than the flexures and thrust-planes. The latter are the result of former horizontal movements of the crust; the normal faults, on the other hand, are vertical displacements due to later movements of direct subsidence. It will be understood, therefore, that reversed faults or overthrusts are practically confined to regions of highly flexed and contorted strata, while normal faults traverse every kind of geological structure. The latter, however, are certainly best displayed in areas of horizontal and moderately inclined strata, while they often form lines of separation between these and contiguous areas of highly disturbed rock-masses.
The amount of downthrow of normal faults is very variable. Sometimes it does not exceed a few feet or yards, in other cases it may reach thousands of feet, so that strata of vastly different ages may be brought into juxtaposition. The smaller faults usually extend for very short distances, while the larger ones may continue for hundreds or even thousands of miles. The course of great faults is usually approximately straight, but not infrequently it is curved. Very often they are accompanied by a series of smaller parallel dislocations; and now and again, in place of one great fault, with accompanying minor dislocations, we may find a series of more or less closely set parallel minor faults. When the downthrow of all these minor faults is in one and the same direction, the result is practically the same as if there had been only one major dislocation with a large downthrow. Another fact may be noted: faults, especially large ones, often split up, as it were, into two or more. A major fault may begin as a mere crack or fracture, with little or no accompanying rock-displacement. But as it continues the amount of downthrow gradually increases until a maximum is reached, after which the displacement usually decreases until finally the fault dies out. In not a few cases, however, the degree of downthrow varies very irregularly.
Frequently faults are intimately connected with folds and flexures. This is shown at once by the fact that large dislocations very often trend in the same direction as the strike of the strata. Now.and again, indeed, when a large fault can be followed to the end, it is found gradually to die out in a fold or flexure. In other words, what is a fault in one place is represented elsewhere by a flexure. It is not hard to see how that should be. Strain or tension must obviously be set up along the margin of a sinking area. If, for example, subsidence should take place within an area of horizontal strata, the horizontal position of the rocks along the margin of the sinking area will be interfered with. The pull or drag of the descending mass will cause the strata of the adjacent relatively stable area either to bend over or snap across. Should the movement be slow and protracted, the rocks will probably at first yield by bending; but as the movement continues they will eventually give way, and a fold will thus be replaced by a fracture. Towards either end of such a fault, therefore, we should expect it to die out into a simple