the case, it would follow that the surface might be very considerably affected-rocks might be shattered and weakly constructed ridges shaken down-so that the anticlinal ridges of a mountain-chain might well have presented, even in the days of its infancy, a broken and ruptured surface.

But, to return to our hypothetical mountain-chain, we shall suppose this consists of a series of parallel ridges which attain their greatest elevation along a line or axis not far removed from the thrust-side of the chain. From this axis the ridges decline gradually in importance in the direction of earth-movement, and eventually die out in a series of gentle undulations. Each of the ridges, we shall suppose, coincides with an anticline, and each of the intervening hollows with a syncline. In a word, we shall take the surface to be a more or less exact expression of the geological structure, the undulations of the ground, however, being less pronounced than those of the strata at considerable depths. The diagram (Fig. 37, page 99), will represent a section across such a chain. It will be observed that all faults and possible intrusions of igneous rock are neglected.

In any series of stratified rocks some are sure to be more porous than others, while all will be traversed by joints or cracks approximately at right angles to the bedding-places. This, then, we shall take to be the case with the rocks of which our young mountainchain is composed; and we shall suppose that the parallel ridges extend in a linear direction for many

miles, gradually declining in elevation towards both ends of the chain. With these conditions of surface, it is obvious that drainage will take place in the direction of the great longitudinal valleys or synclinal troughs, while a set of transverse streams will flow down the slopes of the anticlinal ridges. Many of these will thus become tributary to the rivers making their way along the axial hollows. All the rivers in course of time must cut into the rocks, but it is obvious that the transverse streams will be of a torrential character, and will tend therefore to carve out narrower, deeper, and straighter channels than the larger rivers can excavate in the less inclined, broad axial depressions. Immense quantities of rock-material will be swept down from the anticlinal ridges to accumulate in heaps and sheets in the synclinal troughs, or to be swept away more readily, according as the gradients of the latter are gentle or steep. Erosion, in short, will be carried on most actively upon the anticlinal mountains. This would naturally follow, whatever the character of the geological structure might be, for the erosive action of running water increases with the gradient.

But in all cases denudation is hastened or retarded according as the rock-structure is weak or strong. If, therefore, the mountains of our hypothetical chain be more weakly built than the parallel synclinal troughs, the former will tend to be reduced more rapidly than the latter. This can be shown diagrammatically as in Fig. 38, p. 105. Here we have a section across two

anticlinal mountains and a synclinal valley. The strata consist of a series of more or less porous sandstones separated by intervening layers of impermeable clayrocks. Moreover, they are jointed, and the joints traversing the anticlines tend to open out upwards, while the reverse is the case with those cutting the synclines. Some of these joints may be shrinkagecracks which came into existence during the slow consolidation of the strata, perhaps long before the latter were flexed and folded. But a large proportion no doubt would be produced while the rocks were being bent and doubled up. In whatever way formed, joints are readily permeated by meteoric water, which finds its way down from the surface and soaks into

FIG 38. DIAGRAM OF ANTICLINAL MOUNTAINS

Pervious strata (stippled) and impervious layers (thin lines); jj, joints, cutting strata at right angles; v, valley; ss, springs coming out at junction of pervious and impervious beds.

the porous strata below. Constantly augmented from above, the water thus imbibed is forced to percolate through the porous beds in the direction of the dip. Hence wherever these beds are truncated (as in the valley) the water comes out at the surface as natural springs. Thus in the illustration springs appear at ss, where permeable sandstones are underlaid by im

permeable clay-rocks. The effect of these springs is not hard to understand. They tend to undermine the sandstones, and as the dip of the strata is towards the valley, rock-falls and landslips must continue to take place until the anticline is reduced. Anticlinal mountains separated by a synclinal trough are thus in a state of unstable equilibrium. Sapped and undermined by rain, frost, and springs, their existence

d

FIG 39. SYNCLINAL VALLEY SHIFTING TOWARDS ANTICLINAL AXIS. a, synclinal valley; d, anticline; v, valley, gradually widened in the direction of the arrow.

cannot be prolonged. On the other hand, the strata in the synclinal trough, although consisting of the same materials, will be relatively more durable. Their arrangement favours their preservation; they are not sapped and undermined as in an anticline, but are reduced chiefly by the vertical erosion of the rivers that traverse them.

The anticlines of our mountain-chain are thus not only deeply incised by transverse streams and torrents,

but they are liable all along their flanks to the undermining action of the longitudinal rivers and their allies, —rain, frost, and springs. Quite undisturbed by earthquakes, their destruction by epigene action is, nevertheless, assured. But if the young mountainchain be liable, as all such mountains are, to earthquake-shocks, the demolition of the already weakened anticlines will often be greatly accelerated.

Unsymmetrical anticlines are not less liable to destruction than those we have just been considering. Indeed, their arrangement must lead sometimes to the gradual shifting of a longitudinal river from a synclinal to an anticlinal axis. Thus a river occupying the syncline a (Fig 39), and eventually cutting more or less deeply into the underlying strata, will tend to work its way towards the axis of the anticline d. For it will be observed that the beds of that anticline dip into the valley, while those on the other side dip away from it. The latter, therefore, is a strong structure, and the valley-cliffs will recede relatively slowly in that direction, while rock-falls and landslips will prevail on the side of d. The valley, therefore, will be widened most readily towards d; and, the like conditions obtaining in all the longitudinal valleys of a chain, the time will come when every similarly constructed anticlinal ridge may be reduced.

Many other modifications of the drainage of a mountain-chain will be brought about by the action. of the streams and rivers. Thus a transverse stream, which as a rule works more energetically than a longi