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 snow and frost are continually disintegrating the rocks, and thus gradually lowering the higher peaks. At the base of almost any steep cliff may be seen a slope of débris. This stands at a regular angle—the angle of repose—and, unless it is gradually removed by a stream at the base, gradually creeps up higher and higher, until at last the cliff entirely disappears.

Sometimes the two sides of the valley approach so near that there is not even room for the river and the road; in that case Nature claims the supremacy, and the road has to be carried in a cutting, or perhaps in a tunnel through the rock. In other cases Nature is not at one with herself. In many places the débris from the rocks above would reach right across the valley and dam up the stream. Then arises a struggle between rock and river, but the river is always victorious in the end; even if dammed back for a while, it concentrates its force, rises up the rampart of rock, rushes over triumphantly, resumes its original course, and gradually carries the enemy away.

Sometimes two lateral valleys come down nearly opposite one another, so that the cones meet, as, for instance, some little way below Vernayaz, and indeed, in several other places in the Valais. In this case, or indeed by one, if it is sufficiently large, the valley may be dammed up, and a lake formed.

Dams, indeed, may be due to other causes. In some cases valleys have been dammed by ice—for instance, in the Vallée de Bagnes, in the year 1818; or by rock falls, as in the Valais, in the sixth century.

Almost all river valleys contain, or have contained in their course, one or more lakes, and when a river falls into a lake, a cone, like those just described, is formed, and projects into the lake. Thus, on the Lake of Geneva, between Vevey and Villeneuve, are several such promontories, each marking the place where a stream falls into the lake.

The Rhone itself has not only filled up what was once the upper end of the lake, but has built out a strip of land into the lake.

That the lake formerly extended far up the Valais no one can doubt who looks at the flat ground about Villeneuve. It is clear that the valley must formerly have been much deeper, and that it has been filled up by material brought down by the Rhone, a process which is still continuing.

At the other end the lake the river rushes out fifteen feet deep of, "not flowing, but flying, water, not water neither—melted glacier matter one should call it; the force of the ice with it, and the wreathing of the clouds, the gladness of the sky, and the countenance of time."

It would, however, be a great mistake to suppose that rivers always tend to excavate their valleys. This is only the case when the slope exceeds a certain angle. When