to see these countless drifts adjust themselves in their relative position, where they unite by congelation, and convey the idea of the fall of some mountain which has covered the plain with rocks of every dimension. But it is not this accumulation of ice-drifts in the Rhine which is of itself the cause of danger; it is, on the contrary, the débâcle, or breaking-up, which is often productive of calamitous consequences. When this débâcle commences in the upper part of the river, above the point where the latter is completely frozen, the masses of ice, drifting with the current and unable to pass, are hurled upon those already soldered together; thus an enormous barrier is formed, which the water, arrested in its course, cannot pass over, and hence overflows to the right and left, breaking the dykes, inundating the plains, and spreading devastation and suffering, far and near. The disasters caused by the débâcles of the Rhine have taught the riparian inhabitants to observe attentively the facts which may serve them as a prognostic, and put them on their guard against the irruption of the ice. It is thus that they have been led to observe the grund-eis—that is to say, the ice formed at the bottom of the rivers—for it is this ice which, in becoming detached from the bottom and rising towards the surface, unites itself to the under surface of the masses already in place, and by further embarrassing the discharge, exposes the country to inundation."
Another most effective agent in transporting the pebbles and larger blocks of stone along the course of rivers is shore-ice. During a severe winter masses of thick ice are formed which enclose the larger stones on the bottom of the river towards its edge; these masses are dislodged and carried away by subsequent floods, whether arising from rapid thaws or from rain higher up the river, or from accumulations of ice, such as those described, having formed a temporary barrier across the stream through which the pent-up water eventually burst and carried all before it. The lateral pressure of such dams of ice, with a large body of water behind, must be enormous; and we can readily conceive their crumbling-up any beds of gravel on the banks of the rivers against which they might happen to abut.
But there is still another way in which a severe climate, such as has been supposed, would act upon the rocks, namely, by their being rent and disintegrated by frost. This has been well pointed out by Sir Joseph Prestwich,[1] who has cited numerous instances
- ↑ Phil. Trans., 1864, p. 296.
