of its effects, and mentions having seen a low cliff of chalk, 15 feet high, form a talus or heap of fragments at its foot, 6 feet broad and 4 feet high, in the course of an ordinary winter.
As I am by no means attempting an exhaustive geological essay on this subject, which is indeed hardly needed, I think that enough has been said to show that under conditions such as have been supposed in this hypothetical case, the great subaërial agents—rain and snow, ice and frost—would, in the course of time, enable rivers to excavate their valleys to an almost indefinite extent. Indeed, one can conceive the process being carried on, until what had been rivers became estuaries or arms of the sea; or, until a large island once traversed by rivers became converted into several smaller islands, by the cutting back, and subsequent junction, of its various river-valleys.
Without, however, carrying the excavatory process to such an extreme, let us now consider what would be the condition of our hypothetical river-valley when excavated to a depth of say 100 feet, at a point about midway between its source and the sea. We have already seen that at an earlier period—when the river ran at a higher level by 100 feet than that it is now supposed to occupy—its valley must have been broader, and its bottom strewn with detritus of various kinds, in the shape of gravel, sand, and clay, and, it may be, some larger blocks of stone. In the further process of excavating by agents such as have been described, it has also been seen, that it is in the highest degree improbable that the succeeding floods and other transporting agents should have entirely removed and obliterated the deposits left by those of earlier date. We should, therefore, expect to find, at various heights on the slope of the valley, remains of such beds of detritus, and especially at points such as the junctions of affluents with the river, and the inner side of the bends it makes in its course, which would naturally be the least exposed to the violent invasion of the stream. In these beds we might reasonably search for the remains of the surface and freshwater life of the period; and had there been any amelioration of climate during the process of excavation, a larger proportion of silt and clay, and less of coarse gravel, in the lower and more recent deposits, would testify to the fact. Looking also at the power possessed by rivers of levelling the bottoms of their valleys, during their successive changes of course, we might expect to find in places, tracts of these old valley-bottoms left as terraces on the slopes of the more deeply excavated valleys. The
