There can be little doubt that in any supposable case the changes themselves are of very high antiquity. Now, all the lines where they are on similar slopes, in similar ground, or generally in the same circumstances, present such a resemblance as to entitle us to conclude, that had the ground been uniform throughout, the whole of them would have been equal and similar. Yet lying at different heights, and consequently subject to an equal action of the descent of water, the most universally destroying of the wearing causes, they must, had those wearing causes been active, have shown very different degrees of injury. It is as inconsistent with the action of such causes, as with the ordinary calculation of chances, that an equality so great should have been preserved had they been materially subjected to these causes of destruction. We may therefore consider them as differing but little, even at this distant period, from the condition in which they were left by the subsidence of the water, or by the cessation of whichever of the supposed possible causes we choose to assume as that which produced them. The waste and destruction of hills is by no means therefore, however certain it may be, an operation of great rapidity. We are here furnished with a permanent criterion by which, within certain wide limits indeed, we can estimate it.
But this is not the only important fact pointed out by the same phenomena. We have seen that the lines are formed in two different sets of alluvia. The one of these is found at the upper part of the glen, and consists of sharp fragments that have been subjected to no distant transportation. They are the result of the wearing process acting on the summits of the hills, as is proved by their identity with the natural rock, by their freedom from foreign mixture, by their angular integrity, and by the quantity of fine clay that is mixed with them. This latter circumstance I would
