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CHAPTER XXV.
ANTIQUITY OF THE RIVER-DRIFT.
In order to discuss this subject, it will be necessary to enter into some geological details; as it is evident that the least antiquity that can be assigned to the implements is that of the beds of gravel, sand, and clay in which they occur, and of which, in fact, they may be regarded as constituent portions. Whether they may not in some instances have been derived from beds of even greater antiquity than those in which they are found, is another question, which will subsequently be dealt with; but any one examining the condition of the beds in which the implements occur, will have no difficulty in seeing that they have not been disturbed since their deposit; while in most cases, the colouring of the worked and of the unworked flints they contain is similar, and affords proof of their having long lain together under the same conditions.
That the containing beds have, at all events in most cases, been deposited by fresh water, and not by the sea, is proved by the occasional abundance in them of land and freshwater shells, and the absence of those of marine origin; while their general analogy with the flood deposits of existing rivers, and their almost universal contiguity to them, raises the strongest possible presumption of their existence being due to river action. At the risk of being thought to have prejudged the question, I have, therefore, made no scruple in treating them hitherto as being River-drift. To show that for the most part they are so in reality, and to enable the reader to form some opinion of the manner in which deposits originally formed in and about the beds of streams or lakes, now in some cases occupy the tops of hills, and cover the slopes of valleys, far above the level of any existing neighbouring river, or even at a considerable distance from any stream, it will, I think, be well to state a hypothetical case; and then to compare the actual phenomena with it, and see how far they correspond.
