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704 he determined with anything approaching to precision. Not only have we no trustworthy measure of the rate of excavation of the valleys, which might give us an approximate date for the higher deposits in them, but we are at a loss to know at what epoch their excavation in the lower part of their course ceased, and what may be termed the modern alluvial deposits, which to some extent have partially refilled the old channels, began to accumulate.

That the general configuration of the surface of the country, in Neolithic times, when the ordinary forms of polished stone implements were in use, was much the same as it is at present, is proved by the fact of such implements being frequently found in recent superficial deposits. Were we, in defiance of probability, to assume that the use of these polished implements did not date farther back than two thousand years from the period when we are first made acquainted with this country by history, this would give an additional four thousand years beyond the period necessary for the excavation of the valleys for the date of the older River-drift implements. Such a period as two thousand years is in all probability almost ridiculously small to assign for the duration of the Neolithic and Bronze Periods; but however this may be, there appears, in this country at all events, to be a complete gap between the River-drift and Surface Stone Periods, so far as any intermediate forms of implements are concerned; and here at least the race of men who fabricated the latest of the palæolithic implements may have, and in all probability had, disappeared at an epoch remote from that when the country was again occupied by those who not only chipped out but polished their flint tools, and who were, moreover, associated with a mammalian fauna far nearer resembling that of the present day than that of Quaternary times.

So different, indeed, are the two groups of animals that, as has already been observed, Prof. Boyd Dawkins has shown that out of forty-eight well-ascertained species living in the Post-glacial or River-drift Period, only thirty-one were able to live on into the Prehistoric or Surface Stone Period. Such a change as this in the fauna of a country can hardly have been the work of a few years, or even of a few centuries; and yet we must intercalate a period of time sufficient for its accomplishment between the remotest date