to which we can carry back the Neolithic Period, and the close of the Palæolithic Period as indicated by the low-level gravels. The antiquity, then, that must be assigned to the implements in the highest beds of River-drift may be represented (1) by the period requisite for the excavation of the valleys to their present depth; plus (2), the period necessary for the dying out and immigration of a large part of the Quaternary or Post-Glacial fauna and the coming in of the Prehistoric; plus (3), the Polished Stone Period; plus (4), the Bronze, Iron, and Historic Periods, which three latter in this country occupy a space of probably not less than three thousand years.
A single equation, involving so many unknown quantities, is, as already observed, not susceptible of solution; but various attempts have been made to arrive at some approximate idea of the amount of time it represents. One method has been that of assigning a date for the Glacial Period, deduced from astronomical causes, mainly in connection with the eccentricity of the earth's orbit, as pointed out by M. Adhémar and Mr. Croll. From data thus obtained, Sir Charles Lyell[1] inclines to place it at a period of extreme cold about 800,000 years ago, though Sir John Lubbock[2] would rather accept an epoch of somewhat less severity about 200,000 years removed from our time.
Another and more direct method suggested by Sir Archibald Geikie,[3] following in Mr. A. Tylor's track, is that of estimating the time required for the excavation of the valleys by the amount of solid matter carried down in suspension by various rivers at the present day. He estimates that this amount, if spread over the whole area drained by the rivers, represents, on an average, an annual loss of about 16000 of a foot; but inasmuch as the erosion of the slopes and watercourses is very much greater than that of the more level grounds, the excavation of the valleys must proceed at a more rapid rate, which he assumes to be about 11200 Part of a foot per annum, or one foot in 1,200 years. Such a calculation is, of course, open to various objections, as we may readily conceive the bottom and slopes of a valley to have been so far washed that, under ordinary circumstances, they afford little or no fine earthy matter to be taken up by the rain falling on their surface; and in such a case, the rivers, if turbid, would derive their turbidity from the water delivered from the higher and comparatively
