Page:Early Man in Britain and His Place in the Tertiary Period.djvu/293

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We may obtain a rough, approximation to the relative length of the Pleistocene and Prehistoric periods from the fact that the valleys were cut down by the streams flowing through them; in the former, sometimes as much as a hundred feet, while the work done by the rivers during the latter is measured by the insignificant fluviatile deposits close to the adjacent stream. It may therefore be concluded that the former period was beyond all calculation longer than the latter. The latter, however, may have been of very considerable length, since it includes a series of changes in the fauna, and a series of invasions of different races of men into Europe, which, if measured by simliar changes recorded in history, must have required the lapse of many centuries. In dealing with these questions it is only possible to grasp the relative duration, for the measurement of time absolute in terms of years outside the reach of history is beyond our power. We do not know the length of the interval separating any two events not recorded in history, nor are we possessed of any natural chronometer by which to fix a date in the historical sense. We are dealing merely with time relative, and not time absolute.

The great changes in the fauna and geography of Britain, at the close of the Pleistocene age, render it