Again, in the case of the great fault separating the Silurians of the south of Scotland from the Old Red Sandstone tracts lying to the north, a thickness of the latter strata of probably more than a mile, as we have seen, must have been removed from the ground to the south of the fault before the commencement of the Carboniferous period. And again, in the case of the Lancashire coal-fields, to which reference has been made, nearly two miles in thickness of strata had been removed in the interval which elapsed between the Millstone Grit and the Permian periods. _Time required to effect the foregoing amount of denudation._—To lower the country one mile by denudation would therefore require, according to the rate which we have already established, about 15,000,000 years; but we have seen that a thickness of rock more than equal to that must have been swept away since the Carboniferous period; and even during the Carboniferous period itself more than a mile in thickness of strata in many places was removed. Again, there can be no doubt whatever that the amount of rock removed during the Old Red Sandstone period was much greater than one mile; for we know perfectly well that over large tracts of country nearly a mile in thickness of rock was carried away _between the period of the Lower Old Red Sandstone and the Carboniferous epoch_. Further, all geological facts go to show that the time represented by the Lower Old Red Sandstone itself must have been enormous. Now, three miles of rock removed since the commencement of the Old Red Sandstone period (which, doubtless, is an under-estimate) would give us 45,000,000 years. Again, going farther back, we find the lapse of time represented by the Silurian period to be even more striking than that of the Old Red Sandstone. The unconformities in the Silurian series indicate that many thousands of feet of these strata were denuded before overlying members of the same great formations were deposited. And again, this immense formation was formed in the ocean by the slow denudation of pre-existing Cambrian continents, just as these had been built up out of the ruins of the still prior Laurentian land. And even here we do not reach the end of the series, for the Laurentians themselves resulted from the denudation, not of the primary rocks of the globe, but of previously existing sedimentary and probably igneous rocks, of which, perhaps, no recognisable portion now remains. It is the opinion of Mr. Darwin, and also of Mr. Wallace, that the geological time which elapsed anterior to the Cambrian period was as long as the whole interval from that period to the present day. This is an opinion which I suppose is supported by most geologists. But, to err on the safe side, I shall assume that the time which had elapsed prior to the Old Red Sandstone was not greater than the time which has elapsed since that period. Even on this assumption we have at least 90,000,000 years as a minimum duration of geological time.