One foot in 6,000 years may, therefore, I think, be safely taken as the average rate of denudation of the whole surface of the globe. [Footnote 27: _Student’s Elements of Geology_, p. 91.] _The average rate of denudation in the past probably not much greater than in the present._—The belief has long prevailed that the rate of denudation was much greater in past ages than it is now; but I am unable to perceive any good grounds for concluding that such was the case at any time since the beginning of the Palæozoic period. Various reasons have, however, been assigned for this supposed greater rate; and to the consideration of these I shall now very briefly refer. It has been thought that at some remote epoch of the earth’s history, when the moon was much nearer and the day much shorter than now, the rate of denudation would, owing to the erosive power of the enormous tides which would then prevail, be much greater than at the present day. This, however, is very doubtful. There is nothing in the stratified rocks which affords any support to the idea of great tidal waves having swept over the land, at least since the time when life began on our globe. Such a state of things would have destroyed all animal life. “The Palæozoic sediments,” as Professor A. Winchell remarks, “have been deposited, for the chief part, in quiet seas. The deep beds of limestones and shales are spread out in sheets continent-wide, which testify unmistakably to placid waters and slow deposition.”[28] But high tides, not sweeping over the land, would not increase the rate of denudation to the extent supposed. High tides silt up a river channel more readily than they deepen it. A higher tide would probably produce a greater destruction of sea-coast: it would tend to increase the rate of marine denudation, but this would not materially affect the general rate of denudation. For, as the present rate of marine denudation is to that of subaërial denudation only as 1 to about 1,700,[29] it would take a very large increase in the rate of marine denudation to affect sensibly the general result. Suppose the rate of marine denudation to have been, for example, ten times as great during the Palæozoic age as it is now (which it certainly was not), it would only have shortened the time required to effect a given amount of denudation of the whole earth by 9 years in 1,700, i.e. by little more than one-half per cent. [Footnote 28: _World Life_, p. 265.] [Footnote 29: See _Climate and Time_, p. 337.]