Again, it is assumed that the greater rate of terrestrial rotation in the early ages would produce certain influences which would in turn bring about a greater amount of denudation. The rate of rotation has been slowly decreasing for ages, and in Palæozoic times it must, of course, have been greater than at present. A more rapid rotation would increase the velocity of the trade and anti-trade winds, and would thus tend to augment the action of those meteorological agents chiefly effective in the work of subaërial denudation. Here again the testimony of geology is negative. We have no geological grounds to conclude that the winds of Palæozoic times were stronger than those at the present day. The heat was no doubt greater, and perhaps there was more rain; but, on the other hand, there would be less frost, snow, ice, and other denuding agents. There is one cause which would, perhaps, be more effective than any of the foregoing: viz. the periodic occurrence of glacial epochs. When a country is buried under ice, the erosion of the surface is great. But it must be borne in mind that the influence of rain, rivers, and other denuding agents now in operation would then, in the glaciated regions, be almost _nil_. Besides, the greater part of the materials ground off the rocks would be left on the land; and, when the ice disappeared, it would be found in the form of a thick mantle of boulder clay—a mantle which would protect the rocky surface of the country for thousands and tens of thousands of years from further denudation. This is shown by the fine striæ on the rocky surface, made perhaps more than 50,000 years ago, remaining under the boulder clay as perfect as the day on which they were engraved. But, more than all this, a very considerable part of the 1 foot presently being removed off the country in 6,000 years consists of the loose materials belonging to the glacial epoch, such as sands, gravels, and boulder clay, which are being swept off the surface by rain and river action. Were it not for this, the present rate of subaërial denudation would not be so high as it actually is. Taking all things into consideration, it is, I think, obvious that the average rate of denudation since the beginning of Palæozoic times was probably not much greater than at the present day.