but to a great extent of cherts and sandstones, such as are found at present in the forest-ridge of the Wealden and nowhere else. It must have been brought by water, for the gravels are to a considerable extent rounded and water-worn. This water must have run down-hill and with considerable velocity during floods, from the size of the rolled stones, and it must have come from the south, because the cherts and grits are only found there, and because the levels at which the gravels are found rise in that direction. By following these levels as far as the present surface extends, which is to the southern edge of the green-sand, it is easy to plot out what must have been the continuation of this rising gradient to the south, and what the elevation of the southern range in which these northward-flowing streams took their origin. Prestwich has gone into the question in full detail, and his conclusion is, that the height of this Wealden ridge must have been at least 2800 feet, or in other words, that about 2000 feet must have disappeared by denudation. This is the more conclusive as Prestwich is the highest authority, and he approached the subject with a bias for shortening rather than lengthening the periods commonly assigned for the glacial epoch and the antiquity of man. The present average rate of denudation of continents has been approximately measured by calculating the amount of solid matter brought down by rivers. It varies a good deal according to the nature of the area drained, but the average is about one foot in 3000 years. At this rate the time required for the removal of 2000 feet of the Wealden ridge would be no less than 6,000,000 years; but of course this would be no fair test, as denudation would be vastly more rapid than the present average rate, on hilly ranges and under glacial conditions of climate. It is enough to say that the time required must have been extremely great, and quite ample to fit in with the most extended time required by Croll's theory of the varying eccentricity of the earth's orbit. It is to be noted also, that Prestwich pronounces part of this high level or southern drift to be older than the Westleton pebble drift which forms part of the Upper Pliocene series in Suffolk and Norfolk, and which the Professor has traced over many of our southern counties. If this conclusion is correct, it solves the problem of tertiary man by showing numerous palæolithic implements in a stage older than an undoubted Pliocene bed. The implements found in these high-level southern drifts are all of a very rude type, and the discovery is confirmed by similar implements having been found at corresponding elevations on the chalk downs of Hertfordshire and on the South Downs.[10]