Page:The Ancient Stone Implements (1897).djvu/728

706 unwashed table lands. Or again, the soil may, like the Chalk under ordinary circumstances, be so absorbent that but little of the rainfall flows off from its surface. The calculation has already been made, that a rainfall of 54 inches annually, supposing the whole of it flowed off the land into the sea in a turbid state, containing, like the Mississippi, $1⁄1500$ part of its weight of solid matter, would lower the surface a foot in 450 years; but as has already been observed, we cannot conceive it possible that with such soils as we have here to do with, the constant turbidity should have been anything like so great. And, in fact, the whole system of calculation is one which may be regarded rather as proving the necessity of valleys being in course of time formed by subaërial action, than as giving any definite guide by which to calculate the period requisite for their formation. There can, indeed, be no doubt that the denuding power of the falling rain is greater on the slopes than on the level surfaces; but it seems impossible to assign any proportions to the effects on land lying at different inclinations, of different characters, and under different circumstances as to any vegetable covering. Were the action uniform over the whole surface exposed, of course no alteration beyond a general lowering of the land-surface would result from this cause, and the valleys would remain of precisely the same depth with regard to the adjacent land as they did at remote epochs. Looking at the quantity of brick-earth still left on the slopes of many of our valleys, I am inclined to think that the lowering of the surface has been more general than has been supposed by Sir Archibald Geikie. The presence of these soft and easily denuded beds is also an argument against the excavation of the valleys having progressed in a uniform manner, by heavy rains falling during the period of the year when such beds were soft and unfrozen; and seems rather significant of the excavation of the valley by floods principally occurring at a time when the upper part of the soil was in a frozen condition.

Certainly the whole character of the deposits is more in accordance with their resulting from the occasional flooding of the streams than from any other cause. If this be so, who shall tell at what intervals such floods occurred, and what was the average effect of each in deepening the valleys? That they were of comparatively rare occurrence, and not so frequent that they were foreseen by the men of those days, seems deducible from the number of their implements found in the gravels. For there is much probability that these must have been washed in from