rapidly than that of the hill. With an increased rainfall, such as we have supposed, this removal of solid matter by solution must have been considerable; but still nothing in comparison with that effected by the other denuding agencies which have been mentioned. It is, moreover, to be borne in mind that, as will shortly be seen, until the valleys had been excavated to a considerable depth, the amount of water delivered by the springs would, with the same rainfall, have been far less than at present. The springs would also, to some extent, have been affected by the chalk being in a less porous condition than it now is, owing to its not having lost so much of its substance by the chemical action which has just been described.
Before comparing the actual phenomena with the results of the conditions which have been assumed, it will be well to say a few words as to the probable effects of an amelioration of climate, and a diminution in the rainfall, upon a valley already excavated to an average depth of 100 feet, such as has already been described. It is evident that any transport of materials due to the action of ice, by floating loose stones and pebbles from one part of the bed of the stream to another, would be materially diminished; as would also the number of floods resulting from the thawing of the winter accumulation of ice and snow, and from rain falling on frozen ground. The only remaining principal cause for floods would be the heavy fall of rain during storms or wet seasons; but here, a comparatively slight alteration in the conditions will have made a vast difference in the results. When the valleys were once excavated to a certain depth, the level of the springs or outfalls carrying off the accumulation of water in the absorbent soil, would be proportionally reduced, as would also be the line of permanent saturation in the chalk. The effect of this would be that during any dry interval, the water contained in the upper part of the chalk would gravitate downwards, until it reached the subterranean reservoir of water saturating the chalk; and thus leave the surface soil in the same absorbent condition as it is at present, and capable of receiving a much greater amount of rain than formerly, before any would flow from off its surface.
Even with a constant and excessive rainfall, the result of the continued deepening of the valleys would be to cause more and more to flow off by the springs, and less from the surface; but with the valleys once deepened, a small diminution in the rainfall, or its more even distribution over the whole year, might cause the
