valleys, partly or almost entirely excavated in Pre-Glacial times, gravels belonging to the Glacial Period exist, and tend to complicate the question of the more recent River-drifts.
Any theory of the valleys having been excavated at some remote period in some unknown manner, and then having been filled with gravels derived from an unknown source, and again re-excavated, presents such difficulties that, to my mind, it cannot well be entertained. If, however, such a view be accepted, it seems to add to the time necessary for the excavation of the valleys; as much of the rainfall might find a subterranean vent at a low level through the gravel lining the bottom of the filled-up valleys, and thus keep the upper soil in a more absorbent condition and therefore less liable to erosion.
I must not, however, dwell too long upon this hypothetical case, which perhaps is such as may not have found an absolutely exact analogue in nature, but which may yet, I think, be accepted as a fair typical example of the results which, under the supposed conditions, must, judging from what we know of the action of subaërial causes, in all probability have ensued.
Let us now compare the phenomena as we find them in the gravel-beds of our present river-valleys, with those of the hypothetical case, and we shall, I think, find them coincide in a remarkable manner.
In the first place, the constituent parts of the gravels of the beds of Drift containing Palaeolithic implements are always, petrologically, such as are to be found in the existing river-basins, as they must also of necessity have been in the hypothetical case. This fact, which holds good both in France and England, has been insisted on by Sir Joseph Prestwich, and such insistency cannot be too often reiterated. Where old superficial marine deposits of the Glacial or any other period, consisting of pebbles of various ages and origins, exist within a river-basin, there also will such pebbles be found in its gravels, but the originally derivative character of the pebbles prevents any strong argument being founded upon their presence. Where, however, no such beds exist, the case can clearly be made out. Unless a river traverses a granite or slate country, no granite or slate is found in the Quaternary gravels of its valley: unless it passes over Oolite, Purbeck, or Greensand, no blocks or pebbles of these rocks occur. This fact suffices to prove that the gravels are due to some local cause, such as river-action, and not to any general submergence or supposed
