Should it appear that with a certain given configuration of the land surface, a certain character of rock, a certain climate, and a certain number of years, certain effects must, judging from all analogy, have been produced; and should we in the case of these ancient Drifts find some of the conditions to have existed, and all the phenomena to be in accordance with the hypothesis, we may with some confidence assume that the other original conditions existed also; and build up a connected theory which will account for the whole of the observed results, and will also throw light on their causes, as well as on the duration of time necessary for their operation to have produced such effects. In stating the case, I lay no claim to originality, and do little more than follow in the steps of Sir Charles Lyell, Sir Joseph Prestwich, and others who have made a study of the character and effects of fluviatile action.
As it is in the gravels of Chalk districts that Palæolithic implements have been chiefly, though by no means exclusively, found, let us base the hypothesis on the assumption that an extensive and almost horizontal area of Upper Chalk, covered for the most part with beds of marine clay and shingle, gradually rose from beneath the sea, to an elevation of 200 feet above its level. Let us also assume that the land was elevated at a rate far in excess of that at which any subaërial action, such as rain, frost, or snow, would enable a river flowing over it to excavate its valley to the depth of 200 feet in the space of time required for its elevation to that height. Let us further assume, that the winter climate was some- what more rigorous than that which at present prevails in this country, and that there was a considerably greater annual rainfall. We may also, for the purposes of the argument, take the position of the coast-line as permanent, instead of its constantly receding in consequence of the eroding power of the sea upon the cliffs.
Let us now see what would theoretically be the effect produced by subaërial causes on the river-valleys in this area during an indefinite number of centuries.
Under ordinary circumstances, and with our present amount of rainfall, there is no geological formation less liable to floods than the Chalk, or at all events, its upper portion. It is of so absorbent a nature that it is only in the extraordinary event of the ground being hard frozen at the time of a heavy fall of rain, or of a rapid thaw of snow; or of some inches of rain falling in the course of a few hours, that the soil is unable to absorb the water as fast as it is delivered upon it. The moisture when once in the soil is
