It may be taken as certain, that after the one age ceased and the first men appeared, the beds in which their relics occur have been raised violently, and again depressed and subjected to great flushes and floods of water. The caves have been upheaved, and the gravels are found chiefly along the valleys of our present rivers, but at a much higher level, showing that there was both a higher level of the soil itself and a much greater volume of water. The Straits of Dover were formed during this period. But none of these changes required a very long time; and if we can trace back the later stone age, which shows remains of pottery and other proofs of greater civilization, to the dawn of the historic period not more than 4000 or 5000 years ago, there is nothing in the nature of the changes which, as we have stated, intervened between the palaeolithic and neolithic periods, that need have occupied more than a thousand or two of years. Upheavals of strata and disruptions may be the work of but a short time, or they may be more gradual. And as to the effect of water, that depends on its volume and velocity; no certain rule can be given. Our own direct experience shows that very great changes may take place in a few hundred years. "The estuaries," remarks Mr. Pattison,[1] "around our south-eastern coast, which have been filled up in historical times, some within the last seven hundred years to a height of thirty feet from their sea-level, by the gradual accumulation of soil, now look like solid earth in no way differing from the far older land adjoining. The harbours out of which our Plantagenet kings sailed are now firm, well-timbered land. The sea-channel through which the Romans sailed on their course to the Thames, at Thanet, is now a puny fresh-water ditch, with banks apparently as old as the hills. In Bede's days, in the ninth century, it was a sea-channel three furlongs wide." [Footnote 1: "Age and Origin of Man"--Present-Day Tract Series.] Thus we are in complete uncertainty as to the date of the palaeolithic man, or as to the time necessary to effect the changes in the surface of the earth which intervened between it and the later stone ages. But there is nothing which conflicts with the possibility that the whole may have occurred within some 8,000 years. For the supposition of Mons. Gabriel Mortillet that man has existed for 230,000 years, there is neither evidence nor probability. His theory is derived from an assumption that the geologic changes alluded to occupied an immense time; and the further assumption (if possible still more unwarranted) that the old race which used the chipped stone tools remained stationary for a very long period, and very gradually improved its tools and ultimately passed into the neolithic stage when the art of pottery became known, however rudely.