ments on the banks of the rivers, which, notwithstanding previous catastrophes of the same kind, were constantly placed within reach of the stream when flooded.
Sir Joseph Prestwich[1] has suggested as a possible gauge of the antiquity of the deposits, the natural funnels eaten into the chalk by the action of water charged with carbonic acid, and has cited one at Drucat, near Abbeville, which has been formed since the deposit of the gravel containing flint implements, and is upwards of 20 feet in diameter at top, and probably 100 feet in depth; but here also it seems impossible to introduce a factor by which the time represented can be ascertained. There are, however, features in connection with this case which can only be reconciled with the former high level of the bottom of the adjacent valley, and with its gradual excavation. It will be remembered that similar pipes of erosion, leading in some cases to caverns above them, occur in the Drift-beds of the valley of the Little Ouse.
There is yet another means at our command for forming, at all events, an approximate idea of the time that has elapsed since the deposit of the beds containing the remains of the old Quaternary fauna, inasmuch as at the time of their introduction into this country, if not for a lengthened period afterwards, Britain had apparently not become an island, but was still connected by an isthmus of greater or less width with the Continent. To estimate the time, however, that would be required for cutting through this isthmus and widening the Channel to its present dimensions, is a work from which the mind almost recoils. Even the wearing away of that tract of land to the south of the present Hampshire coast, which must almost of necessity have existed at the time when the Bournemouth flint implement-bearing gravels were deposited, taking the present rapid inroad of the sea on the unusually soft cliffs at Hordwell as a guide, would seem to involve a period of not less than 10,000 years; but inasmuch as the cliffs during a considerable portion of the time must have been of chalk instead of sand and clay, and as a chalk cliff 500 feet high, instead of being worn away at the rate of a yard each year, is said only to recede at the rate of an inch in a century,[2] the actual period necessary for the removal of this tract must probably have been many
