This idea receives some countenance from the fact of the tertiary strata, near their junction with the chalk of the London and Hampshire basins, often consisting of dense beds of sand and shingle, as at Blackheath and in the Addington Hills near Croydon. They also contain occasionally freshwater shells and the remains of land animals and plants, which indicate the former presence of land at no great distance, some part of which may have occupied the centre of the Weald.

Such masses of well-rolled pebbles occurring in the lowest Eocene strata, or those called "the plastic clay and sands" before described (No. 3. b, Tab. [p. 197.]), imply the neighbourhood of an ancient shore. They also indicate the destruction of pre-existing chalk with flints. At the same time fossil shells of the genera Melania, Cyclas, and Unio, appearing here and there in beds of the same age, together with plants and the bones of land animals, bear testimony to contiguous land, which probably constituted islands scattered over the space now occupied by the tertiary basins of the Seine and Thames. The stage of denudation represented in [fig. 259.], [p. 249.], may explain the state of things prevailing at points where such islands existed. By the alternate rising and sinking of the white chalk and older beds, a large area may have become overspread with gravelly sandy, and clayey beds of fluvio-marine and shallow-water origin, before any of the London clay proper (or Calcaire grossier in France) were superimposed. This may account for the fact that patches of "plastic clay and sand" (No. 3. b, Tab. [p. 197.]), are scattered over the surface of the chalk, reaching in some places to great heights, and approaching even the edges of the escarpments. We must suppose that subsequently a gradual subsidence took place in certain areas, which allowed the London clay proper to accumulate over the Lower Eocene sands and clays, in a deep sea. During this sinking down (the vertical amount of which equalled 800, and in parts of the Isle of Wight, according to Mr. Prestwich, 1800 feet), the work of denudation would be unceasing, being always however confined to those areas where land or islands existed. At length, when the Bagshot sand had been in its turn thrown down on the London clay, the space covered by these two formations was again upraised from the sea to about the height which it has since retained. During this upheaval, the waves would again exert their power, not only on the white chalk and lower cretaceous and Wealden strata, but also on the Eocene formations of the London basin, excavating valleys and undermining cliffs as the strata emerged from the deep.

There are grounds, as before stated ([p. 205.]), for presuming that the tertiary area of London was converted into land before that of Hampshire, and for this reason it contains no marine Eocene deposits so modern as those of Barton Cliff, or the still newer freshwater and fluvio-marine beds of Hordwell and the Isle of Wight. These last seem unequivocally to demonstrate the local inequality of the upheaving and depressing movements of the period alluded to; for we find, in spite of the evidence afforded in Alum and White Cliff Bays, of continued depression to the extent of 1800 or 2000 feet, that at the close of the Eocene period a dense formation of freshwater strata was produced. The fossils of these strata bear testimony to rivers draining adjacent lands, and the existence of numerous quadrupeds on those lands. Instead of such phenomena, the signs of an open sea might naturally have been expected as the consequence of so much subsidence, had not the depression been accompanied or followed by upheaval in a region immediately adjoining.

When we attempt to speculate on the geographical changes which took place in the earlier part of the Eocene epoch, and to restore in imagination the former state of the physical geography of the south-east of England, we shall do well to bear in mind that wherever there are proofs of great denudation, there also the greatest area of land has probably existed. In the same space, moreover, the oscillations of level, and the alternate submergence and emergence of coasts, may be presumed to have been most frequent; for these fluctuations facilitate the wasting and removing power of waves, currents, and rivers.

We should also remember that there is always a tendency in the last denuding operations, to efface all signs of preceding denudation, or at least all those marks of waste from which alone a geologist can ascertain the date of the removal of the missing strata within the denuded area. It may often be difficult to settle the chronology even of the last of a series of such acts of removal, but it must be, in the nature of things, almost always impossible to assign a date to each of the antecedent denudations. If we wish to determine the times of the destruction of rocks, we must look any where rather than to the spaces once occupied by the missing rocks. We must inquire to what regions the ruins of the white chalk, greensand, Wealden, and other strata which have disappeared were transported. We are then led at once to the examination of all the deposits newer than the chalk, and first to the oldest of these, the Lower Eocene, and its sand, shingle, and clay. In them, so largely developed in the immediate neighbourhood of the denuded area, we discover the wreck we are in search of, regularly stratified, and inclosing, in some of its layers, organic remains of a littoral, and sometimes fluviatile character. What more can we desire? The shores must have consisted of chalk, greensand, and Wealden, since these were the only superficial rocks in the south-east of England, at the commencement of the Eocene epoch. The waves of the sea, therefore, and the rivers were grinding down chalk-flints and chert from the greensand into shingle and sand, or were washing away calcareous and argillaceous matter from the cretaceous and Wealden beds, during the whole of the Eocene period. Thus we obtain the date of a great part at least of that enormous amount of denudation of which we have such striking monuments in the space intervening between the North and South Downs.

Fig. 265.