deposits in Belgium are of Miocene age-these are the sands of Antwerp and the Bolderberg, which rest unconformably upon the Oligocene (Rupelien) clay, and have a pebbly basement bed which contains flints and rolled septaria; the lower part of these sands is very dark and argillaceous, and contains many shells in the position of life; the higher sands are less argillaceous, but still dark coloured from the presence of glauconite, and the whole group was formerly termed the "black crag."

There is some reason for thinking that similar deposits originally extended across the North Sea into the east of England, for the pebble beds at the base of the Suffolk Crags contain nodules of greenish or reddish-brown sandstone, which enclose casts of some of the "black crag" fossils. Professor Ray Lankester has also recorded teeth of a Mastodon whale and shark (Carcharodon) in the same matrix, and he is of opinion that many of the mammalian bones found in these nodule beds have been derived from strata of Miocene age, of which no other traces now remain.

Pliocene Epoch.-The Pliocene series is divisible into an older and a newer group, and the British beds of this age may be classified as follows:

:

(Forest Bed Group. Newer.{Norwich and Red Crags.

The oldest Pliocene deposits in England are certain ferruginous sands which at Lenham and other places on the North Downs in Kent and Surrey are at or above the level of 600 feet. They were first described by Professor Prestwich in 1857, and correctly referred by him to the Pliocene, but Ray Lankester, "Quart. Journ. Geol. Soc.," vol. xxvi. p. 493. 2 "Quart. Journ. Geol. Soc.," vol. xiv. p. 322.

the imperfect nature of the fossils then found, and the apparent presence of some Eocene species, led others to regard them as Lower Eocene. In 1886, however, Mr. Clement Reid was able to settle the question by obtaining fresh specimens, which proved the beds to be of older Pliocene age. The sands often contain flint pebbles, and include layers of loamy clay and seams of fossiliferous ironstone, and though their aspect is not that of deep-water deposits, yet the perfect and unworn condition of the shells shows that they are not shore deposits, and the fauna seems to indicate a depth of at least 20 fathoms.

The Lenham Sands occur at intervals along the Downs from the heights above Folkestone to Chipstead, near Croydon,' but though they are now found along this narrow tract, it is evident that they are the remnants of a formation which was once far to the north, and also some distance to the south of this line. It would appear, in fact, that though the upheaval of the Wealden area commenced in Eocene times, and though this district was exposed to erosion throughout the times of the Upper Eocene, Oligocene, and Miocene, yet that the great plane of denudation out of which the present surface of the Weald is carved was formed by the waters of the early Pliocene sea. Consequently, if we would restore the surface over which the Lenham Sands were laid down, we must imagine a seafloor stretching northward from the summit of the Chalk escarpment at a high level above the Isle of Sheppey and the estuary of the Thames. Fig. 6 is an attempt to restore this horizon, and, allowing for some subsequent elevation over the Wealden area, to show the position which the base of the Lenham Sands would now occupy if they had not been so largely removed by subsequent erosion. Over

At present, however, fossils have only been found at Lenham, and it is open to doubt whether the Chipstead sands, and those of Headley still further west, are precisely of the same age.

V

Sheppey and Essex they doubtless rested on some member of the Bagshot group, and thence their base-line passed across the outcrops of the Lower Eocene and Cretaceous strata till it rested on the Wealden beds over the central axis of the Wealden area (see fig. 6).

Outliers of similar ferruginous sands occur also at intervals in France and in southern Belgium, the level of their base-line sinking lower and lower to the eastward. Thus, at Cape Blanc-Nez, they are about 500 feet above the sea; at Cassel, near Ypres, their level is 470 feet; above Renaix it is 445; at Gramont 380; on the heights above Brussels it is only 245, and near Diest, though the level varies considerably, their base nowhere rises above a height of 200 feet.1 Northward the base-line sinks still more rapidly, and is less than 20 feet above the sea near Anvers (Antwerp). Yet throughout this range the sands are similar in constitution and contents, proving that the present differences of level are due to subsequent differential movement of the land. Around Diest and Tessenderloo they cover a large area, and attain a thickness of nearly 100 feet; at Antwerp they contain many fossils, and form the zone of Isocardia cor, passing beneath newer Pliocene beds. Northwards, as they pass beneath Holland, their thickness increases, for at Utrecht a deep boring passed through the overlying beds and 410 feet of Diestien Sand without reaching their base, though carried to a depth of 1,208 feet. The range of these beds in Belgium and the north of France is shown in fig. 7, the eastern portion of which is reduced from Vandenbroeck's map.

Returning to England, fine yellow sands, sandstones, and sandy marls of somewhat similar character form the lower division of what is known as the Coralline Crag in Suffolk, and as nearly all the Lenham species occur also in

1 Vandenbroeck, "Bull. Soc. Belge Géol.," &c., tome i. p. 51, and map.