The name 'Thanet Sands' was given by Mr. Prestwich from the fact that they are best exhibited, and marked by organic remains, in part of the Isle of Thanet and adjacent. country.

The series consists of pale-yellow quartzose sand and loam, with an admixture sometimes of greenish particles, and forming, according to Mr. Prestwich, an impure argillaceous greyish greensand. He states that a marked feature is the occurrence of green-coated flints at the base of the deposit, and immediately reposing on the Chalk.

The thickness of the beds is variable: under London it is about 20 feet, in West Kent upwards of 60 feet. Mr. Whitaker has made out the following subdivisions in the Thanet Beds:

(e) Fine sharp light-grey sand, slightly greenish, often iron-shot, with layers of calcareous sandstone here and there; the fossils sometimes silicified; thins eastward, and is almost confined to East Kent, where it attains about 40 feet in thickness, and passes down into—

(d) Bluish-grey sandy marl, weathering to a pale yellowish-grey, often rather hard, with green grains and fossils, more sandy at top; thins westward, and is almost confined to East Kent, where it is the thickest member of the series near and beyond Canterbury.

(c) Fine light-buff sand, mostly soft, with few fossils (only some very obscure remains have been found): it is thickest in West Kent (up to 60 feet or more), where for the most part it forms nearly

the whole of the Thanet Beds, thinning out westward in Surrey, and eastwards in Kent.

(b) Alternations of brown c'ay and loam, without fossils, thin and local (to part of East Kent).

(a) The 'base-bed,' clayey greensand, with unworn greencoated flints, resting on the Chalk, thin (rarely over 5 feet) but constant.

Mr. Whitaker mentions that in every large section the junction between the Chalk and Thanet Beds is even; where, however, there is but a thin capping of the latter, it often fills pipes,' irregular-shaped hollows, that have been formed since the deposition of the beds, by the infiltration of water with carbonic acid in solution.

He doubts there being proof of any eroded surface' of the Chalk (that is to say, a surface worn down before the deposition of the succeeding beds) below the older Tertiary formations in England, whether it is overlaid by the Thanet Beds, or, in their absence, by the Woolwich and Reading Beds: an uneven surface is not of necessity an unconfor

mable one.

The Thanet Beds yield remains of Mollusca, Polyzoa, Crustacea, Entomostraca, Echinodermata, Spongida, Foraminifera, and Plants. Among the more prominent species are the Mollusca Cyprina Morrisii, Pholadomya cuneata, Corbula Regulbiensis, and Aporrhais Sowerbyi.

The beds are well exposed in the cliffs of Pegwell Bay, in those east of Herne Bay, and in pits at Charlton near Woolwich and the south of London; they are met with in deep wells under London, but do not extend further west than Windsor, nor do they occur in the Hampshire Basin.

Their occurrence near Sudbury on the northern side of the London Basin had not been noticed until recently, when Mr. Whitaker drew attention to them.

The Thanet Beds are a marine deposit of shallow water, formed in a sea open to the north.

The deposit of green-coated flints at the base is considered by Mr. Whitaker and Professor Hughes to have been formed after the deposition of the Thanet Sand by the decomposition of the top of the Chalk.

In Berkshire Mr. Whitaker has sometimes noticed a bed of reconstructed chalk immediately overlying the Chalk and capped by Reading Beds. This reconstructed bed, which may be 20 feet in thickness, contains blocks of chalk, scattered lines of flint, and much rubbly chalk. It may possibly have been formed at the period when the Thanet Sands were elsewhere deposited.

Allophane (Hydrous silicate of alumina) occurs at the junction of the Thanet Beds and Chalk at Charlton near Woolwich, and Silicified wood is met with in the Thanet Beds in East Kent.

WOOLWICH AND READING BEDS.

PLASTIC CLAY.

This series, named by Mr. Prestwich from the localities where it is characteristically developed, consists of alternations. of plastic clay, loam, and sands variegated in colour, and of pebble-beds of rolled flint, which are sometimes hardened into pudding-stone.

The thickness of the beds varies from 15 to 90 feet in the London Basin; and in the Isle of Wight from 84 feet in Alum Bay to 163 feet in Whitecliff Bay.

It has been shown, and mainly through the researches of Mr. Prestwich, that three distinct conditions of the Woolwich and Reading series are developed.1

1 See Whitaker, Mem. Geol. Survey, vol. iv. pp. 98, 99.

1. In the Hampshire Basin, and in the London Basin all along the northern outcrop, and on the western part of the southern outcrop from Berkshire through North Hampshire and the greater part of Surrey, this series is generally unfossiliferous. It consists of irregular alternations of clays and sands; the former of many and bright colours, mostly mottled and plastic; the latter also of many colours, both coarse and fine, sometimes with flint pebbles, and now and then hardened into sandstone or conglomerate; loam also occurs.

2. In the eastern border of Surrey, in West Kent, the border of East Kent, and partly in South Essex, we find, with the light-coloured sands, finely-bedded grey clay, mostly crowded with estuarine shells, and often with oyster-shells compacted into rock. Above this there is often (on the south-east of London) a fairly thick bed consisting of thin alternations of sand and clay, or loam; at the base of the shelly clays there is generally a bed of imperfect lignite, and lower down sometimes a pebble-bed.

3. In East Kent we find the simplest state of this formation, which there consists throughout of light-coloured sharp false-bedded sand with marine fossils, differing alike from the estuarine Woolwich Beds and the changeful Reading Beds. One part of the formation is fairly constant. Where it rests on the Thanet Beds this lowest member of the Woolwich and Reading Series is a greensand, more or less clayey, with flint-pebbles and here and there oyster-shells. Where, however, it rests on the Chalk it is usually of a more clayey character; the flints at its base are angular and greencoated, instead of being in the state of perfectly rolled pebbles; as before, there are sometimes oyster-shells in the greensand, and besides there are (somewhat rarely) casts and impressions of other shells in the accompanying roughly laminated grey clays (with green grains). The difference in the condition of the flints in the two cases is just what one

:

would expect in the latter they have been got direct from the underlying Chalk, whilst in the former they must have been carried some distance, and therefore worn. Although sections do not show any unconformability between this formation and the Chalk, yet there may be some unconformability, so gradual that it can be inferred only by the comparison of a series of distant sections. (Whitaker.)

The fossils of the Woolwich and Reading Beds include Mammals, Birds, Reptiles, Fishes, Mollusca, Polyzoa, Crustacea (including Entomostraca), Spongida, Foraminifera, and Plants.

The tapir-like Coryphodon has been met with in these strata, also Turtles among the Reptiles. The Mollusca include Ostrea Bellovacina, Melania inquinata, Cyrena cuneiformis, C. Dulwichiensis, Unio, Paludina, Pitharella, &c.

The Bottom bed is often termed the Oyster-bed from the frequent abundance in it of Ostrea Bellovacina.

At Loam Pit Hill, near Lewisham, the Woolwich Beds have been well exposed: indeed the succession is complete from the Chalk to the London Clay. The thickness of the Woolwich Beds here is about 50 feet, and comprises lightcoloured sand, laminated clay, a shell-bed with Ostrea and Cyrena, and a pebble-bed 12 feet in thickness.

At Peckham the Paludina-bed or Cockle-bed, a thin layer of grey clayey limestone, occurs near the top of the Woolwich Beds; it contains Paludina lenta. At this locality Mr. Rickman first noticed Pitharella and Cyrena Dulwichiensis.

The Hertfordshire pudding-stone is composed of flint pebbles embedded in a siliceous matrix, and so firmly cemented that the pebbles are fractured equally with the matrix.

In the Isle of Wight the beds may be studied in White