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The Eocene strata occupy two distinct tracts, termed respectively the London and Hampshire Basins. The basinlike arrangement of the Chalk strata upon which these newer deposits lie, is, perhaps, most conspicuous in the Hampshire district, which includes the Tertiary beds of Dorset, Hants, the Isle of Wight, and Sussex. In the London Basin the sea-ward extension of the strata on the east is unknown, but wherever observed the beds rest upon a foundation or basin of Chalk. The formation of these Basins is due to upheaval and denudation subsequent to the Eocene period.

Although there is nowhere any marked unconformity in stratification between the Chalk and overlying Tertiary strata due to erosion in pre-Tertiary times, yet the change from deep-sea conditions to those of comparatively shallow-water is abrupt: the “pires,' or irregular-shaped hollows, that have been dissolved out of the Chalk by the action of carbonated water, are features that may have been produced after the deposition of the newer strata. Nevertheless most of the lower Tertiary beds contain pebbles formed of fint evidently due to destruction of the Chalk; and when we also take into consideration the fact that in Denmark and Holland there are passage-beds between the Cretaceous and Tertiary formations which we do not find to be represented in England, the natural conclusion is that in our country these two great groups of rocks are unconformable.

Fig. 19.—General Section of the London Basin, showing the probable ridge of old rocks.

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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 yellow

ish-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 Bels, thinning out west

ward in Surrey, and eastwards in Kent. (6) 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 green

coated 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 unconformable 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 A porrhais 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 wben 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.


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 See Whitaker, Mem. Geol. Survey, vol. iv.

pp. 98, 99.

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