Page:VCH Kent 1.djvu/50

 A HISTORY OF KENT lying to the westward and north-westward of the Weald. But this matter will require re-discussion in the light of evidence recently obtained, which seems to point to the principal drainage-area having lain to the eastward. The method of classification by which the whole of the Hastings Beds are placed in the Lower Cretaceous system, as in the accom- panying map and in Table II., though hitherto generally adopted, has lately given rise to much discussion by reason of the Jurassic affinities of many of the fossils. It is indeed probable that the lower part of the series is the freshwater equivalent of marine deposits in other dis- tricts which have been classed as the uppermost beds of the Jurassic system. This however is a question of technical terminology into which we need not enter further.^ WEALD CLAY The northerly dip soon carries down the Tunbridge Wells Sand beneath the surface in southern Kent ; and to the northward the over- lying Weald Clay occupies a broad belt of low ground stretching from Romney Marsh on the east to the Surrey boundary on the west, with a width varying from about 4 to 6 miles. This thick mass of clay, with a depth reaching 700 feet or more in the west of the county and in- creasing to 1,000 feet in Surrey, represents the continuation of the same freshwater conditions that had previously brought about the deposition of the sandy Hastings Beds, though the presence of a few dwarfed oysters here and there in the uppermost layers of the clay,* indicates that brackish water began to find its way into the area toward the close of the period. This clay represents the muddy detritus from the land, deposited quietly in the deeper and stiller parts of the lake or lagoon. Its great thickness denotes the long-continued prevalence of the freshwater con- ditions ; and also that the lake-floor was sinking gradually during the period, so that the complete infilling of the basin was never effected. Slow subsidence of this kind appears to be in progress at the mouths of many large rivers at the present day, and may perhaps be caused by the gradual depression of the earth's crust by the weight of the sedi- ments accumulated over such tracts. Since the Weald Clay as a whole overlies the Hastings Beds it has been usually assumed that the full sequence has been successively deposited throughout the Wealden district. But we may here note that the maximum thickness of both divisions is not known to occur in the same area ; and after consideration of the evidence from deep borings in Kent and Sussex, and from the field-relations of the equiva- i See Prof. O. C. Marsh, Geo/. Mag. (1896), dec. 4, iii. 8 ; A. S. Woodward, Geo/. Mag. dec. 4, iii. 70 ; A. C. Seward, Nature (1896), liii. 462 ; and G. W. Lamplugh, Geo/. Mag. (1900), dec. 4, vii. 443. Lamplugh, in Summary of Progress of the Geo/offca/ Survey for 1900, p. 116) and in the Isle of Wight {Mem. Geo/. Survey, ' Isle of Wight,' p. 1 5). 8
 * At Hythe in Kent (F. Drew, Quar(. Joum. Geo/. Soc. xvii. 280) ; also in Surrey (G. W.