GEOLOGY lent beds in west Sussex and the Isle of Wight, it appears to the writer that part of the sediments classed as Weald Clay in one district may be equivalent to strata classed as Hastings Beds in another part of the Weald. This point is mentioned because of its practical con- sequence in cases where it is intended to penetrate the Weald Clay by borings for water-supply or other purposes. The Weald Clay is interstratified at intervals with thin bands of sand and silt, and with layers of limestone made up almost entirely of a freshwater shell of the genus Paludina. This limestone, often known as ' Bethersden marble,' from a locality where it was extensively dug, was formerly much used, like the ' Sussex marble ' of similar origin, in ecclesiastical architecture, both as a polished stone and unpolished, as for example in the church towers of Headcorn, Smarden, Biddenden and Tenterden, and in the polished altar stairs of Canterbury Cathedral. The fossils of the Weald Clay resemble those of the Hastings Beds, being chiefly freshwater shells and cyprids, with the teeth and scales of fish and the remains of land plants.^ Where exposed at the surface the clay forms a heavy tenacious soil, expensive and difficult to cultivate. But from its low-lying position much of its outcrop is overspread by alluvium and other superficial deposits, and the tracts thus modified are very fertile and embrace some of the principal hop gardens of the county. The clay itself is dug in many places for brickmaking. Deep borings have shown that this division undergoes the same rapid diminution in thick- ness in its northward underground extension as the Hastings Beds, and that it thins out entirely before reaching the north-eastern border of the county.^ LOWER GREENSAND The invasion of the sea, of which, as already noted, there are slight preliminary indications in the brackish water fauna towards the top of the Weald Clay, appears to have become suddenly accelerated at the close of the Wealden period, so that the long prevalent freshwater conditions were abruptly terminated and the whole district submerged beneath the tides of an encroaching ocean. The marine conditions thus established were thenceforward persistent throughout the remainder of the Lower Cretaceous and the whole of the Upper Cretaceous times. During the first stages of this great period of submergence the Atherfield Clay and Lower Greensand were deposited ; afterwards the Gault Clay and Upper Greensand; and finally the thick white mass of the Chalk. Minor oscil- lations of level during this long submergence were frequent, rendering the sea now deeper and now shallower, and the coast-line sometimes near and sometimes more remote ; and thereby causing modification or change of character in the sediments. Indeed it is probable that during the earlier stages the shore at times approached within the northern limits 1 For description of the scanty vertebrate remains of the Kentish Wealden, see subsequent article 'Palaeontology,' p. 31. 2 See subsequent records of deep boring-sections, pp. 25-8. I q 2
