GEOLOGY Dorset ; but the deposits are very unlike the Dorset type, half the thick- ness being shale. Above these come nearly 1,300 feet of shaly strata, with numerous ammonites and other characteristic fossils of the Kimeridge Clay. Of this enormous mass the upper 700 feet is entirely shale, the lower 600 consisting of alternations of shale, shaly sandstone and shaly limestone. The next 1 1 5 feet of strata are sandy shales and soft sandstones, yielding Ammonites biplex near the base and referable to the Portland series, though little resembling the hard rocks of the isle of Portland. Next above the Portland occur Purbeck rocks to a thickness esti- mated by Topley at 400 feet. Of this the lower 70 feet is only known from the Sub-Wealden boring and from a mine which has since been sunk to work the beds of gypsum proved by that boring to occur at the base of the series. The upper half of the Purbeck series can be examined at the surface to the north and north-west of Battle, where it has also been exposed in bell-shaped pits, opened to obtain a calcareous sand- stone and certain grey and blue limestones formerly much used for lime. The associated strata are mainly shales like those of the Isle of Purbeck, and contain a similar mixture of freshwater and marine fossils, the marine species being mostly stunted and small. The character of the rocks and of their included fossils suggests an estuarine or lagoon origin for these strata, for gypsum is a product of salt lakes and lagoons, and the abundant remains of brackish-water shells and entomostraca, belonging to few species and fewer genera, point to similar conditions. Two ferns and some insect remains have also been found ; but the curious small marsupials and the numerous cycads which occur in Dorset have not yet been discovered in the Purbeck rocks of Sussex. Cretaceous follow the Jurassic strata without a break, estuarine deposits more than 1,000 feet thick, known as Wealden, indicating a continuance of conditions very similar to those which held during the Purbeck period. In their lower part the deposits of the Wealden period consist mainly of sands, the Hastings and Tunbridge Wells sands, with subordinate masses of sandstone, shale or clay ; but above these comes a mass of clay, with little sand, several hundred feet in thickness. So much interest has been excited by the occurrence of remains of the gigantic land reptile Iguanodon in Tilgate Forest* that it is scarcely realized that the Wealden strata are very sparingly fossiliferous. Beds of freshwater shells such as form the well known ' Sussex marble,' or shales rendered fissile by multitudes of the minute valves of a bivalve entomostracan, occasionally occur ; but it is quite possible to. search closely a hundred feet of strata and not find a fossil. Those fossils that occur tend rather to link the Wealden with the Purbeck below than with the Cretaceous above ; but the Wealden fossils have been derived mainly from the lower part of the series, and we have still a most imperfect knowledge of those belonging to the Weald Clay. Besides land reptiles we find one or two small mammals closely allied to those of the Purbeck period, 1 Mantell. Fossils of the South Downs ; or. Illustrations of the Geology of Sussex (410, London, 1822). 5
