A HISTORY OF KENT of the county the borings indicate two or three thousand feet of strata of intermediate age. Where the uppermost Jurassic beds are preserved they show that after the deposition of the Kimeridge Clay, which appears to have accumulated in waters of some depth, the sea became shallow and its bottom covered with sand (' Portlandian '), and at a later stage ('Pur- beck Beds ') its site was occupied by lagoons of brackish water through the increasing influence of the rivers draining from the land ; until finally the freshwater Wealden conditions were established, under which the older surface rocks of the county were accumulated, as previously described. The northward overlap of the freshwater Wealden deposits across the boundaries of the marine Jurassic series, and the further overlap of the Gault and Chalk across the limits of both, are proof that the relative uplift of the northern district must have been repeated at several stages before the deposition of the Chalk. But after the great Upper Cretaceous subsidence the axis of main uplift was shifted farther southward ; and as already shown, the Wealden anticline was raised over the tract in which the Jurassic and Lower Cretaceous rocks had attained their greatest thickness.' The pressure which caused the upward bulging of the Wealden dome appears to have acted laterally from the south, the thick masses of yielding Secondary sediments, confined by the rigid Palaeozoic slope on the north, obtaining relief from the compression by broad undulation. In this glimpse at the foundation rocks of the county we have been enabled to trace the outlines of its evolution backward to the remoter periods of geological time. But it must be remembered that in other regions there are rocks now exposed at the surface of far higher antiquity than the oldest of those reached by the deep borings in Kent, and that although our records have covered a past that is immeasurable by any time-standard within our grasp, they yet fail by many sons to reach backward to the known limits of geological time. The Palaeozoic sediments of Kent must themselves have had a floor on which to rest ; and our knowledge is bounded only by the limitations of our researches. 1 As pointed out by Topley {Quart. Journ. Geol. Soc. xxx. 1 86, and 'Geology of the Weald,' p. 241), the Wealden dome may have been in part built up by this thickening of the Secondary rocks toward its centre, independently of the effect afterwards produced by unequal uplift.
