Page:VCH Kent 1.djvu/75

 GEOLOGY same tracts. It is remarkable also that all the Lower Cretaceous beds, that at the surface stretch, as we have seen, from east to west across the county with only slight modification, are found, when their underground course is traced, to alter their character or to disappear entirely within a few miles to the northward of their outcrop. Deep borings to the north of the Thames have shown that this thinning away of the Second- ary rocks below the Chalk becomes still more pronounced beyond the Kentish boundary. As to the Palaeozoic rocks, beyond the fact that they differ entirely in character in different parts of the county and that the Coal Measures are certainly present at Dover and Ropersole, we have not much definite information, as it has not been found possible to determine the exact age of the lowest beds reached in the Brabourne and Crossness borings on the scanty evidence available. It is clear however that at some time before the deposition of the Mesozoic beds these Paleozoic rocks had formed a land surface, their component strata having been previously disturbed and tilted and brought within the reach of erosive agencies ; so that at the commencement of the Secondary era they had been denuded across the edges and planed down to an uneven floor of diverse composition, upon which the Mesozoic rocks were afterwards deposited. The Triassic conglomerate of the Brabourne section, made up of pebbles of older rocks, bears witness to this ancient epoch of land waste. Early in Secondary times, portions of this land were submerged beneath the sea, and soon the irregular ' Palaeozoic floor ' was buried under the newer sediments, which rested unconformably across the worn edges of the older formations. By unequal movement or tilting, perhaps in gentle stages oft repeated, this floor was raised up northward, so that the Secondary deposits were either unable to accumulate to so great a thickness in that quarter as in the gradually sinking area to the south, or were removed after their accumulation by being brought within the reach of currents and wave-action. Thus may we explain the rapid thinning away northward of all the Secondary rocks below the Chalk, and their great thickness in the more southerly of the Kentish borings and in Sussex. The Jurassic (Lower Mesozoic) beds underlying Kent consist of thick alternations of clays and limestones, the latter frequently showing characteristic round-grained ' oolitic ' structure. These beds, from the Lias upward to the base of the Purbecks, indicate a continuity of marine conditions — at least in the south of the county — and have yielded numerous fossils by which they can be identified and cor- related with beds of the same age in the west of England. The limestones of the ' Corallian ' division, like those of that period in other parts of the country, are crowded with fossil corals, and have probably originated as true coral reefs of the ancient sea. At Crossness the whole of the Jurassic and Lower Cretaceous beds are absent, while at Chatham the attenuated representative of the Lower Greensand rests directly on Oxford Clay although in the south 29