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 



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