GEOLOGY 



in the lower than in the upper part of the Gault ; and the majority of 

 the species are confined to certain ' zones ' or bands which represent the 

 thickness of sediment accumulated during the period that successive 

 species flourished in this part of the ancient sea. The demarcation of 

 these life-zones and their comparison with the time-equivalents in other 

 regions have received much careful study/ 



From the coast the Gault, increasing gradually in thickness, stretches 

 inland in a narrow but uninterrupted belt rarely exceeding a mile in 

 width, to the western limit of the county, where it has a thickness of 

 about 200 feet, or nearly twice that of the Folkestone section. Being 

 more perishable than the underlying and overlying formations, its course 

 is marked by a tract of clayey land forming a depression of the surface, 

 bounded by the bold escarpment of the Chalk on the north, and by the 

 rising ground of the Lower Greensand on the south. It is used in 

 several places for brickmaking, and a band of phosphatic nodules at its 

 base was formerly dug at Cheriton near Folkestone for conversion into 

 chemical manure. 



Upper Greensand. — The upper part of the Gault at East Wear Bay 

 is a light grey or buff-coloured marl in which fossils are comparatively 

 rare. This is capped by 1 o or 15 feet of glauconitic sandy marl, which 

 was originally considered to be the attenuated representative of the Upper 

 Greensand, a division that in Surrey attains a thickness of 150 feet or 

 more. It is now believed however that this glauconitic marl is really 

 the basement bed of the Chalk (' Chloritic Marl'), and that the true 

 Upper Greensand only commences in the extreme west of the county, 

 probably near Brastead, where a firm grey micaceous and siliceous rock 

 resembling the ' Malmstone ' of the Upper Greensand of Surrey may be 

 seen beneath the glauconitic sandy marl.'' According to this view the 

 upper part of the Kentish Gault passes laterally westward into the Upper 

 Greensand of Surrey, the one representing the calcareous mud and the 

 other the fine silt deposited at the same time on different parts of the 

 same sea-floor. For this reason it is urged that the Gault and Upper 

 Greensand should be linked together as a single formation, for which the 

 term ' Selbornian ' is suggested.^ This method of classification has there- 

 fore been adopted here ; but it must be remembered that in questions of 

 this kind the system of nomenclature employed is of little consequence 

 so long as the actual facts of the stratigraphical arrangement be definitely 

 understood. 



CHALK 



From the dawn of history to the present day perhaps the best 

 known fact regarding the rock-structure of England has been that the 

 principal part of the framework of Kent is built up of Chalk. 



1 Our knowledge of the fossils of the Gault and their zonal distribution is principally due to the 

 work of F. G. H. Price {Quart. Journ. Geol. Soc. 1874, xxx. 342, and monograph, 'The Gault,' 

 separately pub. Lond. 1879) ; and of C. E. De Ranee {Geol. Mag. 1868, v. 163). The most recent 

 list of these fossils is contained in the Mem. Geol. Survey referred to on the preceding page. 



* Mem. Geol. Suix'ey, ' The Cretaceous Rocks of Great Britain,' i. 9 1 . 



^ A. J. Jukes Browne, ibid. p. 30. 



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