Page:VCH Kent 1.djvu/55

 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. ^ A. J. Jukes Browne, ibid. p. 30. 13
 * Mem. Geol. Suix'ey, ' The Cretaceous Rocks of Great Britain,' i. 9 1.