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A HISTORY OF SURREY map). The Tertiary deposits thus lie in a shallow trough or basin of Chalk, termed by geologists the London Basin. A trough of this kind formed by strata dipping towards a medial line from both sides is tech- nically known as a syncline. This arrangement of the strata is of course the reverse of an anticline such as we have traced out in the Weald. The dome or anticline of the Weald and the trough or syncline of the London Basin, taken in conjunction, are the governing factors in the geological structure of Surrey. To return to the description of the Chalk at its outcrop ; we shall find that when closely examined its successive parts exhibit slight differ- ences of character, which enable us to distinguish the divisions given in the table on p. 3. Of these, the lowest or Chalk Marl consists, as the name implies, of a marly admixture of calcareous and clayey material. Above this comes rather hard greyish chalk, slightly clayey, and then i oo to 1 50 feet or more of white chalk. The beds up to the top of this ' Lower Chalk' division are marked by the absence of flint; but from this horizon to the top of the formation in Surrey flints in scattered nodules and occasionally in thin continuous layers are everywhere present ; and the whole formation is thus roughly divisible into an upper part containing flints and a lower part without flints, of which the former is always thicker than the latter. As previously mentioned, it is however from the suc- cessive appearance and disappearance of the different species of fossils in the slowly accumulated sediment that we are best able to divide the Chalk into zones ; but at present it is only in certain parts of the county that this has been done, 1 and further research on these lines is greatly to be desired. In the Hog's Back ridge the Chalk has scarcely sufficient width to display the typical down-land scenery, but farther east where the outcrop begins to widen it exhibits the rounded steep-sided hills and deep dry winding valleys which are everywhere so characteristic of a chalk country. The elevation of the Downs also increases eastward through Surrey, their highest ground, 876 feet above sea level, occurring near the eastern boundary of the county. The dry thin soil of the Downs where the Chalk immediately under- lies the surface is generally treeless, and covered only with smooth short turf. But these conditions are largely modified in the Surrey uplands by the presence on the hills of an irregular surface-deposit of clay or clayey earth containing many flints derived from the Chalk, hence termed the ' Clay-with-flints.' This material occurs as a variable sheet which fills all the little pits and hollows in the weathered surface of the Chalk wherever the ground is not too steep for it to rest. Its origin bears a simple explanation. We know that the calcareous matter of the Chalk is slowly taken up and carried away in solution by the downward percolation 1 Consult C. Evans' paper, On some Sections of the Chalk between Croydon and Oxtead, with Observations on the Classification of the Chalk,' Proc. Geol. Assoc., supplement to vol i., 1870 ; also that of G. E. Dibley, ibid. vol. xvi. (1900) pp. 489-496. 12