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 A HISTORY OF SURREY which in the east of the county are about 20 feet and in the west from 60 to 80 feet in thickness. The Malm-rock is peculiar in containing a large amount of colloid silica, soluble in alkaline solutions, this material having been derived from organic sources, chiefly from the siliceous spicules or internal framework of sponges which have lived in the ancient sea. Some beds of this stone are valued for building, while the softer kind is dug for rubbing on hearths. It has been extensively worked at Godstone, Merstham, Reigate and other places, not only in open quarries but also by long galleries driven under the Chalk from the outcrop. These stone-bands give rise to a terrace-like feature at the foot of the Downs. They are overlain by 5 to 1 5 feet of marly greyish- green sand, which forms the top of the Upper Greensand and passes up gradually into the Chalk Marl. The soil of this tract is peculiarly favourable to the growth of hops and for orchards. CHALK The general aspect of the Chalk, which is the next formation to demand our attention, is so familiar in England that description seems almost superfluous. It forms the range of the North Downs, and the bold escarpment marking its southward termination runs from east to west across the county, broken only by the transverse valleys of the rivers Mole and Wey. In the west, between Farnham and Guildford, the northerly dip is so steep that the Chalk is rapidly carried out of sight beneath newer beds, its outcrop where it forms the well-known ridge of Hog's Back averaging only about half a mile in width. But eastward from Guildford the dip lessens and the area of Chalk widens out gradu- ally, until in the eastern part of the county it has a breadth of about seven miles. The most striking characteristic of the formation is its homogenous composition throughout its extensive range, both horizontally and ver- tically. From its lowest to its highest beds with a thickness in some parts of England reaching from 1,000 to 1,500 feet, 1 and from the shores of the English Channel to the shores of the North Sea in East Yorkshire this clean white limestone preserves everywhere the same general characters, with only such minor modifications of structure as require special study to discriminate. The whole of this enormous mass has slowly accumulated at the bottom of an open sea as a calcareous mud, made up for the greater part of the relics of generation after gene- ration of lime-secreting organisms, among which the minute shells of foraminifera usually predominate. The presence of any extraneous mat- erial of other than microscopic dimensions in the formation is extremely rare, so that the discovery some years ago in the Hay ling chalk-pit at Croydon of a boulder of granitic rock along with some other fragments alien to the Chalk and a similar discovery more recently in the Middle 1 See Mem. Geol. Survey, ' The Cretaceous Rocks of Great Britain,' vol. i., by A. J. Jukes- Browne, pp. 1-3. TO