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 



