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 GEOLOGY Chalk at Betchworth have attracted much attention. 1 These stones have probably been rafted from a distant shore either by floating ice or en- tangled in the roots of a fallen tree, whence they have dropped and sunk to the mud of the sea-floor. In this mud the remains of many of the denizens of the sea have been embedded and preserved, so that the Chalk is famous for the beauty and variety of its fossils, which in many parts of the mass can be extracted and cleaned from their matrix with little trouble. Among these fossils we may note the teeth and other hard parts of extinct fish and reptiles ; the shells of Ammonites and several other extinct genera of Cephalopoda ; a few univalve and many bivalve shells, the latter including characteristic species of Inoceramus and Spon- dylus ; the tests of sea-urchins, often in great profusion, the different species and gradual evolution of which have been closely studied, and have afforded a method of dividing the Chalk into zones where the same- ness of composition would otherwise render this task difficult or im- possible * ; abundant traces of sponges whose siliceous spicules have supplied most of the material for the nodules of flint which are a con- spicuous feature in a considerable part of the Chalk ; and the remains of corals, polyzoa, etc. In Surrey the Chalk is thinner than in most parts of its range in England, and in some places has evidently lost a considerable portion by denudation before the deposition of the Eocene beds, 3 especially where, in the south-east of the county, the Blackheath Beds rest directly upon it and the older divisions of the Eocene are absent. Where the last-men- tioned conditions prevail, its present thickness is estimated at about 500 feet ; but as we shall presently see, where penetrated in deep borings far- ther north it was between 600 and 700 feet thick, while in a boring at East Horsley a thickness of 8 17 feet was proved. 4 Its northerly dip carries it down beneath the Tertiary strata along a line running from the vicinity of Farnham, past Guildford, Leatherhead, Epsom and Sutton, to the eastern boundary of the county near Croydon ; and it is not again seen at the surface to the northward of this line in Surrey, except at one spot at the north-eastern boundary presently to be mentioned. It is known, however, from the evidence of numerous borings, to underlie the newer strata throughout the northern part of the county ; and by a reversion in the direction of dip is brought gradually nearer the surface again further northward, until it emerges to form the Chalk range running across the south-midland counties west and north of London (see 1 See R. A. C. Godwin- Austen, Quart. Journ. Geol. Soc., vol. xiv. p. 252 ; and W. P. D. Stebbing, ibid. vol. liii. (1897) PP- 213-220. the zones of Rh. Cuvieri, and M. Cor-anguinumJ Quart. Journ. Geol. Soc., vol. Iv. (1899) pp. 494-548 ; and 'The Zones of the White Chalk of the English Coast,' Proc. Geol. Assoc., vol. xvi. (1900) pp. 289-368. 8 For discussion on this point see Prof. J. Prestwich in ' Geological Inquiry respecting the Water-bearing Strata of the Country around London' (London : Van Voorst, 1851), and W. Whitaker in Mem. Geol. Survey, ' The Geology of the London Basin,' p. 23. Nat. Hist. Club, 1894-95, p. 138. TI
 * See especially the recent papers of Dr. A. W. Rowe, ' On the genus Micraster, between
 * See W. Whitaker, ' Some Surrey Wells (Second Paper),' Trans. Croydon Microscop. and