430 EOCENE. 



(40 feet — see p. 432), and at Purfleet; and they were exposed at 

 Park Hill, Croydon. ^ 



The Thanet Beds are met with in deep wells under London, but 

 they do not extend further west than Windsor, nor do they occur in 

 the Hampshire Basin. Eastwards they occur near Grays Thurrock 

 and West Tilbury. Their occurrence near Sudbury and near 

 Ipswich on the northern side of the London Basin has been 

 noticed by Mr. Whitaker.- 



Allophane (hydrous silicate of alumina) occurs at the junction of tlie Thanet 

 Beds and Chalk at Charlton, near Woolwich, it also occurs at Purley Downs, near 

 Charlton. Silicified wood is met with in the Thanet Beds in East Kent. The 

 beds furnish a good foundry-sand at Charlton. 



WOOLWICH AND READING BEDS. 



This series was named by Prof. Prestwich in 1853, from the 

 localities in Kent and Berkshire, at which it is characteristically 

 developed. It consists of alternations of mottled plastic clay, loam, 

 and sands variegated in colour, together with pebble-beds of rolled 

 flint, which are sometimes hardened into pudding-stone.^ The 

 term Plastic Clay, originally applied to the series, was adopted by 

 T. Webster from the term. Argile plasiique of Cuvier and Brongniart,* 

 and was given on account of the nature of the clay. 



The thickness of the beds varies from 15 to 90 feet in the 

 London Basin ; and in the Isle of Wight from 84 feet in Alum Bay 

 to 163 feet in Whitecliff Bay. The strata sometimes rest on a 

 slightly eroded surface of the Thanet Beds. 



It has been shown, and mainly through the researches of Prof. 

 Prestwich, that three distinct conditions of the Woolwich and 

 Reading series are developed ; these may be noted as follows :^ — 



A. In the Hampshire Basin, and in the London Basin all along the northern 

 outcrop, and on the western part of the southern outcrop from Berkshire through 

 North Hampshire and the greater part of Surrey, this series is generally unfossil- 

 iferous. It consists of irregular alternations of clay and sands; the former of many 

 and bright colours, mostly mottled and plastic ; the latter also of many colours, 

 sometimes with flint pebbles, and now and then hardened into sandstone or con- 

 glomerate ; loam also occurs. 



B. In the eastern border of Surrey, in West Kent, the border of East Kent, and 

 partly in South Essex, we find, with the light-coloured sands, finely-bedded grey 

 clay, mostly crowded with estuarine shells, and often with oyster-shells compacted 

 into rock. Above this there is often (on the south-east of London) a fairly thick 

 bed consisting of alternations of sand and clay, or loam with impressions of leaves ; 



^ H. M. Klaassen, P. Geol. Assoc, viii. 227. 



2 Q. J. XXX. 401 ; Explan. Sheet 47 (Geo!. Surv.), p. 12 ; Geol. Ipswich, p. 6. 

 ^ Q. J. x. 75. The name Woolwich Loam and Sand was used by John Farey 

 in Sowerby's Mineral Conchology, vol. i. 1812. 

 ■* Description Geol. des Env. de Paris. 

 * See Whitaker, Mem. Geol. Survey, iv. 98, 576. 



