LONDON CLAY. 435 



unconformably on the Woolwich Beds, Thanet Beds, or on Chalk. 

 (See Fig. 74.) ^ 



Near Higham, in the Hundred of Hoo, east of Gravesend, a 

 railway-cutting showed London Clay, resting on 12 feet of Old- 

 haven Beds, which there yielded Carditim, Pectimculus, Natica, and 

 Corbida ; beneath, the Woolwich Beds and Thanet Beds were 

 exposed.^ Fossils have also been obtained at Grove Ferry, a 

 few miles inland, near Oldhaven Gap ; these include Cytherea, 

 Aporrhais, etc. 



The pebble-beds are dug for ballast, etc, and near Canterbury beds of ironstone 

 have been worked for iron-ore and used for road-metal. 



LONDON CLAY. 



This formation, so named by William Smith in 1812, from its 

 development around London, consists of stiff brown and bluish or 

 slate-coloured clay, containing layers of septaria or cement-stones. 

 The clay as a rule is of so homogeneous a nature that it affords no 

 indications of stratification ; * then the only guides are the layers 

 of septaria. At its base it displays green and yellow sandy and 

 loamy beds containing flint pebbles, sometimes ' cemented by 

 carbonate of lime into semi-concretionary tabular masses ; ' this 

 division was termed the 'Basement-bed' of the London Clay by 

 Prof. Prestwich.* The top strata are also sandy, passing, in places, 

 by alternations of sand and clay into the Lower Bagshot Beds 

 above. 



The London Clay contains much iron-pyrites and selenite ; and it is believed 

 that the formation of the selenite may be largely due to the decomposition of iron- 

 pyi'ites and the destruction of organic remains, the sulphuric acid resulting from 

 the decomposition of the pyrites uniting with the carbonate of lime of the fossils. 

 Almost all the London Clay fossils found at Sheppey are impregnated with iron- 

 pyrites ; and Dr. Duncan has observed that sulphuric acid itself may result from 

 the decomposition of organisms. Hence the formation of iron-pyrites and selenite 

 may represent the former organisms ; and as the selenite in process of time may 

 disappear, he argues that there is no reason why the purest clay-slate may not 

 have been formed from a fossiliferous clay.^ 



In thickness the London Clay varies from a few feet in Wiltshire, 

 and from 50 to 60 feet in Berkshire, to nearly 500 feet in the south 

 of Essex. The London Clay yields a very extensive series of 

 fossils, and, although specimens are abundant, it is not always easy 

 to find them, as they occur in groups or colonies conspicuous at 



^ T. V. Holmes, P. Geol. Assoc, viii. 59 ; and Report Lewisham and Elack- 

 heath Assoc. 1S81. 



2 W. Whitaker, P. Geol. Assoc, vii. 190. 



^ See Godwin-Austen, Q. J. vi. 81. 



* Q- J- i'i- 354. vi- 255, X. 401, 435. 



^ Q. J. xxii. 19 ; see also O. Fisher, Q. J. xviii. 82 ; Dixon, Geol. Sussex, 

 ed. 2, 58; Bauerman, Metallurgy of Iron, ed. 5, p. 37. 



