230 THE WONDERS OF GEOLOGY. Lect. Ill, 



over Essex, a considerable part of Suffolk, Epping and 

 Hainault forests, the whole of Middlesex, and a portion of 

 Bucks. The tertiary deposits comprised in this series are 

 subdivided as follows : — 



Bagshot Sands, and BracMesham Clays ; — sands, sandstones and 

 foliated marls, and concretionary masses of siliceous sand- 

 stones ; with a medial group of argillaceous beds. 



London Clay; — a tenacious blueish-black clay, with layers of septaria, 

 intercalated sands, shingles and conglomerates. 



With the first division, Mr. Prestwich places the Bracklesham clay ; 

 and with the latter the Bognor rocks : beneath the whole are 

 the mottled clays of Alum Bay, and the Newhaven and Wool- 

 wich beds, all of which are included in the term Plastic clay. 



The site of the metropolis and its environs consists of the 

 London clay, which abounds in marine remains, and consti- 

 tutes the great mass of the materials that fill up this ancient 

 gulf of the ocean. Immediately upon the chalk, how- 

 ever, there are thick beds of sand and mottled clay, called 

 Plastic clay (from the Argile plastique of the Paris basin), 

 in which fresh-water shells, plants, and drifted wood are 

 found in some localities. In other instances, layers of 

 green sand lie upon the chalk, which at Reading contain 

 immense quantities of oyster shells : a similar accumulation 

 of shells occurs near Norwood, and at Headley in Surrey, a 

 few miles from Reigate. At Bromley, in Kent, there is an 

 extensive bed of oyster-shells with pebbles of chalk-flints 

 interspersed, the whole being cemented together by a cal- 

 careous deposit into a remarkable shell conglomerate, 

 which is in much request for grottoes and ornamental rock- 

 work. 



The London clay is found immediately beneath the 

 gravel which so generally constitutes the subsoil of the 

 metropolis ; it is of great extent, and varies from 300 to 

 600 feet in thickness. The blue clay produces a dark, 

 tough soil, with occasional intermixtures of green and 

 ferruginous sands, and variegated clays. It abounds in 



