12 J. L. LOBLEY — CRETACEOUS BOCKS OF ENGLAND. 



the northern edge of the London Tertiary Basin, and the geology 

 of this neighbourhood is consequently very interesting. 



Sections showing the Lower Eocene beds reposing on the Chalk 

 are to be found near Eushey, where the Tertiary "Woolwich and 

 Eeading Series, and the lowest or Basement Bed of the London 

 Clay, may be seen lying on the Secondary Chalk ; and nearer 

 "Watford, Glacial and Post- Glacial beds of gravels and sands are 

 displayed, covering the Chalk "without any intermediate Lower 

 Tortiarics. In these exposures, too, may be seen sections of curious 

 lengthened sinkings in the Chalk, filled with sand, which have been 

 called sand-pipes, telling us of the gradual subterranean erosion 

 or dissolving away by cai'bonated water of the Chalk at those 

 points where the water from above has had more ready access. 

 From the sections of the Lower Tertiaries, organic remains, chiefly 

 fish teeth, may be without difficulty obtained, and in the Chalk 

 will be found the more common of the echinoderms which the IJpper 

 Chalk everywhere contains. 



The great bed of the Chalk and "Upper Greensand underlying the 

 London area forms a vast reservoir for the holding of water, since, 

 lying as it does between the London Clay above and the Gault 

 below, it holds the water which falls where the Chalk or Tpper 

 Greensand forms the surface rock south and north of the Thames 

 Valley. This great reservoir is tapped whenever a well is sunk in 

 the London area to the lower bed of the Chalk. Such Avells are 

 numerous, and give to London a considerable portion of its wator 

 supply, since most of the great breweries and manufactories of 

 London obtain their large supplies of water in this manner. These 

 wells are artesian, since the water rises above the level of the rock 

 which yields it, and even higher than the surface of the ground, con- 

 sequent upon the outcrop of the Chalk at AVatford and Croydon 

 being higher than the top of the wells. Tlie rainfall of tliis district, 

 therefore, contributes in no inconsiderable degree to the water 

 supply of the metropolis. 



To the south-west, north, and north-east the Chalk constitutes 

 the whole of the country for a long distanc(> from Watford, and 

 foi-ms nearly the whole of the County of Hertford, covered, how- 

 ever, in many places by superficial gravels and sands of glacial and 

 post-glacial age, which diversify the vegetation and contribute 

 greatly to the richly wooded and beautiful aspect of this county. 

 The Chalk has been so repeatedly spoken of as a whole, that it 

 may be thought to be unvaried in character throughout. Tliis is 

 not the case, however, several divisions having been described, 

 three of which are conspicuously observable. Tlie Chalk is usually 

 divided into the Chalk Marl, the Lower Clialk, and the Upper 

 Chalk. 



The lowest division, the Chalk Marl, is a transition bed from the 

 Upper Greensand to the Chalk, and contains those green glauconitic 

 or chloritic grains which give the nanu> to the underlying beds. 

 Sometimes these lower strata assume an indurated character, and 

 vaiyiug lithologically yield the well-known Firestone of Godstoue, 



