6o HAMPSTEAD HILL. 



Croydon, thus forming a basin in which repose the later-formed 

 Lower London Tcrtiaries and the still later London Clay. 



Underlying the whole of the Chalk, the Upper Greensand and 

 the Gault extend quite comformably, as geologists say ; that is, 

 they have the same angle and direction of dip, and are conse- 

 quently parallel with the Chalk above. The former of the two, 

 the Upper Greensand, though in the main sandy, has some 

 valuable beds of stone, pardy calcareous, called " Fire-stones." 

 The green grains giving the name " Greensand " consist of a 

 silicate of iron. 



A more important formation is the Gault, which has a thickness 

 in some places of 200 feet, and, being a fine, blue clay, is largely 

 worked, where it is at the surface, for the manufacture of tiles and 

 red bricks. 



It is the basin-like extension of the porous, and therefore 

 water-bearing and water-yielding, Chalk overlying a great bed of 

 impervious clay (the Gault), and covered by a great bed of im- 

 pervious clay (the London Clay), that constitutes this formation 

 the great reservoir of water available for the supply of the 

 inhabitants of the Thames Valley whenever wanted ; and it is this 

 geological structure of the valley of the Thames that gives the 

 conditions necessary for the formation of those " artesian wells " 

 which at present furnish so easily and so continuously an 

 abundant supply of the purest water to our great breweries and 

 other large manufactories. The water of the concave porous 

 chalk strata lying between the impervious beds of clay rises freely 

 in borings at places lower than the level of the super- saturation of 

 the chalk where that rock forms the surface, and where it receives 

 the water from the rainfall of Hertfordshire, Kent, and Surrey. 



