ARTESIAN WELLS. 81 



constantly in the lower part of the series a bed of 

 green sand, which abounds in oyster shells, and 

 pebbles, and in some localities (as near Bromley) 

 passes into a hard rock or shelly conglomerate. 

 Lignite, leaves of land-plants, and freshwater 

 shells are contained in some of these layers ; but 

 in many places there are no remains of this kind 

 found in the eocene beds immediately above the 

 chalk; and the term plastic clay, as indicating a 

 particular group, can only be regarded as of limited 

 application.* 



Artesian wells. — From the alternation of 

 porous sandy strata with stiff impervious clays 

 throughout the London basin, the district around 

 the metropolis is favourable for obtaining water, 

 by means of the borings, termed Artesian wells ; 

 by which perennial fountains are raised from 

 the natural reservoir of water in the lowermost 

 arenaceous deposits. The nature of these springs 

 is easily explained. The beds of sand under 

 the clay are fed by the rain that falls on the 

 uncovered margin of the basin, and the water 

 percolating through the porous strata, gradually 



* The term plastic clay (argille plastiquc) was given to this lowermost divi- 

 sion of the eocene strata in conformity with the nomenclature of the French 

 geologists ; but the beds thus named are less developed in the London than 

 in the Paris basin, and do not with us possess distinctive characters suffi- 

 ciently important to warrant their separation from the other intercalated fresh- 

 water deposits of this formation. 



