236 



THE WONDERS OF GEOLOGY. Lect. III. 



Artesian wells around London. Argillaceous strata are 

 generally found to be dry within, and the blue clay con- 

 fines the water contained in the sands beneath ; the engineer 

 therefore perforates the clay, introduces tubes, and taps the 

 natural tank, and the imprisoned waters rush to the surface ; 

 by this method perennial fountains have been formed at 

 Tooting, Hammersmith, Fulham, &c* 



The wells sunk into the London clay (Lign. 35, 1, rf, d,) do 



3 



^^D 



London. 3 



a b d c b d c 



Lign. 35. — Plan op the Artesian wells near London. 



1. The London clay. 2. Plastic clay and sand. 3 The chalk. 



a, b, b, Borings that reach the chalk, c, c, Wells in the plastic clay, d, d, Wells 

 in the London clay. 



not yield pure water ; but the sandy strata alternating with 

 the clays, afford a supply, the quantity and quality of which 

 depend on the nature of the rock. The borings which 

 reach the sands of the plastic clay (Lign. 35, 2, c, c,) furnish 

 good soft-water ; but the wells that extend to the chalk 

 (Lign. 35, 3, a, b, &,) obtain the most copious springs, the 

 water often rising to the surface in a perennial fountain. f 



* Consult Dr. Buckland's Bridgewater Essay, p. 561 ; and an 

 admirable Essay on Artesian Wells, in the Mining Journal, conducted 

 by H. English, Esq. F.GLS. 



f As the white chalk, into which the Artesian borings in 

 London are generally carried some twenty or thirty feet, is not a re- 

 tentive rock, but on the contrary is so porous that not a natural pool 

 or lake is to be found throughout the chalk districts, except under 

 some peculiar local conditions, a few words must be added, to ex- 

 plain this anomaly. The chalk-marl, which is the first retentive 

 bed of any considerable extent in the cretaceous formation, lies at a 

 depth of many hundred feet beneath the bottom of the London basiu, 

 and therefore could not, in ordinary conditions, form a reservoir that 

 could be reached by artificial means, if the strata were either horizontal, 



