82 GEOLOGY OF THE ISLE OF WIGHT. 



accumulates till a reservoir is formed beneath 

 the central plateau of clay. If this stratum be 

 penetrated, the water must necessarily rise to 

 the level of the fluid in the highest part of the 

 subterranean lake, and may therefore flow to the 

 surface, or even escape in jets, to a considerable 

 height above it. Such is the phenomenon of 

 the Artesian wells in and near London. The 

 impermeable clay confines the water contained 

 in the sand beneath ; the engineer perforates 

 this bed with his borer, and introduces tubes 

 for the passage of the pure element to the 

 surface, and the exclusion of the brackish water 

 that may ooze from the saliferous strata above ; 

 for although the wells sunk into the London 

 clay yield no water, the sands alternating with 

 the argillaceous beds afford a limited quantity. 

 The prevalence of various saline minerals, how- 

 ever, in the clay, such as the sulphates of iron, 

 lime, and magnesia, generally renders the water 

 derived from this source unfit for domestic pur- 

 poses ; and the borings are carried down to the 

 sands of the plastic clay before a copious supply 

 of excellent water can be obtained.* The total 



• Tin' most remarkable instance of success in obtaining a perennial foun- 

 tain from a deep Bource by the process oi boring described in the text, is thai 



of Sri m lit , Dear Paris. An Artesian well is there carried to the extraordinary 

 depth of I. miii I., i, |iassinn through the entire thickness of the eocene strata 



