On Artesian or Overfloxomg Wells. 113 



NW., from between Arras and Lille to Calais : a little to the 

 south of the last of which, tlie Cap-blanc-nez consists of this 

 limestone. 



By far the greater part of the Artesian wells, which are bored 

 in this district, lie to the north of this line, where the newer 

 cover of beds of sand and clay have yet attained no great thick- 

 ness ; and experience teaches us that water is not found till the 

 borer reaches as far as the hmestone, or has penetrated into it. 

 Few wells lie to the south of this line, in the limestone forma- 

 tion itself. But the relations of these last are quite the same as 

 those of the others ; they are found, for example, in valleys of 

 the formation, the bottoms of which are covered with the same 

 masses which form the greater plain ; we even here do not meet 

 with water, till the waterproof stratum of clay, resting on the 

 limestone, has been penetrated. When, which is not unfre- 

 quently the case, water is met with before this, in the beds of 

 sand and loam, its impurity, and the feebleness of its propul- 

 sion upwards, shew that it is derived from quite another source 

 than the pure water of the Artesian wells. 



From these relations, which are elucidated in Gavnier's work 

 by the profiles of several boring works, it is sufficiently evident, 

 that the water, which ascends through the shafts, is always de- 

 rived from the deep-lying points of the limestone strata, from 

 the subterranean slope of the mass of the rock. A farther 

 proof of the Artesian wells deriving their supplies only from 

 this source, is derived from the observation made in several 

 places, as, for example, at Lillers and Bethune, that, when one 

 of two adjoining wells, lying in the same line of direction as 

 that of the limestone formation, is rendered muddy by the pis- 

 ton of the pump, the water of the other is simultaneously milky, 

 from the suspension of minute particles of lime. The origin of 

 the Artesian wells can therefore hardly be doubted. It is well 

 known how numerous and extensive are the fissures, often miles 

 in length, contained in the limestone of this part of France, how 

 quickly the rain-water is absorbed on the high grounds, and 

 how abundantly it re-appears, in the form of springs, at the foot 

 of these hills.* If any proof of this was required from the 



• One of tlie most instructive instances of the passage of water through 

 APRIL JUNE 1830. II 



