On Artesian or Overflowing Wells. 115 



is buried in the rock. M. Garnier is so convinced of the truth 

 of tliis principle, that he only advises the boring of wells in the 

 valleys of those districts whose elevations contain the outgoing 

 of a cavernous limestone. 



Besides, upon a review of the appearances observed in Arte- 

 sian wells, it is evidently sufficient, that an inclined stratum of 

 a fissured or porous limestone be included between two water- 

 proof beds of clay, one of which sets a limit to t-he sinking of 

 the water downwards, and the other which keeps it back from 

 above. The existence of such a cover is evinced by all boring 

 works : a waterproof stratum of clay must always be pene- 

 trated, before reaching the spring-water. But also, it can easily 

 be conceived, that the undermost layer is never wanting; and 

 although, for the most part, some thinner strata of limestone 

 supply its place, yet the strata, which conduct the water, always 

 contain it in crevices, which are much more numerous on the 

 surface than in the centre of the beds : thus there is a demon- 

 stration, as in a boring-work at Blengel, that, even in the lime- 

 stone itself, beds of clay occur. From these circumstances, it 

 is easily explained how we can never hope to sink Artesian wells 

 in granite, gneiss, porphyry, serpentine, &c. Even in schistose 

 mountains, it would not be advisable to sink these wells, be- 

 cause, even if found, it would be very easily impregnated with 

 sulphuretted hydrogen, from the abundance of pyrites occurring 

 in these strata, and thus be unfitted for many uses. Limestone, 

 on the contrary, which is very Insoluble, experience teaches us, 

 yields a very pure water. 



Other districts, where water has been bored for, shew a simi- 

 lar geognostic constitution to the Pas de Calais. M. Garnier 

 notices this, with regard to Boston, in America, and Sheerness *, 

 in England. London, where many sugar-works, distilleries, 

 and breweries, have, for a long time, been principally supplied 

 with water from Artesian wells, lies in the middle of a basin- 

 shaped hollow, the fundamental rock of which is a limestone - 

 belonging to the chalk formation ; which also forms the heights 



• Very pure and clear water was here found at a depth of 550 feet, under 

 the clay in a chalky limestone, which sprung at first 344 feet high, then 

 sunk, and now remains 120 feet under tlic surface of the ground. 



a.2 



