§ 24. ARTESIAN WELLS. 235 



according to the direction and configuration of the over- 

 lying bed of clay. This state of things will continue, till, 

 by an increased supply, the water rises above the level of 

 the basin or channel, and either overflows, or escapes 

 through fissures in the rocks, to another level. If the 

 course of the waters be subterranean, the beds of slightly 

 coherent stone are worn away, and chasms or caverns are 

 formed ; hence the underground rivers and streams, of 

 great extent, which occur in many places (p. 56); but if the 

 water find its way to the surface, a spring bursts forth. 

 This is the nature of all springs, except those which arise 

 from very great depths, such for example as the thermal 

 waters of many countries ; these probably originate from 

 the condensation of steam, evolved through fissures by 

 volcanic agency. Streams impregnated with the mineral 

 substances contained in the strata through which they flow, 

 are called mineral waters. Those in the tertiary strata 

 near Epsom, contain sulphate of magnesia, whence the 

 name of Epsom salt, given to this substance wherever it 

 occurs.* 



24. Artesian wells. — But pervious strata frequently 

 alternate with others which are not so ; and they, likewise, 

 form basins, the areas of which are partially filled with clay, 

 through which water cannot pass : in such a case it is 

 obvious, that the sands beneath the clay, fed by the rain 

 which descends on the uncovered margin of the basin, must 

 constitute a reservoir, and the water gradually accumu- 

 late beneath the central plateau of clay, whence it cannot 

 escape. If this bed of clay be perforated, either by natural or 

 artificial means, the water must necessarily rise to the surface, 

 and may be thrown up in a jet to a considerable height, 

 which will depend on the level of the fluid in the subterranean 

 reservoir ; such is the phenomenon observable in the 



* See my notes on the Geology of Surrey, in Brayley's History of 

 that county. 



