SPRINGS. 531 



structure.' Thus the rain falling on the ground filters through the 

 porous rocks, or finds its way through joints and crevices in denser 

 rocks, which generally exhibit its action in furrows and channels. 

 It sinks until it is arrested by a stratum of clay or marl, and 

 this, although only two or three inches thick, may, if continuous, 

 be sufficient to retain the water, which either collects or issues at 

 the outcrop of the strata on the side of a hill in the form of springs. 

 Caverns and fissures in limestone rocks, such as the Carboniferous 

 Limestone, constitute many underground watercourses. Much rain- 

 water is absorbed by the soil and vegetation ; but on clayey strata 

 it either collects at the surface or forms natural channels, which 

 carry it away in streamlets. The drainage of the land is therefore 

 both superficial and subterranean. Moreover, rivers flowing over 

 pervious rocks have in course of time clayed their own beds and 

 rendered them water-tight ; in other cases, however, notably on the 

 Cotteswold Hills, a considerable diminution in volume of certain 

 streams has been noticed, owing to their loss in passing over 

 porous strata.^ 



The circulation of water above and below ground is an im- 

 portant study, the general direction in many cases being totally 

 different. Hence in well-sinking attention must be paid to the 

 nature and inclination of the strata, and to the circulation of under- 

 ground waters. 



Water may be obtained in valleys where there is an accumulation 

 of gravel resting on clay. It may be obtained on hills and table- 

 lands formed of alternations of pervious and impervious strata, 

 provided the former come to the surface in places. Near an 

 escarpment the water may be drained off by springs, or by the 

 inclination of the strata, while faults may be instrumental in 

 damming up or in draining off supplies. 



The supply of water in wells depends more or less directly upon 

 the rainfall, but the deeper wells, as a rule, are the more constant, 

 because their sources of supply are more widespread. 



The outcrop of a porous rock may receive a considerable 

 quantity of water, which may be retained within it by the presence 

 of clayey beds both above and below ; and should the outcrop be 

 very extensive, and the strata be characterized by a basin-shaped 

 structure, the water will rise (sometimes above ground) to near 

 the level of the outcrop of the water-bearing stratum, when 

 tapped at or near the centre of the basin. Upon this kind of 

 structure the system of Artesian ^ wells depends, and it is 

 exemplified in the London Basin, where many wells are sunk 

 into the Chalk which crops out to the north and south of the 

 metropolis, contains at its base the impervious Chalk Marl, and 



^ See Prestvvich, Water-bearing Strata around London, 185 1 ; De Ranee, 

 Water Supply of England and Wales, 1882 ; Reports on Circulation of Under- 

 ground Waters, Brit. Assoc. 1875-86 ; Reports Royal Comm. on Water Supply, 

 (G. Mag. 1869, p. 414). 



^ J. H. Taunton, Proc. Cotteswold Club, vi. 305. 



^ So named from the old province of Artois (Artesium), in France. 



