GROUND-WATER . 201 



water entering elsewhere. Since the freedom of movement of 

 ground-water is influenced greatly by the porosity of the rock, 

 and since the rock is, on the average, most porous near the sur- 

 face, the movement of ground-water is, on the whole, greatest near 

 the surface and least at its limit of descent. It follows that while 

 the upper part of the ground-water, especially that above ground- 

 water level, moves somewhat freely, the lower part moves more 

 slowly. It is probable, indeed, that movement in the lower part, 

 of the subterranean hydrosphere is extremely slow. 



The amount of ground-water. The porosity of surface rocks 

 varies widely, and the porosity of but few has been determined. 1 

 From such determinations as have been made, it is estimated that 

 the average porosity of the outer part of the lithosphere is some- 

 where between five and ten percent. If the porosity diminishes 

 regularly to a depth of six miles, where it becomes zero, the average 

 porosity to this depth would be half the surface porosity. 2 An 

 average porosity of two and one-half per cent would mean that 

 the rock contains enough water to form a layer nearly 800 feet 

 deep, if brought out to the surface. 3 For the reason indicated in 

 the second foot-note below, it may be that this figure is too large, 

 even for the land. The porosity beneath the sea is probably less 

 than that beneath the land, so that for the earth, this figure is per- 

 haps too high, and is not to be regarded as a measurement. It is 

 the water beneath the surface which justifies the term hydrosphere, 

 as applied to the waters of the earth. 



Fate of ground-water. Most of the water which sinks into the 

 earth reaches the surface again after a longer or shorter journey. 

 Some of it is evaporated from the surface directly; some of it is 



1 For tables see Buckley, Building and Ornamental Stones, Bull. IV, Wis. 

 Surv.; Merrill, Stones for Building and Decoration; and various Survey 

 Reports. 



2 It is probable that the porosity decreases in more than an arithmetic 

 ratio, both because the deeper rocks are not so generally of porous kinds, as 

 at the surface, and because of the pressure which tends to close openings. 



3 Slichter (op. cit., p. 15) estimates that the ground- water is sufficient in 

 amount to cover the earth's surface to a depth of 3,000 to 3,500 feet. Earlier 

 estimates (Delesse, Bull. Soc. Geol., France, Second Series, Vol. XIX, 1861- 

 62, p. 64) gave still higher figures. Fuller, in a recent estimate, places the 

 amount much lower, about 100 feet. Water Supply and Irrigation Paper 

 160, U. S. Geol. Survey. 



