THE WORK OF GROUND-WATER. 221 



much more slowly. It is probable, indeed, that the movement in the 

 lower part of the subterranean hydrosphere is extremely slight. 



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

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

 determinations as have been made are chiefly on building stones, in 

 which the range of porosity varies from a fraction of one percent., in 

 the case of granite, to nearly 30 percent, in the case of some sand- 

 stones. Building stone is perhaps more dense than the average sur- 

 face rock. Furthermore, such tests as have been made do not take 

 account of the larger cracks and openings of rock, for these would not 

 appear in the specimens tested; nor of the mantle rock, which generally 

 contains a large amount of water. From such determinations as have 

 been made it is estimated that the average porosity of the outer part 

 of the lithosphere is somewhere between 5 and 10 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 poros- 

 ity. ^ An average porosity of two and one-half percent, would mean 

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

 deep. AVith an average porosity of 5 percent., this figure would be 

 doubled.^ While these figures are not to be regarded as measurements, 

 they perhaps give some idea of the amount of ground-water. It is this 

 sphere of ground-water which justifies the term hydrosphere, as appHed 

 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 taken up by 

 plants and is passed by them into the atmosphere; some of it issues 

 in the form of springs; some of it seeps out; some of it is drawn out 

 through wells; and much of the remainder finds its way underground 

 to the sea or to lakes, issuing as springs beneath them. A small por- 

 tion of the descending waters enters into permanent combination with 



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

 Merrill, Stones for Building and Decoration, and various Survey Reports. 



' It is probable that the porosity decreases in more than an arithmetic ratio, both 

 because the deeper rocks are not of porous kinds, and because of the pressure which 

 tends to close openings. 



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

 cover the earth's surface to a depth of 3000 to 3500 feet. Earlier estimates gave still 

 higher figures (see Delesse, Bull. Soc. Geol., France, Second Series, Vol. XIX, 

 1861-62, p. 64). 



