34 GROUND-WATER 



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 somewhere 

 between five and ten percent. If the porosity diminishes at a con- 

 stant rate to a depth of six miles (where it becomes zero), the average 

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

 average porosity of 2>% would mean that the rock might contain 

 enough water to form a layer nearly 800 feet deep, if brought out 

 to the surface. 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 those at the surface, and because of the pressure 

 which tends to close openings. For this reason it may be that the 

 figure given above 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, 800 feet is perhaps too high a figure, and is not 

 to be regarded as a measurement. 



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 is taken 

 up by plants and passed by them into the atmosphere, some issues 

 in the form of springs, some seeps out, some is drawn out through 

 wells, and much of the remainder finds its way underground to the 

 sea or to lakes, seeping out beneath them. A small portion of the 

 descending water enters into combination with mineral matter. 

 It does not necessarily follow, however, that the total supply 

 of water is for this reason decreasing. Minerals once hydrated 

 may be dehydrated, the water being set free. Furthermore, con- 

 siderable quantities of water in the form of vapor issue from volca- 

 noes, and some volcanic vents continue to steam long after volcanic 

 action proper has ceased. It is probable that some, and perhaps 

 much of the water issuing from these vents has never been at the 

 surface before. The amount of water reaching the surface of the 

 earth for the first time from volcanoes, may, so far as now known, 



1 Buckley, Building and Ornamental Stones, Bull. IV, Wis. Surv.; Merrill, 

 Stones for Building and Decoration. 



2 Slichter 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: Water Supply and Irrigation 

 Paper No. 67, U. S. Geol. Surv. Earlier estimates gave still higher figures. Fuller, 

 in a recent estimate, places the amount at about 100 feet: Water Supply and 

 Irrigation Paper 160, U. S. Geol. Surv. 



