1906.] GEOLOGY IN RELATION TO WATER SUPPLY. 289 



may reach a maximum of 47.64 per cent. So long as the 

 spheres are of uniform size the porosity of a pile of large mar- 

 bles is the same as that of small ones. In nature the grains 

 are not like marbles, all of one size, and are not tightly com- 

 pacted, so that the porosity of sands and gravels falls below the 

 theoretical maximum and above the theoretical minimum. 

 Chalk will absorb two gallons of water for every cubic foot. 

 Sandstone consists of cemented grains and is so constructed 

 that it may hold 20 to 30 per cent, of its weight of water. 

 There is enough water in the sandstone underlying parts of 

 Minnesota and Wisconsin to make a lake covering those States 

 to a depth of 50 to lOO feet. Sand may contain 30 to 40 per 

 cent, of its volume of water, and some of the loamy soils near 

 the surface of the ground have a capacity for even a larger 

 amount. Not only do sands and sandstones contain water, but 

 every known rock contains a larger or smaller amount. The 

 densest granite known in this country, that from Montello, 

 Wis., which was used for the sarcophagus of Grant's tomb in 

 New York, has a porosity of .237, or about one-fourth of one 

 per cent. 



Ground water is most abundailt a few feet below the sur- 

 face and decreases in amount from that point downward. At 

 great depth no pores can exist ; under the pressure of the over- 

 Iving rock all spaces, cavities, and pores would be closed. The 

 depth to which any open space in rocks may occur is about six 

 miles below the earth's surface. 



So great is the amount of ground water that if it were taken 

 from the surface rocks down to that depth there would be 

 sufficient water to cover the entire earth with an ocean 3,000 

 to 3,500 feet, equal to one-third as much water as in the present 

 oceans. This great amount of water remains in the ground 

 from year to year, and varies in its amount and nearness to the 

 surface with all the factors which determine climate. 



The level below which rock and subsoil are always full of 

 water is the zvater fable. This water table is not a horizontal 

 plane but adjusts itself to the topography. In lakes and 

 swamps it is at the surface. It is at less depth in valleys than 

 on hillsides, but it bends upward and downward with hills 

 and valleys, and in a humid region is never at great depth be- 

 low the surface. Wells sunk below this water table will pro- 

 duce water. In dry seasons the water table recedes from, the 



Agr. — 19 



