GROUND WATERS AND THEIR OCCURRENCE 29 



solidated into rocks, and is represented by the salt waters now 

 found at great depths in the deep wells drilled for oil or gas, in 

 which it is not uncommon, after passing through hundreds of 

 feet of entirely dry rock, to suddenly encounter porous beds 

 filled with salt water. 



Absorptive Capacity of Soils and Rocks. The absorptive 

 capacity of soils and the more porous rocks is enormous. As 

 pointed out elsewhere 80 per cent of the rainfall is probably 

 absorbed by the soils and rocks in the eastern United States and 

 over 90 per cent in much of the West. The greater part of water 

 so absorbed reappears at the surface as seepages and springs to 

 form the streams or feed the ponds and lakes, usually within a 

 few weeks or months of the time it fell upon the surface, although 

 a part may join the deeper waters and reappear at the surface 

 only after years or even centuries of imprisonment in the subter- 

 ranean depths. 



The amount of water in several of the more common classes 

 of rocks is discussed elsewhere, but a statement of the common 

 porosities or the percentage of volume occupied by water when 

 the soil or rock is saturated is given below. 



POROSITY OF SOILS AND ROCKS. 



Per cent. 



Soil and loam 55 



Clay 50 



Sand 30 



Chalk 50 



Sandstone 10 



Limestone and marble 4.5 



Slate and shale 4 



Granite I 



Quartzite 5 



The actual amount of water in the rocks will in most cases be 

 slightly in excess of that indicated by the porosities, since very 

 appreciable quantities also occur in the more or less open pas- 

 sages such as joints, bedding planes, etc., while in limestones con- 

 siderable amounts are often present in solution channels. 



