CHAPTER III 

 THE WORK OF GROUND WATER 



Taking the world as a whole, about 78 per cent, of the rainfall 

 either soaks into the ground or is evaporated, the remainder — the 

 run-off — being carried directly into streams and rivers. The amount 

 of the precipitation which is retained in the soil depends upon 

 (1) the climate, (2) the slope of the ground, (3) the porosity of the 

 soil and rock, and (4) the amount and character of the vegetation. 

 In moist climates the run-off may amount to as much as one half of 

 the rainfall, while in arid regions, on account of the excessive evapo- 

 ration and the dryness of the soil, there may be no run-ofF. That 

 portion of the rainfall which sinks into the soil is called ground water. 

 Once beneath the surface, it continues its descent through the pores 

 and cracks of the rock until it may reach great depths. 



Quantity of Ground Water. — All rocks are more or less porous, even granites con- 

 tain some water; for example, chalk may hold two gallons of water a cubic foot, 

 and sandstones may hold 20 to 30 per cent, of their weight. The total amount of 

 water in the rocks is therefore very large, and it is probable that, if the ground water 

 were squeezed from the rocks, there would be enough to cover the earth with a sheet 

 of fresh water one hundred or more feet deep. Locally, the quantity of underground 

 water may be much greater; as, for example, in Wisconsin and Minnesota, where the 

 underlying sandstone alone contains enough water to form a layer 50 to 100 feet 

 deep. The ground water of any region is not always derived from the local rainfall, 

 but may have had a long subterranean course, as is true of the underground water 

 of the Great Plains, the source of which is in the mountains, many miles distant. 



The Water Table. — The level beneath which the rock is saturated 

 with water is called the water table or the level of underground water. 1 

 This varies greatly in different regions. In humid portions of North 

 America it is from one to forty feet below the surface ; in limestone 

 regions, where the drainage is largely subterranean, such as in por- 

 tions of Kentucky and Tennessee, it may be two to three hundred 



1 "In deep mines in various parts of the world water is found only in the upper levels, 

 within 2500 feet or less of the surface, while below that the mines are dry or even dusty." 

 (Scott.) 



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