196 Introduction to the Study of Science 



or swamps. In regions where there is a heavy rainfall the water 

 level will be found closer to the surface than in arid regions, 

 where it may be very deep. The beds of stream channels are 

 usually lower than the water level during the dry season. Where 

 this is not the case, as in certain sections of the Great Plains, 

 the rivers disappear largely or entirely during the dry season. 



The underground course of water which may be supplied by 

 deep wells depends upon the geological formation of the region 

 through which it flows. In general water sinks through the soil 

 until it reaches a water table formed by clay or rock strata im- 

 pervious to water. This is sometimes called hardpan. Along 

 the surface of this impermeable layer water slowly makes its 

 way until it reaches crevices or porous joints ; then downward 

 it goes until it arrives at another impervious layer. It may flow 

 on this until it emerges on the surface. It may emerge as a 

 spring from the side of a hill ; it may be captured by a deep 

 well and flow or be pumped to the surface. Or it may enter 

 through the floor of a river, lake, or ocean. Its rate of movement 

 may vary from two inches to twenty feet a day, or it may be 

 exceedingly slow. It may remain in the earth for months at a 

 time or even for years. 



Water's solvent power. Upon all rocks with which it comes 

 into contact, water exercises solvent power and takes a toll of 

 mineral substances. Such deep water may therefore sometimes 

 have a peculiar taste or color. If it comes through a limestone 

 region, it will probably contain a great deal of lime and be 

 " hard." If it comes from such very old rocks as the archeozoic, 

 it will be soft, for these rocks do not contain much lime. As 

 there are many salt deposits or rock-bound reservoirs full of 

 saline water in the solid crust of the earth, water will some- 

 times be salty. It may be sulfurous ; or it may contain an 

 amount of ammonia derived from organic matter buried in 

 ancient strata of coal or other material. Examination shows 

 it to be entirely free from bacteria, unless it is contaminated 

 by surface water or seepage through defective sewage pipes. 



