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FIRST YEAR SCIENCE 



layer, its water is held in by the impermeable rocks above 

 and below, and so is under hydraulic pressure. When a 

 hole is made in the upper rock layer (Fig. Ill), the water 

 will flow to the surface and if the pressure is sufficient, it 

 may gush out of the hole. 



Borings of this kind form what are called artesian wells. 

 These are of great importance in many regions where it is 



NATURAL BRIDGE, SAXONY. 



difficult to obtain sufficient surface water. In some of our 

 western states the water from artesian wells has been ob- 

 tained in sufficient quantity for extensive irrigation. 

 Although this water often contains minerals in solution, it 

 is free from surface contamination and is therefore usually 

 healthful for drinking. 



In some places the surface water penetrates into layers 

 of rock which it can dissolve, such as salt or limestone. 

 Here it forms caves and caverns, the solid material which 



