THE WORK OF WATERS UNDERGROUND 119 



this, and in addition that contained in the waters of salt 

 lakes and in the great salt deposits of certain rock forma- 

 tions, was obtained by ground water from rocks that con- 

 tained the constituents of salt. Again, the great formations 

 of limestone were formed either by the accumulation of the 

 shells of marine animals that take lime carbonate from solu- 

 tion in the ocean water, or by precipitation from the over- 

 charged waters of embayments or lagoons of the sea (p. 39). 

 Most of the material of the limestone was dissolved by 

 ground water from the rocks of the land and delivered to 

 the streams, by which it was carried to the sea. These facts 

 help to illustrate the importance of the work of solution 

 done by ground water. The effect of the transfer of dis- 

 solved material to the sea is to lower the land. This is being 

 accomplished by solution alone at an estimated average rate 

 of one foot in about 13,000 years. 



Deposition by ground water. While possibly the major 

 part of the mineral matter dissolved by ground water is 

 carried in solution directly to the sea, vast quantities are 

 deposited below and at the surface (p. 80). Deposition 

 may be brought about in several ways. The water may be 

 overcharged and deposit because of (1) evaporation, (2) a 

 lowering of temperature, (3) a decrease in pressure, (4) a 

 loss of part or all of the gas it contained, or (5) the mixing 

 of waters having different things in solution. In the last 

 case new combinations are likely to be formed, leaving the 

 water overcharged with one or more things, which are de- 

 posited. Certain minute plants also have the power of ex- 

 tracting some things from solution. As already indicated, 

 the zone of greatest cementation lies below the zone of solu- 

 tion, where the ground water is sluggish, and heavily charged 

 with mineral matter. 



Where material is deposited among loose rock particles, 

 the latter may be cemented into firm rock (p. 37). Where 

 deposition from solution occurs on the walls of cracks or 

 fissures, the material forms mineral veins (Fig. 111). Most 



