CHAPTER V. 

 WATER-BEARING FORMATIONS. 



Classes of Rocks. All known rocks contain more or less 

 water, and though many of them will not yield it in useful quan- 

 tities to wells all must be considered in a discussion of water- 

 bearing formations. Many rocks are familiar to everyone, at 

 least by name; others are less widely known and may be re- 

 stricted to very small areas. The classes of rocks commonly 

 recognized and the definitions of the simpler varieties are given 

 below for the benefit of those not familiar with the terms in 

 use. 



In a simple classification rocks may be grouped into three 

 divisions: (i) sedimentary, (2) igneous and (3) metamorphic. 



Sedimentary rocks are formed either of fragments worn from 

 older rocks by the action of rain, wind, frost, etc., and carried 

 by water, wind or glaciers until deposited as beds of clay, sand, 

 gravel, marl, loess, etc. ; or of the remains of corals or of shellfish, 

 such as oysters and clams. When first deposited the materials 

 are loose and unconsolidated, but gradually they become hardened 

 and cemented together, especially when covered by later beds, 

 and eventually form solid rocks. These fragments or grains are 

 commonly rounded, and their form may therefore help to dis- 

 tinguish them from igneous rocks, which may have corners or 

 angles or may include angular crystals. 



Igneous rocks may have come from the earth's interior in a 

 molten state and have been forced into or through the rocks 

 above them or may have overflowed as lava beds at the surface. 

 They are generally made up of or at least include angular crys- 

 tals, which may be recognized by their glistening faces, a feature 

 that is not usually possessed by sedimentary rocks. Many of 



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