36 DOMESTIC WATER SUPPLIES FOR THE FARM 



them are banded and most of them are irregular in mass and oc- 

 currence; locally they may contain vividly colored patches and 

 bands. 



Sedimentary and igneous rocks in many places have been sub- 

 jected to heat and pressure after their deposition. This action 

 may have further hardened them or have so greatly changed them 

 that their original character can scarcely be determined. Rocks 

 thus changed are known as metamorphic rocks. Mica schist and 

 marble are typical examples. As a rule they exhibit a bedded or 

 banded structure. 



Rocks of all of these types occur the world over, and those of 

 one locality may be indistinguishable from those of another. 

 Notwithstanding these similarities, they may have been formed 

 under very different conditions and at periods thousands or even 

 millions of years apart. Resemblances between rocks do not, 

 therefore, mean that they were formed at the same time or will 

 yield the same products, such as coal or oil or water. 



Simple definitions of the more common rocks are given below. 

 Complete descriptions and definitions of rarer varieties may be 

 found in any text-book on geology. 



Unconsolidated Sedimentary Deposits. Gravel, sand and 

 clay are made up respectively of pebbles, sand grains and fine 

 silts worn from older rocks and redeposited in their present form. 



Alluvium is a general term for gravels, sands and clays, or 

 mixtures of these that have been deposited by streams. 



Chalk is a soft white earthy form of limestone. 



Marl is a clay with much intermingled calcareous material. 



Till is a heterogeneous mixture of clay, sand, pebbles and 

 boulders deposited by glaciers. 



Hardpan is a general term which may be applied to any bed 

 which is considerably harder than those with which it is asso- 

 ciated. It is often applied to till, to tough clays or to partly 

 cemented sand or clay beds. 



Consolidated Sedimentary Rocks. Conglomerate is a con- 

 solidated gravel. 



