38 DOMESTIC WATER SUPPLIES FOR THE FARM 



Marble is a crystalline limestone, and gives the same reaction 

 with acid as limestone, marl and chalk. 



True soapstone is a soft, even-grained, greasy-feeling rock com- 

 posed of the mineral talc, but the term is incorrectly applied to 

 any soft, greasy-feeling rock, such as soft shale. 



Schist is a more or less crystalline rock which has a laminated 

 structure, due to the flat crystals of mica or other minerals of 

 which it is composed. 



Gneiss is similar to granite in composition, but has a less per- 

 fect crystalline structure and a banded structure due to the linear 

 arrangement of its crystals. 



Fossils. In many sedimentary rocks remains of animals and 

 plants are found. These generally consist of portions or im- 

 pressions of shells, bones or leaves, and are known as fossils. 

 A bed may be recognized and determined by such remains, which 

 are therefore of great importance in geologic work. In the oldest 

 rocks only low organisms, such as shellfish, are found, but in later 

 rocks, fishes, reptiles and mammals progressively appear, while 

 vegetation shows a corresponding change of its predominant types 

 from microscopic forms to the forest trees of to-day. 



Formations. A rock bed or a succession of beds that are 

 uniform in character throughout a considerable area is termed a 

 formation and is given a geographic name derived from some place 

 or feature in the area where it typically occurs, such as Trenton 

 limestone. The correct identification of formations is very im- 

 portant in underground-water studies, as by this means the 

 structure and position of water-bearing beds is worked out. 



Structures of Rocks. When deposited, sedimentary beds are 

 nearly horizontal, but they may be subsequently thrown into in- 

 clined positions, or bent into arches or troughs, or broken and 

 displaced. It is rather unusual, in fact, to find in the interior of 

 the continents any sedimentary beds which have not been tilted, 

 folded or otherwise disturbed, at least slightly. Some of the 

 terms used to designate the structures that result from these dis- 

 turbances are as follows: 



