322 PRACTICAL LESSONS IN SCIENCE. 



nearly transparent. It is the basis of the different kinds of lime- 

 stone and marble, appearing in a great variety of forms. It is 

 easily scratched with a knife, and effervesces vigorously when 

 treated with any acid, even strong vinegar will decompose it. 



Dolomite is a carbonate of calcium and magnesium. It is per- 

 haps a little harder than limestone, and is not as easily affected 

 by acids, but it is often difficult to distinguish between them. 



Gypsum, the hydrated sulphate of calcium, often forms ex- 

 tensive beds of rock. It is easily cut with a knife, is white, some- 

 times variegated, in color and when burned to drive off water of 

 crystallization becomes plaster of Paris. 



Pyrite, the disulphide of iron, is of a pale brassy yellow color, 

 crystallizing in cubes. It is very common in the neighborhood 

 of coal banks, and is the source of the sulphur in the coal. From 

 its color it is often mistaken for gold. Pyrite and the sulphides 

 of copper, lead, zinc, and other metals are often associated with 

 gold, and are abundant minerals. 



Graphite, nearly pure carbon and the various ores of iron are 

 widely disseminated minerals of value and interest. 



Chloride of sodium, common salt; corundum, nearly pure ox- 

 ide of aluminium; apatite, phosphate of lime, etc.; leucite, abun- 

 dant in some lavas ; olivine, a silicate of magnesium and iron, are 

 all important minerals. 



The minerals mentioned, either singly or variously combined, 

 make up the greater part of the rocks which compose the earth's 

 crust. 



Granite is a crystalline rock composed of thoroughly mixed 

 quartz, orthoclase feldspar and mica, the particles being of near- 

 ly uniform size. It is usually hard, light-colored and can gener- 

 ally be identified by the scales of mica. Sometimes the rock is 

 very fine grained, containing large crystals of feldspar scattered 

 through the mass when it is called porphyritic granite. 



Quartz Felsite or Felsite. This rock consists of a fine granular 

 or homogeneous ground mass, through which are disseminated 

 crystals of quartz, feldspar, mica or hornblende. This rock va- 

 ries widely in structure without much modification in composi- 



