OEIGIX ANIJ FOU.MATIOX OF SOILS. 109 



ant mineral. It is a cotnpound of silica with alumina^ 

 and with one armor e of the alJcdies, and sometimes vrith 

 lime. Mineralogists distinguish several species of feld- 

 spar according to their composition and crystallization. 

 Feldspar is found i:i crystals or crystalline masses xisually 

 of a white, yellow, or flesh color, with a somewhat pearly 

 luster on the smooth and level surfaces which it presents 

 on fracture. It is scratched by, and docs not scratch 

 quartz. 



In the subjt>ined Table are given the minefalogical names 

 and analyses of the principal varieties of feldspar. Ac- 

 companying each analysis is its locality and the name of 

 the analyst. 



Mica is, perhaps, next to feldspar, the most abundant 

 mineral. There are three principal varieties, viz.: Musco- 

 vite, Phlogopite, and Biotite. They are silicates of alumi- 

 na with potash, magnesia, lime, iron, and manganese. 



Mica bears the common name " isinglass." It readily 

 splits into thin, elastic plates or leaves, has a brilliant 

 luster, and a great variety of colors, — Avhite, yellow, brown, 

 green, and black. Muscovite, or muscovy glass, is some- 

 times found in transparent sheets of great size, and is used 

 in stove-doors and lamp-chimneys. It contains much 

 alumina, and potash, or soda, and the black varieties oxide 

 of iron. 



Phloyoplte and Biotite contain a large percentage of 

 magnesia, and often of oxide of iron. 



