ORIGIN AND FORMATION OF SOILS. 113 



Chlorite is a common mineral occurring in small scales 

 or plates which are brittle. It is soft, usually exists in 

 masses, rarely crystallized, and is very variable in color 

 and composition, though in general it has a grayish or 

 brownisli-grcen color, and contains magnesia, alumina, and 

 iron, united with silica. See analysis below. 



LeilCite is an anhydrous silicate of akimina found 

 chiefly in volcanic rocks. It exists in white, hard, 24-sid- 

 ed crystals. It is interesting as being formed at a high 

 heat in melted lava, and as being the first mineral in which 

 potash was discovered (by Klaproth, in 1797). See anal- 

 ysis below. 



Kaolinite is a hydrous silicate of alumina, which is 

 produced by the slow decomposition of feldspar under the 

 action of air and water at the usu.il temperature. Form- 

 ed in this way, in a more or less impure state, it consti- 

 tutes the mass of white porcelain clay or kaolin, which is 

 largely used in making the finer kinds of pottery. It ap- 

 pears in white or yellowish crystalline scales of a pearly 

 luster, or as an amorphous translucent powder of extreme 

 fineness. Ordinary clay is a still more impure kaolinite. 



Steele Mine, N. C. Vesuvius, Summit Ilill, Chaudie'-e 



Eruption of 1857. Pa, Falls. Canada. 



Genth. Rammelsberg. S. W. Johnson. T. S. Hunt. 



Silica, 24.90 .57.24 45.9.3 46.05 



Alumina, 21.77 22.9(5 ,■59.81 38.37 



Sesquioxidc of iron, 4.()0 



Protoxide of iron, 24.21 



Protoxide of manj,'ane8e, 1.15 



Magnesia, 12.78 0.63 



Lime, 0.91 0.61 



Soda, 0.93 



Potash, 18.61 



Water, 10. .59 14.02 14.00 



Talc is often found in pale-green, flexil)le, inelastic scales 

 or leaves, but much more commonly in compact gray 

 masses, and is then known as soapstone. It is very soft, 



