184 GEOLOGY AND CHEMISTRY OF SOILS. 



transparent. The impure varieties contain variable quanti- 

 ties of iron, alumina, manganese and nickel. These varie- 

 ties are found under the names of jasper, flint, chalcedony, 

 rose-quartz, horn-stone, chrysoprase, feruginous quartz, cor- 

 nelian, agate, amethyst, etc. The prevailing color is that of 

 water, or white. There are various shades of red, yellow, 

 green, blue and brown. It is so hard as to scratch glass, but 

 is not scratched in turn. Its lustre resembles glass, and may 

 be known by not being acted upon by any acid, excepting 

 the hydrofluoric. 



2. Feldspar differs from quartz in having a paler white 

 color, and lamella or granula texture. It scratches glass, and 

 is scratched by glass in turn. It is a silicate of alumina, and 

 is composed of silica, 64 parts in 100, alumina 20, potash 

 10 to 14, and traces of lime, oxide of iron and water. 



3. Mica. This mineral, known under the name of ising- 

 glass, exists in thin, shining scales, and in broad tables or plates. 

 It is of various colors. It is transparent, and the laminae 

 are thin, very flexible, elastic and very tough. These char- 

 acters sufl^ciently distinguish it in the rocks. It is a silicate^ 

 and varies in composition. A specimen analyzed by Rose, 

 gave 



0.^6 

 0.56 

 1.39 



A specimen, analyzed by Turner, had 5.49 of lithia, 

 but no oxide of iron. Its composition may be stated gener- 

 ally 



It appears, therefore, to be a compound of the silicate of 

 alumina, potassa, oxide of iron and of manganese. 



4. Talc resembles mica in its thin, shining scales, but 

 may be distinguished from it by its want of elasticity. It is 



