ORIGIN AND FORMATION OF SOILS. 109 
ant mineral. J¢ is a compound of silica with alumina, 
and with one or more of the alkalies, and sometimes with 
lime. Mineralogists distinguish several species of feld- 
spar according to their composition and crystallization. 
Feldspar is found in crystals or crystallinc masses usually 
of a white, yellow, or flesh color, with 2 somewhat pearly 
luster on the smooth and level surfaces which it presents 
on fracture. It is scratched by, and does not scratch 
quartz. 
In the subjoined Table are given the en ee names 
and analyses of the principal varieties of feldspar. Ac- 
companying each analysis is its locality and the name of 
the analyst. 
ORTHOCLASE. ALBITE. OLIGOCLASE. LaBRADORITE, 
Common or potash Soda feldspar. Soda-lime feldspar. Lime-soda 
Jeldspar. Sédspar. 
New Rochelle, N.Y. Unionville, Pa. Haddam, Conn. Drummond, C. W. 
S. W. Johnson. M.C Weld. G. J. Brush. T.S. Hunt. 
Silica, 64.23 66.86 64.26 54.70 
Alumina, 20.42 21.89 21.90 29.80 
Potash, 12.47% — 0.50 0.33 
Soda, 2.62 8.78 9 99 2.44 
Lime, trace 1.79 2.15 11.42 
Magnesia, — 0.48 — —_— 
Oxide of iron, trace — — 0.36 
Water, 0.24 0.48 0.29 0.40 
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,—white, 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. 
Phlogopite and Biotite contain a large percentage of 
magnesia, and often of oxide of iron. 
