ORIGIN AND FORMATION OF SOILS. 119 
coarsely crystalline, of various shades of color, depending 
on the color and proportion of the constituent minerals, 
usually gray, grayish white, or flesh-red. In common 
granite the feldspar is orthoclase (potash-feldspar). A 
variety contains albite (soda-feldspar). Other kinds (less 
common) contain oligoclase and labradorite. 
Gneiss differs from granite in containing more mica, and 
in having a banded appearance and schistose* structure, 
due to the distribution of the mica in more or less parallel 
layers. It is cleavable along the planes of mica into 
coarse slabs. 
Mica-slate or Mica-schist contains a still larger pro- 
portion of mica than gneiss; it is perfectly schistose in 
structure, splitting easily into thin slabs, has a glistening 
appearance, and, in general, a grayish color. The coarse 
whetstones used for sharpening scythes, which are quar- 
ried in Connecticut and Rhode Island, consist of this min- 
eral. 
Argillite, Clay-slaie, is a rock of fine texture, often 
not visibly crystalline, of dull or but slightly glistening 
surface, and having a great variety of colors, in general 
black, but not rarely red, green, or light gray. Argillite 
has usually a slaty cleavage, i. e., it splits into thin and 
smooth plates, It is extensively quarried in various local- 
ities for roofing, and writing-slates. Some of the finest 
varieties are used for whetstones or hones. 
Other Granitic Rocks.—Sometimes mica is absent; in 
other cases the rock consists nearly or entirely of feldspar 
alone, or of guartz alone, or of mica and quartz. The 
rocks of this series often insensibly gradate into each oth- , 
er, and by admixture of other minerals run into number- 
less varieties. 
* Schists or schistose rocks are those which have a tendency to break into 
slahs or plates from the arrangement of some of the mineral ingredients in 
layers. 
