ORIGIN AND FOKMATIOX OP SOILS. 119 



coarsely crystalline, of various shades of color, depending 

 on the color and proportion of tlie constituent minerals, 

 usually gray, grayish white, or flesh-red. In common 

 granite the feldspar is orthodase (potash-feldspar). A 

 variety contains albite (soda-feldspar). Other kinds (less 

 common) contain oligoclase and lalyradorite. 



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 3Ilca-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, €lay-Slate, 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 m,ica is absent ; in 

 other cases the rock consists nearly or entirely o? feldspar 

 alone, or of quartz 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 arc those which have a tendency to break into 

 slabs or plates from the arrangement of some of the mineral ingredients in 

 layers. 



