THE COMPOSITION OF THE EARTH 



43 



gneiss, and exhibits a strong tendency to split into uneven 

 leaves or plates. These plates are often spangled with glisten- 



FIG. 18. Banded gneiss. (Pirsson, Rocks and Rock Minerals.) 



ing flakes of mica, or with needles of hornblende. Indeed, 

 the splitting habit characteristic of schists is due largely to the 

 presence of cleavable minerals in parallel arrangement. Many 

 varieties of schist are recognized. Mica schist is most common, 

 and consists chiefly of quartz and mica, usually with a sub- 

 ordinate amount of feldspar. It is often formed from slates 

 and feldspathic sandstones. In hornblende schist, leaves of 

 imperfect hornblende crystals are separated by other minerals, 

 in many cases by feldspar, quartz, and mica. Basic igneous 

 rocks, when metamorphosed, often become hornblende schists. 

 The latter may be formed also from sedimentary rocks. 



Slate is formed from shale by compression. It is a hard, very 

 fine-grained rock, not obviously crystalline, usually dark- 

 colored, and characterized by a remarkable cleavage, often 

 so perfect that the rock is quarried extensively in parts of New 

 England and in other regions for roofing purposes (Fig. 19). 

 The formation of slate involves far less change than the develop- 

 ment of gneiss or schist. 



