METAMORPHISM 



295 



Fig. 286. Slaty structure and its relation to bedding planes. Two miles south 

 of Walland, Tenn. (Keith, U. S. Geol. Surv.) 



of the compression of such a rock as granite may be to crush it. It 

 then becomes granular or fragmental, and is really a peculiar species 

 of clastic rock (autodastic). By further compression, the fragmented 

 material may be pressed into layers or leaves, much as in the develop- 

 ment of slaty cleavage; but as a result of the nature of the material, 

 the cleavage is less perfect. These changes may be attended by 

 more or less shearing of the material upon itself. The result is a 

 foliated or schistose structure (Fig. 4), the most distinctive feature of 

 highly metamorphic rock. A foliated structure may be developed 

 even in the most massive rocks. Thus granite may be transformed 

 into gneiss which is like a granite in composition, but has a 

 foliated structure, and basalt may be converted into schist, a common 

 term for foliated crystalline rocks. 



The kind of schist produced by metamorphism depends on the 

 constitution of the rock metamorphosed. Basic rocks give rise to 

 basic schists, and acidic rocks to acidic schists. It is obvious that 



