56 



GEOLOGY 



heat ; and thermal effects are mingled with pressure effects, but 

 the thermal effects may be neglected for the moment. The first 

 effect 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 (autoclastic) . By further compression, 

 the fragmented material may be pressed into layers or leaves, much 



Fig. 32. 



ry rendered schistose by pressure. Near Green Park, 

 Caldwell Co., N. C. (Keith, U. S. Geol. Surv.) 



as in the development of slaty cleavage; but as a result of the 

 nature of the material, the cleavage is less perfect. This is often 

 attended by more or less shearing of the material upon itself. The 

 result is a foliated or schistose structure (Fig. 4). A foliated struc- 

 ture may be developed thus in even the most massive rocks. Thus 

 a granite may be transformed into a gneiss which is like a granite 

 in composition, but has a foliated structure; or a basalt may be 

 converted into schist, a common term for foliated crystalline rocks. 

 Porphyritic rock rendered schistose by pressure is shown in Fig. '>-. 



