444 



GEOLOGY. 



of much heat, and thermal effects are mingled with pressure effects, 

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

 stage of the mechanical effect of the compression may be to crush the 



Fig. 366. — Porphyry rendered schistose by pressure. Near Green Park, Caldwell Co., 

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



rock more or less. It thus becomes granular or fragmental, and is really 

 a peculiar species of clastic rock (autodastic) . At a further stage, the 

 fragmented material may be pressed into layers or leaves, much 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, and thus a rude fissility and 

 fohation is developed. The result, including the attendant metamor- 



