THE ORIGIN AND DESCENT OF ROCKS. 443 



usually horizontal or nearly so, the induced cleavage commonly crosses the 

 bedding planes at a high angle (Fig. 364) ; but after the beds are tilted or 



Fig. 365. — Foliated rock. (Ells, Can. Geol. Surv.) 



bent, the lines of pressure take new directions relative to the bedding 

 planes, and the angles between the original bedding and the slaty cleav- 

 ages usually become smaller, and may even disappear in exceptional 

 cases. Limestones, sandstones, and conglomerates are not so easily 

 compressed as mudstones, and they usually take on only an imperfect 

 cleavage normal to the direction of pressure. Often they merely show 

 some little compacting, while the shaly strata between them are con- 

 verted into slate. Obviously the direction of slaty cleavage may be 

 used to determine the direction of the compressing force, and is thus 

 serviceable in dynamic studies. 



Foliation, schistosity. — A more intense apphcation of pressure in a 

 given direction is capable of breaking down and deforming the most 

 resistant rock. This must necessarily be attended with the evolution 



