MATERIALS OF THE EARTH 



55 



cleavage planes developed by pressure (Fig. 31). As the original 

 mud beds were horizontal or nearly so, and as the thrust is most 

 commonly horizontal or nearly so, the induced cleavage com- 

 monly crosses the bedding planes at a high angle. If the beds are 

 tilted or bent before the development of the slaty cleavage, the 



Fig. 31. Slaty structure and its relation to bedding planes. Two miles 

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



angle between the original bedding and the slaty cleavage may be 

 small. 



Limestones, sandstones, and conglomerates are not so easily 

 compressed as mudstones, and they usually take on only an im- 

 perfect cleavage normal to the direction of pressure. 



Foliation, schistosity. More intense pressure in a given direc- 

 tion is capable of breaking down and deforming the most resistant 

 rock. This must necessarily be attended with the evolution of 



