294 MATERIALS AND THEIR ARRANGEMENT 



Fig. 285. Pre-Cambrian fossiliferous slate. Deep Creek Canyon, 16 miles 

 southeast of Townsend, Mont. (Walcott, U. S. Geol. Surv.) 



developed by pressure (Fig. 286). As the original mud beds were 

 horizontal or nearly so, and as thrust is most commonly horizontal 

 or nearly so, the induced cleavage commonly crosses the bedding 

 planes at a high angle. If the beds are tilted or bent before the 

 development of the slaty cleavage, the angle between the bedding 

 planes and the slaty cleavage may be small. 



Limestones, sandstones, and conglomerates are not so easily 

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

 perfect cleavage normal to the direction of pressure. 



Foliation, schistosity. Extreme pressure in a given direction 

 is capable of breaking down and deforming the most resistant rock. 

 This must necessarily be attended with the evolution of heat, and 

 thermal effects are combined with pressure effects. The first effect 



