THE ORIGIN AND DESCENT OF ROCKS. 



441 



from other sources. In the case of intruded lavas there may or may not 

 be exceptional pressure. Thrust usually gives heat as well as pressure, 

 but if lateral thrust acts on rocks near the surface, they may be mashed 

 into new forms without becoming very exceptionally heated, though 

 some rise of temperature is inevitable. 



(1) Slaty structure. — When rocks made up of clastic particles are 

 compressed in a given direction and are relatively free to expand at right 



Fig. 363. — Pre-Cambrian fossiliferoiis slate. Deep Creek Canyon, 16 miles southeast 

 of Townsend, Mont. (Watcott, U. S. Geol. Surv.) 



angles to the direction of pressure, the particles that are already elon- 

 gated tend to take positions with their longer axes at right angles to the 

 direction of pressure, and all particles, whether elongate or not, are more 

 or less flattened in a plane transverse to the direction of pressure. This 



