PHYSICAL CHANGES OF THE OUTER SHELL 71 



Individual faults differ widely among themselves, and to 

 classify or interpret the many kinds is a matter of much 

 complexity. It has been customary, however, to group the 

 majority of them in two divisions : (1) normal faults, and 

 (2) reversed faults. 



FIG. 52. Three systems of joints in a hard, brittle rock. (Weidman.) 



Normal faults are often produced by warping ; they imply 

 a stretching of the outer part of the crust (Fig. 54). The 

 fault planes are almost invariably steeply inclined or vertical. 

 The moved block may have slipped either vertically, diago- 

 nally, or horizontally. It is often possible to ascertain the 

 direction of this slipping by means of the polished grooves, 

 called slickensides, which are produced by the grinding of 

 the one mass of rock over the other. The vertical distance 

 between the broken ends of a given stratum measures the 

 amount of displacement and is called the throw of the fault 

 (Fig. 55). The two sides are designated as the upthrow and 



