THE STRUCTURE OF THE EARTH 



261 



Fig. 252. — Normal fault. The hori- 

 zontal displacement BC is the heave; the 

 vertical displacement AB is the throw; the 

 angle BAC is the hade; AC is the slip. 



Faults 



When beds are displaced along joints, bedding, or fracture planes, 

 the beds are said to be faulted (Fig. 252). The fracture along which 

 the movement occurred, usually 

 is not an open crack. It some- 

 times consists of a single, clean- 

 cut fracture, but more often of a 

 number of closely parallel frac- 

 tures. A fault dies out at its ends 

 and varies along its course in the 

 amount, direction, and character 

 of the displacement. The fault 

 line, moreover, is not always 

 straight and single, but is often 

 irregular and in many cases splits 

 into one or more branches. Faults are seldom vertical, but when 

 followed downward in mines are usually found to be inclined, the 



degree of inclination 

 often varying from 

 point to point. The 

 general inclination of 

 a fault from the verti- 

 cal is called the hade 

 (Fig. 252). 



There are three 

 principal types of 

 faults: (1) normal, 

 (2) reverse or thrust, 

 and (3) vertical. 



Normal or Gravity 



Faults. — A normal 



fault (Figs. 252, 253) 



is the simplest type. 



In faults of this class 



it is convenient to 



consider one side as 



Fig. 253. — Faults in sand, Adirondacks. Since the having moved down 



faulting, lateral movement has occurred and has changed • ,• , r 



the direction of the fault planes so that the footwall an inclined fracture, 



seems to be on the downthrow side. and the other to have 



