GENERAL TERMS 167 



A horse is a mass of rock broken from one wall and caught between 

 the walls of the fault. (See figure 1.) 



The fault strike is the direction of the intersection of the fault sur- 

 face, or the shear zone, with a horizontal plane. 



The fault dip is the inclination of the fault surface, or shear zone, 

 measured downward from a horizontal plane. It is never greater than 

 90 degrees. 



The hade is the inclination of the fault surface, or shear zone, meas- 

 ured from the vertical; it is the complement of the dip. A fault is said 

 to hade to the side toward which it dips. 



The hanging wall is the upper wall of the fault. 



The foot wall is the lower wall of the fault. 



General Classes of local Movements on Faults 



Movements on faults may be classified, according to the character of 

 the local displacement, into translatory and rotatory movements. 



Translatory movements are those in which all straight lines on oppo- 

 site sides of the fault, and outside the dislocated zone, which were paral- 

 lel before the displacement, are parallel afterward. If at a later date, or 

 even at the time of the displacement, the whole region were tilted, the 

 movement would still be considered a translatory movement, so far as 

 the fault is concerned. 



Rotatory movements are those in which some straight lines on oppo- 

 site sides of the fault and outside the dislocated zone, parallel before the 

 displacements, are no longer parallel afterward — that is, where one side 

 of the part of the fault under consideration has suffered a rotation rela- 

 tive to the other side. 



No faults of any magnitude exhibit merely translatory movements 

 over their whole lengths. Faults die out and the displacement is not 

 uniform along them, so that there is necessarily some slight rotation. 

 varying in amount in different parts of the fault's course. Probably the 

 greatest number of faults, certainly all large faults, are of this character. 

 The variations in rotation and translation are permitted by slighl plastic 

 deformations. If, however, we con tine our attention to a small pari o( 

 the fault, we may describe the displacement there as though the rock were 

 rigid; and if the rotation is very small, as if a translatory displacement 

 had occurred, and for conciseness we may use the terms translator!/ 

 fault, or rotatory fault, to describe the pari under consideration. It 

 sometimes happens that the strikes on opposite sides o\' a fault are dill'or- 

 ent; this suggests a rotation, hut it may be due to a local variation o( 

 strike or to an unconformity. 



