NOMENCLATURE 17S 



total shift at right angles to the strata, or the perpendicular displacement 

 of a stratum. It will be seen that when we are dealing with a strike- 

 fault these terms correspond to the terms now in use. The only new 

 word introduced is the word "shift/' which practically defines itself ; and 

 the word displacement is used in the general sense and is not confined to 

 any particular direction. The hade of the fault is measured by the angle 

 between the fault-plane and the vertical, and is not, as some writers have 

 used it, the equivalent of the fault-dip, which is its complement. 



These definitions apply especially to cases of no rotation; where rota- 

 tions occur the nomenclature is not confused. 



A System of Projection 



When we consider faults which are diagonal to the strike of the strata, 

 or strike-faults in which a component of the movement is parallel with 

 the fault-strike, the total shift does not, in general, lie simply in a hori- 

 zontal or a vertical plane, which we might use as a plane of reference; 

 and it is not at all convenient to use an inclined fault-plane for this pur- 

 pose. It is, therefore, necessary to use some method of projection to 

 represent the various movements and the various surfaces we deal with, 

 on a plane surface of reference. Fortunately there is a very simple 

 system of projection, familiar to many geologists, by which this can be 

 done; and this consists in representing the surface by its intersection 

 with a horizontal plane, which we shall call the horizontal plane or the 

 reference plane, and by contour lines drawn upon it. The horizontal 

 plane can not correspond with an irregular topographic surface; but it 

 may be chosen at the mean level of that surface, or at any other con- 

 venient altitude. This system of projection will be readily grasped if we 

 keep in our minds a clear conception of the positions of the various lines 

 and surfaces in space and use the diagrams to work out their quantita- 

 tive relations. 



A plane is represented by its intersection with the reference plane, 

 which is a straight line, and is called its trace, and by a series of parallel 

 straight lines representing its contours. These contours, however, may 

 be omitted if we indicate by the side of the trace the direction and 

 amount of the dip. Thus in figure 2 let tT represent the trace of a plane 

 dipping to the east at an angle represented by 8. The position of any 

 contour can be determined immediately as follows : Draw a straight line 

 ta at right angles to the trace ; lay off the angle 8 equal to the dip and 

 complete the right-angle triangle abt right angled at a; ab will then be 

 the depth of the contour at a distance ta from the trace. This will read- 



