186 RELD ET AL. REPORT OX NOMENCLATURE OF FAULTS 



and oblique-slip faults. An oblique-slip fault, as the most general case 

 of fault movement, may be illustrated by figure 18. The block represents 

 a portion of the earth's crust: its boundary lines have no real existence 

 in nature, but are drawn for the purpose of indicating directions and 

 showing the position of the fault surface. Suppose, then, that a fracture 

 takes place in the surface ABCD, and suppose that the point which was 

 formerly immediately adjacent to A should move by faulting to C; then 

 AC would be the slip or net slip, DC the strike-slip, and AD the dip-slip. 



The fault-plane is completely defined locally if we know its strike and 

 dip and one point of its plane, such as a point of its outcrop. As the 

 movement takes place parallel with the fault-plane, we need only two 

 components of the motion in this plane completely to define the displace- 

 ment; the strike-dip, DC, and the dip-slip, AD, are suitable components ; 

 but it may be convenient at times to use the trace-slip and the perpen- 

 dicular slip. To determine the slip it is necessary to identify the original 

 and final position of a point ; this is frequently impossible, and then the 

 slip can not be found. 



The displacements of strata, dikes, etcetera, are the displacements 

 usually determined, and in some mining operations they are all that is 

 needed. They are described by means of the separations, which must be 

 defined. Figures 7 and 8 show that they frequently give no clue to the 

 slip. If the offset or the normal horizontal separation and the dip of a 

 bed have been found, the stratigraphic separation can be calculated and 

 the relation of the two branches of a stratum becomes known. The ver- 

 tical and horizontal separations can be determined in shafts and drifts 

 of mines. 24 The generalized definitions of throw and heave and of nor- 

 mal and reverse faults find their places here. 



Where the strata near the fault have been distorted by the movement 

 and bent up or down, the displacement of the crust outside the distorted 

 position is not the same as the slip: this leads to the definition of the 

 shift. Xote that in strike faults without a distorted zone the heave 

 shows the amount of horizontal contraction or extension of the crust due 

 to the fault. In the general case this contraction or extension is shown 

 by the normal shift, which, where there is no distorted zone, becomes 

 equal to the corresponding component of the slip. 



Finally, the relations existing in groups of faults should be explained, 

 and such special terms as flaw, horst, graben, etcetera, should be defined. 



24 In a former foot-note references have been given to the methods necessary for the 

 determination of the slip. The subject is too extensive to be treated here. 



