172 



H. F. REID GEOMETRY OF FAULTS 



not been carefully studied, nor any methods described by which these move- 

 ments could be determined ; and a search through geological text-books and 

 books on applied and field geology failed to discover more than a cursory 

 treatment of this subject. I have for this reason thought it worth while 

 to discuss the nature of the observations necessary to determine com- 

 pletely the movement at a fault and to show how this movement can be 

 worked out from the observations. The subject is of very considerable 

 importance to the mining engineer, to enable him to determine the posi- 

 tion of faulted veins, and also to the dynamical geologist, for it is essen- 

 tially necessary to know the actual movement which has taken place at a 

 fault in order to infer from it the character of the forces which caused it. 



Nomenclature 



The confusion of nomenclature makes it necessary to define the mean- 

 ing of the words to be used. The names "normal" and "reversed" faults 

 will be retained in their ordinary geometrical meaning without any in- 

 ferences with respect to the forces 

 producing them. The displace- 

 ment in any direction of any sur- 

 face — such as a stratum, dike, 

 vein, or old fault, or of any line — 

 is the length of the line joining 

 the separated parts (actual or 

 produced, if necessary) of the 

 surface or line, measured in that 

 direction; the perpendicular dis- 

 placement is measured at right 

 angles to the surface or line. The 

 offset of a surface is the horizontal 

 displacement, measured at right angles to the strike of the surface. For 

 the total movement we shall use the word shift, which is represented by 

 the line ah, figure 1. The projection ac of the total shift on a horizontal 

 line parallel with the fault-plane will be called the horizontal shift; and 

 the projection cb, on a line parallel with the fault-plane and at right 

 angles to the former line, will be called the dip-shift. The throw, or 

 vertical throw, is the difference in altitude of the two ends of the shift, 

 and the heave, or horizontal throw, in the vertical plane at right angles 

 to the fault-plane, is the horizontal component of the total shift at right 

 angles to the strike of the fault-plane ; it is also the horizontal projection 

 of the dip-shift. The stratigraphical throw is the resolved part of the 



Figure 1. — Displacement at a Fault 



