184 



H. F. REID GEOMETRY OF FAULTS 



b, whose depth is bd; and a contour on the fault-plane through the orig- 

 inal of b must have the depth bd; therefore lay off bd' = bd through b 

 and parallel with the fault-trace, complete the dip-triangle, and 8" will 

 be the angle of the fault-dip. At the time of the rupture the stratum, 

 the projection of whose intersection with the fault-plane is Im, slipped 

 diagonally down the fault-plane in a direction parallel with the original 

 of ff and at an angle less steep than that of the original of Im. A hori- 

 zontal line at right angles to the fault-trace was lengthened by an 

 amount, /" /', but the intersection of the displaced stratum with the 

 fault-plane (original of V m') will be above the intersection of the fault- 

 plane and the undisturbed stratum (original of Zm), and the fault would 

 therefore be called a reversed fault. 



Case III : Fault in igneous rock. Given the trace and dip of the fault- 

 plane; the traces, offsets, and dips of two dikes, or of one dike and an old 

 fault-plane, or of any two planes which have been disrupted by the fault. 

 The construction in this case is exactly like that in case I. 



Case IV : Given, the azimuth and inclination of strice, and the traces, 

 offsets, and dip of a stratum, or of any plane disrupted by the fault. 

 Let T, t', figure 9, be the traces of the stratum ; ff' the projection, and 8' 



the inclination of the 

 striae, the original of /' 

 being the lower point. 

 Let us find the total dis- 

 placement of some point, 

 d, on the trace, T. 

 Through d pass a line par- 

 allel with the striae; dd', 

 parallel with ff', will be its 

 projection; pass a plane 

 through d, having Td for 

 its trace and containing 

 the original of the line 

 dd' ; its dip will be 8", ob- 

 tained by the method given on page 175. This plane will meet the dis- 

 placed stratum, t' , in the contour h' e', obtained by laying off the dip 8 at 

 c', as shown in the figure, and finding the intersection, <?', of the lines de' 

 and c' e' ; and therefore the original of line dd! will meet the plane in the 

 same contour in the original of the point d'. dd' will therefore be the 

 projection of the shift of d, an&dg' obtained by laying off the angle 8' at 

 d, or by drawing d' g' equal to In! e' 9 will be its amount. 



Figure 



Determination of the Shift, Case IV 



