PARALLEL DISPLACEMENTS 



179 



would yield two independent measures. The measures which can bs 

 made are the displacements of strata, dikes, old faults or veins, and the 

 azimuth and inclination of stria? on the fault-plane ; the azimuth is de- 

 termined by the compass, the inclination by the clinometer. We shall 

 now see how these different measures may be combined to give the com- 

 plete movement at the fault. It must be remembered that we can only 

 determine the movement of one side relative to the other, and it makes 

 no difference which side we regard as stationary. 



Strike-faults. — Geologists in the field often recognize the existence of 

 a fault by the fact that certain strata are repeated or that certain strata 

 are missing. We do not now consider the cases where these facts are 

 due to unconformities or to folds, but we assume for the present that 



i 



_4 *JL 





\.W 



s s' 



x*/x« ' 



^ /X;c 



\ 





'igure 5. — Repeated Strata 



Figure 0. — Missing Strata 



they are due to the existence of a fault. Frequently no other observa- 

 tions are possible ; in this case the only determination which can be made 

 is the stratigraphical throw. If certain strata are missing, it is neces- 

 sary to determine the relative positions of the same stratum on opposite 

 sides of the fault by means of the known thickness of the missing strata. 

 If this cannot be done, we can merely infer which side has dropped 

 stratigraphically relatively to the other ; we cannot determine the amount 

 of the movement. If this can be done, let s. s', in figures 5 and 6, repre- 

 sent the two positions of the repeated stratum, and let 8 be the dip. 

 Then as' = ss'sin $ will represent the stratigraphic throw, and it is quite 

 evident that nothing further can be determined. The same displace- 

 ment could be produced by a slip along the line cb, or along the line 



