Philip Lake — Direction of Faults. 545 



IV. — On a Mkthod of Determining, the Actual Direction of 

 Movement, whether Lateral or Vertical, in FAtrLTS. 



By Philip Lake, M.A., F.G.S. 



N calculating the throw of a fault, it is usual to assume that the 

 whole of the displacement due to the fault has been produced 

 by a vertical motion — i.e. by a motion in a vertical plane at right 

 angles to the direction or strike of the fault — and that there has 

 been no such thing as a lateral movement of one face of the fault 

 upon the other. 



Suppose, for example, that in Fig. 1 the plane of the paper is the 

 plane of the fault, that the heavy line A B represents the outcrop or 

 " trace " of a bed upon one of the faces of the fault, and the light 

 line A' B' the outcrop or " trace " of the same bed upon the other 

 face, the horizontal line being the surface of the ground. It is 

 almost universally supposed (in practice at least) that before the 

 formation of the fault the point A was in coincidence with the point 

 A' (which is now vertically below it), and the point B with the 

 point B' ; and that the line A A' or B B' shows both the direction 

 and amount of the movement. 



A 



i^iy. i. Fig. Z. 



But this may not be true. It is possible that A niay have 

 coincided with A" (Fig. 2) and B with B" ; the movement may 

 have taken place along the lines A A", B B", or, in other words, 

 may have been entirely lateral or horizontal. Or the point A may 

 have coincided with some other point A'" or A""; and in that case 

 the movement must have taken place along the line A A"' or A A"", 

 and must have been partly horizontal and partly vertical. Yet the 

 result is apparently the same ; and it is impossible, therefore, in 

 such a case, to determine the actual direction of the movement. 

 When, however, the fault crosses an anticlinal or a synclinal, the 

 effects of lateral, as opposed to vertical, motion are sometimes so 

 clear that they have already attracted attention. 



Most of the textbooks explain the eifect of a fault upon an anti- 

 clinal on the assumption that the movement has been purely vertical. 

 They show, if the ground be level, that the two outcrops of a bed 

 on the downthrow side of the fault will lie closer together than the 

 two outcrops on the upthrow side ; and they show, moreover, that 

 if the outcrops on the downthrow side be produced across the fault 



DECADE IV. — VOL. IV. — NO. XII. 35 



