Philip Lake — Direction of Faults. 



547 



It is certain, therefore, tbat in some cases an actual, and not 

 merely an apparent, lateral displaceinent has occurred during the 

 formation of a fault. But so far as I am aware, no one has yet 

 sliown how to determine the real direction of the movement and 

 the inclination of this direction to the horizontal. If all the beds 

 affected by a fault dip at the same angle and in the same direction, 

 the problem is indeed insoluble. But if the dip be not uniform, 

 if the beds be in any way bent or folded, the solution is 

 theoretically easy. 



A /> 



c c' 



./^ V 



Fi^^. 



s. 



Method of finding trne direction of movement. 

 (The description applies to any of these four diagrams.) 



Suppose that in any of the four diagrams in Fig. 5 the plane of 

 the paper is the plane of the fault ; that the heavy lines A B, D 

 represent the outcrops of beds (having different dips) upon one 

 face of the fault ; and A' B', C D' the outcrops of the same beds 

 upon the other face. Produce A B and C D to meet in 0, 

 A' B' and C D' to meet in 0'. Then the line 0' lies in the 

 direction in which one face of the fault has moved upon the other, 

 and the length 0' is the amount of the movement. For the effect 

 of the fault is as if the line A B were moved bodily into the position 

 of the line A' B7 and the line C D into the position of the line C D'. 

 Now the point is a point in the line A B, and it must therefore 



a railway line. This alone, however, -would not be conclusive, for railways are but 

 loosely attached to the earth. A similar disturbance of a railway line was produced 

 in the recent Indian earthquake of the I'ith June, 1897; but the figure given by 

 Oldham (Rec. Geol. Surv. India, vol. sxx, pi. xvii) seems to show that the sides of 

 the railway track, as indicated by the lines of vegetation, remained undisturbed. 



^ This does not mean that the 2ioint A is moved to the position A', or B to B' ; but 

 that the line A B (which may be imagined to be indefinitely produced) is moved to 

 the position of the line A' B' (also indefinitely producedj . 



