Rotatory displacements 187 



the intersection of two dikes, so that we can immediately say that a point 

 of this line on one side of the fault has been displaced to a point of the 

 displaced line on the opposite side of the fault. If we can determine the 

 strike and dip of the fault-plane, the whole movement can be determined. 

 This can also he done if we can find the strike, dip, and offset of a dike 

 or other broken plane. 



Potatory Displacements 



SIMPLE ROTATION WITHOUT TRANSLATION 



In this case the fault surface will be a surface of revolution, with the 

 axis of rotation as its axis of revolution ; it is only under these conditions 

 that the movement can take place and contact be maintained between the 

 two sides. The axis is apt either to be at right angles to the fault sur- 

 face or not actually to intersect it at all; for if it should intersect the 

 fault surface in an acute angle, the surface must, in the neighborhood of 

 the point of intersection, envelop the axis like a cone — a form which has 

 never been observed. 



Professor Jaggar has suggested that some faults occur in which one 

 side has rotated as a block about an axis at right angles to the fault- 

 plane. 5 I am not sure that movements of this kind occur on a large 

 scale in nature; certainly the California earthquake fault, which Pro- 

 fessor Jaggar refers to, is not an instance of it; and it is difficult to 

 understand what allowable forces would cause such a rotation. We must 

 remember that the continuity of the rock must exist except at the actual 

 break, and that the displacement of faults is taken up near their ends by 

 plastic or elastic distortion. Moreover, in very large masses the rock 

 can not be expected to act like a rigid body. If the angle of rotation is 

 small, we can decide if the main part of the rock rotates about a single 

 axis by determining the displacement at different parts of the fault by 

 methods already given; then by a comparison of these displacements we 

 can see if they all represent a rotation around the same axis. If they do, 

 the directions of the movements must be all at right angles to the radii 

 drawn from a single axis, and the amounts at different points must be 

 proportional to the distances of the points from the axis. The axis can 

 then be easily determined, as pointed out by Professor Jaggar, by finding 

 the intersection of lines drawn at right angles to the directions of the 

 movements. If the angle of rotation is large, we have merely to deter- 



5 "Economic geology," 1907, vol. ii, p. 60. 



XV— Bull. Geol. Soc. Am., Vol. 20, 1008 



