190 



H. F. REID GEOMETRY OF FAULTS 



new position, and the mass then rotated around an axis through this 

 point, or we may represent the whole displacement as made up of a rota- 

 tion about a properly chosen axis and a translation along that axis ; that 

 is, by a movement similar to that of a screw. The fault surface will be 

 a screw-surface, though it may approximate, or even become, a cylinder 

 or a plane ; in the latter case the movement will reduce to a simple trans- 

 lation or a simple rotation. It may be a question whether a composite 

 displacement of this kind occurs in nature on a large scale; but, as the 



Figure 12. — Rotation and Translation 



method of treating this case is the same as that of finding the axis of a 

 simple rotation, when the observations do not make the direction of the 

 latter immediately evident, we shall take up the more general case. This 

 case is perfectly general, and includes every possible displacement of a 

 rigid mass without distortions. 



It is quite evident that if three points in a body are fixed, the body 

 itself is fixed ; and if the displacements of three points are known, the dis- 

 placement of the body is known. Three points determine a plane and the 



