Ixxxii PT?0.C'EED1>'GS OF THE aEOLOGHCAL SOCTETT. [vol. Ixxvil, 



^vliicli inovemeiit liad taken ])lace, without any indication o£ 

 crushing ov deformation of the friable or plastic material on 

 either side. Tliere was no indication of any such resistance 

 to movement as would liave resulted from friction at the fault- 

 surface, if the upthrow side had been forced upwards by being- 

 thrust against the inclined surface of the fault. On the contrary, 

 the appearance was rather as if the pressure of the overlying on 

 the underlying mass had been temporarily relieved, at the time 

 when the displacement took place. Much the same appearance is 

 frequently presented by normal faults ; sometimes there is con- 

 siderable crushing and deformation close by the fault-surface, such 

 as would reasonabl}'- be expected if the movement had been due to 

 the overlying mass sliding, by its own weight, over the inclined 

 surface of the fault ; at other times, however, no such appearance 

 is met with, and even soft rock, easily bent or broken, lies on 

 either side of the fault-surface almost as uninjured as if the two 

 sides had been separated from, each other w^hen the movement 

 took place. 



We must also consider that very large class of faults, sometimes 

 of great vertical throw, in which the fault-surface is either vertical, 

 or so nearly vertical that it is difficult to decide the direction of 

 the hade. In these, and especially where the throw is large, we 

 can hardly attribute their origin simply to extension or com- 

 pression in a horizontal direction, and we seem compelled to 

 invoke the action of some force acting vertically, or with a very 

 large vertical component, in its direction ; and more than that, it 

 must have been one which acted with much greater effect on one 

 side of the fault than on the other, or, possibly, in opposite 

 directions on opposite sides of the fault. 



Paradoxical as it may seem, this is by no means physically 

 impossible. Some years ago, when discussing the displacements of 

 the ground which took place along the San Andreas Fault, in 

 connexion with the South Californian earthquake of 1906,^ I had 

 occasion to refer to the very complicated stresses which are set up 

 in a bod})^ subjected to compression, or extension, in one direction, 

 but free to change its form in another, and exhibited to you a 

 model illustrating how a force acting in one direction on the body 

 as a whole might set up stresses, and give rise to displacement, 

 within it, in a wholly different direction. The displacements 

 dealt with on that occasion were of comparatively small amount ; 



^ Q. J. Ct. S. vol. IxY (1909) pp. 1-16. 



