HORIZONTAL MOVEMENTS. 55 



therefore make an attempt to infer from the nature of the phenomena the 

 character and direction of a system of forces such as would give rise to the 

 fractures and faults actually observed. In doing so it will be most con- 

 venient to begin with the horizontal movements on the vertical fissure 

 systems. 



HORIZONTAL MOVEMENTS. 



Resolution of Forces, — In figure 1 one of the small vertical prisms is shown 

 in projection on a horizontal plane. The sides are oriented as if the top of 

 the page were to the north, and the arrows show the directions in which the 

 movements of adjoining columns have taken place. These arrows may also 

 be considered as representing in direction and intensity the forces which have 

 produced the motions of the neighboring columns. Regarded as forces, they 

 form two couples which, as drawn, exactl^^ balance one another; so that, 

 though either couple alone would produce rotation, the two conjointly can 

 produce no rotation, but only deformation of the mass regarded as elastic. 



FiGVRE 1— Plan of vertical prism. 



It is quite possible that the couples actually involved in the dynamic action 

 on the Sierra did not exactly balance, but if so the excess of one above the 

 other must have produced an actual rotation about a vertical axis, and only 

 that portion of the stronger couple which was exactly balanced by the weaker 

 couple was employed in producing deformation and rupture of the mass. 



The combination of two balanced couples is known as a simple shearing 

 stress. It is possible for such a stress to exist alone, but, if it did so, it would 

 be unattended by compressive action or by any horizontal or diagonal fis- 

 sures. It is also difficult to see how the precise combination of forces need- 

 ful to produce a horizontal shear by itself could have existed in a geological 

 convulsion. 



The slickensides and other evidences of great pressure in a horizontal 

 direction indicate that the shear must have accompanied a thrust, and it is 

 known that a simple compressive thrust involves shearing action. Now, 

 when both the direction of the fissures and the direction of the relative 

 motions along these fissures are taken into consideration, it can be shown 



