. Bulletin of the Geological Society of America. Geology. 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 arro
. Bulletin of the Geological Society of America. Geology. 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 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 shea
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