If a rupture plane of low dip exist in the earth’s crust emergent at the surface, and if a stress exist in the overlying prism which tends to move it toward the emergent side, the movement is restrained by the friction of the surfaces in contact on the rupture plane. If, further, the stress be due to a pressure applied from without to the cross-section of the prism, then, in order that the prism may move, and so be thrust over the underlying rock-mass, the pressure must exceed the friction on the rupture plane. If the stress be tangential in direction and uniformly distributed in the cross-section of the prism, then the limiting length of the prism which may be moved is determinable; since, when the stress necessary to overcome the frictional resistance becomes greater than the strength of the material of the . . .