Continuing earlier explorations of the geological consequences of a strong elastic lithosphere supported on a weak magma, the mechanical characteristics of a shear thrust fault are examined. Due principally to the differential cooling of the continental and oceanic areas and the higher standing of the former, continental boundaries are regions of potential weakness and the lithosphere there is subject to shear failure with resultant over-thrusting of the continental sector when sufficiently compressed by horizontal forces. The deformations of such a compressed and fractured lithosphere and the fibre stress distribution therein are quantitatively worked out, permitting the determination of maximum depths, heights, widths, figures, gravity anomalies, and fibre stresses. These estimated quantities are found to be strikingly similar to the properties of a geological area on the western coast of Mexico that exhibits juxtaposed ocean deeps, mountain and volcanic chains. The reasons for the simultaneous existence of these remarkable earth features and their special relation to each other are shown to follow immediately from the mechanical characteristics of the lithosphere. The existence of volcanic activity only in regions where there is a definite tendency for the bottom of the lithosphere to be in tension gives a valuable clue as to the basic mechanisms responsible for volcanic activity.