346 A. C. LAWSON ISOSTATIC COMPENSATION CAUSE OF THRUSTING 



direction of stress. The geometric relation of these curved rupture planes 

 to CD is such that any movement on them, affording temporary relief of 

 compressive strain, would develop another strain, which could only be 

 relieved by normal faulting. These curved ruptures would be limited, 

 both as to their extent and the amount of movement on them, by the 

 frictional resistance on their surfaces; and this limit might be reached 

 before they intersect the surface, or, if CD be not too deep, they might 

 actually intersect the surface. In the latter case, or in the former after 

 exposure by erosion, we would have an imbricated structure in the upper 

 block, but no large displacement on the planes of imbrication. The im- 

 bricating ruptures would be distinguished from those developed in a 

 mobile upper block by the reversal of the curvature. In the common 

 case, in which CD is inclined, some of these secondary ruptures might 

 reach the surface, while others, originating at greater depth, might fail 

 to do so. 



Now, since it has been shown that the mobile lower block, by gravita- 

 tive stress, may move past the upper passive block to an unlimited extent, 

 it is apparent that the same stress would have been competent to develop 

 the rupture CD with which, for the sake of simplicity of discussion, we 

 started; for, if we imagine a portion of the earth's crust actuated by 

 gravitative stress so distributed that it tends to move past and under a 

 neighboring passive block, the tendency will result in shear when the 

 stress exceeds the strength of the rocks. Prior to rupture in the shear 

 zone the upper passive block would fail and develop an imbricated struc- 

 ture; but one of the ruptures thus developed, namely, that most nearly 

 coincident with the shear zone, or the boundary between the mobile and 

 passive regions, would be susceptible of extension, owing to the univer- 

 sality of the force below this zone and the incompetence of friction to 

 restrain its operation. This extension of the shear rupture would con- 

 stitute a fault-plane on which relative displacement might take place that 

 would be unlimited by frictional resistance. It would be an apparent 

 overthrust but a real underthrust. 



The extension of the shear rupture would relieve progressively the 

 strain in the mobile block, and the direction of stress in the portion in 

 advance of the rupture would, therefore, progressively change ; and, on 

 the principle that the angle which the plane of rupture makes with the 

 direction of stress tends to be approximately constant, the direction of 

 the rupture would correspondingly change. The latter would conse- 

 quently become curved with the concavity to the right, or toward the 

 direction of stress. This curvature of the upper surface of the mobile 

 block indicates that as it passed the upper passive block it would do so by 



