850 A. C. LAWSOX ISOSTATIC COMPENSATION CAUSE OF THRUSTING 



may, therefore, recognize a relatively viscous condition, in contrast to a 

 rigid condition above. Let GH denote the boundary between the rigid 

 and the viscous condition. Below GH a mass, represented by the prism 

 IJKL, 3 is transferred from the right to the left of CI. If this transfer 

 be effected in a stream of uniform thickness, the flow of which begins 

 throughout at the same time and ends at the same time, then the relative 

 velocity of the latter is inversely proportional to the mass passing the 

 points at which the velocity is compared. Under this assumption the 

 relative velocity of the stream throughout the flow is indicated by the 

 curve MjSTO, which is also the curve of the square root of the momentum 

 of the moving mass. If the transfer is effected by a stream of unequal 

 thickness, then the curve MNO still indicates the relative momentum of 

 the moving mass, but not the velocity. 



As appears in the diagram, figure 9, the maximum mechanical effect of 

 the viscous stream is at the point I, or directly below C, the border line be- 

 tween relief and load in the transfer of mass by erosion. Whatever, then, 

 may be the stress applied to the crust above GH by the viscous stream, 

 that stress (that is, drag effect) would probably have its maximum in the 

 region under C, and we should expect, therefore, the greatest strain and 

 the first manifestation of underthrusting to be located to the left of CI ; 

 but, as has already been indicated, the stress would probably overtake the 

 strength of the rocks at GH before doing so at the surface, and any rup- 

 ture that occurred would be initiated at GH and be extended to the sur- 

 face. Let PQ be such a rupture, on which underthrusting immediately 

 proceeds. The mobile block to the right would be thrust under the passive 

 block to the left either by wedging or by rotation. If by wedging, then 

 partial compensation would result, and the viscous current, the cause of 

 the thrust-stress, would be checked. If by rotation, partial compensation 

 would be also effected by transfer of mass from right to left, and the 

 urgency of the viscous stream would be similarly diminished; but if by 

 rotation, which is the more probable, there is a more rapid relief of hori- 

 zontal strain near the surface than near GH, since the horizontal com- 

 ponent of motion on the curved rupture is large near the surface and 

 small near GH. The strain set up in the passive block above PQ induced 

 by frictional resistance on PQ would be relieved by (1) relatively short 

 thrusts, with the development of an imbricated structure in the region 

 below CF; (2) folding in the stratified rocks above CF; and (3) thrust- 



3 The mass IJKL is less than the mass CDF by the amount which flows out to the 

 right of D to compensate any disturbed balance in that direction, the amount which 

 flows in from the left of E by reason of disturbed balance in that direction, and an 

 amount corresponding to any residual strain. 



