342 A. C. LAWSON ISOSTATIC COMPENSATION CAUSE OP THRUSTING 



and relief. In geological studies it is of importance to discriminate be- 

 tween the mobile and the passive blocks, since when we can do this we 

 know the direction of the origin of stress. In the case of a real overthrust 

 the upper block is the mobile side of the fault, and it would seem that 

 the stress which is the cause of the progressive advance of the block could 

 scarcely be applied otherwise than horizontally and fairly uniformly to 

 its cross-section. Any other non-uniform application of the stress would 

 probably result in a limiting value for AB less than that found above. 



For a real overthrust, therefore, it appears that the limiting length of 

 the overthrust prism in the direction of the force applied is between 20 

 and 30 miles when the thrust-plane is nearly horizontal, and less than 

 this if the thrust-plane is inclined. It follows from this limitation on 

 the length of overthrust blocks that, if the stress which causes the dis- 

 placement persist, relief of renewed strain can only be effected by a new 



Figure 3. — Diagrammatic Representation of a Thrust Block 



AB, surface of the earth ; CD, thrust plane ; EF and GH, successive curving thrusts due 



to failure of thrust block. 



fault rupture situated nearer the origin of the stress. In this way, by a 

 prolongation of the process, an imbricated structure may be produced, 

 the fault blocks overriding each other in orderly succession like tiles on 

 a roof. 



This limitation on the length of an overthrust block, measured in the 

 direction of the thrust, places a still more important limitation on the 

 amount of movement by real overthrusting on a fault plane; for if it be 

 true that 20 to 30 miles is the limiting length of the overriding block, 

 then any displacement relative to the passive block — say, for example, 

 one mile — must affect the whole of the block, except for slight differences 

 in the condition of compressive strain ; that is to say, the end of the block 

 nearer the origin of stress must move approximately as much as the more 

 remote end. But this could only happen if the ground still nearer the 

 origin of stress were also moving, and this implies a frictional resistance 

 to movement in excess of the strength of the rocks, as has been shown in 

 arriving at the limiting value of AB. It seems, therefore, mechanically 



