THE STRENGTH OF THE EARTH'S CRUST 679 



In such viscous flow the tendency would be for a swelHng and burst- 

 ing to appear at the near end rather than a through flowage with 

 a crumphng of the pipe at the far end. 



Finally, the greatest theoretical difliculty is encountered when 

 it is sought to transmit matter from beneath the regions of oceanic 

 marginal sedimentation to beneath the regions of a continental 

 interior. Either directly or indirectly there must be a subcrustal 

 transference going forward all the way between these distant 

 regions; for example, from beneath the Mississippi and Colorado 

 deltas to the fields of erosion in the Rocky Mountains, if a condi- 

 tion of even approximate isostasy is to be maintained throughout. 

 This does not mean of course that an individual ton of plastic 

 rock is transferred a thousand miles to balance a ton of sediment. 

 Each subcrustal unit may be transferred only a mile, but it involves 

 a subsurface movement of matter all the way from the regions of 

 sedimentation to the regions of erosion. 



Now this implies a continuous pressure-gradient, and even under 

 the conception of great crustal weakness, a pressure-gradient which 

 could fold the weak cover-rocks would be far higher than that 

 needed for the movement of a continental glacier. Any large 

 degree of viscous resistance in the zone of undertow would there- 

 fore require, in order to initiate movement, an enormous defect 

 of isostasy under the distant continental interior, an enormous 

 excess under the marginal oceans. After a rejuvenative movement 

 had started, it would be slow, the frictional and deformative resist- 

 ances nearly balancing the deforming force. Therefore inertia of 

 the moving mass could not carry it appreciably beyond the point 

 where the moving force, weakened by loss of head, would just 

 balance the resistances to further movement. It would be ex- 

 pected, in consequence, that a residual pressure-difference would 

 remain, even after a period of restorative isostatic movement. 

 But an inspection of the map of New Method anomalies given in 

 Part II, p. 153, does not show any such anomaly gradients as would 

 comport with this expectation. A vast region of the continental 

 interior extending from Lake Superior to the Rio Grande and west- 

 ward to beyond the front ranges of the Rocky Mountains shows 

 average positive anomalies, indicating an excess of matter, not a 



