666 JOSEPH BARRELL 



limit. Therefore they may be regarded as having not more than 

 a quarter of the value shown in Fig. 13A. 



The suggestion of the existence of opposing modifying factors is 

 to be found in conclusions from the previous parts of this investi- 

 gation — that compensation may be in many places concentrated 

 somewhat in the outer half of the zone as here shown and in other 

 places fade out through a notable distance below. These two 

 variations in the distribution of compensation would modify the 

 stress diagram in opposite directions. 



Modifications of stresses produced by base-leveling. — Consider next 

 the case of complete erosion to sea-level, as shown in Fig. 13B. 

 The rock from the land-column has been deposited as sediment 

 over the sea-column. As the columns are supposed to act as 

 units the sediment is shown as spread uniformly. The lateral 

 stress diagram beneath the bottom of the sediment shows a rate 

 of decrease the same as in case A, but the value of the hydrostatic 

 stress at any depth is diminished by the sum of the depths of 

 erosion and deposition. The lateral stress now changes in sign 

 at a point S and at this depth is a line of no lateral stress. Above 

 this depth the continental segment tends still to spread over the 

 ocean, but less effectively than before; below this depth the oceanic 

 segment now thrusts against the continental crust. 



If the ocean water be eliminated from the diagram and base- 

 leveling should bring both columns to a uniform surface, then the 

 neutral depth 5 advances to the surface and the lateral stress 

 diagram in B is just the reverse in value to A. In that case there 

 is no lateral thrust at the surface, but at all depths below there is 

 an excess pressure against the continent reaching a maximum 

 at the bottom of the lithosphere. This extreme case cannot apply 

 to the ocean except for that limited width over which is built out 

 a continental shelf. To the degree to which the weight of this 

 shelf is supported by the ocean crust beyond, the column beneath 

 the shelf would not operate with its full pressures against the land. 

 The case would apply better to the complete erosion of level- 

 topped plateaus situated within a continent. 



For the lateral pressure within the lithosphere to become effect- 

 ive in a landward undertow would require a lesser rigidity of the 



