1913.] SEE— ORIGIN OF HIMALAYA MOUNTAINS 509 



Call this level of temporary equality of pressure in the two columns the 

 neutral level. As the process of erosion and deposition progresses the neu- 

 tral level will gradually progress upward from its original position at the 

 base of the columns. Eventually if no interchange of mass took place be- 

 tween the columns except at the surface, and no vertical displacement 

 occurred in either column, the neutral level would reach the surface when the 

 process of erosion and deposition became complete and the upper sur- 

 faces of the two columns were at the same» level. During the process of 

 erosion and deposition the excess of pressure in A at any level above the 

 neutral level will continually decrease. Similarly, at any level below the 

 neutral level the excess of pressure in B will continually increase as the 

 erosion progresses and the neutral level will rise. Thus there will be estab- 

 lished a continually increasing tendency for the^ material below the neutral 

 level in B to be squeezed over into A. If the stresses tending to produce this 

 undertow from the lower part of B to A become greater than the material can 

 stand, the flow will take place as indicated by the arrow in the figure. If the 

 material flows without change of volume, as if it were incompressible, the 

 upper part of A and its surface will be raised, the upper part of B and its 

 surface will be lowered, the neutral level will sink and an approximation to 

 the original conditions with complete isostatic compensation will be re- 

 established." 



"This is the general case of isostatic readjustment by the action of 

 gravitation alone. Gravitation tends to produce a deep undertow from the 

 regions where deposition is taking place to the regions where erosion is in 

 progress, in the direction opposite to that of the surface transfer of material." 



" Let us suppose that the isostatic compensation at a given stage in the 

 earth's history is practically complete for a continent, that the process of 

 erosion from the greater part of the continent and deposition around its 

 margins is in progress, and that the process of readjustment by a deep under- 

 tow is in progress." 



The fatal defect in this reasoning consists in the fact that it 

 begs the question^ and does not in any way explain the elevation of 

 the margin of a continent, but only hotv it may maintain its present 

 form by a process of readjustment. This is like a river rising 

 higher than its source, or a man trying to Hft himself by pulling on 

 his bootstraps, or the logician reasoning in a circle. For in order to 

 explain the development of the inequalities of the earth's crust, we 

 must not only explain the adjustment and balancing between ad- 

 jacent parts, but also hoiv the original uplift came about, to give the 

 observed contrast in surface levels. 



Now on the premises used 'by Hayford, it is possible to explain 

 how a given inequality of surface levels, when once existing, can 



