1915-] THE GEOPHYSICAL STANDPOINT. 305 



portional to the space rate of change of pressure. The flows would 

 all be found to be away from beneath high regions toward low 

 regions, from continents toward oceans, from mountains toward 

 valleys. 



After such a picture has been clearly formed assume that the 

 isostatic condition is disturbed by long-continued erosion and depo- 

 sition producing changes in the surface elevations and surface loads. 

 On the same assumptions as to the nature of the viscous flows as 

 before, form a new picture of the viscous flows which would now be 

 in progress. It will be found that under the new conditions the 

 viscous flows near the surface would still be away from high areas 

 and toward low areas, but in general they would be slower than 

 before. At greater depths, however, it will be found that the vis- 

 cous flows would be undertows from regions of recent deposition 

 toward regions of recent erosion. These undertow flows would in 

 general tend to be in the direction opposite to recent surface trans- 

 portation of material. This picture would serve as a first approxi- 

 mation to an understanding of the mechanism of isostatic readjust- 

 ment. The undertows would be found on these assumptions to 

 extend to a considerable depth, certainly more than 122 kilometers. 



Next one should picture the changes in density which would be 

 produced by the viscous flows. The density should be pictured as 

 decreasing in regions from which material is being carried away by 

 the flow and increasing in regions to which the material is being 

 ca rried. It will be noticed as soon as such a picture is formed that 

 every undertow flow at any level tends to equalize pressures at lower 

 levels. This will have a strong tendency to make the prevailing 

 undertows occur at much higher levels than they otherwise would. 



Let it be assumed that the viscous material offers some small re- 

 sistance to shear and still has elastic properties to a slight degree. The 

 condition assumed originally that the pressure at a point depends 

 simply upon the weight of the material above that point will be dis- 

 turbed thereby. Form as clear a conception as possible of these dis- 

 turbances and the modifications of the floAvs produced by them. I be- 

 lieve the modifications will be found to be important, and that they 

 will be found to be such as tend to confine the effects of surface 

 changes of load to a depth which is a small fraction of the radius. 



