THE INTEEIOR OF THE EARTH. 245 



picture as accurately as possible of the viscous flows which would 

 take place on the assumption that at each level the material would 

 flow horizontally from regions of greater pressure to regions of less 

 pressure along lines of maximum rate of change of pressure, and 

 that the time rate of such viscous flows would tend to be propor- 

 tional 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 iso- 

 static condition is disturbed by long-continued erosion and deposition, 

 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 prog- 

 ress. 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 viscous flows 

 would be undertows from regions of recent deposition toward re- 

 gions of recent erosion. These undertow flows would in general tend 

 to be in the direction opposite to recent surface transportation of 

 material. This picture would serve as a first approximation to an 

 understanding of the mechanism of isostatic readjustment. The 

 undertows would be found on these assumptions to extend to a con- 

 siderable 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 

 carried. 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 have elastic properties to a slight degree. 

 The condition assumed originally that the pressure at a point de- 

 pends simply upon the weight of the material above that point will 

 be disturbed thereby. Form as clear a conception as possible of these 

 disturbances and the modifications of the flows produced b}'- them. I 

 believe 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. 



So much for the direct effects of gravity which it seems im- 

 portant to picture clearly. Next study other effects, some of which 

 are inclirecth^ produced by gravity. 

 73839°— SM 1916 17 



