334 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



It is absolutely certain that the masses pushed up, whether by- 

 vertical or horizontal forces, are not extra loads above the surface 

 which limits the depth of isostatic compensation. These masses 

 above sea level are, of course, extra loads on the sea-level surface, 

 but they can not be extra loads on the imaginary blocks of the earth's 

 crust which are resting on the plastic subcrustal material. If they 

 were extra loads, this fact would be easily and clearly indicated by 

 geodetic data in the form of deflections of the vertical and values of 

 gravity. The masses that apj)ear above sea level are compensated for 

 by the deficiency of density in the material lying below them. 



The zone witliin which the compensation of topographic features 

 lies must be of limited depth. If it were otherwise, the computed 

 effect of the compensation would be practically zero and the material 

 above sea level wouhl have full effect on the direction and force of 

 gravity. 



It has been concluded from a study of the deflection data for the 

 United States that the actual deflections of the vertical are on the 

 average not more than about 10 per cent of what they would be if 

 the masses above sea level and the deficiency of the mass in the oceans 

 were not compensated by deviations from normal -densities in the 

 crust below. This is the very strongest evidence possible in favor of 

 isostasy and, also, in favor of the theory that the outer portion of the 

 earth is rigid and strong. This rigid material, which has been found 

 by geodesists to extend to an average depth of approximately 60 

 miles, will resist for extremely long times gravitational forces which 

 tend to make the earth's surface a true spheroid. The gravity data 

 supplement the data derived from the deflection of the vertical in 

 showing the existence of isostasy. 



Since areas of sedimentation and erosion and all plateau and moun- 

 tain regions are now in isostatic equilibrium, it seems reasonably 

 certain that they have been in equilibrium throughout the geological 

 era. If this is true, we must conclude that there has been an actual 

 uplift of the surface in some places and a down-warping in others. 

 These changes in the earth's surface can occur only by vertical move- 

 ments, due to changes in the density of the crustal or subcrustal 

 material, or to the action of horizontal forces. I am inclined to favor 

 the former idea because it is rather difficult to see where horizontal 

 forces of sufficient magnitude could originate. Since the sea-level 

 surface of the earth is at all places at right angles to the direction of 

 gravity, it is difficult to see how any large horizontal component of 

 the gravitational force could come into existence. 



I believe that there has been no collapsing of the outer shell of 

 the earth on a shrinking nucleus. The outer solid shell of the earth 

 must be of the magnitude of 60 miles in thickness, and certainly at 



