OCTOBEE 16, 1914] 



SCIENCE 



535 



anced, the apparently large mass of a 

 mountain-range ought to be compensated 

 by lightness of material in and below it. 

 Button was aware of the fact that this was 

 approximately true regarding the great 

 continental plateaus and oceanic depres- 

 sions ; but he imagined that the balance was 

 delicate enough to show up in a small hill- 

 range of 3,000 to 5,000 feet. 



The data required to test this theory, 

 accumulated during the triangulation of the 

 United States, have been made the subject 

 of an elaborate analysis by J. F. Hayford 

 and W. Bowie. ^ They find that, by adopt- 

 ing the hypothesis of isostatic compensa- 

 tion, the differences between the observed 

 and computed deflections of the vertical 

 caused by topographical inequalities are re- 

 duced to less than one tenth of the mean 

 values which they would have if no iso- 

 static compensation existed. According to 

 the hypothesis adopted, the inequalities of 

 gravity are assumed to die out at some uni- 

 form depth, called the depth of compensa- 

 tion, below the mean sea-level. The 

 columns of crust material standing above 

 this horizon vary in length according to 

 the topography, being relatively long in 

 highlands and relatively short under the 

 ocean. The shorter columns are supposed 

 to be composed of denser material, so that 

 the product of the length of each column 

 by its mean density would be the same for 

 all places. It was found that, by adopting 

 122 kilometers as the depth of compensa- 

 tion, the deflection anomalies were most 

 effectually eliminated, but there stiU re- 



3 J. F. Hayford, ' ' The Figure of the Earth and 

 Isostasy," U. S. Coast and Geodetic Survey, 

 Washington, 1909. "Supplementary Investiga- 

 tion," Washington, 1910. See also Science, New 

 Series, Vol. XXXIII., p. 199, 1911. J. F. Hay- 

 ford and W. BoTpie, ' ' The Effect of Topography 

 and Isostatic Compensation upon the Intensity of 

 Gravity," V. S. Coast and Geodetic Survey Spe- 

 cial Publication No. 10, Washington, 1912. 



mained unexplained residuals or local an- 

 omalies of gravity to be accounted for. 



Mr. G. K. Gilbert,* who was one of the 

 earliest geologists to turn to account But- 

 ton's theory of isostasy, has recently of- 

 fered a plausible theory to account for 

 these residual discrepancies between the 

 observed deflections and those computed on 

 the assumption of isostatic compensation to 

 a depth of 122 kilometers. An attempt had 

 already been made by Hayford and Bowie 

 to correlate the distribution of anomalies 

 with the main features of the geological 

 map and with local changes in load that 

 have occurred during comparatively recent 

 geological times. For example, they con- 

 sidered the possibility of an increased load 

 in the lower Mississippi valley, where there 

 has been in recent times a steady deposi- 

 tion of sediment, and therefore possibly 

 the accumulation of mass slightly in ad- 

 vance of isostatic adjustment. One would 

 expect in such a case that there would be 

 locally shown a slight excess of gravity, 

 but, on the contrary, there is a general 

 prevalence of negative anomalies in this 

 region. In the Appalachian region, on the 

 other hand, where there has been during 

 late geological times continuous erosion, 

 with consequent unloading, one would ex- 

 pect that the gravity values would be 

 lower, as isostatic compensation would 

 naturally lag behind the loss of overbur- 

 den ; this, however, is also not the case, for 

 over a greater part of the Appalachian 

 region the anomalies are of the positive 

 order. Similarly, in the north central 

 region, where there has been since Pleisto- 

 cene times a removal of a heavy ice-cap, 

 there is still a general prevalence of posi- 

 tive anomalies. 



These anomalies must, therefore, remain 



^ ' ' Interpretation of Anomalies of Gravity, ' ' 

 U. S. Geol. Surv. Professional Paper 85-C, 1913, 

 p. 29. 



