Address by Prof. Sir T. H. Holland. 459 



to the conditions that favour deposition and the uplifting of a range results in 

 the increased energy of eroding streams. 



Thus there was a natural desire to see if Button's theory agi-eed with the 

 variations of gravity. If the ups and downs are balanced, the ajiparently 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 plateau and oceanic depressions ; 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 adopting the hypothesis 

 of isostatic compensation, the differences between the observed and computed 

 deflections of the vertical caused by topographical inequalities are reduced to 

 less than one-tenth of the mean values which they would have if no isostatic 

 compensation existed. According to the hypothesis adopted, the inequalities 

 of gravity are assumed to die out at some unifoiTn depth, called the depth of 

 compensation, 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 kilometres as the depth of com- 

 pensation, the deflection anomalies were most effectually eliminated, but there 

 still remained unexplained residuals or local anomalies of gravity to be 

 accounted for. 



Mr. G. K. Gilbert," who was one of the earliest geologists to turn to account 

 Button's theory of isostasy, has recently offered 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 kilo- 

 metres. 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 considered the possibility of an increased 

 load in the Lower Mississippi Valley, where there has been in recent times 

 a steady deposition of sediment, and therefore possibly the accumulation of ma-^s 

 slightly in advance of isostatic adjustment. One would expect in such a case 

 that there would be locally shown a slight excess of gravity, but, on the con- 

 trary, 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 

 expect that the gravity values would be lower, as isostatic compensation would 

 naturally lag behind the loss of overburden ; 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 Pleistocene times a removal of a heavy ice-cap, there is still a general 

 prevalence of positive anomalies. 



These anomalies must, therefore, remain unexplained by any of the obvious 

 phenomena at the command of the geologist. G. K. Gilbert now suggests that, 

 while it may be true that the product of the length of the unit column by 

 its mean density may be the same, the density variations within the column 



1 J. F. Hayford, "The Figure of the Earth and Isostasy": U.S. Coast 

 and Geodetic Survey, Washington, 1909. " Supplementary Investigation," 

 Washington, 1910. 'See also Science, N.S., vol. xxxiii, p. 199, 1911. J. F. 

 Hayford & W. Bowie, " The Effect of Topography and Isostatic Compensation 

 upon the Intensity of Gravity " : U.S. Coast and Geodetic Survey Special 

 Publication No. 10, Washington, 1912. 



2 "Interpretation of Anomalies of Gravity" : U.S. Geol. Surv. Professional 

 Paper 85-C, p. 29, 1913. 



