PRESIDENTIAL ADDRESS, 353 
J. I. 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 uniform depth, called the depth of 
compensation, below the mean sea-level. The columns of crust material stand- 
ing above this horizon vary in length according to the topography, being rela- 
tively 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 kilométres 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 
Dutton’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- 
métres. 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 mass 
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 
may be such as to give rise to different effects on the pendulum, If, for instance, 
one considers two columns of the same size and of exactly the same weight, with, 
in one case, the heavy material at a high level and in the other case with the 
heavy material at a low level, the centre of gravity of the former column, being 
nearer the surface, will manifest itself with a greater pull on the pendulum ; 
these columns would be, however, in isostatic adjustment.” 
*° J. F. Hayford, ‘The Figure of the Earth and Isostasy,’ U.S. Coast and 
Geodetic Survey, Washington, 1909. ‘ Supplementary Investigation,’ Washing- 
ton, 1910. See also Science, New Series, vol. xxxiii., p. 199, 1911. J. F. 
Hayford and 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. 
*1 “Interpretation of Anomalies of Gravity,’ U.S. Geol. Surv, Professional 
Paper 85-C, 1913, p. 29. 
* It is interesting to note that the idea suggested by G. K. Gilbert in 1913 
was partly anticipated by Major H. L. Crosthwait in 1912 (Survey of India, 
Professional Paper No, 13, p. 5). Major Crosthwait in discussing the similar 
gravity anomalies in India remarks parenthetically : ‘ Assuming the doctrine of 
isostasy to hold, is it not possible that in any two columns of matter extending 
from the surface down to the depth of compensation there may be the same 
mass, and yet that the density may be very differently distributed in the two 
1914. AA 
