THE ISOPIESTIC LEVEL 409 



This generally successive decrease in estimated depth with increased 

 number of available data suggests very evidently "the trend of the suc- 

 cessively accumulating evidence," and indicates that the estimates are 

 approaching a limiting value. We are thus justified in believing that the 

 value of 60 kilometers is a fairly close approximation to the truth. This 

 value, it is of some interest to note, also approaches more nearly than 

 those of Hayford and Bowie to Fisher's early estimate of 25 miles = 40 

 kilometers. 



All the estimates of the depth of the isopiestic level which have been 

 discussed in the preceding pages are based on the assumption that the 

 density remains constant throughout the length of the column. If, how- 

 ever, there be some sort of "gravitative adjustment," whereby the density 

 increases with depth, the estimates of the depth of the isopiestic level 

 must be modified; and the modification would depend on the mode and 

 rate of change in density with increase in depth, whether percentually 

 gradual, or by successive discrete steps. Assuming a gradual percentual 

 increase, which seems to be the most plausible for depths of the order of 

 magnitude down to that of the isopiestic level, the centers of gravity of 

 the columns would be lowered, those of the longer and lighter columns 

 more than those of the shorter and heavier ones, so that there would be a 

 virtual lessening of A h. In accordance with the generalization noted on 

 page 406, by which M decreases when A h (for constant AS) decreases, it 

 is to be expected that M also would decrease, thus bringing the isopiestic 

 level nearer the surface. 



It is quite impossible to form any estimate, even of the roughest sort, 

 as to the possible extent of such a diminution in isopiestic depth brought 

 about by gravitative distribution of densities. We know nothing of the 

 rates of change in density with depth or of the downward limit which 

 such a change might approach. Indeed, we are still uncertain whether 

 such gravitative adjustment exists, as advocated by Daly, Bowen, and 

 others, although there is much to be said in its favor. It is a factor in 

 the problem which must be taken into account (when possible), but which 

 has been overlooked by geodesists. 



COMAGMATIC REGIONS AND GRAVITY ANOMALIES 



A few words may be devoted to a somewhat striking illustration of the 

 correspondence between the distribution of igneous rocks into comagmatic 

 regions and the excess or deficiency of gravity, called gravity anomalies. 33 

 Gravity anomalies, or departures from the theoretical value of gravity 



33 H. S. Washington : Jour. Franklin Inst, vol. 190, 1920, p. 813. 

 XXVII— Bull. Geol. Soc. Am., Vol. 33, 1921 



