Nature and Bearings of Isostasy. 285 



through a great part of the earth's body. It is to be 

 concluded, therefore, that the larger relief of the earth 

 is on the average nearly compensated by density varia- 

 tions which occur mostly within 122 kilometers of the 

 surface. 



Another and independent method of investigation is 

 based upon the direct measurements of the intensity of 

 gravity at many stations by means of the pendulum, and 

 comparing the results with the gravity at each station 

 as calculated for latitude and elevation and modified by 

 the influence of the topography of the whole earth and 

 its assumed isostatic compensation. The difference be- 

 tween the observed and calculated values of gravity at 

 each station, adjusted to the hypothesis of isostasy, is 

 known as the gravity anomaly. The results of such an 

 investigation confirm the evidence of the degree of isos- 

 tasy and depth of compensation as obtained from astro- 

 nomic and geodetic measurements. 



Having reached this conclusion, the study of the 

 nature of the deflection residuals and gravity anomalies 

 becomes of the first importance, for they measure the 

 differences at each locality between the actual conditions 

 in the earth's crust and the simple ideal of Hayford's 

 hypothesis. It is this subject especially which the writer 

 has studied. 



The results indicate that individual mountain ranges 

 like the Front Range of the Rocky Mountains may be 

 sustained by the rigidity of the crust, this being found 

 to be true at Pike's Peak; but broader areas, such as 

 the Great Basin, are largely supported by corresponding 

 variations in crustal density. Unit areas two hundred 

 miles across may lie on the average, as previously stated, 

 more than one thousand feet above or below the level 

 giving isostatic equilibrium, but an area as broad as a 

 continent probably lies as a whole at an elevation con- 

 siderably within a thousand feet of the isostatic mean. 

 It appears that the major relief of the earth, that be- 

 tween continent and ocean basin, is as much as nine- 

 tenths compensated, perhaps somewhat more, since the 

 regional departures from isostasy tend to mask the 

 broader approach toward isostasy. The present epoch, 

 however, may, as Willis has observed, be an epoch which, 

 because of recent diastrophic adjustment, shows an un- 

 usual degree of isostatic equilibrium. 



In regard to the differences in the vertical distribution 



