THE STRENGTH OF THE EARTH'S CRUST 



309. 



do not play a large part in causing gravity anomalies. It will 

 be shown later that neither can nucleal heterogeneity below the 

 200 to 300 km. level of isostatic compensation account for a large 

 part. The anomahes represent in greater part real departures from 

 isostasy and, as shown in this section, the limited areas of high 

 anomaly are to be interpreted as implying on the whole a local 

 load in higher ratio to anomaly than do the broad areas of anomaly. 

 The average relation thought to exist is shown then in the following 

 table : 



TABLE XXIII 



An Estimated Average Relation of Anomaly Contours 



TO Contours of Equivalent Rock Masses 



OF Density 2.67 



Upon conversion of a detailed anomaly map, if such existed, 

 into the equivalent topographic contour map by means of such 

 ratios as those given in Table XXIII, the whole United States with 

 its compensated topography previously smoothed out to sea-level 

 would be reconverted into a roughly mountainous country with no 

 notable distinction between what are now the central plains and 

 mountainous border regions of the continent. On to broad plateaus 

 or basins upward of 1,000 feet from the mean elevation would be 

 added higher elevations and depressions. The extreme differ- 

 ential relief would probably be in the neighborhood of two miles 

 though the average departure without respect to sign from the 

 mean surface of the geoid would probably be between 800 and 

 1,000 feet. Though everywhere as irregular as a mountainous 

 country, there would be little or no relief of the mean level of this 

 hypothetical surface above the ocean bottoms and no such broad 

 and high masses as the Cordilleran plateaus would remain within 



