SUBMARINE MOUNTAINS 01 



diately above the vessel. In general this sealevel value was 

 found to differ from that expected from the standard spheroid. 

 This difference we recognize as the free-air anomaly of gravity ; 

 it may be a positive quantity or a negative quantity. Geodetic 

 work in continental regions shows that a positive free-air 

 anomaly at a given station does not necessarily mean excess of 

 attracting matter under the station; nor does a negative free- 

 air anomaly mean deficiency of mass. In order to detect excess 

 or deficiency under a station its free-air anomaly must be "cor- 

 rected" in accordance with the principle of isostasy, briefly 

 described in the first chapter. 



According to that principle, high parts of the earth's surface 

 are, in general, underlain by crust-rock which is less dense than 

 the average superficial rock of the crust; on the other hand, 

 low parts of the solid surface are in general underlain by crust- 

 rock which is more dense than the average superficial rock. 

 In technical language, the earth's relief is "compensated" by 

 the horizontal variation of density. As already noted, this 

 variation in an area as large as the United States is largely con- 

 fined to a layer extending from the surface down to the depth 

 of about 50 miles. At about this level is the "depth of compen- 

 sation." At the depth of compensation the columns of rock or 

 rock plus water, under the surfaces of wide mountain range, 

 low plain, and sea-basin have nearly equal weights. 



The exact mode of horizontal variation of density is not 

 easily determined. While searching for it geodesists have tested 

 different working hypotheses. Besides those bearing the names 

 of Hayford, Airy, and Heiskanen — referred to in Chapter I — 

 Vening Meinesz tried a fourth hypothesis, based on what is 

 called "regional-isostatic compensation." He computed the 

 anomalies derived from all four hypotheses. As a rule he found 

 no great differences in the respective values. Further, it became 

 clear to him that a strongly positive free-air anomaly means 



