FOUNDATIONS OF THE GREAT DEEP 31 



the middle map, specially observing those over the high 

 western Cordillera. 



Evidently, then, the greatest of the earth's reliefs, the pro- 

 jection of continent above sea floor, is compensated for by a 

 difference of density, and therefore weight, for the rock under- 

 lying the two regional types. In effect we can say: the conti- 

 nents are floating high on the earth's body. 



Further, the geodesists have been able to show that the 

 difference of density between sub-continental and sub-oceanic 

 rock is almost entirely confined to the fifty-mile layer at the 

 surface of the earth. At its bottom there begins a thick, world- 

 circling layer or earth-shell, which in the horizontal direction 

 is nearly constant in density and composition. This shell is in 

 an almost perfect hydrostatic state, so that the weight of the 

 50.5 miles of continental crust is essentially counterbalanced by 

 the weight of 47.5 miles of sub-oceanic crust plus the weight of 

 2.5 miles of overlying sea water. The level at which rocky 

 matter is assumed to begin is called the depth of compensation. 



Proof of the mutual balancing, with the direct implication 

 that the major reliefs of our planet are compensated for by 

 systematic variation of density in the horizontal direction, is 

 too technical and lengthy for present description. The prin- 

 ciple involved has been given the name "isostasy," meaning 

 "equipoise" (of vertical column against vertical column) in 

 free translation from the Greek roots. Many different hypoth- 

 eses have been proposed to cover laws for the variation of 

 density. For each "isostatic" hypothesis, necessary corrections 

 are made to the measured value of gravity at each land and sea 

 station, to find the value at sealevel. The result may be called 

 the reduced "observed" value. It is then compared with the 

 intensity of gravity expected at the same point of latitude and 

 longitude on the standard spheroid. The difference of intensity 

 is called the "isostatic anomaly." For a given station the iso- 



