32 THE FLOOR OF THE OCEAN 



static anomalies differ according to the underlying hypotheses. 

 These have been proposed by as many geodesists, whose names 

 have been attached to the corresponding anomalies of gravity; 

 hence we have Hayford anomalies, Airy anomalies, Heiskanen 

 anomalies, and still other types. Now, with any of the hypoth- 

 eses it is found that the depth where the earth-shell in an 

 approximately hydrostatic state begins is only a few scores of 

 miles below the surface of the globe. One of the good evi- 

 dences that this is true is the prevailingly small size of the 

 isostatic anomaly when computed according to a good assump- 

 tion as to the depth of compensation. Examples are given in 

 the three maps of Figure 14, where the numbers without un- 

 derlining represent the isostatic anomalies, here too expressed 

 in milligals. Thus the isostatic data from two oceans and a 

 continent tell the same story as the Bouguer anomalies: the 

 thickness of the strong superficial shell of the earth is of the 

 order of 50 miles, though somewhat greater under the deep sea 

 than elsewhere. 



Some Relevant Facts from Geology. — The geologist has his 

 own independent grounds for belief in an extremely weak 

 earth-shell at moderate depth. He has shown that thousands 

 of feet of rock have been eroded off the continents and that a 

 large part of the resulting detritus is now weighing on the sub- 

 oceanic sectors of the earth. To restore the balance so disturbed, 

 material in depth has been flowing from oceanic region to 

 continental region. The stress causing the flow has at no time 

 been great; hence the flowing material must have a minimum 

 of strength or no strength at all. For this there is only one 

 explanation, namely, high temperature. How high must it be ? 



Some students of the problem believe that the necessary 

 temperature may be far below that where the rock would melt. 

 Others assume a temperature close to that of melting — a view 

 implying an incredibly delicate relation of temperature to the 



