7 8 



THE FLOOR OF THE OCEAN 



FIGURE 44. MAP OF THE NERO DEEP. DEPTHS IN METERS. 



of gravity anomalies. The scale for the anomalies, in milligals, 

 is on the left; the scale for depth of water, in meters, on the 

 right. The strong downward deflection of all the curves is 

 proof of abnormally low intensity of gravity over the deep, 

 and this can mean only local deficiency of attracting matter. 

 The study of other deeps has given essentially similar results. 



The new data suggest that the negative loads rival the posi- 

 tive loads in stressing the sub-Pacific crust. From both sets of 

 observations geologists have been led to assume for the crust 

 an average strength about twice that of granite. 



Here we meet our third principal question: can a 50-mile 

 crust, although red hot at its base, have such a high degree of 

 strength? Manifestly the strength of rock decreases with rise 

 of temperature, and mines and bore-holes amply prove rise of 

 temperature with increase of depth. On the other hand, well 

 controlled experiments, including the best of all, those of 

 Mr. David Griggs at Harvard University, show cool rock to 

 grow much stronger when subjected to an all-sided pressure of 

 a few thousands of atmospheres. Because of the time element 



