FOUNDATIONS OF THE GREAT DEEP 45 



only a little hotter than an oceanic sector. Failing accurate 

 knowledge of the distribution of radioactive elements, and also 

 in view of uncertainty about other sources of underground 

 heat, including that inherited from the earth's infancy, it is not 

 possible to calculate exactly the depth where the sub-oceanic 

 rock begins to show extreme weakness. However, with reason- 

 able assumptions it appears that the strong superficial layer is 

 probably between ten and twenty miles thicker under the 

 ocean than under the continental surface. 



Conclusions 



The next chapter will be concerned with the deep-sea 

 islands, from which we shall also seek information about the 

 earth-shells. Before thus adding to the store of relevant facts, 

 it is expedient to list the principal results so far obtained. 



i. The instrumental records of earthquake waves suggest 

 that under the central Pacific, the Arctic Ocean, and much of 

 the Indian and Atlantic oceans the solid rock, down to the 

 depth of 50 miles or so, has the elastic properties of crystallized 

 basalt. However, the velocities of the waves traversing the 

 middle of the Atlantic and the southwest Pacific indicate the 

 presence of broad veneering patches of rocks like those out- 

 cropping on the continents. 



2. Those suggestions gain support from studies of the figure 

 of the earth and variation of gravity along the surfaces of lands 

 and seas. 



3. All three geophysical methods of diagnosis, like the 

 geological evidence from glaciated regions, make it necessary 

 to postulate a thick layer of exceedingly weak rock, that begins 

 at a depth of about 50 miles. 



4. The weakness of this deep layer implies high temper- 

 ature. Its actual temperature is unknown, but we have con- 



