FOUNDATIONS OF THE GREAT DEEP 33 



earth's internal supply of heat. Neither assumption assures 

 sufficient weakness for the deep layer. According to a third 

 hypothesis, the rock at depth is too hot to crystallize and is 

 therefore in the glassy or vitreous state. In the writer's opinion, 

 founded on a multitude of facts, this hypothesis is the best of 

 the three, even though it is as yet not clearly supported by 

 seismological evidence. If the reasoning is correct, the earth 

 has a true, solid crust resting on a vitreous substratum, which, 

 however stiffened by the high pressure upon it, is not actually 

 solid and endowed with notable strength. Most of the grounds 

 for that opinion are technical and not to be described here, but 

 one of them, combining facts won from geological and geodetic 

 studies, will be outlined. Beforehand, a related consideration 

 may be mentioned. 



According to geophysical theory the earth's crust must be 

 thicker under the ocean than under a continent. All rock 

 species yet tested are somewhat radioactive, the radioactivity 

 being accompanied by the evolution of heat. Because granite 

 and other rocks characteristic of the continents are richer in 

 radioactive elements than the denser rocks of the sea floor, the 

 radioactive furnace is specially efficient under the continental 

 surfaces. We therefore expect the crust, as just defined, to have 

 maximum thickness under the ocean. Such excess of thickness 

 is indicated in the section of Figure 30. On the left is the ocean 

 with its gabbroic or simatic crust; on the right, the continent 

 with its thinner crust; partly sialic and partly simatic. One 

 reason for picturing the substratum in two sub-layers will be 

 given in the next chapter. 



Now, in spite of the somewhat greater thickness of the sub- 

 oceanic crust, any special evidence for a weak, deep layer under 

 the continent can also be regarded as good evidence for a sim- 

 ilar layer under the ocean. There is such special evidence, 

 recently shown to be more than ever trustworthy. To under- 



