]^92 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



upward movements, with retreat of the sea, the formations of wide 

 land masses, and the ridging of strata to form mountain ranges. 

 Thus he forged a link that could unite the continental or epeirogenic 

 movement with orogenic or mountain movement. 



The visible parts of mountains and continents, as well as their lower 

 and hidden portions, or "roots", are made of comparatively light 

 rocks. In order to stand up as they do their roots must be embedded 

 in denser matter, in which they "float" like icebergs in water. A 

 far larger mass must exist below than is visible above, and the bigger 

 the upstanding part the bigger the submerged root. Over the larger 

 area of the ocean floor, on the other hand, the thickness of material 

 of low density must be very slight, and the denser layer must come 

 close to the surface. 



The study of earthquakes, to which the seismology committee of the 

 British Association has made outstanding contributions, has yielded 

 from the times taken in transmission of vibrations through the earth, 

 the best information as to the nature and state of the interior. It 

 has proved that the dense layer is solid at the present time. It is 

 probably no coincidence that the earth is also but just recovering from 

 what is possibly the greatest period of mountain building, if not the 

 greatest negative movement of ocean retreat, that it has ever 

 experienced. 



But solidity cannot be the permanent condition of the substratum. 

 Heat is generated in it by its own radioactivity, but according to the 

 terms of the hypothesis, cannot escape in consequence of the higher 

 temperature generated in the continental rocks which cover it. It is 

 therefore retained in the substratum and stored as latent heat of 

 liquefaction, so that within a period which has been calculated ap- 

 proximately in millions of years, complete melting of the subcrust 

 must ensue. 



The resulting expansion of the liquefied stratum will have at 

 least two eflPects of great importance to us. In the first place the 

 unexpanded superficial layers will be too small to fit the swelling 

 interior. They will, therefore, suffer tension, greater on the ocean 

 floor than on land, and cracking and rifting will occur, with intrusion 

 and extrusion of molten rock. In the second place the continental 

 masses, now truly floating in a substratum which has become fluid 

 and less dense than before, will sink deeper into it, suffering displace- 

 ment along the rift cracks or other planes of dislocation. As a result 

 the ocean waters, unchanged in volume, must encroach on the edges of 

 the continents, and spread farther and farther over their surfaces. 



Thus we have the mechanism which Suess vainly sought, causing 

 positive movements of the oceans, their waters spreading over wide 

 stretches of what was formerly continental land, and laying down 



