SUBMARINE MOUNTAINS 77 



so clearly indicated by the flatness of the lagoon floors. A 

 proper choice between the two explanations of the "drowning" 

 remains an open problem, which illustrates the difficulty of 

 telling where "submergence" means "subsidence." 



In summary, it looks as if the sub-oceanic crust preserves its 

 elastic strength even under the heavy volcanic loads. If this 

 is true, we should expect that similar support should be given 

 to what may be conveniently called negative loads. Just such 

 loads are represented at the ocean "deeps," mentioned in the 

 first chapter. (See page 9.) The long troughs seem to have 

 been formed where the sub-oceanic crust has been bent down 

 by the powerful force of mountain-building: specifically under 

 the horizontal pressure of overriding, continental blocks. Be- 

 neath each resulting trough the material of the substratum was 

 forced away laterally, and sea water was accumulated in the 

 trough itself. Manifestly the outflow of heavy rock-matter and 

 inflow of the much less dense water involved a net deficiency 

 of matter under the trough. Here the upward pressure of the 

 quasi-liquid substratum has been increased, but its tendency to 

 push up the crust toward the original level is resisted by the 

 strength of the crust. So long as this resistance is unimpaired, 

 the crust under the deep carries a negative load. 



The amount of the load can be approximated by measuring 

 gravity at a sufficient number of stations within the region of 

 deformation. This Professor Vening Meinesz has done at the 

 Nero Deep off Guam Island, a map of which appears in Figure 

 44. The maximum sounding in the deep is nearly 10,000 

 meters or 33,000 feet. The submarine of Vening Meinesz fol- 

 lowed the course marked by the broken line. The correspond- 

 ing cross-section (Figure 45) represents the solid crust by shad- 

 ing, and sealevel by the middle horizontal line that bears the 

 designating numbers of the observing stations. The broken-line 

 and dotted curves picture the variations of four different kinds 



