SUBMARINE MOUNTAINS 6j 



volcanic loads on the Pacific floor are breaking the crust and 

 therefore in net subsidence. 



Soundings show the foundations of many barriers and atoll 

 reefs to have shapes essentially like those of the truncated vol- 

 canoes on which the gravity pendulum has been swung. More- 

 over, in a large number of cases the visible island surrounded 

 by barrier reef is clearly the emerged top of a volcano of dom- 

 inantly basaltic composition. It seems highly probable, there- 

 fore, that the reef-veneered volcanic piles also are excess masses 

 weighing heavily on the crust. Nevertheless, the submarine 

 topography spells stability for these weighted areas during hun- 

 dreds of thousands of years and even millions of years. 



The topographic evidence for that stability is easily stated. 4 

 The lagoons inclosed by barrier or atoll reef are wide, and ex- 

 cept for occasional knolls of upgrowing corals, the bottom of 

 each lagoon is almost perfectly horizontal. A few examples 

 will be cited. 



First, we study the barrier lagoon of the Truk Islands, 

 Caroline group (Figure 36). Note its considerable width, 

 given by the scale, and the small variation in the number of 

 fathoms of lagoon depth. 



Similar characteristics are found in the Nomwin and Murilo 

 atolls, also of the Carolines (Figure 37) ; the Mille or Mulgrave 

 atoll (Figure 38); the magnificent atoll of Suva Diva in the 

 Indian Ocean (Figure 39), whose cross-section helps the map 

 to emphasize the flatness of the lagoon floor. 



The cross-sections of Figure 40 still further illustrate the 

 case. The water is shown in solid black; depths in fathoms; 

 respective horizontal scales in nautical miles, each equal to 

 1000 fathoms. The first two sections represent the Seychelles 

 Bank of the Indian Ocean and the Macclesfield Bank of the 

 China Sea. The next two sections were made across banks in 

 the Indian Ocean, bearing reef patches in their central parts 



