GLACIAL-CONTROL THEORY OF CORAL REEFS. 239 



and smoothing the indefinitely varied floor of a lagoon surrounding a 

 sinking island. 



One must conclude that the filling of the "moats," so that nowhere 

 they shall be covered with water more than about 90 m. deep, means 

 an exceedingly slow rate of subsidence; one may doubt that the amount 

 of aggradation, matching the known flatness of the lagoon floors, is a 

 physical possibility even if that process occupied all Miocene and later 

 time. But, on account of the narrowness of reefs, Darwin himself 

 inferred a geologically rapid rate of sinking for them.^^ The subsi- 

 dence theory therefore faces a serious dilemma, and none of its up- 

 holders has yet offered a reasonable explanation of the "extremely 

 slow" (Darwin) filling-up process accompanying a rapid subsidence. 

 Apparently the only conceivable way out of the dilemma is to assume 



50 100 



_6p0 |v) 



Figure 36. Section through a typical coral knoll in the "drowned atoll" 

 Tizard bank, China Sea (after W. U. Moore and P. W. Bassett-Smith), show- 

 ing steepness of the knoll slopes and the distribution of corals. L, live coral; 

 D, dead coral; S, sand. 



that the filling of the "moat" occurred during a long pause in the 

 sinking, while the narrow reef rim is due to a recent renewal of sub- 

 sidence. Since the atolls and barrier reefs of all the world have 

 virtually the same features, it would follow that, throughout the 

 greater part of the tropical Pacific, the Indian ocean, and the East 

 Indian archipelago, there was a simultaneous, long-continued pause in 

 subsidence; and that the pause was followed by a recent, rapid sinking 

 to about the same amount, everywhere in the same world belt. The 

 manifest improbability of the assumption shows the necessity of 

 adding some other one, as yet unimagined, if the older theory is to 

 account for the maximum depth of lagoons.^* 



73 C. Darwin, Coral Reefs, London, 3rd ed., p. 43 (1889). 



74 Darwin did not believe that crustal movement could be uniform even 

 over the comparatively small area represented by the West Indian sea (C. 

 Darwin, Coral Reefs, London, 3rd ed., p. 269 (1889)). How much more 

 improbable is the view that the vastly larger Indian and Pacific areas occupied 

 by atolls and barrier reefs, have been uniformly depressed in recent time, i. e., 

 after the "long pause " ! 



