238 



DALY. 



As the island, of normal profile, sinks, the land detritus rapidly 

 diminishes in volume. The heads of the island deltas retreat farther 

 from the "moat," so that its filling cannot be essentially attributed to 

 outwashing from the central island. 



The slowness of the filling-in process is further shown by the very 

 common steepness of the inner reef slope (Fig. 35). Darwin speaks 

 of such reefs as "like a wall." ^° Describing the Funafuti atoll, 

 Gardiner WTites: "The bottom of the lagoon, if the shoals were re- 

 moved, and the whole elevated, would be a great plain surrounded by 

 a ridge sloping steeply up to a line of perpendicular cliffs broken only 

 at the few ship's channels; on this plain the greatest heights would be 

 from 20-30 feet." ^^ In his great monograph on the INIaldives, 

 Gardiner describes the lagoon slopes of the reefs as "practically per- 

 pendicular" and elsewhere states that this fact "is not consistent with 



La g oon 



50 100 200 



400 



Figure 35. Section illustrating the common steepness of reef edges on the 

 two sides. 



the possibility of the lagoon's having been filled in by detritus washed 

 over their encircling reefs." ^^ Like many coral knolls dotting the 

 lagoon floors, the inner edge of the main reef often shows soundings 

 of 20 to 40 m., at very short distances from the reef edge visible at low 

 tide. In such cases the rate of upgrowth for the living reef must have 

 been greater than the rate of upgrowth for the sand terrace alongside. 

 If the aggrading process is so inadequate at the source of supply, how 

 much more inadequate is it to smooth the vast interior of the lagoon 

 in any reasonable time! (See Fig. 36.) 



As shown by the normal deepening toward the lagoon centers and 

 by the sparseness of the coral knolls, pelagic shells, coral-knoll detritus, 

 and bottom growths would be still less effective in filling the "moat" 



70 C. Darwin, Coral Reefs, London, 3rd ed., p. 67 (1889) 



71 J. S. Gardiner, Proc. Cambridge Fhil. Soc, 9, 434 (1898) 



72 J. S. Gardiner, Amer. Jour. Science, 16, 211 (1903). 



