240 DALY. 



Flatness of Lagoon Floors; Comparison of Depths in Lagoons and on 

 Banks. The comparative flatness of lagoon floors has been empha- 

 sized as a general fact of the first importance. It is a feature expected 

 on the Glacial-control theory and quite unexpected on the older theory, 

 unless the auxiliary hypothesis of a very long pause in subsidence be 

 accepted. With the last suggestion goes the correlative hypothesis 

 that the reef rims now visible at the water surface, above the great 

 platforms, are very modern affairs, relatively high and narrow be- 

 cause formed by a recent renewal of sinking. Before its last assumed 

 sinking, the plateau, now crowned with an atoll, was flatter than at 

 present because then it lacked the existing main reef as well as the 

 coral knolls, also recently upgrown, in the lagoon. Therefore, at the 

 close of the long pause in subsidence, the plateau was nearly or quite 

 as even-topped as if it had been truncated by wave erosion. 



In this connection a passage in Darwin's book is significant: "Di- 

 rectly north of the Laccadives, and almost forming part of the same 

 group, there is a long, narrow, slightly-curved bank, rising out of the 

 depths of the ocean, composed of sand, shells and decayed coral, with 

 from 23 to 30 fathoms (42 to 55 m.) on it. I have no doubt that it has 

 had the same origin with the other atoll-like banks; but as it does not 

 deepen towards the centre, I have not coloured it. " ^^ This is one of 

 the reefless banks (Fig. 37) referred to in an earlier section (page 198). 

 For some reason corals have been unable to raise an atoll crown on 

 this bank and its form could not have been changed essentially since 

 the end of the assumed long pause in subsidence. If all the banks of 

 the coral seas had a similar history, their topography would not sug- 

 gest recent submergence to an amount greater than 55 to 90 m., or 

 possibly 100 m. The principal ground on which the theory of deep 

 subsidence has been founded would be entirely cut away. Yet Dar- 

 win held that this Indian ocean bank did have the same origin as the 

 banks crowned with atoll reefs. 



The subsidence theory was invented chiefly to explain the ground- 

 plans, maps, of the surface reefs; that is, one topographic element was 

 emphasized, and the evidence of submergence is certainly good. But 

 the same principle of questioning the existing topography — por- 

 trayed in charts, full of soundings — suggests as clearly that submerg- 

 ence has been strictly limited. 



A section across the southern part of Suvadiva atoll (Fig. 16) 

 illustrates the common, pronounced break of slope between the reef 



75 C. Darwin, Coral Reefs, London, 3rd ed., p. 247 (1889). 



