208 J. S. Gardiner — Origin of Coral Reefs, 



be enhanced, and more water will consequently have to pass 

 over it. The Maldive plateau is an extreme case both from its 

 size, particularly its length, and from the fact of its being sit- 

 uated practically at right angles to the main currents of the 

 Indian Ocean; viz., those of the monsoons. Even at 75 to 150 

 fathoms there would have been, I believe, a considerably lesser 

 supply of food to its edges than to its center so that on the 

 former more growth would have been seen, and gutters, having 

 been formed as shown above, the foundations would have been 

 laid for the present banks. Still more definitely would these 

 conditions have become established as the banks grew up, and 

 in the second phase of growth the separation of the banks would 

 have been still more marked. 



It follows from the above considerations that, as the banks 

 approached the surface, there would naturally have tended to 

 be a rim formed to each. Each again would have tended to 

 split up into separate reefs in the same manner as the whole 

 plateau had previously itself been severed into separate banks. 

 The depth at which this cleavage took place is of some import- 

 ance as bearing on the depths of the lagoons of the various 

 banks as well as on the depths of the passages into these lagoons. 

 It is not until the rim becomes moderately perfect that the 

 atoll lagoon begins to be hollowed out by solution, and its depth 

 in the first place would to a large extent depend on that of the 

 original shoal before the rim became delinitely determined. 

 JN^ow, as the depth at which the growth of the rim commenced 

 would necessarily vary by decreasing with the reduction in size 

 of the original bank, the lagoons of larger banks would from 

 the start have had a greater depth than those of smaller. 

 Tiladumati-Miladumadulu and Malilos give the depth of 25-30 

 fathoms for large, linearly extended banks, but in Suvadiva, 

 Kolumadulu and Haddumati, large, rounded and somewhat 

 isolated banks, the depth would naturally be expected to be 

 greater. Our present knowledge allows only a guess at the 

 original depth and the amount which it has increased owing 

 to various causes. In any case the Maldives clearly give a series 

 from an immense atoll 50 fathoms deep on the one hand to 

 open banks studded with isolated reefs ; and on the other through 

 atolls of various sizes to separate reefs of a mile or two across, 

 not as yet of atoll form and rising directly from the common 

 plateau. Lastly, even on an open bank it is obvious from the 

 foregoing considerations that some parts of the rim might again 

 be still further broken up and form atolls of a secondary order, 

 a condition which seems to be clearly shown in Mahlos and 

 the other northern banks. 



Having shown how the banks may have, and, considering our 

 evidence, probably have grown up, I pass to the consideration 



