394 



NA TURE 



{Feb. 



Oi 



1888 



graphic Department being not generally available, no 

 opportunity has been afiforded for their examination, but 

 on these their form is highly suggestive. It should also 

 be remembered that few submerged banks have yet been 

 surveyed on a sufficiently large scale and in sufficient 

 detail to show their characteristics. 



I may, however, add that it is well known to seamen 

 experienced in navigation in coral waters that shallower 

 soundings are frequently found on the edges of banks. 



How precisely it comes about that coral is growing on 



the yet deep rims of these large banks, and that little or 

 none is flourishing in the interior, evidence is yet wanting 

 to show. These, however, are the facts, and the result, 

 so far as the necessity for future scooping out is concerned, 

 seems indisputable. I may nevertheless offer a suggestion. 

 This condition of reef is apparently only to be accounted 

 for in two ways. Either by subsidence ; or by assuming 

 that the animals, be they corals or other lime-secreting 

 organisms, that settle on the bank, do, when it gets, by 

 their accumulation, within a certain distance of the 



surface, and under certain conditions of currents and food 

 supply, intercept so much of the food borne in by the 

 currents, that similar life, suitable to that depth zone, 

 cannot be supported in the central area. Thenceforward, 

 the rim alone will grow, and the organisms fitted to live 

 in the successively shallower zones to the surface will only 

 find on it foothold. 



This would be the perfect atoll, but, with less nicely 

 balanced conditions, growth would also take place in 

 patches in the central area, as is so often the case. 



It is to be observed that the depths on the more or less 

 level tops of these large banks, as also in many perfect 

 atolls, is frequently between 30 and 50 fathoms. 



The smaller the bank, the less would be its general 

 depth when the rim is formed, owing to the different pro- 

 portions in which an area and its periphery, over which 

 the non-consumed food must pass, relatively increase, 

 and assuming that the animals that live nearer the 

 surface consume more than those at greater depths. 



I am fully aware that this view of the growth of an 



