154 bulletin: museum of comparative zoology. 



vigorous growth of all lime-secreting organisms wherever the condi- 

 tions of life are most favourable, and less vigorous growth and even 

 death of these organisms where the conditions are unfavourable. 

 A detailed study of the favourable and unfavourable conditions for 

 different species in an existing atoll, seemed to Agassiz a great desider- 

 atum at the present time and I am delighted to learn that this is now 

 being undertaken by American Naturalists under the auspices of the 

 Carnegie Institute. 



In small atolls, where the surrounding reef is very extensive rela- 

 tively to the enclosed lagoon, the lagoon tends to become filled up by 

 the accumulation of coral sand, the deposition carbonate of lime, by 

 the living organisms of the atoll being in excess of that removed in 

 solution and by mechanical means; where the atoll is large, and the 

 encircling reef is — relatively to the size of the lagoon — small, then 

 the lime removed from the lagoon by solution and currents is greater 

 than that deposited by living organisms; hence the lagoon becomes 

 deeper and wider. The lagoon of Diego Garcia appeared to have 

 increased considerably in area in this way between 1837 and 1885. 



It is undoubtedly true that many coral-reef regions have been 

 recently elevated. The circular atoll and barrier reef cannot be 

 accepted as evidence of subsidence; the characteristic features of 

 coral reefs would be very similar in a stationary, in a slowly sinking 

 or slowly rising area, although each would show secondary modifica- 

 tions. It matters not whether the change of sea-level be due to 

 crustal movement, to attraction of elevated continental land, or to 

 the accumulation or the melting of polar ice-masses. 



When coral plantations rise from a submerged bank, the corals 

 and other lime-secreting organisms situated towards the seaward 

 edge would from the first have the advantage; they would hence 

 reach the surface, before the central portions, where the corals would 

 be in a position more or less unfavourable for vigourous growth. A 

 shallow lagoon would thus be formed, which might subsequently be 

 cleared by solution, and mechanical action of many of its living coral 

 plantations. 



The coral atoll, on reaching the surface would, he admitted, in very 

 many cases advance seawards on a talus of its own debris, expanding 

 like a fairy ring, and it seemed to him more than probable that the 

 boring at Funafuti atoll was driven down into such a talus, with an 

 underlying Tertiary base. 



The red earth which is found on coral islands and supplies the food 

 for plant life, is chiefly derived from the disintegration and decom- 



