496 Mr Gardiner, The Coral Reefs of [Mar. 7, 



accepting the theory of subsidence for the moment, the wonder 

 would be that the atoll and barrier reefs have not greater depths 

 than even that of 70 fathoms found on the Saya de Malha bank, 

 north-east of Madagascar. 



I have already, in treating of the Lau Group, referred to the 

 great destruction of the limestone islands, which is continually 

 taking place by means of the wearing and dissolving actions of 

 the sea water. The fact of such a solution occurring here cannot 

 be doubted ; even in the most protected parts of the salt lake of 

 Fulanga, which is most completely inclosed, the rocks everywhere 

 show the undermining action of the sea. The narrow edge of reef 

 under the overhanging limestone does not extend beyond the edge 

 of the limestone, and is evidently also being dissolved. There are 

 many fallen masses of rock, and islets have slipped over completely, 

 while there are no growing shoals, so that it seems as if they were 

 being dissolved away, until they became level with the general flat 

 of the lagoon. Corals and nullipores do not grow in the lagoon, 

 and there is nothing to make up for the continual waste from 

 solution. Murray and Irvine 1 have shown that one part of calcspar 

 is soluble at 10° C. in something over 136,000 parts of sea water ; 

 three corals, taken at the same temperature, belonging to the 

 genera Goniastraea, Melobesia and Oculina were soluble one part 

 in from 12,000 to 43,000 parts of sea water. The soft limestone of 

 an atoll is hence probably about three times as soluble as the hard 

 limestone of the Lau Group, so that the formation by solution in the 

 middle of Fulanga of a lake 10 fathoms deep with only one narrow 

 tidal passage, furnishes a strong argument that solution must be 

 of immense importance in the formation of atoll and barrier reefs. 



The experiments of Reid, quoted by Murray and Irvine 2 , show 

 that, under pressure, carbonate of lime is more soluble in sea water 

 containing carbonic acid gas. He took about 1 gram of the solid 

 nearly pure carbonate of lime, forming the skeletons of different 

 organisms, and tested how much of it would be dissolved by 1 litre 

 of sea water, containing carbonic acid gas. His results are of great 



importance, and his table may be briefly summarised : 



Average 



1 On Coral Eeefs and other Carbonate of Lime Foundations in Modern Seas. 

 Proc. R. Soc, Edin., 1889. 



2 Loc. cit., Table xvi. 



