CORALLINES. 177 



and other organic debris will probably represent a fourth of the total 

 produce in relation to corals. In this manner, taking everything 

 into account, the mean increase of a reef cannot exceed the eighth of 

 an inch annually. According to this calculation, some reefs which are 

 not less than two thousand feet thick would require for their formation 

 a hundred and ninety-two thousand years. 



It is necessary to add, however, that in favourable circumstances the 

 increase of the masses of coral may be much more rapid. Mr. Darwin 

 speaks of a ship which, having been wrecked in the Persian Gulf, was 

 found, after being submerged only twenty months, to be covered with 

 a bed of coral two feet in thickness ; he also mentions experiments 

 made by Mr. Allen on the coast of Madagascar, which tend to prove 

 that in the space of six months certain corals increased nearly three 

 feet. 



We proceed to the theoretic explanation of these curious mineral 

 formations. 



Naturalists and navigators have been much divided in opinion as 

 to the true origin of these madreporic islands. Most of them have 

 admitted that these enormous banks are composed of the mineral 

 spoils and earthy detritus of the madrepores and corals, which, de- 

 veloping themselves in their midst, or upon the bed of the ocean, 

 multiplying and superposing themselves, age after age, and genera- 

 tion after generation, have finally concluded by forming deposits of 

 this immense extent. The growth of the vast madreporic column 

 would be finally arrested by the want of water when its summit 

 approached the level of the sea. It is thus that Forster, Peron, 

 Flinders, and Chamisso have explained the formation of the atolls 

 and madreporic reefs. This opinion has also found a supporter, in 

 our times, in the French admiral, Du Petit Thouars. But he objects, 

 with reason, that the corals cannot live at the prodigious depth of 

 sea at which the base of these islets lie. It has therefore been found 

 necessary to seek for another cause to satisfy the diverse conditions of 

 the phenomena, and explain, at the same time, the strange circular 

 arrangement of these islands, which is almost constant, and which it 

 is essential to keep in view. 



Sir Charles Lyell was of opinion that the base of an atoll was 

 always the crater of an ancient submarine volcano, which, when 



M 



