200 THE OCEAN WORLD. 



I 



exceed the eighth of an inch annually. According to this calculation, 

 some reefs which are not less than 2,000 feet thick would require 

 for their formation 192,000 years. 



It is necessary, however, to add 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 organic 

 formations. 



Naturalists and navigators have been much divided in opinion as to 

 the true origin of these coral islands. Most of them have admitted that 

 these enormous banks are composed of the calcareous remains and 

 earthy detritus of the madrepores and corals, which, developing them- 

 selves in their midst, or upon the bed of the ocean, multiplying and 

 superposing themselves, age after age, and generation after generation, 

 have finally concluded by forming deposits of this immense extent. 

 The growth of the vast madreporic mass would be finally arrested by 

 the want of water when its summit approached the level of the sea. 

 It is thus that Forster, Pe'ron, Flinders, and Chamisso, have explained 

 the formation of the atolls and fringing 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 

 crowned with corals and madrepores, would naturally reproduce this 

 circular wall-like shape formed of heaped-up corals. 



This theory supposes the existence of volcanic craters in the 

 neighbourhood of all the coral islands. It is quite certain that these 

 islands are often found not far from extinct volcanoes; and Sir Charles 

 Lyell has published a ver}' curious map in connection with the sub- 

 ject; nevertheless, the coincidence does not always exist. We have 

 already remarked on the theory by which Mr. Darwin seeks to explain 



