200 THE OCEAN WORLD. 
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 coralislands. 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, Péron, 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 very curious map in connection with the sub- 
ject ; nevertheless, the coincidence does not always exist. We have 
aiready remarked on the theory by which Mr. Darwin seeks to explain 
