CORALLINES. 
179 
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 polypier may he 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 polypiers 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 polypiers 
increased nearly three feet. 
We proceed to the theoretic explanation of these curious mineral 
formations. 
Naturalists and navigators have been much divided m opinion as 
to the true origin of these madrcporic islands. Most of them have 
admitted that these enormous banks are composed of the mineral 
s Poils 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 madrcporic column 
Would lie finally arrested by the want of water when its summit 
approached the level of the soa. It is thus that Forster, Poron, 
D finders, and Chamisso have explained the formation of the atolls 
an d madreporio 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 polypiers cannot live at the prodigious depth of 
sea at which the base of these islets he. It has therefore been found 
Accessary to seek for another cause to satisfy the diverse conditions of 
fhe phenomena, and explain, at the same time, the strange circular 
arrangement of these islands, which is almost constant, and which it 
18 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 
downed with corals and madrepores, would naturally reproduce this 
mrcular wall formed of lieaped-up polypiers. 
This theory supposes the existence of volcanic crateis m the 
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