180 
THE OCEAN WORLD. 
neighbourhood of sill 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 
already remarked on the theory by which Mr. Darwin seeks to 
explain the complicated conditions of the phenomena. The expla- 
nation proposed accounts for the known facts, as well as the present 
appearance of the madrcporic islands. The circular atolls and madre- 
poric banks which are disposed as a sort of girdle, are principally 
formed of porite s, miUepora , and asirea, zoophytes which cannot exist 
at any great depth in the ocean, but which swarm on the rocks at 
some few fathoms only below the limits of the tide. These animals, 
by means of their accumulated debris, soon form a sort of coating 
round the island, which constitutes the littoral reefs : this marginal 
tongue or shoulder, according to Mr. Darwin, is the first stage in the 
existence of a madreporic island. At this point the author intro- 
duces a geological cause, namely, a great subsiding movement of the 
soil, in which the madrcporic colony is sunk under the water. It is 
evident that after submersion the zoophyte will only continue to 
develope itself on the upper surface, and within the limits which its 
nature prescribes. The madrepores exhibiting their greatest vitality 
at the points most exposed to the fury of the waves, it will be near 
the outer edge of the reef that the development will be most rapid. 
If the subsidence of the island thus surrounded should still continue, 
as mountain after mountain and island after island slowly sink beneath 
the water, fresh bases would be successively afforded for the growth of 
the corals, and the outer edge elevated by their continual labour, thus 
transforming the space into a sort of circular lagune. The madre- 
poric deposits would thus form an isolated girdle, and the lagune, 
which occupies the centre, would become deeper and deeper in pro- 
portion to the lowering of the soil. This is the second stage of the 
madreporic isle. 
The existence of the atolls are thus subordinated to two principal 
conditions: the progressive subsidence of the shore washed by the 
sea, and the existence of coral formed of stony polypiers, the growth 
and multiplication of which are extremely rapid. 
It follows from this that madreporic isles cannot exist in all seas ; 
that they only have their birth in the Torrid zone, or at least near 
