178 THE OCEAN WOKLD. 



crowned with corals and madrepores, would naturally reproduce this 

 circular wall 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 

 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 madreporic islands. The circular atolls and madre- 

 poric banks which are disposed as a sort of girdle, are principally 

 formed of porites, mittepora, and astrea, 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 madreporic colony is sunk under the water. It is 

 evident that after submersion the zoophyte will only continue to 

 develop 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 madreporic 

 deposits would thus form an isolated girdle, and the lagune, which occu- 

 pies the centre, would become deeper and deeper in proportion 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 cells, the growth and 

 multiplication of which are extremely rapid. 



