384 AUSTRALIA : 



craters of old submarine volcanoes ; their crests overgrown 

 by the work of these zoophytes, which retained the form 

 while altering the material of the surface exposed. On the 

 other hand, the great superficial extent of some of the atolls 

 and of their contained lagoons, remove from them all cha- 

 racter of volcanic cones ; and their close and peculiar arrange- 

 ment in groups, like that of the Maldives, still further 

 contradict the resemblance. The difficulty of explaining why 

 such numerous detached summits should rise so closely to 

 the same level near the surface, formed another obvious ob- 

 jection to the theory; and a more cogent one arose when it 

 was discovered that the reef-building corals worked only to 

 a limited depth, and could not, below this level, construct 

 the coralline covering to the submarine peaks which the 

 theory supposed. 



Under the failure of these hypotheses, and pressed by 

 other considerations, Mr. Darwin adopted what we have 

 adverted to as the third solution of the question ; viz. that 

 the areas, greater or smaller, on which the coral reefs rest as 

 a foundation the flooring, in fact, of the seas must have 

 undergone progressive subsidence, so as utterly to withdraw 

 islands or large tracts of land below the surface of the ocean ; 

 leaving the coral reefs which encircled or fringed their shores 

 still on the water's level, and therefore under circumstances 

 fitted to sustain their growth and position, even were the 

 sinking of the foundation beneath them further continued. 

 We must admit this hypothesis to be a bold and startling one, 

 and such it appeared in the outset even to the most hardy of 

 our geologists. Yet it has rapidly gained ground, not merely 

 as the only one fulfilling the conditions required, but fur- 

 ther because it well illustrates the different modifications 

 and peculiarities of the coral reefs, and accords at the same 

 time with certain remarkable discoveries which have dis- 

 tinguished the progress of modern geology. Mr. Darwin's 



