PREFA TOR Y NOTE. xxi 



as a mceandrina head becomes perfectly solid by the deposition 

 of lime within its mass. This deposition can only be effected 

 by the infiltration of sea-water. In reckoning the solvent 

 action of sea-water, therefore, account must be taken of the 

 fact that a not inconsiderable proportion of the carbonate of 

 lime held in solution is re-deposited in the form of crystalline 

 limestone. Of this, it seems, Mr. Murray has not taken 

 sufficient account, and has, therefore, overstated the destructive 

 agency of the sea. Secondly, the growth of corals, and the 

 consequent formation of coral-rock within the lagoon, is 

 generally overlooked. 



'Whilst diving for corals at Diego Garcia, I had abundant 

 opportunities of studying the formation of coral-rock within 

 the lagoon, in depths under 2 fathoms. The layers of tolerably 

 compact rock thus formed are of no mean extent or thickness ; 

 they soon become covered with sand, and are thus protected 

 from the solvent action of the water. I have found it impossible 

 to reconcile Mr. Murray's views with what I saw of coral growth 

 within a lagoon. Not only do the more delicate branching 

 species of the madreporatia flourish in considerable numbers, 

 but true reef-building species, porites, mceandrina, pocillopora y 

 and various stout species of madrepora are found there. It 

 is a mistake to suppose that certain species of corals are 

 restricted to the external shores, others to the lagoon. My 

 collections proved that many of the species growing in the 

 lagoon at distances of five miles and upwards from its outlet 

 are identical with those growing on the outer reef. In addition 

 to them are numerous species, such as Seriatopora strida, 

 Mussa corymbosa, Favia lobata, Fungia dentata, and many 

 others that are not found on the outside. The reason is that 

 the last-named are either free forms such as fungia, or are 

 attached by such slender and fragile stems to their supports that 

 they could not possibly obtain a foothold and maintain them- 

 selves among the powerful currents and waves of the open ocean. 



' These various species, numbers of which grow close together, 

 form knolls and patches within the lagoon, and it cannot be 

 doubted that their tendency is to fill it up. Again, in reefs 



