88 THE GREAT BARRIER REEF. 



western. [On the whole, however, he considers that most of the coral formations of the Indian Ocean mark are.ns of 

 elevation rather than of rest or subsidence.] 



"In regard to the explanations of the formation of lagoons l)y solution of the interior parts of the reef, and by the 

 more rapid growth of the corals on its periphery, as being more directly in the track of food-bearing currents, Mr. 

 Bourne observes : — ' Neither of these explanations has completely satisfied me. That sea-water exercises a solvent 

 action upon carbonate of lime does not admit of doubt, and that the scour of tides, combined with this solvent action 

 of the water, does affect the extent and depth of a lagoon, is obvious. But I challenge the statement that the 

 destructive agencies within an atoll or a submerged bank, are in excess of the constructive. It would be nearer the 

 mark to say that they nearly balanced one another. In the first place the carbonate of lime held in solution by 

 sea-water is deposited as crystalline limestone in the interstices of dead corals or coral debris. Anyone who is 

 acquainted with the structure of coralline rock knows how such a porous mass as a Majandrina 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 crystallised 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 two 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 Madreporaria flourish in considerable numbers, 

 but the true reef-building species, Porites, Ma^andrina, Pocillopora, 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 stric/a, AJussa coryiiibosa, Favia !oba/a, Faii^iii dciitata, and many lithers are not found 

 on the outside. The reason is that the last-named are either free forms, such as i''ungia, or are attached by such 

 slender and fragile stems to their supports that they could not possibly obtain a foothold and maintain themselves 

 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 laguon, 

 and it cannot be doubted that their tendency is to fill it up. Again, in reefs which do not rise above the surface, 

 or are awash for the greater part of their extent at low tides, great quantities of debris, torn from the outer slopes, 

 are constantly carried over the rim of the reef, and tend to fill it up. Hence it follows that in a lagoon 

 entirely surrounded by dry land or nearly so, as is the case at Diego Garcia, the tendency to accumulation of 

 material within the lagoon would be less than in submerged or incomplete atolls, for debris cannot be swept over 

 the lagoon, and the only constructive agency is the growth of coral. If the power of solution of sea-water is so 

 great, it must be supposed that in complete or nearly complete atolls the lagoon would be deepening rather than 

 shallowing, yet at Diego Garcia it is obviously shallowing in many places, and has nowhere increased in depth since 

 Captain Moresby's survey in 1837. Indeed, the southern part seems to have shoaled a fathom since that time, and 

 this is the more remarkable since the south-east trade winds are by far tlie most constant and strongest winds there, 

 and tend to accumulate material at the northern rather than the southern end. The fact is that these winds sweep the 



