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APPENDIX II. 
form of crystalline limestone. Of this, it seems, Mr. 
Murray has not taken sufficient account, and has, there- 
fore, 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 madreporaria 
flourish in considerable numbers, but true reef-building 
species, pontes, mceandrina, 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 addi- 
tion to them are numerous species, such as Seriatopora 
stricta, Mussa corymbosa, Favia lobata, Fungia dcntata, 
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 foot- 
hold 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 lagoon, 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, 
