76 Couthouy on Coral Formations 



mense barrier reefs of New Caledonia and Australia, the latter 

 of which, at a distance of thirty or forty miles from the coast, 

 extends in an unbroken chain nearly one thousand miles from 

 north to south. It affords at best, but a very questionable 

 explanation, of a single variety of structure in these wonder- 

 ful edifices, than which nothing more forcibly illustrates the 

 immense results that may ensue from the operation of appa- 

 rently trifling causes, when continued unremittingly through- 

 out a long series of ages. 



It is my belief that, to a certain extent, the corals are limited 

 in their range of growth by temperature rather than depth, 

 and that wherever this is not below 76^ Fahr. there, 

 ccBteris paribus, they will be found to flourish as in the 

 Polynesian seas ; accordingly we find that their principal for- 

 mations are placed within the tropics, and though I have no 

 means of ascertaining at this moment the fact, I apprehend 

 that in the Indian Ocean, as in the Pacific, the saxigenous 

 polypes will be found most abundant and at their greatest 

 depths, in a belt comprising about twenty degrees on each 

 side of the equator. 



But even allowing that they invariably commenced their 

 structures at the extreme depth of twenty fathoms, it is obvi- 

 ous that no reef would attain a thickness of much more than 

 a hundred feet, before the labors of the polypes must cease 

 and themselves perish, in consequence of their exposure to the 

 sun's rays. The question then naturally arises, how are we 

 to account for the existence of coral banks, so greatly exceed- 

 ing this thickness as some are known to do ; if it is thus dis- 

 proved that their polypes build at corresponding depths? 



Mr. Charles Darwin, who accompanied King and Fitzroy, 

 as Naturalist, in their late survey of the southern extremity of 

 our continent, was led by his examination of a lagoon island, 

 (the only one I believe on which he landed,) and a compari- 

 son of the observations of his predecessors on this subject, to 

 frame an hypothesis, which appears to offer us a solution of 

 this problem, at once satisfactory, simple and rational. 



According to the statements lately given by Prof. Lyell, in 

 his lectures before the Lowell Institute, Mr. D supposes the 



