in the Pacific^ ^c. 97 



not by any means, as I conceive, disprove it, but rather afford 

 us the means of determining with some degree of exactitude 

 the amount of re-elevation that has taken place at such islands. 



I was informed by Mr. Samuel Wilson, of Tahiti, who had 

 long resided among the Hervey Islands, that at Mangaia, 

 which is an ancient reef elevated nearly three hundred feet, 

 there are in the central hollow formerly the bed of the lagoon, 

 many scattered patches of coral rock, some of them raised to 

 a height of forty feet. At a certain stage of the elevatory 

 process, it is evident that these patches must have presented 

 an appearance very similar to the lagoon islets to which I have 

 referred. 



So far from the beach as examined, the bottom in some 

 lagoons I have found to be a clean coral sand, composed of 

 a detritus of coral and shells, but in most instances this 

 was covered to the depth of a foot by an exceedingly fine 

 white sedimentary paste or ooze, which on desiccation had 

 every character of common chalk, except in being much 

 more friable. I think it is principally formed by the decom- 

 position of flexible corallines and alcyonia, and the softer 

 Echinodermata, such as Fistularia and its congeners, all of 

 which abound in the lagoons, rather than from that of corals 

 proper. It may also partly arise from the excretions of certain 

 fishes, which feed occasionally on the tender extremities of 

 Madrepores, as readily as on Crustacea, Echini, 6oc. I once 

 collected a quantity of these corallines and kept them in a 

 jar of water till the muscular and fibrous portions were entire- 

 ly decomposed, when an impalpable sediment was deposited, 

 in all particulai's answering to that obtained from the lagoons. 



Not le£LSt among the phenomena attracting our attention in 

 these Paumotus, are the channels which in the majority of 

 them afford a passage from the sea into the lagoon. There is 

 usually but one of any consequence at each island, though 

 there are rare instances of the occurrence of three or even 

 four at different points of the reef. They are almost invaria- 

 bly situated in the leeward reef, but there are occasional ex- 

 ceptions and deviations from this general rule, some of which 

 will be specified presently. 



13 



