CRUISE OF THE PORPOISE. 289 



ruptured, some wholly above the water, others awash, and some 

 again altogether submerged, having various depths of water over 

 them. 



As the coral islands have sand and limestone for their base, it 

 would appear possible to account for the formation of the lagoons by 

 supposing that, after the several portions of the pre-existing continent 

 were separated from each other, the outer edge or line of coral, une- 

 qually worn by the sea, had become more compact in some cases than 

 in others. Thus, while the border of the island resisted in one place, 

 it might be torn asunder in others, and through the washing influx 

 and efflux of the sea, strata underlying the centre might be carried 

 off into the deep sea in the shape of sand and mud, or in solution. 

 The centre, thus undermined, from want of support would cave in, 

 and form the inverted cone or tunnel-shaped lagoon, generally found 

 in the centre of these islands, surrounded by an outer rim, variable in 

 width and elevation. 



Actual observation proves that the reefs and islands are under- 

 going dissolution, for at many points where former navigators have 

 laid down shoals of coral, none now exist. One reef, in particular, 

 noticed by those who visited Tahiti ten years before we did, was 

 found by Captain Belcher, of H. B. M. ship Sulphur, to exist no 

 longer. This oflicer states that he visited and surveyed the place 

 where it is laid down on previous charts, and that it was not to be 

 found. In speaking of Bow Island, he likewise mentions the fact 

 that several of its points had undergone material change, or were no 

 longer the same,* when visited after a lapse of fourteen years. These 

 remarks refer particularly to islets situated within the lagoon. I could 

 myself quote many instances of the same description ; but this would 

 occupy too much space. 



I shall, therefore, take leave of the coral islands, trusting that these 

 few remai'ks may excite a spirit of investigation in others. 



Among other duties assigned Lieutenant Johnson were tidal obser- 

 vations, which were continued uninterruptedly, from the 19th of 

 December, 1840, till his departure from the island ; but unfortunately, 

 the tide-staff was placed in the lagoon, a place not free from objections, 

 because the tide there has but a small rise and fall, and is much 

 influenced by the wind, that blows the water over the reef, giving less 



* See Captain Belcher's remarks on Bow Island — Voj-age around the World in 1836 

 and 1842. 



VOL. IV. 73 



A. 



