62 CORAL-REEFS. 



reefs of the barrier and atoll classes may be seen in the 

 small, but accurately reduced charts on the Plates, 1 and this 

 resemblance can be further shown to extend to every part 

 of the structure. Beginning with the outside of the reef; 

 many scattered soundings off Gambier, Oualan, and some 

 other encircled islands, show that close to the breakers 

 there exists a narrow shelving margin, beyond which the 

 ocean becomes suddenly unfathomable ; but off the west 

 coast of New Caledonia, Capt. Kent 2 found no bottom 

 with 150 fathoms, at two ships' length from the reef; so 

 that the slope here must be nearly as precipitous as off the 

 Maldiva atolls. 



I can give litde information regarding the kinds of corals 

 which live on the outer margin. When I visited the reef 

 at Tahiti, although it was low water, the surf was too violent 

 for me to see the living masses ; but, according to what I 

 heard from some intelligent native chiefs, they resemble 

 in their rounded and branchless forms, those on the margin 

 of Keeling atoll. The extreme verge of the reef, which 

 was visible between the breaking waves at low water, con- 

 sisted of a rounded, convex, artificial-like breakwater, entirely 

 coated with Nulliporse, and absolutely similar to that which 

 I have described at Keeling atoll. From what I heard 

 when at Tahiti, and from the writings of the Revs. W. Ellis 

 and J. Williams, I conclude that this peculiar structure 

 is common to most of the encircled islands of the 

 Society Archipelago. The reef within this mound or 

 breakwater, has an extremely irregular surface, even 

 more so than between the islets on the reef of Keeling 



1 The authorities from which these charts have been reduced, 

 together with some remarks on them, are given in a separately appended 

 page, descriptive of the Plates. 



2 Dalrymple, Hydrog. Ale/n., vol. iii. 



