100 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



Paumotu islands. Sand beaches are thrown up on the inner lagoon side of 

 the narrow low islands ; these vary greatly in length, and the gaps separat- 

 ing them vary also much in width. The gaps are frequently walled off on 

 the sea face by coral shingle beaches, or only partially so, allowing the sea 

 access to the lagoon through very shallow passages dotted with shingle 

 and rocks and sand bars, forming most irregular canals likely to be closed 

 at any time (PI. 61, fig. 2). Or the gaps may be similarly closed on the 

 lagoon side by the formation of sand bars and beaches across the openings 

 (PL 61, fig. 2), as we have described them at Rangiroa, between Avatoru 

 and Tiputa Pass. 



On such steep faces as the sea faces of the Paumotu atolls, the corals 

 living on the outer slope are limited to a very narrow belt, which expands 

 at the passes and spreads into a luxuriant mass over their bottom. In some 

 atolls, judging from the amount of coral shingle derived from the old ledge, 

 they supply but a comparatively small amount of the material which goes 

 to build up the islands and islets and sand bars of the outer rim. In many 

 of the lagoons the corals form very extensive patches, and together with the 

 disintegration of the shoals and ledge flats of the interior, supply, when 

 dead and ground to pieces on the beaches of the lagoon, a large amount of 

 material for the building up of the land rim from the lagoon side. Of 

 course this proposition is not of universal application, as the luxuriant 

 growth of corals on the sea face depends upon a number of local conditions 

 and combinations of conditions which must be taken into account at each 

 atoll. It is not always on the weather side that corals flourish best, but on 

 the lee face of a reef, as, for instance, at Tahiti, where the corals take their 

 greatest development on the lee shore of the island ; and that, on the whole, 

 is usually the case in most of the districts I have examined, the lee sea face 

 presenting the healthiest development of corals. Yet from the constant 

 destruction of these going on on the sea face of the weather side, they 

 supply a far larger amount of material for the making of the land rim than 

 the corals which are growing undisturbed on the lee face. As far as acces- 

 sions are due to Nullipores and corallines, their action is very different from 

 that of the corals ; inside of the lagoon their tendency is to coat every 

 fragment of loose coral, and cover it with a harder coating, to prevent 



