THE TROPICAL PACIFIC 375 



rock act like buttresses and collect coral shingle, and 

 thus are built up little by little, at first small sand bars, 

 then larger bars or islets, which gradually form islands. 

 In the larger atolls, with lagoons many miles across, the 

 trade wind creates a very considerable sea inside the la- 

 goon, and the process goes on in the lagoon as well as 

 on the sea side of the reef flat. Gradually the islands 

 become covered with vegetation, and when the refuse 

 material is very abundant, the land rim becomes more 

 or less solid, forming a series of islands with passages 

 between them. The formation of these land rims can be 

 traced from an atoll like Fakarava, where the islands, 

 especially on the lee side, are widely scattered along the 

 rim of the reef ; or at Rangiroa, where the islands are 

 rather less scattered, until as the process of filling in of 

 the islands continues, there results such an atoll as Pinaki, 

 with a continuous land rim, and one shallow passage into 

 the lagoon. One step further, and we may imagine this 

 atoll entirely enclosed like Niau (which, however, was 

 never planed down to sea level), when it would be merely 

 a question of time for the sand to blow in and fill the 

 lagoon. 



Niau is the only one of the larger atolls of the Pau- 

 motus whose lagoon is entirely cut off from the sea ; in 

 general, all the atolls are now in a condition which allows 

 a vast amount of water to be forced into the lagoons ; 

 this scours them out in the manner described in the pre- 

 ceding chapter. Dana and other writers on coral reefs 

 mention a great number of lagoons as being absolutely 

 shut off from the sea : such descriptions must have been 

 taken from charts, where many atolls are indicated as 

 closed because they have no boat passages. For nothing 

 would be easier than to pass unnoticed, even at a short 



