TAHANEA. 89 



islands north of the western entrance, with a wide lagoon flat and many 

 rocky ledges extending from a mile to a mile and a half into the 

 lagoon. 



The formation of all these islands, more or less at right angles to 

 the line of the outer reef flat, is probably due to the currents which 

 run parallel to the lagoon face of the flats, caused by the hauling of 

 the prevailing winds along the coast lines of the atolls. These currents 

 carry more or less sand, and meeting an obstacle, this acts like a jetty, 

 some of the sand is deposited, and thus a diminutive sand spit is 

 formed which, little by little, forms a series of sand points projecting into 

 the lagoon. The spits increase from either side, from whichever direction 

 the wind may draw, and thus the reef flats are in the first place widened, 

 and in the second place partially covered with sand, leaving the sea free 

 access to the lagoon only through the gaps between the spits and over 

 the sea face of the reef flat, wherever the beach rock or recent 

 conglomerate or coral shingle spread over the old ledge has not 

 been covered with sand. These sand buttresses thus form the begin- 

 nings of small low sand islands on the lagoon face of the atoll ; as 

 they increase in size, they gradually join similar accumulations of 

 beach rock or coral shingle, thrown up in a similar but in a much 

 more active way, on the sea face of the reef platform. 



This phenomenon is well illustrated on the islands and islets to the 

 south of the north point of Tahanea, where not only their growth can 

 be distinctly traced, as has been described, but where also the' manner 

 in which they become clothed with vegetation can be equally well fol- 

 lowed. The ledges and the shoals in the lagoon supply the material for the 

 inner sand spits. On the line of reef flats and on the sea face, the breakers 

 grind the material on its edge and supply the coarser material for the 

 shingle beaches, which may be thrown up to a height of from two to three 

 feet. 



As far as we could see to the eastward, the conditions of the eastern 

 shore did not seem to differ from those of the northern and southern parts 

 of the atoll which we examined. 



