TAHANEA. 87 



platform. On the west face we could see that the islets of the reef flat 

 were placed on the inner lagoon edge, at an angle with the shore line ; 

 we could also readily trace the sand beaches of the lagoon end of the 

 islands, as they gradually pass into the shingle beach characterizing their 

 sea face extremity (Pis. 54, fig. 1 ; 55, figs. 2, 4). These islets and islands, 

 thrown up either from the outside face or from the inside face, alternate 

 with one another, but all have shingle beaches on the sea face ex- 

 tremities and coral sand beaches on the lagoon side ends. The shingle 

 beaches are gradually forced towards the lagoon side, where they meet 

 the sand bars pushed seaward by agencies from the lagoon side, or 

 vice ve)'sa (PI. 55, fig. 2). The currents, both running in and running 

 out, gradually accumulate more and more sand, cover the beach rock 

 boulders and shingle, and spread over the reef flat, and finally form 

 a bar extending quite across it. If the depth of water between the 

 outer and inner edges of the reef flat is too great, two separate islands 

 are formed which will become joined when the channel between them 

 has been filled or bridged over in part, when the whole of the exposed 

 flat, either on the lagoon side or the sea face, has been completely cov- 

 ered by shingle or sand and has formed a continuous island from the 

 lagoon edge of the reef flat to its sea face (Pis. 54, fig. 3 ; 55, figs. 2, 4). 



All along the south shore the I'eef flat or the land rim is so low that 

 at the least sea, water must be driven through the gaps separating the 

 reef flat islands (Pis. 54, 55). In the lagoon the old ledge has been 

 planed off to about high-water level, so that but little material is now 

 supplied from the disintegration of the numerous shoals and ledge flats left 

 in the lagoon ; the greater part of the material is derived from the lagoon 

 edge of the outer reef flats or from some of the higher ledges left in the 

 lagoon. 



Young cocoanut trees grow on some of the smallest and lowest sand 

 islets when not more than one to two feet high, while on larger and higher 

 islets they are more numerous, and by the time the islands extend 

 across the outer reef platform they are well covered with the char- 

 acteristic coral reef vegetation, as well as with cocoanut trees (Pis. 54, 

 fig. 2 ; 55, figs. 1, 2). 



