TIKAHAU. 53 



height of the cocoanut trees seems to show that the elevation of the island 

 belt, like that of Rangiroa, varies but little. Its islands and islets are 

 covered with similar vegetation. They are separated by gaps and open 

 passages, like those at Rangiroa, where the old reef ledge is exposed. 



Tlie island belt must have been formed, like that of Rangiroa, by the 

 throwing up of the sand as dunes or bars and sand islets, upon the inner face 

 of the lagoon flat behind the sea wall of the old ledge. From the smaller 

 size of the lagoon and the proportionally greater mass of available material 

 as sand, the islands and islets formed a more solid land belt concealing the 

 greater part of the old ledge wall or leaving only a few patches of it exposed. 

 The building up of the land belt from the lagoon side in all directions is 

 quite marked in Tikahau. It is still more marked in Matahiva, and culmi- 

 nates, as we shall see, in the filling up of some lagoons in the smaller atolls, 

 as Whitsunday for instance. On the west side of Tikahau there are only a 

 few points where the ancient reef rock ledge is exposed ; it has either been 

 cut away or the inland buttresses are buried in the lagoon sand formed upon 

 the lagoon flat, so that it crops out only occasionally, usually in the gaps, and 

 rarely across the face of an island. The outer shore platform is from 150 to 

 300 feet in width, and the inner lagoon reef ledge from 600 to 1000 feet 

 or perhaps even more. 



Tikahau is elliptical in outline, nearly fifteen miles long and ten miles 

 wide. The position of the southwestern part of the atoll is about 

 three miles more to the westward (Pis. 201 ; 204, fig. 4) than is indicated 

 by Wilkes's sketch in H. 0. Chart 85. 



The southwestern and western face of the land rim is also, as we 

 saw it, far narrower than is indicated on Wilkes's sketch, consisting of 

 a narrow reef flat, bare at low water (PI. 20, figs. 2, 4), with only five 

 small, widely separated islets on the submerged rim. The western part, 

 as we observed it (PI. 204, fig. 4), differed greatly from the drawing given 

 by the U. S. Exploring Expedition. On the west side of the atoll the sea 

 had access to the lagoon through many wide and shallow openings separating 

 the islets forming the land rim. 



It will be noted that the entrances to Tikahau and to Matahiva 

 are on the western face of these islands, while the entrances to Rangiroa 



