50 "ALBATEOSS" TROPICAL PACIFIC EXPEDITION. 



lagoon, or the materials were piled up as islets on the outer flat, or carried 

 away across it, or washed out in solution; the disintegration of the land 

 rim by the waves and the solvent action of the sea all tending to deepen the 

 lagoon and to clear it of islands and bars and ledges. In the larger atolls 

 especially they are cut to a general level from which only here and there 

 islets and bars rise to attest the former existence of larger areas of land 

 within the atoll, as will be shown in the description of atolls like Taha- 

 nea, Anaa, Marokau, and Ravahere, and of atolls in the topography of 

 which islands, islets, and flats play an important part. 



To the southeast of the northwest point of Rangiroa, the great wall of ele- 

 vated ancient coralliferous rock begins, extending towards Taeroere Island, 

 which we had seen, with the wall extending towards it, from the island of 

 Funuarua. On the south side the reef flat is of great width, varying from one 

 to two miles and more. The wall rises from the shore platforni itself, from 

 300 to 400 feet wide, to a height varying from 8 to 15 feet.' At various 

 distances behind the wall, and in the rear of the larger islands and islets 

 which are more or less covered with vegetation, and which abut against the 

 wall, are bars and islets of sand forming on the inner edge of the lagoon 

 reef flat and extending towards the wall, until they finally form a buttress 

 or a narrow island behind the wall, which closes more or less the gaps inter- 

 vening between the lines of bars and islets and islands built up on the outer 

 land belt of the south shore near the northwest point. Some of the bars have 

 very scanty vegetation ; as they grow larger, this increases ; a few cocoanut 

 trees occur, and the larger islands, occupying the whole width of the land belt, 

 are well covered with vegetation and cocoanut trees. On the southwest side 

 the gaps between the islands become of great width ; some of them are several 

 miles across. The great wall is the most prominent feature, and runs appar- 

 ently without a break, not only across the gaps but also across the butts of 

 the islands. In some places the sand beach which has accumulated behind 

 the wall encroaches gradually upon the sea face of the wall itself and con- 

 ceals it so that spurs of the wall only crop out here and there on the face of 

 the beach. The contrast between the brilliant white sand bars standing out 

 against the green water of the shallow edge of the lagoon and the sand 



^ Dana has described what I take to be similar tertiary limestone from Honden and Manihi. 



