THE PAUMOTUS. . 15 



motus would have given us so good a basis for an explanation of the causes 

 which have been at work to bring about tlie existing conditions of the 

 islands and atolls of the group. These have so persistently been represented 

 as low atolls, that with such a preconceived notion strengthened by an examina- 

 tion of the lower islands and atolls of the group, it would have been an easy 

 matter to fail to see in the structure of Rangiroa, Niau, and Makatea the 

 survival of a state of things the history of which can easily be read as demon- 

 strated by what these islands teach, but which it would be difficult to 

 trace from a study of the lower islands alone. It would have been 

 comparatively simple to recognize in the outliers and remnants of the old 

 ledge in the low islands the same tertiary elevated limestone with which we 

 had become familiar in the Fiji, Tonga, and Sandwich Islands, yet we 

 could have formed no conception of the great orographic changes the low 

 islands and atolls of the group had undergone after their elevation and sub- 

 sequent erosion. The old ledge crops out at various points in the narrow 

 cuts which divide the land rim of the lagoon into a number of smaller 

 islands; in these secondary passes the underlying ledge, full of fossil corals, 

 is left exposed. In some of the secondary passes a clear channel extends 

 across from the lagoon to the sea side, through which water flows at high 

 or half tide. In other cases the cuts are silted up with coral sand blown 

 in from the lagoon side. In others the cut is shut off by a high sand 

 bank, or a bank composed of broken fragments of corals, leaving access to 

 the water from the northern shore only ; and finally the cuts are also shut 

 off on the northern side by sand and broken coral banks, the extension of the 

 north-shore outer beach, leaving a depression which at first is filled with salt 

 water and gradually silted up both from the lagoon side and the sea face, and 

 forms the typical north-shore land rim of the lagoon. This building up of 

 the land rim of the Paumotu atolls by the accumulation of material both 

 from the lagoon side and the sea face, is very characteristic of the atolls of 

 that group. It is a feature which I have not seen so marked in any other 

 coral reef district. 



The land rim of the atolls in the Paumotus and Fiji is very stable. There 

 is not much movement except in hurricanes ; and there is no such regular 

 migration of sand as occurs on some islands in Florida. I know nothing to 



