THE PAUMOTUS. 13 



would not differ from other islands of the Pauraotus which we intended 

 to examine. On the south face of Ahe the islands of the low land rim 

 are separated by a series of gaps and of cuts such as we foimd chai'ac- 

 teristic of Rangiroa ; the entrance into Alie differs in no way from those 

 of a number of other islands of the Paumotus. The entrance to the lagoon 

 of Manihi, although somewhat narrower than any we entered, is yet very 

 similar to the entrances into Hao. Ptangiroa, Tikahau, and other islands 

 of the group. The sketches of Tikahau and of Rangiroa, as given by 

 Wilkes, are somewhat incorrect; they give the idea that the land rim is 

 continuous, while in reality it is composed of islands and islets separated 

 by long stretches of reef, bare at low water. The gaps between them 

 allow an immense amount of water to enter the lagoon from either face 

 of the atoll. The description which has been given of many of the 

 Paumotu atolls would give one the impression that they are clo.sed and 

 have only a single well-marked passage. On the contrary, with the excep- 

 tion of a few atolls to which we shall return later, the water flows into 

 the lagoon fi'om either side of the atolls through numerous cuts. 



It was only after an examination of the old ledges exposed in the 

 gaps of the northern land rim of Rangiroa, and especiall}' in the ele- 

 vated wall of coralliferous limestone of the weather coast, that we 

 obtained a satisfactory explanation of the formation of the atolls of 

 the Paumotus, — an explanation further confirmed after we had visited 

 the elevated islands of Niau, Makatea, and the other low Paumotu 

 Islands. It is to the cutting down of the elevations of the old ledge to a 

 general level, and the subsequent building up of the atolls by the material 

 supplied from the reef flats and from the coral slopes, that is due the 

 great uniformity of all these atolls. 



Certainly sub.sidence could not have acted in such a uniform way, and 

 have lowered the level of these numerous atolls, once rai.sed to very 

 different heights, to the same point. We can, of course, imagine subsi- 

 dence to have lowered many of them, but not all, to the same level. 

 The cutting down by erosion and denudation and the planing action 

 of the sea are all factors which we still see going on at all points 

 of the Paumotus, and which readily explain the phenomena as we 



