2-4 EXPEDITION OF THE "ALBATROSS," 1899-1900. 



cuts, which gradually silting up finally allow no water to pass through and 

 merely indicate the former separation of the various parts of the land. 



In the lagoons of atolls of such great length as some of those of the 

 Paumotus, like Rangiroa, Fakarava, Makemo, and Hao, which are between 

 thirty and forty miles long, and others of less dimensions, considerable sea 

 rises under the prevailing trades. The sea and wind generally follow the 

 trend of the shores, both in the lagoon and along the sea face, so that the 

 bars of beach-rock act like buttresses and collect material at their inner and 

 outer extremities, forming the sand-bars and islets which eventually become 

 the land-rim of the lagoon. When the material is not, from local causes, 

 very abundant, or is washed out over the flats, there are fewer islands, and 

 often these are but mere islets or bars for long reaches of the submerged 

 land-rim, forming the characteristic weather-faces of many of the lagoons. 



Many of the lagoons are filled with shoals or ledges awash or a few 

 feet above the sea level. These shoals are parts of the old ledge which 

 have not as yet been eroded, and the disintegration of which has gone 

 far to supply the material for the land of the outer rims of the atolls. In 

 Fakarava there were no less than thirty-six islands and islets and ledges, 

 parts of a former great flat, now broken up, existing parallel to the 

 outer reef-flat about four miles in the lagoon. Similar reef flats exist in 

 Tahanea, where they form a secondary lagoon with two to three fathoms 

 of water, extending nearly the whole length of the western face of the 

 atoll. There are several large islands on this flat, and at high water 

 they would appear, as the islands and islets of Fakarava do, as discon- 

 nected and planted in the lagoon itself. A secondary lagoon also exists 

 in Ravahere, and in Anaa ; in both these atolls the reef flat extends 

 across one extremity of the lagoon, and does not run parallel to the 

 longer line of the land-rim of the atoll. 



The lagoons of these atolls have a general depth of thirteen to twenty 

 fathoms. In some cases they are somewhat deeper, as is stated, but there 

 are no measurements, the greater depths, thirty fathoms or more, being 

 due probably to orogenic conditions. Some of the atolls are quite shallow, 

 as at Matahiva, as w^ell as Pinaki, where the lagoon is not more than 

 two to three fathoms, and Takume, wliere it is from five to six fathoms 



