22 "ALBATROSS" TEOPICAL PACIFIC EXPEDITION. 



cuts, which gradually silting up finally close the gaps, and merely indicate 

 the former separation of the various parts of the land rim. 



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

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

 30 and 40 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, from local causes, 

 the material is not very abundant, or is washed out over the flats, there 

 are fewer islands, and these are often but mere islets or bars for long 

 reaches of the submerged land rim forming the 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 material from the lagoon side, for the land of the outer rims 

 of the atolls. In the Fakarava lagoon there are no less than thirty-six 

 islands and islets and ledges, parts of a former great flat, now broken up, 

 once existing parallel to the outer reef flat. 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 disconnected and planted 

 in the lagoon itself. A secondary lagoon also exists in Ravahere and one 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 13 to 20 

 fathoms. In some cases they are somewhat deeper ; some of the greater 

 depths, to 30 fathoms or more, are probably due to orogenic conditions. 

 Some of the lagoons are quite shallow, as at Matahiva, as well as Pinaki, 

 where they are not more than two to three fathoms, and Takume, where it is 



