24 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 well as Pinaki, where the lagoon is not more than 
two to three fathoms, and Takume, where it is from five to six fathoms 
