26 EXPEDITION OF THE “ALBATROSS,” 1899-1900. 
in great part of pandanus and putu trees and the usual coral-reef vege- 
tation of the Paumotus! and Fijis. 
The only atoll.-we have seen in the Paumotus the lagoon of which is 
entirely shut off from the sea is Niau. In this case the old ledge forming 
the land-rim which surrounds the nearly circular lagoon is about a third 
of a mile in width and sufficiently high, fifteen to twenty feet, to pre- 
vent any sea from having access to it except in case of a cyclone, 
as that of 1878, when the sea washed into the lagoon. The lagoon is 
shallow, of an average depth of about three fathoms, the deeper parts 
perhaps five. The water is brackish, of a density of 1.0216 at 28° C. 
There are no corals living in it, but a species of mullet is found, as 
well as many marine shells, which, like those in the lagoons of San Sal- 
vador, in the Bahamas, are of diminutive size compared to their repre- 
sentatives living outside. The floor of the lagoon is covered with algee. 
The lagoon has probably a slight connection with the sea, the water 
percolating through the limestone ring separating it from the outer reef- 
flat. It is very difficult in this case to decide whether this lagoon has 
been gradually filled up after elevation, or whether it is merely a sink 
formed by solution and atmospheric erosion on a more or less uneven 
limestone surface. 
Dana, and other writers on coral reefs, mention a great number of 
lagoons as being absolutely shut off from the sea. I take it these 
statements are due to their descriptions being taken from charts, many 
of which, as in the case of the Paumotus, are very defective. For 
nothing is easier than to pass at a short distance by the wide and narrow 
cuts which give in so many cases the freest access to the sea to the interior 
of the lagoon, and are described as closed because they have no boat pas- 
sages. I could mention, as instances of such lagoons, those of the atolls of 
Takume, Hikueru, Anaa, etc., which may be said to be closed, yet into 
which a huge volume of water is poured at every tide over low parts of 
the encircling reef-flats. 
The character of the coral reefs of the Paumotus is very different 
from that of other coral-reef regions I have seen. Nowhere have I seen 
? See Dana, Corals and Coral Islands, 8d ed. p. 326. 
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