12 EXPEDITION OF THE “ALBATROSS,” 1899-1900. 
side, through which water flows at high or half tide. In other cases the 
cuts are silted up with coral sand blown in from the lagoon side. In 
others, the cut is shut off by a high sand-bank, or a bank composed of 
broken fragments of corals, leaving access to the water from the northern 
shore only; and finally the cuts are also shut off on the northern side by 
sand and broken coral banks, the extension of the north shore outer beach 
leaving a depression which at first is filled with salt-water and gradually 
silted up both from the lagoon side and the sea-side, and forms the typical 
north-shore land-rim of the lagoon. This building up of the land-rim of 
the Paumotu atolls by the accumulation of sand both from the lagoon side 
and the sea face, is very characteristic of the atolls of that group. It is 
a feature which I have not seen so marked in any other coral-reef district. 
On the lagoon side the slope from the beach is very gradual into 
sixteen and seventeen fathoms, and corals appear to flourish on the lagoon 
slope to six or eight fathoms only, in some cases consisting of Madrepores, 
in others of Porites or of Astraans, and Pocillopores. The corals could 
be seen over the floor of the Avatoru passage down to from nine to ten 
fathoms; and on the sea face Pocillopores covered the outer edge of the 
shore platform. This platform is from 200 to 250 feet wide, and was 
formed by the planing off of the seaward extension of the ledge cropping 
out in the cuts. 
It became very evident, after we had examined the south shore of the 
lagoon, that the ledge underlying the north shore is the remnant of a bed 
of Tertiary coralliferous limestone, which at one time covered the greater 
part of the area of the lagoon, portions of which may have been elevated 
to a considerable height. This limestone was gradually denuded and 
eroded to the level of the sea. Passages were formed on its outside edge, 
allowing the sea access to the inner parts of the lagoon limestone flat. 
This began to cut away the inner portions of the elevated limestone, 
forming large sounds, as in the case of Fiji atolls, and leaving finally. on 
the south side only a flat strip of perhaps 2500 to 3000 feet in width, 
which has gradually been further eroded on the lagoon side and also on 
the sea face, to leave only a narrow strip of land about 1000 feet in width 
and perhaps ten to fourteen feet in height, the material for this land hav- 
