DECEMBER 8, 1899. ] 
depth between 9 and 10 fathoms, shoaling 
near the inner entrance to about 3} fathoms, 
and deepening again to 6 or 7 fathoms, ahd 
gradually passing into 15 to 17 fathoms, 
which is the average depth of the lagoon 
from Avatoru Pass to the south or weather 
shore, a distance of about 13 miles. 
We made an examination of the northern 
side of the lagoon, between Avatoru and 
Tiputa passes. The lagoon beach of the 
northern shore is quite steep, and is com- 
posed of moderately coarse broken coral 
sand at the base, and of larger fragments of 
corals along the upper face, which is about 
5 to 6 feet above high-water mark. These 
coral fragments are derived in part from 
the corals living on the lagoon face of the 
northern shore, and in part of fragments 
broken by the waves from somewhat below 
the low-water mark. The ledge which un- 
derlies the beach crops out at many places 
on the lagoon side of the northern shore ; 
we traced it also along the shores of Avatoru 
Pass, and about half way across the nar- 
row land running between Avatoru and 
Tiputa passes. It crops out also at vari- 
ous points between them in the narrow 
euts which divide this part of the northern 
land of the lagoon into a number of smaller 
islands. These secondary passes leave ex- 
posed the underlying ledge, full of fossil 
corals. In some cases there is left a 
clear channel extending across from the 
lagoon to the northern 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 beach leaving a depres- 
sion which at first is filled with salt water 
and gradually silted up both from the 
SCIENCE. 837 
lagoon side and the sea side, and forms the 
typical north shore land of the lagoon. 
This building up of the land of the Paumotu 
atolls simultaneously both by the accumula- 
tion of sand from the lagoon side and the sea 
face is very characteristic of the atolls of that 
group. Itisa 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 16 and 17 
fathoms, and corals appear to flourish on 
the lagoon slope to 6 or 8 fathoms only, in 
some cases consisting of Madrepores, Pori- 
tes, Astrzeans, and Pocillopores. The corals 
could be seen over the floor of the Avatoru 
Passage down to 9 to 10 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 ex- 
amined the south shore of the lagoon, that 
the ledge underlying the north shore is the 
remnant of the bed, an old tertiary corallif- 
erous 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. This began 
to cut away the inner portions of the ele- 
vated limestone, forming large sounds, as 
in the case of Fiji atolls, and leaving finally 
on the south side only a flat strip of per- 
haps 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 10 to 14 feet in height, 
the material for this land having come from 
the disintegration of the ledge of tertiary 
limestone, both on the sea face and the 
lagoon side. 
