838 
There exist at the lagoon side of both Ava- 
toru and Tiputa passes a number of small 
islets which also consist of this same ter- 
tiary limestone in process of disintegration 
and transformation to coral sand islets; two 
of these we found along our line of soundings, 
the one about 44 miles from the north side of 
the lagoon, and the other about the same 
distance from the south shore. I am told 
that the eastern extremity of the lagoon is 
filled with islets and heads consisting of the 
same limestone rock so characteristic of the 
north and south shores of the lagoon. 
The underlying ledge is not the remnant 
of a modern reef ; its character is identical 
with that of the elevated limestones of Fiji 
which are of tertiary age, and the rock is 
in every respect the same as that I observed 
on many of the elevated islands of Fiji. 
The atoll of Rairoa is in a stage of denuda- 
tion and erosion very similar to that of 
Ngele Levu, in Fiji, only in Ngele Levu 
the elevated limestone attains a height of 
about 60 feet. Our visit to the south shore 
of the lagoon, both on the lagoon side and 
on the sea face, left us no doubt regarding 
the character of the underlying ledge of the 
north shore. As soon as the south shore 
was sufficiently near, as seen from the 
lagoon side, for us to distinguish its char- 
acter, we could see that the entire shore line 
was formed of a high ledge of limestone, 
honeycombed, pitted, and eroded, both by 
atmospheric agencies and the action of the 
Waves in its lower parts both on the lagoon 
side and on the sea face. The great rollers 
of the weather side broke through between 
the columnar masses of the ledge into the 
lagoon,and as far as the eye could reach there 
extended a more or less continuous wall 
(which is described by Danaas he saw it sail- 
ing by in the Vincennes ).. But, on landing, we 
found this wall to be the sea face of the islands 
and islets which dot the weather side for the 
greater part of its length on the southwest- 
ern part of the lagoon. These islands and 
SCIENCE, 
[N. S. Von. X. No. 258: 
islets are entirely composed of coral sand 
and coral fragments, formed from the dis- 
integration of the extension of the elevated 
ledge toward the inside of the lagoon to a 
distance of about 14 to 2 miles; and along 
this very gradual slope of the islands form- 
ing the southern edge of Rairoa, corals 
grow profusely down to 6 or 7 fathoms of 
water, when the bottom runs into hard 
coralline bottom similar to that found on 
all the soundings taken across the lagoon. 
The width of the larger islands is about 
1000 to 1200 feet, the smaller islands and 
islets are less, some of the latter forming in 
reality mere sand buttresses at right angles 
to the great limestone ledge which flanks 
them all on the sea face and connects them 
on the weather side as if by a great wall, 
more or less broken, and shuts off the com- 
munication of the interior of the lagoon 
with the sea on that side. 
The passages between the islands and islets 
illustrate well, only ona larger scale, the for- 
mation of the cuts, more or less silted up, 
which were observed on the northern face of 
the lagoon. Some of these passages are dry 
at low-water, others are partly filled by tide 
pools, others are entirely silted up by lagoon 
sand, only they are lower than the sand- 
blown land of the islands on either side. 
Crossing over to the weather side of the 
southern land of Rairoa, in one of the 
passages between two of the islands we 
came upon the limestone ledge, from 12 to 
14 feet high and about 40 to 50 feet wide, 
which formed the sea face of the islands and 
islets, and extended far to the westward as 
a great stone wall more or less broken into 
distinct parts. We found this ledge tocon- 
sist of elevated limestone as hard as calcite, 
full of corals, honeycombed and pitted, and 
worn into countless spires and spurs, and 
needles and blocks of all sizes and shapes, 
separated by deep crevasses or potholes, re- 
calling a similar scene in Ngele Levu on the 
windward side of the lagoon. In the pas- 
