PRELIMINARY REPORT. 13 
ing come from the disintegration of the ledge of Tertiary limestone, both 
on the sea face and the lagoon side. : 
There exist in the lagoon a number of small islets which also consist of 
this same Tertiary limestone in process of disintegration and transformation 
to coral-sand islets. They are the islets at the lagoon side of both Avatoru 
and Tiputa Passes, the two islets which we found along our line of sound- 
ings across the lagoon, the one about*four and one half 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 Rangiroa is in a stage of de- 
nudation and erosion very similar to that of Ngele Levu, in Fiji, only in 
Ngele Levu the elevated limestone attains a height of about sixty feet. 
Our visit to the southern land-rim of the lagoon, both on the lagoon side 
and on the sea face, left us no doubt regarding the character of the under- 
lying ledge of the north shore. As soon as the south-shore land was suffi- 
ciently near, as seen from the lagoon side, for us to distinguish its character, 
we could see that the entire shore line was formed of a high ledge of lime- 
stone, honeycombed, pitted, and eroded by atmospheric agencies, and under- 
cut by the action of the waves 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 Dana’ as 
he saw it while sailing by in the “ Vincennes”). But in addition to this 
we found, on landing, 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 south- 
Western part of the lagoon. 
These islands and islets of the weather land-rim are entirely composed 
of coral sand and coral fragments, formed from the disintegration of the 
1 Corals and Coral Islands, 3d ed., 1890, p. 369. 
