DECEMBER 8, 1899. ] 
sages the parts of the ledge which had not 
been eroded extended as wide buttresses, 
gradually diminishing in height till they 
formed a part of the lagoon flat and ex- 
tended out below the recent beach rock 
which covered it in short stretches. 
The slope of the sea face of the elevated 
ledge was quite steep and similar to the 
lagoon slope, its upper surface weathered 
by atmospheric and aqueous agencies into 
all possible shapes such as I have men- 
tioned. The slope passed into the shore 
platform which was shaved down as it were 
to a general level surface. On the outer 
edge, within the line of the breakers, were 
growing Pocillopores in great abundance. 
This reef flat or shore platform, as well as 
the reef platform of the north shore, was 
strewn here and there with huge masses of 
the ledge of elevated reef rock torn from its 
outer shore. Similar rocks and bowlders 
occur on the lagoon side of the islands form- 
ing the outer lands of Rairoa; they are 
either torn off from the lagoon face of the 
outcropping ledge, or are parts of the ledge 
which have remained in place and have not 
been planed down to the base level of the 
reef. 
The amount of water which is forced into 
such a lagoon as Rairoa is something colos- 
sal, and when we observe that there are 
but a small number of passages through 
which it can find its way out again on the 
leeward side, it is not surprising that we 
should meet with such powerful currents 
(7 to 8 knots in several cases) sweeping out 
of the passages on the lee sides. 
The islands and islets of Rairoa are 
fairly well covered with low trees and 
shrubs and great groves of palm trees. 
The atolls of Tikehau and Mataiwa, which 
we also examined, present no features which 
we did not meetin Rairoa. The first-named 
atoll shows the same method of formation 
of the land by material piled up both from 
the lagoon side and the sea face; material 
- SCIENCE. 839 
derived from the disintegration of the under- 
lying tertiary limestone which crops out 
here and there along the sea face and the 
inner shores of the lagoon, or forms across 
the southwest face of the lagoon a more or 
less disconnected part of the ring of islands 
and islets encircling that end of the lagoon. 
These islets and islands are irregularly con- 
nected by fragments of the elevated lime- 
stone ledge, attesting its greater extension 
inpasttimes. The outer rings of both these 
atolls are covered with vegetation. We 
could see in the lagoons several rocky islets, 
the fragments of the elevated limestone 
ledge. 
Mataiwa is interesting, as its lagoon is 
quite shallow; it is full of rocky islets, 
remnants of the underlying limestone ledge 
which crops out above the general level, and 
has a very narrow and shallow entrance, 
passable for boats only. Some of its islands 
are wooded and appear to have been formed 
by accretions of sand from the decomposing 
ledges of the lagoon. The outer ring of 
land appears formed by sand banks driven 
in from the sea face and driven out from 
the lagoon side by the action of the waves. 
It is evident that such alagoon as Mataiwa 
could readily be closed to any access to it 
by the sea, as it now has only one very 
narrow aud very shallow boat passage con- 
necting the lagoon with the sea on the lee 
side. 
It was with great interest that we ap- 
proached Makatea, as it is the only high 
elevated island of which Dana speaks as 
occurring in the western Paumotus. For 
though he mentions some others as possibly 
having been elevated 5 to 6 feet, yet he 
considered them, all as well as Makatea 
(Metia or Aurora, of Dana), as modern ele- 
vated reefs. Yet from the very description 
given by him of the character of the cliffs 
and of the surface of Makatea, I felt satis- 
fied that it was composed of the same ele- 
vated coralliferous limestone so character- 
