14 EXPEDITION OF THE “ALBATROSS,” 1899-1900. 
extension of the elevated ledge toward the inside of the lagoon to a dis- 
tance of about one and one half to two miles; and along this very gradual 
inner slope of the islands forming the southern edge of Rangiroa, corals 
grow profusely down to six or seven 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 of the weather land-rim 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 partly shuts off the commu- 
nication of the interior of the lagoon with the sea on that side. 
The passages between the islands and islets illustrate well, only on a 
larger scale, the formation 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 of it. 
Crossing over to the weather side of the southern land rim of Rangiroa 
in one of the passages between two of the islands, we came upon the 
coralliferous limestone ledge, from twelve to fourteen feet high and about 
forty to fifty feet wide at the top, 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 to consist 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 pot-holes recalling a similar 
scene in Ngele Levu on the windward end of the lagoon. In the passages 
the parts of the ledge which had not been eroded extended as wide but- 
tresses, gradually diminishing in height till they formed a part of the lagoon 
flat and extended out into the lagoon below the recent beach-rock which 
covered it in short stretches. 
The sea-face slope of the elevated ledge was quite steep, but otherwise 
similar to the lagoon slope, and its upper surface weathered by atmospheric 
