14 EXPEDITION OF THE "ALBATROSS," 1899-1000. 



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 otherwi.se 

 similar to the lagoon slope, and its upper surface weathered by atmospheric 



