82 "ALBATROSS " TROPICAL PACIFIC EXPEDITION. 



On the northwestern land rim there are a number of small islets con- 

 nected by sand bars on the lagoon side, and running somewhat diagonally 

 towards the reef flat edge, the edges of these bars are separated by 

 negro-heads or bands of beach rock at right angles to the coast line. 



On a nearer view of the cluster of islets inside the lagoon, we could 

 see that many of them were separated by comparatively deep water from 

 the outer reef flat, which is fully half a mile wide. We could see the old 

 ledge outcrops on many islands (Pi. 47, figs. 1, 3), as well as those of 

 beach rock and of recent conglomerate, flanking many of the sand bars 

 and sandy islets or keys. The lower part of it near the foot of the 

 beaches is covered with a recent very coarse conglomerate of shells and 

 corals (PI. 47, fig. 2). On one of the islets the old ledge projects promi- 

 nently at the eastern extremity, the inner extension being covered with 

 sand. On many of the islands the ledges are composed of beach rock and 

 conglomerate which has concealed the old ledge rock cropping out at 

 many of the other islands. The conglomei-ate has also extended over 

 the shallow parts of many of the flats adjacent to the islands. As we 

 steamed south, the old ledge was again observed on the western face of 

 many of the islets, standing like a raised wall, more or less broken, pre- 

 senting on a small scale the elevated wall we had seen at Rangiroa (PI. 

 46, fig. 1). After leaving the cluster of islets, which became gradually 

 more widely separated as we steamed south, we found only a wide reef flat, 

 on the edge of which are large boulders of beach rock or of old ledge. 



The few soundings we made indicate quite deep water between the 

 cluster of islands and the outer reef. The islets are covered with vege- 

 tation just as are the islands of the land rim flat (PI. 47, figs. 3, 4). 



This group of islands, ledges, and shoals is a part of a wide ridge 

 more or less parallel with the basin of the lagoon, of which the higher 

 parts alone remain as islands and bars which have not been eroded or 

 carried off in solution. The reef flat on the sea face of the eastern land 

 rim is full of pot-holes more or less filled with gravel ; these pot-holes 

 are some of them circular, others elliptical or run together as long and 

 deep furrows. The action of the current inside the NuUipore knolls is 

 to scour out a channel along the outer face of the reef platform. This 



