PRELIMINARY REPORT. 15 



and aqueous agencies into all possible shapes such as I have mentioned. 

 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 and NuUipores 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 edge. Similar rocks and bowlders occur on 

 the lagoon side of the islands forming the outer land-rim of Rangiroa ; they 

 are either torn off from the lagoon face of the outcropping ledge, or are 

 outlying 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 Rangiroa 

 is something colossal, 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, seven to eight knots in several cases, sweeping out of the passages 

 on the lee side. 



The islands and islets of Rangiroa are fairly well covered with low trees 

 and shrubs and large groves of palm trees. 



The atolls of Tikehau and Matahiva, which we also examined, present 

 no features which we did not meet in Rangiroa. The first-named atoll 

 shows the same method of formation of the land-rim by material piled up 

 both from the lagoon side and the sea face, — material derived from the 

 disintegration of the underlying 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 an irregular, disconnected part of 

 the ring of islands and islets encircling that end of the lagoon. These 

 islets and islands are more or less connected by fragments of the elevated 

 limestone ledge, attesting its greater extension in past times. The outer 

 land-rings of both these atolls are covered with vegetation. We could see 

 in the lagoons several rocky islets, the remnants of the elevated limestone 

 ledge. 



Matahiva is interesting, as its lagoon is quite shallow ; it is full of rocky 

 islets, remnants of the underlying coralliferous limestone ledge which crops 



