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 Nullipores 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 
