72 "ALBATROSS" TROPICAL PACIFIC EXPEDITION. 



passages crossing the northern beach and which reach far in towards the 

 centre of the island (Pis. 41-43). 



The reef platform of the island is from 50 to 150 feet wide (PI. 43, 

 fig. 4) ; it consists of old ledge planed off on the northwestern face, while on 

 the other side the platform is covered by a mass of boulders all consisting of 

 old ledge rocks thrown up in part from the edge of the reef fiat after being 

 torn off, or in part from the disintegrated old ledge itself (PI. 40, fig. 2). 

 This extends inland to the termination of the bight or shallow wide gap 

 (PL 41, fig. 1) leading to the sink of the central part of the island (PI. 44); 

 channels lead from the bight to the innermost parts of the sink, thus con- 

 necting it with the sea face of Tikei (Pis. 42, 43). 



The sea face beach of Tikei is high (Pi. 40, figs. 3, 4), from 10 to 12 feet. 

 It consists, as at Niau and Rangiroa, of coral rubble or shingle on each side 

 of the part of the reef fiat covered with old ledge boulders. The bight or 

 low gap is crossed by the mass of boulders mentioned above ; they extend 

 from the sea face of the platform a short distance inland in the gap. Behind 

 the boulders, on the inner face, has been thrown up a low rubble beach 

 (PL 41, fig. 1) composed of smaller fragments of recent corals and old ledge, 

 almost closing the mouth of the gap, which is about 500 feet wide and 

 extends fully 1200 feet inland, forming a cul-de-sac (PI. 42), from the 

 extremity of which lead the shallow channels (Pis. 41, fig. 2 ; 43), connect- 

 ing at highest water with the sink which occupies the position of a lagoon 

 in this island (PL 44). 



The central part of the island is low, full of shallow sinks (Pis. 43, 44), 

 connected evidently with one another in the rainy season ; all leading into 

 the main sink, an irregularly shaped depression about a third of a mile in great- 

 est width, with a pool of brackish water in its deepest part from 40 to 50 feet 

 long and about 25 feet wide (PL 44). In the centre of the pool there was 

 an outcrop of the old ledge (PL 44). The floor of these swampy sinks and 

 channels was covered with Algae (PL 43) ; at high-water stages the main sink 

 must have been fully half a mile in length and from 500 to 600 feet wide. 

 The lichens covering the flat resemble those found on the marshy flats of 

 the Florida reefs. 



Here and there we came upon patches of recent conglomerate beach 



