MATAHIVA. 55 



a higher coral sand beach is thrown up, and rapidly closes the cuts and 

 passes leading from the sea face to the lagoon as we had traced it on Rangiroa. 



Great care should be taken to call a lagoon closed only when, as in 

 the case of Niau, the outer ring of land surrounding the lagoon is en- 

 tirely solid. Many atolls are called closed which, while having no 

 boat entrance, yet have many very shallow cuts and passes between the 

 islands and islets of the outer land rim, through which at nearly all 

 stages of the tide a large amount of water passes, and is forced in 

 from certain directions by the prevailing winds. 



On the beach of the northeastern shore a table-topped rock of old 

 ledge deeply undercut was a prominent object. Part of the shore ledge 

 undoubtedly consists of beach rock and modern conglomerate piled up 

 or formed between portions of the old ledge left standing, as on the north 

 face of Rangiroa, but the bulk of the ledge is made up of old elevated 

 reef rock ledge. 



Matahiva is densely wooded, and much coral sand has been blown in 

 between the trees from the outer beaches. At the boat entrance into 

 Matahiva the old reef rock is well exposed on both sides, and through the 

 narrow pass we could see two wooded islets in the rear of the gap fringed 

 in part by the old reef rock ledge. 



The coral sand beaches of Matahiva were from six to eight feet high, 

 and the remnants of the old ledges forming the summit of the land and 

 more or less overwhelmed by sand were from ten to twelve feet. The sea 

 was breaking heavily on the wide platform of the southwest point. 



The condition of Matahiva as well as of Tikahau seems to indicate 

 atolls which are gradually filling from the sand blown into them, both 

 from the decomposition of the ledge of the outer beach and outer reef 

 platform, and from the sand of the lagoon side derived from the disin- 

 tegration of the islets and ledge flats, as well as from the material de- 

 rived from the lagoon shore platform. 



The sketch of Matahiva given by Wilkes on H. 0. Chart 85 is not 

 correct. The atoll is triangular in shape (PI. 204, fig. 4), about seven 

 miles in length, and its position is nearly six miles more to the west- 

 ward than is indicated on the Hydrographic Chart. 



