166 "ALBATKOSS" TROPICAL PACIFIC EXPEDITION. 



Tetiaroa. 



Plates 95, figs. 2,3; 90, fig. 1 ; 202. 



Tetiaroa consists of a number of coral sand islets thrown up on a some- 

 what pentagonal horseshoe-shaped reef flat, about six miles in length ; they 

 are, judging from the character of the reef platform of Maupiti, probably 

 underlaid by a platform of volcanic rocks. Beach rock crops out at the 

 base of the beaches of most of the islets, and between the wooded islands 

 crop up negro-heads of the same or of modern reef rock (PI. 95, fig. 3). 

 There appears to be a somewhat deeper part of the lagoon on the western 

 side. The southern extremity is deeplj^ indented (PI. 96, fig. 1), with a 

 barrier reef along the edge of the platform, from which jut at right angles, 

 far into the lagoon, a couple of .sandy islands separating the deeper part of 

 the lagoon from its shallow eastern side, clearly indicated by the light green 

 color of the water. On the western side of the atoll are thrown up two 

 islands ; the northern side is occupied by along island separated into .smaller 

 parts by wide and shallow cuts, while on the eastern face there are a large 

 island at the .southeastern horn of the atoll (PI. 95, fig. 2). and two smaller 

 islets. The islands are all connected by negro-heads of beach rock, and long 

 ledges of beach rock project from the points of several of the islands. The 

 northeast and the southwest reef flats are well dotted with large beach rock 

 negro-heads. 



The conditions under which these islands and islets have been thrown up 

 on the reef flats seem to be identical with those of the Paumotus, only the 

 nature of the substratum is undoubtedly different. The gaps between the 

 islands are being filled up in the same way as in the Paumotus ; the material 

 for the formation of the islands has in this case been derived from the de- 

 composition and disintegration of the beach rock and of the modern coral 

 conglomerate, formed by the fragments of the corals growing upon the reef 

 flats and slopes, thrown upon the volcanic substratum. In fact, Tetiaroa is 

 the last stage of a series in which Bora Bora stands at one end, Maupiti 

 coming next, then Motu Iti, and finally Tetiaroa.^ In the case of Bora Bora 



1 A. Chart 767. 



