EANGIROA. 41 



found the bottom clear of corals in from nine to ten fathoms. It was 

 only between four to six fathoms that they began to appear in patches. 

 They were mainly rather small heads of two species of Astra^ans. 



At Rangiroa I saw for the first time a feature which turned out to be 

 very characteristic of the structure of the Paumotus as well as of the Society 

 Islands, and explained in a very clear way the mode of formation of the 

 outer land rim islands upon the sea face of the platform of the barrier reefs 

 surrounding the volcanic islands of the Society Islands, or forming the rims 

 of the Paumotu atolls. I had noticed, as we first steamed along the outer face 

 of the Rangiroa atoll from the eastward of Tiputa to Avatoru Pass, that the 

 high outer beach was here and there greatly depressed, and in some instances 

 the depression was sufficient to open out a view into the interior of 

 the lagoou across a shallow passage, thi'ough which in some cases water 

 was flowing from one face of the island to the other, in others the shallow 

 channel reached but a short distance into the island either from the sea 

 face or the lagoon side. These depressions (Pis. 9, 10; 13, fig. 1) were 

 usually indicated on the sea face by stretches of boulders extending on 

 both sides ; and as these were disintegrated they gradually disappeared 

 both to the north and south of the openings, having gone to build up the 

 coral sand and shingle beaches of the sea face of the outer island. There 

 were no less than five such cuts in the island between Avatoru and 

 Tiputa Pass, dividing the island into parts connected at low tide only, and 

 separated at high water by narrow channels, from 75 to 100 feet in 

 width and with one to three feet of water (Pis. 8-11). The first cut to 

 the east of Avatoru Pass was Y-shaped, the main branch extending 

 across from the lagoon to the reef platform nearly in a straight line, 

 the other branch forming a cul-de-sac extending about two thirds of 

 the way across the island from the lagoon face. A high steep coral sand- 

 beach forms a sort of dam across the opening of the cut on the lagoon side 

 (like that of PI. 13, fig. 2). The greater part of its channel has been cut 

 through beach rock and low masses of coral fragments and a conglomerate 

 composed of pieces of recent corals and of corals and smaller fragments 

 washed out from the old ledge platform, which is exposed at several points 

 at the bottom of the main cut. 



