RANGIROA. 43 



opening of the cut was filled with fragments of beach rock and an agglom- 

 eration of fragments of large rocks and masses thrown up from the reef 

 platform. 



At the third cut there is a high dam, composed of coarse coral fragments 

 and sand, from five to six feet high, placed directly across the lagoon open- 

 ing, with a narrow channel leading into the lagoon from a large pool behind 

 the dam, about 60 feet wide and not more than six inches in depth. The 

 channel is partly dry in the middle, and covered with fragments of coral, 

 as are the sides. On the sea face and on the lagoon side the old ledge rock 

 is cut through, and a conglomerate composed of old and recent corals is ex- 

 posed on the flanks of the cut. Across the centre of the cut, on the sea face, 

 a bank has been thrown up, similar to the beach to the east and to the west 

 of the cut, leaving a channel on each side and not quite closing the sea 

 entrance. The channel is deeper on the west side of the central beach. 

 The old ledge is exposed about 150 feet to the south of the sea beach, and 

 is topped here and there with a conglomerate composed of recent and old 

 reef rock. The mixture of this old and recent material makes the deter- 

 mination of the composition and age of any particular stretch of reef rock 

 somewhat complicated. 



Across the lagoon side of the fourth cut extends a dam directly across 

 the channel. This was a shallow depression perhaps 50 feet wide, filled 

 with fine sand to about 100 yards of the outer beach, where the old 

 reef rock ledge cropped out. The sea face channel was open. The sea 

 platform was cut by the sea into numerous tide pools ; the rocks and 

 boulders above the reach of the breakers were pitted and honeycombed 

 in all the cuts which were wholly or partly shut off, showing that for a 

 considerable time no currents had passed over them. The sides of the cut 

 indicated that it was made up, as are the other cuts, of a conglomerate of 

 broken masses of both recent and ancient corals, and fragments of modern 

 beach and old ledge rock. 



The sea face of Rangiroa consists of a steep, coarse coral sand beach 

 made up near the summit of coral shingle, the fragments of which are from 

 two to three inches, with finer material at the base of the beach. This beach 

 is flanked for a distance of 75 to 100 feet by a mass of fragments of old reef 



