144 "ALBATROSS^' TROPICAL PACIFIC EXPEDITION. 



reef flats, especially near the shore. It also gives us a simple explanation 

 of the mode of origin of the lagoon, formed by the scouring of the tides 

 rimning parallel with the shore, and by the solvent action of the sea, rein- 

 forced by submarine erosion. This has gradually changed the wide reef 

 flats, forming at first fringing reefs of great breadth on the northern and 

 southern faces of the island, into such bays as those of Papiete and its 

 eastern and western extensions, and into the lagoons extending from 

 Teavaraa Pass to Phaeton Harbor, and on the southwest side of Taiarapu 

 Peninsula. The amount of silt which the numei'ous rivers of Tahiti 

 bring down into the lagoon of the barrier reefs is very considerable ; we 

 find the bottom of the lagoons covered in great part with volcanic mud 

 and silt, in the central part of the lagoon and in the neighborhood of the 

 shore, but it is mixed more or less with coral sand as we go toward the 

 outer face of the reef flat near the barrier reef. 



On the northern and southern side of the island, as well as the western 

 face where the trades do not strike with any great violence, the reef flats are 

 broad (Pis. 202 ; 208, fig. 3). But on the eastern side, and also off the 

 southeastern point, where the action of the trades is incessant, the reef 

 fiats have been greatly eroded, leaving a wide, deep lagoon, separating the 

 barrier reefs from the main shore. Finally, on the northeastern side, 

 between Artemise Bank and Point Venus, the barrier reef has become 

 broken into isolated and disconnected reef patches, all pointing to the 

 existence of a former wide reef platform (Pis. 208, fig. 2 ; 209). The outer 

 edge of the platform is only indicated now by bars like those off the 

 southern point of the peninsula, covered with from two to four fathoms 

 of water and here and there passes, with from four to twelve fathoms, in- 

 dicating the position of former breaks in the fringing reefs, similar to those 

 still found south of Boudeuse Pass, and as far as the southeastern extremity 

 of the Taiarapu Peninsula. The barrier reefs of the east side are well 

 charted on the B. A. Chart 1382. 



The shore platform of the east face of Tahiti consists mainly of volcanic 

 heads and ledges, outliers standing at a certain distance off shore (Pis. 86 ; 

 208, fig. 2), on which comparatively little coral grows, judging from the 

 nature of the sand to be found on the beaches of the main island, inside 



