182 CARNEGIE INSTITUTION OF WASHINGTON. 



both above and below sea-level, has produced a bold and rugged coast, with 

 nearly vertical cliffs, blow-holes, jagged rocks, etc. 



The reefs practically surrounding the island may be distinguished into 

 (1) submerged reefs or banks, (2) barrier reefs, (3) fringing reefs. In addition, 

 there are isolated reef patches of larger or smaller size. The banks or sub- 

 merged reefs occur off the coast, separated from it by a lagoon-like depression, 

 and simulate true barrier reefs, except that they do not reach to tide-level. 

 Their surface is 2 to 1 1 fathoms below it. It may be supposed that if undis- 

 turbed they may ultimately reach the surface and become true barrier reefs. 

 They are to be found at the northeastern and the southeastern extremities of 

 Tahiti and to a less degree on the northeastern coast of the Afaahite district. 

 Their relation to full force of trade winds and consequently heavy seas is prob- 

 ably worthy of notice. Unfortunately, nothing could be determined as to 

 their composition. They are probably largely nullipores (lithothamnia), but 

 corals may enter more or less into their structure. 



Barrier reefs, rising to the surface and having their rims awash at low-tide 

 intervals, encircle the coast, except at the portions occupied by the banks 

 or where they are separated by the passes. Unusually they are not over 0.25 

 mile, but in some cases, or portions of a reef, they may be 0.5 mile in width or 

 a little over. The upper surface of the reef usually shows five more or less 

 distinct belts or formations: (1) the outer nullipore belt, 250 to 300 feet 

 across; (2) the Sargassum belt, 30 or many more feet across; (3) the Tur- 

 binaria belt, 15 or many more feet across; (4) the inner nullipore belt, 15 or 

 more feet across; and (5) the innermost belt or coral rubble, coral mud, shells, 

 etc., on which may grow a scattered assortment of living coral clumps, 

 remotely or fairly closely placed. 



The seaward edge of the reef descends in a steep slope to 18 to 25 fathoms, 

 after which the descent is even steeper. Unfortunately, the waves pile in 

 on the outer edge of the barrier reef, so that I know nothing of its structure. 

 On the inner edge the reef descends almost vertically to 5 or more fathoms 

 and then in a steep slope to 18 to 25 fathoms, the depth of the lagoon moat. 

 This slope I have been able to study with the glass-bottomed boat of Messrs. 

 R. I. and E. Walker of Papeete, and have observed that it is bare, with only an 

 occasional clump of Porites or other massive coral. The lagoon moat varies 

 from not much over 100 yards to a quarter of a mile in width and expands in 

 places to 0.5 to 0.75 mile. The depth, as mentioned above, uniformly ap- 

 proximates 18 to 20 fathoms. 



Fringing reefs present so many variations that it is necessary to classify 

 them and to distinguish several types. Those on exposed shores are quite 

 distinct from those on protected shores within the lagoon, and the former 

 also vary somewhat among themselves in detail. I find it most satisfactory 

 to discuss several of the exposed fringing reefs as examples of the variation 

 existing among them. 



The fringing reefs of exposed coasts selected for observation divide them- 

 selves naturally into two categories: (1) typical fringing reefs of small extent, 

 not connected with barrier reefs in any way and lying within banks, or sub- 

 merged reefs, farther out and separated from them by a deep depression com- 

 parable to a lagoon moat, and (2) combination fringing and barrier types of 

 reef, but not separated from the land by a lagoon moat. Of the former cate- 



