184 CARNEGIE INSTITUTION OF WASHINGTON. 



reef in its structure, with outer nullipore belt, Sargassum belt, and Turbinaria 

 belt well developed and sloping gently outwards, but without overhang. 

 This barrier portion of the reef is of coral and nullipore structure and is 

 raised 2 feet or a little more above the reef flat within. The reef flat within is 

 beach-rock, nullipore, sand, coral, and cobble structure, and extends inward 

 to the shore with only a narrow, shallow channel running parallel to the shore- 

 line. The edge of the inner reef towards the pass (south) is typical fringing 

 reef, overhanging nullipore belt, and Sargassum and Turbinaria belts within 

 that. The shore of the pass is of large cobbles pitched at a high angle and rolled 

 about by the strong waves. The shore along the reef is of coarse sand. It 

 may be surmised that the sand and cobble debris, cemented by the nullipore, 

 has built up (and outward) the fringing reef to the point of fusing with the 

 outer, probably originally distinct, barrier reef. Such fused reefs are most 

 interesting in the study of the origin of both barrier and fringing reefs. 



As an example of the inner or non-exposed fringing reef, bordering the 

 lagoon moat, such as is general, the reef in the neighborhood of Auae, just 

 west of Papeete, was chosen. The steep outer slopes were examined by aid 

 of the glass-bottomed boat of Messrs. R. I. and E. Walker, while the surfaces, 

 being covered with shallow water, were studied by wading over them. The 

 outer face descends vertically for 6 to 9 fathoms and then slopes abruptly to 

 from 12 to 18 fathoms. The surface of the reef is practically entirely of coral and 

 the slope of coral mud. The reef flat is made up of coral mud nearer shore, then 

 of coral rock and reef rock consolidated by thin crustaceous nullipores, but 

 towards the outer edge shows living growth of various corals often densely 

 aggregated, whose dying tips are also overgrown by a thin crust of nullipore. 



Within the barrier reef, rising from the bottom of the lagoon, are larger or 

 smaller reef patches, isolated, and with vertical walls. Their structure is 

 very similar to that of the outer portion of the protected fringing reefs. The 

 glass-bottomed boat allowed a very satisfactory survey of the fine collection 

 of reef patches in Papeete Harbor. 



The outer reefs, both barrier and bank, are separated from one another by 

 passes, which bear a definite relation to the rivers, both in position and origin. 

 The broader and more open passes are at the mouths of larger rivers, especially 

 of those carrying coarser rock materials when in flood. Tracing down through 

 the larger passes, a distinct channel may be demonstrated leading out to and 

 somewhat beyond the reef edge, a channel probably cut through the older 

 wave-bench (or fringing reef?) when the sea stood at a lower level (about 20 

 to 30 fathoms) than it does at present and kept open by the erosive action 

 of the debris since that time. Even when larger, but less erosive, rivers have 

 their mouths inclosed by reefs, there may be indentations indicating the 

 existence of a former pass, now more or less closed. Typical examples of 

 such indentations of the barrier reef may be seen opposite the Oahu and 

 Tereehia Rivers in the Papara district, an almost closed pass opposite the 

 river emptying by Patere in the same district, and many others. Of the 

 passes between the banks, the Papenu Pass opposite the river of that name 

 is an excellent example. 



A study of the passes, therefore, seems to indicate that they are kept open 

 by erosion caused by the river debris (cobbles and sand) and, although many 

 are now closed, that they exist along depressions caused by the same agencies 

 when the sea-level was much lower than at present. 



