TAHITI. 149 



Mount Diademe rises to the height of nearly forty-four hundred feet, well 

 towards the central part of Tahiti ; it seems to cut off abruptly the ex- 

 tremity of Fautaua Valley (Pis. 88, fig. 1 ; 89, fig. 1). 



The head waters of the Papenu rise from the great central amphitheatre 

 formed by the peaks of Marau, Tetufera, and Orohena ; from its southern 

 and eastern slopes arise the rivers and torrents of the corresponding faces of 

 Tahiti ; from the western faces of Marau, Diademe, Aorai, Orohena, Taiti, 

 Mahutaa, and Ivirairai rise the rivers which find their way to the sea on 

 the western and northwestern faces of Tahiti. The silt brought down by 

 these rivers has formed the wide flats which extend to the west of Papiete, 

 and form the northern coast shelf towards Faa. 



A similar shelf has been formed at Tautira Point, on the north shore of the 

 Taiarapu Peninsula. It is a low tongue of land which extends to the north- 

 ward for nearly a mile from the foot of the mountain, and has been formed 

 by the deposits brought down by the Vaitia River (PI. 208, fig. 1). It is the 

 only iremnant left of the reef platform, which once existed on the north 

 coast of the Taiarapu Peninsula, on each side of Tautira. The great depth 

 of some parts of the lagoon off the eastern and northern faces of the pe- 

 ninsula, is undoubtedly due to original orogenic conditions. The channel to 

 Cook anchorage, off Tautira Point, was probably a deep valley, which directed 

 the river Vitia in its northerly course (PI. 208, fig. 1). Although the north- 

 east trades have eroded a wide platform on the east side of Tahiti, leav- 

 ing here and there only a narrow belt of barrier reef on the outer edge 

 of the reef platform, yet the growth of corals on the outer reef face is 

 sufficiently luxuriant to build up, from the dead material thrown up on this 

 reef flat, an occasional island or islet (PI. 86, fig. 1). These islets are a 

 characteristic feature of the reefs of the east face of Tahiti ; they resemble, 

 only on a more limited scale, the coral islets we observed in other islands of 

 the Society group, such as Bora Bora, Raiatea, and others, where the outer 

 edge of the reef platform is flanked by an almost continuous line of islands 

 and islets composed of coral sand. The islands have been thrown up on a 

 sub-basis of volcanic rock, and here and there an outlier of the volcanic rock 

 can still be traced, giving us a clue to the nature of the underlying platform, 

 and plainly showing that it is only the extension of spurs of the mainland 



