TAHITI. 145 



of the barrier reef line. On some portions of the beach this sand is for 

 the greater part purely volcanic ; on others it is mixed with a little coral 

 .sand, where the platform is wider and there are islets left on it. From the 

 amount of coral sand thrown up on the beach, it is evident that corals 

 flourish on these wide flat.*, not only on the sea face of the shore platform, 

 but also as extensive patches inside the barrier reef and as fringing reefs. 

 The volcanic ledges and rocks of the outer barrier are in part covered 

 by coral boulders thrown up from the sea face as well as from the ledges 

 between them and the shore ; on the slopes of the ledges and in the channels 

 the characteristic corals of the Society Islands flourish. 



The meagre development of the corals on the east face may be due in 

 part to the great amount of detritus brought down the eastern face of the 

 island by the numerous rivers and streams which have furrowed the whole 

 island of Tahiti and cut extensive gorges penetrating to the very centre of 

 the island. Numerous waterfalls can be seen on the eastern slopes of 

 the mountains beginning the work of cutting back into the island. The 

 photographs of Plates 85, 86, 88, 89 ; 202, fig. 1, will give some idea of the 

 extent of the denudation and erosion which have taken place at Tahiti, not 

 only on the east side, but on all the faces of the island, according to their 

 exposure to the prevailing winds and rains. 



An examination of the chart of Tahiti ^ shows conclusively the effect of 

 fresh water streams, with the accompanying silt they bring down in the 

 rainy season, in influencing the growth of corals. The passes in the barrier 

 reef are without exception opposite or near the mouth of a river or of a 

 torrent (PI. 208, figs. 1-5). Aiurua River is opposite Aiurua Pass, Vaitoto 

 River and Vaionifa Pass, Vaitia River and Cook Anchorage, Haavini 

 Valley and Toharoa Pass, Mapuaura River and Faone Pass, Mataea River 

 and Boudeuse Pass, are all rivers opposite passes on the neighboring 

 barrier reefs ; all these gaps are on the weather side of the island. The 

 wide fringing reef flats of the lee side are along reaches of coast without 

 streams or small rivers ; where wide lagoons separate the barrier reef flats 

 from the shore, they are watered by more abundant streams, and the passes 



1 A. Chart 1382. 

 10 



