DEPARTMENT OF MARINE BIOLOGY. 185 



Tahiti Nui is a much eroded, complicated cone with a deep egg-shaped 

 valley occupying its very center and opening out towards the north. From 

 the interrupted curved ridge surrounding the inner portion of this valley 

 (Papenu Valley) arise a number of the higher peaks, and ridges descend 

 thence to the coast. Between and among these ridges are gullies and deeper 

 valleys, each eroded by the stream which it incloses. Around the base of 

 the cone is a broader or narrower flat shelf, or coastal plain, which (alone) 

 is inhabited and cultivated. Taiarapu shows a single slightly curved ridge 

 running lengthwise of the peninsula and somewhat nearer the southwest coast, 

 from which a few ridges run out almost at right angles. Our reconnaisance 

 was almost entirely confined to Tahiti Nui and to the lower slopes (up to 

 about 3,600 feet). The highest peak is Orohena, 7,321 feet, but Aorai, 6,773 

 feet, shares its prominence. There are to be distinguished three belts, the 

 strand belt (coastal plain or "zone madreporique"), the slopes up to about 

 5,500 feet, and the extreme summits of Aorai and Orohena. Each of these 

 belts presents a series of distinct formations. 



The strand belt is fiat and not much over two-thirds of a mile wide at any 

 point, usually not much over 100 yards in width, and at various points is 

 lacking altogether. It is evidently of alluvial origin, sand, pebbles, and 

 humus in various admixtures, probably about 18 feet thick or somewhat 

 over, but investigation as to details is needed. In the great valley of the 

 Papenu River there is a sort of extension of it into the interior of the island, 

 but elsewhere it is strictly coastal. The soil is shallow and overlies rock, 

 either a lava bench or more commonly beach-rock of a buried fringing reef 

 at about the level of the sea. The strand trees are those usual in western 

 Polynesia, but some of these are now very scarce. The cultural plants, such 

 as the coconut, sugar-cane, banana, coffee, and vanilla, are largely confined to 

 this belt. The Chinese vegetable gardens are here, largely on drained swampy 

 lands. The villages are all on this belt, although in earlier times some fair 

 proportion of the population occupied the valley slope up to 2,000 feet altitude. 

 The only conspicuous endemic species on this belt is the Tahitian banyan 

 {Ficus prolixa Forst.), but this whole belt is now overrun with tropical weeds, 

 many of them of comparatively recent introduction. Of these, the members 

 of the Leguminosse are by far most frequent, but several Convolvulacese and 

 a few Compositae and Labiatse occur. The principal formations are cultural, 

 strand, swamp, and thicket, but there are no mangrove associations on Tahiti. 

 On its inner edges this belt meets the cliffs and steep slopes of the older shore- 

 line which mark the beginning of the lower slope belt. 



The lower slope belt is varied into ridge and valley formations of various 

 kinds. The slope formations are fairly uniform and cover poor, shallow, dry 

 soil, formed in position from decomposed soft volcanic rock. The character- 

 istic association is fern thicket, composed largely of Dicranopteris linearis, 

 which covers large areas from base to upper limits. This is mixed with 

 shrubs, or occasionally trees, of Dodonoea viscosa and Metrosideros collinum, 

 but more recently invaded by lantana, guava, and Tecoma stans. The char- 

 acteristic ridges and exposed slopes from base to the upper slope belt are 

 often covered with dense and almost impenetrable thickets of these species. 

 Among the fern thickets, especially, higher up, are found scattered, and often 

 extensive, patches of the "native sugar-cane." 



