TAHITI. 151 



the detriment of the growing corals. These dead fragments, as well as those 

 on the bare reef fiats, were covered and cemented with masses of Nullipores. 



The lagoon slope of the reef flat is much less steep than the sea face (PI. 

 209), and on this slope are stretches of large masses and patches of living 

 corals, extending also on the steep slopes of the shoals or islets along the 

 lagoon. The larger masses are usually species of Astraeans and Porites ; 

 towards the inner part of the lagoon and the harbor, the bottom is covered 

 with fine mud, this becomes gradually composed of larger quantities of coral 

 sand towards the entrance and in the passages across the barrier reef. 



I cannot well reconcile the condition of the outer reef of Papiete, as we 

 saw it, with the account given in the Narrative ^ of the " Challenger," which 

 describes the face of the reef as full of growing corals ; or with the state- 

 ment of the presence of very large blocks on the slope, and of the presence 

 of growing corals to a depth of thirty-five fathoms. 



A great part of the reef flat is bare at low water, and exposed to the heat 

 of the sun during a considerable portion of the day. This evidently affects 

 the growing corals, and may account for the unusual number of large masses 

 which are washed down from the flats both on the sea face and lagoon side 

 of the reef. Owing to the exceptionally clear days and smooth sea on 

 which we examined the reef, we could see the condition of the slope of 

 the sea face with the naked eye down to nearly sixteen fathoms, and with 

 the water glass the most minute details could be traced to eighteen fathoms; 

 down to twenty-two we could see indistinctly that the dead corals were 

 replaced almost entirely by coral sand, only here and there a spur of coral 

 shingle extending into the sandy region. There were comparatively few 

 gullies and cuts along the face of the reef, the slope being unusually uni- 

 form and unbroken by crevasses or caverns or fissures. Nine to ten fathoms 

 represented the average depth to which thriving corals extended ; beyond 

 that they were habitually small, and grew only in limited areas between the 

 greater stretches of fragments of dead corals. 



On the crest of the narrow barrier reef flat AlgoB and coralline Algse, as 

 well as the cementing Nullipores, grow in great abundance ; the last are 

 specially developed on the lagoon slope, covering the whole slope with a 



'■ Challenger Narrative of Voyage, Vol. I. Part II p. 777. 



