Sect. III. REEF-BUILDING CORALS LIVE. 
113 
six fathoms. The pearl-fishers at Yemen and Massana 
asserted that there was no coral near the pearl-banks at 
nine fathoms depth, but only sand. We were not able 
tc institute any more special researches.’ 1 I am, how- 
ever, assured both by Captain Moresby and Lieut. Well- 
stead, that in the more northern parts of the Red Sea, 
there are extensive beds of living coral at a depth of 25 
fathoms, in which the anchors of their vessels were 
frequently entangled. Captain Moresby attributes the 
less depth at which the corals are able to live in the 
places mentioned by Ehrenberg, to the greater quantity 
of sediment there ; the situations, where they were 
flourishing at the depth of 25 fathoms, were protected, 
and the water was extraordinarily limpid. On the 
leeward side of Mauritius, where I found the coral 
growing at a somewhat greater depth than at Keeling 
atoll, the sea, owing apparently to its tranquil state, 
was likewise very clear. Within the lagoons of some 
of the Marshall atolls, where the water can be but little 
agitated, there are, according to Kotzebue, living beds 
of coral in 25 fathoms. From these several facts, and 
considering the manner in which the beds of clean coral 
off Mauritius, Keeling Island, the Maldiva and Chagos 
atolls, graduated into a sandy slope, it appears very 
probable that the depth at which reef-building poly- 
pifers can exist, is partly determined by the extent of 
inclined surface which the currents of the sea and 
the recoiling waves have the power to keep free from 
sediment. 
1 Ehrenberg, Ueber die Natur, &c p. 50. 
