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Proceedings of the Royal Society 
Moresby and Lieutenant Wellstead.* This great variation is, as I 
hold, and as Mr Darwin also held, to be attributed to the difference 
in the local conditions in the various localities in which the observa- 
tions were made. In the case of the Red Sea, Captain Moresby 
himself attributed the less depth at which Ehrenberg found living 
corals to the great amount of sediment. In the majority of 
instances, the observers were led to consider that in passing from 
the living corals to the sand beyond they had necessarily passed the 
lower limit of the reef-coral zone ; but I have already shown that 
if the submarine slope is gradual, living coral may be found in the 
depths beyond the sand. If, however, as is usually the case, the 
slope is somewhat steep, the sand will extend to depths far beyond 
the zone of living coral. It was the limited depth at which reef- 
corals thrive in the Florida seas as compared with depths of the 
zone in other regions that led Professor A. Agassiz to infer that the 
vertical distribution of reef-corals is determined by local causes 
rather than by the general influence of depth. “There seems 
to be,” thus he goes on to remark, “ no simpler explanation of 
the limited bathymetrical range than that of the baneful action of 
the silt held in suspense near all reefs ”t The local causes that 
confine the reef-building corals to such shallow depths in the 
Florida seas are to be found, as Professor Agassiz implies, in the 
unusually large quantities of calcareous sediment contained in the 
water in the vicinity of the reefs and in the gradual character of the 
submarine slope of the rapidly advancing Florida Bank, the muddy 
surface of which is being constantly disturbed by the action of the 
waves and of the tides. 
From the foregoing remarks we may with confidence infer that 
the lower limit of the reef-coral zone is determined rather by local 
conditions than by the general influence of depth. If this be granted, 
there can be but little hesitation in allowing that under favourable 
circumstances, such as a moderate submarine slope and unusually clear 
water, reef-corals may commence to build in depths beyond those 
generally assigned, as for instance, in those of 50 and 60 fathoms. 
We have here, then, a ready explanation of the exceptional depths 
* Dana’s Corals and Coral Islands (1872), p. 115; Darwin’s Coral Beef 
(1842), p. 83. 
+ Memoirs, American Academy of Arts and Sciences , vol. xi. part ii. No. 1, 
1885. 
