of Edinburgh, Session 1885-86. 
885 
If we imagine an island, originally formed from the materials 
ejected from some volcanic vent and bare of coral-reefs, to afford, 
after the extinction of the subterranean fires, the conditions of 
growth on its coasts for reef-building corals, a fringing reef of 
varying width according to the degree of inclination of the sub- 
marine slope will ultimately invest its shores. In course of time, 
the detritus of the corals will collect in a band of calcareous sand 
and gravel on the outer slope of the reef, marking the apparent 
limit of the depths in which the reef-corals are usually stated to 
thrive. But the vertical and horizontal extension of such a band 
of detritus will be mainly determined, as my observations have 
shown, by the presence and position of submarine declivities and 
by the degree of inclination of the slope. In such a zone of sand 
and gravel corals will not thrive ; but if the submarine slope has a 
very gradual inclination, as in the case of the barrier-reef of 
Choiseul Bay, the lower limit of this zone of detritus may lie 
within the depths in which reef-building corals flourish, and a line 
of barrier-reef begin lying parallel with the fringing reef, but 
separated by a deep channel.* 
On the other hand, should the submarine slope have a more 
rapid descent, the lower limit of the belt of detritus may extend 
far beyond the depths in which reef-corals can thrive ; in such 
a case no barrier-reef will form, and the original fringing reef 
will continue to grow outwards on its own talus. On this view 
the occurrence of barrier-reefs and of fringing-reefs on different 
parts of the coast of the same island may be readily explained 
as due to the different degrees of inclination of the submarine 
slope. 
pointed out that the explanation of the circumstance that the Florida 
peninsula had been formed by a succession of barrier-reefs, instead of by a 
continuous fringing-reef, was to be found in the fact that, since corals will not 
grow on muddy shores or in water upon the bottom of which sediment is 
collected, the favourable conditions can only be obtained some distance from 
the shore. There, as he remarked, a barrier-reef would be formed, limited on 
one side by the muddiness, and on the other by the depth of the water ( vide 
American Journal of Science, 2nd series, vol. xxiii. p. 46, and Nature, 
October 14, 1880). This view seems to have attracted scarcely any attention 
since it was first proposed ; but the circumstance that I arrived independently 
at the same conclusion with reference to the barrier-reefs of the Solomon Group 
is one that lends it very powerful support. 
* Such a reef covered by 5 fathoms of water lies south of Choiseul Bay. 
