902 
Proceedings of the Royal Society 
may cite the instances of the great Australian barrier-reef and 
of the barrier-reef of Bougainville Strait described in this 
paper (page 20). Such plateaux have a very gradual slope, and, 
provided that their outer margins are within the limits of the reef- 
coral zone, they would afford the most favourable conditions for 
reef -growth. 
This explanation of the origin of barrier-reefs in no way affects 
the views at present held by Mr Murray, Professor A. Agassiz, 
Professor Semper, and others, concerning the formation of atolls. 
Atolls of small size may be produced in the manner referred to in 
paragraph three on page 41. Those of large size would assume their 
form, whilst still submerged, on account of the condition of the 
food-supply favouring the growth of coral at the circumference. 
When such a reef has reached the limits of breaker-action, the sand 
and detritus accumulating in its centre would repress in a greater 
degree the growth of the coral ; and finally, after the reef has reached 
the surface, the lagoon would be further deepened by the solvent 
action of sea-water and by the organic degradation of the dead 
coral.* 
Nor is my view of the origin of barrier-reefs inconsistent with 
that held by Mr Murray and other naturalists. The chief point of 
difference is that I do not consider that the agencies of solution, 
diminished food-supply, organic degradation, and tidal scour, are 
the determining causes of the formation of the lagoon-channel, but 
that they are auxiliary causes which come into play after the reef 
has begun to grow at that distance from the shore where the suitable 
conditions for reef-growth exist. 
On the formation of fringing-reefs, my observations throw no 
additional light. These reefs, when occurring alone, often characterise 
steep submarine slopes, but they may accompany barrier-reefs on 
coasts where the slope is more gradual. 
8. The statement that lagoons and lagoon-channels are some- 
times deeper than the zone in which reef-corals thrive, is founded 
* Mr H. 0. Forbes, in his recent account of Keeling Atoll, refers to the 
welling up of dark sulphureous water in the lagoon, by which the corals, 
molluscs, fish, and other organisms were killed over a large area of the basin 
(A Naturalist’s Wanderings in the Eastern Archipelago, 1885, p. 22). Such 
an agency can scarcely be exceptional, and probably takes a part in the forma- 
tion of lagoons. 
