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Proceedings of the Boyal Society 
heights varying from a few feet to several hundred feet above the 
sea, which I have already treated in a previous paper. 
2. That the numerous detached submerged coral shoals in these 
seas, which represent in fact the early condition of a coral reef, are 
not able, without the aid of a movement of elevation, to raise them- 
selves to within the constructive power of the breakers. Being 
arrested in their upward growth at depths varying between 5 and 
10 fathoms, according to the exposed or protected character of their 
situation, they form flat shoals of no great size. 
3. That atolls of small size, i.e., a mile or so across, do not assume 
their characteristic form until they have reached the surface. A 
small flat-topped shoal is first brought by upheaval to or above 
the sea-level. Lateral extensions or wings grow out on either side, 
so as to ultimately form a horse-shoe reef. Such a reef presents its 
convexity against the prevailing surface-currents, to which in truth 
it owes its shape. The southern island of the Three Sisters was 
evidently produced in this manner (vide p. 16). In some atolls, 
as in the instance of Oima Atoll (p. 21), one or more islets of 
volcanic rock have served as points from which the reef begins to 
grow, guided in its direction by the prevailing surface-currents. 
The foregoing remarks do not refer to large atolls. On account of 
their large extent they would probably have assumed their form 
beneath the surface, since, according to the principle laid down by 
Mr Murray, they would then have a relatively smaller periphery for 
the supply of food and sediment to the interior than would he 
possessed by the small submerged shoals above described.* 
4. That the characters of the seaward slope of a reef are as 
follows : — For the first 70 or 80 yards from the margin of the reef- 
flat there is usually a gradual slope, largely bare of living coral, 
which terminates at a depth of from 4 to 5 fathoms in a rapid 
descent to a depth varying generally between 12 and 18 fathoms. 
It is this declivity that constitutes the growing edge of the reef, 
and the sand and gravel, produced by the constant action of the 
breakers, collect at its base. 
5. That where there is a rapid submarine slope, i.e., more than 
10° or 12°, as is usually the case, the sand and gravel, produced in 
the manner just referred to, will extend far beyond the depths in 
* Proc. Roy. Soc. Ectin vol. x. p. 505. 
