Sect. II. 
RATE OF GROWTH. 
95 
jecture that the corals on the submerged knolls within 
the Chagos atolls have analogous habits with those of 
the lower zone outside Keeling atoll, receives some sup- 
port from a remark by Captain Moresby, namely, that 
they have a different appearance from those on the reefs 
in the Maldiva atolls, which, as we have seen, all rise to 
the surface : he compares the kind of difference to that 
of the vegetation under different climates. I have 
entered at considerable length into this case, although 
unable to throw much light on it, in order to show that 
coral-reefs situated in different places or at different 
depths, whether forming the ring of an atoll or the 
knolls within a lagoon, need not all be supposed to 
have an equal tendency to upward growth. The infer- 
ence, therefore, that one reef could not grow to the 
surface within a given time, because another, not 
known to be covered with the same species of corals, 
and not known to be placed under exactly the same 
conditions, has not within the same time reached the 
surface, is unsound. 
Section II. 
On the Rate of Growth of Coral-reefs. 
The remark made at the close of the last section, 
naturally leads to this division of our subject, which 
has not, I think, hitherto been considered under a 
right point of view. Ehrenberg 1 has stated that in 
the Ked Sea, the corals only coat other rocks in a 
1 Ehrenberg, as before cited, pp. 39, 46, and 50. 
