490 
KEELING ISLAND 
CHAP. 
stayed a long time in the lagoon, examining the fields of coral 
and the gigantic shells of the chama, into which if a man were 
to put his hand, he would not, as long as the animal lived, be 
able to withdraw it. Near the head of the lagoon, I was much 
surprised to find a wide area, considerably more than a mile 
square, covered with a forest of delicately branching corals, 
which, though standing upright, were all dead and rotten. At 
first I was quite at a loss to understand the cause ; afterwards 
it occurred to me that it was owing to the following rather 
curious combination of circumstances. It should, however, first 
be stated, that corals are not able to survive even a short 
exposure in the air to the sun’s rays, so that their upward limit 
of growth is determined by that of lowest water at spring tides. 
It appears, from some old charts, that the long island to 
windward was formerly separated by wide channels into several 
islets ; this fact is likewise indicated by the trees being younger 
on these portions. Under the former condition of the reef, a 
strong breeze, by throwing more water over the barrier, would 
tend to raise the level of the lagoon. Now it acts in a directly 
contrary manner ; for the water within the lagoon not only is 
not increased by currents from the outside, but is itself blown 
outwards by the force of the wind. Hence it is observed that 
the tide near the head of the lagoon does not rise so high 
during a strong breeze as it does when it is calm. This 
difference of level, although no doubt very small, has, I believe, 
caused the death of those coral-groves, which under the former 
and more open condition of the outer reef had attained the 
utmost possible limit of upward growth. 
A few miles north of Keeling there is another small atoll, 
the lagoon of which is nearly filled up with coral-mud. 
Captain Ross found embedded in the conglomerate on the 
outer coast a well-rounded fragment of greenstone, rather 
larger than a man’s head ; he and the men with him were so 
much surprised at this, that they brought it away and preserved it 
as a curiosity. The occurrence of this one stone, where every 
other particle of matter is calcareous, certainly is very puzzling. 
The island has scarcely ever been visited, nor is it probable 
that a ship had been wrecked there. From the absence of any 
better explanation, I came to the conclusion that it must have 
come entangled in the roots of some large tree : when, how- 
