1836.J DEAD CORAL-GROVES. 453 



forest of delicately branching corals, which, though stand- 

 ing 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 explana- 

 tion, 1 came to the conclusion that it must have come 

 entangled in the roots of some large tree : when, how- 

 ever, 1 considered the great distance from the nearest land 

 the combination of chances against a stone thus beinr 

 entangled, the tree washed into the sea, floated so far, 

 then landed safely, and the stone finally so embedded as to 

 allow of its discovery, I was almost .ifraid of imagining 

 a means of transport apparently so improbable. It was 



