486 CHARLES DARWIN 



ably more than a mile square, covered with a forest of deli- 

 cately 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 de- 

 termined 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 ob- 

 served, 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 

 has 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. Cap- 

 tain 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 ab- 

 sence of any better explanation, I came to the conclusion that 

 it must have come entangled in the roots of some large tree : 

 when, however, I considered the great distance from the 

 nearest land, the combination of chances against a stone thus 

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

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



