Ch. VI. DISTRIBUTION OF CORAL-REEFS. 
171 
the shores and in the midst of the lagoons, from 12 to 
30 inches above the sea-level, with the tips of their 
branches dead. He also refers to masses of coral-rock 
which he thinks could not have been carried into their 
present positions and subsequently been w r ater-worn, 
whilst the land stood at its present level. Nevertheless 
it might, I think, have been anticipated that many 
atolls would have presented the above appearance, if 
they had long remained at a stationary level. The sea, 
after the land had at some former period subsided a few 
feet, would have continued for a long time breaking 
over the whole reef, even after the living corals had 
grown up to their full height on the outer margin. The 
waters of the lagoon would thus have been disturbed and 
raised, so that shells and corals, from being bathed by 
the troubled waters, could have existed at a greater 
height than that at which they could exist after the 
reef had been raised by the agglutination of fragments 
and sand, and after islets had been formed on its sur- 
face. Even the mere outward growth of a reef, and the 
consequent increase of its breadth, by checking the 
inward rush of the breakers, would tend to lower the 
level in the lagoon at which corals and shells can live. 
We have seen that at the Keeling Islands there are 
fields of rotten coral with the tips of their branches pro- 
jecting above the surface of the lagoon, — the result of 
the tides not rising so high as formerly (as is said to be 
the case by the inhabitants), from the closing of the 
channels between the islets on the outer reef, and from 
the lagoon being partially choked up by the growth 
