CORAL-REEFS. 163 



Turning now to the red colour ; as on our map, the areas 

 which have sunk slowly downwards to great depths are 

 many and large, we might naturally have been led to con- 

 jecture, that with such great changes of level in progress, the 

 coasts which have been fringed probably for ages (for we have 

 no reason to believe that coral-reefs are of short duration), 



having taken place ; and I think Mr. Couthouy has not borne in mind 

 the indisputable fact, that corals, when constantly bathed by the surf, 

 can exist at a higher level than in quite tranquil water, as in a lagoon. 

 As long, therefore, as the waves continued at low water to break 

 entirely over parts of the annular reef of an atoll, submerged to a small 

 depth, the corals and shells attached on these parts might continue 

 living at a level above the smooth surface of the lagoon, into which the 

 waves rolled ; but as soon as the outer edge of the reef grew up to its 

 utmost possible height, or if the reef were very broad nearly to that 

 height, the force of the breakers would be checked, and the corals and 

 shells on the inner parts near the lagoon would occasionally be left 

 dry, and thus be partially or wholly destroyed. Even in atolls, which 

 have not lately subsided, if the outer margin of the reef continued to 

 increase in breadth seaward (each fresh zone of corals rising to 

 the same vertical height as at Keeling atoll), the line where the 

 waves broke most heavily would advance outwards, and therefore 

 the corals, which when living near the margin, were washed by 

 the breaking waves during the whole of each tide, would cease being 

 so, and would therefore be left on the backward part of the reef stand- 

 ing exposed and dead. The case of the madrepores in the lagoons 

 with the tops of their branches exposed, seems to be an analogous fact, 

 to the great fields of dead but upright corals in the lagoon of Keeling 

 atoll; — a condition of things which I have endeavoured to show, has 

 resulted from thp lagoon having become more and more enclosed and 

 choked up with reefs, so that during high winds, the rising of the tide 

 (as observed by the inhabitants) is checked, and the corals, which had 

 formerly grown to the greatest possible height, are occasionally exposed, 

 and thus are killed : and this is a condition of things, towards which 

 almost every atoll in the intervals of its subsidence must be tending. 

 Or if we look to the state of an atoll directly after a subsidence of some 

 fathoms, the waves would roll heavily over the entire circumference of 

 the reef, and the surface of the lagoon would, like the ocean, never be 



