OTHER CORAL STRUCTURES 279 



about any land whose submarine slope gives this necessary 

 foundation. 



In this way a fringing reef will be formed, and it will be able 

 to extend seawards as far as its submarine base stretches within 

 the required limits, and after its land base is exhausted, upon a 

 bank of its own talus. But, near the shores of the land that 

 it fringes, the water will commonly carry a heavy load of sedi- 

 ment, which has been washed from the land by wave action ; 

 and so a near-shore limit will be set upon the growth of the 

 coral colonies. This is the most simple explanation of the 

 formation of barrier reefs, and since it has been agreed upon 

 by two investigators, who did not advance the action of 

 " sedimentation " as a factor in causing atoll formation, it 

 gains an added weight. The agency that causes barrier-reef 

 formation would seem to be almost certainly the same as that 

 which caused the kindred structure of the atoll, and Darwin re- 

 cognised this fact, for it would seem absurd that two separate 

 conditions should be involved to account for two such similar 

 structures. At the least, it may be said that it would appear 

 illogical to imagine barrier reefs to be formed by •' sedimen- 

 tation," and the nearly allied atolls to be formed by the directly 

 opposed action of " solution." 



I therefore imagine that the workings of " sedimentation " 

 upon the living coral colony, and upon the living coral reef, 

 will account for all the different coral structures that are met 

 with. 



The famous theory of "subsidence" also did this, and it 

 did it in a most complete and masterly way ; and it is natural 

 to inquire if in a fully developed atoll such as Cocos-Keeling 

 there is any sure sign that subsidence has not taken place or 

 caused its features. It has been said by the supporters of this 

 theory, that subsidence, like a conflagration, destroys its own 

 traces ; but in reality it is not so. We have seen, in the 

 course of the development of the atoll, that the first formation, 

 after the reef has come to the surface, is the broad flat layer 

 of the barrier. It has also been insisted, in the description of 

 the barrier, that the breccia stratum is an unbroken layer from 



