ORGANIC AGENCIES. 105 



fectly. The corals do not build up perpendicularly,, but 

 in a steep slope. The barrier, and much more the atoll, 

 is therefore smaller than the original fringe. If, there- 

 fore, the subsidence continues, the atoll will grow smaller 

 and smaller, the separate islets will close together, join 

 each other, and finally close the lagoon. Then the lagoon 

 will close in upon itself and form the lagoonless island, 

 and, last of all, this also will probably disappear. 



As corals grow best on the outside of the reef, they 

 will not occupy the channel formed by recession of the 

 volcanic island ; or, if they do, they are soon drowned out 

 by subsidence. The channel, however, in case of barriers, 

 or lagoon in case of atolls, will be partly filled by debris 

 carried into it in both cases from the reef, and in the case 

 of barriers also from the volcanic island. Fig. 59 is an 



FIG. 59. Ideal section diagram showing the formation of an atoll ; I", I", sea-level 

 when reef was a fringe ; I', I', when it was a barrier, and I, I, the present sea- 

 level. 



ideal section embodying all these facts. In this figure, 

 for convenience of illustration, instead of the sea-bottom 

 sinking, the sea-level is represented as rising. 



Murray's Theory. The subsidence theory, however, 

 is not now universally accepted. Agassiz first showed, by 

 study of the reefs of Florida in 1851, that barriers are 

 formed without subsidence. Murray, traveling in the 

 same region as Darwin, concluded that both atolls and bar- 

 riers may be formed without subsidence. He supposes 

 that atolls are built up by corals on banks previously 



