26 Proceedings of the Royal Physical Society. 



sidence can be based on the steepness of the seaward face of a 

 reef and the thickness of the calcareous mass of the reef itself. 



3. The depth of some lagoons and lagoon-channels fur- 

 nishes probably the strongest argument in favour of Darwin's 

 views. Occasionally a depth of forty fathoms is reached, and 

 as this is beyond the depth at which reef-builders ordinarily 

 live, it has been regarded as a proof that subsidence has 

 taken place. 



This third difficulty is thus met by the opponents of sub- 

 sidence. We must remember, they say, that from the very 

 conditions of their growth, patches of coral tend to assume 

 an annular or atoll- like form, because the outer parts grow 

 vigorously, while the central portions eventually die. Where 

 the coral patches coalesce, and extend along a bank or 

 shore, it is their outer or seaward faces that flourish. The 

 inner parts, as they are more and more cut off from the food 

 supply, gradually die. While the outer face of the reef grows 

 seawards, the inner margin is attacked, partly by the solvent 

 action of the carbonic acid of sea-water, partly by wind- 

 waves, and the tidal-scour sweeps away much fine detritus 

 through gaps in the reef. In this way, the lagoon-channel 

 is widened and deepened. In a perfect atoll — that is, an 

 unbroken annular reef of coral — the lagoon could not be 

 deepened by any mere abrasion of the dead coral and removal 

 of the detritus in suspension, but solution by carbonic acid 

 would still come into play. It is further to be borne in 

 mind that small lagoons are shallow, and are being filled up, 

 and that it is only the large ones, encircled by nearly con- 

 tinuous reefs, where the corals in the lagoon and along its 

 margin are dead, and where the effects of solution may be con- 

 ceived to have been longest in operation, that the depth of the 

 lagoon descends below the limits at which reef-builders live. 



I do not regard this solution of the difiiculty as wholly 

 satisfactory. Of the fact that dead calcareous organisms are 

 attacked and carried away in solution in sea-water, there 

 cannot be any question, and this process must be of great 

 geological importance. Whether the solvent action is suflft- 

 cient to account for the exceptional depth of some lagoons 

 is still, I think, open to inquiry. It seems to me not im- 



