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growth of the corals on the outer edges of the reef which are most exposed to the 

 action of the surf, and the sweeping of the coral material out of the lagoon through 

 the agency of oceanic currents, and the dissolving action of the carbonic acid con- 

 tained in the sea water. The deep canals which divide the barrier reefs from the 

 neighboring mainland are formed in the same way. The enormous magnitude of 

 the reefs which the theory of subsidence demands is nowhere realized. Neither 

 among modern reefs nor among geological formations do we find any traces of such 

 gigantic masses of coral rock. We are thus in face of a fascinating and important 

 scientific problem, which still remains to be solved, a problem which was long 

 thought to have found its solution. After Darwin's and Dana's subsidence 

 theory had been generally for many years accepted as beautiful and com- 

 pletely satisfactory, we are told that it is not always applicable, and that much 

 simpler causes suffice to explain the phenomena. It is obvious, then, that we have 

 an ample supply of possible explanations of coral reefs, and it is most probable that 

 among the many scattered reefs in the world, in one case one set of factors have 

 played the chief part, in another case a slightly different set, and further, a 

 detailed and exhaustive study of any particular reef would probably reveal 

 natural processes of no small importance which have not as yet been taken into 

 account. 



MOUTHS OF MADREPORE. 



