THE FLOKIDA REEFS. 77 



here brought forward. Mr. Murray's " observations of the reefs 

 at Tahiti ^ support the view that they have been built from the 

 shore seawards, and that the lagoons have been and are still 

 being formed by the removal of the inner and dead portions 

 of the coral reef by the solvent action of sea-water. . . . The 

 food supply for the masses of living coral on the outer slope 

 of the reef is brought by the oceanic currents sweeping past 

 the islands, — a fact which explains the more vigorous growth 

 of the reef on the windward sides. It is maintained by Mr. 

 Murray that the whole of the phenomena of the Tahiti reefs 

 may be fully explained by reference to the processes at present 

 in action, and without calling in the aid of subsidence, as is 

 done by Darwin and Dana ; and it is further argued that the 

 form of atoll and barrier reefs generally can be explained on 

 the same principles." 



Undoubtedly, Darwin's theory of reef formation presents a 

 sound and admirable exposition of the grander causes which 

 have brought about the elevation or subsidence of large tracts 

 to a level favorable for coral growth ; but at the time he wrote 

 upon this subject, the formation of extensive limestone banks, 

 built up by the animals living on the bottom, and constantly 

 strengthened and increased by the attendant phenomena of 

 winds and currents, was little understood. These facts have 

 been brought into notice and emphasized by recent deep-sea 

 explorations. Darwin, however, when examining maps of the 

 West Indies, had been struck by the probable connection be- 

 tween the areas of deposition of the great banks marked upon 

 the charts and the course of the sea-currents. He naturally ex- 

 plained the steep slopes, abruptly dropping from comparatively 

 shallow plateaux to great depths, by what is known to occur 

 wherever great masses of sediment are found, and he therefore 

 considered these plateaux to be submerged mountains. Such 

 they are, in a certain sense ; not wholly built, however, as Dar- 

 win supposes, of sediment, but in great part also of the remains 

 of the innumerable animals living and dying upon them. The 

 nucleus of these banks has probably been formed around the 

 shores of promontories subjected to the most active play of the 

 great oceanic currents. 



^ Voyage of the " Challenger," Narrative of the Cruise, p. 781. 1885. 



