1888.] and the Coral Formations of the Indian Ocean. 451 



Speaker's Bank is very little larger than Peros Banhos ; its lagoon is 

 far shallower, having a maximum depth of 24 fathoms. 



These considerations have led me to discredit the solution theory 

 as an explanation of lagoons and lagoon channels, and other objections 

 have been lately urged with great force by Captain Wharton. The 

 conclusion which I reached, after carefully considering the conditions 

 of submerged banks of atoll form, is that the ring-shape of the outer 

 reef is to be explained by the peculiarly favourable conditions for 

 coral growth found on the external slopes. Although corals may and 

 do flourish in lagoons, they are only found in knolls and patches, and 

 are always liable to be smothered when, by a change in the tidal 

 currents, sand is thrown down upon the place where they are grow- 

 ing. On the external slopes, however, corals grow in extraordinary 

 abundance, and chiefly those massive forms whose skeletons take so 

 conspicuous a share in the formation of coral rock. If once it is 

 admitted that the periphery of the reef offers peculiarly favourable 

 conditions to the growth of reef -forming corals, it follows that, as 

 the reef rises to the surface its external parts will outstrip the more 

 internal, and will reach the surface first, forming a rim around a 

 central depression or lagoon. This elevated rim will be as marked a 

 feature in submerged as in complete atolls. Not long after I had 

 arrived at this conclusion, and whilst the earlier part of this paper 

 was writing, Captain Wharton published a letter on coral formations 

 in ' Nature,' in which he arrives at identical conclusions. His know- 

 ledge of coral islands is so extensive that his views have great 

 authority, and I am extremely pleased to find that his opinions on 

 this subject are the same as those which I have formed. The only 

 point in which I differ with him concerns the explanation of the 

 favourable conditions on the external slopes. 



Following Agassiz, Murray, Guppy, and others, Captain Wharton 

 supposes that the favourable conditions consist in the increased food 

 supply brought by the superficial currents of the ocean. This I 

 cannot believe to be a complete explanation. The quantity of food 

 present must of course determine the existence of coral polypes in 

 any particular locality, as it does that of all other animals, but it 

 cannot be considered to "be the chief favouring cause of coral growth 

 on the external shores of an atoll for several reasons. If the prime 

 cause of luxuriant coral growth is an abundant food supply, and if, as 

 we may assume for the present, the food consists in the minute pelagic 

 animals borne in ocean currents, there must always be a definite rela- 

 tion between ocean currents and coral formations. Some authors 

 (Agassiz and Murray) have gone so far as to say that coral reefs are 

 only formed in the track of great ocean currents. This is hardly the 

 case. In the Pacific a study of the chart shows that atolls and barrier 

 reefs are formed irrespective of currents, and some large groups, such 



