1888.] and the Coral Formations of the Indian Ocean. 459 



may be expected to reach, in time a condition like that of Peros 

 Banhos. It is probable that a large bank like the Great Chagos 

 Bank, when it reaches the surface, can never give rise to a continuous 

 strip of land, but must consist of a chain of islets separated by chan- 

 nels of some depth and by tracts of submerged reef. The islets and 

 tracts of reef in either case would be bounded by deeper channels, 

 and these channels, swept by strong currents, would become wider 

 and deeper, for corals could not thrive in them. After a time the 

 islets would become so far isolated, and the entries into the lagoon 

 would become so large and numerous, that oceanic conditions would 

 prevail in the lagoon, and then there would be around each separate 

 islet or piece of reef all the necessary conditions for the formation of 

 a new atoll. The currents would strike upon one side of the islet 

 or reef, sweep round it, and give a backwash at the further side ; the 

 corals would nourish in the circumferential parts of the reef sur- 

 rounding the islet, and new atolls with shallow lagoons would be 

 formed. 



In Tilla-dou-Matte the lagoons of the secondary atolls are tolerably 

 deep. In this case they must have been formed before any land 

 reached the surface. Applying the same reasoning as in the former 

 case, it can readily be understood how in the case of the Great Chagos 

 Bank, which has wide and deep breaches in many places, the isolated 

 reefs as they grow to the surface must tend to assume an atoll form. 

 An examination of the chart shows that this is the case. The Great 

 Chagos Bank in the course of time will rise to the surface as an atoll 

 composed of secondary atolls or atollons, similar to, but on a smaller 

 scale than, Tilla-dou-Matte atoll. The explanation of atollons in 

 the centre of a large lagoon in which oceanic conditions have been 

 established, is quite obvious. 



In conclusion, I may sum up by saying that the strength and direc- 

 tion of currents appears to me to be the main influence on coral growth ; 

 that the behaviour of currents on meeting an obstacle with sipping 

 shores, explains the superabundant growth of corals on the outer 

 slopes of a reef, whether submerged or awash; that the growth of 

 corals on the periphery of a bank being in great excess of the growth 

 in its interior portions is sufficient to explain the formations known 

 as atolls and barrier reefs without the aid of the solution theory pro- 

 posed by Mr. Murray, and ably defended by him in a recent number 

 of ' Nature.' I have shown that Mr. Murray has overestimated the 

 effects of solution in neglecting the compensating action of the re-pre- 

 cipitation of the carbonate of lime held in solution, and the formation 

 of coral rock within the lagoon through the agency of coral growth 

 there. I have also shown that the role played by currents is not 

 what is supposed; that the carriage of food by currents must be 

 considered of subsidiary importance in estimating their effect, though 



