1898.] Funafuti, liotuma and Fiji. 497 



The extent to which solution has been going on is very great in 

 the Lau Group of Fiji ; and Reid has thus shown how much 

 greater the action is under pressure. Given a bank at a suitable 

 depth, nothing further is required to demonstrate how its lagoon 

 has been formed ; the edges being covered with living organisms 

 would be protected while the depth of the lagoon would be regu- 

 lated by the amount of decay of animal matter giving off carbonic 

 acid gas, by the amount of carbonate of lime deposited on its floor, 

 and by the extent to which the currents passing in and out of the 

 lagoon renew its bottom layers of water. At the same time, in 

 large atoll and barrier reefs, I would emphasize the importance 

 also of these currents in removing sand and silt. Both effects of 

 the current depend largely on the number, size, and depth of the 

 passages ; of these the depth is of most importance. Considering 

 how rarely a depth of over 50 fathoms is obtained within barrier 

 reefs, and how common depths up to that are, there is probably to 

 be found a mean, below which increase is greater than solution, 

 and above which solution has the upper hand. The lagoon of 

 Exploring Group of Fiji has depths up to 100 fathoms, with a 

 passage equally deep, to which in this particular case the depth 

 is no doubt due ; this case, however, should be considered in con- 

 nection with the raised limestone islands of Lau, the reefs of which 

 will be treated of later. 



Considering that, in Fiji, coasts can be found with no reef, or 

 with a broad fringing reef, or with a far distant barrier reef, and 

 indeed with every intermediate condition between these, and that 

 there are many atoll-reefs such as the Charybdis, the Horse-shoe and 

 the Mumbolitha, I can only reiterate Murray's conclusions, summed 

 up by Professor Bonney in these words 1 : "That when coral 

 plantations build up from submarine banks they assume an atoll 

 form owing to the more abundant supply of food to the outer 

 margins and the removal of the dead coral-rock from the interior 

 portions by currents and the action of the carbonic acid (gas) dis- 

 solved in the water." 



" That barrier reefs have built out from the shore on a foundation 

 of volcanic debris, or on a talus of coral blocks, coral sediment and 

 pelagic shells, and the lagoon channel is formed in the same way 

 as a lagoon." 



As mentioned before, my opinion is that the reef is formed 

 rather by the growth of calcareous algre of the genus Litho- 

 thamnion than by either the direct building action of corals, or 

 the consolidation of their fragments. Hence the formation of a 

 rim would be due rather to the constant replacement of the sea 

 water outside the reef by fresh sea water renewing the supply 



1 Coral Reefs. By Charles Darwin, p. 286, 1889. 



