130 CORAL-REEFS. 



small islands (or more, according to the number of the hills) 

 included within one annular reef. Let the island continue 

 subsiding, and the coral-reef will continue growing up on its 

 own foundation, whilst the water gains inch by inch on the 

 land, until the last and highest pinnacle is covered, and 

 there remains a perfect atoll. A vertical section of this atoll 

 is shown in the woodcut by the dotted lines; — a ship is 

 anchored in its lagoon, but islets are not supposed yet to have 

 been formed on the reef. The depth of the lagoon and the 

 width and slope of the reef, will depend on the circum- 

 stances just referred to under barrier-reefs. Any further 

 subsidence will produce no change in the atoll, except 

 perhaps a diminution in its size, from the reef not growing 

 vertically upwards ; but should the currents of the sea act 

 violently upon it, and should the corals perish on part or on 

 the whole of its margin, changes would result during sub- 

 sidence which will be presently noticed. I may here 

 observe, that a bank either of rock or of hardened 

 sediment, level with the surface of the sea, and fringed 

 with living coral, would (if not so small as to allow the 

 central space to be quickly filled up with detritus) by 

 subsidence be converted immediately into an atoll, without 

 passing, as in the case of a reef fringing the shore of an 

 island, through the intermediate form of a barrier-reef. If 

 such a bank lay a few fathoms submerged, the simple 

 growth of the coral (as remarked in the third chapter) 

 without the aid of subsidence, would produce a structure 

 scarcely to be distinguished from a true atoll ; for in all cases 

 the corals on the outer margin of a reef, from having space 

 and being freely exposed to the open sea, will grow 

 vigorously and tend to form a continuous ring whilst the 

 growth of the less massive kinds on the central expanse, 

 will be checked by the sediment formed there, and by that 



