360 C0RAL8 AND CORAL ISLANDS. 



is most favorable, not exceeding perhaps a sixteenth of an 

 inch a year, or five feet in a thousand years. And yet such 

 limestones probably form at a more rapid rate than those 

 made of shells, because the animals are to a larger extent 

 calcareous or make proportionally larger calcareous secretions ; 

 and in addition they have the property of rapid multiplica- 

 tion by budding. The mollusks that grow and multiply most 

 rapidly and have proportionally the largest shells are the 

 Lamellibranchs or bivalves, among which the oyster is a fa- 

 mous example ; and the Brachiopods were once the full equals 

 or the ordinary bivalves. Large banks of bivalves seldom 

 occur in regions of corals, the species there being to a great 

 extent Gasteropods (or univalves) ; and hence the contributions 

 of shells to coral reefs from mollusks are small compared 

 with the extent of the beds which, by themselves, they make 

 on other coasts. The coral seas of Florida nowhere have 

 shore shell-beds like those of St. Augustine in northern 

 Florida outside of the coral-reef seas. There is reason for 

 this in the fact that these bivalves that grow in large banks live 

 in beds of ordinary sand or mud, such as reef-regions do not 

 generally supply. 



XII. LIMESTONE CAVERNS. 



The elevated coral limestone, although in general a hard 

 and compact rock, abounds in caverns. They may be due in 

 part to open spaces, or regions of loose texture, in or between 

 the strata. But in most cases they are a result of solution 

 and erosion by the fresh waters of the land, or the waves and 

 currents of the ocean, subsequent to the elevation. 



On the island of Metia, many caverns open outward in 

 the coral limestone cliff and in some were large stalactites, as 

 stated on page 194. 



