3IO CORALS AND CORAL LSLANDS. 



make proportionally larger calcareous secretions ; and in addi- 

 tion they have the property of rapid multiplication by bud- 

 ding. The mollusks that grow and multiply most rapidly, 

 and have proportionally the largest shells, are the Lamelli- 

 branchs, or bivalves, among which the oyster is a famous ex- 

 ample ; and the Brachiopods were once the full equals of the 

 ordinary bivalves. Large banks of bivalves seldom occur in 

 regions of corals, the species there being to a great extent Gas- 

 teropods (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 those bivalves that grow in large banks live in beds 

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

 rally 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 cur- 

 rents 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 157. 



In the raised coral rock of Oahu (p. 290) there are several 

 long winding horizontal chambers, some of which are the 

 sources of subterranean streams that open out on the shores 

 between the layers of the rock, or from the mouths of caverns. 

 These running waters, and others trickling from above, are 

 obviously the eroding agents that have made the caves. 



As briefly remarked on page 157, caverns are still more re- 

 markable on the island of Atiu, on which the coral reef-rock 

 stands at about the same height above the sea as on Oahu. 



