GEOLOGICAL CONCLUSIONS. 397 
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 
of 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. 
