a2, CORALS AND CORAL ISLANDS. 
between Double-Headed Shot Key, Salt Key, and Anguilla 
Key as a very mnstructive combination of the phenomena of 
building and destruction. He says: ‘* The whole group is a 
flat bank covered by four or five, and occasionally six, fathoms 
of water, with fine sandy bottom, evidently corals reduced to 
odlite, the grains, which are of various sizes, from fine powder 
to coarse sand, mingled with broken shells, among which a 
few living specimens are occasionally found. The margin of 
the Bank is encireled on several points by rocky ridges of 
the most diversified appearance, and at others edged by sand- 
dunes. A close examination and comparison of the different 
Keys show that these different formations are in fact linked 
together, and represent various stages of the accumulation, 
consolidation, and cementation of the same materials. On 
the flat top of the bank the loose materials are pounded 
down to fine sand; in course of time this sand is thrown up 
upon the shoalest portions of the Bank, and it is curious to 
notice that these shoalest parts are its very edge, along which 
corals have formed reefs which have become the basis of the 
dry banks. The foundation rock, as far as tide, wind, and 
wave may carry the coarser materials, consists of a conglom- 
eration of coarser odlitic grains, rounded fragments of corals, 
or broken shells, and even larger pieces of a variety of corals 
and conchs, all the species being those now found living upon 
the Bank, among which Strombus gigas is the most com- 
mon; besides that, Astrea [ Orbicella| annularis, Siderastrea 
siderea, and Maandrina mammosa prevail. The shells of 
Strombus are so common that they give great solidity and 
hardness to the rock. The stratification is somewhat irregu- 
lar, the beds slanting toward the sea at an angle of about 
seven degrees. Upon this foundation immense masses of 
Strombus, dead shells, and corals have been thrown in banks, 
a 
