214 CORALS AND CORAL LSLAND8. 



reefs of the Pacific much more than the reefs of the coast of 

 Florida. 



u The whole group of banks and keys embraced between 

 Double-Headed Shot Key, Salt Key, and Anguilla Key (all 

 on the Salt Key Bank), is a very instructive combination of 

 the phenomena of building and destruction. 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 oolitic, 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 encircled 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 accumula- 

 tion, 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, consist of a conglomeration 

 of coarser oolitic 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 common ; 

 besides that, Astrcea (Orbicelld) annularis, Siderastrcea si- 

 devea and Mceandrina mammosa prevail. The shells of 

 Strombus are so common that they give great solidity and 

 hardness to the rock. The stratification is somewhat irregu- 



