vaughan: geologic history of coral reefs 27 



cene oolite, is approximately as 1 to several thousand, or, as a 

 constructive agent, chemical precipitation has been several thou- 

 sand times more effective in forming Umestone than corals. 



topographic relations of the FLORIDA BARRIER REEF 



The living barrier reef of Florida extends as a broken chain 

 from Fowey Rocks at the north to off Key West near Sand Key 

 at its southwest end. Luxuriant reefs also occur around the Tor- 

 tugas atoll. The principal reefs lie just within the 10-fathom 

 curve and stand on the seaward margin of a platform. They are 

 separated from the main line of keys by Hawk Channel, which 

 has a maximum depth of about 7 fathoms, and the keys in their 

 turn are separated from the mainland by a series of shallow bays 

 and sounds. The platform on which the reefs stand extends 

 around the entire coast of Florida. Northward of Fowey Rocks 

 the reefs disappear but the platform continues. The platform 

 also exists on the west side of the peninsula of Florida but 

 bears no reefs. It is evident that the platform is independent of 

 corals and that it owes its existence to agencies other than those 

 dependent upon the presence of coral reefs. 



BUILDING OF THE MARQUESAS AND THE TORTUGAS 



There are two rival hypotheses for the formation of atolls, one 

 of these attributes them to the submarine solution of the inte- 

 rior of a mass of limestone; the other accounts for them by 

 constructional agencies. In order thoroly to test the solution 

 hypothesis the results of four lines of invetigations were brought 

 to bear upon it, and all are concordant. (1) All the bays, sounds, 

 and lagoons within the Florida reef and key region are filling 

 with sediment; (2) Drew's investigations of denitrifying bacteria 

 show that chemical precipitation of calcium carbonate is taking 

 place in the lagoons; (3) the chemical examination by R. B. 

 Dole of samples of sea-water flowing into and out of the Tor- 

 tugas lagoon, collected twice daily for a lunar period, show that 

 altho both carbonate and bicarbonate radicles are in solution 

 uncombined carbon dioxide is not present, and that the water 

 possesses no capacity for further solution of calcium carbonate 



