354 £• M. KINDLE 



either side of the equator strongly tends to confine the formation 

 of considerable deposits of limestone to the tropical and subtropical 

 seas. 



Much has been accomplished in recent years toward delimiting 

 the factors responsible for the areal limitations on limestone for- 

 mations. The more recent work which has been done on the bio- 

 chemical factors chiefly concerned in the formation of limestone 

 has been summarized by Vaughan.' 



In the Florida-Bahama region * occur vast areas of chalky 

 mud. These give place to non-calcareous sediments farther north. 

 Dr. Vaughan's work on the Florida coast has shown that the 

 bottom deposits now forming inside the Florida Keys varies from 

 quartz sand to nearly pure calcareous ooze. He writes : 



Silica is abundant in the form of sand in the northern portion of Biscayne 

 Bay, it becomes rarer toward the southwest and is present in small quantities 

 as far as Big Pine Key. Toward the southwest as the silicious material becomes 

 rarer, calcium carbonate becomes progressively more abundant, occurring 

 as a flocculent sediment or ooze over practically the entire region from the lower 

 portion of Biscayne Bay to the Gulf end of Florida Bay.^ 



The important discovery by the late Dr. G. H. Drew^ of the 

 part played by marine bacteria in producing chemical conditions 

 favorable to the precipitation of calcium carbonate gave the first 

 clue to the mode of origin of these incipient limestones of the Florida 

 Keys. The environment considered especially favorable for these 

 denitrifying bacteria includes a tropical or subtropical chmate with 

 ''drainage into the sea of a well- wooded country composed of cal- 

 careous rock and the soluble organic calcium salts would be pre- 

 cipitated as calcium carbonate by the action of the bacteria. "'• 



Coral reefs. — The contribution of corals to limestone formation 

 is made chiefly in the form of fringing reefs some of which, like the 

 Great Barrier reef of AustraUa, are several hundred miles in length. 



'T. W. Vaughan, "Chemical and Organic Deposits of the Sea," Btdl. Geol. 

 Soc. Amer., XXVIII (191 7), 933-44. 



^ T. W. Vaughan, "A Contribution to the Geological History of the Floridan 

 Plateau," Carnegie Inst, of Washington, Pub. ijj (1910), p. 119. 



3 G. H. Drew, Year Book Carnegie Inst. No. 10 (1911), p. 125. 



4 Ibid., p. 139. 



