11-i The Middle Devoxiax Deposits of Maryland 



stone not greatly unlike the Onondaga to be developed in the future. Dr. 

 Vaughan's work on the Florida coast has shown that the bottom deposits 

 now forming inside the Florida Keys vary from quartz sand to nearly 

 pure calcareous ooze. He wn-ites : 



'• Silica is abundant in the fonn 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 tioeculent sediment or ooze over practically 

 the entire region from the lower portion of Biscayne Bay to the Gulf end 

 of Florida Bay." ' 



The recent important discoveiy by Dr. G. H. Drew ° of the part played 

 by marine bacteria in producing chemical conditions favorable to the 

 precipitation of calcium carbonate gives a clue to the mode of origin of 

 these incipient limestones of the Florida Keys. We may infer that where 

 limestones were dejDosited during Onondaga time as in the central states 

 the deposition may have been due to the presence of such bacteria com- 

 bined with conditions atfording a limited supply of clastic sediments. 

 The environment considered especially favorable for these denitrifying 

 bacteria at present includes a tropical or subtropical climate with " drain- 

 age into the sea of a well-wooded country composed of calcareous rock, 

 and the soluble organic calcium salts would be precipitated as calcium 

 carbonate by the action of the bacteria." ' 



^ Vaughan, Thomas Wayland: A contribution to the geological history of 

 the Floriclian plateau; Carnegie Institution of Washington, Publication 133, 

 1910, p. 119. 



- Year Book, Carnegie Inst., No. 10, 1911, p. 125. 



' Drew, G. H., iUd., p. 139. 



