Mansfield — Western Phosphates of United States. 597 



Vaughan. 21 Drew has shown that in these regions deni- 

 trifying bacteria are very active and are precipitating 

 enormous quantities of calcium carbonate largely in the 

 form of aragonite. Vaughan shows that this chemically 

 precipitated calcium carbonate forms spherulites or small 

 balls, which, by accretion, may become oolitic grains of 

 the usual size, or it may accumulate around a variety of 

 nuclei to build such grains. He reaches the deduction 

 that all marine oolites originally composed of calcium 

 carbonate, of whatever geologic age, may confidently be 

 attributed to this process. Drew's studies of the distrib- 

 ution of denitrifying bacteria have shown them to be most 

 prevalent in the shoal waters of the tropics. Combining 

 the results of Drew and Murray, Vaughan considers that 

 great limestone formations, whether composed of organic 

 or chemically precipitated calcium carbonate, were laid 

 down in waters of which at least the surface temperatures 

 were warm if not actually tropical. 



Among the deductions from the above data which may 

 serve as a tentative working hypothesis for the origin of 

 the Western phosphates may be mentioned the following : 



1. The phosphatic oolites and their matrix were prob- 

 ably deposited originally as carbonate of lime in the form 

 of aragonite. 



2. The waters were probably shoal and of warm or 

 moderate rather than of cold temperature. 



3. The lands bordering the depositional area were low 

 and furnished little sediment to the sea. Thus far the 

 supposed depositional conditions agree with known mod- 

 ern conditions in the Florida region. 



4. The phosphatization of the oolitic deposit was prob- 

 ably subsequent to its deposition rather than coincident 

 with it, for Drew shows that the activities of denitrifying 

 bacteria reduce the nitrate content of the sea water and 

 hence the growth of marine plants and of animals 

 dependent upon them. Such conditions are favorable for 

 the deposition of the carbonate but not of the phosphate 

 of lime. 



5. Cooler temperature in the waters of deposition, 

 perhaps induced by changes in the character or direction 



21 Vaughan, T. W., Preliminary remarks on the geology of the Bahamas, 

 with special reference to the origin of the Bahaman and Floridian oolites, 

 Carnegie Inst. Washington, Pub. 182. Papers from the Tortugas labora- 

 tory, vol. 5, pp. 47-54, 1914. 



