230 THE WASTE AND CONSERVATION OF PLANT FOOD. 



origiu, but does not fiud any peculiar significance in the restriction 

 of these deposits to the western ridge of the Florida Peninsula. 



As this region evidently constituted a long, narrow peninsula during 

 early Miocene times it is a reasonable tentative hypothesis that dur- 

 ing this period guanos were deposited from which was derived the 

 material for the phosphatization of the limestone either at the same 

 time or soon after. 



Darton closes liis paper by saying that the phosphate deposits in 

 Florida will require careful, detailed geologic exi)loration before their 

 relations and history will be fully understood. 



According to Dr. N. A. Pratt the rock or bowlder phosphate had its 

 immediate origin in animal life, and the phosphate bowlder is a true 

 fossil. He supposes the existence of some species in former times in 

 which the shell excreted was chiefly phosphate of lime. The fossil bowl- 

 der therefore becomes the remains of a huge foraminifer whicli had 

 identical composition in its skeleton with true bone deposits or of 

 organic matter. 



Perhaps the most complete exposition of the theory of the recovery 

 of waste phosi)hates, with esi)ecial reference to their deposition in 

 Florida, has been given by Eldridge,^ of the United States Geological 

 Survey. Eldridge calls attention to the universal presence of phos- 

 phates in sea ^ater and to the probability that in earlier times, as during 

 the Miocene and Eocene geologic periods, the waters of the ocean con- 

 tained a great deal more phosphate in solution than at the x)resent time. 

 He cites the observations of Bischof, whicli show the solubility of dif- 

 ferent phosphates in waters saturated with carbon dioxide. According 

 to these observations apatite is the most insoluble form of lime phos- 

 phate, while artificial basic slag jihosphate is the most soluble. Among 

 the very soluble phosphates, however, are the bones of animals, both 

 fresh and old. Burnt bones, however, are more soluble than bones 

 still containing organic matter. Not only are the organic phosphates 

 extremely soluble in water saturated with carbon dioxide but also in 

 water Avhich contains common salt or chloride of amMionium. The pres- 

 ence of large quantities of common salt in sea water would, therefore, 

 tend to increase its power of absorbing lime phosphates of organic 

 origin. It is not at all incredible, therefore, to suppose that at some 

 remote period the waters of the ocean, as indicated by these theories, 

 were mucli more highly charged with phosphates than at the present 

 time. 



According to Eldridge, the formation of the hard rock and soft 

 phosjihates may be ascribed to three periods : First, that in which the 

 primary rock was formed; second, that of secondary deposition in the 

 cavities of the primary rock; third, that in which the deposits thus 

 formed were broken up and the resulting fragments and comminuted 

 material were redeposited as they now occur. 



'A Preliminary Sketch of the Phosphates of Florida, by George H. Eklriflge, 

 author's edition, 1892, page 18 et seq. 



