30 AGRICULTURAL ANALYSIS 



of lime already widely and uniformly distributed throughout the 

 mass of the original rock, or from both, is a difficult question. 

 In any event, the evidence indicates the effect of the percolation 

 of surface waters highly charged with carbonic and other acids, 

 and thus enabled to carry -down into the mass of the limestone 

 dissolved phosphate of lime, to be redeposited under conditions 

 favorable to its separation. Such conditions might have been 

 brought about by the simple interchange of bases between the 

 phosphate and carbonate of lime thus brought together; or by 

 the lowering of the solvent power of the waters through loss of 

 carbonic acid. The latter would happen whenever the acid was 

 required for the solution of additional carbonate of lime, or 

 when, through aeration, it should escape from the water. The 

 zone of phosphate deposition was evidently one of double 

 concentration, resulting from the removal of the soluble car- 

 bonate thus raising the percentage of the less soluble phosphate, 

 and from the acquirement of additional phosphate of lime from 

 the overlying portions of the deposits. 



"The thickness of the zone of phosphatization in the eocene 

 area is unknown, but it is doubtful if it was over 20 feet. 

 In the miocene area the depth has been proved from the phos- 

 phates in situ to have been between six and 12 feet." 



The deposits of secondary origin, according to Eldridge, 

 are due chiefly to sedimentation, although some of them may 

 have been due to precipitation from water. This secondary de- 

 position was kept up for a long period, until stopped by some 

 climatic or geological change. The deposits of phosphates thus 

 formed in the Florida peninsula are remarkably free from iron 

 and aluminum in comparison with many of the phosphates of 

 the West Indies. 



The third period in the genesis of the hard rock deposits em- 

 braces the time of formation of the original deposits and their 

 transportation and storage as they are found at the present time. 

 The geological time at which this occurred is somewhat uncertain 

 but it was probably during the last submergence of the peninsula. 

 In all cases the peculiar formation of the Florida limestone 

 must be considered. This limestone is extremely porous and' 



