THE WASTE AND CONSERVATION OF PLANT FOOD. 231 



"The first of these stages began probably not later than the close of 

 the older Miocene, and within the Eocene area it may have begun much 

 earlier. Whether the primary phosphate resulted from a superficial 

 and heavy deposit of soluble guanos, covering the limestones, or from 

 tlie concentration of phosphate of lime already widely and uniforndy 

 distributed throughout the mass of the original rock, or from both, is 

 a ditticult question. In any event, the evidence indicates the effect of 

 the percolation of surface waters, highly charged with carbonic and 

 eartli acids, and thus enabled to carry down into the mass of the lime- 

 stone dissolved [)liosphate 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 hapi)en 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 ]djosphate of lime from the over- 

 lying portions of the deposit. 



"The thickness of the zone of phosphatization in the Eocene area is 

 unknown, biit it is doubtful if it was over 20 feet. In the Miocene area 

 the depth has been proved from the i)hosphates in situ to have been 

 between G and 12 feet." 



The deposits of the secondary origin, according to Eldridge, are due 

 chiefly to vsedimentation, although some of them may have been due to 

 precipitation from water. This secondary deposition was kept up for a 

 long ])eriod, until stopped by some climatic or geologic change. The 

 deposits of phosphates thus formed in the Florida peninsula are 

 remarkably free from iron and aluminum, in comparison with many 

 of the i)hosphates of the West Indies. 



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

 the period of formation of the original deposits and their transporta- 

 tion and storage as they are found at the present time. The geologic 

 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, therefore, easily 

 penetrated by the waters of i)ercolation. A good illustration of this is 

 seen on the southwestern and southern edges of Lake Okeechobee. In 

 following down a drainage canal which had been cut into the southwest 

 shore of the lake I saw the edge of the basin, which is composed of 

 this porous material. The appearance of the limestone would indicate 

 that large portions of it had already given way to the process of solu- 

 tion. The remaining portions were extremely friable, easily crushed, 

 and much of it could be removed by the ordinary dredging machines. 

 Such a limestone as this is peculiarly suited to the accumulation of 

 phosphatic materials, due to the percolation of the water containing 

 tbem. Thf. solution of the limestone and consequent deposit of the 



