MINERAL DEPOSITS OF FERTILIZERS 27 



Rock Phosphates. Most of the mineral phosphates are 

 probably derived from various animal sources. Sea birds, 

 nesting on grounds above rocks in which limestone or 

 calcareous materials of some kind constitute the major part, 

 will supply the phosphates. When rain falls upon such nesting 

 grounds, the drainage waters contain considerable quantities 

 of soluble phosphates, such as K 2 HPO 4 , which, coming into 

 contact with the calcium carbonate of the limestone, convert 

 the latter into calcium phosphate, potassium bi-carbonate 

 being leached out. The drainage follows the lines of the, 

 fissures in the limestone, and subsequently pockets of calcium 

 phosphate will be formed in the limestone or chalk. In some 

 cases the deposits of phosphates may be concretions which 

 have formed in chalk or calcareous soils. In other cases, 

 rock phosphates are partly water-washed guano, in which 

 the soluble parts have been removed, and the less soluble 

 fractions, consisting largely of calcium phosphate, left be- 

 hind. These materials are frequently washed by rivers 

 or running water, and perhaps transported considerable 

 distances as gravel. They may have even undergone a 

 certain amount of separation from other alluvial ingredients, 

 according to their degree of density or hardness. The theory 

 that all phosphates are of organic origin is demonstrated by 

 the test that when ground very finely, or dissolved in acids, 

 they all give the same " paraffin oil " smell. They are, 

 therefore, not likely to be very ancient or very much meta- 

 morphosed. In the frontispiece is a map which shows the 

 chief phosphatic deposits worked within recent years. In 

 populous districts, where the demand for phosphates is great, 

 diligent search for deposits has often been successful. In 

 unpopulous regions, it must not be assumed that there are 

 no deposits because they are not marked on the map, but 

 rather that search may not have been sufficiently diligent, 

 and that the future may reveal deposits, as yet unsuspected. 



As a general rule, phosphates are dug in a similar way to 

 gravel, but sometimes they are washed out by jets of water 

 under high pressure. They are usually dressed over screens, 

 and sometimes even lixiviated. By these means the 



