BY H. J. JENSEN, D.SC. 71 



phosphates of Hme (dicalcic phosphate), magnesium, and 

 ammonium leached out by meteoric water containing carbon 

 dioxide. The sokible phosphate solution filtering down 

 come into contact with carbonate of lime, either in the form 

 of limestone or calcite, and alters it to the phosphate, 

 liberating at the same time carbon dioxide. 



The origin and mode of occurrence of guano has 

 already been referred to. 



High grade apatite and phosphorite should contain 

 from 70 % to 80 % tricalcic lime phosphate. 



The occurence of phosphorite deposits at the junction 

 of two formations of a diiiEerent character is not unusual^ 

 especially when the lower rock is fossiliferous limestone ; 

 and its occurrence where there is an unconformitv 

 between two formations or a thinning out of a fossiliferouf* 

 stratum is also well known. It is very often associated 

 with arragonite and not seldom with calamine. 



GENERAL CONSIDERATIONS. 



From the foregoing description of occurrences of 

 phosphate rocks (not considering the apatite and guano 

 deposits), it will be seen that phosphorite may originate 



(1) from the insoluble residue of leached guano ; 



(2) from the action of ammonium phosphate 

 derived from guano beds on limestone ; 



(3) as a breccia from the erosion of partly phos- 

 phatised limestone ; 



(4) from the leaching of bone beds ; 



(5) from the decomposition of plant remains in 

 dried up swamps ; 



(6) from the leaching of coral limestone, forami- 

 niferal limestone, molluscan limestone, and 

 brachiopod limestone. In such cases the phos- 

 phate deposits may be in pockets either on the 

 surface of the limestone under the soil, or under- 

 laying the limestone on its junction with the 

 subjacent formation. 



Aluminous phosphates originate by the action of 

 ammonium phosphate from guano, decomposing organic 

 matter, bones, etc., on trachytic or basaltic rocks or kaolin. 



A notable observation is that phosphates often occur 

 at the unconformable junction between two different 

 geological horizons. Thus phosphates occur at the junction 



