114 PHOSPI/ATIC MANURES [CHAP. 



Mineral Phosphates. 



With the recognition that the fertih'sing value of 

 bones lay in the phosphate of lime they contained, 

 which we may conclude had become the accepted 

 opinion about 1840, attention began to be directed to 

 mineral sources of phosphate of lime, — apatite and 

 phosphorite, the existence of which had long been 

 known to mineralogists. Acting on an analysis of 

 Proust's, Professor Daubeny and Captain Widdrington 

 made an expedition in 1843 to Estrcmadura to find a 

 bed of phosphatic rock there reported. They dis- 

 covered the deposit and secured enough of it for a few 

 field experiments in England in the following year, 

 but difficulties of transport prevented anything more 

 than small quantities being exported until a much later 

 period. The immediate demand for such material was 

 satisfied by the discovery in 1845 by Professor Ilenslow 

 of the bed of coprolites lying at the base of the Green- 

 sand near Cambridge. 



These coprolites — small rounded nodules of impure 

 phosphate of lime, mixed with fragments of bone and 

 shell, shark's teeth, etc. — were at one time regarded as 

 fossilised dung, but are now considered to be pebbles of 

 carbonate of lime in which the carbonic acid has been 

 replaced by phosphoric acid by long contact with 

 material containing organic matter. They occur at 

 various horizons in the newer secondary and tertiary 

 rocks, e.g., at the base of the Upper Greensand and at the 

 base of the Gault, and in the Crag, where it rests upon 

 the London Clay. Shortly after their discovery these 

 deposits began to be worked for coprolites at various 

 places in Suffolk, near Cambridge, and at Potton in 

 Bedfordshire. The output reached 50,000 tons or so 

 in the early eighties of the last century, but then rapidly 



