PHOSPHATIC MANURES 213 



associated with larger or smaller quantities of calcium 

 carbonate. 



Quantities of phosphatic nodules are also found in 

 the Suffolk Crag and in various parts of the Green Sand. 

 They closely resemble the coprolites in appearance, and 

 were at one time thought ~fco be of the same nature, 

 but are now known to be concretionary. They are also 

 very similar in chemical composition, but generally con- 

 tain a larger proportion of iron and alumina. They have 

 been called pseudo - coprolites to distinguish them from 

 those of fsecal origin, but the term coprolite is generally 

 applied without discrimination to all kinds of phosphatic 

 nodules. The composition of coprolites is necessarily 

 somewhat variable, but they usually contain from 50 to 

 60 per cent, of tricalcic phosphate. The following 

 analysis shows the proportions of various ingredients 

 found in a particular sample: 



Per cent. 



Tricalcic phosphate 54'0 



Calcium carbonate ....... 28' 1 



Oxide of iron and alumina ...... 7'4 



Silica ' 0-7 



Organic matter ........ 2*0 



Water 3'9 



Not estimated . 3'9 



100-00 



Coprolites are found chiefly in Cambridgeshire, Suffolk 

 and Bedford, and to some extent in Norfolk, Buckingham 

 and Essex. They are also found in France, in the 

 neighbourhood of Boulogne, in Eussia, and some other 

 countries. Like apatites and phosphorites they are used 

 for the manufacture of superphosphates, and as much 

 as 30,000 tons have been produced in England in a 

 year, a single "acre yielding sometimes as much as 300 



