152 MANURES. VII. 



are believed to be the fossilised excrements or intestinal de- 

 posits of extinct animals which fed upon fish. 



They were formerly of great importance and are still largely 

 used. Their composition varies considerably, the chief con- 

 stituents being 



Calcium phosphate ... 50 to 65 % 



Calcium carbonate ... 20 to 25% 



Silica, &c. ... ... 10 to 20% 



They are sometimes used raw in a finely divided state, but 

 generally converted into " superphosphate." 



Of mineral phosphates or phosphorites there are many 

 varieties, the most important being Norwegian, Canadian, 

 Sombrero, Belgian, Carolina, Florida, and Somme. These 

 are only occasionally employed in the raw 7 state, but are 

 mainly used in the preparation of " mineral superphosphates." 

 They all consist essentially of more or less impure apatite, 

 some containing calcium chloride, others calcium fluoride, and 

 some both compounds. In the process of manufacture of 

 superphosphate the first variety is much preferred, for reasons 

 which will be mentioned shortly. 



Another factor of importance in gauging the value of a 

 mineral phosphate is the proportion of iron and aluminium 

 which it contains. 



The extent and growth of the phosphate mining industry 

 may be seen from the following table, giving the total produc- 

 tion in tons of raw phosphates in 1880 and 1890" : 



Bulletin No. 15, United States Dept. of Agriculture, 1893. 



