214 SOILS AND MANURES 



tons. The English coprolites, especially those from Cam- 

 bridgeshire and Suffolk, are generally richer than the 

 French. 



Phosphatic guanos. The true phosphatic guanos, like 

 the true coprolites, are of organic origin. They are 

 derived from guano by the gradual removal of the volatile 

 and soluble matters (p. 295), and consist of the insoluble 

 phosphatic residue. Traces of alkalis and sometimes even 

 of nitrogen are occasionally present, but the quantities 

 of these substances are negligible and the material is to 

 be regarded simply as a native phosphate. Phosphatic 

 guanos are usually very rich. Commercial samples con- 

 tain from 70 80 per cent, of tricalcic phosphate. They 

 are comparatively easily ground and dissolved, and as 

 they contain but little iron and aluminium, are very 

 suitable for the manufacture of high class superphos- 

 phates. Apart from these qualities, they are of no greater 

 value than apatites and phosphorites of similar composi- 

 tion notwithstanding their organic origin. 



They are found in several of the Pacific and West 

 Indian Islands, and in various parts of North and South 

 America. 



SUPERPHOSPHATES. 



The native phosphates, it has been said, are not largely 

 used as manures, in the natural state. Their action is 

 too slow, and it is found more profitable to hasten it by 

 treatment with sulphuric acid. The products are called 

 superphosphates. The name is a purely conventional 

 one and does not imply, as might perhaps be supposed, 

 that they contain more phosphoric acid than the native 

 or other phosphates. The treatment with sulphuric acid 

 does not alter the quantity of phosphoric acid present. 

 AH that it does is to convert the insoluble tricalcio 



