MANURES. 297 



phous and can easily be reduced to fine powder. Some occur 

 in great rock masses, others as comparatively small nodules 

 mixed with useless earthy matters. Some contain 70, 80 

 or even as much as 90 % tricalcic phosphate e.g. Canadian 

 and Florida phosphates, others have only some 30 to 40 % 

 e.g. Belgian. In colour they differ but when ground 

 most of them have a yellowish brown appearance. 



The chief impurities present in the natural phosphates 

 are silica and silicates, carbonate fluoride and chloride of 

 calcium, and compounds of iron and aluminium chiefly 

 oxide and phosphate. The value of a deposit for the making 

 of superphosphate is determined partly by its richness in 

 phosphate, and partly and to a considerable extent by the 

 nature of the impurity present. The ease with which it- 

 can be reduced to powder is also a consideration. Closely 

 related to the above phosphatic rocks are the phosphatic 

 guanos. These deposits, found chiefly on islands in the 

 Pacific and in the West Indies, are derived from the 

 bodies and excrements of sea birds and may contain from 

 60 to 80 % phosphate of lime. They are similar in their 

 origin to the nitrogenous guanos but their organic matter 

 has almost entirely disappeared, and through the action 

 of water they have lost all the soluble substances they 

 once contained. 



In the Egyptian deposits the presence of fish teeth, 

 fragments of bone, etc. points to their being derived from 

 great accumulations of fish remains. The different layers 

 vary considerably in the amounts of phosphate they contain 

 but 40 to 50 % may be taken as a fair average. The 

 stuff is of too low grade for profitable exportation but in 



