MINERAL DEPOSITS OF FERTILIZERS 45 



between these various forms of lime since the cost of carriage 

 by rail and cart may easily equal the cost of the material 

 at the kiln. Table 8 gives the composition of lime from a 

 few special sources. 



TABLE 8. 

 COMPOSITION OF LIME. 



The limestone in County Durham, being situated on the 

 Permian series, exhibits a great variation in the amount of 

 magnesia. Limestone and lime containing a large percentage 

 of magnesia are not suitable for agricultural purposes. 

 There is much evidence of a negative character to show that 

 magnesium limestones are not harmful, but a few positive 

 results which show harm are quite sufficient to justify the 

 fears, which most farmers have, of such materials. 



Gypsum. Gypsum is derived chiefly through the de- 

 position of calcium sulphate from salt water, in which case 

 the gypsum will contain a certain amount of common salt. 

 Gypsum may be derived from the action of sulphuric acid 

 penetrating limestone and converting the latter into calcium 

 sulphate ; with this origin calcium carbonate may be present 

 as an impurity. The sulphuric acid responsible for this 

 conversion may have been derived from pyrites, after 

 oxidation, or even from the decomposition of organic sulphur 

 compounds. Gypsum occurs in many parts of the world, 

 and is comparatively easily mined. The bulk of the gypsum 

 mined in England is contributed by Nottingham. Nova 

 Scotia is also a large producer. A typical gypsum contains 

 about 90 % to 95 % of crystallized calcium sulphate, and 

 3 % to 5 % of calcium carbonate, with i % or 2 % of 

 silica. vSome deposits of gypsum contain a little;anhydrite, 



