THE COMPOSITION OF THE SOIL 99 



For practical purposes it is necessary to have resort to 

 simpler but less exact methods. 



By the method of mechanical analysis described in the 

 appendix the particles of soil can be sorted out into fractions, 

 each falling within certain specified limits of diameter ; but 

 unfortunately there is no international agreement as to the 

 limits, so that the same words are used in different countries 

 with different meanings. Some of the commoner units are 

 given in Table XXVIII. 



The justification of the -002 mm. limit for clay adopted in 

 Britain is that a tolerably sharp change in the physical pro- 

 perties of bodies does occur at or about this point. 1 



The difference between the British and the American units 

 can be considerably reduced by splitting the fine silt fraction 

 into two, viz. O'Oi to 0*005 mm - diameter, and 0*005 to 

 0-002 mm. diameter. This does not greatly lengthen the 

 analysis, and it should always be done 2 when there is any 

 likelihood of comparison with American results. There still 

 remain small differences in the fine sand and silt which in many 

 cases would not greatly affect the discussion. 



If none but pulverising forces had been at work during 

 soil formation the soil particles would be identical in composi- 

 tion with the original rock. But weathering and leaching have 

 always wrought changes, and in extreme cases only the most 

 resistant minerals have survived unaltered. A succession of 

 grades may therefore be expected, shading off imperceptibly 

 one into the other : from the extreme grade where the changes 

 have been at a minimum and the soil particles, both coarse 

 and fine, are complex in composition : through the intermedi- 

 ate grades where more change has occurred, but a number of 

 minerals can still be found in the coarse particles : to the other 

 extreme where the coarse particles consist almost wholly of 

 silica, everything else having gone. The case of minimum 

 change is exemplified in arid soils. The intermediate case is 

 seen in soils derived direct from igneous or old rocks and 



1 R. Zsigmondy, Colloidal Chemistry, trs. E. B. Blair, 1917. 



2 See p. 355 for method. 



7* 



