191 



Bureau of Soils,* but is far from being in general use. Thus Wolff and 

 Schone designate as " fine earth " all the soil that passes through a sieve 

 with holes 3 mm. in diameter : Knop applies the same term to that which 

 passes a J mm. mesh sieve. Whitney and some other American chemists 

 generally take 2 mm. Hilgard and others again take -| mm. 



There is an equally great variety in the methods of classifying the 

 several finer soil grades : thus W T ollnyf classifies the several grades as 



follows : 



Diameter of particles. 



Stones over 10 mm. 



Coarse gravel 5 10 mm. 



Medium gravel 2 ,, 



Fine gravel 1 



Coarse sand '5 



Medium sand '25 '5 



Fine sand '1 '25 



Coarse silt '05 '1 ,, 



Medium silt '025 '05 



Fine silt '005 '025 



Clay '0001 '005 



The following is Hilgard's complete classification : 



Diameter of particles. 



Coarse grits 1 3 mm. 



Fine grits "5 1 



Coarse sand '4 ,, 



Medium sand '3 ,, 



Fine sand '16 



Finest sand '12 ,, 



Dust sand '072 



Coarsest silt '047 



Coarse silt '036 



Medium silt '025 



Fine Silt '015 ,, 



Finest silt '008 



Clay t 



While there has been, of late years, a gradual approach to uniformity 

 of nomenclature, a sufficiently wide variation still exists to render the use 

 of mere names valueless unless the method of analysis is also distinctly 

 stated. 



Clay, for instance, is mostly determined by a process of sedimentation. 

 In the investigations recorded in these pages, the soil water has been 

 allowed to stand for 24 hours at a time, before drawing off the clay water, 

 but many a soil analyst would terminate the sedimentation within a much 

 shorter period; three hours, for instance, or one hour, or perhaps only 

 fifteen minutes. Obviously results thus obtained are not comparable 

 amongst themselves; and, as the finely divided clay exercises a most im- 

 portant influence on the agricultural value of the soil, it is very necessary 

 to know how its proportion has been determined, and to be assured that 

 what is called clay does not actually also include silt. 



The importance of silt and clay as component parts of any soil is one 

 of the chief reasons for the systematic mechanical analyses of soils. Not 

 only does the clay bind together soil particles which would otherwise col- 

 lapse into drift sands, but its own fineness of texture renders soils which 



* Except for the coalescing of the " silt " and " fine tilt " into one grade. 

 f Vide Experiment Station Record, vi, p. 762. 



