i] NATURAL HISTORY OF THE SOIL 3 



these continuous disintegrating processes. But the 

 action of these processes is exceedingly slow, or the 

 particles would have disappeared entirely, or have 

 become reduced to an impalpable dust. The fact 

 that they survived proves them to be very resistant 

 and indicates that they are not likely to undergo any 

 appreciable change during the short period of time 

 that interests the agriculturalist. Over longer periods 

 of time, however, the different particles show different 

 degrees of resistance, the most resistant being the 

 grains of quartz and the least resistant the more 

 complex combinations of silica and oxides of iron, 

 aluminium, potassium and other metals. The latter 

 have therefore suffered more than the quartz and 

 have been reduced to much finer dimensions. Thus 

 if a soil is separated out by mechanical analysis into 

 portions, the particles of which fall within certain 

 definite limits of size, it will be found that the 

 coarsest particles of all the stones and gravel 

 represent complex rock material, the coarse particles 

 of the fine earth (the so-called coarse sand, fine sand 

 and silt) are practically pure silica, while the finest 

 particles (the clay and to a less extent the fine silt) 

 contain not only silica, but oxides of iron, aluminium 

 and of other metals as well. Further, the top 8 inches 

 of soil that has been exposed to weathering processes 

 for very long periods of time contains practically as 

 much coarse material (silica) as the subsoil which has 



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