SOIL FORMATION AND STRUCTURE 5 



by water or by ice to regions often far removed from the 

 place of their original formation. Thus river sands and 

 gravels and the mud of estuaries and salt marshes have 

 been transported by rivulet, stream, and river from higher 

 levels to be deposited where the slower flow of the water 

 has let them settle. Great tracts of country are covered 

 with glacial drift of gravel or clay, planed off higher levels 

 and carried to where the melting face of the ice-sheet stood 

 for the moment. In drier climates soils may be transported 

 by wind, a case known in this country only in the relatively 

 small movements of the white sand dunes. 



In such cases the soil may bear no relation to the under- 

 lying rock. A prolonged geological history may intervene 

 between the initial degradation of the rock mass and the 

 final colonisation by an advanced plant community, yet the 

 various intermediate stages and the final result are essentially 

 the same as when the soil is found in situ. 



Soil Structure. — The chief solid constituents of a fertile 

 soil are : {a) quartz and mica, in the form of sand and the 

 finer silt ; ih) clay, the very finely divided colloidal silicates 

 of aluminium derived from felspar and chalk ; {c) humus, 

 organic matter, also colloidal, derived from the remains of 

 dead plants ; (d) sometimes a proportion of calcium carbo- 

 nate from chalk or limestone. The mass is moist and when 

 in good condition crumbles in the hand. 



This crumby property is an expression of the fact 

 that the soil is not a simple mixture of its various 

 solid constituents moistened with water, but that it has a 

 definite structure. That this is so is demonstrated very clearly 

 either by shaking the soil with water or by drying it out 

 completely. In the former case drying the sediment of 

 mud does not restore it to its proper condition ; in the latter 

 it is difficult to wet the dusty mass thoroughly, and when 

 this has been done only a sticky paste is produced. The 

 soil crumbs have been destroyed, and simple wetting or 

 drying, though it may restore the original degree of moisture, 

 does not reconstruct the crumbs. In agricultural practice 

 this fact is of great importance, for a soil if worked when 



