9S THE ORIGIN AND NATURE OF SOILS [chap. 



thousandth of an inch in diameter (o-ooo2 mm.), there 

 is rarely as much as 30 per cent. In some very wet and 

 heavy working soils, of which No. 8 is an example, the 

 percentage of true clay is not especially great, but it is 

 associated with high percentages of the fractions next in 

 fineness — the fine silt and the silt — and these, in the 

 absence of coarse sand, are almost as retentive of water 

 and clay-like as clay itself. Such a soil does not shrink 

 so much on drying, but it lies wet for a long time ; it is 

 excessively greasy and sticky after rain, and when 

 worked down to a fine tilth will run together after rain 

 and cake on the top in a fashion special to itself. The 

 difficulty of working all heavy soils is much mitigated if 

 they contain an appreciable quantity of carbonate of lime, 

 because that substance constantly dissolves in the soil 

 water which has gathered carbon dioxide from the 

 decaying humus, and thus forms a solution of bicarbonate 

 of lime which keeps the clay particles flocculated. On 

 some formations marls occur containing a considerable 

 proportion of very fine-grained carbonate of lime mixed 

 with clay ; such soils are very sticky and work heavily, 

 but they crumble readily when dry, and never lie so wet 

 as the true clay soils. Lastly, the sandy soils vary very 

 much in character ; some sandy soils, in which the fine 

 sand and the silt are the predominant fractions, are 

 amongst the most valuable of soils for tillage purposes. 

 They are light, and so are cheaply worked ; no time is 

 lost, because they are fit for the plough almost immedi- 

 ately after rain, and yet for all the quickness with which 

 they dry, they do not lose their power of supporting the 

 crop with water during a drought. Such soils may 

 contain no more than 10 per cent, of clay, but owe their 

 good qualities to the general predominance of fractions 

 of medium coarseness. The really poor sands, which 

 are often so light (in the farmer's sense) as never to have 



