Types of Soil 33 



though calcareous sands occur. Where surface-moisture 

 accumulates, e.g. owing to the occupation of certain kinds 

 of vegetation, acid peaty humus is formed, apparently 

 largely owing to the deficiency of lime. Very fine-grained 

 sandstones weather to a soil indistinguishable physically 

 from clay, and bearing the same kind of vegetation. 



2. Loam. This is formed of a mixture of sand and 

 clay and results from the weathering of impure sandstones 

 and similar rocks and from mixed alluvial deposits. It 

 is more retentive of water than the purer sands, and 

 where a fair proportion of lime is present forms the best 

 agricultural soils. 



3. Clay. The purer clays are mainly formed of hy- 

 drated aluminium silicate, are typically fine-grained, and 

 very retentive of moisture. They crack when dry and 

 form close heavy soils. Clay deposits, bearing typical 

 clay soils, form the " rock " of much of the midlands of 

 England. When clay is very deficient in lime it may, 

 like sand, if kept wet, accumulate an acid peaty humus. 



4. Siliceous soils of the older rocks. The older, 

 non-calcareous rocks, which have become indurated and 

 compacted by pressure, though composed of the same 

 lithological constituents as the more recent sediments, 

 often jrield soils which have somewhat different characters 

 from those of the newer sediments. Though technically 

 they may be classified into sands, loams and clays, ac- 

 cording as the particles are mainly coarse, a mixture of 

 coarse and fine, or mainly fine, yet they often differ 

 in respect of the vegetation they bear from the soils 

 produced by more recent deposits. This is true of the 

 majority of the Paleozoic and also of the metamorphic 

 and igneous rocks. Thus the older Palaeozoic shales and 

 slates, which correspond lithologically with the more 

 modern clays, do not bear the same vegetation as the 

 clays, and the same may be said of the quartzites and 

 many of the older sandstones and grits, which correspond 



T. 3 



