334 SOIL TYPES [chap. 



may be manifest even within the confines of a single 

 field. But, in the main, each soil type has a well-defined 

 area, within which it presents a reasonably constant 

 composition and texture, and though the boundaries 

 cannot be laid down with the precision of the outcrop 

 of a stratum, the zone of transition from one type to 

 the other may be indicated with approximate accuracy. 

 The basis upon which any soil survey must be 

 constructed is the origin of the soils ; each geological 

 formation, for example, will give rise to a distinct type 

 of soil if it has been formed in situ ; should the 

 weathered material have further undergone transport 

 by water, two or more types may have been constructed 

 by the sorting action of the water. It is also well 

 known that a geological formation may change very 

 considerably in lithological character in passing from 

 the lower to the upper portions of the bed. For 

 example, stiff as the London Clay is, the upper beds 

 become increasingly sandy in character, so that it is 

 not easy to draw a line of demarcation between the 

 soil arising from these beds and those due to the Bag- 

 shot Sands above. The lower beds of the Gault Clay 

 are also very pure, and give rise to a stiff clay deficient 

 in calcium carbonate ; the upper beds become marly, 

 and form soils indistinguishable from those due to the 

 contiguous Chalk Marl. Similarly, a geological forma- 

 tion may show a progressive change of character in 

 passing into a different area, which change will be 

 reflected in the soils derived from them. For example, 

 the Great Oolite limestones of the Cotswolds shade 

 off into sandstones in Northamptonshire, and the Hythe 

 beds of calcareous sandstone in East Kent become pure 

 coarse-grained sandstones in West Surrey. However, 

 in the main, geological origin may be taken as the 

 basis of a soil survey, to which must be added the 



