336 SOIL TYPES [chap. 



the river valleys, where alluvium of varied epochs and 

 rain-wash of not very different origin may be hope- 

 lessly intermingled. In many cases, however, where 

 the outcrop of the originating formations is broad, and 

 where the gradients of the country are slight, the soil 

 may be extremely constant in composition over a wide 

 area, so that the survey has only to notice such minor 

 variations as the grading from a lighter to heavier type 

 as one descends a slope, or the occasional influx of 

 drift material by creeping from a neighbouring area. 

 The further work of a field survey will be the selection 

 of typical samples of soil and subsoil for detailed 

 examination and analysis in the laboratory, the collec- 

 tion of such data as the distance to, and nature of 

 ground water, and any particulars which may be 

 available locally, as to special features in the working 

 of each type of soil, or in the growth of its crops ; 

 the nature and character of such deposits as " brick 

 earth " in each district can also be reported. The 

 samples for detailed analysis should be taken where a 

 general survey of the district indicates the soil as most 

 likely to be typical of the formation and free from 

 admixture with drift and other accidental intrusions. 

 At the same time, since the soil in the main will by no 

 means be so pure as the typical samples, a much larger 

 number of samples should be taken and subjected to 

 a less detailed examination, by way of ascertaining 

 within what limits the normal variation of the soil is 

 confined. 



The number of type samples to be taken must be 

 entirely decided by an examination of the circum- 

 stances ; in continental areas, where deposition has 

 been very uniform over wide districts, one or two 

 samples may be sufficient to characterise an extensive 

 soil type ; in other cases the local variations may be so 



