Causes of Fertility or Barrenness of Soils. 173 
Silica G4 
Alumina 20 
Potass 11 
Lime (traces). 
Oxitlc oi' iroti 1*75 
Water 275 
99-50 
It is found impossible to classify soils geologically, because 
the beds vary so extremely, and the soil has so frequently been 
derived from various sources ; therefore we have recourse either 
to physical or chemical distinctions : the former being most 
easily recognised are usually adopted. 
I. — Mechanical distinctions of Soils, as cohesiveness or 
POROUSNESS, coarseness OR FINENESS OF GRANULATION. 
We are in the habit of speaking of soils as light or stiff, 
loamy or marly, yet from such descriptions little correct know- 
ledge of their real nature can be gained. It is true that the 
primary forms of soil, such as stiff cLiy, pure sand, chalky or 
peaty soils, may be well expressed by such terms ; but in nature 
we have most frequently to deal with combinations of tlie above 
in the most varied proportions ; very frequently the distinctive 
characters are completely masked from observation, so that the 
appearance alone would often lead to very erroneous judgments 
on the qualities of soils. A good classification based upon the 
physical condition and chemical combinations of the various 
ingredients is much wanted. In the absence of such I shall 
adopt the faulty method of dividing all soils into four groups, 
bearing the names of their chief constituents : — 
Argillaceous or Clay soils, so called because clay {aryilla), or 
alumina, forms the principal ingi-edient. Sir Humphry Davy 
considers the term should only be applied to such as contain 
above l-6th of impalpable matter which does not effervesce in 
the presence of an acid. This division is again divided many 
times according to the presence of other substances, such as sand, 
lime, or peat, causing variations from the original type. 
Silicious soils are those which contain a large amount of silica 
or sand in an uncombined state. Davy considers the term ap- 
plicable only to soils containing 7-8 ths of their bulk of sand. 
Perhaps this is going too far ; we may with more propriety allow 
upwards of 70 per cent, as the minimum quantity. 
Calcareous soils contain a large proportion of carbonate of 
lime, above 20 per cent. ; and lastly. 
Humous or Peaty soils, which include all that contain above 
5 per cent, of vegetable matter. Such soils generally result from 
