the desired degree of filth. Moreover, clay is very retentive of
water, gases, and soluble plant-foods, which are important factors
in successful agriculture. Soils, in fact, are classified according
to the amount of clay that they contain. Hilgard suggests the
following classification:--


Very sandy soils 0.5 to 3 per cent clay
Ordinary sandy soils 3.0 to 10 per cent clay
Sandy loams 10.0 to 15 per cent clay
Clay loams 15.0 to 25 per cent clay
Clay soils 25.0 to 35 per cent clay
Heavy clay soils 35.0 per cent and over


Clay may be formed from any rock containing some form of _combined
silica _(quartz). Thus, granites and crystalline rocks generally,
volcanic rocks, and shales will produce clay if subjected to the
proper climatic conditions. In the formation of clay, the extremely
fine soil particles are attacked by the soil water and subjected to
deep-going chemical changes. In fact, clay represents the most
finely pulverized and most highly decomposed and hence in a measure
the most valuable portion of the soil. In the formation of clay,
water is the most active agent, and under humid conditions its
formation is most rapid.

It follows that dry-farm soils formed under a more or less rainless
climate contain less clay than do humid soils. This difference is
characteristic, and accounts for the statement frequently made that
heavy clay soils are not the best for dry-farm purposes. The fact