is, that heavy clay soils are very rare in arid regions; if found at
all, they have probably been formed under abnormal conditions, as in
high mountain valleys, or under prehistoric humid climates.

_Sand.--_The sand-forming rocks that are not capable of clay
production usually consist of _uncombined silica _or quartz, which
when pulverized by the soil-forming agencies give a comparatively
barren soil. Thus it has come about that ordinarily a clayey soil is
considered "strong" and a sandy soil "weak." Though this distinction
is true in humid climates where clay formation is rapid, it is not
true in arid climates, where true clay is formed very slowly. Under
conditions of deficient rainfall, soils are naturally less clayey,
but as the sand and silt particles are produced from rocks which
under humid conditions would yield clay, arid soils are not
necessarily less fertile.

Experiment has shown that the fertility in the sandy soils of arid
sections is as large and as available to plants as in the clayey
soils of humid regions. Experience in the arid section of America,
in Egypt, India, and other desert-like regions has further proved
that the sands of the deserts produce excellent crops whenever water
is applied to them. The prospective dry-farmer, therefore, need not
be afraid of a somewhat sandy soil, provided it has been formed
under arid conditions. In truth, a degree of sandiness is
characteristic of dry-farm soils.

The _humus _content forms another characteristic difference between
arid and humid soils. In humid regions plants cover the soil
thickly; in arid regions they are bunched scantily over the surface;
in the former case the decayed remnants of generations of plants