nd the lines more sharply drawn, with the result
that the geologist is perhaps not so likely to venture into problems
which he is not qualified to handle.

The limits to geologic work here discussed are not necessarily limits
separating scientific from non-scientific work. The study and
determination of the qualities of rocks necessary for commercial
purposes is fully as scientific as a study of the qualities commonly
considered in purely geologic work, and the results of technical
commercial investigations may be highly illuminating from a purely
geological standpoint. When a field of scientific endeavor has been
established by custom, any excursion beyond traditional limits is almost
sure to be regarded by conservatives in the field as non-scientific, and
to be lightly regarded. The writer is fully conscious of the existence
of limits and the necessity for their recognition; but he would explain
his caution in exceeding these limits on the ground of training and
effectiveness, rather than on fear that he is becoming tainted with
non-scientific matters the moment he steps beyond the boundaries of his
traditional field.

SOILS AS A MINERAL RESOURCE

Soils are not ordinarily listed as mineral resources; but as weathered
and altered rock of great economic value, they belong nearly at the head
of the list of mineral products.

ORIGIN OF SOILS

Soil originate from rocks, igneous, sedimentary, and "metamorphic" by
processes of weathering, and by the mixing of the altered mineral
products with decayed plant remains or _humus_. The humus averages
perhaps 3 or 4 per cent of the soil mass and sometimes constitutes as
much as 75 per cent. Not all weathered rock is soil in the agricultural
sense. For this purpose the term is mainly restricted to the upper few
inches or feet penetrated by plant roots.

The general process of soil formation constitutes one of the most
important phases of katamorphism--the destructive side of the
metamorphic cycle, described in Chapter II. Processes of katamorphism or
weathering, usually accompanied by the formation of soils, affect the
surface rocks over practically all the continental areas.

The weathering of a highly acid igneous rock with much quartz produces a
residual soil with much quartz. The weathering of a basic igneous rock
without quartz produces a clay soil without quartz, which may be high
in iron. Where disintegration has been important the soil contains an
abundance of t