great areas (p. 149), which may likewise
be considered as forming mineral provinces; for them the term
"petroliferous provinces" has been used. The list might be extended
indefinitely. Knowledge of such group distributions of minerals is a
valuable asset to the explorer, in that it tends to localize and direct
search for certain classes of ores in certain provinces; also, within a
province, it tells the explorer what is to be normally expected as
regards kinds and occurrences of mineral deposits. In searching for
minerals of sedimentary origin, the explorer will use stratigraphic
methods in following definite sedimentary horizons. In searching for
ores related to igneous intrusions he will naturally hunt for the
intrusions, and then follow the periphery of the intrusions for
evidences of mineralization, taking into account possible features of
zonal arrangement of minerals about the intrusives (see pp. 42-44), and
the preference of the ores for certain easily replaced horizons like
limestones, or for certain planes or zones of fracturing.

Just as minerals may be grouped by provinces, they may be grouped by
geologic ages. Such groupings are especially useful in the case of
minerals which are closely related to certain stratigraphic horizons,
such as coal, oil, and iron. The greater number of the productive coal
deposits of the United States are of Carboniferous age, and the
distribution of sediments of this age is pretty well understood from
general geologic mapping. The Clinton iron ores all follow one general
horizon in the lower-middle Paleozoic. The Lake Superior iron ores are
pre-Cambrian, and over three-fourths of them occur at one horizon in the
pre-Cambrian. Gold deposits of the United States were formed mainly in
the pre-Cambrian, the early Cretaceous, and the Tertiary. Copper
deposits of the United States were formed chiefly in pre-Cambrian,
Cretaceous, and Tertiary time. While there are many exceptions and
modifications to general classifications of this sort, they seem to
express essential geologic facts which can be made very useful in
localizing exploration.

CLASSIFICATION OF MINERAL LANDS

In recent years there has been considerable development of the practice
of classifying mineral lands in given areas for purposes of exploration
and valuation, or for purposes of formulation and administration of
government laws. This has been done both by private interests and by the
government. These classificati