by determining the angle of the holes, by identifying
and interpreting the samples, by studying bedding, cleavage, and other
structures as shown in the samples, and determining the attitude of
these structures in the ground, by determining when the horizon is
reached which is most promising for mineral, and by determining when the
hole shall be stopped. With a given set of surface conditions, the
problem of locating and directing a drill hole to secure the maximum
possible results for the amount expended requires the careful
consideration of many geologic factors,--and, what is more important,
their arrangement in proper perspective and relationship. Faulty
reasoning from any one of the principal factors, or over-emphasis on any
one of them, or failure to develop an accurate three-dimensional
conception of the underground structural conditions, may lead to failure
or extra expense. Success or failure is swiftly and definitely
determined. The geologist is usually employed by the company financing
the drilling; but in recognition of the importance of his work, some of
the large contracting drill companies now employ their own geologists.
The technique of the geologic interpretation and direction of drilling
has become rather complicated and formidable, and has resulted in the
introduction of special college courses in these subjects.

The desirability of public registration of drilling records is discussed
on another page (pp. 305-306).

QUANTITATIVE ASPECTS OF GEOLOGIC EXPLORATION

In recent years there has been a tendency to reduce the geologic factors
in exploration to some kind of a quantitative basis. While these factors
may be very variable and very complex, their net effect frequently may
be expressed in terms of quantitative averages. In various mines and
mining districts where operations are of wide extent, local quantitative
factors have been worked out which are useful in predicting results from
proposed explorations in undeveloped portions. Figures of this sort may
be useful and practical guides in planning any given exploration, its
cost, and its probable outcome.

Quantitative methods are illustrated in the general account of Lake
Superior iron ore exploration in a later section.

Curves of production from oil wells and from oil districts have been
found to have certain characteristic features in common which are often
used in predicting the future output and life of a given well, property,
or distri