n by
the addition or subtraction of the intervals between them. For instance,
knowing the succession, an outcrop of a certain sandstone may indicate
that the marking horizon is 200 feet below, and the structural contour
is then drawn accordingly. Observations of strike and dip at the surface
are helpful; but where the beds are but slightly flexed, small
irregularities in deposition may make strike and dip observations
useless in determining major structures. It is then necessary to have
recourse to the elevations of the marking horizons.

In the selection of key horizons, knowledge of the conditions of
sedimentation is very important. For example, some of the oil fields
occur in great delta deposits, where successive advances and retreats of
the sea have resulted in the interleaving of marine and land deposits.
The land-deposited sediments usually show great variations in character
and thickness laterally and vertically; and a given bed is likely to
thin out and disappear when traced for a short distance, rendering
futile its use as a marker. The marine sediments, on the either hand,
show a much greater degree of uniformity and continuity, and a bed of
marine limestone may extend over a large area and be very useful as a
key horizon.

Over large areas outcrops and records of previously drilled water and
oil wells may not be sufficient to give an indication of structure; it
then becomes necessary to secure cross sections by drilling shallow
holes to some identifiable bed, and to determine the structure from
these cross sections, in advance of deeper drilling through a favorable
structure thus located. The cooperative effort of the Illinois State
Survey and private interests, cited on page 306, is a good illustration
of this procedure. This method is only in its infancy, because
well-drilling has not yet exhausted the possibilities of structures
located from surface outcrops.

The so-called anticlinal structures, which have been found by experience
to be so favorable to the accumulation of oil, are by no means
symmetrical in shape or uniform in size. They may be elongated arches
with equal dip on the two sides, or one side may dip and the other be
nearly flat. In a territory with a general dip in one direction, a
slight change in the angle, though not in the direction of dip,
sometimes called an arrested dip, may cause sufficient irregularity to
produce the necessary trapping conditions. In other cases the anticline
m