ogic series where oil is associated with coal. Where
the coal is in one series and the oil in another, separated by
unconformity (indicating different conditions of development), the
principle may not hold, even though there is close geographical
association.

The oil and gas distillates migrate upward under gas pressure and under
pressure of the ground-water. If there are no overlying impervious beds
to furnish suitable trapping conditions, or conditions to retard the
flow, the oil may be lost. The conditions favorable for trapping are
overlying impervious beds bowed into anticlines, or other structural
irregularities, due either to secondary deformation or to original
deposition, which may arrest the oil in its upward course. A dome-like
structure or anticline may be due to stresses which have buckled up the
beds, or to unequal settling of sediments varying in character or
thickness; thus some of the anticlinal structures of the Mid-Continent
field may be due to settling of shaley sediments around less
compressible lenses of sandstone which may act as oil reservoirs, or
around islands which stood above the seas in which the oil-bearing
sediments were deposited and on the shores of which sands capable of
acting as oil reservoirs were laid down. Favorable conditions for
trapping the oil may be furnished by impervious clay "gouges" along
fault planes, or by dikes of igneous rock. Favorable conditions may also
be merely differences in porosity of beds in irregular zones, determined
by differences either in original deposition or in later cementation.

The thickness of oil-producing strata may vary between 2 or 3 feet and
200 feet. The porosity varies between 5 and 50 per cent. In sandstones
the average is from 5 to 15 per cent. In shales and clays, which are
commonly the impervious "cap-rocks," porosity may be equally high, but
the pores are too small and discontinuous to permit movement.

When the impervious capping is punctured by a drill hole, gas is likely
to be first encountered, then oil, and then water, which is usually
salty. The gas pressure is often released with almost explosive
violence, which has suggested that this is an important cause of the
underground pressures. It has been supposed also that the pressures are
partly those of artesian flows. The vertical arrangement of oil, gas,
and water under the impervious capping is the result of the lighter
materials rising to the top. In certain fields, oil and g