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are often sent to various sites around the world to assess their
contents, determining whether or not they can be productively
mined for valuable substances ranging from garnets to coal. They
also work in the petroleum industry, assessing potential oil sites
and assisting with their maintenance.
As oil is not a quick distillate resulting from sudden heat as
that of a volcano; it comes from the slow distillation of organic
material in source rocks that were laid down as sediments on the
bottom of ancient seas. It is not found in underground lakes filling
vast caverns or in streams flowing in subterranean channels; it is
found in the minute pores or voids of reservoir rock such as
sandstones and some limestones. It does not collect in the troughs
and low places of such rocks; it accumulates in traps in their
higher parts, held there by water that fills the pore spaces of the
rest of the reservoir rock.
The three essentials for a commercial accumulation of oil
are, therefore, source beds, reservoir beds, and traps, and these are
the things for which an oil geologist looks. The source beds will
probably be shales or limestones which when they are deposited as
mud or ooze on the sea floor millions of years ago, contained an
abundance of organic remains, either animal or vegetable or both.
The reservoir beds will probably be sandstone, but may be one of
the porous varieties of limestones or its cousin dolomite. The
structure or trap will probably be an arch or uphold in the strata,
formed by the folding or settling of the earth's crust in the course
of some crustal readjustment, or a place where the continuity of
the reservoir bed is interrupted or terminated.

Degrading forces

The face of nature is not a fixed and static thing. Rain, snow,
frost, wind, and vegetation, the plastic surgeons of geologic time,
are busy remolding it. Frosts and roots shatter the rocks within
their reach. Wind carries sand and acts as a sand blast, wearing

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