ike
sandstones or cavernous limestones, where, under suitable conditions,
they have become trapped.

The original organic material is believed to have been plants of low
order and animal organisms (such as foraminifera) which were deposited
as organic detritus with mud and marl in the bottoms of ponds, lakes,
estuaries, and on the sea bottom,--in both salt and fresh waters.
Bacteria are supposed to have played a part in the early stage of
alteration, sometimes called the biochemical stage. When the organic
matter was buried under later sediments and subjected to pressure,
physical conditions were responsible for further volatilization or
distillation. This stage is called the geochemical stage. There is much
in common as to origin between coal, oil shales, and petroleum.
According to White,[22]

whether the ingredient organic matter, be it plant or animal,
will be in part transformed to coal of the ordinary type, to
cannel, to oil shale, to the organic residues in so-called
bituminous shales and carbonaceous shales, or to petroleum and
natural gas, is dependent upon the composition of the
ingredient organic debris, the conditions of its accumulation
or deposition, and the extent of the microbian action.

White has further developed the important principle that, in the
geochemical stage of development, both coal and oil react to physical
influences in much the same way; and that therefore when both are found
in the same geologic series, the degree of concentration of the coal,
measured by its percentage of carbon, may be an indication of the stage
of development of the oil. More specifically where the coal contains
more than 65 to 70 per cent of fixed carbon, chances for finding oil in
the vicinity are not good (though commercial gaspools may be found),
probably for the reason that the geochemical processes of distillation
have gone so far as to volatilize the oils, leaving the solid residues
in the rock. White also finds that the lowest rank oils, with
considerable asphalt, are found in regions and formations where the
coal deposits are the least altered, and the lighter, higher rank oils,
on the whole, where the coal has been brought to the correspondingly
higher ranks; in other words, up to the point of complete elimination of
the oil, improvement in quality of the oil accompanies increased
carbonization of the coal. The principle, therefore, becomes useful in
exploration in geol