and gas from the organic
matter in the rocks, and the squeezing out of the oil, gas, and water
into adjacent rocks, such as coarse round-grained sandstones and porous
limestones, which are more resistant to change of volume under pressure.
Migration, concentration and segregation of the oil, gas, and water is
supposed to be brought about, partly through the effect of capillary
forces--the water, by reason of its greater capillary tension, tending
to seize and hold the smaller voids, and thus driving the oil and gas
into the larger ones--and partly through the action of gravity.

White also suggests that the process may go further where the parent
carbonaceous strata are of such thickness and under such load of
overlying rocks that they undergo considerable interior adjustment and
volume change before yielding to stress by anticlinal buckling,--than
where the strata yield quickly. It is not clear to the writer that the
interior adjustment assumed under this hypothesis is necessarily slowed
up or stopped by anticlinal buckling. Interior stresses are inherent in
any sedimentary formation, when settling and consolidating and
recrystallizing under gravity, and these may be independent of regional
thrusts from without.

The first oils evolved by pressure from the organic mother substance are
probably heavy, the later oils lighter, and the oils from formations and
regions where the alteration is approaching the carbonization limit are
characteristically of the highest grade. This is the reverse of the
order of products obtained by heat distillation. Whether there is also a
natural fractionation and improvement of the first heavy oils as they
undergo repeated migrations is not known.

=Inorganic theory of origin.= Another theory of the source of oil has
had some supporters, although they are much in the minority. This is the
so-called "inorganic" theory, that oil comes from magmas and volcanic
exhalations. In support of this theory attention is called to the fact
that igneous rocks and the gases associated with them frequently carry
carbides or hydrocarbons; that many oil fields have a suggestive
geographic relationship with volcanic rocks; and that certain of the oil
domes, as for instance in Mexico, are caused by plugs of igneous rocks
from below. It has been suggested that deep within the earth carbon is
combined with iron in the form of an iron carbide, and that from the
iron carbide are generated the hydrocarbons of the