oil, either by or
without the agency of water. Iron carbide is magnetic, and significance
has been attached to the general correspondence between the locations of
oil in the western United States and regions of magnetic disturbance.

It seems not unlikely that some inorganic theory of this sort is
necessary to explain the ultimate source of oil or of the substances
which become oil, but the evidence is overwhelming that organic agencies
have been mainly responsible for the principal oil pools now known.

=Oil exploration.= A simple geographic basis for oil exploration is the
fact that the major oil fields of the world are situated between 20 deg. and
50 deg. north latitude, and that thus far there are no major oil areas
within the tropics or within the southern hemisphere. This broad
generalization may have little value when exploration is carried
further. It has also been suggested that the geographic distribution of
oil corresponds roughly with the average annual temperatures, or
isotherms, between 40 deg. and 70. deg.[25] It is thought that this present
distribution of temperatures may indicate roughly the temperatures of
the past when the oil was accumulated; and the inference is drawn that
there was some sort of limitation of areal deposition within these
temperature limits. If this be true, the only reasons why the southern
hemisphere is not productive are the relatively small size of the land
areas and the lack of exploration to date.

In approaching broadly the problem of oil exploration, the geologist
considers in a general way the kinds and conditions of rocks which are
likely to be petroliferous or non-petroliferous. Schuchert[26]
summarizes these conditions for North America as follows:

1. The impossible areas for petroliferous rocks.

(_a_) The more extensive areas of igneous rocks and especially
those of the ancient shields; exception, the smaller dikes.

(_b_) All pre-Cambrian strata.

(_c_) All decidedly folded mountainous tracts older than the
Cretaceous; exceptions, domed and block-faulted
mountains.

(_d_) All regionally metamorphosed strata.

(_e_) Practically all continental or fresh-water deposits; relic
seas, so long as they are partly salty, and saline lakes are
excluded from this classification.

(_f_) Practically all marine formations that are thick and uniform
in rock character and th