of the sources of oil which is likely to become important in the
future is oil shales,--that is, shales from which oil product can be
extracted by distillation. These have already been referred to on
previous pages. Such shales are now mined only in Scotland and in France
to a relatively small extent, but there are immense reserves of these
shales in various parts of the world which are likely to be drawn upon
when commercial conditions require it. In the United States alone it is
estimated that the oil shales are a potential source of oil in amounts
far greater than all the natural petroleum of this hemisphere.[30] The
solution of the problem of extraction of oil from shales is fairly well
advanced technically, and the problem has now become principally one of
cost. In order to recover any large amount of oil from this source,
operations of stupendous magnitude, approximately on the scale of the
coal industry, must be established. As long as there are sufficient
supplies of oil concentrated by nature to be drawn upon, it is unlikely
that oil shale will furnish any considerable percentage of the world's
oil requirements. With the great increase in world demand for oil,
however, which may very possibly outstrip the available annual supply in
the future, and particularly with the increase in the United States
demand relative to domestic supplies, exhaustive surveys of the
situation are being made with a view to development of oil shales when
warranted by market conditions.

Oil shales are sedimentary strata containing decomposed products of
plants and animals. Locally they grade into cannel coal, with which they
are genetically related. They may be regarded as representing the kinds
of sediments from which the oil of oil pools has in the main originated.

The most extensive of the oil shales of the United States are found in
the Eocene beds of northwestern Colorado, northeastern Utah, and
southwestern Wyoming, and in the Miocene beds of northern Nevada. The
largest known foreign deposits occur in Brazil and Russia.

NATURAL GAS

ECONOMIC FEATURES

Natural gas is used both for lighting and for fuel purposes. In the
United States it has become the basis of a great industry, the value of
the product ranging above that of lead and zinc. The United States is
the largest producer of natural gas. Other producers are Canada, Dutch
East Indies, Mexico, Hungary, Japan, and Italy. Nearly all producing oil
fields furnish als