e may be due
to upward escape of oil material through joints in the rocks capping a
reservoir, and productive pools may be found directly below such
showings. In other regions similar surface indications may mean that the
stratum in the outcrop of which they are found is oil-bearing; but
accumulations of oil, if present, may be several miles down the dip, at
places where the structural conditions have been favorable. In still
other cases the seepage may have been in existence for such a long time
as to exhaust the reservoir. It must also be remembered that gas seeps
are common in sloughs and marshes where vegetation is decaying, and may
be of no significance in the search for petroleum.

[29] Arnold, Ralph, Conservation of the oil and gas resources of the
Americas: _Econ. Geol._, vol. 11, 1916, pp. 321-322.

[30] Oil shales may also be made to yield large quantities of fuel and
illuminating gas, and of ammonia (see pp. 101-102).

CHAPTER IX

MINERALS USED IN THE PRODUCTION OF IRON AND STEEL (THE FERRO-ALLOY
GROUP)

GENERAL FEATURES

Iron and steel and their alloys are the most generally used of the
metals. The raw materials necessary for their manufacture include a wide
variety of minerals.

Iron is the principal element in this group; but in the manufacture of
iron and steel, manganese, chromium, nickel, tungsten, molybdenum,
vanadium, zirconium, titanium, aluminum, uranium, magnesium, fluorine,
silicon, and other substances play important parts, either as
accessories in the furnace reactions or as ingredients introduced to
give certain qualities to the products.

Nearly all parts of the world are plentifully supplied with iron ores
for an indefinite period in the future, but their abundant use has thus
far been confined mainly to the countries bordering the North
Atlantic,--the United States, Germany, and England,--which, possessing
ample coal supplies, have had the initiative to develop great iron and
steel industries. China has abundant coal, moderate quantities of iron
ore, and a large population, but a low per capita consumption of iron
and steel products. Development of its iron and steel industry is just
beginning. Japan has neither coal nor iron in sufficient quantities, and
hence the Japanese effort in recent years to control the mineral
resources of China and other countries. As a result of the war Germany
has been largely deprived of its iron ores, and France may assume
somewhat the rank i