274 ECONOMIC GEOLOGY

huge works necessary, is still economically impracticable.
Wind may be used for pumping and for small sources of
power, but it is too uncertain for many industrial purposes.
Subterranean heat is being tapped in areas of volcanic
activity, and solar heat in dry tropical countries; but these
sources are handicapped by the fact that they cannot be
utilized on a large scale except near great markets, with cheap
supplies of ores, fluxes, and other raw materials. There is
no apparent alternative to coal as the source of power for
the primary industries, except in limited areas.