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.