Throughout the world coals occur in scattered localities in each of the major post-Silurian stratigraphic subdivision. The workable coal seams are, however, principally included in the sediments of three ages, (1) the Upper Carboniferous-Permian, (2) the Upper Triassic-Jurassic, and (3) the Upper Cretaceous-Middle Miocene. These considerable coal formation in the world were coincident with the development of the land plants.
Coal deposits exist in many countries of the earth. However, the largest coal deposits are concentrated in some countries on the northern hemisphere, viz. the Union of Soviet Socialist Republics, the United States of America, the People's Republic of China as well as a few European countries. On the southern hemisphere, only two countries have important coal deposits, viz. Australia and Republic of South Africa.
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